Two drumlike shearers relentlessly tear away at a 600-foot wall of underground Appalachian coal, knocking large chunks onto a conveyor belt which carries the black mineral to the surface.
With dozens of carbide-tipped steel teeth, each one as big as a dagger, the shearers have little trouble eating into the coal as they move down the long face.
This is lonwall mining, a century in the making and a significant step in automation of the coal industry. Today, longwall has evolved into the most modern, efficient, and safe mining technique going.
It is also the most expensive. A single system designed to clear out a 600 -by-6,000-foot seam that is 5 feet thick costs from $6 million to $8 million, generally limiting its use to the larger coal companies. But with only a seven-member crew to keep it running, longwall can more than quadruple coal production rates, churning out some 1,400 tons of coal on a single shift. Some longwalls have established records ranging from 12,000 to 17,000 tons in a three-shift day.
This kind of efficiency is the reason the US coal industry has steadily increased the number of longwall mines in operation. Currently, about 5 percent of US coal is mined by longwall. Estimates of longwall production by 1985 range from 15 to 25 percent.
"There's no question it's a lot more efficient," says Norman Brooks, a vice-president of the coal division of MAPCO Inc. The longwall system's 1,000- to 1,400-ton-per-shift rate compares with the 200 to 300 tons that can be mined during a shift using continuous mining machines, he said.
Still, Mr. Brooks points out, longwall mining "can't be used everywhere." Coal seems that are too thin, those that have too many irregular up and down curves, or seams with gas pipes running through them cannot be mined this way, he explained.
In the longwall system, a block of coal is cut away 30 inches at a time. The two barrel-like shearers ride together on a track, cutting away at the coal face. A steel canopy protects the machine from rocks which fall from the roof of the cavern left behind.
When the shearers move ahead, so do the "self-advancing" roof supports, all pushed forward by hydraulic rams. The rock over the coal seam has to be of a type that will easily break up behind the supports. Otherwise the roof will come down in huge, single-slab sections, putting too much pressure against the supports.
Besides being safer and more productive, longwall also removes all the coal from a mine, unlike traditional mining, which must leave pillars of coal in a honeycomb fashion to hold up the roof for the safety of miners.
Longwall mining began in Europe, and until a few years ago US companies relied on European imports of the machines. The first longwall mines were opened in the late 1800s, with a long row of miners using picks and shovel to cut away at a wide coal face. As they moved forward, wooden roof supports were put up behind them. "The first mechanical piece of equipment was a conveyor," said Timothy Hoak, an engineer with Eickhoff America Corporation, a subsidiary of Eickhoff of West Germany, which distributes the parent firm's shearers in the United States. There is some debate over whether the first longwall system was set up in Germany or Britain, Mr. Hoak said, "but we say it was Germany, of course."
"Most of the longwall experience is in Europe, so there are not a lot of longwall people here yet," he said. But the US is catching up. For example, after spending 16 years representing overseas longwall manufacturers, Joy Manufacturing Company, of Pittsburgh, now builds its own equipment, Robert E. Kinter, a Joy spokesman, said.
While it has been "kind of slow" to move into longwall mining, the US coal industry has been taking to it much more enthusiastically in the last four or five years, Mr. Kinter said.
The reason for this icnreased interest can be found in several developments introduced in 1975 and '76. The first was a roof support that offered better protection. At about the same time, heavy-duty conveyors and chains were introduced, along with double-drum shearers, replacing the single-drum shearers.
Before these improvements came along, fewer than 60 longwall systems had been installed in the 15 years since their introduction in the US. From 1976 through 1980, more than 40 were added. By the end of this year, the number of longwall mines is expected to top 120. Some coal analysts estimate there will be over 200 longwalls in operation by 1985, although the Department of ener gy has put the total for that year at fewer than 150.