Advancements in CO2 methanation: A comprehensive review of catalysis, reactor design and process optimization

Renewable electricity, production/storage and distribution of green hydrogen and carbon dioxide emission reduction are just three of the practices needed for a net zero-emission world. The increasing amount of renewable energy produced requires the development of versatile technologies capable to store the excess renewable electricity produced. Hydrogen production through electrolysis, despite not being still economically viable, can be considered a mature technology. Different strategies to convert H2 into more volumetric dense fuel are under development through Power-to-Chemicals (PtC) process. Among these, CO2 methanation offers the advantage of a wide infrastructure available for the distribution and use of methane as chemical and fuel for both heat and power generation. The latter approach is also called Power-to-Gas (PtG) process. These technologies in the last year have been the focus of research, both public and private. In particular, direct methanation of biogas obtained from anaerobic digestors represents a challenge and an interesting opportunity due to the possibility to avoid the CO2 separation step at the moment needed for biogas purification to biomethane. Since CO2 separation is an additional cost, this route can be the key for economic sustainability of the process. In this review we focus on the main aspects involved in the design of a methanation plant, with a particular focus on the biogas methanation, starting from the reaction thermodynamics and kinetics. Afterwards, light is shed on the most interesting catalysts reported in the literature, with a focus on Ni-based catalysts and considering the support role in the reaction. Different kinetic approaches currently available and promising reactor types, including different simulation models, which are becoming increasingly fundamental in the reactor design and scale-up phase, are reported. Finally, the last chapter contains the most interesting and industrially relevant ongoing projects on direct biogas methanation.