The Efficient Utilization of Carbon Dioxide in a Power-to-Liquid Process: An Overview

Abstract

As the global climate crisis escalates, reductions in CO₂ emissions and the efficient utilization of carbon waste resources have become a crucial consensus. Among the various carbon mitigation technologies, the concept of power-to-liquid (PTL) has gained significant attention in recent years. Considering the lack of a timely review of the state-of-the-art progress of this PTL process, this work aims to provide a systematic summary of the advanced PTL progress. In a CO₂ capture unit, we compared the process performances of chemical absorption, physical absorption, pressure swing adsorption, and membrane separation technologies. In a water electrolysis unit, the research progress of alkaline water electrolysis, proton exchange membrane water electrolysis, and solid oxide water electrolysis technologies was summarized, and the strategies for improving the electrolysis efficiency were proposed. In a CO₂ hydrogenation unit, we compared the differences of high-temperature and low-temperature Fischer–Tropsch synthesis processes, and summarized the advanced technologies for promoting the conversion of CO₂ into high value-added hydrocarbons and achieving the efficient utilization of C₁–C₄ hydrocarbons. In addition, we critically reviewed the technical and economic performances of the PTL process. By shedding light on the current state of research and identifying its crucial factors, this work is conducive to enhancing the understanding of the PTL process and providing reliable suggestions for its future industrial application. By offering valuable insights into the PTL process, this work also contributes to paving the way for the development of more efficient and sustainable solutions to address the pressing challenges of CO₂ emissions and climate change.

1. Introduction

In recent years, global CO₂ emissions have been on the rise due to the excessive combustion of coal, petroleum, and other fossil fuels. According to the report of the International Energy Agency, global energy-related CO₂ emissions increased to 36.8 billion tons in 2022, which resulted in a series of crises such as global warming, glacier melting, and ocean acidification [1]. Consequently, it is imperative to capture the emitted CO₂ in the atmosphere and employ clean renewable energy to replace conventional fossil-based energy. Power-to-liquids (PTL) processes, integrating CO₂ capture and water electrolysis technologies, have received widespread attention and are conducive to converting fluctuant renewable energy (e.g., wind and solar energy) into sustainable liquid fuels [2].

Typically, PTL processes consist of three sub-units: CO₂ capture, water electrolysis, and CO₂ hydrogenation. CO₂ capture technologies mainly obtain their CO₂ from carbon-intensive industries such as fossil fuel power plants and cement plants, or directly from ambient air [3]. Generally, CO₂ capture technologies include pre-combustion, oxygen-rich combustion, and post-combustion. Pre-combustion capture technology captures CO₂ derived from the flue gas in power plants or other industrial plants that rely on fossil fuels, which is primarily applicable to high-concentration CO₂ (around 30–40%). However, its complex process design and high capital investment are its main limitations. Oxygen-enriched combustion technology reduces the accumulation of particulate matter in flue gas. However, expensive O₂ supply systems and its potential safety risks are its main challenges [4]. Post-combustion carbon capture technology refers to the selective enrichment of CO₂ from flue gas using chemical or physical methods. Post-combustion capture technology has been widely applied in power plants, steel plants, and chemical plants due to its flexible operation and high technology maturity [5]. Hence, this study mainly focuses on post-combustion capture technology.

H₂ is an important raw material that can be produced from fossil energy and renewable energy [6]. More specifically, most H₂ is generated via coal gasification and natural-gas-reforming technologies in the industry, which have a low cost, controllable production scale, and high technical maturity. However, energy-intensive and carbon-intensive technologies can result in massive CO₂ emissions [7]. Therefore, in recent years, some researchers have been dedicated to developing sustainable and clean H₂ generation technologies. Among the various H₂ generation technologies, water electrolysis is framed as a promising technology for producing green H₂ [8]. This is mainly because water electrolysis technologies eliminate the dependence on fossil energy and show a strong competitiveness compared with traditional H₂ generation technologies in terms of CO₂ mitigation [9]. Therefore, this work concentrates on water electrolysis technologies for H₂ generation. The principle of these technologies is to apply a stable direct current to electrolyze water and obtain H₂ on the cathode side and O₂ on the anode side. Generally, water electrolysis technologies mainly consist of alkaline water electrolysis, proton exchange membrane water electrolysis, anion exchange membrane water electrolysis, and solid oxide water electrolysis [10]. We will summarize the recent progress of these four water electrolysis technologies in the following content.

CO₂ hydrogenation to high value-added liquid fuels and chemicals has been considered to be an important alternative to fossil energy [11]. Among the various liquid fuels and chemicals, syncrude has received wide attention and can be further upgraded to diesel, gasoline, and jet fuel, which can contribute to further optimizing existing energy structures. Generally, the hydrogenation of CO₂ to liquid fuels is mainly composed of methanol-mediated and modified Fischer–Tropsch synthesis (FT)-based pathways [12]. For the methanol-mediated pathway, CO₂ and H₂ are converted into methanol, which is further converted into syncrude via a series of catalytic reactions, such as dehydration, oligomerization, cyclization, and hydrogen transfer reactions [13]. For the modified FT-based pathway, CO₂ and H₂ are firstly transferred to the intermediate CO via a reverse water–gas-shift (RWGS) reaction, which is then converted into hydrocarbons including methane, C₂–C₄ paraffins, C₂–C₄ olefins, and long-chain hydrocarbons (i.e., syncrude) [14]. Compared to the methanol-mediated pathway, the modified FT-based pathway usually has a relatively simple process configuration and adjustable product selectivity. Therefore, we mainly concentrate on the research progress of the modified FT-based pathway.

Up until now, considerable efforts have been devoted towards the development of PTL processes to efficiently convert waste CO₂ into sustainable liquid fuels and store intermittent renewable energies in clean chemical energies. However, considering the lack of a comprehensive review of the promising PTL processes, conducting an appropriate summary of the advanced PTL process is necessary. With this in mind, we have critically and comprehensively summarized the recent research progresses of the emerging PTL processes. More specifically, for the CO₂ capture unit, we summarized the research processes of chemical absorption, physical absorption, pressure swing adsorption, and membrane separation technologies. For the water electrolysis unit, alkaline water electrolysis, proton exchange membrane water electrolysis, solid oxide water electrolysis technologies, and anion exchange membrane water electrolysis are introduced in detail. For the CO₂ hydrogenation unit, we compared the differences of high-temperature and low-temperature Fischer–Tropsch synthesis processes, and summarized the advanced technologies to promote the conversion of CO₂ into high value-added hydrocarbons and achieve the efficient utilization of light hydrocarbons. Finally, we analyzed the technical and economic performances of the PTL processes and provided some specific strategies to further enhance the process feasibility of the PTL processes. Overall, this work provides a systematic and comprehensive summary of the emerging PTL process.

2. Power-to-Liquid Technology

As mentioned above, PTL processes mainly consist of CO₂ capture, water electrolysis, and CO₂ hydrogenation units. Therefore, in the following content, we will analyze and summarize their research progress and current challenges.

2.1. CO₂ Capture Technologies

Generally, post-combustion capture technologies mainly include chemical absorption, physical absorption, pressure swing adsorption, and membrane separation. In the following content, we will provide a detailed review of these four technologies considering their principles, process designs, and applications.

2.1.1. Chemical Absorption Method

The chemical absorption method is based on reversible chemical reactions and uses alkaline solvents as absorbents to capture CO₂. The principle is that the absorbent reacts with CO₂ in the absorption tower to capture the CO₂, and then undergoes a reverse reaction to desorb the CO₂ using the absorbent under high-temperature and low-pressure conditions [15]. Some common absorbents used in these chemical absorption methods include an alcohol amine solution, hot potassium alkali solution, and ammonia solution. Currently, it has been widely recognized that the chemical absorption method using alcohol amine is the most mature, so it has received widespread attention from researchers.

