Direct utilization of biogas without storage

Within the framework of the project DEUS 21 funded by the BMBF the Fraunhofer IGB is operating a semi-centralized plant for the anaerobic purification of water with integrated membrane filtration in Knittlingen. The plant generates biogas from the organic compounds contained in the wastewater and kitchen waste from approximately 170 households added via a vacuum system. The quantities of biogas achieved per day range between 8000 and 10,000 liters. As the methane content in this biogas lies between 60–70 percent, it constitutes a valuable energy source the utilization of which can reduce the demand from fossil energy carriers.

The biogas quantities generated in conventional biogas plants are considerably larger than the biogas generated at the Knittlingen plant. Plants for the utilization of relatively small amounts of biogas have therefore not been available on the market. Similarly small plants for the utilization of natural gas can not be utilized for biogas due to the different composition and the lack of accreditation. To be able to utilize the biogas with a larger aggregate, it would have to be stored in a reservoir. This is critical for explosion prevention reasons and because of its location in a residential area. In addition, for pressure storage it would be necessary to process the biogas. Therefore the Fraunhofer IGB has developed an aggregate together with C-deg GmbH in Kiel which allows for the energetic utilization of small quantities of unpurified biogas.

As biogas can not safely be provided at all times due to the small size of the wastewater plant as well as potential variations in the filling level in the reactors, combustion is handled by means of a support flame which is fed with natural gas. Moreover, combustion is fed with an exhaust air flow from the filtrate ventilation of the wastewater treatment plant. This air flow also contains 2–3 percent methane which escapes the wastewater treatment plant dissolved in the filtrate. Yet, due to the high greenhouse effect potential the methane must be prevented from escaping into the atmosphere. At the same time the exhaust air flow transports oxygen and substances causing an odor nuisance such as hydrogen sulphide and ammonia, for combustion. A controlled combustion process ensures that the best possible conversion rate for these pollutants is achieved. Therefore, the flow of supply air varies depending on the amount of oxygen required, which in turn depends on the biogas amounts added. The exhaust gas generated by the combustion process is fed to a heat exchanger in which the heat energy of the incinerated gas is transferred to the heating circuit of the wastewater purification plant to heat the mesophilic digestion of the solid matter and the regenerate stream of the plant for nitrogen recovery.

The main advantage of the incineration of the biogas and the process air is the elimination of the greenhouse gas methane and the reduction of pollutants such as hydrogen sulphide and ammonia. The plant also shows that even small quantities of biogas can be used in a useful way for the generation of heat. The addition of biogas can occur under atmospheric pressure. The biogas is converted in the time and the quantity in which it is created. There will therefore be no need for compressors, gas processing plants and gas tanks. The biogas plant will also not have any problems with negative pressure as is the case with active biogas suction. These advantages facilitate the easy and cost-efficient connection to various biogas plants.

Technical data

• Controlled combustion depending on biogas supply,
  nominal capacity: 6 kW
• Efficiency of heat exchanger: 60–70 percent
• Temperature combustion chamber: 1000–1200 °C

Within the scope of the project presented here it was possible to develop a reliable prototype which can be further optimized during future implementation. If necessary, the technical design allows for a modification of the heat exchanger. In this way, aggregates such as a Stirling engine can be used to generate additional electricity. It is also possible to utilize heat exchangers with an increased efficiency. The heat output of the burner can be adapted to meet the relevant requirements. In the case of biogas plants which continually produce biogas in sufficient quantities a natural gas flare may not be necessary.

Funding
We would like to thank the German Federal Ministry of Education and Research (BMBF) for funding the project “Decentralized Urban Infrastructure Systems DEUS 21”, promotional reference 02WD0850.

Project partner
Fraunhofer Institute for Systems and Innovation Research ISI, Karlsruhe, Germany; City of Knittlingen, Germany; Eisenmann Maschinenbau KG, Holzgerlingen, Germany; EnBW Energie Baden-Württemberg AG, Karlsruhe, Germany; Kerafol GmbH, Eschenbach, Germany

Further information
www.deus21.de