生物质热解非均相二次反应多环芳烃生成机制及动力学研究

Polycyclic aromatic hydrocarbons (PAHs) are of great research interest not only due to being soot precursors and particulate matter constituents but their carcinogenic, mutagenic, and teratogenic properties as persistent environmental contaminants. It has been reported that biomass combustion is a major contributor of PAHs emissions in China. During thermochemical conversion of lignocellulosic biomass, PAHs are mainly formed from secondary reactions during pyrolysis process. A few studies have investigated the yields of PAHs from secondary gas phase pyrolysis of both biomass feedstocks and individual constituents of biomass. Yet none has been carried out to study the formation of PAHs from heterogeneous secondary reactions (HESRs) among primary biomass pyrolysis products. The HESRs, which involve some volatiles being subject to additional thermal reactions before quenching or burnout, can take place in most practical apparatus and processes of biomass thermochemical conversion, namely, combustion, gasification and pyrolysis for biofuels..The aims of this proposal are to investigate the behavior of primary pyrolysis products during the heterogeneous secondary reactions and the mechanistic formation of 16 priority PAHs, as well as to establish a kinetic model for the formation of PAHs during biomass pyrolysis. A rig will thus be setup to combine a biomass pyrolyser with secondary gas-solid rectors where primary volatiles from biomass pyrolysis undergo secondary reactions directly. Both gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC) will be used for characterize the components of bio-oil vapor before and after HESRs, with emphasis on PAHs. Radical traps and electron spin resonance (ESR) will be employed to study free radicals produced among HESRs leading to the formation of PAHs. The effects of primary pyrolysis products, temperature, residence time and catalysis of char in the secondary reactions on PAHs formation will be investigated. Based on the experimental work, the kinetics of PAHs formation can be obtained and PAHs formation can be simulated with a developed mathematical model. The outputs of this work can provide insights into the fundamental mechanisms of HESRs during biomass pyrolysis and a tool for predicting PAHs emissions from various practical biomass conversion systems.

作为烟尘颗粒物的前驱体，多环芳烃是一种具有高致癌致畸和致突变特性的大气污染物。在生物质热化学转化利用过程中，热解产物间的二次反应是多环芳烃形成的主要途径。虽然热解产物非均相二次反应是生物质热化学转化过程中普遍存在的现象，但目前还没有研究揭示非均相二次反应过程多环芳烃生成机制，因而无法对生物质热解、燃烧过程中的多环芳烃排放进行预测和模拟。本项目采用自由基阱-电子自旋共振分析、结合气相色质联用和高效液相色谱等分析手段，研究生物质一次热解蒸气和焦炭在非均相二次反应中的行为和变化，重点研究固体表面对二次反应过程中多环芳烃生成的催化机制，分析热解一次产物、反应温度和焦炭物理化学性质等对16种优先控制的多环芳烃生成的影响规律，阐明生物质热解过程中非均相二次反应多环芳烃生成机制，获得多环芳烃生成宏观动力学参数并建立能够描述二次反应中多环芳烃生成的数学模型，为预测和控制多环芳烃生成和排放提供理论依据。

多环芳烃是一种具有高致癌致畸和致突变特性的大气污染物，是燃烧排放烟尘的前驱体。本项目围绕生物质热解过程中非均相二次反应这一重要环节中以多环芳烃为主的热解产物开展研究，旨在揭示这一过程中多环芳烃生成机制。针对这一目标，本项目建立了生物质热解实验台，开展热解及二次反应实验研究及产物分析，建立了典型多环芳烃的检测分析方法。利用实验装置和热裂解仪研究了生物质热解过程中气相产物中典型多环芳烃的生成规律，获得了相应的生成动力学模型。对生物质热解反应及有固体催化剂存在条件下热解反应过程开展了研究，分析了产物分布，研究了可测得的典型多环芳烃的生成规律。研究了非均相热解过程中焦炭固体及催化剂性质变化。研究表明生物质热解产生的呋喃和糠醛等物质通过Diels-Alder反应并经脱水反应形成芳香烃化合物，在焦炭或催化剂存在条件下，生物质热解产生的含氧有机产物更易于脱氧形成芳香烃化合物。为了更好地认识生物质热解二次反应过程及相关性质，建立热解过程中生物质及焦炭热物性参数的测量方法，获得了生物质热解的二次反应热随反应进程变化规律。本项目的研究结果为进一步研究生物质热解过程及产物调控打下基础，为生物质热解燃烧的多环芳烃排放预测和模拟提供依据。