Insights into urea-induced modulation of lignin pyrolysis behaviors and product distribution

Abstract

This study systematically investigated the influence of urea, an active nitrogen, on the pyrolysis behavior of lignin. Thermogravimetric analysis (TGA) and kinetic analysis were employed to elucidate the synergistic effect of reaction pathway modulation and product distribution optimization between urea and lignin. The results indicated that urea significantly reduced lignin pyrolysis activation energy (E), with a maximum reduction of 254.14 kJ/mol (lignin+50 % urea) and average energy decreasing to 87.28 kJ/mol. Meanwhile, it induced a transition from the nth-order reaction to the geometric contraction. Moreover, urea effectively inhibited the formation of complex phenolic and hydrocarbon substances, elevating the proportion of simple phenols in bio-oil to about 40 % while promoting formation of pyridine-N and pyrrole-N structures in bio-char, which increased the nitrogen content from 2.00 % to 2.71 %. The experiments also found that NH₃ and free radicals derived from urea decomposition could participate in the free-radical reactions of lignin, promoting the cleavage of C-O and C-H bonds and hydrogenation and deoxygenation, thereby achieving the regulation of product selectivity. Urea offers an efficient and feasible technical route for the directional conversion of lignin into high-value chemicals with promising application prospects.