Coal combustion in boilers: A mature technology facing new constraints

Abstract

Three techniques are being used currently for combustion of coal in boilers: stoker firing, pulverized-coal combustion, and cyclone furnaces. Combustion characteristics of these techniques are discussed from the viewpoint of the impact of modern-day demands on combustion of coal. Constraints imposed by three demands are singled out: adaptation of state-of-the-art techniques to major changes in coal quality; environmental protection requirements; and the need to develop modifications of existing techniques allowing rapid increased utilization of coal. Specifically, the discussion is about: (i) Implications of the requirement to hold the emissions of three major pollutants—SOx, NOx, and flyash—within prescribed U.S. standards. (ii) Effects of increased and decreased ash content, respectively, on combustion characteristics. (iii) Combustion of coal/oil dispersions, allowing coal to be burned in boilers designed for combustion of oil. An attempt is made at identifying gaps in the technical knowledge of the existing technologies, and some research needs are suggested. In addition, a brief mention is made of the ways in which two new (developing) technologies—fluidized-bed combustion and coal-fired magnetohydrodynamic generators—are expected to deal with modern-day constraints.