Gas-Phase Pyrolysis Characteristics of Hydrocarbon Fuels and Their Potential Impacts on Combustion Operational Performance

摘要

Fifteen hydrocarbon fuels comprising a wide range of chemical compositions were thermally stressed under subcritical pressure to evaluate their thermal decomposition characteristics and potential correlations to combustion operational performance. The fuels were heated up to 650 °C (1202 F) at a pressure of 0.27 MPa (40 psig) at several flow rates in a near-isothermal flow reactor. Thermal decomposition propensity of the fuels was estimated via gas-phase yields [i.e., liquid-to-gas (LTG) conversions], with quantitation of major gaseous products. Fuel thermal decomposition levels were limited to minimize the formation of secondary products from primary pyrolysis species, while they were kept sufficiently high to ensure statistical significance. As anticipated, higher temperatures and lower flow rates (i.e., increased residence time) increased the extent of thermal decomposition. Results show that for the same test conditions, the highly branched fuels and blends underwent the highest degree of thermal decomposition, while fuels containing cyclic and aromatic compounds were the most resistant to decomposition. It was observed that for an equivalent level of LTG conversion, the fuels with a high degree of branching yielded the highest concentrations of hydrogen, methane, and branched species, while linear alkanes produced the highest concentrations of C_(2) species. Correlations of gas-phase pyrolysis species to previously determined lean blowout (LBO) limits for these fuels in a model single-nozzle combustor show that fuels with higher relative yields of ethylene and ethane produced improved LBO performance, while those with increased methane yields performed poorer. These results agree with previous research conducted under supercritical conditions with several of the same fuels, further demonstrating that specific low molecular weight pyrolysis products are strong indicators of relative fuel LBO performance. Although not a major topic of this study, the impacts of a cetane-improving additive on the LBO performance of in -C_(12) and its effects on pyrolysis gas products were investigated. Correlations of the parent fuel composition to subcritical (i.e., gas phase) pyrolysis species selectivity, LTG conversions, and the corresponding implications on fuel LBO performance are discussed.