Carbon dioxide emission, mitigation and storage technologies pathways

ABSTRACT The Paris Agreement was aimed at reducing the global temperature to no more than 2ºC while continuing efforts to keep it to no more than 1.5 ºC to combat climate change and its detrimental effects. Scientists are more confident than ever that the damage already being caused by climate impacts will get worse as global warming progresses, with increasing adverse effects on human health, ecosystems, and economies. Therefore, to alleviate the anthropogenic emissions of carbon dioxide (CO2) into the atmosphere, carbon capture, utilisation and storage (CCUS) is considered the most urgent and viable approach. This work studied extensively the likely sources of CO2 emissions, via anthropogenic or non-anthropogenic; and the current trends in technology to mitigate CO2. Also, the work provides an extensive discussion on CO2 storage and sequestration and briefly discusses the possible utilisation of CO2. By combining the benefit of direct capturing of CO2 and utilization, with long-term CO2 storage, CCUS is thus an appealing and profitable idea for meeting CO2 reduction targets.


Carbon emissions
Carbon dioxide (CO 2 ) emission is the principal greenhouse gas that poses a threat to the environment on a worldwide scale (Zhang et al., 2019).Of all the greenhouse gases (CO 2 , CH 4 , N 2 O and fluorinated gases), CO 2 accounted for 80% of total global emissions (Ochedi et al., 2021).These emissions are caused mainly by human activities like burning fossil fuels for energy and transportation.Energy consumption has increased exponentially as a result of industrialisation's explosive growth in order to satisfy the ever-growing demands of humanity.Currently, fossil fuel combustion accounts for 85% of all energy needed, eventually leading to CO 2 emissions into the atmosphere and an increase in the concentration of CO 2 (Younas et al., 2020).Even in estimates anticipating substantial increases in energy efficiency paired with the rapid expansion of wind, solar, and bio-fuelled power, the combustion of fossil fuels stays dominant for a long time (C.Xu & Hedin, 2014;Song et al., 2019).Aside from the combustion of fuel, other naturally occurring activities, such as eruptions of volcanoes, are also contributing factors to this rise in CO 2 concentration (Shen et al., 2019).Oil and gas treatment, and chemical processes in the industry.
Sources of carbon emission can be generally classified based on the (non-)involvement of human beings in their generation.Carbon emission sources are either anthropogenic (or human) or non-anthropogenic (or natural) sources.Anthropogenic sources can be further classified based on the five identified Greenhouse gases (GHG) producing economic sectors, which include energy systems, industry, buildings, transport, agriculture, forestry and other land uses (AFOLU) (Lamb et al., 2021).Figure 1 shows that the top three contributory sectors are the energy systems sector with 33%, followed by agriculture, forestry and land use sectors with 22%, while industries contribute 24% of the global carbon emission.

Non-anthropogenic carbon emissions
Natural sources of GHG emissions include forest fires, oceans, and volcanoes.Forest fires are often caused by drought, heat, and lightning, and 90% of the gases emitted from such occurrences are CO 2 (Yue & Gao, 2018).According to the Copernicus Atmosphere Monitoring Service (2022), 409.7 megatonnes of CO 2 were released during the Australian wildfires between September 2019 and January 2020.Global fires cause immediate direct carbon emissions of about two gigatons (Gt) annually to the atmosphere (Van Der Werf et al., 2017).Some of the CO 2 released is reabsorbed by new vegetation growth in the burnt area in the short term.In the long term, however, the net CO 2 uptake level is a function of the use to which the land of the burnt area is put.
Oceans simultaneously serve as carbon sources and sinks-they contribute greatly to regulating the concentration of greenhouse gases in the atmosphere.Approximately 6.12 Gt of CO 2 -eq is released annually by global oceans into the atmosphere (Elliott & Angell, 1987), 60% of which comes from the equatorial Pacific Ocean alone (Yue & Gao, 2018).Large amounts of CO 2 are also injected into the atmosphere alongside water vapour and sulphur dioxide gas during volcanic eruptions.Global CO 2 emission rates for all degassing land and underwater volcanoes are estimated to lie between 0.13 gigatons to 0.44 gigatons per year (USGS, n.d.).This is, however, low compared to CO 2 emissions from human activities (USGS, n.d.).

