H2Tech - Q2 2021 - 26

PATHWAYS FOR SUSTAINABLE HYDROGEN

SPECIAL FOCUS

CO2, unconverted methane and a small amount of inerts. To
maximize H2, the synthesis gas is cooled and shifted in a watergas shift reactor to convert CO and water to H2 and CO2.
In a gray H2 scheme, the shifted syngas is separated in an H2
pressure swing adsorption (PSA) unit to generate a high-purity
H2 stream and a low-pressure tail gas stream that is sent to the
reformer furnace as fuel, with additional natural gas to supply
heat for the endothermic SMR reaction. Accordingly, all the
carbon from the natural gas exits the system as CO2 in the furnace stack.
To reduce the carbon emissions of an existing gray H2 asset,
CO2 can be captured from three locations:
1. Shifted syngas
2. PSA tail gas
3. Flue gas.
The cost of CO2 capture depends on the pressure and concentration of the CO2 in the source stream (TABLE 1), plus the
product specifications for the H2 and CO2. The most costeffective location to remove CO2 is from the pre-combustion
streams. The CO2 can be removed by a variety of means, including solvent-based absorption, PSA or cryogenic fractionation.
The option that provides the lowest overall cost of CO2 captured is cryogenic fractionation, which also achieves additional
high-purity H2 yield (FIG. 1). In this option, the H2 PSA tail gas
is compressed, dried, condensed and fractionated, resulting in a
high-purity liquid CO2 stream. Combining separation and liquefaction in a single unit operation saves utilities when a liquid
product is required.
Recent advances include further separation of the CO2 fractionation overhead in a second, smaller PSA unit that operates
with a novel process cycle that enables recovery of 90% of the
Flue gas
Natural gas
Water

Export steam
Shifted syngas

SMR

remaining H2. Overall, 99% H2 recovery from the SMR is possible with this scheme. This additional H2 recovery offsets investment in CO2 capture, reducing the net cost of carbon captured
to $20/t-$40/t. This retrofit does not require any revamp to the
existing H2 PSA, can be operated " on " or " off " without impacting the SMR operation, is solvent-free, has a smaller footprint
than an amine unit, requires no steam usage in the CO2 recovery steps, and is guaranteed to meet high-purity CO2 product
specifications with 99+% CO2 recovery. This combination of
technologies has been selected for a large U.S. CCS project for
clean H2 production at Wabash Valley Resources LLC in West
Terre Haute, Indiana.
An alternative option for CO2 capture from the PSA tail gas is
a CO2 PSA unit. A CO2 PSA unit can be installed on the shifted
syngas or the H2 PSA tail gas, although the latter is preferred primarily due to a simpler revamp and ease of operation in the event
that the CO2 capture unit is bypassed (FIG. 2). The CO2 PSA is
the lowest CAPEX and OPEX carbon-capture option and can
remove 99% of the CO2 in the pre-combustion stream, but the
extracted product is low pressure and low purity, requiring drying and liquefaction, or contaminant polishing via catalytic oxidation, followed by drying and multiple stages of compression to
be transport-ready.
The third option for CO2 recovery is amine-based solvent capture of the shifted syngas. This established technology can achieve
99% CO2 removal from the shifted stream (FIG. 3). However, this
option requires the use of steam for solvent regeneration. The
carbon emissions associated with steam generation erode the net
benefit. Furthermore, by removing the CO2 upstream of the H2
PSA, the overall H2 recovery will be eroded. This deficit could
be mitigated with PSA adsorbent reload and cycle modification;
however, such changes would make it very difficult to continue
operation if the CO2 removal unit were bypassed.
TABLE 1. Gray H2 SMR process stream pressures and
CO2 concentrations

H2 PSA
Tail
gas

Air

Fuel gas

Pre-combustion

CO2 fractionation
system

CO2 content, mol%
Pressure, barg

Post-combustion

Shifted syngas

PSA tail gas

Flue gas

12%-18%

50%-60%

15%-22%

20-30

0.3-0.5

0.1

High-purity CO2 liquid

TABLE 2. Comparison of CO2 capture options for blue H2

Additional high-purity H2 product

Cryogenic
fractionation CO2 PSA
on tail gas
on tail gas

FIG. 1. SMR retrofit CO2 capture option 1.
Flue gas
Natural gas
Water

SMR

Export steam
Shifted syngas

H2 PSA
Tail
gas

Air

Fuel gas

CO2 PSA

> 99%

90%-99%

CO2 phase

Liquid

Gas

Ultra-high-purity
CO2

Yes

Steam required

Yes

H2 yield

FIG. 2. SMR retrofit CO2 capture option 2.

Q2 2021 | H2-Tech.com

Amine on
ﬂue gas

% CO2 recovery
from stream

Burner revamp
CO2 to drying and compression

Amine on
shifted
syngas

Yes

+10%

No change

-1%

No change

CAPEX/OPEX

Medium

Low

Medium

High

Cost of CO2
captured, $/t

20-40

35-50

45-60

70-100

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H2Tech - Q2 2021

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