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 Presented at 33rd International Symposium on Combustion Tsinghua 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 deposition Flue gas cleaning Flame properties Heat transfer Char burnout Combustion efficiency Experimental data with error quantification Validated mechanisms Validated models with error quantification Oxy-fuel Combustion Retrofit Design

Laboratory Combustor Primary Coal feeder 3.8 m Secondary 1.2 m Heat exchanger #1 - 8 Flue gas 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 Sampling port This work: Uses once- through CO 2 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 Stoichiometric ratios (SR) at different flue O 2 concentrations Case Summary

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 O 2 concentration Air-firing has higher BC than oxy-firing as flue O 2 → 0 Difference becomes slight at higher flue O 2 Utah coal Air Oxy 27%O2 Oxy 32%O2

Average BC concentration data for the Utah coal BC decreases with increasing O 2 level Except for the uptick at 3% flue O 2 for oxyfuel case with 32%O 2 (black ▲) At very low O 2 levels, oxy-coal combustion appears to yield lower BC concentrations No significant differences between the two oxy-coal cases are observed (except at 3%O 2 ) 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 O 2 → 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 O 2, but slightly increased soot concentrations at high flue O 2. Compare to: Total soot (BC) via PA

LOI Data Utah coal PRB coal LOI generally decreases with increasing O 2 Exception: LOI at 3%O 2 is higher than that at 2%O 2 for some cases Similar to BC and ultrafine data At low O 2 concentrations, air-firing cases have higher LOI At high O 2 concentrations, oxy-firing cases have higher LOI

Utah coalPRB 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 O 2 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 O 2 levels?

Discussion: Evolution of bimodal psd in ultrafine range As flue gas O 2 concentration decreases, the smaller d p mode decreases while the larger d p mode increases. Three possibilities: 1. Coagulation As flue O 2 ↓, the number of ultrafines ↑, coagulation rate ↑ (dependent upon N 2 ), 2. Soot Oxidation Oxidation may cause the larger soot aggregates to break up into multiple small particles. As flue gas O 2 ↓, there is less oxidation, increasing the second mode while decreasing the first mode. 3. Sulfates/H 2 SO 4 Subsequent research has indicated that the first peak may be high in sulfates, possibly condensed H 2 SO 4, which is diminished at low O 2 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 O 2 level UFPs have two modes: ～ 30 nm, >100 nm First mode decreases while second mode increases with decreasing flue gas O 2 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 O 2 levels (e.g., 3%O 2 ), 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- NT , and the National Natural Science Foundation of China under Award Number 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) 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)