Anthropogenic carbon emissions
The activities of man have contributed significantly to the level of CO 2 emissions, which has risen by 50% since the dawn of the industrial revolution (NASA, 2022).The demand for services such as food, shelter, business, transportation, telecommunication, and entertainment has greatly increased, with an attendant rise in energy demand.Burning of fossil fuels such as coal, natural gas and oil for industrial processes and transportation is not abating.The carbon content of coal is very high, so coalfired power plants emit more CO 2 for every unit of electricity generated than petroleum or natural gas.Fossil fuel-fired power plants produce around 33%-40% of the global CO 2 emission.For instance, a typical 500 MW power plant emits over 2 million tons of CO 2 annually (Yoro & Daramola, 2020).Despite the push for greater penetration of renewable energy sources into the energy mix worldwide, electric power generation from fossil fuels has remained an attractive choice.For example, in 2018, 65% of total electricity generation was from fossil fuels.The inherent variability of solar and wind power output and the high cost of energy storage is possible factors contributing to this.
The challenge of urbanisation, driven by growing population and income, is causing the demand for goods and services to increase.With an increase in urban population by about 1.3 million people per week and a daily expansion of about 102 square kilometres in urban area size, urban areas account for more than 45% of the carbon footprint globally (Grubb et al., 2022).Massive land use changes, deforestation, large-scale farming activities, and degradation of soils cause a disruption of the natural balancing mechanisms in CO 2 level control.Also, the chemical processes involved in cement production for the construction industry release a vast amount of CO 2 into the atmosphere.Agricultural activities, burning biomass and fossil fuels also influence levels of methane and nitrous oxide in the atmosphere.Various industrial processes and refrigeration also increase the volume of fluorinated gases present in the air.
Due to greater awareness in the developed economies of the world of the magnitude of the challenge posed by climate change, efforts are being made to consciously reduce their carbon footprint of various human activities; this is not the case in many developing economies where such concerns are rarely put into consideration.

Energy systems
The largest percentage of CO2 emissions comes from the energy system.Carbon-dioxide emissions from fossil fuels grew at an annual rate of 1.1% per year between 2015 and 2019 and accounted for approximately twothirds of global anthropogenic GHG emissions (IPCC, 2022).Coal, oil and natural gas accounted for 44%, 34% and 22%, respectively, of energy sector CO2 emissions within the period.Within the energy system, the electricity sector contributed about 36% of carbon emissions in 2019, while industry and transport followed with 22% and 21%, respectively (Clarke et al., 2022).Recent trends indicate a continuous increase in all these sectors despite advances in the deployment of renewable energy, battery storage systems and electric cars.

Agriculture, forestry and other land uses (AFOLU)
Major drivers of carbon emission in the AFOLU sector include conversion of natural ecosystems to agriculture, deforestation, land degradation and urbanisation with the resultant effect on carbon sinks available.Smith et al. (2014) reported that land area available for agricultural activities had been on the decline since 2000.This includes land for the cultivation of temporary and permanent crops, pasture, fallow land and grazing (FAO, 2021).Similarly, a decline of about 178 Mha in available global forest area was recorded within the thirty years from 1990 to 2020 (FAO, 2020).
Although cities, towns, villages and human infrastructure occupy about 1% of global land, urban clusters have expanded approximately 2.5 times since 1975 (Lwasa et al., 2022).In the developing world, massive rural-to-urban migration occurs, and the population in urban areas is rising.Expansion in urban areas takes its toll on the landscape, leading to deforestation and areas of vegetation converted to use for infrastructural development (Richards & Friess, 2016).

Transport industry
The global transport system grew by 73% between 2000 and 2018, resulting in higher transport emission levels, contributing about 15% of the worldwide carbon emission.Most automobiles and jets are run on fossil fuels, thereby contributing to the high levels of atmospheric CO 2 and leading to global warming and climate change (Okedere, Adeboye, et al., 2021;Okedere, Elehinafe, et al., 2021).The use of alternative fuels (such as natural gas, biofuels and synthetic fuels in running vehicles), electric cars, fuel cell technology and improvement in refuelling and charging infrastructure are major steps being taken towards decarbonisation of the transport sector (Jaramillo et al., 2022).

Buildings
Waste disposal, burning of fossil fuels, and the utilisation of refrigeration and cooling devices contribute to GHG emissions in buildings (Yoro & Daramola, 2020).The building sector contributed 21% of global GHG emissions that year.This comprised 57% of indirect CO 2 emissions from offsite electricity and heat generation, 24% of direct CO 2 emissions produced on-site and 18% from the production of cement and steel used in building construction and repair (Cabeza et al., 2022).Energy demands for heating and cooling of the building space, water heating, refrigeration, cooking and cleaning also contribute to carbon emissions from buildings.

