Co-firing of coal and rice straw pellet in a circulating fluidized-bed reactor

Abstract

This paper presents the experimental investigation of co-firing of coal with rice straw pellet in terms of combustion efficiency, gaseous emissions (CO, NOx and SO2) and occurrence of bed agglomeration. The influence of excess air ratio (EA) and mass share of rice straw in the mixed fuel were investigated in a lab-scale circulating fluidized bed (CFB) reactor. The fuel feed rate was fixed at 14 kg/hr, while the air supply was varied to achieve EA 1.28-1.94. Rice straw pellet was mixed with coal at 25% and 50% mass fraction. The results show that increasing EA generally reduced CO emission. However, at excessively high EA, CO emission started to rise due to the insufficient residence time for complete combustion and the cooling effect. The CO emission clearly reflected the combustion efficiency trend. Higher CO emission was also found when increasing biomass, which may be influenced by the lower combustion temperature and the incomplete combustion of biomass volatiles. NOx and SO2 emission strongly depended on N and S content in the fuel that is the higher rice straw addition the lower NOx and SO2. At lower EA, the observed lower NOx emission could be the effect of reduction by CO and carbon present; while the opposite effect was found at high EA. SO2 emission was more temperature dependent and continuously decreased with increasing EA. System pressure monitoring and visual observation of the spent bed showed no occurrence of bed agglomeration even at 50% addition of rice straw pellet. However, the size distribution of spent bed moved slightly towards a larger particle size range indicating a tendency of agglomeration.