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Development of an affordable heat treatment process for wood

Final ReportSummary - TORCHWOOD (Development of an affordable heat treatment process for wood)

Current SOA large scale drying and chemical treatment processes for wood are highly expensive (in the region of EUR 500 000 to EUR 15 000 000) and have cycle times of between 24 and 80 hours to dry and treat the wood. Such processes typically utilise a variety of heating systems such as oil, steam, electric or gas heating in an inert gas or water vapour atmosphere. Whilst such systems are attractive to large scale enterprises, the ability of small and medium-sized enterprises (SMEs) to afford such technology and introduce the appropriate handling and environmental controls and provide the volumes necessary (typically 50 000 m per year), make them an impractical and unviable solution.

Increasing restrictions on the use of chemical preservatives (Council Directive 76/769/EEC) and the fact that preservatives such as creosote and pentachlorophenol should not be applied to the interior of dwellings, make the need for a cost effective, efficient system which is affordable to SMEs across Europe.

This new process will give SMEs the ability to heat treat wood species such as spruce, pine, birch, aspen and beech and plywood. These woods are used in outdoor constructions such as garden furniture, windows, doors, etc. and also indoor products and applications such as saunas and kitchen furniture. This access to alternatives to traditional softwoods, which are modified to increase their stability, hydrophobicity and resistance to disease, will further enhance the competitiveness of this group of SMEs.

Project context and objectives:

The overall project objective is to research and develop a microwave process that will be capable of heat treating wood uniformly in order to change its physical properties without causing any damage to the wood.

The project will also develop a natural oil formulation for the treatment of wood to give it increased durability and strength.

A list summarising the scientific and technological objectives is as follows:

Scientific objectives:
To gain sufficient extension to the current knowledge of:

- the effects of heating wood to high temperatures (approx. 200 degrees Celsius) in a short period of time (approximately 5 - 10 minutes) using microwave technology;
- a natural oil formulation, based on natural oil based products, such as linseed oil, rapeseed oil, sunflower oil and soybean oil. This formulation will then restore the strength to the heat treated wood;
- six key wood types (spruce, pine, birch, aspen, beech and plywood) permeability to several natural oils (linseed, rapeseed, sunflower, soybean) when treated with 1000 kW of microwave energy to ensure complete penetration of oil into heartwood.

Technological objectives:
- Heat treat wood from 27 mm square section to 150 mm square section or 50 x 400 mm x 4 m long planks. The consortium will develop a system which can deal with stacked wood to maximize the throughput from each machine.
- Use up to 1000 kW of microwave energy to provide a direct heat source for the treatment of wood. This will allow cost effective treatment in a highly controlled manner where the user can adjust the level of microwave energy, the time of treatment and where the treatment is focused.
- Develop an oil which when applied to the wood will dry within a few hours (or minutes if possible), penetrate to the heart of the wood, restore the impact bending strength and durability.
- Produce at least 4 m3 of wood per day at a commercially viable price of EUR 100 / m3 and be manufactured for less than 50 % of the current market cost for heat treatment equipment.

Project results:

The main objective of the TORCHWOOD project is to research and develop a heat treatment and oil application system for the treatment of wood in different dimensional sections. This is to be achieved using a microwave drying process capable of heating wood uniformly in order to reduce its moisture content whilst maintaining its mechanical properties and appearance. The application of oil post heat-treatment is necessary to allow the wood to retain its natural mechanical properties whilst exhibiting the durability and preservation stability of pressure treated, chemically preserved wood. The project will develop an oil formulation from sustainable sources so addressing legislation regarding the increasing restrictions on the use of chemical preservatives such as creosote and pentachlorophenol. Microwave drying of wood has the potential to reduce moisture content beyond current SOA and yield greater outputs through decreased cycle times. The main economic objective of the project is to develop a system capable of producing treated wood for a commercially viable price.

The technical work programme over the period 1 September 2008 to 28 February 2011 has involved the following:-

Work package (WP)1: Enhancement of scientific knowledge
WP 2: Development of a natural oil formulation to restore woods mechanical strength
WP 3: Development of the microwave delivery system and chamber
WP 4: Development of an oil delivery system
WP 5: Integration of TORCHWOOD technology
WP 6: Pre-production validation of TORCHWOOD technology.

