Chinese scientists are on the verge of successfully producing clean fuels from underground coal deposits
After 40 years of failure, Chinese scientists are on the verge of successfully producing clean fuels from underground coal deposits. Lanhe Yang and colleagues at the China University of Mining and Technology in Jiangsu Province, and Beijing, recently field-tested improved methodology demonstrating that shaftless underground coal gasification (UCG) could be economic.
UCG converts coal into a combustible mixture consisting of hydrogen, carbon monoxide, and a little methane called syngas. Syngas is used for industrial heating, power generation, or manufacture of synthetic fuels and other chemicals. It can be processed to remove any CO2 content, providing a source of clean energy with minimal greenhouse gas emissions and no environmentally disfiguring slag heaps. Being underground, UCG is safer, more efficient, and provides a higher energy return than coal extracted by traditional mining methods. It can also access previously unreachable coal seams. With the world’s coal reserves currently running at around 10 trillion metric tonnes, UCG is an attractive transformational technology until other environmentally friendly and sustainable energy sources come on stream. And because firing occurs underground, it is easier to sequester any carbon dioxide produced.
Using know-how borrowed from the oil and gas industries, wells are drilled into the coal seam; one for injecting oxidants (water/air or oxygen mixtures) and another some distance away to bring the product gases to the surface. Coal at the base of the first well is heated to its normal ignition temperature, but by carefully regulating the flow of oxidants the coal does not burn. Instead, it separates into syngas which is drawn out the second well (see image).
To work, UCG needs a cavity to be excavated out of a seam of un-mined coal. This becomes the gasification reactor, and means humans have to engineer the underground reactor chamber. So far this has made UCG uneconomical. Yang’s team have made UGC work without digging out a reactor cavity. Called shaftless UCG, it has been studied since the 1960’s especially in China, but without much success until now.
The researchers drilled several bore-holes into the coal seam. The seam was rapidly ignited electrically and air blown through it in a process called forward and backward pushing through. This controls the fire source within the coal seam, which in turn maximises the composition of the syngas mixture that exits from another bore-hole. Gasification is a two-stage process that fires the seam to an ideal temperature with air, then blasts steam into it periodically in ever larger amounts and over greater time intervals. It ensures the temperature does not drop too quickly and slow the endothermic reaction between the incandescent coal and steam: this has to be closely monitored to achieve maximum coal-to-syngas conversion efficiency. ’We have shown our pushing through technology could make UCG economical,’ said Yang. Meanwhile, the race for practical clean coal technology is hotting up with the opening earlier this year of a UCG plant in South Africa, and the recent announcement of a two-year partnership deal between BP and the Lawrence Livermore National Laboratory to cooperate on the development of UCG technology.
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L Yang et al, Energy & Fuels, 10.1021/ef700231p