Combining Biomass Gasification and LOHC Mixed Gas Hydrogenation for High Purity Hydrogen Production and Storage

Abstract

Hydrogen produced by steam gasification of biomass contains a complex mixture of byproducts, such as carbon oxides and aromatic tar species. We propose purification of such mixtures and subsequent hydrogen storage using liquid organic hydrogen carrier (LOHC) systems. Our concept includes the direct use of crude biomass gasification product gas for the LOHC hydrogenation step without previous tar removal. While hydrogen is bound to the aromatic LOHC component and to aromatic tar species, gaseous impurities like CO₂, CH₄ and CO remain in the gas phase. The hydrogenated LOHC mixture is then purified by removing hydrogenated tar species with different vapor pressure and adsorption properties via distillation or adsorption. These removed saturated tar compounds may serve as biofuels or biofuel additives. The biogenic component remaining in the LOHC system after purification can act themselves as LOHC molecules in subsequent hydrogen storage cycles. To demonstrate this concept, we investigated the Pd-catalyzed hydrogenation of benzyltoluene that was premixed with model tar compounds in relevant concentrations. Our experiments demonstrate that tar compounds with up to three rings do not affect the rate of the LOHC hydrogenation. The same is true for heteroaromatic, oxygen-containing tars. Nitrogen-containing tars, however, were found to lead to a moderate but reversible deactivation. Rapid deactivation of the catalyst was observed with sulfur-containing tars and tar compounds with more than three aromatic rings. The latter strongly adsorb to the Pd/Al₂O₃ catalyst, a fact that could be shown via thermogravimetric temperature-programmed oxidation.