Quantum light source for eco-friendly production of biogas

Until now, a very common by-product of gasification -- water vapor -- has been a particular headache. To control gasification efficiently, it is important to know the water content of the product gas as accurately as possible. However, conventional methods make it difficult to measure the water content. In a collaboration between process engineering and photonics at TU Wien (Vienna), this problem has now been solved using a very special type of light source: terahertz radiation from a quantum cascade laser. State-of-the-art quantum technology now supports environmentally friendly biomass recycling.

Conventional measurements are not enough

"Many chemical components of the product gas can be detected using infrared light," explains Florian Müller, who is researching renewable carbon systems as part of the CO2Refinery PhD programme at the Institute of Chemical, Environmental and Biological Engineering at TU Wien. "Different molecules absorb different wavelengths of infrared light. By measuring which part of which wavelength is absorbed by a sample, it is possible to determine whether the sample contains a certain substance or not."

However, this is hard to do with water vapor, a by-product which is particularly important for the gas production process. "When you convert biomass into gases, you end up with a complicated gas mixture that contains not only water vapor but also many different hydrocarbons," says Florian Müller. And some of them absorb infrared radiation at exactly the same frequencies as water. This means that it is not possible to say exactly which substance is responsible for the absorption, and therefore the water content in the product gas cannot be accurately determined. You can cool a sample of gas and then measure the amount of condensed water -- but this takes time. It is not possible to react quickly to fluctuating water concentrations, and this makes efficient operation difficult.

TU Wien develops terahertz radiation sources

At the same time, however, Michael Jaidl was conducting research at the Institute of Photonics at TU Wien on laser beams in the terahertz range, i.e. radiation with a wavelength slightly longer than the infrared radiation commonly used today for spectroscopic measurements. Michael Jaidl and Florian Müller are old friends who have known each other since school days -- and so they came up with the idea of combining their research areas.

Michael Jaidl was able to show that frequencies in the terahertz range can be found that are specifically absorbed only by water molecules and not by the many other substances that exist in significant concentrations in the product gas of a biomass gasification plant. The problem of detecting water vapor can therefore be solved by using terahertz radiation instead of the usual infrared radiation.

Terahertz radiation is difficult to produce. At TU Wien, tricks from quantum technology are being used to produce quantum cascade lasers -- tiny semiconductors with a tailor-made geometric structure on the nanometre scale that ensures that only radiation of a very specific wavelength is emitted when an electrical voltage is applied. This quantum cascade laser requires its own cooling, but the two researchers have succeeded in developing a compact, portable device that can reliably measure the water content in hot product gases using a terahertz beam.

First successful tests

"A key advantage of our method is that it provides reliable results over a wide range of water vapor concentrations and temperatures," says Michael Jaidl. "This is because the terahertz radiation we use is particularly strongly absorbed by water vapor -- this allows us to use a more compact setup. Another major advantage of the compact design is that the temperature in the measuring cell does not fluctuate as much, which reduces the risk of errors."

The fact that the new method works perfectly was demonstrated in gas production experiments using waste wood at the Getreidemarkt campus at TU WIen. Now the two researchers and their teams want to improve their technology even further: firstly, to make it even more handy and user-friendly, and secondly, to investigate whether other components of the product gases can be reliably detected using terahertz technology.