AN EMISSIONS MODEL FOR A BUBBLING FBC USING DETAILED CHEMICAL-KINETICS - SIGNIFICANCE OF DESTRUCTION REACTIONS

An emissions model has been developed by coupling the fluid dynamics o f a bubbling fluidised-bed combustor with the 8-reactions heterogeneou s and 340-reversible-reactions homogeneous chemistries. The model is u sed to explain the observed emission trends of nitric and nitrous oxid es as a function of bed temperature, excess air and operating pressure . Reduction of nitrogen oxides in the bed results in only a small frac tion of the nitrogen oxides formed by coal oxidation being emitted fro m the bed. At low temperatures the concentration of CO in the bed is h igher than at high temperatures; this enhances NO reduction via the ca talytic NO/CO destruction reaction. Increases in N2O destruction rates at high temperatures lead to reductions in its emissions. As the exce ss air ratio is raised, the bed char loading falls, with a consequent fall in the reduction of nitrogen oxides over bed char and higher emis sions. At elevated operating pressures both the char loading and the r esidence time of gases in the bed govern the emissions. The emissions of nitric oxide fall inversely with pressure, whereas the emissions of nitrous oxide, a product formed by NO destruction on char, pass throu gh a maximum. Emissions of nitrogen oxides can be explained in terms o f a competition between the formation and destruction reactions. This study shows that the destruction reactions control the net emissions o f nitrogen oxides. In turn, this dominance of the destruction reaction s over formation suggests that the optimal operating conditions should be evaluated in order to maximise the destruction of nitrogen oxides within the bed.