The influence of air staging on the ‘NO’ emission from a fluidised bed coal combustor

Abstract

Experiments have been carried out on a 30 cm×30 cm square sectioned fluidised coal combustor to investigate the influence of air staging on the NO emission. In this operational mode the bed, primary stage, is maintained at sub-stoichiometric conditions whilst the balance of the combustion air (secondary air) is injected into the freeboard. The experimental results showed that the NO emission is substantially reduced for staged operation when compared to conventional operation. The extent of NO reduction for staged operation increased with the proportion of secondary air injected, and with bed temperature. At 800°C bed temperature, a reduction of 33% in the NO emission was achieved if 25% of the combustion air was injected into the freeboard. The elutriation loss from the combustor for staged operation was greater than that observed for conventional operation, and increased with the fraction of secondary air injected; the CO emission for staged operation was lower than that in conventional operation. Measurement of the axial NO concentration profile in the bed and freeboard showed that the NO emission in the vicinity of the coal feed is comparable for staged operation to that in conventional operation, but is reduced below that of conventional operation by chemical destruction higher in the bed and freeboard. NO reduction experiments were carried out in a 7.5 cm diameter batch combustor to investigate the nature of the chemical destruction of NO in staged operation. Results of these experiments showed that coal, coal volatiles and char will reduce NO emissions; in particular char in the absence of oxygen was found to be very effective in reducing NO emissions especially at bed temperatures of 800°C or higher and with char sized 500 μm or less.