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Abstract

The global warming is directly related to the increased greenhouse gas emissions from both natural and anthropogenic origins. There has been a drastic rise in the concentration of CO₂ and other greenhouse gases since the industrial revolution primarily due to the intensifying consumption of fossil fuels. With the need to reduce carbon emissions and mitigate global warming certain strategies relating to carbon capturing and sequestration are indispensable. This paper comprehensively describes several physicochemical, biological and geological routes for carbon capture and sequestration. The trend of the increase in greenhouse gases over the years is illustrated along with the global statistics for fossil fuels usage and biofuels production. The physicochemical carbon capturing technologies discussed include absorption, adsorption, membrane separation and cryogenic distillation. The algal and bacterial systems, dedicated energy crops and coalbed methanogenesis have been vividly explained as the biological routes for carbon sequestration. The geological carbon sequestering route centers on biochar application and oceanic carbon storage. A systematic survey has been made on the origin and impact of greenhouse gases along with the potential for sequestration based on some fast-track and long-term sequestration technologies.

Introduction

Today, climate change and global warming are two of the hot topics for discussion at the global environmental panorama. Climate change is a long-lasting and irrevocable shift in the weather conditions recognized by the variations in atmospheric temperature, precipitation, air quality, the wind, and other indicators. The climate change has led to the experiencing of several extreme weather events worldwide, which are mostly attributed to anthropogenic global warming. Some of these unusual and unseasonal weather events include, but are not limited to, heat and cold waves, melting of ice cover, a rise in the sea level, drought, floods, violent storms, and tropical cyclones. The Earths climate is seasonal and naturally variable on all timescales. Since the yesteryears, the increased concentrations of GHG (greenhouse gases) have led to the induced greenhouse effect resulting in the warming of the planetary surface.

According to IPCC, climate change can be defined as “any change in the climate over time, whether due to natural variability or as a result of human activity.” Likewise, the UNFCCC (United Nations Framework Convention on Climate Change) defines climate change as “change in the climate that is attributed directly or indirectly to human activity thereby altering the composition of the global atmosphere and natural climate variability observed over comparable periods of time” [1]. Since the time of industrial revolution, there have been dramatic changes in the global agriculture, material manufacturing, transportation and infrastructure. The rapid urbanization has led to an increase in the emissions of the popular GHGs viz. CO₂ (carbon dioxide), CH₄ (methane) and N₂O (nitrous oxide). In addition to CO₂, CH₄, and N₂O, the GHGs also include SF₆ (sulfur hexafluoride), O₃ (ozone), water vapor, hydrofluorocarbon, and perfluorocarbon groups of gases.

A major proportion of the GHGs in the atmosphere is due to anthropogenic reasons. The increased consumption of fossil fuels, use of chlorofluorocarbons in refrigerants, solvents, foam blowing agents and spray propellants are accountable not only for increased GHGs but also in the depletion of ozone layer. The industrial practices such as processing of minerals, metals, chemicals, solvents together with the production and utilization of halocarbons, and SF₆ also contribute to this effect. The GHGs aid in the greenhouse effect by trapping the outgoing infrared radiation from the Earths surface and adding the heat to the net energy input of lower atmosphere [2]. Figure 1 is a graphical illustration of the natural and anthropogenic greenhouse effects. The higher levels of CO₂, CH₄ and N₂O cause depletion of the ozone layer along with the thickening of the layer of GHGs that trap the heat within the atmosphere making the Earths surface warmer. The anthropogenic greenhouse effect has been recognized to adversely impact the global climate and functioning of the oceanic and terrestrial ecosystems by altering the temperature and rainfall patterns.

Figure 1. Graphical illustration of natural and anthropogenic greenhouse effects.

The attributions of CO₂, CH₄, and N₂O towards global warming can be considered to be 60%, 15%, and 5%, respectively [3]. While the concentration of CO₂ and CH₄ is increasing at the rate of 0.4–3% per year, N₂O is rising by 0.2% annually [4]. The Intergovernmental Panel on Climate Change (IPCC) has estimated a global rise in Earths temperatures by 0.6°C over the last century. However, it can be predicted that the temperature will rise from 1.4°C to 5.8°C in the next two centuries if the anthropogenic emissions of GHGs keep enduring. The month of April 2015 was by far the warmest month on record since 1880s. The global average land surface temperature in April 2015 was 1.11°C above the twentieth century average [5].

The rapid global industrialization has led to the unprecedented consumption of fossil fuels including coal, petroleum and natural gas that releases surplus amounts of CO₂ into the atmosphere. The world human population is expected to grow from 7.3 billion today [6] to 9.2 billion by 2050 [7]. The per capita consumption of energy also increases with the gradual increase in population. The total energy use in 2012 was 524 Quadrillion Btu (quad); however, it is expected to rise by 60% by 2030 [8]. The use of petroleum and other liquid fossil fuels was 85.7 million barrels per day in 2008 with projections for an increase up to 112.2 million barrels per day by 2035 [9]. Figure 2 shows the trend of fossil fuels consumption since 1980. The use of petroleum increased from 156 quads in 2000 to 181 quads in 2012 [8]. Similarly, the demands for coal and dry natural gas also rose to 147 and 124 quads, respectively in 2012. It entreats for a smarter and sustainable way to manage the energy demand for the growing world population. In such a scenario, energy efficiency and conservation along with decarbonizing our energy sources are essential [10].

Figure 2. Worldwide consumption of fossil fuels from 1980 to 2012 (Data source: [8]).

Climate change is already having significant impacts on the ecosystems, communities and global economy irrespective of any particular geographical region. The increased GHG emissions also threaten the human health due to freshwater shortages, smog, acid rain and other ecological disturbances. The government, policy makers and individuals should act in synergy for mitigating GHG emissions and subsequently lowering their impacts, risks, and associated vulnerabilities. Although the climate change is irreversible, yet it can be alleviated by curbing the GHG emissions (especially CO₂ and CH₄) with strategies to capture and sequester the carbon.

Many papers have reported the potential of CCS (carbon capture and sequestration) to mitigate global warming. Among these many reports, the information about the routes of carbon capturing is scattered, which makes it difficult to evaluate the efficiency of one technology over the other. This paper attempts for a comprehensive review of the underlying principles and current trends in the field of CCS to deter the global warming caused by GHGs. The review is focused on careful integration of technologies for sequestering CO₂ through physicochemical, biological and geological processes. However, it is important to note that the CCS technologies discussed in this paper are either under developmental stage or at a precommercial scale. The CCS technologies have not yet been fully integrated in full-scale commercial operation due to restrained CO₂ capturing efficiency, high costs and lack of regulatory framework.

Current Trend of Greenhouse Gas Emissions

The current top ten CO₂-emitting countries are China, USA, India, Russia, Japan, Africa, Germany, South Korea, Iran, and Saudi Arabia (Fig. 3). China and India rank as the first and third largest CO₂-emitting countries because of their escalating demands for fossil fuels, which are increasing at the rates of 3.5% and 3.9% per year, respectively [11]. As some developed nations such as USA, Russia, Japan, and Germany are in the list of high CO₂ emitters, the Kyoto Protocol places a heavier burden on the principle of “common but differentiated responsibilities.” The Kyoto Protocol is an international pact linked to UNFCCC that sets international GHG emission reduction targets. According to the Kyoto Protocol (an international environmental treaty proposed by UNFCCC on December 11, 1997 in Kyoto, Japan), it is mandatory for industrialized nations to reduce their anthropogenic GHG emissions by 5.2% from their 1990 levels within the commitment period of 2008–2012.

Figure 3. Top ten CO2-emitting countries in 2012 (Data source: [8]).

The “Doha Amendment to the Kyoto Protocol” was adopted in Doha, Qatar, in 2012. During its first commitment period (2008–2012), 37 industrialized countries including the European Union committed to reducing GHG emissions by 5% from their 1990 levels. In the second commitment period (2013–2020), the parties agreed to reduce the GHG emissions by 18% below their 1990 levels by 2020. The arrangement of parties in the second commitment period is different from the first, with Canada, Japan, and Russia withdrawing their commitments from the Protocol in 2011. The Canadian government withdrew from the Kyoto Protocol after ratification in December 2011. Although, Canada was committed to restraint its GHG emissions to 6% below 1990 levels, it showed 17% higher emissions in 2012. The penalties of $13.6 billion for not achieving the targets led to its withdrawal from the treaty [12]. The GHG emissions are debatable for the continual increase as two of the largest CO₂ emitters, namely, Russia and Japan also walked out of the Kyoto agreement.

Recently (November–December 2015) in Paris, France, the United Nations Climate Change Conference, that is, COP21 or CMP11 was held as the 21st annual session of the Conference of the Parties (COP) to the 1992 UNFCCC and the 11th session of Meeting of the Parties to the 1997 Kyoto Protocol. In this global meeting, 200 nations agreed to cut their carbon emissions to set a goal of curbing global warming to <2°C compared to the preindustrial levels [13]. The GHG emissions have increased by 80% since 1970 and 30% since 1990, totaling 49 gigatonnes (Gt) of CO₂ equivalent in 2010 [14]. According to COP21, the parties will purse efforts to reduce the global GHG emissions by 40–70% by 2050 compared to 2010 levels and drop to zero emissions by 2100.

