生物质大颗粒热厚模型研究

Biomass is one of the most important alternatives for fossil fuels in the future. But because of its own fiber properties, its particle size tends to be larger compared with pulverized coal particles. As a result, the traditional isothermal particle model (i.e., thermal thin model) cannot accurately characterize the temperature changes of large particles during the heating process. Based on the above analysis and the concepts of stratification and discretization, this project will develop the one, two, and three-dimensional non-isothermal particle models (i.e., thermally thick models) for the large biomass particles, which consider the intraparticle heat conduction, resolve the temperature gradient within the particle and explore the variation of internal void fractions as well as the influence of internal and external flow fields on the reaction characteristics. Meanwhile, the model parameters are optimized by the experimental data and thus the detailed thermally thick model will be constructed. Furthermore, the moisture evaporation, devolatilization and char gasification processes during the thermochemical conversion of biomass will be based on the local temperature inside the particles. Therefore, the successful establishment of thermally thick model will greatly improve the prediction accuracy of the conversion process of large biomass particles. Finally, the thermally thick models proposed will be integrated into the CFD-DEM model already developed by the applicant and be used to explore the influences of key operating conditions on biomass pyrolysis and gasification characteristics in a fluidized-bed reactor, which will help optimize the process parameters and can also be beneficial to the large-scale industrial application of bioenergy.

生物质燃料是未来化石能源的重要替代品之一。但是由于自身的纤维属性，和煤粉相比，其粒径往往偏大。因此，传统的等温热薄模型不能准确表征大颗粒在加热过程中的温度变化。基于此，本项目将采用分层和离散的思想，分别开发适用于生物质大颗粒的一维、二维和三维非等温热厚模型，考虑颗粒内部热传导，求解颗粒内部温度梯度，并解析颗粒内部孔隙率变化和颗粒内外流场对反应特性的影响，同时通过实验数据优化模型参数，进而建立详细颗粒热厚模型。在此基础上，生物质热化学转化过程中的水分蒸发、脱挥发分和炭气化等子过程，均将基于颗粒内部的局部温度进行，因此，热厚模型的建立将大大提高对生物质大颗粒转化过程的预测精度。最终，将开发的热厚模型和课题组已有的CFD-DEM模型集成，并针对生物质大颗粒流化床热解和气化过程，探究重要操作参数对流动与反应特性的影响，给出优化的操作条件范围，为生物质能的大规模工业应用提供定量数据支持。

本项目以可再生的生物质新能源为研究对象，针对已有计算流体力学（CFD）方法中热薄颗粒模型存在的缺点与不足，遵循从简单到复杂的研究规律，分别开发了适用于生物质大颗粒的一维和高维非等温热厚模型。首先，采用分层和离散思想，根据生物质的实际转化过程，将颗粒从外到内分为灰、炭、干和湿四层，每一层赋予不同的物性参数（如密度、热传导系数和化学组成等），并且每一分层内再离散为n个计算单元层，解析颗粒内部热传导，求解颗粒内部温度梯度，建立了适用于生物质大颗粒的一维非等温热厚模型；接着，基于颗粒簇思想开发了二/三维大颗粒热厚模型，解析颗粒内外流场和内部孔隙率变化，生物质热化学转化过程中的水分蒸发、脱挥发分和炭气化等子过程，均基于颗粒内部的局部温度进行，大大提高了对大尺寸生物质颗粒热化学转化过程的预测精度；之后，采用构建的热厚模型深入探究了生物质大颗粒的热解和气化过程；同时，将热厚模型和CFD方法集成，系统分析了热薄和热厚生物质颗粒在气流床、流化床、转鼓等多种反应器中的运动、热解和气化特性，研究了多个关键操作参数（如温度、反应器形状）的影响规律；另外，还实验研究了松木、柚木和螺旋藻的热解特性以及藻类生物炭的燃烧性能，测定了过程的反应动力学参数。系列成果在能源、化工和力学领域的一流期刊如CNF、E&F、BT、CES、I&ECR、PT、IJMF、POF等正式发表论文24篇，加深了对生物质大颗粒运动规律和热化学转化机理的理解和认识，并为优化相关过程控制提供了指导建议。