Review of recent process developments in the field of carbon dioxide (CO₂) capture from power plants flue gases and the future perspectives

ABSTRACT

A comprehensive review of recent process developments in the field of post-combustion carbon dioxide (CO₂) capture from power plant flue gases is presented in this article. Different types of technologies for post-combustion CO₂ capture namely: Absorption, Membrane, and Adsorption (AMA), were evaluated based on their CO₂ recovery, energy efficiency, and cost. The study examines the fundamentals of each process, including their advantages and limitations, and highlights the recent advancements made in these areas. Specifically, the paper provides an overview of developments in each process area and discusses the development of new process configurations and the optimisation of existing ones with a view to identify the optimal process route. The two-stage-hybrid configurations were identified as the optimal process configurations that will meet the required needs in terms of energy efficiency, cost savings, and the desired CO₂ purity and recovery of ≥95mol% and ≥97mol% respectively. However, techno-economic analyses are still needed to identify the best configuration. Thus the review concludes by emphasising the need for further research and development on techno-economic analyses to identify the best configuration in the two-stage-hybrid options for post-combustion carbon capture technology to be viable for commercialisation.

Highlights

• Climate change results from global warming caused by CO₂ emissions, mainly from fossil fuel power plants.

• Post-combustion CO₂ capture technologies involve absorption, adsorption, and membrane separations.

• The CO₂ capture process configurations can be single-stage, multi-stage, or hybrid

• The combined two-stage-hybrid configurations were found to be the optimal process route for post-combustion CO₂ capture in terms of CO₂ recovery/purity, energy efficiency, and cost savings.

KEYWORDS:

• Global warming
• greenhouse gases (CO₂)
• carbon capture
• climate change
• hybrid process configurations

Nomenclature

AMA	=	Absorption, membrane, and adsorption processes
ADS	=	Adsorption
CCS	=	Carbon capture and storage
CO	=	Carbon monoxide
CO2	=	Carbon dioxide
DEA	=	Diethanolamine
GHG	=	Greenhouse gases
IRR	=	Internal rate of return
K	=	Temperature in degrees Kelvin
KOH	=	Potassium hydroxide
LCOE	=	Levelised cost of electricity
LVC	=	Lean vapour compression
MDEA	=	Methyl diethanolamine
MEA	=	Monoethanolamine
MOF	=	Metal organic frameworks
NH3	=	Ammonia
NO	=	Nitrogen oxide
NO2	=	Nitrogen dioxide
NO3	=	Nitrogen trioxide
NOx	=	Nitrogen oxides
oC	=	Temperature in degrees Celsius
ppm	=	Concentration in parts per million
PSA	=	Pressure swing adsorption
PZ	=	Piperazine
REG	=	Regeneration
RSP	=	Rich solvent preheating
SO2	=	Sulphur dioxide
TRL	=	Technology readiness levels
TSA	=	Temperature swing adsorption
VPSA	=	Pressure swing adsorption with the use of a vacuum for regeneration
VTSA	=	Temperature swing adsorption with the use of a vacuum for regeneration

1. Introduction

Climate change, sea level rise, atmospheric heat waves, shore floods, land droughts, and other environmental challenges are caused by global warming. During the Katowice Climate Change Conference in December 2018, it was declared that anthropogenic carbon dioxide (CO₂) emissions are directly related to global warming. Specifically, it was reported that world energy-related CO₂ emissions would increase to 43.2 billion metric tons in 2035, resulting in a global CO₂ concentration of approximately 450 ppm and a 2°C increase in the global mean temperatures with severe environmental and economic catastrophe (Etheridge et al. 1996; Peters et al. 2017). As a result, ocean carbonate chemistry will undergo serious changes leading to ocean acidification (Le Quéré et al. 2018).

Thus, control of anthropogenic emissions of greenhouse gases (GHG) is one of the most critical environmental challenges (Rau et al 2007). As shown in Table 1, fossil fuel-fired power plants are the primary emission sources of greenhouse gases (CO₂, CO, NO_x, SO_x) worldwide, of which CO₂ is the major culprit.

Table 1. Flue gas composition from industrial sources (Harkin, Hoadley, and Hooper 2009).

