An experimental study of the effect of water content on combustion of heavy fuel oil/water emulsion droplets

Experiments of isolated high asphaltene droplets of heavy fuel oil/water emulsion were performed to investigate the non-steady behavior of the burning droplets. High-resolution video methods allowed monitoring of the various combustion stages. A highly radiant (1000 degreesC/s) oxidizing environment was necessary to enhance the volatility differential of the high asphaltene fuel, thus reducing uncertainties because of the presence of the fiber. Data on size and temperature histories were obtained and coke residues were analyzed by a Scanning Electron Microscope. A lower and upper bound for ignition time delay was established. The error defined as the time lag between these two limits never exceeded 10 ms, which is the maximum time for soot to form in the flame after actual ignition. The ignition time delay of emulsions was longer than for an ordinary heavy fuel oil (HFO) droplets of the same size. The peak temperature of emulsions occurred much earlier in time. The steeper temperature rise seen in the emulsions for portions of their combustion history is evidence of both soot reduction and the extent of burnout of the cenospheres, which is an important aspect in the reduction of pollutant emissions. The occurrence of swelling, disruptive boiling, splashing and the formation of coke were clearly identified by three characteristic combustion times. The emulsion droplets showed swellings of considerable magnitude relative to that of HFO. Coking of the solid phase took place by a dramatic eruption during the last moments of the droplet lifetime when the molecular structure of the cenosphere appeared to be molded. Coke particles formed from emulsions were more void with thinner and fragile shells suggesting that an amorphous-like molecular structure could have developed as opposed to the more compact shell structure observed for HFO residues, which were harder and more reticent to burning. Excess burnout time or the ratio of burnout time of the emulsions was dependent on the water concentration, indicating that less oxidation time was required for coke particles from emulsions than from heavy fuel oil. (C) 2001 by The Combustion Institute.