Coal Age

FEB 2016

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country, many operations can also run into gas post-mining as the progression of longwall mining results in roof movement and, in some cases, floor heaves, and that gas must be handled as well, according to Thakur. There are two techniques to achieve that, including vertical gob wells and cross-measure boreholes, a European-based technology measure mainly used in smaller and slower moving longwalls. "When the floor heaves, sometimes 25% of the gas in gob comes from the floor," he said. "Many mines [in history] have had disasters because of this floor gas." In general, the wider the panel, the high- er up it breaks. In one case from his expe- rience, he confirmed gas production from up to 1,000 ft from the surface at a long- wall with a 1,000-ft face. "When you plan degas of the gob, you have to allow for these things," he added. The Bottom Line A typical longwall liberates 8 million ft 3 of gas daily, and on average 55-60% of g a s f r o m t h e g o b c a n b e c a p t u r e d . Assuming bleeders are kept in the safe zone below 1.5%, and with bleeder air managing 3.2 million-4.4 million ft 3 /day, the ventilation air needed in such a sce- nario is 150,000 to 200,000 ft 3 per minute (cfm). Gas emissions on the longwall face primarily depend on the gas content of de-gassed coal and the rate of mining (Thakur uses a rate of 70 ft/day), but it is clear that those ventilation figures are a tall order. So, just how wide can a longwall face be in the Pittsburgh No. 8 seam and still mine safely? Thakur said that the criteria for an optimum face length/panel width are the tailgate's total gas flow and venti- lation air and a methane (CH 4 ) concen- tration of below 1%, ideally 0.8%. "Based on my experience, if you make it too long and you don't have enough air…you may be legal, but go 6 in. up and you may not," he said. "It's very important to check; it can't go unabated." Using the gas layering index to deter- mine one's gas layering number (GLN) is important as well. With a typical degas emission of about 300 cfm at an average of 10 ft, one would need 39,000 cfm of air, a number, that some mines can't achieve — a "tragedy," according to Thakur, who rec- ommends cutting down the face from 1,600 ft, in some cases, to a more manage- able size. He also strongly stresses the use of an internal return with regulator. With faces at their current range of depths and length, the result is often less air and more emissions in tailgate. Fans are helpful to an extent, but a better financial selection for operations to make is degasification, which will cut down on quickly rising costs of powerful fans and ventilation plans focused on them. As seen in the chart, Thakur offers an estimation of an optimum face width for the Pittsburgh No. 8 seam using methane emissions and total ventilation air; the seam, again, is considered to be moder- ately gassy. His general recommenda- tions include a face length of 700 to 750 ft in highly gassy mines, 1,250 to 1,320 ft in moderately gassy mines and those classi- fied as highly gassy can exceed 1,500 ft. Another advantage to degasification, should an operator choose to collect and market it, is additional revenue to offset degassing and overall mining costs. "You clean that gas, put in in a pipeline, you can make some money…even at today's rates," Thakur noted. Without question, however, whether or not a longwall mine chooses to sell its gas, he encourages that management listen to its technical experts, especially when many mines waste millions to find a result that matches the initial recommendation of said expert. Over the years, he said, he has watched many mines do just that. "There's a famous [movie] saying…'a man gots to know his limitations.' We all need to know our limitations," Thakur said. "Panels need to be brought down to this level. My message to you: we have the technology today to make sure that mines don't blow up. People don't need to die because of gas anymore." Note: This article is an adaptation of a pre- sentation given by Murray Energy Corp. Coal Degasification Group Manager Dr. Pramod Thakur at the Longwall USA Exhibition and Conference held during June 2015. m i n e v e n t i l a t i o n c o n t i n u e d 30 www.coalage.com February 2016 Optimum width of face in Pittsburgh No. 8 seam, according to Thakur's calculations. Thakur's recommendations for optimum face lengths considering their respective levels of gas.

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