"In fact, when you burn coal, you emit more acid rain pollution, more air pollution, more smog and more global warming pollution which threatens the entire planet, " said Becker, global warming program director for the Sierra Club.

CNN: Energy debate may soon turn on 'cleaner' coal

But the government is becoming increasingly concerned about the problems that pollution brings, such as sickness from filthy air and arid soil from acid rain, which has made it keen to boost the use of renewables and increase energy efficiency.

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