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Special solution for clean bio-methane

Böblingen/Germany, November 27th 2018 – The little plant that packs a big punch: A state-of-the-art plant produces clean bio-methane. The fermentation residue is used as an agricultural fertilizer.

The plant is characterized by unusually compact dimensions. The relatively small site is situated next to Härnösand, a Swedish town with 25,000 inhabitants, 400 km north of Stockholm. Its main features are a 340 m3 steel digester with an external 160 m3 steel tube hygienization unit arranged in line to it. The Härnösand Energi & Miljö AB (HEMAB), a communal supplier and disposer of electricity, district heating, water and waste, had called for bids on a plant capable for the treatment of 5,000 to 6,000 tons of substrate a year from the communal biowaste collection. The goal was to provide environmentally friendly, sustainable biowaste disposal in accordance with EU requirements and national regulations.

Separate digestion and hygienization stages
Eisenmann’s bid, featuring detailed process engineering solutions, stood out from the competition, winning Eisenmann the contract for the construction of the plant. One of the factors for the successful implementation of the technology was the experience that the engineers had accrued with plants built previously in Finland and southern Sweden, which have been processing 44,000 and 20,000 t/a of biowaste respectively for several years now. This experience gave the engineering experts special insight into the unique demands of the fermentation process and how to handle them. The Swedish biowaste from the communal collection is mainly food waste. At the typical process temperatures of 55 °C, needed for an hygenization within the digestion, this generates a lot of ammonia, leading to the risk of process inhibition. That’s why the digestion and hygienization stage are separated in this plant to be able to operate the digestion at 38 °C.

The biogas plant that was commissioned right on 2016, is a compact solution with two horizontal steel digesters operating based on the plug-flow method. The plant is designed for very low ambient temperatures of down to -30 °C and for a biogas flow volume of 90 Nm3/h. The biowastes are delivered by trucks and stored on side. Continuous (24/7) gas production requires an evenly metered infeed. That’s why the biowaste is loaded into a fully automated buffer storage and passed through a shredder into the digester.

The plant features industry standard controls, continuous monitoring of the fill level, temperature, gas flow rate and pressure values, and logging of all relevant process parameters. All this adds up to reliable automated operation. The biogas obtained is transported to the external 480 m3 gas storage tank, eventually reaching the on-site treatment plant, where the final product – compressed bio-methane – is fed into mobile gas storages before being delivered to local vehicle filling stations.

The fermentation process
Using a screw conveyor, the shredded but unsorted material is transported to the horizontal steel digester. A plug screw brings the substrate down below the liquid level in order to ensure a gas-tight seal. The fully closed process almost entirely prevents odor and methane emissions. In the horizontal reactor, the mass passes through a mesophilic system at 38 °C, undergoing the process steps of hydrolysis, acidogenesis, acetogenesis and methanogenesis.

A robust paddle agitator mixes the material at low speed. It generates a homogeneous mixture and prevents the formation of swimming and sediment layers. The continuous movement and sophisticated agitator design ensure that the resulting gas is very effectively expelled from the substrate. The agitator features speed and torque monitoring, frequency control and permanent lubrication via automatic grease presses.

Such horizontal digesters are increasingly coming out ahead in biogas production – and with good reason. The used technology designed by Eisenmann offers both microbiologically and mechanically optimal conditions for the efficient conversion of organic materials. The result is high gas yields and smooth operation. The slim geometry of the digester, which measures 30 m long and 3.8 m in diameter serves to reduce inefficient short circuits in the substrate to a minimum. The system allows for an optimum retention time in order to ensure perfect offgassing at a very highorganic loading rate. Just how effectively does the plant in Härnösand work? The unusually high biogas yield speaks for itself – up to 200 m3 per ton of substrate.

The hygienization stage
The downstream hygienization process at HEMAB is also performed using a 15 m long horizontal unit set up in a row directly after the digester. It reliably hygienizes the material at a temperature of 60 - 70 °C. Background: The liquid residues are intended for use as an agricultural fertilizer and should pose no risk to people, animals or plants. That’s why they first need to be rendered hygienically safe. The required hygienization is successfully implemented, as demonstrated by a first direct process test in accordance with the strict German Ordinance on biowastes (BioAbfV 2012). The first one of two required tests has already been successfully passed at HEMAB’s installation. The hygienization stage boasts hard-wearing equipment, homogeneous mixing and efficient processes.

A modular assembly for every occasion
Eisenmann biogas plants are perfect for substrates high in solid materials, since they are not sensitive to interfering substances and require little or no additional external liquids and water. This minimizes the digester volumes, as well digestion residue and boosts efficiency and profitability.

Eisenmann has made a name for itself internationally for its anaerobic digestion technology, its innovations and also as a specialist in small-scale solutions. The German company’s intelligent modular assembly concept allows for custom-tailored combinations. The volumes of the digester modules run from 270 to 2,000 m3. Eisenmann has amassed plenty of expertise, having built nearly 100 anaerobic digestion plants in countries like the US, Finland, Sweden, Italy, Switzerland, Germany, Czech Republic and Poland. These plants have throughputs of 5,000 to 160,000 t/a and process substrates such as energy crops, biowaste from sorted collection, organic fraction of household wastes, sewage sludge, green waste and agricultural and industrial residues. Eisenmann’s service offerings also include extensions, inspections, maintenance and repair work, remote diagnostics, on-call service and full service.

Eisenmann offers
• Customer-specific concepts starting at 5,000 t/a
• Fast construction times thanks to modular design
• Top gas yields, minimized recirculation volumes
• Continuous dry digestion with high dry substrate (DS) content
• High volume load with optimum retention time
• Minimized short circuit currents
• Closed system prevents odor emissions
• Expertise based on nearly 100 completed biogas plants

About Eisenmann
Eisenmann is a leading global industrial solutions provider for surface finishing, material flow automation, thermal process technology and environmental engineering. The family-run enterprise is headquartered in southern Germany and has been advising customers across the globe for more than 65 years. It designs and builds flexible, energy- and resource-efficient systems that are tailored to customer requirements, and support state-of-the-art manufacturing and intralogistics. Today, Eisenmann has a workforce of approximately 3,000 worldwide, with 27 locations in 15 countries in Europe, the Americas and the BRIC countries. In 2017, Eisenmann generated annual revenues of 723 million euros.

The compact bio-methane plant in Härnösand, Sweden.
The compact bio-methane plant in Härnösand, Sweden.

The compact bio-methane plant in Härnösand, Sweden.

Functionality of the system.
Functionality of the system.

Functionality of the system.



Sven  Heuer


Eisenmann SE


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+49 7031 78-1522

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