Hydrocarbons for Hydrogen Generation - Hydrogen Generation, Storage, and Utilization - page 10

Usually, following reforming, the resulting syngas is sent to one or more
shift reactors in which the water-gas shift reaction takes place and generates
more hydrogen:

CO H O CO + → + H

2 .

(2.2)

2 2

In practice, this shift reaction is often conducted in two stages: first in a
high temperature shift reactor operating in the 350-475°C range that does
most of the conversion and then a low temperature reactor operating in the
200-250°C range that brings the CO concentration to less than a few % by
volume. Lower temperature reaction requires more active catalysts.
The final step is hydrogen purification, which can be accomplished using
PSA systems or palladium membranes, producing hydrogen with up to
99.999% purity. CO removal is a major step and can be done using prefer-
ential oxidation based on the following reaction:

CO + O = CO

2 .

1 2

(2.3)

2

This reaction is strongly favored over hydrogen oxidation, thereby allow-
ing preferential removal of CO to the level of a few part per million.
Methane steam reformers have been built over a large range of sizes and
types, including conventional, compact “fuel cell type,” plate-type, and mem-
brane reactors, as summarized in an excellent review article [1]. We will not
get into details of reformers here. Briefly, Figure 2.2 shows a schematic of
a small-scale methane steam reformer designed for fuel cell applications that
involved convective heat transfer. Compared with conventional tube reform-
ers, such “fuel cell type” reformers can operate at lower pressure and tem-
perature with lower cost materials, reducing the overall cost associated with
the operation. This type of reformers is commercially available.

2.3 PARTIAL OXIDATION

Another important method for H 2 production is partial oxidation of hydro-
carbons. Figure 2.3 shows a flow chart on the comparison between the steam
reforming and partial oxidation methods [1]. While both methods are based
on thermochemistry for hydrogen generation, they differ mainly in the source
of oxygen, that is, water for steam reforming and oxygen gas for partial
oxidation.
For example, methane can be partially oxidized to produce CO and H 2
based on the following reaction:

2 CH + → O 2 CO + 4 H .

(2.4)

4 2 2