Industry	Composition
	CO2	H2O	O2	N2	NO	NOx	SO2	SO3	CO
Power Plant Flue Gas Composition Component Value Rotary Air Heater Inlet	15.39 vol%		4.55 vol%	–	1749ppm	1.9 ppm	214 ppm	0.5 ppm	99 ppm
Coal-fired power plant	10–11 vol %	20–23 vol%	4–5 vol%	-	-	150-250 ppm	120-200 ppm	-	-
Gas-fired power plant	4 mol%	8 mol%	12 mol%	76 mol%	-	-	-	-	-
Cement Plant	17.8 mol%	18.2 mol%	7.5 mol%	56.5 mol%	-	250 mg/Nm3	25 mg/Nm3	-	1470 mg/Nm3
Coal-fired power generation plant	7–15 mol%	5–15 mol%	2–12 mol%	65–75 mol%	-	400 ppm	400 ppm	1–12 ppm	-

Thus the increase in global atmospheric concentration of CO₂ and world energy demand has highlighted the importance of low-carbon energy systems/power plants. Table 2 shows the attributes of such power plants in comparison to fossil fuel plants.

Table 2. Comparison of various power production technologies (Qadir et al. 2015).

Technology	Attributes
	Cost	Environmental friendliness	Power on demand	Resource availiability
Coal	low	low	Yes	high
Natural gas	low/medium	low/medium	Yes	high
Hydroelectric	low	high	Yes	low/medium
Biomass	low/medium	high	Yes	low/medium
Wind	medium	high	No	medium
Solar thermal energy	very high	high	Yes	high

While some attributes are met by some technologies, none of the technologies meet all the attributes of sustainable power generation in terms of low cost, environmental friendliness, power on demand, and resource availability. Although it is expected that there will be a gradual switch to other alternative low-carbon energy systems (He et al 2018) like renewable energy resources (biomass, solar, wind, etc), fossil fuels power plants still remain the major electricity producer because they present several advantages over other sources of energy such as:

• Fossil fuel plants can respond quickly to short-term changes in peak demand for power

• Provide backup when other sources of energy, wind or solar, are used for electricity production; produce energy in larger quantities

• Lower cost than those obtained from emerging renewable sources

• Provide the flexibility to meet short and long-term changes in the demands.

Thus, it is important to develop technologies for fossil fuel plants that can assist in the reduction of CO₂ emissions from fossil fuels. The available options for reducing CO₂ from conventional power plants are:

• Fuel switching from carbon-intensive fuels (e.g. coal) to less carbon-intensive fuels (e.g. natural gas) or non-fossil fuel energy alternatives such as nuclear, biomass, solar energy, wind, etc.

• Capturing and storing the CO₂ emitted from the fossil fuel combustion

It is expected that the implementation of all the options mentioned above will provide longer-term benefits to global warming. However as shown in Table 2, renewables though meet the requirements for environmental friendliness, they are still high in cost and further research are needed to bring them as viable alternatives to fossil fuels. Carbon capture technologies have the potential to allow continuous use of fossil fuel since it is regarded as a realistic immediate and medium-term approach for drastically mitigating CO₂ emission from fossil fuel power plants until renewables take over (Dave et al 2009). Thus CO₂ capture is considered as a viable option to minimise the emissions of greenhouse gases. (Soltanieh, Azar, and Mohammad 2012, Petrakopoulou and Tsatsaronis (2013); Bekun, Alola, and Sarkodie 2019). Different AMA technologies are widely used for CO₂ capture. Conventional chemical absorption is a mature technology for CO₂ separation (Haas et al 2014).

However, it is also energy intensive which can result in a high incremental cost and a significant environmental impact. Membrane technology has already been commercialised and documented as a competitive technology for CO₂ capture (Brunetti et al 2014). However, there are still challenges to the applications of membranes for CO₂ capture such as capacity limitations of membrane separation performance and poor membrane stability and short lifetime when exposed to a gas stream containing the impurities of acid gases such as SO₂ and NOx. Though adsorption has good energy savings and low cost, the throughput is low. Great efforts have recently been made to bring these AMA technologies for post-combustion CO₂ capture to higher technology readiness levels (TRL) for enhanced recovery potential, energy, and cost reduction benefits. Thus, this paper reviews these AMA technologies to identify future perspectives and research directions to make post-combustion carbon capture viable for full-scale commercialisation.

