Activated carbons from lignin: kinetic modeling of the pyrolysis of Kraft lignin activated with phosphoric acid

A phenomenological kinetic model has been developed for the pyrolysis at low heating rates of lignin activated with phosphoric acid. The model is based on thermogravimetry (TG) and differential thermogravimetry (DTG) data from pyrolysis experiments and assumes that lignin carbonization proceeds through a set of pseudo-first-order reactions. These reactions are a simplified description of the multiple reactions involved in the process. TG experiments were performed in nitrogen atmosphere for lignin (L) impregnated with 85% phosphoric acid (PA) at mass ratios (PA:L) from 1.0:1.0 to 1.75:1.0, a typical heating rate of 10 °C/min and a maximum carbonization temperature of 650 °C, including isothermal stages at 150 and 300 °C in the temperature programs for some of the experiments. Analysis of the TG and DTG curves led to a kinetic model that includes an initial reaction step between lignin and phosphoric acid, water formation from the dehydration of the excess of phosphoric acid to P2O5, pyrolysis of lignin to carbon and volatiles, evaporation of water and P2O5 and finally, partial volatilization of the carbon to light gases. Activation energies and the other parameters of the model were adjusted from experimental data. Activation energies were 26.0 kJ/mol for water desorption, 72.0 kJ/mol for the dehydration of phosphoric acid to phosphoric pentoxide, 95.0 kJ/mol for the volatilization of P2O5, 47.7 kJ/mol for the carbonization of the activated lignin and 106.3 kJ/mol for the pyrolytic release of light gases from activated carbon. The model provides a good representation of the thermograms regardless of the phosphoric acid to lignin ratio and the temperature profile along the reaction.