Page 307 - Biofuels Refining and Performance

286 Chapter Nine

reactions, which cools the gas. In a well-designed POX reformer with
controlled preheating of the reactants, the overall reaction is exother-
mic and self-sustaining. Both catalytic (870–925 C) and noncatalytic
(1175–1400 C) POX reformers have been developed for hydrocarbon
fuels. The advantage of POX reforming is that it does not need indi-
rect heat transfer, resulting in a compact and lightweight reformer.
Also, it is capable of higher reforming efficiencies than steam reformers
[3, 6].
Autothermal reforming combines SR with POX reforming in the pres-
ence of a catalyst that controls the reaction pathways and thereby deter-
mines the relative extents of the POX and SR reactions. The SR reaction
absorbs part of the heat generated by the POX reaction, limiting the
maximum temperature in the reactor. This results in a slightly exother-
concentration. The
mic process, which is self-sustaining, and high H 2
ATR fuel processor operates at a lower operating cost and lower tem-
perature than the POX reformer, and is smaller, quicker starting, and
quicker responding than the SR.
Most of the natural hydrocarbon fuels, such as natural gas and
gasoline, contain some amount of sulfur, or sulfur-containing odorants
are added to them for leak detection. As the fuel cells or reformer cat-
alysts do not tolerate sulfur, it must be removed. Sulfur removal is
usually achieved with the help of zinc oxide sulfur polisher, which
removes the mercaptans and disulfides. A zinc oxide reactor is oper-
ated at 350–400 C to minimize bed volume. However, removing sulfur-
containing odorants such as thiophane requires the addition of a
hydrodesulfurizer stage before the zinc oxide polisher. Hydrogen (sup-
plied by recycling a small amount of the natural gas-reformed prod-
uct) converts thiophane into H S in the hydrodesulfurizer. The zinc
2
oxide polisher easily removes H S.
2
To reduce the level of CO in the reformat gas, it must be water gas
shifted. The shift conversion is often performed in two or more stages
when CO levels are high. A first high-temperature stage allows high
reaction rates, while a low-temperature converter allows a higher con-
version. Excess steam is used to enhance the CO conversion. In a
PEMFC, the reformate is passed through a preferential CO catalytic oxi-
dizer after being shifted in a shift reactor, as a PEMFC can tolerate a
CO level of only about 50 ppm.
A fuel processor is an integrated unit consisting of one or more of the
above stages, as per the requirements of a particular type of fuel cell.
High-temperature fuel cells such as the SOFC and MCFC are equipped
with internal fuel reforming and hence do not require a high-
temperature shift, or low-temperature shift stage. The CO removal
stage is not required for the SOFC, MCFC, PAFC, and circulating AFC.
For the PEMFC, all the stages are required.