Research on modeling the lignin waste molecular structure and pyrolytic reactions using ReaxFF MD method

Abstract

The lack of clarity in characterizing the chemical structure model of lignin waste hinders the comprehensive understanding of its pyrolysis reaction mechanism. The characterization results revealed that the de–alkalized lignin sample contained larger amounts of aliphatic and aromatic carbons compared to carbonyl carbons. Additionally, the aliphatic carbon molecules exhibited a higher presence of oxygenated carbons. The degree of aromaticity (f_a) was determined to be 63.93 %. The resulting single–molecule structural model exhibited a molecular formula of C₃₃H₂₆O₁₁ and a molecular weight of 598.56. Furthermore, the evolutions patterns and formation pathways of primary volatile components (H₂, CH₄, CO, and CO₂) in pyrolysis reactions were revealed. The number of H₂ molecules exhibited a positive correlation with the rise in pyrolysis reaction temperature. CH₄ molecules exhibited a pattern of initially increasing and subsequently decreasing. The number of CO molecules exhibited a positive correlation with the rise in pyrolysis temperature. Conversely, the number of CO₂ molecules demonstrated an initial increase followed by a decrease as the pyrolysis temperature increased. Finally, the generation pathway analysis of pyrolysis products shows that the H₂ molecule is composed of two hydrogen atoms bonded together that have been dissociated from the carboxyl group. Alternatively, it can be produced through hydrogenation reactions involving hydrogen atoms detached from the carboxyl group and methyl groups. CH₄ is primarily generated through the reaction between –CH₃ and free hydrogen ions. CO is primarily produced through the cleavage of the carbonyl group. CO₂ is primarily produced through the cleavage of carboxyl and ester groups.