Underground coal gasification must also have carbon capture and storage

Trials of UCG (Underground Coal Gasification) 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.

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.

An assessment by the World Energy Council puts the proportion of global coal that is readily recoverable at 15 to 20 per cent of the total, which Gordon Couch of the International Energy Agency’s Clean Coal Centre puts at 18 trillion tonnes. Potentially, UCG could unleash the energy from the other 80 to 85 per cent – enough to supply the world, at current requirements, for 1000 years.

Industrialists may salivate at the idea of burning all that coal, but for the climate the prospect is truly terrifying. The Intergovernmental Panel on Climate Change recently reckoned that the world needs to limit total emissions of carbon, from now on, to less than half a trillion tonnes just to keep global warming below 2 °C. Most climate analysts agree even burning a large fraction of conventional fossil fuel reserves would produce unacceptable warming, let alone what could be released by UCG.

Burning dilemma

What to do? Either we have to leave the fuel in the ground, or develop a global industry for capturing CO2 at the source and storing it out of harm’s way. In the case of UCG that would mean capturing the CO2 produced both when the coal is burned underground and when the resulting methane is burned in power stations. Climate scientists such as Myles Allen at the University of Oxford argue that carbon capture and storage (CCS) is the only practical way forward. And this is where UCG has something to offer. Burning coal in situ leaves huge voids that are ideal places for burying captured CO2. And the infrastructure created to bring coal gas to the surface, purify it and deliver it to power stations would be ideal for carrying the CO2 away again.

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