During cold test and ignition of boiler, when the primary air flow rate doesn’t reach the critical flow rate, the air is not distributed evenly in the bed of materials and the distribution and voidage of particles size of materials in the bed are not even too. The flow of steam is larger at the place with small resistance, and it is less at the place with big resistance, at this moment large amount of air passes through the bed of materials from the place with small resistance, but the other place is still in fixed bed status, this phenomenon is called channeling.  If the bed of materials is severely uneven or the air distributor is not arranged reasonably, the air nozzle is blocked locally, even the primary flow rate exceeds the critical flow rate, the bed of materials will be channeled. The channeling is divided into penetration –channeling and local channeling.  The penetration channeling means the channeling is through the whole bed of materials, the local channeling means the channeling only happens in local height of the bed of materials. The channeling in the hearth bed will not only make fluidization quality of the materials be lower and bed of materials easily coke, but also affect the stability of heat transmission and flaming inside the boiler, The main reasons of channeling are:
1. The primary air temperature during operation is too lower and does not reach the design requirement.
2. The bed of materials is too thin or severely uneven or cokes in the boiler bed.
3. The coal supplied is too wet, the air for distributing coal and for returns is not adjusted reasonably, causing stacking phenomenon at the coal supply inlet or at the returns position.         
4. The air distributor is not designed reasonably, the air nozzle is too less and the pitch is too big.