Fine particle emissions from residual fuel oil combustion: Characterization and mechanisms of formation

The characteristics of particulate matter (PM) emitted from residual fuel oil combustion in two types of combustion equipment were compared. A small commercial 732 kW rated fire-tube boiler yielded a weakly bimodal particulate size distribution (PSD) with over 99% of the mass contained in a broad coarse mode and only a small fraction of the mass in an accumulation mode consistent with ash vaporization. Bulk samples collected and classified by a cyclone indicate that 30% to 40% of the total particulate emissions were less than 2.5lm aerodynamic diameter (PM2.5). The coarse mode PM was rich in char, indicating relatively poor carbon burnout, although calculated combustion efficiencies exceeded 99%. This characteristic behavior is typical of small fire-tube boilers. Larger, utility-scale units firing residual oil were simulated using an 82 kW rated laboratory-scale refractory- lined combustor. Particulate matter emissions from this unit were in good agreement with published data including published emission factors. These data indicated that the refractory-lined combustor produced less total but more fine particulate emissions, as evident from a single unimodal PSD centered at ~0.1 lm diameter. Bulk cyclone segregated samples indicated that here all the PM were smaller than 2.5 lm aerodynamic diameter, and loss on ignition (LOI) measurements suggested almost complete char burnout. These findings are interpreted in the light of possible mechanisms governing the release of organically bound refractory metals and may have particular significance in considering the effects of fuel oil combustion equipment type on the characteristic attributes of the fine PM emitted into the atmosphere and their ensuing health effects.