The Environmental Protection Agency, under extreme pressure from the coal industry and Senate Majority Leader Robert C. Byrd (D-W.Va.), has decided to relax proposed air pollution standards for new coal-fired power plants.

The decision, which has not been publicly released, represents a complete "cave-in" to industry and a betrayal of President Carter's commitment to clean air, environmentalists charged yesterday.

But a coal company spokesman said industry generally is pleased with the decision because it represents a "balance" between eastern and western coal interests and does not require excessive expenditures for cleanup equipment.

The coal plant standard is one of the hottest regulatory controversies in the Carter administration. With an estimated price tag of at least $4.4 billion a year, EPA rules proposed in September were attacked by White House economic advisers as inflationary. Energy Secretary James Schlesinger said the rules would inhibit coal production.

However, Interior Secretary Cecil D. Andrus sided with environmentalists and favored and strict rules to protect the pristine air of the West's vast national parks, to discourage strip mining in the West and to protect eastern coal mine jobs.

Administration sources said yesterday tht EPA Administrator Douglas Costle has decided to require partial "scrubbing" of sulfur emissions on a sliding scale of 70 to 90 percent, depending on the sulfur content of the coal burned. His original proposal was to require all utilities to remove 90 percent of the sulfur.

Costle also has decided to retain the current emissios ceiling of 1.2 pounds per million BTUs, measured as a monthly average. EPA had been considering a standard almost twice as strict.

The ceiling decision came after two weeks of what one Senate source called "hard-ball arm-twisting" by Byrd and other coal state senators. Byrd summoned Costle and White House adviser Stuart Eizenstat, strongly hinting that the administration needs his support on the strategic arms limitation treaty (SALT) and the windfall profits tax, according to Senate and administration sources.

Byrd also has persuaded a score of senators to sign a letter to President Carter arguing that a "stringent emission standard" of 8 pounds per million BTUs "would have dire consequences for the coal industry and . . .would discriminate against Midwestern and Eastern coal, which is high in sulfur content."

Byrd and Sen. Wendell Ford (D-Ky.) cite a recent National Coal Association study claiming that 85 to 99.9 percent of the reserves of certain companies in northern West Virginia, Ohio, Illinois, Kentucky and Indiana would be barred from use under a strict ceiling.

Richard Ayres, a Natural Resources Defense Council attorney, called the study "a gross exaggeration" and "an illegal, last-minute attempt to alter the standard after the close of the public commetn period."

Costle's decision to adopt a "weak" standard will result in "serious long-term health problems," he said, given the nation's projected doubling of coal use by 1980. Many of the 200 proposed new power plants could end up emitting three times as much pollution as current plants, he added.

"The East especially is already heavily polluted," Ayres said. "If you allow those additional emissions you get tens of thousands more deaths, sickness from respiratory disease and acid rain," which kills fish and crops.

However, an administration official who asked not to be identified defended Costle's choice of partial scrubbing, saying that some computer studies show it could decrease emissions natinally because it would prompt the use of low-sulfur coal. Also, he added, it will encourage dry scrubbing, a cheaper, but relatively untested technology.

The official said utilities are likely to criticize the relaxed standard as too expensive. But he said its projected cost of $3.2 billion a year by 1995 is only 2 percent of the industry's $175 billion revenue requirements. It will, he said, add 95 cents to the consumer's average monthly utility bill in 1995-as compared to a $1.50 cost for full scrubbing.

The official added that EPA, under constant pressure from industry, economic and energy officials, decided "we can't fight on every issue." He added, "I don't think anyone in the administration is unaware that Byrd can help us or hurt us on a variety of administration priorities . . .The administration needs support from coal state senators on SALT and especially on the windfall profits tax. Doug [Costle] is part of the administration and he doesn't want to harm other administration objectives."

EPA disagrees with Byrd that a strict ceiling would devastate the high sulfur coal industry. However, the official said, "precluding the use of coal reserves doesn't fit with the image of the administration encouraging coal use. There're a lot of symbols involved in this."

Costle's decision is certain to be challenged in Congress, which instructed EPA in the 1977 Clean Air Act amendments to require "best available control technology," i.e. full scrubbing. Costle and Schlesinger testified in favor of best available technology-a fact that is now prompting accusations of hypocrisy by environmentalists.

Environmental Defense Fund attorney Robert Rauch, calling Byrd's role "outrageous," said he will file suit. "A powerful senator is given information by a private party and forces Costle to back down. It makes a complete sham of rule-making procedures, which are supposed to be above board."