FORMATION OF INTERNAL CIRCULATION IN A FLUIDIZED-BED BOILER AND CONTROL OF OVERALL HEAT-TRANSFER COEFFICIENT

Research was conducted on an internally circulating fluidized-bed boiler as to formation of internal circulation and control of overall heat transfer coefficient. The boiler features a fluidized bed divided into a main combustion cell and heat exchange cells located on both sides of the combustion cell and separated by inclined partition walls. Silica sand was used to form a revolving flow in the main combustion cell and a circulation between the main combustion cell and heat exchange cells as the fluidized-bed material. It was found experimentally that adequate circulation of the fluidized-bed material between the main combustion cell and the heat exchange cell, which was needed to recover the required quantity of heat from the immersed heating surface, is obtained when the stable fluidized-bed height exceeds the height of the inclined partition walls and when the volume of fluidizing air supplied to either side of the main combustion cell exceeds a certain limit. It was also found that the descending rate of the moving bed in the heat exchange cell can be controlled by changing the amount of circulating air supplied from the lower part of the heat-exchange cell. Measurement of overall heat transfer coefficient values verified that this coefficient varies almost linearly with the change in fluidization velocity in the heat exchange cell in the range of U/U(mf) = 0 to 2. Thus a flexible load control method to control the internally circulating fluidized - bed boiler is established.