A Review of Short-Rotation Forestry Harvesting in Europe

Raffaele Spinelli, CNR/IRL, Florence, Italy

Pieter Kofman, Danish Forest and Landscape Research Institute, Vejle, Denmark

Introduction

In Europe, the interest in energy forestry dates back to the '70s. The sudden shock of the energy crisis pushed national governments to investigate all the alternatives to fossil fuels - including energy forestry.

Since then the scenario has changed a lot, and new needs have been incorporated into the energy forestry concept. In many Countries, the major reason for using biofuels is the obligation put on utilities to reduce the emission of carbon dioxide. This coincides with a wish of the European Union to reduce the agricultural surplus.

Another important change - and a necessary implication of development - has been the subdivision of energy forestry in a number of specialized sectors. Short rotation forestry is one of such sectors, and it ranks among "recovery of forest residues", "thinning to energy" or "upgrading traditional energy forestry". This paper deals with the harvesting of short rotation energy forests, describing how Europe copes with the specific problems it poses.

Short Rotation Energy Coppice

As a definition "short rotation forestry" is rather ambiguous. For a forester it may describe a common poplar plantation: its 12 years rotation certainly proves short when compared to that of a "normal forest", cut after 70-140 years. Adding the attribute "energy" gives a marginal help only. Nobody can prevent us to tag 12 year old poplar for the energy market. Indeed, something similar already happens in some Countries. Similarly, the term "energy coppice" might indicate any conventional coppice grown for fuelwood.

The crop we are dealing with is just another thing. This paper concerns the harvesting of a certain type of short rotation energy forest, which happens to be the most common in Europe - and the one with the largest potential for expansion. We are talking about specialized energy forests, grown according to the "grassland" concept. Extremely dense stands, harvested at 3-4 years intervals and regenerated from the stools, which are expected to survive 5 rotations at least (fig.1).

Figure 2 shows a synthetic description of the most important such crops growing in Europe.

At present, short rotation forestry is still at the experimental stage. In Sweden, however, the experiment is carried out at full-scale proportions, with 11,000 hectares of short rotation willow planted and managed on a commercial basis. More willow is available in Denmark and in Britain. Austria claims several thousands of hectares of energy poplar, and many experimental plantations have been established in Germany. Robinia is being planted in Italy by Regional Management Agencies, who are keeping an eye on poplar and eucalyptus as well.

All these crops are very similar to each other in terms of rotation, density and yield. One may even say that they are regional versions of the same concept, each version being adapted to the local climate and terrain. Southern SRF - for example - resorts to species needing less water, which are grown at lower densities, yield less and produce drier biomass. Geography and climate, however, do not account for all differences. Some of them can simply be explained by the growers' preferences. This is especially true for the planting system. Willow growers generally adopt the twin-row system, with a spacing of 0.75 m in the twin-row and 1.50 m between the twin rows. On the contrary, people growing Poplar and Robinia seem to prefer single rows 1 m apart. The distance along the row is subject to large variation, and it is generally between 0.5 and 1.0 m. Of course, these "details" have a strong impact on harvesting technology and performance.

Systematics of SRF Harvesting Equipment

SRF compares neither to conventional forestry nor to common agriculture. The crop is a completely new one, posing completely new problems. The question of how to harvest these new crops was raised already 10 years ago. Manual system, conventional forestry equipment and unmodified agricultural machinery were all tried, but the experiments generally met with little success. Fortunately, much progress has been made since then, thanks to the efforts of many European scientists and to the confidence put by the National Governments and the European Union in the potential of SRF. Many projects have been carried out, investigating a number of alternative solutions. More projects and networks are still going on, and we expect substantial progress in the near future. So far, the most impressive result of these efforts is the production of a number of SRF harvesters, able to cope with a variety of different conditions. In the following pages we propose a systematization of SRF harvesting equipment, based on the harvester's functions, on its derivation and on its locomotion. This system provides vital, synthetic information that allows framing the operational scenarios connected to any given harvester.