2. Review of CO₂ post-combustion capture technologies

2.1 Absorption

The primary method for CO₂ recovery is the absorption of the CO₂ in the flue gas of a power plant using solvent. This process is then followed by the desorption of the CO₂ and the regeneration of the solvent for reuse, as shown in Figure 1. The captured CO₂ is injected into the deep geological formation or underground storage site that has been previously utilised, such as gas and oil wells, mines, offshore drilling wells, and so on (Figueroa et al. 2008). Also, captured CO₂ is used in other industries as a raw material. For instance, CO₂ is used in the fertiliser industry to produce urea; in food/beverage as a safe preservative; in the petroleum industry to enhance oil recovery and in chemical industries for aethers and alcohols (methanol and ethanol) synthesis (Edrisi, Mansoori, and Dabir 2016).

Figure 1. Process diagram for CO₂ capture by absorption.

The most common solvents that are often employed for CO₂ scrubbing processes are the amines namely: monoethanolamine (MEA) and diethanolamine (DEA) (Romeo et al 2020). Other recent solvents include potassium carbonate (Barbera et al. 2022), aqueous ammonia (NH₃) absorbent (Siddiqui et al. 2020), and piperazine (Dubois and Thomas 2017). Researchers have studied absorption methods/process configurations suitable for carbon capture in terms of absorption capacity, CO₂ recovery, energy efficiency/consumption, operating conditions, and costs (Darde et al 2010).

Among the absorption methods, several techniques can be listed, due to the method of regenerating the solvent (Ayittey et al. 2021; Dubois and Thomas 2017) namely, Lean vapour compression (LVC), rich solvent preheating (RSP), and their combination (LVC + RSP). Compared with the conventional MEA CO₂ capture process, the modified process configurations yielded higher carbon capture levels at reduced reboiler duties. Pertaining to the carbon removal efficiency, the LVC, RSP, and LVC + RSP were able to improve the capture rate by 1.61%, 1.17%, and 2.79%, respectively. Further, the modifications were able to reduce the total equivalent electrical penalty on the power plant by 12.94%, 5.24%, and 14.48%, respectively. However, the addition of extra equipment such as a compressor, flash vessel, extra pump, and extra heat exchanger in the modified configurations, increased the capital and operational costs of the capture plant.

Similarly, in terms of process configurations, we have a single stage (Barbera et al. 2022), using only one solvent MEA, NH₃, or KOH or using blends of three different solvents, namely: monoethanolamine (MEA), piperazine (PZ), and methyl diethanolamine (MDEA) (Dubois and Thomas 2017), double stage using two different solvents of MEA and DEA in series (Chauvy et al. 2022). Using mixed amines solvent and innovative configuration reduced the energy consumption; however, it will lead to extra equipment such as a compressor, flash vessel, extra pump, and extra heat exchanger that would increase the capital and operational costs of the capture plant (Petrakopoulou et al 2012). Finally, a hybrid configuration (Reza et al. 2022) involves an ammonia absorption refrigeration system for CO₂ liquefaction (Yu, Jingwen, and Shujuan 2015).

The main challenge in the absorption method is to reduce the energy required for the regeneration of the solvent. The thermal energy required to regenerate the solvent in post-combustion CCS is typically provided with low-pressure steam extracted from the plant, which leads to some efficiency penalty. Thus, external energy sources were employed in the form of renewables, such as wind (Siddiqui et al. 2020), solar (DinAli and Dincer 2019), and electrolysis (Chauvy et al. 2022), all aimed to reduce energy penalties. Nonetheless, renewable technologies are considerably more competitive in terms of levelised cost of electricity (LCOE) when they are deployed as a hybrid system in conjunction with other thermal energy and CO₂ utilisation technologies so that it provides a small to moderate fraction of the total energy required for the carbon capture plant.