For the chemical absorption method using alcohol amine, the main solvents include monoethanolamine (MEA), diethanolamine (DEA), and n-methyldiethanolamine (MDEA), etc. [16,17]. Among them, organic amine absorbents represented by MEA aqueous solutions have been widely used as standard industrial absorbents for CO₂ capture and separation processes due to their low cost, relatively fast kinetics, high mass transfer rate, and easy recovery [18]. In the CO₂ absorption process, the specific chemical reactions between MEA and CO₂ are shown in Equations (1)–(7). At present, some researchers are committed to further improving the CO₂ capture rate, so that the CO₂ in the flue gas can be fully recovered. The operating pressure, packing materials, and absorbent are the main factors affecting this CO₂ capture rate [19]. As the operating pressure increases, the CO₂ capture rate significantly increases, but the operating cost also increases. Moreover, for the chemical absorption processes, the common packing materials mainly include porous plate packing and disk packing, such as saddles, sulzer, pall rings, and rasching rings. Fillers with a suitable porosity and surface area should be selected, which can also increase the CO₂ capture rate. In addition, ideal CO₂ absorbents should combine well with CO₂ to generate a sufficiently high capacity. Generally, it would be better for the absorption capacity to be more than 2 mol CO₂/kg absorbent [20].

$${\mathrm{H}}_2\mathrm{O} \rightleftharpoons { \mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{O}{\mathrm{H}}^{-}$$

(1)

$${\mathrm{CO}}_2+\mathrm{O}{\mathrm{H}}^{-}⟶{\mathrm{HCO}}_3^{-}$$

(2)

$${\mathrm{HCO}}_3^{-}⟶{\mathrm{CO}}_2+\mathrm{O}{\mathrm{H}}^{-}$$

(3)

$${\mathrm{HCO}}_3^{-}+{\mathrm{H}}_2\mathrm{O} \rightleftharpoons {\mathrm{CO}}_3^{2-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$$

(4)

$${\mathrm{MEAH}}^{+}+{\mathrm{H}}_2\mathrm{O} \rightleftharpoons \mathrm{MEA}+{\mathrm{H}}_3{\mathrm{O}}^{+}$$

(5)

$$\mathrm{MEA}+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O} ⟶{ \mathrm{MEACOO}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$$

(6)

$${\mathrm{MEACOO}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}⟶ \mathrm{MEA}+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$$

(7)

Although the chemical absorption method using an MEA solution is currently a relatively mature CO₂ capture technology, its high energy consumption and low CO₂ loading are still its main obstacles. Conducting process optimization and adopting more advanced absorbents have been considered to be important pathways. For example, for process optimization, Li et al. set up intercoolers and heaters for the absorber and stripper, and optimized the operating parameters simultaneously, resulting in a reduction of 13.5% in the total energy consumption [21]. Although improving the process can reduce this energy consumption, this comes with relatively high investment costs. Moreover, the selection of effective absorbents has gained widespread attention. The traditional CO₂ loading of the MEA solution is about 0.45 mol CO₂/mol absorbent [22]. However, the addition of efficient absorbents (e.g., MDEA, DPTA, and TEPA) can significantly improve this CO₂ loading. More specifically, the CO₂ loading of a mixed MEA–TEPA solution is up to 1.32 mol CO₂/mol absorbent. In addition, the application of efficient absorbents can remarkably reduce the heat load. For example, the heat load of the mixed MEA–MDEA solution is reduced by more than 50% compared to the traditional MEA solution, saving the energy consumption during the solvent regeneration process [23]. In addition to the aforementioned absorbents, Ma’mun et al. evaluated the process performances of CO₂ capture technology using 2-((2-aminoethyl) amino) ethanol (AEEA). Compared to MEA, AEEA has a lower regeneration energy consumption, higher CO₂ absorption rate, and absorption capacity [24,25]. Recently, cyclic diamines, especially piperazine (PZ), have been considered as improvements to these absorbents due to their faster reaction rate, stronger absorption capacity, and higher heat resistance and oxidation resistance [26,27]. However, considering the higher volatility of piperazine compared to MEA, its application in CO₂ absorption is more expensive, and it is still in the research and development stage [28].

In addition to its high energy consumption, the MEA solution also has other drawbacks, such as a low absorption rate, strong corrosiveness, and poor thermal stability. Therefore, researchers have developed some emerging absorbents to address these problems. Idem et al. conducted CO₂ absorption experiments on an MEA + MDEA aqueous solution and single-component MEA aqueous solution, respectively. The results showed that the addition of MDEA significantly reduced the regeneration energy consumption [29]. Ramazani et al. investigated the effect of six additives on the CO₂ absorption performances of an MEA solution. At an operating temperature of 313.15 K, the molar ratios of the additives to MEA were set to 0.25, 0.66, and 1, respectively. The results showed that the addition of TEPA to MEA increased the pH value of the solution, increased the absorption rate, and enabled the maximum loading capacity and absorption efficiency [30]. Mondal et al. measured the vapor–liquid equilibrium data of CO₂ absorption using a mixture of polyamine bis (3-aminopropyl) amine (DPTA) and MEA in an aqueous solution, and found that its CO₂ loading capacity was 0.7 mol CO₂/mol antioxidants [31]. Apaiyaku et al. explored the potential of high-concentration AMP-PZ-MEA solvents to capture CO₂ from solvent precipitation, density, viscosity, and CO₂ absorption capacity, and found that, compared to an MEA solvent, they had an increase of 25% in their CO₂ loading [32].

2.1.2. Physical Absorption Method

The principle of the physical absorption method is that the solubility of the CO₂ in the absorbent varies with changes in operating pressure and temperature. Therefore, by changing the operating pressure and temperature between the CO₂ and absorbent, CO₂ is more easily absorbed by the absorbent. Compared to the chemical absorption method, the physical absorption method generally has a lower energy consumption. According to Henry’s law, the operating pressure, operating temperature, and absorbents could significantly affect the absorption performances. More specifically, the solubility of the CO₂ in the absorption increases with an increase in the operating pressure and a decrease in the operating temperature [33,34,35,36]. In addition to the effects of the operating conditions, the selection of high-performance absorbents is also important. According to the differences in these absorbents, the common physical absorption methods in the industry include Rectisol, Selexol, and Fluor, etc.

Table 1. Comparison of various physical solvents used in acid gas separation.

Physical Absorption Method	Major Solvent	Operating Requirements, Advantages, and Limitations:
Rectisol	Methanol	(1) Deep refrigeration. (2) Water washing of effluent streams to prevent high solvent losses.
		Limitations: Higher selectivity for H2S over CO2.
Selexol	Polyethylene glycol dimethyl ether	(1) Highest CO2 solubility among physical solvents. (2) Operate at a broad temperature range: from 0 to 175 °C. (3) Low cost than Rectisol.
		Limitations: Higher viscosity than most physical solvents.
Fluor	Propylene Carbonate	(1) Higher vapor pressure than Selexol. (2) Low solvent losses. (3) Operating temperature is below 65 °C.
		Limitations: Poor sulfur resistance.

summarizes their industrial applications and respective advantages [37].

Rectisol mainly removes the CO₂ and H₂S from flue gas by using methanol as the adsorbent at low temperatures. At present, Rectisol has been widely used in a series of purification devices to remove the CO₂ from the flue gas derived from coal-fired power plants. Selexol mainly uses polyethylene glycol dimethyl ether as an absorbent to remove the CO₂ from tail gas. Polyethylene glycol dimethyl ether has the advantages of a high boiling point, non-corrosiveness, and non-degradability, and has been widely used to remove the CO₂ from natural gas and fertilizer production [38,39]. Fluor uses propylene carbonate as the solvent to absorb the CO₂ from flue gas. However, it should be noted that this method might not be suitable for treating flue gases containing sulfides [40,41].

For physical absorption, it is important to select solvents according to their operating conditions and feed composition. During the absorption process, it is necessary to minimize the loss of valuable components and remove unnecessary acidic gas components [33]. At present, the main focus of the physical adsorption method is on solvent development, and ideal physical solvents have the following characteristics:

(a)

A high solubility and selectivity for CO₂ and H₂S;

(b)

A low saturated vapor pressure;

(c)

A high chemical stability;

(d)

A low cost;

(e)

A low toxicity and minimal environmental impact.

