Hydrocarbon Processing - June 2022 - 76

Carbon Capture/CO2
* Pre-combustion: If H2
is burned
in a fired heater, the flue gas
contains no CO2
; therefore,
no CO2 capture is required for
the heater. The fuel is sent to a
reformer, followed by a watergas
shift reaction to convert
the fuel into H2
and CO2
. The
CO2 is then captured at the
reformer outlet (higher % CO2
and higher pressure makes CO2
easier to capture than from
flue gas) by using a physical
solvent in an absorber (see " A "
in FIG. 6). The process, with the
CO2
reformer's outlet to a pressure
swing adsorption (PSA) unit
that recovers pure H2
. The PSA
tail gas goes to the fuel system.
Reforming is an endothermic
reaction requiring heat. In an SMR
unit, 60% of CO2
is generated by
the reformer/water-shift reaction,
and 40% is from the reformer's
firebox, which generates the heat
required for the reaction. The CO2
absorber on the reformer's outlet
will recover approximately 60% of
the produced CO2
. To recover the
remaining amount, a second CO2
absorber (marked " B " in FIG. 6) on
the firebox flue gas stream would
need to be added to obtain a 90%
CO2
fuel the firebox with H2
produces no CO2
capture. An alternative is to
-which
but reduces H2
production. Installing a single,
large CO2
absorber on the firebox
flue gas (B) may be more cost
effective. An alternative to the
reformer is to use an auto thermal
reformer (ATR) to produce H2
.
An ATR oxidizes (burns) part
of the CH4
feed in the reactor to
generate the required reaction
heat, thereby eliminating the
need for a firebox. The ATR
uses oxygen from an ASU as an
oxidant, so there is no nitrogen
in the outlet of the reactor. The
remainder of the ATR outlet is H2
and CO2
, which can be separated
like a basic reformer, using a CO2
absorber or a PSA unit. Using
partial oxidation of methane with
carbon capture is another route
to produce H2
for fueling a fired
heater. FIG. 6 shows a high-level
76 JUNE 2022 | HydrocarbonProcessing.com
Mitigation
schematic of the three methods to
capture CO2
from combustion.
CO2 is captured and removed from
process streams by either chemical absorption
or physical separation. Chemical
absorption involves a chemical reaction
between the amine solvent and
CO2
, which is
equivalent to 2 Gj/t CO2
. Physical separemoved,
proceeds from the
ration can include either adsorption,
absorption, cryogenic separation, or
dehydration and compression. Physical
adsorption uses solid surfaces like zeolites,
metallic oxides, alumina or activated
carbon. After the CO2
the physical adsorbent, the CO2
is a challenge (direct air capture
requires processing 214 times
larger volume of air to remove
the same amount of CO2
as flue
, and requires the regeneration of
the solvent-1 t steam/1 t CO2
gas capture, requiring large air
contactors, like cooling tower
units with air flowing up through
packing where it contacts a
solvent or solid adsorbent)
* Cryogenic separation of CO2
from H2
on the outlet of a
steam methane reformer
* A process for growing algae
to absorb CO2
from air
is captured by
is then
released by cycling temperature in temperature
swing adsorption (TSA), pressure
in a PSA unit, or vacuum in vacuum
swing adsorption. Physical absorption
utilizes liquid solvents that are regenerated
by flashing the solvent to a lower
pressure in the regenerator, which does
not require steam, thus resulting in less
energy being used.32
Significant research is being conductcapture
costs in CCUS.
ed to reduce CO2
The U.S. Department of Energy's Carbon
Negative Shot initiative seeks to reduce
the cost of CO2
removed from the
atmosphere to less than $100/t by 2030
via either direct air capture or by helping
forests, agriculture and energy crops
capture and store CO2
. Additional ongoing
CCUS research includes:
* Absorption with chemical solvents
(hindered amines) to lower costs
by reducing the energy required
in the solvent regenerator for
aqueous solvent regeneration
* Absorption with physical solvents,
which can be regenerated with
vapor flashing and without
requiring steam (resulting
in lower energy usage)
* Utilization of other non-aqueous
or water lean solvents to reduce the
energy required for regeneration
* Adsorption onto solids, hydrated
solid sorbents and metals with
thermal or vacuum swing
adsorption (solid adsorbents
can withstand higher gas
temperatures than solvents and
can avoid solvent degradation)
* Direct air capture, as the low
CO2
concentration in air at 420
ppm (0.042%) vs. flue gas at 9%
(photosynthesis), and then
converting algae oil to biofuels.
Large-scale CCUS installations using
chemical solvent absorber
are proven to be effective with 1-MMtpy
to 3-MMtpy CO2
capture trains
[e.g., Boundary Dam in Saskatchewan,
Canada; Petra Nova in Houston, Texas
(U.S.); QUEST in Alberta, Canada
on a H2
pipeline and underground storage. To
prevent carbonic acid corrosion in CO2
pipelines, there are tight specifications
on oxygen at 50 ppm and on water at 20
ppm. Because of the high capital expenditures
of CO2
and CO2
capture units, pipelines
storage areas, a " hubs and clusvolumes
for an economically
and then to eiters "
concept is necessary to obtain sufficient
CO2
efficient project.
There will be a need for regional CO2
pipelines to collect CO2
ther provide it as a feedstock or deliver it
to storage. This activity will see competitor
companies working together to jointly
reduce their CO2
emissions. Several CCS
pipeline projects have been announced
in the UK and near Rotterdam/North
Sea. In the U.S., discussions are ongoing
for a CCS network to be developed in the
Houston Ship Channel to capture up to
100 MMtpy of CO2
by 2040 and to store
it offshore in the Gulf of Mexico. Additionally,
Talos Energy has announced
plans to develop CO2
pipelines and storage
along the Mississippi River corridor
in Louisiana and on the Texas Gulf Coast.
CO2
storage can include underground
saline reservoirs, depleted oil
and gas reservoirs, porous rock, subsea
or basalt. When CO2
is stored, the operator
must measure, monitor and verify the
steam methane reformer (1.1
MMtpy); and LaBarge, Wyoming (U.S.)
in a sour gas treating plant (6 MMtpy).
CO2
must be compressed to 100 bar for
technology
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