CO2 utilization: Turning greenhouse gas into fuels and valuable products

Journal of Environmental Management 260 (2020) 110059

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Research article

CO2 utilization: Turning greenhouse gas into fuels and valuable products M.N. Anwar a, *, A. Fayyaz a, N.F. Sohail b, M.F. Khokhar b, M. Baqar a, A. Yasar a, K. Rasool c, A. Nazir d, M.U.F. Raja b, M. Rehan e, M. Aghbashlo f, M. Tabatabaei g, h, i, j, A.S. Nizami a a

Sustainable Development Study Centre, Government College University, Lahore, Pakistan Institute of Environmental Sciences and Engineering, National University of Sciences and Technology Islamabad, Pakistan c Qatar Environment and Energy Research Institute (QEERI), Hamad Bin Khalifa University, Qatar Foundation, P.O. Box 5825, Doha, Qatar d Department of Environmental Science and Policy, Lahore School of Economics, Lahore, Pakistan e Center of Excellence in Environmental Studies (CEES), King Abdulaziz University, Jeddah, Saudi Arabia f Department of Mechanical Engineering of Agricultural Machinery, Faculty of Agricultural Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran g Faculty of Plantation and Agrotechnology, Universiti Teknologi MARA (UiTM), 40450, Shah Alam, Selangor, Malaysia h Biofuel Research Team (BRTeam), Karaj, Iran i Microbial Biotechnology Department, Agricultural Biotechnology Research Institute of Iran (ABRII), Agricultural Research, Education, and Extension Organization (AREEO), Karaj, Iran j Faculty of Mechanical Engineering, Ho Chi Minh City University of Transport, Ho Chi Minh City, Viet Nam b

A R T I C L E I N F O

A B S T R A C T

Keywords: Climate change CO2 utilization Desalination Algal biofuel Renewable energy Greenhouse gas

This study critically reviews the recent developments and future opportunities pertinent to the conversion of CO2 as a potent greenhouse gas (GHG) to fuels and valuable products. CO2 emissions have reached an alarming level of around 410 ppm and have become the primary driver of global warming and climate change leading to devastating events such as droughts, hurricanes, torrential rains, floods, tornados and wildfires across the world. These events are responsible for thousands of deaths and have adversely affected the economic development of many countries, loss of billions of dollars, across the globe. One of the promising choices to tackle this issue is carbon sequestration by pre- and post-combustion processes and oxyfuel combustion. The captured CO2 can be converted into fuels and valuable products, including methanol, dimethyl ether (DME), and methane (CH4). The efficient use of the sequestered CO2 for the desalinization might be critical in overcoming water scarcity and energy issues in developing countries. Using the sequestered CO2 to produce algae in combination with waste­ water, and producing biofuels is among the promising strategies. Many methods, like direct combustion, fermentation, transesterification, pyrolysis, anaerobic digestion (AD), and gasification, can be used for the conversion of algae into biofuel. Direct air capturing (DAC) is another productive technique for absorbing CO2 from the atmosphere and converting it into various useful energy resources like CH4. These methods can effectively tackle the issues of climate change, water security, and energy crises. However, future research is required to make these conversion methods cost-effective and commercially applicable.

1. Introduction The recent increase in global temperature is closely linked to the increased anthropogenic emissions of carbon dioxide (CO2) in the last century ( Kumar et al., 2018a, 2018b). Global energy consumption is expected to increase by over 48%, reaching 815 quadrillions Btu by 2040 vs. the 2012 baseline (Vooradi et al., 2018). Fossil fuels, having the largest contribution to this energy paradigm, are the most significant source of CO2 emissions (Vooradi et al., 2018). Furthermore, the

conversion of raw materials in industries such as aluminum, textile, pulp and paper, refineries, cement, iron, and steel as well as landfills (Ouda et al., 2016) also result in additional CO2 emissions to the atmosphere. CO2 alone accounts for around 77% of the total greenhouse gases (GHGs) emissions (Ellabban et al., 2014). The natural removal of CO2 through forests and oceans is not enough to remove the excessive amount of CO2 present in the atmosphere. Renewable energies such as hydropower, wind and solar energy can serve as alternatives to fossil fuels mitigating this challenge (Aghbashlo et al., 2018b), however these sources depend upon geographical, nocturnal, and seasonal variations.

* Corresponding author. E-mail address: [email protected] (M.N. Anwar). https://doi.org/10.1016/j.jenvman.2019.110059 Received 5 September 2019; Received in revised form 23 December 2019; Accepted 31 December 2019 Available online 21 January 2020 0301-4797/© 2020 Elsevier Ltd. All rights reserved.

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Nomenclature Table/Box AD BiR CaO Ca (OH)2 CCS CCU CCUS CH4 CO2 CRI DAC DME DMR FAME FFA GHG GT IEA IRR

IRS LCA LCO2 LDPE LECZ LED LPG MEA MTBE NaOH NGCC NREL PBR RO SHS ST TAG TDS TRL U.S DOE WHO

Anaerobic Digestion Bioreforming Calcium Oxide Calcium Hydroxide Carbon Capture and Storage Carbon Capture and Utilization Carbon Capture, Utilization, and Storage Methane Carbon Dioxide Carbon Recycling International Direct Air Capturing Dimethyl Ether Dry Methane Reforming Fatty Acid Methyl Ester Free Fatty Acids Greenhouse Gases Gas Thermal International Energy Agency Internal Rate of Return

To achieve the goal set forth by the Paris Climate Accord and to limit mean global temperature increase to 2 � C and CO2 concentration to 450 ppm, a sustainable mechanism to sequester 800 Gt of CO2 from the at­ mosphere during 2010–2150 is an imperative need (Mac Dowell et al., 2017). At present, the world has a CO2 utilization potential of 3.7 Gt/yr, which is only 10% of the global CO2 emissions. However, through coupling industries with suitable carbon capture and storage (CCS) projects, the CO2 sequestration capacity could be increased (Koyt­ soumpa et al., 2018). Such integration could also result in the simulta­ neous production of fuels and many value-added products. This global initiative could offer an estimated annual market of 0.8–1.1 trillion dollars on carbon-based products by just utilizing the 10% CO2 (Koyt­ soumpa et al., 2018). Therefore, carbon capture, utilization, and storage (CCUS) could be considered a medium-term substitute to reduce global CO2 emissions (Anwar et al., 2018). Technological improvements to minimize energy consumption as well as retrofitting the existing plants for CO2 utilization could be a promising approach in the upcoming years (Vooradi et al., 2018). According to Europe’s energy road map towards a low carbon energy system, there must be 40% reduction in GHGs emissions by 2030, and this could only be achieved by employing CCS technologies in fossil fuelbased power generation systems (P� erez-Fortes et al., 2016). In other words, according to the 2030 Climate and Energy Policy Framework, the target for 2030 is to reduce the CO2 emissions of the power sector origin, from the 2013 baseline of 3400 Mt CO2 to 1550 Mt CO2, and to reach an emission-free electricity production system eventually (P�erez-Fortes et al., 2016). Carbon capture and utilization (CCU) symbolizes a new economic market by allowing the stakeholders to use captured CO2 as raw material to generate fuels and value-added products through other processes. The basic methods of CCU are explained in Table 1. The utilization routes could be the synthesis of fuels, chemicals and materials including methanol (P� erez-Fortes et al., 2016), dimethyl ether (DME) (Olah et al., 2008), methane, formic acid, polyurethanes, carbonates, ammonia, and urea as well as utilizing the physicochemical properties of CO2 in other processes (Van-Dal and Bouallou, 2013). The captured CO2 can be employed in biological conversions, including direct photo-conversion of CO2 (Park et al., 2015), bacterial CO2 fermentation, and algae biorefinery (Adeniyi et al., 2018; Nizami et al., 2017). From the technical point of view, CO2 as a raw material in different synthesis reactions could be used by either incorporating the whole CO2 moiety into organic backbones to yield exothermic reactions or by

