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.

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.

The material balance in CFB furnace has been proved important for the performance of the boiler. The bed inventory of CFB boiler is composed of fuel ash and sorbent additives. It is necessary for the engineer to know the ash size formation character of fuel before make a proper design of the boiler and the auxiliary system. Traditionally, the only way to know the ash formation information is by the fuel combustion test in pilot CFB furnace. Because of the high cost of the test and the insufficient information from the test, such test was rarely done. The present investigation is proposing a simple laboratory test procedure instead of the pilot CFB combustion. The validation of the method is introduced in present paper. Two kinds of coals include the coal from Gardanne 250MW CFB were tested by present method and the result is compared with the data from Gardanne boiler.

It was confirmed that the combustion action in dense bed is different between the bubbling bed and circulating fluidized bed by the measurement of the gas profile distribution and combustion fraction distribution along the furnace height. The bed quality results in the mass transfer resistance increasing, which effects the combustion reaction in dense bed. The combustion reaction in dense bed is in reduce condition. The combustion occurs all along the furnace even in the cyclone. A comprehensive model is proposed here to predict the axial gas concentration profile in the dense be, which is considered the mass transfer resistance between the solid and gas in dense bed. Calculations indicated that the resistance has a great effect on the combustion in the dense bed. Experimental data and results of the model are in good agreement.

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.