Renewable fuels must replace fossil fuels shortly. One such energy source is biogas.
What is Biogas?
Biogas is an alternative to fossil fuels that contains methane gas, a flammable gas we can use as an energy source for heat, transportation, and electricity production. Like in cows, biogas is produced by bacterial digestion of biological substrates, but unlike cows, we can create biogas on a large industrial scale. The substrates used can be in the form of biological waste products from agriculture and aquaculture industries.
How is hydrogen involved?
The biological digestion is achieved within an anaerobic chamber known as a digester and requires a consortium of many different bacteria. Just as in nature, the bacterial consortium within the digester is dynamic. Their dominance (or lack of) within this ecosystem can change concerning the chemical environment of the system. One such chemical that can become present within the digester is hydrogen, which can be produced in low amounts. Although at first, this may seem to be an added benefit as hydrogen can also be used as a fuel, it is not. This is because once the dissolved hydrogen level within the liquid-phase of the digester reaches a level higher than 40 nM (a tiny amount), it starts to inhibit the production biogas.
Therefore, in the case of anaerobic digestion, the accumulation of dissolved hydrogen in the liquid-phase can cause the reduction of activity for bacteria that are essential for biogas production. This can allow other bacteria to dominate, deteriorating the biogas production. It is therefore favourable to have a warning if such an event may occur soon.
Multiple biofuel tanks. Photo: Colourbox
How do we sense hydrogen in anaerobic digesters?
Although there are many sensor technologies available for detecting hydrogen, they require placement at the top of the digester in the gas-phase, as opposed to the liquid phase. For such gas-phase sensors, calculations can be made from the detected hydrogen concentration to determine the dissolved hydrogen concentration within the liquid phase. It is a semi-complex equation involving something known as Henrys Law, but we can leave this out for now. The problem with this method is that the transfer of dissolved hydrogen from liquid to gas is very slow. Therefore, once you have detected an increase in hydrogen in the gas-phase, it may already be too late!
How can we measure hydrogen in the liquid phase?
The low concentration of dissolved hydrogen required to cause inhibition of biogas production makes it hard to measure. Therefore, what is needed are hypersensitive sensors that can be placed within the liquid-phase of the digester. One possibility is to develop optical fiber based sensors that can utilise surface plasmon resonance. These have an increased sensitivity and can be used within the liquid-phase of the digester to collect data in real-time.
Surface plasmon resonance is a complex physical property of certain metals. A simplified explanation would be that specific wavelengths of light (colours) interact with the surface electrons of metals such as gold when the incident at a particular angle. In the case of an optical fiber coated with gold, the light travelling down the fiber will interact with these surface electrons, and cause a dissipation of a specific wavelength of light. By adding a coating of another metal over the gold coating, the particular wavelength of light dissipation can be modulated respective of the state of the second metal coating, resulting in a colour change of the light observed at the end of the optical fiber.
Surface Plasmon Resonance in an Optical Fiber. The oscillation of surface electrons of gold will interact with wavelengths (colours) of light that oscillate at the same frequency before (A) and after (B) hydrogenation.
To measure hydrogen, we, therefore, require a metal layer that is sensitive to hydrogen. Palladium is such a metal. It readily interacts with hydrogen to form palladium hydride. This change from palladium to palladium hydride causes a change in the specific electron oscillation on the surface of the gold layer that dissipates light. Therefore, we can have a colour change of the light monitored at the end of the optical fiber representative of dissolved hydrogen concentration within the liquid-phase of the anaerobic digester (Figure 1).
Future application
It is hoped that these sensors will allow the early detection of dissolved hydrogen in the anaerobic digesters used for biogas production. This will increase the stability, and therefore efficiency, of the biological process, improving the validity of biogas as a widely used, future renewable energy source.
Photographs in this blog entry by Colourbox
This blog post was written by Jacob Lamb, Postdoctoral researcher at NTNU – Institute of Electronic Systems / ENERSENSE.
See also Lamb’s personal website.