Local bed to wall heat transfer coefficient in the furnace of a 75t/h CFB boiler was measured with a heat flux probe and bulk density sampling probe which were developed by Tsinghua University. The test result shows the strong relation between local heat transfer coefficient with the local bulk density and the bed temperature. The result was compared with the data from literature. According to the experimental data, a semi-empirical model was developed to predict the local heat transfer coefficient based on the bed temperature and bulk density. The prediction of the model is basically matching the published data.

Char combustion is on a special reducing condition in the dense bed of a circulating fluidized bed combustor. Experimental findings were used to develop comprehensive mathematical model to simulate the hydrodynamic and combustion processes in a circulating fluidized bed combustor. In the model, gas-solid interaction was used to account for the mass transfer between the bubble phase and the emulsion phase in the dense bed, which contributes to the reducing atmosphere in the dense bed. A core-annular structure was assumed in the dilute area rather than a onedimension model. The submodels were combined to build the comprehensive model to analyze the combustion in CFBC and the effect of operating parameters on the coal combustion. The model predictions agree well with experimental results.

A comprehensive model for predicting the sulfur retention performance in circulating fluidized bed combustors was developed which involves the different residence times, the wide particle size distribution and the different forms of sulfur in the coal. In addition, the reductive decomposition of CaSO4 is highlighted. The simulation results from the model show that the sulfur contents, the bed temperature, the sorbent particle size distribution and the sorbent activity or the maximum conversion rate can significantly influence the sulfur retention performance in CFB combustors.

The effect of boundary layer reactions on the conversion of char-N to NO, N2O and HCN was studied by a coupled experimental/theoretical approach. In the experimental part, the evolution of HCN at conditions of a batch fluidized bed reactor was studied. The results show that the char-N conversion to HCN is very low under normal oxidizing conditions. This explains why it was not detected by early studies of this system. The results also suggest that the release of HCN is related to gases released when the char is oxidized rather than to late devolatilization reactions. The product distribution is modeled using heterogeneous reactions on the char surface coupled to homogeneous reactions in the gas phase. Three cases were analyzed. In Case A, only the heterogeneous production of N2O and NO was considered. Case B considered HCN and NO heterogeneous production. Finally, Case C combined cases A and B by allowing the heterogeneous production of NO, N2O and HCN. Cases A and B predict most of the qualitative trends for the char-N conversion to N2O reported in the literature. However, Case A fails to predict the experimentally observed release of HCN when oxidation around the particle is suppressed. Among the three cases, Case C is the one that closely predicts experimental observables. Finally, the predictions obtained by the model are used to define further experiments, particularly on the evolution of gas concentration in the boundary layer with char burnout, that can be useful in order to obtain more insight in the mechanism of nitrogen oxide formation during char oxidation.

Separator is one of the key components of circulating fluidized bed (CFB) boiler, which directly influence the performance of the CFB boiler. Only when the separator supplied enough circulating solids, the bed can work under the material balance and energy balance condition. The square cyclone with curved inlet has shown the unique advantages such as compact size, no expansion joint, excellent reliability and lower operating cost. So it can fulfill well the scaling up of CFB boiler. A series of tests have been continued on the large-scale cold model test system for characteristic dimension 700mm square cyclone by changing the length of the vortex fonder, diameter of vortex finder and the length of vertical part of square cyclone. The pressure drop in the square separator and the fraction collection efficiency under different operation conditions were measured. The results of the experiment showed that the square cyclone with curved inlet has high collection efficiency and low pressure drop. The cut size d50 is around 20μm～30μm and the critical size d99 is around 80μm～160μm. The collection efficiency may be the function of the length of vertical part, diameter of the vortex finder, the length of the vortex finder and the speed of air at inlet.