Finally, in an attempt to offset the energy penalties associated with the absorption method of carbon capture, resource recovery of the captured carbon was done to convert the captured carbon into valuable products such as synthetic natural gas (Chauvy et al. 2022); urea synthesis fertiliser, formaldehyde (Edrisi, Mansoori, and Dabir 2016), In the different production rate, the proposed resource recovery plants have a rate of returns (IRR) above 28%, while the IRR of conventional plants is almost near 20%. Nevertheless, in addition to more process units involved, the environmental impacts of resource recovery plants are higher than conventional natural gas production facilities. Chauvy et al.(2022) implemented an advanced CO₂ capture process using mixed amines solvent and an innovative configuration that includes an optimised heat integration with the CO₂ conversion to hydrogen through water wind-based electrolysis. Simulation results with CO₂ captured from cement plants flue gas showed an overall reduction in CO₂ emissions by 76% (Kirchner et al 2020). Similarly, the fossil resource scarcity is drastically reduced by over 80% due to the heat integration between the CO₂ capture and the CO₂ conversion processes.

2.2 Adsorption

Adsorption is the deposition of gas molecules on a solid surface, thereby creating an adsorbate film on the adsorbent’s surface. The separation performance is based on different interaction forces between adsorbent and adsorbate (Veneman et al. 2014). Separation by gas adsorption processes is mainly controlled by thermodynamics (adsorption affinities) and kinetic effects (diffusion rate) (Iribarren et al 2013). Typically, CO₂ adsorption has been associated with specific surface area values (Yang et al. 2013), pore size (Wilson et al. 2021), and micropore volume (Alhassan et al. 2016) of the adsorbent materials. Adsorbent materials that are used for carbon capture include zeolites (Muriithi, Petrik, and Doucet 2020), metal organic frameworks, MOFs (Assen et al. 2021), and activated carbons derived from Amazonia nutshells (Serafin et al. 2022), pomegranate peels (Ouzzine, Serafin, and Sreńscek-Nazzal 2021), waste tea (Rattanaphan, Rungrotmongkol, and Kongsune 2020). Researchers have conducted studies on adsorption methods/process configurations suitable for carbon capture in terms of adsorption capacity, CO₂ recovery, energy efficiency/consumption, operating conditions, and costs.

Among the adsorption methods, several techniques can be listed due to the method of regenerating the adsorbent (Sjostrom, Sharon, and Holly 2010). Among the popular method is temperature swing adsorption (TSA) (Yang et al. 2013). In the TSA cycle shown in Figure 2, CO₂ is selectively removed by adsorption at low temperatures and then recovered by adsorption at high temperatures while the adsorbent is regenerated for reuse (Lee and Park et al 2015).

Figure 2. Process diagram for CO₂ capture by absorption (TSA).

Similarly, other adsorption processes include pressure swing adsorption (PSA), temperature swing adsorption (Bahamon, Daniel, and Vega 2016) with the use of vacuum for regeneration (VTSA), and pressure swing adsorption with the use of vacuum for regeneration (VPSA) (Mason et al 2011). Similarly, in terms of process configurations, we have a single stage (Yang et al. 2013) that decreases energy consumption from 114.53 to 28.23 kJ/mol when the initial concentration of CO₂ rises from 15% to 75%; two stages (Dhoke et al. 2021; Liu et al. 2022; Wang et al. 2013) that give high CO₂ purity (> 95%) and recovery ratio (> 90%).

The benefits of multi-stage CO₂ adsorption under the vacuum-pressure swing adsorption (VPSA) or vacuum-temperature swing adsorption (VTSA) include the high CO₂ purity of the final product and the high CO₂ recovery (Dhoke C, et al. 2021). For example, two successive vacuum-pressure swing adsorption (VPSA) configurations applied to CO₂ separation from flue gas showed a high CO₂ purity (>95%) and recovery ratio (>90%) (Wang et al. 2013). Liu et al. (2022) evaluated the thermodynamic performance (Energy consumption and exergy efficiency) of two-stage VPSA and compared with a single-stage VPSA cycle. Results indicated that energy consumption decreases from 114.53 kJ mol−1 to 28.23 kJ mol−1 when the initial concentration of CO₂ rises from 15% to 75%. Thus introduction of the second stage significantly elevates the concentration of product gas with less energy input. Santori et al. (2018) proposed a thermally driven, negative-carbon adsorption process for capture, purification, and compression of carbon dioxide from the air. The process is based on a series of batch adsorption compressors of decreasing size to deliver a compressed carbon dioxide stream to final storage (Rau et al 1999). Thermodynamic analysis of the process shows that, by exploiting the equilibrium properties of commercial and non-commercial materials, carbon dioxide can be produced at specifications appropriate for geological storage (Renforth and Henderson 2017). In their work, (Zhang et al. 2022) designed a thermal energy-driven multi-stage adsorption system based on temperature swing adsorption for direct air capture. Analysis of the process showed that the CO₂ purity in the adsorbent bed can exceed 90% and the pressure can exceed 14 bar with zeolite 13X at a regeneration temperature of 368 K (Mores et al 2019). Exploration of performance regarding specific energy, exergy efficiency, and the influence of the mass of adsorbent on the purity and specific energy were also been evaluated.

