摘要
在煤热解过程中,侧喷嘴下行床内气固流动及热量传递行为对其反应性能有着至关重要的作用.本文通过计算流体动力学CFD模拟,采用半焦为热载体,构建了考虑气体-煤、气体-半焦对流传热及煤-半焦碰撞导热三种传热机制的传热模型.详细考察了操作参数半焦密度、颗粒粒径和气速对床内流动行为和热量传递特性的影响,以期为下行床的推广及优化提供一些定性指导.结果表明:降低半焦密度、颗粒直径及气速有利于提高煤颗粒固含率.对煤升温过程影响敏感度由高到低的顺序是:半焦密度＞气速＞颗粒直径.在热传递过程中,当半焦密度为900kg/m3、颗粒直径为0.4mm及气速为10m/s时最有利于气-固相间对流传热及固-固碰撞导热.其中气体-半焦对流传热占主导,其次为气体-煤对流传热,煤-半焦碰撞导热占比较小.
Abstract
In the process of coal pyrolysis,the gas-solid flow and heat transfer behavior in the downer bed with side nozzle play a crucial role in its reaction performance.A heat transfer model considering three heat transfer mechanisms:gas-coal,gas-char convective heat transfer,and coal-char collision heat conduction,was constructed using char as the heat carrier through computational fluid dynamics CFD simulation.A detailed investigation was conducted on the effects of operating parameters such as char density,particle size,and gas velocity on the flow behavior and heat transfer characteristics in a downer bed,in order to provide some qualitative guidance for the promotion and optimization of downer beds.The results indicated that reducing the char density,particle diameter,and gas velocity was beneficial for improving the solids holdup of coal particles.The influence sensitivity to coal heating process from high to low was:char density＞gas velocity＞particle diameter.In the process of heat transfer,a char density of 900kg/m3,particle diameter of 0.4mm,and gas velocity of 10m/s,were most favorable operating conditions for gas-solid phase convective heat transfer and coal-char collision heat conduction.Among them,gas-char convection heat transfer was dominant,followed by gas-coal convection heat transfer,and collision conduction of coal-char accounts for a relatively small proportion.
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