Indus River System Life Cycle Assessment Liquid CO2 Low-density Polyethylene Low Elevation Coastal Zones Light Emitting Diodes Liquid Petroleum Gas Monoethanolamine Methyl Tertiary-butyl Ether Sodium Hydroxide Natural Gas Combined Cycle National Renewable Energy Laboratory Packed Bed Reactor Reverse Osmosis Switchable Hydrophilicity Solvents Solar/Thermal Triacylglyceride Total Dissolved Solids Technology Readiness Level U.S Department of Energy and Department of Defense World Health Organization

including only a portion of it to other C1 or Cn molecules producing endothermic reactions. Endothermic reactions are the most common ones and can be used to store renewable energy by using the excess electric energy to integrate CO2 reduction and water splitting, e.g., converting CO2 into an H2 carrier. The consumption of energy in CCU is hence essential and must be considered when performing the emissions balance (Olah et al., 2008). Most of the CCU technologies in which the captured CO2 is converted into a product can only delay the CO2 emis­ sions into the atmosphere. This delay depends upon how and when the product is consumed. However, it should also be noted that CCU im­ pedes not only CO2 discharge into the atmosphere but also reduces the raw materials (feedstock) required to produce the same product and therefore, prevents the release of different emissions in case of using conventional production pathways (Al-Mamoori et al., 2017). Another limitation of CCU is economic unfeasibility. Through an integrated, multi-product, and retrofit approach this barrier could also be overcome (Fern� andez-Dacosta et al., 2018). CCU is still in its infancy, and partic­ ular issues, including poor product selectivity, low activation rates of CO2, poor stability of the material, and underdeveloped technologies are the main factors hindering its application at a commercial level (Olajire, 2018). This article is aimed at critically reviewing the recent developments and future opportunities pertinent to the conversion of CO2 to market­ able fuels and valuable products. This study presents a thorough over­ view of how various seawater desalination technologies can utilize the captured CO2 and hence, proving a panacea to fighting the exacerbation of CO2 emissions into the atmosphere and groundwater scarcity at the same time. Moreover, techno-economic analysis, existing policies and future prospects of different CCU technologies are presented. The ambition is to enlighten the scientific community, policy-makers, gov­ ernment agencies, and other stakeholders about the importance of CCU to achieve the Global Paris Climate Accord by lowering global CO2 emissions. A multi-product CCU system, in contrast with a single prod­ uct approach, is also highlighted. CCU is at varying levels of develop­ ment ranging from the laboratory- (e.g., photo-catalysis) to the demonstration-(e.g., synthesis of methanol by direct hydrogenation), and pilot-scale (e.g., synthesis of syngas) projects. Therefore, the pros­ pects for each CCU alternative are at different time horizons.

2

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Table 1 Different Methods for the utilization of CO2.

Table 2 Factors affecting the algal growth.

Studies

Key objectives

Key findings

Process products

Parameters

Ranges

Abdelaziz et al. (2017)

In this study, three processes are compared techno economically, to find the optimum method to produce methanol by flue gas. Firstly, flue gas is directly converted into methanol. Second process includes the separation of water from the flue gas before conversion and in the last CO2 is separated from flue gas before the processing. Two algal strains, Chlorella sp. and Chlorococcum sp. were grown in an industrial wastewater and flue gas supply to observe the biomass production, CO2 fixation and nutrient removal efficiency. A techno-environmental assessment has been performed for the dimethyl ether production by capturing and without capturing CO2.

Water removal process is reported as an optimum method to produce economical methanol. 0.625 tons of methanol can be produced by using per ton of CO2 from the flue gas.

Methanol

pH

7–8

Yadav et al. (2019)

Schakel et al. (2016)

Yang et al. (2018)

Al Ketife et al., 2019

Post hydrothermal wastewater and activated carbon granules was used as nutrient for the cultivation of Chlorella vulgaris to produce methane. A large-scale experimentation was conducted to estimate the breakeven selling price of algal biofuel using flue gas as a carbon source and wastewater as a nutrient source.

Light

A biomass increment of 1.74 folds have been observed with the CO2 fixation of 187.65 mg L 1 d 1 along with 90% removal of nitrogen.

Lipids and carbohydrates rich biomass

Climate change potential was evaluated. A prominent reduction of 7% in CCP has been observed in the case of capturing CO2. A massive amount of methane i.e. 67.7%–228 mL/g CODremoval, has been observed using these nutrient sources.

Dimethy ether (DME)

The breakeven selling price of biomass estimated was $0.544 per kg resulting into $0.9 L 1 for extracted biomass.

Bio-crude

Temperature

33 μmol m μmol m 2 s 20–30 � C

Salinity

20-24 gl-1

Micronutrients (Iron and Manganese) Essential trace elements (Cobalt, Zinc, Boron, Copper and Molybdenum) Macronutrients (Nitrogen/ phosphorous) Carbon dioxide

30–2.5 ppm

References

2 1

s

1

400

4.5–2.5 ppm 25 mg/l for Nitrogen and 2 mg/l for phosphorous 1.83 kg CO2 for 1 g algal production

Sharma et al. (2018) Singh and Singh (2015) Singh and Singh (2015) Adeniyi et al. (2018) Juneja et al. (2013) Juneja et al. (2013) Mostert and Grobbelaar, 1987 Wu et al. (2018)

algae can fix inorganic and organic atmospheric carbon, respectively. Whereas some mixotrophic species of algae can fix both organic and inorganic atmospheric carbon (Singh and Olsen, 2011). Some micro­ algae are photoorganitrophic, also named as photoheterotrophic, pho­ toassimilation, and photometabolism, which use a light source and fix organic carbon. The difference between mixotrophic and photo­ heterotrophic is in their energy sources (Mata et al., 2010). Algae also require less water, and as mentioned earlier, they absorb more CO2, e.g., algal biomass can efficiently absorb 1.83 kg CO2/kg biomass (Wu et al., 2018). Municipal and agricultural wastewaters contain 10–100 mg/L and 1000 mg/L of nitrogen and phosphorous content, respectively (Mathimani and Pugazhendhi, 2018). Thanks to their ability to absorb phosphate, nitrate, and ammonium ions, algae can also be employed for wastewater treatment (Voloshin et al., 2016). Algal oil, along with the algal biomass residue after oil extraction can be utilized as energy sources. Algal oil can be converted into biodiesel (Ali et al., 2017) while the biomass can be transformed into syngas through co-combustion and ultimately into electricity (Chang et al., 2017). Waste gasses from industries and power stations, i.e., flue gases containing a high concentration of CO2, provide an opportunity for the increment of photosynthetic activity of algae (Faried et al., 2017). The liquid form of CO2 is advantageous in terms of the ease of transport and storage. Several sources of liquid CO2 can be used. For instance, the reforming phase of ammonia production releases flue gases containing CO2. Monoethanolamine (MEA) that is an absorbing agent can absorb and separate the CO2. The post-combustion capture process by MEA can also be employed to collect CO2. If the algae production site is in the vicinity of a flue gas production source, the flue gas can be directly injected into open algae production ponds, hence reducing the harmful environmental impacts and production costs (Collotta et al., 2018). Flue gas can be provided for the cultivation of microalgae as a carbon source. Many studies have been reported for the cultivation of microalgae using flue gas. Nitrogen and sulphur oxides in the flue gas reduce the pH of the medium and affect the growth of microalgae. Therefore, they should be removed before the injection of flue gas. However, few microalgal strains such as Nannochloris sp. and Nannochloropsis sp. are not affected by 50 ppm of SO2 supply along with 15% of CO2 (Zhu et al., 2014). Compared to conventional petroleum-based fuels, the utilization of algal biofuels can reduce the net carbon emissions by 78% (Ali et al., 2017). This is attributed to the fact that the carbon emissions associated with the combustion of biofuel take place within a closed carbon cycle because terrestrial plants and algal growth can reabsorb them. As mentioned earlier, algae biomass can be transformed into biofuels employing fermentation, transesterification, pyrolysis, anaerobic digestion (AD), hydrotreatment, and direct combustion (Adeniyi et al.,

Methane

2. Utilization of CO2 to produce biofuels from algae Algae is a promising renewable source of energy because of its high oil and protein contents. It can be used as regular feed for shrimps and fish, to produce bio-fertilizers, to treat wastewater treatment, and for CO2 sequestration (Adeniyi et al., 2018; Mathimani and Pugazhendhi, 2018). Several factors significantly affect algal growth, including pH, light (quantity and quality), salinity, temperature, nutrient availability, oxygen, CO2, as well as biotic elements like bacteria, viruses, and fungi (Table 2). Algae can grow on saline or marginal lands and provides employment and financial benefits (Ali et al., 2017). Algae, in com­ parison with the terrestrial plants, have a greater capacity to produce neutral fats and higher photosynthetic efficiency and biomass produc­ tivity (Voloshin et al., 2016; Su et al., 2017). These organisms can produce 30 folds more oil per unit land than terrestrial oilseed plants and require 49–132 times less space than soybean (Mathimani and Pugazhendhi, 2018; Singh and Olsen, 2011). On the other hand, green algae such as Chlorophyta can fix 10–50 times more CO2 than terrestrial plants (Mata et al., 2010). The autotrophic and heterotrophic species of 3

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2018). Historically, the 1950s oil crisis in the USA triggered research in marine algae energy (Su et al., 2017). Overall, algal biofuels offer numerous environmental benefits vs. conventional biofuels including a reduction in GHGs emissions, reduced carbon footprint, more energy reliability, more economic viability, reduced land-use change (LUC), and production of other value-added products (Naqvi et al., 2018; Mathimani and Pugazhendhi, 2018).

system (Leite et al., 2013). Various methods could perform harvestings such as flocculation, dissolved air floatation, and centrifugation (Sun et al., 2011). For algal oil extraction to generate biodiesel, the oil is extracted from the biomass by mechanical cell disruption and solvent extraction at high temperatures. The solvent is used to separate water and remaining biomass from triacylglyceride (TAG) and is then sepa­ rated from TAG generating algal oil of over 99.5% purity. The biomass and water fraction are subsequently used to generate biogas through the AD process (Sun et al., 2011).