2.1.3. Membrane Separation Method

Membrane separation is an emerging method for physically separating the CO₂ from flue gases through a semi-permeable membrane [42], which achieves gas separation according to the difference in the permeation rates of the various gas components on the surfaces of polymer membranes. Compared to traditional chemical absorption technology, membrane separation has a lower energy consumption, lower cost, smaller footprint, and simple process configurations [43,44].

The membranes include inorganic membranes and organic membranes. For the CO₂ capture process, inorganic membranes can screen out the CO₂ from mixed gas according to molecular size, whereas organic membranes selectively permeate this CO₂ due to the specific interaction of the CO₂ and the functional groups attached to the membrane surface. Inorganic membranes primarily include zeolite, metal organic framework, carbon molecular sieve, ceramics, and some metal oxides (alumina, titania, and zirconia) [45]. Inorganic membranes can withstand high temperatures of 400 °C or even 500 °C. In addition, they also have a high mechanical stability, but their cost is high. Organic membranes are polymer composite membranes, such as polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF). Among them, the hollow fiber membrane in the PP membrane has been commercialized due to its cheap cost [46,47]. However, the PP membrane may degrade at temperatures from 100 to 150 °C, which is the biggest obstacle to its industrialization. Among the above four membranes, PTFE can withstand high temperatures of 150 °C, but its cost is relatively high, which is not suitable for industrialization [48].

In Figure 1, the CO₂ in feed gas reaches the pores of the membrane, diffuses to the penetration side, and is finally enriched on the penetration side [49]. For membrane separation technology, the capacity of the CO₂ capture is determined by the selectivity and permeability of the membrane. Koros et al. found that creating thin selective layers on permeable porous carriers can improve the selectivity and permeability of the membrane, further enhancing this CO₂ capture capacity [50,51]. In addition to the selectivity and permeability of the membrane, the CO₂ concentration in feed gas, the pressure difference between the feed side and permeate side, and the membrane separation method all affect the separation effect of the membrane.

The difference in the CO₂ concentration on both sides of the membrane is the main driving force for membrane separation. Increasing the CO₂ concentration in the feedstock makes this separation easier. Studies have shown that, when the CO₂ capture rate is 90%, the energy consumption for the solvent absorption and membrane separation method is comparative when the CO₂ concentration in the feed gas is 10 mol%. However, with a continuous increase in the CO₂ concentration, the energy consumption for the membrane separation is much lower than that in the solvent absorption method. Therefore, the solvent absorption method is generally used for low-concentration CO₂, while membrane separation is used for medium- and high-concentration CO₂ [53]. In addition, the membrane separation method is also an important factor for the separation effect of the membrane. A single membrane system is generally used in the early stages of membrane separation, but when the CO₂ capture rate reaches 90%, the CO₂ purity is only 50%. If the CO₂ purity is greater than 90%, the selectivity of the membrane needs to be at least greater than 200, which can result in high capital and operating costs [54]. Furthermore, in Table 2, it can be seen that, when the CO₂ capture rate is 90%, if the CO₂ purity increases from 95% to 98%, the capture cost significantly increases. Therefore, the CO₂ purity required by the subsequent process has a significant influence on the cost of the membrane separation.

2.1.4. Pressure Swing Adsorption

Among the carbon capture methods, the absorption method is currently the most mature. However, the absorption method inevitably has problems such as a high energy consumption and the corrosion of its process equipment. Therefore, the adsorption method, represented by pressure swing adsorption (PSA), is developed [55]. PSA technology generally uses adsorbents with microporous or mesoporous structures as a fixed bed [56]. Table 2 shows some common adsorbents and some specific parameters, advantages, and disadvantages of PSA. Herein, the common adsorbents mainly include zeolite 13X and zeolite NaY, in which zeolite 13X has a relatively strong CO₂ adsorption capacity.

The ultimate goal of PSA technology now is to capture CO₂ with minimal energy consumption. Based on a traditional PSA design, Ammar et al. used a compact plate to transfer the heat generated during the CO₂ capture process to the desorption process, ensuring a more effective heat dissipation from the adsorption bed and reducing the energy required for the regeneration steps [57]. However, in the PSA process, the partial pressure of the CO₂ desorption is close to the ambient pressure and the absorption pressure can reach at least 7 bar, which means that the feedstock needs to be compressed, causing additional energy consumption [58]. Subsequently, researchers have developed vacuum swing adsorption (VSA) to solve this problem. VSA was industrialized in 1986 and is considered to be the most likely alternative to PSA, due to its low energy consumption and the long life of its adsorbent. In the VSA process, the partial pressure of the CO₂ in the feed gas is generally lower than 1.5 atm [59] and the desorption pressure is lower than 10 kPa [60]. Regardless of VSA and PSA, even if the 13X adsorbent with the best adsorption performance is chosen, the single-stage process has a recovery rate of 90% and the CO₂ purity is only 76% [61]. Therefore, it would be better to choose a two-stage process. The first stage is mainly to obtain a higher CO₂ capture rate and the second stage is to increase the CO₂ purity, so that the final CO₂ capture rate can reach 90% and the CO₂ purity is 95% [62]. Yu et al. used a two-stage VSA method to capture the CO₂ from flue gas. This process combined dynamic and equilibrium control separation processes with reflux. The first stage was dynamic control separation, which removed most of the N₂ from the dry flue gas, increasing the CO₂ concentration by 54.1%. This stage greatly reduced the subsequent treatment volume. The second stage adopted balance control separation and the final CO₂ purity was 95.1%, with a recovery rate of 92.9% [63].

At present, there are two critical problems in PSA or VSA. First of all, although the process configuration of PSA is simple, the area of its fixed bed reactor is much larger than that in solvent absorption and membrane separation [62]. Researchers are currently studying the feasibility of moving bed reactors in PSA [64]. In addition, the steam in flue gas has a negative effect on the CO₂ adsorption. On the one hand, many adsorbents are hydrophilic, such as 13X zeolite. On the other hand, flue gas generally contains 8% to 10% steam, and even after pretreatment, its steam concentration can reach up to 5% [65]. Steam and CO₂ can form competitive adsorption on the adsorbent, reducing the adsorption capacity of the adsorbent for the CO₂ [66]. There are two methods to solve this problem. The first option is to use hydrophobic adsorbents, such as MOF and amine functional adsorbents [62]. The second option is to use two fixed beds. The adsorbent of the first fixed bed uses Al₂O₃ or silica gel to absorb the steam and the adsorbent of the second fixed bed uses zeolite 13X to absorb the CO₂ [67].

Table 2. Comparison of four CO₂ capture methods [45,53,60,61,68,69,70,71,72].

Capture Method	Chemical Absorption Method	Physical Absorption Method	Membrane Separation Method	Pressure Swing Adsorption Method
Absorbent/Adsorbent	MEA/MDEA	Methanol	Inorganic/organic membrane	Zeolite/molecular sieve/activated carbon/alumina/MOF
Inlet CO2 concentration	<20%	>20%	>28%	20–60%
Outlet CO2 concentration	≥99	≥95	≥99.9	Single-stage: 76
				Multiple-stage: >95
Capture rate	90%	90%	90%	90%
Technology maturity	High	High	Pilot stage	High
Processing capacity	High	High	Low	Medium
Operational difficulty	Low	Low	High	Medium
Cost ($/t CO2)	50–65	40–60	23–47	32–34
Advantages	Mature technology.	Mature technology.	Low energy consumption.	Low energy consumption.
	2. High CO2 purity.	2. High CO2 purity.	2. High mass transfer efficiency.	2. Low operating costs.
	3. Large processing capacity.	3. Large processing capacity.	3. Low operating costs.	3. Simple process.
Disadvantages	High regeneration energy consumption.	High regeneration energy consumption.	Relatively immature technology.	H2O has a negative effect on the adsorbent.
	2. Low absorption rate.	2. Amine solutions degrade easily.	2. Relatively high fixed cost of inorganic membrane.	2. The PSA/VSA unit covers a relatively large area.
		3. Amine corrodes the equipment.	3. Organic membrane is not resistant to high temperatures and will swell and age.

