Practical Application of Joule Heating to the Sterilization of Plantation Grown Pinus radiata Logs
Pinus radiata (D. Don) log exports are one of New Zealand’s major foreign revenue earners, with 12.8 million cubic metres, valued at nearly NZ$1.7 billion, shipped in 2011. Most trading partners require logs to be treated before shipment from NZ, to prevent the inadvertent import of unwanted pests. This is carried out by the use of the fumigants methyl bromide (MeBr) or phosphine. MeBr is an ozone depleting substance and has been phased out for all but quarantine and phytosanitary purposes. The New Zealand Environmental Protection Agency requires that, by 2020, MeBr used in New Zealand is recaptured after use. Alternative phytosanitary treatment methods are required, with heat being one option. Trials using electrical Joule heating were begun at the EPECentre in 2007. Early results indicated that the technique was worthy of deeper investigation. Further EPECentre funded work, with later support from the Stakeholders in Methyl Bromide Reduction and the Ministry for Primary Industries Primary Growth Partnership, has shown that the technique could be used to successfully treat logs in accord with importing country requirements. The work is now funded as part of a six year STIMBR-Ministry of Business, Innovation and Employment co-funded research programme, led by Scion. A test rig, incorporating novel segmented electrodes and an automated data acquisition and energy control system, has been built and commissioned in the HV laboratory at UC. This rig is powered by a single phase Foster Regulator (FR) which provides 0 to 400V at 0 to 500A, within a 100kVA envelope. The FR output is stepped up by a 200kVA, 240V:11kV transformer, to provide up to 11kV across the log, at up to 11A. The automated energy control system drives the FR to provide maximum power to the log, within the equipment constraints, while integrating the active power supplied until a preset quantity of energy has been injected into the log, to raise its temperature by the desired amount. The rig has successfully been used to treat 3.3m long, 0.5m diameter logs. About 30 to 40kWh of energy per m3 are required. Thus, on a 16 hour per day, all year round operation, around 400 to 500GWh, from about 80MW of generation, would be required on a nationwide basis (2012 data). The paper presents and discusses electrical and log temperature data from the rig, along with planned future developments and a sneak preview of how a wharf-located production machine might operate.