Bronkhorst
Application note

Power to gas

‘Power to gas’ refers to the conversion of electricity to gas. A well-known example is using electricity for the electrolysis of water to generate hydrogen, with oxygen as a by-product. This hydrogen can be used as it is – e.g. to feed fuel cells - but it can even go further. If the hydrogen subsequently reacts with carbon dioxide to methane, the latter can be introduced to the natural gas network. In countries like Germany and the Netherlands, there is an existing infrastructure to transport natural gas trough pipes from the place where it is extracted or processed to the place where it is used – for central heating in houses or for the stove in the kitchen. 

There are several German companies developing technologies to have bacteria excrete methane based on hydrogen and carbon dioxide as their ‘food’. This can be considered as a natural power-to-gas application for storing green electricity and a treatment technology for process gases containing carbon dioxide (CO2) as well. Nature meets technology! The methane (CH4) is to be fed into the natural gas network which is already present. These companies requested Bronkhorst’s distributor, Wagner Mess- und Regeltechnik, to supply mass flow instruments to provide the bacteria with enough hydrogen and CO2 as well.
 

Natural power to gas

Application requirements

A major aim of the customers is to optimise the efficiency of the bacteria to convert hydrogen into methane. The process is first tested on a small scale in order to develop the optimal bacteria type. Optimisation parameters for this process are H2 and CO2 flow, type of bacteria, temperature and pressure. To this end, a solution is needed to accurately and reproducibly supply hydrogen (and carbon dioxide) to the bacteria-containing reaction vessel.

Important topics

  • Reproducibility
  • Operating under ATEX Zone 2 conditions
  • Automation


Process solution

The Bronkhorst solution to supply hydrogen to the reaction vessel is an IN-FLOW gas mass flow controller with a typical flow rate ranging from 10 to 50 ln/min hydrogen. The reactor vessel is operated under anaerobic conditions at a temperature of 65 °C and pressures between 1 and 10 bars. Most type of bacteria feel most comfortable at atmospheric pressure, others at a higher pressure. The bacteria metabolise hydrogen and carbon dioxide and convert them into methane. The lab in which the experiments are conducted is classified as a hazardous area ATEX zone 2, therefore, the instruments need an ATEX zone 2 approval.  

The Bronkhorst instrument is controlled via Profibus. A setpoint is given, and the actual value is measured and recorded. Data are logged so that the test values are documented. Experiments typically run for 48 hours and automation is important here. When an error occurs, such as a serious deviation between the setpoint and the actual value, actions have to be taken - even overnight. The advantage of electrically controlled mass flow controllers is that the hydrogen supply can be stopped easily in case of a failure or emergency, without human interference. 

Flow scheme Power to Gas
Flow scheme

As part of the process to determine at which conditions the bacteria metabolise best, the reproducibility of the test results and hence of the mass flow controllers is important.

Although the process scheme looks quite simple with two gas flows going in, the pressure maintained at a certain value and one gas flow going out, some end-of-pipe processing is required. At the backside a moist methane flow leaves the bacteria-containing vessel, containing among others HCl in ppm range that will damage transport pipes and will have to be removed. Mercaptan must be added to give natural gas its distinctive smell, and then it is ready to be fed into the natural gas network.

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