Study On The Pyrolysis Mechanism Of Lignin And Its Model Compounds

Due to the depletion of fossil fuels and severe environmental issues, the utilization of biomass resources has gained widespread attention. As one of the important renewable resources, the utilization of biomass can save fossil energy, which can help reduce greenhouse gas emissions and mitigate climate change. Besides, biomass is an important raw material for producing energy and chemical products. According to the pyrolysis characteristics of biomass, selective pyrolysis of its organic components(cellulose, hemicellulose and lignin) is an promising technique. As a natural complex aromatic polymer, lignin comprises three phenylpropane units(p-hydroxyphenyl, guaiacyl and syringol units) which are linked by various chemical bonds. Pyrolysis as an important thermochemical method for using lignin, and it is also the foundation of the gasification/catalytic pyrolysis. Studies of the lignin pyrolysis mechanism can understand the pyrolysis characteristics and pathways for specific products formation, thus providing a theoretical basis for research on selective pyrolysis for high-quality bio-oils and chemical products. In this study, pyrolysis characteristics of lignin was investigated from the following four aspects : i) analysis the influence of extraction methods and structure types on the pyrolysis behaviors of lignins; ii) explore the potential values of lignins extracted from different agricultural wastes; iii) interprete the pyrolysis mechanism of the lignin dimer model compounds and the influence of oxygen-substituted on the.phenethyl phenyl ethers;iv) Explore the interaction of biomass molecular fragments.The structures of functional groups in klason lignins extracted from cotton stalk and walnut shell by klason method(labeled as KCSL and KWSL, respectively), and milled wood lignin extracted from walnut shell via Bj?rkman method(labeled as BWSL), were analyzed with Fourier transform infrared spectroscopy(FTIR). The pyrolysis behaviors were studied by pyrolysis-gas chromatography-mass spectrometry(Py-GC/MS). The results were compared with commercial alkali lignin(CAL). The pyrolysis results from Py-GC/MS suggested that the products distribution of various lignins are almost the same, however, the yields are in great difference because of the extraction methods and structure types. Meanswhile, plenty of catechol derivatives are found in the lignins extracted from walnut shell and cotton stalk. The modification of MWL is mainly attributed to the rapture of benzyl aryl ether linkages, whereas the cleavage of ether bonds at the ?-and ?-positions on the propanoid side chain is the marked feature of klason lignin.TG-FTIR and Py-GC/MS were employed to investigate the pyrolysis behaviors of lignins extracted from different agricultural wastes. The results showed that compared with the woody lignin, the phenylpropane unit present in the non-woody lignin would enhance the ability of condensation and lead to the formation of biochar. Compared with commercial alkali lignin(CAL, 6.7%), a high proportion of catechol type compounds was obtained in the volatiles of the peanut shell(37.5%) and cotton stalk(27.6%) lignins. Furthermore, the amount of styrene and 4-hydroxy-3-methoxystyrene has an important link with the content of hemicellulose in raw materials.To elucidate the pyrolysis mechanism of a ?-O-4 type lignin dimer compound, analytical pyrolysis-gas chromatography/mass spectrometry(Py-GC/MS) and in-operando FTIR experiments were performed to reveal the distribution of the pyrolytic products under different temperatures and monitor the changes of functional groups of volatile products and main gases during the pyrolysis process. The pyrolysis process of phenethyl phenyl ethers is composed of C?-O, C?-C? homolytic and concerted mechanism, and C?-O homolytic reaction dominate the whole process. The oxygen substitution of benzene ring shows an significant effect on the pyrolysis rate and product selectivity. The ?-OH can increase the dissociation energy of C-C shift reaction and prompt C-C shift reaction.To understand the coupling thermal degradation characteristics of biomass molecular fragment, the interaction of ?-O-4 model compound and sugar units was studied by Py-GC/MS, in-operando FTIR and sealed reaction device. In the pyrolysis process, the ?-O-4 model compound has an important influence on glucose, this effect mainly reflects the following aspects: promot the formation of aldehydes, ketones and furan and suppress the yields of anhydrosugar. Meanwhile, in medium temperature glucose can accelerate the reaction rate of C?-O homolysis and promot the C?-O homolysis reaction and concerted mechanism.