An Overview of Economic Analysis and Environmental Impacts of Natural Gas Conversion Technologies

sustainability

Review An Overview of Economic Analysis and Environmental Impacts of Natural Gas Conversion Technologies

Freida Ozavize Ayodele 1,* , Siti Indati Mustapa 2, Bamidele Victor Ayodele 2 and Norsyahida Mohammad 2

1 Department of Accounting and Finance, Faculty of Business and Management, UCSI University, Kuala Lumpur, 1 Jalan Menara Gading, Taman Connaught, Cheras 56000, Kuala Lumpur, Malaysia 2 Institute of Energy Policy and Research, Universiti Tenaga Nasional, Jalan IKRAM-UNITEN, Kajang 43000, Selangor, Malaysia; [email protected] (S.I.M.); [email protected] (B.V.A.); [email protected] (N.M.) * Correspondence: [email protected]

Received: 14 October 2020; Accepted: 19 November 2020; Published: 4 December 2020

Abstract: This study presents an overview of the economic analysis and environmental impact of natural gas conversion technologies. Published articles related to economic analysis and environmental impact of natural gas conversion technologies were reviewed and discussed. The economic analysis revealed that the capital and the operating expenditure of each of the conversion process is strongly dependent on the sophistication of the technical designs. The emerging technologies are yet to be economically viable compared to the well-established steam reforming process. However, appropriate design modifications could significantly reduce the operating expenditure and enhance the economic feasibility of the process. The environmental analysis revealed that emerging technologies such as carbon dioxide (CO2) reforming and the thermal decomposition of natural gas offer advantages of lower CO2 emissions and total environmental impact compared to the well-established steam reforming process. Appropriate design modifications such as steam reforming with carbon capture, storage and utilization, the use of an optimized catalyst in thermal decomposition, and the use of solar concentrators for heating instead of fossil fuel were found to significantly reduced the CO2 emissions of the processes. There was a dearth of literature on the economic analysis and environmental impact of photocatalytic and biochemical conversion processes, which calls for increased research attention that could facilitate a comparative analysis with the thermochemical processes.

Keywords: capital investment cost; operation expenditure; economic analysis; natural gas conversion; environmental sustainability; natural gas reforming

1. Introduction Natural gas is a fossil fuel-based energy resource that is abundant in nature [1,2]. It is a mixture of various components, predominantly methane [3]. Natural gas has wide applications in the industries, residential buildings for heating, electricity generation, as a transportation fuel, and for various commercial purposes [4–7]. The use of natural gas for electric power generation oﬀers several advantages compared to other fossil fuels [8]. It has high fuel eﬃciency and its cleaner compare to coal and petroleum [9]. Hence, less CO2 is emitted when natural gas is utilized for power generation compared to coal and petroleum [10]. Consequently, the global consumption of natural gas for power generation has increased over the years [11]. Due to its vital role in the global energy mix, the International Energy Outlook in 2016 projected that the global consumption of natural gas will rise

Sustainability 2020, 12, 10148; doi:10.3390/su122310148 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 10148 2 of 18 to 203 TcF in 2040 [11]. Compared to other primary energy sources, natural gas accounted for a large proportion of global primary energy consumption [12]. Besides being used as a primary source of energy, the value chain of natural gas can be improved by converting it to various chemical products for sustainable use [5,13]. To achieve this, methane, a vital component of natural gas, is often used as a feedstock to produce value-added chemicals and chemical intermediates such as methanol and syngas [14–16]. Additionally, a large proportion of global hydrogen consumption is produced from the conversion of methane by the steam reforming technique [17]. The conversion of methane to value-added products can be achieved using various technological pathways such as reforming, fermentative and photocatalytic processes [18,19]. An extensive review of the nano-catalytic conversion of natural gas to liquid fuels and petrochemical feedstocks has been reported by Gharibi et al. [20]. The authors reported that natural gas can be converted directed or indirectly to various products using different types of nano-catalysts. The direct conversions include pyrolysis, methane dehydro-aromatization, and direct oxidative conversions. While the indirect methods involve the reforming of methane using different types of oxidants such as carbon dioxide, oxygen, and steam. A review reported by Gür [21] revealed the prospects of efficient electricity generation by the conversion of methane in solid oxide fuel cells. The study revealed that the electrochemical conversion of methane in solid oxide fuel offers a great potential for efficient generation of electricity [22]. The efficient conversion of natural gas to value-added products is key to its sustainable utilization. However, it is expedient to consider the economic viability and environmental sustainability of some of the emerging conversion technologies. Although there are several review papers on the conversion of natural gas to various products, none of these studies focused on the economic analysis and environmental implications of the technological processes used for natural gas conversion. This review delves into the literature on the economic feasibility and environmental impacts of various technological processes employed for natural gas conversion to provide a quick overview of the potential economic feasibility and environmental impact of the processes. The literature considered in this review was published between 1998 and 2020.

