Comparing CO₂ Storage and Utilization: Enhancing Sustainability through Renewable Energy Integration

Abstract

Addressing the environmental challenges posed by CO₂ emissions is crucial for mitigating global warming and achieving net-zero emissions by 2050. This study compares CO₂ storage (CCS) and utilization (CCU) technologies, highlighting the benefits of integrating captured CO₂ into fuel production. This paper focuses on various carbon utilization routes such as Power-to-Gas via the Sabatier reaction, indirect production of DME, and Power-to-Fuel technologies. The maturity of these technologies is evaluated using the Technology Readiness Level (TRL) method, identifying the advancements needed for future implementation. Additionally, global regulations and policies surrounding carbon capture and storage are reviewed to provide context for their current status. The study emphasizes the potential of CCU technologies to reduce future CO₂ emissions by converting captured CO₂ into valuable fuels and chemicals, thus supporting the transition to a sustainable energy system. The findings indicate that while CCS technologies are more mature, promising CCU technologies can significantly contribute to reducing greenhouse gas emissions if green hydrogen becomes more affordable. This research underscores the importance of further technological development and economic evaluation to enhance the feasibility and adoption of CCU technologies in the pursuit of long-term environmental sustainability.

1. Introduction

1.1. Climate Change and Global Warming

Anthropogenic climate change has become a critical issue, causing detrimental effects on global climate, ecosystems, and species across terrestrial and marine environments [1,2,3]. Climate change includes various aspects such as global warming, shifts in weather patterns, changes in precipitation, more frequent extreme weather events, sea level rise, and alterations in ecosystems and wildlife. Global warming refers specifically to the increase in Earth’s average surface temperature due to the accumulation of greenhouse gases (GHGs) in the atmosphere. This phenomenon is primarily driven by human activities such as burning fossil fuels, deforestation, and industrial processes, leading to a measurable rise in temperatures.

The average global temperature has increased by approximately 1.2 °C above pre-industrial levels, and it is projected to rise further if greenhouse gas emissions are not significantly reduced. The current action plan aims to keep the increase in average global temperature below 1.5 °C [4]. In 2022, CO₂ emissions approached 40 billion tons per year (40 × ${10}^9$ t/yr). When considering all GHGs converted to their carbon dioxide equivalent (CO₂e), global yearly emissions are estimated to be around 50 billion tons per year (50 × ${10}^9$ t/yr) [5]. The objective is to cut GHGs to 1990 levels by 2030, effectively halving current annual CO₂ emissions (to ≈22 × ${10}^9$ t/yr), and achieve net-zero emissions by 2050 [6,7]. However, estimates suggest that 2030 global emissions will far exceed the current goal [8]. Therefore, this ambitious goal necessitates a multifaceted approach, incorporating advancements in renewable energy, carbon capture and storage (CCS), carbon capture and utilization (CCU), and energy efficiency technologies.

1.2. Global Regulatory and Policy Frameworks: Recent COP Outcomes

The Paris Agreement, adopted at the 21st Conference of the Parties (COP21) in December 2015, is a landmark international treaty to combat climate change and accelerate actions towards a sustainable low-carbon future. The Agreement’s central goal was to limit global temperature rise to well below 2 °C above pre-industrial levels, with efforts to cap the increase at 1.5 °C. To achieve this, countries committed to nationally determined contributions, which outline their plans to reduce GHG emissions and adapt to climate impacts. The Paris Agreement also emphasized the importance of climate finance, technology transfer, and capacity building to support developing countries’ climate efforts.

Key outcomes from COP22 to COP25 included the focus on implementing the Paris Agreement, highlighted by the Marrakech Partnership for Global Climate Action and the detailed rulebook established in the Katowice Climate Package. The Talanoa Dialogue and the Gender Action Plan were significant from COP23, promoting inclusive discussions and gender-responsive policies. COP24 emphasized a just transition to low-carbon economies and underscored the urgent need for action following the IPCC’s 1.5 °C report. COP25, while struggling with agreements on carbon markets advanced the Santiago Network for loss and damage and underscored youth engagement in climate action.

COP26 (Glasgow, 2021) reaffirmed the Paris Agreement’s goals. COP26 was the first to explicitly mention coal, oil, gas, and fossil fuels as the primary causes of global warming. The COP26 agreement included the intention to “phase down” rather than shut down coal power plants, with CO₂ emissions to be neutralized by CCS technologies despite their high costs [9]. COP27, held in Sharm El Sheikh, Egypt, in autumn 2022, continued these discussions. A week before the conference, the United Nations Environment Program (UNEP) reported that there was no credible pathway to limiting global warming to 1.5 °C, and mitigation efforts since COP26 had been “woefully inadequate” [10]. The agreement aimed to uphold the goal of limiting the temperature increase to 1.5 °C. The COP27 implementation plan emphasized comprehensive strategies incorporating CCS and CCU technologies [11]. COP28, held in Dubai in 2023, further advanced the climate agenda by instituting a more rigorous global review process, setting specific targets for emission reduction and renewable energy expansion, and emphasizing the transition from fossil fuels. Governments recognized the importance of emerging renewable technologies, such as low-emission hydrogen and CCS, in achieving environmental targets [12].

1.3. Captured CO₂ Storage and Utilization

CCS is becoming an essential approach to reducing GHG emissions. CCS consists of storing CO₂ in a suitable geological sink, which is kept for an extended period. The options are mainly underground: exhausted oil and gas fields, deep coal beds, aquifers, and salt caverns. In some of these cases, CO₂ might react chemically with minerals in the rock. Thus, CO₂ control over geological time is needed [13,14].

As the world’s largest CO₂ emitter, China has set ambitious targets to reach its carbon peak by 2030 and achieve carbon neutrality by 2060. The EU has updated its regulatory framework, including Directive 2009/31/EC on the geological storage of CO₂, facilitating CCS in sub-seabed geological formations. This directive enforces stringent permitting, monitoring, reporting, and verification standards to ensure CO₂’s permanent isolation in CCS projects.

In the United States, the Bureau of Ocean Energy Management (BOEM) oversees leasing, licensing, and regulation within federal waters on the Outer Continental Shelf, developing a legal framework for offshore CCS facility leases and applying CCS technologies. Japan has introduced regulations under the London Protocol for licensing seabed CO₂ storage projects, managed by the Ministry of Economy, Trade, and Industry alongside the Ministry of the Environment. These regulations mandate a comprehensive submission of storage process information and an environmental impact assessment before licensing. In Australia, CCS projects in the Commonwealth marine area are governed by the Offshore Petroleum and Greenhouse Gas Storage Act of 2006, allowing CO₂ storage in deep geological formations beyond state jurisdictions [15].

CCS is expected to be the predominant method for large-scale CO₂ reduction due to its long-term and secure storage capacity. While CCS is expected to lead in mitigating climate change, CCU is vital, as it reduces emissions and creates economic value from CO₂, encouraging broader adoption and innovation in carbon management.

CCU consists of converting the captured CO₂ into useful products, such as fuels, chemicals, building materials, and other industrial products. Converting the captured CO₂ into valuable chemicals requires H₂ production, preferably green H₂, produced exclusively with renewable electricity via water electrolysis. Consequently, some CCU technologies present the challenge of producing much hydrogen at a low and competitive price.

Transforming the energy sector is crucial to achieving climate goals. Although the share of electricity produced from renewable sources has increased, it often remains below average demand in most countries. To bridge this gap, flexible carbon or gas power plants match production with demand, primarily due to the difficulty of storing excess electricity generated from renewable sources. This reliance on energy sources such as natural gas led to Europe’s 2021–23 energy crisis [16].

It is relevant to remember that clean energy sources like wind, solar, or hydro-energy depend strongly on weather conditions. Thus, the energy system cannot rely strongly on them. That is why a power system with a high fraction of renewable energy sources may suffer intermittent overproduction, also called a surplus of electricity production (SEP). Sometimes, the SEP may exceed the technical capacity for usage or exportation, creating the critical electricity excess production (CEEP) issue. CEEP represents a low-cost electricity source that has to be instead stored or dissipated [17,18].

A new approach involves significantly increasing energy production from renewable sources to exceed demand, thereby reducing dependency on fossil fuels. The surplus of green electricity, which can vary with weather conditions, would be used to produce green H₂. This transition from relying on natural gas for demand surges to utilizing renewable excess represents a sustainable step forward. It aligns energy production with environmental goals, enhances the energy system’s resilience, and reduces fossil fuel dependency. Accordingly, energy storage technologies, classified as Power-to-Power, Power-to-Heat, Power-to-Fuel, and Power-to-Chemicals, also play a crucial role in utilizing captured CO₂. These technologies use various energy sources, including electrochemical, chemical, mechanical, and thermal storage [18].

