Fuel Processing for Fuel Cells - Institut für Technische Chemie und ...

Author's personal copy
22 Torsten Kaltschmitt and Olaf Deutschmann
The combination of SR and POX is called ATR:
C x H y O z þ nO 2 ðx
zÞþH 2 O ! xCO 2 þ x z þ y
H 2 ; DH 0 298 0: (15)
2
In reality, the reactions proceed via a complex network of elementary
reactions and are often both in the gas phase and on the catalyst. Indeed,
most of the fuel processing devices use heterogeneous catalysis, that is,
the chemical conversion is carried out on the surface of a solid material,
the catalyst. However, since most reforming processes are conducted at
temperatures above 400 C and sometimes even above 1000 C (POX,
ATR), homogeneous conversion in the gas phase can accompany the
catalytic conversion. The potential products of gas-phase reactions may
be quite different from the ones obtained over the catalyst. The formation
of methane and light olefins are of particular interest. The latter ones may
undergo molecular growth processes leading to polyaromatic hydrocarbons
(PAHs) and finally to soot particles and coke layers on the catalysts
and the solid walls of the apparatus and pipes (Kaltschmitt et al., 2011).
SR is the highly endothermic conversion of any hydrocarbon species
by steam into a mixture of carbon monoxide and hydrogen, the synthesis
gas. The product stream usually also contains unconverted steam and
some fuel and carbon dioxide, the latter due to consecutive WGS. Even
though SR requires the supply of steam and heat, it has been the primary
route for the production of hydrogen and synthesis gas. A major advantage
of SR in comparison to POX and ATR is the fact that no oxygen is
needed, and therefore nitrogen does not need to be pumped through the
process in case oxygen is provided in form of air.
POX offers a possibility to run the fuel processor without additional
devices for providing heat and steam; aside from the fuel, only oxygen
(air) is needed. Using noble metal catalysts such as rhodium—this process
is called CPOX—the fuel can be converted within milliseconds. By recycling
the steam containing exhaust gas of the fuel cell stack, the CPOX
reformer could be operated as an ATR reformer. For start-up, the C/O
ratio may be decreased (C/O