Published by Farmers Guide, December 2007.

The central generating turbine at Avedøre 2 is driven by steam produced by burning coal, oil and wood chips as well as straw.

Big bales fuel the world's largest biomass boiler

AS A part of the drive for greener energy, Denmark's farmers help keep the country's power stations running by supplying big bales for burning in specially designed boilers. Farmers Guide visited one power station to see how this works in practice.

Big square bales, including many made by Massey Ferguson balers, are key to efficient biofuel energy production at the Avedøre 2 power station, which is located on the coast south of Copenhagen, Denmark.

Operated by Danish Oil and Natural Gas, Avedøre 2 is one of the world's most energy efficient combined heat and power (CHP) plants. It utilises up to 94% of the energy in the fuel for electricity and heat generation - meeting the heat demands of 200,000 households and supplying electricity to 1.3 million homes. The power station's biomass boiler plant is the largest in the world.

The power station is designed specifically as a multi-fuel plant, which allows it to use a number of different fuels from natural gas and oil through to a range of biofuels, including straw.

Last year the plant consumed 172,000t of bales - including rape, cereals and ryegrass - from 500 different farms in eastern Denmark. Every day, Avedøre 2 handles 65 lorry-loads of 24 half-tonne bales.

The straw-fired biomass plant consists of a boiler, straw store, ash separator and a system for handling the bottom and fly ash. The straw store holds enough bales to run the plant for two to three days, with deliveries arriving from Monday until noon on Saturday, every week.

The whole biomass side of the plant - cranes, straw lines and feeding system - are designed exclusively to handle bale dimensions of 1.2m x 1.2m x 2.5m long square bales, the bale size that reflects years of experience from farmers and contractors baling straw for industrial and other uses.

The plant will accept bales with moisture contents of up to 24% and farms are expected to store them under cover until they are required. Bales are then transported to Avedøre 2 on lorries stacked with 24 bales laid across the bed in two layers. On arrival, the trucks are unsheeted on a special gantry. Then the truck moves to the unloading area where the bales are weighed and ultra-sonic sensors are used to check the moisture content.

If the moisture content is OK, an over-head crane lifts off a layer of 12 bales in one go. The bales are then stacked in the storage area in a particular pattern, which is critical because from now on all the handling is carried out automatically.

Two special straw 'tables' feed the bales onto four straw lines that convey the bales into the power generation process. The first job is to remove the strings, which are cut and stripped off before the bale feeds into contra-rotating peg rollers that loosen the material before it is blown into the boiler and burned.

The steam generated by the biomass boiler is directed to the central generating turbine at Avedøre 2, which makes much better use of the energy in the fuel compared with using a separate steam turbine. Coal, wood chips and natural gas are also used to provide heat to produce steam to drive the generator.

Straw economics

Managers at Avedøre 2 would not reveal how much they pay farmers for their bales, other than to say that everyone is paid on a dryweight basis and that an element of the payment relates to the length of on-farm storage.

A recent report from the Danish Technological Institute, however, quoted the 2006 straw price for supplies to the country's 55 co-operatively owned district heating plants (which supply hot water for heating to homes) as £34/t.

The same report reveals that of Denmark's annual straw production of 5.5 million tonnes, 2.3 million tonnes was ploughed back in; 1.9 million tonnes was used on farms and 1.3 million tonnes was used for energy.

The split for straw used for energy production was 460,000t for central power plants; 330,000t for individual farm and household heating; 260,000t for district heating plants; and 250,000t for CHP plants (like Avedøre 2).

Like many agricultural outputs, transport can represent a major cost for farmers selling straw. Those using Massey Ferguson's latest MF2190 big square baler can, however, expect to produce bales up to 20% heavier than from the company's current baler range, cutting the number of journeys required to transport the straw by a fifth.

The Avedøre 2 power plant is on
the coast near Copenhagen.