Development and Optimisation of a Low-Temperature Drying Schedule for Eucalyptus grandis (Hill) ex Maiden in a Solar-Assisted Timber Dryer

摘要

The Brazilian furniture industry consumes about 45 million m³ of sawnwood per year which is mainly supplied by deforestation of the tropical rainforest. At the same time, fast growing eucalypt species are produced on almost 3 million ha for the production of wood pulp and charcoal. Meanwhile, several Brazilian companies try to substitute the expensive natural woods by hardwood from eucalypt trees for the production of high quality sawnwood. However, eucalypt wood has to be dried very carefully under controlled conditions to prevent drying defects. Ambient air drying is not suitable since missing control causes high losses and long drying times. Beside this, the low wood moisture content required in the furniture industry cannot be achieved. Artificial drying technologies reduce the drying time, the timber can be dried to a low wood moisture content and the quality can be improved. However, sophisticated high-temperature dryers cause high investments. Locally manufactured timber dryers do not allow an adequate control of the drying process. Furthermore, the required slow drying process is increasing the thermal and electrical energy consumption causing high drying costs. To overcome the existing problems, the Institute of Agricultural Engineering in the Tropics and Subtropics of the University of Hohenheim (ATS) developed in close co-operation with the German company THERMO-SYSTEM Industrie- & Trocknungstechnik Ltd (THS), Alfdorf and the Brazilian forest company CAF Santa Barbara Ltda (CAF) a solar-assisted dryer for sawnwood with integrated solar collector and biomass backup heating system. Aim of this research work was to analyse the newly developed solar dryer and to develop a suitable drying schedule which allows the economical production of high quality sawnwood for the furniture industry. Therefore, a prototype of the greenhouse type dryer was installed under subtropical climate in Brazil. Due to the low thermal insulation of the solar dryer and missing experience with the extremely sensitive Brazilian varieties of Eucalyptus grandis, a new type of drying schedule had to be developed. The new schedule considers not only the general drying demands of eucalypt sawnwood but also the system immanent characteristics of the solar dryer and the ambient air conditions. An oscillation of the drying air temperature according to the ambient air allowed to speed up the drying process and reduces the condensation of water on the cover without a negative impact on the timber quality. In more than 80 drying tests with 16 000 m³ of eucalypt sawnwood the schedule was tested and improved. Based on experiments and information from literature, the course of the temperature, relative humidity and velocity of the drying air and the mode of remoistening was systematically optimised. Thereby, the influence of the changing drying conditions on the drying time, the timber quality, the energy consumption and the drying costs were analysed. With the final version of the drying schedule, 27 mm thick boards could be dried in the solar dryer from a medium wood moisture content of 60 to 12 % d.b. in 27 days. This drying time was about 20 % higher than in a high temperature dryer. However, a drying time of at least 60 days was required to reduce the moisture content to about 20 % d.b. at ambient air drying. The electric energy consumption in the solar dryer was reduced to about 20 kWh per m³ dried eucalypt sawnwood. This is only 20 % of the energy usually consumed in a high temperature dryer. The thermal energy consumption was 1.2 GJ per m³ which is about 60 % less than the energy required in conventional high temperature dryers. The low thermal and electrical energy consumption combined with the considerable lower investment costs for the solar-assisted timber resulted in average drying costs of 7.90 Euro per m³. This is only half of the costs caused by drying 27 mm thick eucalypt hardwood in a high temperature dryer. For an economic evaluation, a sensitivity analysis was done for the most important cost parameters. The electrical energy costs, the currency exchange rate and the interest rate for credits were found to be the main influencing parameters on the Brazilian market conditions. However, solar drying was generally more cost efficient than conventional high temperature drying.In the framework of this research work, it was proved that Brazilian eucalypt timber can be dried economically to a low moisture content of 10 to 12 % at a high quality level by applying the developed drying schedule in the optimised solar-assisted dryer. Meanwhile, approximately 35 000 m³ of eucalypt hardwood is dried annually in two solar-assisted drying plants contributing significantly to the protection of the natural rain forests.