The effects of cofiring natural gas and coal on sulfur retention in ash

Abstract

Under certain conditions, cofiring natural gas with coal has been shown to reduce SO x emissions beyond the reduction expected from simple replacement of sulfur-bearing coal. This enhanced reduction of sulfur emissions, known as sulfur leveraging, is believed to result from increased sulfur capture in coal ash. In this research, experiments with three coals, using size ranges from 90 to 106 μm and 125 to 150 μm, and furnace temperatures of 1300 and 1550 K, demonstrate the extent of sulfur leveraging through increased sulfur retention in ash when cofiring coal and natural gas. Leveraging is shown to be affected by residence time (through particle size) and furnace temperature, while original sulfur form (pyritic or organic) and coal sorbent capacity are shown 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 SO 2 to more reactive SO 3, as caused by the natural gas flame. Results from numerical modeling of the furnace environment, particle combustion, and the evolution of sulfur to SO 3 support the experimental findings.