Design Analysis And Numerical Simulation Of Industrial Scale Pressurized Oxy-fuel Combustion Circulating Fluidized Bed Boiler

With the rapid development of the world economy and industrialization,CO₂ emissions have risen sharply.The greenhouse effect has increasingly serious impact on human living environment.China is a big coal country.Coal is still one of the main energy sources in the future development because of its rich reserves and low price.As the largest source of CO₂emissions,coal-fired power plants are urgent to develop CO₂ emission reduction technology.Among many CO₂ emission reduction processes,O₂/CO₂ combustion circulating fluidized bed technology combines the advantages of oxy-fuel combustion and circulating fluidized bed combustion,which is considered to be one of the most promising technologies to reduce CO₂ emission of coal-fired power plants.Pressurized oxy-fuel combustion circulating fluidized bed technology can effectively utilize the condensation heat of water vapor in the tail flue gas,and reduce the compression energy consumption of CO₂ to a certain extent,which has attracted the attention of researchers all over the world.There has been some progress in the research of pressurized oxy-fuel combustion circulating fluidized bed technology at home and abroad,but there are relatively few reports on industrial scale units.Based on the scheme design of industrial scale pressurized/atmospheric oxy-fuel combustion circulating fluidized bed,a comprehensive CFD model of pressurized/atmospheric oxy-fuel combustion circulating fluidized bed is established.By the method of theoretical analysis and numerical simulation,the change rule of heating surface arrangement of circulating fluidized bed boiler,gas-solid flow,heat and mass transfer and coal particle combustion burning characteristics in furnace under different pressures are discussed.In order to analyze the influence of oxygen concentration and pressure on flue gas circulation ratio,boiler structure,external heat exchanger and heat absorption distribution on boiler heating surfaces,the design of 75t/h industrial scale pressurized/atmospheric oxy-fuel combustion circulating fluidized bed boiler was carried out,and the calculation method of ash balance and flue gas composition of circulating fluidized bed boiler under oxy-fuel combustion conditions was proposed.Under the condition of pressurized/atmospheric oxy-fuel combustion,with the increase of the average oxygen concentration and pressure at the furnace inlet,the heat absorption share of each heating surface of the boiler can be adjusted by pulling out the furnace,adding external heat exchanger,platen superheater,etc,so that problems such as the difficulty of furnace temperature control,low heat transfer temperature difference,steam over-temperature,and high economizer boiling degree are improved and each heating surface of the boiler can be operated within the reasonable parameter range.A 75t/h oxyg-fuel combustion circulating fluidized bed CFD is established based on the DDPM and the KTGF model under the Euler-Lagrange framework.Through the simulation results,the gas-solid flow,heat and mass transfer,and combustion conditions in the 75t/h circulating fluidized bed are predicted,and explore the flow and combustion characteristics of coal particles of different sizes.The numerical simulation results are in good agreement with the design values,indicating that the CFD comprehensive model established in this paper has a certain accuracy.The simulation results predict the structure of the classical annular core flow model in the furnace.The central area is the core area with lower solid volume fraction,and the solid velocity upward,the side wall area is the annular area with higher solid volume fraction,and the solid velocity downward.The gas-solid flow and combustion characteristics of coal particles with different particle sizes are quite different.Small particle size coal particles can escape from the furnace outlet quickly,the burning rate is faster,and the burning time is shorter,while large particle size coal particles stay longer,the burning rate is slower,and the burning time is longer.Further,a comprehensive CFD model of 75t/h pressurized oxy-fuel combustion circulating fluidized bed is established.Through the comparative analysis of simulation results and design values,it is verified that the CFD model established in this paper has a certain accuracy.Through numerical simulation,the gas-solid flow and combustion characteristics in the pressurized oxy-fuel combustion circulating fluidized bed are predicted,and the effects of different pressures and combustion atmosphere on the gas-solid flow and combustion characteristics are analyzed.With the increase of pressure,under the same Fr _P number of particles,the concentration distribution of solid particles tends to be uniform.There is little difference between the concentration of particles in the side wall area and the center area.With the increase of pressure,the volatilization of coal particles and the combustion rate of coke are accelerated.The combustion rate of large coal particles is significantly increased,and the burnout time is significantly shortened.Under different primary and secondary air ratios,there is a big change in the combustion conditions in the dense phase zone.