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首页> 外文期刊>Acta Polytechnica Scandinavica. Mechanical Engineering Series >Numerical modeling of the drying, devolatilization and char conversion processes of black liquor droplets
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Numerical modeling of the drying, devolatilization and char conversion processes of black liquor droplets

机译：黑液液滴干燥，脱挥发分和焦炭转化过程的数值模拟

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In this work a detailed physical single particle combustion model for black liquor was developed. As a difference to previous models, intra-particle mass transfer during drying and devolatilization was considered, in addition to heat transfer. Relevant chemical reactions and experimentally observed physical combustion phenomena e.g. swelling were implemented. The model was widely tested against experimental data. Char conversion mechanisms were studied in laboratory reactor and furnace conditions addressing the relevant reaction mechanisms needed for developing simplified particle combustion sub-models for use in combination with CFD (Computational Fluid Dynamics). The model succeeded well in predicting experimentally observed combustion behavior. Release rates and yields of mass and carbon were well predicted for the cases studied. The model also succeeded well in predicting particle temperatures during combustion. Values for particle thermal conductivity, devolatilization heat, swelling and shrinkage parameters could be obtained by sensitivity analysis and experimental verification. A novel char conversion mechanism was found by the model, referred to as auto-gasification. Char conversion may take place already during devolatilization as H{sub}2O and CO{sub}2 flow out from the particle interior and pass through the hot, pyrolyzed particle surface. This auto-gasification mechanism could not be fully validated experimentally. However, an excellent correlation with experimental data was obtained when this mechanism was included. It was shown that char conversion mechanisms for black liquor essentially differ from those for other fuels. It was observed that at typical recovery boiler temperatures and gas compositions dominating char conversion mechanisms are H{sub}2O gasification, CO{sub}2 gasification and carbonate reduction, under non-convective conditions. When convective effects are present, the role of direct char oxidation increases. For high slip velocity conditions, the overlapping of devolatilization and char oxidation is an issue as O{sub}2 can reach the particle surface more easily. The concept of envelope flame under furnace conditions should be re-evaluated. The results from this study suggest that the development of a simplified CFD particle combustion model requires the proper understanding of physical and chemical processes taking place in the particle during combustion. In order to transfer the experimental observations to furnace conditions, the relevant mechanisms that take place need to be understood before the important ones can be selected for CFD-based modeling.
机译：在这项工作中，开发了详细的黑液物理单颗粒燃烧模型。与以前的模型不同，除传热外，还考虑了干燥和脱挥发分期间的颗粒内传质。相关的化学反应和实验观察到的物理燃烧现象肿胀得到实施。该模型已针对实验数据进行了广泛测试。在实验室反应堆和熔炉条件下研究了焦炭转化机理，解决了开发简化的颗粒燃烧子模型与CFD（计算流体动力学）结合使用所需的相关反应机理。该模型成功地预测了实验观察到的燃烧行为。对于所研究的案例，可以很好地预测释放速率以及质量和碳的产率。该模型还成功地预测了燃烧过程中的颗粒温度。颗粒热导率，脱挥发分热，溶胀和收缩参数的值可以通过敏感性分析和实验验证来获得。该模型发现了一种新颖的焦炭转化机制，称为自动气化。在脱挥发分过程中，由于H {sub} O和CO {sub} 2从颗粒内部流出并穿过热的，热解的颗粒表面，因此焦炭转化可能已经发生。这种自动气化机制无法通过实验完全验证。但是，当包括该机理时，与实验数据具有极好的相关性。结果表明，黑液的焦炭转化机理与其他燃料基本不同。观察到，在非对流条件下，在典型的回收锅炉温度和气体组成中，主要的焦炭转化机理为H {sub} 2O气化，CO {sub} 2气化和碳酸盐还原。当存在对流作用时，直接炭氧化的作用增加。对于高滑移速度条件，脱挥发分和炭氧化的重叠是一个问题，因为O {sub} 2可以更轻松地到达粒子表面。炉膛条件下的包络火焰概念应重新评估。这项研究的结果表明，简化CFD颗粒燃烧模型的开发需要对燃烧过程中颗粒中发生的物理和化学过程有适当的了解。为了将实验观察结果转化为熔炉条件，在为基于CFD的建模选择重要的机理之前，需要了解发生的相关机理。

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