Thermochemical Conversion of Lignocellulosic Biomass: a Thermodynamic Study Based on Isothermal and Adiabatic Reactors

Abstract

In this paper a comparative study is presented between thermochemical routes of pyrolysis (PR), Supercritical water gasification (SCWG), steam reforming (SR), oxidative reforming (OR) and autothermal reforming (ATR) aiming hydrogen production from lignocellulosic biomass (LB). For this, thermodynamic approaches were used, based on Gibbs energy minimization (minG) and entropy maximization (maxS) methods, in order to represent isothermal and adiabatic reactors, respectively. To carry out the simulations, the GAMS 23.9.5 software and the CONOPT3 solver were used. The results obtained demonstrated that the regions of greatest hydrogen formation were at high temperatures (= 900 K) and at the lowest pressures tested (5 bar for PR; 230 bar for SCWG and 1 bar for the other thermochemical routes). Furthermore, it was verified which type of operation (adiabatic or isothermal) is most favourable for hydrogen production. The adiabatic operation proved to be more important and productive for SCWG and the isothermal operation proved to be efficient for all other thermochemical routes studied. It was also found that adiabatic reactors resulted in higher H2 productivity for SR and PR processes; and isothermal reactors resulted in better quality of gas produced for the OR, ATR and SCWG processes. SR presented the best result for the quality of H2 (47% in the product stream at 1 bar and 1000 K) produced. Exothermic reaction behaviours were observed for ATR, OR and for SCWG reactions, the other thermochemical routes presented endothermic characteristics in most of the operational ranges studied. The proposed thermodynamic models proved to be fast and effective in the calculations carried out in this paper, with computational times of less than 5 seconds in all performed simulations.