Rheological Properties and Ignition and Combustion Characteristics of Biochar–Algae–Water Slurry Fuels

This study examined the rheological properties, ignition, and combustion characteristics of biochar–algae–water (BAW) slurry fuels. A pine sawdust biochar with a median particle size (D₅₀) of approximately 12 μm and algae Chlorella vulgaris in dry powder form with D₅₀ of approximately 19 μm were used. The BAW slurries with a constant solid loading of 40 wt % and algae/biochar ratio varying from 0 to 0.2 by weight were prepared. The apparent viscosity was measured using a Haake VT550 cone-and-plate viscometer. The stability of the slurries was characterized using a “drop rod” method. Ignition and combustion characteristics of the slurries were studied using a suspended single-droplet technique. A single droplet of a slurry fuel with a diameter ranging from 0.5 mm to 1.5 mm was suspended on a silicon carbide fibre and burned in air at 1023 K in an electrically heated tube furnace. The ignition and combustion processes of the droplet were recorded using a CCD camera at 200 fps. The ignition delay time, burnout time, and burning rate were determined. The BAW slurries showed shear-thinning flow behavior. The slurries had higher viscosity and greater stability at higher algae proportion in the solid. The ignition and combustion process of BAW slurries followed the sequence of water evaporation, devolatilization, ignition, and combustion of the solid residue. The combustion of the residual solid was diffusion controlled under the experimental conditions and the burning rates of the BAW slurry droplets ranged from 0.15 to 0.25 mm² s⁻¹.