Valorization of Tunisian olive pomace by steam gasification: thermodynamic study using Mathematica(C) and Aspen-plus®

Biomass steam gasification is a promising technique for generating hydrogen-rich gas. This paper reports on and discusses the results of a thermodynamic analysis of Tunisian olive pomace steam gasification intended to produce hydrogen-rich syngas and/or syngas with predefined H-2/CO molar ratios. A non-stoichiometric approach is applied using MatheMatica(C) and/or alternatively the process simulator aspen-plus (R). For high-pressure cases, where deviation from ideal-gas behavior becomes significant, the Soave-Redlich-Kwong equation of state (SRK EoS) is selected for the calculation of the residual Gibbs' free energy and the compressibility factor ( Z ) of the gas mixture. Developed thermodynamic models are validated against experimental data from literature. Gas-phase equilibrium composition and syngas quality (H-2/CO ratio) are evaluated for a large range of the operating parameters temperature ( T ), pressure ( P ), and steam/biomass molar ratio ( r = S/B ratio). Results indicate that at 1000 K, syngas production (H-2 + CO) decreases from 80 to 30% as pressure is increased from 1 to 250 bars. At atmospheric pressure and for temperatures increasing from 800 to 2000 K, this behavior is inversed. Concurrently, carbon conversion is promoted. A higher S/B ratio (e.g., between 0.3 and 3.5) results in increased hydrogen production and carbon conversion, while the CO production is decreasing. Furthermore, by atmospheric pressure and for temperatures between 500 and 1000 K, carbon conversion reaches 100% for a S/B ratio varying between 0.5 and 1.5. These findings underline the particular role of the S/B ratio as a key parameter for the quality of producer syngas. To reach a target syngas quality however (e.g., 2 or 3: 2 for the Fischer-Tropsch process, fuel cell applications and methanol synthesis, and 3 for SNG production), one has to find a compromise between conflicting effects of the S/B ratio. On the one hand, larger values of this parameter lead to a decrease of the energy efficiency of the process. Lower values on the other hand may result in incomplete char conversion. As illustration of such a compromise we found, for the optimal conditions for a H-2/CO ratio of 2, P = 1 bar, T = 1000 K, and S/B ratio, r = 0.5.