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Transient pulsed plasma for pollution remediation and energy conversion applications
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Transient pulsed plasma for pollution remediation and energy conversion applications
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Content
TRANSIENT PULSED PLASMA FOR POLLUTION REMEDIATION AND ENERGY
CONVERSION APPLICATIONS
by
Sisi Yang
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(PHYSICS)
December 2020
Copyright 2020 Sisi Yang
Dedication
This thesis is dedicated to my beloved parents for their constant support.
献给我亲爱的父母以感谢他们长久以来的支持
ii
Acknowledgements
In the first place, I would like to express the gratitude to my academic advisor Prof. Stephen
B. Cronin for his guidance. Steve has always been a passionate scientist, which sets an inspiring
example for all of us. In addition to the advisable suggestions for science, I sincerely appreciate
his understanding and encouragement all these years, that has been a great support when I have
“bad” luck with the research.
I would also like to thank Prof. Stephan Hass, Prof. Aiichiro Nakano, Prof. Wei Wu and
Prof. Han Wang for being my dissertation committee members and qualifying exam committee
members.
After that, my sincere thanks go to all my colleagues. They are very generous on sharing
knowledge for scientific discussion, and they have been an important part of my personal life. All
the precious memories in the spare time help me to achieve a colorful life and maintaining the
work-life balance.
Meanwhile, I want to thank all my best friends from childhood even though we don’t see
each other often due to the long distance, which instead makes it possible whenever I need the
support from a friend, there is always someone that I feel totally comfortable with, so that I can
confide all the feelings, even though you already have your own busy lives.
Last but not the least, my gratitude goes to my boyfriend Bo Wang and my beloved parents.
They have been taking care of me from all aspects. I feel so fortunate to have them in company in
the different phases of my life, so that I can always regain the faith to step forward after setbacks.
iii
Their support and love make it possible that I always know I have the option to live for myself
with the least concern.
iv
TABLE OF CONTENTS
Dedication ......................................................................................................................................... i
Acknowledgements ........................................................................................................................ ii
List of Tables ................................................................................................................................ vii
List of Figures .............................................................................................................................. viii
Abstract ......................................................................................................................................... xii
Chapter 1 Introduction ..................................................................................................................... 1
1.1 Pollutants (PM, NOx/SO2/CO2): Sources, Effects and Removal Technologies ..................... 1
1.1.1 Particulate Matter (PM) ................................................................................................................................. 2
1.1.2 NOx (i.e., NO and NO2) ................................................................................................................................. 4
1.1.3 Sulfur Dioxide (SO2) ..................................................................................................................................... 5
1.1.4 Carbon Dioxide (CO2) ................................................................................................................................... 6
1.2 Transient Plasma .................................................................................................................... 7
1.3 Electric Discharges in Aqueous Solutions ............................................................................. 8
1.4 CO2 Reduction ....................................................................................................................... 9
1.5 Electrostatic Precipitator (ESP) ........................................................................................... 11
1.6 Li-ion Batteries .................................................................................................................... 12
Chapter 2 Transient Plasma-based Remediation of Nanoscale Particulate Matter in Restaurant
Smoke Emissions ........................................................................................................................... 15
2.1 Abstract ................................................................................................................................ 15
2.2 Introduction .......................................................................................................................... 15
2.3 Experimental Details ............................................................................................................ 17
2.4 Results and Discussion ........................................................................................................ 20
2.5 Conclusion ........................................................................................................................... 24
Chapter 3 Transient Plasma-enhanced Remediation of Oil-based Particulate Matter via
Electrostatic Precipitation .............................................................................................................. 26
3.1 Abstract ................................................................................................................................ 26
3.2 Introduction .......................................................................................................................... 26
3.3 Experimental Details ............................................................................................................ 29
3.4 Results and Discussion ........................................................................................................ 31
v
3.5 Conclusion ........................................................................................................................... 37
Chapter 4 Plasma-Enhanced SO2 Remediation in a Humidified Gas Matrix: A Potential Strategy
for the Continued Burning of High Sulfur Bunker Fuel ................................................................ 39
4.1 Abstract ................................................................................................................................ 39
4.2 Introduction .......................................................................................................................... 39
4.3 Experimental Details ............................................................................................................ 42
4.4 Results and Discussion ........................................................................................................ 45
4.5 Conclusion ........................................................................................................................... 49
Chapter 5 Plasma-Enhanced NOx Remediation using Nanosecond Pulsed Discharges in a Water
Aerosol Matrix ............................................................................................................................... 51
5.1 Abstract ................................................................................................................................ 51
5.2 Introduction .......................................................................................................................... 51
5.3 Experimental Details ............................................................................................................ 54
5.4 Results and Discussion ........................................................................................................ 57
5.5 Conclusion ........................................................................................................................... 60
Chapter 6 Recycling Diesel Soot Nanoparticles for Use as Activated Carbon in Li Ion Batteries
....................................................................................................................................................... 61
6.1 Abstract ................................................................................................................................ 61
6.2 Introduction .......................................................................................................................... 62
6.3 Experimental Details ............................................................................................................ 64
6.4 Results and Discussion ........................................................................................................ 66
6.5 Conclusion ........................................................................................................................... 69
Chapter 7 CO2 Reduction to Higher-order Hydrocarbons by Plasma Discharge in Carbonated
Water .............................................................................................................................................. 71
7.1 Abstract ................................................................................................................................ 71
7.2 Introduction .......................................................................................................................... 72
7.3 Experimental Details ............................................................................................................ 73
7.4 Results and Discussion ........................................................................................................ 75
7.5 Conclusion ........................................................................................................................... 79
Bibliography .................................................................................................................................. 80
Appendix A: Characterization of Electronic Propertied of Dual-gate Carbon Nanotube (CNT)
Field Effect Transistor (FET) Devices ......................................................................................... 102
vi
Appendix B: Emission Spectra of High V oltage Discharge Generated in Liquid Nitrogen ........ 109
vii
List of Tables
Table 1.1. Examples of chemicals and sources of air pollutions.
1-7
................................................ 1
Table 1.2. Classification for common sources of NOx emission.
20, 21
............................................. 4
Table 1.3. Examples of possible half-reactions of electrochemical CO2 reduction.
76
.................. 10
viii
List of Figures
Figure 1.1. (a) Common sources of particulate matter.
9, 10
(b) Typical composition of diesel
particulate matter.
10
.................................................................................................................. 2
Figure 1.2. Examples of particle size distributions of (a) diesel engine exhaust, (b) wood-burning
smoke, (c) hamburger cooking smoke and (d) soybean oil aerosols measured by scanning
mobility particle sizer (SMPS) spectrometer. .......................................................................... 3
Figure 1.3. Total U.S. greenhouse gas emission in 2018.
50
............................................................ 7
Figure 1.4. (a) A photograph of electrical discharge generated in DI water. Plasma emission spectra
of (b) atomic oxygen, (b) hydroxyl radical, (c) atomic hydrogen. .......................................... 9
Figure 1.5. Schematic diagram of a basic electrostatic precipitator. ............................................ 12
Figure 1.6. Illustration of the basic components of a Li-ion battery and the operation principles.
............................................................................................................................................... 13
Figure 1.7. Basic components of the LMO cathode of a Li-ion battery. ...................................... 14
Figure 2.1. (a) Schematic diagram of the experimental setup used to test the transient pulsed
plasma reactor. In this configuration, the plasma reactor is installed in parallel to a kitchen
ventilation system including a charbroiler, hood, duct, and blower. Here, only a fraction of
the full flow is passed through the reactor. (b) Photograph of the transient plasma (high
electron energy, low-temperature plasma), (c) typical particle size distribution, and (d) output
characteristics of the nanosecond pulse generator. ................................................................ 18
Figure 2.2. Typical waveform of the TPS Model 20X pulse generator obtained with a 50W load.
............................................................................................................................................... 19
Figure 2.3. Particle size distributions taken 15 minutes apart for different flow conditions of (a)
2.5 m/s and (b) 0.25 m/s. ....................................................................................................... 20
Figure 2.4. Particle number densities measured with and without the plasma treatment for different
flow conditions of (a) 2.5m/s and (b) 0.25m/s. The integrated peak areas are indicated in the
figures. Here, the pulse generator was operating at a peak voltage of 17kV , pulse repetition
rate of 1200 Hz, and continuous power of 75W. ................................................................... 21
Figure 2.5. Relative particle mass measured with and without the plasma treatment for different
flow conditions of (a) 2.5m/s and (b) 0.25m/s. The integrated peak areas are indicated in the
figures. Here, the pulse generator was operating at a peak voltage of 17kV , pulse repetition
rate of 1200 Hz, and continuous power of 75W. ................................................................... 22
Figure 2.6. Integrated particle numbers (i.e., total particle number) plotted as a function of pulse
repetition rate taken under low flow conditions. ................................................................... 22
Figure 2.7. (a) Particle number densities measured for various pulse generator input voltages. (b)
Integrated peak areas (i.e., relative particle mass) plotted as a function of pulse generator input
voltage taken under low flow conditions. .............................................................................. 23
ix
Figure 3.1. (a) CAD drawing of the plasma-based reactor for restaurant particulate emissions
remediation. (b) Photograph of the plasma discharge at the output port of the reactor system.
(c) Schematic circuit diagram of the experimental setup. ..................................................... 30
Figure 3.2. Waveform of a TPS 30X pulse generator. .................................................................. 31
Figure 3.3. Particle size distributions produced using PAO-4 oil in an aerosol particle generator.
These spectra were taken 30 minutes apart using a scanning mobility particle sizer (SMPS)
spectrometer. .......................................................................................................................... 31
Figure 3.4. Particle size distributions obtained with PAO-4 under applied DC voltages of (a) 5kV
and (b) 10kV both with and without the 30kV nanosecond pulsed plasma (pulse repetition
rate of 200Hz and electrically continuous power of 30W). ................................................... 33
Figure 3.5. Particle size distributions under 10 kV and 5 kV DC voltage only using PAO-4 oil in
an aerosol particle generator. ................................................................................................. 34
Figure 3.6. Particle size distributions obtained with soybean oil under applied DC voltages of (a)
2.5kV and (b) 5kV both with and without the nanosecond pulsed plasma running at a peak
voltage of 30kV , pulse repetition rate of 200Hz, and continuous power of 30W. ................. 36
Figure 4.1. (a) Schematic diagram of the experimental setup used to test the transient pulsed
plasma reactor. (b) Typical output characteristics of nanosecond high voltage pulse generator
(USC-patented technology). (c) Photograph of the transient plasma (hot electron, low-
temperature plasma) formed by the high voltage nanosecond pulse. .................................... 44
Figure 4.2. Photographs of (a) the plasma discharge across a 5mm gap on a glass slide and (b, c)
glass-slide flow cell. (d) Plasma emission spectra taken in DI water and in 0.1M KOH. .... 45
Figure 4.3. Temperature dependent SO2 remediation study on approximately 600 ppmV synthetic
SO2 in a humidified air matrix. .............................................................................................. 47
Figure 4.4. (a) Plasma emission spectra of OH radicals observed from high voltage discharge in
aqueous solution. (b) SERS-enhanced vibrational spectrum of SO2-plasma exposed Ag
nanoparticles. ......................................................................................................................... 49
Figure 5.1. Possible chemical pathways for NO remediation via plasma-based treatment. ......... 53
Figure 5.2. (a) Schematic diagram of the experimental setup used to test the transient pulsed
plasma reactor. (b) Typical output characteristics of nanosecond high voltage pulse generator.
(c) Photograph of the transient plasma (hot electron, low-temperature plasma) formed by the
high voltage nanosecond pulse approach. ............................................................................. 56
Figure 5.3. NO and NOx gas concentrations with and without the plasma discharge under both dry”
(i.e., without water aerosol) and “wet” (i.e., water aerosol) conditions. (a) low plasma density
and (b) high plasma density. .................................................................................................. 58
Figure 5.4. (a) Plasma emission spectra of (a) OH radicals and (b) atomic oxygen observed from
high voltage discharge in aqueous solution. (c) SERS-enhanced spectrum of NO-plasma
exposed Ag nanoparticles. ..................................................................................................... 59
x
Figure 6.1. (a, b) High-resolution transmission electron microscope (HRTEM) images and (c)
scanning mobility particle sizer (SMPS) spectra of diesel soot particles (i.e., particulate
matter). ................................................................................................................................... 65
Figure 6.2. Current-voltage curves taken from (a) diesel engine exhaust and (b) Super-P
®
(commercially available activated carbon, typically used in Li-ion batteries). ..................... 67
Figure 6.3. (a) Discharging capacity of LMO electrodes containing Super P carbon, annealed soot
carbon, and unannealed soot carbon. The composite electrodes were cycled at 1C rate. (b)
Impedance plots of the electrodes before and after 100 cycles at 1C rate. ............................ 69
Figure 7.1. (a) Schematic diagram of the experimental setup used to take the in-situ plasma
emission spectra. (b) Typical output characteristics of nanosecond high voltage pulse
generator. Plasma Emission generated with copper tape electrodes in DI water and carbonated
water at (c) 13 kV and (d) 28 kV . .......................................................................................... 74
Figure 7.2. Plasma emission spectra of (a) C2 species, (b) O, (c) OH and (d) H radicals observed
from high voltage discharge in aqueous solution. ................................................................. 76
Figure 7.3. Cryogenic
1
H NMR spectra taken before and after 30-min plasma discharge in CO2-
saturated water of (a) formic acid and (b) acetic acid. (c) Liquid ion chromatography
measurements taken before and after 30-min plasma discharge in CO2-saturated water for
oxalate. ................................................................................................................................... 78
Figure A.1. Schematic diagram of a dual-gate CNT FET device. .............................................. 103
Figure A.2. (a) Linear plot and (b) semi-log plot of the source-drain current as a function of gate
voltage Vg = Vg1 = Vg2 at a constant bias Vbias = 0.2 V taken from a p-channel CNT FET. 103
Figure A.3. Semi-log plot of source-drain current plotted as a function of bias voltage at Vg1 = -
Vg2 = ± 10V , ± 8V , ± 6V , ± 4V . ............................................................................................ 104
Figure A.4. Semi-log plot of source-drain current plotted as a function of bias voltage at Vg1 = -
Vg2 = (a) -10~0V , (b) 0~10V ,. .............................................................................................. 104
Figure A.5. (a) (b) (c) (d) (f) Semi-log and (e) linear plot of source-drain current plotted as a
function of bias voltage at Vg1 or Vg2 = 10 V . ...................................................................... 105
Figure A.6. (a) (b) (c) (d) (f) Semi-log and (e) linear plot of source-drain current plotted as a
function of bias voltage at Vg1 or Vg2 = -10 V . .................................................................... 105
Figure A.7. (a) (b) Linear and (c) (d) semi-log plot of source-drain current plotted as a function of
bias voltage at Vg1 or Vg2 = 8 V . .......................................................................................... 106
Figure A.8. (a) (c) Linear and (b) semi-log plot of source-drain current plotted as a function of
bias voltage at Vg1 or Vg2 = -8 V . ......................................................................................... 106
Figure A.9. (a) (c) Semi-log and (b) linear plot of source-drain current plotted as a function of bias
voltage at Vg1 or Vg2 = 6 V . .................................................................................................. 107
Figure A.10. (a) Linear and (b) semi-log plot of source-drain current plotted as a function of bias
xi
voltage at Vg1 or Vg2 = -6 V . ................................................................................................ 107
Figure A.11. (a) Linear and (b) semi-log plot of source-drain current plotted as a function of bias
voltage at Vg1 or Vg2 = 4 V . .................................................................................................. 107
Figure A.12. (a) Linear and (b) semi-log plot of source-drain current plotted as a function of bias
voltage at Vg1 or Vg2 = -4 V . ................................................................................................ 108
Figure A.13. (a) Linear and (b) semi-log plot of source-drain current plotted as a function of bias
voltage at Vg1 or Vg2 = 2 V , 0 V . .......................................................................................... 108
Figure A.14. (a) Linear and (b) semi-log plot of source-drain current plotted as a function of bias
voltage at Vg1 or Vg2 = -2 V , 0 V . ......................................................................................... 108
Figure B.1. Emission spectra of high voltage discharge generated in liquid nitrogen displayed in
different wavelength regions. .............................................................................................. 109
xii
Abstract
This dissertation work presents several investigations related to the applications of transient
pulsed plasma on the remediation of common air pollutants including particulate matter (PM),
nitrogen oxides (e.g. NOx), sulfur dioxide (SO2) and carbon dioxide (CO2). Air pollution has
getting more and more attention from the public due to its growing tendency and harmful effects
on human health and natural ecosystems. Non-thermal plasma has been used for developing
pollution control technologies because of its substantially high energy electrons which reduce the
energy requirement for pollution removal. These research work presented in the dissertation
provide the promising results on pollution remediation and may inspire more possibilities for
applications of transient pulsed plasma.
Chapter 1 starts with an introduction of environmental effects and current removal
technologies of related pollutants, it provides some background information that can be helpful to
understand this dissertation work. The basic concepts of transient pulsed plasma will be discussed
in this chapter. It is then followed by an introduction of the properties of electric discharge
generated in liquid and the principle of electrostatic precipitators (ESPs). Then, the basic structural
components and functions of Li-ion battery will be briefly discussed to help understand Chapter 6
of this dissertation work.
Chapter 2 presents the work of transient plasma-based remediation of PM from restaurant
smoke emissions by characterizing of particle size and relative mass distribution. The effectiveness
and scalability of the technology are discussed for its practicability on higher flow rate and larger
xiii
systems.
In Chapter 3, we give a description of a follow-up research effort focused on the
remediation effects on oil-based nanoparticles by electrostatic precipitation with the assistance of
transient plasma. The results show a three-order-magnitude enhancement in the reduction of
nanoparticles, which provides a new approach in the design of electrostatic oil aerosol pollution
devices.
