Page 306 - Biofuels Refining and Performance

Fuel Cells 285

3. Downstream processing—It involves reformate gas alteration by
converting carbon monoxide (CO) and water (H O) in the fuel gas
2
reformate to hydrogen (H ) and carbon dioxide (CO ) through the
2
2
water gas shift reaction, selective oxidation to reduce CO to a few
parts per million, or removal of water by condensing to increase the
H concentration.
2
A schematic showing the different stages in the fuel-processing system
is presented in Fig. 9.14. Major fuel-processing techniques are steam
reforming (SR), partial oxidation (POX) (catalytic and noncatalytic),
and autothermal reforming (ATR). Some other techniques such as dry
reforming, direct hydrocarbon oxidation, and pyrolysis are also used.
Most fuel processors use the chemical and heat energy of the fuel cell
effluent to provide heat for fuel processing. This enhances system
efficiency.
Steam reforming is a popular method of converting light hydrocarbons
to hydrogen. In SR, heated and vaporized fuel is injected with super-
heated steam (steam-to-carbon molar ratio of about 2.5:1) into a reac-
tion vessel. Excess steam ensures complete reaction as well as inhibits
soot formation. Although the steam reformer can operate without a cat-
alyst, most commercial reformers use a nickel- or cobalt-based catalyst
to enhance reaction rates at lower temperatures. Although the water gas
shift reaction in the steam reformer reactor is exothermic, the com-
bined SR and water gas shift reaction is endothermic. It therefore
requires a high-temperature heat source (usually an adjacent high-
temperature furnace that burns a small portion of the fuel or the fuel
effluent from the fuel cell) to operate the reactor. SR is a slow reaction
and requires a large reactor. It is suitable for pipeline gas and light dis-
tillates using a fuel cell for stationary power generation but is unsuit-
able for systems requiring rapid start and/or fast changes in load.
In POX, a substoichiometric amount of air or oxygen is used to par-
tially combust the fuel. POX is highly exothermic, and the resulting
high-temperature reaction products are quenched using superheated
steam. This promotes the combined water gas shift and steam-reforming

Air

Gaseous Sulfur High- Low- CO
fuel Reactor removal temperature temperature removal
high °C
350°C shift shift 200–260°C
260–370°C 200–260°C
Water
Figure 9.14 A fuel-processing system.