Mass flow and variability in screw feeding of biomass powders — Relations to particle and bulk properties

•Feeding variability at long time steps is positively correlated to angle of repose.•Feeding variability at short time steps correlates positively to compressibility.•Mass flow is negatively correlated to biomass powder compressibility.

Biomass powders often have high cohesiveness, low bulk density and poor material flow characteristics which cause interruptions and variations in feeding systems. In this study, a range of biomasses – commercial charcoal, torrefied Norway spruce stem wood, non-treated Norway spruce stem wood, and reed canary grass – were milled (screen size: 1 mm) using two different methods; cutting mill and hammer mill, to form eight types of biomass powders. The powders were analyzed for loose bulk density, Hausner ratio, compression ratio, angle of repose and for size and shape distributions. Size and shape were determined by mechanical sieving and optical particle size and shape analysis. Additionally, yield loci and wall yield loci were determined through standard bulk solid testing methods. Screw feeding properties of the eight biomass powders were determined by feeding the materials in a twin screw feeder — at constant rpm and at a constant feeding rate of 1 kg/h. Correlation analysis and principal component loadings were used to describe relations between material properties and feeding characteristics. When materials were fed at a constant rpm, feeding variability was closely correlated to the powder's angle of repose (long time) and Hausner and compression ratios (short time).

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