Coke
formation is an obstacle in using hydrocarbons as the coolant
in hypersonic flight vehicles. In this paper, effective inhibition
of coke deposition was realized by the addition of wall catalytic
steam reforming, and the corresponding mechanism was revealed. The
anticoking tests were evaluated during the thermal cracking and catalytic
steam reforming processes of an endothermic hydrocarbon fuel under
3.0 MPa and outlet temperature from 600 to 680 °C. The amount
and properties of coke deposited in the thermal cracking with and
without steam reforming were investigated on the basis of their temperature-programmed
oxidation profiles and scanning electron microscopy. The results show
that the mass percentages of filamentous and amorphous cokes deposited
during thermal cracking without steam reforming are 20.32 and 79.68%,
respectively. The amount of coke deposited in a bare reactor is nearly
twice that deposited in a reforming catalyst-coated reactor, and the
coke formation rate in the former case is 8 times that in the latter
case. The absence of filamentous deposits during catalytic steam reforming
is ascribed to the catalyst layer on the inner surface, which prevents
contact between the hydrocarbon fuel and active metal sites. Filamentous
coke formation is therefore totally inhibited. Moreover, catalytic
steam reforming also inhibits amorphous coke deposition. Analyses
of the gaseous products and residual liquids from thermal cracking
of jet fuel show that the monocyclic and polycyclic aromatic hydrocarbon
contents decrease significantly under catalytic steam reforming. The
large amount of hydrogen generated from the wall catalytic steam reforming
reaction suppresses dehydrogenation, Diels–Alder, and condensation
reactions; therefore, coke deposition decreases.
A comprehensive understanding of the thermal cracking behavior of hydrocarbon fuels is important for thermal protection applications and investigations into the combustion of thermally cracked fuels. In the present study, n-dodecane is selected as a surrogate for aviation kerosene and it is subjected to a series of thermal cracking experiments at supercritical pressure. According to variations in chemical heat sink, fuel-conversion rate, and gas-production rate, the thermal cracking of ndodecane is divided into three regions: primary, secondary, and severe. In the primary cracking region, the fuel-conversion rate is lower than 13%, and the liquid products contain only chain alkanes and alkenes. Owing to the mass fraction of main products being proportional to the fuel-conversion rate, a one-step global reaction kinetics is constructed. The secondary cracking region is characterized by rapidly increasing chemical heat sink, fuel-conversion rates, and gas-production rates with increasing fuel temperature, and the appearance of monocyclic aromatic hydrocarbons (MAHs) and cycloalkenes. A kinetic model containing three reactions is proposed for this region. This also considers the thermal decomposition of chain alkanes and alkenes, which result in the formation of MAHs and cycloalkenes. Severe cracking is observed for fuel-conversion rates above 71% where a rapid increase in the concentration of monocyclic and polycyclic aromatic hydrocarbons (PAHs) occurs. The increasing rate of chemical heat sink slows in this region which is characterized by the formation of MAHs, PAHs, and coke. A three-dimensional numerical model is built for the primary and secondary cracking regions, taking the effects of the flow, heat transfer, and thermal cracking of n-dodecane into consideration. Predicted values for the outlet temperature, fuel-conversion rate, and distribution of the main species in all tested cases agree well with the experimental results, validating the numerical model and kinetics for the primary and secondary thermal cracking of n-dodecane.
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