Functions

SRF harvesting consists of four main operations: cutting, collection, extraction and comminution. The main functional difference between harvester types is the number and the type of operations that they can perform. In order of growing integration, we find the following functional types (figure 3):

Cut-only harvester. The harvester cuts the stems, laying them in windrows or heaps. Cut stems are then collected by a separate unit, which delivers them to a chipper. As an alternative one can use a chip/forwarder to collect, chip and extract in one pass.

Cut-and-bundle harvester. The harvester cuts the stems and collects them in bundles. Bundles are dropped on the field, like hay bales. They are later collected by a separate unit, most often a conventional forwarder or a farm tractor with forestry trailer.

Cut-and-extract harvester. The harvester cuts the stems, collecting and loading them over a deck of some sort. It then takes its load to the field edge or to any suitable landing. Chipping is the only operation delegated to a separate unit.

Cut-and-chip harvester. The harvester cuts, collects and comminutes the crop, delivering the chip at the field edge. In alternative, the extraction can be delegated to chip shuttles, to keep the harvester going. Chip shuttling is used preferably when the extraction distance is large.

All these four functional types are actually represented by some existing machines. However, cut-and- extract and cut-and-chip are by far the most common types.

Origin

SRF harvester can be classified according to their origin - i.e. the machine they derive from. Up to now, SRF harvesters have been derived from one of the following machines:

Cuttings harvesters. Some SRF harvesters are the blown-up version of cuttings harvesters, which are normally used in poplar and willow nurseries. This origin is especially common for cut-only and cut-and- extract harvesters. Of course, no cut-and-chip harvester sprouted from this source.

Forage harvesters. In the early days of SRF harvesting, unmodified forage harvesters where tried, with encouraging results. It is then logical that a whole generation of cut-and-chip harvesters has been obtained by modifying mass-produced combine harvesters. In fact, two of the best SRF harvesters now available - the Austoft and the Claas - derive respectively from a sugar cane and a forage harvester.

Prototype. In this case the SRF harvester is built from scratch. The majority of SRF harvester models have this origin. Of course, designers have often used mass-produce prime movers for their machine, so that building from scratch only applies to the harvesting device proper.

An harvester's origin involves certain consequences. The modification of a mass-produced unit - for example - is generally more reliable in its base components than a machine built from scratch. Besides, using mass-produced modules allows reducing production costs. On the other hand, modifying existing equipment requires a certain amount of compromise, which may involve renouncing the perfect match between crop and harvester characteristics.

Locomotion

Finally, one can class SRF harvesting according to the way they are connected to their prime mover. Again, there are three options:

Towed harvesters. The harvester comes in the form of a trailer, which can be towed by any conventional prime mover - most often a farm tractor. Towed harvesters are generally light and simple. They are designed for the part-time user, who wants a cheap device, easy to connect and disconnect to his multi- purpose prime mover. Limited mobility and low productivity are the most frequent constraints of towed harvesters.

Carried harvesters. The harvester comes as a module, which can be mounted on a wide range of prime movers. These machines are often heavier and more expensive than towed models, but they also offer better mobility and higher productivity. Connecting the harvesting module to the prime mover may require some time.

Self-propelled harvesters. They offer the best in terms of both mobility and productivity. On the other hand, they prove the most expensive due to the immobilization of the prime mover, often very expensive itself. Self propelled harvesters are thought and designed for the full-time contractor, who can provide his harvester with a sustained volume of work.

As we can see, locomotion provides a good deal of information on harvester capabilities and on its potential user. More can be inferred from the power of the prime mover and the overall weight of the whole harvesting unit.

An Overall Picture

Figure 4 gives information about a number of SRF harvesters that have been built and tested in Europe in the last 5 years. Most of these harvesters are still in use and many are undergoing further development. Some of them are used in commercial operations: this is the case with the Austoft, the Claas, the Dansalix, the Hvidsted and the Fröbbesta. Other ones did not work well and were dropped,

* Note: in the case of towed and carried harvesters, the figure applies to the harvesting trailer only, excluding the prime mover since no money was available for their improvement. They are the Biber, the Gandini, the ESM 901 and the Bender I, this one replaced by the better Bender II.

