Scrubbing coal for emission control.

Scrubbing Coal For Emission Control

The demand for electric power in the United States will grow at an annual rate of 2.4 percent throughout the 1990s, according to government forecasters. With coal already comprising 80 percent of our fossil fuel resources and coal-fired facilities supplying 57 percent of our electric power, it is safe to assume that coal will remain a viable option for meeting this demand well into the next century. At the same time, electric utilities are likely to have to comply with increasingly stringent emissions standards. As in the early 1970s, when passage of the Clean Air Act caused research on pollution control technologies to accelerate, electric utilities lately have been stepping up their search for effective and economical ways of further reducing emissions.

A portion of this work is directed towards refining a system for cleaning combustion gases known as the wet scrubber - a processing facility built at the back of the main power plant that traps sulfur dioxide before the flue gas reaches the atmosphere. Although the process underlying wet scrubbing was developed just after the turn of the century, the first devices were not built until the 1930s in Great Britain.

Most of the scrubbers in use today are based on concepts worked out during the 1960s. In the United States, the first full-scale scrubber began operating in 1967 at a coal-fired power plant owned by Union Electric of Missouri. Since then, utilities, suppliers, architectural and engineering firms, and the federal government have taken part in developing the technology.

The earliest scrubbers were dogged by numerous problems, such as corrosion and plugging, whose persistence reflected the slow development of the technology. And though many of these operational problems were eventually solved, most of today's scrubbers remain expensive and difficult to operate and maintain. For 90 percent of scrubber operations in the United States, handling and disposing of the waste product - a wet, pasty sludge - is a costly and complex undertaking. But above all, there is the question of efficiency: running a scrubber system typically drains away between 3 and 5 percent of the power produced by a generating plant.

Scrubbers come in several varieties, differentiated by three major characteristics: whether the process results in a wet or a dry product, whether it employs a slurry or a solution to absorb the [SO.sub.2], and whether it produces a salable or a throwaway end product. Whatever the type, scrubbers belong to a technology known as flue-gas desulfurization (FGD), the traditional method by which pollutants are removed from the coal after combustion. In the past few years, refinements in FGD have resulted in more efficient and reliable scrubber designs that remove more [SO.sub.2] at lower cost and produce a salable, or at least a more easily disposable, end product.