Chapter 4 and 5 report plasma enhanced remediation of SO2 and NOx (i.e., NO and NO2)
in the environment of water vapor or water aerosols. Reaction pathways and spectroscopic
evidence are discussed to explain the enhancement of the removal effects. As new regulations for
the emissions of SO2 and NOx are announced for better pollution control, our results represent a
promising approach to effectively enhance the total remediation in diesel exhaust.
A successful method to recycle diesel soot nanoparticles are discussed in Chapter 6. Here,
diesel soot particles are reused as conductive additives in lithium manganese oxide cathode in Li-
on batteries. The electrochemical characterization of the recycled diesel soot shows a better
performance than the commercially available activated carbon (i.e., Super P®) in the presented
configuration of Li-ion batteries.
Chapter 7 presents a project of CO2 reduction to higher order hydrocarbons by plasma
discharge in carbonated water. The plasma emission spectra exhibit Swan bands, which correspond
to C2 species, the cryogenic NMR spectra show peaks of formic acid and acetic acid, both of which
indicate the possibility of converting greenhouse gas to energy dense fuels via plasma discharges.
1
Chapter 1 Introduction
1.1 Pollutants (PM, NOx/SO2/CO2): Sources, Effects and Removal Technologies
The environmental problems caused by air pollutants have significant influences on the
basic living requirements of all the species, animals and plants in the ecosystem. A variety of
emission sources can produce harmful pollutants into the atmosphere, which lead to health
problems such as heart diseases, lung cancers and respiratory diseases.
1
As is closely related to the
progress of human society, many researchers and organizations have devoted to investigating the
sources, transmission and control technologies of these air pollutants. In the vast majority of
countries throughout the world, there are specific agencies that are monitoring and regulating these
air pollutants. The United States Environmental Protection Agency (EPA) has revised the national
air quality standard for fine particles (2006, 2012), ground-level ozone (2008, 2015), sulfur dioxide
(2010), nitrogen dioxide (2010) and lead (2008) based on the new scientific results to restrict the
pollution emissions.
2-6
The classifications and leading sources of common air pollution problems
are categorized in Table 1.1.
Air Pollutant Problems Chemicals Sources
Acid rain SO 2, NO x, HCl Burning of fossil fuels, industry exhaust
Greenhouse effect CO 2, CH 4, N 2O, O 3, CFCs Burning of fossil fuels for transportation
Ozone Depletion O 3 Manufactured chemicals
Toxic gases NH 3, CO, VOCs Industry exhaust
Eutrophication NO x, NH 3 Diesel exhaust, industry waste
Haze PM 10 and PM 2.5 Combustion of gasoline products
Table 1.1. Examples of chemicals and sources of air pollutions.
1-7
2
1.1.1 Particulate Matter (PM)
Particulate matter (PM), is a complex mixture of various suspended solid particles and
liquid droplets in air, which includes both organic and inorganic products, such as dirt, dust, soot
and smoke. These particulates can originate from a variety of activities, including household
services, transportation, agriculture and industrial processes.
8
The common sources of PM are
presented in Figure 1.1a, and Figure 1.1b shows a typical composition of diesel particulates.
9, 10
PM has been designated as a Group 1 carcinogen by the International Agency for Research on
Cancer (IARC), and long-term exposure to the ambient PM has been shown a close relationship
to lung cancers.
11-13
Many scientific works have reported the harmful health effect of PM on the
premature cardiovascular and respiratory deaths in metropolitan area.
14-16
Figure 1.1. (a) Common sources of particulate matter.
9, 10
(b) Typical composition of diesel particulate matter.
10
The size of PM ranges from few nanometers to few microns for different sources. Figure
1.2 shows some examples of particle size density distribution of diesel exhaust, wood smoke and
oil-based smoke measured by a scanning mobility particle sizer (SMPS) spectrometer. The
particulates are further divided into coarse (PM10 - PM2.5) and fine particles (PM2.5) based on its
aerodynamic diameter. PM10 generally indicates inhalable coarse particles with the aerodynamic
41%
25%
13%
7%
14%
Carbon
Unburned oil
Sulphate and water
unburned fuel
Ash and others
(a) (b)
25.60%
19.40%
12.70%
11.20%
9.50%
8.10%
7.70%
4.30%
1.50%
Energy industries
Road transport
Agriculture
Manufacturing/construction
Other transport
Other non-energy
Household and Services
Industrial processes
Other emissions
3
diameter below 10 µm, and PM2.5 refers to inhalable fine particles with the aerodynamic diameter
smaller than 2.5 µm. These fine and ultra-fine particulates with complex composition can get deep
into human body and even into the bloodstream, which can cause serious health problems.
17
The
main commercially available control technologies for particulate emissions include electrostatic
precipitators (ESPs), fabric filters, wet particulate scrubbers, mechanical/inertial collectors and
high temperature, high pressure (HTHP) particulate control.
18
The PM emission can also be
reduced by improving combustion efficiency to decrease the productions of incomplete
combustion. Use of cleaner fuels like low-ash distillate oil is also a substitution for residue oil.
19
Figure 1.2. Examples of particle size distributions of (a) diesel engine exhaust, (b) wood-burning smoke, (c)
hamburger cooking smoke and (d) soybean oil aerosols measured by scanning mobility particle sizer (SMPS)
spectrometer.
100
0.0
5.0E4
1.0E5
1.5E5
2.0E5
Particle Number Density (#/cm
3
)
Diameter (nm)
0% Loading
30% Loading
60% Loading
100% Loading
Diesel Exhaust
10 100
0.0
5.0E4
1.0E5
1.5E5
Particle Number Density (#/cm
3
)
Diameter (nm)
Wood Smoke
100
0.0
5.0E4
1.0E5
1.5E5
2.0E5
Particle Number Density (#/cm
3
)
Diameter (nm)
Soybean Oil
100
0.0
5.0E6
1.0E7
1.5E7
2.0E7
Particle Number Density (#/cm
3
)
Diameter (nm)
Hamburger Cooking
(a) (c)
(b) (d)
4
1.1.2 NOx (i.e., NO and NO2)
Nitrogen Oxides are a mixture of compounds which are composed of oxygen and nitrogen,
including nitric oxide (NO), nitrogen dioxide (NO2), nitrogen trioxide, nitrous oxide (N2O), etc.
The two most common and relevant oxides of nitrogen in air pollution are NO and NO2, which are
collectively known as NOx. NO is a colorless and odorless toxic gas, which is not soluble in water
and a highly reactive chemical that can be converted to NO2 when exposed to oxygen. NO2 is a
reddish-brown gas at room temperature with an acrid smell, it’s water-soluble and can react with
the chemicals in the atmosphere to cause many other pollution problems, such as acid rain.
Sources Examples
Natural Lighting, forest fire
Biogenic Agriculture fertilization, nitrogen fixing plants
Industrial
Thermal NO x Natural gas combustion (i.e., N 2 and O 2)
Fuel NO x Transportation fuel combustion (coal, diesel fuel, fossil fuel, etc.)
Prompt NO x Formation by molecular nitrogen in the early stage of combustion
Table 1.2. Classification for common sources of NO x emission.
20, 21
During the combustion of most types of fuels, NOx can be formed directly from the N2 and
O2 in the air or the nitrogen compounds in the fuel. Besides, it can also be from natural and biogenic
sources. Table 1.2 presents the common sources of NOx in the air. NOx can create a series of
environmental and health concerns.
20, 21
It can react with O3 or other chemical resulting in the haze
smog over cities.
22-24
Breathing in an environment of high concentration of NOx will cause
irritation of the respiration system, eyes and skin, and long-term explosion will lead to severe
respiratory diseases such as asthma.
25-27
NOx can also react with O2, H2O and other chemicals to
5
form acid rain and contribute to eutrophication.
28-32
The pollution control of NOx emission is
mainly two types of approaches: combustion control and post-combustion control. Combustion
control includes use of cleaner fuel, improvement of combustion efficiency (low-NOx burner, fuel
gas re-circulation), water/stream injection.
33
The main methods for post-combustion control are
using selective non-catalytic reduction (SNCR) systems and selective catalytic reduction (SCR)
systems.
34
1.1.3 Sulfur Dioxide (SO2)
Sulfur dioxide (SO2) is a colorless toxic gas with a pungent odor. Most SO2 pollution comes
from the combustion of fuels containing sulfur, such as coal, fossil fuels and oil, which are widely
used in trains, large ships and some diesel equipment. SO2 and other sulfur oxides can react with
the chemicals in air that contribute to acid rain and haze which will harm both urban and wilderness
areas.
35, 36
Short-term exposure to SO2 may cause severe expiratory dyspnea and increase the
incidence rate of respiratory diseases such as asthma. At a high level of SO2, it will result in
immediate irritation and burning of eyes, nose and throat. Many studies have shown that long-term
exposure to SO2 may cause dysfunction of lungs, hyposmia, bronchitis and headache.
37-39
SO2 can
also react with other chemicals to form fine particles that can penetrate deep in human body and
contribute to PM pollution.
40, 41
The strategies of controlling emission of SO2 include use of cleaner
fuel (low-sulfur fuel), reduction of sulfur in the feed, improvement in combustion technologies
and use of end-of-pipe control treatment of flue gas.
42
The International Marine Organization has
reduced the global limit for sulfur in fuel used on board ships from 3.5% to 0.50% m/m (mass by
6
mass), which will be responsible to 77% reduction of SO2 emission from ships, which is about 8.5
million tons per year.
43
In petroleum industry, the sulfur content of fuel is reduced by beneficiation,
which can remove 50% of pyritic sulfur, and fluidized-bed combustion (FBC) will emit less sulfur
oxides than other combustion technologies.
44
The two major methods for SO2 emission control are
sorbent injection and flue gas desulfurization which correspond to 70-90% of the total SO2
reduction.
45
Limestone forced oxidation scrubber and lime spray dryer scrubber are the
commercially available options for SO2 post-control.
34, 46
1.1.4 Carbon Dioxide (CO2)
Greenhouse gases, which include carbon dioxide (CO2), methane (CH4), nitrous oxide
(N2O) and fluorinated gases, have caused increasing concerns in the human society since these are
responsible for the climate change over the 150 years. CO2 emission is counted about 81% of the
total U.S. greenhouse gases in 2018.
47
In the past 250 years, the concentration of CO2 in the
atmosphere has increased from 280 ppm to 379 ppm.
48
In the biological carbon cycle, CO2 enters
the atmospheres by combustion of fossil fuels, solid waste and other biological materials, and it is
absorbed by the plants through photosynthesis. Due to the human activities, the amount of CO2
emission has exceeded the capacity of the current ecosystem, which causes the rising average
temperature of earth’s climate system. Besides, high levels of CO2 can also affect human health
by causing drowsiness, sweating, increased heart rate and blood pressure.
49
The largest source of
CO2 emission involving human activities is burning of fossil fuels for electricity, heat and
transportation. Figure 1.3 indicates the detailed percentage of each source of the total greenhouse
7
emission in the United States in 2018.
50
The main CO2 control technologies include CO2 capture
and sequestration, coal-to-gas conversion and heat rate improvement.
34
Figure 1.3. Total U.S. greenhouse gas emission in 2018.
50
1.2 Transient Plasma
Transient plasma, also categorized as non-thermal plasma, is a non-equilibrated plasma, in
which the electron energies are considerably higher than those of the components of the
background gas. Most electrical energy is received by those energetic electrons instead of being
transformed into the gas heat. Thus, the temperature of the electrons (10
4
~10
5
K) are much higher
than the temperature of the ions and gas molecules (300~10
3
K).
51
In the gas ionization processes
of transient plasma, the background gases are dissociated, excited or ionized by the collision with
these energetic electrons which are accelerated under high electric field, resulting in the production
of radicals, ions and more electrons. Other than the traditional approaches to initiate the gas
ionization such as high DC voltages and spark, the transient plasma discusses in this thesis is
generated by nanosecond pulsed power, in which the electric field collapse before a substantial
amount of current can flow, hence it stays in the transient, formative phase of an arc and consumes
28%
27%
22%
12%
10%
1%
Transportation
Electricity production
Industry
Commercial and Residential
Agriculture
Other
8
far less energy in the creation of the plasma.
In the formation of transient plasma in air, also for non-thermal plasma, vast active species
are generated at atmospheric pressure and room temperature, which allows various chemical
reactions that normally requires high temperature. The energetic electrons and those highly
reactive species such as atomic hydrogen (H), atomic oxygen (O), hydroxyl radicals (∙OH) can
participate in lots of reactions with other molecules in the atmosphere. As a result, there are a lot
of applications involving non-thermal plasma, such as thin film deposition, plasma etching,
plasma-catalysis interactions and decontamination of medical equipment.
52-55
In environmental
and biological field, non-thermal plasma can be used for removal of NOx, SO2 and particulate in
diesel exhaust, plasma assisted combustion, water treatment, air pollution treatment (VOCs, CO2
reduction), ozone production, etc.
56-61
1.3 Electric Discharges in Aqueous Solutions
Electrical discharges generated in liquid (most in water solution) is delivering high energy
to the aqueous medium which result in its accumulation and discharge ignition, in which the
plasma is created between two electrodes that are emerged in the water phase. Due to the relatively
high dielectric strength of the aqueous medium, it requires much higher voltage to generate a
plasma. Many studies use the point to plate electrode configuration to localize the electric filed to
increase the field intensity.
62-69
The free electrons are accelerated by the electric field applied on
the point, then collide with other molecules, which initializes the ionization. The high electric field
can also lead to the overheating and boiling of the aqueous medium near the points, where bubbles
9
are forms and then propagate in the solution.
70
The main active species generated in the plasma in
aqueous phase include hydroxyl radicals(∙OH), hydrogen peroxide (H2O2), atomic oxygen(O) and
atomic hydrogen(H).
66, 67, 71, 72
Figure 1.4 shows a photograph of an electric discharge generated in
water and spectroscopic evidences of some related radicals generated during the discharge. These
high reactive species can oxidize harmful organic compounds (e.g. benzene) and also react with
inorganic compounds (e.g. CO2).
61, 64, 73, 74
Figure 1.4. (a) A photograph of electrical discharge generated in DI water. Plasma emission spectra of (b) atomic
oxygen, (b) hydroxyl radical, (c) atomic hydrogen.
1.4 CO2 Reduction
With the rising level of the ambient CO2 in the atmosphere, people have performed large
770 775 780 785
2000
2500
3000
3500
4000
Wavelength (nm)
Counts
Atomic
Oxygen
645 650 655 660 665
9.0x10
3
1.2x10
4
1.5x10
4
1.8x10
4
2.1x10
4
Counts
Wavelength (nm)
H
920 925 930 935
0
500
1000
1500
2000
Wavelength (nm)
Counts
OH
(a) (b)
(d) (c)
DI Water
Plasma
Emission
10
numbers of studies on carbon capture and utilization. The electrochemical reduction of carbon
dioxide is a promising field of converting CO2 into useful products using electrical energy. The
first research of electrochemical reduction was conducted in 19
th
century using a zinc cathode with
the reduction product of CO. The mechanism of electrochemical CO2 reduction has been greatly
studied since then. The most difficult step in this process is the generation of the CO2
-
intermediate,
which has an extremely high energy barrier of -1.90V vs. NHE at pH 7.
75
Some possible half-
reactions of electrochemical CO2 reduction and the corresponding potentials are listed in Table
1.3.
76
The main paths of electrochemical CO2 reduction are divided into three groups based on
electrode materials and final products. The first group using Hg, Pb, Bi, which don’t bind with
CO2
-
, normally has formate or formic acid (HCOOH) in the final products. The second group of
metals, such as Au, Ag and Zn, will coordinate with CO2
-
and produce CO. The last path using
copper electrodes can reduce CO2 to higher energy-dense products such as hydrocarbons.
77-81
Half-reactions of CO2 reduction E
o
(V vs. NHE at pH 7)
CO 2 + e
-
→ COO
-
-1.90
CO 2 + 2H
+
+ 2e
-
→ HCOOH -0.61
CO 2 + 2H
+
+ 2e
-
→ CO + H 2O -0.53
CO 2 + 4H
+
+ 2e
-
→ HCHO + H 2O -0.48
CO 2 + 6H
+
+ 6e
-
→ CH 3OH + H 2O -0.38
CO 2 + 8H
+
+ 8e
-
→ CH 4 + 2H 2O -0.24
2CO 2 + 12H
+
+ 12e
-
→ C 2H 4 + 4H 2O 0.06
2CO 2 + 12H
+
+ 12e
-
→ CH 3CH 2OH + 3H 2O 0.08
Table 1.3. Examples of possible half-reactions of electrochemical CO 2 reduction.
76
Considering the high energy barrier of generating CO2
-
intermediate, which is also the rate-
limit step of CO2 reduction, technologies using non-thermal plasma are gaining increasing interest
11
due to its highly energetic electrons and reactive species generated in plasma, which provide a
strategy for overcoming the reaction barrier at room temperature. Plasma-based CO2 reduction
mainly includes (packed-bed) dielectric barrier discharges (DBDs), microwave plasma, gliding arc
(GA) discharges, spark discharges and nanosecond-pulsed discharged. The work discussed in the
thesis using the nanosecond pulsed transient plasma to achieve the CO2 conversion to higher order
hydrocarbons, which will be discussed with more details in Chapter 7.
