Flow measurement by converting biogas to green gas

The Netherlands is facing the major task of transitioning to a sustainable society, with much lower emissions of the greenhouse gas carbon dioxide, and without natural gas of fossil origin from the province of Groningen (NL). Solar and wind are going to play an even greater role in electric power generation and the related electrification of society. The Hanze University of Applied Sciences in Groningen is conducting research and Bronkhorst's flow instruments are playing a role in this.

The role of biogas within the energy transition

In this energy transition, there is a major role for ‘green gas’ – this is biogas upgraded for the Dutch natural gas grid. A prerequisite is that enough of it becomes available. We are speaking to Gert Hofstede, a lecturer-researcher at the Hanze University of Applied Sciences in Groningen, who has immersed himself in the matter. Flow instruments play an important role in his research into reprocessing biogas into green gas.

Biogas is a mixture consisting mainly of methane (CH4, 50-60%) and carbon dioxide (CO2, 40-50%) and is made by fermentation (‘anaerobic decomposition’) of organic biomass such as roadside grass or vegetable, fruit, and garden waste. Biogas is considered a renewable energy source because it is produced from organic waste but contains too much carbon dioxide for practical use, for example to be added to the natural gas grid. The process of upgrading biogas to green gas is mostly done by removing the carbon dioxide present but can also be done by converting the excess carbon dioxide into methane. We ask Gert how this can be done.

Biogas into green gas

‘My PhD research is about breaking down biomass as efficiently as possible, to eventually turn it into bioenergy. There is an abundance of biomass, but we cannot do anything with much of it. My job is to make bioenergy from these ‘leftovers’. Biogas has a ‘poor’ quality, and we want to upgrade this gas to green gas. For that, we pulled a 'trick' by building a bioreactor to which we add green hydrogen. I have specialised micro-organisms in the reactor that convert H2 and CO2 into methane (4H2+CO2 CH4 +H2O). So, suddenly you have much more methane (almost 2x as much) from the same amount of biomass. Of course, ‘green’ hydrogen – generated by electrolysis of water with electricity from the sun or wind – is best. But... what is a practical way of adding hydrogen to the bioreactor so that you know exactly how much you add?’

‘Upgrading biogas means going from a methane content of just 60% to over 88-90%, after which the quality is compatible with low calorific Dutch natural gas and can be injected into the natural gas grid. If you ‘bubble’ hydrogen directly from a gas cylinder with a pressure valve into the liquid containing the micro-organisms, you have no idea how much you put in. What's more: hydrogen dissolves badly in 40°C water with sludge, and does not get to the micro-organisms properly, so it does not work.’

Flow meter used for biogas setup

Adding hydrogen

Gert continues: ‘Eventually, for the lab-scale reactor, we came up with a clever solution of putting a silicon tube in the reactor, and then using a mass flow controller to add hydrogen. The hose was filled with hydrogen that diffused through the wall of the hose, adding very small bubbles that were barely visible – but they were effective. By also stirring the reactor, the micro-organisms could reach the available hydrogen. This went very well on a small scale – but scaling up would require kilometres of hose. Time for the next step: the trickle-bed reactor.’

‘In a trickle-bed reactor, the micro-organisms are immobilised: they are trapped on a porous support with a very large surface area, along which liquid or gaseous components are transported. The micro-organisms we use consist of many different species, and depending on the hydrogen to carbon dioxide ratio, we only facilitate those that can make CH₄ from CO₂ and H₂. Theoretically, you would expect four molecules of hydrogen to one molecule of carbon dioxide to make methane, but in practice you need a little less hydrogen, because carbon dioxide is also used to grow the micro-organisms. So, the ratio is highly important.’

Mass flow controllers for hydrogen

‘We started with one mass flow controller calibrated for a hydrogen/carbon dioxide ratio of 80/20 volume %. It soon turned out that in doing so, we were introducing too much hydrogen, facilitating the wrong micro-organisms, at the expense of methane production. So, we thought: we must be able to add the two gases, hydrogen and carbon dioxide, separately – allowing us to add any ratio we want. We bought a separate mass flow controller (from the EL-FLOW Select series) for hydrogen and one for carbon dioxide.’

Gert is now also using a third mass flow controller, to add nitrogen, for example. This lowers the partial pressure of hydrogen, which appears to improve the stability of the process. His research is now in a further scale-up phase, in which he will scale up the reactors he has built from lab scale to demo scale and even larger.