Basic Research On Particle Flow,Heat Transfer And Combustion Of Coal And Biomass Pellet In Fluidized Bed

Biomass pellet is widely used because of its high density,easy storage,transportation and management.The co-combustion of coal and biomass pellet can reduce pollutant emissions and increase energy density,which combines the advantages of two different energy sources.Co-combustion of coal and biomass in fluidized bed is a promising technology for thermal power generation.Because of the limitation of traditional experimental methods,numerical simulation has become an important way to study fluidized beds.In this paper,the flow,heat transfer and co-combustion of coal and biomass pellet in fluidized bed were studied by means of experiment and numerical simulation,providing reliable parameters and information guidance for industrial application.The combustion characteristics of coal and biomass pellet were studied by thermogravimetric test.The effect of biomass blending ratio on ignition temperature,burning rate,burnout temperature and combustion index was studied.Combustion performance of coal is improved by the added biomass.The synergistic effect between biomass and lignite is greater than that between biomass and bituminous coal.By analyzing the different combustion stages of coal and biomass through thermodynamic results,the appropriate combustion model function is selected.The first-order chemical reaction model is suitable for the release and combustion of volatile matters,and the carbon combustion of bituminous coal.The blending of lignite and biomass belongs to the diffusion model.A small test facility for coal and biomass pellet was set up to measure the pressure fluctuation signal in fluidized bed,and the minimum fluidization velocity was determined by pressure drop.The minimum fluidization velocity formula was deduced and the suitable operation air velocity was selected according to the distribution of biomass.The effects of biomass ratio and air velocity on flow patterns were studied.The periodicity and stability of flow in fluidized bed were investigated by using the recursive plots.The frequency characteristics of coal and biomass pellet were studied by Hilbert-Huang Transform method,and the energy and frequency distribution of intrinsic modal function were analyzed.An experimental device for the flow and heat transfer of coal and biomass pellet was set up.The particle temperature,particle flow and bubble variation in the process were recorded by infrared thermal imaging and high-speed camera.The biomass pellet was constructed by virtual sphere element,and the concept of particle volume coefficient was introduced to modify the gas-solid interaction force.The improved DEM-CFD model was coupled with heat transfer model to simulate the flow and heat transfer of coal and biomass,and the simulation results were compared with the experimental results.The values of particle temperature,mixing index and heat transfer were extracted to study the effects of air velocity and biomass addition on these characteristics.A fluidized bed device for the co-combustion of coal and biomass pellet was developed to study the effects of biomass adding on pollutant emission,bottom ash slagging rate,heavy metal morphology and environmental risk.The concentrations of sulfur and nitrogen in biomass pellete are low.When burned with coal,sulfur dioxide and nitric oxide are obviously reduced.Alkali metals and alkaline earth elements in biomass can inhibit the volatilization of trace heavy metals during coal combustion,especially for chromium,nickel and vanadium.Ash slagging indexes have certain predictive effect on the coal combustion with biomass,and the possibility of slagging can be effectively reduced by co-combustion.In order to carry out risk assessment more comprehensively and accurately,it is often necessary to take into account different evaluation methods,which not only study the impact of the total concentration of heavy metals,but also the sensitivity and mobility of heavy metals.A coupled Euler-Euler model with chemical reaction was used to simulate the co-combustion of coal and biomass pellet.The distribution of temperature field,carbon monoxide,sulfur dioxide and nitric oxide concentrations in fluidized bed were analyzed,compared with the experimental results.When the mixing ratio is less than 10%,simulation results are in good agreement with the measured values.As the blending ratio is more than 10%,the deviation rate between simulation and experimental results is large,about 15%.