Of course, more harvesters are being developed in these days, to cover a larger range of harvesting conditions. At present, however, there are models to satisfy most needs. Specific harvesters are available for the part-time user and for the full-time contractor. There are models for farm tractors, so that individual farmers can harvest their plantations on their own. Some models can cope with difficult terrain, even during the wet season. This is the case of tracked harvesters, such as the Austoft, the Hvidsted or the ESM 901. Concerning high-mobility harvesters one point must be stressed, however: their special capabilities are no use, if biomass extraction is delegated to conventional units. In this case, the harvester will cope with the soft or sloping terrain, but the chip-shuttle will get stuck. Some attention should be paid to the auxiliary units as well. So far, studies have concentrated on harvesting and extraction has been neglected. Things are changing, however. In Britain, for example, a Caterpillar Challenger tractor has recently been tested along the Austoft harvester, operating on soft terrain.

This paper analyses in detail the Austoft and the Claas harvesters. These two machines have attracted considerable attention in Europe, and have been studied extensively by several teams. The Authors themselves have carried out a series of trials aimed at evaluating them in both Danish and Italian conditions. Most of the work was done in the scope of the European Project EU-AIR2-CT94-1102, “Harvesting and Storage Technologies Essential for the Establishment of Short Rotation Coppice as an Economic Source of Fuel in Europe.” However, similar trials have concerned most harvesters, and data are available for the majority of the models listed in figure 4. Yet, there are several reasons for giving a special place to the Austoft and the Claas in this paper.

The first one is that they are cut-and-chip harvesters. Cutting and chipping in one single pass is still the most effective option, as far as harvesting logistics are concerned. Harvesting and chipping in two separate stages may be more sensible in a storage perspective, but it is still disproportionately expensive and therefore totally impractical.

Secondly, the Austoft and Claas harvesters are some of the most mature products now available. They result from the modification of mass-produced machines and are much more reliable than those models that have been built from scratch. These ones can be more innovative and have bigger potential - at least in some cases - but their development is longer and requires much effort and money. As a consequence, the majority of harvesters built from scratch are not mature yet, and often require considerable improvement.

Finally, both the Austoft and the Claas are the only SRF harvesters used in large commercial operations. They are owned by actual entrepreneurs who harvest several hundreds hectares each year. This can be of special interest to the US reader, who generally regards agriculture as a large-scale, industrial activity.

The Austoft 7700

The Austoft 7700 is a self-propelled sugar cane harvester, adapted to SRF harvesting. The machine consists of a 179 kW tracked prime-mover with a cutting-conveying header in the front, a comminuting device in the middle and a belt conveyor in the rear (figure 5). All functions are hydraulic. Mobility benefits from the long steel tracks and the hydrostatic transmission.

Figure 5 - The Austoft 7700 cut-and-chip harvester

The cutting head consists of two disc saws placed side by side, which can harvest two rows at a time: stems are pushed forward by an adjustable pushing bar and are directed to the saws by two vertical feeding augers. An horizontal feeding roller mounted on the tip of a pushing bar also contributes to correct feeding. Once they are cut, stem butts jump upwards and horizontally into the infeed mechanism and are taken to the comminuting device. This is a two-blade propeller, which cuts 5-10 cm chips and let them fall down to the bottom end of a ladder conveyor. The ladder conveyor is attached to the rear of the chassis and can be rotated 170·, so that it can direct chip flow to shuttle units approaching the harvester both to the left and to the right side.

Figure 6 shows the average productivity recorded in 5 trial programmes.

All studies agree in describing the Austoft harvester as a sturdy, reliable machine. Some blockages of the infeed mechanism have been recorded in Britain, and attributed to the thick weed layer. In any case, the Austoft showed the lowest downtime rate of all machines tested in the British trials.

General agreement is also on the high mobility of the tracked harvester. The machine can negotiate both steep and soft terrain. In Denmark it harvested the wettest spots without any trouble. In Britain and Italy, it harvested wet slopes of over 20% gradient, uphill, downhill and sideways. Such a good performance on slopes is due to the low center of gravity, which falls in the center of the harvester. In turn, this explains the backwards and forwards pitching recorded when traveling at speed.

