A comprehensive review on lignin pyrolysis: Mechanism, modeling and the effects of inherent metals in biomass

•Effects of critical factors on lignin pyrolysis are summarized.•Estimated energies by different DFT methods for the initial reactions are compared.•Effects of different inherent metals with different existing forms are summarized.•Kinetic modeling is discussed on structure representation and reaction network.•Recommendations are suggested for experimental and simulation research on lignin pyrolysis.