UNDERGROUND COAL GASIFICATION

The DoE's Laramie Energy Research Center in Wyoming has been researching that point for a number of years. The general idea is this: Underground coal deposits are first rubblized by detonating explosives, then ignited. Air, or possibly oxygen and steam, is forced down into the fire through a pipe to produce combustible gases such as hydrogen, carbon monoxide and methane, and perhaps also raw material for synthesizing fuels (such as carbon dioxide, which can be reacted with hydrogen to produce methanol). These gases are collected from the burning coal by wells with pipes of 6 to 12 in diameter.

Tests at the Hanna, Wyo., site during 1975 76 showed that low BTU product gas can be produced with high process efficiency by pumping air into the ignited coal bed. The resulting gas contains about 150 BTU per cubic foot, compared to about 1,100 BTU for natural gas (methane); but that, of course, merely means that the energy is more dilute - sufficient amounts of it can be obtained by burning a sufficiently great volume.

Trouble developed at the third site tested at Hanna, Wyo., last summer. The lack of ground water caused the output gases to be too hot, and the experiment had to be terminated, since there was a risk of material failure and possibly even injury to operating personel.

On the other hand, if the coal is too wet, underground gasification might not work at all or so researchers feared until recently, when the Lawrence Livermore Laboratory (of the U. of Calif., but working for the DoE) performed another test on wet coal at the Hoe Creek site near Gilette, Wyo., where the coal contains about 3 times as much water as at Laramie.

But it worked, and hopes now run high for in situ (in place) coal gasification, for the wet coal at Hoe Creek is typical of much of the country's deepcoal reserves. Hoe Creek is part of the Powder River basin, a 7,000 square mile area holding 1 trillion tons of coal - a quarter of the country's reserves outside Alaska - in deposits locked 500 to 1,000 feet below the surface. Coal gasification could make such deep deposits accessible, though the economics are still far from clear yet.

With air pumped down, the resulting gas again had an average heating value of about 150 BTU/cft, but in a two-day experiment last November, researchers replaced the air flow with steam and pure oxygen to improve the quality of the output gas. It was the first US application of steam oxygen technology, and the experiment produced a medium quality gas with 265 to 300 BTU/ cft.

Further tests at both Hanna and Hoe Creek will continue this year. Experiments at Hanna are to demonstrate, among other points, an automated process control system, and the LLL plans a 3 month test at Hoe Creek with the exclusive use of steam and oxygen, hoping to upgrade the medium BTU gas to pipeline quality at the surface.

[More: Coal Age, Dec.77; LLL news release 12/27/77; "Clean fuels from coal," Power Engineering, Oct. 77]