Browsing by Subject "Methanation"

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Biologische Wasserstoffmethanisierung in Hochdruck-Rieselbettreaktoren für Power-to-Gas-Konzepte
(2018) Ullrich, Timo; Jungbluth, Thomas
In order to achieve climate protection targets, intermittent and decentralised energy sources such as wind power and photovoltaics will be expanded in the future. However, the power grids are not designed for the large-scale expansion and connection of different decentralised and fluctuating generation plants. This represents a major challenge for grid stability and requires an increasing expansion of energy storage. Power-to-Gas technology, a process for converting electrical energy into chemical energy, will play a central role in this process. In this two-stage process, hydrogen is first produced by electrolysis, which then reacts with carbon dioxide to form methane. It can be stored and transported in the natural gas grid almost indefinitely and can be used flexibly in a wide variety of applications. In addition to the chemical-catalytic methanation of hydrogen, there is also the biological methanation process. Characteristic features are a flexible load change behaviour and a marked robustness regarding the educt gas composition. Compared to chemical-catalytic methanation, however, the gas flow rates are significantly lower, which is the greatest challenge of this process. For this reason, the aim of this work was to optimize the performance of trickle-bed reactors for biological hydrogen methanation. The focus was on improving the gas-liquid-mass-transfer as described in the literature, but not yet which has not yet been investigated in the context of this promising concept. In an automated and continuous test plant, the operating pressure was initially varied in stages of 1.5, 5 and 9 bar in the first publication. With increasing pressure, conversion rates were improved and gas quality increased by 34%. Furthermore, the circulation of the process liquid to the trickling bed of the reactors was paused for periods up to 1440 min in the second publication. As the circulation pause rose, there was a noticeable increase in all performance parameters with maximum methane contents > 97 Vol.-%. Finally, different temperature levels of 40 - 55 °C were also examined. In spite of the continuous increase in gas volumes in the three publications, the performance parameters increased again. Overall, the combined optimization measures more than doubled the output with an MFR of 4.28 ± 0.26 m3 m-3 d-1 to 8.85 ± 0.43 m3 m-3 d-1, while simultaneously increasing the methane content in the product gas. Periodical analyses of the process liquid, especially the acid concentrations, as well as the stable conversion rates indicated a stable biological process in all experiments. The tests were done with three identical reactors, underlining the high degree of reproducibility. It was noticeable that the microorganisms quickly adapted to the changing operating parameters within a maximum of 24 hours. The performance increases could thus be related to the successful increase in the gas-liquid-substance exchange rate and not to a changed microorganism concentration or selection. The studies have also revealed further optimisation potential. In particular, the properties of the process liquid with regard to pH and nutrient composition should be the subject of further investigations. Thus, the present study not only successfully demonstrated the goal of increasing performance; with stable and uncomplicated operation over several months and a wide range of operating parameters, it also demonstrated that trickle bed reactors for the biological methanation of hydrogen are a reliable, flexible and thus promising concept in the context of power-to-gas applications.