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Animal Waste to Energy in Denmark

by 5m Editor
20 October 2009, at 12:00am

For the Danish agriculture sector, the production of biogas through anaerobic digestion is more than just a means of producing cheap renewable energy, writes ThePigSite senior editor, Chris Harris.


Fangel biogas plant in Denmark.

Biogas is a multifunctional technology that also provides a security of energy supply, sustainable agriculture development, environmental protection and rural development.

At present, the industry has a total of 22 centralised biogas plants and 60 farm-scale plants that together process a total of 2,192,864 tonnes of manure and organic residues a year.

The process allows for a sustainable agricultural production system with the recovery of the nutrients and energy content from the organic matter and manure.

The anaerobic digestion produces a digested biomass that can be redistributed to the farms as fertiliser that helps protect the water supply and has less odour. The system also produces power and heat and even fuel for transportation, while at the same time helping to mitigate global warming.

The Fangel biogas plant was built by Bigadan as a turn-key project in 1988/89 as part of an initiative by local farmers to improve the environment when handling slurry. The plant has been owned by Bigadan since 2001.

It receives animal slurry from local farmers and industrial waste mainly from local food processing companies. The plant processes 240 tonnes of waste a day with 80 per cent of it as cow and pig manure and the rest as food waste.

The digestate is heated for an hour to ensure that any salmonella is killed and in all the plant produces 12,000 cubic metres of biogas a day with 65 per cent of it as methane. The gas is burnt to produce 1.4 megawatts (MW) of electricity per hour and 1.7MB of heat.

The electricity is sold to the grid and half the heat is used in the plant and the rest is sold on.

The manure comes from farms within a 15-kilometre radius of the plant and the final digestate and liquid slurry is given back to the farmers to use on their fields.

The plant collects from between 18 to 20 farms and brings in between 12 and 14 loads of manure and food waste each day. The plant itself operates on a five-day basis. At the weekends, the plant ensures that there is enough manure and waste in holding tanks to keep the plant running continuously.


Digesters at the Fangel Biogas Plant.

The plant has two digesters – one built when the plant was originally commissioned in 1988 and the second one in 1999.

The slurry and waste is delivered into a receiving tank and then passed into a mixing tank and then on to pasteurisation in two tanks – one 600 cubic metres in capacity and the second 800 cubic metres capacity. The mix is pasteurised at 70°C before being pumped into the digester. The slurry is heated from recycled heat from the process.

The gas that is drawn off from the digester is cleaned before it goes through to be burnt in the generator. Any extra gas goes through to a storage tank that has the capacity to store up to 1,500 cubic metres of gas for three hours. The system works automatically, sending the correct amount to the generator and the rest to storage. Any excess gas can be burnt off through an emergency valve.


Storage tanks at Frangel biogas plant

The wet digestate, which is 30 per cent solids and 70 per cent liquid, is then pumped into storage tanks and the solids and liquid are separated in a centrifuge. The liquid goes into further storage before being transferred to tankers to be returned to the farmers for use on the fields.

The dry solids are pelleted in a system that has been introduced over the last year to form a fertiliser that is then sold back to the farmers. In all, the plant produces 2,000 tonnes of fertiliser pellets a year. The pellets can also be burnt as another fuel.

While half the heat that is produced in the system is recycled to heat the slurry, the plant also provides the local village with hot water at a constant temperature of 29°C.


Pelleted dry solids used for fertiliser
Karsten Buchhave, the CEO of Bigadan, who runs the plant, said there is a great potential in Denmark to increase the number of anaerobic digestion plants producing biogas as only about five per cent of the total animal waste volume is used in the current plants.

"The political intention is to double the amount of animal waste that is used in these plants," he said.

"The authorities want 50 per cent of the manure to go through biogas plants by 2020."

He said that biogas is the most active technology from the view of mitigating climate change factors.

October 2009