1.5 Electrostatic Precipitator (ESP)
Electrostatic precipitation technologies have been widely used in filtration devices for
cleaning of industry process flue gases, combustion exhaust, residential air, etc. An electrostatic
precipitator (ESP) are capable to remove fine particles in flowing medium with a high collection
efficiency and a low pressure drop. As shown in Figure 1.5, the basic setup of an ESP contains a
discharge electrode, which generally has a sharp feature, and a collecting electrode, which mostly
has a broader surface. By applying a high DC voltage of several thousand volts between the two
electrodes, corona discharges are formed, which will ionize the gas near the discharge electrode,
generating ions and electrons. These ions can attach to the suspended particles in the flue gas, these
charged particles will move towards the collecting electrodes due to the electrostatic force and rest
on the electrodes, then the collected particles will be removed from the electrodes and disposed in
a safe manner. ESPs can be characterized by the shape of the collecting electrode, the direction of
gas-flow and the electrode geometry, which are correlated to the categories of cylindrical and plate
type, vertical gas-flow and horizontal gas flow, one stage and two stage, respectively.
82
They can
12
also be classified as dry and wet type ESP based on whether water is used or not on the collecting
electrode. The dry type ESP are used for removal of ash or cement in a dry environment, such as
air in ventilation and air condition systems, while the wet type ESP are usually used to treat wet
particles such as oil, acid and resin, that have a relatively high resistivity or corrosivity.
83-87
Figure 1.5. Schematic diagram of a basic electrostatic precipitator.
1.6 Li-ion Batteries
Li-ion battery is an advanced rechargeable battery technology that has been extensively
used in portable electronics, power tools, hybrid and electric cars due to its light weight, high
energy capacity and long cycle life. As the lightest metal element, lithium has the highest
electrochemical potential, which provides the largest energy capacity per weight. Same as other
batteries, a Li-ion battery has five components, which are a cathode (positive electrode), an anode
(negative electrode), a separator, an electrolyte and two current collectors (positive and negative).
Figure 1.6 shows the basic components and operation principals of a Li-ion battery. Lithium can
be stored in the cathode and anode. In the discharging or charging process, lithium ions can migrate
+ -
HVDC
Collecting
Electrode
Discharge
Electrode
Ionization Zone
(+) (+)
13
between the two electrodes in the electrolyte, which contains a dissociated lithium conducting salt.
The separator, which isolates the two electrodes from each other, is typically a microporous
polymer membrane, so that the lithium ions can exchange between the two electrodes. The
migration of lithium ions creates free electrons, the electrical current then flow to the power
devices through the current collectors.
Figure 1.6. Illustration of the basic components of a Li-ion battery and the operation principles.
The most commonly studied anode materials are graphite and lithium titanium oxide (LTO).
Graphite is a low-cost material, which has high electrical conductivity, high Li diffusively,
moderate cycle life and relatively low volume change during charging/discharging process, but
commercially available graphite has a limited volumetric capacity (330-430 mAh/cm
3
).
88-91
LTO
can provide a long cycle life, excellent thermal stability, high volumetric capacity and high safety,
but it has a relatively low energy density.
91-94
As shown in Figure 1.7, the positive electrode
e
-
e
-
e
-
e
-
e
-
e
-
e
-
e
-
e
-
e
-
Li
+
Li
+
Charging
Discharging
e
-
e
-
Anode Cathode
Separator
Current
Collector
Current
Collector
Electrolyte
14
(cathode) of Li-ion batteries is composed of multi-metal oxide materials with lithium, polymer
binders and carbon additives. Common cathode materials include lithium cobalt oxide (LiCoO2 or
LCO), nickel cobalt aluminum (NCA), nickel manganese cobalt (NMC), lithium manganese oxide
(LiMn2O4 or LMO) and lithium iron phosphate (LFP).
95
The cathode material that is discuss in
Chapter 6 of the thesis is LMO, which is a safer and cheaper material than the traditional material
LCO, but it has a limited cycle life. Polymer binders serve as glues that connect the particles of
active materials to provide sufficient electrical conductivity, durable mechanical strength and keep
the integrity of the electrode. The carbon additives are introduced to create the conductive network,
which influence the electrode capacity and battery performance. The practical performance of
carbon additives has only been studied with commercially active carbons.
96, 97
In Chapter 6, we
study the electrochemical performance of annealed diesel soot particles as carbon additives, which
provides the possibility of converting an abundant pollution to a valuable electrode material.
Figure 1.7. Basic components of the LMO cathode of a Li-ion battery.
Separator
Current Collector
Cathode
LiMn
2
O
4
(LMO)
Polymer
Binder
Carbon
Additives
15
Chapter 2 Transient Plasma-based Remediation of Nanoscale Particulate
Matter in Restaurant Smoke Emissions
This chapter is similar to Yang et al., published in Environmental Research
2.1 Abstract
Recent studies have shown that nanoscale particulate matter produced in commercial
charbroiling processes represents a serious health hazard and has been linked to various forms of
cancer and cardiopulmonary disease. In this study, we propose a highly effective method for
treating restaurant smoke emissions using a transient pulsed plasma reactor produced by
nanosecond high voltage pulses. We measure the size and relative mass distributions of particulate
matter (PM) produced in commercial charbroiling processes (e.g., cooking of ham- burger meat)
both with and without the plasma treatment. Here, the plasma discharge is produced in a 3”
diameter cylindrical reactor with a 5–10 ns high voltage (17 kV) pulse generator. The distribution
of untreated nanoparticle sizes is peaked around 125–150 nm in diameter, as measured using a
scanning mobility particle sizer (SMPS) spectrometer. With plasma treatment, we observe up to a
55-fold reduction in relative particle mass and a significant reduction in the nanoparticle size
distribution using this method. The effectiveness of the nanoscale PM remediation increases with
both the pulse repetition rate and pulse voltage, demonstrating the scalability of this approach for
treating particulate matter at higher flow rates and larger diameter reactors.
2.2 Introduction
During the past couple of decades, the adverse health effects of particulate emissions have
16
been firmly established by many toxicological studies.
98-101
In epidemiological reports, these
ultrafine particulates have been linked to premature cardiovascular and respiratory deaths in
metropolitan areas, as well as lung cancer.
102-105
A 1993 study published by Dockery et al. has been
cited more than 4,600 times as of the time of this writing, demonstrating the broad impact of this
problem.
106
Since 1997, the South Coast Air Quality Management District (SC-AQMD) in Southern
California has regulated smoke emissions from chain-driven (i.e., conveyor-belt) charbroilers
under RULE 1138.
107
These emissions consist of oil aerosol particles centered around 100 - 200
nm in diameter that are generated from the charbroiling of fat contained within the meat being
cooked. In these chain-driven charbroilers, high temperature catalysts are placed just a few inches
above the cooking surface and provide effective mitigation of the oil aerosol pollutants. These
chain-driven charbroilers are typically found only in large fast-food restaurants. However, a vast
majority of restaurant smoke emissions (~85%) originate from open-underfire charbroilers. In
New York City, these open-underfire charbroilers emit an estimated 1,400 tons of PM annually.
The New York Department of Health estimates that more than 12% of the PM2.5-attributable
premature deaths can be attributed to these charbroiler emissions.
108
If all restaurant charbroilers
in the New York metropolitan area were equipped with pollution control technologies, a substantial
number of these premature deaths could be prevented through reduced PM2.5 concentrations.
It should be noted that the high temperature catalysts that are used for chain-driven
charbroilers are not suitable for treating open-underfire charbroilers. Here, the exhaust hood is
17
approximately 1 m away for the hot cooking surface. As such, the exhaust cools down substantially,
by the time it reaches the hood, and would thus require additional heating of the catalyst in order
to function properly. Dr. Karavalakis and coworkers at the University of California at Riverside
has recently performed a comparative study of three pollution control technologies for removing
PM from commercial meat cooking operations using the South Coast Air Quality Management
District (SCAQMD) Method 5.1 testing procedure.
109
These technologies include filtration,
electrostatic precipitation (ESP) and steam injection. A similar study was carried out in Korea by
Lee et al.
110
. For cooking applications that produce a large amount of grease particles (e.g.,
hamburger charbroiling), filter-based approaches become cost-prohibitive, as expensive filters
must be replaced frequently. Also, with filter-based approaches, 2-3 filters are typically configured
in series, resulting in a considerable pressure drop which, in turn, requires high power blowers to
be utilized in order to achieve the necessary flow rates for kitchen ventilation compliance. The
accumulation of grease in electrostatic precipitation systems also poses a potential fire hazard, and
frequent cleaning of the collection plates is required.
2.3 Experimental Details
In the work presented here, we utilize a transient pulsed plasma to reduce nanoscale PM
produced in a commercial charbroiling process. Here, the plasma-based flow reactor consists of a
3 ft-long, 3 inch-diameter stainless steel cylindrical anode with a 4-wire array of cathode center
electrodes, as shown in Figure 2.1. The overall footprint of the system is approximately 0.5’ × 3.5’.
The plasma is produced using a TPS Model 20X pulse generator operating at a peak voltage of
18
17kV , pulse repetition rates up to 2000 Hz, and continuous powers up to 80W. A typical waveform
from this pulse generator is plotted in Figure 2.2. While radio frequency (RF)-based plasma
reactors have been investigated for remediation of diesel exhaust for several decades,
111-117
the
nanosecond pulsed plasma used here consumes far less energy in the creation of the plasma. At a
peak voltage 17kV , our system delivers a transient power of 4.76 MW. The transient nature of the
plasma necessitates that very little current is drawn in its creation. That is, once the streamer is
created, the applied electric field collapses before a substantial amount of current (and hence
electric power) can flow.
Figure 2.1. (a) Schematic diagram of the experimental setup used to test the transient pulsed plasma reactor. In
this configuration, the plasma reactor is installed in parallel to a kitchen ventilation system including a charbroiler,
hood, duct, and blower. Here, only a fraction of the full flow is passed through the reactor. (b) Photograph of the
transient plasma (high electron energy, low-temperature plasma), (c) typical particle size distribution, and (d)
output characteristics of the nanosecond pulse generator.
33 kV
(a)
Model 40X
(b)
Model 30X
Time (nsec)
Time (nsec)
Output Voltage (kV)
Output Voltage (kV)
Time (nsec)
Output Voltage (kV)
17kV
Blower
(a)
(d)
Cathode
Anode
(a) (b) (c)
(b)
Particle Density
Diameter
10 100
(c)
19
Remediation experiments were carried out at the Center for Environmental Research &
Technology (CE-CERT) test kitchen in Riverside, CA. Here, the transient plasma reactor was
installed in the CE-CERT facility, as illustrated in Figure 2.1. Particle distributions were measured
using a scanning mobility particle sizer (SMPS) spectrometer (TSI Model 3776) with a
condensation particle counter (CPC) over the range from 14-685 nm. The scanning time of each
dataset was 120 s while the aerosol flowrate of the SMPS was set at 0.3 LPM, and the sheath
flowrate was 3 LPM. Hamburgers (75% lean, 25% fat) were cooked for 4.5 minutes per side
continuously for 3 hours during this study. 15 patties were cooked at a time on a grill that was 25”
× 30” in area. A total of 375 patties were cooked during this study. Baseline particle distributions
(i.e., histograms) were measured using the SMPS without a plasma exhibit highly stable
distributions, as shown in Figure 2.3.
Figure 2.2. Typical waveform of the TPS Model 20X pulse generator obtained with a 50W load.
20
Figure 2.3. Particle size distributions taken 15 minutes apart for different flow conditions of (a) 2.5 m/s and (b)
0.25 m/s.
2.4 Results and Discussion
Figure 2.4 shows the particle number densities measured with and without the plasma
treatment for two different reactor flow conditions: 2.5 m/s and 0.25 m/s (790 and 79 LPM,
respectively). For these datasets, the original untreated particle distributions peaked around 125-
150 nm diameter. With plasma treatment, a significant drop in the particle number was observed
along with the emergence of a narrow distribution centered around 30-40 nm. The integrated area
of the dominant peak shows a factor of 1.7X reduction in PM number density (i.e., 4.62/2.71=1.7X)
at high flow rates (2.5 m/s) and a 10-fold reduction in PM at low flow rates (0.25 m/s), as shown
in Figure 2.4b.
100
0.0
0.2
0.4
0.6
0.8
1.0
Normalized Particle Number Density
Diameter (nm)
Baseline 0.25
15 min
100
0.0
5.0x10
6
1.0x10
7
1.5x10
7
Particle Number Density (#/cm
3
)
Diameter (nm)
Baseline 2.5 m/s
15 min
(a) (b)
21
Figure 2.4. Particle number densities measured with and without the plasma treatment for different flow
conditions of (a) 2.5m/s and (b) 0.25m/s. The integrated peak areas are indicated in the figures. Here, the pulse
generator was operating at a peak voltage of 17kV , pulse repetition rate of 1200 Hz, and continuous power of
75W.
Since smaller diameter nanoparticles have substantially lower mass than larger diameter
nanoparticles, it is more appropriate from a regulatory perspective to plot the particle mass instead
of number density. Figure 2.5 shows the relative mass density obtained by multiplying the particle
number densities in Figure 2.4 by the radius cubed. Here, we observe a 2.4- and 55-fold reduction
in relative PM mass for flow rates of 2.5 and 0.25 m/s, respectively. In this representation, the
narrow distribution of particles centered around 30-40 nm is negligible compared with the larger
diameter particles, because of the diameter-cubed mass relation.
100 200 300 400 500 600
0.0
5.0x10
6
1.0x10
7
1.5x10
7
2.0x10
7
2.5x10
7
3.0x10
7
Particle Number Density (#/cm
3
)
Diameter (nm)
Without Plasma
With Plasma
Area
cont.
= 4.62E+9
Area
pulser
= 2.81E+9
Area
peak 1
= 9.59E+7
Area
peak 2
= 2.71E+9
1
2
100 200 300 400 500 600
0.0
5.0x10
6
1.0x10
7
1.5x10
7
2.0x10
7
Particle Number Density (#/cm
3
)
Diameter (nm)
Without Plasma
With Plasma
Area
cont.
= 2.713E+9
Area
pulser
= 2.65E+8
(a) (b)
22
Figure 2.5. Relative particle mass measured with and without the plasma treatment for different flow conditions
of (a) 2.5m/s and (b) 0.25m/s. The integrated peak areas are indicated in the figures. Here, the pulse generator
was operating at a peak voltage of 17kV , pulse repetition rate of 1200 Hz, and continuous power of 75W.
The nanoparticle distributions were also measured as a function of the pulse repetition rate.
Figure 2.6 shows the integrated particle number plotted as a function of pulse repetition rate, which
decreases linearly with increasing repetition rate. Here, each pulse delivers approximately 40.2 mJ
of energy. To first order we assume that the total power is proportional to the pulse repetition rate.
These results demonstrate that this approach can be scaled up to treat higher flow rates at higher
pulse repetition rates.
Figure 2.6. Integrated particle numbers (i.e., total particle number) plotted as a function of pulse repetition rate
taken under low flow conditions.
100 200 300 400 500 600
0.0
2.0x10
13
4.0x10
13
6.0x10
13
8.0x10
13
Relative Particle Mass Density (a.u.)
Diameter (nm)
Without Plasma
With Plasma
Area
cont.
= 2.73E+16
Area
pulser
= 4.96E+14
100 200 300 400 500 600
0.0
4.0x10
13
8.0x10
13
1.2x10
14
1.6x10
14
Relative Particle Mass Density (a.u.)
Diameter (nm)
Without Plasma
With Plasma
Area
cont.
= 6.99E+16
Area
pulser
= 2.87E+16
(a) (b)
2.4X
55X
0.0 0.5 1.0 1.5 2.0
0.0
2.0x10
7
4.0x10
7
6.0x10
7
8.0x10
7
1.0x10
8
1.2x10
8
Integrated Particle Number (#/cm
3
)
Pulse Repetition Rate (kHz)
jdfhg
23
The particle distributions were also measured as a function of voltage dependence, as
shown in Figure 2.7. Here, again, a monotonic decrease is observed in the integrated area of the
PM peak distribution (i.e., relative PM mass), with an overall reduction of 40x observed at a pulse
peak voltage of 17,830 V . These input voltages correspond to pulse energies of approximately 10,
20, 40, and 50 mJ. These results further demonstrate the scalability of this approach for treating
higher flow rates and larger diameter systems with higher pulse voltages.
Figure 2.7. (a) Particle number densities measured for various pulse generator input voltages. (b) Integrated
peak areas (i.e., relative particle mass) plotted as a function of pulse generator input voltage taken under low
flow conditions.
Plasma-based treatment of diesel engine exhaust has been demonstrated by many groups
for both PM and NOx remediation, including a large effort at the Ford Motor Company,
nanosecond pulsed plasmas consistently outperform conventional RF-based plasmas. As
mentioned above, this transient plasma draws very little current in creating the plasma since the
applied electric field collapses once the plasma is formed and, thus, very little current (and hence
electric power) can flow. Matsumoto et al. reported a comparison of the NO removal efficiency of
150 200 250 300 350 400
0.0
4.0x10
15
8.0x10
15
1.2x10
16
1.6x10
16
Integrated Particle Mass (a.u.)
Pulse Input Voltage (V)
100 200 300 400 500 600
0
1x10
13
2x10
13
3x10
13
4x10
13
5x10
13
Relative Particle Mass Density (a.u.)
Diameter (nm)
150 VDC 2 kHz
250 VDC 2 kHz
350 VDC 2 kHz
400 VDC 2 kHz
(b) (a)
24
nanosecond pulse discharge technologies with pulsed corona discharge and dielectric barrier
discharge (DBD) reactors, which dissipate a substantial amount of energy as heat.
118
The
nanosecond pulse discharge produces a “cold” plasma in which the electron energies are around
30 eV (T=10
5
K), while the vibrational modes of the molecules remain close to room temperature.
These highly energetic (or “hot”) electrons enable new chemical pathways through the formation
of charge-free radicals and highly reactive species, including atomic oxygen and ozone, which are
known to break down grease into CO, CO2 and other smaller hydrocarbons.
