Before the bed materials form into turbulent bed during fluidization, the primary air for fluidization moves mainly upward in the bed of material in form of “bubble”, at the upper part of the bed of materials, small bubbles come together to form big bubbles. The bubbles tail carry part of fine particles, which is equivalent to every sparse fluidized air mass, in the bed of materials around the bubbles, there is gas flowing at lower speed, but the particle concentration is thicker than that of bubble, if the bubbles are not distributed evenly or are too big in the bed of materials, the fluidization will be abnormal. If the bubbles are too big or concentrated to move upward, the fluidization surface layer will fluctuate greatly, so the boiler will not operate stably. In fact it is normal that the bubbles are available in the bed of materials, but if the bubbles are too big or concentrated to upwell, which belongs to abnormal fluidization. If the bubbles in the bed of materials aggregate and combine to approach the width , the bed of materials will be divided into several layers, one layer is thicker bed materials, one layer is sparse bed materials, and “Air” moves upward, when a certain height is reached, the bed materials collapse, the solid particles drop like rain. This phenomenon is called slugging. If slugging happens in the boiler, the air pressure fluctuates violently and the combustion is not stable. The damage is that the lower part of the bed of materials easily cokes. If there are tubes imbedded in the bed, they will be worn quickly. The slugging phenomenon occurs during fluidization between bubbling bed and turbulence , therefore the fluidization status between bubbling bed and turbulence bed is called unstable fluidization status. The boiler shall be avoid to operate under this status. The occurrence of abnormal bubbling and slugging is mainly related with the factors of unreasonably designing of air distributor and air nozzle, excessive coarse bed materials and too thin bed of materials.