Volatile organic compound emissions when drying wood particles at high dewpoints

Emission of volatile org. compds. (VOCs) during drying of wood particles for particleboard manuf., or for prodn. of refined biofuels, is a potential environmental problem. The aim of this work is to study the emissions of VOCs during drying, and the influence on the emissions by different drying parameters. The expts. have mainly been done in a batch fluidized bed dryer, but measurements have also been done in a, pilot scale, continuous fluidized bed dryer. The parameters studied are air flow rate through the bed, air temp., air dew point, and bed height. Since the present trend in biofuels and particle drying is towards very high dew points during drying, some measurements were made at dew points up to 95°, corresponding to a water content of 3.2 kg water per kg dry air. The emissions were measured in two different ways, online during drying with a flame ionization detector and through wood analyses of the terpene content in the wood particles before and after drying. Substantial work was made in developing the latter method as well as developing the drying equipment to be able to produce a wide range of conditions of the drying medium. It was shown that the VOCs emitted, during wood particle drying, mainly consist of terpenes. The behavior of the emissions was typical, at first contact with the drying gas a sharp peak in the release rate was detected. After that, the release rate stabilizes and declines slowly to a point where the material temp. increases and results in a second small peak in the release rate. Typically 80%-90% of the initial terpene content in the material was emitted during drying to low moisture content and it is not possible to significantly influence this release by changing the drying conditions. Up to 50% of the emitted terpenes are released during a very short period after contact with the drying medium. This fact could be utilized by employing a 2-step drying process. The fact that the VOC concn. in the inert gas gets much higher when drying at higher dew points could be taken into consideration when dealing with closed loop applications where the inert gas after the dryer is burnt