Hydrogen production from biomass coupled with carbon dioxide capture: The implications of thermodynamic equilibrium

In this work we report on the consequences of thermodynamic equilibrium for hydrogen (_H2)$({\mathrm{H}}_2)$ generation via steam gasification of biomass, coupled with in situ carbon dioxide (_CO2)$({\mathrm{CO}}_2)$ capture. Calcium oxide (CaO) is identified as a suitable sorbent for CO₂ capture, capable of absorbing CO₂ to very low concentrations, at temperatures and pressures conducive to the gasification of biomass. The proposed process exploits the reversible nature of the CO₂ capture reaction and leads to the production of a concentrated stream of CO₂, upon regeneration of the sorbent. We develop a thermodynamic equilibrium model to investigate fundamental reaction parameters influencing the output of H₂-rich gas. These are: (i) reaction temperature, (ii) reaction pressure, (iii) steam-to-biomass ratio, and (iv) sorbent-to-biomass ratio. Based on the model, we predict a maximum H₂ concentration of 83%-mol, with a steam-to-biomass ratio of 1.5 and a Ca-to-C ratio of 0.9. Contrary to previous experimental studies, this maximum H₂ output is reported at atmospheric pressure. Model predictions are compared with an experimental investigation of the pyrolysis of pure cellulose and the reactivity of CaO through multiple CO₂ capture and release cycles using a thermogravimetric analyser, coupled with a mass spectrometer (TGA–MS). On this basis, we demonstrate the applicability of thermodynamic equilibrium theory for the identification of optimal operating conditions for maximising H₂ output and CO₂ capture.