Studies on circulating fluidized bed (CFB) boilers have being conducted at the Tsinghua University (TH) for about two decades and much of works are done to link the fundamentals with practical application. A full set of design theory was developed and some key elements of this theory are presented in this paper. First, a classification of state of the solid-gas two-phase flow in CFB boiler is given. TH's studies validated that a CFB boiler can be generally described as the superposition of a fast bed in the upper part with a bubbling bed or turbulent bed in the bottom part. A concept model of material balance for the open system of CFB boiler was developed and later improved as a more comprehensive 1-D model taking ash formation, particle attrition and segregation in bed into account. Some results of the models are discussed. Then the concept of State Specification of a CFB boiler is defined and discussed. The State Specification is regarded as the first step to design a CFB and a base to classify different style of CFB boiler technologies for various CFB boiler manufacturers. The State Specification adopted by major CFB boiler makers is summarized and associated importance issues are addressed. The heat transfer model originally developed by Leckner and his coworkers is adopted and improved. It is further calibrated with experimental data obtained on the commercial CFB boiler measurements. The principle, improvements and application of the model are introduced. Some special tools developed for heat transfer field test are also given. Also, combustion behaviors of char and volatile content are studied, and the combustion difference between a CFB boiler and a bubbling bed is analyzed. The influence of volatile content and size distribution is discussed. The concept of vertical distribution of combustion and heat in CFB boiler furnace is introduced and discussed as well. In the last, the suggested design theory of CFB boiler is summarized.

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.

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.