Soot, Unburned Carbon, and Ultrafine Particle Emissions from Air and Oxy-Coal Flames
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Transcript of Soot, Unburned Carbon, and Ultrafine Particle Emissions from Air and Oxy-Coal Flames
Soot, Unburned Carbon, and Ultrafine Particle Emissions
from Air and Oxy-Coal Flames
William J. MorrisDunxi Yu
Jost O. L. Wendt
Department of Chemical EngineeringUniversity of Utah, Salt Lake City, UT 84112
Presented at 33rd International Symposium on CombustionTsinghua University, Beijing, China. August 1-6, 2010
Outline
Introduction Oxy-fuel impacts upon retrofit
Objectives Down-flow oxy-coal combustor (nominal 100kW) Sampling and analyses
Soot Ultra-fine particles Loss on ignition of total ash sample
Results Discussion Conclusions
Oxy-fuel Combustion Impacts upon Retrofit
(Adapted from: Stromberg, 2004)
Flame Ignition
SOx, NOx
Heat transfer…
Fouling, Slagging, Ash partitioning
…Ultra-fine particles
Burnout
Soot
This work
Objectives of this research
Determine effects of oxy-firing on
Ultrafine Particles Soot Unburned Carbon
Ash depositionFlue gas cleaning
Flame propertiesHeat transfer
Char burnoutCombustion efficiency
Experimental data with error quantification
Validated mechanisms
Validated models with error quantification
Oxy-fuel CombustionRetrofit Design
Laboratory Combustor
Primary
Coal feeder
3.8 m
Secondary
1.2 m
Heat exchanger #1 - 8
Flue gas1. Maximum capacity: 100 kW2. Representative of full scale
units:1. Self sustaining combustion2. Similar residence times
and temperatures3. Similar particle and flue
gas species concentrations3. Allows systematic variation of
operational parameters
Sampling port
This work: Uses once-through CO2 to simulate cleaned flue gas recycle with all contaminants and water removed.
Future work: Will use recycled flue gas.
Sampling & Measurement
Ultrafine particles: A Scanning Mobility Particle Sizer (SMPS) was used
to determine ultrafine particle size distribution (psd). Black carbon or “soot”:
Real time and continuous black carbon measurements were performed using a Photo-acoustic Analyzer (PA).
Bulk ash: 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
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
Utah coal
Air
Oxy 27%O2
Oxy 32%O2
Average BC concentration data for the Utah coal
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)
Error bars in this work: Standard
deviation
Ultrafine Particle Size Distributions
Two particle modes: ~ 30 nm, >100 nm
The smaller mode decreases while the larger mode increases as flue gas O2 → 0
Utah coal
Air Oxy 27%O2
Oxy 32%O2
Integrated SMPS mass concentrations (15-615nm) Utah Skyline
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 .
Compare to:Total soot (BC) via PA
LOI DataUtah coal PRB coal
LOI generally decreases with increasing O2
Exception: LOI at 3%O2 is higher than that at 2%O2 for some cases
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
Utah coal PRB coal
Comparison of soot and LOI
Only a weak correlation is observed between soot and LOI (unburned char + soot) for the coals and conditions presented here.
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
Lower local temperatures can diminish soot formation from coal tars
Discussion: Why is soot diminished in oxy-fuel cases compared to air, at low O2 levels?
Discussion: Evolution of bimodal psd in ultrafine range
As flue gas O2 concentration decreases, the smaller dp mode decreases while the larger dp mode increases. Three possibilities:1. Coagulation
As flue O2 ↓, the number of ultrafines ↑, coagulation rate ↑ (dependent upon N2),
2. Soot Oxidation 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.
l Sulfates/H2SO4
Subsequent research has indicated that the first peak may be high in sulfates, possibly condensed H2SO4, which is diminished at low O2 levels.
Conclusions
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 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)
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
Financial support from the Department of Energy under Awards DE-FC26-06NT42808 and DE-FC08-NT0005015 , 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.
Photoacoustic Analyzer (PA)
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)