Industries
Industrial sector carbon emissions represented the second largest source of all direct anthropogenic emissions in 2019, contributing 24% of the total emissions.The chemical industry consumes large amounts of various chemical substances from fossil feedstock, and these basic chemicals produce tens of thousands of derivative end-use chemicals (Bashmakov et al., 2022).To reduce the contribution of this sector to the total GHG burden, energy efficiency standards need to be strengthened while promoting efficient heating and cooling use.The development of industrial processes with low carbon footprints should also be encouraged.This includes strategies that promote material efficiency, a circular economy which emphasises reuse, remanufacture and recycling, energy efficiency and fuel switching.

Technologies to mitigate CO 2
Around 200 nations came together to adopt the Paris Agreement in 2015, which was aimed at drastically reducing emissions of greenhouse gas to reduce global temperature to no more than 2ºC while continuing efforts to keep it to no more than 1.5 ºC in order to combat climate change and its detrimental effects.The IPCC report of this year, 2022, emphasises how climate change impacts affect people's lives and means of subsistence around the world.According to the report, scientists are more certain than ever that the damage already being caused by climate impacts will get worse as global warming progresses, with increasing negative effects on human health, ecosystems, and economies (IPCC, 2022).Due to these worldwide concerns, strict global CO 2 emission rules have been enacted.
Globally, scientists and researchers concurred that a reduction in CO 2 emissions into the atmosphere is highly desirable (Lee & Park, 2015;Tseng et al., 2015).Therefore, to alleviate the anthropogenic emissions of CO 2 into the atmosphere, carbon capture, utilisation and storage (CCUS) from major emission sources (e.g.coal-fired power plant, cement, iron-steel industry) and injected into rock formations (e.g.saline formations, unmineable coal seams) or utilising it (for chemical synthesis, fertilisers and recovery of oil) is seen as the most urgent and viable approach (Fayemiwo et al., 2018;Omoregbe et al., 2020).Combating global warming can be accomplished in four ways: by directly mitigating emissions of greenhouse gas, indirectly via advancing renewable energy engagement, by using energy more effectively, or by enacting a variety of climate policies (Omoregbe et al., 2020).This session focused on reducing direct carbon dioxide emissions via carbon capture (Rabiu et al., 2021).Four primary technology strategies are being developed for CO 2 capture systems (CCS), namely, pre-combustion, oxyfuel combustion, chemical looping combustion (CLC), and post-combustion (Gibbins & Chalmers, 2008;Sreenivasulu et al., 2015).

Pre-combustion CO 2 capture technology
In a pre-combustion technology, prior to combustion, CO2 is removed from the fuel.Syngas, primarily consisting of carbon monoxide and oxygen, is produced from the reaction of fuel with oxygen (or air or steam).In a shift converter (referred to as a catalytic reactor), CO reacts with steam to produce mixed gas (syngas), which contains 20-40% CO 2 and 60-80% H 2 (Song et al., 2019).The physical or chemical absorption process is then used to separate the CO 2 yielding a fuel rich in hydrogen.Pre-combustion has the benefit of producing H 2 , which is an ideal green energy source and is helpful in a variety of applications, including the chemical industry, aerospace industry, fuel cells, gas turbines, boilers, etc. (Jansen et al., 2015) Process streams that involve a higher concentration of CO 2 , hightemperature range and elevated pressure such as 15-60% by volume), 200-400 ºC and 2.7 MPa, respectively, do make use of pre-combustion capture techniques (Theo et al., 2016).Even though pre-combustion capture is typically more efficient and less costly than postcombustion capture for coal plants, there are still many obstacles to the commercial application of gasification.

Oxyfuel combustion capture technology
The technology of oxyfuel combustion uses pure oxygen instead of air for the combustion of carbonaceous fuel (Niu et al., 2013).The CO 2 concentration in the flue gas will be extremely high (over 80%) because the oxidant (O 2 ) is devoid of significant air constituents like N 2 , while the water vapour content can be removed with ease, thus making the CO 2 purification easier.Nevertheless, the presence of impurities like oxides of sulphur and nitrogen, and particulates, will necessitate pretreatment before CO 2 is processed.The high cost of providing oxygen for oxyfuel combustion and high temperatures of combustion may be considered drawbacks of this technology.

