Soot, Unburned Carbon, and Ultrafine Particle Emissions from Air and Oxy-Coal Flames

1. 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

2. 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

3. Oxy-fuel Combustion Impacts upon Retrofit Fouling, Slagging, Ash partitioning … Ultra-fine particles This work Flame Ignition SOx, NOx Heat transfer … Soot Burnout (Adapted from: Stromberg, 2004)

4. 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

5. Laboratory Combustor Coal feeder Primary Flue gas Secondary 1.2 m 3.8 m Heat exchanger #1 - 8 • Maximum capacity: 100 kW • Representative of full scale units: • Self sustaining combustion • Similar residence times and temperatures • Similar particle and flue gas species concentrations • Allows systematic variation of operational parameters 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 port

6. Fuels Utah coal: bituminous coal PRB coal: sub-bituminous coal

7. Test Cases Case Summary Stoichiometric ratios (SR) at different flue O2 concentrations

8. 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 gravimetricloss-on-ignition (LOI) analyzer.

9. Sampling & Measurement (Continued) Ultrafine Particles Soot Unburned Carbon (collected on AdvantechCellulose Acetatefilters:C045A090C)

10. Black Carbon (BC) Data Air • 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 Oxy 27%O2 Oxy 32%O2 Utah coal

11. 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

12. Ultrafine Particle Size Distributions • Two particle modes: ～30 nm, >100 nm • The smaller mode decreases while the larger mode increases as flue gasO2 → 0 Air Oxy 27%O2 Oxy 32%O2 Utah coal

13. Compare to: Total soot (BC) via PA 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 concentrationsat high flue O2 .

14. LOI Data PRB coal Utah 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

15. 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. Utah coal PRB coal

16. Discussion: Why is soot diminished in oxy-fuel cases compared to air, at low O2 levels? • 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

17. 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: • Coagulation • As flue O2 ↓, the number of ultrafines ↑, coagulation rate ↑ (dependent upon N2), • 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. • 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.

18. 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

19. 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.

20. 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.

21. Thanks for your attentionQuestions?

22. 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)