7 June 2002


The prospect of an

expanding market for wood

chips to fuel power stations

is attracting new interest

in machinery to harvest

short rotation coppice,

says Mike Williams

IF the government is to meet its commitment to produce 10% of Britains electricity supplies from renewable energy sources by 2010, a big expansion in the acreage of energy crops such as short rotation coppice or SRC will be essential.

Prospects for real growth in the market for biomass for power stations have improved since the announcement of £30m worth of government funding to encourage production of crops for fuel. Establishment costs for crops such as miscanthus grass and trees planted for SRC production are high and they demand a long-term commitment by growers.

But last years funding news suggests the government is aware that substantial grant aid will be needed if acreage targets are to be met.

Progress is also being made in developing new power stations equipped to burn woodchip fuel, aided by capital grants financed from the government and EU. The first of the new generation SRC fuelled power stations is Project Arbre at Eggborough, Yorks, operated by First Renewables. It was commissioned earlier this year and will burn more than 800 tonnes of dry woodchip every week at full capacity to produce 10 megawatts of electric power, enough to meet the needs of 33,500 people.

Some of the Project Arbre fuel needs will be met by chipped forestry biomass and the rest will come from 1450ha (3600 acres) of SRC willow grown on farms within a 45-mile radius of the plant.

More power stations are on the way. First Renewables is planning a second power station at Grimesthorpe, Yorks with 35 megawatts capacity, Energy Power Resources expects woodchip to be on the list of fuels for a new biomass power station near Corby, Northants, and Border Biofuels has received planning consent for a new power station near Carlisle with capacity to burn 150,000 tonnes/year of energy crops.

Others are still at the discussion stage, with delays in some cases due to local planning problems, but within the six years or so many UK contractors and farmers could be within the supply area for one of a network of biomass burning power stations.

The biggest question is the harvesting operation, and at this stage the options include self-propelled forage harvesters with special wood cutting headers, and purpose built machines which may be either trailed or self-propelled. Another possibility is using a self-propelled sugar cane harvester, and this is the option chosen by First Renewables to harvest fuel for Project Arbre.

Winter harvest

Coppice wood is harvested in the winter, and advantages of the forage harvester approach include the fact that SRC provides additional out-of-season work for machines that already earn their keep harvesting silage. Apart from the header units, they represent tried and tested technology which many contractors are already using, and they have already shown their capability working with SRC grown for demonstration and trials purposes.

Forage harvesters also have potential disadvantages. Critics say they are too big and heavy for a crop that will sometimes be grown on a relatively small scale and is harvested when ground conditions are likely to be difficult. Another problem is that a forage harvester converts freshly harvested willow or poplar stems into small wood chips with a high moisture content, and the chips ferment during prolonged storage unless they are dried or kept in ventilated containers.

A tractor and trailer are needed with the forage harvester to move the freshly produced chips to the storage point, and this means more wheels working in tricky conditions. This also adds to harvesting costs, and it slows the job down unless there are two tractor/trailer teams to keep the harvester working non-stop, says Damian Culshaw, energy crops development manager for Biomass Energy, a subsidiary of Edinburgh based Border Biofuels.

Biomass Energy has been working with the Danish owned Nordic Biomass company in a joint project to develop a new self-propelled machine that takes a totally different approach to SRC harvesting. The machine is the Mantis, based on a much modified Morooka dumper with rubber tracks and a two-row saw action cutting unit at the front. A conveyor moves the cut stems and lays them horizontally in a container mounted behind the cab with the contents off-loaded at the headland by a hydraulically operated sideways tipping action.

The first Mantis harvester has been working this winter. It is powered by a 140hp Komatsu engine and carries a 4t load. As well as using it on their own crops, Border Biofuels believes the harvester could attract interest from contractors, and a production version will sell for about £180,000. It could also have export potential, especially in Sweden where SRC production is expanding.

"The next version of the Mantis will have a bigger container," says Mr Culshaw. "Increasing the capacity to 10t or 15t would allow the machine to harvest reasonably long rows without stopping. The container on the first machine fills too quickly and wastes time driving to and from the headland to be emptied. But the concept appears to be right, and leaving the harvested stems in heaps on the headland has several advantages. It provides ideal natural drying conditions, and the heaps can be left until the wood is needed at the power station."

Compressed stems

"We think this is likely to be cheaper than using a forage harvester and a tractor and trailer. The most efficient arrangement is probably to transport the stems from the headland and chip them at the power station, and we hope to develop a method of compressing the stems to allow a bigger payload to be carried."

One situation where a self-propelled forage harvesters cut-and-chip based system would be cheaper than using the Mantis is when the crop is to be taken straight from the harvester to the power station, says Mr Culshaw, and he predicts a need for both systems as the energy crop market expands.

Bob Smith, Fuel Supply Manager for First Renewables, says their experience with harvesting whole stems was not satisfactory. Instead they use an Australian built Austoft self-propelled sugar cane harvester which reduces the freshly cut rods to large chips or billets.

"They are about 50mm long, which is much bigger than the chips you get from a forage harvester. The Austoft chips are also a much more even size, and it is easy to ventilate them while they are being stored.

"We also like the Austoft harvester because it is very ruggedly made and it is mounted on tracks.n

Above: The Australian-built Austoft self-propelled harvester reduces the cut rods to large

chips or billets.

Left: Aerial view of the Project Arbre site at Eggborough, York.

Below: First outing for the Mantis harvester – an increase for its carrying capacity is planned.

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