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Schaaf et al. Energy, Sustainability and Society ( DOI 10.1186/s13705-014-0029-1 REVIEW Open Access Methanation of CO2 - storage of renewable energy in a gas distribution system Tanja Schaaf*, Jochen Grünig, Markus Roman Schuster, Tobias Rothenfluh and Andreas Orth Abstract This article presents some crucial findings of the joint research project entitled «Storage of electric energy from renewable sources in the natural gas grid-water electrolysis and synthesis of gas components». The project was funded by BMBF and aimed at developing viable concepts for the storage of excess electrical energy from wind and solar power plants. The concept presented in this article suggests the conversion of CO2-containing gases into methane in a pressurized reactor using hydrogen produced via electrolysis. The produced gas can be upgraded to synthetic natural gas (SNG) and fed into the well-developed German natural gas grid. This concept benefits from the high storage capacity of the German gas grid and does not require any extensions of the current gas or power grid. The reaction heat released by the exothermic methanation reaction leads to a temperature rise of the gas in the fixed bed catalyst of the reactor. The conversion of carbon dioxide is limited in accordance to the chemical equilibrium which depends strongly on temperature and pressure. For maximum carbon dioxide conversion, it is convenient to split the methanation into several stages adding cooling sections in between. This article focuses on the methanation process and its transfer onto an industrial scale evaluating the different plant capacities and feedstock mixtures used. The methanation takes place in a staged fixed bed reactor. This staged reactor concept is an in-house development based on know-how from the sulfuric acid production technology. Keywords: Chemical storage; Fixed-bed staged reactor; Methanation Review via an electrochemical way. Furthermore, electrical energy Background can be converted into chemical energy by transferring it Huge efforts are currently being made to replace the into fuels such as hydrogen, synthetic natural gas (SNG), conventional energy sources such as nuclear power or or methanol. fossil fuels by renewable energy sources such as wind A comparison of the described storage technologies or solar energy. However, most of the renewable energy with regard to their storage capacity and their character- sources cannot provide base load electric power due to istic charge/discharge times is shown in Figure 1 [1]. A their intermittent nature (e.g., wind energy). To over- crucial requirement for storage technologies in the con- come this problem, storage systems have to be inte- text of the present article (storage of energy from renew- grated in the power grid. able resources) is an elevated storage capacity combined There are various possibilities of storing this excess with high charge/discharge periods. Only chemical second- energy: A direct storage of the electrical energy can for ary energy carriers such as hydrogen and carbon-based instance be realized with supercapacitors and supercon- fuels (SNG) fulfill this requirement. Storage technologies ducting magnetic energy storage (SMES). By the use of such as flywheels and batteries, on the other hand, are pumped storage power plants, electric energy is converted strongly limited regarding their capacity and charge dur- to potential energy. Compressed air reservoirs or flywheel ation. Therefore, these technologies are only used for com- energy storage systems buffer electricity as mechanical en- pensating short-term fluctuations by supplying electric ergy, whereas batteries enable storage of electricity via energy over a time period of several minutes up to several chemical energy, which is however convertible in electricity hours. Besides high capacity and charge/discharge cycles, * Correspondence: [email protected] the process efficiency is another important characteristic. Outotec GmbH, Oberursel, Germany © 2014 Schaaf et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Schaaf et al. Energy, Sustainability and Society ( Page 2 of 14 Figure 1 Charge/discharge period and storage capacity of different electricity storage systems. CAES, compressed air energy storage; PHS, pumped hydro storage; SNG, substitute natural gas. Figure 2 illustrates the efficiency losses of described typical shown in surveys that additional 3,600 km of high-voltage power to gas process. power lines are needed until 2020 to eliminate the trans- The conversion of electrical energy into hydrogen port bottlenecks in Germany. Up to now, only 200 km of causes efficiency losses of about 20% due to the internal new power lines have been built because of tedious plan- cell resistance which will be realized as heat losses. The ning and approval processes and because of strong oppos- subsequent transformation of hydrogen in the methana- ition to high-voltage power lines and