基于二次反应修正的富油煤快速热解过程模拟

Simulation of fast pyrolysis process of tar-rich coal based on secondary reaction supplementation

  • 摘要: 富油煤具有产油潜力大的资源属性,与粉煤快速热解焦油收率较高的工艺特点相匹配。将富油煤作为快速热解原料可以获得较高的焦油收率,但依旧面临着焦油中重质组分较高的挑战。挥发分之间的二次反应是影响快速热解焦油品质的重要因素,深入分析热解二次反应的机理与特点是解决焦油低品质问题的前提与基础,然而目前缺少描述在反应器高温环境内气相挥发分之间二次反应的有效动力学模型,及其与计算流体力学进行结合以模拟真实热解反应器中二次反应作用的应用案例。从化学渗透脱挥发分(Chemical Percolation Devolatilization, CPD)模型出发,在简化煤分子结构及一次反应网络的基础上进一步构建焦油及轻质气体内部组分相互转化、焦油歧化生成气固产物的二次反应体系。采用OpenFOAM开源软件计算考虑二次反应修正的固定床快速热解过程中产物演变规律,探究了热解载气温度、颗粒粒径以及挥发分停留时间对富油煤热解产物分布的影响。结果表明:考虑二次反应修正的热解动力学模型能描述焦油产率随温度升高先增加后减小的趋势,焦油产率在载气温度为873 K下达到最高值10.1%。颗粒粒径主要影响热解的升温速率与挥发分的停留时间,从而间接影响焦油产率与品质。当粒径减小为48 μm(300 目),焦油产率可以进一步提升至12.4%。随着停留时间的缩短,二次反应进行的程度减小,焦油总产率与品质也会有所提高,焦油产率相对增加将近20.1%,焦油轻组分质量分数由0.621提升至0.677。研究结果有望为富油煤快速热解焦油产物的品质调控提供理论指导。

     

    Abstract: Tar-rich coal has the resource attribute of high tar production potential, which matches with the process characteristic of high tar yield in fast pyrolysis of pulverized coal. The use of oil-rich coal as the raw material for fast pyrolysis can achieve high tar yield, but still faces the challenge of too much heavy component in tar. The secondary reactions between volatile components is an important factor affecting the quality of fast pyrolysis tar, and an in-depth understanding of the mechanism and characteristics of the pyrolysis secondary reaction is a prerequisite and foundation for solving the problem of low tar quality. However, there is a lack of effective kinetic models describing the secondary reactions between gas-phase volatiles within the high-temperature environment as well as application cases coupling them in the simulation of pyrolysis processes in real reactors. Starting from CPD (Chemical Percolation Devolatilization) model, a secondary reaction system was further constructed based on the simplification of the coal molecular structure and the primary reaction network. The secondary reaction involved the interconversion of the internal components of tar and light gas, and the disproportionation of tar to generate gas-solid products. Using a two-dimensional fixed-bed reactor model as a framework, the product evolution in the fast pyrolysis process considering the secondary reaction modification was calculated based on OpenFOAM open source software. The effects of pyrolysis carrier gas temperature, particle size and volatile residence time on the pyrolysis product distribution of the tar-rich coal were investigated. The results showed that the pyrolysis kinetic model with secondary reaction modification was able to describe the trend of increasing and then decreasing tar yield with increasing temperature, and the tar yield reached the highest value of 10.1% at a carrier gas temperature of 873 K. The particle size mainly affected the rate of pyrolysis and the residence time of volatiles, which indirectly affected the tar yield and quality. When the particle size was reduced to 48 μm (300 mesh), the tar yield could be further increased to 12.4%. As the residence time was shortened, the degree of secondary reaction decreased, and the total yield and quality of tar was also improved. The tar yield increased by nearly 20.1%, and the proportion of light components in tar increased from 0.621 to 0.677. This results are expected to provide theoretical guidance for the quality control of the tar products from the rapid pyrolysis of tar-rich coal.

     

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