2.1. Algae cultivation, harvesting, and dewatering

2.2. Conversion of algal biomass into biofuels

Algae grow efficiently under the proper source of light, pH, tem­ perature, source of carbon and other nutrients (Rashid et al., 2014; Faried et al., 2017; Qari et al., 2017). CO2 of industrial flue gas origin can be supplied to open ponds by spargers and sumps to avoid out gassing (Sun et al., 2011). With light, algae grow efficiently under a specific spectrum, adequate photoperiod, and color of light. Blue is a higher energy light and can cause photo-inhibition. Whereas, red light, due to its efficiency to excite chlorophyll a and b in microalgae, is more desirable. The respiration is initiated in darkness, and under such cir­ cumstances, light-emitting diodes (LED) can serve as an efficient source of light for photosynthesis. Nitrogen in the form of nitrate, nitrite, urea, and ammonia is also an essential component for algal cultivation; regulating the growth metabolites and protein synthesis. Phosphorous supplementation is also of importance as this element plays a crucial role in energy conversions in algal cells. The optimum nitrogen to phos­ phorous ratio is 20: 1 (Rashid et al., 2014). Hydrogen ions (represented by pH) are also an essential factor for algal biomass production. pH range of 7–9 can well support algal growth with values ranging between 8.2 and 8.5 regarded as optimum conditions. Optimum temperature conditions for algal growth ranges between 20 and 24 � C (Faried et al., 2017). Trace elements, including molybdenum, cobalt, boron, and iron, can also increase the efficiency of algal biomass production. For instance, supplementation of molybdenum has been reported to increase algal growth by 37.9%; cobalt by 30%, boron by 27.6%, and manganese by 20.7% (Rashid et al., 2014). Several types of photo-bioreactors can be employed to produce algae (Fig. 1). Among these, vertical column photo-bioreactors are inexpen­ sive, easy to operate, and compact while still regarded as the most efficient ones and can be used for large-scale production of algae (Faried et al., 2017). It should be noted that efficient conversion to algae biomass into energy carriers takes place when dry biomass is used. Therefore, the dewatering of algal biomass is of critical importance, and it requires a considerable amount of energy (Asomaning et al., 2016). This further highlights the significance of implementing an efficient harvesting method to achieve a cost-effective algal biofuel production

There are several methods for the conversion of algal biomass into biofuels, i.e., transesterification, gasification, AD, direct combustion, fermentation, and pyrolysis (Fig. 2). A brief description for each of these methods is elucidated below. Transesterification is employed for the conversion of algal oil into fatty acid methyl ester (FAME) also known as biodiesel (Aghbashlo et al., 2018a). Throughout this process, the long-chain fatty acids (C14–C24) contained in algal oil react with an alcohol (methanol or ethanol) in the presence of a catalyst (mostly KOH or NaOH) to produce their corresponding FAMEs and glycerol as by-product (Eq. (1)) (Adeniyi et al., 2018; Tabatabaei et al., 2019). Two methodologies can be employed for biodiesel production from algal biomass: 1) oil extraction and transesterification and 2) In situ transesterification. Triglycerides þ Methanol →Glycerol þ Methylesters ðbiodieselÞ

(1)

Gasification involves the processing of biomass to syngas, i.e., carbon monoxide, methane, and hydrogen. These gases can then be used to produce fuels. This process requires dried algal materials, and therefore, additional costs are imposed by the dewatering process (Asomaning et al., 2016). Anaerobic digestion (AD) converts algal biomass into biogas that can be used for heat and/or electricity generation (Naqvi et al., 2018; Aghbashlo et al., 2019). AD involves four stages for the conversion of moist algal biomass into biogas in anaerobic digestion tanks (Tabatabaei et al., 2020a), as shown in Eq. (2). During the first stage, i.e., hydrolysis, the polymers in the algal cell are hydrolyzed into their constituting monomers such as sugars, fatty acids, and amino acids (Tabatabaei et al., 2020b). During the second stage, i.e., acidogenesis, volatile fatty acids (VFAs) are generated through the action of acidogenic bacteria using the products of the preceding stage. Acetogenic bacteria then convert acids into acetic acid, hydrogen, and CO2 (acetogenesis stage). In the final step (methanogenesis), acetic acid, and hydrogen and CO2, are converted into methane by acetoclastic and hydrogenotrophic methanogens,

Fig. 1. Major microalgal cultivation systems. 4

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Fig. 2. Conversion methods of algal biomass into biodiesel (Collotta et al., 2018; Adeniyi et al., 2018).

respectively (Adeniyi et al., 2018).

recovery of conventional organic solvents have potential associated economic costs. Researchers have been investigating the potential of different alternatives to phase out this limitation. Boyd et al. (2012) reported the utilization of switchable to reduce the problem associated with the use of volatile, chlorinated, or flammable conventional organic solvents. Switchable hydrophilicity solvents (SHS) are designed to deal with the wet samples. The unique quality of these solvents is the ability to switch between hydrophilic and hydrophobic forms. SHS are tertiary amines, N, N-dimethylcyclohexylamine, which are usually hydrophobic when they encounter water in the presence of air. Still, by the addition of CO2, they can be effectively turned into bicarbonate salts and become hydrophilic. This CO2 can be stripped off, by simple bubbling an inert gas through this solvent to achieve original hydrophobicity. SHS can be recovered efficiently using carbonated water instead of distillation. World’s largest algal biofuel production center is situated in Western Australia. In this case study, a hybrid life cycle assessment using input and output analysis of an algal biofuel plant in comparison with a conventional plant was performed. The algal plant, with an area of 740 ha, was selected 50 km away from the power plant and bio-crude oil refinery sites. This algal bio-crude production plant was compared to the conventional crude oil-producing plant. The socially, economically, and environmental sustainability was elucidated using input and output equations. The results have shown that algal bio-crude production is feasible, environmentally friendly, and socially profitable as compared to conventional crude oil production. Its carbon sequestration rate is 1.5 tons per ton of the bio-crude output, with a carbon emission of 0.5 tons. So, it is a negative carbon process sequestering 1 ton of CO2 per ton of bio-crude production. The economic analysis proves that one million tons of the bio-crude output will generate approximately 13,200 new employment along with an economic stimulus of $4 billion (Malik et al., 2015).

Algal biomass ðpolymersÞ → Monomers ðe:g:; sugars; amino acids; etc:Þ →Volatile Fatty Acids ðVFAsÞ→Acetic acid þ Hydrogen þ Carbon dioxide →Methane (2) The simplest way of thermal conversion, i.e., direct combustion, can also be applied for the efficient conversion of algal biomass into energy (Azizi et al., 2017). However, before the combustion process, a pre­ treatment process, i.e., dewatering of algal biomass is necessary (Ade­ niyi et al., 2018). Co-combustion of algal biomass with coal can also significantly reduce the GHGs footprints of the process as compared with the combustion of coal alone. As mentioned earlier, the major limitation of this process is the requirement to reduce the moisture content of the algal biomass to below 50% (Adeniyi et al., 2018). Pyrolysis is the thermochemical conversion of algal biomass into biooil. It occurs in the absence of oxygen with the temperature range of 300–800 � C producing bio-oil, non-condensable gases, and solid biochar (Azizi et al., 2017). Higher pyrolysis temperatures increase gaseous fractions and reduce the solid mass. The optimum temperature for the efficient production of bio-oil is around 400 � C (Veselovskaya et al., 2018). Fermentation is a biochemical reaction through which the car­ bohydrates present in algal biomass could be converted into bioethanol (Panahi et al., 2019). Algal carbohydrates produce two and five folds more ethanol as compared to sugarcane and corn, respectively. Chlorella vulgaris is an efficient microalga with 65% ethanol production efficiency mainly because of its high starch content. Process efficiency can be further enhanced by employing techniques milling, saccharification, and liquefaction (Adeniyi et al., 2018). 2.3. Preliminary assessment and limitations of algal fuels

3. Fuels and chemicals production using the captured CO2

Life cycle assessment (LCA) is a valuable tool to monitor the cradle to grave analysis of a product or service to identify its environmental im­ pacts. It enhances the efficiency of the process, by reusing the coproducts formed and elevating lipid extraction, to make it sustainable (Dutta et al., 2016). Algal biofuel synthesis from wastewater containing nitrogen and phosphorous nutrients is environmentally friendly and cost-effective as compared to the conventional methods of petroleum fuel production and freshwater-based algal production (Collotta et al., 2018). Collotta et al. (2018) conducted LCA for the growth of algae by different CO2 and nutrient sources and has reported the production approach as most environmental friendly involving wastewater as a nutrient source and flue gas by cement industry as a CO2 source. Yang et al. (2011) reported that wastewater use for algal production could reduce the utilization of phosphate, nitrogen, and other nutrients. However, processes of extraction, drying, distillation, and the

