Scale Down of Reforming Processes for Hydrogen Production from Renewable Sources

Abstract

Hydrogen has attracted much interest as energy carrier, both as feed for fuel cells or as an environmentally friendly fuel for automotive and a transition from a fossil fuel to a hydrogen-based economy has been forecasting for the 2050s. Nowadays, major issues concerning a wide use of hydrogen are related to the lack of a proper H2 grid and problems related to hydrogen storage (mainly due to the low energy density and safety issues). Scale down of the production, allowing a decentralized hydrogen production (close to the end-user), or production on-board of vehicles can bypass these problems.Traditionally, hydrogen is widely used in the chemical industry and it is mainly produced by natural gas (NG) reforming, an endothermic process that demands a large heat input and, due to the equilibrium limitations, high temperature to obtain large conversion. Furthermore, this process requires the integration of different units (the reformer itself, the water gas shift reactor and separation units to recover hydrogen)The challenges for scale down are mainly related to the success in reducing the operating temperature, simplifying the reforming process with integrated units and using liquid feedstock. The use of renewable sources both as feedstock for the reforming and to supply the energy required for endothermic process is also advisable for process sustainability.If reforming is carried out at moderate temperatures (400-500°C), a significant gain in material costs can be achieved; furthermore, at these temperatures, concentrating solar power equipment can be used as energy source for sustaining the hydrogen-producing endothermic reaction, thus reducing the carbon footprint of the process. On the other hand, thermodynamics limit the conversion at 30-40%. Therefore, two approaches have been considered.The first one, carried out with the financial support of the METISOL project of the Italian Environment Ministry, is to exploit the possibility to produce an enriched methane mixture (EM) with a hydrogen content between 10 -30% v/v; such a mixture can be used as feedstock in the traditional natural gas IC engines to improve the engine efficiency and to reduce the emission of green house gases; furthermore EM can be stored and distributed by the standard NG storage systems or medium pressure grid [1]. Indeed if the process is aimed to EM production, hydrogen does not have to be separated from unreacted CH4, but separation units to remove carbon dioxide are required [2].The second one is to use membrane reactors (MR), comprising a palladium or palladium-silver membrane, to overcome the thermodynamic limitation by separating hydrogen from the reacting mixture. Indeed, these membranes have an infinite selectivity for hydrogen and two goals can be achieved: a) the hydrogen separation shifts the reaction towards the product and allow to reach high conversion even at low temperature, b) hydrogen with a very high degree of purity, as required for fuel cells, is obtained. These considerations are at the basis of the CoMETHy (Compact M...