During the past one and half century, there has been a consistent increase in CO₂ concentration by 33% with the rate expected to rise by 4% per year [15]. Figure 4 illustrates the trend of GHG emissions since 1750. The current atmospheric concentration of CO₂ (404 ppm) compared to its 1750 levels (278 ppm) shows a dramatic increase in 265 years [16]. The CO₂ emission from fossil fuels combustion in 2014 was 36 Gt [17]. CO₂ accounted for 77% of the total anthropogenic GHG emissions in 2004 as its annual emissions have grown between 1970 and 2004 by 80%, that is, from 24 to 38 Gt, respectively [18]. CH₄ and N₂O are 20 and 300 times as potent as CO₂, respectively in retaining the atmospheric heat. The levels of CH₄ increased from 700 ppb in 1750 to 1814 ppb in 2013 indicating a 61% rise. Similarly, N₂O concentration grew by 17%, that is, from 270 ppb in 1750 to 326 ppb in 2013.

Figure 4. Worldwide greenhouse gas emissions from 1750 to 2013 (Data source: [16]).

Despite the anthropogenic sources, the sources of natural GHG emissions are also of parallel consideration. Soil can be considered both as a chief source of atmospheric CO₂ and also a storehouse for carbon storage. Soil contributes a fraction of the total emission of CO₂ through soil respiration (from the plant root and heterotrophic respiration) [19]. Carbon released through soil respiration accounts for around 10% of the total atmospheric carbon pool [20]. Agricultural methods of manure management, field burning of agricultural residues, and rice cultivation are also classified as the sources of GHG emissions [21]. About 80% of CH₄ is produced biologically from rice cultivation, wetlands and sediments, landfills, enteric microbial fermentation and organic waste composting under conditions of low redox potential [22]. About 35% of the annual N₂O emission is from agriculture and change in land use [23]. N₂O is biologically generated through denitrification by heterotrophic microorganisms in oxygen deficient environments, nitrification by autotrophic and heterotrophic nitrifying microorganisms, and dissimilation of nitrate to ammonium by heterotrophic microorganisms in aerobic conditions [24].

The natural catastrophic events such as volcanic eruptions, forest fires and hydrothermal vents also release significant amount of GHGs into the atmosphere. The CO₂ released from volcanoes can be from the erupting magma and degassing of unerupted magma (i.e., recycled sub-ducted crustal materials and decarbonation of shallow crustal materials). They can restore the lost CO₂ from the atmosphere and oceans through weathering of silicates, carbonate deposition and burial of organic carbon. The CO₂ typically makes up to 10 mol% of the total volcanic gas emissions [25]. A hydrothermal vent is a fissure in Earths surface from which geothermally heated water and hot gas bubbles explode. The dissolution of the gases causes local increases in water density and acidity resulting in sequestration of CO₂ [26]. The emissions from forest fires also have a direct impact on the regional and global carbon cycles by increasing CO₂ levels and affecting carbon sequestration by forests. However, compared to the land use changes (3.4 Gt CO₂ per year) and vehicular emissions (3 Gt CO₂ per year), volcanoes emit less CO₂ (up to 0.44 Gt) [27].

Climate change is predicted to influence biodiversity and agriculture to a large extent. The changes in climate system is unambiguous as evident from: (1) the increase in global average temperatures of soil, air, and water bodies; (2) widespread melting of snow, ice and glacier; (3) shorter freezing seasons of lake and river ice; (4) decrease in permafrost extent; and (5) rise in the average sea level [18]. The climate change and unseasonal weather conditions have also led to habitat shifting and alterations for several ecological species, for example, desertification, coral bleaching, tundra thawing, etc. The rising temperatures cause the meltdown of mountain ice caps, snow, glacier and permafrost. This results in a shift or loss of many species from their natural habitat. The behaviors of adaptation, endangering and extinction are the anticipated in several plant and animal species. The bird migration, egg-laying, leaf-unfolding, as well as the poleward and upward shift in plant and animal species are most common examples [18]. Community shift between fishes, coral reefs, aquatic plants and animals, plankton, algae, and marine bacteria are also associated with rising water temperatures, changes in ice cover, salinity, and oxygen availability.

The emission trading is an economically sensitive strategy at the national and international level for reducing the concentrations of GHGs, particularly CO₂. Carbon credits, carbon trading and carbon markets are some of the CO₂ emission trading approach. A carbon credit is a tradable permit or certificate representing the right to emit one ton of CO₂ or the mass of another GHG with one ton CO₂ equivalent. Carbon trading allows the industries (that cannot practicably reduce CO₂ emissions) to buy credits from those industries that have already reduced their emissions more than the committed level [28]. However, each carbon credit is worth one metric ton of CO₂. The carbon trading market is an economic approach as the cost of emission reduction is higher than the cost of credits. Alternatively, the industries can invest in reforestation projects for removing CO₂ from the atmosphere biologically via photosynthesis and carbon fixation [29]. Although this approach is typically called reduction rather than credit, the process is monitored over time and units are measured in tons of CO₂. Upon the reduction in other GHGs, carbon equivalents can be earned and traded.

Carbon Capturing and Storage Technologies

Carbon capture and storage (or sequestration), often abbreviated as CCS, is the process of capturing CO₂ from large-scale emitters such as fossil fuel refineries, power plants and product manufacturing industries, and transporting it to a storage site thus preventing its re-entry into the atmosphere. The cost of CO₂ separation and compression (to 11 MPa) is estimated to be $30–50 per ton CO₂, whereas transportation (for every 100 km) and sequestration cost is about $1–3 per ton CO₂ [30]. There are several routes available for CO₂ removal from the atmosphere or industrial flue gas stream. These routes can be categorized into physicochemical, biological and geological as shown in Figure 5.

Figure 5. Routes of carbon capture and sequestration.

The physicochemical route involves the application of absorption, adsorption, gas separation membranes and cryogenic distillation. This route is most popular for capturing CO₂ from industrial flue gas streams. The CO₂ can be isolated from the absorbent material for compression and transportation. The biological route includes algal systems and energy crops that can utilize CO₂ from the atmosphere for photosynthesis and fix carbon in the form of carbohydrates (in lignocellulosic plants) and polysaccharides (in algae). The carbohydrates and polysaccharides can be converted to hydrocarbon fuels via thermochemical (e.g., pyrolysis, torrefaction, liquefaction and gasification) and biochemical (e.g., fermentation and anaerobic digestion) conversion technologies [31]. Coalbed methanogenesis is also included in biological route owing to the involvement of methanogenic bacteria that lead to the biodegradation of coalbed and generating biogenic CH₄ instead to CO₂ that would otherwise be released upon coal combustion. The geological route involves storing carbon in the soil in the form of biochar or through underwater storage.

Carbon dioxide capture can be done in three ways, such as, postcombustion, precombustion, and oxy-fuel combustion [32-34]. The fossil fuels (e.g., coal and natural gas) are subjected to pyrolysis, gasification or reforming reactions in precombustion method to produce clean gas fuels such as H₂ that on combustion produces water [35]. The carbon present in the fuel gets converted to CO₂, and the effluent gas from reforming or gasification majorly comprises of H₂ and CO. The complete conversion of carbon content into CO₂ leads to its higher concentration with high pressure of the CO₂-rich effluent stream that implies lower operating costs for the separation process. High-pressure steam or pure oxygen is a prerequisite for reforming and partial oxidation to generate H₂ from fossil fuels.

The postcombustion process can capture CO₂ on-site from the flue gases emitted as a result of fossil fuel combustion. As the CO₂ concentrations in the flue gas are usually ≤15 vol%, indirect mode of operation (introducing a new phase to capture the required component) is employed to separate and sequester CO₂ [36]. The postcombustion process includes absorption, adsorption, membrane separation, and cryogenic distillation used in the capturing CO₂ from the flue gases. Each of these technologies has been vividly discussed in the following section. Table 1 summarizes different aspects of the pre-, post- and oxy-fuel combustion processes.

Table 1. Summary of CO₂ capturing technologies

Parameter	Precombustion	Postcombustion	Oxy-fuel combustion
CO2 concentration	15–40 vol%	4–14 vol%	75–80 vol%
Acid gases	Sulfur compounds need to be removed.	Contains NOx, SOx, COS and H2S.	NOx absent but gas desulfurization is required.
Combustion medium	Steam/air is required for gasification to generate CO2.	Air is used.	High purity oxygen for combustion.
Equipment size	Medium size equipment.	Large size equipment required with high investment.	Low size equipment.
Temperature and pressure	Low temperature and high pressure (depends on the process employed).	Flue gas need to be cooled and pressure depends on CO2 capture process.	Cryogenic temperature for separation of O2. High temperatures are obtained for oxy-fuel combustion hence the flue gas is recycled to reduce the temperature.
Potential	Integrated gasification combined cycle and turbines which can effectively use H2-rich syngas.	Can be applied to the existing coal combustion plants.	Novel cycles have been employed in synergy with integrated gasification combined cycle.
Pros	Low energy penalty than postcombustion processes. Regeneration can be achieved by altering pressure and temperature.	Small concentrations of CO2 can be captured. Retrofit technology.	Efficiency of CO2 capture reaches 100%. No presence of harmful NOx.
Cons	Drying of syngas and its treatment prior to CO2 capture. High investment and capital costs.	High operating and regeneration costs. High solvent losses.	High capital and operating cost. Annexing to the existing plants is difficult.
State of the art	Integrated gasification combined cycle and ammonia production plants are running currently.	Amine scrubbing plants (reaction with monoethanolamine) are in practice. Power plants currently employ this technology	Efficient CO2 separation.