3.1. Chemical utilization of CO2 and syngas production CO2 can be directly converted into a variety of value-added chem­ icals by exothermic and endothermic reactions. This practice signifi­ cantly reduces the use of virgin raw materials, alleviates the energy crisis, and reduces the emission of CO2 into the atmosphere (Rafiee et al., 2018; Aresta and Dibenedetto, 2004). Syngas is produced as an inter­ mediate product in the reforming process in which low-value fuel or materials are converted into value-added fuels or chemicals. It com­ prises carbon monoxide and hydrogen, and sometimes a small fraction of water and CO2 is also present (Ayodele et al., 2015). Reforming can be in the solid-state (pyrolysis or gasification of biomass, waste, and coal) or gaseous state (conversion of natural gas to syngas). 5

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3.2. Methanol production using captured CO2

2016). It’s high oxygen content results in the enhanced combustion process and reductions in CO and particulate matters (Cai et al., 2016). Due to the similar properties of DME with liquid petroleum gas (LPG), it can be easily manufactured using LPG infrastructure with slight modi­ fication. DME can also be used as a replacement for different chemicals like propylene, ethylene, and chlorofluorocarbons (Saravanan et al., 2017). DME holds a prime position in the CCU framework because of the merits mentioned above. DME can be produced through two pathways; directly from syngas, and through methanol synthesis followed by dehydration of two meth­ anol molecules, as shown in Eqs. (3) and (4), respectively.

Methanol, wood alcohol, is used for the manufacturing of several industrial chemicals, including formaldehyde, acetic acid, methyl tertiary-butyl ether (MTBE), and DME (P� erez-Fortes et al., 2016). It can be produced from recycled CO2 of flue gas origin or directly from the CO2 captured from the atmosphere. MeOH can be used as a suitable fuel in direct gasoline blending. Hydrogen can be stored in MeOH, and it can be transformed into olefins that is a precursor to manufacturing hy­ drocarbons (Olah, 2005). As a fuel, MeOH can be mixed with gasoline and may also be employed in fuel cells. The manufacturing of MeOH is attractive in developing economies as a liquid fuel to replace conven­ tional energy sources (P�erez-Fortes et al., 2016). Conventionally, MeOH is produced from the catalytic conversion (usually using Cu/ZnO/Al2O3 as a catalyst) of synthesis gas, mostly derived from natural gas (Cai et al., 2016). MeOH can be produced from CO2 in two catalytic routes: one-step direct hydrogenation of CO2 or two-step CO2 conversions into CO and further hydrogenation of CO (Albo et al., 2017). Higher alcohols, methane, methyl formate, and DME are produced as by-products. MeOH can also be produced alternatively by electrochemical reduction and oxidation of CO2 and H2O, respectively, and can be customized to generate different products by appropriately selecting reactant con­ centrations and operational parameters (Van-Dal and Bouallou, 2013). Conventional MeOH synthesis using natural gas or coal may cause water shortages and increased GHG emissions (Yang and Jackson, 2012). It is vital to find substitute methods to synthesize MeOH, other than using fossil fuels as feedstock. Utilization of captured CO2 can overcome these problems. Effective reductive transformation of CO2 to MeOH can convert the harmful GHG into a renewable and valuable product (Dinca et al., 2018). MeOH production at a commercial scale with captured CO2 and H2 as raw materials have been conceptually studied considering technological, financial, and environmental metrics by theoretical simulations (P�erez-Fortes et al., 2016). The studied plant could produce an estimated 440 kt/yr of MeOH with lower specific capital cost as compared to conventional plants; however, the proposed plant is not financially viable for the higher cost of captured CO2. Methanol synthesis from captured CO2 is becoming popular. Iceland and Japan have already established different plants combining CO2 and renewable H2 (Quadrelli et al., 2011). Carbon Recycling International (CRI) started the operation of the first commercial demonstration plant with a capacity of about 5 Mt/yr of MeOH production in Iceland in 2011 to improve plant economics for larger plants and to gain operation expertise. CRI also has a pilot plant operating since 2007. CRI is involved in the H2020 project, whose aim is to use surplus and intermittent renewable energy sources to produce chemicals and fuels from CO2 captured from coal-fired power plants (Faberi et al., 2014). This study will focus on the deployment of fast response electrolyzers. Mitsui Chemicals Inc., in 2008, built a pilot plant to synthesize MeOH from CO2 and H2 in Osaka, with a capacity of around 100 t/yr of MeOH to produce olefins and aromatics. The installation uses CO2 emitted from factories and H2 obtained from water photolysis. These plants allow us to conclude a Technology Readiness Level (TRL) of 6–7 for MeOH pro­ duction from recycled CO2. However, thorough research and develop­ ment are still required to design competitive CCU-process (Haunschild, 2015). According to the International Energy Agency (IEA), global MeOH production capacity has been increasing with an average annual rate of about 10% since 2009. China holds not only the world’s 50% MeOH production capacity but the world’s 50% MeOH consumption capacity, too (P�erez-Fortes et al., 2016).

CO þ 2H2 →CH3 OH ðHΟ ¼

90:4 Kj = molÞ

CH3 OH þ CH3 OH →CH3 OCH3 þ H2 O

(3) (4)

Indirect synthesis by using different catalysts, e.g., ZnO–Al2O3 and Cu/ZnO/Zeolites are used in CO2 hydrogenation for DME production. Zhang et al. (2014) observed 30.6% CO2 conversion with 15% DME production over optimum concentration of Cu/ZnO/Zeolite catalyst. Acidic zeolite, when used in combination with Cu/ZnO, could improve the reaction efficiency and DME selectivity. Different technologies and reactors are used in the synthesis of DME from CO2. Luu et al. (2016a, 2016b) used two types of reforming processes for the synthesis of DME: dry methane reforming (DMR), and bio reforming (BiR). Dry methanol-reforming is preferred over the other method due to its opti­ mum H2/CO ratio of 1. While BiR has H2/CO2 ratio of 2 results in the production of high heat capacity by the product (H2O) that makes the recovery of DME energy-intensive process. However, this disadvantage can be alleviated by utilizing steam in the distillation column re-boiler, making this process feasible. When these reaction routes are compared with conventional auto-thermal reforming, on average 6.5% reduction in CO2 emissions is observed. Luu et al. (2016a, 2016b) further investigated the application of solar energy for DME synthesis. Solar energy provides the temperature for the synthesis of syngas. Two alternative routes could be used, 1) solar reformer is integrated with non-solar reformer, and 2) solar reformer is coupled with waster gas shift reactor. Different simulation results showed that the later configuration shows a 20% reduction in CO2 emissions. Dadgar et al. (2016) studied the effect of a copper-based catalyst on the direct synthesis of DME concerning methanol forma­ tion and dehydration. In indirect synthesis, conversion of syngas to methanol is temperature-dependent, and thus temperature could affect DME production. Under similar operating conditions, complete CO2 conversion to methanol was observed in a water gas shift reactor. DME production is expected to increase to 20 million tons/yr in 2020 and to serve as a potential renewable fuel. Therefore, DME could not only bridge the energy gap but could also contribute to climate change mitigation by using captured CO2 as feedstock (Dadgar et al., 2016). 3.4. Methane (CH4) production using captured CO2 High stability and low reactivity of CO2 make its conversion to fuels or value-added chemicals difficult. However, with the aid of certain catalysts, this problem can be removed (Wannakao et al., 2015). Park et al. (2015) investigated the photocatalytic conversion of CO2 to CH4 using a double layer of TiO2/Cu–TiO2 and TiO2/TiO2 films. Results showed a two-fold increase in CH4 production over TiO2/Cu–TiO2 layer than compared with TiO2/TiO2 layer. Jiao et al. (2015) used three-dimensional microporous Au-supported TiO2 for the photoreduc­ tion of CO2 to CH4. The slow light effect of the three-dimensional structure increased the absorption of solar radiation, and introducing Au nanoparticles extended light absorption from UV to visible region, yielding high catalytic conversion of CO2 to CH4 under visible light. Another study reported the use of Ce-doped TiO2 nanoparticles as catalyst and Pd nanoparticles as cocatalyst. Results showed the pro­ duction of 220.61 μmol/g of CH4 under visible irradiation. The

3.3. Dimethyl ether (DME) production DME is a colorless, non-toxic, and environmentally benign gas (Semelsberger et al., 2006). Because of its higher cetane number than diesel, it can be used as an additive in diesel engines (De Falco et al., 6

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Journal of Environmental Management 260 (2020) 110059

improved catalytic activity could be ascribed to the synergistic effect of Ce and Pd (Li et al., 2017). Hydrogenation of carbon oxides to methane was performed in ammonia plants to purify syngas. This could yield carbon-neutral fuel (methane) as well (Rafiee et al., 2018). CO2 can also be converted to methane by biological processes like the use of methanogens, as shown in Eq. (5). A 70 fold increase in the production of methane was observed using the activated culture of methanogens obtained from anoxic enrichment of waste activated sludge. A mixture of H2 and 13C was used, demonstrating that the source of methane was CO2 (Yasin et al., 2015). CO2 þ 3H2 → CH3 OH þ H2 O

The final step in the DAC is the compression through which the CO2 separated or obtained by kiln exhaust is compressed by mixing with a pure CO2 stream at 1 bar pressure (De Jonge et al., 2019). 3.6. Methane generation by DAC Atmospheric CO2 capture is an effective alternative for curbing the atmospheric concentration of this GHG and for producing useful prod­ ucts such as methane (Fig. 4). Solvents having a high potential of CO2 absorption capacities can be used for this purpose. By wet impregnation, an inorganic composite sorbent of K2CO3/Al2O3 can be prepared. This sorbent is usually preferred over others because it does not demand any pre-treatment and can absorb CO2 from the atmosphere directly. Ruthenium catalyst prepared by mesoporous alumina was used to catalyze the reaction (Veselovskaya et al., 2018). Potassium carbonate forms bicarbonates by reacting with atmospheric ultra-dilute CO2 along with water vapors (Eq. (10)). Inside the pores of alumina, CO2, water, and other sorbent’s components react to form potassium dawsonite (Eq. (11)). The sorbent is recovered by thermal regeneration to obtain the pure stream of CO2.

