Power boiler design

Abstract Boilers consist of structural elements including furnace, waterwall, superheater, economizer and air-preheater, fuel supply system, water treatment and supply system, emission control system, and so on. This chapter overviews these principal structural and functional elements and processes as well as the various steam-power systems, mainly focusing on coal as a typical fuel. This is mainly because other representative fuels, that is, oil and gas, are relatively easily treated, compared with coal-firing boilers, in boiler furnace and combustion-related events, for example, emission of nitrogen oxide (NOx), sulfur oxide (SOx), and particulate matter (PM). The coal system needs much more complicated elements owing to the existence of a variety of contents and ash from the aspect of efficient and high performance of boiler and the environmental issues. Thermal power generation is strongly linked to fossil fuel resource production. Countries rich in oil, natural gas, and coal resources use these as base power sources in combination with other energy sources, such as nuclear power and renewable energy, to meet the demand for their electricity. On the other hand, in countries with scarce fossil fuel resources, imports are premised, but long-term stability of electricity has been established by proper combination of fossil fuels, nuclear power, and renewable energy in consideration of economic efficiency, supply stability, and environmental friendliness. Fossil fuels used for boilers of utility and industrial power plant are categorized into gaseous, liquid, and solid. Natural gas, liquefied natural gas, liquefied petroleum gas, refinery gas, by-product gas such as coke oven gas, blast furnace gas (BFG) are gaseous fuels. Key points of design for gaseous fuel firing boiler are achievement of lower excess air firing and lower NOx production, suppression of combustion oscillation, prevention of backfire, and so on. BFG is low-calorific gas, so special consideration for combustion is needed. Crude oil, residual oil, heavy oil, and so on are liquid fuels. Key points of design for liquid fuel firing boiler are achievement of finer oil droplets by atomizing, lower excess air firing, lower NOx production, and lower unburnt carbon. Residual oil from refinery process tends to be high viscosity and to highly contain sulfur and some heavy metal elements such as vanadium, so special consideration for fuel handling, combustion, corrosion, and long-term boiler performance that is easy to deteriorate due to ash deposition. Coal, coke, wood, municipal, and industrial wastes are classified to solid fuels and their combustion characteristics including ash behavior widely vary. Representative solid fuel is coal that enables to mine world-widely so various kinds of coal have been effectively utilized for power generation for long time with continuous technical development. Among fossil fuels, coal is superior in terms of economy and supply stability. State-of-the-art technologies for ultralow-NOx combustion, high strength material for higher steam temperature, flexible operation, wider coal applicability, etc. have been developed and applied to the latest coal-fired boilers that contribute stable electricity supply. From these backgrounds, coal-fired power now accounts for about 40% of global power generation. Nowadays, coal-fired thermal power is also important in covering the fluctuations caused by increasing renewable energy.