Results showed that the specific energy declines and the optimal exergy efficiency increases from 0.205% to 0.238% as the regeneration temperature grows from 368 K to 373 K. The values of adsorbent mass in the second and third beds for optimal purity and specific energy were calculated. It means that a thermally powered carbon separation device can not only meet the requirements of direct air capture but also get the optimal purity of carbon dioxide or specific energy by altering the operation parameters. C. Dhoke et al. (2021) reviewed several reactor configurations that were proposed for adsorption-based CO₂ capture. The fundamental behaviour of adsorption in different gas–solid contactors (fixed, fluidised, moving, or rotating beds) and regeneration under different modes (pressure, temperature, or combined swings) were discussed, highlighting the strengths and limitations of different combinations of gas–solid contactors and regeneration mode. In addition, the estimated energy duties in published studies and the current technology readiness level of the different reactor configurations were reported. Other aspects, such as the reactor footprint, the operation strategy, suitability to retrofits, and the ability to operate under flexible loads are also discussed. In terms of future work, the paper emphasised that the key research need is a standardised techno-economic benchmarking study to calculate CO₂ avoidance costs for different adsorption technologies under standardised assumptions. Qualitatively, each technology presents several strengths and weaknesses that make it impossible to identify a clear optimal solution. Such a standardised quantitative comparison is therefore needed to focus on future technology development efforts.

2.3 Membrane

A membrane is a barrier with the ability to control the permeation rate of chemical species through the membrane. In essence, a membrane is a discrete, thin film that moderates the permeation of chemical species that meet it. In separation applications, the goal of the process is to allow the passage of one mixture component freely through the membrane while hindering the permeation of other components (Shah et al 2021). A membrane-based post-combustion CCS can be easily retrofitted to fossil fuel power plants to significantly reduce the CO2 emissions produced by the combustion as shown in Figure 3. Thus, in CO₂ capture, the goal is to allow CO₂ to permeate while hindering the other components of the flue gas. This is because CO₂ at a relative size of 2.87A is the smallest in the flue gas stream, except for H₂0 at 2.65A, which is condensable.

Figure 3. Process diagram for CO₂ capture by membrane separation.

Different membranes such as polymer membranes, microporous organic polymers (MOPs), mixed matrix membranes (MMMs), carbon molecular sieve membranes (CMSMs), and inorganic (ceramic, metallic, zeolites) membranes can be used for CO₂-related separation (He et al. 2017b); (He et al. 2017a). However, there are several challenges, such as energy-intensive operation and the high cost of the CO₂ separation system, which affect the reliability and the cost of electricity generation. For instance, the power plant flue gas needs to be treated to remove membrane contaminants, including water (by dehydration), which further increases the cost of CO₂ separation from flue gas with available technologies. Thus, single-stage membrane separation process can only achieve a purity of 68.2 mol% CO₂ and a recovery of 42.7% in the permeate (Giordano et al. 2017), (Karaszova et al. 2019)

3. Future perspectives

What is clear from the review of current CO₂ capture methods is that a standalone method, be it absorption, adsorption, or membrane process, can never viably achieve the desired carbon capture requirements/specifications, as a result of mitigating factors associated with individual processes such as energy penalties, severe operating conditions, costs, efficiency, and low-capacity throughput. For instance, the performances of single-stage adsorption methods (PSA, VSA, TSA, and TVSA) are shown in Table 3.

Table 3. Comparison of CO₂ separation processes (Zhang et al. 2022).

