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KING COAL is ready for a comeback. The dirtiest, most polluting fossil fuel on the planet seemed doomed in a world of cheap gas and worries about global warming. But now it could be set to return, and in a form that smacks more of medieval hellfire than an energy source fit for the 21st century.

Environmentalists have not yet heard about the plans. But when they do, they will be up in arms. Because, under the guise of offering a 鈥渃lean鈥� technique for unlocking the energy from coal reserves, engineers are actually concocting a way to increase massively the amount of coal the world can get its hands on. If even a fraction of it gets burnt to satisfy our hunger for energy, awesome quantities of carbon could be poured into the atmosphere, unleashing runaway global warming across the planet. The question is: do the engineers have an answer that could make exploiting this energy source a realistic possibility?

The idea is to do away with coal mines and burn the coal where it lies, far underground. Light permanent fires in seams too deep for conventional mining and, engineers hope, they can siphon off the resulting vapours as fuel for gas turbines at the surface. Underground burning could unlock the power of hundreds of billions, perhaps trillions, of tonnes of otherwise inaccessible coal. Worldwide, there is growing enthusiasm for such schemes, and in Britain engineers will this year select a coal seam somewhere deep beneath the English countryside as the site for a trial that they hope could revive the country鈥檚 moribund coal industry.

So has coal finally got out of jail? Or, as environmentalists will argue when they hear the news, are we headed back on the road to a climatic Armageddon?

Geologists have long known that there鈥檚 far more coal underground than there is oil or natural gas. Worldwide, these unexploited reserves contain several trillion tonnes of carbon. Until now, the prospects of using most of that coal seemed remote because of the cost of mining, and the emergence of cheaper, cleaner sources of energy.

But underground coal gasification (UCG) promises to change all that. It offers the prospect of 鈥渃lean, cheap, greenhouse-friendly energy鈥� says Cliff Mallett of the CSIRO, the government research agency in Australia鈥攁 country that incidentally has some of the world鈥檚 largest coal reserves.

The trick with UCG is to ignite fires in underground coal seams and keep them burning for years at a stretch by pumping air, or oxygen and steam, down a borehole (see Diagram). The high temperature and pressure this creates partially oxidises the coal, breaking it down into a mixture of CO₂, along with combustible gases鈥攎ainly methane and hydrogen鈥攚hich can be collected at the surface and burnt in a gas turbine to generate electricity.

Coal gasification has its origins back in 19th-century Europe and North America. Those first supplies of gas for heating and cooking came from coal or town gas manufactured in gasworks. But over the course of the past century, coal gas for domestic use was superseded by natural gas tapped from oil reserves underground. More recently, natural gas has also been taking over coal鈥檚 role as the major fuel for electricity generating stations as well, because it is a more concentrated and easily handled fuel source and contains fewer contaminants.

Now the coal industry is hitting back. One idea is to build a new style of gasworks that converts coal into something like natural gas by heating it in pressurised containers. But the notion has yet to pan out. While coal is more abundant than natural gas, it is currently no cheaper, and converting it to gas makes things more expensive still. In the US, the federal government set aside $1.8 million in the 1980s and 1990s to underwrite commercial projects, without great success. A study in 2000 by the US General Accounting Office found that many of these projects had foundered and much of the money had gone unspent, mainly because the energy companies involved couldn鈥檛 see the technology translating into profits.

This hasn鈥檛 stopped the current administration from trying again. In an effort to counter charges that it was turning its back on the greenhouse debate, it has approved an extra $2 billion to be spent in the coming decade on pump-priming demonstrations of 鈥渃lean-coal鈥� projects at power stations to 鈥渦se our vast coal reserves cleanly and economically鈥�. It鈥檚 a pattern that has been repeated all around the world.

Yet nobody has been able to make gas derived from coal commercially competitive with natural gas. After two decades of clean-coal research in the US, only two coal gasification plants are in operation.

By carrying out the gasification underground, however, engineers now believe they can halve the cost. They no longer need to dig expensive mines or employ miners to extract the coal. Nor do they have to build expensive equipment to contain the gasification process: the subterranean rock itself acts as the containment vessel. Len Walker, an Australian pioneer of the technology, says that underground coal gasification 鈥渋s uniquely placed to combine the low fuel cost of coal with the efficiency of use in power generation of natural gas鈥�.

It offers other advantages, he says. No more nasty spoil heaps since the ash and tar from burning need never come to the surface. And because all the combustion gases come up through a single pipe, unwanted components such as CO₂ can be removed cheaply鈥攁nd perhaps poured back underground into the cavities left behind after the burning. Such claims are questionable, since it has never actually been done.

There are also technical difficulties with the method, of course. Keeping the coal burning may not be as easy as it sounds. Without careful control of events below ground, and a series of holes bored parallel to the seam, the fires may go out. Or the mixture of gases coming to the surface may be highly variable in both quantity and quality.

The key is to maximise the amount of methane released and minimise the production of unwanted gases such as oxides of nitrogen, sulphur and carbon. If that can be achieved, then coal engineers believe that once natural gas becomes scarce, as it will over the coming decades, UCG could really come into its own.

Mining engineers already know a fair amount about getting the most out of underground coal gasification. The technology harks back to the 1930s, when Stalin鈥檚 coal engineers began using it in shallow mines in the Soviet Union. The largest complex, in the mines of Uzbekistan, is still in operation today converting up to half a million tonnes of coal a year into gas, albeit rather inefficiently. Inspired by the Soviet projects, in the 1950s Britain operated a small gasification plant in a coal seam at the Newman Spinney mine in Derbyshire.

