Impacts of water vapor and AAEMs on limestone desulfurization during coal combustion in a bench-scale fluidized-bed combustor

Limestone is normally added into fluidized bed boiler furnace as sulfur sorbent. Water vapor, alkali and alkaline earth metals (AAEMs) within coal have great influence on desulfurization mechanism. In this paper, the impacts of water vapor and AAEMs on limestone desulfurization during coal combustion are studied. In the experiments, Jincheng anthracite (JCA) and Zhundong lignite (ZDL) were used as fuel with limestone addition under different concentrations of water vapor in a bench-scale fluidized-bed combustor (FBC). SEM, ICP-AES and XRD techniques were employed to analyze the micro-morphology and chemical compositions of deposited ash to understand the mechanism. The results show that water vapor and AAEMs within coal accelerate the limestone desulfurization during coal combustion. ZDL has a stronger propensity of self-desulfurization than JCA. The grain size of coal ash grows with water vapor addition, and this phenomenon is further intensified under the condition of desulfurization or ZDL combustion. The mechanism is probably that water vapor and AAEMs improve the solid state diffusion during desulfurization and improve growth of ash grains.