THE EFFECTS OF COFIRING NATURAL-GAS AND COAL ON SULFUR RETENTION IN ASH

Under certain conditions, cofiring natural gas with coal has been show n to reduce SOx, emissions beyond the reduction expected from simple r eplacement of sulfur-bearing coal. This enhanced reduction of sulfur e missions, known as sulfur leveraging, is believed to result from incre ased sulfur capture in coal ash. In this research, experiments with th ree coals, using size ranges from 90 to 106 mu m and 125 to 150 mu m, and furnace temperatures of 1300 and 1550 K, demonstrate the extent of sulfur leveraging through increased sulfur retention in ash when cofi ring coal and natural gas. Leveraging is shown to be affected by resid ence time (through particle size) and furnace temperature, while origi nal sulfur form (pyritic or organic) and coal sorbent capacity are sho wn to have little effect. Results from sorbent activation studies, SEM images, and N-2 adsorption measurement of total surface area indicate that the effects of a natural gas flame on ash sorbent reactivity and ash surface area are minimal. Results also indicate that the primary mechanism for sulfur leveraging is the gas phase conversion of SO2 to more reactive SO3, as caused by the natural gas flame. Results from nu merical modeling of the furnace environment, particle combustion, and the evolution of sulfur to SO3 support the experimental findings.