MODELING OF NOX EMISSIONS FROM PRESSURIZED FLUIDIZED-BED COMBUSTION -A PARAMETER STUDY

Simulations with a mathematical model of a pressurized bubbling fluidi zed-bed combustor (PFBC) combined with a kinetic model for NO formatio n and reduction are reported. The kinetic model for NO formation and r eduction considers NO and NH3 as the fixed nitrogen species, and inclu des homogeneous reactions and heterogeneous reactions catalyzed by bed material and char. Simulations of the influence of operating conditio ns: air staging, load, temperature, fuel particle size, bed particle s ize and mass of bed material on the NO emission is presented and compa red to results from the literature. In general, the trends predicted b y the model are in good agreement with the experimental observations. A rate of production analysis for the nitrogenous species is used to i dentify the important reactions for formation and reduction of NO. Acc ording to the kinetic model, the reduction of NO by CO catalyzed by be d material is very important, especially at low temperatures where the CO concentration in the bed is high. The sum of the reduction of NO b y char and by CO catalyzed by char increases with increasing temperatu re, but is lower than usually attributed to these reactions. In the te mperature range 973-1273 K, 20-30% of the fuel-N in the form of NH3 is oxidized catalytically to N-2 over bed material and so this reaction is important for a low conversion of fuel-N to NO. The importance of t he homogeneous oxidation of NH3 to NO and reduction of NO by NH3 incre ases with increasing temperature. The sensitivity of the simulated NO emission with respect to hydrodynamic and combustion parameters in the model is investigated and the mechanisms by which the parameters infl uence the emission of NO is explained. The analysis shows that the mos t important hydrodynamic parameters are the minimum fluidization veloc ity, the bubble size, the bubble rise velocity and the gas interchange coefficient between bubble and dense phase. The most important combus tion parameters are the rates of CO and CH4 combustion and the CO/(CO + CO2) ratio from char combustion. (C) 1997 Elsevier Science Ltd.