Determining Thermal Transport Properties for Softwoods Under Pyrolysis Conditions

The thermal properties of biomass over a range of pyrolysis temperatures have been measured using a Transient Plane Source (TPS) instrument and a differential thermal analyzer (DTA). In this study, thermal property measurements were made on six softwoods: subalpine fir, Douglas fir, Engelmann spruce, loblolly pine, lodgepole pine, and ponderosa pine. Results from this method show that the average thermal conductivity for these softwoods decreases by almost a factor of 3, from 0.198 to 0.091 W/(m K), as the wood goes from ambient conditions to a pyrolysis temperature of 453 °C. Over the same temperature range the average thermal diffusivity increases from 0.313 to 0.427 mm2/s, and the specific heat decreases from 1.58 to 0.93 kJ/(kg K). Investigation of the anisotropic nature of heat transport through lignocellulosic biomass found that heat transport is generally three to four times faster along the grain of the wood than across the wood pores, and studies on the rate at which thermal conductivity and diffusivity change with temperature revealed only a slight increase over 50–300 °C. It has also been shown that the thermal conductivity of a material correlates strongly with the density throughout the pyrolysis regime. This correlation with density has been shown before for the moisture content of green wood but not through the range of material changes associated with pyrolysis. The direct measurement of these anisotropic thermal properties has the ability to enhance modeling of lignocellulosic biomass pyrolysis and provide new insight into heat transfer through a naturally occurring lignocellulosic material.