Numerical simulation of SNCR (selective non-catalytic reduction) process in coal fired grate boiler

Abstract

The paper describes numerical simulations of flow with combustion in a coal-fired grate boiler. The work focuses on the NOx reduction process utilizing SNCR (selective non-catalytic reduction) technology. An integration of 12-step reduced mechanism with commercial CFD (computational fluid dynamics) code was presented. The SNCR reaction chemistry was fully coupled with the turbulent fluid mechanics and heat transfer via user-defined interface. The reduced mechanism was compared to 7-step global model which is commonly used in CFD codes. The comparison was done in the first step via plug-flow calculations. A better performance of 12-step reduced mechanism was demonstrated. Second stage of comparison was done with the CFD code. Final NO prediction at the boiler outlet for both mechanisms is similar and sufficiently accurate in relation to measurement. Nonetheless contours of NO concentrations suggest different NO rate in the initial stages of reduction. A similar trend was observed for secondary reactants NH3, HNCO. Large differences in calculated values at the outlet were observed. Disparity in secondary pollutants predictions is also observed. The general trend in NH3, N2O, HNCO and CO predictions was similar as in the plug flow calculations. These results suggest necessity of more detailed verification in a pilot or industrial scale to evaluate which mechanism is more accurate for coupled fluid dynamics computations.