Cool flame partial oxidation and its role in combustion and reforming of fuels for fuel cell systems

Abstract

The purpose of this review was to integrate the most recent and relevant investigations on the auto-oxidation of fuel oils and their reforming into hydrogen-rich gas that could serve as a feed for fuel cells and combustion systems. We consider the incorporation of partial oxidation under cool flame conditions to be a significant step in the reforming process for generation of hydrogen-rich gas. Therefore, we have paid particular attention to the partial oxidation of fuels at low temperature in the cool flame region. This is still not a well-understood feature in the oxidation of fuels and can potentially serve as a precursor to low NO x emissions and low soot formation. Pretreatment, including atomization, vaporization and burner technology are also briefly reviewed. The oxidation of reference fuels ( n-heptane C 7H 16, iso-octane C 8H 18 and to a lesser extent cetane C 16H 34) in the intermediate and high temperature ranges have been studied extensively and it is examined here to show the significant progress made in modeling the kinetics and mechanisms, and in the evaluation of ignition delay times. However, due to the complex nature of real fuels such as petroleum distillates (diesel and jet fuel) and biofuels, much less is known on the kinetics and mechanisms of their oxidation, as well as on the resulting reaction products formed during partial oxidation. The rich literature on the oxidation of fuels is, hence, limited to the cited main reference fuels. We have also covered recent developments in the catalytic reforming of fuels. In the presence of catalysts, the fuels can be reformed through partial oxidation, steam reforming and autothermal reforming (ATR) to generate hydrogen. But optimum routes to produce cost effective hydrogen fuel from conventional or derivative fuels are still debatable. It is suggested that the use of products emanating from partial oxidation of fuels under cool flame conditions could be attractive in such reforming processes, but this is as yet untested. The exploitation of developments in oxidation, combustion and reforming processes is always impacted by the resulting emission of pollutants, including NO x , SO x , CO and soot, which have an impact on the health of the fragile ecosystem. Attention is paid to the progress made in innovative techniques developed to reduce the level of pollutants resulting from oxidation and reforming processes. In the last part, we summarize the present status of the topics covered and present prospects for future research. This information forms the basis for recommended themes that are vital in developing the next generation energy-efficient combustion and fuel cell technologies.