Table 2. Comparison of four CO₂ capture methods [45,53,60,61,68,69,70,71,72].

Capture Method	Chemical Absorption Method	Physical Absorption Method	Membrane Separation Method	Pressure Swing Adsorption Method
Absorbent/Adsorbent	MEA/MDEA	Methanol	Inorganic/organic membrane	Zeolite/molecular sieve/activated carbon/alumina/MOF
Inlet CO2 concentration	<20%	>20%	>28%	20–60%
Outlet CO2 concentration	≥99	≥95	≥99.9	Single-stage: 76
				Multiple-stage: >95
Capture rate	90%	90%	90%	90%
Technology maturity	High	High	Pilot stage	High
Processing capacity	High	High	Low	Medium
Operational difficulty	Low	Low	High	Medium
Cost ($/t CO2)	50–65	40–60	23–47	32–34
Advantages	Mature technology.	Mature technology.	Low energy consumption.	Low energy consumption.
	2. High CO2 purity.	2. High CO2 purity.	2. High mass transfer efficiency.	2. Low operating costs.
	3. Large processing capacity.	3. Large processing capacity.	3. Low operating costs.	3. Simple process.
Disadvantages	High regeneration energy consumption.	High regeneration energy consumption.	Relatively immature technology.	H2O has a negative effect on the adsorbent.
	2. Low absorption rate.	2. Amine solutions degrade easily.	2. Relatively high fixed cost of inorganic membrane.	2. The PSA/VSA unit covers a relatively large area.
		3. Amine corrodes the equipment.	3. Organic membrane is not resistant to high temperatures and will swell and age.

2.1.5. Comparison of Four CO₂ Capture Methods

The solvent absorption methods (chemical absorption and physical absorption methods) are the most mature, and their CO₂ purity is also the highest. However, their high solvent regeneration energy consumption cannot be ignored. Adopting an advanced solvent has been proven to be an effective method. Membrane separation and PSA methods are superior to solvent absorption methods in terms of their operating costs and energy consumption, but they are still a long way from large-scale applications. For membrane separation methods, membrane degradation and high-temperature resistance are the bottlenecks to its development. With the investment of large amounts of funds into membrane research, novel membranes may solve these issues. PSA has rigorous requirements for its feed materials. As mentioned before, the steam in the feed can form competitive adsorption with the CO₂ on the adsorbent, significantly reducing the adsorption capacity of the adsorbent. In addition, maintaining the vacuum conditions of the desorption process can consume more energy, but the energy consumed is far lower than that in the solvent absorption method.

2.2. Water Electrolysis Technologies

For PTL processes, H₂ generation is a critical step. Among the promising approaches to H₂ generation, water electrolysis technology, which utilizes renewable electricity to split water molecules into H₂ and O₂, presents enormous advantages [9]. Up until now, water electrolysis technologies have included alkaline water electrolysis (AWE), proton exchange membrane water electrolysis (PEM), solid oxide water electrolysis (SOEC), and anion exchange membrane electrolysis (AEM), according to the types of electrolyzer.

2.2.1. Alkaline Water Electrolysis

AWE technology uses an alkaline electrolyte solution (e.g., sodium hydroxide and potassium hydroxide) for electrolyzing water to H₂ and O₂, and the relevant reactions are shown in Table 3. The commonly used electrolytes include potassium hydroxide (KOH) aqueous solution, sodium hydroxide (NaOH) aqueous solution, and ionic liquids. The KOH aqueous solution is a strong alkali, with a concentration typically between 25 and 30 wt% [73,74]. Under the same concentration and volume, the KOH aqueous solution can generate more H⁺, promoting H₂ and O₂ formation. Compared to the KOH solution, the NaOH solution may have significant cost advantages, but its conductivity is lower. Ionic liquids, such as electrolytes, have the advantages of a low vapor pressure, non-volatility, and a high chemical and thermal stability, but their higher viscosity and cost limit their commercialization [75]. At present, AWE technology has achieved large-scale industrial application because of its relatively mature technology, simple process, and low equipment cost. However, its electrolysis efficiency is relatively low, with values of 51–60% (based on lower heating value). Hence, improving the electrolysis efficiency is the key to the current development of AWE. The factors that affect the electrolysis efficiency mainly include the temperature and current density.

From the perspectives of kinetics and thermodynamics, as the temperature increases, the electrolyte ionizes more ions, and the movement rate of these ions accelerates. Therefore, increasing the temperature is also beneficial for improving the electrolysis efficiency. However, an increase in the temperature can lead to more energy consumption [76]. In addition to increasing the temperature, some optimizations can improve the electrolysis efficiency. For example, Lei et al. proposed acidic/alkaline amphoteric electrolysis technology. With this technology, the cathode and anode electrolytes were H₂SO₄ and KOH, respectively. It was found that the productivity of H₂ increased by more than four times and the energy consumption decreased by 30 to 35% [77]. Zhao et al. designed an IFP electrolytic cell and tested the energy consumption and H₂ purity under two operating conditions: a fused fluid flow path (FFP) and independent fluid flow path (IFP). The results showed that, after purification, the purity of the H₂ reached 99.99%. In addition, due to the improvement in the separation problem, the H₂ generation efficiency of the IFP electrolytic cell increased by 17.46% under the same conditions. Under the same generation of H₂, the energy consumption was reduced by more than 3.29% [78].

In addition to the aforementioned issues, the risks of combustion and explosion due to the mixture of H₂ and O₂ are crucial for an electrolysis system. Generally, the concentration of the H₂ in the O₂ needs to be lower than 2% [79]. In order to solve this issue, Haug et al. studied the effects of the electrolyte concentration, electrolyte temperature, and electrolyte temperature. It was found that the average flow rate of the electrolyte decreased from 0.00768 to 0.00325 L·s⁻¹, resulting in a decrease in the anode H₂ content from 1.636 to 0.701 vol%. When the electrolyte concentration increased from 28.9 to 34.2, the H₂ content at the anode decreased from 1.350 to 0.911 vol%. When the electrolyte temperature increased from 50 to 80 °C, the anode H₂ content decreased from 1.348 to 1.090 vol%. Therefore, a high temperature, high alkali concentration, and low alkali flow rate can reduce the H₂ content in the anode and improve the O₂ purity [76].

2.2.2. Proton Exchange Membrane Electrolysis

PEM technology uses an acid electrolyte solution for splitting water into H₂ and O₂ using the proton exchange membrane. Different from the diaphragm used in AWE technology, the proton exchange membrane only allows for H⁺ to pass through, separating H⁺ and O²⁻. At the same time, it can also transfer H⁺ protons and serve as a carrier for anode and cathode catalysts [80]. As an essential module, the PEM cell mainly includes two bipolar plates, a proton exchange membrane, and two gas diffusion layers (GDL). Bipolar plates are usually nickel-plated and are primarily used as a flow channel to ensure a uniform and stable water flow. They are connected in series to the circuit to establish a complete current path between the cathode and anode. The GDLs generally use porous materials to transport the H₂O, H₂, and O₂.

