Clamping is an important part of managing the feedstock supply for AD plants. Jane Carley gets some tips from the experts.
With 10 anaerobic digestion (AD) plants across the east and north of the country, and a demand for 400,000 tonnes of feedstock per year, Future Biogas director of farming Innes McEwen must work closely with farmers and contractors to ensure feedstock of the highest quality is grown for the plants, and harvested and clamped for optimum energy output and minimum losses.
He says: “For Future Biogas this involves a lot of forward planning – we want to make best use of the feedstock without being left with maize or grass over, so we may have to allow access to different parts of the clamp at various times of year.
“We also monitor the harvest and then keep an eye on the clamp throughout the year to check how well compacted it is, and assess energy output and dry matter losses.”
The clamp governs the pace of the harvest, much as it does in any silaging operation, he adds.
Feedstock manager Seb Edwards expands on this: “Logistics dictates the forager can’t be utilised to 100% of its potential – the clamp would be swamped and the operator would be too busy pushing the crop up to roll it properly. It’s a matter of getting the right amount of clamp machinery to gain the optimum practical utilisation of the forager.”
While the actual choice and brand of machinery is left to the contractor, Mr McEwen has one or two preferences.
“I am wary of using loading shovels to compact the clamp – they can be very good in the right hands, but the reverse is also true.”
Mr Edwards concurs, but adds: “Loading shovels can have their use on old grass to flick or manoeuvre the drier crop around the clamp, while a machine with a blade is better for maize.
“We also recognise a loader is a versatile machine for a contractor who also works in applications such as muckspreading where the ability to spread its weight is required – but on the clamp we need that concentrated weight.”
High capacity tractors such as the Claas Xerion and Case Quadtrac have been successfully used, and rear-mounted rollers such as the Silapactor are also useful, he says.
The size of the operation dictates the amount of compaction machinery needed: “A tractor such as the Xerion can push five times its own weight per hour, so one 18t Xerion would be sufficient if 100t/hour is coming in. Any more and you would need a second machine, although this could be a smaller tractor with a buckrake to push the crop up while the Xerion is dedicated to rolling,” says Mr Edwards.
The type of feedstock also has an influence on the compaction machinery needed, says Mr McEwen. “You may have enough weight for grass or early season maize, but be aware the product gets drier later on. We found the rye was dry this year and needed more weight to compact it.”
Well-planned, slow, careful clamp building is key, he advises. Logistics between field and clamp are an important consideration and may be partly governed by movement restrictions imposed as part of planning conditions, as well as by economics.
As a rule of thumb, Mr Edwards suggests two tractors and trailers are sufficient for a clamp which is less than a mile from the field, but at more than three miles away, six tractors and trailers would be needed.
“In practice there is usually little difficulty in sourcing enough tractors and trailers,” Mr McEwen says. “We have also successfully moved crop using a transfer system in the field and HGVs on the road.”
Future Biogas’s contractors also use transfer systems such as the Ropa Maus – originally designed as a sugar beet loader – or walking floor ejector trailers to fill lorries at the headland, a system which finds favour with growers looking to control traffic and cut compaction.
“Lorries are also more economical for longer journeys – costs for a tractor and trailer are £40/hour, while at £55/hour a HGV can transport twice the load,” says Mr McEwen.
High forage quality and a short chop length – best achieved with a specialist biogas drum on the forager – have a big effect on the success of the digestion process and thus dictate the suitability of the harvesting machine.
“We need a product with a short chop which is consistent throughout the clamp as this provides an increased surface area for digestion. It does differ between forage harvesters, but regular servicing and knife sharpening are more important than the brand,” Mr McEwen says.
This is especially significant when harvesting hybrid rye, he adds. “We only have a two-week harvest period – this year the rye seemed to turn particularly quickly. Dry matter was holding, but cellulose decreased and lignin levels came up, which makes it harder to manage in the clamp.”
The business also uses its own teams to cover the clamps. “I felt it was a distraction for harvest contractors – the last thing you want to be doing after 13 hours of harvesting is covering a clamp. The clamp covering team comes in at 8am and focuses purely on that,” he adds.
Digestate application procedures vary widely across the sites, from being part of the main contractor’s role to a task for specialist application firms.
“For some of our grower groups, the opportunity to apply digestate to their land is part of the attraction but there is no definitive method of application. We recently took some of our farmers to Germany to study a wide range of methods, which can depend on the location, the landlord’s requirements and the amount of land available for spreading. We are constantly looking at new technology, such as the Mississippi Digestate Dryer which concentrates the nutrients and produces aluminium sulphate,” adds Mr McEwen.