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ENGLISH ISSUE BIOGAS JOURNAL | AUTUMN_2023 Horse Manure: Tempting, but Tricky “We change the substrate 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,” Adrian Bartels explains (left). On the right is the co-partner Jens Boedecker. An increasing number of biogas plant operators are tapping into the tremendous potential of horse manure. However, this manure cannot be used in agricultural facilities simply at the drop of a hat. There are two options: upgrading a wet fermentation system for co-digestion with this challenging material, or investing in specialized plant technology such as dry fermentation. This is a report from research and practice. Author: Christian Dany We estimated a disposal fee of 5 to 10 Euros per ton,” Adrian Bartels explains. However, because the demand for horse manure is constantly increasing, it is becoming more and more difficult to achieve this fee in renegotiations. Bartels operates a biogas plant specialized in horse manure near Hanover. Similarly, Herbert Königs from Neuss am Rhein says almost the same: “Small equestrian farms with less than 20 horses are often willing to pay a fair price for the disposal of manure. In the case of large horse farms, we face strong competition from mushroom production.” Mushroom substrate, for example, consists mainly of horse manure, and with the high fertilizer prices, more arable farms are now becoming interested in horse manure. The demand from the other side also plays a significant role: According to the new fertilizer regulation, horse farms must be able to store the manure for two months on a dung pile, or they must reach an agreement with a buyer. Transport Distance: Considering the Cost-Benefit Ratio All the horse manure biogas producers who were interviewed operate a container service for their equestrian farms. “It’s a challenging business,” says Königs. It requires large containers. His company, Königs Pflanzenenergie GmbH & Co. KG, has 50 cubic meters of storage space, but due to the low bulk density, it only has room for 8 to 20 tons. Depending on the distance, the cost-benefit ratio always has to be taken into consideration. This is why the horse manure digesters that were surveyed also include some self-deliveries. However, our survey clearly shows that horse manure, often referred to as a sleeping giant among energetically usable residual materials, is a giant that is slowly moving! An estimated 1.2 million horses are kept in Germany. Each of them produces between 17 and 21 tons of manure per year. The University of Hohenheim has found that if only half of this potential, approximately 20 million tons, is converted into biogas, it could save 7.8 million tons of corn silage, equivalent to a cultivation area of 156,000 hectares. As enticing as this potential may sound, using horse manure as a biogas substrate is challenging. Long straw used as bedding material contributes to dry matter content in the manure, reaching up to 50 percent. It is poorly digestible by microorganisms, can clog pumps, and lead to floating layers in the fermenter. Sawdust used as bedding is unsuitable for the digestion of the manure. PHOTOGRAPH: LANDVOLK NIEDERSACHSEN 42
BIOGAS JOURNAL | AUTUMN_2023 ENGLISH ISSUE PHOTOGRAPH: IZES/DR. JOACHIM PERTAGNOL Front and rear view of the Bioenergie Herforst bunker silo biogas plant in the Eifel. Guiding Horse Facilities to Provide Substrates Free of Contaminants Foreign objects such as horseshoes, halters, or bale straps pose a risk to the plant technology. “We have to advise those responsible at horse farms,” is something that was repeatedly heard. This ranges from raising awareness to issuing warnings, and even terminating the supply relationship if the manure is not free of contaminants. In general, the decentralized generation of smaller quantities requires sophisticated collection logistics, with efforts made to avoid longer interim storage periods, as the material can also degrade aerobically. The legal situation has been favorable since the EEG 2012 (Renewable Energy Sources Act), but it is impractical to poor for older facilities. If horse manure is to be used in agricultural biogas plants with wet fermentation processes, preparing the substrate and, if necessary, separating foreign objects is indispensable. Dr. Benedikt Hülsemann from the University of Hohenheim favors preparing the substrate directly at the biogas plant and using it immediately. Experiments were conducted at the Hohenheim research facility using a cross-flow shredder. The device, manufactured by MeWa, was originally designed for material shredding. It features chain-based impact tools and is relatively resistant to foreign objects. The shredding process increases the surface area and improves the flowability of the substrate. With a chosen container diameter of 900 millimeters, the input power is 55 kilowatts (kW e ). Preparation of Dung Accelerates Decomposition and Increases Methane Yield Both in the laboratory and on a practical scale, the researchers at Hohenheim were able to demonstrate that the mechanical preparation of the fiber components from the straw results in accelerated decomposition and can achieve a methane yield increase of up to 26 percent. The power consumption of the cross-flow shredder ranged from 13.8 to 20.5 kilowatt-hours (kWh) per ton of fresh matter, which corresponded to 3 percent of the electricity generated with the substrate. In a current project, researchers at Hohenheim are testing a novel ball mill made by Biokraft Energietechnik GmbH. In this setup, a rotating drum is placed on an inverted truck chassis. Steel balls, lifted upwards in the drum, fall onto the substrate and grind it. According to project team member Rene Heller, during a horse manure experiment, the ball mill was able to increase the specific methane yield by more than 37 percent. While final results are not available yet, a significantly positive energy balance is expected. Under the motto “Manure instead of corn” systems for substrate preparation have spread in recent years. Other methods involving mechanical size reduction include cutting mills, shredders, and extruders. In addition, the market offers various physical processes such as thermal pressure hydrolysis or ultrasound treatment, as well as biological methods. In the case of biological methods, enzyme preparations are the preferred option. According to manufacturer information, they reduce viscosity, making the treated substrate more liquid, ensuring stirring capability, and preventing floating layers. The effectiveness of these preparations is demonstrated by flow experiments conducted by the manufacturers. For example, the Berlin-based company Biopract GmbH has developed an inclined channel test for this purpose. Together with his family, Martin Oing implements three measures for substrate preparation in Schöppingen, which is located in the Münsterland region of Germany, where they operate a biogas plant with a rated capacity of 700 kW e . With more than 1,000 horse farms, the Münsterland is one of the most horse-dense regions in Europe. Oing uses manure from approximately “The addition of enzymes has resulted in a significant increase in the proportion of manure and a reduction in the use of silage maize” Martin Oing 43
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