Desulfurization Characteristics of Carbide Slag during Circulating Fluidized Bed Coal Combustion

Abstract

At low loads, the desulfurization efficiency of circulating fluidized bed (CFB) boilers tends to decrease due to the deviation of low bed temperature from the optimal temperature required for limestone desulfurization. However, carbide slag, a waste product rich in Ca(OH)2 and possessing a low decomposition temperature, can be an economically viable alternative to limestone. In this study, the desulfurization characteristics during coal combustion with carbide slag addition were studied both in a fixed-bed reactor and a 30 kW CFB reactor. The effect of different reaction conditions on the desulfurization performance was evaluated. Results showed that the addition of carbide slag decreased SO2 emission effectively. A higher oxygen content prompted the improvement of the desulfurization efficiency. The carbide slag showed the highest desulfurization efficiency at about 800 °C. The desulfurization performance deteriorated when the temperature was beyond 800 °C. To explain this phenomenon, the micromorphology of carbide slag raw particle, calcined particle, and sulfated particle was observed by scanning electron microscopy. More holes formed on the surface of the particles after calcination. Besides, the outer surface of the holes formed more crystal grains with the increasing temperature. CaSO4 products covered the outer surface of the crystal grains after desulfurization. The crystal grain gaps on the surface of carbide slag were easily blocked at high temperatures, which resulted in the low desulfurization efficiency.