Comprehensive process simulation of a biomass-based hydrogen production system through gasification within the BECCS concept in a commercial two-stage fixed bed gasifier
Karim Rabea, Stavros Michailos, Kevin J. Hughes, D.B. Ingham, Mohamed Pourkashanian
Abstract
Hydrogen production through biomass gasification coupled with carbon capture has the potential to be a net negative emission process. Among the different designs of biomass gasifiers, the two-stage fixed bed gasifier has proved its ability to produce high quality syngas with minimum tar content at an industrial scale. However, it has not been investigated for hydrogen production. Hence, the current study is the first attempt to assess, through process modelling, the technical feasibility of hydrogen production in a 10 MWth two-stage gasification system using wood chips as feedstock. Mass and energy balances have been established in the Aspen Plus and MATLAB software. In contrast to most models in the literature, which were based on the equilibrium approach, the proposed system utilizes reliable kinetic models for the gasifier operation and the main downstream processes. An extensive validation of the gasifier kinetic model has been carried out and then a sensitivity analysis, which has revealed that the optimum steam-to-biomass ration (SBR) is 0.8 and 1.2 for the air-steam and the oxy-steam gasification systems, respectively. Further, the optimum steam-to-CO ratio (S/CO) for the water gas shift reactors (WGSRs) is 4, under which an overall 82.9% conversion of CO has been achieved. The results show that the 10 MWth two-stage gasifier can attain a specific hydrogen yield of 81.47 gH2/kg dry biomass. Based on the carbon footprint assessment, the process is net negative with an emission factor of −1.38 kgCO2-eq/kg biomass. Further, heat integration has also been conducted and it was found that the energy conversion efficiency of the whole system is 49.6%. This study is important since it provides a reliable data source for biomass-based hydrogen production through gasification in a commercial two-stage gasifier that can dictate operational strategies of pilot and demo plants.