Low temperature sorption-enhanced hydrogen production from natural gas using variable-volume, batch-membrane reactors
Anderson, David M.
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Natural gas is an attractive fuel choice for distributed power generation due to the availability of a well-developed production and distribution infrastructure worldwide, including both intra-continental compressed gas pipeline network and inter-continental liquefied delivery. With its favorable 4:1 hydrogen to carbon ratio, natural gas also has compelling potential to become an economically viable “transition” fossil fuel to a low carbon energy future. This doctoral dissertation proposes a novel approach to producing hydrogen – the ultimate clean, carbon-free fuel – from natural gas for use in fuel cell vehicles and residential fuel cell based power generation systems with possibility for energy efficient on-board/on-site CO2 capture. A variable-volume batch membrane reactor, termed CHAMP-SORB, is comprehensively investigated, aiming to dramatically reduce the operating temperature and steam to methane feed ratios to levels that enable economically feasible hydrogen production at the point-of-use. In the CHAMP-SORB reactor, a batch of steam and methane is introduced into a piston-cylinder reaction chamber, where a steam reforming catalytic reaction occurs that produces H2 and CO2. To circumvent thermodynamic limitations and produce a purified H2 stream, both reaction products are continuously removed from the reactor by selective H2 permeation through a dense palladium-silver membrane and selective CO2 uptake onto a solid porous adsorbent. A series of first principle models with increasing complexity, including (i) a purely thermodynamic analysis of the CHAMP-SORB operating cycle to define optimal reactor temperature and feed composition, (ii) a purely kinetic model in the absence of any transport limitations to define the ideal limit on the rate of achieving high fuel conversion and hydrogen yield efficiency, and (iii) a comprehensive heat/mass transport-kinetics model were developed to understand the fundamental physical and chemical principles governing the CHAMP-SORB reactor process. A complimentary experimental study was also performed to investigate the combined reaction/separation processes in CHAMP-SORB and validate the fundamental understanding of the key operating principles revealed through modeling efforts.