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Permeability of bulk wood pellets with respect to airflow Yazdanpanah, Fahimeh

Abstract

Data on the resistance of wood pellets to air flow are required for the design and control of ventilation and drying of bulk pellets in storage. In this study, pressure drops versus air flows were measured for several sizes of wood pellets, with diameter of 6 mm and lengths varying from 4 to 34 mm. Air flow rates ranging from 0.014 to 0.80 m s⁻¹ were used in the experiment. The maximum pressure drop measured was 2550 Pa m⁻¹. Three predictive models - Shedd, Hukill-Ives, and Ergun equations that relate pressure drop to air flow in bulk granular materials were used to analyze the data. The Ergun equation was found to provide the best fit to the data. Aeration of bulk pellets in storage requires a low airflow. The airflow range used for low permeability tests was from 0.0002 to 0.0220 m s⁻¹. The corresponding measured pressure drop ranged from 0.18 to 8.30 Pa m⁻¹ for low permeability tests. Three models were investigated and compared for the low permeability data. The increase in moisture content over a wider range of airflows (0.0042 to 0.7 148 m ⁻¹) showed a slight decrease in the resistance to airflow due to increased moisture content. Broken and fines are produced when pellets are handled. The resistance to air flow for wood pellets was measured in the presence of fine materials. Fines were defined as broken pellets passed a 4 mm sieve. The average geometric diameter of the fines was 0.75 mm. The pressure drop for pellets mixed with fines ranged from (2.0 to 191.2 Pa m⁻¹) and (7.9 to 1779.0 Pa m⁻¹) for 1% and 20% fines content (mass basis) respectively. Coefficients of Hukill and Ives’ equation for pellets were estimated as a function of percent fines content.

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