Novel Micropyrolyis Index (MPI) to Estimate the Sooting Tendency of Fuels

A new fuel quality indicator called micropyrolysis index (MPI) is proposed for quantifying sooting tendencies of fuels in research laboratories. The MPI has been designed with the following characteristics: (i) oxygen independence, since sooting tendencies can change dramatically with varying oxygen concentration, (ii) reduced sample volume that can be utilized on a laboratory scale, (iii) improved repeatability over traditional methods, and (iv) use of common laboratory equipment so that the method may easily be implemented in multiple laboratories. MPI measurements were conducted by pyrolyzing 20 μL of fuel across a bed of R-Al2O3 beads at a temperature of 850 °C. Subsequently, the carbon-containing beads were transferred to a clean reactor where the deposited carbon was burned via temperature programmed oxidation (TPO) in order to quantify the amount. The amount of deposited carbon provides an indication of the tendency of a particular fuel to form soot when pyrolyzed. The observed tendency is independent of operating conditions, such as flame temperature and stoichiometric oxygen/fuel ratios, which greatly affect the measurements in other techniques used to estimate sooting tendency. An MPI value of 5 was assigned to the amount of carbon deposited from n-octane, and a value of 20, to the value corresponding to decalin. These two compounds were used as reference. Repeatability tests showed that MPI measurements had a standard deviation of 1.02 MPI at MPI ) 94.7 which scaled down to 0.11 at MPI ) 3.3. MPI values of pure nonaromatic hydrocarbons were found to correlate well with traditional threshold soot index (TSI) values. However, MPI was found to be more differentiating than TSI among the different hydrocarbon families: n-alkanes, isoalkanes, and cycloalkanes. A quantitative structure-property relationship (QSPR) model was developed in order to predict the MPI of other pure compounds not measured experimentally and further expand the database. Blending issues with aromatics are discussed, as well as the effects of aromatic content in real feeds.