There's been much too much fudging about what means "clean": It's well within present capabilities to reduce emissions such as carbon monoxide, sulfur compounds and nitrogen compounds from a coal-fired operation. There's NO known way to eliminate the CO2 that's the result of burning carbon compounds. Probably the most promising approach--still a long way off--is to genetically engineer bacteria or algae that consume CO2. But then you'd have to deal with the vastly increased volume of those microrganisms, which could easily produce a macro problem.

Unless there's a major breakthrough in solar or nuclear fusion technology, the only sensible approach would be to reduce energy use. Ideas like a network of rail travel combined with easy rental of short-range vehicles located at stations might work, but would require massive re-education.