Chemical Engineering January 2015 - 44

TABLE 2. h2-Co-mixTurE for
diffErEnT produCTs
Feature Report
will the final gas be used for? Depending
on requirements, producers
will look to create a mixture with a
specific ratio of H2 to CO, and each
technology will produce a different
result (Table 2). In general, SMR is
the most flexible in this regard. It
can yield the highest H2 content, but
also enables different ratios for applications
in the chemicals and metallurgy
sectors. ATR and GasPOX,
on the other hand, result in a syngas
with a larger CO content, due to
the higher temperatures and the O2
used in the process.
The following section discusses
in more detail the important points
to consider when designing a system
to generate the following products:
H2, CO and synfuels.
Hydrogen. To generate H2, the
feedstock chosen will ideally exhibit
an inherently high hydrogen-tocarbon
ratio, such as natural gas. It
is also possible to produce H2 from
more carbon-heavy fuels like naphtha;
in this case, the heating value
of the feed helps to extract H2 from
the steam during the reforming process.
However, the overall levels of
H2 will be lower and the additional
steam must be taken into account
in plant design.
For each of the four reforming
technologies discussed here, the syngas
generated will contain differing
levels of H2. With SMR, the resultant
gas exhibits the highest H2-to-CO
ratio (up to 5.5). At the other end of
the scale, the ratio with GasPOX is
in the range of 1.5-2.0. Of course, it
is possible to change this balance by
adding a CO shift unit downstream
of the reforming process.
Carbon monoxide. Where the required
end product is CO or oxogas,
GasPOX can deliver the highest
CO levels. CO can, however, also be
generated using SMR: in this case,
almost all the carbon in the feed is
converted to CO, by taking the CO2
and unconverted CH4 remaining
in the reformed gas and recycling
it back to the feedstream, thereby
lowering the H2-to-CO ratio from
around 4.5 to 3.0. Moreover, if CO2
is available from an external source,
and more than 50% of the CH4 feed
molar flow is replaced with CO2,
then it is possible to produce H2 and
CO in the reformed gas at a ratio
of 1. However, it is important to observe
the thermodynamic limit in
order to avoid carbon formation in
the reforming stage.
Synfuels. When manufacturing
synthetic fuels, CO2 in the syngas
will also be converted to fuel. As
such, the optimum syngas composition
is expressed in terms of a
stoichiometric number, which describes
how much CO and CO2 will
react with H2 to form methanol or
Fischer-Tropsch products. The optimum
stoichiometric number can
be achieved by deploying more than
one technology simultaneously. A
combination of SMR and ATR - a
process known as combined reforming
- is often chosen to produce
methanol and synthetic fuels. When
using the combined method, the
steam reformer is operated at higher
pressure (~40 bars) and lower temperature
(~750°C). The lower level
of methane conversion in the SMR
stage (primary reforming) is then
compensated by the downstream
ATR system (secondary reforming).
The required stoichiometric number
can be achieved by adjusting the
SMR and ATR feed flow ratio, and
by separating H2 from the purge gas
following methanol synthesis and
recycling it back to the upstream
syngas generation system.
Combining syngas streams. Another
option for large-scale syngas
production is to combine syngas
streams from different sources -
for example, from entrained-flow
coal gasification and from a methane
reforming unit - to achieve
the required product flowrates.
This scenario allows the individual
syngas production units to run at
maximum efficiency, while the mixing
process ensures the optimum
product-ratio adjustment.
Conditioning and purification
Syngas generation does not end
with the reforming process, as the
gas created will always contain several
unwanted components. First of
all, even where a CO shift unit has
been deployed, it is never possible to
achieve complete conversion to H2
Hydrogen
100% H2 Hydrogen
100% H2 Ammonia
75% H2 SNG
67% H2 MeOH
≤ 67% H2 Fischer Tropsch /
synfuels
~62% H2 Direct reduction gas
60% H2 DME
50% H2 Oxoalcohol
26-41% H2 Power (integrated
gasification combined
cycle)
100% CO Pure CO (acetic acid)
100% CO Polyurethanes
(MDI/TDI)
Carbon monoxide
due to the chemical equilibrium at
the necessary reaction temperature.
Moreover, methane is not fully converted
during reforming, and all nitrogen
in the feed and in the oxygen
stream will end up as N2 in the reformed
gas. And of course, the CO2
created as a byproduct will need to
be removed in the majority of cases.
A number of downstream gas-conditioning
and product-separation systems
can be used to produce pure
gases or gas mixtures. The methods
chosen will vary depending on the
required end product.
The cost and effort of CO2 extraction
will depend on the level of CO2
present and the syngas flow, and the
CO2 fraction in the reformed gas
will vary in line with the generation
technology deployed (Table 3). There
are two main reasons for this: differences
in the carbon-to-hydrogen
ratio for individual feedstocks, and
variances in the outlet temperature
of the reforming unit in question.
When it comes to product separation,
there are three main options
downstream of methane reforming:
a cryogenic or cold box unit, polymer
membranes, or pressure swing
adsorption (PSA). The choice of one
technology over the other will be influenced
by a number of factors. For
cryogenic processes, for example, it
is important to keep in mind that a
higher reforming temperature - as
in a GasPOX plant - will contribute
to higher methane conversion
and lower methane slip. As a result,
when separating CO from a synChemiCal
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