A couple of decades later, American scientists proposed using small nuclear bombs to reduce coal to gas. Perhaps not surprisingly, that idea was quickly dropped in favour of using propane to fire seams during a decade of experiments in the Rocky Mountains of Wyoming. This research was not a conspicuous success either, says Robert Lyman of the Wyoming State Geological Survey. The fires would not spread because the cracks and small cavities in the coal became clogged with tar.

The true successors to Stalin鈥檚 engineers are at work today in Australia, where Walker and his Linc Energy company have hired experts from Uzbekistan to help produce gas from a seam 140 metres below Chinchilla in Queensland. The team has generated gas continuously since early 2000, burning more than 20,000 tonnes of coal in the process. Walker calls it 鈥渂y far the longest burn in the history of UCG development in the Western world鈥�. Now the seam is about to go commercial, delivering gas to a small power station at prices he says are half those of a surface coal gasification plant and competitive with local natural gas sources.

Walker鈥檚 initiative is undoubtedly a commercial coup for the clean-coal lobby. But UCG will really make an impact when it鈥檚 used on much deeper coal reserves. This became clear in the 1990s, during European research at Alcorisa in Spain, in a seam more than half a kilometre down. Michael Green, the British mining engineer responsible for the project, says the successful recovery of gas at the site was a major breakthrough. Not just because it showed gasification could be done at these depths, but because it also revealed that high pressures deep underground help coal burn more easily and produce more methane, boosting the value of the gases reaching the surface.

The success at Alcorisa fired the enthusiasm of energy strategists, particularly in the British government. They believe that the country鈥檚 deep coal reserves, which till now were of no commercial interest, might at last become an asset. Officials at the Department of Trade and Industry (DTI), shrugging off concerns from their counterparts in the environment department, swiftly began their own national research programme.

According to Mark Armitage, an engineer in charge of the 拢20 million programme, it has 鈥渢he potential to provide security of future energy supplies long after oil and natural gas have been exhausted鈥�. The British Geological Survey says there is far more coal untapped beneath the British sector of the North Sea than in all its onshore mines. The aim eventually is to attract established operators to start drilling for coal rather than oil, says Armitage. As North Sea oil and gas run out, they could be replaced in the energy business by North Sea coal.

Other countries share this excitement, notably the US, where energy security has become a major political issue. Bob Schock, one of the Lawrence Livermore pioneers who now works for the laboratory鈥檚 Center for Global Security Research, says it 鈥渃ould double or triple the accessible US coal reserves鈥�. Mallett in Australia agrees. 鈥淭he amount of coal suitable for use in UCG,鈥� he says, 鈥渋s much greater than the reserves available to conventional mining.鈥� China and India, which hold the world鈥檚 largest coal reserves, are also taking an interest.

If these grand plans work out, what will they mean for the environment? The most immediate risk from UCG is that it can poison underground water sources. Test burns to date have generated cancer-causing benzenes and phenols: one Rocky Mountain site required a $5 million clean-up. Even if the toxic chemicals do not spread, they will poison water within the coal seams themselves. As Armitage points out, British coal seams are classified as aquifers that might in future be needed to supply drinking water.

Such concerns partly explain the DTI鈥檚 extreme sensitivity in discussing potential testing sites for UCG in Britain. It has refused to approve any public statements by its advisers and consultants, but New Scientist understands that an initial list of 10 possible sites for its experimental 60,000-tonne burn has now been whittled down to four by consultancy WS Atkins. The final choice is expected this summer.

But the overriding issue is whether reviving the coal industry in this way can be made compatible with tackling the world鈥檚 biggest environmental problem: global warming. The British government, among others, classifies underground gasification as a clean-coal technology with emissions levels substantially lower than those from traditional coal burning. But there is considerable, and perhaps deliberate, confusion about what advantages UCG can bring.

CO₂, the biggest culprit in global warming, can be released into the atmosphere by UGC in two ways: in the gas stream itself, and from the gas turbines where it is produced from burning methane. Engineers routinely promise that it will be easy to remove CO₂ from the gas stream as it comes to the surface, and that it can then be buried back in the spaces left by the burnt coal.

This has not yet been tested, however, and even if it works it will only reduce emissions to levels comparable with burning natural gas鈥攖ypically half those from conventional coal burning. But to make the process truly clean would mean capturing the CO₂ that鈥檚 produced in the turbines, and burying that as well. If it were, says Green, the process would in theory be about as clean as other sources of renewable energy鈥攑rovided of course the CO₂ remains permanently underground. But that would make coal gasification much more costly than other energy sources, including wind and solar power, and it is hard to see how it will ever be a serious runner. What it boils down to is that the 鈥渃lean-coal鈥� label is about to be used to justify the unlocking of vast untapped coal reserves. And however cleanly it can be burnt, the sheer volume of the stuff means it threatens to unleash a carbon time bomb.

According to the International Energy Agency, known 鈥渞ecoverable鈥� coal reserves worldwide are around a trillion tonnes. But underground gasification is likely to raise that to around 6 trillion tonnes. Burning the lot would increase the total amount of CO₂ that鈥檚 poured into the atmosphere from fossil fuels to 20 times current levels and this would be slow to clear. The effects on climate would be far greater than the worst predictions by climate scientists.

Energy analyst Michael Grubb of Imperial College, London, says 鈥済as and oil alone cannot do serious damage to the climate. But coal can.鈥� Whatever the local, short-term benefits of adopting UCG, in the long run liberating even a fraction of the carbon stored in the world鈥檚 subterranean coal reserves could create one hell of an environmental nightmare.