Peat consists of partially decomposed plant debris. It is considered an early stage in the development of coal. Peat is distinguished from lignite by the presence of free cellulose and a high moisture content (exceeding 70 percent). The heat content of air-dried peat (about 50 percent moisture) is about 9 million Btu perton. Most U.S. peat is used as a soil conditioner. The first U.S.electric power plant fueled by peat began operation in Maine in 1990.