Producing fuels from CO₂ and green H₂ has two benefits over CCS. First, it reduces CO₂ emissions to the atmosphere and transforms the CO₂ molecules into renewable fuels. Second, it stores excess renewable electricity at peak production [19]. However, carbon utilization requires, on many occasions, intermediate storage before further transforming CO₂ into valuable products. Some processes might require CO₂ transportation from the capturing facility to the usage facility, so intermediate CO₂ storage is needed before injecting CO₂ into the transportation equipment. Furthermore, intermediate CO₂ storage can also be applied after the CO₂ capture unit to offer a constant flow and condition modification before being introduced into the usage facility. Thus, CCU and CCS can be integrated into carbon capture storage and utilization (CCUS). CCUS includes CO₂ capture, CO₂ transport, CO₂ storage, and final usage. This route requires a considerable amount of energy, which decreases the efficiency of industrial facilities. Many CCUS technologies are expensive and not economically competitive, which caused the main challenge for implementation [20].

Nevertheless, due to the rise of carbon emission tariffs, this situation has changed in recent years, particularly in Europe, through the European Emissions Trading Systems (ETS). The price increased from about EUR 25/t in 2020 to over EUR 80/t in December 2021 [15].

Table 1. CO₂ emission tariff auction price in different markets (July 2024) [21].

Markets	Carbon Price
EU ETS	EUR 72.4/t
New Zealand (NZD)	NZD 51.5/t
Australia (AUD)	AUD 33.5/t
California (USA)	USD 44.0/t
South Korea	USD 6.5/t
China	USD 12.6/t

shows the carbon prices in different markets per tonne of carbon emissions.

The CCU perspective might provide an opportunity to use CO₂ effectively and is a promising way to obtain economic and environmental incentives such as green credits and decrease operative costs, reducing the CO₂ emission tariffs and opening secondary business lines in several industries [22].

This review analyzes the usage of captured CO₂ generated as a by-product of industrial processes instead of isolating the CO₂ in storage for a long time. The pathways and technologies available to capture CO₂ are explained in the previous review published by this research team in [23]. The paper reviews the options for storage and utilization of CO₂, assuming that the CO₂ has been captured. The manuscript provides a comprehensive analysis and comparison of CCS and CCU technologies, emphasizing the potential of integrating renewable energy sources to enhance sustainability. It highlights the benefits of converting captured CO₂ into valuable fuels and chemicals. The study includes a detailed assessment of these technologies’ technological maturity, economic feasibility, and environmental impact using the Technology Readiness Level (TRL). The primary objective of the manuscript is to evaluate and advocate for the transition from traditional CO₂ storage methods to more innovative CO₂ utilization techniques.

2. CO₂ Storage

CCS involves the sequestration of CO₂ in geological formations, ensuring its long-term isolation from the atmosphere. The primary storage options are underground, including depleted oil and gas reservoirs, deep coal seams, aquifers, and salt caverns. Large amounts of CO₂ can be permanently stored in geological formations such as depleted oil or gas fields, unmineable coal beds, aquifers, or saline caves without practical use. Within these geological settings, CO₂ may undergo chemical reactions with the surrounding minerals, necessitating meticulous oversight to ensure its stability and containment over geological timescales [13,14].

By 2100, an estimated 2700 Gt of global storage capacity is needed to reach the climate goals [24]. The global capacity for CCS is approximately 40–50 million tons per year (≈0.1% of the current emissions). The IEA underscores the necessity for CO₂ storage, growing from today’s capacity to over 5 Gt/yr by mid-century, making it a global industry essential for emission reduction across the energy system. The potential storage capacity globally far exceeds the required amount, with estimates ranging between 8000 and 55,000 Gt, showcasing the vast capability for CO₂ sequestration to support deep emission reductions and carbon removal efforts [25].

Nearly 200 new CCS projects were announced during 2022, meaning 60 new CCS facilities since 2021, including projects with transportation and storage facilities with no CO₂ capture. These plants’ number and CO₂ storage capacity are displayed in Figure 1. The most mature CO₂ storage technologies are presented in this section.

When selecting the appropriate geological formation for CCS, several requirements must be considered; rock permeability, thickness, and porosity are critical factors [27]. Also important are storage capacity, the potential for leakage, proximity to the CO₂ source, socio-political conditions, and hydrological aspects. Common challenges in CCS implementation include improving efficiency, ensuring long-term safety, navigating government regulations and costs, selecting suitable sites, preventing leakages, and gaining public acceptance. During the carbon storage process, failures in caprock integrity, significant container faults, and valve malfunctions can cause leakages, leading to groundwater pollution. Therefore, advanced sealing and pressure management technologies must be developed to mitigate these risks [28].

While not covered in this review, carbon sequestration in natural sinks, such as the soil of reforested forests, is a crucial strategy for mitigating climate change. Reforestation enhances soil carbon storage capacity, as trees absorb CO₂ during photosynthesis and transfer carbon to the soil through their roots and leaf litter. This process not only captures atmospheric CO₂ but also improves soil health and biodiversity. Studies estimate that global reforestation could sequester up to 205 Gt of CO₂ by 2100. This approach complements technological solutions in achieving carbon neutrality and promoting sustainable land management.

2.1. Enhanced Oil and Gas Recovery

Enhanced Oil Recovery (EOR) and Enhanced Gas Recovery (EGR) involve extracting oil and gas from crude oil fields that cannot be exploited using standard methods. The three primary EOR techniques are gas, chemical, and thermal injection. Gas, chemical agents, or heat is applied to the retained oil or natural gas to facilitate extraction. Currently, the primary use of captured CO₂ is for EOR and EGR [23].

Despite utilizing captured CO₂ to extract oil and natural gas, these technologies also serve as storage methods. In CO₂-EOR, CO₂ is injected into a crude oil reservoir to enhance oil extraction. The injected CO₂ increases the field pressure, providing the driving force to extract residual oil and natural gas (see Figure 2). CO₂ is typically injected in its supercritical phase, which lowers the viscosity of the displaced oil from the rock pores in a single-phase drainage process. The pressure is generally around 75 bar, and the temperature is approximately 70 °C to ensure suitable miscible conditions within the oil field.

CO₂-EOR is currently one of the most profitable ways to add value to captured CO₂. Economic incentives exist to recover and recycle as much CO₂ as possible, thereby reducing the operational costs of the process [29,30]. This makes CO₂-EOR a valuable technology for enhancing hydrocarbon recovery and providing a viable means for CO₂ sequestration.

CO₂-EOR aims to extract crude oil while storing some of the injected CO₂ in depleted oil and gas fields. Consequently, EOR does not significantly contribute to the abandonment of fossil fuels and is associated with substantial water usage. Therefore, the net impact on emissions depends on the source of the CO₂ and the end use of the recovered oil and gas. It is crucial to evaluate whether this technique results in a net negative emission solution [30,31]. These considerations are essential in transitioning to renewable energy, enhancing CO₂ utilization, and fostering a circular carbon economy.

Figure 2. Enhanced Oil Recovery illustration. Adapted from [32].

Figure 2. Enhanced Oil Recovery illustration. Adapted from [32].

CO₂-EOR has been practiced for several decades, reaching a TRL of 9. The first industrial CO₂ injection into a gas field started in 1996 in Norway’s Sleipner Vest gas field. The captured CO₂ was injected into saline aquifers near the production gas jacket with more than 20 Mt CO₂ injected [15]. There have also been past pilot projects in the US, concretely in the Gulf of Mexico [30]. There are several challenges for offshore CCS-EOR [33]:

• Higher CAPEX and OPEX compared to the onshore CCS-EOR.
• Limited CO₂ supply. Transport to the offshore facility might increase the overall cost.
• Limited space for retrofitting new equipment and pipelines. Replacing systems with new technology increases the cost since much more equipment needs to be replaced.

2.2. Enhanced Coal Bed Methane Recovery

CO₂-enhanced coal bed methane recovery (CO₂-ECBMR) is a technique that involves injecting CO₂ into deep coal beds to enhance the recovery of methane from the coal’s porous structure. In this process, CO₂ diffuses into the coal’s pore layers and chemically binds to sulfur, demonstrating an affinity almost twice as effective as CH₄. The costs associated with CO₂ injection and storage can be partially offset by the methane recovered from abandoned coal mines [34].

In conventional coal bed methane recovery (CBMR), methane production efficiency is around 50% due to the rapid reduction in reservoir pressure. However, methane recovery from unconventional CBMR projects is currently only about 6–9%. CO₂-ECBMR enhances methane production by lowering the partial pressure of methane in the free gas phase while maintaining overall reservoir pressure [35]. According to Liang et al. [36], more than 60% of high-rank coal gas from closed pores contributes to CH₄ and CO₂ injection.

