Kinetic evaluation of high-pressure pyrolysis of biomass using distributed activation energy model

In this study, pressurized non-isothermal decomposition of rice straw (RS) and pine bark (PB) biomass from room temperature to 800 °C at heating rates of 5, 10 and 20 °C min⁻¹, and under different pressures 1, 10, 20 and 30 bar, is investigated in inert ambience. The final residue obtained from biomass pyrolysis increases from 31.2 % to 38.4 % and 33.5–39.5 % for RS and PB respectively with increasing the pressure from 1 bar to 30 bar. Biochar yield was almost unaffected with increase in heating rate from 5 to 20 °C min⁻¹ irrespective of biomass type. For both the biomasses, the evolution time of gases (CO and CO₂) increases with pressure. In the present study, a distributed activation energy model (DAEM) is developed to describe the mass loss and differential mass loss profiles under different conditions. Hemicellulose, cellulose and lignin are the pseudocomponents for RS, while for PB, an additional pseudocomponent, tannin, was included to represent the extractives. Importantly, the effect of pressure on the kinetics is captured by increasing the activation energy of lignin decomposition without altering the rate parameters of other pseudocomponents. High-pressure pyrolysis conducted in CO₂ atmosphere shows that RS and PB exhibit drastic increase in mass loss after 640 °C and 700 °C, respectively, due to the Boudouard reaction that increases the CO formation through the reaction of CO₂ with the biochar. The DAEM was thoroughly validated by predicting the mass loss profiles at different heating rates and in the presence of CO₂ ambience.