Kiln drying operations in the spotlight

EVERY TWO to three years, the DryTech technology series provides local wood producers with an independent and practical update on new tools, kiln designs and schedules to improve their drying operations.
The latest program, DryTech 2008 runs in Melbourne on 26-27 November and again for New Zealand companies, in Rotorua on 1-2 December 2008. New and emerging technologies will be profiled by leading technologists and wood products companies drawn from Australia, New Zealand, the USA, Canada and South Africa.
In a presentation titled “Innovations in Solid Wood Processing-Kiln Drying Technologies” at a Wood Innovations program in September, Steve Riley, drying group leader at Scion identified a number of key challenges currently being addressed by researchers, product and service suppliers to the industry and kiln drying operations.
These included:
(1) A shift to greener processing. Drying consumes 70% of the energy used from forest to finished product. Much of this energy is expelled from kiln vents as low temperature water vapour. Utilising this energy means treating vast quantities of water. Energy costs and emission awareness are increasing, as is the awareness and regulation of water use – especially in Australia.
A number of initiatives including modifications to HT schedules, new kiln designs that reduce evaporated water emissions and a newly-patented process using super-critical carbon dioxide for the selective removal of water and solutes from the lumens of green wood are being worked on and introduced to the industry.
(2) Understanding the in-kiln drying process. For all the wood science research that has been done in the last century there are still significant unknowns that have huge impacts on the drying process. For example, basic wood properties and moisture transport dynamics under in-kiln conditions are not fully understood. Unique testing systems for evaluating the basic mechanical properties of wood under in-kiln environments are currently being developed.
(3) Thermal modification. The variable nature of instability (large dimensional change with humidity changes) in softwoods limits their use in some markets. The low durability of softwoods has been traditionally met with preservation treatments, but there is now decreasing market acceptance of the use of chemicals.
As a consequence, various European companies have promoted high temperature heat treated wood (i.e. PlatoTM, ThermowoodTM) and several commercial heat treatment systems are now available and being used.
(4) Impacts of scanning technologies on drying. Not adding cost to inappropriate material is a key factor in improving kiln efficiencies. This presents the challenge to develop systems that identify low grade and distortion-prone material and to sort timber into similarly behaving groups. This approach also presents the challenges of remediating low grade material and developing appropriate drying methods for individual sorts.
The benefits of green density sorting have been demonstrated in terms of improved drying uniformity. Improved green density sorting with microwave sensing is a possibility. Reducing degrade from twist in pith-in material using simple stack wedges to promote reverse twist has also been demonstrated but not widely used.
(5) Developments in moisture sensing. Despite increased adoption of in-kiln capacitance meters most users still rely on time or “hot checking” to determine drying end point. All methods are density dependent and rely on empirical calibration. The uniformity dilemma (i.e. high final MC leads to lower distortion but high standard deviation: low MC leads to low standard deviation but higher distortion) still complicates end point decision. Also, while the use of handheld non-contact type meters is common because of ease, they are limited by their density dependence and inaccuracy.
Recent research has included a large comparison of commercial in-line meters being undertaken in addition to a comprehensive Australian study on handheld meters. A ‘hands off’ version of the Dryzone in-kiln capacitance meter has been developed and Scion is currently developing a microwave based in-kiln sensor that gives density independent MC readings spatially dispersed across a stack over the full MC range.
These and many more developments, innovations and practical tools that can be adopted by local kiln drying operations – both softwood and hardwood – will be detailed in a practical series of workshops, managed exhibitions and discussion groups as part of the DryTech 2008 Australasian series. For more information on DryTech 2008 visit www.drytechevents.com