Review of hydrogen-enriched gas production from steam gasification of biomass: The prospect of CaO-based chemical looping gasification

Global warming, climate change and energy security issues are the forces driving the fossil fuel based energy system towards renewable and sustainable energy. Hydrogen as a clean energy carrier is believed to be the most promising source to replace fossil fuel. Biomass gasification with the presence of steam offers a feasible, sustainable, and environment-friendly option as well as a favorable alternative for higher hydrogen yields and for large-scale hydrogen production which can satisfy the need of hydrogen in the future. However, the process suffers from the problem of undesirable CO2 and tar formation. Calcium oxide (CaO) has been acknowledged as a catalyst to produce hydrogen-rich gas and has currently gained broad attention due to its cheapness and abundance. Nevertheless, the deactivation of CaO after carbonation reaction is challenging for continuous hydrogen production and economical perspective. To conquer such challenge, the concept of CaO-based chemical looping gasification (CaO-based CLG) has emerged recently. Additionally, due to its energy-efficient and environment-friendly aspects, the CaO-based CLG using biomass as feedstock is gaining more attention in recent years. This study first presents a review on conventional steam gasification of biomass without catalysts for producing hydrogen-rich product gas. The effects of key variables, such as biomass characteristics, gasifier temperature, steam-to-biomass ratio (S/B) and equivalence ratio (ER), on hydrogen-enriched gas production are discussed based on recent researches and developments. Then the use of CaO in biomass steam gasification for hydrogen production with in situ CO2 capture and tar reduction is described. The prospect of CaO-based CLG using biomass fuel is also discussed as a promising process for renewable, sustainable and environment-friendly hydrogen production.