Calcium enhanced hydrogen production with CO2 capture

Abstract

Abstract The addition of a calcium-based sorbent to a standard methane reforming catalyst alters the equilibrium and permits both the reforming and water-gas shift reactions to approach completion in a single step at a temperature approximately 200 K lower than currently used. Depending on reaction conditions greater than 95% CO 2 capture is possible and the product may contain 98+% H 2 (dry basis) and only ppm levels of CO and CO 2 . The requirements for final H 2 purification are thereby minimized for some applications and eliminated for others. While dolomite and limestone sorbent precursors have received the majority of attention because of their widespread availability and low cost, some researchers have studied synthetic calcium-based materials in an effort to improve sorbent durability. Regardless of the source, process economics requires that the sorbent be regenerable and used in many reaction-regeneration cycles. Regeneration is accomplished using temperature swing in an atmosphere of pure CO 2 or a mixture of CO 2 and steam from which pure CO 2 can be produced by condensing the H 2 O. Circulating fluidized-bed reactors are appropriate for large-scale operations while dual fixed-bed reactors with alternating reaction-regeneration functions may be appropriate for small-scale operations. No supplemental energy is required in the H 2 production reactor as the combined reforming, shift, and carbonation reactions are essentially thermally neutral. Supplemental energy is required in the regeneration step but overall energy reduction of 20–25% has been estimated. Other advantages of sorption-enhanced H 2 production include a reduction in capital cost due to process simplification, replacement of high temperature, high alloy steels in the reformer with less expensive materials of construction due to the lower operating temperature, total elimination of the shift reactor(s) and shift catalyst(s), and reduction of carbon deposition in the reformer. The concept is equally applicable to H 2 production from syngas from coal or biomass, with the added advantage that the need for reforming catalyst is eliminated.