Comparison of global ammonia chemistry mechanisms in biomass combustion and selective noncatalytic reduction process conditions

The success of computational fluid dynamics (CFD)-based combustion modeling is strongly dependent on several submodels which are required to close the Favre-averaged conservation equations for mass, momentum, and energy and transport equations for scalar quantities. Due to the requirement of high computer capacity, global mechanisms are frequently applied to model chemical reactions in industrial- scale CFD. The present study compares the performance of five global ammonia chemistry mechanisms in the conditions typical of the biomass combustion in fluidized beds. A special emphasis is given to the modeling of the selective noncatalytic reduction (SNCR) process. A modification of the standard k-epsilon model is used to model turbulence, the eddy dissipation combustion model and the eddy dissipation concept are used to model turbulence-chemistry interaction, and the finite-volume method (discrete ordinates) together with the weighted sum of gray gases model are used to model radiative heat transfer. A simplified approach is used to consider the bubbling bed in the overall CFD model. Experimental data available on the temperature and the concentrations of NH3 and NO are presented to validate the predictions to a certain extent. The results are shown to be strongly dependent on the chemistry model. Global chemistry models typically perform well under the conditions for which they were derived, but under other conditions, they may fail badly. Models suitable for modeling the SNCR process are identified, and it is shown that the correct emission trends can be predicted as a function of the SNCR process load. Due to the different conditions in the lower part near the bubbling bed and the upper section of the freeboard, a combination of more than one model may be a good approach in modeling the overall process.