Particle size measurement by static and dynamic image analysis for processed woody biomass crops

Abstract. Various uses of biomass crops require particle size specification, notably for length and shape. The traditional method for length characterization is mechanical sieving. However, recent studies have shown that biomass density, sample preparation, sieving time and vibration frequency can influence results. Different willow biomass sources were analyzed; they included wood chips, wood mulch and wood chunks. Samples were initially analyzed with three laboratory mechanical sieves: the Ro-Tap® (RX-30) from Tyler, the Porta-Screen® (PS-4) from Gilson and the ASABE standard for forage particles (S424.1). These screening systems have different vibration frequency and directional movement (horizontal or vertical). All sieve openings are smaller than 32 mm (on the side). Variables studied included vibration time (1 to 15 min) and abrasion after multiple passages of the same sample (ten times at 15 minutes per passage). Concurrently, certain particles were measured in two dimensions by static image analysis using Matlab software and by dynamic image analysis using the Camsizer by Horiba. Chips, mulch and chunks had an average length of 13.8, 8.8 and 14.3 mm, at 50% cumulative mass, and 21.9, 19.0 and 24.9 mm, at 90% cumulative mass, respectively, after mechanical sieving. Multiple sieving of the same particles, up to 10 times, caused abrasion with 2 to 5% of the mass. The duration time of sieving, between 1 and 15 min, also caused an increase in the proportion of particles in the finer screens and the pan. Static and dynamic image analysis results indicated that the real length was on average 15% longer for chips, 116% longer for mulch and 21% longer for chunks, than lengths measured with sieves. Because mulch particles are more oblong than chips or chunks, more long mulch particles tip over across finer screens. Mechanical sieving alone cannot provide an accurate measurement of length of biomass particle.