Impacts of Anisotropic Porosity on Heat Transfer and Off-Gassing during Biomass Pyrolysis

Abstract

The pore structure of biogenic materials imbues the ability to deliver water and nutrients through a plant from root to leaf. This anisotropic pore granularity can also play a significant role in processes such as biomass pyrolysis that are used to convert these materials into useful products like heat, fuel, and chemicals. Evolutions in modeling of biomass pyrolysis as well as imaging of pore structures allow for further insights into the concerted physics of phase change-induced off-gassing, heat transfer, and chemical reactions. In this work, we report a biomass single particle model which incorporates these physics to explore the impact of implementing anisotropic permeability and diffusivity on the conversion time and yields predicted for pyrolysis of oak and pine particles. Simulation results showed that anisotropic permeability impacts predicted conversion time more than 2 times when the Biot number is above 0.1 and pyrolysis numbers (Py 1 , Py 2 ) are less than 20. Pore structure significantly impacts predicted pyrolytic conversion time (>8 times) when the Biot number is above 1 and the pyrolysis number is below 1, i.e., the "conduction controlled" regime. Therefore, these nondimensional numbers reflect that when internal heat conduction limits pyrolysis performance, internal pyrolysis off-gassing further retards effective heat transfer rates as a closely coupled phenomenon. Overall, this study highlights physically meaningful opportunities to improve particle-scale pyrolysis modeling and experimental validation relevant to a variety of feedstock identities and preparations, guiding the future design of pyrolyzers for efficient biomass conversion.

Measurement of Transport Properties of Woody Biomass Feedstock Particles Before and After Pyrolysis by Numerical Analysis of X-Ray Tomographic Reconstructions

“…However, the geometric models investigated in the aforementioned study neglected important features such as ray cells and pits, which are revealed by the current work to play a dominant role in facilitating transport in the radial and tangential directions. Additional recent work by some of us has empirically investigated the importance of anisotropic permeability and similarly concluded that large ratios of longitudinal to radial permeabilities must be employed in numerical models in order to agree with experimental results (Pecha et al, 2021). However, this study did not include dramatic evolution of radial porosity throughout the pyrolytic process that we observe here, due to lack of quantitative microstructural information.…”

“…Four total samples were analyzed for this study: a native hardwood red oak particle measuring 13.5 × 3.2 × 3.2 mm, a The red oak dowels were pyrolyzed at 500 °C under He in a modified Frontier micropyrolyzer as described in (Pecha et al, 2021) with a char yield of 19% after 90 s. The two Douglas fir samples (25-35 mg) were pyrolyzed in a horizontal spoon reactor under He at 500 °C as described by (Christensen et al, 2017) for 5 min with char yields averaging 25%. At least 20 particles were pyrolyzed for each Douglas fir sample and random char particles were chosen for XCT imaging.…”

“…Principal permeabilities were extracted from these simulations for application in particle models that utilize a pseudo-homogeneous assumption for internal pore structure (Di Blasi, 1998;Gentile et al, 2017;Pecha et al, 2021). For each particle, three simulations with dominant flow along each axis were set up with a pressure inlet and an outlet along the dominant flow direction and wall boundaries on the transverse boundaries.…”

“…High-fidelity models of biomass conversion and biochar utilization require accurate transport models to enable scale-up and to mitigate technoeconomic risk of industrial deployment. Combined modeling and experimental works have previously elucidated the importance of directional permeability of biomass during pyrolysis (Di Blasi, 1998;Pecha et al, 2021). Other factors such as density and orientation in a gas stream also impact pyrolysis product quality and yield and have been investigated in studies such as in Kumari et al (2022) and Pecha et al (2017), but were not examined in this study as these properties can be measured without detailed information of the 3D microstructure of the material.…”

“…X-ray Computed Tomography (XCT)-based techniques and the application of numerical analyses described here provide accessible, direct answers to these knowledge gaps. Refined measurements of tortuosity and void fraction further enable accurate determination of effective diffusivities, which in turn allow for more robust assessments of reaction vs. diffusion limitations in gas-solid (Pecha et al, 2021) and liquid-solid systems (Luterbacher et al, 2013;Thornburg et al, 2020). Mechanical approaches have also been used to measure some microstructural properties.…”