Study On Condensation Heat Transfer Characteristics Of High Temperature Pyrolysis Gas

In the graded and quality utilization of coal,coal pyrolysis produces high temperature pyrolysis gas,which needs to recover its sensible heat,but because the pyrolysis gas contains tar,the traditional direct spraying method can not recover the heat,resulting in energy waste.The development of new waste heat recovery technology of high temperature pyrolysis gas is one of the keys of coal pyrolysis combustion co-production technology,and its core is to study the condensation heat transfer characteristics of high temperature pyrolysis gas in heat exchanger.The experimental device of condensation heat transfer containing non-condensable gas vapor is established.Four typical condensate forms of drop,drop-streak,film and streamlet were found in the experiment,and the influence of the change of condensate morphology on condensation heat transfer was analyzed in combination with the literature.The effects of non-condensable gas mass fraction and mixed steam inlet velocity on wall temperature fluctuation and heat transfer coefficient were investigated.The experimental data are used to verify the reliability of the Euler Wall Film(EWF)model used in the simulation.Using biphenyl with high content in tar washing oil as condensation component and CO,CH4 and H2 as non-condensable components as model compounds of high temperature pyrolysis gas,the condensation heat transfer characteristics of biphenyl in condensate tube were simulated and studied by EWF model.The flow characteristics and wall heat flux distribution of liquid tar under the inlet temperature of pyrolysis gas,inlet velocity and inlet temperature of cooling medium were studied,and the differences between single-stage condensation and stage condensation were compared and analyzed.The results show that when the inlet velocity of pyrolysis gas increases from 2m/s to 3.5m/s,the liquid film velocity along the wall increases,the liquid film thickness decreases by 25%,the average heat flux density increases by 23%,the pressure drop in the tube increases,and the distance between the initial position of the condensation point and the inlet increases.When the inlet temperature of the pyrolysis gas increases from 530K to 620K,the thickness of the liquid film along the wall increases by 26%,the average heat flux density increases by 21%,the pressure drop in the tube increases,and the distance between the initial position of the condensation point and the inlet increases.When the inlet temperature of the cooling medium increases from 295K to 325K,the thickness of the liquid film along the wall decreases by 32%,and the average heat flux decreases by 30%.When the inlet temperature of the cooling medium increases from 325Kto 335K,the distance between the initial position of the condensation point and the inlet increases from 0mm to 28mm.Considering the variation of liquid condensation component viscosity and thermal conductivity with temperature,it is found that it has an important effect on the discharge of condensate and wall condensation heat transfer coefficient.It is verified that fractional condensation is more suitable for waste heat recovery of high temperature pyrolysis gas,which provides important guiding value for the engineering design and development of condensation heat exchanger.Based on Nusselt theory and boundary layer theory,the condensation heat transfer model of high temperature pyrolysis gas in condensation tube is established by considering liquid film fluctuation effect and suction effect.The reliability of the model is verified by experiments and classical model data,compared with the EWF model,and the literature shows that the model is closer to the actual situation,and the condensation heat transfer of high temperature pyrolysis gas should be enhanced from the point of view of reducing liquid film thermal resistance and diffusion layer gas thermal resistance,which provides theoretical guidance for the development of a general model of condensation heat transfer of high temperature pyrolysis gas.