An experimental investigation into the ignition and combustion characteristics of single droplets of biochar water slurry fuels in air

The ignition and combustion behaviour of single droplets of slurry fuels made from a coconut shell biochar and a pine sawdust biochar was experimentally investigated. The two types of biochar water slurry fuels were prepared with water content varying from 50 to 70wt% and the median size (D50) of the biochar particles being approximately 10μm. Single droplets of the slurry fuels with diameters ranging from 0.5mm to 1.5mm were suspended on a silicon carbide fibre and burned in air at 1023K in an electrically heated tube furnace. The ignition and combustion processes of the droplets of the slurry fuels were recorded using a colour CCD camera at 200fps. The ignition delay time, burnout time and burning rate based on the d2 law were determined. It was found that the ignition delay time slightly increased first and then decreased as the water content increased from 50wt% to 70wt% for the coconut shell biochar slurry fuels and from 60wt% to 70wt% for the pine sawdust biochar slurry fuels, indicating that the effect of water content on the ignition delay time was a trade-off between the water evaporation time and the heating time of residual solid after water evaporation. The combustion of residual solid after ignition followed a shrinking sphere model and was controlled by external oxygen diffusion to the outer surface of the residual solid and a shallow region below the outer surface. The burning rate of the residual solid calculated based on the d2 law ranged from 0.07 to 0.15mm2s−1, which was also independent of the initial droplet size. A slurry fuel with a higher water content showed a slightly greater burning rate as the voids of inter-particles after water evaporation became larger for slurry fuels with higher water contents.