Power-to-gas (PtG) is an energy exploitation concept that began to expand during the last decade. It fits into the broader topic of sector integration, whereby the electricity and gas sectors are combined.

As nonprogrammable renewable energy sources (e.g., wind and photovoltaic) gradually gained a foothold in the energy market, in order to manage the energy surplus at certain times of the day and the deficit at others, storage methods began to be considered to a greater extent. In this context, PtG aims to be a viable technology for long-term storage in the form of chemical energy. In this case, in fact, the energy surplus of renewables is directed to an electrolyzer to produce green hydrogen, which can then be made to react with carbon dioxide to obtain the energy carrier methane.

Although the capture of carbon dioxide is, to date, a very expensive process ^[1], it allows the production of a renewable fuel that has a net zero impact on the environment, as is the case when using biomass. In addition, the energy carrier obtained has undoubted value since it can be used within the existing transport infrastructure for multiple purposes such as the production of electricity and heat, or for mobility.

The growing interest in PtG plants and in the involved processes is testified in various review papers. Specifically, in ^[2], a review of PtG and power-to-hydrogen plants focuses on technical characteristics, it analyzes the possible operation of the various components, and it evaluates how they are actually operated. Moreover, in ^[3], a selection of laboratory, pilot, and demonstration PtG projects is analyzed with particular focus on the methanation process. In ^[4], the authors consider plants producing hydrogen or other synthetic fuels and they assess the state of the art limited to technological and economic aspects. In ^[5], reference is made to PtG demonstration projects in Europe. The focus is on the technologies adopted for the various phases. Then, the analysis is widened to power-to-x projects in ^[6] and other categorization criteria, e.g., carbon dioxide and electricity sources, are added. Finally, in ^[7], a similar analysis is extended worldwide.

2. Power-to-Gas Technical Backgroundology

Power-to-gas basically consists of three stages: electrolysis, carbon dioxide capture, and methanation ^[8]. Despite this, plants of this type do not have a well-defined architecture. In fact, for each of the mentioned stages, various technologies can be adopted, which can be very different from each other. One example is the methanation phase, which can be catalytic or biological ^[9], or the carbon dioxide capture phase, which can be mechanical, chemical, or thermal ^[10]. For this reason, it is considered appropriate here to provide an overview of the technologies that can currently be used in plants of this type.

2.1. Electrolysis

The aim of the electrolysis step is the production of hydrogen. It is an electrochemical process in which the splitting of water molecules occurs. The electrolysis cell consists of two electrodes, the negatively charged one in which the reduction reaction takes place (namely, the cathode) and the positively charged one in which the oxidation reaction takes place (namely, the anode). The charge carriers vary depending on the technology used, but the overall reaction that occurs is generally as follows ^[11]: where the water phase can be liquid or in vapor form depending on whether a low- or high-temperature electrolyzer is used, while the molecular hydrogen and oxygen phases are gaseous. Electrolyzers are mainly classified by their operational temperature, i.e., high- or low-temperature. Specifically, among low-temperature electrolyzers, the main types are alkaline and polymer electrolyte membranes (PEM). These electrolytic cells work at temperatures up to 90 °C. High-temperature technologies are, in contrast to the former, still under development. The main high-temperature electrolytic cell is the solid oxide electrolyte cell (SOEC), which is capable of working at temperatures between 700 °C and 900 °C ^[12]. From a PtG perspective, high-temperature electrolyzers establish the way for a further approach, that of co-electrolysis. In this case, in addition to steam, carbon dioxide is also split, and syngas (i.e., a mixture of carbon monoxide and hydrogen) is produced. Syngas can be further processed to produce synthetic fuels, including methane. As stated in ^[13], in a power-to-gas system, using an SOEC in co-electrolysis rather than steam-electrolysis can lead to an increase in the total system efficiency due to the greater heat available from the methanation of syngas than from the methanation of carbon dioxide alone.

