The rotary dryer adopts a drying mode in which the sludge is in direct contact with the drying hot air. The drying hot air is provided by a gas (oil or biomass) hot blast stove, and the amount of hot air can be controlled according to the heat demand for sludge drying. With the rotation of the cylinder, the sludge is lifted to the center of the cylinder by the lifting plate, and is gradually scattered to form a material curtain. The high-temperature air flows through it to preheat the sludge, evaporate water, and reduce the moisture content, thus achieving the expectation The purpose of drying.