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 framework’s 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.