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)(H2) generation via steam gasification of biomass, coupled with in situ carbon dioxide (CO2)(CO2) 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.