IN 1959, electric utilities accounted for 44 percent of the coal consumed in the United States. Their share last year was 86 percent.Because of the nation's growing appetite for electricity, coal use jumped from 10 quadrillion British thermal units in 1959 to last year's record 19 quads - a quarter of total US energy consumption - even as sales of coal to residential, commercial, and industrial users dwindled and railroads switched entirely to oil. Coal's growing importance in the US energy mix can be measured a variety of ways. r Since 1984 coal has been the largest source of domestic energy production, passing 33 percent in 1990. (Because of energy imports, coal accounts for only 23 percent of domestic consumption.) r Coal production last year reached a record 23 quads, with the excess going to exports that earned $3 billion. r Fifty-six percent of electrical generation comes from coal - almost three times more than nuclear, over five times more than natural gas or hydroelectric, and 16 times more than oil. r The US has the world's largest coal reserves - 25 percent of the total. Ninety percent of the nation's store of fossil fuels is coal. The US has 6,000 quads of recoverable reserves, 10,500 quads of reserves potentially mineable with existing technology, and identified deposits of 89,000 quads. "We are the Saudi Arabia of coal," says John Grasser, a spokesman for the National Coal Association. Additional demand for coal will come from new power plants and replacement of old ones. By 2000, more than 25 percent of generating capacity will be 50 years old or more and ready to retire. And electricity generation, which accounted for 36 percent of total energy consumption in 1990, could rise to 45 percent by 2010. A midrange projection in the national energy strategy puts US coal consumption up another 65 percent by 2010. How close it comes - and how it gets there - will be affected by economic and environmental criteria. The 1990 Clean Air Act Amendments require coal fired-plants to slash sulfur dioxide and nitrogen oxide emissions. Most plants not already in compliance are in the Midwest and were built before 1977. They lack "scrubbers" and rely on locally mined, high-sulfur coal. One way they can comply is to spend $250 million for a scrubber that will swallow 5 percent of their energy output. "Yeah, they work," Mr. Grasser says. But scrubbers are "yesterday's technology." Another way to comply is to switch to low-sulfur coal found in the western US. Most coal is still mined east of the Mississippi, but its share of production has eroded from 95 percent to 61 percent over the past quarter-century. Continued switching will cost jobs in states like Illinois and Ohio, producers of high-sulfur coal. In response, those states are developing economic incentives to ensure that scrubbing is the lowest-cost means of compliance for their own utilities. Already, average sulfur content of coal used at utilities is 25 percent lower than 10 years ago because of switching to western coal and the growing practice of washing some of the sulfur out of eastern coal. In turn, plant emissions of sulfur dioxide are down 20 percent. Newer, cleaner-burning plants assist in lowering the average. The Department of Energy launched a $5 billion program in 1986 to develop highly efficient "clean coal" technology for new plants. Thirty projects have been funded so far. One, a "pressurized fluidized bed combustion" system in Brilliant, Ohio, captures more than 90 percent of coal's sulfur emissions. "We think we can come up with even better ways to burn it," Grasser says.