Evaluation of the coke formation potential of residual fuel oils

Abstract

Previously, engineering control and prediction of coke particulate emission has been hampered by the apparent inadequacy of the existing heavy fuel tests that were used as coking indicators, specifically: asphaltene contents (heptane and pentane), viscosity, specific gravity, and the Ramsbottom and Conradson Carbon Residues. A new measurement, the Coke Formation Index (CFI) has been established as a more accurate measure of the coke particulate formation propensity of residual oils. This parameter, combined with previously reported results concerning particle diameter dependence on initial droplet conditions, will allow more accurate prediction of heavy oil combustion particulate emissions than was previously possible. This improved understanding of the factors affecting cenospheric coke formation and oxidation will increase the potential for control of coke particulate emission. The CFI is measured using a free, isolated droplet combustion apparatus, closely simulating the conditions experienced by droplets in a real boiler and yet separating the coke formation and oxidation processes. The CFI has been evaluated for 41 different residual fuels. From these experiments the Coke Formation Index (CFI), or ratio of the particulate mass to the initial droplet mass, has been established as principally related to fuel properties. Correlation exists between the CFI and the fuel asphaltene content or the Ramsbottom or Conradson Carbon Residues, however there is also considerable variation. Close similarity to the carbon formation results reported by others using electrically heated foils is observed; however, the CFI technique also provides particle size, morphology and composition information not generated by other coke formation tests. Results suggest that, from a single experiment with only a small mass of fuel, the resultant particle size and mass distribution can be predicted for the spray distribution of full scale burners firing the same fuel.