The oxy-fuel combustion process is the combustion of fuels with pure oxygen to generate huge quantities of energy at high temperatures [37]. The demand of pure O₂ for the combustion requires its separation from air, which is quite expensive and requires cryogenic distillation. Water and CO₂ are the typical products of the oxy-fuel combustion process. The exhaust gases obtained at high temperatures are cooled and further recycled back to reduce the temperature of the flue gases. As a result, the exit stream contains higher concentrations of CO₂ for capture and storage. The economy of the process depends on separating O₂ from N₂, which is very expensive [38]. The typical techniques employed to separate CO₂ from the flue gas includes absorption, adsorption, membrane separations, and cryogenic distillation [34, 35].

Chemical looping combustion, first developed by Richter and Knoche [39], is similar to oxy-fuel combustion where O₂ for combustion is supplied by using a metal oxide. By using a metal oxide, the exit flue gas is free of N₂ resulting in high concentrations of CO₂ making it viable for easy capturing. Chemical looping combustion is usually operated in two fluidized-bed reactors where the fuel comes in contact with metal oxide carrier (that supplies O₂ for combustion). Further, the metal oxide is transferred to a regenerative bed where it is reoxidized. The supply of O₂ from the metal oxide carrier depends on its oxidation and reduction cycles. The potential metal oxide carriers are copper, manganese, iron and nickel [40]. However, some technical issues encountered during this process are deactivation of metals, inefficient metal oxide–fuel contact and sulfur poisoning.

Physicochemical Routes for CO₂ Capture and Separation

Absorption

Sorption is a composite process involving absorption and adsorption. Sorption studies can evaluate the surface areas and pore structures of solid materials providing information on the sorption abilities of adsorbates by porous or nonporous solids [41]. Absorption is a gas-liquid operation where the gas components are absorbed into the new liquid phase. The solute is recovered from the solution either by reversing the process conditions or by other mass-transfer operation. The solubility requirements along with constraints such as nonvolatility, low viscosity, nontoxicity, nonflammability, and chemical stability restrict the choice of absorbent solvent to capture gases from effluents.

The capturing of gas molecules from the exhaust gases can be achieved by physical or chemical means. In the physical process, higher concentrations of flue gases at high pressures to capture CO₂ in a suitable solvent by dissolution process. The gas molecules dissolve into the chosen solvent at the given operating conditions to capture harmful gas effluents including CO₂, NO_x, SO_x, and H₂S. Henrys law comes into picture for the absorption of gases into the liquid solvents (partial pressure α concentration of the component in the liquid phase). High pressures and low temperatures are recommended for greater dissolution of gases in liquid solvents while the solvents are regenerated by altering the process temperature and pressure. An additional advantage of physical absorption process is that the solvents are capable of absorbing H₂S, carbonyl sulfide (COS) and hydrocarbons along with CO₂ [42].

The physical absorption of CO₂ by the solvent mixture of dimethyl ethers and polyethylene glycol solution is known as the Selexol process [43]. The typical formula for Selexol solvent is CH₃(CH₂CH₂O)_(3–9)CH₃ with pressures of 1.5–14 MPa and temperatures of 1–25°C [42, 44]. In the Rectisol process, methanol is employed as the absorbing solvent at −10 to −70°C and 3–8.1 MPa pressure [45]. The solubility of CO₂ in methanol is five times that of water at ambient temperature, although it can be enhanced by 8–15 times at temperatures lower than −0.15°C [46]. In addition to the methanol and Selexol solvents, other solvents used to capture CO₂ are N-methyl-2-pyrrolidine, morpholine, and propylene carbonate. Purisol process involves N-methyl-2-pyrrolidine as the solvent at −20 to −40°C and 1 MPa pressure. Similarly, Fluor process uses propylene carbonate as the solvent at 3–7 MPa and temperatures below 25°C [42]. Since the physical absorption processes require low temperatures (i.e. <25°C) and high pressures, the exit flue gas should be cooled and pressurized to the desired operating conditions. The solubility of CO₂ is usually high in Fluor process and appropriate for the gas streams where the partial pressure of CO₂ is greater than 0.4 MPa [42, 47].

Chemical absorption is one of the traditional techniques in which CO₂ from flue gases reacts with the solvent in an absorption column. The exhaust gas from power plants is obtained at high temperatures and cooled to temperatures below 50°C before feeding it to an absorption column. The cooled gas stream is allowed to pass through the absorption column in which nearly 85–90 vol% of CO₂ is absorbed. The CO₂-rich stream is fed to a regenerator or stripping column. The absorption of CO₂ operates at moderate conditions, that is, 0.1 MPa and 40–50°C while the regeneration of absorbent solvents takes place at 0.2 MPa pressure and high temperature (100–120°C) [47]. The solvents react with CO₂ present in the effluent gas and form a stable product in the absorber that is further passed to a stripping tower where the solvent can be regenerated by altering the temperature and pressure. The CO₂ lean stream solvent is recycled back to the absorption unit, and the energy intensity for recovering the solvent is high.

Table 2 summarizes the current worldwide status of research and development on several physicochemical CCS technologies. It is noteworthy that the technologies mentioned in this table are not fully implemented on a commercial scale; hence limited practical data is accessible on the fate of captured CO₂. The first choice of a feasible solvent is aqueous alkanolamines. The widely used alkanolamine to capture CO₂ from the gaseous stream is monoethanolamine. The reactivity of amines with CO₂ decreases with the increase in alkanol group (e.g., RNH₂ < R₂NH₂ < R₃NH₂) [43]. The reactions between amines and CO₂ can be considered similar to the reactions between a weak acid and a weak base. The zwitterions are formed as a result of the reactions between primary and secondary amines reaction. The zwitterions further form carbamates that can be reversed at rising temperatures. The carbamates thus formed are unstable in the presence of tertiary amine solvent, and bicarbonate ions become the main product. The carbamates undergo hydrolysis to form free amine molecules and bicarbonates from which the captured CO₂ is recovered in the regenerator.

Table 2. Current status of different physicochemical CO₂ capturing technologies

CO2 capture process	Separation technique	Status of research and development
Physical absorption	Rectisol, Selexol, etc. Mostly integrated gasification combined cycle.	Summit Power Group, LLC, Seattle, USA (Texas Clean Energy Project) Don valley, Yorkshire, UK (Don Valley Power Project) Nuon Power, Buggenum, The Netherlands (Integrated gasification combined cycle plant) Elcogas, Puertollano, Spain (Integrated gasification combined cycle plant)
Chemical absorption	Amine, chilled ammonia, and amino acid salt solvent.	SaskPower, Saskatchewan, Canada (Boundary Dam Carbon Capture Project) TransAlta Corporation, Alberta, Canada (Project Pioneer Keephills 3 Power Plant) American Electric Power, Ohio, USA (Mountaineer Power Plant) Dow Chemicals, West Virginia, USA (South Charleston plant) PGE Bełchatów Power Station, Łódz´ Voivodeship, Poland Hazelwood Power Station, Victoria, Australia (Hazelwood Carbon Capture Project) Abu Dhabi Future Energy Company Masdar, Abu Dhabi, UAE (Siemens PostCap™ technology)
Adsorption	Pressure swing adsorption and Pressure-temperature swing adsorption	Under developmental stage.
Cryogenics	Cryogenic distillation	Air Products and Chemicals, Inc., Pennsylvania, USA
Membrane separation	Polymeric, inorganic and mixed membranes	Schwarze Pumpe power station, Spremberg, Germany (Oxy-fuel technology) CS Energy: Callide Power Plant A, Queensland, Australia (Callide Oxy-fuel Project) OxyCoal, UK (Oxy-fuel technology)
Chemical looping combustion	FeO, CuO, MnO, and NiO	Less large-scale demonstration plants.

The application of these solvents has constraints to capture CO₂ from the flue gas streams. The acidic gases such as SO_x and NO_x pose serious risks to these amine solutions due to their capability to form heat-stable salts that can be corrosive to the reactor in the power plants. The inert gases such as nitrogen and argon do not account for any safety or operational issues during the pipeline transport process or final storage. If not removed, 1 mol% of nitrogen can increase the energy requirement as well as CAPEX (capital expenditure) and OPEX (operational expenditure) in the transport chain with by 1% [48]. The presence of inert gases tends to reduce the volume of actual CO₂ in the transport pipeline and require the same compression energy. In addition, NO_x tends to react with some amine-based solvents and increase the overall reclaiming cost [49].

Solvent losses usually occur at high temperatures due thermal degradation and carbamate polymerization. The absorption usually happens at low temperatures while the regeneration of solvent involves high temperatures, and the solvent loss occurs in the stripping or regeneration tower. Furthermore, in the presence of oxygen, these amines degrade into undesired compounds demanding oxygen-free gas stream. To overcome these issues associated with amine solutions, mixed aqueous amines are employed to enhance their chemical characteristics to capture CO₂.

The economics of the process depends on solvent regeneration and compression of the captured CO₂ for transporting. The energy required for solvent regeneration is 3.2–4.2 GJ/ton of CO₂ and comprises of nearly 60% of energy consumption [50]. Additionally, to capture a ton of CO₂, nearly 1.4 kg of monoethanolamine is required. In a study by Knudsen et al. [51], different process upgrades were employed to a 1 ton/h of CO₂ capture test facility operating on flue gas slipstream from a coal-fired power plant. Nearly 25% saving in regeneration energy was achieved with monoethanolamine (3.7 GJ/ton CO₂), process improvements and novel solvents. The improved process configurations and better solvents can reduce the overwhelming power demand of CO₂ removal by amine scrubbing from 0.37–0.51 MWh/ton CO₂ to 0.19–0.28 MWh/ton CO₂, equivalent to 20–30% of a power plant supply [52].

