Abstract:

The photothermal heat source can solve problems such as low elemental utilization in low and medium coal gasification processes, but the reaction characteristics are different from those of conventional coal gasification processes due to changes in the properties of the heat source and the heat transfer method, and the introduction of the mixing agent for CO₂ dissipation will have an impact on the temperature field in the reactor. In order to study the above problems, this paper constructed a lignite fixed-bed gasification reactor with a photothermal heat source using the multi-field coupling software Comsol Multiphysics, and investigated the influence laws of the temperature distribution and product composition as well as the changes of the heat and mass transfer behaviors in the reactor with different thermal conductivity characteristics of the gasification mediums steam, carbon dioxide and different ratios of the mixing gasification agents. As the proportion of CO₂ added to the gasification agent increased from 0 to 100%, the viscosity and density of the fluid continued to increase, resulting in an increase in the viscous force between the fluids and a decrease in the momentum transfer, and the convective heat transfer coefficient between the gas-solid phases decreased from 26.3W/(m²·K) to 22.7W/(m²·K). Compared with steam gasification, the addition of CO₂ reduced the particle gasification temperature and the reaction rate of coal gasification, but the increase of CO₂ concentration was conducive to increase the reaction rate of semi-coke CO₂ reduction. Under the combined effect, the complete gasification time of coal coke particles did not change significantly. In addition, the optical heat saving heat medium heating mode made the reactor bed temperature curve overall "funnel-shaped", which led to the extension of the particle gasification time in the center by 70min, and after the mixing of CO₂, the reduction of thermal conductivity made the temperature curve appeared obvious "lag effect". Finally, by adjusting the proportion of CO₂ added to the gasification medium, it was found that when the proportion of CO₂ added increased to 40%, the gasification medium exhibited the behavior of exogenous CO₂ absorption and its CO₂ absorption reached 0.013g/g. As the gasification temperature increased, the reaction rate of CO₂ gasification increased, which ultimately showed an increase in CO₂ consumption and the effective syngas yield, but a decrease in the H₂/CO ratio.

Key words: coal gasification, gasification medium, heat transfer, temperature distribution, reaction rate

摘要：

光热热源可解决中低价煤气化过程中元素利用率低等问题，但由于热源属性和传热方式改变，反应特性与传统煤气化过程不同，为消纳CO₂引入的混合气化剂会对反应器内的温度场产生影响。为研究上述问题，本文利用多场耦合软件Comsol Multiphysics，构建了光热热源的褐煤固定床气化反应器，研究了不同导热特性的气化介质水蒸气、二氧化碳、不同比例混合气化剂下反应器内温度分布和产物组成以及反应器内热质传递行为变化。结果表明，随着气化剂中CO₂添加比例由0增大至100%，由于CO₂加入带来了流体黏度、密度不断增大，使得流体间黏滞力增强，动量传递减少；气/固相间对流换热系数从26.3W/(m²·K)降至22.7W/(m²·K)。相较于水蒸气气化，CO₂的加入使得颗粒气化温度有所降低，煤气化反应速率下降，但CO₂浓度的增加又提高了半焦CO₂还原反应的速率，煤焦颗粒完全气化的综合时间没有明显改变。此外，光热储蓄热介质加热的方式使得反应器床层温度曲线整体呈“漏斗状”，导致炉内中心位置颗粒气化时间延长了70min，且在掺混CO₂后，传热效率的降低使得温升曲线出现明显的“滞后”效应。通过改变CO₂在气化剂中的添加比例发现，当CO₂添加比例增至40%（体积分数）时，气化炉表现出消纳外源CO₂的行为，其CO₂消纳量达0.013g/g；且随着气化温度的增加，CO₂气化反应速率升高， CO₂的消纳量增加，有效合成气产量增加，但H₂/CO下降。

关键词: 煤气化, 气化介质, 传热, 温度分布, 反应速率