The storage of wood chips with a high moisture content under inadequate conditions may adversely affect their quality. Elevated microbial activity in the material leads to a loss of dry weight, greenhouse gas emissions, and heating up of the heap, which in extreme cases may significantly lower the quality of wood chips or even cause spontaneous ignition. During the storage of large quantities of wood chips, it is necessary to decrease moisture content from 60–90% to less than 40% for transportation, and to less than 25% for effective combustion. One way of reducing moisture content is microwave drying.
The study involved short-term microwave drying of three kinds of wood chips (pine, maple, and shiny cotoneaster) divided into several size fractions. The fractions were obtained using a separator with mesh diameters of 3.15 mm, 8 mm, 16 mm, 31.5 mm, 45 mm, and 63 mm (according to PN-ISO 565:2000 and PN-ISO 3310-2:2000). Wood chip samples were exposed to microwaves in a SHARP R-200 oven for 30 s, 60 s, and 90 s. Subsequently, a VIGOcam V50 thermographic camera was used to record the surface temperature of the samples, and the loss of moisture was determined by comparing sample weight before and after microwave exposure.
The study showed that due to the selective nature of the process, the power of the microwave generator and exposure time should be adjusted taking into consideration the size fraction and weight of wood chip samples. Microwave irradiation heated the wood chips to over 100°C, with unfractionated samples reaching lower maximum temperatures. Smaller wood chips were found to lose moisture more slowly. Thermographic analysis showed that temperature differences within individual samples decreased with the degree of sample homogeneity.