CO₂-ECBMR has been adopted for many years, with several commercial extraction sites worldwide, primarily in the USA. White et al. highlighted several issues with this practice, such as environmental health and safety, storage integrity, and potential capacity [37]. Unfortunately, the high CO₂ absorption capacity can lead to swelling of the coal matrix, which reduces coal permeability and pore size [38].

The estimated current capacity for Enhanced Coal Bed Methane Recovery (ECBMR) in terms of CO₂ sequestration is poorly documented. If scaled up to commercial levels, a well-optimized ECBMR project could target CO₂ sequestration capacities of 1–2 million tons per year or more, depending on the size and characteristics of the coal bed. Global theoretical storage capacity for CO₂ in coal beds could range from hundreds to thousands of Gt of CO₂ [35].

2.3. Aquifer Storage

Captured CO₂ can be stored in aquifers through injection wells (see Figure 3). Depending on the output pressure of the capture facility, a compression system is needed on-site. The depth range depends on the aquifer’s geological formation. This system consists of an injection well, an injection pump if the pressure needs to be increased, monitoring in the excellent cellar, and different monitoring systems spread out on the surface of the anticipated delineation of the CO₂ plume [39].

This method has been used in Europe since 1953 for natural gas storage. Previously, town gas containing 50–60% H₂ was stored in aquifers. The company E.ON has operated natural gas storage in Hähnlein, Germany, since 1960, while Dong Energy (now Ørsted) started the operation of natural gas storage in Senlille, Denmark, in 1989. The TRL of this method is 9 [40].

Figure 3. Model of CO₂ storage in saline aquifers. Adapted from [41].

Figure 3. Model of CO₂ storage in saline aquifers. Adapted from [41].

Due to the CO₂ injection, the pressure within the reservoir increases. To balance that pressure, a specific volume of water can be extracted and brought to the surface through a separate production well. This produced water could potentially be utilized in a geothermal system, thereby enhancing the economic feasibility of the overall system. However, pressure management is complex due to the lack of regulatory guidelines on permissible pressure increases. Also, managing pressure involves handling large volumes of water at the surface, presenting logistical and environmental challenges [39].

Injecting CO₂ into a saline aquifer can impact the petrophysical properties of the reservoir rocks through complex geochemical reactions. The CO₂ dissolves in the brine and subsequently precipitates as carbonate minerals, partially deleting the stored CO₂. Continuous CO₂ injection and the resultant lower brine pH increase the risk of corrosion. Furthermore, carbonate precipitation reduces the porosity of the saline aquifer, thereby diminishing the system’s energy storage capacity [42].

Once the CO₂ is stored in the aquifers, it can be extracted for further use. However, pumping the CO₂ out of the aquifers for further utilization produces moderate results. Some studies demonstrate that 33–50% can be pumped up, but a CO₂ recovery of 50% is required to obtain favorable conditions. Additionally, part of the CO₂ is lost due to several factors. For example, CO₂ can react with minerals and be mineralized. Also, if the temperature drops below 10 °C, the CO₂ can form hydrates with water, and the CO₂ can be dissolved in the formation water [43,44].

2.4. Salt Cavern Storage

Since the 1970s, natural gas has been stored in salt caverns in the USA and Europe, with more than 300 such caverns utilized in Europe alone. Town gas and hydrogen (H₂) have also been successfully stored in salt caverns under similar operational conditions, albeit with different material and safety measures at the surface [40]. Due to its maturity and proven reliability, this technology has achieved a Technology Readiness Level (TRL) of 9.

Salt caverns are artificial cavities in salt deposits, typically found in salt domes. The process involves drilling a well into the salt dome and injecting water to dissolve the salt, thereby creating a cavity with resilient walls that resist reservoir degradation [45]. While these saline caverns have no other commercial value, they can be repurposed for CO₂ storage. However, storing CO₂ in salt caverns has historically been more expensive than other technologies such as aquifers and EOR [29,46,47]. Despite the higher costs, CO₂ recovery for utilization after storage is feasible. Various trapping mechanisms exist when CO₂ is injected into a saline cavern, providing different levels of security and efficiency for long-term storage; see

Table 2. CO₂ trapping mechanisms in saline caves [48,49,50].

Mechanisms	CO2 Trapping Phase	Description
Mineral	Reacted solid phase	Dissolved CO2 reacts with minerals based on Fe, Ca or Mg to form carbonates
Hydrodynamic	Supercritical fluid	Undissolved CO2 is trapped by cap rocks with low permeability
Solubility	Dissolved liquid phase	CO2 is dissolved in the brine water
Residual	Gas phase	CO2 displaces water from the rock pores

.

Estimates suggest that the global theoretical capacity for CO₂ storage in salt caverns could be tens to hundreds of Gt. However, at the moment, the actual amount of CO₂ being stored in salt caverns is relatively small compared to their theoretical capacity. Several pilot and commercial projects have been established for CO₂ storage in salt caverns. The Statoil Sleipner project, for instance, stores about 1 Mt CO₂/yr in a 1 km deep cavern in the Utsira Sand formation in the North Sea, with a storage capacity of 660 million cubic meters [51,52,53]. Another notable example is the Latrobe Valley commercial project, which started in 2015 in Victoria, Australia, with a capacity of 13 Mt CO₂/yr [54].

In Lille Torup, Denmark, seven salt cavern storage sites are located at depths between 1000 and 1700 m. These caverns are 300–350 m high and have 40–60 m diameters, with an estimated total capacity of about 0.3 Mt of supercritical CO₂ per cavern [55]. The caverns are naturally tight due to the unique properties of rock salt, which can be structured in salt diapirs, bedded salt, and salt domes. Optimal pressure for storage depends on the cavern depth and the type of fluid or gas being stored, commonly ranging from 65 to 180 bar at depths of around 1000 m [40]. It is recommended to store CO₂ in the dense liquid or supercritical phase [47].

One of the main drawbacks of this technology is the large amount of wastewater produced. In particular, during the creation of the cavern, significant quantities of brine are pumped to the surface. Proper handling of the brine is crucial to avoid environmental impact. This is typically achieved by mixing the brine with seawater before discharge into the sea [40,46,47].

2.5. Deep Ocean Storage

Oceans, covering more than 70% of the Earth’s surface, are the largest natural carbon sinks. There are two primary processes for ocean CO₂ storage: (1) ocean fertilization and (2) direct CO₂ injection [56].

Ocean fertilization is a biological CO₂ storage method that promotes the growth of oceanic organisms, such as phytoplankton, which integrate CO₂ into their metabolisms through photosynthesis. By stimulating the growth of these organisms, more CO₂ is absorbed from the atmosphere and sequestered in the ocean’s biological systems. This process enhances the natural biological pump, which transports carbon from the surface ocean to the deep ocean, where it can be stored for centuries or longer.

In the direct CO₂ injection method, captured CO₂ is transported to the ocean via pipeline or ship. CO₂ can be injected in various forms, including gas, liquid, solid, or supercritical fluid, as shown in Figure 4. CO₂ can also be injected into gas hydrates, crystalline solids formed from water and gas. Although promising, this technology is not yet ready for widespread practical application [56]. At depths greater than 3 km, CO₂ becomes liquefied and is sequestered at the ocean bed due to its higher density compared to seawater [47,57].

Sheps et al. [58] proposed criteria to compare CO₂ storage systems, highlighting that ocean CO₂ sequestration offers large capacity and medium permanence, with a 30–500-year time scale. It is considered an irreversible process at a relatively low cost. However, this approach is more controversial than other storage methods due to potential environmental impacts.

At depths greater than 3 km, CO₂ becomes denser than seawater, facilitating its sequestration at the ocean bed. The theoretical maximum capacity for CO₂ storage in the deep ocean is vast, estimated to be thousands of Gt. One major concern is the impact on seawater chemistry. Injected CO₂ reacts with seawater to form carbonic acid (H₂CO₃) and hydrogen ions (H⁺), leading to ocean acidification. This acidification reduces the seawater pH and can significantly harm marine ecosystems, affecting species such as coral, shellfish, and plankton sensitive to pH changes [56,59,60]. Although the vast volume of seawater could theoretically buffer these changes over time, the localized and long-term impacts on marine life could be severe, with potential consequences for the broader marine food web and biodiversity [56].

3. CO₂ Utilization Technologies

As previously discussed, the CCS approach alone does not solve the global warming problem. Even technologies like EOR fail to help us reach the net-zero CO₂ emissions target, as they lead to the production of more fossil fuels. Moreover, techniques like deep ocean storage can harm natural media. Additionally, CCS has geological constraints since there is not enough for permanent storage at current CO₂ emission rates, as illustrated in Figure 5. This means that we need to look at alternatives to CO₂ storage.

