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1、Chapter 8 Electric Power Generation,Section 1 Coal-Fired Power Plants,Text,New Words and Expressions,Exercises,End,Authors, Organization, Address of a Paper,Section 1 Coal-Fired Power Plants,In fossil-fuel power plants, coal, oil, or natural gas is burned in a furnace, the combustion products heat w
2、ater, converting it to steam, and the steam drives a turbine which is mechanically coupled to an electric power generator. A schematic diagram showing a typical coal-fired power plant is given in Fig.8-1. In very brief outline, the operation of the plant is as follows. Coal is taken from storage and
3、 fed to a pulverizer (or a mill), mixed with preheated air, and blown into the furnace, where it is burned. The furnace contains a complex of tubes and drums,Section 1 Coal-Fired Power Plants,called a boiler through which water is pumped, its temperature rising in the process until it evaporates int
4、o stream. The steam passes on to the turbine, while the combustion gases (flue gases) are passed through mechanical and electrostatic precipitators which removed upward of 99% of the solid particles (ash) before being released to the chimney or stack.,Section 1 Coal-Fired Power Plants,Fig.8-1 Basic
5、principles of a coal-fired power plant,Section 1 Coal-Fired Power Plants,The unit just described, with pulverized coal, air, and water as an input and steam as a useful output, is variously called a steam generating unit, or furnace, or boiler. When the combustion process is under consideration the
6、term furnace is usually used, while the term boiler is more frequently used when the water-steam cycle is consideration. The steam, at a typical pressure of 3500 psi and a temperature of 1050F, is supplied through control and stop (shutoff) valves to the steam turbine. The thermal energy of the stea
7、m is converted into,Section 1 Coal-Fired Power Plants,mechanical energy by the turbine. The exhaust steam from the turbine is cooled in a heat exchanger called condenser, and as feed-water, is pumped back to the boiler again. The control valve permits the output of the turbine-generator unit (turbog
8、enerator) to be varied by adjusting steam flow. The stop valve has a protective function; it is normally fully open but can be “tripped“ shut to prevent overspeed of the turbine-generator unit if the electrical output drops suddenly (due to circuit-breaker action) and the control valve does not clos
9、e.,Section 1 Coal-Fired Power Plants,Fig.8-1 suggests a “single-stage“ turbine, but in practice a more complex “multi-stage“ arrangement is used to achieve relatively higher thermal efficiencies. The schematic diagram of a representative “multi-stage“ arrangement is given in Fig.8-2. Here four turbi
10、nes with different steam pressures are mechanically coupled in tandem. In routine outline, high-pressure steam from the boiler (superheater) enters the high-pressure (HP) turbine. Upon leaving the HP turbine it is returned to a section of the boiler (reheater) and then directed to,Section 1 Coal-Fir
11、ed Power Plants,the intermediate-pressure (IP) turbine. Leaving the IP turbine, steam (at lower pressure and much expended) is directed to the low-pressure (LP) turbines. The exhaust steam from the LP turbines is cooled in a condenser, and as feed-water, is reheated (with steam extracted from the tu
12、rbines) and pumped back to the boiler.,Fig.8-2 A representative “multi-stage“ turbine,Section 1 Coal-Fired Power Plants,The net efficiency of a coal-fired plant is usually less than 40% (i.e., less than 40% of the chemical energy in the coal is converted into electrical energy). Although this figure
13、 compares favorably with the industry average of approximately 30%, more than 60% of the fossil-fuel energy is converted to waste heat! To raise the efficiency and save the primary energy sources, the cogeneration system has long been used, where electricity and steam (or hot water) for industrial u
14、se or space heating are produced simultaneously . In such,Section 1 Coal-Fired Power Plants,systems, the overall energy efficiencies have been claimed to be as high as 60% to 65%.Some problems associated with the use of coal-fired power plants are that mining and transportation of coal present safet
15、y hazards and other social costs. Coal-fired plants share environmental problems with some other types of fossil-fuel plants: these include “acid rain“ and the “greenhouse“ effect.,Return,New Words and Expressions,coal-fired a. 燃煤的 furnace n. 炉，炉膛，燃烧室 pulverizer n. 磨煤机，粉煤机 preheated a. 预热的 boiler n.
16、 锅炉，蒸发器，蒸汽发生器 flue gas 烟气，废气，排烟 electrostatic a. 静电的 precipitator n. 除尘器,New Words and Expressions,electrostatic precipitator 电除尘 chimney n.（高）烟囱，烟筒 coal hopper 煤斗 burner n. 燃烧器，喷燃器 cooling tower 冷却塔 feed water pump 给水泵 psi 磅/平方英寸（压强单位） shutoff n. 关闭，切断，断路,New Words and Expressions,exhaust steam 乏汽 heater exchanger 热交换器 condenser n. 凝结器，冷凝器 feed-water 给水 turbogenerator n. 涡轮发电机 single-stage turbine 单级汽轮机 thermal efficiency 热效率 multi-stage turbine 多级汽轮机,
