Study On The Laws Of Lignin Pyrolysis Based On The Interpretation Of Volatile Matter And Char Structure

Lignin,as the only large-scale renewable source of aromatic compounds,is a potential raw material for the production of commodities and special chemicals.Pyrolysis technology is one of the most promising thermochemical conversion technologies with great potential for the valorization of lignin.And the pyrolysis process serves as the initial stage for all thermochemical conversions.Therefore,in-depth study of lignin pyrolysis laws is of utmost importance for achieving high value utilization of lignin thermal conversion.This study focuses on lignin as the research subject and aims to investigate the relationship between lignin pyrolysis volatiles and char structure.Specifically,it examines the release characteristics of lignin pyrolysis volatiles,the evolution behavior of carbon skeleton,pore structure,surface functional groups of char,and the pyrolysis reaction mechanism of lignin.Firstly,the chemical composition and structural characteristics of lignin were examined,and the volatile release characteristics of lignin during rapid pyrolysis at temperatures ranging from 200 to 800℃were investigated.Below 350℃,the cleavage ofβ-O-4 andα-O-4 linkages primarily contributes to the release of simple substituted aryl compounds.Simultaneously,the further decomposition of C-O,C-C,and C=O in depolymerized radical fragments leads to the formation of stable mono-phenolic compounds such as guaiacol（350℃→500℃）.The enhanced demethoxylation reaction results in increased yields of H-type phenols.Higher temperatures promote the formation of aromatic monomers,generating BTEXs and other aromatic hydrocarbons through demethylation and demethoxylation reactions（>500℃）.Additionally,secondary reactions and the decomposition of high-molecular-weight compounds contribute to the formation of polycyclic aromatic hydrocarbons（>700℃）.Fractal theory and deconvolution methods were applied to provide a quantitative mathematical description of the microcrystalline structure,pore properties,and surface morphology of char,along with proposing the evolution law of the synergistic development of carbon microcrystals and pore structure.Through ¹³C NMR deconvolution analysis,it was found that the relative abundance of aromatic carbon structures in char exponentially increased with temperature.The content of aliphatic carbon-oxygen species（Cal-O）in char decreased from 16.25%to 4.13%within the temperature range of 200℃to500℃.The macromolecular structure of char transformed gradually from disorder to order,primarily characterized by a reduction in the aromatic layer spacing（d002）from 3.99（?）to3.73（?）and an increase in stacking degree（Lc）from 7.88（?）to 8.63（?）,as well as an increase in aromaticity（f _a^X）from 75.19%to 96.94%.Strong negative linear correlation was observed between f _a^X and d002（R²=0.965）.The fractal dimension DFHH-1 of the pore surface was around 2.3 at temperatures below 350℃,but increased to approximately 2.7at 600℃,indicating the transition from two-dimensional to three-dimensional pore surfaces.When the temperature reached 600℃,the fractal dimensions DFHH-2 and DW of the pore structure reached their peaks,indicating the most complex and non-uniform pore structure at this temperature.Furthermore,a ternary quadratic polynomial function was established to correlate the fractal dimensions（DFHH-1,DFHH-2,and Dw）of the pores with specific surface area（SSA）,total pore volume（TPV）,and carbon content（Cdaf）.Based on the correlation between volatilization characteristics and the char structure evolution,the pyrolysis law of lignin was been revealed.The results showed that the ratios of H/C and O/C atomic ratios in char evolved in stages with the pyrolysis temperature.The results showed that the ratios of H/C and O/C atomic ratios in char evolved in stages with the pyrolysis temperature.When the pyrolysis temperature exceeded 250℃,functional groups such as OH,aliphatic hydrocarbon-CHn,C=O,and CH3O-gradually broke and recombined,resulting in a continuous decrease in their contents.The g-factor types of free radicals in char transitioned from O-centered radicals（g-factor<2.0030）to C-centered radicals（g-factor>2.0040）.The evolution process of aromatic structure was quantitatively described,and it was found that the formation of condensed ring systems began at 350℃.With increasing temperature,the aromaticity（f _a^F）continued to increase,and smaller aromatic rings gradually transformed into more ordered rings（≥6 rings）.Furthermore,a cubic polynomial function was established to correlate the f _a^F with the H/C atomic ratio,while the ring condensation degree showed a good positive linear correlation with the H/C atomic ratio.The development sequence of condensed ring structures in char was determined as 1×2,2×3,4×4,and 4×5,corresponding to 350℃,500℃,600℃and800℃,respectively.Finally,a three-stage process for the evolution of char structure was proposed,including bond cleavage and reformation,aromatization,and graphitization of carbonaceous materials.