CCU is an innovative approach that focuses on repurposing captured CO₂ for various applications rather than storing it. The objective is to purify and utilize CO₂ in industrial processes or to transform it into valuable products such as chemicals, materials, or fuels. Capturing CO₂, converting it into fuels, and subsequently using these fuels delays the ultimate release of CO₂ into the atmosphere.

This approach primarily reduces the consumption of fossil fuels, thereby decreasing overall CO₂ emissions. It promotes the concept of an artificial CO₂ cycle, which involves capturing CO₂, converting it into valuable products, optionally storing it temporarily, using it, and then capturing it again. While this cyclic process helps mitigate CO₂ emissions, it is essential to recognize that it does not eliminate them. Consequently, CCU should be integrated with other strategies to achieve comprehensive emission reduction targets.

3.1. Non-Energy Applications

The global consumption of CO₂ as a chemical reagent has risen in recent years. It was about ≈150 Mt/yr by 2000 and increased to ≈250 Mt/yr by 2022. In 2015, the major markets were urea for fertilizer production and EOR; see Figure 6a. Based on an average year-on-year growth rate of 1.7%, global CO₂ consumption is expected to rise in the years to come, as shown in Figure 6b. The three major CO₂ markets are the USA (33%), China (22%), and Europe (16%) [62,63].

The CCU approach is expected to produce revenues, reducing pressure on energy costs, and it can create economic benefits and job opportunities [64]. The Global CO₂ Initiative shows an annual market on CO₂-based products of USD 0.8–1.1 trillion using about 10% of the global CO₂ emissions. Table 3 displays how CO₂ is already used in various sectors, such as oil, power, chemical, food, steel, and pharmaceutical industries.

The highest potential for direct CO₂ utilization lies within the oil and chemical industries. Urea production represents a significant demand area, estimated at up to 300 Mt/yr. Polymer processing and synthesizing renewable fuels, such as methanol, are key fields for CO₂ utilization. The cement industry also demonstrates substantial potential, with an estimated demand of approximately 300 Mt/yr. The food sector has moderate potential for CO₂ utilization, primarily in applications such as packaging, decaffeination, horticulture, and beverage carbonation [65,66].

Table 3. State of the art for CCU [67,68,69,70,71,72].

Potential	Process/Product	Sector
High	Oil extraction	Enhanced Oil, Gas and Coal Recovery
		Stimulation/Fracturing of oil and gas
		Polymer Processing
		Chemicals and Fuels (Methanol, methane, CO, fertilizers and derivatives)
Medium	Food processing	Coffee decaffeinating
		Wine production
		Beverage carbonation
		Dry ice production
		Horticulture (greenhouses)
		Food packaging and preservation
Medium	Mineralization	Calcium and magnesium carbonate for use in cement
		Calcium bicarbonate
		Bauxite residue treatment
		CO2 concrete curing
Medium	Power	Working medium in CO2 cycles
		Heat pumps
Low	Steel	Bottom stirring agent in basic oxygen furnaces
		Injection to metal casting
		Hardening sand cores and molds
		Chilling medium
Low	Pharmaceutical	Chemical synthesis
		Supercritical Fluid Extraction
		Product transportation
		Inerting
Low	Pulp and Paper	pH reduction during washing
Low	Energy crops	Algae cultivation
Low	Other	Electronics (printed circuit manufacture)
		Pneumatic (working medium in hand tools and equipment)
		Fire extinguishers, fire suspension
		Urea production
		Flavors, Fragrances
		Blanket Products
		Aerosols and propellants
		Soda ash production for glass industry
		Welding (shield gas)
		Dry gas cleaning
		Refrigerant gas
		Water treatment

Table 3. State of the art for CCU [67,68,69,70,71,72].

Potential	Process/Product	Sector
High	Oil extraction	Enhanced Oil, Gas and Coal Recovery
		Stimulation/Fracturing of oil and gas
		Polymer Processing
		Chemicals and Fuels (Methanol, methane, CO, fertilizers and derivatives)
Medium	Food processing	Coffee decaffeinating
		Wine production
		Beverage carbonation
		Dry ice production
		Horticulture (greenhouses)
		Food packaging and preservation
Medium	Mineralization	Calcium and magnesium carbonate for use in cement
		Calcium bicarbonate
		Bauxite residue treatment
		CO2 concrete curing
Medium	Power	Working medium in CO2 cycles
		Heat pumps
Low	Steel	Bottom stirring agent in basic oxygen furnaces
		Injection to metal casting
		Hardening sand cores and molds
		Chilling medium
Low	Pharmaceutical	Chemical synthesis
		Supercritical Fluid Extraction
		Product transportation
		Inerting
Low	Pulp and Paper	pH reduction during washing
Low	Energy crops	Algae cultivation
Low	Other	Electronics (printed circuit manufacture)
		Pneumatic (working medium in hand tools and equipment)
		Fire extinguishers, fire suspension
		Urea production
		Flavors, Fragrances
		Blanket Products
		Aerosols and propellants
		Soda ash production for glass industry
		Welding (shield gas)
		Dry gas cleaning
		Refrigerant gas
		Water treatment

3.2. CO₂ Applications in the Energy Sector

The US Department of Energy actively supports research into CCU and reuse technologies [73]. Similarly, the European Commission has launched several initiatives under the Horizon 2020 framework, including projects such as BIOTEC, LCE 25, SPIRE, and NMBP 19 and 20, to explore CCU pathways for the production of chemicals, intermediates, and alternative fuels [74].

There is significant potential for CCU in the power and fuel sectors, with applications spanning coal, biomass, natural gas, waste incineration, and small power plants, as well as nuclear plants [64,65,73,75,76].

As previously mentioned, expanding renewable technologies is crucial for reducing greenhouse gas (GHG) emissions. However, the expansion of renewable energy sources intensifies the fluctuation of supply and demand curves due to the unpredictability of weather-dependent energy production. Power systems with a high fraction of energy produced by renewable sources, such as wind or solar, may experience periodic surpluses in electricity production (SEP). In some cases, SEP exceeds the capacity of existing transmission lines, making it impossible to export the surplus. This unexportable SEP is termed critical electricity excess production (CEEP) and represents low-cost electricity that must either be stored or dissipated [17].

The term “green H₂” refers to hydrogen produced via water electrolysis using renewable energy, preferably from CEEP. Green H₂ is recognized as a versatile energy carrier to address decarbonization and support the renewable energy transition. H₂ can be applied in the chemical sector and the steel industry and can also be used as a direct fuel for transportation or electricity production via fuel cells [77]. In addition to its use as a fuel, H₂ can be combined with CO₂ (preferably captured CO₂) to produce a wide range of energy carriers through fuel synthesis processes suitable for both mobile and stationary applications [78]. Currently, CO₂ hydrogenation enables the production of chemicals such as methane, methanol, ethanol, and formic acid, which can serve as alternatives to fossil fuels or feedstocks for producing complex chemical compounds.

The group of technologies that transform electrical energy into valuable chemicals using H₂ is known as “Power-to-X”, where X represents various chemical products, gases (e.g., methane), liquid fuels, or electricity (see Figure 7). The Power-to-Gas approach involves using green H₂ to produce CH₄. Although CH₄ is a carbon-based fuel, it offers several advantages over H₂. For instance, the storage and distribution of H₂ present significant technical challenges, whereas there is an existing infrastructure for methane transportation and distribution via pipelines, liquefied natural gas (LNG) carriers, and gas turbines for converting methane into heat and electricity. The produced Synthetic Natural Gas (SNG) can be injected into the natural gas network [79,80,81].

3.2.1. The Catalytic Sabatier Reaction

The Sabatier process combines CO₂ with H₂ to produce CH₄:

$${4\mathrm{H}}_2+{\mathrm{CO}}_2\leftrightarrows {\mathrm{CH}}_4+2{\mathrm{H}}_2\mathrm{O}$$

(1)

$$\Delta H_{298\mathrm{K}}=-165\mathrm{kJ}/\mathrm{mol}$$

This reaction is a linear combination of the reverse direction of the steam reforming, Equation (2), and the water–gas shift (WSG), Equation (3), reactions [82]:

$$\mathrm{CO}+3{\mathrm{H}}_2\leftrightarrows {\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}$$

(2)

$$\Delta H^o=-206.2\mathrm{kJ}/\mathrm{mol}$$

$${\mathrm{CO}}_2+{\mathrm{H}}_2\leftrightarrows \mathrm{CO}+{\mathrm{H}}_2\mathrm{O}$$

(3)

$$\Delta H_{298\mathrm{K}}=41\mathrm{kJ}/\mathrm{mol}$$

The French chemist Paul Sabatier and his co-worker Abbé Senderens discovered this catalytic methanation process at the beginning of the 20th century. The technology has been mainly applied to remove carbon oxide traces from feed gas for ammonia synthesis [83]. Nowadays, the conversion of CO₂ to CH₄ is gaining interest due to its application in areas such as biogas upgrading, to transform the bio-CO₂ into additional biomethane [84]. The Sabatier synthesis has a TRL of 9 [84].

