The Pentagon papers are being burned, under guard, where they will produce a lot more heat and hot water for the Defense Department.

Daily, more than 10 tons of classified documents, ones that cannot be pulverized into paper pulp and recycled, are being burned in a small metal barn a coal lump's throw from the easternmost side of the Pentagon.

The new incinerator, beside the Pentagon's coal-fired power plant, not only solves a 20-year-old "secrets" disposal problem for the Pentagon and numerous downtown Washington military offices, but provides nearly 25 per cent of the heat and hot water needed for more than 22,000 Pentagon employees. It will save the Defense Department an estimated $250,000 a year.

The six-months-old incinerator already is a roaring success with Defense officials, not only because it saves money evaporating secrets, but because it creates virtually no air pollution.

"EPA (the Environmntal Protection Agency) checked it and it far surpassed their greatest expectations," Pentagon security chief Sam Carmell proudly proclaimed. "There are no visible emissions. You can't see a thing coming from the two stacks except occasional heat waves."

It's also more secure to truck secret documents 100 yards to a guarded incinerator than to haul them around town, said Carmell.

"Remember that Air Force truck that crashed on 14th Street Bridge and spilled classified documents all over the bridge and floating down the Potomac?" he said.

For the past 20 years the Defense Department daily has been trucking up to 10 tons of classified computer print-outs, typewriter ribbons, films, plastic tapes and other non-recyclable paper products to assorted incinerators around Washington, and guarding them until they were burned. Officials became desperate last year because most of the incinerators are being closed by EPA for causing air pollution. Carmell said. "And the we found this."

"This" is a 20-ton-a-day inclinerator built by a Mechanicsville, Va., company. Air Pollution Control Products Inc. The company now has incinerators in 34 U.S. cities "such as Salem," Va.; Orlando, Fla., and Hot Shrings, Ark . . . mostly cities under 150,000 population and most converting trash and garbage into steam that they use or sell," said company vice president Lee Wiles. The incinerators also can be made to produce electricity or even air conditioning, Wiles said.

The Pentagon incinerator, which operates only eight hours day at half its capacity, produces between .05 and .08 grams of particulate matter per cubic meter of flu gas, according to Wiles. A spokesman for EPA said this week that he believes the Pentagon plant actually was inspected for EPA by environmental officials of the Army stationed at the Army's Edgewood (Md.) Arsenal, but that its emission levels appeared to be below the level EPA requires for large new incinerators - which is no more than .18 grams of particulates per cubic meter.

The $212,000 cost of the incinerator - the building, a security vault and some minor road work cost another $106,000 - should be recouped in the firsty year since it until now it has cost about that to haul classified trash to nearby incinerators, such as those at Andrews Air Force Base and Alexandria, and to pay incineration fees.

At least an additional $50,000 a year should be saved in coal costs at the Pentagon's heating plant next door, because steam produced by the incinerator saves five tons of coal a day, which costs about $50 a ton. The material burned, primarily computer printouts "which aren't pulpable," said Carmell, is soaked with home-heating fuel oil until combustion temperatures reach up to 1,800 degrees, when the material - fed continuously into the incinerator - burns itself. The maximum fuel oil use is 12 gallons an hour, said Wiles.

While the Pentagon produces most of the incinerator fodder, other Defense Department offices around town also contribute, including the Forrestal Building, Buzzards Point and buildings in Arlington and Alexandria. The Pentagon's recyclable paper wastes are turned into pulp inside the Pentagon and then trucked to Richmond, Va., says Carmell, where a paper plant turns them into things like paper towels, napkins and toilet paper.