Review of eucalypt wood processing issues
By Dave Cown, August 2016.
Download SWP-T016 (pdf)
The vast majority of eucalypt plantations worldwide (at least 20 million ha) have been established as fast-growing exotic biomass forests for pulpwood and energy in tropical and sub-tropical environments and have been a major success in that regard.
The Australian hardwood industry was founded on the utilisation of eucalypts, and has now moved on from a long history of unseasoned utility timber to an industry aiming for higher-value sophisticated solid wood products for appearance and structural uses, since eucalypt forests can achieve high growth rates and the wood is known to potentially have superior performance characteristics for many applications. Almost any conventional sawing system can be applied to eucalypts provided there is an understanding of the material properties of the logs, but eucalyptus logs are undeniably more difficult to process economically, mainly due to the common presence of growth stresses, high wood shrinkage and associated internal defects. Nevertheless, it is possible to produce timber of high appearance and strength quality although economic yields cannot be guaranteed for various reasons.
Prior to about 2000, most conversion research on plantation eucalypts was ad hoc, without carefully considering production issues or market requirements. It has now been accepted that material properties should be carefully considered when planning all processing activities from felling and log-making to primary log breakdown, sawing and drying. Research scientist, Dr Russell Washusen (formerly a CSIRO world expert in eucalypt utilisation), has concluded that one of the biggest remaining challenges in processing young eucalypt plantations is the limitation of the adverse effects of growth stresses and shrinkage in both sawing and drying, and both must be carefully considered in any new enterprise.
Production lines that can be considered consist of frame-saw lines, double-arbor-multiple-circular sawlines, single to quad-band lines or even rotary veneer peeling for the production of laminated veneer lumber. The basic approach to sawing eucalypts in South America (where most eucalypt sawn timber is produced) is a twin saw unit and re-sawing the centre cant with a multi-saw. This basic principle applies over a very wide range of throughputs, from about 3,000 m3/year to over 100,000 m3/year. The larger mills use more sophisticated, faster equipment with more saws, such as quad band break-down units and combined optimisation technology. Modern technology may not be particularly well suited to the NZ situation where log supply is erratic at best. According to one of the most comprehensive reports on eucalypt processing, even technological advances such as scanning, sawing and drying methods, can clearly help to improve the efficiency and recovery levels, but cannot in themselves overcome material-related defects such as knots, grain deviation, shrinkage, tension wood and gum veins. To take full advantage of the potential of the eucalypts, it is necessary to understand the specific material characteristics of the species considered.
Conventional wood drying methods need to be altered from softwood schedules to provide greater control over the timber drying conditions, moisture content, and the effectiveness of re-conditioning treatments. Such control, coupled with improvements in sawing accuracy and sawmill efficiency, should improve product recovery and quality and reduce processing costs.
Eucalypts in general have a reputation of superior strength and stiffness and this has been confirmed in NZ studies. The reputation of some species for high in-ground durability, based mainly on testing of old natural forest material, has perhaps been exaggerated for plantation products (in fact treated radiata pine is superior in both above-ground and in-ground situations). Young material from the same species is unlikely to show the same level of heartwood development and resistance to decay as the old-growth on which many of the Australian durability classes are based. Those that have been shown to perform best in this regard are either absent or very scarce in NZ, due mainly inappropriate site conditions.
The eucalypt situation in NZ has been fluid, with several phases where groups of species have been recommended based on the evidence of small experimental plantings – often on farms and in shelterbelts. As elsewhere, NZ research has been heavily focussed on genetics, siting and silvicultural issues rather than wood processing techniques. After many studies in NZ during the1980‘s, it was concluded that for solid wood products, emphasis should be given to the ash eucalypt group (and E. nitens) as well as E. saligna and E. botryoides. Later on, more emphasis was given to the stringybarks. However, few sawmillers have become thoroughly familiar with the specialised techniques required to successfully process eucalypts. For better success in the future, operators must be trained in these specialised techniques and this will be best achieved in specialist eucalypt processing plants with a consistent log supply. The fact that most of the eucalypt plantation resource is very varied, fragmented and not specifically grown for solid wood products, limits the chance of success.
One of the major current issues for New Zealand is that decisions regarding choice of genetic stock, siting and silviculture are often made by landowners, decades ahead of utilisation considerations without robust information on wood properties, technical behaviour, future market conditions, or appropriate processing technology. Australian researchers have stressed the importance of appropriate silvicultural treatments involving both thinning and pruning. On the other hand, a few small-scale NZ studies have confirmed that favourable results are possible in situations where good silviculture has been applied and sawing methods have been well researched and applied. However, the merchantability of other material unsuitable for sawmilling is likely to be critical to overall financial success. This is linked to silvicultural treatments which produce sufficient high grade pruned sawlogs, the acceptability of knotty timber from unpruned logs and the merchantability of thinnings.
Establishment of plantations aimed at solid wood products involves longer rotations, higher costs and greater biological and market risks than simply growing for pulpwood or energy crops. Several species show outstanding ground-durability properties, and fast growth rates, but there is often a trade-off in terms of susceptibility to pests and diseases. However, the profitable management of young eucalypt resources on an industrial scale has been elusive, and remains a major challenge for the wood processing industry around the world.
Some eucalypt species are reputed to exhibit very high in-ground durability, and this has become a sought-after property in order to limit the use of chemicals in wood processing for agricultural uses. While this sounds attractive, great care must be taken to verify the growth, health and durability of young material from selected species grown under NZ conditions. A robust long-term researchstrategy must be followed, including the establishment of “graveyard” trials in different environments.
There can be no doubt that eucalypt plantations are potentially capable of producing large stems with DBH to around 60 cm on relatively short rotation under good conditions, and the challenge is to process them profitably into a range of useful products. In some areas, the phenomenal growth rates recorded (e.g. up to 70 m3/ha/annum) have helped the overall economics. However, eucalypt processing is a specialist skill, not to be taken lightly. Those undertaking it should ensure that the logs used have been well tended (thinned and pruned) and that they have read at least some of the voluminous literature from the more comprehensive research completed in Australia.
Developing a robust eucalypt processing industry requires a range of skills. Growers need to be aware of the best establishment, silviculture and harvesting techniques while processors should be aware of the behaviour characteristics of the species they are dealing with, including defect occurrence, growth stress development (significantly affecting conversion rates), and changing market requirements.
Value-chain R&D similar to past targeted efforts is essential to maintain progress.
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