Setting fire to coal underground could answer our energy prayers, or start an environmental disaster on a bigger scale than ever before
IF YOU thought shale gas was a nightmare, you ain't seen nothing yet. A subterranean world of previously ignored reserves is about to be opened up. These are the vast coal deposits that have proved unreachable by conventional mining, along with gas deposits around them. To the horror of anyone concerned about climate change, modern miners want to set fire to these deep coal seams and capture the gases this creates for industry and power generation. Some say this will provide energy security for generations to come. Others warn that it is a whole new way to fry the planet.
A primitive version of the technology behind this Dantean inferno of underground coal gasification (UCG) has already been running for 50 years in the former Soviet republic of Uzbekistan. Some 300 metres beneath the plains east of Tashkent, Stalin's engineers and their successors have been burning a seam of brown coal that can't be mined conventionally. There are two well heads on the surface: one pumps air down to fan the flames while the other retrieves a million cubic metres of combustion gases a day. Scrubbed of coal dust, cooled and compressed on site, the gases are then sent down a pipeline that snakes across the countryside to a sprawling power station on the outskirts of the industrial town of Angren, where they are burned to generate electricity.
A deadbeat town in a forgotten rust-belt backwater of the former Soviet Union is an unlikely test bed for a cutting-edge technology. But if it can be scaled up successfully, the Australian engineers who bought the operation seven years ago think it could transform the world's energy markets, open up trillions of tonnes of unmineable coal and provide a new carbon-based energy source that could last a thousand years.
With trials of UCG under way globally from China to Queensland, and South Africa to Canada, the stakes are high. Not least for the atmosphere. Without a way to capture all the carbon and store it out of harm's way, it could raise the world's temperature by 10 degrees or more. Is this burning desire for fossil fuel pushing us towards disaster?
Until recently, only reserves with rich concentrations of coal, oil and natural gas were exploited – but not any more. With those reserves approaching exhaustion, the hunt is on to tap huge volumes of , particularly natural gas, or methane. With these we could keep the lights on, power vehicles, deliver feedstock for the chemicals industry, and quite possibly heat the planet, for centuries to come.
In the past decade, the focus has been on shale gas: methane tightly trapped in tiny pores and fractures in shale, a sedimentary rock made up of mud and clay mixed with minerals such as quartz. Capturing that gas required two crucial new technologies. Horizontal drilling launched from conventional vertical wells can penetrate for up to 3 kilometres along shale beds. And hydraulic fracturing, or fracking, blasts high-pressure water into the shale to fracture the rock and release the gas. As well as opening up the shale, these technologies open the door to a wide range of alternative sources of methane. They can release methane trapped within coal seams, for example, notably in the coalfields of Wyoming and Montana. Methane is often produced as seams develop, as the coal becomes compacted and heated deep underground. The gas has always been the bane of coal mining, but if collected and pumped to the surface, it becomes an asset.
According to the International Energy Agency's latest estimates, some 400 trillion cubic metres of economically recoverable methane lies trapped in coal and shale beds around the world. It roughly doubles estimates of how much gas miners may be able to get their hands on. But that is just the start. There might be even more gas down there in different rock strata, much of which has migrated from coal seams over millions of years. And why limit the plan to existing gas? The real prize, the miners say, is to create yet more methane by setting fire to the huge amount of unmineable coal lurking underground.
Setting fire to coal and capturing the gaseous emissions has long been routine above ground. Till half a century ago, many of us got our gas for heating and cooking from gas works that ignited and "gasified" coal. The combustion converts the carbon in the coal to carbon dioxide while providing heat for subsequent reactions in which the CO2 reacts with steam to produce hydrogen, carbon monoxide and methane.
In most countries, gas works have been superseded by natural gas from oil fields. But now the idea is to turn coal seams into underground gas works. That, say proponents of the idea, exploits coal once thought too deep, too costly or too dangerous to exploit. It also saves time and money in mining, and land isn't spoiled by mines and waste dumps – not to mention the costs and environmental hazards of conventional gas works. Any nasty by-products can be left below ground (see diagram).
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