Recently, a novel solvent from amine family namely piperazine (cyclic diamine-C₄H₁₀N₂) has been found to enhance the performance both in capturing of CO₂ and maintaining its stability at higher temperatures and aerobic environment with minor solvent losses [53]. The piperazine solvent has a limited solubility in water; hence high temperatures are usually employed for absorption of CO₂. Aqueous ammonia solution has been found to be an alternative to the conventional amine solutions to capture CO₂ due to its inexpensiveness and low reaction heat. Due to the high volatility of ammonia, there is a high probability for its escape into the gas stream creating a technical impediment. The temperature employed in chilled ammonia process ranges from 0 to 20°C where the formed reaction products precipitate in the absorber. An advantage of ammonia solution is its ability to capture the acid gases such as SO_x, NO_x and Hg. The stripping tower operates at 50–200°C in the pressure range of 0.2–13.7 MPa [54].

Amino acid salts are also found to be beneficial as CO₂-absorbing solvents because they are nonvolatile (eliminating solvent loss via gas phase), nontoxic, nonexplosive, odorless, and biodegradable [55]. Recently, Siemens in partnership with Abu Dhabi Future Energy Company (Masdar) in UAE developed a proprietary postcombustion CCS technology called PostCap™. The Siemens PostCap™ technology involves selective absorption of CO₂ from flue gas using amino acid salt solvent and subsequent desorption to gain high purity CO₂. The technology was successfully validated with more than 9000 operational hours in a CO₂-capturing pilot plant adapted to a coal-fired and gas-fired power plant. The PostCap™ technology, capturing 1.8 million tonnes of CO₂ per year, is planned for use in enhanced oil recovery in the local depleted fields [55]. By capturing and transporting the CO₂ emitted from a steel-making process, the enhanced oil recovery project is targeted to benefit from approximately 5 million tonnes of CO₂ captured per year [49].

Another postcombustion process of CO₂ capture is the combination of carbonation and calcination. In carbonation, CaO reacts with CO₂ to form CaCO₃, whereas in the regenerator CaCO₃ decomposes to CaO and CO₂. After regeneration, CO₂ is stored while CaO is recycled saving the operating cost. While carbonation takes place at 600–700°C, calcination occurs at temperatures greater than 900°C [56]. The carbonation and calcination occur simultaneously in absorption and regeneration fluidized-bed towers. Although it has been proved to be economical than other CO₂ capture processes (i.e., amine-based process), a primary concern for this calcium looping is the decay in the activity of the absorbent.

Recently, ionic liquids have also gained interest in CO₂ capture. Ionic liquids are salts in the liquid state made of ions and short-lived ion pairs. The thermophysical properties such as low vapor pressure, high polarity, and thermal stability with nontoxic nature make ionic liquids as promising solvents in postcombustion CO₂ capture [57]. Depending on the ionic liquid, CO₂ can be both physically and chemical absorbed. The anionic ionic liquids exhibit higher absorbing capacity over the cationic ionic liquids. Amine-based ionic liquids can be synthesized to react with CO₂ to form complexes. The absorption capacity can be enhanced by loading the synthesized ionic liquids onto stable supports (e.g., silica gel) for chemical absorption [58].

Adsorption

The shortcomings of absorption process such as lower gas-liquid contact area, low CO₂ loading, and absorbent losses have shifted the attention toward adsorption. Adsorption is both a pre- and postcombustion CO₂ capture technology that involves a fluid-solid interface for effective capturing of CO₂. It is a gas-liquid operation, in which the molecules from the gas stream are adsorbed on the surface of the solid adsorbent. The component that is adsorbed is termed as the adsorbate while the solid that adsorbs is known as the adsorbent. The fluid molecules adhere to the surface of adsorbent either by physical forces or chemical reactions. While physical adsorption is called physisorption, the chemical adsorption is known as chemisorption [41]. Since adsorption is a surface phenomenon, the properties of solid adsorbents such as surface area, polarity, and porosity along with surface-reactive species play key roles in determining an ideal adsorbent to capture CO₂ from flue gases. The criteria for selecting effective adsorbents are: (1) low cost; (2) high availability; (3) fast reaction kinetics (i.e., adsorption and desorption); (4) low heat capacity; (5) high CO₂ loading and selectivity; and (6) chemical and thermal stability both during adsorption and regeneration. As the adsorption process is an exothermic process, it favors low temperatures, whereas desorption process (i.e., regeneration) is favored at high temperatures.

Physical adsorption is based on the interactions between the molecules of CO₂ and the solid adsorbent surface. The gas solute molecules of CO₂ preferentially interact and adhere to the adsorbent surface if the intermolecular forces between the adsorbent and gas molecules dominate the existing interactive forces between the gases. The heat of adsorption for physical adsorption process ranges from −20 to −50 kJ/mol [59]. The typical operating conditions are with partial pressures of CO₂ close to 0.1 MPa at normal temperatures (i.e., 25°C) due to the exothermic nature of the adsorption process.

Carbonaceous materials, microporous and mesoporous zeolites, chemically surface-modified polymeric materials along with metal-organic frameworks are found to be the potential adsorbents [47, 59]. Carbonaceous materials such as activated carbon, carbon nanotubes, and carbon molecular sieves can capture CO₂ by physical adsorption. Although with high thermal stability, activated carbons have low adsorption capacities. For improved adsorption characteristics by the carbon-based materials, a high surface area is desired along with chemical modification of adsorbent surface with basic groups to enhance the acidic nature CO₂. Activated carbons exhibit fast kinetics rendering less time for adsorption and desorption cycles. The major drawback of activated carbons is lower selectivity toward CO₂. Microporous carbon molecular sieves with narrow pore size distribution and high pore volume increase both selectivity and loading capacity. Carbon nanotubes (both single- and multi-walled) are growing in demand due to their selectivity toward CO₂ compared to other adsorbents.

Both natural and synthetic zeolites have been applied to capture CO₂ from flue gas streams. Among natural zeolites, mordenite, ferrierite, clinoptilolite, and chabazite exhibit an improved performance in the separation of CO₂ from N₂. The crystalline structure of zeolites results in narrow pore size distribution (i.e., 0.5–1.2 nm), which acts as molecular sieves for separation of gas mixtures [60]. The performance of natural zeolites with the high concentration of sodium and large surface area exhibit greater adsorption capacities with enhanced adsorption kinetics [61]. Conventional zeolites are synthesized by altering the silica-to-aluminum ratio and substitution of other metals (e.g., alkali and alkaline earth metals) which lead to rendering charge inside the pores [47, 59]. The structure of adsorbents can be modified by incorporating the reactive basic sites with amine or alkali/alkaline earth metals and enhance the loading capacities. The strong interaction between the basic groups with acidic CO₂ not only improves the adsorption but also increases the selectivity of CO₂ over other gases.

The adsorption capacities of zeolites depend on the pore size and characteristic ions within the pores. The charge induced by the cations in zeolite enhances the selective adsorption of oppositely charged molecules such as CO₂ (quadrupole moment 14.3 × 10⁻⁴⁰ C.m²)[62]. The adsorption kinetics on zeolites has been found to be quick in attaining the equilibrium loadings within a few minutes. The poor selectivity of zeolites toward CO₂ has directed to explore the incorporation of various cations into the adsorbent structures. The performance of synthetic adsorbents depends on the optimization of operating conditions (i.e., low temperature and high partial pressures), basicity (i.e., type of cations that are incorporated into the pore structure of zeolites), pore size, and pore volume.

Another class of materials named as metal-organic frameworks has received considerable attention to capture CO₂ due to high surface area and flexibility in altering the pore structures and surface properties [63]. The solid networks with the metal ion or cluster vertices coupled with organic spacers are usually known are metal-organic frameworks. The choice of organic linkers in the cluster of metal networks provides an opportunity to tune pore structure and its shape to alter CO₂ selectivity, kinetics and carbon loading capacities. Although metal-organic frameworks demonstrate high loading capacities of pure CO₂, yet in the presence of other gases the CO₂ adsorption is reduced dramatically. A considerable amount of research is being focused for improving the selectivity, stability, and recycling of metal-organic frameworks for multiple cycles. The physical adsorbents have poor selectivity and small capacities relatively at low pressures.

Unlike physical adsorption, chemisorption involves the reaction of CO₂ with the reactive groups on the surface of adsorbent, and the heat of adsorption is in the range of −60 to −90 kJ/mol [59]. Chemisorbents are classified into two classes, primarily amine-based adsorbents and alkali metal-based adsorbents [47, 59]. Amine-based adsorbents are synthesized with the help of impregnation and grafting methods. In impregnated adsorbents, weak interactions exist between the support and amine, whereas grafted amine exhibit strong covalent binding. Based on the interactions, the impregnated adsorbents demonstrate low thermal stability compared to the grafted amine-based adsorbents. Among alkali metal-based adsorbents, the carbonates of calcium, potassium, sodium and lithium have demonstrated better performance toward CO₂ capture. Although lithium-based adsorbents show high adsorption capacities, their high diffusional resistance hampers the commercial application [64, 65]. Sodium and potassium-based carbonated chemisorbents have showed high CO₂ adsorption capacities of 9.43 and 7.23 mmol/g, respectively [59]. The moderate adsorption and desorption cycle temperatures (60–200°C) make them potential adsorbents for carbon capture in the flue gases in the postcombustion processes. Microporous materials are competent with other adsorbents due to their high surface area. The polyphenylene material (PPN-4) showed adsorption capacities close to saturation due to its high surface area of 6460 m²/g [66].

