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ENGLISH ISSUE BIOGAS JOURNAL | AUTUMN_2023 The Ökobit bunker silo fermentation biogas plant in the Saarland. This is where biogas is produced from horse manure in three bunker silo digesters and then converted into electricity in a combined heat and power plant (CHP) with an electrical output of 80 kW e . 300 horses, amounting to 10 to 15 tons per day, along with some cattle manure. To achieve this, the Oings have equipped their fermenter with a MeWa cross-flow shredder and add a daily dose of 1.2 liters of enzymes. “The addition of enzymes has resulted in a significant increase in the proportion of manure and a reduction in the use of silage maize,” says Oing. Despite the positive effect of mechanical cutting and enzymes, the situation was unsatisfactory: “The substrate in the digester was so thick that the agitators ran practically all the time.” So, last year the Oings installed a PlurryMaxx wet grinder. The device extracts digester content through a bypass system and pulverizes it with a rapidly rotating blade hammer. “The system is designed in such a way that the entire input goes through the wet grinder once,” explains Oing, “which allowed us to reduce agitation by more than a third.” The power consumption of the PlurryMaxx is more or less compensated by this saving. A similar scenario, and yet somewhat different, is the setup of the processing system at Königs Pflanzenenergie. In Neuss, a fermenter was “adjusted” to horse manure two years ago, using a cross-flow disintegrator equipped with movable beaters and a counter-blade. A magnetic separator is installed upstream, which is fed through belts. As Herbert Königs explains, the processed substrate is mixed with recirculated liquid during the injection process. In addition, one liter of enzymes is added every day. One difference to Oing is the fermentation temperature: while Oing operates “The TS (Total Solids) content in liquid digesters should not be more than 14 percent” Dr. Hans Oechsner their biogas plant mesophilically at approximately 40 degrees Celsius (°C), Königs runs the horse manure fermenter at 52 °C to optimize the degradation rate. Through this combination of measures, along with sugar beet pulp and silage maize as co-substrates, a very high proportion of horse manure can be used in the digester. Dr. Hans Oechsner from the University of Hohenheim recommends not using more than 50 percent horse manure in wet fermentation: “The TS (Total Solids) content in liquid digesters should not be more than 14 percent.” In areas surrounding cities with a high population of horses, Oechsner considers biogas plants specialized in horse manure to be beneficial. “In these cases, one could consider dry fermentation plants that can easily handle the high TS (Total Solids) content and foreign matter in the substrate,” says the agricultural scientist. These plants operate with cyclically operated digester boxes, into which the fermentation substrate is feed in and after digested extractedusing a wheel loader. The substrate stack is sprayed with a liquid percolate, a by-product during digestion. “The permeability of the substrate has to be ensured by structural material,” says Oechsner, pointing out that this process requires careful attention by the operator. The coarse straw in horse manure is usually sufficient for this purpose. It is crucial to avoid material slumping: “If the percolate does not reach certain areas, ‘dead heaps’ can form, leading to acidification.” In addition, a quarter to a third of the fermented material must always be mixed into the new substrate as a starter culture, which adds to the workload. Gas from the Garage There is only one specialized facility like that in Germany which specifically focuses on horse manure: Adrian Bartels operates it together with Jens Boedecker in Lehrte near Hanover. The plant, owned by Babö GmbH, mainly consists of five parallel garage digesters, a percolate tank with a gas storage unit, and a 75 kW e combined heat and power (CHP) plant. “We could produce a little more gas,” says Bartels. However, during its commissioning in 2017, 75 kW e was the maximum limit for a small-scale manure plant. The plant, built by the Swiss manufacturer Renergon, processes the manure from 250 to 300 horses, amounting to 3,500-4,000 tons per year. “We change the substrates every Monday and Thursday. That means: we open the gate, remove the material, mix a few shovels into the new substrate, and put the material back in,” explains Bartels. Approximately 130 m3 of substrate then remains PHOTOGRAPH: ÖKOBIT GMBH 44
BIOGAS JOURNAL | AUTUMN_2023 ENGLISH ISSUE in the airtight enclosed garage for 18 to 21 days, during which time the volume shrinks to about one-third. This is the way the process runs through the five digesters. The substrate is irrigated with percolate every two hours on average. The liquid travels into a drainage channel through the substrate via a slope in the bottom of the digester. From there, it is pumped back to the percolate tank. “In principle, the plant is simple in design,” says the farmer. It is equipped with two pumps, an agitator in the percolate tank, and five irrigation nozzles in each digester. The heating is done solely with the percolate, which is directly heated by the CHP unit through heat exchangers. No heating pipes are installed. In addition to horse manure, the residents of Lower Saxony use approximately 10 percent of cattle manure and small amounts of grass clippings. According to Bartels, the digestate is stored and then applied as a top fertilizer on the farm’s