The development of autothermal microchannel reactor technology for hydrogen-based gas processing

Abstract

Key challenges associated with the production, transport, storage and continuous use of hydrogen (H₂), generated from renewable energy, is the natural intermittency of some renewable resources such as solar photovoltaic and wind, and the physical properties of H₃ that complicates its handling in the industry, i.e. its low volumetric energy density and high flammability. The work presented in this thesis demonstrates the use of process intensifying microchannel reactor technology for the thermo-catalytic processing of renewable H₂ via attractive energy carriers: (i) the decomposition of ammonia (NH₃) to form H₂, as well as (ii) the synthesis of methane (CH₄) using renewable H₃ as feedstock (CO₂ methanation). Furthermore, these processes require heat management strategies for effective autothermal operation. The NH₃ decomposition reaction is endothermic and a coupled exothermic process (NH₃ oxidation) is demonstrated to provide the heating requirements for H2 release. Conversely, the CO₂ methanation process is exothermic, and cooling is required to reach equilibrium favouring reaction temperatures that promote the conversion of H₂ and CO₂ towards CH4. Extensive experimental investigations were carried out into these thermally coupled processes, and reported on for NH₃) decomposition and for CO₂methanation. Then an evaluation of a compact methanation demonstrator unit incorporating the microchannel-based reactor was carried out, followed by computational fluid dynamic (CFD) modelling to evaluate associated heat and mass transport properties in the microchannel reactor. The scale of the microchannel reactors investigated here was such that NH3 up to a flow rate of 6 NL min-1 was processible towards H₂ at a high conversion rate (99.8%), and corresponding to an equivalent H₃ fuel cell power of 0.71 kWe, while total methanation flow rates of up to 7 NL min-1 were used to demonstrate CO₃ methanation (90.5% CO₂ conversion). Respective thermal efficiencies of 75.9% and 76.6% were obtained at the recommended steady-state operating points, which were close to thermodynamic equilibrium. These thermal efficiencies are deemed remarkable, considering the compact- and R&D-scale reactor technologies investigated herein. Overall, the work described in this thesis contributes to the development of micro-engineered reactors that are multifunctional catalytic conversion units and heat exchangers, and which support modular technologies for the processing of renewable H₂.