Research Of Heat Transfer Model In Rotary Ash Coolers

Rotary ash cooler has been widely applied in the Circulating fluidized bed (CFB)boilers due to its particle size flexibility and reliability without agglomeration.However, a rotary ash cooler is still designed empirically, because of the leakage of aproper heat transfer calculation model.In this thesis, a cold test rig of rotary ash cooler was constructed to investigatethe dynamic angle of repose of the bulk solid bed, which is of great importance toassure the heat transfer area between ash and wall. Based on the heat transfer analysisin a rotary ash cooler by the heat transfer characteristics of ash particles as well as theflow, a whole one-dimensional heat-transfer model was suggested with the reburningof residual char in the ash, where, the unsteady heat conduction and wide sizedistribution of ash particles was considered.The experiment results from the cold test rig showed that the dynamic angle ofrepose was independent from the rotational speed, but it generally increased with theincreasing of ash size, roller diameter and ash filling percentage. The results providedsome key parameters in the suggested model. And the predictions by the presentmodel agreed well with the measurements of five rotary ash coolers of five differentcommercial boilers. The comparison confirmed the reliability of the suggested model.With this model, the performance of a rotary ash cooler was predicted further. It wasfound that the convective heat transfer of ash-wall dominates in the total heat transfer.In addition, residual char reburning and unsteady heat conduction of ash particleshave great effect on the ash particle temperature at the outlet of the cooler in the caseof the high carbon content and large ash particle size. And the heat released fromresidual char is more than that from physical heat at the entrance of the roller.However, it decreases rapidly to zero after a short distance within1m.Finally, a new concept of heat-transfer average size d_Hof ash particles wasproposed to simplify the calculation as well as its formula, which represents the heattransfer area of the particle.