Morphological and effective transport properties of fixed beds of wood chips: Toward realistic modeling of low-temperature pyrolysis

A detailed understanding of heat transfer and flow is critically important for the pyrolysis of carbonaceous feedstock in fixed bed reactors. In this paper, multimode transport phenomena are simulated by direct pore level simulation on representative digitized samples to retrieve the heat-transfer and pressure-drop properties of highly porous fixed beds of oak-wood chips, which exhibit anisotropic solid-phase tortuosity. A comparison of the direct pore level simulation and simple models of porous media with simple morphologies shows that simple models cannot be used to determine the transport properties of a material with a complex morphology, such as fixed beds of wood chips. Because of drying and devolatilization, the effective thermal conductivity of wood chips decreases with increasing temperature. Heat conduction passes preferentially through the fluid because wood chips have very low thermal conductivity. This study also shows that the shrinking and cracking phenomena resulting from pyrolysis lead to a decrease in the porosity and solid-phase tortuosity and an increase in the specific area. At a high Reynolds number, the pyrolysis also diminishes the permeability but increases the inertial resistance to flow and the interfacial heat transfer.