Page 531 - Advanced thermodynamics for engineers

524 CHAPTER 21 FUEL CELLS

21.6.1 STEAM REFORMING
Widely used throughout the petrochemical industry, SR combines a vaporised hydrocarbon with steam
over a catalyst (often nickel supported on alumina) at high temperatures (700–1000 C).

C x H y þ 2xH 2 O4xCO þ y 2 þ 2x H 2 0CO; CO 2 ; H 2 ; H 2 O (21.67)

During SR, the water-gas shift reaction also takes place, thereby converting the hydrogen in water
directly to hydrogen gas.

CO þ H 2 OðgÞ4CO 2 þ H 2 (21.68)
The most common design of SR for industrial use is the tubular reformer, a furnace containing several
tubes filled with catalysts through which the reactants pass. The reaction inside the tubes obtains heat
from a flame external to the tubes. Tubular reformers present one main drawback for small PEM
systems: they are not easily miniaturised.

21.6.2 POX REFORMING
In a POX reaction, the hydrocarbon is reacted with an amount of oxygen below the stoichiometric
amount:

C x H y þ x=2O 2 4xCO þ y=2H 2 (21.69)
During POX, a small fraction of steam is added both to prevent carbon deposition on reformer surfaces
and to reduce carbon monoxide production.

CO þ H 2 OðgÞ 4CO 2 þ H 2 0CO; CO 2 ; H 2 ; H 2 O (21.70)
The primary disadvantage of the POX reaction is that it may waste a significant portion of the energy
in the fuel as heat if this heat is not recovered. Its primary advantages are that
1. it is exothermic and therefore self-sustaining, and
2. it can be used on fuels with a variety of chemical components because catalysts are not
required.

A POX reaction may be either noncatalytic or catalytic.

21.6.3 AUTOTHERMAL REFORMING
AR combines the processes of SR and POX. By combining the hydrocarbon with a mixture of oxygen
and steam, the reformer uses the exothermic reaction of the hydrocarbon and oxygen to provide heat
for the endothermic reaction of the hydrocarbon and steam.

C x H y þ zH 2 O þ x z=2 O 2 4xCO 2 þ z þ y=2 H 2 0CO; CO 2 ; H 2 ; H 2 O (21.71)