The Air Force wants to build at its Malmstrom base in central Montana the first piece of what it hopes will be a nationwide network of facilities that would convert coal into cleaner-burning synthetic fuel.

Air Force officials said the plants could help neutralize a national security threat by tapping into the country's abundant coal reserves. And by offering itself as a partner in the Malmstrom plant, the Air Force hopes to prod Wall Street investors - nervous about coal's role in climate change - to sink money into similar plants nationwide.

''We're going to be burning fossil fuels for a long time, and there's three times as much coal in the ground as there are oil reserves,'' said Air Force Assistant Secretary William Anderson. ''Guess what? We're going to burn coal.''

Tempering that vision, analysts say, is the astronomical cost of coal-to-liquids plants. Their high price tag, up to $5 billion apiece, would be hard to justify if oil prices were to drop. In addition, coal has drawn wide opposition on Capitol Hill, where some leading lawmakers reject claims it can be transformed into a clean fuel. Without emissions controls, experts say coal-to-liquids plants could churn out double the greenhouse gases as oil.

''We don't want new sources of energy that are going to make the greenhouse gas problem even worse,'' House Oversight Committee Chairman Henry Waxman, D-Calif., said in a recent interview.

The Air Force would not finance, build or operate the coal plant. Instead, it has offered private developers a 700-acre site on the base and a promise that it would be a ready customer as the government's largest fuel consumer.

Bids on the project are due in May. Construction is expected to take four years once the Air Force selects a developer.

Anderson said the Air Force plans to fuel half its North American fleet with a synthetic-fuel blend by 2016. To do so, it would need 400 million gallons of coal-based fuel annually.

With the Air Force paving the way, Anderson said the private sector would follow - from commercial air fleets to long-haul trucking.

Coal producers have been unsuccessful in previous efforts to cultivate such a market.

The Air Force's involvement comes at a critical time for the industry. Coal's biggest customers, electric utilities, have scrapped at least four dozen proposed coal-fired power plants over rising costs and the uncertainties of climate change.

That would change quickly if coal-to-liquids plants gained political and economic traction under the Air Force's plan.