d to trap these pollutants. Baghouses work like giant vacuum cleaners, drawing coal emissions through giant fabric bags that trap the fly ash inside. Electrostatic precipitators [36] use discharge electrodes [37] (electrically charged parts of an electric circuit) to trap ash particles. In an electrostatic precipitator the electrodes are located between long, positively charged collection plates. As the fly ash passes between these collection plates, the discharge electrodes give each particle a negative charge. These negatively charged particles are then attracted to and held by the positively charged collection plates.
15-3 Clean Coal Technology [38]
Since 1986 the United States government and private industry have been working together to develop cleaner and more efficient ways to harness the energy in coal. This joint effort, known as the Clean Coal Technology Demonstration Program, includes several technologies, such as fluidized bed coal combustion [39], furnace sorbent injection [40], and advanced flue-gas desulfurization [41].
Fluidized bed coal combustion burns coal in a limestone bed that transfers heat to water, generating steam. This steam is pressurized and used to turn a turbine shaft, which subsequently drives an electric generator. The limestone absorbs sulfur dioxide emitted by the coal, thus reducing the amount of acid gases released during combustion.
A process called furnace sorbent injection removes acid gas from coal emissions at less cost than expensive scrubbers. A sorbent is a highly absorbent material, such as powdered limestone [42]. It is injected into furnaces, where the powdered limestone reacts with the acid gases emitted by the burning coal. The used powder is siphoned away through the furnace outtake and is captured (with fly ash) in a baghouse or electrostatic precipitator.
A process called advanced flue-gas desulfurization also removes acid gas from burning coal without expensive scrubbers. Emissions from burning coal are piped into a container called an absorber, where the acid gases react with an absorbing solution (such as a mixture of lime, water, and oxygen). This reaction forms gypsum, a soft white mineral valuable as an ingredient [42] in cement.
NOTES TO THE TEXT
[1] environmental issues：环境问题
[2] rock waste：废石
[3] landscape：景观，景色
[4] topographic features：地形特征
[5] subsidence：沉陷，下沉
[6] indiscriminately：无差别地
[7] acid drainage：酸性排水
[8] sulfur-bearing compounds：含硫混合物
[8] alkaline compounds：碱性混合物
[9] heavy metals：重金属
[10] groundwater：地下水，潜水
[11] pollute：污染
[12] land reclamation：采区复田
[13] waste disposal：废石处理
[14] stringent：严格的，必须遵守的
[15] greenhouse gases ：温室气体
[16] global warming：全球变暖
[17] coal combustion：煤的燃烧
[18] greenhouse effect：温室效应
[19] botanical greenhouse：植物的温室
[20] indust
