The primary air required by fuel combustion passes three strokes in the air preheater and enters to the primary air chamber and then enters to the hearth via the water-cooing air distributor. For the uniformity of air distribution and the requirement of method of ignition, the primary air chamber is provided with partition for separating the primary air chamber into two chambers with equal air pressure. At the back side of the air chamber, one ignition device is provided respectively, the primary air is fed into the primary air chamber from the air passage and ignition device at two sides respectively. The secondary air passes two strokes in the air preheater and is arranged in three layers around the combustion chamber, and enters the hearth slantly via 21 tuyeres.