Study On The Pyrolysis Characteristic Of Aspen Lignin And Pyrolysis Mechanism Of Lignin Model Compounds

Lignin widely exists in the lignocellulosic biomass. It has the dual properties of the aromatic and the aliphatic, and can be used with modification. Compared to carbohydrates, lignin has the higher calorific value and can replace fossil energy to a certain extent as a renewable energy, which could effectively reduce carbon emission and reduce pollution. In the conversion and utilization of lignin, pyrolysis is an effective and industrial promising method. It can converte lignin into gas, bio-oil and char. Bio-oil obtained from pyrolysis can be further upgraded to liquid fuels and value-added bio-chemicals.As an excellent short-cycle paper timber, Populus tomentosa Carr. is widely used in the pulp and paper industry, which is the most important source of industrial lignin. So in this paper, alkali lignin(AL) and ethanol organosolv lignin(OL) isolated from Populus tomentosa Carr. were selected as raw materials. Elemental analysis, higher heating value(HHV), FT-IR spectroscopy, gel permeation chromatography(GPC), and quantitative 31P-NMR were used for the structure characterization. The thermogravimetric characteristics and the releasing laws of the volatiles were analysed on Thermogravimetric Fourier transform infrared(TG-FTIR). Pyrolysis coupled with gas chromatograph–mass spectrometer(Py-GC/MS) was employed to study the flash pyrolysis characteristics. Products obtained from flash pyrolysis were classified, and the formation laws of some products were summaried. Furthermore, a horizontal closed pyrolysis system was built to investigate the pyrolysis characteristics of AL and OL. The composition and the formation rules of the three-phase product(gas, bio-oil and char) were discussed. This is the first time to study of the pyrolysis characteristics of AL and OL comprehensively and systematically.In order to explain the lignin pyrolysis process and the formation laws of the pyrolyic products further, lignin model compounds were chosen to simulate the pyrolysis process. Lignin monomeric model compounds(p-coumaric acid, ferulic acid and sinapic acid) were selected as the primary products to simulate the secondary pyrolysis. Moreover, changes on the substituents and aromatic nucleus in the pyrolysis process were discussed. Dehydrovanillin was synthesized and screened as the 5-5’ biphenyl lignin dimer model compound to discuss the changes on the 5-5’ biphenyl linkage and the relationship with coke formation during pyrolysis. Guaiacylglycerol--guaiacyl ether was used as-O-4 lignin dimer model compound to simulate the degradation of-ether bond in lignin. In addition, the subsequent degradation processes of the pyrolytic products were discussed. Owing to involving the two major connection types(5-5’ biphenyl linkage and-O-4 linkage) in lignin and covering the primary and secondary reactions in the pyrolysis process, the above three types of lignin model compounds could simulate the lignin pyrolysis process well.