| The circulating fluidized bed (CFB) combustion technology with the advantages of extensive fuel flexibility, low NOx emission, high desulfuring efficiency and good load adjustable capability and so on, has become the new generation clean coal combustion technology. With the development of CFB combustion technology, the capacity of a CFB boiler has become larger and larger. Until now, the largest 460MW CFB boiler was built and the design of 600-800MW super-critical CFB boilers had been complete in overseas. In China, the development work of 600MW CFB boiler is carried out. Current large CFB boiler usually uses the structural arrangement that several separators are laid around one or two furnaces. So the main problems we faced are as the number of separators of a large CFB boiler increases more and more, the arrangement of separators becomes more difficult. And the flow rate of flue gas to each separator is very different. Also, larger furnace can result in bad penetration of the Second Air. Therefore, the risk of scale-up of CFB boiler becomes higher. The existing arrangement schemes of foreign large-scale CFB boiler are granted patents. So a new domestic CFB boiler's scaled up scheme which has independent intellectual property is needed to solve those problems.By summarizing and analyzing the structure forms of CFB boiler, a new type of CFB boiler arrangement scheme called"furnaces around separator"was put forward by Chongqing University. The largest difference with other schemes is identified that a square cyclone separator with double inlets is laid between two furnaces, and a new integrated external heat exchanger (EHE) is used under the separator. Thus, this scheme has many advantages comparing with traditional arrangement schemes such as low request for the penetration of the Second Air. The risk of scale-up can be effectually reduced by adopting module combination. And every module can be independently controlled such as the coal supply. In order to verify the feasibility of this new CFB boiler arrangement scheme, gas-solid flow in the solid circulation system of this boiler is investigated. The main research works include the gas-solid flow characteristics of system operation and operation stability of this new boiler, gas-solid flow characteristics of the EHE and the performance of the square separator with double inlets.Experimental study on the gas-solid flow characteristics of system operation and operation stability of this new boiler was conducted. Preliminary results indicated that the fluidization characteristics of the two furnaces were quite adjacent. The disturbance between the two furnaces was small and the whole solid circulation system could run steadily. The square separator separated solids efficiently and the integrated external heat exchanger could successfully discharge the solids from the standpipe into both of the furnaces. The solid circulation system had excellent characteristics of pressure balance.A new type of pneumatically controlled EHE was developed and used in this new CFB boiler scheme. The major feature of this EHE was that the solids flowed to two heat transfer chambers could be controlled. Thereby the distribution of the quantity of heat transfer could be easily adjusted. Simultaneously, the EHE and recycle device were connected in order to recycle the solids back to two furnaces steadily. Experiments completed in the visible cold EHE test rig showed that the solid mass flow rate to the EHE or the loop seal, to two heat transfer chambers, to two exports of solids could be controlled by the aeration airflow into each chamber and geometrical dimensions like the height of partitions. That was because the change of fluidizing velocity of each chamber led to the change of particle entrainment rate and pressure distribution in the EHE. In addition, an empirical correlation between the solid mass flow rate and pressure drop across an orifice was proposed and a new orifice discharging coefficient was defined according to the experimental data and theoretical analysis.The performance of the square separator with double inlets is a key factor of the success of this new CFB boiler arrangement scheme. Experiments on the performance and gas-solid flow of the square separator have been conducted in a cold test rig with a separator cross section 400mm×400mm. Experimental results indicated that with the inlet velocity of 22.4m/s and the inlet solid concentration of 4.9g/m3, the cut size was 15μm, the critical size was 75μm, and the pressure drop coefficient was 1.7. The performance was also affected by the inlet velocity and solid concentration. Especially, the instantaneous separation occurred at the corner was very significant for the improvement of the collection efficiency with high inlet solid concentration for CFB boiler.On the basis of experimental study, the gas-solid flow in the square separator was numerically simulated. The calculated grade collection efficiency and pressure drop have reasonable agreement with the experimental data. And numerical simulation was used to optimize the configuration and dimension of the square separator. The flow field in square separator shows a feature of Rankine eddy and local vortex exits at the corners. By optimization of the configuration and dimension, the cut size is about 6μm. The pressure drop of the square separator is small and is not considered to be the main factor of the performance.Experimental and numerical results indicated that the control characteristics of solid flow rate in EHE and the performance of square separator with double inlets met the operation requirements of this new CFB boiler arrangement scheme well, but it will be verified when this new CFB boiler is scaled up. A new 35t/h industrial boiler was designed according to the above results. First, the laboratory bench scale CFB boiler arrangement scheme will be scaled up to industrial boiler. Some test will be conducted on the industrial boiler. Those tested data and experience will be used to scale up the new CFB boiler to a larger commercial boiler.
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