2.2. Carbon Dioxide Capture

One of the most critical points of the entire process is undoubtedly obtaining carbon dioxide. Carbon capture and utilization, in fact, currently represents one of the most expensive processes for a power-to-gas plant. Carbon dioxide can be taken from combustion processes or obtained as an industrial byproduct (it is worth highlighting here the presence of anaerobic digestion processes for biogas production). Another way could be to take atmospheric carbon dioxide directly from the atmosphere through a process known as direct air capture (DAC). This allows power-to-gas to be implemented even if there is no access to a specific plant producing carbon dioxide. There are various methods that can be adopted for its capture. The main ones involve absorption (which can be chemical or physical), adsorption, or membrane systems. Cryogenic and algae-based methods are also being investigated ^[10]. The lower the concentration of carbon dioxide in the gas from which it is taken, the higher the cost of separation. This is why the use of atmospheric carbon dioxide, although it could be very useful, is currently extremely expensive ^[1] since its concentration is 420 ppm ^[14]. Despite this, as shown below, in practice there is no shortage of examples of plants making use of this technology. Another possibility, as yet unexplored in this field, could be to use air present in buildings which, as shown in ^[15], reaches values of more than 500 ppm.

2.3. Methanation

The methanation stage is the final stage by which the desired gas is obtained. Reactors that can carry out methanation are divided into two large groups, catalytic and biological. In the catalytic ones, thermochemical conversion takes place, assisted by catalysis to facilitate the kinetics of the process. This procedure is known as the Sabatier process. What takes place is the reaction between hydrogen and carbon dioxide that have already been obtained by the methods described above. Specifically, the reactions that occur are as follows: where the first two are the hydrogenation reactions of carbon dioxide and carbon monoxide, respectively, the third is the reverse water gas shift reaction, and the last is the Boudouard equilibrium reaction ^[11]. As can be seen, hydrogenation reactions are highly exothermic, which leads, according to Le Chatelier’s principle, to the need to cool the reactor. Methanators generally work at temperatures between 300 °C and 550 °C and at pressures ranging from 1 bar to 100 bar ^[16]. The materials used for catalysis are usually active metal particles (e.g., Ni, Fe, Ru) dispersed on a support consisting of a metal oxide such as alumina ^[17]. The most common types of catalytic methanator for these purposes are fixed-bed, fluidized-bed, and the so-called three-phase methanator. In the last, the solid catalyst (e.g., a powder) is suspended in a stable and inert liquid, and the third phase is represented by the processed gas. Fixed-bed reactors are adiabatic due to the poor capacity of the bed to conduct heat; this is the reason why there is often a need for a cooling section between the stages. To solve this problem, another solution is to use structured-type reactors such as monolith or honeycomb-type reactors, in which the internal metal structure is designed to facilitate heat transfer ^[18]. In addition, it is considered important to highlight the existence of studies on other types of methanators, such as sorption-enhanced reactors, in which water removal is carried out by adsorption directly inside the reactor, or the so-called photocatalytic methanation reactors, in which reactions are assisted by either solar or artificial light ^[9]. In biological-type reactors, the fundamental idea is very different from those mentioned above. In this case, catalysis is carried out by microorganisms, thus it is often referred to as a biocatalytic process. Operating conditions can range between 0 °C and 122 °C and between 1 bar and 10 bar ^[9]. The microorganisms used are the autotrophic hydrogenotrophic methanogens (archaeal bacteria), already known to be used in the methanogenic phase of the anaerobic digestion for the production of biogas. Based on this close relationship with anaerobic digesters for biogas production, there are two macrotypes of biological methanators, in situ and ex situ ^[16]. In the former, the hydrogen produced by electrolysis is injected directly into the digester to increase the total methane yield by converting the carbon dioxide present in the biogas. In the latter, a separate reactor is used in which hydrogen and carbon dioxide are fed under stoichiometric conditions. In the ex situ applications, the carbon dioxide input does not necessarily come from a biogas plant and the methanator can be a continuously stirred tank reactor, a trickle-bed reactor, or a reactor with hollow fiber membranes.