Simulation And Optimization For Oxy-coal Combustion Thermodynamic System And Its Heat Transfer Performance Analysis

As one of the most efficient carbon capture,utilization and storage technologies in coal-fired power plants,Oxy-coal combustion technology is based on the principle where the mixture of recycled flue gas and oxygen is used as the combustion-supporting gas for burning pulverized coal in O₂/CO₂ atmosphere.The concentration of CO₂ in dry flue gas can reach 95%or higher.At present,it is of great feasibility to convert the conventional power plant to Oxy-coal combustion power plant.Many domestics and foreign research institutions have carried out much research work on evaluating Oxy-coal combustion tech-economic property.So far,dozens of pilot-scale Oxy-coal combustion power stations have been built all around the world.There have not been commertial Oxy-coal combustion power plant built yet because of its weak economic level.Because the O₂/CO₂ atmosphere is formed in the Oxy-coal combustion instead of the air atmosphere,the difference of physical properties between CO₂ and N₂ leads to the difference of combustion,emission and flue gas physical properties.The applicability of conventional air-combustion calculation method is reduced under Oxy-coal combustion conditions.Because the experimental steps of Oxy-coal combustion power plant are relatively complex and still in the pilot-scale test,the thermodynamic system of industrial-scale power plant is established to simulate its operating performance.Moreover,several optimization measures are put forward to improve its feasibility.Additionally,simulation results provide valuable reference for the implementing the Oxy-coal combustion technology.Taking a domestic315MWe coal-fired power plant as the reference,this paper simulates the thermodynamic system transforming the conventional coal-fired power plant into the Oxy-coal mode,using Aspen Plus software.The thermodynamic system contains the air separation,pulverized coal combustion,steam Rankine cycle,flue gas compression and purification subsystem.Then,these subsystems are coupled with heat integration.The simulation results show that the net efficiency is reduced from 36.1%to22.7%.The optimum Oxygen concentration in the Oxygen product is obtained by optimizing the air separation single system.The waste heat produced by compressing air and flues gas is used to heat the feed water.Hence,the amount of the regenerative steam extraction is reduced,so that the amount of work is increased.Additionally,the rotating machinery,such as air and flue gas compressor,pump and fan,are driven by extra steam turbine directly,so as to reduce the auxiliary power consumption.Through coupled optimization,the higher auxiliary power consumption resulting from the air separation and flue gas treatment can be effectively reduced.As a result,the net efficiency is increased by about 5.1 percentage.Because the density,specific heat capacity,heat conductivity,viscosity and other physical properties of flue gas would change as a result of the difference of triatomic gas concentration between conventional and Oxy-coal combustion modes.And the radiative and convective heat transfer feature would change remarkably.In the Oxy-coal combustion mode,the heat transfer feature of utility boiler are of great significance for designing the convection heating surfaces of Oxy-coal combustion power plant.Much research work on the radiative heat transfer performance in O₂/CO₂ atmosphere has been done,but there is still much room for research on convective heat transfer performance.In this paper,the coal combustion model is established using Aspen Plus with the reference of315 MWe power plant boiler.The composition and state parameters of flue gas are obtained by simulating the combustion process.The physical properties of flue gas are estimated by PR-BM physical property method.On this basis,the heat transfer performance of convection heating surface in boiler flue is analyzed by combining heat transfer theory calculation with CFD analysis.The results show that the performance of convection heat transfer of boiler is enhanced by burning Zhundong coal in O₂/CO₂atmosphere.The average convection heat transfer coefficient of each wall increased by about 21.4W/?M²·K?.The outlet steam temperature of high temperature reheater was increased by about 11.3K.In order to verify the effect of burning Zhundong coal on the convective heat transfer performance of boiler in the O₂/CO₂ atmosphere,a test system of slagging,ash deposition and heat transfer based on DTF?Drop Tube Furnace?is designed and built.The general layout of the system is designed in detail.In the design of the heat transfer test section,the numerical simulation of the flow field uniformity was carried out by adding the guiding plate and rectifying the grid with CFD software,and the convection heat transfer conditions in the air and Oxy atmosphere were compared.The results show that the flow velocity distribution can be more uniform when the guiding plate is arranged in the bend and horizontal transition section and the rectifying grid is set at the inlet of the heat transfer test section.It can be found that the convection heat transfer of the snake-shaped tube increases about 1700W,the convection heat transfer coefficient increases slightly,and the outlet air temperature increases about55?.