A 30-year-old mining technique is becoming all that’s keeping a group of U.S. coal producers from joining their competitors in bankruptcy. Coal, already locked in a battle with cheap natural gas, now faces federal environmental rules that threaten to reduce its share of power generation to the lowest in 66 years. Companies from Illinois to Northern Appalachia are responding by leaning more heavily than ever on longwall-mining, a technology that’ll be used to produce a quarter of America’s coal this year, up from 19 percent in 2013. Investors are backing the miners who rely on the efficient approach. Think of a giant deli slicer with multiple revolving blades that cuts coal from a seam into slices. Eighty percent of analysts covering Foresight Energy LP and CNX Coal Resources LP, both known for […]