Processing Ash and Slag Wastes from Thermal Power Stations. Part 2

Abstract

For existing coal-fired power plants, current methods of utilizing ash and slag waste may be considered in addressing new environmental and economic risks. However, for new power sources, environmental considerations are much more important in selecting the coal-combustion technology. Technology based on a circulating fluidized bed is sometimes cited as the most promising approach to environmentally sound coal combustion. It permits significant decrease in emissions of sulfur and nitrogen oxides beyond the boiler, but is of no help in processing ash and slag waste. For waste disposal, a different approach is recommended for new plants or the upgrading of coal-fired plants: instead of coal combustion in a gas flux or a fluidized bed, combustion in bubbling slag melt. Such methods are described. The basic characteristics of pulverized-coal combustion and combustion in slag melt are compared. Two basic approaches to the development of coal-based power generation are proposed: coal combustion with supercritical steam parameters; and gas generation with a hybrid (steam + gas) power-generating cycle based on gasification of solid fuels. The electrical efficiency of steam-based plants may be increased from 30–36 to 44–45% with supercritical steam parameters; and to 50–55% when using a hybrid steam–gas cycle. The proposed industrial system for coal gasification in slag melt increases the overall electrical efficiency. The environmental and economic efficiency of carbon gasification is demonstrated. It is simple to produce components from slag by casting. The cast slag–coal components are of considerably higher quality than analogous cement–sand components with added fly ash. The ease of switching from one type of casting to another permits rapid response to market demand.