Literature research was undertaken to better understand current scientific knowledge regarding the drying of wood. Dielectric properties, moisture content (mc), material density, fibre saturation points and field patterns were identified as being key parameters in any drying process. When the drying process begins the material temperature increases, if the mc exceeds the fibre saturation point the water in the cells will be forced out of the wood due to increasing interior water and vapour pressure during the heating process. Charing of the material (thermal runaway) will occur if the thermal conductivity is low and the dielectric properties increase resulting in an uncontrolled rate of temperature rise, this effect may well occur toward the end of a process as there is less mc in the material and a rapid pyrolysis takes place in the interior of the material. The magnetron unit is determined as being the best method of applying a microwave field as this can be done in a uniform pattern.

Lab-scale drying trials were undertaken on a range of species of wood (oak, beech, pine and spruce) using both conventional techniques (oven) and lab-scale microwave equipment. Initial tests confirmed that microwaving timber at high power outputs and short dosage times causes localised thermal runaway as anticipated in the literature reviews resulting in charring, burning and cracking of the material. Subsequent testing proved that using lower power and temperatures over longer periods of time gives an effective and efficient drying and also causes less damage. This set of trials also indicated that drying wood with microwave energy appears to cause almost no colour change which can be considered as a positive attribute as there is a demand for paper products in some markets such as furniture and flooring.

Microwave drying regimes were further developed for oak and a target mc set of 10 % which meets the criteria for 'dried' wood in standard EN322 which covers wood quality for building applications, dried wood can be used internally and it is currently not possible to purchase oak for this application. In order to determine the suitability of microwave drying as a commercially viable method for rapid drying of timber products, it was essential to compare mechanical properties and drying times between conventional processes and the developed microwave drying profile. Literature values are of no use in making such comparisons as timber is a highly variable material so in order to complete a statistically accurate set of results large numbers of samples were tested over several months in both conventional and micro-wave techniques.

The trials showed that to achieve a final mc of about 10 % in a conventional oven, 120 hours of drying is required at a maximum temperature of 60 °C whereas for microwave drying the total cycle time is 72 hours including a total illumination time of 53 minutes.

The data obtained can be extrapolated up for larger material dimensions and species.

An outline design for the prototype microwave chamber was discussed and agreed by the consortium and an outline design produced with the capacity to hold 4 off 150 mm² x 2500 mm sections of oak. Bulk drying trials were undertaken in the lab-scale equipment in order to determine the bulk drying characteristics in a microwave field.

Modelling work determined the optimum design of the antenna giving consideration to key parameters such as micro-wave field uniformity, limitation of internal volume (to prevent heat loss) material loading requirements and need for sensors and ports within the equipment. The antenna chosen for the technology is a slotted waveguide, the configuration of which can be tailored to provide a uniform field along its length and a good match to the generator ensuring that maximum energy is transferred uniformly to the workload. For the prototype system, each antenna can be driven at a maximum power of 3kW.

The microwave chamber is designed around four equi-spaced waveguides each powered by a 3 kW magnetron. The system is designed to be fully automatic and incorporates sensors to monitor air-flow, temperature and humidity. The system has a re-circulating air pump with one inlet and two outlets to establish a uniform flow through the chamber without loss of humidity which is monitored and vented if required. Two additional ports are provided to vent or supply air from atmosphere. A vacuum pump is included to allow low pressure environments to be evaluated for drying performance.

Research into natural oil formulations has been conducted. Five candidate natural oils were chosen (linseed - crude, soybean - neutral, soybean - refined, rapeseed - neutral, rapeseed - refined). A number of additives were investigated to aid the delay of oil oxidation. Formulations were made up for each possible combination in concentrations of 300, 400, 500 and 1000 mg/lt. These were tested in-line with industrial standards as issued by the International Oil Council and the following formulations recommended to the consortium:

1. soybean oil (neutral) combined with BHT (Butylated Hydroxy Toluene), concentration 500 mp/lt;
2. soybean oil (neutral) combined with TBH-Q (T-Butyl HydroQuinone), concentration 400 mp/lt;
3. soybean oil (neutral) combined with TBH-Q (T-Butyl HydroQuinone), concentration 500 mp/lt;
4. soybean oil (neutral) combined with BHT (Butylated Hydroxy Toluene, concentration 400 mp/lt.

Extensive testing has been undertaken to determine the best method of application these were the following:

1. Spraying: Pre-heated wood samples were sprayed with the two formulations until completely covered. The oil mass uptake and oiled area data was collected at wood temperatures of 20 oC, 40 oC, 100 oC and 200 oC.
2. Soaking: Wood samples were pre-heated to 40oC before soaking in the two formulations to two different depths, 10 cm and 30 cm, and for different durations - 4 hours, 8 hours and 24 hours.

