Sorption-Enhanced Hydrogen Production: A Review

In the sorption-enhanced hydrogen production process, hydrocarbon reforming, water gas shift, and CO 2 separation reactions occur simultaneously in a single reaction step over a reforming catalyst mixed with a CO 2 sorbent. Transferring CO 2 as it is formed from the gas to the solid phase shifts the normal equilibrium restrictions and allows both the reforming and water gas shift reactions to approach completion. Depending on reaction conditions, the product (dry basis) may contain as much as 98% H 2 and only ppmv levels of CO and CO 2, thereby minimizing the final H 2 purification step or even eliminating it for some applications. A number of CO 2 sorbents have been studied including calcium-based oxides, K-promoted hydrotalcite, and mixed metal oxides of lithium and sodium. The sorbent is consumed during H 2 production so that the process is intrinsically unsteady state. Process economics requires that the sorbent be regenerable and used in many reaction−regeneration cycles. Regeneration may occur via temperature swing, pressure swing, or a combination. Much of the current research is devoted to testing and improving sorbent multicycle durability. Both circulating fluid-bed reactors and dual fixed-bed reactors with alternating reaction−regeneration functions have been proposed to provide overall steady state H 2 production.