Burning coal

Burying carbon dioxide for cleaner coal

Could a new approach to underground coal gasification, with carbon dioxide injected back underground, make for carbon-neutral coal?

In July 2010 a pan-European team of researchers and engineers revealed an underground coal gasification project with a difference. Akin to past schemes, coal will be burned underground to produce a combustible gas stream.

The gases will then be extracted via a production well with the recovered hydrogen and methane being used to generate power. The difference with the new project is that the carbon dioxide will then be compressed and injected back underground, either filling the space where the coal had previously been or into a nearby unmineable coal seam.

Dr Yong Sheng, a lecturer in structural engineering at the University of Leeds and project leader for the university, is excited. This is the first European project to combine underground coal gasification with carbon capture and storage.

'Compared to traditional mining, UCG is cheaper, and makes the most of [deep] coal seams that might otherwise be impossible to exploit,' he says. 'And by injecting the CO2 back into the coal seam, the process becomes almost carbon-neutral and more cost effective as you don't need to transport the gas.'

The project will focus on the Dobrudzha coal deposits in north east Bulgaria, where coal is buried 1,200m down. The project team is currently gathering data on coal-seam geology and composition, overlying strata geology, hydrology and more, to model how a commercial UCG and CCS operation will proceed at such a deep site.

For his part, Sheng is developing a model to predict how the coal will burn within the seam during gasification and what will happen to the cavity left behind. 'We will predict the speed at which the cavity grows and chemists will then convert this into the energy efficiency of the gasification process,' he explains.

At the same time, fellow project researchers are modelling safety and environmental effects, while others evaluate costs. Digging deep cuts the risk of surface subsidence and water contamination, with seams less likely to adjoin potable aquifers, but deep-drilling operations are complex and costly.

'The total value of this project is £3m, enough for a feasibility study but not enough to start a pilot project,' Sheng explains. 'However, after this study it is more than likely that Overgas, [project coordinator and coal deposit owner] and the European Union will invest in a pilot test.' And if so, Sheng is confident we will see Bulgarian power stations fired with gas from this UCG project in the next five years.

Industry growth

Sheng is not alone in his enthusiasm, at least for underground coal gasification. Both Dr Marcos Millan-Agorio, lecturer in chemical engineering at Imperial College London, and Rohan Courtney, chairman of UK-based UCG consultants, believe we will soon see many more schemes as myriad licences are granted for UCG exploitation around the world.

'UCG didn't compete when oil and gas was cheap,' says Courtney. 'Today security of supply and environmental [climate change] issues are also more relevant while drilling technology has moved at a pace and allows you to control, with extreme precision, the coal you are gasifying.'

As Millan-Agorio adds: 'The technology has potential to be an important part of the energy mix... especially in countries such as China, India and Australia, which have huge deep-coal reserves.'

Indeed for Courtney, UCG is a must-have for such nations. As he says, coal that is unmineable using traditional mining techniques, makes up around 90 per cent of the world's entire coal resource. 'Add up all the other energy resources, from oil and gas to [present] solar and wind generation, and these still don't match the quantity of coal that could be gasified.'

What's more, Courtney isn't afraid to dig deep, even though many UCG projects to date have only recovered coal from depths of around 500m. 'We haven't gone incredibly deep as we're still picking the 'low-hanging fruit', but there is no reason to believe UCG wouldn't work... the oil sector has reached 5,000m,' he points out. 'Importantly, if you go deeper than 500m, the chance of subsidence is zero and you avoid issues with contaminated water.'

What about carbon capture and storage? While UCG has its clear proponents, others have doubts over storing the CO2 underground, especially in the cavity left behind by the burnt coal. As Millan-Agorio puts it: 'Is storing CO2 in the empty seam really an option? After operations, water filtration will take place; that cavity doesn't stay an empty place.'

Sevket Durucan, professor of mining and environmental engineering at Imperial College, London, echoes Millan-Agorio. Also the expert reviewer for the Intergovernmental Panel on Climate Change (IPCC) report on CO2 capture and storage, he says: 'Undoubtedly underground coal gasification will create fractures above the coal seam, so if you store CO2 in the cavity you must ask, will that CO2 stay or will it leak?'

Durucan is determined that drilling engineers must control subsidence and prevent fractures from propagating to the surface, even in sites more than 800m deep – the depth at which CO2 would convert to liquid, reducing the risk of leaking. He adds: 'Even with this, that cavity will not take up much gas, as the gas will only compress to the pressure at which it is injected underground.'

Instead, Durucan believes the CO2 would be better stored in nearby unmineable seams either too thick or thin to be mined economically; the second option for the Dobrudzha project. Here, the injected CO2 would accumulate on the surface of the coal and, according to Durucan, 'orders of a magnitude more would be held'.

But still he has doubts. 'If made to work, such a UCG-CCS project would make a good closed system but personally, I would prefer to store CO2 in alternative underground sites such as saline aquifers; coal seams are way down on the list.'

Despite the challenges Sheng remains confident that in-situ CO2 storage at a UCG site is the future. As he points out, by 2020, all power plants across Europe will have to incorporate some system of carbon capture and storage to meet EU targets on greenhouse gas emissions. His project will be making the most of coal seams that might otherwise be impossible to exploit while removing the high cost of transporting CO2 to a separate storage site.

Courtney concludes: 'You've already got an empty cavity down there. Why not put the CO2 back?' *

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