As the core component of the water electrolysis cell electrode, the proton exchange membrane directly determines the performance and service life of the water electrolysis cell, so it is crucial within the entire equipment. At present, the industrialized application of the proton exchange membrane is the perfluorinated sulfonic acid (PFSA)-type proton exchange membrane (Nafion membrane), developed by DuPont in the 1960s. It has the advantages of a high proton conductivity, strong chemical stability, and high mechanical strength [81]. PEM technology has the advantages of a compact structure, high current density, fast response speed, and small footprint, which is moving it from development to industrialization, with its commercialization becoming increasingly mature. However, there are still problems, such as a low electrolysis efficiency and membrane degradation. Typically, operating temperature, film thickness, and current density can significantly affect the electrolysis efficiency. More specifically, the effect of the operating temperature on PEM is similar to that in AWE technology. Increasing the temperature can improve the electrolysis efficiency. For example, López et al. found that, at higher temperatures and lower pressures, the efficiency of the electrolytic cell can reach its maximum and better adapt to power fluctuations [82]. The thickness of the membrane has a significant impact on the electrolysis efficiency. A decrease in the membrane thickness leads to an increase in the electrolysis efficiency. For example, Toghyani et al. found that the current density could increase by more than 48% and the electrolysis efficiency could increase significantly if the thickness was reduced from 200 to 50 mm under a working voltage of 1.6 V. However, the reverse diffusion of the H₂ to the anode also increased, which is unfavorable. The effect of the current density on the electrolysis efficiency is the most significant among all of the influencing factors. The contribution rate of the anode current density to the electrolysis efficiency can be as high as 67.5%. By increasing the anode current density, the electrolysis efficiency of PEM can be significantly improved [83]. However, with an increase in the current density, the permeability of the H₂ to the anode increases. If the volume fraction of the H₂ in the O₂ exceeds 2%, some security issues, such as explosions, may occur.

Membrane degradation is an unavoidable problem in PEM, which mainly occurs on the cathode side. The mechanism of membrane degradation is quite complicated. Firstly, O₂ is attached to the anode. Under the action of the catalyst, H₂O₂ is easily generated through the two-electron reaction. The hydroxyl (HO⁻) and hydroperoxy (HOO⁻) produced by the H₂O₂ decomposition can attack the chemical structure of the membrane with different steps, releasing hydrofluoric acid (HF). Therefore, the fluorine emission rate (FER) is a critical elevation indicator of the membrane stability [84,85]. Due to the long experimental period and high membrane degradation cost, the detailed mechanism of membrane degradation is still under continuous research [86].

2.2.3. Solid Oxide Water Electrolysis

SOEC technology utilizes solid oxide as an electrolyte to decompose water into H₂ and O₂ at high temperatures. Unlike AWE and PEM technologies, SOEC technology can generally be carried out at temperatures above 800 °C. At high temperatures, water molecules are decomposed into H⁺ and O²⁻, which are then separated through a solid oxide electrolyte membrane, transferring the H⁺ and O²⁻ to the anode and cathode, respectively, ultimately obtaining pure H₂ and O₂.

It is essential to choose the suitable electrolytes for SOEC technology. Researchers have developed various ionic electrolytes, such as yttria-stabilized zirconia (YSZ), zirconia-based, and LaGaO₃-based ionic electrolytes with a perovskite crystal structure. It is worth noting that, when the electrolysis temperature exceeds 700 °C, YSZ has a better stability and lower cost than other electrolytes [87]. The cathode material is generally Ni/YSZ, which has a higher catalytic activity and lower cost than other materials [88]. The anode generally uses LSM (La_0.6Sr_0.4MnO₃)-YSZ or LSF (La_0.8Sr_0.2FeO₃)-YSZ, but LSF-YSZ has a better catalytic performance [89]. In addition, Table 3 summarizes the performance parameters involved in SOEC technology.

Compared to other water electrolysis technologies, SOEC technology has the advantages of a high electrolysis efficiency. This is mainly because its high operating temperature further promotes an electrolytic reaction in the thermodynamics and kinetics in comparison to low-temperature electrolysis. In addition, it also has the advantages of flexible operating modes and a wide fuel adaptability. In the future, SOEC is expected to become an important technology for connecting the development of multi-energy systems. However, expensive electrolytic cells and its high energy consumption are still the main obstacles to its industrialization.

Syngas, a raw material for producing basic chemicals, has played a crucial role in the industry. Currently, syngas is generally obtained through SMR and RWGS, as shown Equation (8). These processes have a high energy consumption and complicated subsequent purification steps. Recently, H₂O and CO₂ co-electrolysis technology based on SOEC has gained wide attention and been recognized as an important alternative pathway, as shown in Equation (9). Compared to traditional CO₂ dry electrolysis, co-electrolysis technology dramatically improves the conversion of CO₂ into CO due to the promoting effect of H₂ and overcomes the soot deposition suffered in CO₂ dry electrolysis [90]. The schematic diagram is presented in Figure 2. There is no doubt that H₂O obtains electrons at the cathode, obtaining H₂ and O²⁻. The principle of CO₂ electrolysis can be divided into two types, according to the electrode material. If the cathode is Ni/YSZ, CO may be obtained through RWGS [91]. On the contrary, Yue and Irvine et al. found that CO₂ may obtain electrons, generating CO via investigating the co-electrolysis based on an LSCM/GDC cathode [92]. Compared to high-temperature steam electrolysis and CO₂ dry electrolysis, H₂O and CO₂ co-electrolysis exhibits a more complex reaction mechanism in the whole system, but has significant merits. For instance, co-electrolysis can generate rich syngas directly, simplify the process configuration, and remarkably reduce the electricity energy requirements [93].

In addition, the electrolysis temperature in SOEC technology is at least 800 °C, which results in more energy consumption and higher operating costs compared to other water electrolysis technologies. However, if the electrolysis temperature is lowered to between 300 and 400 °C, the ability of YSZ to transfer O²⁻ is significantly reduced. How to balance the electrolysis temperature and ion conductivity of YSZ is the key to reducing the SOEC electrolysis temperature in the future [93].

RWGS reaction:

$${\mathrm{CO}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)\rightleftharpoons \mathrm{CO}\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right) {\mathsf{\Delta }}_{\mathrm{r}}{\mathrm{H}}_{\mathrm{m}}^{\mathsf{\theta }}=42.1 \mathrm{kJ}/\mathrm{mol}$$

(8)

The co-electrolysis of CO₂ and H₂O:

$$\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)\stackrel{\mathrm{electricity}}{\to } \mathrm{CO}\left(\mathrm{g}\right)+2{\mathrm{H}}_2\left(\mathrm{g}\right)+\frac{3}{2}{\mathrm{O}}_2\left(\mathrm{g}\right) {\mathsf{\Delta }}_{\mathrm{r}}{\mathrm{H}}_{\mathrm{m}}^{\mathsf{\theta }}=525.1{ \mathrm{kJ}·\mathrm{mol}}^{-1}$$

(9)

2.2.4. Anion Exchange Membrane Electrolysis

AEM is a key component of H₂ generation equipment and a typical organic cationic polymer, composed of cations on the polymer skeleton and its covalent attachment skeleton. Since Leng et al. first proposed the AEM concept in 2012 [95], many researchers have been devoted to the development of AEM. The principle of AEM is the same as that of AWE. AEM technology has shown a strong competitiveness due to the application of non-noble catalysts compared to PEM and AWE technologies, and become an alternative technology to replace PEM and AWE technologies in the future. However, some key technical challenges need to be further explored and solved, such as strong alkaline-resistant membrane materials.

Since AEM electrolysis is still in the early stage of research, there are few reports on the electrolysis performance of AEM. Table 3 summarizes some of the performance parameters of AEM. The current study mainly focuses on the development of catalyst materials and anion exchange membranes. Choosing the appropriate non-precious metal catalyst can directly affect the investment cost of AEM electrolysis. According to the literature, catalysts such as graphene, Ni-Fe alloys, and Cu_0.7CO_2.3O₄ have been used as anode catalysts, and catalysts such as CuCoO_x and Ni-Mo have been used as cathode catalysts [96]. With the further development of these catalysts, the problem may be solved. In addition, the anion exchange membrane has a lower chemical stability under alkaline conditions, which seriously hampers the industrial application of AEM. The main reason for this is that hydroxide easily breaks the main chain of the polymer in an alkaline environment, reducing the mechanical strength and conductivity of the membrane. At present, commercial membranes, such as PEEK, PESU, and PPO, have this problem. Therefore, many researchers are devoted to studying the mechanism of membrane degradation in order to solve this problem fundamentally [97,98].

