Combustion characteristics of heavy oil-water emulsions

The combustion of heavy oil and its emulsions with water was investigated in experiments on a semi-industrial scale. Two comparisons between heavy oil and oil-water emulsion flames are presented that, due to the different initial conditions of the spray, provide complementary information. Reported results include spatial distributions in the flame of temperature and species concentrations (O₂, CO, UHC, NO_x) as well as gaseous and solid emissions in the flue gases. The measurements inside the emulsion flame display a remarkable improvement in the combustion process with respect to that of the neat oil with poor atomization; differences are much less important if a fine spray is achieved with the heavy oil. Solid emissions are significantly reduced in the emulsion tests and the morphology of the particle samples demonstrates the fragmentation of the drops and/or the coke particles initially formed. The flame temperatures are reduced by ∼65 K. The heat absorbed by the water injected in the emulsion and enhanced radiative heat transfer due to the higher particle number density could explain this difference. The spatial distribution of NO_x indicates that a significant reduction is obtained in the final part of the flame; this may be attributed to a decrease in the rate of thermal-NO formation as a consequence of lower gas temperatures. No measurable difference in NO_x concentration is found in the inner core of the flames.