2. Natural Gas Conversion Technologies The conversion of natural gas can be done using various technological routes broadly classify as thermochemical, biochemical, and photochemical in Figure1. The thermochemical route entails the use of thermal energy and catalytic materials for the conversion of natural gas to various products. These methods include methane reforming using steam, carbon dioxide, and oxygen. Recently, research focus is shifting to the advanced form of reforming such as tri-reforming of methane, chemical looping reforming, and photocatalytic reforming [23–28]. Amongst the thermochemical processes, the steam methane reforming is the most established and presently being employed for a large proportion of global hydrogen production [17,29,30]. Other emerging reforming technologies using carbon dioxide and oxygen are seriously receiving research attention due to their perceived advantages over the well-established steam methane reforming [31]. One of such advantages is the prospect of the methane dry reforming to help mitigate greenhouse gases through the utilization of carbon dioxide. Besides, it is a potential technological route for producing syngas used as chemical intermediates for Fischer Tropsch synthesis. The various reforming processes have been investigated using transition metal catalysts such as Ni, Co, Pd, Pt Cu, Ru, and Rh [32–35]. Among the various catalysts, Ni-based catalysts have been reported to displayed high activity for the steam reforming reaction [36]. Besides, it has a very low cost compared to other metal catalysts. However, it is very prone to catalyst deactivation by sintering and carbon deposition [37]. The stability of the Ni-catalysts during the reforming reaction can be improved using promoters. The use of noble metals in small composition has been reported to be effective in improving the stability of Ni-catalysts during reforming reaction. Rategarpanah et al. [38] reported the influence of noble metal on the activity of Ni-Cu/MgO-Al2O3 catalysts in the thermo-catalytic conversion of methane. The unpromoted Ni-Cu/MgO-Al2O3 was found to be unstable during the Sustainability 2020, 12, 10148 3 of 18 thermo-catalytic methane conversion. The addition of the noble metal promoters created a synergistic effect with the Ni-Cu/MgO-Al2O3, thereby preventing the sintering and deactivation of the catalysts and enhancing its stability with time on stream. Besides the use of promoters, a well-synthesized support material could significantly improve the activity and stability of Ni catalysts as demonstrated by Kumar et al. [24]. The stability of Ni-catalyst synthesized on Al2O3, ZrO2, TiO2, SBA-15, MgO, and CeO2-ZrO2 was investigated in tri-reforming of methane at 800 ◦C[24]. The Ni/SBA-15 was found to display the highest stability throughout the 10 h time on stream. The high stability of the Ni/SBA-15 can be attributed to the confinement of the Ni-species within the pores of the SBA-15 thereby preventing sinteringSustainability and 2020 deactivation, 12, x FOR PEER [39 REVIEW]. 3 of 19

