Clean coal-fired power plants have been touted as a remedy for an environmentally challenged age, offering the promise of turning cheap but dirty coal into a pollution-free energy source. Don’t turn off those wind turbines yet.

At least eight clean coal plants, more than a third of those on the drawing board, have been canceled, delayed or rejected by regulators this year.

Developers cite soaring construction costs, technology hurdles and uncertainty about regulation of greenhouse gases.