Mechanization of Short Rotation Intensive Culture Forestry (in the UK)

Damian Culshaw, Energy Technology Support Unit, Oxfordshire, United Kingdom

SUMMARY

Wood as a fuel is currently used in the UK in the domestic sector and in the wood processing industry although there is now an incentive to generate electricity from it. Short rotation intensive forestry crops are being considered as an energy crop on land set aside from agriculture. A 2- TO year cycle coppicing system is being developed using willow and poplar. Agricultural techniques and machines will be used and the husbandry techniques will be similar to those employed in Sweden.

Work on the development of mechanization of the crop is being funded by the UK Government's Department of Trade and Industry (DTI) under a program managed by ETSU. Work on harvesting machines has already started and work on transport and supply is due to start shortly. Modelling of the drying and storage of bulks of wood chips has already been advanced leading to some understanding of the processes involved. This work continues and will be applied to short rotation crops.

INTRODUCTION

In the UK, wood as a fuel is used mainly in the domestic sector of the energy market. Other applications for wood as a fuel are in the wood processing industry. In situations where wood residue is a disposal problem then it is often economic to use it on-site for generating process steam for kilning; examples of this application exist.

Short rotation forestry in the U.K. is seen as a pure energy crop and is not planted for paper pulp manufacture as it is the U.S.A., although some traditional coppiced woodland of hazel and sweet chestnut is still harvested for hardwood pulp in the south of England. Another use for all types of short rotation forestry is for stabilizing soil embankments around roads and waterways.

The advantage of energy cropping in a short rotation forestry system over using by-products from conventional forestry is that energy cropping allows the production of wood fuel in a concentrated area. The crop can then be sited close to the heat or power plant without experiencing the high transport costs which are often incurred when using forest residues as a fuel. A further incentive which applies throughout Europe is that some agricultural land is now surplus to requirements for food production and cropping of energy is seen as a way of usefully employing some of this resource. Short rotation forestry crops usually grown as coppice fit into the existing agricultural systems better than traditional forestry crops.

The research to date has covered a number of areas including clonal selection, spacing, disease control and harvesting. Willow and poplar are the clear leaders from a yield point of view in the British climate and the trail plots are based on these two species. In 1991, 5 commercial demonstration plots at various sites around the south of England were established by a group of farmers brought together by ETSU. These growers will act as market centers in their locality and will generate wood fuel markets and encourage other local farmers to contribute by growing extra material. Each of the initial growers is committed to plant at least 10 ha, each has planted around 6 ha so far with the final area due to be planted in spring 1994. In addition to these research plots other areas are planted around the country and the total area is around 100 ha.

Cultural Practices

(Plate 1. Salix Maskiner Four Row Step Planter From Sweden) Cuttings are planted at around 10,000/ha into a pre-prepared seedbed using a mechanical planter. A modified cabbage planter can be used although other mechanized techniques have been developed such as the Salix Maskiner step planter from Sweden .

(Plate 2. Fröbbesta Cuttings Harvester from Sweden) Herbicide is applied in the early stages of growth when the crop is particularly susceptible to competition from weeds. After one year of growth, the sticks are cut back to ground level to encourage coppicing and to provide cuttings for subsequent plantings. This can be done by hand although Swedish machines have been developed to carry out this operation. Following the 1 year cut back, the crop is allowed to mature for a full rotation which in the U.K. is between 2 and 5 years. Crops are then cut at the end of each rotation when weed treatment may be applied. For planning purposes yields of 10 to 12 dry tonnes of wood per ha per year can be expected on a reasonably fertile site.

Economics

If short rotation forestry must compete for land with agricultural crops, the opportunity cost associated with not growing food crops would, in most cases, mean that the crop would not be economic. Within the European Union, reform of the Common Agricultural Policy has lead to a need to 'set aside' some of the land previously used for food production. The growing of short rotation coppice on set aside land has been agreed so this overcomes the opportunity cost problem.

Another incentive for wood fuel in Britain is given to generators of electricity from non-fossil fuel sources. Such generators are receiving premium prices and a guaranteed market under a system known as the 'Non Fossil Fuel Obligation' (NFFO) designed to stimulate development of these technologies. Although no schemes are yet producing power from short rotation forestry, projects are being planned under the current round of this scheme. The results of the current competitive bidding process will be known at the end of 1994.

UK DEVELOPMENT EFFORT ON THE MECHANIZATION OF SHORT ROTATION COPPICE

Two projects are currently being conducted under the ETSU managed program which are relevant to this conference. These are on the harvesting of short rotation forestry and on the drying and storage of the crop. A further project on transport and supply logistics is also due to start shortly and this will cover all the biomass crops. The work on harvesting and on drying and storage which has already started is described below.

Harvesting

The development of effective mechanization from harvest through to the utilization plant is critical to reduce the delivered cost of the fuel. Costs must be reduced to levels similar to those for comparable agricultural crops if the coppice is ever to compete with fossil fuels while still giving some return to the growers. The British program aims to do this in the following ways:

• Provide data on the best of the harvesting machines available in the UK and Sweden under British conditions.
• Demonstrate to the industry the best of the systems available.
• Provide data on the costs of comminution of coppice material which has been stored as sticks.
• Adapt or modify existing machines developing them so that they operate effectively under British conditions.