2.2.5. Comparison of Four Electrolysis Technologies

SMR technology is the most widely used in industry, but its by-products may generate a large amount of CO₂, aggravating global warming [99]. It is necessary to develop alternative technologies to produce green H₂. Water electrolysis technology with close to zero CO₂ emissions may be the best solution to this problem. In the above four water electrolysis technologies, AWE has been identified as the most likely technology for large-scale industrial realization [100]. However, as shown in Table 3, AWE also has a relatively low electrolysis efficiency compared to other technologies. Moreover, the AWE cell response time is longer, which means that, when the input renewable energy fluctuates, the AWE cell cannot respond in time, which may cause cell damage. PEM technology, which has the advantages of a high electrolysis efficiency, high operating pressure, convenient maintenance, and fast response, is attracting more and more attention. However, as shown in Table 3, it can be seen that PEM needs precious metal catalysts, which significantly increases the cost of PEM cells. According to the literature, the cost of a PEM catalyst is about 70 $/kW at present. With the continuous development of PEM electrolysis technology, the catalyst cost may be reduced to 18 $/kW, which is acceptable to a certain extent. In addition, proton exchange membranes, such as PFSA, may release environmentally harmful HF during the recovery process [97,101]. SOEC has the highest electrolysis efficiency out of the four electrolysis technologies, but its high-temperature operation consumes much energy, hindering its large-scale commercialization.

We consulted with many pieces of research and found that most researchers only compared three other technologies except AEM. This was mainly because AEM is still in the theoretical development stage and lacks technical and economic parameters. Table 3 summarizes the technical parameters related to AEM that can be found in the existing literature. It can be found that AEM has significant advantages over the other three technologies. The electrolyte of AEM uses a 1 wt% K₂CO₃ solution. The electrolyte also chooses distilled water, ultrapure water, and a 0.1 mol·L⁻¹ KOH solution, which reduces the corrosion of the membrane by the strong alkaline solution [98]. AEM generally uses transition metal catalysts instead of noble metal catalysts, reducing its economic cost.

Among the four electrolysis technologies, AWE has the lowest investment cost and most mature technology, making it the most suitable solution for preparing H₂ [102]. PEM electrolysis costs are relatively high, but it is possible for them to be reduced to 500 EUR/kW by 2050 [103]. SOEC has the highest investment cost among them due to its high operating temperature. However, Hauch et al. estimated that, with the rapid development of SOEC technology, its cost could be reduced to 530 EUR/kW by 2050 [104]. The AEM economy has not been reported in the literature. The current four electrolysis technologies are not economically comparable with SMR. However, with the large-scale use of renewable energy, lower electricity prices, higher carbon taxes, and government financial support, the four electrolysis technologies may replace SMR.

Table 3. Comparison of four electrolysis technologies [79,80,98,105,106,107,108,109].

	AWE	PEM	SOEC	AEM
Operation conditions
Charge carrier	OH−	H+	O2−	OH−
Cathode materials	Ni	Ni	Ni-YSZ	Ni-Fe-Co or NiMo-NH3/H2
Cathode catalyst	Pt	Pt, Ir, Ru	Ni	CeO2-La2O3
Anode materials	Ni	Ni or C	LSM-YSZ or LSF-YSZ	Ni-Fe-Ox or Fe-NiMo-NH3/H2
Anode catalyst	Pt	Pt	LSM-YSZ or LSF-YSZ	Co3O4
Electrolyte	KOH	H2SO4	YSZ	1 wt% K2CO3
Isolation medium	PPS/PES a	PFSA b	N.A.	QAPS c
Operation parameters
Pressure of work (bar)	10–30	20–50	1–15	30
Electrolytic temperature (°C)	60–80	50–80	700–900	50–80
Current density (A/cm2)	0.25–0.45	1–2	0.3–1.0	0.2–0.5
Flexibility
Start-up time	5 min	<10 s	15 min	N.A.
Efficiency
Commercial system electrolysis efficiency (LHV)	51–60	60–65	76–81	64
Specific energy consumption (kWh/Nm3)	5.0–5.9	4.6–5.0	3.7–3.9	4.7
H2 purity (vol%)	99.5	99.99	99.9	99.99
Durability
Life time (kh)	55–120	60–100	8–20	N.A.
Economic parameters
Investment cost (EUR/kW)	800–1500	1400–2100	>2000	N.A.
Maintenance costs (% of annual investment cost)	2–3	3–5	N.A.	N.A.
Anode reaction	4OH− → 2H2O + O2 + 4e−	2H2O → 4H+ + O2 + 4e−	H2O + 2e− → H2 + O2−	4OH− → O2 + 2H2O + 4e−
Cathodic reaction	4H2O + 4e− → 2H2 + 4OH−	2H+ + 2e− → H2	2O2− → 4e− + O2	4H2O + 4e− → 2H2 + 4OH−
Total reaction	2H2O → 2H2 + O2

^a Polyphenylene-sulfide membrane (PPS) and polyether sulfone membrane (PES) are composite materials based on ceramic materials and microporous materials, which can be used as the diaphragm between the cathode and anode of AWE. ^b Perfluorosulfonic-acid (PFSA) proton exchange membrane represented by DuPont’s Nafion^® membrane. ^c A quaternary ammonium polysulfone membrane represented by the A-201^® membrane of Tokuyama Corporation of Tokyo, Japan.

Table 3. Comparison of four electrolysis technologies [79,80,98,105,106,107,108,109].

	AWE	PEM	SOEC	AEM
Operation conditions
Charge carrier	OH−	H+	O2−	OH−
Cathode materials	Ni	Ni	Ni-YSZ	Ni-Fe-Co or NiMo-NH3/H2
Cathode catalyst	Pt	Pt, Ir, Ru	Ni	CeO2-La2O3
Anode materials	Ni	Ni or C	LSM-YSZ or LSF-YSZ	Ni-Fe-Ox or Fe-NiMo-NH3/H2
Anode catalyst	Pt	Pt	LSM-YSZ or LSF-YSZ	Co3O4
Electrolyte	KOH	H2SO4	YSZ	1 wt% K2CO3
Isolation medium	PPS/PES a	PFSA b	N.A.	QAPS c
Operation parameters
Pressure of work (bar)	10–30	20–50	1–15	30
Electrolytic temperature (°C)	60–80	50–80	700–900	50–80
Current density (A/cm2)	0.25–0.45	1–2	0.3–1.0	0.2–0.5
Flexibility
Start-up time	5 min	<10 s	15 min	N.A.
Efficiency
Commercial system electrolysis efficiency (LHV)	51–60	60–65	76–81	64
Specific energy consumption (kWh/Nm3)	5.0–5.9	4.6–5.0	3.7–3.9	4.7
H2 purity (vol%)	99.5	99.99	99.9	99.99
Durability
Life time (kh)	55–120	60–100	8–20	N.A.
Economic parameters
Investment cost (EUR/kW)	800–1500	1400–2100	>2000	N.A.
Maintenance costs (% of annual investment cost)	2–3	3–5	N.A.	N.A.
Anode reaction	4OH− → 2H2O + O2 + 4e−	2H2O → 4H+ + O2 + 4e−	H2O + 2e− → H2 + O2−	4OH− → O2 + 2H2O + 4e−
Cathodic reaction	4H2O + 4e− → 2H2 + 4OH−	2H+ + 2e− → H2	2O2− → 4e− + O2	4H2O + 4e− → 2H2 + 4OH−
Total reaction	2H2O → 2H2 + O2

^a Polyphenylene-sulfide membrane (PPS) and polyether sulfone membrane (PES) are composite materials based on ceramic materials and microporous materials, which can be used as the diaphragm between the cathode and anode of AWE. ^b Perfluorosulfonic-acid (PFSA) proton exchange membrane represented by DuPont’s Nafion^® membrane. ^c A quaternary ammonium polysulfone membrane represented by the A-201^® membrane of Tokuyama Corporation of Tokyo, Japan.

