Soot, Unburned Carbon, and
Ultrafine Particle Emissions
from Air and Oxy-Coal Flames
William J. Morris
Dunxi Yu
Jost O. L. Wendt
Department of Chemical Engineering
University of Utah, Salt Lake City, UT 84112
Presented at 33rd International Symposium on Combustion
Tsinghua University, Beijing, China. August 1-6, 2010
Outline
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Introduction
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Objectives
Down-flow oxy-coal combustor (nominal 100kW)
Sampling and analyses
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Oxy-fuel impacts upon retrofit
Soot
Ultra-fine particles
Loss on ignition of total ash sample
Results
Discussion
Conclusions
Oxy-fuel Combustion Impacts upon Retrofit
Fouling, Slagging,
Ash partitioning
Ultra-fine particles
…
This work
Flame Ignition
Soot
SOx, NOx
Burnout
Heat transfer
…
(Adapted from: Stromberg, 2004)
Objectives of this research
Determine effects of oxy-firing on
Ultrafine Particles
Ash deposition
Flue gas cleaning
Soot
Flame properties
Heat transfer
Unburned Carbon
Char burnout
Combustion efficiency
Experimental data with error
quantification
Validated mechanisms
Validated models with error quantification
Oxy-fuel Combustion
Retrofit Design
Laboratory Combustor
Coal feeder
Primary
Secondary
1.2 m
3.8 m
1. Maximum capacity: 100 kW
2. Representative of full scale units:
1. Self sustaining combustion
2. Similar residence times
and temperatures
3. Similar particle and flue
gas species concentrations
3. Allows systematic variation of
operational parameters
Heat exchanger #1 - 8
Sampling port
Flue gas
This work: Uses oncethrough CO2 to simulate
cleaned flue gas recycle
with all contaminants and
water removed.
Future work: Will use
recycled flue gas.
Fuels
Utah coal: bituminous coal
PRB coal: sub-bituminous coal
Test Cases
Case Summary
Stoichiometric ratios (SR) at different flue O2 concentrations
Sampling & Measurement
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Ultrafine particles:
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Black carbon or “soot”:
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A Scanning Mobility Particle Sizer (SMPS) was used
to determine ultrafine particle size distribution (psd).
Real time and continuous black carbon
measurements were performed using a Photoacoustic Analyzer (PA).
Bulk ash:
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The ash was characterized using a hot foil
gravimetric loss-on-ignition (LOI) analyzer.
Sampling & Measurement (Continued)
Ultrafine Particles
Soot
Unburned Carbon
(collected on Advantech Cellulose
Acetate filters:C045A090C)
Black Carbon (BC) Data
Air
Oxy 27%O2
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Oxy 32%O2
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Utah coal
BC concentration varies with
flue gas O2 concentration
Air-firing has higher BC than
oxy-firing as flue O2 → 0
Difference becomes slight at
higher flue O2
Average BC concentration data for the Utah coal
Error bars in this
work: Standard
deviation
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BC decreases with increasing O2 level
 Except for the uptick at 3% flue O2 for oxyfuel case with 32%O2 (black ▲)
At very low O2 levels, oxy-coal combustion appears to yield lower BC
concentrations
No significant differences between the two oxy-coal cases are observed
(except at 3%O2)
Ultrafine Particle Size Distributions
Air
Oxy 32%O2
Oxy 27%O2
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Two particle modes: ～30 nm, >100
nm
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The smaller mode decreases while
the larger mode increases as flue
gas O2 → 0
Utah coal
Integrated SMPS mass concentrations
(15-615nm) Utah Skyline
Compare to:
Total soot (BC) via PA
Ultra-fines via SMPS
• Most of the ultra-fines are soot
• Oxy-firing leads to significantly decreased soot concentrations at low
flue O2 , but slightly increased soot concentrations at high flue O2 .
LOI Data
Utah coal
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LOI generally decreases with increasing O2
 Exception: LOI at 3%O2 is higher than that at 2%O2 for some cases
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PRB coal
Similar to BC and ultrafine data
At low O2 concentrations, air-firing cases have higher LOI
At high O2 concentrations, oxy-firing cases have higher LOI
Comparison of soot and LOI
Utah coal
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PRB coal
Only a weak correlation is observed between soot and LOI
(unburned char + soot) for the coals and conditions
presented here.
Discussion: Why is soot diminished in oxy-fuel
cases compared to air, at low O2 levels?
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Oxy-firing conditions inhibit the transport of O2 to the
particle and the diffusion of pyrolysis products to the
environment, which would lead to lower temperatures
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Lower local temperatures can diminish soot formation
from coal tars
Discussion: Evolution of bimodal psd in
ultrafine range
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As flue gas O2 concentration decreases, the
smaller dp mode decreases while the larger dp
mode increases. Three possibilities:
Coagulation
1.
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As flue O2 ↓, the number of ultrafines ↑, coagulation rate ↑
(dependent upon N2),
Soot Oxidation
2.
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Oxidation may cause the larger soot aggregates to break up into
multiple small particles. As flue gas O2 ↓, there is less oxidation,
increasing the second mode while decreasing the first mode.
Sulfates/H2SO4
3.
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Subsequent research has indicated that the first peak may be
high in sulfates, possibly condensed H2SO4, which is diminished
at low O2 levels.
Conclusions
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Oxy-coal combustion may diminish soot formation
at low stoichiometric ratios when compared to air
fired combustion
Ultrafine particle emissions from coal combustion
consist mostly of soot or black carbon
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Soot and UFPs decrease with increasing O2 level
UFPs have two modes: ～30 nm, >100 nm
First mode decreases while second mode increases with
decreasing flue gas O2 concentration
Soot emissions can be important due to their effects on human
health and climate change.
Effects of retrofit from air to oxy-coal on soot in the combustor
are also important for predictions of radiation heat transfer in
the furnace
Conclusions (Continued)
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At higher O2 levels (e.g., 3%O2), loss-on-ignition
(LOI) of the ash can increase under oxy-coal
conditions, relative to air.
Soot emissions, measured by PA, do not correlate
significantly with LOI, but do correlate with total
amount of ultra-fine particles, indicating that the PA
measures soot and probably not unburned char
particles.
Acknowledgements
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Financial support from the Department of Energy
under Awards DE-FC26-06NT42808 and DE-FC08NT0005015 , and the National Natural Science
Foundation of China under Award Number
50720145604
Jingwei (Simon) Zhang, Ph.D., Department of
Chemical Engineering, University of Utah
David Wagner, Ryan Okerlund, Brian Nelson,
Rafael Erickson, Institute for Clean and Secure
Energy, University of Utah.
Thanks for your attention
Questions?
Photoacoustic Analyzer (PA)
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Real-time measurement of
soot (black carbon)
concentration
Measurement of light
absorption at a laser
wavelength of 1047 nm
No filter artifacts
Rapid measurement, labor
saving
Providing information on
transient conditions
large dynamic range
(Arnott et al, Environ Sci Technol, 2005)