The regeneration of the applied material after the capture of CO₂ is a critical challenge to overcome the energy constraints imposed on the overall process economics. Different regeneration processes such as pressure swing, temperature swing, vacuum swing, hybrid (temperature and pressure), and electric swing adsorption are in practice [56]. The pressure swing adsorption operates at high pressures while desorption occurs at atmospheric pressures. In contrast to pressure swing adsorption, vacuum swing adsorption involves the adsorption and desorption processes at atmospheric pressure and vacuum, respectively [56]. High-temperature flue gas from the power plants is expected to reduce the temperatures for the implication of adsorbents to capture CO₂. Moreover, the flue gas often needs to be pretreated for removing any gases that can poison the adsorbent or reduce the selectivity by undesirable binding to the active sites.

Temperature swing and electric swing adsorptions are other regeneration processes where the adsorption capacities are altered with the change in temperature. Steam or hot air is used as the regeneration medium. In electric swing adsorption, the temperature of adsorbent is increased with the help of electricity (Joule effect) to liberate the adsorbates from adsorbents [56]. In situ heating of adsorbents in electric swing adsorption results in lower energy demand, fast kinetics, and dynamics without constraints on flow rate or heat transfer unlike temperature swing adsorption and vacuum swing adsorption [67]. The demand for longer regeneration time (i.e., in hours) in the case of temperature swing adsorption process compared to seconds in pressure swing adsorption makes the latter an economical option. A combination of pressure and temperature swing adsorption has also been attempted for CO₂ capture [47]. Adsorption also finds application in precombustion processes especially in steam reforming of hydrocarbons and water-gas shift reaction [68]. With the capture of CO₂, the equilibrium shifts toward the right and thereby enhances the productivity of H₂.

Membrane separation

Selective membranes have found versatile applications in the separation of gas mixtures such as in CO₂ capture from natural gas, separation of H₂ from CO₂ in synthesis gas and fuel cells. The two key parameters that decide the performance of the membranes are permeability and selectivity. The volume of gas passing through the membrane per unit area in unit time is termed as permeability. On the other hand, selectivity is defined as the ratio of permeability of the chief component to the other component in mixture. In the postcombustion processes, the flue gas stream comprises of NO_x and SO_x that have adverse impacts on the membranes. Prior to the application of membranes for separation of CO₂ from N₂, it is mandatory for the flue gas to be free of impurities. The flue gas needs to be cooled and compressed to create sufficient pressure differential for CO₂ transport.

The performance of the membranes depends on the driving force, that is, pressure differential (∆P) for the transfer of the solute from feed to permeate side. The flux of a component (J_i) across the membrane of thickness (δ) can be obtained by Ficks law as shown in equation (1) [63].

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://test.scipedia.com:8081/localhost/v1/":): J_i=\left(\frac{P_i}{\delta }\right)\Delta P

(1)

There is always a constraint on the pressure differential in terms of economics and selectivity since the increase in pressure decreases the purity of the CO₂ stream on the permeate side. The mechanism for transfer of the solute from feed to permeate depends on the type of membranes. For example, in porous membranes the solute passes through the membrane by pore diffusion. In contrast, in nonporous and facilitated transport membranes, the solute follows the solution-diffusion mechanism and reacts with carrier molecules [69].

A variety of organic (i.e., polymeric), inorganic, mixed membrane, and hybrid systems can be applied to capture CO₂ in postcombustion processes [63, 69]. Different polymeric organic membranes are synthesized for separation of CO₂ from N₂. The organic membranes such as polyimides, polysulfones, polyarylates, polyacetylenes, polycarbonates, polyaniline, etc. exhibit satisfactory permeability (85–450 Barrer) and selectivity (5–55) [63]. The polymeric membranes with reactive carrier molecules (either amines or carbonates) are flexible to fabricate providing high selectivity due to charging. In a recent investigation, researchers have developed a polyvinylamine membrane with CO₂ permeance of 1000 GPU (Gas Permeance Unit) and selectivity of 200 [70]. Moreover, with materials having a selectivity of 50 and permeance of 1000 GPU, the CCS cost has been estimated to be as low as $23/ton [71].

Inorganic porous membranes such as zeolites, microporous silica, and carbon follow the pore diffusion mechanism for CO₂ transport [69]. Inorganic membranes have low CO₂ permeance with high selectivity. Microporous silica membranes exhibit comparable permeance and selectivity but are restricted for application due to the pore blocking with water and poisoning by SO_x [63, 69]. Membranes with inorganic particles dispersed in the continuous matrix of polymers, known as mixed matrix membranes, have also gained importance. The inorganic materials enhance the permeance and selectivity with good mechanical and thermal stability. In addition, the polymeric materials provide defect-free films at low processing cost. Research efforts are being invested to synthesize thin mixed matrix membranes with high inorganic loading to improve both permeance and selectivity. A hybrid membrane system is the combination of membrane and absorption process to enhance the selective capturing of CO₂. Hydrophobic membranes are usually employed to avoid the interactions between gas and absorption liquid. CO₂ from the gas stream diffuses through the membrane and gets absorbed into the liquid.

Similar to postcombustion processes, membranes are also applied to precombustion processes to selectively separate CO₂ from H₂. High purity CO₂ can be attained with CO₂-selective membranes. Polymeric rubber materials with ether groups, polyethylene glycol polymeric blends and polyethylene oxide block copolymers are considered for high selectivity of CO₂ in the precombustion process [69]. Cross-linked hydrophilic membrane matrix composed of stationary and mobile carriers facilitates high CO₂/H₂ selectivity showing excellent mechanical stability and processing flexibility.

The performance of the facilitated membranes depends on the carrier molecules as well as the highly reactive and stable carriers. Mixed matrix membranes for precombustion processes are targeted to improve CO₂ selectivity and enhance the stability characteristics. The incorporation of nanofillers in polymeric matrix offers high sorption of CO₂ and increases free volume leading to high selectivity and permeability. Carbon nanotubes are found to be potential fillers with high separation and excellent mechanical stabilities [72]. Although mechanical stability can be easily achieved with mixed matrix membranes, yet their low CO₂ separation efficiency is still a challenge to overcome.

The membrane separation processes usually have high energy (i.e., electricity) requirement. For instance, in the case of a target energy requirement of 2 GJ/ton for heat regenerated process, nearly 0.5–0.7 GJ/ton of heat is allowed for membranes [73]. Ho et al. [74] attempted to pressurize the flue gas stream from a coal-fired power plant up to 0.15 MPa while maintaining the permeate stream pressure at 0.008 MPa. About 35% reduction in the cost was achieved in this trail study. Compared to U.S. $82/ton CO₂, the capture cost was U.S. $54/ton CO₂ using membrane separation at the pressurized feed conditions.

Cryogenic distillation

Cryogenic separation of flue gases is based on the freezing points of the components to condense the desired product by reducing the temperature. From the flue gas mixture, CO₂ is separated by cooling the gas mixture below −73.3°C at atmospheric pressure to obtain it in liquid form [75]. The required cryogenic temperatures can be obtained by employing compressor, multi-stage heat exchangers, Joule–Thomson valve and cold traps. The composition of CO₂ in flue gas plays a key role on the operating temperature since the greater purity of CO₂ is attained at lower de-sublimation temperatures. For example, 2% CO₂ is obtained at de-sublimation temperature of −116°C, whereas 15% CO₂ is obtained at −99.9°C [63].

Currently, cryogenic separation has been performed in two ways for postcombustion processes. In the separation method proposed by Clodic and Younes [76], CO₂ is de-sublimated to solid CO₂ on the fins of heat exchangers, which is further heated and pressurized to obtain liquid CO₂ in the recovery stage. In another method by Tuinier et al. [77], packed beds are used for de-sublimation of CO₂. CO₂ is recovered from the packing material by feeding fresh gas stream to increase the temperature and enhance the concentration of CO₂ recovered from the packed bed. Though the cryogenic process is energy intensive, it does not involve any additional chemicals in the separation process.

Cryogenic distillation finds its application for the separation of O₂ from the air. The air separation unit employs cryogenic distillation for separation of O₂ from N₂ and other inert gases (mainly Ar). The energy requirement is proportional to the required purity of O₂. The condensation temperatures less than −182°C are applied to air for obtaining a pure stream of O₂ for oxy-fuel combustion [34]. Due to the oxy-fuel combustion, CO₂ concentration in flue gas stream reaches up to 89 vol% that is very high compared to other combustion processes. The efficiency of oxy-fuel CO₂ capture technology majorly depends on the method to obtain O₂. Typically, O₂ purity of 99.5 vol% is in demand, and an excess of 15% is supplied to the power plants for complete combustion [78]. The unreacted excess O₂ is recycled back, and the product stream is condensed to remove the product water and obtain pure CO₂ for compression and storage.

Some new routes to obtain the cold duty required for cryogenic separation of gases have been deployed. The cold duty from liquefied natural gas at its sites has been found to be an option to decrease the economics of cryogenic processes [56]. Cryogenic distillation is also applied in the separation of acid gases from natural gas. Controlled freeze zone process separates CH₄ and acid gases in a single unit [79]. CO₂ and other sulfur containing compounds along with heavier hydrocarbons result in the liquid product providing an economical route for transport and sequestration.

