Fly ash recirculation by bottom feeding on a circulating fluidized bed boiler co-burning coal sludge and coal

Coal sludge is a by-product of coal washing, whose production is huge. Burning it in Circulating Fluidized Bed (CFB) boiler can both recover the heat and reduce the pollution. However, the combustion performances of CFB boilers burning coal sludge are generally not high because the particle size of fuel is small and its resident time in the furnace is short. With the purpose of improving combustion efficiency and reducing pollutions, fly ash recirculation by bottom-feeding (FARBF) technology was applied to a 75 t/h CFB boiler burning mixture of coal sludge and coal. Industrial experiments were carried out to investigate the influence of fly ash recirculation rate on combustion performance and pollutant emission characteristics of the boiler. Results show that with FARBF, the dense bed temperature drops while the furnace exit temperature increases, the temperature distribution in the furnace becomes uniform. Under the condition of 100% maximum continuous rate (MCR), the combustion efficiency increases from 92% to 95% and the desulfurization efficiency increases from 83% to 90% as the fly ash recirculation rate increases from 0 t/h to 8 t/h. As the recirculation rate increases, the emissions of NO and CO decrease, but the particulate matter emission increases. The present study indicates that FARBF technology can improve the combustion performance and reduce pollution emissions (except for particulate matter emission) for CFB boilers burning coal sludge, and it can bring significant economical and environmental benefits.