(FORTUNE Magazine) – GOOD NEWS for humans and other species worried about acid rain, beyond the shadow of a trout: this year several utilities are breaking ground by adapting an old technology to burn coal cleanly. The process, called atmospheric fluidized-bed combustion, offers the prospect of cleaner emissions at lower cost than can be achieved with the methods now used. It could become the mainstay of new and refurbished coal-fired plants. Since World War II most coal has been burned in pulverized form in hot-air furnaces at about 2,500 degrees F., producing not only steam that drives turbines but also nitrogen and sulfur oxides, both linked to acid rain. Utilities have been able to capture up to 90% of the sulfur emitted by installing cumbersome devices called scrubbers that clean the exhaust gas before it goes out the chimney. But scrubbers are expensive -- they can amount to one-third the cost of building a power plant -- and virtually ineffective against nitrogen oxides. Most scrubbers also produce a wet sludge that is difficult to dispose of. In conventional fluidized-bed boilers, different-size chunks and grades of coal are mixed with inexpensive limestone. Air is forced in, sometimes at such high velocity that the coal and limestone float around like ping-pong balls. The limestone reacts chemically with the coal to capture as much sulfur as a scrubber would. The result is a dry waste product, which may have some use as a building material. Even better, the temperature remains about 1,000 degrees lower than in a pulverized furnace, reducing the quantity of nitrogen oxide produced. The fluidized-bed concept was born in Germany in the 1920s, but the utility industry did not try to make the process economical at a power plant until the 1970s, when oil prices hit the roof. A pilot plant in Paducah, Kentucky, run by the Tennessee Valley Authority since 1982, demonstrated the commercial potential of the technology for coal. Now the idea is being applied to larger plants. This summer, at its Black Dog facility in Burnsville, Minnesota, Northern States Power (1985 assets: $4.1 billion) will ''light off'' a $52-million fluidized-bed boiler. The Colorado Ute Electric Association ($1.2 billion) is spending $87 million to install a fluidized-bed boiler and triple the capacity of its Nucla plant by August 1987. The TVA ($21.5 billion) has broken ground on a brand-new $205-million power plant, also in Paducah. And Wisconsin Electric ($2.3 billion) plans to retrofit four separate units in Oak Creek, Wisconsin, with the new technology. When the $380-million project is finished in five to six years, Oak Creek will be the largest fluidized-bed combustion station in the world. Says Charles Linderman, manager of the fossil fuels program at the Edison Electric Institute, an industry trade association in Washington, ''These plants can lead us to the environmentally benign utilization of coal.'' Nearly all of the utilities doing the pioneering have been helped along by money from the Electric Power Research Institute, an industry-funded outfit in Palo Alto, California; from other private companies; or from the federal government. Industry experts say no further subsidy for building conventional fluidized-bed plants is needed. A government and industry anti-acid-rain program that President Reagan discussed in March with Canadian Prime Minister Brian Mulroney (see Other Voices) may include funds to help develop so-called pressurized fluidized-bed systems, in which the air is forced upward under even greater pressure. This is a more efficient way to burn coal, but as yet the utilities that are working on the systems -- Wisconsin Electric and Ohio Power Co. -- have not proved they can harness the gases produced.