The catalytic Sabatier process is well understood and used for industrial applications such as ammonia production. As the reaction is exothermic, higher temperatures will thermodynamically hinder the process. Therefore, it is necessary to increase the pressure to ensure conversion. Typical operating conditions are between 300 and 800 °C with a pressure of up to 20 bar [85]. The process requires a pretreatment to remove impurities such as H₂S and siloxanes [86].

One of the challenges for the methanation process is catalyst development [80]. Several metals may be used as catalysts for the methanation reaction: ruthenium (Ru), rhodium (Rh), cobalt (Co), and nickel (Ni). However, Ni is considered the most optimum catalyst choice, owing to the good CH₄ selectivity, high activity, and low price [87].

The highest CH₄ yield is obtained at high pressure and low temperature. A yield of 98% is obtained with a temperature below 300 °C and at 10 bar. The yield decreases drastically at temperatures above 600 °C, depending on the pressure [88]. Nevertheless, a high-pressure operation is not economical, and a low-temperature operation requires a highly active catalyst. The methane yield at 99.5% is possible with an energy efficiency of around 77%, as seen in Figure 8:

The excess heat can be used in other parts of the process to improve energy efficiency. Currently, there are different configurations for the catalyst reactors. Generally, fixed-bed reactors present the advantages of easy utilization; thus, it is the most frequently used reactor [85]. To optimize the catalyst behavior and heat management, there are different methanation processes based on several fixed-bed reactors in series with intermediate cooling [89]. More recent research has used four fixed-bed reactors in series with integrated air cooling systems with intermediate heaters to pre-heat the gas mixture [90]. New reactors are being developed, such as microreactors and three-phase methanation [80,81,91].

3.2.2. The Biological Sabatier Reaction

This process presents the advantages of minimal chemical applications and lower energy requirements. Despite being known for many years, it is still at the demonstration level [92]. It can be accomplished using two main processes: an in situ reactor and a separate reactor. The in situ process is typically used in anaerobic digestion systems to increase the proportion of CH₄ over CO₂ of biogas by injecting H₂ directly into the digester. This process does not need a second reactor and reduces the investment, but total CO₂ conversion is very difficult [87]. Alternatively, a separate biological reactor can convert captured CO₂ from biogas or any other source. This option presents the advantage of allowing the conditions to be adjusted according to the requirements of the hydrogenotrophic methanogens. However, the main challenge for this reactor design is the H₂ supply for the microorganisms because of the limit rate.

Biological methanation is a simple way to upgrade biogas but harms the anaerobic process due to some inhibition mechanisms. For example, CO₂ removal may increase the pH value. Thus, in situ, biological methanation may require co-digestion with acidic substrates or pH control to keep the pH within appropriate levels [93]. The heat excess from the biological methanation is not high enough to overheat the anaerobic digestion process but is still high enough to be worth recuperation for other uses [81].

Biological methanation operates at temperatures of 30–60 °C and atmospheric pressure and, in contrast to the catalytic process, has a high tolerance against pollutants in the feed gas. Previous studies [94,95] showed mesophilic and thermophilic anaerobic digestion to convert CO₂ to CH₄ by adding H₂. At thermophilic temperature (55 °C), bio-conversion of CO₂ and H₂ produced results 60% higher than at mesophilic temperature (37 °C), providing higher efficiency for biogas upgrading. The CH₄ produced can reach around 90–95% of CH₄ [93], the rest being a mixture of H₂ and CO₂. Due to the exothermicity of the biological reaction, the overall energy efficiency is around 80% [86].

Despite biological methanation having low kinetics and flexibility, its use is increasing due to the rise of biogas production [80,81,91]. Consequently, it is possible to conclude that a biological process is best suited for small to medium-sized plants, preferably biogas production plants. In contrast, catalytic processes seem more suitable for larger plant sizes. The fixed-bed reactor is a well-known and well-suited technology for large-scale methanation plants (>100 MW) [87].

3.2.3. Dry CO₂ Reforming

Dry CO₂ reforming is an attractive solution to produce syngas from CO₂ and CH₄. The produced syngas can be further used to produce fuels and chemicals via fossil-free petrochemical methods.

$${\mathrm{CH}}_4+{\mathrm{CO}}_2\to 2\mathrm{CO}+2{\mathrm{H}}_2$$

(4)

$$\Delta H_{298\mathrm{K}}=247\mathrm{kJ}/\mathrm{mol}$$

The reaction is endothermic and requires a catalyst, typically Ni-based. The catalyst may suffer from fouling due to carbon deposition from undesired reactions such as:

$${\mathrm{CH}}_4\to \mathrm{C}+2{\mathrm{H}}_2$$

(5)

This fouling can be avoided by the interaction of a wide size distribution of Ni with the metal oxide surface as a support material [96,97,98]. The selection of MgAl₂O₄ as a metal oxide can slow the formation of carbon deposits [99,100,101,102]. However, higher catalytic activity has been acquired with the usage of rhodium (Rh), platinum (Pt), iridium (Ir), and lead (Pb) [103]. This reaction can achieve a conversion efficiency of up to 98% at 600–800 °C and 1 atm [30].

The redox cycle with CeO₂ offers an attractive thermodynamic conversion over a metal oxide catalyst. In this cycle, the metal oxide takes oxygen from the CO₂, forming CO. Secondly, the catalyst is regenerated by releasing oxygen. The CeO₂ catalyst forms H₂ from water at 900–1000 °C and offers an efficiency of 19–22%. Even though CeO₂ is the most commonly studied catalyst for this reaction, this technology has not yet been applied commercially and is still not sufficiently mature [104,105,106].

3.2.4. Methanol Production

Methanol is a suitable energy carrier for the fuel and chemical industry due to its flexible properties [81,107,108]. The main route for methanol production is steam reforming of natural gas, but it can also be produced from non-fossil H₂ by water electrolysis, biomass conversion, or solar conversion [109,110,111].

Methanol is also the base to produce many other chemicals. It can be the basis for transportation fuel production, such as polyoxy dimethyl ether, dimethyl ether, and higher-alcohol-content products like butanol [112,113]. It can also be applied as a gasoline additive [86,109,114]. Since methanol has a high octane rating, it can also be used as a gasoline substitute in car engines [113].

Methanol can be produced from syngas. This synthesis takes place at 250–300 °C and 50–100 bar, with the use of Cu/ZnO as catalyst together with Al₂O₃ as promoter [107,108,109,110,111,115].

$$\mathrm{CO}+2{\mathrm{H}}_2\leftrightarrows {\mathrm{CH}}_3\mathrm{OH}$$

(6)

$$\Delta H^o=-90.5\mathrm{kJ}/\mathrm{mol}$$

Methanol can also be synthesized from CO₂ by including an intermediate step of converting CO₂ to CO by the reverse water–gas shift (RWGS) reaction, resulting in:

$${\mathrm{CO}}_2+3{\mathrm{H}}_2\leftrightarrows {\mathrm{CH}}_3\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}$$

(7)

$$\Delta H^o=-49.5\mathrm{kJ}/\mathrm{mol}$$

The CO₂ hydrogenation reaction produces water as a by-product; thus, a third of the H₂ is converted to H₂O. This water conversion ratio is much higher than in commercial methanol synthesis from syngas. Furthermore, the thermodynamic properties of direct methanol production from CO₂ are not as favorable as those from CO. The methanol yield from CO₂ at 200 °C is about 40% less, while the yield from CO is higher than 80%. This is mainly due to the Cu/ZnO catalyst and Al₂O, which makes the methanol production from syngas easy, showing lower reactivity from CO₂ at temperatures below 250 °C.

Despite the reactivity increase by raising the temperature, this leads to the formation of CO and H₂O by RWGS. Consequently, additional H₂ is consumed, and the methanol production decreases [109]. Therefore, the efficiency depends enormously on the temperature and can be between 30 and 92%, while higher temperatures drop the efficiency rate. The excess heat can be used in high-temperature electrolysis, improving the overall efficiency of the system [116]. The efficiency for methanol production by direct hydrogenation of CO₂ is reported to be around 79% with a methanol selectivity above 99.8%; see Figure 9.

