Two ways this could occur - they lift emissions caps/operational limitations on diesel generators (usually even power plants have them for emergencies) or its some wishy washy pleaz build something quick before winter. Former is more likely.
@Tohsaka saw that. Hardly a revolutionary idea. Natural gas burns cleaner, though from a CO2 standpoint I'm guessing they're about the same. What they really need to is go nuclear, but heaven forbid we listen to the science.