Chemical-looping combustion (CLC) capture technology
Chemical-looping combustion (CLC) technology employs a metal oxide (Fe, Mn, Cu, Ni, and Co) as an oxygen carrier to deliver only oxygen to the fuel from the combustion air, thereby excluding other gases (Fan et al., 2012;Rydén, 2015).The oxidation takes place in an air reactor, followed by the reduction of the metal with a hydrocarbon fuel in a fuel reactor for metal regeneration, thereby releasing only CO 2 and water (Wang & Song, 2020).After that, the metal is returned to the air and fuel reactors to begin a new cycle.Both the exothermic air and the fuel reactor are capable of producing heat and power at higher temperatures (Wang & Song, 2020).CLC is more cost-effective than oxycombustion because it does not require an expensive air separation unit, but the procedure is also more complex and necessitates additional research.

Post-combustion capture technology
CO 2 is removed after combustion at low pressure using solid or liquid sorbents in a post-combustion technology (Sreenivasulu, et al., 2015).The exhaust gases, which often comprise CO 2 , N 2 , and some oxygenated compounds (O 2 , NO 2 , and SO 2 ), are initially treated to get rid of any particulate matter and any oxides of sulphur and nitrogen that might be present.Postcombustion capture is the most mature and extensively used CO 2 capture system, owing to its flexibility and the ease with which it can be adapted to existing fossil fuel power plants (Fayemiwo, Chiarasumran, Nabavi, Loponov, et al., 2019;Kutorglo et al., 2019).Although this technology is the most mature, the low carbon capture efficiency attributable to low CO 2 concentration in the flue gas is a significant drawback of choosing this technology.

Absorption
Among the aforementioned CO 2 capture options, liquid absorption technology is the most advanced, particularly in the chemical and petroleum industries (Yang et al., 2014).Depending on the absorbent and CO 2 interaction nature, absorption can occur via physical (primarily used for pre-combustion CO 2 capture) and chemical absorption (applicable to post-combustion CO 2 capture).In physical absorption, CO 2 is absorbed into solvents such as aqueous amine (MEA or DEA) and amine blends at high pressure and low temperature, while solvent regeneration is accomplished through heating, pressure reduction or both (Kim et al., 2013).Chemical absorption is the process of absorbing CO 2 using chemical solvents.When heated, the solvents form a weakly bonded intermediate compound with CO 2 , releasing the trapped CO 2 .Despite being one of the most well-established CCS processes, amine-based absorption has a drawback in that it consumes a lot of energy and uses caustic, poisonous, and corrosive solvents that may evaporate or degrade, making the method ineffective and polluting the environment (Chowdhury et al., 2013;Fayemiwo, Chiarasumran, Nabavi, Loponov, et al., 2019).

Adsorption
An affordable and energy-efficient potential CCS technique that is applicable to both pre-and postcombustion CCS and could also be retrofitted to any power plant is adsorption technology.A variety of adsorbents, including silica, alumina, activated carbon, zeolites, and polymeric compounds, are available for use in different application techniques (Jansen et al., 2015;Theo et al., 2016;Fayemiwo et al., 2019;Wang & Song, 2020).These materials are each tailored to specific application domains and have unique pore structures, surface areas, and surface functional groups.Several lowcost adsorbents, including carbon materials (activated carbon, graphene, carbon nanotubes), metal-organic frameworks (MOFs), and zeolites (zeolite 13×, zeolitic imidazolate frameworks), nitrogen-enriched polymerbased materials have been used in CO 2 capture.Carbons are much more stable in the presence of water than zeolites.Carbons perform better under high pressure but have lower CO 2 capacities at low pressure when compared to zeolites.Although zeolite MOFs have excellent regeneration stability and high surface area, they are deemed non-cost-effective.In spite of the fact that all these adsorptions collections and more have been employed in CCS, each has inherent limitations due to poor selectivity, poor adsorption capacity, and dehumidification requirements.

Membranes separation
Membrane techniques, providing an alternative to chemicals, are one of several technologies under consideration for post-combustion carbon capture (Dai et al., 2016).The membrane process may lower carbon capture costs while posing few environmental risks.The comparison of the membrane process and conventional CO 2 separation methods revealed that the membrane systems offer meaningful improvements in adaptability, dependability, and modularity.More so, their energy requirements are typically lower or, at the very least, comparable to absorption (Low et al., 2013).Improvements to the membrane's transport characteristics are still needed to achieve purities and recoveries that are on par with conventional operations in a single step.There are three types of membranes: inorganic, organic, and hybrid membranes, depending on the characteristics of the materials used for fabrication.Although inorganic membranes have good mechanical stability and can operate at high temperatures, their ability to be scaled up is constrained by their expensive fabrication (Zhu et al., 2012).Organic membranes have a lot of benefits, including low production costs, simplicity in synthesis, superior separation performance and good mechanical stability.However, their low thermal stability makes them less useful for post-combustion CO 2 capture.

