It almost sounds too good to be true – power generation by microbial fuel cells utilising wastewater and residual products from the pretreatment of straw! But that is precisely the object of a new project launched by the Biosystems Department and the Fuel Cells and Solid-State Chemistry Department on Risø DTU.
Denmark could lead the way
Microbial fuel cells are still a small field of research. Worldwide, there are perhaps a total of four groups within this field, but increasing focus on renewable energy and on reducing CO2 emissions has suddenly made this project more interesting.
The combination of improved utilisation of plant materials and the simultaneous elimination of wastewater is perfect. If the project can assist in increasing knowledge about this type of fuel cells and related issues, Denmark could become the leading country within research into this new and attractive energy source.
The microbial fuel cell
A microbial fuel cell consists of two vessels separated by a membrane. The two vessels are connected by two electrodes allowing the electrons to travel and power to be generated.
The basic principle is that nutrients are added to one of the vessels, e.g. sugar and acetic acid. These are converted by micro-organisms that exist naturally in wastewater. The decomposition creates hydrogen ions and free electrons. The hydrogen ions travel through the membrane into the second vessel where they combine with oxygen or nitrate (NO3-) to form the residual products water or nitrogen.
This provides for a simple and sustainable system which utilises the metabolic conversion of the microorganisms to generate power.
Schematic of setup.
Non-toxic transport substances
The micro-organisms from the wastewater will often attach to the electrode itself, making it quick and easy for them to get rid of their excess electrons, but they will also be present in the solution proper. There is therefore a need for a substance to transport the excess electrons from the decomposition in the aqueous solution to the electrode.
Normally, this type of transport substances, also referred to as mediators, are toxic and thus not appropriate for use. However, senior scientist Anne Belinda Thomsen had the idea of using the substance that assists in making the straw rigid, i.e. lignin. Lignin is the main component of the humus present in the ground, and it is a well known mediator.
“We conduct research in second-generation bioethanol at Risø DTU, and during the pretreatment of straw, an aqueous solution is formed, containing cellulose, hemicellulose, acetic acid and lignin. The cellulose (C6 sugars) can be used directly in the ethanol production, but hemicellulose (C5 sugars) must be fed more energy in order to be utilised. It is therefore interesting to see whether hemicellulose and acetic acid might be used for power generation instead,” explains Anne Belinda Thomsen.
First milestone reached
Over the past year, Anders Thygesen, post doc on the project, has managed to build small microbial fuel cells that are currently being tested. He measures how constant volumes of glucose (C6 sugar) and acetic acid are converted by microorganisms from wastewater with and without the so-called mediator.
“So far, it would seem that acetate is able to create a much higher voltage than glucose, and this is good news to us, because we would rather use the glucose to produce second-generation bioethanol,” explains project scientist Anders Thygesen and continues, “With a transport substance added, I have observed a voltage of approx. 0.6 V, and this is sustained for considerably longer using acetate as a nutrient compared to glucose.”
As with other fuel cells, they would have to be interconnected to create a system generating the desired 220 V. Over the next two years, Anders will be conducting experiments with different concentrations and substances from the pretreatment of straw instead of the test substances with which he is currently working.
For further information
The name of the project is “Microbial fuel cells used in direct conversion of lignocellulosic waste to energy” and is supported by the Danish Agency for Science Technology and Innovation. It is a cross-scientific project that utilises the knowledge gained through research at both DTU and Risø DTU, including Risø’s in-depth knowledge about fuel cell techniques.
If you want to know more, please contact project scientist Anders Thygesen on tel. +45 4677 4279 or by email: anders.thygesen@risoe.dk, or senior scientist Anne Belinda Thomsen on tel. +45 4677 4164 or by email: anne.belinda.thomsen@risoe.dk. |