Methanol synthesis from CO₂ has become technically competitive with the syngas route due to the development of practical reactors and effective catalysts [22,109,111]. Several companies offer commercial solutions, such as Luigi, Mitsubishi, and Haldor Topsoe [109]. Iceland’s George Olah Renewable Methanol Plant, run by Carbon Recycling International (CRI), is one of the largest commercial production plants, with 4000 tons/y. The facility reacts H₂ produced from water electrolysis using electricity from a geothermal power plant and CO₂ from an integrated capture system [78,107,110,117]. The TRL of this technology is estimated to be eight and could soon reach nine [108].

3.2.5. Fischer–Tropsch Process

The Fischer–Tropsch process consists of the hydrogenation of CO to produce a variety of higher hydrocarbons (C₅₊), mainly alkanes and alkenes. The mixture is called syncrude and can be refined into fuels such as diesel or gasoline [114,118,119]. These fuels can be used in internal combustion engines, releasing fewer emissions compared to conventional gasoline or diesel, as Fischer–Tropsch fuels have a lower concentration of N₂, aromatics, and sulfur [120,121].

The CO applied for Fischer–Tropsch fuel production can be obtained by various pathways, such as CO₂ SOEC electrolysis, biomass gasification producing bio-syngas, and from RWGS reaction from CO₂ [114,116,118,120,122]. The resulting syngas is fed to the synthesis reactor to produce a mixture of long-chain hydrocarbons; see Reaction 8:

$$\mathrm{CO}+2{\mathrm{H}}_2\to -{\mathrm{CH}}_2-+{\mathrm{H}}_2\mathrm{O}$$

(8)

$$\Delta H^o=-158.5\mathrm{kJ}/\mathrm{mol}$$

This reaction is a simplified version of several simultaneous reactions. The -CH₂- is the carbon chain from which higher hydrocarbons are generated (C₁–C₇₀₊) [123]. Gasoline consists of a hydrocarbon chain between C₄ and C₁₂, while the diesel carbon chain is in the range of C₁₀–C₂₀ [124]. The mixture and the carbon chain length depend on operation conditions, catalyst, and the syngas composition [81,118,119,120]. This process can produce other fuels, like aviation biofuels, which have gained relevant interest due to the possibility of reducing emissions in the aviation sector. The synthetic diesel produced by this technology can also be applied in the shipping sector. Thus, all transportation sectors can reduce GHG emissions by integrating this approach.

The Fischer–Tropsch process can operate at different temperatures. The low- temperature Fischer–Tropsch process occurs between 210 and 260 °C, while the high-temperature Fischer–Tropsch process operates at 310–340 °C. The operation temperature determines the fuel properties and the carbon number. High-temperature Fischer–Tropsch fuels are mainly olefins and aromatics, whereas low-temperature Fischer–Tropsch fuels consist primarily of paraffin. Thus, the high-temperature Fischer-Tropsch process is more suitable for gasoline production due to the high octane number of aromatics. The low-temperature Fischer–Tropsch process is applied for diesel production because of the high cetane number of paraffinic compounds [125,126]. Typically, this process applies fixed- or fluidized-bed reactors operating at 10–60-bar pressures with iron-based or cobalt-based catalysts together with a supporter, usually Al₂O₃ [120,127].

The total efficiency of the Fischer–Tropsch process to produce hydrocarbon with a carbon chain of C₅₊ is typically in the range of 60–90% with a selectivity of about 85% [81,118,128]. The Low Fischer–Tropsch process selectivity for diesel production is much lower, about 26%; see Figure 10. This result includes waxes and by-products.

The Fischer–Tropsch product is then separated and refined to obtain the final fuel. The excess heat can be used elsewhere in the system to improve efficiency. The surplus heat is estimated to be 6.57 MJ steam/kg CO₂. These data are in the range of the heat demand of 5.4–7.2 MJ/kg CO₂ at 95 °C for CO₂ capture from air. The excess heat can also be applied to the electrolysis process, increasing the final efficiency [112].

The Fischer–Tropsch process or Power-to-X was developed by Franz Fischer and Hans Tropsch in the Kaiser Willhelm Institute for Coal Research at Mülheim (Ruhr) in Germany in the 1920s from hydrocarbon production from coal gasification products [130,131]. Germany applied this process during the Second World War to extend the Nazi’s fuel. This approach has obtained considerable interest in producing sustainable synthetic liquid fuels [132]. Sunfire inaugurated a demonstration plant in Dresden (Germany) in 2014. The facility produces synthetic diesel using the Fischer–Tropsch process. The excess heat of the process is applied to heat the Solid Oxide Electrolysis Cell (SOEC) to produce H₂, improving the overall efficiency [116]. This process is entirely commercial, and a variety of fuels are made all over the world. Nevertheless, most facilities use non-renewable feedstock with CO₂ from emitting processes. The Fischer–Tropsch process integrated with renewable CO₂ pathways is not very mature yet. That is why the TRL is between 5 and 9 [133].

3.2.6. CO₂ Electrochemical Reduction

CO₂ electrochemical reduction (CO₂ECR) can generate valuable products such as CO, methanol, ethanol, formic acid, oxalic acid, methane, and formaldehyde based on a specific reaction promoted by catalysts and solvents in an electrolysis cell. This reaction’s selectivity and faradaic efficiency depend on several parameters: pH, electrode potential, selected electrolyte, and catalyst. The generation of H₂ from water is one of the main challenges of CO₂ECR due to the energy cost. Nevertheless, this can be avoided using aprotic or non-aqueous solvents and a cathode with minimal H₂ over-potential. However, more reliable, scalable, and cost-effective electrolyzer designs are still required for the future application of CO₂ECR [134]. Kibria et al. [135] disclose a Life Cycle Assessment (LCA) comparing CO₂ECR for fuel and feedstock production. They conclude that CO₂ECR must be combined with low-carbon electricity production for lower GHG emission benefits.

CO₂ECR reactions occur at the electrolysis cathode surface where CO₂ is reduced to form products like HCOOH, CO, or OH⁻. In contrast, oxidation reactions such as oxygen evolution, amine oxidation reaction, and H₂ oxidation reaction occur at the anode depending on the electrocatalyst and the anode electrolyte [136,137].

The CO₂ molecules are adsorbed to the cathode to an active anion radical form at 25 °C and 1 atm pressure. The electrolyzer’s ion exchange membrane acts as a barrier between the cathode and anode, allowing the transport of selective ions while reducing the transport of all other products, avoiding the reverse oxidation of the product to the anode. An electrolyte solution, such as KOH or NAHCO₃, conducts electricity, delivering and dissolving CO₂ to the cathode. On the other hand, electrons are transferred by a power source with enough voltage from the anode to the cathode [134]. The CO₂ECR includes several steps:

(1)

CO₂ transport from gas phase to electrolyte bulk.

(2)

CO₂ diffusing through the electrolyte to the electrolyte/cathode interface.

(3)

CO₂ adsorption at the cathode.

(4)

Intermediate product production at the cathode, such as CHO, COH, COOH, and CO.

(5)

Intermediate electrons coming from the cathode catalyst.

(6)

Product desorption from the electrode.

(7)

Reduced products transfer into liquid phases or bulk gas.

The electrolysis design controls the amount of electrolyte required for the reaction. The cell design affects the electrolyte requirement, gas diffusion, cathode area, and other parameters to assure the reaction performance at a low cost [30]. The ion exchange membrane, type of catalyst, electrodes, and electrolyzer cell configuration influence the CO₂ECR process, including CO₂ conversion rate, long-term stability, Faraday efficiency, and energy efficiency. CO₂ECR allows a modular design in combination with CO₂ capture and CO₂ utilization technologies since it operates at mild conditions. To reduce CAPEX in the entire process from CO₂ capture to CCU, the CO₂ECR process should be operated at a high Faraday efficiency, CO₂ conversion rate, current density, and energy efficiency [134]. Roh et al. [138] analyzed the potential of ten products from electrochemical CO₂ reduction at TRL 2.

3.2.7. Photocatalytic CO₂ Reduction

Photocatalysis is an advanced oxidation process that converts inorganic and organic pollutants into non-hazardous compounds. This method utilizes photon irradiation to accelerate redox reactions, effectively transforming solar energy into chemical energy and lowering the activation energy required for chemical transformations. It is recognized as an advanced, environmentally friendly, and cost-effective technology for reducing pollutants in wastewater treatment applications [139]. CO₂ photocatalytic reduction (CO₂PCR) involves using a semiconductor catalyst to convert CO₂ into valuable products. In photocatalysis, photons of light with energies equal to or greater than the semiconductor’s band gap energy (${\mathrm{E}}_{bg}$) excite electrons (e⁻) and create holes ($h^{+}$) in the semiconductor’s conduction and valence bands, respectively [30]. CO₂PCR is noted for its energy efficiency compared to CO₂ electrochemical reduction (CO₂ECR), as it operates under milder conditions.

