Performance Optimization of Pd-Ag Based Catalytic Membrane Reformer for ‘On-Site’ Hydrogen Production via Methanol Steam Reforming

Abstract

Hydrogen being the most abundant element on the earth has not been exploited to its full capacity till now. Despite having the highest energy-to-weight ratio (141.9 kJ g-1) among other parallel competitors like gasoline (47.5 kJ g-1) and diesel (44.5 kJ g-1), the potential of hydrogen is still underutilized. The commercial exploitation of hydrogen as a renewable energy carrier has been the subject of significant effort over the past two decades. Safe storage and transportation are the primary impediments to commercialization. In order to overcome these challenges, producing and utilizing H2 at the application site (on-site) is deemed to be advantageous. However, hydrogen is perpetually present in its compounded forms, including alcohols, water, coal, biomass, and other hydrocarbons. In order to extract hydrogen from these sources, a variety of methods are employed, including steam reforming (SR), autothermal reforming (ATR), partial oxidation (POX), pyrolysis, gasification, and fermentation. Steam reforming of hydrocarbons is the most prevalent commercial method owing to its high efficiency (70-85%). Hydrogen produced using these methods is always accompanied by gases (CO2, CO, CH4, N2, and O2). Membrane technology plays a crucial role due to its operational flexibility, ability to produce ultra-pure hydrogen, and compatibility with simultaneous reaction-separation processes, hence enhancing overall efficiency. Furthermore, they are quite practical for small-scale and portable applications and produce high-purity hydrogen at the cost of minimal energy and space. Pd-based dense composite membranes have always been the favorite for efficient hydrogen separation. From a practical standpoint, a Pd-based membrane offers high hydrogen permeability with ideally infinite selectivity. Additionally, it can operate at high temperatures and pressure for extended periods while consuming little energy and requiring little maintenance. Among different sources of H2 generation, methanol is the simplest alcohol and has the potential to revolutionize the process of producing hydrogen onboard/onsite because of its abundance of availability, low cost, high hydrogen density, environmental friendliness, and ease of transport and storage under ambient conditions. Moreover, its energy density is 80% higher than that of liquid hydrogen, with a gravimetric and volumetric hydrogen storage capacity of 12.5 (wt%) and 99 kg m-3, respectively. There is more hydrogen in a cubic meter of methanol (99 kg m-3, atm) than there is in a cubic meter of liquid hydrogen (70 kg m-3, 20 K), formic acid (53 kg m-3, atm ), H2 compressed cylinder (39.9 kg m-3, 70000 kPa), and even AlH3 based metal-organic framework (85 kg m-3, achieved in practice). Its low reforming temperature (523-623 K) due to the absence of a C-C bond also lowers the possibility of coke production at high temperatures. The presence of the –OH functional group is also a good attribute, as it reduces the amount of water consumed in the steam reforming reaction to produce hydrogen, hence reducing the steam reforming process temperature. To fully utilize hydrogen's potential, it must be separated from methanol due to its lower heating value (119.9 MJ kg-1) which is roughly 6.5 times higher than methanol's (18.1 MJ kg-1). Consequently, methanol can serve as a future ‘Hydrogen Carrier’ that can be readily accommodated by the existing infrastructure. Hence, to integrate the simultaneous methanol steam reforming (MSR) and H2 separation in a compact device, novel membrane reformers (MR) devices are currently being conceptualized that integrate hydrogen production and separation into a single reactor unit. The fundamentals of MR are derived from Le Chatelier's principle, which states that the continuous removal of the H2 through the membrane shifts the thermodynamic equilibrium towards the desired product resulting in enhanced conversion.