2007
MICROWAVEAPPLICATOR FOR INTENSIVE TIMBER RADIATION TO MODIFY WOOD STRUCTURE
(Conference paper. September 3-6, 2007, Oradea, Romania, 11 th International Conference on Microwave and High Frequency Heating, Proceedings, pp. 335-338)
G. Torgovnikov, P. Vinden
Cooperative Research Centre for Wood Innovations, University of Melbourne,
Creswick, Victoria 3363, Australia
ABSTRACT
The practical application of microwaves for wood modification with the aim to increase wood permeability for liquids and gases requires high intensity power, applied in short bursts to provide the required degree of the modification. Industrial facilities require high microwave (MW) power levels in order to deal with large volumes of timber. These requirements demand the development of special MW applicators. New high power MW installationoperating at 0.922 GHz (power 30-300 kW) has been built for sawn and round timber modification. To obtain the desired energy distribution within the timber, a special round MW applicator (diameter 350mm) suitable for various wood species, has been designed, built and tested. A study of the energy distribution within the timber enabled the calculation of specific MW power levels for round wood modification and required MW energy consumptions. Research results indicate that the new MW technology can be applied to softwood and hardwood species. The costs of MW timber processing are acceptableto industry and provide good opportunities for the commercialization.
KEYWORDS: microwave wood modification, timber, microwave plant, microwave intensity, applicator.
INTRODUCTION
The low permeability of many wood species, particularly hardwoods, causes problems during timber processing. The shortcomings include: very long drying times, large material losses after drying, expensive drying processes.Impregnating timber with preservatives and resins can be extremely difficult.Therefore it is essential for the timber industry to have a MW technology that can provide an increase in wood permeability, reduce internal stresses, and minimize down-grade due to drying defects. Intensive microwave wood processing provides a several thousand-fold increase in wood permeability in radial and longitudinal directions [1, 2]. This can be achieved in species previously found to be impermeable to liquids and gases. MW wood modification allows: improved timber preservative treatment, fast lumber drying, stress relief in timber and manufacturing of the new material “Vintorg”.
The practical application of MW energy to wood requires high intensity power applied in short bursts to provide the required degree of modification. These requirements demand the development of special MW applicators. To arrive at the required quality of wood modification for different applications, the MW equipment must have the ability to control parameterssuch as: MW intensity (flux), energy absorbed by wood, mode of energy application to wood (pulse or continuous),vector electric field strength “E” orientation relative to wood grain, energy distribution within the timber cross section, MW applicator configuration and speed of timber through theapplicator.
The high MW intensity round Applicator 350A was built for 0.922 GHz and was used to study operating factors such as: energy distribution within round timber and data to allow the calculation of the MW timber processing and plant productivity.
EXPEREMENTAL
Some 270 green debarked logs of different species (mainly Radiata pine heartwood and Blue Gum) measuring 128-200 mm in diameter and 2400 - 4100 mm in length with moisture contents in the range of 20 to 40 % for softwoods and 80-110% for hardwoods were used in experiments. The oven dry density was in the range 370-450 kg/m3 for softwoods and in the range of 600-700 kg/m3 for hardwoods.
The 300 kW MW plant (frequency 0.922 GHz) was used for experiments (Fig.1) and is capable of handling logs with diameter - 100-300mm, length - 4700 mm; output -0.5-2.5m3/h; MW power - 30-300kW; feed speed - up to 8.5m/min.
Fig. 1. 300 kW MW experimental Plant for timber modification.
The key part of every MW plant is the applicator which must provide the required energy distribution within the timber. Applicator 350A (Fig. 2) was used for round timber processing. It has aluminium cylindrical body diameter 350 mm and three radiator inlets (waveguides with open ends 60x200 mm) through which MW energy was supplied to the applicator.
Fig. 2. Three port MW Applicator 350A for timber modification
Vector electric field strength “E” orientation was parallel to wood grain. During experiments, timber is held rigid in the applicator and can be transported through the applicator with controllable speeds. Energy distribution within the timber was determined by measuring the temperature at different points along the timber cross section and along the log by means of thermocouples after MW heating the wood to around 80 -100oC.