Harvesting speed is largely variable. It was lower where the crop was thicker, such as in some Danish, British and Swedish stands. In Italy, where the crop was thinnest, harvesting speed exceeded 8 km/hr. This confirms the assumption that the Austoft may benefit from a more powerful engine.

Excessive stool height was recorded everywhere. The Danish and British reports indicate a large variability in stump height: from 0 to 38 cm. This can be explained by the backwards and forwards pitching of the harvester at speed, mentioned in the same report. Besides, cutting height adjustment is generally inaccurate: the system should either changed or coupled to a precision gauge, acting automatically.

Stool damage was frequent and severe in all trials. Bush blades are held responsible for it. Both the British and the Swedish studies compared bush blades with conventional circular saws, concluding that the use of circular saws substantially reduces damage severity. Circular saws were tested in Italy with the large-size poplar, but they proved too flexible and were soon replaced. The Italian study identifies a further cause of stool damage in the peculiar cutting height adjustment system. Cutting height adjustment is obtained by tilting the whole chassis upwards or downwards, so that any variation of the cutting height will also result in a change of the cutting angle. If the cut is too high, the saws will work on a horizontal plane and the stools will be hit by the lower of the two fixing plates that sandwich the blade.

All studies indicate that the Austoft creates a very limited soil disturbance. Rutting was absent in most cases, and whenever recorded it was blamed on the chip shuttle fleet. Harvesting losses are limited, seldom exceeding 4% of the standing crop. Chip quality was found mediocre in most studies. All reports mention the presence of numerous oversize particles in the Austoft chip.

On-road transportation can be a problem. The Austoft 7700 is a tracked vehicle and cannot travel on roads. Moving between different harvesting sites requires a deep loader and involves a certain amount of delays. Small, scattered stands are unsuitable for the Austoft system.

In general, the Austoft system requires a number of support units, whose movements must be carefully planned and supervised. The logistics of such system can be rather complicated, and are made more difficult by the excellent machine performance. This does not mean that the system cannot be managed, on the contrary. But its management requires skilled professionals and careful planning. In fact, the Austoft system is designed for full-time contractors, who should be skilled enough to use it effectively.

The Claas Jaguar 695

The Claas SRF harvester consists of a conventional forage harvester, equipped with a new header for harvesting short rotation coppice. The header is fitted to the standard faceplate of the Claas Jaguar, so that all owners of this Claas model can expand their job range to SRF harvesting (figure 7). The prime mover is powered by a 230 kW engine and has hydrostatic transmission on all four wheels.

Figure 7 - The Claas Jaguar cut- and-chip harvester

The special header consists of two counter-rotating disc saws, placed side by side and separated by a crop divider. Horizontal feeding rolls are also provided, both before and after the saws. A pushing bar is mounted on top of the assembly cover and can be adjusted hydraulically.

The base machine is fitted with its standard processing drum, with 12 knives instead of the usual 24. The 12-blades configuration produces 28 mm long chip, which is accepted by heating plants, but it is still rather short for optimum conversion.

The many hydraulic functions of the SRF header exceed the capacity of the standard hydraulics of the Jaguar Mega. For this reason, the SRF version is fitted with auxiliary oil pump and tank.

Figure 8 shows the average productivity recorded in 5 trial programmes. The Claas harvester is a very productive piece of equipment, given the right site conditions. Productivity largely varies from Country to Country. The Danish and Swedish figures are the highest. In Italy and in Britain, Claas productivity was comparably lower. This can be explained by the lower stand stocking, and especially by the different row system. Both in Italy and in Britain, the Claas harvested one row only, and harvesting speed did not substantially increase if compared with double-row harvesting.

The Claas harvester is generally described as a reliable machine. However, both the British and the Italian report mention a large incidence of infeed jams. Blockages exceed respectively 40% and 55% of the net cycle time. The problem is certainly related to the row system. The Claas is designed for harvesting double-rows and dealing with single rows may result in attacking the crop at wrong angle. As a consequence, stems tend to jam between the blades and the crop divider, the central height skid or the vertical spacer.