Hydrate-based separation
The selective partitioning of the target component between the hydrate and vapour phases forms the basis for the hydrate-based separation.However, a major disadvantage of the hydrate-based process is that in order for gas hydrate to form, high pressure is necessary.In addition, other major barriers identified to the successful application of hydrate-based separation techniques are the rate of hydrate formation, gas selectivity, and gas storage capacity (Pera-Titus, 2014).

Cryogenic
Cryogenic is an alternative CO 2 separation method and is less energy intensive.It is a physical process that separates CO 2 at very low temperatures.It makes it possible to directly produce liquid CO 2 at low pressure for storage or sequestration rather than compressing gaseous CO 2 to extremely high pressure, which saves energy during compression.Through a series of compression, cooling, and separation steps, the components of gas mixtures are divided during the cryogenic separation procedure (Safdarnejad et al., 2015).The operation and design feasibility of all these steps can be guaranteed because they all involve highly developed technologies in the chemical industry.Because the cryogenic separation process uses no chemicals, secondary pollution is prevented.This process has some configurations that store energy in the form of liquefied natural gas (G.Xu et al., 2014).
Research and development of CO 2 capture technologies have significantly advanced, but commercialisation is still far from being economically viable.Thus, next-generation technology development of CO 2 capture materials necessitates a thorough investigation into the interaction between intrinsic properties, materials, and process performance.

CO 2 geological formations
Carbon storage is one of the crucial stages in any carbon capture and sequestration project.CO 2 has the capability to be stored underground for a significant period, but before the injection of CO 2 starts, proper characterisation of storage sites is critical to avoid the risk that is related to leakage during and after the storage (Ajayi et al., 2019;Bachu, 2008;Rabiu et al., 2017).CO 2 is soluble in water, i.e. it dissolves in water, becomes denser than water, and is therefore stored by solubility trapping.Its solubility decreases with brine salinity and temperature but rises with pressure (Ajayi et al., 2019;Rabiu et al., 2017).Depending on reservoir temperature and pressure, CO 2 exists in four stages, i.e. gas, liquid, supercritical and solid (hydrate).CO 2 is commonly stored in supercritical conditions because a considerable amount of CO 2 can be sequestered.At this state, CO 2 is above the critical point, i.e. temperatures higher than 31.1°Cand pressures greater than 7.3 MPa (Ajayi et al., 2019;Rabiu et al., 2017).In addition, at low temperatures and high pressures, CO 2 forms a solid hydrate denser than water.The CO 2 hydrate would have been the best option for carbon storage, but it is not thermodynamically stable.Hence, more research is required in this area of CCS.All these characteristics of CO 2 and other measures play an important role in selecting storage sites in CCS technology (Cao et al., 2020;Kakouei et al., 2016;Kampman et al., 2014).Some of the examples of geological formation options in which CO 2 can be stored are depleted hydrocarbon reservoirs, saline aquifers, and basalts (Ajayi et al., 2019;Rabiu et al., 2020), and are explained in the following section.

Storage in depleted hydrocarbon reservoirs
Depleted hydrocarbon reservoirs have the potential of storing CO 2 because they have been successfully used for hydrocarbon storage for geological timescales (Abidoye et al., 2015;Aminu et al., 2017).CO 2 sequestration in depleted hydrocarbon reservoirs is more advantageous than other storage techniques because of its economic value, i.e. it can be used to improve oil recovery and also can be used to sequester CO 2 and therefore mitigate atmospheric emission through carbon sequestration (Cao et al., 2020;Rabiu et al., 2017).

Storage in deep saline aquifers
The saline aquifer is a ubiquitous and promising medium for CO 2 storage (Almayahi et al., 2022;Fang & Li, 2014;Rabiu et al., 2017).Saline aquifers do not have any economic value because of the presence of a high concentration of salts and therefore making them unfit for irrigation and drinking.For instance, the salt concentration in saline aquifers is between 25 and 225 g/L (Alcalde et al., 2018;De Silva et al., 2015;Surampalli et al., 2015).Because of the nature of the saline aquifer, being available worldwide, and its large capacity, more research on its storage process and improvement are required from researchers in the area.

