煤矿科技英语——15. ENVIRONMENTAL ISSUES
A new-type building material Xinke mortar is put into production in Zhuanghe, a county-level city under Dalian. As the developer Liu Zhenguo explained, the new - type mortar with coal fly ash as main raw material is a new generation of all conventional mortars , lime, coal fly ash and calcium carbide powder. The new product, making full use of industrial slag is toxin-free ,odour-free ,environmental friendly and energy-saving, thus reducing pollution.(Dalian Daily p.B2.May 24,2002.)
Because significant volumes of earth must be displaced to mine coal, coalmines and the resulting rock waste  can harm the environment. Furthermore, burning coal releases environmentally harmful chemical compounds into the air.
15-1 Mining and Mining Waste
Surface mining has resulted in a great deal of damage to the landscape . Many surface mines have removed acres of vegetation and altered topographic features , such as hills and valleys, leaving soil exposed for erosion. Longwall mining, which allows the mine to collapse, results in widespread land subsidence , or sinking. Coal and rock waste, often dumped indiscriminately  during surface and underground mining processes, weathers rapidly, producing acid drainage . Acid drainage contains sulfur-bearing compounds  that combine with oxygen in water vapor to form sulfuric acid. In addition, weathering of coalmine waste can produce alkaline compounds , heavy metals , and sediments. Acid drainage, alkaline compounds, heavy metals, and sediment leached from mine waste into groundwater  or washed away by rainwater can pollute  streams, rivers, and lakes.
Today, enterprises in many countries must secure government permits before mining for coal. In the United States, mining companies must submit plans detailing proposed methods for blasting, road construction, land reclamation , and waste disposal . New land reclamation methods, driven by stringent  laws and regulations, require coal mining companies to restore strip-mined landscapes to nearly premined conditions.
15-2 Burning Coal
The burning of coal produces environmentally harmful emissions. Some gases produced from burning coal, such as carbon dioxide, are known as greenhouse gases  because they trap the Earth’s heat like the roof of a greenhouse and may contribute to global warming . Other emissions from coal combustion  can lead to air and water pollution.
A. Greenhouse Effect 
Earth absorbs much of the heat energy radiated from the Sun. The planet then reradiates this heat back into the atmosphere. Carbon dioxide and some other gases that are naturally present in the atmosphere prevent much of the heat from escaping back into space, maintaining Earth at a temperature that can support life. These gases are known as greenhouse gases because they trap the Sun’s heat in much the same way as the glass roof of a botanical greenhouse . However, the immense quantity of fossil fuels burned during the world’s rapid industrialization  over the last 200 years has raised levels of carbon dioxide in the atmosphere by about 28 percent. This dramatic increase in atmospheric carbon dioxide, coupled with continuing depletion  of the world’s forests, which absorb carbon dioxide, has led many scientists to predict a heating of the atmosphere on a global scale. Such a global warming could disrupt  weather patterns, cause
the polar ice caps  to melt, and possibly lead to other environmental problems.
Today, many industrial countries are working to reduce emissions of greenhouse gases. One proposal is to establish a system requiring companies that create greenhouse gases to pay to emit carbon dioxide above a specified level. This payment could take several forms, including purchasing the rights to pollute from a company with carbon dioxide emissions below the specified level; purchasing forests, which absorb carbon dioxide, and keeping them from being developed; or paying to upgrade a plant in a developing country, thus lowering that plant’s carbon dioxide emissions.
B. Acid Rain 
Another environmental problem is acid rain, which forms from sulfur contained in coal. As coal burns, the sulfur combines with oxygen in the air to form sulfur dioxide. As sulfur dioxide is released into the atmosphere, this compound reacts with atmospheric moisture , forming sulfuric acid. This acidic moisture eventually falls back to Earth in the form of precipitation  known as acid rain. Environmental studies indicate that acid rain damages crops and forests as well as streams, lakes, and rivers.
The U.S. Clean Air Act, implemented in 1970 and revised in 1970 and 1990, is the federal law regulating air pollution in the United States. This legislation has significantly reduced emissions of sulfur oxides, known as acid gases. For example, the Clean Air Act requires facilities such as coal-burning power plants to burn low-sulfur coal. High-grade coals (coals with a higher heating value) generally contain more sulfur than low-grade coals such as lignite and subbituminous coal. Therefore, certain processes have been developed to remove sulfur-bearing compounds from high-grade coal prior to burning. The Clean Air Act also requires use of pollution-trapping equipment such as air scrubbers  (devices installed inside plant smokestacks  to remove sulfur dioxide from coal emissions). In addition, revisions to the Clean Air Act in 1990 established a system that allows coal-burning power plants to buy and sell sulfur emission permits with one another. This system tries to establish a financial incentive  to lower sulfur emissions by rewarding  power plants that reduce emissions below federal levels. Power plants that cut their sulfur emissions below the permitted levels can sell permits to burn coal to companies that exceed federal levels. Companies that reduce emissions reap financial rewards while polluters must pay an extra cost to pollute.
C. Fly Ash 
The burning of coal releases ashes known as fly ash into the atmosphere. Fly ash contains toxic metals  such as arsenic  and cadmium . In the United States the Clean Air Act requires that fly ash be removed from coal emissions. As a result, antipollution devices such as air scrubbers, baghouses , and electrostatic precipitators are use
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  use discharge electrodes  (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 
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 , furnace sorbent injection , and advanced flue-gas desulfurization .
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 . 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  in cement.
NOTES TO THE TEXT
 environmental issues：环境问题
 rock waste：废石
 topographic features：地形特征
 acid drainage：酸性排水
 sulfur-bearing compounds：含硫混合物
 alkaline compounds：碱性混合物
 heavy metals：重金属
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