(5)

3.5. Direct air capturing (DAC) of CO2 DAC involves the absorption of the CO2 from the atmosphere by a chemical sorbent (Fig. 3). This CO2 is then separated into a thermal regeneration unit, and the pure stream of CO2 can be utilized for various purposes (De Jonge et al., 2019). DAC allows two major benefits, first is the efficient removal of CO2 from the atmosphere, and second, it helps with the production of high energy fuels like electrolytic hydrogen and solar fuels (Keith et al., 2018). DAC is a diverse arch of science since it allows many significant advantages, including solar fuel production. Solar energy is used to produce many valuable products. This process is like the basic phenomenon of photosynthesis: the photocatalyst act as a green plant –converting CO2 into fuel in the presence of visible or ul­ traviolet (UV) light (Fu et al., 2019). DAC is comparatively cost-effective since there is no need to spent transportation cost of CO2 for a long distance, which is approximately $10 per ton (Lackner et al., 2001). DAC has a positive carbon efficiency since it absorbs CO2 from the atmo­ sphere efficiently (De Jonge et al., 2019). The DAC system comprises an air contactor unit having 4 rows coupled with 40 modules. Each module is individually designed with a PVC packing having a layer of active hydroxide sorbent like sodium hydroxide to provide a large surface area (De Jonge et al., 2019). A suitable sorbent should be environmentally friendly, cheap, and reus­ able for multiple recycling loops (Lackner et al., 2001). Air is introduced into the air contactor unit from outside by fans to maintain a pressure drop. This forms a saturated sorbent (bicarbonate) by reaction with CO2 (Eq. (6)). The bicarbonate is then treated with calcium hydroxide (Ca (OH)2) to produce calcium carbonate and regenerate saturated sorbent (Eq. (7)). The NaOH is again transferred to air contractor, and the Ca2CO3 is heated at high temperatures in an oxyfuel kiln to obtain CO2 and Calcium oxide (CaO) (Eq. (8)). This CaO reacts exothermically in the presence of water to form Ca (OH)2 that can be used in the regeneration process (Eq. (9)). 2NaOH þ CO2 →Na2 CO3 þ H2

(6)

Na2 CO3 þ CaðOHÞ2 → 2NaOH þ Ca2 CO3

(7)

Ca2 CO3 → CaO þ CO2

(8)

CaO þ H2 O→ CaðOHÞ2

(9)

K2 CO3 þ CO2 þ H2 O →2KHCO3

(10)

K2 CO3 þ CO2 þ H2 O þ Al2 CO3 → 2KAlCO3 ðOHÞ2

(11)

Methane can also be yielded by a two-step process. The first step is the generation of hydrogen by electrolysis of water, and next is the conversion of the captured CO2 along with the synthesized hydrogen into methane, as shown in Eqs. (12) and (13), respectively (Vese­ lovskaya et al., 2018). H2 O → H2 þ O2

(12)

CO2 þ H2 → CH4 þ H2 O

(13)

3.7. Seawater desalination by CO2 in Low Elevation Coastal Zones (LECZ) Because of unique features of coastal zones, e.g., subsistence re­ sources, ample recreational and trade opportunities, these areas have been subjected to development pressures resulting in substantial envi­ ronmental and socio-economic changes. The gravity of the situation can be comprehended because about 60% of the world population resides within the 100 km distance of coastlines (Yu et al., 2019). Least devel­ oped countries and South Asian countries (Bangladesh, India, Pakistan, Sri Lanka, Maldives) have even denser Low Elevation Coastal Zones (LECZ) with 382 people/km2. In India, 64 million people live in LECZ, and a three folds increase is expected (216 million people) by the year 2060. Bangladesh LECZ population is anticipated to increase by two folds from 63 million to 109 million people by the same year. Pakistan is currently ranked third among the Asian countries in terms of LECZ population. However, its LECZ population of 4.6 million in 2000 is ex­ pected to increase by six folds reaching 30 million by the year 2060, which would be alarming. Overall, the global population in LECZ is likely to increase from 625 million in 2000 to 949 million people by

Fig. 3. Direct air capturing coupled with methane generation (Younas et al., 2016; De Jonge et al., 2019). 7

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Journal of Environmental Management 260 (2020) 110059

Fig. 4. Seawater desalination mechanism employing captured CO2 gas.

2030 and 1.4 billion by 2060 (Table 3). Coastal populations usually have an ample supply of seawater, but high salinity between 35,000 and 45,000 ppm makes it unfit for drinking and other purposes (Tavakkoli et al., 2017). Therefore, these pop­ ulations depend upon surface and groundwater for drinking and do­ mestic usages, and given the increasing population, the supply and demand gap for freshwater is widening. Pollution, over-exploitation, and climate change have also exacerbated water scarcity. It is esti­ mated that the frequency of floods will increase by five folds due to storm surges due to sea-level rise by 38 cm, deteriorating the situation even further (McGranahan et al., 2007). Desalination of seawater can solve this problem, hence it is essential to analyze the potential of seawater desalination by using captured CO2 considering the arguments presented above.

measures are not taken to improve it. About 450 million people in 29 countries do not have access to secure and clean freshwater supply (Hanjra and Qureshi, 2010). About 71% of the world’s population lives in water-stressed conditions for at least one month per year, and this growing risk has led to sociopolitical instability (Hanjra and Qureshi, 2010). According to the World Health Organization, the permissible limit of salinity in water is 500 ppm and for individual cases, up to 1000 ppm while surface water mostly has a salinity of up to 10,000 ppm (Zhou and Tol, 2005). Whereas seawater usually has a salinity between 35,000 and 45,000 ppm (Tavakkoli et al., 2017). The global demand for water desalination products and services was estimated to worth be $13.4 billion in 2015. Globally, more than 11,000 desalination plants in 150 countries provide freshwater to around 300 million people with capacity growth of about 8% annually (Cotruvo, 2005). The desalination markets are more dominant in the Middle Eastern countries (Al-Anezi and Hilal, 2006). However, water scarcity has now become a global issue, thus many other countries have also adapted this option. Desalination is performed in two ways: distillation and reverses osmosis (RO). Distillation is a heat-based process predominantly used to treat seawater in areas equipped with abundant sources of heat with a large volume of water output requirements. Reverse osmosis is a membrane-based process used in the treatment of brackish water where low flow rate and high cost are involved (Morad et al., 2017).

3.8. Global trend in seawater desalination technologies The world is facing water scarcity driven by environmental pollu­ tion, climate change, and an increase in water demands because of population growth (Dadson et al., 2017). Recent studies by World Bank show that water scarcity could cost up to 6% of GDP by 2050 if proper Table 3 2000 baseline population in millions living at Low Elevation Coastal Zones (LECZ) and forecast for 2060 per region/sub-region/country (Neumann et al., 2015). Country

Asia China India Pakistan China, India, Bangladesh, Indonesia and Viet Nam Saharan Africa Senegal Southern Africa, Europe Northern America Russian Federation Global

3.9. Desalination of seawater by CO2

Population living in low elevation coastal zones (millions) 2000

2060

461 144 64 4.6 353 24 2.9 0.5 50 24 3.51 600 Approx.

729 200 216 30 745 174 19 1.7 56 46 3.55 1100 Approx.