Hart and Gnanendran [80] have reported a cryogenic separation technology called CryoCell^® that uses the distinctive solidification property of CO₂ as the basis of its separation from natural gas. The CryoCell^® technology reduces water and chemical consumption as well as any corrosion-related issues. The cost savings in a CryoCell^® plant are associated with gas treatment, CO₂ disposal, reduced electricity consumption, elimination of solvent pumping, and abridged heat requirement for amine reboiler duty.

Biological Routes for CO₂ Capture and Sequestration

Algal and bacterial systems

Microalgae, macroalgae, and cyanobacteria can be used to utilize the flue gas CO₂ as their carbon source along with sunlight and other nutrients to produce biofuels, value-added chemicals and byproducts. For the flue gas with 4% CO₂ and flow rate of 0.3 L/min, the carbon fixation rate of 15 g carbon/m² per day by microalgae has been attained [81]. The biofuels produced from algae are considered to be carbon-neutral as the CO₂ emitted from their combustion is consumed by fresh algae and plants during photosynthesis. Algae are simple photosynthetic aquatic organisms with estimated 300,000 species [82], that is, a diversity much greater than that of terrestrial plants. Microalgae photosynthesis can also result in the precipitation of CaCO₃ that is a potential long-lasting carbon sink [83].

The algal cells are made up of polysaccharides, especially hemicellulose (xylose, rhamnose, arabinose, mannose, and glucose); glucosamine; vitamin; proteins; fatty acids; lipids; and amino acids [84, 85]. Some typical green algae include Botryococcus braunii, Chlorella spp., Chlamydomonas reinhardtii, and Dunaliella salina. Algae also include diatoms such as Phaeodactylum tricornutum and Thalassiosira pseudonana as well as heterokonts such as Nannochloropsis and Isochrysis spp. [82]. Algae are used in BTL (biomass-to-liquid) and BTG (biomass-to-gas) conversion systems. They are attractive feedstock for the production of biofuels such as biodiesel, bioethanol, biobutanol, biogasoline, methane, hydrogen, and jet fuels. The lipids or the oily portion of algae are extracted and converted to biodiesel. Botryococcus braunii, although slow growing, is a rich source of hydrocarbons and ether lipids as it contains 60 wt% lipids within its cell wall [86]. The annual yield of oil from algae per unit area is estimated to be between 4.6 and 18.4 L/m² (5000 and 20,000 gallons per acre), which is about 30 times greater than the best oil-producing crop [87, 88].

The algal biomass is harvested and processed to release triacylglycerides for transesterification to produce biodiesel. However, extensive research is being done in recovering the lipids from the intracellular location of algae by energy-efficient and economical ways [89]. In addition, the primary consideration should be to convert most of the carbon from the algal biomass to biofuel with potential high-value byproducts. Biodiesel is one of the primary fuels of interest from algae because of: (1) its higher productivity than other plants; (2) higher accumulation of large amounts of triacylglycerides; and (3) lower cultivation cost with no arable land requirement. Today the top five biodiesel-producing countries in the world are USA, Germany, Argentina, Brazil, and France (Fig. 6).

Figure 6. Top five biodiesel-producing countries in 2012 (Data source: [8]).

After lipid extraction from algal cells, the green waste residue obtained is carbohydrate content of algae that can be fermented into bioethanol or biobutanol. Potts et al. [90] have demonstrated the production of butanol from macroalgae (Ulva lactuca) through fermentation by Clostridium beijerinckii and C. saccharoperbutylacetonicum. From the reducing sugar content of 15.2 g/L in the algal hydrolyzates, about 4 g/L of butanol was recovered as a result of fermentation. Furthermore, Ellis et al. [91] have also demonstrated the production of butanol from wastewater algae via fermentation using C. saccharoperbutylacetonicum. Algae such as Gelidium amansii, Laminaria japonica, Sargassum fulvellum, and Ulva lactuca have been used as raw materials for ethanol production by Kim et al. [92]. Escherichia coli KO11 employed for the fermentation of algal hydrolyzates resulted in ethanol yield of about 0.4 g/g of carbohydrate. The top five bioethanol-producing countries in 2012 were USA, Brazil, China, Canada, and France (Fig. 7).

Figure 7. Top five bioethanol-producing countries in 2012 (Data source: [8]).

Although algae utilize CO₂ during the growth cycle, their anaerobic digestion can result in the production of CH₄ as a gaseous fuel. The solar energy and CO₂ stored in the algal biomass in the form of carbohydrates could be released as CH₄ through anaerobic digestion. Yen and Brune [93] performed anaerobic co-digestion of Scenedesmus spp. and Chlorella spp. along with waste paper to generate CH₄. It is also suggested that the conversion of lipid extracted-algal biomass into CH₄ can recover more energy than the energy from the lipids alone [94]. Thermochemical liquefaction of microalga Spirulina using subcritical and supercritical ethanol has shown to produce high-quality bio-oil due to esterification of organic acids, ethanol, fatty acid methyl/dimethyl, and ethyl esters [95].

Although the benefits of algal cultivation are binary, particularly for CO₂ capturing and biofuel/biochemical production, yet there are some challenges to be addressed. Some disadvantages of microalgal cultivation in carbon sequestration are the high operating cost that makes the algal biofuels more expensive than fossil fuels. Sunlight is another limiting factor for algal growth. The artificial light source in the photobioreactors can be an alternative to sunlight, but their installation consequently adds to the overall production cost. Nevertheless, as the microalgal cultures also result in commercially viable products such as natural pigments, dyes, food additives, health supplements, antioxidants, and bioactive compounds [96], their high market value may offset the high capital investment. Conversely, macroalgae have gained more attention than microalgae due to their higher biomass yield and utilization of flue gas as a direct source of CO₂, which significantly reduces the production cost [97]. The macroalgae such as Gracilaria chilensis, Hizikia fusiforme, and Porphyra yezoensis demonstrated nearly 2–3 times increased growth at elevated levels of CO₂ compared with the atmospheric CO₂ levels [98]. The use of flue gases containing 12–15% CO₂ is also found to maintain the desired pH range optimal for the macroalgal growth [99].

Cyanobacteria, also known as blue-green algae, are photoautotrophic bacteria that use CO₂ as the carbon source along with light and water as energy and electron sources, respectively. Cyanobacteria are photosynthetic in nature and require anaerobic growth conditions and sunlight. In addition to CO₂ fixation, cyanobacteria can also fix N₂ with the aid of nitrogenase enzymes. A few cyanobacteria that have been used for carbon fixation include Anabaena [100], Aphanothece microscopica Nägeli [101], Chlorogleopsis [102], Fischerella [103], and Synechococcus elongates [104]. Synechococcus is a promising cyanobacterium for CO₂ mitigation because of its high CO₂ uptake rate (i.e., 0.025 g/L/h or 0.6 g/L/day) at a cell concentration of 0.286 g/L [97]. Equation (2) gives the overall reaction of photosynthesis by cyanobacteria.

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://test.scipedia.com:8081/localhost/v1/":): {\mbox{CO}}_2+H_2O+\left(\mbox{sun}\right)\mbox{light}\rightarrow {\left({\mbox{CH}}_2O\right)}_n+ Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://test.scipedia.com:8081/localhost/v1/":): O_2

(2)

Cyanobacteria are also referred to as microbial fuel cells due to the ability to generate H₂ as a result of their metabolic pathway. A few cyanobacteria that can produce H₂ belong to the genus Anabaena, Aphanocapsa, Calothrix, Chlamydomonas, Chroococcidiopsis, Cyanothece, Gloebacter, Gloeocapsa, Microcoleus, Microcystis, Microcystis, Nostoc, Oscillatoria, Rhodovulum, Synechococcus, and Synechocystis [97, 105]. Both nitrogenase and hydrogenase enzymes in cyanobacteria can aid in biological H₂ production using CO₂ as the carbon source.

Dedicated energy crops

A dedicated energy crop is a non-food plant variety grown for the primary purpose of harvesting energy or biofuel production. The energy crops are mostly lignocellulosic in nature, and their cultivation is relatively less expensive with low-maintenance farming. The energy crops are used to generate biofuels such as bioethanol, biobutanol, bio-oil, biodiesel, and jet fuels, or combusted to generate electricity and heat. The energy crops can be woody (e.g., willow and poplar) or herbaceous in origin (e.g., Miscanthus, elephant grass, switchgrass, and timothy grass). These crops can be genetically modified for faster growth, greater biomass yields as well as less water and nutrient requirements. In addition to biofuel production, energy crops have tremendous potentials in carbon sequestration. In a study, Miscanthus and switchgrass have shown high yields with low production cost and less environmental impacts compared to conventional agricultural crops [106].

The pyrolysis of an energy crop can result in the production of combustible bio-oil and gases as well as biochar. Nanda et al. [107] and Mohanty et al. [108] have performed the carbon and energy (calorific value) balance of biochar and bio-oil from the pyrolysis of an energy crop – timothy grass. Timothy grass contains 43.4 wt% carbon (C), 6.1 wt% hydrogen (H), 1.3 wt% nitrogen (N), 0.1 wt% sulfur (S), and 45.4 wt% oxygen (O) [109]. The biochar from fast pyrolysis of timothy grass contained 63.7 wt% C, 3.6 wt% H, 1.9 wt% N, 0.04 wt% S and 30.8 wt% O [108], whereas bio-oil from the same process contained 49.2 wt% C, 9.3 wt% H, 2.2 wt% N, 0.9 wt% S, and 38.4 wt% O [107]. However, slow pyrolysis biochar from timothy grass contained 67.5 wt% C, 2.3 wt% H, 1.9 wt% N, 0.1 wt% S and 28.2 wt% O [108], whereas slow pyrolysis bio-oil contained 44.9 wt% C, 8.4 wt% H, 1.8 wt% N, 0.5 wt% S and 44.4 wt% O [107]. As the biochar had more carbon than the biomass, it can make the biorefinery process carbon-negative upon integration with soil amendment for carbon sequestration [110]. The thermochemical conversion of energy crops can also result in energy dense bio-oil and synthesis gas. The calorific value of timothy grass biomass was determined to be 15.9 MJ/kg [109]. While fast pyrolysis bio-oil from timothy grass had high yields, it also demonstrated higher calorific value (23.8 MJ/kg) compared to low yielding slow pyrolysis bio-oil (20.2 MJ/kg) [107].

