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.

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.

Coal gasification is one of the key clean coal projects in China. A partial coal gasification of CFBB co-generation cycle process is suggested by Tsinghua University, which takes the hot circulating ash as the heating source to pyrolyze the coal in a steam fiuidized gasifier to generate the coal gas and tar. The coal gas is the fuel of the gas turbine. The char is burnt in the furnace to generate steam. In this process, One of the key technology is to reduce the yield of tar and get more gas. In this paper, the experiment investigation of two main components in tar: benzene and toluene catalytic cracking over three kind of circulating ash particles from CFBB in a fixed bed reactor, is introduced. And the catalytic mechanics is also discussed. The experimental results show circulating ash promote the cracking reactions of tar, the cracking reactions active energy of the reactance decrease compared with that over quartz sand. The temperature needed for same conversion ratio is decreased 80 C to 160 C for different ashes. The cracking gas product is more for ash than for sand. The solid product is coke for both case, which deactivates the ash. While the activity of the ash can be regenerated by burning the coke on the surface of ash particles. The total yield of gas increases.

Since the first pilot 75 t/h water cooled separator CFB boiler was in operation in 1996, a hot test program was been conducted by Tsinghua University and several boiler works, so as to promote the scaling of CFB boiler. The present paper introduces the test program and the primary results, such as the separator efficiency, material balance, solid bulk density, heat transfer along the height of the furnace, et al. Besides the traditional test technology, some new developed apparatus was used in the hot test. According to the experience obtained from the boiler, the improvement measure was taken for the second and third boiler, which were in commercial operation in 1998. The first operation experience of the second and third boiler proves the effect of the improvement. Considering the experience and the test program, the scaling up design of water cooled separator was done. The present paper also introduces the design consideration and feature of the 130 t/h and 220 t/h CFB boiler.