Thermodynamic Calculation And Numerical Simulation Of Converter Flue Waste Heat Boiler

Researches on the mechanism of radiation and convection that coupled with heat transfer in the boiler and the reasonable design of waste boiler structure is of the ultmost importance on the recovery of the heat of flue gas at the range of800℃to1000℃. Exploratory research has been done on the waste heat recovery of flue gas in the converter and a scheme of subsection has been proposed. Relative calculations on the low-temperature and high-temperature of the converter have been made to investigate the effects of the structure of gas flue, gas velocity, thermal resisitance of fouling, and saturated stream pressure at the exit on the heat exchange. The feasiblity of recovering90t flue gas has been studied, too. Numerical simulation was used to study the effects of the thickness of fouling on the heat exchange and the results were compared with that of thermadynamical calculations to explore the distribution of flow field in the different flues, which aimed at analyzing the feasibility of the research of recovering the heat waste in converter gas flue and optimizing the structure of the converter. The main works and results in this paper are listed as follows:(1) According to the characteristics of converter gas and dedusting process, a sectional type study on the heat recovery was proposed for the first time. Evporative cooling and memberane tube wall with a vertical cylindrical structure was determined for the low-temperature and high-temperature of the converter.(2) A thermodynamcal calculation model for the waste heat boiler was developed. Semi-automotize programming by Excell was utilized to design and check the thermodynamical calculation.(3) Effects of the structure of heating surface, flue gas velocity, fouling resistance, saturation vapor pressure on the heat exchange at different thermal calculation conditions were investigate. It was found that the diameter of converter influenced the heat exchange remarkably. The increase in diameter was proved favorable to rediation heat transfer but unfavorable to convection heat transfer. The total heat transfer coefficient changes as a curve with a single peak with the diameter of the menberane water tube wall or the number of tubes. The intensity of heat transfer reached the minimum at the minimal total heat transfer coefficient. The effects of external diameter or numeber of tubes at constant diameter of converter can be neglected. It was flue gas velocity but not the lengh of tubes influecned the convection heat transfer greatly. However, it ncreased the heat exchange area. (4) The thermodynamic calculation analysis of the low and high temperature converter waste heat boiler revealed that radiation heat transfer played a dominant role in medium temperature converter waste heat boiler and the radiation heat transfer ability affected the final heat transfer ability. For low temperature converter waste heat boiler, the ability of the convection heat transfer imprved so that the radiation heat transfer and convection heat transfer should be considered at the same time. The influence of parameters on the total heat transfer should be determined according the proportion of the convection heat transfer in the total heat transfer.(5) Feasibility analysis of the waste heat recovery of the90tons converter has proved the feasibility of the scheme.(6) The existence of dust has a strong effect to the heat transfer and the greater thickness of the dust, the worse the heat transfer and the higher the temperature of the flue gas at the exit. The trend was that the temperature of the exit flue gas was approximate linear increasing as the dust thickness increased. For90tons of high temperature converter waste heat boiler, when the thickness of the ash layer is equal or greater than5mm, the effect of dust to heat exchange can not be ignored.(7) The total heat transfer by thermodynamic calculation was twice as the numerical simulation data, but the variation tendency of the heat exchange amount with the fouling was the same. The raditation heat transfer was not very different by thermodynamic calculation and numerical calculation.(8) The flow field of the gas flue of the moderate temperature converter is analysed by numerical simulation:the flow field distribution of the upright gas flue is much more uniform. The end of the tube bending structure affects the velocity field and the pressure field of the flue gas severely that the flow field distribution is ununiform. The pressure on the outboard of the bending tube is larger and the velocity is smaller. The velocity on the inner side of the bending tube increase fiercely that boundary layer separation phenomenon occurs at the inner side of the gas flue exit. The pressure is negative and has a strong backflow and the flow here is vortex flow.For the temperature field of the flue gas in the gas flue of the converter, the temperature in a thin layer near the wall was lower and at the center of the mainstream area was higher so that a temperature gradient was formed. At the exit of the flue gas, the strong backflow of the flue gas made a small low temperature area at the exit of the inner side of the bending tube and the flow here was vertex flow.