生物质热解过程中挥发分与焦的多相交互反应机理研究

Volatile-char interactions is an important phenomenon in all existing biomass pyrolysis processes. It affects almost every aspect of biomass pyrolysis, including the structure and reactivity of char, the volatilization of Alkali and Alkaline Earth Metallic species (AAEMs), the composition and distribution of bio-oil, etc. The current studies are mainly focusing on the primary pyrolysis process and its mechanism. While for the practical pyrolysis process, for instance, after scaling up a pyrolyzer, the secondary reactions, especially the interactions between volatiles and char, are inevitably present, and have significant effect on the final product formation and distribution as well. However, it is still rarely reported, and its mechanism keeps unclear. Therefore, this study will focus on the volatile-char interactions, a common fundamental issue during biomass pyrolysis. Both experiment studies and quantum chemistry calculations will be carried out. The effects of volatile-char interactions on the formation and distribution of the final biomass pyrolysis products will be investigated. The migration, transformation, and catalytic mechanism of the active sites on char surface to the volatile cracking, and the synergetic mechanism between carbon-based active sites and metal-based active sites on the char surface will be explored. The response mechanism and transformation pathway of the active components and chemical structures in volatiles during the interactions will be revealed as well. Finally, quantum chemistry will be introduced to explain the detailed mechanism of volatile-char interactions from the molecular level. The results from this study could provide scientific guidance and theoretical support for the design, scaling up and operation of the biomass pyrolysis process.

挥发分与焦的交互反应是生物质热解过程中一种非常重要且普遍存在的现象，几乎影响热解过程的方方面面，包括焦结构与反应性、碱金属及碱土金属挥发、生物油组成与分布等，是热解过程必须考虑的重要因素。当前围绕生物质热解的研究更多关注一次热解构及其过程机理，而实际热解过程，如热解炉放大后，二次反应特别是挥发分与焦之间的交互反应则不可避免，其对热解最终产物形成与分布有重要影响，目前仍鲜见报道。鉴于此，本项目针对挥发分与焦的交互反应这一生物质热解过程中的共性基础问题，通过实验研究与量子化学理论计算相结合，深入研究交互反应对生物质热解最终产物形成与分布特性影响机理，探索交互反应中焦表面活性位的迁移转化、催化机理与耦合作用机制，揭示挥发分中的活性组分及结构在交互反应中的响应机制与转化路径，以期从分子层面掌握挥发分与焦的多相交互反应机理。项目预期成果可为生物质热解工艺的设计、放大及运行优化提供科学依据和理论基础。

挥发分与焦的交互反应是生物质热解过程中的共性基础问题。本项目从交互反应对生物质热解最终产物形成与分布特性影响出发，建立了焦表面物理化学结构与挥发分特征组分/结构转化的响应机制，阐明了交互反应的主要转化路径与特征产物，并探讨了交互反应调控实现生物质选择性热解的方法与途径。主要发现包括：1）实验证实在400-600℃挥发分与焦之间确实存在显著交互反应，使得不凝气产率增加而生物油产率下降。焦表面官能团和芳香环系统是交互反应的活性位点，通过氢键或芳香环π-π堆积促进挥发分在焦表面裂解。2）焦表面官能团在交互反应中被大量消耗，并发生脱氢缩合。-OH和-COOH可促进吡喃环开环和脱羧、脱羰、脱水反应，导致脱水糖和芳烃产率降低，而线性酮类和酸类产率增加。-NH2可抑制木聚糖开环并促进其环收缩形成糠醛，其还可通过氢键和芳烃-氢键协同效应促进苄苯醚（BPE）Cα-O和Caryl-O键断裂。3）H2O和CO2活化均可增加焦表面活性位点数量，大幅提高其比表面积和孔容积，并降低孔径至2.5-3.5nm，显示出良好的择形催化效果。H2O活化焦可实现BPE完全转化并高选择性生成苯酚和甲苯，而CO2活化焦可促进醛、酮、酸类发生脱水、脱羧、低聚等反应转化为酚类，并增强对烷基或烷氧基酚类侧链的断裂形成更多芳香烃。4）焦中K和Na在交互反应中表现出相似效果，其氢氧化物和碳酸盐对挥发分存在很强裂解作用，而氯化盐和硫酸盐对交互反应影响较小，Mg和Ca的氯化盐和硫酸盐对交互反应也无显著影响。5）通过调节交互反应程度，可显著改变生物油的产率、组成及平均分子量，耦合酸洗脱灰和降低交互反应程度，即便是堆积固定床生物质热解也可获得富含脱水糖的高产率生物油。本项目研究结果有助于深入掌握挥发分与焦的交互反应机理及其调控机制，对于实现生物质定向热解及精准调控具有重要意义。本项目共发表SCI论文24篇，申请发明专利8项，以本项目为基础，项目负责人获得国家自然科学基金面上项目（52276196）资助，并入选江苏省特聘教授和六大人才高峰等重要人才项目。