During MW wood modification the temperatures inside the wood was in the range of 120-150oC depending on species. MW power applied to the logs was in the range from 30 to 150 kW measured by power meters during timber processing. The average MW intensity (flux) in the radiator cross section 60x200 mm was in the range of 0.08-0.42 kW/cm2. The required log speed was provided by a variable speed drive. Vapours released from the wood during modification were removed from the applicator by high speed 90-110oC air flow.
RESULTS AND DISCUSSION
Experiments with Applicator 350 A enabled the estimation ofenergy distribution withinlog cross sections and along the log radius.After MW heating 128 mm diameter, 28% moisture content logs the maximum temperature was found in the centre of the log ( Fig. 3) opposite the centre of the radiator. And the heated zone spreads along the log up to 800 mm in both directions from the centre of the radiator.
Fig.3 .Temperature distribution along the log (D=128 mm, MC=28%, oven dry density 401 kg/m3) after heating during 20 second in Applicator 350A. MW power was supplied via 3 radiators 10 kW each. 0 – plane of the radiatorscentre
Study of the temperature distribution within the logs showed that practically all applied energy was absorbed in a 1600 mm log length. The central 200 mm zone absorbs 35% and the 400 mm zone 60% of supplied MW energy. MW modification takes place mainly in the 200 mm zone. In order to produce good modification of heartwood in softwoods with moisture content 25-40% and oven dry density 400-450 kg/m3 the minimum required specific energy release is 9000 kW/m3. The MW energy consumption required for modification must be 100-110 kWh/m3.
Experiments with hardwood logs (Blue Gum, Flooded Gum, Stringybark species) with diameter 90 – 200 mm, oven dry wood density 600-700 kg/m3 and moisture content 90-110 % showed that for MW full log cross section modification the specific energy release in the modification zone must be 8000 - 12000 kW/m3. Required MW energy consumption is 190 -250 kWh/m3.
MW plants (with round Applicator) parameters for log processing calculated on the base of the experimental data is presented in Table 1.
Table 1. Parameters of MW plant with round Applicator for log modification
Log characteristics / Output m3/h / MW energy consumption, kWh/m3 / Specific energy release, kW/m3 / MW power, kW / Timber speed, m/h
Softwoods(heartwood, D=130mm, MC=30%, OD density 400 kg/m3) / 1
4 / 110 / 9000 / 110
440 / 75
300
Hardwoods (D=150-170 mm, MC=90-110 %, OD density 600-700 kg/m3) / 0.5
1
2 / 250 / 9000 / 125
250
500 / 25
50
100
About 270 logs have been MW modified in Applicator 350A and impregnated with preservatives. Results of preservative treatment are acceptable for commercial application. The costs of MW modification for heartwood of softwood logs are AU$ 29-35 /m3, for hardwoods – AU$37-43 /m3 at electricity cost AU$ 0.1/kWh. These costs include capital (equipment) costs, labour, electricity, maintenance, magnetron replacement, floor space costs, but do not include electrical connections, mechanical installation and taxes.
CONCLUSIONS
The study of timber modification process in Applicator 350A allowed us to determine the rational parameters of modification and to give practical recommendations for designing special applicators. Tests of the round Applicator 350A showed that the lengths of modified zones vary depending on timber properties and sizes in the range 0.2-0.4 m. Zones of modification absorb from 35 to 60% of supplied MW energy. Minimum specific MW power release required for timber modification is 8000-9000 kW/m3. MW energy consumption for softwood (heartwood) log modification is 100-110 kWh/m3, for hardwoods – 190-250 kWh/m3.
After processing in Applicator 350A the log impregnation with preservatives was acceptable for commercial applications. Economic calculations showed that the costs of softwood modification are AU$29-35 /m3, for hardwoods – AU$37-43 /m3 at electricity cost AU$ 0.1/kWh.These costs are acceptable for industry and for customers and provide good opportunities for commercialization of new MW technology.
REFERENCES
[1] Vinden P., Romero J., Torgovnikov G. 2004. A method for increasing the permeability of wood. US Patent No 6,742,278.
[2] Torgovnikov G., Vinden P. 2002. Microwave Method for Increasing the Permeability of Wood and its Applications. In the book: ”Advances in Microwave and Radio Frequency Processing”, Springer –Verlag,Germany, pp. 303-311.