Turning coal into clean-burning gases in the ground can avoid the environmental impact of mining coal and halve the cost of managing its carbon-dioxide emissions. But while a few pilot tests of such underground coal gasification (UCG) are moving towards small-scale commercial operations in Australia, China, and South Africa, much research is needed to improve the control of UCG operations and to prove their environmental safety, according to a report issued last month by the Clean Air Task Force, a nonprofit environmental consulting firm based in Boston. “It’s tricky business–you don’t just go out and drill a well and declare victory,” says report author Julio Friedmann, who is carbon management project leader for Lawrence Livermore National Laboratory in California.
The chemistry of UCG is akin to that of gasification power plants, such as the coal-upgrading project announced last month for China’s Pearl River Delta, in which heat and pressure turn coal into a combustible mixture of carbon monoxide and hydrogen known as syngas. UCG exploits drilling technology to engineer the coal seam itself into an underground gasification reactor. Wells drilled into the coal seam supply air or oxygen, and sometimes steam, to burn some of the coal, generate heat and pressure to gasify more coal, and then deliver the resulting syngas to the surface.
UCG got started in the former Soviet Union and reached commercial scale by the 1950s. One such plant in Angren, Uzbekistan, continues to generate up to 18 billion cubic feet of syngas per year. But Soviet production peaked in the 1960s as production of cheaper natural gas ramped up. The U.S. conducted 33 UCG pilot projects in the 15 years following the first Arab oil embargo.
The U.S. work demonstrated an improved method that enhanced the quality of the syngas developed by Lawrence Livermore in the 1980s. Whereas earlier UCG efforts used one horizontal well to connect distant air injection and syngas removal wells, the improved method uses parallel injection and syngas removal wells 20 to 30 meters apart that descend vertically to the seam and then horizontally through the seam for several hundred meters to several kilometers.
Gasification begins with combustion of the coal between endpoints. As a cavity forms and the coal is emptied out, the gasification front is pulled progressively back toward the vertical wells. Friedmann says that the technique made UCG a more reliable process, but the U.S. UCG programs were nevertheless wound down by the early 1990s, as U.S. natural-gas production exploded and energy prices crashed.
China kept working on UCG, however, as a means of accessing deeper coal seams, and now has the world’s largest research effort. The most advanced project is a pilot project that’s been running since 2007 in Wulanchabu, Inner Mongolia.