MOUNTING concern over acid rain - rapidly emerging as environmental villain No. 1 - is prompting a stepped-up search for ways to curb the pollution at a major source: the power-plant stack.
The aim is to find new ways to clean up nose-searing emissions of sulfur and nitrogren oxides from plants that burn low-grade coal.
Both ingredients, when mixed with moisture in the air, are believed to cause a good deal of the acid rain affecting everything from forests to fish.
At present, devices called scrubbers are the front line of defense acid-rain-causing pollutants. These can be effective; most snare more than 90 percent of the sulfur dioxide in coal emissions.
But they are also expensive - often costing more than 10 percent of a plant's capital outlays - and do little to whisk-broom away offending nitrogen oxides. Most, too, produce large amounts of gray-white sludge as a byproduct - itself a growing pollution problem.
As a result, scientists are looking for cleaner ways to squeeze electricity from coal. Two novel techniques are gaining ground:
* Fluidized bed combustion. The idea here is that if the fuel can be burned more efficiently to begin with, little removal work will have to be done afterward on emissions. Many combustion technologies are being devised, but the fluidized bed method is a leading contender.
It involves burning crushed coal in a bed of limestone suspended in air. The limestone absorbs the sulfur pollutants. At the same time, because the boiler burns at a low temperature, little nitrogen oxide is produced. Moreover, a fluidized-bed furnace will handle many different fuels, from wood chips to a wide variety of coal grades.
''It is going to be the standard of the future,'' contends Howard Feibus, a US Department of Energy (DOE) coal specialist.
The technology has long been used in industry. At least 70 boilers and incinerators are in operation at oil plants, chemical firms, universities, and other sites across the country. But none have yet been built on a utility-grade scale.
A pilot plant is being tested by the Tennessee Valley Authority in Paducah, Ky. A larger commercial-grade boiler is to be built at the same site. At least two other utilities - one in Minnesota and another in Colorado - are planning demonstration models.
A fluidized bed is no magic wand, however. For one thing, the economics and technology remain to be proven on a large scale - which, of course, the demonstration projects will go a long way toward doing. There have been some problems with clogging and incomplete combustion in certain models, although advanced versions are now being built. Still, even if the utility tests prove successful, widespread commercial use probably won't get under way until the early 1990s.
* Perhaps even farther down the road is an unusual clean-up method - this one focusing on emissions, - that bombards pollutants with electron beams. Backers of the idea claim it can remove both the sulfur and nitrogen oxides - and turn leftover residues into salable products.
The technique works by passing flue gases coming off the plant boiler through a powerful electron beam in a reactor. In a game of molecular billiards, the electrons collide with certain particles in the gases, making the pollutants more ''reactive'' and easier to capture. Another agent is added, often ammonia, to help solidify the pollutants and produce a waste product usable in fertilizers.
Backed by federal funding, Indianapolis Power & Light Company is building a small E-beam pilot plant that will treat less than 1 percent of its flue gases starting in 1985. The system was developed by the Ebara International Corporation, a Pennsylvania firm.
Another tiny test plant, this one in Kentucky, is already using an electron gun to zap emissions. It is a slightly different technology and uses calcium instead of ammonia to remove acids. Westinghouse is working on still another version which, it says, will be less energy intensive than some of the others. ''We expect this to be the best available control technology,'' said Ira Kanter, a scientist at Westinghouse's R&D center in Pittsburgh. Still, the first small-scale model won't be off the lab bench until the end of next year.
Testing remains to be done to see if E-beams will be simple and cheap enough for full-scale utility use - and if they are, thus, a viable option for the 1990 s. ''It isn't here today and probably won't be tomorrow,'' says Kurt Yeager, a coal combustion specialist at the Electric Power Research Institute, of the E-beam method.
Other technologies, of course, are being devised that may eclipse these methods. Much research, for instance, is going into cleaning up coal before it even goes into the power plant. Conventional ''washing'' techniques can remove about one-third of the total sulfur content. But other ideas - from bombarding bits of coal with microwaves to bathing it in sulfur-eating bacterial solutions - should eventually improve on that. Also, plant operators could switch to higher-grade coals or other fuels.
How fast these options will emerge from the lab will depend as much on politicians as on the scientists. Efforts in Congress to impose tougher controls on plant emissions - a move opposed by the Reagan administration, which believes more research needs to be done into the causes of acid - were recently scuttled by the House. But the issue is certain to resurface.
Federal regulations calling for dramatic reductions would probably favor now available chemical scrubbers.
But eventually experts see utilities turning to several different techniques to avoid fouling the skies. Sums up Dr. John Deutch, dean of the School of Science at the Massachusetts Institute of Technology and a former director of DOE's Office of Energy Research: There won't be just ''one silver bullet coming along'' to solve the problem.