An experimental study of rheological properties and stability characteristics of biochar-glycerol-water slurry fuels

The rheological behaviour and stability characteristics of biochar-glycerol-water slurry fuels were experimentally investigated, emphasising the effect of glycerol addition. A pine sawdust biochar with a size fraction of < 32 μm and a median particle size (D₅₀) of approximately 12 μm was used. A biochar-glycerol-water slurry fuel was prepared by dispersing biochar particles in a premixed glycerol/water solution as the suspending medium with glycerol content varying from 0 to 100 wt%. 2 wt% lignosulfonic acid sodium salt (LASS) (on the basis of dry weight of biochar) was added to the suspending medium as a dispersant. The effect of glycerol content on the rheological properties and stability characteristics of the slurry fuels were studied. The yield stress was measured with a Brookfield vane viscometer and the dependence of apparent viscosity and shear stress on the shear rate was characterised using a Haake VT550 cone and plate viscometer. The stability characteristics of the slurry fuels were characterised by using a “drop rod” method. As glycerol content increased, the maximum biochar loading decreased from 50 to 10 wt% while the calorific values of the slurry fuels first increased and then decreased, reaching a maximum of 21 MJ kg^− 1 at 90 wt% glycerol content. For a given biochar loading, increasing glycerol content increased the viscosity of the suspending medium and therefore the viscosity of the slurry fuels, displaying higher yield stress, more apparent shear-thinning behaviour and higher stability due to the enhanced flocculation of biochar particles.