April 26, 2007 | General

Biogas Systems Work On Food Residuals Streams (Germany)

BioCycle April 2007, Vol. 48, No. 4, p. 60
For the United States and Canadian markets, a German firm with more than 20 operational sites sees a large potential for codigestion plants.
Jorgen Fink

OUR company, Hese Biogas GmbH, is one of Germany’s leaders in biogas, with special focus on miscellaneous food residuals streams that often are codigested with animal slurry (manure). The firm mainly concentrates on larger scale plants (> 500 kW electricity production). In the German market, this includes energy crop plants like corn silage. Abroad, we are mainly focusing on food waste digestion and codigestion of animal slurry with food waste.
People with knowledge of the German market know that anaerobic digestion is booming due to the very attractive feed-in tariff for electricity. Most of the plants being built are fueled with energy crops, mainly corn silage because there is an extra bonus on the electricity price when the biogas plants are using energy crops. Hese Biogas’ experience with corn silage digestion may not be of direct interest to the U.S. market at present, but there is an increasing interest in the U.S. to digest residues from corn ethanol plants where our experience can transfer over. However, Hese Biogas’ strength is our experience in digestion of different food waste streams inclusive of codigestion of food waste with animal slurry.
We have more than 20 large-scale plants in operation. This article describes four different plants built on different concepts. The plants, all based in Germany, are: Werlte – Codigestion of food waste with animal slurry; Stellinger Moor – Digestion of food waste from restaurants and food producers with pretreatment of the waste (removal of plastic and other inert material); Brensbach – Codigestion of food waste with animal slurry and after-treatment of the liquid fraction to clean water standard; and Plätz I & II – Energy crop plant with codigestion of corn silage with animal slurry
The Hese concept is based on mesophilic digestion. Thermophilic is available, but mesophilic digestion is more robust with changes in the input streams and is therefore seen as a more reliable process for large-scale food waste and codigestion plants. The digesters are made of glass enameled steel plates with a vertical slow rotating mechanical mixer. We also include gas storage that is normally built as a double membrane roof on top of the storage tank for the digested material coming from the digester. This also allows the minor gas production in this tank to be collected and used.
Plants taking in any food waste of animal origin (i.e., “Animal By-Products”) also include a prepasteurization step where the biomass is heated to minimum of 158 °F for an hour. This is in line with the new European Union (EU) regulations. The heating is done by specially designed heat exchangers that are very efficient and have a low risk of blocking even with the high dry matter food waste streams. For plants dealing with food waste and the like, a biological desulphurization unit is included to reduce sulfur and other harmful particles in the gas to a minimum, and thus not harm the gas engines. Any pre or post-treatment is done according to the expected input streams and the required need of the users of the digested material (i.e., biofertilizer).
The Werlte biogas plant started operation in 2002. The main parameters are as follows: Capacity – approximately 120,000 tons/year; Input material – animal slurry and liquid food waste including fat; Digester capacity – 1.7 million gallons; Mesophilic digestion with pasteurization at minimum of 158°F for one hour; Power capacity – 2.5 MW.
The plant receives slurry from approximately 100 farmers. The food waste comes from different food companies. The plant has been set up as a new company, initially with the farmers as participants. The supplying farmers also use the liquid digestate as biofertilizer on their fields. Additional storage tanks for the end product have been built as part of the project.
Electricity is sold to the public grid. There are future plans for cleaning the biogas to upgrade it to natural gas standard and then pipe it to the natural gas grid. This codigestion concept has been used in Europe for more than 20 years in countries like Germany, Denmark, Sweden and is now spreading to many other countries in Europe.
The Stellinger Moor biogas plant started operations in 2005. The main parameters are as follows: Capacity – approximately 22,000 tons/year; Input material – catering and food waste; Digester capacity – about 770,000 gallons; Mesophilic digestion with pasteurization at minimum of 158°F for one hour; Power capacity – 1.0 MW; Pretreatment – maceration and separation of packaging material and other inert material by the Dewaster.
The Stellinger Moor plant is the first to be built with Hese Biogas’ newly designed Dewaster that mechanically separates the waste (by extrusion) into a clean organic pulp that can be pumped to the digestion process, and a residual waste stream that can be incinerated or the like. There also is a metal separator. After unloading into a bunker, the waste is macerated and transported on a conveyor belt where the metal separator is located before reaching the Dewaster. At the Stellinger Moor plant, the residual waste is combusted at the nearby waste incineration plant. The clean organic pulp is pumped to the pasteurization unit; after pasteurization for one hour, it is pumped to the digester.
Electricity is sold to the public grid; the excess heat is piped to the nearby waste incineration plant, which is connected to the district heating network of Hamburg and all their excess heat is sold to consumers in Hamburg.
The Brensbach plant started operating in 2005. Cleaning of the liquid fraction of the digestate started in 2006. The main parameters are as follows: Capacity – approximately 77,000 tons/year; Input material – animal slurry and food waste; Digester capacity – about 1.3 million gallons; Mesophilic digestion with pasteurization at minimum of 158°F for one hour; Power capacity – 1.4 MW; Mechanical separation of the output from the digester and cleaning of the liquid fraction to clean water by ultra filtration and reverse osmosis.
The Brensbach plant is a traditional codigestion plant treating manure and food waste. It is supplemented with our newly designed treatment process that upgrades the liquid fraction into clean water, which is then discharged to the nearby stream. This process can be used in areas where there is limited land available for spreading the liquid end product from the facility.
The Plätz plant is a typical energy crop plant. It is built with two separate lines to maximize electricity sales from the plant. Operations began in 2005. The main parameters are as follows: Capacity – approximately 38,000 tons/year in total (2 plants each at 19,000 tons/year); Input material – corn silage and animal slurry; Digester capacity – 660,000 gallons/year/plant; Mesophilic digestion; Power capacity – 537 kW/plant.
The Hese design allows using high dry matter material because corn silage is mixed with digestate from the digester and recirculated back into the digester. Furthermore, the heat exchanging is done by recirculation from the digester back into the digester to avoid blocking of pipes and the heat exchanger. The corn silage is further macerated before being mixed and pumped into the digester. In order to have a stable digestion process, up to 20 percent animal slurry is added to the corn silage. Normally the energy crop plants include extra storage facilities for the end product. In order to get the full nutrient value of the biofertilizer, and to comply with the EU nitrate directive, this material is only allowed to be spread at certain times of the year when crops can take up the nutrients applied to the land.
Electricity is sold to the public grid at the special energy crop tariff, which is a higher tariff than the normal tariff. Normally there is no market for the heat, but at the Plätz plant the farmer is using the excess heat during part of the year to support the growing of asparagus.
In the U.S. and Canada, we see a large potential for codigestion plants with or without the liquid cleaning of the end product. The need for the liquid cleaning depends very much on the location of the facility, and whether or not there are farmers nearby who can use the liquid biofertilizer. Thus the cost of cleaning has to be compared to the cost of trucking the liquid biofertilizer over long distances.
There is also an increasing potential for different dedicated food waste plants that could use our technology. We don’t see our energy crop concept being applied to the North American market directly. However, we believe there is a growing market for residues from corn based bioethanol production. Hese has started to get involved in the bioethanol residue market in Germany. This experience, combined with our corn silage plants, may be applicable to the U.S. market.
Jorgen Fink is with Hese Biogas GmbH, and can be reached directly at fink@hese-biogas.com. Hese Biogas is based in Geisenkirchen, German (www.hese-biogas.de).

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