own fields. As it was one of the first plants in Germany, Babö GmbH obtained a favorable price from the equipment manufacturer. “Wet fermentation with substrate processing would have cost roughly the same,” Bartels estimates. His garage digester plant has an electricity self-consumption rate of only 3 percent, which is significantly lower than an upgraded wet fermentation system. However, it should be noted that more operating hours with diesel consumption for the farm loader need to be considered. Nevertheless, he is very satisfied with his plant and is convinced that he made the right decision. A horizontal fermenter handles higher TS (Total Solids) content Frank Bauer from Schrozberg has found a kind of compromise solution: a plug-flow fermenter made by the manufacturer Novatech. The 900 m³ horizontal fermenter with a slowly rotating longitudinal agitator can handle TS (Total Solids) levels of up to 30 percent, as is typical for this type of digester. Bauer raises turkeys and keeps 35 horses. He wanted a robust technology to utilize his manure. He currently gets manure from more than 200 horses, and combined with turkey and some cattle manure, the solid manure fraction amounts to approximately 50 percent. The biogas plant, built in 2007, still consists of three cylindrical tanks and two CHP units with a combined capacity of 500 kW e . Bauer directly feeds renewable raw materials into one of the cylindrical digesters. The material from the plug-flow fermenter is pumped into a secondary digester and then into the digestate storage. However, Bauer also cannot avoid pre-processing the horse manure. “I didn’t want an expensive and potentially unreliable technology directly in the fermentation process,” he says. That is why he uses a mobile biomass shredder. Although it is primarily designed for woody green waste, it works perfectly fine with horse manure as well. “If I use horse manure that is not shredded, I immediately notice an increase in power consumption,” explains Bauer, adding, “30 liters of diesel are enough to shred around 200 tons of horse manure. After shredding, the manure pile steams, and I can see that energy is being lost.” The manure then has to be brought to the plant as quickly as possible. Bauer is convinced that horse manure is “very good for the biology” because, unlike poultry manure, it does not involve nitrogen inhibition. Bunker Silo Digesters A novel, cost-effective type of dry fermentation plant is being developed in the current research project FeBio (Solid-State Biogas Plant): a bunker silo biogas plant. “The bunker silo digester is comparable to a long garage embedded in the ground without a roof, which is covered with a tarpaulin after being filled,” explains Eike Ziegler, a developer at the participating biogas plant manufacturer Ökobit GmbH. This type is based on the work of Hans Wolfertstetter from Upper Bavaria, who has built several of these plants under the concept of the “Chiemgau Model”. Following the Chiemgau model, a plant was also established in the Eifel region, which now serves as a basis for standardization and technical improvements for Ökobit and the FeBio project. Technical challenges in the bunker silo design include the practical opening and closing of the cover tarp. According to Ziegler, this is resolved by using a rolling and gas-tight fastening system. In addition, the pipelines for percolate irrigation should be easily removable to provide space for filling the fermenter with a wheel loader. However, the modular plant should be kept as simple and compact as possible. “The aim is to achieve electricity generation costs of around 18 cents/kWh e and investment costs of less than 8,000 euros/kW e of installed capacity for the core plant,” states Christoph Spurk, Managing Director of Ökobit. The company intends to offer this type of plant as a cost-effective builder-owner model. The groundbreaking ceremony for the FeBio pilot plant recently took place in Saarland. Biogas will be produced here out of horse manure in three bunker silo digesters and converted to electricity using an 80 kW e CHP (Combined Heat and Power) unit. With regard to profitability, only one of the interviewed users of horse manure digestion is skeptical, while two are satisfied and one is even enthusiastic. Horse owners can be new business partners to biogas plant operators and offer new opportunities, such as providing the service of spreading horse manure digestate. Adrian Bartels explains that some of his equestrian centers get straw for horse stall bedding from him. Horse Manure Mobilizes Waste Collection Trucks Meanwhile, Königs Pflanzenenergie is working on its concept of “Mobility with Horse Manure”. The company has been operating a biogas purification and injection system since 2010. A significant portion of the biomethane now comes from horse manure, which presents good opportunities for marketing it as a fuel. According to Herbert Königs, a Bio-CNG (bio methane compressed natural gas) refueling station is currently being built directly at the biogas plant. It has already signed a contract for refueling waste collection trucks. Biomethane from horse manure allows for additional income through the sale of greenhouse gas reduction credits. The European Clean Vehicles Directive now requires public authorities to use alternative fuels, and according to Königs, “With Bio-CNG, we are far ahead of other technologies as a diesel substitute.” Author Christian Dany Freelance Journalist Gablonzer Str. 21 · 86807 Buchloe 00 49 82 41/911 403 christian.dany@web.de 45
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