Towards a quantitative understanding of NOx and N2O emission formation in full-scale circulating fluidised bed combustors

A mathematical tool is being developed for studying the nitrogen oxide emission formation in circulating fluidised bed combustors. The model is based on detailed homogeneous and heterogeneous chemical kinetics and a simplified, reasonable description of CFB hydrodynamics with presumed temperature distribution (Kilpinen et al, 1999a). With the model different fuels and fuel mixtures can be compared in regard to their nitrogen oxide emission formation tendency at typical CFBC conditions. In this paper the structure of the CFBC model and its submodels are shortly described in present form. The CFBC model is tested for nitrogen oxide prediction at normal air staging conditions in a 12 MW CFB with bituminous coal and wood chips as the fuel, respectively. Comparisons of modelling results with detailed gas concentration profiles measured inside the furnace are made. The relative importance of homogeneous and heterogeneous reactions on NO and N2O concentration profiles is illustrated based on a quantitative reaction rate analysis at different parts in the combustor. The importance of effects of radical removal on particle surfaces, and thus, a decreased CO burnout and, simultaneously, enhanced rates of catalytic bed/char reactions on nitrogen oxides’ destruction are discussed