NEW YORK--Patriot Coal Corp. filed for Chapter 11 bankruptcy on Monday, the largest casualty of the worst U.S. market for the power-plant fuel in decades. U.S. demand for coal has slumped this year as utilities favored cheaper natural gas, as prices of that fuel hit decade lows amid a supply glut. Coal miners, including Patriot Coal, have slashed production and laid off workers in an effort to match supply to weakening demand. Coal accounted for 35% of U.S. electricity generation during the first four months of the year, according to the Energy Information Administration, down from 44% during the same...