a b s t r a c tThe Power-to-Gas (PtG) process chain could play a significant role in the future energy system. Renewable electric energy can be transformed into storable methane via electrolysis and subsequent methanation.This article compares the available electrolysis and methanation technologies with respect to the stringent requirements of the PtG chain such as low CAPEX, high efficiency, and high flexibility.Three water electrolysis technologies are considered: alkaline electrolysis, PEM electrolysis, and solid oxide electrolysis. Alkaline electrolysis is currently the cheapest technology; however, in the future PEM electrolysis could be better suited for the PtG process chain. Solid oxide electrolysis could also be an option in future, especially if heat sources are available.Several different reactor concepts can be used for the methanation reaction. For catalytic methanation, typically fixed-bed reactors are used; however, novel reactor concepts such as three-phase methanation and micro reactors are currently under development. Another approach is the biochemical conversion. The bioprocess takes place in aqueous solutions and close to ambient temperatures.Finally, the whole process chain is discussed. Critical aspects of the PtG process are the availability of CO 2 sources, the dynamic behaviour of the individual process steps, and especially the economics as well as the efficiency.

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Three ionic liquids {butyl-trimethyl-ammonium bis(trifluoromethylsulfonyl)imide [N 1114 ][BTA], 1-methyl-1propyl-piperidinium bis(trifluoromethylsulfonyl)imide [PMPip][BTA], and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [EMIM][Tf]} and two heat-transfer oils [dibenzyltoluene (DBT) and polydimethylsiloxane (trade name X-BF)] were evaluated for use in the three-phase methanation and the biogas purification processes. The density, viscosity, and surface tension of these liquids were measured and modeled as a function of the temperature. The solubilities of H 2 , CO, CO 2 , and CH 4 in these five liquids were also obtained under different pressures and temperatures. Additionally, the criteria required for each of the two processes considered were identified: the three-phase methanation process requires a thermally stable liquid with a low vapor pressure and a high H 2 , CO 2 and CO solubility, while the biogas purification process requires a highly selective CO 2 solubility liquid at ambient temperature. From the evaluation of both the experimental data and the process requirements, the most suitable liquid for each of the aforementioned processes was identified. For the three-phase methanation process, the two ionic liquids [N 1114 ][BTA] and [PMPip][BTA] and the two heat-transfer oils DBT and X-BF met the minimum requirements, while [EMIM][Tf] showed promising potential for the biogas purification process.

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Bei der Methanisierung ist eine effektive Abfuhr der Reaktionswärme eine zentrale Herausforderung. Die Dreiphasen-Methanisierung ist unter diesem Aspekt durch einfach zu gestaltende Energieabfuhr und Temperaturregelung vorteilhaft und erlaubt eine isotherme Fahrweise. Durch die hohe Wärmekapazität der Suspensionsflüssigkeit kühlt der Reaktor langsam aus und bietet sich daher für die Power-to-Gas (PtG) Prozesskette mit ihrer schwankenden Eduktgas-Bereitstellung an. Verschiedene Flüssigkeiten und Betriebsbedingungen wurden bezüglich ihrer Eignung für die Dreiphasen-Methanisierung untersucht.The high reaction heat of the methanation reaction requires an effective heat removal. For this purpose a three-phase reactor could be advantageous as the heat can easily be removed by external heat exchangers. Moreover, the temperature control is simplified, and the reactor can be operated isothermal. Especially for the Power-to-Gas process a three-phase reactor is very promising. Due to the high heat capacity of the liquid phase, it cools down slowly and hence buffers the effect of fluctuating feed stream. The applicability of several liquids and different operating conditions for three-phase methanation was investigated.Abbildung 2. Vergleich ausgewählter Suspensionsflüssigkeiten (IL = ionisches Fluid, PFPE = Perfluoropolyether); je größer die Fläche für eine Flüssigkeit ist, desto größer ist deren Eignung.Abbildung 5. Druckeinfluss auf die Umsätze von CO bzw. CO 2 im DN 25 Metallreaktor (d S = 50 -100 lm).