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Show A NUMERICAL STUDY OF THE EFFECTS OF BIOMASS COAL COFIRING ON UNBURNED CARBON AND NOx EMISSIONS Dinesh Gera1, Mahendra P. Mathur2, Mark C. Freeman2, Allen L. Robinson3 'Fluent, Inc., 3647 Collins Ferry Road, Morgantown, WV 26505 2Federal Energy Technology Center, 626 Cochran Mill Road, Pittsburgh, P A 15236 3Carnegie Mellon University. Mechanical Engineering Department, Pittsburgh, P A 15213 ABSTRACT A biomass particle combustion model is developed and implemented in the commercial computational fluid dynamic code F L U E N T to examine the effects of cofiring biomass and coal on unburned carbon and N O x emissions in a pilot scale pulverized fuel combustor. The coal/biomass combustion is simulated using the two-mixture-fraction, probability density function (PDF) approach and finite rate chemistry formulation. The coal devolatilization is simulated using the two-competing-rates Kobayashi model, and the char oxidation is modeled as the kinetics controlled surface reaction. The biomass devolatilization is incorporated using an Arrhenius-type, first order kinetic rate model. The biomass char oxidation is controlled by diffusion-limited surface reaction, and it is modeled as a constant density process. Model predictions were found to be in reasonably good agreement with the experimental data from the Sandia National Laboratory's Multifuel Combustor. Some exploratory simulations are also performed to predict the unburned carbon and N O x emissions in the CERF, a pilot scale 150 kW, combustor at the Federal Energy Technology Center. 1.0 INTRODUCTION Increasing renewable biomass energy is a potential strategy to reduce net C02 emissions. Cofiring biomass with coal in existing coal-fired boilers represents a promising avenue for increasing biomass fuel utilization. In addition, previous studies have shown that biomass has the potential to be an effective reburn fuel for reducing N O x emissions - offering the potential for reduced C O ; and N O x emissions (Zamansky, et. al, 1998). The large particle size and high moisture content of most biofuels, however, raises concerns over unburned carbon in terms of boiler operability and the marketing of ash. Minimizing unburned carbon is important when evaluating biomass fuel sourcing/processing requirements for boiler-specific applications, where variations in furnace temperature, flow profiles, boiler design (physical dimensions and heat release) and load swings significantly impact the residence time requirements for combustion. This is especially true for reburn applications in which the biomass is injected as a reburning fuel high in a boiler under fuel rich conditions for N O x control. Finally, specifying the capital and operating costs for fuel preparation (processing) and handling are critical factors that will influence the economic viability of a cofiring project. Due to the stiff competition and stringent emission regulations in the power industry, fuel utilization and the environment have become major concerns. Researchers are focusing on ways to improve fuel efficiency and reduce emissions in the pulverized coal combustion units. Important factors affecting fuel efficiency are the design of ports, size and number of burners, and the placement of burners to achieve good mixing with combustion air. High temperatures on one hand promote efficient heat transfer, but on the other hand generate higher levels of N Cy An environment-friendly combustor design should keep an optimum balance between the capital investment and the amount of pollution generated per process unit from the plant operation. The Federal Energy Technology Center (FETC) biomass co-firing program includes joint tests with researchers at Sandia National Laboratories (SNL) and the National Renewable Energy Laboratory (NREL) (Freeman, et al., 1998). The FETC is conducting pilot-scale biomass cofiring tests with emphasis on slagging/fouling, carbon burnout (fly ash LOI), and emissions (Freeman, et al., 1997). S N L and N R E L are acquiring more fundamental data related to ash deposition and kinetics, including 100% biomass comparison tests'to identify synergies and explain behavior between coal and biomass combustion (Robinson, et al., 1997). W e are in the |