Compiled By Glenn Hasek Coal's reign as king of electricity production in the United States may soon be over. A new study released by New York-based RDI predicts only 4% of the new power generation capacity needed by 2015 will be fired by coal. About 52% of current electric output is generated by burning coal. The study, "New Coal-Fired Generation: The Race is On," says natural gas will be the fuel of choice for 94% of the new electric generating capacity and renewable energy (hydro, solar and wind) will account for 2%. The study says overbuilding of gas-fired plants in the next few years combined with the high capital cost of coal technology will be a disincentive to coal plant developers. Siting risks also will negatively impact coal plant development. RDI -- a research unit of Platts, an energy information services provider -- says the equivalent of 592 500-megawatt plants will be built in the U.S. by 2015. Platts is a division of The McGraw-Hill Cos.