CO2 naturally reacts with water under high pressures and low tem­ peratures to produce crystalline CO2 hydrates. These conditions are naturally present in oceans at specific depths. These hydrates have a three-dimensional, hydrogen bounded, cage-like structure like ice crystals and can entrap CO2 molecules (Izquierdo-Ruiz et al., 2016; Demirbas et al., 2016). Rejection of all other materials is an attribute of hydrates that form pure crystalline aggregates. The crystalline structure of CO2 hydrates depicts a pressure-induced transition between ortho­ rhombic and cubic hexagonal forms (Yang et al., 2012). Their physical properties show a remarkable relevance with the composition and ge­ ology of icy bodies present in the solar system, where a conducive environment, low temperature, high pressure, and water, favor their formation. These hydrates, being denser than water, sink to the seafloor and are stored for a more extended period without escaping back to the 8

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Journal of Environmental Management 260 (2020) 110059

atmosphere (Bachu and Adams, 2003). CO2 hydrates formed are negatively buoyant and thus sink, making these suitable for CO2 sequestration. However, for the desalination of seawater, a positively buoyant hydrate plume is required (Max et al., 2008). It can be obtained by injecting liquid CO2 below a depth of 1000 m in the ocean where the temperature is slightly above 0 � C (Izquier­ do-Ruiz et al., 2016). Liquid CO2 is dense and can be readily delivered to such depths by solar-powered, motor-operated vertical delivery pipes. Injection of liquid CO2 to such ocean depths will produce less harmful effects as compared with the injection of gaseous CO2. CO2 injection at this hydrates’ stability zone with high pressures and low temperatures, results in outward forms of hydrant shells, 4–10 μm thick, on the water surface (Al-Anezi and Hilal, 2006). These shells rise and are collected before the unstable hydrate zone. These solid crystals (round in shape) can easily be separated from saline water. Pure water can be obtained by shifting the temperature and pressure to ambient, with the recycling of CO2 and can be used in the next cycle. As no additional chemical (except CO2) is added, thus membrane separation is not required (Max et al., 2008). In areas of low population density like islands, a portable marine desalination assembly can be used at 300 m depth. It can be fixed, semifixed, or mobile. It also eliminates the problem of acidification of sur­ rounding water, as it is an enclosed system; some effects may be ex­ pected at mid-ocean depths though (Max et al., 2008). The only energy cost involved is the pumping of CO2 to such depths, and in the return system, freshwater naturally rises because of its low density than its surrounding saline water. Residual water due to its high density takes the dissolved CO2 towards the ocean floor. As the water above 400 m is lighter than the water containing dissolved CO2, it prevents its entry back into the atmosphere instead makes it sink as a mass (Max et al., 2008). To make this process commercially viable, some challenges need to be overcome such as the CO2 hydrates formed have less extractable water, and sometimes a slurry is obtained in the form of broken shells. These shell fragments contain residual brine, and washing alone cannot eradicate them. Centrifugation is required that renders the process un­ economical (Yang et al., 2012). To eliminate these problems, a hydrate-forming substance, pre-dissolved CO2, is delivered above the hydrate-forming zone, but conditions are suitable for hydrates stability. This substance causes nucleation of hydrates, and when CO2 is injected outward, the growth of CO2 hydrates is observed with thicknesses of up to a few centimeters. These Hydrants do not collapse and produce portable freshwater (Max et al., 2008). This method not only helps to store captured CO2 in deep saline aquifers but also helps in the desalination of seawater to produce clean water. The revenue generated from desalinized water sales can offset the cost for capturing, purification, and transportation of CO2 from source (flue gas of power plants) to sink (saline aquifer). The purification in­ tensity depends upon the end use of this water (Bachu and Adams, 2003). For instance, if TDS levels are above 500 ppm, then it is suitable for hydraulic fracking, cooling tubes, agriculture, and vice versa. TDS levels of 30–500 ppm are acceptable for residential drinking (Greenlee et al., 2009). For estimation of the unit cost of different production, models have been used, starting from brine having TDS of 23,000 ppm to purified water with 500 ppm, using a two-stage CO2 desalination plant. The cost comes out to be $1.36 per 1000 gallons for agriculture use. For residential use with a TDS of 21 ppm, the price is $3.17 per 1000 gallons (Al-Mamoori et al., 2017). The availability of the CO2 source near the desalination site is essential. A typical coal-fired power plant produces around 1360 m3 of CO2 per hour that has the potential to produce about 8 million gallons of freshwater (Max et al., 2008). The purity of this water depends upon the captured CO2. Different solvents, scrubbers, dry sorbents like lithium silicate, and CO2 strippers are being used for separation of CO2 from stack exhaust. This kind of desalination plant would be commercially viable if constructed in a geographical region where both CO2 sources

and saline aquifers or brine are close to each other. This will eliminate the transportation cost of captured CO2 (Max et al., 2008). For instance, The Karachi city of Pakistan could be a potential site for such projects as CO2 sources in the form of different industrial units, and deep saline aquifers are next to each other. Further research and advances in tech­ nology like the use of CO2 from a gas-fired power plant could further reduce the cost of desalination as no purification of the exhaust stream would be required. 4. Techno-economic and policy analysis 4.1. Techno-economic analysis of the synthesis of chemicals A case study presented by Roh et al. (2018), used a computer-aided tool ArKa-TAC for the simulation of CCU considering CO2 emitted from the iron/steel industry as a source. CO2 was separated using MEA, and three steps were involved in converting this captured CO2 into acetic acid. Dry forming of methane involves the synthesis of CO by the reac­ tion of CO2 with methane (Eq. (14)). The CO in the CO hydrogenation step is converted into methanol (Eq. (15)). Later, this methanol further reacts with CO to form acetic acid (Eq. (16)). CH4 þ CO2 →2H2 þ 2CO

(14)

CO þ 2H2 þ 2CH3 OH

(15)

CH3 OH þ CO→ CH3 COOH

(16)

This system of acetic acid synthesis was employed in four countries (the US, South Korea, China, and the UK) to examine the sustainability metrics. This case study found this conversion method as economically feasible, resulting in lower CO2 emissions compared to the conventional techniques to produce acetic acid. However, the CO2 emitted is higher than the CO2 consumed. Total operating cost was approximately 618.9 M$/yr in South Korea, 203.8 M$/yr in the US, 302.7 M$/yr in China, and 470.0 M$/yr in the UK. A techno-economic analysis was conducted on methanol production from captured CO2 and H2 in Europe. Process flow modeling was employed for TEA, and the production rate of the plant was approxi­ mately 440-kiloton methanol/yr. The capital cost of this CCU plant is much lower as compared to the conventional methanol production plant; i.e., €451.16/ton.yr vs. €846.73/ton.yr. However, the cost esti­ mated for raw materials was higher with the former plant. This analysis also provided the details of CO2 emission reductions when using the CCU approach leading to the net reduction in CO2 emission of 2.71 MtCO2/yr (P� erez-Fortes et al., 2016). Hydrocarbon’s reforming reactions produced synthetic gas (syngas). A membrane reactor (MR) was used for CO2 reforming of methane that incorporates the functions of a reactor and separator. The economic analysis of MR and a conventional packed bed reactor (PBR) was carried out using Aspen HYSYS. Capital and operating costs for both were calculated to produce a unit H2 production. The cost for MR was comparatively lower (6.48 $kgH2 1) as compared to con­ ventional PBR (11.18 $kgH2 1) (Kim et al., 2018). 4.2. Techno-economic analysis of desalinization The desalination method and feed water significantly affect the process cost. Morad et al. (2017) presented a comparison between seawater treatments by conventional RO desalination system and argued that the cost ranged from 0.4 to 3 $/m3 but was reduced to half when brackish water was used as feed. The cost rose to 15 $/m3 if renewable energy sources were utilized. However, this cost was offset by the environmental benefits. If the same water was treated with a developed desalination plant-with the addition of a vacuum pump-the cost would be 0.031 $/L equaling the cost of a conventional system (Morad et al., 2017). Welle et al. (2017) presented environmental and economic modeling 9

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Journal of Environmental Management 260 (2020) 110059

for water desalination using solar/thermal (ST) and (RO). Seawater desalination on an average cost around 0.5–1 $/m3 and inland desali­ nation cost of 0.40–1.80 $/m3. Brine disposal is another problem for inland desalination, and it could widen the benefits gap from 0.10 to 0.30 $/m3. More water recovery is required that increases the mainte­ nance cost and membrane scaling to reduce the small brine volume (McCool et al., 2013; Stuber, 2016). ST uses less electricity as compared to RO, and so, the damage due to air emissions is significantly higher for RO, i.e., 0.05 $/m3 for ST vs. 0.16 $/m3 for RO (Welle et al., 2017). The gas thermal (GT) system uses natural gas and results in significant human health and environmental damages of $0.36/m3 as compared with ST and RO. Above mentioned processes use fuel and electricity as primary sources of energy and thus increases the emissions of GHG and criteria pollutants, which leads to human health issues and environmental damages. This can be evident from CO2 damages associated with ST ($31/acre-ft), RO ($103/acre-ft), GT ($363/acre-ft). PM 2.5 damages indicate the two-thermal systems; GT ($23.40/acre-ft) as most polluting with ST($2.75/acre-ft) being least (Welle et al., 2017). NOx damages for ST ($3.15/acre-ft), RO ($10.50/acre-ft) and GT ($39.90/acre-ft) in­ dicates the same trend. However, SO2 damages are higher for RO ($71.90/acer-ft) and equal for ST ($21.60/acre-ft) and GT ($22.70/acre-ft). Unit cost for desalination using conventional technology is 1.4 $/m3 for ST and 0.5 $/m3 for RO (Van der Bruggen, 2003; Fritzmann et al., 2007). Tavakkoli et al. (2017) presented a techno-economic analysis for the membrane desalination system integrated with a waste heat source and reported a reduction of treatment cost from 5.70 $/m3 to 0.74 $/m3. The considerable variation in the cost estimates by different studies could be attributed to various factors, including energy sources, the salinity of feed water used, and plant capacity.

