Gas Solid Heat Transfer Process And Application Research In Vertical Tank For Sinter Waste Heat Recovery

In view of the inevitable drawbacks including the high air leakage,the low efficiency of waste heat recovery and low quality of heat carrier(cooling air)of existing sinter annular cooler for waste heat recovery,the vertical tank for sinter waste heat recovery presented by imitating the structure and process of coke dry quenching furnace is a high efficient recycling technology of sinter waste heat.The electricity generation capacity per ton of ore is expected to double than the current domestic average index.The vertical tank technology for sinter waste heat has been incorporated into the Iron and Steel Industry Adjust Upgrade Planning of China(2016-2020)as one of the energy saving and emissions reduction technology of demonstration and promotion.The coupling of heat transfer and flow between cooling air and sinter in vertical tank is one of the key scientific problems to determine the feasibility of vertical tank technology for waste heat recovery.As known from the three variable relationships of waste heat recovery system,the structural and operational parameters of vertical tank determine the gas flow and gas-solid heat transfer processes in sinter bed layer of vertical tank,and then determine the quantity and quality of outlet heat carrier of vertical tank,as well as the recovery rate of waste heat and electricity generation capacity per ton of ore of vertical tank system.Combined with the study of gas flow resistance characteristics,the quantitative relationship of structural and operational parameters of vertical tank with the waste heat recovery index is clarified through the study of gas-solid heat transfer process in vertical tank.The suitable structural and operational parameters of vertical tank are determined,which lays a theoretical foundation for the subsequent project implementation and technical renovation.The situation of simply depending on the coke dry quenching furnace to determine the structural and operational parameters of vertical tank in the present engineering is also changed.The vertical tank is a typical vertical furnace from the thermos-technical angle.The gas-solid heat transfer in sinter bed layer of vertical tank belongs to the gas-solid counter-flow heat transfer in vertical moving bed with large particles,and approximates to the steady state heat transfer.At present,the related numerical calculation of gas-solid heat transfer in sinter bed layer is mainly based on the unsteady heat transfer model of local thermal non-equilibrium,and there are little studies on the numerical calculation of gas-solid heat transfer in sinter bed layer of vertical tank.The required gas-solid heat transfer coefficient and resistance characteristic coefficients in the gas-solid heat transfer model of sinter bed layer,as well as the judgment of gas flow state are still based on the universal empirical correlations in the packed beds with uniform particles,and there are little studies on the gas-solid heat transfer coefficient and resistance characteristic coefficients,as well as the judgment of gas flow state in sinter bed layer.The experimental investigation of gas-solid counter-flow heat transfer process in vertical sinter bed layer is still blank.Therefore,based on the existing correlational studies,the basic rule,the main influence factors and influence laws of gas-solid heat transfer in sinter bed layer of vertical tank are studied through the methods of experiment and numerical calculation,and the suitable structural and operational parameters of vertical tank are determined.The main research work and results are as follows.(1)Overcoming the influence of edge effect,the experimental platform of gas flow resistance characteristic in sinter bed layer is designed and built.The influences of gas superficial velocity and sinter particle diameter on the gas flow resistance characteristic are investigated.The judgment of gas flow state in sinter bed layer is determined,and the experimental correlation of gas flow resistance is fitted.The results show that in the actual production condition,the gas flow pressure drop of unit bed layer height increases as a quadratic relationship with increasing the gas superficial velocity,and decreases as exponential relationship with the increase of sinter particle diameter.There is no Darcy flow state in bed layer with sinter particles,and in the non-Darcy flow area,the critical particle Reynolds numbers marking transition from Forchheimer flow to turbulent flow increases as a third order relationship with increasing the sinter particle diameter.(2)The experimental platform of gas-solid heat transfer in sinter bed layer is designed and built.The influences of gas superficial velocity and sinter particle diameter on the gas-solid heat transfer coefficient are investigated,and the experimental correlation of gas-solid heat transfer Nusselt number in sinter bed layer is fitted.The technical difficulties including the continuous inclosed discharge and the heating of sinter particles are conquered.The small test platforms of vertical tank are designed and built,the roasting areas are 1m2 and 5.6m2,and the design throughputs are 5t/h and 25t/h respectively.The influences of cooling air flow rate,blanking quantity and inlet temperature of sinter on the gas-solid heat transfer process in vertical tank are investigated.The results show that,the greater the gas superficial velocity in bed layer is,the smaller the sinter particle diameter is,and the higher the sinter temperature is,the greater the gas-solid heat transfer coefficient in bed layer is.The smaller the cooling air flow rate is,the greater the sinter blanking quantity is,and the higher the sinter inlet temperature is,the higher the outlet temperature of cooling air in vertical tank is.For a given sinter inlet temperature,the smaller the ratio between the water equivalent of cooling air and sinter water equivalent is,namely the smaller the gas-solid water equivalent ratio is,the higher the outlet temperature of cooling air is.