Study On The Bed Material Size Distribution Dynamic Balance In CFB Boilers’ Start-up Process

Recently, power demand increases rapidly, China’s energy consumption still bases on coal, while coal plant is the main part of power field. Environment protection laws are stricter and plant coal’s quality changes, which promote circulating fluidized bed (CFB) combustion technology’s application in power plants. The new combustion technology could be suitable for various fuels, change load over a wide range, exhaust small amount of pollutants and so on.The bed material size distribution dynamic balance is studied, in order to provide theory reference and feasible suggestions for CFB boilers’mass concentration distribution, start-up optimization and automatic starting system.According to "one inlet and two outlets" mass balance system and CFB boiler start-run process, mass dynamic balance model is built, which includes coal ash model, fly ash model, slag model and attrition model. Coal ash model is based on intrinsic ash size distribution of coal. In fly ash model, grade collection efficiency is the function of cut size (d50) and critical size (d99), mass flow rate is gained from saturated entrained flow rate above TDH (transport disengaging height). Ash particles become thinner because of attrition. This phenomenon is put into consideration in slag model, and attrition rate is in direct proportion with u0-umf.Through analysis software about CFB bed material of start-up size distribution dynamic balance, it is studied in this thesis that the material size distribution changes with time of CFB boiler bed’s start-up process. Then, boiler parameters’effect on mass size distribution is analyzed. These parameters include coal, grade collection efficiency, size distribution of primary bed material, bed inventory, fluidized air velocity and so on. Results show that these factors’influence is different in some degree. Fluidized air velocity’s effect is so little that could be neglected, in the situation that flow state is not changed. The start time decreases with the decrease of bed inventory, but bed inventory has little effect on mass size distribution. Grade collection efficiency relates with separator’s design parameters, inlet mass concentration and running temperature. The bed material size distribution is optimized and the start-up process speeds up with the decrease of cut size and critical size. Primary bed material size distribution is better, the start-up is faster, but this factor has little influence on steady-state mass size distribution.Mass transport system should be used to supply suitable particles for boiler to change bed material’s size distribution, if the mass size distribution could not be stable in short time, when the ability of coal changing into ash is not good. These measures are to make boiler’s start-up process more rapidly and decrease energy consumption of start-up process.Mass concentration distribution is important for CFB boiler’s start-up and running. In boiler’s start-up, bed material’s size distribution influences concentration distribution, circulated ash and heat load speed up process, and then determines start-up control law. So it is very important for CFB boiler start-up mass size distribution in theory and project value.