119
These high reactive
species drive chemical reactions that are fundamentally different from those of standard
equilibrium chemistry. In addition, it is possible that the plasma induces the formation of smaller
nanoparticles, which appears as a distinct peak in the spectra, corresponding to newly nucleated
particles. Also, due to the limitations of our measurements (i.e., SMPS), we are unable to analyze
the possible formation of PM with diameters smaller than 14 nm. The main difference between
plasmas created in diesel engine and restaurant exhaust is the temperature, which is less than 100
o
C
for restaurant smoke. While the same amount of energy is required to produce a plasma in both
applications, the higher temperatures associated with diesel engine exhaust lead to arcing at lower
thresholds, which ultimately limits the power/plasma density that can be achieved in these two
applications.
2.5 Conclusion
In conclusion, these preliminary measurements show the effectiveness of transient pulsed
plasmas to provide substantial remediation of PM produced by commercial charbroiling processes
25
(e.g., cooking of hamburger meat). Using a SMPS spectrometer, we observe the distribution of
untreated nanoparticle sizes to be centered around 125-150 nm diameter. A 55-fold reduction in
relative particle mass is observed with plasma treatment, as well as a significant reduction in the
nanoparticle size distribution. Here, the remediation of nanoscale PM increases with pulse
repetition rate and pulse voltage, demonstrating that this general approach can be scaled up to treat
higher flow rates and larger systems. This transient plasma-based approach provides a new method
for breaking down oil-based PM that is fundamentally different from UV and/or ozone approaches,
which are effective in treating odor but not PM.
120-126
Here, we believe that the formation of active
free radicals in the plasma, such as atomic oxygen, break down the grease particles into CO, CO2
and other smaller hydrocarbons similar to the mechanism by which plasmas break down polymer
films.
119
26
Chapter 3 Transient Plasma-enhanced Remediation of Oil-based Particulate
Matter via Electrostatic Precipitation
This chapter is similar to Yang et al., published in Particuology
3.1 Abstract
It is now recognized that nanoscale particulate matter (PM) represents a substantial health
hazard for our society, including PM from restaurant smoke. In this study, we explore the use of a
transient pulsed plasma in conjunction with an applied DC bias to treat oil aerosols that closely
resemble restaurant (i.e., charbroiler) smoke emissions. For polyaromatic olefin PAO-4 and
soybean oil, we found that a three-order-of-magnitude reduction in particulates (i.e., 99.9%
remediation) can be achieved with this system. Here, the plasma discharge is produced in a 4”
diameter cylindrical reactor with a 5-10 nanosecond high voltage (30 kV) pulse generator together
with applied DC bias voltages up to 10 kV . The distribution of nanoparticle sizes is measured using
a scanning mobility particle sizer (SMPS) with diameter centered around 225 nm. Here, the main
mechanism of remediation occurs in a two-step process in which the oil nanoparticles are first
ionized by the free electrons and free radicals in the plasma and then the charged particles are
swept out to the sidewalls of the reactor by the applied DC potential. We believe this general
approach opens up new degrees of freedom in the design of electrostatic oil aerosol pollution
control devices.
3.2 Introduction
The wide acceptance of the serious health effects associated with nanoscale particulate
27
matter (PM) produced by fossil fuel combustion has led to a wide range of nanotoxicology studies
of the environmental emissions from restaurants in commercial cooking processes (e.g.,
charbroiling).
102-106, 127-129
In 1997, the South Coast Air Quality Management District (SC-AQMD)
in Southern California passed RULE 1138, which regulates PM emissions from chain-driven (i.e.,
conveyor-belt) charbroilers.
130
This PM is made up of oil aerosol particles approximately 100 -
200nm in size, which are produced from fat-containing meat during the cooking process. In
response to this ruling, these chain-driven charbroilers are now outfitted with high temperature
oxidation catalysts located just above the hot cooking surface, mitigating these nanometer-scale
oil aerosol particles. While chain-driven charbroilers are primarily used in large fast-food
restaurants, a majority (~85%) of total restaurant smoke emissions are produced by open underfire
charbroilers.
131, 132
In a 2016 report, there was an estimated 1,400 tons of particulate matter
produced annually in New York city originating from these open-underfire charbroilers. According
to the Department of Health and Mental Hygiene, it is estimated that more than 12% of premature
deaths due to PM2.5 (i.e., particles ≤2.5µm) are attributed to open underfire charbroiler
emissions.
108, 133
By equipping all of the restaurant charbroilers in New York city with effective
pollution control technologies, an estimated 88% of these premature deaths can be reduced by
limiting the PM2.5 concentrations in the region.
133
With the exception of New York city, San Francisco and San Joaquin, particulate matter
(PM) emissions from open underfire charbroilers are largely unregulated and account for 94% of
all restaurant emissions in these regions.
134
A typical charbroiler produces around 10 lbs/day of
28
particulates at volumetric flow rates above 1600 ft
3
/min. Per hamburger, this corresponds to about
5 grams of particulate matter (PM).
135
It is important to point out that the high temperature
oxidation catalysts in chain-driven charbroilers cannot be used to treat open underfire charbroilers
because, in this configuration, the ventilation hood is typically more than 1 m above the cooking
surface. Here, the exhaust is relatively cool, and the catalyst would need to be heated separately in
order to provide effective remediation. Currently available methods for removing oil aerosol
particulates include electrostatic precipitation (ESP), wet scrubbers, and filtration.
109, 110
For heavy
grease cooking, such as hamburger charbroiling, filtration is cost-prohibitive and requires daily
maintenance. In this approach, several filters are arranged in series, creating a large pressure drop
making it necessary to use a high-power fan in order to achieve sufficiently high flow rates to reach
kitchen ventilation compliance.
While there have been many reports characterizing harmful emissions from various
cooking processes
127-129, 136-140
, there have been relatively few reports on techniques for
remediating these harmful pollutants.
109, 110, 141, 142
Lee et al. reported emission rates and removal
efficiencies of particulate matter by electrostatic precipitators (without plasma) in under-fired
charbroilers with efficacies between 55%-97%.
110
Researchers at the University of California,
Riverside's commercial test cooking facility performed a comparative study of three remediation
technologies using the South Coast Air Quality Management District (SCAQMD) Method 5.1. Of
the three devices studied, a dual-stage filtration system, a device based on evaporative cooling and
electrostatic ionization, and an electrostatic precipitator, only the electrostatic precipitator provided
29
remediation above 80%.
109
Chang et al. reported plasma-based removal of gaseous polycyclic
aromatic hydrocarbons from cooking fumes.
141
In 2019, Yang et al. reported first results on plasma-
based remediation of nanoscale particulate matter in restaurant smoke emissions, however, without
an applied DC bias.
142
In the work reported here, we use a nanosecond pulse plasma in conjunction with a DC
bias, which provides an electrostatic field across the reactor and produces a higher plasma density
than the nanosecond pulsed plasma alone. When the DC bias voltage is added to the nanosecond
pulse, the voltages sum additively, resulting substantially higher peaks fields. For example, a 30kV
nanosecond pulse applied with an 10kV DC bias will provide a peak voltage of 40kV . Here, the
remediation occurs in a two-step process where the nanoparticles are ionized by free radicals in
the plasma and then swept out to the sidewalls by the applied DC bias.
3.3 Experimental Details
In the study presented here, we use an oil aerosol generator from Aerosol Technologies
International (ATI, Inc), which is created by forcing compressed air through a Laskin nozzle.
143-
145
Our plasma-based flow reactor consists of a 4 ft-long, 4 inch-diameter stainless steel cylindrical
anode with a 25mil single-wire cathode arranged in a coaxial geometry, as illustrated in Figure 3.1.
This system has electrical feedthroughs on either end of the reactor, one for supplying high DC
voltages and the other for high voltage nanosecond pulses, as indicated in Figure 3.1a. Figure 3.1c
shows a circuit diagram illustrating how the DC bias is configured together with the nanosecond
pulse generator. In this configuration, a high voltage capacitor protects the nanosecond pulse
30
generator from the high voltage DC power supply, and an inductor protects the DC power supply
from the nanosecond high voltage pulses. The plasma is produced using a TPS Model 30X pulse
generator operating at a peak voltage of 30kV , a pulse repetition rate of 200Hz, and a continuous
power of 30W. A typical waveform of the nanosecond high voltage pulse is plotted in Figure 3.2.
Here, the generation of plasma is assisted by 10kV DC power supply capable of supplying up to
30W of continuous power. Particle size distributions (i.e., histograms) were measured using a
scanning mobility particle sizer (SMPS) spectrometer (TSI Model 3938) with a condensation
particle counter (CPC), capable of measuring particle distributions over the range from 14 - 685
nm. Baseline particle distributions (i.e., without plasma) exhibit highly stable distributions, as
shown in Figure 3.3.
Figure 3.1. (a) CAD drawing of the plasma-based reactor for restaurant particulate emissions remediation. (b)
Photograph of the plasma discharge at the output port of the reactor system. (c) Schematic circuit diagram of the
experimental setup.
(b)
Pulser
Feedthrough
DC Voltage
Feedthrough
4’
(a)
High Voltage
Nanosecond
pulse generator
Capacitor
(high pass filter)
Inductor
(low pass filter)
DC bias
voltage
Reactor
(c)
Plasma
discharge
31
Figure 3.2. Waveform of a TPS 30X pulse generator.
Figure 3.3. Particle size distributions produced using PAO-4 oil in an aerosol particle generator. These spectra
were taken 30 minutes apart using a scanning mobility particle sizer (SMPS) spectrometer.
3.4 Results and Discussion
Figure 3.4a shows the particle size distributions taken under an applied DC voltage of 5kV
both with and without the nanosecond pulse generator running at a peak voltage of 30kV , pulse
repetition rate of 200Hz, and continuous electrical power of 30W. A comparison of these two
distributions shows a 12-fold reduction of total PM concentration (i.e., 92% remediation). Here,
(a)
Model 40X
40 kV (b)
Model 30X
Time (nsec)
Time (nsec)
Output Voltage (kV)
Output Voltage (kV)
200 300 400 500 0 100 600
30 kV
100 200 300 400 500
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
Particle Number Density (#/cm
3
)
Diameter (nm)
t = 0
t = 30 minutes
5.8X
32
the integrated areas are indicated in the plot corresponding to the total particle concentrations both
with and without the transient pulsed plasma. Similarly, Figure 3.4b shows the particle size
distributions taken with an applied DC voltage of 10kV both with and without a nanosecond pulse
generator, exhibiting a 1500-fold reduction in PM concentration (i.e. 99.9% remediation). It should
be noted that the particle distributions taken with 5kVDC and 10kVDC only, without the
nanosecond pulse generator, are nearly identical to the untreated baseline data (i.e., no remediation)
plotted in Figure 3.5. Here, we believe the main mechanism of remediation occurs in a two-step
process in which the oil-based nanoparticles are first ionized by the high energy electrons and free
radicals in the plasma and then the charged particles are swept out to the sidewalls of the reactor
by the applied DC bias.
33
Figure 3.4. Particle size distributions obtained with PAO-4 under applied DC voltages of (a) 5kV and (b) 10kV
both with and without the 30kV nanosecond pulsed plasma (pulse repetition rate of 200Hz and electrically
continuous power of 30W).
100 200 300 400 500
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
2.5x10
5
Area
5 kV only
= 7.31E+7
Area
5 kV + Plasma
= 5.90E+6
Particle Number Density (#/cm
3
)
Diameter (nm)
5 kV only
5 kV + Plasma
100 200 300 400 500
10
1
10
2
10
3
10
4
10
5
Area
10 kV only
= 6.84E+7
Area
10 kV + Plasma
= 4.64E+4
10 kV only
10 kV + Plasma
Particle Number Density (#/cm
3
)
Diameter (nm)
(a)
(b)
12X
1500X
34
Figure 3.5. Particle size distributions under 10 kV and 5 kV DC voltage only using PAO-4 oil in an aerosol
particle generator.
We can make a rough estimate for the time required for a 225 nm-diameter nanoparticle to
be swept out to the sidewalls of the reactor by assuming an electrostatic force of qE accelerating a
nanoparticle of mass 𝜌∙
!
"
𝜋(
#
$
)
"
, resulting the formula 𝑡 = (
%&'#
!
"()
, where 𝑑, 𝜌 and 𝑞 are the
diameter, density, and charge of the nanoparticle, respectively, E is the electric field, and R is the
radius of the reactor. For a 4”-diameter reactor under an applied DC bias of 10kV , a 225 nm-
diameter nanoparticle with a charge of one electron (i.e., minimum charge) and mass density of
0.819 g/ml (density of PAO-4), this “sweep out” time is 79 msec. The timing and kinetics of the
soybean oil particles are expected to be quite similar to those of the PAO-4.
We have also performed a separate set of measurements using soybean oil rather than PAO-
4. The soybean oil more closely resembles the oil-based nanoparticles that are generated by the
charbroiling of hamburger meat and is often used as a surrogate grease generator following the UL
100 200 300 400 500
0.0
2.0x10
4
4.0x10
4
6.0x10
4
8.0x10
4
1.0x10
5
1.2x10
5
1.4x10
5
1.6x10
5
1.8x10
5
Particle Number Density (#/cm
3
)
Diameter (nm)
Baseline
10 kV only
5 kV only
a
35
1046 standard method.
146
However, it is also worth noting that these soybean oil grease aerosol
particles are generated at room temperature. Figure 3.6 shows particle size distributions taken with
applied DC voltages of 2.5kV and 5kV both with and without the nanosecond pulsed plasma. For
a DC bias of 2.5kV , we observed a 21-fold reduction in PM concentration (i.e., 96% remediation).
For a DC bias of 5kVDC, we observe a 1260-fold reduction in PM concentration (i.e., 99.9%
remediation). The improved remediation obtained with a DC bias of 5kV compared to that of
2.5kVDC can be attributed to the increased electric fields that are achieved when adding the 30kV
peak pulse voltage. Also, at higher DC biases, the plasma density is higher and fills a more
substantial volume of the reactor. It should be noted that the distributions observed with 2.5 and
5kVDC bias only (i.e., without the nanosecond pulse generator) are nearly identical to the
untreated data (i.e., no remediation). As was the case for PAO-4, the main mechanism of
remediation, here, occurs in a two-step process in which the oil nanoparticles are first ionized by
the free electrons and free radicals in the plasma, and then the charged particles are swept out to
the sidewalls of the reactor by the applied DC potential.
36
Figure 3.6. Particle size distributions obtained with soybean oil under applied DC voltages of (a) 2.5kV and (b)
5kV both with and without the nanosecond pulsed plasma running at a peak voltage of 30kV , pulse repetition
rate of 200Hz, and continuous power of 30W.
It should be noted that the transient nature of the plasma dictates that very little current is
drawn in creating the plasma. That is, once the plasma is created, the applied electric field collapses
100 200 300 400 500
10
1
10
2
10
3
10
4
10
5
Area
5 kV only
= 3.63E+7
Area
5 kV + Plasma
= 3.43E+4
Particle Number Density (#/cm
3
)
Diameter (nm)
5 kV only
5 kV + Plasma
100 200 300 400 500
1x10
4
2x10
4
3x10
4
4x10
4
5x10
4
Area
2.5 kV only
= 1.74E+7
Area
2.5 kV + Plasma
= 8.27E+5
Particle Number Density (#/cm
3
)
Diameter (nm)
2.5 kV only
2.5 kV + Plasma
21X
1260X
(a)
(b)
37
before a substantial amount of current (and hence electric power) can flow. Because of its transient
nature, this is a non-thermal cold plasma with electron energies around 30 eV (T=10
5
K) and
vibrational modes of the molecules remaining near room temperature. These “hot” electrons are
responsible for providing new chemical pathways by forming charge-free radicals and highly
reactive species in the plasma, which include atomic oxygen and ozone, driving chemical reactions
in a fundamentally different manor than standard equilibrium chemistry. Previously, Yang et al.
showed that oil aerosol particles produced during the charbroiling process were shifted to smaller
diameters when treated with transient pulsed plasma (without DC bias), likely due to chemically
active free radicals, such as atomic oxygen, which break down the grease particles into CO, CO2
and other hydrocarbons similar to the plasma-induced break down of polymer films.
142
Future
studies are needed in order to obtain a complete understanding of the chemical pathways associated
with this aspect of the remediation process. As mentioned above, our plasma-based flow reactor
consists of a 4 ft-long, 4 inch-diameter stainless steel cylindrical. Assuming a pressure drop of 1-
2 inches of water, this reactor will operate up to volumetric flow rates of 850-1250 CFM. Typical
commercial kitchen ventilation systems start around 2000-3000 CFM, thus requiring 2-4 of these
reactors in parallel.
3.5 Conclusion
In conclusion, we demonstrate the effectiveness of a transient pulsed plasma to improve
the remediation efficiency of oil-based nanoparticles through electrostatic precipitation by several
orders of magnitude (99.9% remediation). Here, a non-thermal (i.e., cold) plasma is generated
38
using 30kV pulses with 5-10nsec rise times. The high energy electrons and free radicals in this
plasma ionize these oil-based particulates, which are subsequently swept out to the sidewalls of
the reactor under an applied DC electric field on msec timescales. In addition to providing an
electrostatic field across the reactor, the applied DC voltage sums additively with the nanosecond
high voltage pulse creating an ionizing plasma over a larger volume of the reactor than that created
with the nanosecond high voltage pulse alone. In this proof-of-principle demonstration, oil-based
nanoparticles are generated using a Laskin nozzle aerosol generator using polyaromatic olefin
PAO-4 and soybean oil, which is a common surrogate for restaurant smoke emissions. This
combined approach presents new design considerations in the treatment and optimization of oil-
based aerosol particulates produced by restaurant smoke emissions, which present a substantial
health hazard to our society.
