news / 2016-10-10

Utilising surplus electricity for biological methanation

The tubular reactor forms the centrepiece of the test rig for biological methanation. (Source: DBI Freiberg)

Surplus electricity can be used to generate hydrogen by means of electrolysis. Part of this hydrogen can be directly mixed with the natural gas and fed into the natural gas network. The remainder can then be used for downstream methanation. In the test, a newly developed biological process has shown promising results. The system is suitable for small, decentralised installations and requires little energy.

The power grid of the future needs large-scale storage systems. These will enable the sometimes large amounts of surplus electricity generated from wind turbines and photovoltaic systems to be stored for future needs. A technical option for this are Power-to-Gas plants. These generate hydrogen using the surplus electricity. This can be added to the natural gas up to a maximum concentration. The hydrogen that cannot be deposed of in this way can then be methanised, whereby carbon dioxide –for example from the exhaust gas from conventional power plants – is added to the hydrogen. This then creates synthetic methane in a chemical or biological process. This can be fed into the gas network without limiting the quantity and replaces natural gas there.

 

In collaboration with partners as part of the BioRePow research project, scientists from Gastechnologisches Institut gGmbH from Freiberg in Saxony have developed a biochemical methanation method based on a fermentative approach. For this purpose, the researchers from Saxony have developed an innovative tubular reactor that manages without electricity-intensive components such as circulation, agitators and gas separation. This increases the energy efficiency of the new process. In the tubular reactor, the methanation takes place by feeding CO2 and H2 through it once. The process is suitable for small and decentralised systems, but can also be scaled up as desired.

Permanent operation of the tubular reactor

The facility in Freiberg is known as an ex-situ concept. Here the hydrogen, together with the CO2, is converted using fermentation in a separate reactor. If, on the other hand, hydrogen were to be fed into an existing biogas reactor, this would be known as an in-situ concept. Two factors are decisive for the quality of the methane formation: the microorganisms’ living conditions such as the nutrient supply and temperature must be optimal. Gas solubility is the second aspect. The research project is therefore focussing on the nature of the gas supply and the resulting higher biological availability.

 

On the test rig, the scientists in Freiberg have investigated the influence of basic parameters such as the temperature, pressure, composition and volume flow of the feed gas. The results have been incorporated into a mathematical model. With the optimised parameters determined there, the researchers have adapted the gas supply system with a view to achieving high conversion rates and methane levels. The H2 and CO2 only spend about three seconds in the tubular reactor, and a conversion rate of more than 80 per cent has already been achieved in the first project phase. The conversion rate is based on the proportion of the source gases (H2 and CO2) actually converted into methane, whereby the tubular reactor is operated continuously. It is not necessary to re-circulate the starting gases through the reactor. The reaction proceeds exothermically and thermophilic mixed cultures are used.

Project participants

In addition to Gastechnologisches Institut, Fraunhofer UMSICHT and Engler-Bunte-Institut from Karlsruhe are also working on the new reactor. The German Federal Ministry for Economic Affairs and Energy has funded the project as part of the Cooperative Industrial Research (AiF) programme. It forms part of the programme entitled “Development of innovative, highly efficient technologies for the processing of biogas/biomethane across the entire value-added and exploitation chain (inTeBi)”.

Project management

DBI - Gastechnologisches Institut gGmbH Freiberg
09599 Freiberg

Web: www.dbi-gti.de
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