Biogas production


Bioextruder tunt material cycle


ADAP Biogas in Ahrenshagen near Rostock already built up a closed biogas cycle in 2005. In the ADAP cattle breeding group of companies, this starts with feed cultivation and extends to the supply of residential buildings with heat. At the heart of methane gas production is a bioextruder from Lehmann-UMT. Bioenergy is now a central component of the energy transition in Germany, which guarantees safe, controllable performance in the electricity and heat sector. In 2020, renewable energies provided more than 250 billion kWh of electricity, which could cover over 45% of the electricity demand, according to the Bundesverband Bioenergie. Biomass contributed one fifth to this electricity generation from renewable energies. For these 50.9 billion kWh, biogas including methane, sewage and landfill gas contributed 33.5 billion kWh, solid biomass 11.2 billion kWh and the biogenic share of the waste 5.8 billion kWh, the association continues. For example, greenhouse gas emissions of about 30.4 million tons of CO2 could be saved with the use of biomass in the electricity sector.


Closed cycle


ADAP cattle breeding has been pursuing this approach to energy production for almost twenty years: After the group of companies was founded in 1991, ADAP Biogas was also integrated into the network in 2005, where biogas has since been produced and recycled. In a closed cycle system that ranges from feed production to cattle breeding to the utilisation of waste products, ADAP cattle breeding with 800 cows and 500 young cattle is responsible for the main business of the association - cattle breeding and milk production - while ADAP Biogas generates electricity and heat in the secondary business, reports Richard Schulz, Operations Manager of ADAP Biogas. The starting material used for biogas production is cattle hard dist from our own holdings as well as silage from our own cultivation, which does not meet the quality requirements for feed. As thumb figures, almost 300 standard cubic metres of methane are to be expected per cow, almost 6000 per hectare of silage maize. "We work with two biogas plants that went into operation in 2005 and 2006 respectively," says the plant manager. After the biomaterial has been delivered, it comes into a doser that mechanically splits the material and feeds it to a bioextruder via a conveyor belt. So that the system is not damaged by foreign objects, the belt works with a stone filter system and a metal detector, which sort out corresponding substances. "After the biomass was hydrothermally developed in the bioextruder, it is entered into two fermenters with 2000 m³ of storage space each," says Schulz. For the bacterial decomposition processes, she stays here for 30 days and then goes to a Nachgärer again for 90 days. The methane, which forms during the decomposition processes in the fermenters and the Nachgärer, is finally withdrawn and fed to electricity generation by cogeneration. The empty biomaterials, from which methane no longer develops, come into a repository, from where they are applied as fertiliser to the fields or serve as bedding for the cows.


Motor of the circulatory system


From the beginning, the bioextruders from Lehmann-UMT were decisive for the degree of methane extraction in Ahrenshagen. The basic idea of the bioextrusion process developed in Jocketa is the hydrothermal digestion of particularly ligninated substances for energetic use. In order to be able to break down these substances efficiently, the lignin structures are broken up and thus the cellulose and hemicellulose chains are exposed and made accessible to microorganisms. In the bioextruders, in which two non-uniform, interlocked worms work in opposite directions, the biosubstrate is exposed to high stresses with rapidly changing pressure and temperature peaks. As a result, the material is plasticised, compacted and fibred at the same time. By squeezing, relaxing, heating and cooling the substrate in the rapid alternation between the snails, water repeatedly evaporates in the biomass - the cells are fibred by steam explosion. The split fibres now have a much larger surface area that can be better attacked by bioorganisms. The result is a significantly increased biogas yield. As soon as the fibred biomass in the Ahrenshagen fermenters is exposed to the metabolic activity of the microorganisms, the formed enzymes receive direct access to the substrate, which in the first step ensures an accelerated separation of the cellulose and hemicellulose structures into various sugars. The second step leads to an increase in the gas formation rate and the degree of degradation.


24/7 in action


Neben dem Aufschluss ligninhaltiger Biomasse und der erhöhten Methanerzeugung bringt das Bioextrusionsverfahren eine Reihe weiterer Vorteile mit sich. „Dazu gehört, dass das Substrat weniger aufschwimmt und sich gut im Behälter verteilt. Zugleich ist eine verbesserte Rohr-, Ventilpassier- und Pumpfähigkeit sowie eine Einsparung von Rührenergie zu konstatieren, weil das extrudierte Substrat in Mittellage geht und sich gut verteilt“, so Schulz. Außer der höheren Geschwindigkeit beim Biomasse­abbau, die aus der größeren Materialoberfläche resultiert, ist von verbesserten Reaktions- und Milieubedingungen auszugehen. „Vor allem verkürzen sich die Verweilzeiten bei erhöhten Ausfaulgraden.“ ADAP Biogas profitiert von diesen Vorteilen mittlerweile seit bald zwei Dekaden. Zugleich ist von einer hohen Anlagenverfügbarkeit auszugehen. „Natürlich gibt es hin und wieder Störungen durch Fremdkörper, das kann man nicht aus­schlie­ßen“, betont der Betriebsleiter. „Dadurch können die Module, die für das mechanische Zerkleinern der Biomasse zuständig sind, mehr verschleißen.“ Allerdings könne man für die Module auf der Materialeingangsseite sicher von Standzeiten von über 1200 Betriebsstunden ausgehen, für die hinteren – weniger beanspruchten – von deutlich mehr. „Die Extruder sind bei uns täglich 24 Stunden im Einsatz. Dabei laufen sie pro Stunde circa 40 bis 45 Minuten. Und das seit Jahren.“ ADAP Biogas arbeitet mit einem zwei Bioextruder vom Typ MSZ B 55, der bei einer Anlagenleistung von 2 x 55 kW stündlich bis zu 8 t Biomasse durchsetzt. Mit den Ausgangsmaterialien von ADAP lässt sich hierbei im Vergleich zu nicht extrudierter Biomasse die Methangasbildung um circa 30 % steigern.


The circle is closing


Currently, most of the biogas produced in Germany is electricityed directly at the place of origin, according to the Federal Association of Bioenergy. This type of use was decisively promoted by the Renewable Energy Act. The vast majority of on-site power plants would use combined heat and power plants for power generation, which consist of an internal combustion engine and a generator. Gas-oline and ignition jet engines are mainly used for the power generation of the biogas. This is also the case in Ahrenshagen: "After the methane has been removed from the fermenters and the replenisher, it first runs through desulphurization and then through an activated carbon filter, with which mechanical suspended solids are filtered out before it is finally provided cleanly to a gas-otto engine," Schulz describes. Through combustion, this drives a generator to generate electricity.


Decisive for efficiency


"With the bioextruder centrally in the process chain, the biogas plant generates an electrical output of 590 kW and a thermal output of 612 kW annually," the plant manager continues. Both forms of energy are used by the ADAP network itself to about 30%. The much larger shares are fed into the public power grid or used for heating a daycare centre in the municipality of Ahrenshagen and three apartment blocks with 45 residential units. The savings in heating oil achieved here amount to about 50,000 l per year. The Lehmann-UMT bioextruder has a very decisive part in this with the increased methane gas formation that efficiency is seen over the process, the fact that the primary energy contained in the biomass can be used particularly efficiently.


LMEngineering GmbH
Jocketa - Bahnhofstraße 34, 08543 Pöhl, Germany

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