Three bulk density measurement methods provide different results for commercial scale harvests of willow biomass chips

Bulk density is a key attribute of biomass feedstocks that affects both conversion processes and logistics. Reported bulk density for fresh willow chips ranges between 195 and 393 kg m-3 and it is unknown if standardized methods using several kg of chips scale effectively to collection vehicles that contain several Mg. The objective of this study was to compare three bulk density methods for 53 commercially harvested willow loads and compare bulk density in three different collection wagons. An ISO standard ‘bucket’ method was compared to actual and effective bulk density measurements. Mean as-received bulk density using the standard ISO method was 262 kg m-3 (σ = 25) and was not correlated with wagon load measurements of actual or effective bulk density. The actual bulk density in a small (10 m3) collection vehicle was 302 kg m-3 (σ = 32), which was significantly greater than the 214 kg m-3 (σ = 28) that was measured in two larger (30 m3) collection vehicles. In addition, actual bulk density in wagons was higher than the effective bulk density (247 kg m-3 (σ = 24)) observed for the truck deliveries. Collection wagon geometry and design impacted these results, which may have implications for modeling these systems. In conclusion, the variation in bulk density values is important to understand and represent in modeling of harvesting and logistics systems in order to capture what is actually occurring in commercial scale operations.