CO₂ is a very inert and stable molecule due to its high C-O bond energy (750 kJ/mol), necessitating elevated pressures and temperatures for thermocatalytic conversion. However, CO₂ photocatalytic reduction (CO₂PCR) can be optimized using semiconductor materials to produce valuable products such as CH₄, CH₂O, HCOOH, or methanol [140]. Additionally, the CO₂PCR process can generate C2 products from C-C coupling reactions, including CH₃CH₂OH, C₂H₄, and C₂H₆ [141,142,143].

Current research focuses on enhancing the energy efficiency of the CO₂PCR process on semiconductors. TiO₂-based semiconductors are the most extensively studied due to their high photoactivity. Other semiconductor materials, such as CuO and Cu₂O, have also garnered significant attention. Further studies have explored materials like Ta₂O₅, MgO, Ga₂O₃, In₂O₃, Bi₂MoO₆, and Bi₄Ti₃O₁₂. These materials can be doped with metals such as Fe, Cu, Pd, Cr, Pt, Co, Ni, Ag, or Ru to reduce charge recombination. Additionally, several combined semiconductor systems have been extensively studied to improve process performance in recent decades. These include TiO₂/polydopamine, CeO₂/3D g-C₃N₄, CsPbBr₃/Bi₂WO₆, and MXene/Bi₂WO₆ [139].

The design of a hybrid system based on photoelectrochemical cells combined with a photovoltaic cell has shown promising results by separating the oxidation reduction reactions into two subsystems, thereby increasing process optimization. Additionally, this hybrid approach maintains high solar energy conversion efficiency into fuel without requiring precious materials. Despite these advancements, the photocatalytic reduction of CO₂ currently has a low Technology Readiness Level (TRL) of 3–4 [139,144].

The first step in the CO₂ photocatalytic reduction (CO₂PCR) process is to capture and absorb light to generate efficient photogenerated electrons (e⁻) and holes (h⁺). This photogenerated e⁻ must possess an adequate reduction potential to achieve the desired product selectivity and initiate the specific reduction reactions. However, the process still faces several challenges, including insufficient light absorption, photocorrosion, and low reaction kinetics. The CO₂PCR process begins with the adsorption of CO₂ onto the photocatalyst’s surface, followed by hydrogenation, deoxygenation reactions, and the formation of C-O bonds with the photocatalyst’s surface atoms. The sequence of hydrogenation and deoxygenation reactions is crucial in determining product selectivity. The adsorption of CO₂ can follow three pathways, which dictate the reaction route:

(a)

Carbon coordination.

(b)

O₂ coordination.

(c)

Mixed coordination.

Therefore, understanding the fundamental chemical pathways is essential for significantly improving the CO₂ photocatalytic reduction (CO₂PCR) process. This understanding offers two potential advantages: (1) lowering the key thermodynamic or kinetic barriers of the intermediate reactions to increase the CO₂ conversion rate, and (2) regulating the process to generate the desired product. Potential intermediates can significantly impact the reaction pathways, influencing product selectivity [139,143]. Two proposed pathways are prominent in the CO₂PCR process: the formaldehyde and carbene pathways. The formaldehyde pathway reduces CO₂ to produce CH₄, while the carbene pathway can reduce CO₂ to produce either CH₄ or methanol, depending on the specific conditions. The formaldehyde pathway follows the reaction outlined in Equation (9)

$${\mathrm{CO}}_2\to \mathrm{HCOOH}\to {\mathrm{H}}_2\mathrm{CO}\to {\mathrm{CH}}_3\mathrm{OH}\to {\mathrm{CH}}_4,$$

(9)

while the carbene pathways follows Reaction (10):

$${\mathrm{CO}}_2\to \mathrm{CO}\to \mathrm{C}\to {\mathrm{CH}}_3\to {\mathrm{CH}}_3\mathrm{OH}/{\mathrm{CH}}_4.$$

(10)

Methanol is formed faster than methane with aqueous photocatalysis [145]. This is due to the formation of bicarbonate, carbonic acid, and carbonate species, resulting in the generation of HCOOH and final reduction to CH₃OH. The methane production may also produce heavier hydrocarbons like CH₃COCH₃ and CH₃COOH [146].

3.2.8. CO₂ Fuel Cells

Molten carbonate fuel cells (MCFCs) can capture and convert CO₂ into heat and power [147]. Thus, CO₂ fuel cells can also be considered a CO₂ capture technology. These fuel cells require H₂ at the anode and use CO₂ and O₂ as input at the cathode. At the cathode catalyst (Equations (12)), CO₂ reacts with O₂ generating ${\mathrm{CO}}_3^{2-}$ that migrates through the membrane towards the anode, as shown in Equation (12), and reacts with H₂ to produce CO₂ and H₂O [148]. Consequently, this method allows the simultaneous purification of CO₂ containing currents and the production of electrical energy.

$$\mathrm{A}\mathrm{n}\mathrm{o}\mathrm{d}\mathrm{e}:{\mathrm{CO}}_3^{2-}+{\mathrm{H}}_2\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+2e^{-}$$

(11)

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{d}\mathrm{e}:2{\mathrm{CO}}_2+{\mathrm{O}}_2+2e^{-}\to 2{\mathrm{CO}}_3^{2-}$$

(12)

CO₂ in flue gases from power plants or industries can be directly fed into an MCFC, as illustrated in Figure 11. The MCFC electrolyte plays the role of a membrane that allows capturing CO₂ from the flue gas with additional heat and power generation. The anodic outflow of CO₂ and H₂O can be separated by water condensation. MCFC process performance depends on the CO₂ concentration. Therefore, as standard flue gases present a relatively low CO₂ concentration, between 4 and 15%, CO₂ recirculation may be used. Removal of flue gas impurities is required to avoid undesired electrode processes.

The generated heat and power can also be recovered and utilized in a power plant or other applications [148]. A combined cycle power plant integrated with molten carbonate fuel cells (MCFCs) can capture up to 90% of the CO₂ in the flue gas while generating additional heat and power, thereby increasing the overall efficiency of the process. This technology is considered to have a Technology Readiness Level (TRL) of 5 [149].

3.2.9. Dimethyl Ether and Dimethyl Carbonate Synthesis

Dimethyl ether (DME) is an organic compound primarily used as an aerosol propellant and a reagent in producing dimethyl sulfate and acetic acid compounds. There are two routes for its production: the direct route (TRL 4), which converts syngas using a bi-functional catalyst (Cu/ZnO/Al₂O₃), and the indirect route (TRL 9), which involves the dehydration of methanol. Although the indirect route is more commonly employed, the direct route is considered more economically and thermodynamically feasible [81,108]. DME serves as an alternative fuel to conventional diesel. Its combustion produces no aromatics or sulfur, resulting in lower emissions of NOx, SOx, CO, and particulate matter compared to traditional diesel [107,150,151,152]. Additionally, DME can be utilized as liquified petroleum gas (LPG) for heating purposes [150,153].

Dimethyl carbonate (DMC) is a valuable intermediate in organic synthesis and a fuel additive. Its diverse applications stem from its favorable chemical and physical properties [154,155]. DMC can be used in green adhesives and as a gasoline additive, owing to its high oxygen content compared to other chemicals [156,157]. Other applications include the production of antibacterial medicines, veterinary drugs, and insecticides [158,159,160]. One of the most critical future applications for DMC in the energy sector is its use as a battery electrolyte, which can enhance conductivity, current density, and battery life [161]. There are several methods for the synthesis of DMC: the phosgene method, oxidative carbonylation of methanol, transesterification method, and direct DMC production from methanol [162,163,164]. The direct route faces two main challenges. First, the reaction’s inherent limitations and the high inertia of CO₂ necessitate the use of an effective catalyst. Second, the efficiency and reusability of the selected catalyst are critical factors that influence the overall feasibility and cost-effectiveness of the process [165]. The direct DMC production (TRL 3) is explained in Equation (13):

$${\mathrm{CO}}_2+{\mathrm{CH}}_3\mathrm{OH}\to \mathrm{DMC}+{\mathrm{H}}_2\mathrm{O}$$

(13)

4. Discussion

The main target set by the Intergovernmental Panel on Climate Change (IPCC) is to limit global temperature increase to 1.5 °C by 2050. Achieving this goal requires reducing global net CO₂ emissions to zero emissions by 2050.

CCS plays a crucial role in that target by capturing and securely storing CO₂ emissions, thereby preventing them from entering the atmosphere. This technology is essential for mitigating emissions from challenging sectors that need decarbonization, complementing renewable energy adoption, and enhancing overall carbon reduction strategies. On the other hand, CCU offers a promising approach to help meet that target by using CO₂ by-products from industrial processes within the facilities of the capturing companies. This avoids the costs associated with emission tariffs and provides opportunities to generate valuable products, thereby opening new business lines in various industries.

