Kinetic modeling of isothermal degradation of lignin under conventional pyrolysis and oxidative pyrolysis conditions

The reaction kinetics of gas, tar and real char (r-char, defined herein as the solid residue from reacted lignin), as well as the grouped phenolic compounds from conventional isothermal pyrolysis and oxidative pyrolysis of lignin in a tubular flow-reactor over the temperature range of 200-500 oC were studied using the lumping techniques. Two semi-global models were proposed to describe the evolution kinetics of conventional pyrolysis and oxidative pyrolysis of lignin, respectively. The identical set of pseudo-first order kinetic parameters were derived for both models by taking into account the temperature-dependent r-char fraction and the initial dynamic heating ramp. The model simulations well reproduced the evolution of various products from lignin degradation under both inert and reactive conditions. Furthermore, the observed experimental discrepancies of char formation between pyrolysis and oxidative pyrolysis of lignin was resolved by including the additional lumped reactions of oxygen-induced r-char formation and its subsequent oxidation in the oxidative model. In addition, the result revealed that significant amount of lignin decomposition occurred during the initial heating ramp, especially for oxidative pyrolysis at higher temperatures.