The biobattery: Turning sewage sludge into electricity and engine oil

Biogas plants are an important element for decentralized energy supply. They produce electricity from renewable resources and can compensate for highly fluctuating wind and solar energy. There are already 8,000 plants in operation in Germany with an electrical output of 3.75 gigawatts in total, that is the equivalent to roughly three nuclear power plants. However, the plants have several disadvantages too: they only process a limited range of organic substances and are in competition with the cultivation of food plants.

Producing electricity, oil, gas and biochars

Scientists from the Fraunhofer Institute for Environmental, Energy and Safety Technology UMSICHT have now succeeded in considerably improving the efficiency of biogas plants. The biobattery process developed by them not only supplies electricity and heat but also high quality products such as gas, oil and vegetable carbon. These can be utilized as required, for example to produce electricity, as marine or aviation fuel, as an admixture for fuels or as a fertilizer. If further processed they even provide basic substances for the chemical industries.

The biobattery is modular and consists of a pool of environmentally-friendly technologies such as biogas plants, thermal storage, carburetors and engines to produce electricity. The heart of the concept is thermo-catalytic reforming (TCR®). With this the experts convert carbons out of organic material, for example fermentation residues from biogas plants and bioethanol production, industrial biomass waste, sewage sludge, straw, scrap wood or animal excrement. The result: oil, gas and biomass cokes. "The particular advantage of the biobattery is that we can utilize a number of raw materials which would otherwise have to be disposed of often at great cost," explains Professor Andreas Hornung, Director of UMSICHT at the Institute Branch in Sulzbach-Rosenberg.

Pilot plant processes biogenous residues

Researchers are demonstrating that this also works in practice at a pilot plant that utilizes roughly 30 kg of fermentation residues every hour. The raw materials first pass through a sluice without oxygen into a continuously rotating screw. There the material is heated up and broken down into biochar and volatile vapours. The vapours are reheated and then cooled down again. In the process it condenses into a liquid, which contains biooil and water. Researchers separate the high quality oil to reuse it. The resulting gas is purified and collected.

The liquid, gaseous and solid products can be reutilized in various ways. The oil can either be processed into marine and aviation fuel or used in a combined heat and power plant, as can the gas, to produce electricity and heat. The separated process water contains numerous short-chain biodegradable carbon compounds. It can be fed back into the biogas plant to increase the methane yield. The biochar is ideal as a soil conditioner.

But does the biobattery also work efficiently?

"The plant converts more than 75 percent of the energy efficiency into high quality energy sources in a robust, continuous process. The efficacy can be improved even more if mobile latent heat accumulators are used," explains Hornung. One particular advantage of the biobattery is that the system can be gradually expanded. "This is very interesting for operators in terms of finance. High investments are not required at the start, as our profitability analyses prove," explains Hornung. Susteen Technologies GmbH, a spin-off from UMSICHT, is already implementing the biobattery concept in practice together with cooperation partners in Germany and abroad at major pilot plants.