Currently, the majority of captured CO₂ is destined for CCS, with about 95% of the CO₂ captured being geologically stored. Only a small fraction, less than 5%, is destined for CCU, primarily for producing synthetic fuels and chemicals. Nowadays, one of the primary uses for captured CO₂ is EOR, as depicted in Figure 6a, mainly due to the economic return it provides.

CCU faces several challenges that hinder its widespread adoption. The primary obstacles include high initial capital investment, technological barriers, and the need for substantial advancements in catalyst efficiency and reactor design to achieve high conversion rates and selectivities. To make CCU economically feasible, it is essential to focus on reducing the costs of key processes, such as hydrogen production via electrolysis, by leveraging economies of scale and advancements in renewable energy technologies. Additionally, developing efficient and scalable technologies for CO₂ conversion and creating supportive regulatory frameworks and financial incentives can further promote the adoption of CCU.

By 2050, the expectation is that the role of CCU will grow as technologies mature and become more economically feasible. However, CCS is still projected to be the dominant method for managing captured CO₂ due to its large-scale capacity and longer-term storage solutions [5].

Figure 12 displays the technological development based on the Technology Readiness Level. The matrix shows the energy sector’s maturity of storage and utilization technologies. The TRL scale is divided into three sections: research (TRL 1–3), development (TRL 3–6), and deployment (TRL 7–9). The figure shows that Sabatier methanation and methanol production are well-understood technologies with a TRL of 9 and 8, respectively. Figure 12 shows that both CCU and CCS include technologies that have reached TRL 9. This means that these approaches are scientifically validated and ready for deployment at a commercial scale, providing reliable solutions for reducing CO₂ emissions.

Some of the CCU technologies still require further research [168]. For example, the electrochemical reduction of CO₂ necessitates a highly selective and efficient catalyst and a substantial energy supply. Therefore, more research is needed to better understand the catalyst’s working mechanism, degradation, and morphological changes over time to enhance its longevity and efficiency [169,170]. Photocatalytic CO₂ reduction faces ongoing challenges related to photocatalyst activity, scalability, and limited efficiency. Developing more advanced photocatalysts with higher activity and stability is crucial to making this process more viable on a larger scale. In the case of the biological CO₂ conversion, the technology is currently hindered by slow microorganism growth rates and the inefficiency of biological processes under industrial conditions. Enhancements in microbial engineering and process optimization are necessary to increase the rate of CO₂ conversion. Furthermore, advancements in the purification and separation stages are required to make these processes economically feasible and scalable.

Beyond technical challenges, regulatory approval and public acceptance are critical factors that need to be addressed to successfully implement future Carbon Capture and Utilization (CCU) systems [28]. Public perception can be influenced by clear communication of CCU technologies’ environmental and economic benefits. Developing robust regulatory frameworks that ensure safety, efficiency, and environmental protection will be essential.

Integrating CCS and CCU into a comprehensive Carbon Capture, Utilization, and Storage (CCUS) framework can address these requirements and provide innovative solutions to reduce GHG emissions. CCUS enhances the overall efficiency and effectiveness of CO₂ management and supports the transition towards a circular carbon economy by converting waste CO₂ into useful products. CCUS is, therefore, an integrated approach to mitigate CO₂ emissions that involves the three main stages: capturing CO₂ at the emission source, utilizing the captured CO₂ in various industrial applications, and securely storing the remaining CO₂ in geological formations.

Considering Figure 6a,b, and assuming that 33% of the global CO₂ demand will be used for EOR, the amount of CO₂ available for CCUS in the energy sector is projected to be about 100 Mt/yr by 2030 and approximately 140 Mt/yr by 2050.

The SNG production will increase together with the increase in global CO₂ demand. Thus, the projected SNG share will slowly replace some demand for natural gas and offer the opportunity to use CO₂ that would otherwise go to EOR projects. Theoretically, natural gas demand could be supplied with the methanation of approximately 8–10 Mt/yr of captured CO₂, a small fraction of the current emissions. The energy transition will probably continue to rely on natural gas due to the fluctuation of the performance of clean energy technologies. Methanol would hopefully represent about 30% of total gasoline demand by 2050. Likewise, methanol and electric cars will replace gasoline engines because many countries will ban selling internal combustion engines after 2035.

Shifting from CCS technologies like EOR to CCU technologies like SNG and methanol production involves several potential challenges and limitations. The current market price disparity is a significant limitation, with hydrogen being approximately 5–6 times more expensive per kWh than synthetic natural gas. This economic disincentive makes using green hydrogen for methane or methanol production less attractive. Additionally, the initial capital investment for SNG and methanol production facilities is substantial. These projects require advanced technologies and specialized infrastructure, such as high-efficiency electrolyzers for hydrogen production and reactors for the Sabatier process or methanol synthesis, which can be cost-prohibitive. Technological barriers include the need for continuous advancements in catalyst efficiency, reactor design, and process integration to achieve high conversion rates and selectivities, which are crucial for economic viability. Furthermore, the scalability and integration of these technologies into existing energy systems pose additional challenges that must be addressed to realize their full potential.

The International Energy Agency (IEA) explores three World Energy Outlook 2022 scenarios based on government policy assumptions to meet IPCC targets. The Stated Policies Scenario (STEPS) follows the current trajectory, the Announced Pledges Scenario (APS) assumes that all government targets will be met on time, and the Net Zero Emission scenario (NZE) aims to limit the global average temperature increase to 1.5 °C [171].

Table 4. Common fuels’ share in the future energy sector. “bcm/yr” stands for billion cubic meters per year and “Mb/d” stands for million barrels per day.

Year	2030			2050
Scenarios	STEPS	APS	NZE	STEPS	APS	NZE
Gas demand (bcm)	4456	4069	3666	4661	3568	2681
SNG share (%)	1.12	1.22	1.36	1.50	1.96	2.61
Gasoline demand (mb/d)	23.2	20.6	-	19.3	8.2	-
Methanol share (%)	6.6	7.4	-	11.1	26.2	-

presents the global projected natural gas and gasoline demand for 2030 and 2050 under these scenarios, along with the theoretical share of SNG and methanol in the future energy market.

5. Conclusions

The technologies for CO₂ capture, utilization, and storage are crucial for achieving the net-zero emission target and limiting global temperature rise to 1.5 °C by 2050. CCUS technologies offer diverse and valuable approaches to managing CO₂ emissions. In addition to CCUS, other strategies are essential for meeting climate goals. These include promoting renewable energy sources, increasing energy system efficiency, and adopting non-CO₂ emitting technologies such as nuclear energy. Each approach complements the others, creating a multifaceted strategy for mitigating climate change and ensuring sustainability.

The maturity of CCS technologies, particularly EOR, EGR, aquifers, and cavern storage, is well established. These technologies have proven effective and reliable for long-term CO₂ sequestration. In contrast, CCU technologies, though promising, are still emerging. CCU technologies that are technically ready (TRL 9) are heavily dependent on the availability of affordable green H₂. Currently, green H₂ is generally more valuable than the potential CCU products (such as CH₄, CH₃OH, DME), which economically limits the development and widespread adoption of CCU. Addressing this economic constraint is crucial for the advancement of CCU.

The Power-to-Gas approach through the Sabatier methanation process can store substantial energy over the long term and provide ancillary services to stabilize the future Smart Energy System. By increasing the production of SNG, the share of renewable energy in the transportation, district heating, and electricity sectors can be significantly increased, facilitating a more sustainable energy transition. However, the efficiency of electrolyzers, which are essential for hydrogen production, is susceptible to fluctuations in renewable energy input. Methanol and DME production processes offer sustainable and renewable alternatives to gasoline and diesel. Both methanol and DME are commercially viable and poised to play essential roles in the future transportation sector. Additionally, Fischer–Tropsch fuels present various possibilities for producing gasoline, diesel, and even jet fuel through renewable pathways. Dimethyl carbonate is another promising chemical with multiple applications, including its use as an electrolyte in lithium-ion batteries.

The future energy system must integrate CCS and CCU technologies to reduce GHG emissions and to effectively ensure energy supply. This integration facilitates a more comprehensive approach to managing CO₂, including capturing, converting, storing, and reusing CO₂. CCUS must address several technical and economic challenges. Key among these is the integration of renewable technologies to ensure a constant flow of affordable green hydrogen for utilization routes.

Further research is required to establish effective performance indicators based on techno-economic analyses and Life Cycle Assessments (LCAs) to evaluate CCUS technologies comprehensively and scientifically. These efforts are critical for overcoming existing challenges and advancing the deployment of these technologies.