The soil contains over 2000 Gt of carbon, and the vegetation cover helps in sequestering up to 3 Gt of carbon per annum making it a significant natural carbon pool [97]. Since, the carbon captured in living vegetation is estimated to be around 550 Gt, the trees work as a good carbon sink. The energy crops are lignocellulosic in composition as they contain cellulose, hemicellulose and lignin. The biofuels produced from energy crops are considered carbon-neutral (or near-zero CO₂ accumulation) as the CO₂ emitted from their combustion is recycled by new plants for photosynthesis. In addition, energy crops do not compete with cash crops in terms of arable lands as they can grow in low-value soil without the need for intensive agricultural practices [11].

The carbon-neutral biofuels generated from energy crops can also offset some amount of fossil fuel-derived CO₂. In addition, the carbon sequestered into the soil represents supplementary benefit for CO₂ mitigation that no alternative energy source could provide [111]. Figure 8 gives the trend for the worldwide production of biofuels, especially biodiesel and bioethanol. The continual proliferation in the generation of biofuels is a good indication for the alleviation of CO₂ emissions from fossil fuels [112-114].

Figure 8. Worldwide production of biofuels from 2000 to 2012 (Data source: [8]).

The physiological stimulus of plants to CO₂ has received considerable attention because CO₂ is a substrate for photosynthesis. The chief indications of elevated CO₂ on plants include: (1) stimulated photosynthesis; (2) accumulation of non-structural carbohydrates; (3) reduced tissue N₂ concentration; and (4) increased root-to-shoot ratio [115]. In plants, a substantial portion of photosynthate (i.e., 12–54% of carbon fixed by photosynthesis) is allocated to stimulate the root growth [116]. Much of this additional photosynthate is made available to soil microorganisms as root exudates. Hence, elevated atmospheric CO₂ levels could have a significant impact on the energy flow through microbial food webs in the soil. Nevertheless, studies have shown that the rhizosphere of the plants grown under elevated CO₂ contained 60% of soluble carbon [117]. In most terrestrial energy crop systems, much of the total carbon is below the ground because the soil contains two-third of the total terrestrial carbon in the biosphere [118].

As a response to elevated CO₂, the carbon assimilation in plants can increase through enhanced carbon fixation. This can also lead to higher carbon storage in the soil which is believed to be the largest and most stable terrestrial carbon pool. The high CO₂ levels lead to greater CO₂ fixation in energy crops through photosynthesis and higher carbon storage in the soil through soil-microbial-plant interactions. In addition, soil stores 2–3 times more carbon compared to the atmosphere. The temperature sensitivity of soil organic matter also determines the amount of soil carbon storage and emission over time. The long-term rise in atmospheric CO₂ may stimulate biomass production from energy crops through enhanced root growth resulting in greater carbon input in the soil through rhizodeposition and root exudates [119].

It should also be noted that increased soil microbial activity can lead to a loss of carbon from the soil by respiration and decomposition. However, increased CO₂ concentration can suppress soil organic matter degradation as microorganisms would primarily decompose plant root exudates as readily available carbon source before gearing up to utilize other organic matter from the soil via the priming effect [120]. The soil microorganisms are the chief regulators of soil organic matter and any alteration in its availability impedes their priming effect. In a study with an increase in CO₂ concentration, there was about 85% increase in rhizosphere bacterial population and 170% rise in respiring rhizosphere bacteria [121].

The natural decomposition and mineralization processes transforming soil organic matter into inorganic forms are affected by soil temperature and moisture content [122]. Soil temperature and moisture affect the soil microbial activity at microscale levels, which eventually impacts the cycling of soil organic carbon at local, regional and even global scenarios. The soil organic carbon decomposition and mineralization should be minimized to make an energy crop management system efficient for carbon sequestration. Soil disturbances such as change in the soil temperature, moisture, nutrients, and mineral matter maintain an optimal level of soil organic carbon and prevent its premature decomposition. The farming practices such as tillage is another soil disturbance factor that can disrupt soil aggregates that otherwise occludes soil organic matter [123]. Tillage can increase aeration and land surface area for triggered bacterial activity leading to the decomposition of soil organic matter.

Coalbed methanogenesis

The world coal reserve has dominated as one of the primary sources for fossil fuels. As mentioned earlier, there are significant consequences related to the direct combustion of coal concerning the GHG emissions. By reducing or eliminating the need for coal combustion, carbon emissions can be reduced dramatically, considering that a 500-megawatt coal-fired power plant produces 3 tonnes of CO₂ per year. This could potentially result in a net CO₂ emission reduction of 25% or more in the developed countries. A small but growing body of work related to coalbed methanogenesis exists today [124] including studies on methane-producing microbial communities from different coal seams in USA [125-127], Canada [128], China [129], and India [130].

Biogenic CH₄ is generated through anaerobic microbial process of methanogenesis and constitutes a significant fraction of the natural gas reserves. It is generally accepted that biogenic coalbed CH₄ is a product of coal biodegradation by methanogenic archaea and syntrophic bacteria inhabiting coal beds [129]. These microorganisms artificially stimulate the regeneration of biogenic coalbed CH₄. There have also been reports of field trials conducted in USA where stimulants have been added to coal seams to enhance biogenic CH₄ production [131]. Although the understanding of microbial coal conversion remains rudimentary, it is thought that both acetoclastic and hydrogenotrophic methanogens produce CH₄ by consuming small molecules generated from in situ biodegradation of coal components by other microorganisms [132]. It is reported that acetotrophic, hydrogenotrophic, and methylotrophic methanogenesis are the pathways for production of biogenic CH₄ in coalbed reservoirs [133, 134].

A study on the bioconversion of coal using core-flooding experiments revealed that coal can be economically converted into CH₄ and other value-added products [135]. Along with CH₄ generation, the methanogens produce CO₂, which can be pumped into a depleted oil reservoir for enhanced oil recovery [136], thereby enabling an efficient mechanism for geological carbon sequestration. However, anaerobic subsurface coal bioconversion science is in its infancy. While methanogenesis seems to be a feasible approach, other possibilities for biofuel generation are currently largely untapped. The understanding of microbial life in such coal seams, often refer to as the “microbial dark matter”, is still being pursued vigorously due to the advancements of DNA sequencing and genomic analysis. New pathways for in situ bioconversion of coal to CH₄ would shed more light toward the overall reduction in GHGs.

Geological Routes for Carbon Sequestration

Biochar amendment

Increasing the soil carbon content is one of the best strategies to sequester carbon for longer timescales because more than 80% of organic carbon is preserved in the soil [137]. Biochar is one of the promising bioresources rich in recalcitrant carbon, thus making it a long-standing carbon pool. Biochar, a byproduct of biomass pyrolysis and gasification, is obtained along with crude bio-oil and gas components (e.g., H₂, CO, CO₂ and CH₄) [108, 138]. It comprises mostly of the recalcitrant aromatic forms of carbon that cannot be readily oxidized and released into the atmosphere as CO₂ [139]. The occurrence of biochar in the Amazonian terra preta soil is a historical indication that biochar can preserve carbon in the soil for hundreds to thousands of years. This makes biochar application a carbon-negative approach for CO₂ sequestration.

Biochar can also capture CO₂ through carbon sequestration and exclude NO_x and SO_x from flue gas through air purification. Biochar applied to soil can also help decrease emissions of N₂O and CH₄ whose respective potencies are 300 and 23 times higher than CO₂ [140]. The carbon sequestration by biochar is different to that of afforestation, conversion into trees, and no-tillage agriculture. While agricultural areas switched to no-tillage farming may lose carbon capturing efficiency within two decades, forestlands start maturing over the years and release CO₂ [141]. These divergences make biochar a long-term carbon sink for reducing CO₂ emissions. The non-charred carbonaceous materials such as dead plant or animal wastes decompose in the soil to release carbon slowly over time that might take a few decades. However, the complete loss of carbon from biochar is very slow and difficult to estimate suggesting it to preserve carbon from centuries to millennia.

Integrating pyrolysis with carbon sequestration can help a biorefinery earn carbon credits. In other words, pyrolysis of biomass converts about 50% of the biomass carbon to bio-oil and gases, while the remaining 50% of carbon is stabilized in biochar [142]. Although burning of bio-oil and combustible gases (H₂ and CH₄) for energy can release CO₂, it is also consumed by plants for photosynthesis making the process carbon-neutral. Consequently, amending biochar to the soil can capture the stable biomass carbon for longer durations making the entire refinery process carbon-negative [11]. This helps in CO₂ abatement and earning carbon credits. In Brazil, carbonization of sugarcane bagasse has shown to optimize the pyrolysis process by producing biochar for use in household and industrial applications [143].

