Power-to-X (P2X) and Power-to-Gas (P2G) are generic terms used for technologies that convert grid and/or renewable electricity into carbon-neutral synthetic fuels such as hydrogen, synthetic natural gas, liquid fuel or chemicals. These can be used in sectors that are difficult to decarbonise or, unlike electricity, can be stored for later use.

2.1 Decarbonising / Conversion of CO₂

Microwave plasma dissociation and conversion of carbon dioxide (CO₂), the main greenhouse gas, has been considered in the context of Carbon Capture and Utilization (CCU) approaches. CCU is the sequestration of CO₂ and its conversion into energy carriers or value-added chemicals. Many different routes are being developed and investigated including CO₂ fixation, e.g., inorganic carbonates, growing algae for biofuel/biodiesel production, dry reforming of methane (CH₄) – eq. 1, the direct conversion of CO₂ into carbon monoxide (CO) and oxygen (O₂) – eq. 2.

(1) CO₂ + CH₄ ↔ 2CO + 2H₂

(2) CO₂ ↔ CO + ½ O₂

The resulting CO can then be further processed through the water-gas shift reaction, eq. 3, to hydrogen (H₂), important feedstocks for the synthesis of ammonia (NH₃), liquid fuels such as synthetic diesel, methanol, etc.

(3) CO + H₂O ↔ CO₂ + H₂

2.2 Hydrogen for fuel and chemical synthesis

Molecular H₂ is considered to be the most important fuel for the energy transition. Today, more than 96 % of H₂ is produced from fossil resources such as coal, natural gas, and oil. To reduce the environmental impact, new alternative solutions are being developed, such as water electrolysis using electricity from renewable resources, such as wind energy, solar energy, geothermal energy, biomass etc.⁽⁴⁾ An alternative technology for the production of H₂ is the direct microwave plasma conversion of CH₄ contained in biogas or in natural gas, since both feedstocks contain mainly CH₄, i.e. up to 60-70 vol% for biogas and >80 vol% for natural gas. In this process there are no CO₂ emissions, the electricity/energy input is much lower than for water electrolysis and the co-products, i.e. carbon black (C) obtained mainly in the plasmalysis process at atmospheric pressure – eq. 4 and the C₂ fraction - acetylene (C₂H₂) and ethylene (C₂H₄) - obtained at moderate pressures – eq. 5, are products with great industrial value.

(4) CH₄ ➟^{~atmospheric pressure}➟C + 2H₂

(5) 2CH₄ ➟ ^{50 - 150 mbar} ➟ C₂ H₂ + 3H₂

In these reactions, the H₂ produced is classified as either green H₂ or turquoise H₂, depending on the feedstock, biogas or natural gas, respectively.

The microwave power generated by the microwave generator must be stable to ensure plasma stability and process and equipment reliability. The use of the latest industrial microwave generator technology allows very close control of the plasma process and improved energy transfer to the CH₄ feedstock, resulting in improved process control and efficiency and higher product quality. In addition, MUEGGE's microwave plasma reactors can operate at low and high pressures and can be easily scaled up by placing multiple plasma sources in series or in parallel, thereby reducing the development time to full-scale production – Figures 1a-c and Figure 2. As the transport and storage of H₂ is costly, the high modularity and the rapid, inertia-free on/off of the plasma associated with microwave plasma reactors offers the possibility of building plants that can be operated in situ and on demand.

2.3 Nitrogen fixation - fertilisers

Nitrogen is one of the basic elements responsible for the growth of living organisms and is essential for the production of many chemicals such as fertilisers, medicines, explosives and dyes. More than 99% of the world’s nitrogen is in the form of atmospheric N₂, which makes up ~78% of the air. However, N₂ is chemically inert and therefore inaccessible to most of the organisms and must be converted to a reactive form through a process called nitrogen fixation. The main practice for introducing nitrogen into the soil is through nitrogenous fertilisers, i.e. urea and nitrates derived from NH₃.
The chemical process that has been used for over 100 years to extract fixed nitrogen from the atmosphere to meet the needs of a growing population is the Haber-Bosch NH₃ synthesis process. The Haber-Bosch process is a catalytic process in which N₂ extracted from the air is reacted with H₂ at pressure ~200 bar and temperatures ~500 ⁰C. Today, the annual amount of nitrogen fixed by the Haber-Bosch process has reached 130 million tonnes per year, about 29% of global fixation⁽⁵⁾. The Haber-Bosch process consumes almost 1 - 2% of the world's total energy production.
As an alternative technology, more environmentally friendly and less costly, atmospheric pressure microwave plasma assisted nitrogen fixation is generally achieved by the reaction of N₂ with O₂ to produce nitrogen oxides (NO_x), usually a mixture of nitrogen dioxide (NO₂) and nitrogen monoxide (NO). In this case, the raw input gas for the plasma is atmospheric air. The reactions to produce NO are favoured by high temperature processing due to the high dissociation energy of N₂, eq. 6.

(6) N₂ + 3/2 O₂ ↔ NO + NO₂

Process parameters such as gas composition, flow rate and temperature are of great importance in NO_x production, and the precise parameter tuning associated with microwave plasmas allows control of the energy input required for the chemical reactions and the NO₂/NO ratio.

In another nitrogen fixation process, microwave plasmas can achieve catalyst-free conversion of N₂/air and H₂O under ambient conditions. The most studied configuration uses atmospheric-pressure plasma to generate plasma on the surface or inside of water⁽⁶⁾, which can produce NH₃ (i.e., ammonium ions, NH₄⁺), and other forms of the fixed nitrogen, such as nitrite (NO₂^–) and nitrate (NO₃^–).