2.3. CO₂ Hydrogenation to Liquid Fuels Technologies

At present, CO₂ and H₂ can be converted into hydrocarbons via the methanol-mediated and modified FT-based pathways. As highlighted above, this work mainly focuses on the modified FT-based pathway, due to its simple process configuration and adjustable product selectivity. Generally, modified FT-based pathways are mainly divided into high-temperature Fischer–Tropsch synthesis (HTFT) and low-temperature Fischer–Tropsch synthesis (LTFT) processes [110]. Generally, the targeted syncrude obtained via HTFT and LTFT can be further purified and upgraded to liquid fuels, including gasoline (C₅–C₁₂), naphtha (C₅–C₁₂), and diesel (C₁₂–C₂₀) [111]. The differences between the HTFT and LTFT processes mainly lie in their operating conditions and catalysts. For the HTFT process, its operating temperature is about 300~330 °C. CO₂ and H₂ are converted into C₁–C₄ paraffins, C₂–C₄ olefins, and C₅₊ hydrocarbons over Fe-based catalysts. In addition, HTFT products may contain oxygen-containing compounds and aromatic compounds, but almost no residues, such as paraffin wax [112,113]. For the LTFT process, its operating temperature is about 200~280 °C and CO₂ is hydrogenated over Co-based catalysts. Herein, Co-based catalysts are suitable for synthesizing straight-chain alkanes in a range of liquid waxes, especially for C₅₊ hydrocarbons with a high selectivity [10,114]. It is worth noting that a Co-based catalyst has no catalytic activity for CO₂, so it is necessary to convert CO₂ into CO, which can be achieved via the RWGS reaction [115]. In addition, Co-based catalysts are prone to methanation at high temperatures [116]. By adding K to Co-based catalysts, the methanation reaction can be effectively suppressed and the selectivity of the C₅₊ hydrocarbons can be increased [117].

For HTFT and LTFT processes, the reaction temperature, superficial gas velocity, particle diameter, and bed height affect the composition of the product. Among the various factors, the influence of the reaction temperature is the most significant. Given that the endothermic RWGS reaction generally competes with the exothermic FT synthesis reaction, an increase in the reaction temperature can accelerate the reaction rate, whereas the yield and the selectivity of the C₅₊ hydrocarbons start to decrease. This is mainly because an increase in the operating temperature promotes the RWGS reaction, resulting in a higher CO selectivity [118]. Moreover, considering that both the HTFT and LTFT reactions are strictly limited by the thermodynamic equilibrium, therefore, there are considerable amounts of unreacted CO₂ and CO in the unreacted syngas derived from the outlet of the FT syntheses reactor, which can be recycled to the previous FT synthesis reactor in order to improve the production of the syncrude. Generally, the CO₂/CO ratio in the recycled unreacted syngas could affect the process performances. With this in mind, Jun et al. investigated the effects of the CO/CO₂ ratio in unreacted syngas on the catalytic behavior in an FT synthesis reaction over K/Fe-Cu-Al catalysts. The results indicated that a high CO₂/CO ratio is beneficial for efficient CO₂ conversion and prevents carbon disposition, providing good guidance for the subsequent process design [119].

As mentioned above, the CO₂ conversion in HTFT and LTFT reactions is relatively low. Therefore, considerable efforts have been devoted toward the development of strategies for promoting the transformation of CO₂ into high value-added hydrocarbons, such as water removal and multiple FT synthesis reactors in series. More specifically, water is one of the primary byproducts of the FT synthesis reaction and its accumulation can significantly reduce the driving force of the reaction, easily resulting in catalyst deactivation [120]. To address these issues, Hyeon et al. removed the water in situ by using a thermally rearranged polybenzoxazole membrane (See Figure 3), and found that the selectivity of the targeted low-carbon olefins was improved by 2~6% [121]. The aforementioned membrane reactors are still in their research and development stage and have a long way to go before their large-scale deployment. With regard to multiple FT synthesis reactors in series, these have become the research emphasis in industry and academia. Meiri et al. adopted three reactors in series with condensation devices in order to convert CO₂ into targeted low-carbon olefins. The results indicated that the CO₂ conversion was significantly increased by more than 20% [122]. Kamkeng et al. proposed two improved FT process configurations (including a three-stage reactor in series and a single reactor with recycling) to facilitate off-site water removal (See Figure 4). They found that the application of three reactors in series notably improved the CO₂ conversion by 36% and the gasoline production by 27% [123].

For HTFT and LTFT synthesis reactions, the obtained crude products also include light hydrocarbons (i.e., C₁–C₄ paraffins and C₂–C₄ olefins), which are generally considered to be inert components during the recycling process [124]. Therefore, achieving an efficient utilization of these light hydrocarbons has become the emphasis. In traditional HTLT and LTFT processes, light hydrocarbons can be converted into CO and H₂ via high-temperature reforming technology, which are then recycled into the FT synthesis reactor, together with the unreacted syngas. However, this energy-intensive reforming process inevitably generates considerable carbon emissions. Hence, it is necessary to develop more clean and sustainable processes. Recently, some emerging processes have been proposed to further achieve the upcycling of these light hydrocarbons. In light hydrocarbons, C₂–C₄ hydrocarbons are important platform chemicals for the petrochemical industries [125]. Therefore, in the work of Kim et al., methane was separated from light hydrocarbons, using the distillation method to obtain high value-added C₂–C₄ hydrocarbons (See Figure 5). Therein, the methane was sent to the combined heat and power unit to generate utilities on site [126]. Mahouri et al. also conducted similar work. They sent light hydrocarbons to the energy recovery unit to generate sufficient steam or electricity for internal use [114]. Adelung et al. considered light hydrocarbons as fuel and sent them to the burner to obtain high-temperature steam, which could be used to preheat the energy-intensive RWGS reactor and other heaters, further reducing the external utility consumption [127]. Moreover, emerging co-electrolysis technology has been proposed. For example, Cinti et al. proposed an efficient CO₂ hydrogenation to liquid fuel process, coupled with SOEC water electrolysis technology. Therein, light hydrocarbons were recycled into the previous high-temperature SOEC cell to obtain syngas [128]. In addition to the aforementioned methods, in our previous work, an integrated process coupled with HTFT and methanation technology was proposed. As shown in Figure 6, unreacted syngas and light hydrocarbons were sent to the subsequent methanation unit to produce high-caloric synthetic natural gas (SNG). The hybrid process achieved the co-production of syncrude and high-caloric SNG, further improving the carbon utilization efficiency [129,130]. Furthermore, light hydrocarbons consist of considerable light olefins, which can be further converted into high value-added C₅₊ hydrocarbons via an oligomerization reaction over modified HZSM-5 catalysts. In the previous work, we proposed a CO₂ hydrogenation process integrating HTFT and olefin oligomerization technologies (See Figure 7). The results indicated that the application of an oligomerization reactor further enhanced the syncrude production and improved the energy efficiency [131,132].

3. Technical and Economic Analysis of PTL Processes

Technical and economic analyses (TEA) of the PTL process are crucial for the development and commercialization of these PTL processes. However, variations in regional and temporal factors, as well as fluctuations in government policies and regulations, have led to significant disparities among the different PTL processes [133]. This section addresses this gap by evaluating the existing PTL processes from technical and economic perspectives.

3.1. Technical Analysis of PTL Processes

Numerous researchers have dedicated significant efforts to enhancing the liquid fuel production and process efficiency. Furthermore, adopting efficient water electrolysis has been proven to be crucial for improving the technical performances of the emerging PTL processes.

Generally, the stack efficiency and operating conditions of the electrolyzer in water electrolysis technologies have significant effects on the technical performances of the PTL process.

