| Our energy structure taking coal as primary energy determined that we must solve the air pollution caused by burning coal. Circulating fluidized bed (CFB) combustion technology is widely recognized as one of the most commercialized clean coal technologies, However, there are still some problems that affect the Economy and Reliability when CFB being actual operation, such as less combustion efficiency and higher auxiliary power than pulverized coal fired boilers under the same fuel, and serious abrasion in the furnace. In this condition, the development of CFB boilers with high-efficiency, low-auxiliary power and abrasion seems particularly important.There is a poor-oxygen region in the CFB burning center and this region make the gas and solid couldn't mix well, resulting the high carbon-content of fly ash, post-combustion phenomena in the high temperature separator and so on. The volume of the oxygen-poor region has deeply relationship with the bed inventory and is one of the key constraints to the further development of CFB technology. The number of bed inventory is largely responded by the bed pressure drop.Bed pressure drop has profoundly influence on gas-solid dynamic characteristics, combustion characteristics and transfer characteristics. Several positive effects have appeared when CFB boilers running in Low bed-pressure-drop. First, the solid density at the second air outlet is deduced and the penetration depth of secondary air is increased. The gas-solid mixing in the upper zone is improved and this is good to improving the combustion efficiency. Second, the energy consumption of fan falls down due to the primary fan head getting down, and then the auxiliary power is diminished. The third is that the abrasion of the dense zone can be lightened. The availability of CFB unit is improved in this way. However, there are also some negative effects when bed pressure getting down. To begin with, particles residence time shorten result from cycle material quantity decreasing. This factor can reduce the combustion efficiency. In addition, the lower pressure drop, the less heat savings, which can cause the thermal inertia of CFB gets down. Using mechanism modeling method,the effect of bed pressure drop on the CFB combustion characteristics has been analyzed, aiming to find the best pressure drop and achieve the optimal burning of CFB boiler.In this research, a static mathematical model is established by taking a 75 t/h CFB boiler as its object. Based on "three zones core-annular structure", the core area is divided into oxygen-poor region and oxygen-rich region according to the secondary air penetration depth in this model. Mass balance equation of each gas component is established respectively in these two regions. in this way, the gas component content of each region can be obtained, especially the oxygen content. Based on this, we can get the carbon-burning rate of each region and the sum of this two rates is the carbon-burning rate of the core area. Bed pressure drop is introduced to this model as an active operation parameter. Under a given particle size distribution, the mass concentration of furnace is a function of both bed pressure drop and fluidized velocity and this function is obtained by fitting experimental data. Through Visual Basic numerical calculation, the influence of bed pressure drop and fluidized velocity on the volume of oxygen-poor region, furnace temperature, combustion fraction distribution and combustion efficiency is analyzed. The calculated results are compared to the actual test data which was made out by Shanxi Electric Power Research Institute at the 150 t/h CFB boiler in Datuhe Second Thermal Power Plant, and they are agree well with each other. This can provide some theoretical support for the performance prediction of low bed pressure drop CFB boiler. |
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