Numerical study of pulverized coal-litter biomass blend combustion and pollutant emissions in a swirl burner.

摘要

Combustion of pulverized coal-biomass blend is one of the most attractive new technologies to the electric power industry to reduce fuel cost and pollutant emissions. In this work the biomass considered is broiler litter called litter biomass or LB. Two tasks were performed: (1) formulation study of a mixture fraction PDF method for pulverized fuel combustion modeling and its simplification for three mixture fraction problems, (2) numerical study of pulverized coal-LB blend combustion and pollutant emissions in a swirl burner.; In the study of the mixture fraction PDF method, a system of arbitrary number mixture fractions is considered. General equations and procedures to solve turbulence mean chemical properties based on fast chemistry assumption are obtained. In the numerical study which is the major part of this work, the PCGC2 model for pulverized fuel combustion is modified by incorporating a moisture vaporization model, considering more char reactions, implementing adiabatic boundary condition, and including PO₂ and P₄O ₁₀ calculations. The generalized mixture fraction PDF method is reduced to three mixture fractions which track primary air, fuel mixture offgas, and vaporized moisture, respectively. The PCGC2 program is significantly modified.; Numerical predictions are conducted and a fairly good agreement with experimental data is achieved for the TAMU 100,000 BTU/hr (30 kW) boiler burner facility. The major features of moisture vaporization and its effects on flame structure and species emissions are successfully captured. The reaction 2C+O ₂ → 2CO is generally the only important char reaction whereas the steam-char reaction is found important at some near-burner locations where water vapor concentration is high and O₂ concentration is low. At furnace exit most phosphorous emissions are in the form of PO₂ at a few hundred ppm while emission of P₄O₁₀ is very low (a few ppm).; Parametric study is carried out numerically to determine effects of key parameters on combustion behavior and pollutant emissions. It is found that increasing the moisture level in fuel blend delays combustion causing longer flame length, lower burnout, more CO, more P₄O₁₀, less PO₂, and less NO emissions. Changing the moisture level in biomass only has negligible influence on combustion and pollutant emissions if the co-firing ratio is low. Increasing the swirl number leads to stronger recirculation flow near the burner, higher burnout, lower CO, slightly lower P₄O ₁₀, slightly higher PO₂, and higher NO emissions. Increasing excess air level causes stronger recirculation flow, stronger fuel-air mixing, a shorter flame length, higher burnout, lower CO, higher P₄O ₁₀, lower PO₂, and higher NO emissions.