Combustion characteristics of crude oils for gas turbine applications by DCN measurements and NMR spectroscopy

Operating combined cycle gas turbines and/or low speed diesel engines on light petroleum crudes rather than distillate or heavy fuels produced from them avoids the carbon emissions associated with refining while continuing to achieve high energy conversion efficiencies. However, crude assays generated for refinery feed stock characterization do not include an ignition propensity property indicator, and the wide distillation range of crudes disfavors the use of the Calculated Carbon Aromaticity Index or Cetane Index used to characterize heavy or distillate fuel ignition quality. Here, the derived cetane number (DCN) of a single (whole) light crude is reported and the liquid volume fractions and DCNs of light end distillation fractions of this and three other light crudes are compared. Using ASTM D86 distillation method, four crudes were distilled into four light fractions as typically reported in crude assays: 1) light naphtha; 2) heavy naphtha; 3) kerosene; 4) light gas oil. DCNs were obtained using an ignition quality tester (IQT) and ASTM D6890 procedures. The DCNs of the fractions 1) – 4) for all crudes increased from 23 to 35 for the lowest boiling point fraction to 50–60 for the highest tested, in the range of the DCN measured for one of the whole crudes (52.6). The large DCN variation with distillation temperature suggests that preferential vaporization is likely relevant to pre-vaporized/premixed gas turbine combustion behaviors near operational limits. 1H and 13C NMR spectra were also acquired for the distilled samples. A QSPR regression developed using a Scheffe simplex-polynomial descriptor indicates that n-paraffinic CH2 groups play the most significant role in determining crude global ignition propensity. Results suggest that the reactivity potential of crude oils can be estimated reasonably well by employing a well-trained QSPR prediction tool for DCN, based upon key chemical functional groups determined from 1H and 13C NMR spectral analyses.