An experimental study of the effects of asphaltenes on heavy fuel oil droplet combustion

Experiments of isolated heavy fuel oil droplets containing high asphaltene concentrations were carried out by the captive droplet technique. High-resolution video methods were used to monitor the transients experienced by the burning droplets. The suspended droplets confined in a highly radiative oxidizing environment were exposed to a maximum heating rate of 1000 degrees C/s. Data on size and temperature histories were obtained for different initial droplet diameters. A critical artificial lower bound for ignition time delay was determined by applying classical thermal theory to the monitored temperature histories. The ignition temperatures so predicted were comparable in magnitude to the observed values within experimental uncertainty. The occurrence of disruptive boiling, swelling, splashing and the formation of coke were clearly identified by three characteristic combustion times. Under the experimental conditions to which the droplets were subjected, disruption behavior was found to be attributed to the volatility differential of the high asphaltenic fuels rather than the presence of the suspended fiber. Coking of these asphaltenic fuels was accompanied by a dramatic eruption effect during the last moments of the droplet lifetime when the molecular structure of the cenosphere is molded. Excess burnout time or the ratio of burnout time of a high to a low asphaltenic fuel oil was strongly dependent on the initial droplet diameter. The experimental results strongly indicate that less oxidation time was required for coke particles that were low in asphaltenes. In boiler and furnace operation this means that greater residence times are required to oxidize conospheric residues if the heavy fuel oils have a substantial asphaltene content compared to droplets of the same size but with less aromaticity. (C) 1999 Elsevier Science Ltd. All rights reserved.