More than half a dozen large U.S. coal companies have filed for bankruptcy in the past year, a signal that the one-time king of American energy is fading as it faces competition from cheap natural gas and renewable-energy sources while reckoning with the retirement of coal-fired power plants.

At least seven coal producers—including Westmoreland Coal Co., Cloud Peak Energy Inc., Blackhawk Mining LLC and Blackjewel LLC—have filed for chapter 11 bankruptcy since October 2018 and more companies could be following them to court....