OPTIMIZING CONTROL OF NOX AND SO2 EMISSIONS IN THE FBC PROCESS

A marked reduction of SO2 and NOx emission levels can be achieved by means of process-control methods. The sulphur capture by limestone additions into the bed can be optimised by the selection of appropriate process conditions-eg bed temperature and the ratio of primary:secondary air. Effective techniques for NOx reduction in the furnace include operation at relatively low temperature, operation at low excess-air values, staged combustion and flue-gas recirculation. The amount of fuel in the bed also has an effect on NOx emissions. The means of reducing emissions are partly interrelated, and they also have an influence on plant efficiency. Because of the complexity of the pollution-forming process, the static models for optimisation were based on process experiments conducted on two coal-fired industrial boilers. Generalised distance function was used as the cost function in the optimisation. The resulting constrained optimisation problem was solved by means, of the quadratic penalty function method, with Powell's method used as the optimisation algorithm. The results indicate that emission rates can be reduced by the choice of process operating conditions.