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English Issue Biogas Journal | Spring_2021 Denmark totally committed to organic residues Copenhagen Only residues such as stable manure, stover and potato peelings are used at the plant in Ringe. Denmark is among the growing markets for biogas. In the period from 2012 to 2020, the biogas production of our neighbours to the north increased more than threefold, currently reaching about 4.1 billion kilowatt hours. Biogas is primarily used to generate electricity; however, it is expected that the percentage that is treated and fed into the gas grid will increase. The ambitious goals that Denmark has set for 2035 also include a complete shift away from fossil fuel sources and the decarbonisation of 70 percent of the gas grid. Notably, in addition to agricultural biogas plants, there are quite a few very large plants at industrial scale. By Thomas Gaul In terms of biogas production, Denmark has always followed its own path. For example, energy crops have never played the same role as substrate as they have here up to now. Instead, from the very beginning, Denmark committed to using residues and made the circular economy a priority. This demonstrates that cattle manure and pig slurry can serve as a source of nutrients for the bacteria in the digester, even on a large scale. Up to now, substrate in Danish biogas plants was still allowed to contain up to 25 percent maize, but this figure has now been reduced to 12 percent. In future, the plants will have to manage with just six percent maize. The Nature Energy Midtfyn biogas plant in Ringe demonstrates how this looks in practice. In three digesters, each with a volume of 8,500 cubic metres, 360,000 tonnes of biomass are processed annually. Previously, this also included a percentage of maize silage and grain meal. Stable manure, stover and the like instead of maize But now only alternative biomass such as stable manure, stover or potato peelings is used. The amount of agricultural residues totals 75 percent; 25 percent is biowaste from households, restaurants and cafeterias. To prevent unpleasant smells, collection takes place in a closed hall. After the residues are unloaded into an underground container, a crane with a gripper is used to feed the biomass into the chopping process because it is important to open up the cell walls of the material before fermentation. The treatment is performed with a hammer mill in a drum in which heavy steel chains rotate and chop the material. Together with slurry from animal husbandry, the material is pumped into the first digester. After a retention time of 20 days, the substrate is pumped into the second digester, where it remains for another ten days. The process takes place at a temperature of 52 degrees Celsius in the digesters. The lorries that pick up the slurry from the agricultural operations take the liquid fermentation residues back with them as return freight. The biogas is processed and fed into the gas grid. They have also discovered an application for the CO 2 removed during biogas treatment: It is used to produce beer. Biogas from fish residues In a recently expanded plant in Herning, after pasteurisation they also ferment slaughterhouse waste and fish residues that arrive from Norway by ship. In addition, 650 tonnes of slurry flow daily into the four digester towers of Herning Bioenergi A/S where it is transformed into biogas. Two of the towers have a volume of 3,500 cubic metres each and the other two 8,000 cubic metres each. In Herning, gas is not converted to electricity in combined gas and heat power plants. Neither is the biogas specially treated in photos: Thomas Gaul 42
Biogas Journal | Spring_2021 English Issue order to feed it into the natural gas grid. Instead, Herning Bioenergi has two large, direct customers – and they belong to Arla Foods, a global dairy company with cooperative ownership including over 13,500 dairy farmers from Sweden, Denmark, Germany, Great Britain, Belgium, Luxembourg and the Netherlands. A 21 kilometre long gas pipeline connects the biogas plant in Herning to the first farm in Naviro. From there, another 6 kilometre long pipeline runs to the Arla production site in Videbæk. Only when it reaches these two production sites the biogas is burned in a total of three combined heat and power plants (CHP). The electrical power generated is used on site for the base load supply as well as for heat. But even though all three CHP plants achieve an output of about three megawatts with cogeneration, the demand for the two milk processing operations is many times greater. “Because the base load is so high and we only contribute only a small percentage of it, we can be sure that the biogas is really used,” explains Olav Hald, plant manager of the biogas plants in Herning. This offtake security creates a basis for continuous operation with optimum biological processes. Lots of cows and a grit washer Torben Pedersen currently has 2,000 cows in Holsted. From a small operation at the beginning, the farm has grown continuously over the years. Each cow produces an average of 40 litres of milk per day. The yields of 800 hectares of forage maize, 400 hectares of grass and 200 hectares of grain are used to feed the cattle. When they are in their stalls, the cows stand on a bed of sand. “Since then I haven’t had to deal with hoof problems anymore,” Torben Pedersen reports. The disadvantage here, though, due to the herd size, is the huge demand for sand. “Per week I need 160 tonnes,” says Pedersen. This not only poses a logistics problem, but it is also a cost issue. In 2016, in order to separate the sand from the slurry, Pedersen invested in an innovative grit washer. He uses it to reprocess the sand so that it can be used in the stalls again. The separated slurry is delivered to a biogas plant. By regulating the dry matter and flow meter, the grit washer is continuously supplied with the slurry-sand mixture. A cyclone separator divides the slurry from the sand. The sand settles out in the grit washer, where it is rinsed with clean water and removed by the screw conveyor. After it is dried, the sand can be reused. The desanded slurry flows from the grit washer through a drum screen, where it is separated into a liquid portion and a solid phase. About 10 to 12 cubic metres of raw slurry can be pumped into the grit washer per hour. Slurry acidification is quite common An important issue for our neighbours to the north is the acidification of slurry. There are various practical methods for adding sulphuric acid to slurry. The goal is to decrease the outgassing of ammonia by reducing the pH value. This eliminates the need to immediately incorporate it into the soil. In general, the pH value is set at 6 if the slurry will be distributed within the next 24 hours. The pH value setting is reduced to 5.5 if the slurry will be distributed within a period of up to three months. The addition of the sulphuric acid is carried out in the storage container. Reportedly, the acidified slurry does not damage the concrete. But the acid can also be added directly before distribution, either when the container is filled or directly during the spreading process. To do so, the front tractor hydraulics are used to pick up the acid tank, usually an IBC container, and the acid is dosed continuously as the slurry is spread. Johan Solmer, a farmer with an operation near Sonderborg in southern Denmark, decided to add acid to the slurry while it is still in the tank. “That gives me more flexibility during spreading in the spring,” he says, explaining his decision. The slurry is used to fertilise 250 hectares of grain and rapeseed. All of the field work is done with his own machines. Only the slurry transport and a few other small tasks are performed by a contractor. The slurry is produced by 750 breeding sows. The piglets remain on the farm until they reach a weight of 30 kilogrammes and then they are exported. Johan Solmer doesn`t need to handle the 96% sulphuric acid solution himself. A service provider delivers it in a tank lorry. A hose connection is used to supply the acid to an agitator. The agitator is mounted on a high-performance tractor and can BIOGASANALYSIS FOS/TAC SSM 6000 automatic titrator for the determination of VOA, TAC and VOA/TAC the classic for the discontinuous analysis of CH 4 , H 2 S, CO 2 , H 2 and O 2 with and without gas preparation SSM 6000 ECO * proCAL for SSM 6000 fully automatic calibration without test gas for NO x , CO und O 2 , several points of measurements GAS ANALYSIS EQUIPMENT BIOGAS ANALYSIS EQUIPMENT WATER ANALYSIS EQUIPMENT AGRICULTURAL EQUIPMENT www.pronova.de PRONOVA Analysentechnik GmbH&Co.KG Groninger Straße 25 I 13347 Berlin, Germany Tel +49 (0)30 455085-0 I info@pronova.de * 43
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