Cold Model Experiment Of Returning Ash Heat Exchanger Device Used For Industrial CFB Boilers

The circulating fluidized bed (CFB) boiler combustion technology as the new generation clean coal combustion technology has made a rapid development in both home and abroad. Its advantages mainly including extensive fuel flexibility, low NOx emission, high desulfurization efficiency in furnace and good load adjustable capability and so on. Because coal will play a main role in the energy structure of our country at present and in quite a long time, and the new situation of gradually increasing energy tension and worse coal quality, the circulating fluidized bed (CFB) combustion technology, which can burn various inferior fuels and is an environment-friendly and highly efficient technology, has become an important direction of coal-fired industrial boiler. However, the height of small CFB boiler is limited, and there is no sufficient space for heating surface arrangement in furnace, which has lead to some problems of industrial CFB boiler for a long time, such as unreasonable layout heating surface, worn off buried pipes, high manufacturing costs, complex structure and so on. When the quality of coal degenerates, the problems will be more serious. The poor quality of coal is mainly reflected in the decrease of calorific value, increase of ash content, raise of sulfur content, increase coal gangue and other poor quality waste fuel. Foreign small and medium sized boilers generally use oil or gas as fuel, so there is little research of the coal-fired industrial boiler. As a large number of coal-fired industrial CFB boilers (The capacity of evaporation is less than 35t/h) are applied in our country, our research team has proposed a new device for industrial CFB boilers: setting a returning ash heat exchanger, and moving buried pipes or parts of water wall heating surface which were originally arranged in the furnace into the returning ash heat exchanger device. These will reduce buried pipes'wearing, and meanwhile maintaining the furnace in fast fluidized bed or turbulent fluidized bed, which will improve the heat transfer or burning intensity in furnace, and then reduce the cost of the manufacturing boiler ultimately.In this paper, in order to solve the problems encountered in the engineering practice, we design a new type of returning ash heat exchanger, and carried out much cold test study in the CFB boiler model system. The cold test study is mainly on the relationship between structure parameters, operating conditions with the gas-solid flow characteristics, pressure balance, material cycling balance characteristics and some other patterns. The main study results showed that:①The newly designed returning ash heat exchanger for industry CFB boiler is feasible, gas-solid flow stability, and the overall fluidization effect are good;②The returning ash heat exchanger chamber's import and export structures are very import for gas-sold flowing in it. The test analyzed the relationship between structure parameters of returning ash heat exchanger chamber and gas-solid flow characteristics in it, obtained from the area ratio of bottom orifice, height ratio of returning ash orifice;③Increase the air speed of current-sharing chamber will be good for material flow in standpipe, and the air speed of returning ash heat exchanger chamber effects the material flowing in whole device much more, also including pressure drop in standpipe. It will achieve the best operation purpose when adjusting the air speed reasonably;④Adding the lateral supply air will help to promote the material flowing in standpipe, which can reduce the height of material level effectively, and relieve the situation of the insufficient height of standpipe caused by the limited height in industrial CFB boiler. Also lateral supply air can replace some of current-sharing chamber air be used, which can lead to the obvious effect of energy-saving;⑤Setting return air pipe in the top of returning ash heat exchanger chamber will help to improve the pressure disturbance leaded by operating air speed increased, and reduce the height of effective material level. Return air flow into furnace which was served as secondary air will be conducive to gas-solid flow mixing in the furnace, which will play an important role in combustion and the efficiency of desulfurization and so on;⑥When changing load, the circulation system connected by returning ash heat exchanger device responded quickly, which is more steady and quicker than U-valve in reducing load.