39
Chapter 4 Plasma-Enhanced SO2 Remediation in a Humidified Gas Matrix: A
Potential Strategy for the Continued Burning of High Sulfur Bunker Fuel
This chapter is similar to Schroeder et al., published in Fuel
4.1 Abstract
We report a substantial enhancement in the removal of gaseous SO2 by discharging a
transient nanosecond pulsed plasma in a water vapor-saturated gas mixture. With the plasma alone
(i.e., “dry”), the SO2 remediation is limited to approximately 15% reduction in SO2 (i.e., ∆SO2 =
65 ppm). In presence of water vapor, we observe 84% remediation (∆SO2 = 500 ppm) during
plasma discharge due to the availability of OH radicals. Here, there is a synergistic effect of adding
water vapor to the gas mixture in which the plasma excites highly reactive OH radical species that
drive a two-step reaction process: SO2 + OH → HSO3 and the subsequent reaction of HSO3 + OH
→ H2SO4, which precipitates out in the aqueous phase. The efficacy of this approach increases as
we increase the temperature of the gas matrix, indicating the relatively low barriers of this reaction,
which is consistent with the OH-driven reaction pathway, and it also increases with plasma density,
thus demonstrating the scalability of this approach. Plasma emission spectroscopy as well as
Raman scattering spectroscopy provide spectroscopic evidence of the OH radical species, further
substantiating the OH reaction intermediate mechanism. This approach provides a promising
mitigation strategy for the continued use of high sulfur fuels (i.e., bunker fuel).
4.2 Introduction
Sulfur dioxide (SO2) is a toxic gas produced as a by-product of the burning of fossil fuels
40
containing sulfur compounds. These high sulfur-containing fuels are almost exclusively used by
the international shipping industry, and the international Marine Organization (IMO) has set a
global limit for sulfur in fuel oil used on board ships of 0.50% m/m (mass by mass) starting 1
January 2020. The previous global limit for sulfur content of ships’ fuel oil was 3.5% (i.e., heavy
fuel oil). Currently, heavy fuel oil (also known as “Bunker Fuel”) comprises 4% of every barrel of
crude oil, which corresponds to 10,000 tons of sulfur emissions globally each day. The new 2020
global limit of 0.50% corresponds to an 85% reduction in SO2, which we believe can be achieved
with our plasma-based approach.
While many research groups (including our own) have demonstrated highly effective NO
remediation using plasma-based processes,
147-152
SO2 treatment remains challenging. In diesel
exhaust, this problem is exacerbated because of the presence of NO, which rapidly consumes a
vast majority of the oxygen radicals in the plasma, via the reaction NO + O → NO2. As such, the
NO remediation reaction serves as a competing reaction pathway, which rapidly consumes most
of the plasma-generated radicals. While existing technologies exist for reducing NOx efficiently
(i.e., selective catalytic reduction (SCRs)), effective methods for removing SO2 are still very much
lacking. SO2 wet scrubber technologies are limited by the low solubility of SO2 in water, which is
several orders of magnitude lower than the solubility of H2SO4. Therefore, one strategy for SO2
remediation is to first convert SO2 to H2SO4 and then capture in H2O using a “wet scrubber” with
nearly unity capture.
Yamamoto’s group investigated a single-stage wet plasma reactor for the simultaneous
41
removal of NOx, SOx, and particulates by flowing Na2SO3 and NaOH solutions along the inner
wall of the reactor.
153
However, the reaction pathways and temperature dependences of these
reactions remain poorly understood. While SO2 is more soluble in water than CO2, there are several
equilibrium processes that occur between SO2 and various hydrogenated and oxygenated species
(e.g., HSO3
-
). We have the following reactions/equilibria:
SO
$
*
(aq) + H2O(l) = H
+
(aq) + HSO
"
+
(aq) (A)
HSO
"
+
(aq) = H
+
(aq) + SO
"
$+
(aq) (B)
2HSO
"
+
(aq) =S
$
O
,
$+
(aq) + H2O(l) (C)
SO2(g) = SO
$
*
(aq) (D)
SO2(g) + H2O(l) = H
+
(aq) + HSO
"
+
(aq) (E)
Once equilibrium is reached with these back reactions, the remediation is limited, and no
further SO2 can be removed from the system. This plasma-based approach enables us to
circumvent the standard SO2(g)/H2SO4
o
(aq) equilibria, thus, enhancing the SO2 remediation
process.
In the work presented here, we demonstrate a method for improving SO2 remediation using
plasma discharge in in a heated, plasma-driven reactor in the presence of water vapor. We present
a systematic study of this reaction as a function of temperature and plasma density. A comparison
of SO2 reduction carried out under wet and dry conditions was performed in order further
understand the synergistic roles of water vapor and plasma discharge. We also present
spectroscopic evidence of the OH radicals, in order to substantiate the hypothetical OH-driven
42
reaction pathway, which represents an important, short-lived reaction intermediate species.
4.3 Experimental Details
In the work presented here, we utilize a transient pulsed plasma discharge in a coaxial
reactor. As illustrated in Figure 4.1, the plasma-based flow reactor consists of a 3 ft-long, 2 inch-
diameter stainless steel cylindrical anode with a single-wire cathode center electrode. The plasma
is produced using a TPS Model 20X pulse generator (Transient Plasma Systems, Inc.) operating at
a peak voltage of 17kV , pulse repetition rates up to 2000 Hz, and continuous powers up to 800W.
Here, the plasma density is varied by adjusting the pulse repetition rate. A typical waveform
produced by this pulse generator is plotted in Figure 4.1b. While radio frequency (RF)-based
plasma reactors have been investigated for remediation of diesel exhaust for several decades,
111-
117
the nanosecond pulsed plasma used here consumes far less energy in the creation of the plasma.
The transient nature of the plasma necessitates that very little current is drawn in creating the
plasma. That is, once the streamer is created, the applied field collapses before a substantial amount
of current (and hence electric power) can flow. Because of its transient nature, this is a cold plasma,
in which the electron energies are around 30 eV (T=10
5
K), while the vibrational modes of the
molecules remain at room temperature. These “hot” electrons enable new chemical pathways to
be explored in the formation of energetic intermediate species that are otherwise not possible to
make through standard equilibrium chemistry. At a peak voltage 17kV , our system delivers a
transient power of 4.76 MW. SO2 concentrations were measured using a Horiba portable gas
analyzer (model PGA-350) with a sample rate of 0.5L/min after passing through a water knock
43
out. This model detects SO2 using non-dispersive infrared absorption with a measurement
repeatability of ±1% relative to the full measurement scale of 1000 ppm or ±10 ppmV absolute.
Our synthetic gas mixture was prepared by mixing neat SO2 gas with compressed dried air to a
concentration of 600 ppm by volume. The flow rate of the synthetic gas mixture through the reactor
was 4.75 slpm. Water vapor was injected into the reactor using an ultrasonic nozzle at a flow rate
of 0.42 mL/min. This is a heated reactor, in which the sidewalls of the reactor are kept above 100
o
C,
meaning that all H2O stays in the gas phase during the residence time in the reactor. The total
reactor volume is 2.2 L resulting in residence times ranging from approximately 16-22 seconds.
After exiting the heated reactor, the sample is transferred by a short (~18 inches) heated PTFE line
to a borosilicate Greenburg-Smith type impinger with a straight stem (no bubbling plate)
submerged in an ice bath. The design allows for the removal of moisture while minimizing
negative bias resulting from dissolved SO2 by limiting the surface area where interaction may
occur between condensed water and the gas sample. The condensation of sulfuric acid leading to
corrosion in flue gas cleaning operations is a well-known phenomenon that is heavily dependent
upon the composition of the flue gas matrix and the specific dynamic thermodynamic parameters
of the cleaning system.
154
Both the gas composition and dynamic thermodynamic conditions
experienced throughout the system result in difficulty estimating the acid dew point. The
experimental system used here mitigated the potential for sulfuric acid aerosol formation and
condensation by operating at ambient pressures and minimizing system temperature differentials
prior to moisture removal.
44
Figure 4.1. (a) Schematic diagram of the experimental setup used to test the transient pulsed plasma reactor. (b)
Typical output characteristics of nanosecond high voltage pulse generator (USC-patented technology). (c)
Photograph of the transient plasma (hot electron, low-temperature plasma) formed by the high voltage
nanosecond pulse.
Figure 4.2 shows photographs of our experimental setup in which the high voltage cable
from our nanosecond high voltage pulse generator is attached to a glass slide-based reactor, which
consists of two parallel copper electrodes separated by an approximately 5mm gap. The plasma
(c)
(a)
45
discharge can be seen here in this image as purple light (corresponding to the C-B transition in
nitrogen).
155, 156
The glass-slide based reactor has a low profile and fits easily underneath our high
numerical aperture microscope objective lens, enabling plasma emission spectra in the visible
wavelength range to be obtained with high collection efficiency, as illustrated in Figure 4.2c.
Figure 4.2d shows a peak around 927 nm, which corresponds to the triple wavelength of the OH
radical line at 309 nm, as reported by Sato and Coworkers.
67, 70-72
Figure 4.2. Photographs of (a) the plasma discharge across a 5mm gap on a glass slide and (b, c) glass-slide flow
cell. (d) Plasma emission spectra taken in DI water and in 0.1M KOH.
4.4 Results and Discussion
Figure 4.3a shows a plot of the absolute SO2 removal (in units of Dppm as measured by
volume) brought about by the plasma discharge. As a comparison, we measure the removal
920 925 930 935
0.0
5.0x10
3
1.0x10
4
1.5x10
4
2.0x10
4
2.5x10
4
3.0x10
4
3.5x10
4
920 925 930 935
0
500
1000
1500
2000
2500
3000
3500
30s 35% 0.1M KOH
30s 45% DI H2O
Wavelength (nm)
Plasma Emission Intensity
(d)
(c)
46
efficacy in dry SO2 in air (i.e., without water vapor injection). Here, we see that only about 65 ppm
(or 15%) of the SO2 is removed with plasma discharge and is largely independent of plasma density.
This behavior reflects the limitation in the availably of OH radicals without the injection of water.
That is, there are simply not enough OH radicals in the plasma to remediate all of the SO2 and,
therefore, much of the plasma’s energy goes into generating oxygen radicals which drive the back
reactions to SO2. Figure 4.3a shows our results taken at moderate plasma densities (i.e., up to 100
J/L), and Figure 4.3b shows results taken at high plasma densities (i.e., up to 145 J/L). Here, we
observe a dramatic increase in the presence of water injection, yielding 84% (500 ppm) removal
of SO2 in the humidified gas matrix.
47
Figure 4.3. Temperature dependent SO 2 remediation study on approximately 600 ppmV synthetic SO 2 in a
humidified air matrix.
In order to substantiate our hypothesis that OH radicals drive the intermediate steps in this
SO2 remediation reaction, we performed in situ plasma emission spectroscopy of our nanosecond
pulsed plasma discharge with water, which shows a clear peak around 927nm, as shown in Figure
15 30 45 60 75 90
0
50
100
150
200
250
300
350
400
450
500
190° C, Wet Fit Curve
160° C, Wet Fit Curve
130° C, Wet Fit Curve
110° C, Wet Fit Curve
110° C, Dry Fit Curve
SO2 Removal (abs. D ppmV )
Energy Density (J/L)
Dry
15 30 45 60 75 90 105 120 135
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Dry
190° C, Wet Fit Curve
160° C, Wet Fit Curve
130° C, Wet Fit Curve
110° C, Wet Fit Curve
110° C, Dry Fit Curve
SO
2
Removal (abs. % diff.)
Energy Density (J/L)
(b)
(a)
48
4.4a.
70
These spectra were taking using a micro-spectrometer described in Figure 4.2. This feature
is associated with charge neutral OH radicals, which correspond to highly chemically active, short
lived species, as reported by Sato et al.
70
These radical species produce several oxidizing agents,
such as ozone and hydrogen peroxide that can be detected chemically.
67, 71, 72, 157, 158
This pulsed
discharge approach has been utilized for water purification via the oxidation of volatile organic
compounds (VOCs).
70,62, 64, 67, 69, 158, 159
Figure 4.4b shows surface enhanced Raman scattering
(SERS)-enhanced vibrational spectra of H2O/SO2-plasma exposed Ag nanoparticles. Sharp peaks
observed around 624 and 928cm
-1
are in agreement with SO3
2-
species, which correspond to the
deprotonated HSO3 reaction intermediate.
160-162
These spectroscopic signatures provide further
evidence of the OH-driven reaction pathway proposed for this water vapor-enhanced, plasma-
driven process. This approach circumvents the standard SO2(g)/SO
$
*
(aq) equilibria and overcome
the relatively low solubility of SO2 in water, which is several orders of magnitude lower than the
solubility of H2SO4. The H2SO4 can then be removed in H2O with nearly unity capture and
subsequently titrated, as is typical done in wet scrubbers.
This plasma-enhanced SO2 remediation process may enable ships to burn high sulfur fuels
while meeting the IMO SOx 2020 emissions standards. The price of low sulfur bunker fuel ($540
per metric ton) is typically 30-40% more expensive than that of high sulfur bunker fuel ($400 per
metric ton).
163, 164
For a typical ship burning100 tons of fuel per day, this price differential equates
to a cost savings of $5M per year, providing a large economic incentive to implement this plasma-
based technology.
49
The results presented here were carried out using compressed air. In actual flue gas,
however, the elevated temperature and presence of other species including NO, NO2 and
particulate matter PM can play a significant role. On one hand, the higher temperatures will shorten
radical lifetimes (e.g., atomic O, O3, and OH), the presence of PM has been shown to increase
NOx remediation by providing a surface that influences the reaction kinetics. In addition, the
presence of NOx will also compete for the radicals generated by the plasma including atomic O
and OH. However, efficient methods for removing NOx currently exist (e.g., diesel oxidation
catalysts) and may be used in conjunction with the plasma-enhanced SOx scrubber demonstrated
here.
Figure 4.4. (a) Plasma emission spectra of OH radicals observed from high voltage discharge in aqueous solution.
(b) SERS-enhanced vibrational spectrum of SO 2-plasma exposed Ag nanoparticles.
4.5 Conclusion
In conclusion, we report the synergistic effect of discharging nanosecond pulsed plasma in
a humidified SO2 gas matrix. Here, a substantial enhancement in the remediation of gaseous SO2
920 925 930 935
OH
Wavelength (nm)
Plasma Emission Intensity
(a)
50
is produced beyond that achieved with the plasma discharge alone (i.e., “dry”) or with the water
vapor alone (i.e., no plasma). Together, the water vapor-enhanced, plasma-driven process produces
a 84% reduction in SO2, whereas the dry, plasma-driven process only produces a 15% reduction
in SO2. Here, the main mechanism of SO2 remediation reaction lies in the availability OH radicals,
which drives the following reaction processes: SO2 + OH → HSO3 and HSO3 + OH → H2SO4.
Spectroscopic evidence of the short-lived, highly reactive OH radical is obtained through plasma
emission spectroscopy and SERS-enhanced Raman spectroscopy. The SO2 removal efficacy
increases with increasing temperature, reflecting the relatively low barriers of this reaction, and it
also increases with plasma density demonstrating the scalability of this approach.
51
Chapter 5 Plasma-Enhanced NOx Remediation using Nanosecond Pulsed
Discharges in a Water Aerosol Matrix
This chapter is similar to Schroeder et al., published in Fuel Processing Technology
5.1 Abstract
We report nitrogen oxides remediation using a transient pulsed plasma discharge in a water
aerosol gas matrix using nanosecond high voltage pulses. While there have been many studies
showing highly efficient conversion of NO to NO2 using plasma-driven processes, the total
removal of NOx (i.e., NO plus NO2) is severely limited by the backreaction of NO2 to NO, which
is also driven by the highly reactive radical species in the plasma. By injecting water aerosol into
the gas matrix, we are able to selectively drive a plasma-based reaction which minimizes the
backreaction of NO2 to NO. Here, the synergistic effect of the water aerosol and plasma discharge
enables enhanced NOx removal by creating OH radicals which, in turn, drive NO2 to HNO3, which
is highly soluble in water. In the presence of water aerosol, the plasma discharge results in a 100%
reduction in NO and a 98% reduction in total NOx. Spectroscopic evidence of the short-lived,
highly reactive OH radical is obtained through plasma emission spectroscopy and the vibrational
signatures of the NO2
-
and NO3
-
intermediates are observed using SERS-enhanced Raman
spectroscopy. We show that the NOx remediation increases with plasma power density
demonstrating the scalability of this general approach.
5.2 Introduction
In the combustion of fossil fuels, NO and NO2 are produced as harmful pollutants giving
52
rise to smog and acid rain. Several research groups (including our own) have shown effective NOx
(i.e., NO, NO2) remediation using various plasma treatment approaches.
149-152
However, the
detailed chemical pathways associated with these plasma-based process are complex and not fully
understood. Figure 5.1 illustrates the multitude of the possible chemical pathways in this
remediation process. Here, the major products are NO2, N2O, N2O5, N2, HNO2, HNO3 (indicated
in boxes in Figure 5.1), and the radicals assisting each reaction include various excited states of O,
O3, OH, N, NO, and HO2 (shown next to the corresponding arrow in Figure 5.1).
165-167
The reaction
of NO with oxygen radicals are believed to be the dominant reactions for plasma-based
remediation. The back reaction of N with NO2 and the reaction of O3 with NO2 both replenish NO.
While RF-based plasma reactors have been investigated for pollution remediation for several
decades, transient plasmas produced by high voltage nanosecond pulses consume far less energy
in the creation of the plasma than conventional RF sources.
168
The transient nature of the plasma
necessitates that very little current is drawn in creating the plasma. That is, once the streamer is
created, the applied field collapses before a substantial amount of current (and hence electric power)
can flow. Because of its transient nature, this is a cold plasma in which the electron energies are
extremely high, while the vibrational modes of the molecules remain close to room temperature.
