http://www.news.uiuc.edu/scitips/02/0703fuels.html
It is well-known that switching from coal to natural gas would significantly reduce carbon dioxide emissions. The carbon content of natural gas is only 60 percent that of coal per unit of primary energy content. The higher efficiency of state-of-the-art natural gas turbines over older, coal-fired power plants could reduce emissions by an additional 30 percent.