In North America, log conditioning during winter for stranding and veneer peeling purposes is traditionally conducted using water ponds. Some disadvantages associated with pond thawing of logs have been observed by the composite wood industry. Some of the disadvantages include lack of flexibility in terms of wood species, moisture content, and log diameter; reduced strand or veneer quality (due to uneven cooking caused by differences in log size); changes in wood chemical characteristics (due to acid accumulation in the ponds); and pollution (due to micro-organism growth).
A trial with radio frequency (RF) heating for log thawing was conducted in 2004 at the Laboratoire des technologies de l’énergie (LTE) of the Institut de recherche d’Hydro-Québec in Shawinigan, Québec. A total of 10 batches of frozen aspen log (-20oC) were treated with a pilot-scale RF kiln installed at the LTE. Each batch contained 9 to 11 logs (4 feet long) with three levels of log diameters (5-6, 10-12, or >12 inches) and two levels of moisture content (MC) (60% or 100%). In addition to log diameter and MC, power density based on wood volume in the batch (15, 22.5, or 30 kW/m3) was used as another main variable of RF treatment. The main objective of this preliminary study was to explore a fast and environmentally friendly method for conditioning aspen logs for use in OSB production.
The main findings of this preliminary study indicate that, compared with conventional pond heating, RF heating could greatly reduce log thawing time for those aspen logs with diameters over 5 inches. The energy consumption for RF heating was similar to that of conventional log conditioning with water spray. The quick log thawing method seems to be more promising for veneer peeling, a process that normally requires larger diameter logs. In the study, the energy transfer efficiency was defined as the percentage of calculated energy in the wood (heat absorption by log) over the measured energy consumption of the RF equipment (total energy input by RF generator) when heating logs from -20oC to about 0oC for all logs in the batch. An increase in RF power level from 7.5 to 20 kW significantly improved energy transfer efficiency from 23% to 64% among 10 batches of logs. Test results reveal that the energy transfer efficiency very closely mirrored RF generator energy conversion efficiency. This means that the best energy efficiency results can be obtained when operating the generator near maximum power, resulting in less energy being lost when the main power was converted to RF power.
The tests also revealed that at 22.5 and 30 kW/m3, lower MC wood thawed faster compared with higher MC wood. This is consistent with the fact that higher MC wood has a higher mass and a higher average heat capacity. Some uneven heating and thawing rates were noticed within a given log and between logs in any given batch. The uniformity was influenced by RF power density, log MC variation, and log piling patterns. Adjustments to the spacing between logs and the distance between the log pile and RF electrode seemed to somewhat improve the uniformity of heating and thawing.
Based on this preliminary study, it is believed that RF treatment is a promising alternative to traditional pond conditioning for log thawing. To investigate the potential of this method, additional research would be necessary. Recommendations for further study are: (1) explore log thawing at greater RF power densities; (2) explore different piling strategies and electrode geometries to improve heating uniformity; (3) evaluate the wood chemical and mechanical properties and their impact on OSB properties; (4) test other wood species or mixed species; and (5) explore the feasibility of RF treatment of wood for plywood manufacturing.
Acknowledgements
First of all, we would like to thank Mr. Sylvain Chenard, technician at the Laboratoire des technologie de l’énergie of Hydro-Québec, for his hard work, dedication and technical expertise. Special thanks is extended to Mr. Kuiyan Song, visiting professor from Northeast Forestry University, Harbin, China, and Mr. Antoine Henry, technician at the Eastern Division of Forintek, for their assistance in the preparation and characterization of material, and for conducting experiments at Hydro-Québec, Shawinigan.