In Sweden energy cropping is more advanced: they have around 9,000 ha of willow coppice planted with further plantings scheduled for 1994. As a result, their harvesting machines are more developed than those in the Britain and there exists the possibility of transferring this technology. However, transfer of the technology may not be as straight- forward as might be hoped because of climate and differences in the end use. Also, a significant proportion of the British crop is likely to be poplar and not willow.

Harvesting machines which currently exist fall into two categories, those which in a one pass operation 'cut and chip' the coppice material and those which simply cut the material leaving the product as a pile of full length shoots (sticks). A full review of the harvesters which are available resulted from the early part of the study (Ref. 1).

There are two commercially available cut and chip machines operating in Sweden :

An unmodified Claas Jaguar self propelled forage harvester fitted with a header developed by Claas themselves. This is currently being developed but is expected for the 1995 harvest. Several hundred of these forage harvesters operate within Britain, harvesting grass and maize for silage in the summer and autumn. These machines, therefore, would be available for harvesting coppice in the winter. Plate 3, Claas Jaguar Forage Harvester With an Earlier Model of the Header, shows the harvester with the 1993 prototype header.

The other cut and chip machine is a self propelled sugar cane harvester imported from Australia and known as the Austoft machine (Plate 4. Austoft Sugar Cane Harvester).

Several stick harvesters also exist both in Sweden and in this country. These range from simple tractor mounted hedge cutters through to Swedish self propelled machines which will cut the sticks and carry them to the end of the row.

The project on harvesting, which is part of the DTI's program managed by ETSU, is co-funded and being carried out by the Forestry Authority. The first harvest under this project was conducted during January 1994. The Austoft sugar cane harvester and the Fröbbesta tractor-trailed stick harvester (Plates 4 and 5, Fröbbesta Coppice Harvester from Sweden (1992 prototype)) were brought over from Sweden and tested under British conditions. Both were found to be capable of handling poplar crops of up to three years old as well as the willow for which they were developed. The Austoft sugar cane harvester which runs on steel tracks coped remarkably well on a very wet, sloping clay site in Northern Ireland.

Drying and Storage

Since 1989, drying and storage work has been a part of the DTI program managed by ETSU. The work has been conducted by Silsoe Research Institute (SRI). The work is directed towards coming to an understanding of the behavior of wood chips as they are dried and stored. This will enable us to make some sense of the experimental data coming from all sources. The same techniques as are used for the drying and storage of grain have been employed. The characteristics of forestry wood chips rather than grain have been quantified and the computer models used for predicting the behavior of grain have been adapted accordingly. Some limited validation of the models has been carried out under the project already completed.

Various low cost techniques for storing wood chips have been devised using the models and the theoretical understanding which has been gained has allowed interpretation of some of the experimental work reported in the literature. Until this modelling work, individual experiments were valid for the conditions in which they were tested, but extrapolation of results from one experiment or set of conditions to another was not possible.

The objectives of the current study are to establish effective methods for the storage and drying of coppiced willow and poplar intended for fuel. This means that storage will continue without excessive deterioration by microbial action and will be at an acceptable cost. A further objective of the current project is to show how these methods can be implemented in practical situations.

The program to fulfil these objectives will :

• Determine the relevant properties of coppice wood chips which will be different to those for forestry residues on which data exists; also data is needed in order to model the behavior of coppice wood chips in waste heat dryers.
• Design storage regimes for coppice wood chips which can be tested at full scale.
• Test the optimal storage regimes suggested by the models at full scale, monitoring all the data needed to further validate the model and to give the operating costs, storage losses and relevant quality parameters.
• Update the models, refine the techniques and repeat the process for two more storage periods using a variety of on farm stores taking the opportunity to demonstrate the techniques to the industry.
• Produce an information pack for industry on how to store wood chips.
• Examine the storage of coppice wood as whole shoots.
• Model the drying of wood chips by use of waste heat from power generation plants.

The expectation is that sufficient understanding of the behavior of stored coppice chips will be achieved, to predict with some confidence, the performance of any practical store or drying technique. This will result in reliable data which can be used to advise the industry on the cost and performance of any particular store before and after commissioning. The results of this study will also be applicable to any chip store anywhere in the world by feeding in the relevant in the U.K.

CONCLUSION

Short rotation intensive culture forestry is being developed in Britain for energy use and may be used for electricity generation in the future. Under the U.K. development program, the first harvesting trial has been successful with Swedish machines performing well under wet British conditions even on poplar. Work on drying and storage of short rotation forestry crops is proceeding. The work is based on an understanding of the drying and storage processes which are now incorporated into a computer model. This means that the work can be applied to any store anywhere in the world.

REFERENCES

1. Technical Development Branch Report no.1/94. This is available from the British Forestry Authority, Technical Development Branch Headquarters, Ae Village, Dumfries, DG1 1QB phone 44 387 86264.

File posted on March 5, 1996; Date Modified: February 21, 1999