51,
57, 169
These “hot” electrons enable new chemical pathways to be explored in the formation of
energetic intermediate species that are otherwise not possible to make through standard
equilibrium chemistry.
53
Figure 5.1. Possible chemical pathways for NO remediation via plasma-based treatment.
While many groups have shown that plasma treatment can convert NO to NO2 extremely
efficiently, this approach generally is not effective in removing NO2.
118, 170-172
In a comparison by
Matsumoto, the NO removal efficiency of nanosecond pulse discharges can be as high as
0.75mol/kWh, which is considerably higher than that obtained with (microsecond) pulsed corona
discharges (0.35 mol/kWh) and dielectric barrier discharge (DBD) reactors (0.2 mol/kWh).
118
Huiskamp et al. reported NO removal using sub-nanosecond transient plasma with an efficiency
of 2.5 mol/kWh when starting conditions of the synthetic gas matrix, a crucial parameter affecting
remediation outcomes, included no measurable quantity of NO2.
173
Aside from this work by
Huiskamp, a handful of papers report NOx remediation values (i.e., NO+NO2), however, these
papers do not discuss reaction pathways or report any spectroscopic evidence of the reaction
intermediates.
174-182
Khacef et al. reported a NOx reduction pathway involving C3H6, which reacts
DNO
(b)
(a) (c)
b
54
with O2 to yield peroxy radicals (HO2) that efficiently converts NO to NO2.
174, 175
In the work presented here, we demonstrate a synergistic effect of injecting water aerosol
with a transient plasma discharge to push this reaction to HNO3 via the creation of OH radicals.
Here, we perform a comparative study of plasma discharge with (i.e., “wet”) and without (i.e.,
“dry”) injection of water aerosol, systematically at different plasma densities. Plasma emission
spectra and Raman scattering spectra are taken in order to verify the OH and NO3
-
intermediates.
5.3 Experimental Details
Here, a transient plasma is created using a nanosecond pulse discharge in a coaxial reactor
consisting of a 3 ft (0.9 meter) long, 2 inch (5 cm) diameter stainless steel cylindrical anode with
a single-wire cathode center electrode, as depicted in Figure 5.2. A Transient Plasma Systems
Model 20X pulse generator is operated with a peak output voltage of 17kV (Figure 5.2b) at pulse
repetition rates up to 2 kHz and continuous powers up to 0.8 kW. Figure 5.2b shows a plot of a
typical pulse waveform generated using this system. Figure 5.2c shows a single pulse image of the
transient plasma discharge in a similar co-axial reactor. Radio frequency (RF)-based plasmas have
demonstrated remediation of diesel exhaust for several decades,
111-117
including a large effort at
the Ford Motor Corporation using conventional RF plasmas. However, the recent availability of
solid-state nanosecond high voltage pulse generators, which consume less energy than
conventional RF plasmas, opens up the possibly of driving these NOx remediation processes more
efficiently. At a peak voltage 17kV , this system delivers a peak power of 4.76 MW. Figure 1a
illustrates the reactor configuration in which a water aerosol (i.e., approximately 100nm diameter
55
nanoparticles) is injected into the reactor using an ultrasonic nozzle in the direction counter to the
flow of NOx to be treated. At the output of this reactor, NO and NOx concentrations are measured
using a portable gas analyzer (Horiba Model PGA-350), which samples at a flow rate of 0.5L/min.
In the experiments reported here, our synthetic gas mixture was prepared by mixing neat NO gas
with compressed dried air at 500 PPM by volume. This model of gas analyzer detects NO and NOx
using chemiluminescence detection with a measurement repeatability of 0.5% relative to the full
measurement scale of 500 ppmV or ± 2.5 ppmV absolute.
56
Figure 5.2. (a) Schematic diagram of the experimental setup used to test the transient pulsed plasma reactor. (b)
Typical output characteristics of nanosecond high voltage pulse generator. (c) Photograph of the transient plasma
(hot electron, low-temperature plasma) formed by the high voltage nanosecond pulse approach.
(c)
(a)
Electrical
Feedthrough
Nozzle
Direction of
Gas Flow
Electrode
57
5.4 Results and Discussion
Figure 5.3 shows a plot of the NO and NOx concentrations taken with (“wet”) and without
(“dry”) water aerosol injection. Figure 5.3a and Figure 5.3b show the results obtained at plasma
energy densities of 18 J/L and 45 J/L, respectively. The starting NO concentrations ranged from
approximately 150 to 325 ppmV , while the starting NOx concentrations ranged from approximately
160 to 390 ppmV . Here, we observe relatively little plasma-induced remediation in the dry gas
mixtures: 40% for ∆NO and only 4% for ∆NOx. However, a dramatic increase in NO and NOx
remediation is observed the presence of water aerosol injection: 100% for ∆NO and 98% for ∆NOx.
These remediation values correspond to 0.54 and 1.1 mol/kWh for the wet reactor and 0.45 and
0.81 mol/kWh under dry conditions. This marked improvement in remediation efficacy
demonstrates the synergistic effects of adding water together with plasma discharge, which
increases the availably of OH radicals far beyond that attained with the plasma alone or the water
aerosol alone. This plasma-enhanced remediation mechanism is comprised of a two-step process,
in which the NO is converted to NO2 by atomic oxygen radicals in the plasma followed by the
rapid conversion of NO2 to HNO3 via the OH radicals created by discharging the plasma in the
presence of water aerosol. This second step minimizes the backreaction of NO2 to NO, greatly
improving the efficacy of this approach. The resulting HNO3 is highly soluble in the water aerosol
matrix is captured with near unity efficiencies and subsequently titrated. We would like to point
out that our initial NO concentrations in Figure 5.3 are somewhat different under dry and wet
conditions. Huiskamp et al. observed a strong dependence on the initial concentrations of NO,
58
however, these data were taken under mildly moist conditions and they observe an increase in NO2
at these energy densities. Nevertheless, we observe a clear enhancement in NOx remediation with
water aerosol injection.
173
Figure 5.3. NO and NO x gas concentrations with and without the plasma discharge under both dry” (i.e., without
water aerosol) and “wet” (i.e., water aerosol) conditions. (a) low plasma density and (b) high plasma density.
In order to corroborate the hypothesis that OH radicals drive the NO2 → HNO3 step in this
NOx remediation process, in situ plasma emission spectroscopy was performed using nanosecond
pulsed plasma discharge with water, as shown in Figure 5.4a. Here, a sharp peak is observed at a
wavelength around 927nm, which corresponds to charge neutral OH radicals that are short lived,
highly chemically active species, as reported by Sato et al.
70
These OH radical species are known
to produce various oxidizing agents, such as hydrogen peroxide and ozone, as used in water
purification and the decomposition of volatile organic compounds (VOCs).
,62, 64, 67, 69-72, 157-159, 183
Here, however, we believe the OH radicals are used directly to drive NO2 to HNO3, as described
0
50
100
150
200
250
300
350
400
450
NO Dry NO Wet NOx Dry NOx Wet
18 J/L Energy Density
Gas Concentration (ppm)
5.4%
28%
79%
52%
0
50
100
150
200
250
300
350
400
450
NO Dry NO Wet NOx Dry NOx Wet
45 J/L Energy Density
Gas Concentration (ppm)
40%
100%
4%
98%
(a) (b)
59
above. Figure 5.4b shows a plasma emission spectrum centered around 777nm, which corresponds
to light originating from atomic oxygen species, responsible for the first step in this remediation
process (i.e., NO → NO2). Figure 5.4c shows surface enhanced Raman scattering (SERS)-
enhanced vibrational spectra of H2O/NO2 plasma exposed Ag nanoparticles. Sharp peaks observed
around 822 and 1053 cm
-1
are in agreement with previous reports of NO2
-
and NO3
-
species, which
correspond to the deprotonated HNO3 reaction intermediate.
160-162
These spectroscopic signatures
provide further evidence of the OH-driven reaction pathway proposed for this water aerosol-
enhanced, plasma-driven process. This approach circumvents the standard NO(g)/HNO
"
*
(aq)
equilibria and overcomes the relatively low solubility of NO in water, which is several orders of
magnitude lower than the solubility of HNO3. The HNO3 is then removed in H2O with nearly unity
capture and subsequently titrated, as is typical done in a wet scrubber configuration. The
synergistic effect of OH for NOx removal has been discussed previously by Huiskamp et al.,
however, the effect was minimal since only mildly moist air was used and not the aerosols used
here.
173
Figure 5.4. (a) Plasma emission spectra of (a) OH radicals and (b) atomic oxygen observed from high voltage
discharge in aqueous solution. (c) SERS-enhanced spectrum of NO-plasma exposed Ag nanoparticles.
600 800 1000 1200 1400
NO
-
2
Raman Intensity (a.u.)
Raman Shift (cm
-1
)
NO Wet (in situ)
NO
-
2
NO
-
3
920 925 930 935
OH
Wavelength (nm)
Plasma Emission Intensity
770 775 780 785 790
Wavelength (nm)
Plasma Emission Intensity
atomic
oxygen
(b) (a) (c)
60
5.5 Conclusion
In conclusion, we demonstrate a synergistic effect in the remediation of toxic nitrogen
oxide pollutants (i.e., NO and NO2) by discharging a nanosecond pulsed transient plasma discharge
together with injection of a water aerosol into the gas matrix. Previous studies have shown high
efficiency conversion of NO to NO2 via plasma-based processes but poor removal of total NOx
(i.e., NO and NO2) due to the rapid backreaction of NO2 back to NO. By comparing the plasma-
based remediation with and without injection of water aerosol, we are able to isolate a reaction
pathway that minimizes the backreaction of NO2 to NO by increasing the availability of OH
radicals. This results in a two-step process, whereby NO is first converted to NO2 by atomic oxygen
radicals, and then NO2 is subsequently converted to HNO3 via OH radicals. Here, the key reactants
OH and O are evidenced by plasma emission spectroscopy, while the vibrational signatures of the
HNO3
-
intermediate species are observed using SERS spectroscopy. Using this synergistic
approach, we observe a 98% reduction in NO and a 100% reduction in total NOx due to the plasma
discharge in the presence of water aerosol. Without the plasma discharge, the NOx removal
efficiency is generally limited by the low solubility of NO and NO2 in water. Here, we provide a
way around this limitation by rapidly driving NO2 to HNO3, before it can be converted by the
backreaction to NO.
61
Chapter 6 Recycling Diesel Soot Nanoparticles for Use as Activated Carbon in
Li Ion Batteries
This chapter is similar to Yang et al., under reviewed by Resources, Conservation & Recycling
6.1 Abstract
We report the successful capture and reuse of diesel exhaust soot particles as a conductive
additive in lithium manganese oxide (LMO) cathodes in Li-ion batteries. This approach enables
an abundant toxic pollutant to be converted into a valuable material for energy storage devices.
This study consists of an initial characterization of the diesel soot particles, a high-temperature
annealing step to remove residual organics and unburned hydrocarbons, and characterization of
electrochemical performance in Li-ion battery configuration. LMO composite electrodes are
fabricated by mixing LMO particles with conductive carbon and binders. The performance of
diesel soot particles as conductive additives is compared with the commercially available activated
carbon (i.e., Super P
®
carbon). The current evolution of the composite electrode with recycled
diesel soot particles demonstrates better performance than the electrode containing the Super P
®
carbon. Based on high-resolution transmission electron microscope (HRTEM) images and particle
number densities distributions via scanning mobility particle sizer (SMPS) spectrometer, we find
that these diesel soot nanoparticles follow a narrow log-normal distribution centered around
100nm in diameter and consist of highly porous amorphous carbon, which provide a large surface-
to-volume ratio, making them ideal candidates for electrode materials in Li ion batteries.
62
6.2 Introduction
The wide acceptance of the serious health effects associated with nanoscale particulate
matter (PM) produced by fossil fuel combustion has led to a wide range of nanotoxicology studies
of the environmental emissions from the combustion of fossil fuels, cigarette smoke, and even
emissions from commercial cooking processes (e.g., charbroiling).
102-106, 127-129
During the past
couple of decades, the adverse health effects of diesel particulate emissions have been firmly
established by many toxicological studies.
98-101
In epidemiological studies, these ultrafine
particulates have been linked to premature cardiovascular and respiratory deaths in metropolitan
areas, as well as lung cancer.
102-105
A 1993 study published by Dockery et al. has been cited more
than 4,600 times as of the time of this writing, demonstrating the broad impact of this problem.
106
Diesel-fueled engines and vehicles, which are the major sources of diesel PM that accounts for
about 8% of the PM2.5 in outdoor air, emit more than 70 tons of PM per day in California alone
(25,000 tons of PM per year).
184, 185
The Air Resources Board (ARB) and U. S. EPA state that an
estimated 3,500 premature deaths per year that are affiliated with respiratory and cardiovascular
diseases can be attributed to diesel PM in California alone.
186
Several technologies have been developed to treat diesel particulate emissions, including
diesel particulate filters (DPFs), wet scrubbers, and electrostatic precipitators (ESP).
187-189
Many
of these have been developed in response to increasingly stringent air quality regulations, such as
those imposed by the U.S. Environmental Protection Agency (EPA), California Air Resources
Board (CARB), and local air quality air management districts (AQMD). Most recently, there has
63
been a popular trend towards zero-emissions systems. However, for most practical applications,
the combustion of diesel fuel will remain a dominant source of power, requiring more advanced
pollution control devices to be developed. Diesel particulate filters provide a relatively cheap
solution that works well for small size engines. Over time, however, these filters become clogged,
which creates engine backpressure and require regeneration at regular maintenance intervals.
The main drawback of electrostatic precipitators is that they eventually need to be cleaned out,
which presents the inevitable problem of how to dispose of this now highly concentrated hazardous
carcinogenic material.
Li ion batteries have become a ubiquitous part of our society, often going unnoticed. As the
global Li ion battery capacity continues to grow at its current rate, innovations in high surface area
materials will be needed in order to sustain this grown and meet the economic pressures of supply
and demand. By 2030, it is estimated that the global Li ion battery capacity will exceed 1 TWh,
and cheap rechargeable batteries remain a key enabling technology for electronic vehicles and
solar farms. The present composite electrodes in commercialized batteries consist of three main
components: active materials, conductive additives, and polymer binders. Active materials
intercalate with Li ions during charging / discharging the Li-ion battery. Common active materials
in commercialized cathode electrodes are lithium iron phosphate (LFP), lithium manganese oxide
(LMO), lithium cobalt oxide (LCO), and lithium nickel manganese cobalt oxide (NMC). These
cathode electrodes, however, have a poor electrical conductivity. For example, the electronic
conductivity of LMO and LFP are ~10
-6
and 10
-9
S/cm, respectively.
190, 191
Carbon additives play
64
an important role in creating conductive networks in composite electrode structures in order to
improve battery performance, and the impact of conductive carbon on the capacity profile of
electrodes is significant. Previously, the contribution of the conductive carbons on the practical
capacity of these electrodes was studied, but only using commercially-available conductive
carbons.
96, 97
In this study, we evaluate the electrochemical performance of conductive carbon
derived from diesel soot particles.
6.3 Experimental Details
Here, we present a proof-of-concept study demonstrating the successful conversion of
diesel engine exhaust soot particles to a high-surface area electrode material for rechargeable Li
ion batteries. We characterize the size and shape of these using HRTEM and SMPS, and then
anneal the material in a pretreatment step before building a coin-cell lithium ion battery using this
material. In this controlled study, the performance of commercially available activated carbon
material is compared to pre-annealed and post-annealed diesel soot material. We characterize the
current-voltage characteristics as well as the charging/discharging performance of these three
prototype LMO composite electrodes.
In our experimental setup, diesel soot particles are collected and then deposited on TEM-
compatible substrates. The particle size, shape, and clustering are characterized using high-
resolution transmission electron microscopy (HRTEM). Figure 6.1 shows HRTEM images of
nanoparticle size distributions in the 20-200 nm diameter range, as measured using a scanning
mobility particle sizer (SMPS) spectrometer. These particulates are then annealed at 250
o
C in air
65
for 4 hours in order to purify them for use as activated carbon.
Figure 6.1. (a, b) High-resolution transmission electron microscope (HRTEM) images and (c) scanning mobility
particle sizer (SMPS) spectra of diesel soot particles (i.e., particulate matter).
Composite LMO electrodes were fabricated to characterize the electrochemical
performance of the activated carbons. The composite electrodes were made by combining lithium
manganese oxide (LiMn2O4, LMO, electrochemical grade, Sigma Aldrich) with carboxymethyl
cellulose sodium salt binder (CMC, Aldrich) and conductive carbon additives in an 8:1:1 mass
ratio. The LMO powders were used as-is, and their particle size is around 3 microns.
192
Conductive
carbons were either commercially available Super P
®
carbon (Timcal), annealed or unannealed
soot material. The CMC polymer binder was dissolved in deionized water in a 1:50 weight ratio.
The prescribed amount of LMO and conductive carbon additives were added to the binder solution.
The slurry was mixed for 30 minutes using a Thinky centrifugal mixer at 2000 RPM. The slurry
was cast on an aluminum foil current collector using a doctor blade and dried under room
conditions. The electrolyte for all electrochemical testing consisted of ethylene carbonate (EC,
Anhydrous, 99%, Sigma Aldrich) and dimethyl carbonate (DMC, Anhydrous, >99%, Sigma
Aldrich) at a 1:1 volume ratio. 1 M LiPF6 (98%, Sigma Aldrich) was added to the EC: DMC
100
0.0
4.0E4
8.0E4
1.2E5
1.6E5
Particle Number Density (#/cm
3
)
Diameter (nm)
Low Engine Load
High Engine Load
(a) (b)
(c) 50 nm 10 nm
66
solution. The electrolyte was mixed with a magnetic stirrer in a glove box for one day. ACR2032
coin-cell was fabricated consisted of an LMO composite cathode as the working electrode, lithium
metal anode as the counter electrode, and Celgard polyethylene film as the separator. The
composite electrodes were cycled either via cycling voltammetry at 50 µV/sec or galvanostatically
at 1C rate between 3.5-4.5 V for 5 cycles.
