Study On The Selective Discharging And Cooling System Of The Bottom Ash In CFB Boilers

The large-scale circulating fluidized bed (CFB) boiler technology has been rapidlydeveloped and more mature in recent years, and becomes the main way to achieve thehigh efficiency and clean utilization on the low grade fuel in China. However, there areseveral problems during the large-scale process of development and application. Thefunctional failure of the bottom ash treatment system, the bottom ash heat recovery andthe abrasion of heating surface in furnace are the three most prominent problems, whichseverely restrict the availability factor and operational reliability of CFB boilers. Theessence of the problems is that the particle size distribution (PSD) of bed materialdeviates the scope of design after the boiler combusting a great amount of low gradefuel. Moreover, the selective discharging on bottom ash is the most efficient, direct andflexible way to regulate the PSD of bed material. Consequently, based on studying theformation mechanism and regulating method of the bed material PSD in CFB boilers,it’s very important to develop a new fluidized bed bottom ash cooler or dischargingdevice and investigate the thermal economy, cooling capacity and recovery efficiency ofthe bottom ash heat recovery system.In the present work, the formation mechanism and regulating method of the bedmaterial PSD was firstly discussed. Then the thermal economy, cooling capacity andrecovery efficiency of the bottom ash heat recovery system were studied. Furthermore,the thinking of particle separation and stepped cooling for the wide distribution bottomash was introduced, and a series of invention patents about fluidized bed bottom ashcooler and matching technologies were applied. Finally, the technical feasibility of theselective ash discharging device (SAD) and the compound fluidized bed ash cooler(CFBAC) were investigated, as well as the experimental study and application of theCFBAC. The main works and innovations can be list as follows:①Modeling study on the regulating effect of the selective discharging of bottomash and exhausting the circulating material on the PSD of the bed materialIt was concluded that the selective discharging ash was the best method to regulatethe PSD by analyzing the formation mechanism and regulating method of the bedmaterial. Based on the one-dimensional static model established for the mass balance ofa CFB boiler, three conditions (unreturning the fine particles in the bottom ash,returning the fine particles and dishcharging the circulating material) were calculated. The results indicated that selective discharging on bottom ash and exhausting thecirculating material have just the opposite regulating effects.②Systemic research on the thermal economy, cooling capacity and recoveryefficiency of the bottom ash heat recovery systemThe unit thermal economic indicators of three CFB power plants in China (150MWe,300MWe and600MWe) were derived based on heat balance calculation andanalysis on the principled thermal system in turbine heat acceptance condition, takingthe influence of two different bottom ash heat recovery modes into account. In addition,the heat recovery efficiencies of different recovery modes, the cooling capacities ofdifferent cooling medium and the proper proportion of water and air were investigated,and some related suggestions have been made.③Experiment study on a selective bottom ash discharging device for CFB boilersThe selective bottom ash discharging device can be used to achieve the selectivedischarge on the bottom ash. Through the experiments about the gas-solid flowcharacteristics conducted in a visible cold test rig, it was concluded that fluidzingvelocity, separation partition height, initial PSD of the test bed material could influencethe separation effect. The fluidizing velocity matching relationship of main bed andseparation chamber was also obtained.④Development of the particle separation and stepped cooling thinking for thewide distribution bottom ashA series of patent technologies about fluidized bed ash cooler with the thinking ofparticle separation and stepped cooling were applied, such as “CFBAC”,“doubleseparating fluidized bed ash cooler” and “double spouted-bed fluidized bed ash coolerand its return air system”. Especially, for developing the CFBAC, experimental studyand numerical simulation about the gas-solid flow characteristics were conducted in asmall scale visible cold test bed and a semi-industrial cold test rig, respectively.According to the experiment results, the CFBACs were industrially applied in a150MWe CFB unit and a300MWe CFB unit, respectively. The application results showedthat the CFBAC had a well separation effect, an excellent adaptability on particle sizeand a strong cooling capacity, all parameters could meet the design requirements. TheCFBAC could be one direction of the future CFB boiler bottom ash cooler.