An experimental study of the ignition and combustion characteristics of single droplets of biochar-glycerol-water slurry fuels

The ignition and combustion behaviour of single droplets of biochar-glycerol-water slurry fuels was experimentally investigated. A pine sawdust biochar with a median particle size (D50) of approximately 12 µm was used. The slurry fuels with a constant biochar loading of 42wt% and glycerol/water ratios varying from 0 to 0.5 by weight were prepared. A single droplet of a slurry fuel with diameter ranging from 0.5 mm to 2.0 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 droplets were recorded using a colour CCD camera. The ignition delay time, burnout time and burning rate based on the d2 law were determined. It was found that all the slurry fuels exhibited heterogeneous ignition of biochar residue (agglomerate) following water and glycerol evaporation. The addition of glycerol significantly reduced ignition delay time, shortened burnout time and increased burning rate of the droplets. This effect was enhanced with increasing glycerol/water ratio in the slurry fuels. It was also found that the glycerol was not completely evaporated before ignition and the remaining glycerol burned concurrently with the biochar on or near the agglomerate surface. The combustion of agglomerate after ignition was controlled by external oxygen diffusion to the outer layer of the agglomerate under the conditions tested.