Research On Separation Characteristics Of Circulating Loads In Medium Speed Mill Of Coal-fired Power Plant Using Pulsating Fluidized Bed

Coal has the highest proportion in national energy consumption,which is closely related to the country's resource endowment of"rich coal,lean oil,and less gas".The abundance,reliability,low cost and cleanability of coal resources determine that the primary energy consumption is dominated by coal will not change in a short time.The increase in coal demand in the power industry is the main reason why coal consumption remains high.Since the coal used in power plants is mostly inferior coal with high ash content,showing the properties of high gangue content,great ash discharge,low heat generation,and poor grindability.Thus,the coal needs to be ground several times to meet the particle size requirements of pulverized coal,which causes the mass of the circulating loads inside the mill to be far greater than the mill output.A large amount of coal-based minerals with great hardness,heavey density and inferior grindability are enriched in the circulating loads,which causes the wear of the mill equipment and the increase of grinding energy consumption.At meanwhile,these minerals are also the fundamental source of environmental pollutants such as SO₂,NO_X,smoke and heavy metals produced by coal-fired power plants.If the mineral components in circulating load can be removed by coal processing,the coal can be desulfurized,reduced ash and upgraded during the grinding process and before combustion,which will significantly reduce the internal cycle rate and wear of the mill and increase the grinding energy efficiency and the reduction of pollutant emissions from coal-fired power plants.In this thesis,a laboratory-scale gas-solid fluidized separation bed was used to carry out a research on the off-line dry separation with circulating loads.The cyclic“grinding-classification”operation of the industrial mill was simulated by the self-made coal mill in the laboratory,and the preparation of circulating loads and pulverized coal were completed under laboratory conditions.Coal-based minerals such as quartz,kaolinite,illite,pyrite,etc.are mostly enriched in circulating loads,but the degree of dissociation of the minerals is relatively high.This creates favorable conditions for the cyclic load to be processed by density in the gas-solid fluidized bed.Through comparison,the circulating loads is the best separating object in the coal pulverizing system,and the dry beneficiation of the circulating loads should be operated based on high-density gangue discharge.The fluidization of the circulating load in a steady gas flow fluidized bed is similar to that of conventional Geldart B particles.With the increase of the fluidization gas velocity,the fluidized bed flow pattern appears in sequence of fixed bed,critical fluidization and bubbling fluidization.However,in a pulsating fluidized bed,the flow pattern can be divided into intermittent fluidization at low pulsation frequency(0.5-2.5Hz),piston-like fluidization at medium pulsation frequency(3-4.5 Hz),and virtual conventional fluidization at high frequency(5-6 Hz).When circulating load is separated in a steady flow fluidized bed,the best combination of operating parameters is fluidization time 10min,gas velocity 5.5 cm/s and initial bed height 90 mm.The tailing ash content increases from 48.11%of the feed to 78.35%,the tailing yield is 42.86%.The corresponding ash segregation degree and overall separation efficiency are 25.75%and 51.69%,respectively.In comparison,the separation performance of a pulsating airflow fluidized bed is better than that of a steady flow fluidized bed.Under the same separation time and bed height,the best combination of parameters for pulsating airflow is the gas velocity of 4.0 cm/s and the pulsating frequency of 6 Hz,which corresponds to 80.43%of tailing coal ash and 44.58%of tailing yield.The ash segregation degree and overall separation efficiency are as high as 28.83%and 57.12%,respectively.Through a simplified force analysis of particles in free space,it is believed that the reason for the better separation performance of a pulsating fluidized bed may be that particles have greater longitudinal displacement under the effect of pulsating airflow.The results of the separation in the gas-solid fluidized bed show that the off-line dry separation with circulating load is feasible.In view of the fact that there is no practically used coal milling process for cyclic load separation,this paper proposes a new process that diverts the circulating load to the outside of the coal mill and uses a pulsating fluidized bed for separation.The CPFD method of computational particle dynamics is used to simulate the micro-scale fluidization process of a steady flow fluidized bed and a pulsating fluidized bed.Through the optimized Wen-Yu/Ergun drag model,the distribution of particles in the separation fluidized bed and the gas-solid two-phase flow law are studied.Compared to steady airflow,the effect of pulsating flow makes the movement pattern of bubbles more orderly.The velocity direction of particle movement is very regular and basically follows the periodic vertical up and down movement of pulsating air flow.For the separation fluidized bed,the reduction of the degree of chaos in bed will weaken the back-mixing effect of the stratified particles,which is beneficial to the stratification process.The simulation results also proved that the simulated particles can be efficiently classified according to density in the gas-solid fluidized bed.There are 73 figures,17 tables,208 reference articles,and 1 appendix in this thesis.