Clean biogas – measurable everywhere

The market for biogas is growing. According to the Swiss Federal Office of Energy, Switzerland fed 471 gigawatt hours of this fuel into the natural gas grid last year – roughly twice the amount fed in ten years ago. This comes with an increase in the need to measure impurities in the biogas quickly and reliably, because strict quality criteria apply to this green gas.

Researchers at PSI’s Center for Energy and Environmental Sciences have now come up with a solution to this problem. The analytical method they have developed can simultaneously detect the two most critical impurities in biogas: sulphur compounds and siloxanes. They have now presented their method in the journal Progress in Energy.

Nationwide production

More than 160 biogas plants throughout the country produce this valuable gas mixture from waste, slurry and manure. In addition, hundreds of wastewater treatment plants use digesters to process sewage sludge and turn it into sewage gas, a subtype of biogas with a similar composition.

This green gas consists of methane (50 to 75 percent) and carbon dioxide (at least 25 percent). Removing the carbon dioxide produces biomethane, which can be fed into the natural gas grid. However, biogas – and therefore biomethane – can contain many impurities, at concentrations of a few parts per million. “Despite their tiny concentration, these cause huge problems,” says Ayush Agarwal, who analysed biogas as part of his doctoral thesis at PSI and is the first author of the study.

Organic sulphur compounds, for instance, are notorious contaminants: they form when bacteria break down proteins containing sulphur atoms. Siloxanes, on the other hand, are silicon-containing compounds, which are used in shower gels, for example, to produce a pleasant sensation on one’s skin. These siloxanes are flushed down the drain with the shower gel and end up in the wastewater treatment plants – and ultimately in the biogas.

Pure poison for fuel cells

When biomethane is burned to produce energy – for example, in a gas boiler – the siloxanes undergo an extremely undesirable reaction. They form silicon dioxide, a component of sand and one of the most stable compounds on Earth. “This clogs up the burner, meaning that the system needs more energy to heat the same amount of water,” explains Agarwal. Like a kettle with a build-up of limescale.

Until now, both siloxanes and organic sulphur compounds have also prevented the use of biomethane in fuel cells. Fuel cells produce electricity from gases with a high energy content, but sulphur compounds are pure poison for them. Consequently, fuel cells cannot currently run on biomethane. Impurities also interfere with the process of concentrating biogas to form biomethane, which can then be fed into the gas grid. In short, “even small traces of siloxanes and organic sulphur compounds are harmful.”

Measuring to improve

In Switzerland, as in the rest of Europe, strict limits apply to sulphur compounds and siloxanes in biogas. Meeting these limits is a prerequisite for feeding biomethane into the public gas grid and for operating biogas plants as a source of fuel.

Larger biogas plants have cleaning systems that remove unwanted substances from the gas. Operators use analytical equipment to measure the composition of their biogas, allowing them to assess the effectiveness of their cleaning systems. Having sound analytics is therefore essential for the entire biogas system to function properly: “You can only improve something if you can measure it,” says Agarwal.

As part of his doctoral thesis at PSI’s Center for Energy and Environmental Sciences, Agarwal developed a robust analytical method that can detect siloxanes and organic sulphur compounds simultaneously – down to traces of fifteen parts per billion, that is to say: just fifteen molecules of the impurity in every billion molecules, a truly tiny amount.

Boost for the energy transition

The researchers at PSI have also developed a practical solution for small biogas plants without on-site analysers. Biogas samples can be collected using a mobile device that dissolves the gases in a liquid. Even trace amounts of impurities have been shown to remain in the liquid for at least 28 days – long enough to send the samples to an analytical laboratory to be quantified.

The universal applicability of the analytical method means that it can be used extensively throughout the country, thereby encouraging the use of biogas. “This is a good example of how we conduct applied research at PSI, providing concrete solutions to current challenges,” says Christian Ludwig, also a researcher at the Center for Energy and Environmental Sciences at and co-author of the study.

How the method works

First, a gas chromatograph separates the components in the biogas. Next, these are recorded one by one using a method called “inductively coupled plasma mass spectrometry” (ICP-MS), in which the components of the sample are vaporised, broken down into their atomic constituents and converted into charged particles. Finally, the mass spectrometer records the isotopes of the individual elements and quantifies them.

The trick is that the device only records very specific, previously selected elements while ignoring all others. This allows sulphur and silicon to be detected even in very small quantities, alongside a host of other compounds present in the biogas. “To our knowledge, this is the first method of its kind that can measure sulphur and silicon compounds simultaneously,” says Agarwal.

Text: Brigitte Osterath

About PSI

The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of future technologies, energy and climate, health innovation and fundamentals of nature. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 2300 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 450 million. PSI is part of the ETH Domain, with the other members being the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Laboratories for Materials Science and Technology) and WSL (Swiss Federal Institute for Forest, Snow and Landscape Research).

Contact

Dr. Ayush Agarwal
ayush.agarwal@psi.ch
[English]

Prof Dr Christian Ludwig
PSI Center for Energy and Environmental Sciences
Paul Scherrer Institut PSI
+41 56 310 26 96
christian.ludwig@psi.ch 
[German, English]

Original publication

Simultaneous quantification of siloxanes and condensable sulfur compounds in biogas for energy applications
Ayush Agarwal, Laura Torrent, Julian Indlekofer, Sylvain Bouchet, Lucy P. Culleton, Serge M.A. Biollaz, Christian Ludwig
Progress in Energy, 27.11.2025 (online)
DOI: 10.1088/2516-1083/ae1923