Adsorbent	Process	CO2 (%)			Specific energy Consumption, MJ/kgCO2
		Feed	Purity	Recovery
13X	PSA	10.5	99	80	2.8
5A	TSA	10	94	85	6.46
5A	TSA	10	95	81	3.23
Mg-MOF-74	TSA	15	97.36	86.5	1.921
13X	TVSA	0.04	95	35	2.2
13X	TSA	0.04	90.51	86,9	11.92

Table 3 shows that single-stage adsorption processes could not reach the desired purity and recovery levels of CO₂ capture specified at ≥95% and ≥97% respectively for climate change mitigation. Similarly for different single-stage absorption processes shown in Table 4, the recovery level is still below the desired minimum, even though the purity levels were exceeded in all cases but with heavy energy penalties in all the process routes.

Table 4. Attributes of various absorption carbon capture methods (Ayittey et al. 2021).

System performance	CO2 capture level, %	CO2 product purity, vol%	Reboiler temp, ^oC	Pump duty, MJ/kgCO2	Compressor duty, MJ/kgCO2	Preheating duty, MJ/kgCO2	Reboiler duty, MJ/kgCO2	Equivalent work, MJ/kgCO2
MEA base case	85.96	99.99	121.2	0.042	–	–	3.98	0.649
LVC	87.34	99.27	120.7	0.064	0.082	–	2.76	0.565
RSP	86.97	99.99	121.1	0.042	–	0.22	3.54	0.615
LVC + RSP	88.36	99.65	121	0.064	0.082	0.22	2.47	0.555

Attempts to solve these shortfalls of single-stage methods led to the development of multi-stage processes.

Giordano et al. (2017) examined the energy demands and capture costs of single and double-stage membrane systems at different CO₂ permeate purity using Polyactive 1500 polymer as a membrane. The membrane system also showed stable performance over 740 hr. continuously, and they also reported that membrane processes were well suitable for post-combustion CO₂ capture, and a CO₂ purity of 68.2 mol% in the permeate and a recovery of 42.7% can be achieved at the tested condition in a single-stage process. They recommended that a two-stage pilot membrane system should be demonstrated to document the technology feasibility related to the energy consumption and the required membrane area. They also opined that the engineering challenge of upscaling envelop modules needs to be addressed. In general, membrane systems are too expensive to compete head-to-head with amine plants. In principle, the combination of membranes for bulk removal of carbon dioxide with amine units as polishing systems offers a low-cost alternative to all amine plants for many streams. However, this approach has not been generally used because the savings in capital costs are largely offset by the increased complexity of the plant, which now contains two separation processes.

Jiayou Xu et al. (2019) conducted a parametric study of a two-stage membrane with a crossflow configuration to find proper feed pressure and membrane selectivity with a focus on decreasing energy usage, required membrane area, and the cost of CO₂ capture. They suggested that by considering a membrane with low CO₂ /N₂ selectivity in the first stage and with high CO₂ /N₂ selectivity in the second stage, the CO₂ capture cost was significantly reduced with targets of 70% CO₂ separation, 95 mol% CO₂ purity.

A technical challenge for cost-efficient CO₂ capturing by the membrane process is the low CO₂ partial pressure (typically 0.15 atm.) in power plant flue gas, which implies a low thermodynamic driving force for CO₂ separation. To reach the desired CO₂ capture rate and CO₂ purity, the permeability and selectivity of available membranes are not adequate for a single-stage membrane process (Xu et al. 2019), (Karaszova et al. 2019). To address this challenge, much research is devoted to both the development of new membranes and the improvement of membrane process designs. Zhang, He, and Gundersen (2013) presented a thermo-economic analysis of a two-stage membrane system for post-combustion CCS in a 600 MW coal-fired power plant Versteeg, Peter, and Edward 2011. They simulated the membrane process using PRO/II software, and they observed that selecting a membrane with high selectivity leads to a reduction in energy consumption and an increment in the required membrane area with the targets of 70% CO₂ separation and 95 mol% CO₂ purity. However, multiple stages will increase the energy penalties, investment, operational, maintenance, and material costs; material degradation, corrosion problems; and environmental concerns. Membrane systems are generally too expensive to compete head-to-head with amine plants. In principle, the combination of membranes for bulk removal of carbon dioxide with amine units as polishing systems offers a low-cost alternative to all amine plants for many streams.

