Energy-efficient frequency converters for bioenergy plant

The world's largest bioenergy plant already processes an annual 450,000 tons of manure and 100,000 tons of other biomass such as deep litter, potato pulp and dairy waste products, generating 18.4 million cubic meters of biogas each year. At the same time, the facility is assisting agriculture by reducing the volume of nutrients released into watercourses and fjords in the region, and contributing to both local energy generation that respects the environment, and increased employment.

This has been achieved thanks to a design with local businesses and agriculture in mind. A 14km pipeline leads directly to agricultural sources in the west, and another directly to Arla Foods in Holstebro.

Control is performed from a central control room with an ICS system that collects all operational data and provides an overview. The facility is large enough to employ a full-time laboratory technician.

Measuring instruments monitor everything, including the output to input ratio. There is a night-time alarm, which sends an SMS message to an operative at home, from where the majority of errors can be adjusted using a computer.

150 frequency converters with PROFIBUS on everything from conveyor screws and agitator tanks, to pumps and heating plants provide an energy-efficient system, plus the ability to perform centralised control and monitoring. To ensure consistent programming with the other plants in the region, the choice went to Danfoss VLT Automation Drive from 0.75kW for most conveyor screws, up to 45kW for the heating plant. Electrician Martin Lorentzen explains: "The most important things have been to be able to control the pressure and flow in the slurry system, and to make sure that agitators power up smoothly, and that revolutions can be reduced. This is why we opted for a relatively simple program."

The control room measures drive data such as frequency, power consumption, output, ramp up and ramp down time. Data from all the Vestforsyningen plants can be compared. All frequency converters are centrally fitted in the control room because of the aggressive environment at the facility. This means that protection class IP21 has been sufficient. As the plant's physical dimensions are considerable and cover a large area, it was essential that the frequency converters could support up to 150m of cable, which only Danfoss was able to offer without extra filters.

To prevent unplanned downtime, a Danfoss Service Agreement has been signed to ensure a proactive service, and the arrival of technicians on site within 5 hours of a call.

Today, Maabjerg BioEnergy is 90 per cent owned by Vestforsyningen, but it functions as an independent unit with seven employees. The ICS system collects all the operational data and provides an overview. Everything is managed by measuring instruments, including the input to output ratio.

A boon to agriculture and industry

For farmers, the plant is also a type of purifying facility. They provide the slurry and the plant returns it, de-gassed, to be used as fertiliser on the fields. Residual fluid from the plant has a high nitrogen content, which provides a higher value as manure. Farmers are relieved of the burden of waste, which is a factor which limits herd sizes. The normal rule is the application of no more than 75 per cent slurry as fertiliser. If the product comes from Maabjerg BioEnergy, however, the content can be as high as 85 per cent; this also helps keep more people employed on the land. A filtration system recovers phosphates and calcium which are added to straw, allowing for alternative uses in the Djursland peninsula. In Western Jutland, phosphates and calcium are pollutants, as the soil is rich in them.

The primary output of the plant, however, is gas. One method of transport is the 17km underground pipeline to Vinderup.

Arla Foods now provides waste products to Maabjerg, rather than using them in animal feeds. As filtration processes have gradually become smarter, there are fewer nutrients in the waste product and this means it is more advantageous to use it for gas production. There is also the benefit to the environment, in that waste products do not have to be transported great distances: they can be pumped directly to Maabjerg BioEnergy. This therefore reduces Arla Foods' CO2 emissions.

To learn more about VLT AutomationDrives, please go to www.danfoss.com.