Herz et al. suggested a PTL process with SOEC technology; meanwhile, advanced heat integration and byproduct recirculation methods were applied to further the energy-saving potential of the system. The results revealed that the overall process efficiency was improved by more than 7% [134]. Markowitsch et al. evaluated the technical performances of PTL processes configured with PEM and SOEC technologies. It was concluded that the application of SOEC technology with a higher stack efficiency had a higher PTL efficiency, improving it by 10% [135]. Moreover, considering the lack of comprehensive comparisons of the effects of different water electrolysis technologies (i.e., AWE, AEM, PEM, and SOEC) on the technical performances of PTL processes, we proposed four hybrid PTL/PTG processes coupled with different water electrolysis technologies (See Figure 8) and compared their process performances in terms of their carbon efficiency, energy efficiency, net CO₂ reduction rate, and exergy efficiency. It was found that the hybrid PTL/PTG process coupled with SOEC technology had the highest energy (54.26%) and exergy efficiencies (70.74%), due to having the highest stack efficiency. However, the hybrid PTL/PTG process coupled with AEM technology had the highest carbon efficiency (75.04%) and net CO₂ reduction rate (70.58%), due to its relatively mild operating conditions [8]. In addition to the aforementioned water electrolysis technologies, co-electrolysis technology has shown a strong application potential in PTL processes [128,136]. In the work of Marchese et al., they used a co-electrolysis device instead of an electrolyzer and RWGS reactor to generate syngas with a suitable ratio of H₂ to CO (See Figure 9). The technical analysis indicated that the plant efficiency of the PTL process coupled with co-electrolysis technology had a higher plant efficiency of 81.1% compared with to the traditional PTL process [137]. Pratschner et al. proposed a similar PTL process integrated with co-electrolysis technology (See Figure 10). However, it should be noted that the CO₂ was mainly derived from the flue gas in the cement plant, biogas-upgrading plant, and biomass CHP plant. The PTL efficiencies were found to be 54.7~63.8% and the carbon efficiencies were found to be 66.4~88.6%, which was mainly dependent on the specific CO₂ sources [138]. At present, a co-electrolysis system at the Dresden plant was successfully launched and put into trial operation (>500 h), which has established a reliable foundation for the subsequent smooth operation of PTL plants [139].

3.2. Economic Analysis of PTL Processes

Economic analyses are an important tool for evaluating economic feasibility and profitability. Currently, considerable efforts have been devoted to conducting detailed economic assessments of PTL processes and providing deep insights for the further application of these PTL processes.

Colelli et al. evaluated the economic performances of direct and indirect PTL processes (See Figure 11). Therein, in the direct route, CO₂ was converted into hydrocarbons over Fe-based catalysts. In the indirect route, CO₂ was transformed into CO, which was subsequently hydrogenated to hydrocarbons over Co-based or Fe-based catalysts. The results suggested that the product cost was 460~1435 EUR/bbl for the direct route and 752~2364 EUR/bbl for the indirect route [140]. The product costs of the direct and indirect routes are both much higher than the market prices of gasoline, diesel, and jet fuel.

Generally, the price of green H₂ is the dominant factor in the product cost of PTL processes [10,126,141]. Therefore, reducing the price of green H₂ in the PTL process plays a decisive role in the overall economy of the process. Zhao et al. found that PTL processes could show a strong economic competitiveness compared to traditional fossil energy when the H₂ price was reduced by 40% based on the existing energy policy in China [142]. Moreover, Adelung et al. also studied the influence of the H₂ price on the net production costs (NPC) of PTL processes. The results showed that, when H₂ prices decreased from 7.6 to 2.3 EUR/kg, the NPC was reduced from 5.47 to 1.81/kgC₅₊ [143]. In fact, this H₂ price is closely related to the selection of the water electrolysis technology. More specifically, in our previous work, we compared the total production costs of four PTL/PTG hybrid processes coupled with different water electrolysis technologies. It was found that the PTL/PTG hybrid process coupled with AEM technology had the lowest total production cost [144]. Meanwhile, according to the work of Adelung, adjusting the full load hours of the electrolyzer could reasonably reduce this net production cost [145]. Moreover, selling the O₂ byproduct could further reduce the product cost of PTL processes. For example, Fasihi et al. conducted a detailed analysis of the economic performance of the PTL process using hybrid PV-wind electricity. The minimum selling price of the syncrude was 79~135 $/bbl when the byproduct O₂ was put up for sale [146]. Adopting more advanced PTL processes is also beneficial for their economic feasibility. In our previous work, the economic performances of direct and indirect PTL/PTG hybrid processes were also evaluated. The total product cost was observed to be 202~211 $/bbl for the direct route, while the total product cost for the indirect route was determined to be 215~240 $/bbl [129,130,147]. According to the aforementioned economic results, the total product cost in our previous work was significantly lower than that in the work of Colelli et al. This may mainly be because the high-calorie SNG further improved the added value of the products. In addition to the aforementioned factors, the large-scale deployment of renewable energy, a high carbon tax, and strong government incentive policies can enhance the economic viability and stimulate the development of PTL processes.

4. Conclusions and Outlook

CCU technologies have demonstrated immense potential for converting CO₂ into value-added materials, chemicals, and fuels, making a significant contribution to reducing greenhouse gas emissions and fostering a more sustainable and circular economy. Among the various CCU technologies, the concept of PTL offers a pathway for the production of sustainable liquid hydrocarbon fuels, utilizing both CO₂ emissions and renewable energy sources. The PTL process consists of three sub-units: CO₂ capture, H₂ generation, and CO₂ hydrogenation. Each sub-unit involves a range of technologies, and the selection and integration of these technologies play crucial roles in determining the performance of a hybrid PTL process. In this study, we conducted a comprehensive and authoritative review of the technologies employed in PTL processes.

Through an in-depth analysis of these sub-units, we gained valuable insights into the advancements, challenges, and potential improvements in each technology. For the CO₂ capture sub-unit, several viable methods were identified, specifically solvent absorption, membrane, and PSA technologies. Each technology has its own strengths and limitations. Solvent absorption, although a mature method, faces cost challenges in solvent regeneration. Membrane separation and PSA exhibit a lower energy consumption and lower operation costs, positioning them as promising alternatives to solvent absorption. Further search and development efforts should focus on enhancing the efficiency and cost-effectiveness of these CO₂ capture technologies. In the H₂ generation sub-unit, AWE technology has emerged as the most mature method, albeit with a lower electrolysis efficiency. Both PEM and SOEC have demonstrated relatively higher electrolysis efficiencies, with SOEC achieving an impressive efficiency of up to 81%. While PEM and SOEC technologies are on the brink of commercialization, AEM has shown promise due to its low cost and high electrolysis efficiency. Future research should aim to overcome the current limitations and promote the practical application of AEM technology. Regarding the CO₂ hydrogenation sub-unit, the F-T process stands as the most popular technology. However, it faces challenges related to its high energy consumption and low energy efficiency. To address these issues, process improvements can be implemented, such as utilizing light hydrocarbons produced as fuel and redirecting them to the burner for high-temperature steam generation, thus reducing the overall energy consumption. Additionally, exploring the integration of PTL/PTG mixed processes can improve the carbon utilization efficiency by generating natural gas from the light hydrocarbons produced.

Notwithstanding its potential benefits, the current technological level of the PTL process presents challenges in terms of its production and product costs, hindering its competitiveness against traditional fuel production. To this end, academia and industry are actively focusing on improving its energy efficiency and developing cost-effective pathways. The integration of the PTL process with SOEC technology offers a promising approach to this improvement in energy efficiency. In this regard, the heat generated during the PTL process can be utilized to produce the steam required by SOEC technology, optimizing the energy utilization and further enhancing the overall efficiency of a PTL system. Additionally, excess heat can be employed to regenerate solvents in the CO₂ capture process or generate power via the Rankine cycle. These integrated strategies contribute to cost reduction and improve the competitiveness of PTL liquid fuel in the market. Moreover, the industrial application of PTL heavily depends on reducing its production costs, with the CO₂ capture and H₂ production being the primary cost drivers. To achieve this, various measures can be implemented, such as adopting new absorbents, membrane separation, and PSA methods for the CO₂ capture, and introducing policy changes related to CO₂ emissions, including carbon taxes and incentive measures. Improving electricity prices, the electrolysis efficiency, and electrolytic cell capital expenditure are critical factors for reducing the H₂ production costs.

In summary, addressing the current cost challenges in PTL processes is essential for their widespread adoption in the fuel production sector. The integration of SOEC technology, alongside measures for reducing the production costs in CO₂ capture and H₂ production, presents a promising way forward. By continuously optimizing the process and adopting favorable policies, the PTL process holds great potential in providing a sustainable and competitive alternative to traditional fuel production, contributing to global efforts towards a more environmentally friendly and economically viable energy landscape.