6.4 Results and Discussion
Figure 6.2 shows the current density evolution of composite LMO electrodes prepared with
different conductive additives plotted as a function of voltage during the 5
th
cycle at 50 µV/sec. As
expected, two distinct current peaks are clearly observed during delithiation and lithiation in the
Super P-containing electrode. These current peaks correspond to the well-known two-phase
transformation in the LMO electrode structure.
193
Two pairs of oxidation/reduction peaks in the
annealed soot-containing electrode indicate that there is no additional redox reaction associated
with the annealed soot particles. The current peaks for annealed soot particle containing electrode
precede the current peaks for the Super-P containing electrode by ca. 0.1 V after during delithiation.
The interval between the oxidation and the corresponding reduction potentials are also greater for
the annealed soot particle-containing electrode, indicating that there is polarization within the
electrode due to the larger resistance in the annealed soot particle containing electrode. While the
annealed soot particles demonstrate larger resistance in the electrode, the maximum current density
observed in the annealed soot particle-electrode is similar to that of the Super P–containing
electrode. The capacity of the electrode is calculated by taking the integral of current with time.
67
The charge/discharge capacity for the LMO electrode containing Super P carbon and annealed soot
particles were around 70 mAh/g. The theoretical capacity of the LMO electrode is 148 mAh/g.
Figure 6.2b shows the current evolution of the electrode prepared with unannealed soot particles.
Less distinct current peaks with lower current density values are observed in the unannealed soot
particle-containing electrode, which exhibits highly unstable current responses in cyclic
voltammetry.
Figure 6.2. Current-voltage curves taken from (a) diesel engine exhaust and (b) Super-P
®
(commercially
available activated carbon, typically used in Li-ion batteries).
Figure 6.3a shows the discharging capacity of the composite electrodes cycled 100 times
at 1C rate. The initial discharge capacities of the electrode containing the Super P carbon and
annealed soot carbon were 3.1 and 21.8 mAh/g, respectively. The discharge capacity of the
electrode containing the Super P carbon and annealed soot carbon became 9.5 and 15.0 mAh/g
after 100 cycles, respectively. The electrode containing unannealed soot carbon demonstrated very
low capacity, which was less than 1 mAh/g. Electrochemical impedance spectroscopy (EIS) of the
3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5
-20
-10
0
10
20
30
Current Density (mA/g)
Voltage (V)
Unannealed Diesel Soot
3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5
-40
-30
-20
-10
0
10
20
30
40
Current Density (mA/g)
Voltage (V)
Annealed Diesel Soot
Super P
(a) (b)
68
composite electrodes containing different conductive carbons was performed before and after
cycling with a sinusoidal amplitude of 5 mV from 100 kHz to 10 MHz, as shown in Figure 6.3b.
EIS measurements were conducted using Biologic potentiostat equipped with acquisition software
EC-EC-lab®. The high-to-mid frequency region obtained from EIS is represented as Nyquist plots
with real (Z′) and imaginary (-Z′′) parts of the impedance plotted on the x- and y-axes, respectively.
The radius of the semicircle at high frequency (low Z′) can be associated with the charge transfer
resistance of the electrode.
194
Before cycling, the charge transfer resistance of electrodes
containing the annealed soot carbon and Super P carbon is much lower than the electrode
containing unannealed soot carbon. Since impedance measurements are highly sensitive to the
surface area, it is more informative to compare the change in electrode resistance with cycling.
Overall, electrochemical resistance increased with cycling in all electrodes. The resistance in the
cycled electrode containing unannealed soot carbon increased dramatically compared to that
observed before cycling. The electrodes containing annealed soot carbon exhibit an increase in
impedance with cycling. In the case of the Super P carbon, the electrode resistance increased with
cycling, more significantly than the electrode containing annealed soot carbon.
69
Figure 6.3. (a) Discharging capacity of LMO electrodes containing Super P carbon, annealed soot carbon, and
unannealed soot carbon. The composite electrodes were cycled at 1C rate. (b) Impedance plots of the electrodes
before and after 100 cycles at 1C rate.
6.5 Conclusion
In conclusion, we demonstrate that diesel exhaust soot particles can be recycled and used
as activated carbon in Li ion batteries. Here, an abundant carcinogenic pollutant is converted into
a value-add material that can be used in Li ion energy storage devices. The diesel soot nanoparticles
follow a log-normal distribution centered around 100nm, as characterized by electron microscopy
and scanning mobility particle size spectroscopy. Once collected, high-temperature annealing is
used to remove residual organics and unburned hydrocarbons from the diesel soot particles.
Composite LMO electrodes containing the recycled material performs better than commercially
available activated carbon (i.e., Super P
®
) typically used in Li ion batteries. The unannealed diesel
soot particulate material, however, performs much worse than the annealed material with highly
unstable current-voltage characteristics and low discharging capacities. These diesel soot
particulates consist of highly porous amorphous carbon, which provide a large surface-to-volume
0 2000 4000 6000
0
2E3
4E3
6E3
8E3
0 100 200 300 400 500
0
100
200
300
400
500
Imaginary Impedance (W)
Real Impedance (W)
Imaginary Impedance (W)
Real Impedance (W)
Unannealed Diesel Soot
Annealed Diesel Soot
Super P
Dashed: Before Cycling
Solid: After 100 Cycles at 1C
0 20 40 60 80 100
0
5
10
15
20
25
Discharge Capacity (mAh/g)
Cycle Number
Unannealed Diesel Soot
Annealed Diesel Soot
Super P
Scan Rate: 1C
(a) (b)
70
ratio, making them ideal candidates for electrode materials in Li ion batteries.
71
Chapter 7 CO2 Reduction to Higher-order Hydrocarbons by Plasma
Discharge in Carbonated Water
This chapter is similar to Yang et al., under reviewed by ACS Energy Letters
7.1 Abstract
By discharging nanosecond high voltage pulses in CO2-saturated water, we observe CO2
reduction to higher order hydrocarbons, including acetic acid, formic acid, and oxalate. Here, the
plasma emission spectra exhibit Swan bands, which correspond to C2 (diatomic carbon radical)
species, indicating that in addition to reducing CO2 (i.e., a high barrier reaction), C2-species are
formed, which presents the exciting possibility of converting a notorious greenhouse gas into
energy dense hydrocarbon fuels. In our electrode configuration, nanosecond pulses of 28kV are
needed in order to generate a plasma discharge in DI water, whereas in carbonated water this
threshold voltage is lowered to 13kV . In order to characterize various hydrocarbon products formed
in this process, cryogenic NMR spectroscopy and liquid ion chromatography are performed ex situ.
Here, we observe clear peaks corresponding to formic acid (CH₂O₂) and acetic acid (CH₃COOH),
which correspond to a C2-hydrocarbon species. We have also observed the presence of oxalates
(i.e., C2O4
2−
) at a concentration of approximately 150g/L using liquid ion chromatography, which
also correspond to C2 species and readily forms oxalic acid (C2H2O4) in water. Plasma emission
spectroscopy exhibits spectral signatures associated with OH radicals, atomic hydrogen and atomic
oxygen due to the plasma discharge in water, each of which represent short-lived, highly reactive
intermediate species that facilitate (and compete with) the CO2 to hydrocarbon conversion.
72
7.2 Introduction
With increasing levels of greenhouse gases in our atmosphere, CO2 reduction processes are
of broad interest due to their ability to mitigate global warming. Many research groups (including
our own) have focused on electrochemical and photoelectrochemical CO2 reduction, involving
various metal electrodes, molecule catalysts, and plasmon-enhanced approaches.
75, 195-207
The
reduction of CO2 with H2O to various hydrocarbons is a complex reaction system requiring up to
8 electrons and many intermediate species, some of which have extremely high energy barriers.
The mechanism for electrochemical CO2 reduction was first proposed by Bockris et al.
208-210
The
high overpotential required for this reaction was attributed to the formation of the (CO2)
-
intermediate, CO2 + e
-
® (CO2)
-
, which is the rate limiting step of CO2 reduction. The highly
energetic species provided by the plasma (i.e., non-equilibrium approach) provide a strategy for
overcoming the reaction barrier associated with the (CO2)
-
reaction intermediate. Dielectric barrier
discharge has been reported for gas phase conversion of carbon dioxide to syngas and
hydrocarbons at atmosphere pressure with or without catalyst.
211-213
Heijkers et al., Masaharu et al.
and Mitsingas et al. reported the decomposition of CO2 by microwave plasma in carrier gasses of
N2, Ar or He.
214-216
Pulsed plasmas have also been used to drive gas phase CO2 reduction reactions
and subsequent conversion to hydrocarbon fuels (syngas).
217-220
While there have been many papers reporting CO2 reduction to higher order hydrocarbons
in the gas phase
74, 221-223
, to our knowledge none have shown CO2 reduction in carbonated water
73
by plasma discharge. Rumbach et al. showed that CO2 can be reduced in an aqueous medium using
a plasma-based electrode to produce oxalate and formate.
224
Here, the plasma discharge was
generated in the gas phase (above liquid) and highly energetic electrons were injected into the
solution, thus, producing solvated electrons. Generating a plasma discharge in aqueous solutions
requires significantly higher voltages that in the gas phase because of the relatively high dielectric
strength of the aqueous medium. Common approaches to lowering the discharge threshold include
using a needle electrode, a gas diffuser or pressurization to create bubbles, enabling the discharge
to be initiated within a small bubble on the electrode surface and then propagate into the aqueous
phase.
63-66, 70, 225-228
In the work presented here, we demonstrate CO2 reduction to higher order hydrocarbons
by discharging a nanosecond pulsed plasma in carbonated (i.e., CO2-satuarated) aqueous solution.
Here, the carbonated water gives rise to small bubbles which lower the voltage needed to generate
the plasma and also provides a relatively high concentration of CO2 in the aqueous solution.
7.3 Experimental Details
In the work presented here, the transient pulsed plasma is produced on a glass slide-based
reactor with a high voltage pulse generator (Model 30X, Transient Pulsed System, Inc.), which
produces pulses with peak voltages up to 30 kV , pulse rise times of 5-10 nanosecond, and repetition
rates up to 2 kHz. A typical waveform is shown in Figure 7.1b. The in-situ plasma emission spectra
are taken with an inVia
TM
Raman micro-Spectrometer (Renishaw, Inc.). The cryogenic NMR
spectroscopy and liquid ion chromatography are performed ex situ before and after 30-min plasma
74
discharges in 10 ml of carbonated DI water.
Figure 7.1. (a) Schematic diagram of the experimental setup used to take the in-situ plasma emission spectra.
(b) Typical output characteristics of nanosecond high voltage pulse generator. Plasma Emission generated with
copper tape electrodes in DI water and carbonated water at (c) 13 kV and (d) 28 kV .
Figure 7.1 shows a schematic diagram of our experimental setup for observing plasma
emission spectroscopy (Figure 7.1a) and several photographs of the plasma discharge using copper
tape electrodes deposited on a glass slide in DI water and CO2-satuarated DI water at 13kV (Figure
7.1c) and 28kV (Figure 7.1d). Here, two electrodes, separated by a 5mm gap, are attached to the
13kV
Plasma
Emission
Carbonated Water
13kV
DI Water DI Water
28kV
Plasma
Emission
Carbonated Water
28kV
Plasma
Emission
Objective
Lens
Carbonated
Water
Teflon
Stand
Glass
Slide
Cu
Electrode
Plasma
-50 -40 -30 -20 -10 0 10 20 30 40 50
0
1
2
3
4
Voltage (kV)
Time (ns)
10nsec
rise time
(a)
5-10nsec
Rise Time
30
22.5
15
7.5
0
-50 -40 -30 -20 -10 0 10 20 30 40 50
0
1
2
3
4
Voltage (kV)
Time (ns)
10nsec
rise time
(b) (a)
(b) (c)
(c) (b)
-50 -40 -30 -20 -10 0 10 20 30 40 50
0
1
2
3
4
Voltage (kV)
Time (ns)
10nsec
rise time
(a)
5-10nsec
Rise Time
30
22.5
15
7.5
0
(b)
(a)
(c) (d)
(b)
1 cm
75
bottom side of a 1mm-thick glass slide and immersed in the carbonated and non-carbonated water
electrolyte, while the top surface of the glass slide remains in air. This “flipped” geometry enables
plasma emission spectra to be taken efficiently using a glass-corrected objective lens. In DI water,
28kV pulses are needed in order to generate a plasma, which is much higher than the voltages
required to create a plasma in the gas phase (~5kV)
229
because of the relatively high dielectric
strength of the aqueous medium, which has a breakdown field 5-6 times higher than that of air.
The copper electrodes are patterned with sharp tips to provide additional field enhancement to help
initiate the plasma. In carbonated water, we can achieve plasma discharge at 13kV because of the
presence of bubbles. The aqueous solution can potentially provide a higher density of reactants
than gas phase reactions.
7.4 Results and Discussion
Figure 7.2 shows plasma emission spectra of our transient pulsed plasma obtained in a
CO2–saturated aqueous environment taken with a high numerical aperture objective lens, as
illustrated in Figure 7.1a. Here, we observe several sets of Swan bands, which correspond to C2
(diatomic carbon) species
230, 231
, indicating that, in addition to reducing CO2 (i.e., a high barrier
reaction), C2-species are formed, presenting the exciting possibility of converting an abundant
greenhouse gas into an energy dense hydrocarbon fuel (see Figure 7.3). From a pulsed discharge
in water, we observe several features associated with the OH, H, and O radicals, which correspond
to highly chemically active species.
67, 71, 72, 157, 232
One of the main challenges in producing
hydrocarbons using a plasma is that the atomic oxygen rapidly drives the back reaction to carbon
76
to CO and CO2. Figure 7.2b shows the plasma emission spectra corresponding to atomic oxygen.
Here, the discharge in carbonated water is 30X higher than that in DI water, due to the presence of
bubbles which assist in the plasma charge processes, thereby, generating a higher density plasma
under the same voltage conditions. Figure 7.2c shows the plasma emission spectrum of OH
radicals formed under plasma excitation in aqueous solution.
233
Figure 7.2d shows the plasma
emission spectra of charge neutral atomic hydrogen in electrolytes with various pH values,
illustrating our ability to control the H radical concentration, and hence reaction selectivity, during
plasma excitation.
Figure 7.2. Plasma emission spectra of (a) C 2 species, (b) O, (c) OH and (d) H radicals observed from high
voltage discharge in aqueous solution.
420 440 460 480 500 520 540 560 580
0
500
1000
1500
2000
2500
3000
Counts
Wavelength (nm)
C
2
(1,0)
Swan band
at 473 nm
C
2
(0,0)
Swan band
at 516 nm
C
2
(010)
Swan band
at 562 nm
770 775 780 785
2000
2500
3000
3500
4000
4500
Water
Carbonated water
Wavelength (nm)
Counts
0.0
2.0E4
4.0E4
6.0E4
8.0E4
1.0E5
1.2E5
1.4E5
30X
Atomic
Oxygen
645 650 655 660 665
1x10
4
2x10
4
3x10
4
4x10
4
5x10
4
6x10
4
12X pH=13
pH=7
Counts
Wavelength (nm)
0.1M H
2
SO
4
5s
DI Water 30s
0.1M KOH 60s
H
pH=1
6X
920 925 930 935
0
500
1000
1500
2000
Wavelength (nm)
Counts
OH
(a) (b)
(d) (c)
77
In order to quantify the hydrocarbons produced by the plasma discharge in carbonated
water, we performed cryogenic NMR spectroscopy of various products in water (i.e., hydrocarbons)
and liquid ion chromatography ex situ. Figure 7.3 shows cryogenic NMR spectra taken before and
after plasma discharge in CO2-saturated water. Here, we observe clear peaks corresponding to
formic acid, and acetic acid, which corresponds to a C2-hydrocarbon species. We have also
observed the presence of oxalates (i.e., C2O4
2−
) at concentrations of approximately 150g/L using
liquid ion chromatography, which also correspond to C2 species. Here, we believe the reaction
pathways take place via hot electrons generated in the bubbles, which are subsequently injected
into the aqueous solution forming solvated electrons, which have an extremely high reduction
potential.
234, 235
This reaction can potentially follow the pathway CO2 → CO2
-
and then CO2
-
+
CO2
-
→ (CO2
-
)2 for oxalate and CO2
-
+ H
+
→ CO2H for formate. Here, the one-electron reduction
to CO2
-
represents a high barrier intermediate that is enabled by the high energy electrons in the
plasma and subsequent generation of solvated electrons.
224
This is essentially homogeneous
chemistry in the aqueous phase driven by solvated electrons, which have an extremely low (i.e.,
strongly) reducing potential (-2.87 V vs SHE or only 1.41 eV below vacuum). These solvated
electrons, while difficult to form, can have relatively long lifetimes and have been used to drive
difficult reactions such as ammonia formation, which transforms N2 and water to NH3 without a
catalyst surface, as well as CO2 reduction to CO.
234-237
78
Figure 7.3. Cryogenic
1
H NMR spectra taken before and after 30-min plasma discharge in CO 2-saturated water
of (a) formic acid and (b) acetic acid. (c) Liquid ion chromatography measurements taken before and after 30-
min plasma discharge in CO 2-saturated water for oxalate.