process to estimate material throughput. The goal was to produce 10 million gallons of TAGs/yr. Flue gas was used as a source for purified CO2, while spargers and sumps were used to supply gas to open ponds without leakage. Nitrogen and phosphorous fertilizers were applied as nutrients. Harvesting was done in three steps to increase the algal biomass from 0.7 g/L to 200 g/L. Flocculation was followed by dissolved air floatation and then by centrifugation. High-pressure homogenizers at high temperatures were used for the mechanical cell disruption to extract the materials of interest from algal cells. The solvent phase was separated from the mixture of water and biomass using disk stack centrifuge. TAGs were finally stripped off from the solvent and recycled to produce algal oil. The capital direct cost was $227 million, capital indirect was $216 million, whereas the operating cost reportedly stood at $43.2 million (Sun et al., 2011). Capital cost refers to the cost required for equipment, facilities, and land. Capital indirect is the cost needed for the building, field expense, permitting, and several other essential contingencies. Operating cost is the sum required for the utilities, ma­ terial, debt services labor, disposal, water, and maintenance (Sun et al., 2011). Judd et al. (2017) reported cost reduction by up to 35–86% by using wastewater as a nutrient source and CO2 of flue gas origin as a carbon source during algae production. Thus, multiple production plants over a stretched geographical location having both flue gas source and waste­ water sources should be installed. This will not only address air pollu­ tion, global warming, and water pollution but also yield economic savings. An economic analysis of different systems for biofuel production was carried out in which open ponds were analyzed to produce TAGs or FAMEs, photo-bioreactors to produce FAMEs or Free fatty acids (FFAs) and light-emitting diode (LED) photo-bioreactors to produce TAGs. In open ponds, the depth of pond varied from 6 to 15 in and land varied from 1 to 20 ac. Centrifugation and gravity settling were used for the dewatering of microalgae to produce TAGs, and afterward, the associ­ ated cost was estimated. In the case of FAMEs production, the thermal drying process was used for the dewatering of microalgae. For micro­ algae production in photo-bioreactors, low-density polyethylene (LDPE) bags were suspended in water basins, and CO2 and nutrients were bubbled into the designed system. Multiple tubes having a constant surface area were employed to increase biomass production. Thermal drying was used for dewatering, and the biomass was converted into FAMEs by transesterification. Conditions were kept similar to produce FFAs by solar photo-bioreactors. However, after dewatering, the biomass was hydrolyzed into FFAs. The oil was extracted, centrifuged for solid separation and gravity decanter for separation of layers. LED’s were used to enhance photosynthesis by increasing the light. Four W LED array having the efficacy of 60 LM/W and spectral luminous effi­ ciency of 0.7 produced an irradiance of 286 W/m2. The perforated ex­ tensions were attached to the shaft and used to provide CO2 and nutrients to the system. This produced more biomass in a less surface area. Electromagnetic pulses were used to gain oil from the algae, and anaerobic digestion was used to produce methane. The TAGs produced by open pond cost 7.5 $/kg whereas, in the LED photo-bioreactors, the TAGs production costs stood at 33 $/kg. In the open ponds, the FAMEs production cost was 4 $/kg while in the solar photo-bioreactors, the cost was estimated at 25 $/kg. The FFAs production by solar photobioreactor costs 29 $/kg. This case study concluded that open ponds were more economically viable options (Amer et al., 2011). Chlorella sp. of algae was selected due to its fast growth and higher potential to tolerate ammonia. Algae were grown in photo-bioreactors, and municipal wastewater was used to meet the nutrient demands of algae. Following the growth stage, pyrolysis was used for the conversion of algal biomass into syngas and biochar. The major costs incurred were related to cultivation, equipment, and operational expenses. Results showed a feasible selling price of 2.23 $/gallon of biofuel. The internal rate of return (IRR) of the process could reportedly be increased by up to 18.7% (Xin et al., 2016).

4.3. Techno-economic analysis of algal fuels The burning of fossil fuels is still the cheapest way of energy pro­ duction. Therefore, the comparison of the price paid at a pump for biofuels and fossil fuels is inadequate. Biofuel requires a meta-economic analysis that incorporates every direct and indirect cost associated with its production. The actual cost of diesel or gasoline is much higher than that of the cost paid by consumers. Ultimately, consumers must pay for the hidden additional production costs in the form of higher taxes. Particularly in the US, fossil fuel industries are supported by massive subsidies of 50 billion USD over a decade. The higher hidden costs associated with the external damages, health impacts, excluding ecosystem, climate change, security, and infrastructure are reported to be 120 billion USD in 2005. The cost of climate change can be estimated by calculating the cost required for adapting to these changes. However, this prediction still does not cover the real cost as it will not include the mitigation cost (Leite et al., 2013). For the cost-benefit analysis of biodiesel and bioethanol production, Aspen Plus software has been used by Wu et al. (2018). The pretreat­ ment process comprises harvesting of algal biomass, biomass dewater­ ing, cell disruption, and finally, oil extraction. In this process, hexane is generally used for lipid extraction, and ultrasonication is used for car­ bohydrate extraction after a microalgal disruption is achieved. Tri­ aceylgricrides (TAG) is further transformed into biodiesel by transesterification with NaOH generally used as a catalyst. Glycerol produced could also be used to produce bioethanol. In a study, 1616 ha land was considered for the open pond cultivation of algae, to produce 10,000 kg/h TAGs and 1430 kg/h carbohydrates. The capital and operating costs were estimated at 0.0076 billion USD and 0.0011 billion USD, respectively. The total estimated cost for the manufacturing and total capital investment of biodiesel synthesis was reportedly 0.0021 billion USD/yr and 0.0022 billion USD, respectively (Wu et al., 2018). In an investigation by the National Renewable Energy Laboratory (NREL), Aspen Plus simulations were used for the modeling of the 10

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4.4. Techno-economic analysis of DAC

biofuel project. Moreover, in 2010, US DOE invested USD 9 million in a project on Sustainable Microalgae Biofuel Group in Arizona Mesa to produce algae by novel recyclable methods (Su et al., 2017). In Australia, the federal government and clean energy council aimed to produce 20% of the country’s electricity by renewable resources. It is reported that in 2011, biofuel production in the US, Australia, and the UK was 971.7, 8.7, and 9 thousand barrels/d, respectively (Azad et al., 2015). Pakistan faces poor scenarios in the development and establishment of the renewable energy sector due to low investments, poor manage­ ment, institutional weakness, and lack of policies. In recent years, Pakistan has tried to improve its renewable energy sector by increasing the use of biofuel blends, allocating the provincial along with a federal budget to the renewable energy sector, subsidies to biofuel users and awareness campaigns (Shah et al., 2018). In Pakistan, different policies and new projects about biomass energy are in progress in collaboration with different countries. Alternative Energy Development Board has the target of producing 5% energy from renewable sources. They are working in collaboration with New Zealand to generate 30 MW of renewable energy by biogas near Karachi city. The government of Pakistan has recently invested 356 million for importing new 1400 biogas plants (Naqvi et al., 2018). Many experiments have been conducted globally that aims to utilize CO2 for different biofuel production. Gonçalves et al. (2016) have calculated the optimum CO2 concentration required to produce maximum algal biomass. Moreover, they have determined the maximum nutrient removal from wastewater by using microalgal strains. The op­ timum concentration that has resulted in the maximum production of algal biomass is 5.35 � 0.34% (v/v) coupled with the nutrient removal of approximately 100%. In another case, Chen et al. (2018) synthesized bio-lipids by the cultivation of Chlorella sp. C2. The nutrient-rich ash and flue gas from a power plant were provided as nutrient sources. High biomass yield of 31 g L 1d 1 along with 99.11 mg L 1 d 1 lipid pro­ duction has been reported. Many other species to produce biodiesel such as Schizochytrium limacinum has been reported (Johnson and Wen, 2009). Methanol can also be one of the significant products of CO2 utilization. In a case study, CO2 from flue gas has been used to produce methanol (Almahdi et al., 2016). In a techno-economic analysis, it has been reported by Asif et al. (2018) that 1.2 billion USD is required for the abatement of 3.2 million tons CO2 from a 600 MW coal power plant by converting it into methanol. Desalination has long been the largely relied source of clean water in the Middle East and related arid environments; however, the climatic changes coupled with overpopulation have enhanced the reliance on desalination for clean water provision worldwide. The situation be­ comes more feasible when we look at the population, three billion people, living within 200 km coastline since we need to provide these people with clean drinking water in the future (Creel, 2003; Shahabi et al., 2015). The fact that approximately 1.2 billion population worldwide is facing the water scarcity issue further makes seawater desalination need of the hour (Verdaguer et al., 2018). The seawater desalination has also witnessed sharp growth in the previous two decades with an increase of over three folds in production capacity as well (Mata-Torres et al., 2017; Pinto and Marques, 2017). However, conventional desalination plants are energy-intensive; hence, they not only are costly but also emit GHGs emissions. These issues can be tackled, using desalination through CO2 hydrates formation (Molinos Senante and Gonzalez, 2019). The other negative footprints of conventional seawater desalination include construction-related damages and noise at land and brine discharge issues. Salinity and pH changes, eutrophication and heavy metals accumulation affect marine ecology. Looking at the locations of water-scarce regions, Middle East, Western North America, North Af­ rica, and Eastern Spain, Northern China, and Australia, who are finding it challenging to meet freshwater supply to their habitants one can easily see their proximity to seashore. At coastal range allow this region to