Then enters hybrid configuration which offers a viable option. Some important performance indicators of two amine-functionalised sorbents and three different metal organic frameworks have been analysed for carbon capture (Leonzio, Fennell, and Shah 2022). Results showed a significant elevation in the concentration of CO₂ recovery to 95 mol% with less energy input. For multi-stage adsorption, the influence of the mass of adsorbent on the purity and specific energy has also been evaluated (Zhang et al. 2022). Results show that the specific energy declines from 11.92 MJ /mol CO2 to 8.73 MJ /molCO2, and the optimal exergy efficiency increases from 0.205% to 0.238% as the regeneration temperature grows from 368 K to 373 K from two-stage to three-stage process. It means that a thermally powered carbon separation device can not only meet the requirements of direct air capture but also get the optimal purity of carbon dioxide or specific energy by increasing the number of stages. But multi-stage adsorption where carbon dioxide can be purified to the desired purity and recovery levels of ≥95 mol% and ≥97 mol% respectively will incur heavy energy penalty as a result of multi-stage compression and refrigeration to maintain the temperature at 0°C (Romeo et al 2019). Thus, operating at a higher temperature with lesser adsorption (but without a vacuum operation) is better than using a hybrid system to upgrade CO₂ capture. Adsorption methods seem promising due to the relatively low operating costs, high recovery levels, and purity of the captured carbon dioxide. However, its low-capacity throughput implies it cannot be used as a standalone process in commercial power plant carbon capture.

Thus, the hybrid configuration is the preferred optimal process route for carbon capture to be viable for commercialisation if the desired purity and recovery of 95mol% and 97mol% respectively are to be achieved. Hence, we propose two-stage-hybrid configurations as the desired capture methods to follow for viable and commercialisable carbon capture. For instance, using two-stage adsorption set up in series will give high recovery purity (>95%) at 0°C, which requires refrigeration involving vacuum operation which is both capital and energy intensive. Thus it is better to operate at 20°C with lesser adsorption, and use a hybrid system to upgrade the CO₂ capture to the desired purity (Zahid et al. 2014). So, in the conceptualised framework, instead of using two adsorbers in series operating under vacuum pressure (with exorbitant energy penalties) as shown in Figure 4(a), the two adsorbers were configured to operate under moderate pressures (with low energy penalties) (Rao, Rubin, and Berkenpas 2004), which, of course, will give low percent recovery (68% purity) but then upgraded to the desired purity (95% recovery) by incorporating additional unit, in this case, membrane (Figure 4(b)).

Figure 4. (a) A two-stage adsorption configuration, (b) A two-stage adsorption-membrane hybrid configuration.

Thus, the two-stage-hybrid configuration is the way to go for carbon capture to be techno-economically viable. However, based on three technologies for carbon capture, namely Absorption (AB), Membrane separation (MS), and Adsorption (AD), all the possible process routes/configurations are shown in Figures 5(a–f)

Figure 5. (a) A two-stage absorption-absorption hybrid configuration, (b)A two-stage absorption-membrane hybrid configuration, (c) A two-stage absorption-absorption hybrid configuration, (d) A two-stage adsorption-membrane hybrid configuration, (e) A two-stage membrane-absorption hybrid configuration, (f) A two-stage membrane-adsorption hybrid configuration.

Each configuration has energy penalties, technical, and economic consequences that should be evaluated in determining the optimal design configuration that is most technically and economically viable for commercialisation. Thus, a detailed techno-economic analysis is required to determine which option will be the optimal process route to be adopted for carbon capture. This will form the topic for the next article.

4. Conclusion and recommendation

This review highlights process developments in CO₂ capture from power plants’ flue gases and the future perspectives. Different types of technologies for CO₂ capture were reviewed in terms of CO₂ recovery, energy efficiency, and cost. Regarding post-combustion CO₂ capture from flue gases, only three technologies have worldwide applications: Absorption, Membrane, and Adsorption (AMA) processes. Therefore, this review focused on configurations in AMA processes, such as single-stage, multi-stage, and hybrid configurations, to identify the possible optimal configurations for post-combustion CO₂ capture. It was found that the two-stage-hybrid configurations fulfilled the required needs in terms of energy efficiency, cost savings, and the desired CO₂ purity and recovery of 95mol% and 97mol% respectively thus recommended as the direction for future research. However, techno-economic analyses are still needed to identify the best configuration in the two-stage-hybrid mix that will make post-combustion carbon capture technology viable for commercialisation.

Disclosure statement

No potential conflict of interest was reported by the authors.