Coal cleans itself up for power generation

Press, 26 April 1982, Page 16

Coal cleans itself up for power generation

From

“The Economist,” London

Despite growing pessimism about replacing petrol with synfuels made from coal, many utilities are bullish about using coal gasification for electricity generation. They reckon “com-bined-cycle” • power plants, which use coal gasification as a first step in electricity production, will be cheaper and more efficient than conventional ' coal-fired power stations. For a rather special reason: gasification technology owes its competitiveness entirely to tough anti-pollution laws.

That is reason enough to spur development of coal-gasi-fication, combined-cycle power plants. A consortium of American utilities and power-station builders plus Japanese utilities is now spending over S3OOM to build a commercial-sized plant called the Cool Water project. Calculations made by America's utility-financed Electric Power Research Institute (E.P.R.1.), and the largest single contributor to the consortium, show why.

E.P.R.I. researchers, reckon the capital costs of the com-bined-cycle plant will be about $B5O per kilowatt of generating capacity, about 3 per cent less than a conventional coal-fired power station. More important, the combined-cycle plant will convert about 36 per cent of the energy in the coal it consumes into electricity. That is about 3 per cent more than can be managed by a conventional power station built to meet American environmental standards.

If the extra equipment needed to meet environmental laws could be eliminated, however. the advantages would be

reversed. The cost of a conven-, tional coal-fired power station would drop by 30 to 40 per cent, and its energy-converting efficiency would climb to 38 per cent. The combined-cycle plant gains its extra efficiency by generating electricity in" two stages. About half of the power comes from the heat of coal gasification. Gasification requires temperatures of about 1300 deg C. and, by running the hot coal gas through heat exchangers, the heat can be used to generate steam for turbine generators. The remainder of the station’s power is then generated by burning the coal gas in other turbines, which use the expanding gases directly to produce electricity. The advantage of this twostep process is that gasified coal can be purified much more easily than the waste gases of coal combustion. Most power stations today use “fluegas scrubbers” to clean pollutants from the exhaust gases of coal combustion. Gasification produces a smaller volume of gas — as little as 1 per cent as much. Also, manv pollutants are in a chemical form that is intrin-

sically easier to clean. Sulphur, for example, becomes hydrogen sulphide in gasified coal, a chemical which engineers have long known how to remove from gas. In conventional flue gas, the sulphur becomes sulphur dioxide, which is nastier. Even the ash left by coal gasifications is less noxious, unlike that of coal combustion, it need not be treated in “sludge ponds” to remove soil pollutants. The ease with which gasified coal can be cleaned up could save land, water, and chemicals like the limestone used in flue gas scrubbing (see table). But there are big obstacles to be overcome before such savings become reality. The coal gasifier needed for" combinedcycle power plants operates at extremely high temperatures and nobody is sure how long the materials used in the gasifier will stand the combined strains of heat and pressure. More important the com-bined-cycle power plant requires "a variety of relatively new technologies to work together reliably. Only experience with commercial-sized plants like Cool Water can prove that they will.