Mobility is a problem. The Claas must be operated on flat, firm ground. In Middle Sweden this is not a problem, since the soil is frozen during the harvesting season. However, the climate of Southern Sweden, of Denmark and of Britain is considerably milder and frozen soil conditions are not assured every year. In this case, the Claas will prove too heavy to negotiate the wet sites where willow is grown. Both the Danish and the British reports highlight this problem. In Italy, the problem is slope.

Everything goes well when harvesting poplar, which is grown on flat, sandy soils. On the other hand, nobody even tried to take the Claas on the slopes where Robinia grows.

Limited cross-country capabilities are somewhat compensated by high road mobility. The Claas harvester is not dependent on a deep-loader for its transportation, especially if the fields are grouped within a few kilometers radius.

Another asset of the Claas machine is the possibility of converting any conventional Jaguar forage combine into an effective SRF harvester. This will allow better depreciation for the base machine. However, this advantage should not be overestimated. The SRF conversion kit is rather expensive, since it includes not only the header, but also the additional oil pump and tank.

Most studies report stool heights above the 10 cm limit. However, the Claas can cut lower and more regularly than the Austoft. The cutting height adjustment device is more effective.

British studies mention minimal stool damage, the lowest recorded with the range of machinery tested. On the contrary, heavy stool damage was observed in Italy. The fact can be explained by the different tree species. Poplar is less flexible than willow, and the bending stress applied by the Claas to the stems to be cut is more likely to result in deep stool splits, such as those observed in Italy. Besides, the Italian poplar was planted on small ridges, resulting from inter-row tillage. The Claas had to raise the cutting height to avoid grazing the soil, but the combined height of the stool and the ridge was enough for scratching the harvester’s belly in many occasions. This might have resulted into further stool damage.

Only the Italian report mentions extensive soil damage, which is blamed on the chip shuttle units rather than on the Claas itself. Very little soil disturbance was recorded in Denmark and Britain. In Sweden, concrete-like frozen soil prevented all problems. Harvesting losses are slightly higher than those recorded for the Austoft. All studies agree that Claas chip is regular, yet too small.

A limit of the Claas harvester is its dependency on a precise row spacing. This means that Claas harvesting must be planned at the establishment. Even if everyone agrees that harvesting must be planned since the establishment stage, reality often differs from optimum theoretical rules and the fact is that many stands are established giving little consideration to all following stages. In this case, the limited flexibility of the Claas system offers a little edge to bad planning.

Conclusions

Harvesting short rotation energy coppice requires special equipment. In Europe, large efforts have been done to design, build and tests suitable machinery. A number of harvesters have been produced, and some are already employed in large-scale commercial operations.

The Austoft and the Claas are such harvesters. Both work fine and achieve high harvesting productivity. The Austoft is sturdier and enjoys better off-road mobility, whereas the Claas can travel on-road and inflicts less damage to the stools.

However, a number of problems still have to be solved. Some concern machine design only, and can be tackled by mechanical engineers. Others involve the crop/machine interaction and must be faced by growers and engineers together. The most important among them are off-road mobility and crop spacing.

Most SRF harvesters have limited cross-country mobility. As a consequence, soil bearing capacity becomes crucial to SRF harvesting. If the soil is too soft, most machines will bog down. The Austoft is the only harvester that can negotiate soft soil, but its capability has no use if the support fleet will eventually get stuck. Then the point is the careful selection of the sites where one will plant short rotation crops. Either one refrains from planting in the wettest spots or new machinery will have to be designed.

Irrational spacing is detrimental to work efficiency. It will slow down most machines and stop some of them. More thought must be given to correct field design. The double-row system works fine for most harvesters, even if it is not the best for all of them. The Claas cannot harvest effectively single rows, if the inter-row is smaller than 1.5 m. Even so, the productivity will be greatly reduced. In fact, the 75/125 cm twin-row system was designed especially for the Claas. However, this spacing will result to narrow for the same Claas harvester, when fitted with wide tires. For the time being it is advisable to conform with the internationally agreed 75/150 cm twin-row system.

Literature

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File posted on March 17, 1998; Date Modified: February 21, 1999