Kinetic modeling study on the potential of staged combustion in gas turbines for the reduction of nitrogen oxide emissions from biomass IGCC plants

The potential for reduction of nitrogen oxides in gas turbine combustors was studied by detailed chemical kinetic modeling under ideal flow conditions. The investigation focused on turbines burning biomass-derived gasification gas from an air-blown integrated gasification combined cycle plant. The aim was to give detailed information about the parameters that favor reduction of NOx emissions, providing a solid background for designing an air-staged, low-NOx gas turbine. The potential and limitations of the detailed chemical kinetic modeling as a predictive tool for simulating the process were discussed. Instantaneous, delayed, and back-streamed air/fuel mixing models were tested to study the effect of mixing on the emissions. Predictions showed that the nitrogen chemistry was mainly affected by temperature and pressure: low temperatures of about 900-1000 degrees C and high pressures of about 10-20 bar favored fuel nitrogen conversion to N-2. At atmospheric pressure, an increase in the number of air addition stages increased the conversion to N-2, but at higher pressure the reduction was more efficient with three-stage addition than with either one- or six-stage addition. The conversion efficiency of NH3 to N-2 increased with the inlet NH3 concentration, but the final NOx emission calculated in ppm(v) increased as well. NOx emission often was higher when HCN replaced ammonia in the gasification gas. The main paths for fuel-NH3 conversion to NOx and N-2 were predicted to occur via intermediate formation of amino radicals (NHi). Another important conversion path to N-x was shown to proceed via a H2NO intermediate. Models accounting for delayed mixing led to more realistic predictions, showing the effect of CH4 in the gasification on increased NOx emission by means of its CHi radicals.