Parameter study of a model for NOx emissions from PFBC

Simulations with a mathematical model of a pressurized bubbling fluidized bed combustor (PFBC) combined with a kinetic model for NO formation and reduction are presented and discussed. The kinetic model for NO formation and reduction considers NO and NH3 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 bed inventory on the NO emission is presented and the trends are compared to experimental data from the literature. In general, the trends predicted by the model are in good agreement with the experimental observations.

The sensitivity of the simulated NO emission with respect to combustion parameters in the model is investigated. The analysis shows that the most important combustion parameters are the rates of CO and CH4 combustion, but the CO/(CO+CO2) ratio from char combustion is important too. A rate of production analysis for the nitrogenous species is used to identify the important reactions.

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 total reduction of NO by char and by CO catalyzed by char increases with increasing temperature, but is lower than usually attributed to these reactions. 20-30 % of the fuel-N in the form of NH3 is oxidized catalytically to N2 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 NH3 to NO and reduction of NO by NH3 increases with increasing temperature.