考研英语历年真题来源报刊阅读100篇(10)

来源：文都教育 | 时间：2008-07-05 | 阅读：1997 次 | [大中小] [收藏] [划词 ]

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Can coal be clean? NO.10

Can coal be clean?

Coal has several advantages as a fuel. It is abundant. It is widely distributed: countries that are short of other fossil fuels, such as Germany and South Africa, have mountains of it. As a result, it is cheap. Even though the price has risen in the past few years, it is still less expensive to run a power plant on coal than on almost anything else.

But coal is also dirty. It releases lots of soot and various noxious chemicals as it burns, and so has fallen out of favour in many Western countries. Worse, coalfired plants produce roughly twice as much carbon dioxide per unit of electricity generated than those that run on natural gas.

The obvious solution is to make coalfired generation cleaner. And that's what utilities in Western countries have been doing for years, to comply with ever stiffer airpollution standards. Reducing emissions of carbon dioxide, however, is another matter. In Britain, as in most rich countries, the average efficiency of coalfired power stations is about 35%. But Mitsui Babcock, an engineering firm, says its most recent designs can achieve efficiencies as high as 46%. It reckons that switching from an old design to a new one can cut fuel consumption and emissions by 23%.

Many methods can reduce the various emissions produced by coalfired power stations, so that they are at least no worse than gasfired stations. But technologies also exist to make coal cleaner still, by filtering out carbon dioxide from the flue gas and storing it somehow. This is theoretically possible, but expensive. Moreover, unlike modifications that improve efficiency, there are no savings to be had by adding carboncapture technology to a power plant. As a result, no such plants have been built.

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How does carbon capture work? Most utilities are eyeing one of three basic designs. The simplest, and easiest to bolt on to existing plants, treats carbon dioxide like any other pollutant, and extracts it from the flue gas. Many firms already use this "amine scrubbing" approach to remove carbon dioxide from natural gas, for example. But it is not so practical for largescale uses, since the amines are expensive, as is heating them to release the captured carbon dioxide.

"Oxy fuel" plants sidestep the difficulties of separating oxygen and nitrogen in the flue gas by burning coal in pure oxygen rather than air. The resulting flue gas is almost pure carbon dioxide. But the energy used to separate oxygen from air before burning is almost as great as that needed to filter out nitrogen afterwards, leading to a similar loss of efficiency.

The third approach, called "integrated gasification combined cycle" (IGCC), also requires oxygen, but for use in a chemical reaction rather than for burning. When heated in oxygen, coal reacts to form carbon dioxide and hydrogen. An amine solution then absorbs the carbon dioxide, while the hydrogen is burnt in a modified furnace. The amine scrubbing is cheaper than usual, since the reaction generates carbon dioxide in a more concentrated form. Engineers are also experimenting with membranes that would allow hydrogen to pass, but not carbon dioxide. There are four IGCC demonstration plants operating in America and Europe, although none currently captures carbon dioxide permanently; instead, it is simply released into the atmosphere. AEP's planned new plants will follow a similar design.

George Bush is a believer, at any rate. In 2003 he unveiled a subsidised scheme to build a zeroemissions IGCC plant called "Future Gen" by 2013. The European Union, for its part, is giving money to utilities dabbling in oxy fuel, among other schemes. Handouts from the taxpayer are needed, power firms argue, since the technology in question is still young. But it is hard to believe that it will ever grow up unless subsidies give way to stronger measures, such as longterm caps or taxes on carbondioxide emissions. The technology to eliminate such emissions from coalfired plants exists, but it will not be adopted without regulatory incentives from governments.

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