US utilities are extending the lives of their coal-fired power plants, as delays in obtaining cleaner replacements and strong electricity demand drive fears of shortfalls on the grid.

Three coal generators this month joined a growing list of plants whose planned retirement date has been postponed, bringing to at least eight the number of deferrals this year. The delays are a setback, at least temporarily, to the country’s efforts to wean itself off the dirtiest fossil fuel.

“While this is a difficult decision, it is necessary to maintain the reliable electricity service our communities have come to expect,” Javier Fernandez, chief executive of the Omaha Public Power District, said this month as he deferred the conversion of two Nebraska coal-fired units to natural gas by three years. The public utility pinned the extension on delays bringing new gas and solar projects into service.