(3)The gas-solid heat transfer process in sinter bed layer of vertical tank is solved by FLUENT software.The double energy equations of local thermal non-equilibrium used to solve the gas-solid steady state heat transfer process in vertical tank are derived on the basis of the theory analysis of porous medium and gas-solid heat transfer.The gas flow state and flow model in sinter bed layer of vertical tank under the actual production condition are determined.The cooling section of vertical tank is selected as the computational domain.The computational domain is divided to the structured grid through ICEM CFD,and the grid independence verification of computational model is conducted.The experimental results of gas flow resistance coefficients and gas-solid heat transfer coefficient in sinter bed layer,as well as the convection item of describing the particle descendingvelocity in the solid energy equation in the form of user defined functions are interpreted and compiled to the gas-solid heat transfer model of sinter bed layer in vertical tank.The small test results are used to verify the numerical calculation model.As known from the comparison analysis between the simulation results and test data,the mean deviation between the calculated results of outlet cooling air temperature in vertical tank and test results is 4.92%,and the maximum deviation is within 6%.(4)According to the calculation model mentioned above,the vertical tank of waste heat recovery matched with the 360m2 sintering machine in an iron and steel company is taken as the research object.The numerical calculation of gas-solid heat transfer process in sinter bed layer of vertical tank is conducted,and the basic rule,the main influence factors and influence laws of gas-solid heat transfer in vertical tank are analyzed.The results show that,with the increase of height of cooling section,the actual flow velocity of cooling air in bed layer gradually increases,and reaches the maximum value at the exit of cooling section.The increasing range of flow velocity of cooling air per unit height becomes smaller and smaller.With the increase of height of cooling section,the gas static pressure in vertical tank gradually decreases.The gas static pressure is basically the same for a given height,and the increasing range of gas flow pressure drop per unit height becomes bigger and bigger.Due to the existence of center hood,the sinter temperature and cooling air temperature at the center area of vertical tank are lower than that at the same height.With the decrease of inlet flow rate of cooling air,increase of inlet temperatures of sinter and cooling air,the outlet temperatures of cooling air and sinter gradually increase.The increase of inner diameter and height of cooling section result in the increase of outlet temperature of cooling air and the decrease of outlet temperature of sinter.With the increase of inlet temperature of sinter,the inner diameter and height of cooling section,the heat quantity and heat exergy of recycled air gradually increase.With the increase of inlet flow rate of cooling air,the heat quantity of recycled air gradually increases,while the heat exergy of recycled air first increases,and then decreases.When the inlet flow rate of cooling air is 180kg/s,the heat exergy of recycled air reaches the maximum value,42.4MW.With the increase of inlet temperature of cooling air,the heat quantity of recycled air gradually decreases,while the heat exergy of recycled air first increases,and then decreases.When the inlet temperature of cooling air is 353K,the heat exergy of recycled air reaches the maximum value,43.33MW.Therefore,the ideal heat exergy of outlet cooling air in vertical tank could be obtained through adjusting the inlet flow rate and inlet temperature of cooling air.(5)The structural and operational parameters of vertical tank for 360m2 sintering machine mentioned above are optimized and analyzed by the method of orthogonal test design.The outlet enthalpy exergy of cooling air in vertical tank is determined as the orthogonal test index.The influence rules of structural and operational parameters of vertical tank on the outlet enthalpy exergy of cooling air are investigated,and the suitable structural and operational parameters of vertical tank are obtained.The results show that,the outlet enthalpy exergy of cooling air gradually increases with increasing the sinter inlet temperature and the inner diameter of cooling section.With the increase of inlet flow rate and temperature of cooling air,the outlet enthalpy exergy of cooling air first increases,and then decreases.When the inlet flow rate of cooling air exceeds a given limited value,the outlet enthalpy exergy of cooling air first increases,and then decreases with increasing the height of cooling section.Under the condition of single vertical tank,the inner diameter and height of cooling section are 9m and 8m respectively,and the inlet flow rate and temperature of cooling air are 180kg/s and 353K.The outlet enthalpy exergy of cooling air in vertical tank is 41.27MW.Under the condition of double vertical tank,the inner diameter and height of cooling section for vertical tank are 6.4m and 8m respectively,and the inlet flow rate and temperature of cooling air are 85kg/s and 343K.The outlet enthalpy exergy of cooling air for two vertical tank is 42.6MW.