Low-seam longwall mining

As the world’s major coal mining nations reach ever higher levels of production, their coal reserves increasingly are in ever lower seams. With technological breakthroughs, longwall mining in these low seams with shearers is no longer is a dream, but a reality. “Nowhere is this more welcomed news than in China,” explains Randy Cooper, Joy Mining Machinery’s Global Product Line Manager – Longwall Systems. “China is unique in the world in that, every possible ounce of coal must be extracted. In underground mining, nothing compares to the ability of the longwall shearer to accomplish this.”

“Compounding the problem for coal operators in China is that to fully maximize recovery of all underground reserves possible, they must first mine their thinner seams that usually overlay the thicker seams,” Joy’s Cooper explained. “Historically, these thicker seams have been mined first, causing subsidence to travel upward, fracturing the thinner seams above and rendering them unrecoverable. These previously sacrificed overlying thinner seams now must be taken first, and that is where the question is being asked: Shearer or plow for low-seam longwalling? With recent technological developments by Joy Mining Machinery, we can answer that resoundingly: Shearer.”

According to Cooper, most modern plow systems generally can produce 1.8 Mt/y of clean coal. “Under the same circumstances, a high power shearer—even taking some additional rock to maintain minimum working height—will mine more than twice that, resulting in higher clean tonnes per-man-year and much lower costs per tonne,” Cooper said.

Significant developments

Cooper sites two significant developments that have made this possible: First, Joy developed the 42-mm Broadband low profile chain to replace the flat type 42-mm chain. With no loss of strength, this significantly lowered the AFC on which the shearer rides. Secondly, Joy developed the adjustable shearer haulage downdrive that lowered the chassis height of the model 7LS1A shearer.

“The Joy 7LS1A shearer is a very powerful, high production machine,” Cooper said. “It is equipped with dual 65 kW traction drives and dual 420 kW ranging arms. Installed on this special 42-mm pan, it has a total machine height of 1 m, allowing comfortable extraction heights down to 1.4 – 1.5 m.”

When the two longwall mining systems are compared, two things stand out on the side of the shearer, according to Cooper: Power and better in-seam mining. “Under difficult conditions, successful mining directly correlates to the mining machine’s capability to adjust up when seam height increases and the power to mine through rock to maintain minimum heights when the seam pinches down,” Cooper said. “These are two attributes a plow just doesn’t have, and these two attributes are particularly important in low-seam longwall mining.

“With the shearer ranging arms, it is possible for the machine operator to adjust to the seam height; taking all the coal, leaving none. With a plow, because it can’t cut rock, the top cutter must be adjusted for the lowest point in the seam.  What this means to the mine operator,” Cooper explained, “is that if the low point on the face is 1.1 m and the high point is 1.5 m, then 0.4 m of coal is being left behind during every pass across the longwall face; something absolutely forbidden in China and totally frowned upon everywhere else.”

Cooper added that the shearer’s cutting ability also is particularly important in the head and tailgates where extra height is needed for power boxes, pumps and belt conveyors, as well as set up and panel move activities. The shearer, he said, can grade very smoothly from the higher gate development entries to the lower running seam. “That’s something a plow just can’t do,” he said.

Additional considerations

In shearers versus plows, Cooper said there are considerations other than cutting performance that need to be given the proper attention when making comparisons. “Two of these issues where the shearer is clearly is superior to the plow are dust suppression and immediate methane detection.” On a shearer, water sprays on the machine body and cutting drums immediately suppress dust generated in cutting, while at the same time cooling the bits. A methane detector positioned on the downwind end of the chassis continuously monitors methane concentrations on the face and immediately during the cutting process.

“With a plow,” he said, “none of this is possible as there is no way to hook water to the cutter body—therefore no immediate dust suppression, making coal float dust absolutely horrendous, and no bit cooling or spark suppression at the bit tip. Also, there is no way to install a methane monitor on the plow cutter body. What you have, then, is a plow operating at high speed, creating hot bits in an atmosphere of dust and no way to immediately detect sudden methane liberation.”

“As we go forward,” Cooper said, “Joy will continue evolving the low-seam shearer for applications in ever-lower seams, applying developments in sophisticated control and automation technology to operator-less longwall mining, taking the shearer where man cannot go and operate easily, comfortably and safely.