Abstract: An air preheater, including a flue; an air channel; a first segment for producing secondary air; a second segment for producing primary air; a third segment for anti-condensation of ammonium bisulfate; and a fourth segment for curing of ammonium bisulfate. The flue and the air channel are disposed on the downstream of a denitration device. The first segment, the second segment, the third segment and the fourth segment are disposed in that order along the gas flow direction. The first segment, the second segment, the third segment and the fourth segment each include a phase-change heat exchanger. The phase-change heat exchanger includes a heat absorption segment disposed in the flue, a heat release segment disposed in the air channel, an ascending tube and a downcomer which are configured to connect the heat absorption segment and the heat release segment.