Modelling of NO x emissions from pressurized fluidized bed combustion—a parameter study

Abstract

Simulations with a mathematical model of a pressurized bubbling fluidized-bed combustor (PFBC) combined with a kinetic model for NO formation and reduction are reported. The kinetic model for NO formation and reduction considers NO and NH 3 as the fixed nitrogen species, and includes homogeneous reactions and heterogeneous reactions catalyzed by bed material and char. Simulations of the influence of operating conditions: air staging, load, temperature, fuel particle size, bed particle size and mass of bed material on the NO emission is presented and compared to results from the literature. In general, the trends predicted by the model are in good agreement with the experimental observations. A rate of production analysis for the nitrogenous species is used to identify the important reactions for formation and reduction of NO. According to the kinetic model, the reduction of NO by CO catalyzed by bed material is very important, especially at low temperatures where the CO concentration in the bed is high. The sum of the reduction of NO by char and by CO catalyzed by char increases with increasing temperature, but is lower than usually attributed to these reactions. In the temperature range 973–1273 K, 20–30% of the fuel-N in the form of NH 3 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 the homogeneous oxidation of NH 3 to NO and reduction of NO by NH 3 increases 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 influence the emission of NO is explained. The analysis shows that the most important hydrodynamic parameters are the minimum fluidization velocity, the bubble size, the bubble rise velocity and the gas interchange coefficient between bubble and dense phase. The most important combustion parameters are the rates of CO and CH 4 combustion and the CO (CO + CO 2) ratio from char combustion.