VOC extraction from softwood through low-headspace heating

Volatile organic compounds (VOCs), principally terpenes, released during wood drying face present or potential regulation. Two approaches are reported to control VOC release: heating green wood, or irradiating it with microwave energy, both in a low-headspace environment where evaporation is minimized. Low-headspace heating of green flakes (for OSB manufacture) releases VOCs but proportionately much less water. Hence, it is feasible to extract and collect the VOCs from green wood prior to drying, and to then dry it with lowered emissions. Irradiating flakes with microwave leads to contrasting behavior. Water is released with very little VOC loss if the flakes are microwaved in an open container. Microwaving under low-headspace conditions removes the VOCs, but retains the water in the wood. The water trapped in the wood because of the low-headspace restriction drives the VOCs out of the hydrophobic regions (e.g., resin canals) where they are principally located, into hydrophilic zones. Movement out of the hydrophilic environment and out of the wood is then quite rapid. Hence, either VOC or water release can be targeted by adjusting the headspace during microwaving. Most of the VOCs lost during drying originate from the surface, and low-headspace microwaving releases this surficial material. Hydrogen isotope exchange work shows that microwaving increases water access to the exchangeable protons in dry, or partially dry, wood tissue. The terpenes are carried out with the small amount of steam generated. Similar results are obtained with low-headspace radiofrequency (RF) irradiation of lumber; RF treatment does not induce a significant change in strength.