IADC 2005 - 3rd Inter-American Drying Conference, August 21-23, 2005

Paper III-4

MICROWAVE WOOD STRAND DRYING: ENERGY CONSUMPTION, VOC EMISSION AND DRYING QUALITY Siqun Wang, Assistant Professor Guanben Du, Visiting Scientist Yang Zhang, Post-Doctoral Research Associate Tennessee Forest Product Center, Department of Forestry, Wildlife and Fisheries, University of Tennessee, Knoxville, TN37996-4563, USA Fax 865 946 1109; Phone 865 946 1120; E-mail: [email protected]

ABSTRACT Conventional drying system for wood strands operates now at high drying temperatures. The high temperatures result in high volatile organic compounds (VOCs) emission. By reducing drying temperatures and transferring energy directly into the moist wood without wasting heat of the surrounding environments, microwave drying reduces energy required and decreases thermal degradation of the wood material, thereby reducing VOCs emission. In this work, the temperature and moisture content changes under different microwave drying conditions were investigated. The effects of microwave drying on emission of VOCs were evaluated. Extractives were analyzed using Gas Chromatography / Mass Spectrometry (GC/MS). The result showed that increasing the microwave power input or decreasing sample weight results in high drying temperature, high drying rate, and short drying time. Different strand geometry and initial moisture content resulted in different warm-up curves, but did not affect final moisture content. Comparing with conventional drying, microwave drying resulted in low VOC emissions. Effect of microwave drying on the strand surface energy was investigated as well. Keywords: Microwave, drying, wood, strand, moisture content, VOC emission, energy, surface energy. INTRODUCTION New environmental regulations in the United States impose stricter controls on wood composite board manufacturing facilities. Volatile organics released from drying furnish for particleboard account for about 75 percent of the total volatile organic compounds (VOCs) emission during particleboard manufacturing (Boswell and Hunt 1991). All wood used in composite board production must be dried from an average of 60% to 90% moisture content (on a dry weight basis) to approximately 3% moisture content before entering the production line. Conventional drying systems for wood strands currently employ a rotary drum dryer that shoots a raw flame through a 20’-30’ rotating drum while tumbling the wood product around.

IADC 2005 - 3rd Inter-American Drying Conference, August 21-23, 2005

Paper III-4

Product scorching and air emission problems, particularly with nitrogen oxides (NOx) and VOCs, are prevalent because the rotary drum operates at up to 1000 oF as it reduces moisture from 50-60% to 2-4%. Such high temperatures produce VOCs both from extractives contained in the wood and from thermal degradation of the wood material itself. It is common practice to dry wood furnish in panel mills to very low moisture levels to ensure that press “blows” caused by a build up of steam pressure in the panel do not occur in the subsequent gluing step. VOCs emission increases when wood is almost dry (