Basalt rock
Basalt can potentially convert CO 2 into solid carbonates (Gadikota et al., 2014;Kelemen et al., 2019;Sissmann et al., 2014).Generally, CO 2 storage in basalt formations seems to be the best storage option because they are safe.Basalts contain minerals such as calcium, aluminium, manganese and iron silicates, and these minerals would thermodynamically react with water and CO 2 to form carbonates and permanently lock the CO 2 underground.In addition, basalts form a strong impermeable caprock which will prevent CO 2 leakage from contaminating the atmosphere or groundwater (Ayub et al., 2020;Kakouei et al., 2016).

Trapping mechanisms
The trapping mechanisms can be used to study the quantification of CO 2 in the subsurface.There are four primary trapping mechanisms (solubility, structural, capillary, and mineral trappings) in CO 2 sequestration, and these are very important in monitoring the CO 2 leakage during and after storage (Table 1) (Rabiu et al., 2017).The types trapping mechanisms and the processes involved in CO 2 storage are summarized in Table 1.

Risks in CO 2 sequestration technology
The characterisation of the storage site is very important before any CCS project starts in order to avoid the risks involved in CO 2 storage.The dangers and risks associated with CO 2 storage in geological formations may be categorised into four, as represented by Table 2 and Figure 2 (Damen et al., 2006).

Estimation of CO 2 capacity in geological Storage sites
The estimation of CO 2 storage size is the amount of the calculated CO 2 that can be injected and stored in a geological formation.Various techniques for CO 2 storage capacity estimation have been explored by different authors (Alcalde et al., 2018;Bachu  This mechanism involves trapping CO 2 through an impermeable caprock that serves as a seal above the sequestration site.After the injection stops, CO 2 tends to move upward because of the density difference between CO 2 and water; and therefore, gets trapped by structural trapping.

Capillary Trapping
This trapping mechanism is also known as residual trapping.This trapping method involves two processes, i.e. imbibition and drainage, and the difference between the two processes is known as hysteresis.Capillary trapping is very important in CO 2 sequestration.

Solubility Trapping
The dissolution of CO 2 and other flue gases in pore water is called solubility trapping.The CO 2 reacts with pore water and becomes denser than the pore water, and therefore sinks and is trapped by solubility trapping.The amount of CO 2 gas that can disintegrate into brine water depends on various factors such as salinity, pressure and temperature.On the one hand, the solubility of CO 2 increases with increasing pressure; on the other hand, it decreases with increasing temperature and salinity.
Once the CO 2 is dissolved in water, it will become denser or heavier than water, and therefore, the dissolved CO 2 will not be able to move upward again due to buoyancy force.

Mineral Trapping
This mechanism involves the reactions of CO 2 , water and rock minerals (i.e.calcium, magnesium and aluminium).After the reaction, it would be converted into carbonates which can serve as raw materials in cement industries.Mineral trapping is attractive because it is safe; in other words, it can store CO 2 permanently.However, the conversion process is prolonged; hence, more research on the improvement of the conversion rate is required (Ayub et al., 2020;Benson & Cole, 2008;Kampman et al., 2014).Some materials or chemicals that can speed up the rate of chemical reactions have been suggested by various researchers (Ayub et al., 2020;Gadikota et al., 2014;Kampman et al., 2014).
Table 2. Risks in CO 2 sequestration (Aminu et al., 2017;Damen et al., 2006;Khatiwada et al., 2012;Rabiu et al., 2017) Types of the associated risk in CO 2 storage Description of the process CO 2 leakage This is the migration or movement of CO 2 from the sedimentary storage basin to other formations, such as groundwater reservoirs, from where it may escape into drinking water or the atmosphere.The contaminated groundwater and the polluted atmosphere negatively affect aquatic animals and humans.

Methane (CH 4 ) leakage
The injection of CO 2 may cause the displacement of methane in some storage sedimentary basins to other formations and perhaps leaks into the atmosphere or groundwater.