The primary drawback associated with DAC is its cost. Multiple studies concluded the cost of DAC ranges from 30 to 1000 $/ton of CO2 removed (De Jonge et al., 2019). However, this cost can be offset by the related environmental benefits and useful products yielded by CO2 ac­ quired through DAC. An industrial-scale DAC was designed to capture CO2 at a rate of 1 Mt/yr. Potassium hydroxide was used as a sorbent in a contactor chamber, and regeneration of the sorbent was accomplished in the pellet reactor by calcium carbonate. Natural gas (8.81 GJ) merely or the combination of natural gas and electricity (5.25 GJ and 366 kWh) were used to operate the capturing unit. The purified CO2 obtained was 1.3–1.5 tons. The estimated levelized cost, i.e., the sum of levelized capital cost, energy cost, and non-fuel operational and maintenance cost, was estimated at 94–232 $/ton of CO2 (Keith et al., 2018). In the designed DAC system, the CO2 is captured from the atmo­ sphere at a rate of 1 ton/d. The system can be operated on air so that it can remove at least 10% CO2 present in the air. In this way, the removal of 1 ton/d can be achieved. Electricity was the primary form of energy to regulate the system. Natural gas combined cycle (NGCC), wind and geothermal resources were used to generate electricity. Water becomes a very efficient input when CO2 is captured by an aqueous solution. The water treatment unit receiving water from the ocean, freshwater, and saline aquifers can be employed to supply purified water to the capturing unit. The captured CO2 is compressed at a certain pressure in the compression unit used after capturing unit. This case study concluded that to achieve the target of 300 $/ton CO2, high second law absorbing efficiencies and cost-effective capturing devices should be employed (Simon et al., 2011). 5. Suitability of existing policies and future outlook The concentration of primary CO2 as GHG is on the rise and is ex­ pected to reach 450 ppm in 2035 while scientists believe that with a 77–99% probability, this will lead to a further increase in global tem­ perature by over 2 � C that would have detrimental impact on the global economy and public health (Watts et al., 2018a, 2018b). The conven­ tional approaches to reducing this concentration by capturing CO2 from primary sources and switching to renewable energy resources have not been successful as suggested by the ever-rising concentration of CO2. This inaction on previous strategies coupled with their low efficiencies, even if implemented, have further highlighted the need for DAC for efficient removal of CO2 from ambient air. IPCC, in its assessment re­ ports, has laid strong emphasis over the DAC to combat climate change. However, to effectively curb global warming, DAC is needed to be installed at a comparable scale to the consumption of petroleum, i.e., in a range of around 4.3 Gt in 2016 (Santori et al., 2018). It is argued that climate action to achieve long-term sustainable development goals will be more expensive by approximately 140%, without the aid of CCS. The DAS becomes even more imperative because fossil fuels will still be the source of the world’s 60% primary energy by 2040 despite the recent boom in multiple renewable energy resources. Therefore, until 2050, there are minimal chances of de-fossilization of many industrial, power production, and other heat-intensive operations. This potentially underlines the importance of urgency in using CCS through DAC to avoid the catastrophic climatic fate of the planet Earth (Fasihi et al., 2019; Andri�c et al., 2017). Canada plays a significant role in the consumption of biofuels. It has implemented certain policies related to biofuels at both provincial and federal levels facilitating the development of the bioenergy sector. Subsidies and investments have been offered in recent years to promote the consumption of biofuels. The oil crisis in Brazil in the 1970s was addressed by the National Bio-alcohol Program (Scaife et al., 2015). The U.S Department of Energy (US DOE) and Department of Defense have funded many projects for research and development of algal biofuels since the 1970s. In 2004, the US DOE-funded USD 3.5 million in an algal 11

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harvest the benefit of seawater desalination. Although, some of these regions are already using conventional desalination technologies; however, these operations are posing a considerable economic burden to these areas. In addition to this frequent drought owing to climate change have also triggered the need for alternative climate independent resource with less GHGs emissions. Both these issues can be managed by using seawater desalination utilizing captured CO2 (Lattemann and €pner, 2008; Scott et al., 2011; Heihsel et al., 2019). Ho Europe and the United States lack the potential to produce soy, rapeseed, and corn, so they must produce algae to meet their energy demand (Bibi et al., 2017). The developments in the field of algal biofuel are based on its production in a more sustainable, feasible, and cost-effective way. Fermentation and direct combustion to produce en­ ergy by algae are the most environment-friendly methods. Genetic tools should be used to produce the algal strains that have a high growth rate and lipid contents that can increase the rate of algal biofuel production (Adeniyi et al., 2018). Production of strains that have higher biomass can also reduce the cost of biofuel production, approximately 40% (Bibi et al., 2017). Though several methods have been used, but the most feasible, environment-friendly and cost-effective way is providing saline water or wastewater coupled with the use of CO2 in flue gas for algae production. Therefore, further in-depth research and development work should be carried out to make this system commercially viable (Bibi et al., 2017).

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6. Conclusions Global warming and associated climate change pose a severe threat to the ecosystem and human survival globally. The primary culprit behind global warming is the ever-increasing atmospheric concentration of CO2. Therefore, we must choose carbon sequestration to meet the goals set in Paris agreement by reducing the CO2 concentration. The use of fossil fuels is also putting the burden on nations’ economies as well as increasing the local air pollution, such as winter smog episodes. Over­ population has also exacerbated the water scarcity issues across the globe. Climate change has led to sea-level rise affecting the coastal population. All these problems can be solved through single panacea: Carbon Capture, Storage, and Utilization (CCU). CCU can be used for biofuel and other valuable chemical production, hence offering an alternative to conventional fossil fuels helping the economic and envi­ ronmental conditions of the country along with a generation of huge employment opportunities. Furthermore, CCU can be employed for the desalination of seawater, hence providing clean water and mitigating sea level rise. Multiple studies have been conducted for technoeconomic and techno environmental analysis of CCU, and results are quite encouraging. However, further in-depth research and develop­ ment work is required to make these technologies commercially viable. References Al-Anezi, K., Hilal, N., 2006. Effect of carbon dioxide in seawater on desalination: a comprehensive review. Separ. Purif. Rev. 35 (3), 223–247. Al-Mamoori, A., Krishnamurthy, A., Rownaghi, A.A., Rezaei, F., 2017. Carbon capture and utilization update. Energy Technol. 5 (6), 834–849. Abdelaziz, O.Y., Hosny, W.M., Gadalla, M.A., Ashour, F.H., Ashour, I.A., Hulteberg, C.P., 2017. Novel process technologies for conversion of carbon dioxide from industrial flue gas streams into methanol. J. CO2 Util. 21, 52–63. Adeniyi, O.M., Azimov, U., Burluka, A., 2018. Algae biofuel: current status and future applications. Renew. Sustain. Energy Rev. 90, 316–335. Aghbashlo, M., Tabatabaei, M., Hosseinpour, S., 2018a. On the exergoeconomic and exergoenvironmental evaluation and optimization of biodiesel synthesis from waste cooking oil (WCO) using a low power, high frequency ultrasonic reactor. Energy Convers. Manag. 164, 385–398. Aghbashlo, M., Tabatabaei, M., Hosseini, S.S., Dashti, B.B., Soufiyan, M.M., 2018b. Performance assessment of a wind power plant using standard exergy and extended exergy accounting (EEA) approaches. J. Clean. Prod. 171, 127–136. Aghbashlo, M., Tabatabaei, M., Soltanian, S., Ghanavati, H., 2019. Biopower and biofertilizer production from organic municipal solid waste: an exergoenvironmental analysis. Renew. Energy 143, 64–76. Al Ketife, A.M., Almomani, F., Muftah, E.N., Judd, S., 2019. A technoeconomic assessment of microalgal culture technology implementation for combined

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