Joule Heating for Phytosanitary Treatment of Export Logs
There is a growing desire (and an international treaty commitment) to end the release to atmosphere of Methyl Bromide, which is currently used as a fumigant for phytosanitary biosecurity treatment of logs and other exports. Joule heating (or using the log as a giant resistor) shows great promise as an alternative and some early work was funded by MAFBNZ (now MPI). There is still uncertainty over the range of variations in electrical behaviour of different logs (of various timber species and moisture content), which may determine the limits of applicability of the technique.
The on-going project (initially funded under the STIMBR PGP and more recently under the STIMBR, Scion, MBIE Market Access Programme) has resulted in the design and construction of a full-size test rig encompassing a pair of special HV electrodes that allow the current distribution going into and out of the log to be measured, and a closed loop controller that can supervise the injection of up to 100kW into a log (driving the Foster controlled transformer in the HV laboratory) whilst integrating the energy supplied to terminate the test automatically.
This equipment allows live tests to be conducted in which the instantaneous power, current distribution, total energy input and temperature distribution can be monitored and logged (pun intended) to determine correlation between these parameters under different heating regimes.
Additionally a computer model of the Joule heating process in timber, incorporating the electrical and thermal mechanisms that take place is being developed. Research and development of instrumentation using the electromagnetic field around the log to estimate hot-spot formation is also underway.
"The timber industry needs simple, ruggedly-built, easily maintained-in-the-field, machinery that will whip logs through as fast as possible and require a minimal amount of technical expertise, or computerised controls, or manpower, to maintain quality control. I think your approach is excellent.” – Grant Knight, MAF