1.8 2.0 2.2 2.4 2.6 2.8
0
2x10
5
4x10
5
6x10
5
8x10
5
1x10
6
Counts
ppm
Before Plasma
After Plasma
Acetic Acid
(CH
3
COOH)
7.8 8.0 8.2 8.4 8.6 8.8
-1x10
5
0
1x10
5
2x10
5
3x10
5
4x10
5
5x10
5
Counts
ppm
Before Plasma
After Plasma Formic Acid
(HCOOH)
1 2 3
0.0
50.0
100.0
150.0
200.0
Oxalate Concentration (g/L)
# of run
Before Plasma
After Plasma
Oxalate
(C
2
O
4
-
)
(a)
(b)
(c)
79
7.5 Conclusion
In conclusion, we have demonstrated CO2 reduction to C1 and C2 hydrocarbons by
discharging nanosecond high voltage pulses in carbonated (i.e., CO2-saturated) water. After
discharging plasma for 30 minutes, we observe the formation of acetic acid, formic acid, and
oxalate through possibly due to hot electron-based injection of solvated electrons. Swan bands are
exhibited in the plasma emission spectra, confirming the formation of C2 (diatomic carbon radical)
species. A substantially lower threshold voltage is needed to generate a plasma in carbonated water
than in DI water, due bubbles formed on the electrode surface. Cryogenic NMR spectroscopy
reveal clear peaks corresponding to formic acid (CH₂O₂) and acetic acid (CH₃COOH), and liquid
ion chromatography shows the presence of oxalates (i.e., C2O4
2−
) after discharging plasma in
carbonated water. Electrodes mounted on a glass slide in a “flipped” geometry enable plasma
emission spectra to be obtained in situ, which show spectral signatures of OH radicals, atomic
hydrogen, and atomic oxygen, representing intermediate species that facilitate (and compete with)
the conversion of CO2 to hydrocarbons.
80
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Appendix A: Characterization of Electronic Propertied of Dual-gate Carbon
Nanotube (CNT) Field Effect Transistor (FET) Devices
All the curves shown in this section were taken with a p-channel CNT dual-gate FET device
with the design shown in Figure A.1. Figure A.2 shows an example of the I-Vg curve of the device
with Vg = Vg1 = Vg2, Vbias = 0.2 V in (a) linear scale and (b) semi-log scale. Figure A.3 shows a
semi-log plot of I-Vbias curves of the device that show the reversible rectifying behavior with Vg1
= -Vg2, the gate values are specified in the legend. Figure A.4 shows the semi-log plots of I-Vbias
curves of the device with Vg1 = -Vg2 = (a) -10~0 V , (b) 0~10 V . Figure A.5 to Figure A.14 includes
the detailed information for the I-Vbias curves of the device at different gate voltages of Vg1 = -
10V~10V , Vg2 = -10V~10V . The curves are divided into 10 groups, each group represents a fixed
value of one gate value. (e.g. Vg1 or Vg2 = -10V for Figure A.5). The values of gate voltages are
specified in each plot. The gate conditions that show the formation of a p-n junction are plotted in
semi-log scale for further illustration.
103
Figure A.1. Schematic diagram of a dual-gate CNT FET device.
Figure A.2. (a) Linear plot and (b) semi-log plot of the source-drain current as a function of gate voltage V g =
V g1 = V g2 at a constant bias V bias = 0.2 V taken from a p-channel CNT FET.
Pt
Si Si
3
N
4
-10 -5 0 5 10
0.0
5.0E-7
1.0E-6
1.5E-6
2.0E-6
2.5E-6
I
bias
(A)
V
g
(V)
V
bias
= 0.2 V
-10 -5 0 5 10
1E-10
1E-09
1E-08
1E-07
1E-06
V
bias
= 0.2 V
I
bias
(A)
V
g
(V)
(a) (b)
104
Figure A.3. Semi-log plot of source-drain current plotted as a function of bias voltage at V g1 = -V g2 = ± 10V , ±
8V , ± 6V , ± 4V .
Figure A.4. Semi-log plot of source-drain current plotted as a function of bias voltage at V g1 = -V g2 = (a) -10~0V ,
(b) 0~10V ,.
-2 -1 0 1 2
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
I
bias
(A)
V
bias
(V)
10V -10V -10V 10V
8V -8V -8V 8V
6V -6V -6V 6V
4V -4V -4V 4V
(a)
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
I
bias
(A)
V
bias
(V)
10V -10V
9V -9V
8V -8V
7V -7V
6V -6V
5V -5V
4V -4V
3V -3V
2V -2V
1V -1V
-0V 0V
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
1E-10
1E-9
1E-8
1E-7
1E-6
I
bias
(A)
V
bias
(V)
-10V 10V
-9V 9V
-8V 8V
-7V 7V
-6V 6V
-5V 5V
-4V 4V
-3V 3V
-2V 2V
-1V 1V
-0V 0V
(a) (b)
105
Figure A.5. (a) (b) (c) (d) (f) Semi-log and (e) linear plot of source-drain current plotted as a function of bias
voltage at V g1 or V g2 = 10 V .
Figure A.6. (a) (b) (c) (d) (f) Semi-log and (e) linear plot of source-drain current plotted as a function of bias
voltage at V g1 or V g2 = -10 V .
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
10V -10V
10V -8V
10V -6V
10V -4V
10V -2V
10V 0V
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
10V -10V
10V -8V
10V -6V
10V -4V
10V -2V
10V 0V
10V 2V
10V 4V
10V 6V
10V 8V
10V 10V
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-2.0E-6
-1.5E-6
-1.0E-6
-5.0E-7
0.0
5.0E-7
1.0E-6
Ib (A)
Vb (V)
10V 0V
10V 2V
10V 4V
10V 6V
10V 8V
10V 10V
8V 10V
6V 10V
4V 10V
2V 10V
0V 10V
-2 -1 0 1 2
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
0V 10V
-2V 10V
-4V 10V
-6V 10V
-8V 10V
-10V 10V
-2 -1 0 1 2
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
10V 10V
8V 10V
6V 10V
4V 10V
2V 10V
0V 10V
-2V 10V
-4V 10V
-6V 10V
-8V 10V
-10V 10V
-2 -1 0 1 2 3
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
10V -10V
10V -8V
10V -6V
10V -4V
10V -2V
10V 0V
0V 10V
-2V 10V
-4V 10V
-6V 10V
-8V 10V
-10V 10V
(a) (c) (e)
(b) (d) (f)
-2 -1 0 1 2
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
-10V 10V
-10V 8V
-10V 6V
-10V 4V
-10V 2V
-10V 0V
-2 -1 0 1 2 3
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
-10V 10V
-10V 8V
-10V 6V
-10V 4V
-10V 2V
-10V 0V
-10V -2V
-10V -4V
-10V -6V
-10V -8V
-10V -10V
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
-3x10
-6
-2x10
-6
-1x10
-6
0
1x10
-6
2x10
-6
3x10
-6
Ib (A)
Vb (V)
-10V 0V
-10V -2V
-10V -4V
-10V -6V
-10V -8V
-10V -10V
-8V -10V
-6V -10V
-4V -10V
-2V -10V
0V -10V
-2 -1 0 1
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
0V -10V
2V -10V
4V -10V
6V -10V
8V -10V
10V -10V
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
-10V -10V
-8V -10V
-6V -10V
-4V -10V
-2V -10V
0V -10V
2V -10V
4V -10V
6V -10V
8V -10V
10V -10V
-2 -1 0 1 2 3
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
-10V 10V
-10V 8V
4V -10V
6V -10V
8V -10V
10V -10V
(a) (c) (e)
(b) (d) (f)
106
Figure A.7. (a) (b) Linear and (c) (d) semi-log plot of source-drain current plotted as a function of bias voltage
at V g1 or V g2 = 8 V .
Figure A.8. (a) (c) Linear and (b) semi-log plot of source-drain current plotted as a function of bias voltage at
V g1 or V g2 = -8 V .
-1.5 -1.0 -0.5 0.0 0.5 1.0
-1x10
-6
-8x10
-7
-6x10
-7
-4x10
-7
-2x10
-7
0
2x10
-7
4x10
-7
6x10
-7
Ib (A)
Vb (V)
8V -8V
8V -6V
8V -4V
8V -2V
8V 0V
8V 2V
8V 4V
8V 6V
8V 8V
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
8V -8V
8V -6V
8V -4V
8V -2V
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
1E-12
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
0V 8V
-2V 8V
-4V 8V
-6V 8V
-8V 8V
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-1x10
-6
-5x10
-7
0
5x10
-7
1x10
-6
Ib (A)
Vb (V)
8V 8V
6V 8V
4V 8V
2V 8V
0V 8V
-2V 8V
-4V 8V
-6V 8V
-8V 8V
(a)
(b)
(c)
(d)
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-2x10
-6
-1x10
-6
0
1x10
-6
2x10
-6
3x10
-6
Ib (A)
Vb (V)
-8V 8V
-8V 6V
-8V 4V
-8V 2V
-8V 0V
-8V -2V
-8V -4V
-8V -6V
-8V -8V
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
-8V 8V
-8V 6V
4V -8V
6V -8V
8V -8V
-1.0 -0.5 0.0 0.5 1.0
-2x10
-6
-1x10
-6
0
1x10
-6
2x10
-6
3x10
-6
Ib (A)
Vb (V)
-8V -8V
-6V -8V
-4V -8V
-2V -8V
0V -8V
2V -8V
4V -8V
6V -8V
8V -8V
(a) (b) (c)
107
Figure A.9. (a) (c) Semi-log and (b) linear plot of source-drain current plotted as a function of bias voltage at
V g1 or V g2 = 6 V .
Figure A.10. (a) Linear and (b) semi-log plot of source-drain current plotted as a function of bias voltage at V g1
or V g2 = -6 V .
Figure A.11. (a) Linear and (b) semi-log plot of source-drain current plotted as a function of bias voltage at V g1
or V g2 = 4 V .
-1.5 -1.0 -0.5 0.0 0.5 1.0
1E-11
1E-10
1E-9
1E-8
1E-7
Ib (A)
Vb (V)
6V -6V
6V -4V
6V -2V
6V 0V
-1.0 -0.5 0.0 0.5 1.0
-2x10
-7
0
2x10
-7
4x10
-7
Ib (A)
Vb (V)
6V 2V
6V 4V
6V 6V
4V 6V
2V 6V
-1.0 -0.5 0.0 0.5 1.0
1E-11
1E-10
1E-9
1E-8
1E-7
Ib (A)
Vb (V)
0V 6V
-2V 6V
-4V 6V
-6V 6V
(a) (b) (c)
-1.5 -1.0 -0.5 0.0 0.5 1.0
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
-6V 6V
-6V 4V
4V -6V
6V -6V
-1.5 -1.0 -0.5 0.0 0.5 1.0
-2x10
-6
0
2x10
-6
Ib (A)
Vb (V)
-6V 6V
-6V 4V
-6V 2V
-6V 0V
-6V -2V
-6V -4V
-6V -6V
-4V -6V
-2V -6V
0V -6V
2V -6V
4V -6V
6V -6V
(a) (b)
-1.5 -1.0 -0.5 0.0 0.5 1.0
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
4V -4V
4V -2V
4V 0V
0V 4V
-2V 4V
-4V 4V
-1.5 -1.0 -0.5 0.0 0.5 1.0
-2.0E-6
-1.5E-6
-1.0E-6
-5.0E-7
0.0
5.0E-7
1.0E-6
Ib (A)
Vb (V)
4V -4V
4V -2V
4V 0V
4V 2V
4V 4V
2V 4V
0V 4V
-2V 4V
-4V 4V
(a) (b)
108
Figure A.12. (a) Linear and (b) semi-log plot of source-drain current plotted as a function of bias voltage at V g1
or V g2 = -4 V .
Figure A.13. (a) Linear and (b) semi-log plot of source-drain current plotted as a function of bias voltage at V g1
or V g2 = 2 V , 0 V .
Figure A.14. (a) Linear and (b) semi-log plot of source-drain current plotted as a function of bias voltage at V g1
or V g2 = -2 V , 0 V .
-1.5 -1.0 -0.5 0.0 0.5 1.0
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
-4V 4V
4V -4V
-1.0 -0.5 0.0 0.5 1.0
-3x10
-6
-2x10
-6
-1x10
-6
0
1x10
-6
2x10
-6
3x10
-6
Ib (A)
Vb (V)
-4V 4V
-4V 2V
-4V 0V
-4V -2V
-4V -4V
-2V -4V
0V -4V
2V -4V
4V -4V
(a) (b)
-0.8 -0.4 0.0 0.4 0.8
-1.5E-6
-1.0E-6
-5.0E-7
0.0
5.0E-7
1.0E-6
1.5E-6
2.0E-6
2.5E-6
Ib (A)
Vb (V)
2V -2V
2V 0V
2V 2V
0V 2V
-2V 2V
0V 0V
-0.5 0.0 0.5
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
2V -2V
2V 0V
(a) (b)
-0.5 0.0 0.5
-2x10
-6
-1x10
-6
0
1x10
-6
2x10
-6
3x10
-6
Ib (A)
Vb (V)
-2V 2V
-2V 0V
-2V -2V
0V -2V
2V -2V
0V 0V
-0.8 -0.4 0.0 0.4 0.8
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
Ib (A)
Vb (V)
2V -2V (a) (b)
109
Appendix B: Emission Spectra of High Voltage Discharge Generated in Liquid
Nitrogen
Figure B.1 shows the emission spectra of high voltage discharge generated in liquid nitrogen with
a 30X TPS pulse generator at input voltage of 150V and repetition rate of 200Hz and 400Hz. These
spectra are taken with Ocean Optics USB2000+ spectrometer.
Figure B.1. Emission spectra of high voltage discharge generated in liquid nitrogen displayed in different
wavelength regions.
200 300 400 500 600 700 800
0
1000
2000
3000
4000
5000
6000
Intensity
Wavelength (nm)
150V 200Hz
150V 400Hz
360 380 400 420 440 460 480
0
1000
2000
3000
Intensity
Wavelength (nm)
150V 200Hz
150V 400Hz
480 500 520 540 560 580 600
0
1000
2000
3000
4000
5000
6000
Intensity
Wavelength (nm)
150V 200Hz
150V 400Hz
650 700 750 800 850
0
1000
2000
Intensity
Wavelength (nm)
150V 200Hz
150V 400Hz
(a) (b)
(c) (d)
Abstract (if available)
Abstract
This dissertation work presents several investigations related to the applications of transient pulsed plasma on the remediation of common air pollutants including particulate matter (PM), nitrogen oxides (e.g. NOₓ), sulfur dioxide (SO₂) and carbon dioxide (CO₂). Air pollution has getting more and more attention from the public due to its growing tendency and harmful effects on human health and natural ecosystems. Non-thermal plasma has been used for developing pollution control technologies because of its substantially high energy electrons which reduce the energy requirement for pollution removal. These research work presented in the dissertation provide the promising results on pollution remediation and may inspire more possibilities for applications of transient pulsed plasma. ❧ Chapter 1 starts with an introduction of environmental effects and current removal technologies of related pollutants, it provides some background information that can be helpful to understand this dissertation work. The basic concepts of transient pulsed plasma will be discussed in this chapter. It is then followed by an introduction of the properties of electric discharge generated in liquid and the principle of electrostatic precipitators (ESPs). Then, the basic structural components and functions of Li-ion battery will be briefly discussed to help understand Chapter 6 of this dissertation work. ❧ Chapter 2 presents the work of transient plasma-based remediation of PM from restaurant smoke emissions by characterizing of particle size and relative mass distribution. The effectiveness and scalability of the technology are discussed for its practicability on higher flow rate and larger systems. ❧ In Chapter 3, we give a description of a follow-up research effort focused on the remediation effects on oil-based nanoparticles by electrostatic precipitation with the assistance of transient plasma. The results show a three-order-magnitude enhancement in the reduction of nanoparticles, which provides a new approach in the design of electrostatic oil aerosol pollution devices. ❧ Chapter 4 and 5 report plasma enhanced remediation of SO₂ and NOₓ (i.e., NO and NO₂) in the environment of water vapor or water aerosols. Reaction pathways and spectroscopic evidence are discussed to explain the enhancement of the removal effects. As new regulations for the emissions of SO₂ and NOₓ are announced for better pollution control, our results represent a promising approach to effectively enhance the total remediation in diesel exhaust. ❧ A successful method to recycle diesel soot nanoparticles are discussed in Chapter 6. Here, diesel soot particles are reused as conductive additives in lithium manganese oxide cathode in Li-on batteries. The electrochemical characterization of the recycled diesel soot shows a better performance than the commercially available activated carbon (i.e., Super P®) in the presented configuration of Li-ion batteries. ❧ Chapter 7 presents a project of CO₂ reduction to higher order hydrocarbons by plasma discharge in carbonated water. The plasma emission spectra exhibit Swan bands, which correspond to C₂ species, the cryogenic NMR spectra show peaks of formic acid and acetic acid, both of which indicate the possibility of converting greenhouse gas to energy dense fuels via plasma discharges.
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Asset Metadata
Creator
Yang, Sisi
(author)
Core Title
Transient pulsed plasma for pollution remediation and energy conversion applications
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Physics
Publication Date
10/28/2020
Defense Date
08/12/2020
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
CO₂ cycling,non-thermal plasma,OAI-PMH Harvest,pollutant removal,transient,waste recycling
Language
English
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Electronically uploaded by the author
(provenance)
Advisor
Cronin, Stephen Burke (
committee chair
), Haas, Stephan (
committee member
), Nakano, Aiichiro (
committee member
), Wang, Han (
committee member
), Wu, Wei (
committee member
)
Creator Email
merry.sisi.yang@gmail.com,sisiyang@usc.edu
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https://doi.org/10.25549/usctheses-c89-386091
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etd-YangSisi-9072.pdf
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Yang, Sisi
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Tags
CO₂ cycling
non-thermal plasma
pollutant removal
transient
waste recycling