Prediction of air pressure resistance during the ventilation of wood chips as a function of multiple physical fuel parameters

Abstract

Air pressure resistance during ventilation was measured experimentally for a high variety of conventional wood chip samples that had been produced using various raw materials, chippers and chipper settings (n = 95). Physical fuel properties such as bulk density or particle size distribution were determined according to international standards for solid biofuels. In addition, particle length and particle shape were determined using a continuously measuring image analysis device. Pressure resistance (in Pa m−1) was determined using a custom-built flow cylinder (2 m height, 0.4 m3 filling volume) and related to air flow rate.Pressure resistance increased exponentially with air flow rate for all samples. Thereby, pressure resistance was highly variable among wood chip samples, ranging from 10 to 107 Pa m−1 at low air flow rates (0.1 m3 s−1 m−2) and from 197 to 1240 Pa m−1 at higher air flow rates (0.5 m3 s−1 m−2). Four common equations to describe pressure resistance (Ramsin, Shedd, Hukill & Ives, Ergun) were compared. Multiple linear regression analysis was applied to express pressure resistance as a function of selected fuel quality parameters. Sensitivity analyses of the resulting equations were performed. Results indicate that pressure resistance strongly depends on bulk density, particle size and particle shape as these parameters determine the number, size and shape of free air voids within wood chip fillings.