Numerical modeling of coal-feedlot biomass blend combustion and nitric oxide emissions in swirl burner.

摘要

Co-firing coal biomass blends in utility burners is emerging as a new and cost-effective technology to reduce pollution and associated costs. It has been found in recent studies that the interactions between coal and biomass help to maintain combustion stability and reduce harmful pollutant emissions. Nevertheless, the fundamental understanding of pollutant formation and destruction in blend combustion is still in its infancy and more work is needed in this direction.; In the work presented here, the biomass used is feedlot animal waste henceforth called feedlot biomass or FB. A computer code, PCGC-2 is modified to include the effects of FB co-firing on NOx emissions. The modifications include addition of a third mixture fraction in the NOx model to track biomass off-gas and inclusion of ammonia in the NO reaction scheme since biomass is believed to release more ammonia during devolatilization process. The combustion model is based on mixture fraction-equilibrium chemistry approach. A postprocessor is used to calculate NOx levels after calculating the major variables (temperature, major species, density, etc.).; Sensitivity analyses are carried out to determine the effect of key parameters on combustion behavior and NO emissions. The data is compared with the experimental measurements obtained in the small-scale 30 kW (100,000 Btu/hr) boiler burner facility at Texas A&M. In general, good agreement is found. The key sensitivity parameters are excess air percentage (0--20%), swirl number (0--3), moisture in feedlot biomass (6--40%) and particle size of feedlot biomass (100 mum--600 mum). NO emissions increase (190 ppm to 320 ppm for coal, 209 ppm to 315 ppm for blend) with increasing excess air percentage. Swirl plays an important role by changing the mixing pattern close to the burner mouth. It is found that NO increases with increasing swirl number (145 ppm to 305 ppm) except at 0% excess air where NO decreases from 140 ppm to 125 ppm as swirl number is increased from 0 to 0.6 and then NO increases from 125 ppm to 202 ppm as swirl number is increased from 0.6 to 1.4. Moisture does not play a major role. Its effect is almost negligible due to low co-firing ratio. Biomass particle size, on the other hand, has a significant effect on combustion behavior and NO emissions. Small particles burn faster and produce more NO close to the burner. Large particles take more time to heat up and release volatiles. These volatiles are thus released further downstream where an oxygen rich zone conspires to produce more NO. It was found that large particles produce more NO than small or medium sized particles. The radially averaged NO levels at burner exit depend on the peak radially averaged NO levels inside the combustor. Higher the peak value of NO inside the combustor, higher the exit NO since heterogeneous reduction kinetics is same.; The technology of co-firing coal and feedlot biomass in pulverized fuel furnaces shows good promise. No flame stability problems are encountered for smaller co-firing ratios. NO emissions are also reduced compared to the coal only firing.