Compared with many alternatives, a coal-fired power plant being built here will be anything but clean. It will emit several times as much smog-causing pollution and heat- trapping gas as the the same utility's new plant, fired by natural gas, a few miles away. But it is the coal-fired plant, not the gas-burning plant, that is being subsidized by the US government, under a 15-year-old program that has spent $1.8 billion to promote so-called clean-coal technology in 38 projects.