COOL FLAME EVAPORATION FOR DIESEL REFORMING TECHNOLOGY

Abstract

The separation of the evaporation from the catalytic reforming process is crucial for fuel processing. Unfavourable mixtures lead to degradation by local hot spots in the sensitive catalysts and formation of unwanted byproducts. Cool flames offer a complete and residue-free evaporation of liquid hydrocarbon mixtures. The conditions whether cool flames can be stabilised or not is related to the heat management of the vaporizer. To this examinations were conducted in a flow reactor to investigate stable cool flame operation under reformer conditions. A validation of the vaporizer is examined in combination with different reforming technologies. INTRODUCTION The mixture generation constitutes a key factor in terms of the quality of a reforming reaction. Inhomogeneties increase the tendency for soot to be formed and favour hot spots in the sensitive catalysts. Thus efforts are made to establish a homogenous educt mixture prior to the reforming zone. Therefore liquid fuels, such as diesel, first have to be atomised and vaporised into microscopically sized droplets. Temperatures of 400 °C, however, are required for the evaporation, since the upper boiling point of diesel and industrial gas oil (IGO) lies at around 380 °C. With direct vaporisation into a pre-heated air stream the vaporisation can be provided by saturation. Already with temperatures about 150 °C to 200 °C a complete evaporation can be achieved (1). However in technical applications the validity of this theoretical calculated temperature is restricted by non-ideal heat and mass transfer as well as a limited time scale. Commonly the evaporation takes place in temperature areas and under residence times where auto-ignition can occur. One model to describe the ignition process is the thermal ignition-model where an auto-ignition arises from the self heating produced by exothermic low-temperature reactions. The question whether it comes to an explosion or not is related to the heat release in comparison to the heat losses of the system. Generally the heat release R& rises exponentially with the temperature governed by the Arrhenius equation [1] whereas the heat losses L& can be described by linear temperature behaviour due to the Newton law of cooling [2]. The impellent temperature difference is given by the mixture temperature T and the wall temperature TW. f T R E r c e A U V Q R ⋅ ⋅ ⋅ ∆ = = − ) ( n & & [1] ) ( W T T V A V Q L l − ⋅ ⋅ = = α & & [2] In a certain temperature range a divergence from the Arrhenius behaviour can be observed for several fuels including diesel or IGO. This temperature region is characterised by a decrease of the heat release with an increase of the temperature. It is also referred as the Negative Temperature Coefficient (NTC). The key to this characteristic lies in the lack of stability of the links formed within the chain reactions as a result of oxygen being absorbed (2). temperature T he at r el ea se R ,