Introduction
The rising concentration of atmospheric CO2 serves as a
contributor to changing climate patterns across the
globe.1 Indeed, since the industrial revolution, the
concentration of CO2 in the atmosphere has nearly
doubled as a result of burning energy-dense carbon sources like coal,
oil, and gasoline.2 A key challenge in the coming
years will be to find ways to curb the release of CO2 to
the atmosphere.3 If it is economically viable, capture
of CO2 from point sources like coal-fired power plants
has potential to be a crucial part of the global transition to a
carbon-constrained world.4 Materials advances in
sorbents,5-9 solvents,10-13 and
membranes14-17 over the past two decades have been a
chief focus of research on CO2 capture from coal flue
gas. Moreover, new process designs and intensification techniques have
been considered to reduce the inherent energy consumption associated
with these processes. Post-combustion CO2 is a
challenging separation as the separation system must contend with high
volumetric flowrates of CO2-dilute flue gases that are
saturated with water and laden with acid gases.18,19Specifically, the flue gas derived from coal-fired power plants contains
water, oxygen, and trace acid gases (SOx and
NOx), all of which must be considered when engineering
an effective CO2 capture system.20,21The challenges of both scale and composition must be accounted for when
considering different methods of separation.
These issues with scale lead to the commonly held opinion that for
CO2 capture processes from dilute point sources any form
of pretreatment, whether adjusting temperature, pressure, or removal of
additional contaminants or water,22 will result in
impractical economics.23,24 In this paper we will
reexamine this assumption. Inspired by the work of Hasse et al. and
others, who proposed a sub-ambient membrane process that required
considerable flue gas pretreatment, the work discussed here focuses on
the design of pretreatment systems for any separation process that
relies on pressure driving forces (in particular, membranes and pressure
swing adsorption).25-29 The critical consideration in
any sub-ambient pressure-driven CO2 capture process is
the removal of heat from the feed. External refrigeration cycles have
been shown to be extremely expensive,30 so the main
portion of what makes the process proposed by Hasse et al. economically
viable is their ability to run the sub-ambient process without the need
for any external cooling utility.
It is helpful to simplify the process we considered into two distinct
sub-processes: one that provides cooling through compression and
expansion of flue gas components, and a second that produces the
purified CO2 to pipeline specifications (see Figure 1).
The expansion of the N2 product provides cooling to both
the feed after compression and the CO2 product stream,
as shown in Figure 1. The expansion of the N2 product is
carried out on a stream that is already moderately cold through heat
integration and removes the heat of compression. This also allows for
the cooling of the CO2 product prior to its
liquefaction.
In this work, we consider the economic viability of sub-ambient
pressure-driven separation of CO2 from coal-fired flue
gas. Upstream pretreatment systems are discussed in detail, including
those used for removal of water from the flue gas, compression of the
flue gas, and cooling to the target conditions as well as downstream
systems used for energy recovery from waste streams, product
liquefaction, and pipeline delivery. In addition, sensitivity of the
process energetics and economics to variation in these process variables
are discussed. The process frameworkâs viability is considered for the
case of thermally-modulated fiber sorbents31-33 for
application in a pressure swing adsorption (PSA) process, mainly
focusing on how separation performance (productivity and purity) impacts
the economics of post-combustion CO2 capture. Three
exemplar thermally modulated fiber sorbents (zeolite 13X and the MOFs
UiO-66 and MIL-101(Cr)) are considered to provide insight into the
sorbent properties that are most desired for such a process
configuration.
Figure 1. Simplified flowsheet describing the sub-ambient
CO2 capture process. Blue and purple arrows indicate
N2-rich and CO2-rich products of
separation. Red arrows indicate the flow of heat. The gold box refers to
a N2-enriched Open Refrigeration Cycle. The navy
box shows the CO2 separation and purification.
Compressors and expanders are shown as one stage for clarity, and
possible boiler feedwater (BFW) integration is noted to be supplemented
with cooling water.