Rheological behaviour and stability characteristics of biochar-water slurry fuels: Effect of biochar particle size and size distribution

- Biochar-water slurry fuels with biochar loading of up to 52 wt% were prepared.
- Yield stress decreased first and then increased as D50 increased.
- Viscosity increased with increasing size ratio of coarse to fine fractions.
- Slurries prepared using smaller particles were more stable.
- Slurries with bimodal size distribution were more stable at larger fine fractions.

The effect of particle size and size distribution on the rheological properties and stability characteristics of biochar-water slurry fuels was investigated. The biochar-water slurry fuels were prepared by dispersing biochar samples in water as the suspending medium. Slurries with varied median particle size (D50) of biochar, unimodal size distribution of biochar with varied D50, and bimodal size distribution with varied size ratio of coarse to fine fractions (λ) and varied fine particle fraction content (ξ) were prepared. The yield stress of the slurries prepared was measured using a Brookfield vane viscometer while the dependence of the apparent viscosity and shear stress on the shear rate was characterised using a Haake VT550 cone and plate viscometer. The stability of the slurry fuels was characterised using a “drop rod” method. At a given biochar loading, the yield stress of the slurry fuel first decreased slightly with increasing D50 for D50 < ~ 15 μm and then increased dramatically with further increasing D50 due to the stronger water uptake capacity of larger biochar. The biochar of bimodal size distribution reduced the viscosity and thus increased maximum biochar loading. The viscosity was further reduced as λ decreased, being more profound at smaller ξ. The slurry fuels prepared with finer biochar were more stable. The stability of slurry fuels of bimodal particle size distribution was greater at higher ξ for a given λ.