All trials used oak samples with dimensions of 300 mm x 20 mm x 20 mm. The wood samples were analysed for oil uptake in terms of mass increase and area of coverage across the cross-section after sectioning each wood sample in three places. The coverage area was determined using image analysis. Flexural analysis was carried out on oil soaked wood samples using the three point test. The three points bending flexural test provides values for the modulus of elasticity in bending Ef, flexural stress sf, flexural strain ef and the flexural stress-strain response of the material. The main advantage of a three point flexural test is the ease of the specimen preparation and testing. The flexural analysis revealed consistent results across all wood samples. The mechanical strength of oil treated samples was on par with naturally dried wood.

Soaking results in consistently better oil uptake than spraying. Moreover, the process conditions for soaking are easier since the wood only needs to be pre-heated to 40 oC. Since this is approximately the temperature at which the wood will exit the microwave treatment, soaking is easier to achieve and will likely lead to consistent results.

As a result the TORCHWOOD project will use a combination of microwave treatment and soaking in oil for at least 8 hours at a depth of at least 30 cm.

The oil soaked samples show very good mechanical strength as evidenced by the results from flexural analysis.

An oil dip-tank, prototype microwave chamber and basic material handling system have been manufactured and sited in a dedicated test area at UK Heri where microwave trials are being conducted which will be immediately followed by oil-soak trials. Once these are complete, the results will be made available to the project partners who are seeking to take the product to market.

The activities in the TORCHWOOD project have been conducted by a consortium consisting of 10 organisations from 8 different European countries. The project has been coordinated by Tecsabois SA.

Potential impact:

Socio-economic and societal implications:

Microwave drying trials will continue at UK Heri after the project has finished in conjunction with Tecsabois who wish to undertake drying trials on a number of other wood species and a range of sizes. The results will be published in research papers written and presented by UK Heri.

The developments made by the project will contribute significantly to the European wood processing industry across the European Union (EU) through the ability to supply dried material at significantly reduced moisture content (quality), reduced processing costs (EUR 100 / M³), and increased capacities (880 m³ / anum). This will help protect jobs within the European wood processing industry and combat imports from Asia and America.

Dissemination activities:

The project has been disseminated at five events:

1. The international Forestry Fair 2010 (Czech Republic)
2. A dissemination event for SMEs at the Athens Chamber of Commerce (Greece)
3. 5th Furnidec Business Show 2011 (Greece)
4. COST-FP0904-Workshop 2011 (Switzerland)
5. 15th Epiplo Design Exhibition (Greece).

Exploitation of results:

Since there are a number of competing technologies within the timber market and because the SMEs want to utilise other possible market opportunities outside this primary market, protection of the vital technology will be sought through patenting the total product concept. The combined concept with microwave heat-treatment, oil formulation (specific to micro-wave dried wood) and oil application method will be strong. The patent will detail the specific oil formulations as developed in the project, the design of the microwave chamber and control system and the oil application system. It is not felt that any material handling system is protectable as each end-user's needs will be different and a handling system designed for each application. The SME partners will not apply for a patent until the results from the microwave trials are known, though a draft patent application is being prepared in advance of this data.

Natural oil formulations will offer the market an alternative oil for the preservation of wood that contains no harmful substances. This means that these oils can be applied to building products for interior use as well as external. Tecsabois are interested in this market as they produce materials for interior use. Further research could be conducted into the penetration of oil into the wood when unheated and some research could be undertaken to determine / modify the formulations for the staining and decorative appearance of wood rather than a preservative.

The microwave system offers an alternative drying process to current SOA. Whilst of similar cost (EUR 300 000) to that of a drying chamber, it offers a number of advantages. The cycle time is greatly reduced (from 300 days to 100 days) giving a capacity increase of 884 m³ per annum, a cost saving on processing of EUR 100 / m³ and a greatly reduced moisture content (about 10 % content) which means that oak can be used for interior use. Tecsabois have identified a number of applications where this can be applied and sell oak as fully dried wood. The system can be adapted to be modular (heat-treatment and oil application separate), combined, continuous throughput or standard operation (batch production). Geert, ICS, Tecsabois and Paide have the know-how to be able to adapt the system for bespoke requirements of the customer.

Project website:

Contact details:
Mr Jean-Baptise Saget
Managing Director
Tecsabois SA
Route De Saint Florent
Tel: +33-(0)23-8361204
Fax: +33-(0)23-8292247