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 mm 2 /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.
Understanding "Where?" and "How much?" oxidation has occurred in a nuclear graphite component is critical to predicting any deleterious effects to physical, mechanical, and thermal properties. A key factor in answering these questions is characterizing the effective mass transport rates of gas species in nuclear graphites. Effective gas diffusion coefficients were determined for twenty-six graphite specimens spanning six modern grades of nuclear graphite. A correlation was established for the majority of grades examined allowing a reasonable estimate of the effective diffusion coefficient to be determined purely from an estimate of total porosity.The importance of Knudsen diffusion to the measured diffusion coefficients is also shown for modern grades. Knudsen diffusion has not historically been considered to contribute to measured diffusion coefficients of nuclear graphite.
Unitless9,10,13Open porosity of a porous material Unitless 9-11 A constriction factor for changing pore cross-section along the length of a pore. Unitless 9,11 Symbol Description Units Equation(s) Geodesic tortuosity. The ratio of the geodesic path length of a pore relative to the Euclidean distance traveled across the porous material. Unitless 9,11 〈 〉 Arithmetic mean gas molecule velocity assuming a Boltzmann energy distribution. / 12 Boltzmann constant ⁄ 12 Temperature 12 Knudsen diffusion coefficient for species A / 13
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