Ground movement/Seismicity
The injection of CO 2 under high pressure may cause some natural disasters, such as microearthquakes, and consequently complicate the storage process.Therefore, proper characterisation is very germane during the storage site selection process.Brine displacement This is the process whereby the brine from the storage reservoir flows to other formations, such as groundwater formations.The brine that has already been contaminated by CO 2 would also contaminate the groundwater.et al., 2007).The CO 2 estimation techniques can be divided into two approaches, i.e. dynamic and static, and they can be used to estimate the storage capacity of all the geological formations (Ajayi et al., 2019;Aminu et al., 2017).Various chemical and physical trapping mechanisms (structural, solubility, capillary and mineral trapping) continuously occur in saline aquifers at multiple rates and timescales.These mechanisms make the storage capacity estimation complex due to the complication of the processes involved in the trapping processes (Aminu et al., 2017).There are different types of methods in the calculation of CO 2 capacity in saline aquifers, depleted oil and gas reservoirs, and coal seams, and they have been discussed by different researchers (Ajayi et al., 2019;Alcalde et al., 2018;Almayahi et al., 2022;Bachu et al., 2007).Calculations of CO 2 capacity in saline aquifers, coal seams and depleted hydrocarbon reservoirs are currently feasible in the literature; however, calculations of the CO 2 capacity in basalts and other mineral rocks are not in the literature.
Hence, studies on the estimates of capacity of mineral rocks are required.

Global CO 2 sequestration projects
There are current and planned CCS projects worldwide (Figure 3).The Sleipner project, Norway and the Weyburn enhanced oil recovery Canada are focused in this review.

Sleipner
Sleipner project commenced the CCS project in 1996, and it was the first fruitful commercial CCS project (Figure 4).The project was managed by Statoil and was anticipated to store about 20 Mt of CO 2 .The storage formation has a permeability of 1-8 D and a porosity of 35-40%.The monitoring tools used in the project are time-lapse, seismic surveys, soil gas and satellite remote sensing (Adefila & Yan, 2013;Damen et al., 2006;Gaasbeek et al., 2014;Hosa et al., 2010;Yang et al., 2014).

Weyburn
The project was managed by European Union, Canadian, US governments and some other oil and gas companies.The aim of the project was to economically and effectively enhance the recovery of more oil and, at the same time, storing CO 2 in the reservoirs for a geological period.(Ajayi et al., 2019;Aminu et al., 2017).The monitoring tools used in the quantification and monitoring of CO 2 are seismic images (which are used to map the CO 2 saturation) and geochemical methods (Hosa et al., 2010;Khatiwada et al., 2012).

Monitoring and characterisation of geological CO 2 storage site
CO 2 monitoring and characterisation are important before, during and after the injection process.CO 2 storage is safe in geological formations if the storage site is properly characterised, and it depends on a number of chemical and physical mechanisms (structural, solubility, capillary and mineral trapping).Rabiu et al. (2017) studied the behaviour of the CO 2 phase and concluded that supercritical CO 2 injection could be conveniently stored in the subsurface without compromising the technology.Additionally, the CO 2 will be in supercritical fluid at depths of 650-1000 metres (Rabiu et al., 2017;Salvi & Jindal, 2019) 4. Re -utilisation of CO 2