Carbon credits are based in the ratio of 1:1 in relation to the tons of CO₂ stored or removed through carbon sequestration. Nearly 8 Gt of CO₂ is being accumulated per year through burning of fossil fuels [144]. This represents around 8–10 billion carbon credits being created every year. At present, CO₂ trading price by Chicago Climate Exchange is set at U.S. $4/ton [141]. In such a scenario, the carbon credit economy would be comparable in size to the current fossil fuel economy. Hence, the Earths capacity for storing biochar would be limitless. For evaluating the carbon budget and carbon credit of an integrated bioenergy system, a few factors should be taken into consideration such as: (1) GHG emissions associated with the production of biomass; (2) CO₂ emissions and energy input during biomass harvest, transportation, pretreatment, and conversion; and (3) energy input in product processing and upgrading, flue gas purification, and biochar handling. These factors could address the amount of GHG emissions and carbon balance in each step of the overall biomass conversion chain with carbon sequestration potential by biochar.

Biochar also has several advantages to the soil upon its amendment. Biochar enhances the available water holding capacity, plants' root development, and soil faunal population [145]. The micropores in biochar allow the diffusion of air and water, thereby reducing the overall bulk density of the soil. Biochar, when mixed with the soil, has the increased internal surface area for better adsorption of soil nutrients and humus. Biochar retains substantial quantities of mineral matter in the soil to make them available to plants through their roots. Lehmann [139] suggested two main routes through which biochar can reduce soil or groundwater pollution, that are: (1) by retaining nutrients and minerals in the soil, thereby lowering their chances of leaching into groundwater; and (2) by improving nutrient availability in the soil thus reducing the use of chemical fertilizers.

Oceanic storage and mineralization

The oceanic CO₂ storage has an expected retention rate of several hundreds of years [146]. The formations of underground carbonate minerals prolong the residence time reducing the chances of CO₂ escape into the atmosphere [147]. The ocean contains approximately 40,000 Gt of carbon compared to 750 Gt of carbon in the atmosphere and 2200 Gt of carbon in the terrestrial ecosystem [56]. The carbon sequestration in oceans is expected at the rate of 5 Gt of carbon per year by 2100 [148]. In the scenario of projected climate change where high CO₂ saturation in oceans may enhance the sequestration rate due to greater carbon fixation by marine photosynthetic organisms, higher temperature might also decrease the solubility of CO₂ in ocean water [97].

Fung et al. [149] performed a series of experiments with the National Center for Atmospheric Research (NCAR) – Climate System Model (CSM1.4) to understand the influence of land and ocean as repositories and sinks of CO₂. The CSM1.4 model included the modified terrestrial biogeochemistry model (i.e., Carnegie–Ames–Stanford Approach or CASA) and the modified Ocean Carbon Intercomparison Project 2 (i.e., OCMIP-2) oceanic biogeochemistry model. The modeling results suggested that carbon sink strengths vary with CO₂ emission rates. In simple words, the terrestrial and oceanic carbon storage capacity can decrease with rapid emissions of CO₂ and other GHGs. According to the model, the magnitude of oceanic carbon storage will depend on the oceanic circulation and sensitivity of marine ecosystem processes at changing climatic conditions (i.e., high temperature and elevated CO₂ levels).

The CO₂ after being separated from the flue gas mixture is compressed to liquid or supercritical fluid state for storage deep under the oceans via pipeline [56]. The CO₂ can be transported and injected into the ocean as dry ice or through vertical injection, inclined pipe transfer, or pipe towed by ship [150]. The longevity of CO₂ captured and injecting in deep reservoirs under the ocean is around thousands of years. Under supercritical conditions, the CO₂ is less dense than water and tends to migrate up the storage formation. Water above its critical temperature (T_c > 374°C) and critical pressure (P_c > 22.1 MPa) is called supercritical water. Likewise, CO₂ above its critical temperature (T_c > 31°C) and critical pressure (P_c > 7.4 MPa) is called as supercritical CO₂. In the event of increasing pressure or weakening of the geological reservoir, the highly pressured CO₂ inclines to escape causing uncertainties on its deep water storability [151]. Recently, saline aquifers were found to be attractive for geological CO₂ storage under water [152, 153]. However, the geological storage of CO₂ under the high-pressure system is under the developmental stage.

The inorganic carbon content in the oceans worldwide is estimated to be around 38,000 Gt with up to 2 Gt of carbon being sequestered annually [97]. The carbon sequestration in oceans can also occur naturally through several ways such as: (1) photosynthesis by deep water aquatic plants as well as photosynthetic bacteria and algae; (2) decomposition of dead aquatic plants and animals; and (3) carbonates formation. The carbon fixed by marine plants and microorganisms is also sequestered upon their decomposition and vertical flux for settlement in the ocean bed.

The photosynthesis by marine plants and microorganisms in the sunlit region and the turbulent diffusion of ocean water ensure the constant flux of carbon from the oceanic surfaces [154]. The biological carbon fixation and subsequent sequestration are enhanced through phytoplankton in the ocean by supplementing additional nutrients such as nitrogen and iron. Such a scheme can also help to earn carbon credits in response to the increased CO₂ emissions. The carbon fixed by the top surface photosynthetic organisms (e.g., aquatic plants, phytoplankton, algae, and photosynthetic bacteria) is eventually sequestered upon their death and descending to the ocean floor. However, any ecological disturbances and mutations in the plankton and photosynthetic microorganisms as a result of the additional nutrients cannot be ignored. The tumbling of surplus quantities of biomass formation from photosynthetic organisms and plankton can also lead to anaerobic digestion and CH₄ formation counteracting carbon sequestration [146].

The solubility pump is a technology used to dissolve CO₂ in ocean water for carbonate formation. This involves the conversion of CO₂ to inorganic carbonates through mineralization. The mineralization process offers an opportunity for the durable storage of CO₂, although it is subject to natural weathering process over longer durations. After the carbonates are generated, their complexes are carried away via water current towards the benthic ocean regions where they linger for longer periods [155]. In the oceans, the carbon is stored in the form of carbonates, especially CaCO₃ and MgCO₃, and their formation is summarized in equations (3) and (4).

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://test.scipedia.com:8081/localhost/v1/":): {\mbox{CO}}_2+{\mbox{CaSiO}}_3\leftrightarrow {\mbox{CaCO}}_3+\right. Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://test.scipedia.com:8081/localhost/v1/":): \left. {\mbox{SiO}}_2

(3)

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://test.scipedia.com:8081/localhost/v1/":): {\mbox{CO}}_2+\mbox{MgSi}\leftrightarrow {\mbox{MgCO}}_3+\right. Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://test.scipedia.com:8081/localhost/v1/":): \left. {\mbox{SiO}}_2

(4)

The carbonate formation acts as a long-term source of carbon that is driven by the metabolic activities by photosynthetic organisms including cyanobacteria and algae [156]. The formation of carbonates especially CaCO₃ in the form of shells is also found in coral reefs by many aquatic organisms such as benthic molluscs, echinoderms, corals, and plankton belonging to the group coccolithophore. However, the particulate organic matter and dissolved organic matter in oceans have a tendency to undergo microbial mineralization releasing most of the organic matter to dissolved inorganic carbon [156]. However, a small proportion of particulate organic matter escapes mineralization as it reaches the sediment where organic carbon remains buried for hundreds of years [157].

The geological CO₂ storage can also have some limitations in the case of unforeseen CO₂ leakage. Due to asphyxiation, CO₂ poses life-threatening risks for aquatic organisms as it can accumulate locally due to its density. Higher CO₂ levels may also change the water pH causing acidification of groundwater, thereby dissolving many toxic heavy metals [158]. The ocean acidification by CO₂ leakage has a direct impact on the growth of corals. It also alters the chemical speciation and biogeochemical cycling of many elements and compounds. Ocean acidification also lowers the saturation states of CaCO₃, which has adverse impacts on the shell-forming marine organisms such as plankton, corals, mussels, snails, clams, etc. [159]. The broader implications and adaptation of marine organisms against increasing CO₂ is not well understood for ocean ecosystems; hence future research is required to advance the innocuous geological CO₂ storage approach.

Conclusions

Carbon sequestration is considered as a feasible approach to mitigate the increased greenhouse gas emissions that lead to global warming and climate change. The currently available technologies for carbon capture and sequestration can be classified under physicochemical, biological and geological routes. The physicochemical route includes some fast-track techniques for the separation of CO₂ from flue gas mixtures with the use of absorbents, adsorbents, gas separation membranes and cryogenic distillation. In contrast, the biological route involves a long-term approach such as carbon fixation through the cultivation of microalgae, macroalgae, cyanobacteria, and energy crops. The biofuels derived from energy crops (e.g., temperate grasses and short-rotation woody biomass) are accounted as carbon-neutral as the plants recycle the CO₂ released from biofuel combustion. Coal, a geological fossil fuel resource, is also subject to biogenic degradation by methanogenic bacteria resulting in CH₄ generation. This biogenic CH₄ could be considered as a clean fuel gas compared to the CO₂ that would otherwise be released upon coal combustion. Finally, the geological storage of carbon includes amendment of biochar into the soil, pumping of CO₂ deep into the oceans, and mineralization of CO₂ in the form of carbonates in seawater. Although these routes seem to be promising, yet their commercial applications are required for implementation toward carbon credits and carbon trading. The current challenge is to develop an international regulatory framework that would help decide the viability of these carbon sequestering technologies based on a long-term lifecycle assessment.

Acknowledgments

The authors thank Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support toward this renewable energy research.

Conflict of Interest

None declared.

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