Study of microwave hardwood board modification and drying: experiments and technical-economic assessment

The very low permeability of many hardwood species causes problems during lumber drying. These include very long drying times, drying defects, large material losses after drying, high energy consumption, and expensive drying processes. Jarrah (Eucalyptus marginata) is a species which very difficult to dry. Microwave (MW) wood modification and MW drying can provide an increase in wood permeability and solve some hardwood drying problems. Two 60 kW MW conveyor plants at frequencies 2.45 and 0.922 GHz were used for experiments. The study of MW Jarrah (29x87mm) board drying after pre-drying MW modification showed that it is possible to dry boards in the moisture content range from 70 to 10% during 147-159 hours (6 -7 days) using the cycle method of MW power application. Such intensive MW drying does not bring perceptible damage to the wood and provides low variability of moisture content in the board cross sections. The shorter drying times reduce associated capital, space, energy and labor costs whilst the reduction in drying defects can increase yields. A short time of MW drying provides an opportunity to develop conveyor systems for hardwood sawn timber drying. Economic assessment of MW Jarrah board drying showed that a specific drying cost ranges from US$ 50 to $277/m3 at electricity prices from US$0.08 to $0.32/kWh. The electricity costs form the largest share in total specific drying costs - 44 to 82%. Jarrah boards are expensive product and MW drying costs can be acceptable for industry taking into consideration savings provided by fast drying and wood loss reduction.