Direct usage
Direct use of CO 2 can be employed in enhanced oil recovery (EOR) through CO 2 flooding, particularly for waterflooding-depleted reservoirs (Rafiee et al., 2018).
Injecting CO 2 into depleted oil reservoirs is an efficient technique because it not only improves oil recovery but also reduces CO 2 emissions because some of the injected CO 2 is stored permanently in the depleted reservoir rather than being pumped back to the surface after EOR (Zhao et al., 2020).CO 2 EOR is an environmentally appealing Carbon Capture and Storage (CCS) option because it lowers CO 2 emissions, boosts reserve production and oil recovery, and ensures the nation's energy supply.Similarly, CO 2 has also been employed in enhanced gas recovery (EGR) and enhanced geothermal systems (EGS).Other direct utilisation of CO 2 includes beverages industries for carbonated drinks, algae farms for photosynthesis, as a shielding gas in welding, fire extinguisher, dry ice and process fluid etc. Table 3 shows summary of direct utilization of CO 2 .
For producing methane, methanol, ethanol, and other substances that can be used as chemicals, fuel for fuel cells, or hydrogen sources for electricity, chemical conversion techniques can be used to convert the captured CO 2 from CCS plants as a relatively pure and lowcost feedstock (Ola et al., 2013).Taking into account the scarcity of oil, the market for methanol is more promising as it can be used to replace gasoline in vehicles while  (Aminu et al., 2017).also addressing the issue of erosion.This approach generates significantly more advantages and values in terms of economics, the environment, and energy in addition to increasing combustion efficiency and reducing CO 2 emissions (by about 45% compared to gasoline) (Huang & Tan, 2014).Although, the approach has some drawbacks which include the high activation of CO 2 and the requirement for a significant energy input.
Present-day environmental engineering uses microorganisms for CO 2 biomitigation in flue gases from coalfired power plants.Due to their greater efficiency compared to plants, microalgae are increasingly being used for CO 2 biofixation (Wang & Lan, 2010).Utilizing microbes, CO 2 can be fixed by growing them in various cultivation systems.Autotrophic microbes use CO 2 from the environment or other sources (such as flue gases) for cell growth and the synthesis of extracellular products (Zahed et al., 2021).Autotrophic microorganisms use CO 2 fixation to synthesize materials for cells and extracellular products in this process (Daneshvar et al., 2022).This led to the CO 2 being stored as organic biomass, which can then be refined to produce a variety of chemicals and biofuels.
Of recent, enzyme-based systems have become more popular due to their ability to perform complex reactions with a high yield to produce desired chemicals (Ünlü et al., 2021).Enzymatic methods provide an environmentally friendly and longlasting alternative for CO 2 conversion compared to various conventional methods, such as chemical, electrochemical, and photochemical methods (Alpdağtaş et al., 2022).Enzymes that have been employed in CO 2 conversion includes: carbonic anhydrase (Jo et al., 2020), pyruvate decarboxylase (Giri et al., 2021), Carbon dioxide reductase (Gao et al., 2020), Remodeled nitrogenase (Fixen et al., 2016), Carbon monoxide dehydrogenase (Contaldo et al., 2021) and Formate dehydrogenases Calzadiaz-Ramirez and Meyer (2022).Although research on enzyme-based and enzyme-integrated studies has yielded promising results, it is not economically feasible.It is therefore a great necessity to obtain large quantities of enzymes at low cost.
CO 2 could also be utilized in the artificial production of starch and sugar with greater efficiency and fewer negative environmental effects since there is no need for nitrogen fertilizers, nor herbicides or pesticides for artificial carbohydrates synthesis (Cai et al., 2021;Martínez et al., 2021).Artificial carbohydrate synthesis from CO 2 has aided in supplementing or replacing industrial agriculture for starch and sugar production.This thus help to serve as a food and life support system to lessen the risk of existential doom in the event of a global agricultural calamity brought on by super volcanic eruptions, climate change, or nuclear winter, especially in remote bunkers (O'brien et al., 2022).A schematic representation of direct and indirect utilization of CO 2 is presented in Figure 5.  Alper and Orhan, 2017;van Heek et al. (2017) to cultivate microalgae (using the flue gases from fossil fuel power plant) Microalgae can be used as a feedstock to make biofuel in addition to consuming CO 2 .most species double their biomass volume in less than 24 hours as a result of their rapid growth.
The harvested microalgae also have tremendous potential for producing food, chemicals, energy, and essential nutrients.

Conclusion
Various anthropogenic activities (e.g.burning fossil fuels and transportation) result in CO 2 emissions into the atmosphere, leading to significant global warming and climate change concerns.Several approaches have been considered for mitigating CO 2 emissions, among which carbon capture, utilisation and storage (CCUS) are considered viable for meeting CO 2 emission reduction.The work highlighted the possible source of CO 2 emissions and the technology available for direct CO 2 capturing.The review highlighted the feasible options for carbon sequestration in sedimentary basins such as depleted hydrocarbon reservoirs, saline aquifers, coal seams and basalts.It is suggested that CO 2 storage can proffer the solution to the problem of climate change even though some challenges need to be addressed to ensure the safety of the technology.It discusses some difficulties in storage site assessment measures, CO 2 behaviour in the subsurface, and CO 2 storage capacity procedures that require additional attention.Furthermore, some of the developments of the major global CO 2 storage projects and potential CO 2 utilisations were pointed out.
The following are the obstacles to CO 2 use: (1).CCS techniques cost implications; (2).Energy penalty in conversion of CO 2 ; (3).Scale up for commercial purpose; (4).Societal and economical motivator.Thus, CO 2 utilization strategies should emphasize the use of CO 2 for environmentally friendly processes, the synthesis of chemicals from CO 2 that are useful in industry, and recycling CO 2 using renewable energy to preserve carbon sources.

Disclosure statement
No potential conflict of interest was reported by the authors.

Table 1 .
Types of trapping mechanisms in geological carbon sequestration

Table 3 .
Potential direct utilization of CO 2