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.

FuelsUtah coal: bituminous coalPRB coal: sub-bituminous coal

Test Cases

Stoichiometric ratios (SR) at different flue O2 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 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.

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)