Synergistic reduction of SO2 emissions while co-firing biomass with coal in pilot-scale (1.5 MWth) and full-scale (471 MWe) combustors

Abstract

The objective of this study was to understand the synergistic effect of sulfur dioxide (SO2) emission reduction during the combustion of biomass-coal blends in comparison to pure coal by recording the gaseous SO2 concentrations, analyzing the composition of combustion ash by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), and simulating and verifying the fate of sulfur by a thermodynamic equilibrium software (FactSage). In Set A experiments, five combinations of coal and steam-exploded biomass (100/0, 75/25, 50/50, 25/75, 0/100 in percentages by mass) were co-fired in a 1.5 MWth pilot-scale combustor. In Set B, pure coal (100/0) and an 85/15 percent mass blend of coal and torrefied biomass were co-fired in a 471 MWe utility boiler. Woody biomass contains lower amounts of sulfur compared to coal and the linear lowering of SO2 emissions as a result of co-firing is called the dilution effect. For Set A tests, FactSage predicted, and ash analysis confirmed that calcium was primarily responsible for the synergistic emission reduction (beyond dilution effect) in all the blends, except in pure biomass, where potassium was responsible. During pure coal combustion, 41.1% of the sulfur in the system was absorbed in the ash, compared to 97.0% during pure biomass testing. In Set B tests, the 85/15 coal-biomass blend, showed a 28.1% reduction in SO2 emissions along with a 22.1% reduction in the lime slurry utilization at flue gas desulfurization (FGD) towers compared to pure coal, which is equivalent to saving 7.4 L of lime slurry per MWe-hour. The results suggest that blending woody biomass with coal can help coal-fired utility boilers meet environmental emission standards and reduce desulfurization costs. These results are important since the current literature is deficient in research conducted at suspension-fired full-scale boilers to reduce SO2 emissions by co-firing coal and biomass blends.