(268a) Catalytic Autoignition of Higher Alkane Partial Oxidation on Rh-Coated Foams

Catalytic reforming
of heavy hydrocarbon fuels (e.g., gasoline, diesel, or jet fuel) to produce a
hydrogen-rich reformate has generated great interest for NO_x
abatement in diesel engines and electricity production in fuel cells. 
Transportation fuels are attractive because of their high energy density and
widespread distribution infrastructure.  To reduce start-up time for mobile
fuel processors, it may be favorable to start the fuel processor in catalytic
partial oxidation (CPO) mode (exothermic) and then add in water to transition
to autothermal reforming mode (thermally neutral).

In this work, the
ignition behaviors of higher alkane fuels (i-octane, n-octane, n-decane, and
n-hexadecane) on Rh-coated foams were investigated under CPO conditions to
better understand the surface processes governing lightoff.  In particular, the
minimum surface autoignition temperature as well as the ignition delay time as
a function of initial surface preheat temperature were determined for each fuel
in a near adiabatic reactor.  Through online mass spectrometry it is
demonstrated that steady-state production of syngas (CO and H₂) can
be attained within 5 s after admitting large alkanes (i-octane, n-octane,
n-decane, or n-hexadecane) and air into a short-contact-time reactor by using
an automotive fuel injector and initially preheating the Rh-coated catalyst
above the respective catalytic autoignition temperature for each fuel.  Minimum
catalytic autoignition temperatures on Rh were ~260°C for n-octane and 240°C for i-octane and
n-decane.  In contrast, catalytic autoignition of n-hexadecane indirectly
occurred at temperatures (> 220^oC) lower than those of the other
fuels investigated because of exothermic homogeneous chemistry that preheated
the catalyst (30-60^oC) to a temperature (~280^oC)
sufficient for surface lightoff.

The
ignition kinetics for the large alkanes were also determined and compared with
those of methane.  The dominant energetic step for large alkane surface
ignition is hypothesized to be oxygen desorption at saturation coverage as has
been suggested for methane.  The step(s) controlling surface ignition possessed
an apparent activation energy of ~78 kJ/mol that was not significantly
different between fuels (p > 0.05).  However, a significant difference was
found between the ignition preexponential for methane, O(10⁴
s^-1), and the other large alkanes, O(10⁶ s^-1). 
Additionally, the effect of carbon surface coverage on lightoff was examined
and found to significantly affect the ignition kinetics.