their pylons in the tion step reduces the efficiency down to 64%. For the population [3,4]. whole process from renewable energy to the storage in the gas grid, an efficiency of approximately 63.6% can be Project description reached [2]. At this stage, the highly developed natural The joint research project «Storage of electric energy from gas grids in Germany can be used for the transport of renewable sources in the natural gas grid-water electrolysis excess energy. The German network for natural gas of- and synthesis of gas components» focused on a concept fers a huge storage capacity taking into account the high for the conversion of electrical energy from renewable energy density of compressed methane. Moreover, the sources into methane (power to gas). A schematic over- gas grid allows for a nearly time-independent storage. view of the whole concept is shown in Figure 3. Compre- The natural gas grid is hence very well suited for balancing hensive studies and investigations into this concept taking the transport bottlenecks of the power grid caused by ex- into account technical and economical aspects have been cess energy production in the northern part of Germany carried out by a consortium consisting of various scientific and an elevated energy demand in southern Germany. In research institutes and industrial partners. contrast to this, a mere expansion of the German power This article focuses on the synthesis step (see Figure 3), grid cannot efficiently reduce the bottlenecks: It has been thus the conversion of carbon dioxide containing gases Figure 2 Sankey diagram for ‘power to gas’ storage technology. Schaaf et al. Energy, Sustainability and Society ( Page 3 of 14 Figure 3 Concept for the storage of renewable energy in a gas distribution system. to SNG. This reaction shall be carried out in a pressur- into processes where carbon dioxide and hydrogen are used ized methanation reactor. Carbon dioxide is taken from as a feedstock, such as the Fischer-Tropsch synthesis, the biogenous or other sources. The produced gas can be methanol synthesis, or the methanation [8-10]. upgraded to SNG to be fed into the natural gas grid. The methanation of carbon dioxide is an exothermic The present article is basically split into two main catalytic reaction and is typically operated at tempera- parts: In the first part, the results of a detailed experi- tures between 200°C and 550°C depending on the used mental investigation into the methanation process are catalyst: presented. A lab-scale pilot plant was used to determine þ ⇆ þ Δ ¼ − = ð Þ both the suited catalysts, which are primarily designed CO2 4H2 CH4 2H2O rH 165 kJ mol 1 for CO methanation, and the optimum operation condi- A two-step reaction mechanism is assumed for this re- tions for the methane synthesis step for CO2 in a fixed action. In the first step, carbon dioxide and hydrogen are bed adiabatic reactor. This data cannot be found in the converted to carbon monoxide and water via the water- literature. gas shift reaction: The second part then addresses the scaling-up of the methanation process from the lab-scale to an industrial CO2 þ H2⇆CO þ H2OΔrH ¼ 41 kJ=mol ð2Þ scale. Different plant concepts have been compared to each other taking into consideration technical and economical In the subsequent reaction, methane is formed from aspects. This part also includes rough cost estimations. carbon monoxide and hydrogen: CO þ 3H2⇆CH4 þ H2OΔrH ¼ −206 kJ=mol ð3Þ Methanation process Fundamentals Besides methane and water, also higher saturated hy- The methanation reactions of carbon monoxide and car- drocarbons can be found in the products [11]. The most bon dioxide were discovered at the beginning of the stable C2+ hydrocarbon is ethane and it is formed ac- 19th century [5]. Nowadays, these reactions are widely cording to applied in ammonia synthesis plants to remove traces þ : ⇆ : þ O Δ H ¼ − = of carbon monoxide that acts as a catalyst poison for CO2 3 5H2 0 5C2H6 2H2 r 132 kJ mol the ammonia synthesis catalyst [6]. Also refineries and ð4Þ hydrogen plants make use of the methanation reactions for the purification of hydrogen (removing the carbon Under certain process conditions, carbon precipitation monoxide) [7]. With today’s increasing interest in produ- can occur according to following reaction mechanism: cing SNG from (cheap) coal via gasification, the percep- CO2 þ 2H2⇆C þ 2H2OΔrH ¼ −90 kJ=mol ð5Þ tion of the methanation process has changed completely: Originally considered as a pure gas cleaning technology, A suitable quantity to estimate which of the above prod- it is today viewed at as a major chemical synthesis ucts can be expected at a certain process temperature process. Another reason for the increasing importance is the Gibbs free energy ΔG. Figure 4 summarizes the of the methanation process is the need for storing excess development of the Gibbs enthalpy with temperature electrical energy from renewable sources.

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