Champions of coal say the superabundant fossil fuel can be made environmentally friendlier by refining it with chemicals – a “clean coal” technology backed by a billion dollars in U.S. government tax subsidies annually. But refined coal has a dirty secret. It regularly fails to deliver on its environmental promises, as electric giant Duke Energy Corp found. Duke began using refined coal at two of its North Carolina power plants in August 2012. The decision let the company tap a lucrative federal subsidy designed to help the American coal industry reduce emissions of nitrogen oxides – also known as NOx, the main contributor to smog and acid rain – along with other pollutants. In nearly three years of burning the treated coal, the Duke power plants collected several million dollars in federal subsidies. But the plants also pumped out more NOx, not less, according to data from the U.S. […]