Experimental study on the effect of limestone on SO2 and NO emission characteristics during coal/coke combustion

Abstract

Adding limestone to coal combustion reduces SO2 emissions but sometimes increases NO emissions, posing a challenge for synergistic SO2 and NO control in circulating fluidized bed (CFB) boilers. Limestone in the CFB furnace can participate in the conversion of sulfur and nitrogen compounds from coal and its devolatilized coke. This study compared the effects of limestone on SO2 and NO emission characteristics during coal/coke combustion to explore the potential for controlling both emissions. Combustion experiments were conducted in a fixed-bed reactor under different reaction parameters including Ca/S mole ratio, oxygen concentration, and temperature. Results showed that SO2 emissions decreased and NO emissions increased with increasing Ca/S mole ratio for both fuels. Comparing the emissions from fuels with and without limestone addition, a lower oxygen concentration decreased the desulfurization efficiency and the increment of NO emissions caused by limestone for both fuels. Temperature had opposite effects of limestone on coal or coke combustion. For coke, higher temperatures improved desulfurization efficiency and decreased the increment of NO emissions caused by limestone. For coal, the effects were reversed. To explain the impact of temperature, the sulfur and nitrogen migration paths affected by limestone were analyzed based on the XPS results of the fuels. Thiophene-S and N-Q became the main sulfur and nitrogen components in coke at high temperatures. The conversion of thiophene-S into SO2 is slow, and the limestone is calcined more fully during coke combustion, improving the utilization rate of limestone. N-Q tends to be converted into NH3. Further, NO is reduced by NH3 over the catalyze of CaO at low oxygen conditions. This reaction process decreases the increment of NO emissions caused by limestone.