Efficient Kiln Drying of Quality Softwood Timber
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis is a study of the kiln drying of radiata pine with a primary focus on the change in wood colour that occurs during this process. The energy efficiency of the drying process has also been examined using computer modelling. The aim of this work was to develop guidelines for commercial wood dryers who wish to produce high quality appearance grade timber in a competitive commercial environment. The colour change in radiata pine wood during kiln drying is mainly caused by sap compounds accumulating at the wood surface and reacting to form coloured compounds. The initial research involved drying experiments designed to determine the relationship between this colour change and the kiln schedule and also measure the accumulation of colour forming compounds. The kinetics of the colour change reaction were also measured using two methods, one in-vitro and the other using small samples of wood. From these experiments a colour change equation was developed that predicts the rate of colour formation based on the drying conditions and this was incorporated into a kiln stack model along with an energy efficiency model. The combined model was used to simulate the drying process to find schedules optimised for energy use and wood quality. The model was also used to simulate the energy efficiency of different humidity control configurations for wood drying kilns. A kiln micro-sensor system was also developed for use in kiln diagnostics and control with the particular aim of identifying areas in wood drying kilns with adverse drying conditions. The recommendation to kiln operators wishing to reduce colour change is to not exceed 70? and to use lower relative humidity schedules with a wet bulb depression of 15-20?. Operating at lower humidity can increase the energy used by the kiln so it is also recommended that kiln designers incorporate heat recovery into the humidity control mechanisms of the kiln.