FigureFigure 1. 1.Schematic Schematic representationrepresentation of natural gas gas conversion conversion technologies. technologies. The use of advanced reforming processes such as coupling chemical looping reforming The various reforming processes have been investigated using transition metal catalysts such as incorporated with CO splitting reactions has been reported [40]. This advanced reforming process could Ni, Co, Pd, Pt Cu, Ru,2 and Rh [32–35]. Among the various catalysts, Ni-based catalysts have been offreporteder an advantage to displayed of mitigating high activity CO 2foremissions the steam in re aforming cyclic two-step reaction syngas [36]. Besides, production. it has Additionally,a very low thecost integration compared of to solar other power metal catalyst units withs. However, chemical it loopingis very prone reforming to catalyst has beendeactivation proven by to sintering help meet theand high carbon energy deposition required [37]. for theThe reactionstability [of41 ].the The Ni-catalysts use of a ceria during oxygen the reforming carrier for reaction solar enhanced can be chemicalimproved looping usingmethane promoters. reforming The usewas of noble reported metals to increasein small syngascomposition selectivity, has been methane reported conversion, to be andeffective reactor in performance improving the [41 stability]. In a similar of Ni-catalysts study, Wang during et al.reforming [42] employed reaction. the Rategarpanah use of CeO2-ZrO et al.2 [38]-CuO oxygenreported carriers the influence to catalyze of syngasnoble metal production on the byactivity chemical of Ni-Cu/MgO-Al looping reforming.2O3 catalysts The addition in the ofthermo- Cu and Zrcatalytic to the CeO conversion2 leads to of the methane. distortion The of theunpromoted lattice thereby Ni-Cu/MgO-Al resulting in2O better3 was oxygenfound to mobility. be unstable duringThe biochemicalthe thermo-catalytic conversion methane entails conversion. the use of Th microorganismse addition of the (methane-oxidizing noble metal promoters organisms) created to converta synergistic methane effect to methanol. with the Ni-Cu/MgO-Al The methanotrophic2O3, thereby bacterium preventing is commonly the sintering used and for thedeactivation conversion of of methanethe catalysts to methanol. and enhancing Studies haveits stab shownility with that time the biochemical on stream. Beside methanes the conversion use of promoters, to methanol a well- offers thesynthesized advantages support of high material conversion could effi significantlyciency. More improve so, the biochemical the activity processand stability is more of environmentallyNi catalysts as friendlydemonstrated compared by Kumar to the et thermochemical al. [24]. The stability process. of Ni-catalyst Besides, synthesized the presence on of Al toxic2O3, ZrO impurities2, TiO2, SBA- in the natural15, MgO, gas and composition CeO2-ZrO could2 was investigated inhibit catalytic in tri-reforming activity during of methane thermochemical at 800 °C [24]. methane The Ni/SBA-15 conversion. Onwas the found contrary, to display the presence the highest of impurities stability does throughout not impede the 10 the h biochemicaltime on stream. process. The high Henard stability et al. of [43 ] revealedthe Ni/SBA-15 that methanotrophic can be attributed bacterium to the confinement has a high of capability the Ni-species of bio-converting within the pores methane of the toSBA-15 lactate, anthereby industrial preventing chemical sintering platform. and One deactivation of the major [39]. constraints with the bioconversion process is the low yieldThe of use the of products. advanced Moreover, reforming there processes is often thesuch challenge as coupling of insu chemicalfficient geneticlooping tractability reforming of microorganismsincorporated with that CO can2 splitting be used toreactions convert has methane. been reported [40]. This advanced reforming process could offer an advantage of mitigating CO2 emissions in a cyclic two-step syngas production. The use of abundant solar energy resources could help in overcoming the high thermal energy Additionally, the integration of solar power units with chemical looping reforming has been proven required for the thermochemical process. The photochemical conversion process enables the conversion to help meet the high energy required for the reaction [41]. The use of a ceria oxygen carrier for solar enhanced chemical looping methane reforming was reported to increase syngas selectivity, methane conversion, and reactor performance [41]. In a similar study, Wang et al. [42] employed the use of CeO2-ZrO2-CuO oxygen carriers to catalyze syngas production by chemical looping reforming. The addition of Cu and Zr to the CeO2 leads to the distortion of the lattice thereby resulting in better oxygen mobility. The biochemical conversion entails the use of microorganisms (methane-oxidizing organisms) to convert methane to methanol. The methanotrophic bacterium is commonly used for the conversion of methane to methanol. Studies have shown that the biochemical methane conversion to methanol offers the advantages of high conversion efficiency. More so, the biochemical process is more Sustainability 2020, 12, 10148 4 of 18 of methane to hydrogen and syngas using a photocatalyst that can be excited under visible or ultraviolet irradiation. The photocatalytic reaction often occurs at a very low temperature compared to the thermochemical conversion process. Shoji et al. [44] demonstrated that strontium titanate supported rhodium nanoparticles (Rh/STO) was efficient in the photocatalytic conversion of natural gas to syngas. The Rh/STO photocatalyst exhibited high efficiency in promoting the conversion of methane under ultraviolet light irradiation. An H2:CO ratio close to 1 was obtained from photocatalytic reforming the methane, making it suitable as a technological route to produce oxygenated fuel by FTS using cobalt-based catalysts. Comparatively, each of the natural gas conversion processes has its pros and cons which can be evaluated using the life cycle assessment and life cycle cost analysis. The review of selected published articles on the various technological routes used for converting natural gas to value-added chemicals and chemical intermediated based on their life cycle assessment and life cycle cost analysis are presented in the next section.

3. Economic Analysis of Natural Gas Conversion Technologies

3.1. Emerging Technologies Performing an economic analysis for emerging technologies is crucial in determining whether such a process is economically viable in terms of profitability in the eventuality of a scale-up process [45]. Prior to the economic analysis of the process, technical design, and analysis of the process are often performed [46]. The details of the design specifications of each of the process units in the production process are employed to carry out detail economic analysis of the process [47]. The economic analysis is usually performed as a function of the process cost which consists of the capital expenditure (CAPEX) or the investment cost and the operating expenditure (OPEX) [48]. The capital expenditure consists of all the costs associated with buying fixed assets with a useful life above one year. The economic analysis of various technologies used for natural gas conversion to valuable products has been reported in the literature, as summarized in Table1.

3.1.1. Thermal Decomposition of Methane Due to the unique nature of the thermal decomposition of methane, the economic analysis of the process is strongly dependent on the design specification of the process [49]. One of the key economic indicators of hydrogen production by various natural gas conversion processes is the production cost. As reported by Mueller-Langer et al. [50], the hydrogen production cost can be estimated from the cost of materials, fixed assets, maintenance, water, electricity, and labor. A process that involves the emissions of CO2 might incur an additional cost known as the emissions tax. The mass and energy balance of each of the unit processes for the thermal decomposition of methane as well as the reactor design is crucial in estimating the avoidable CO2 emissions, product carbon, hydrogen production, and cost of equipment. The economic analysis of direct thermal decomposition of methane to hydrogen has been reported by Keipi et al. [51]. Based on the assumption of small-scale production, the study revealed that the production of hydrogen by thermal decomposition of methane has a great potential to be economically competitive with steam methane reforming which is a matured technological route for hydrogen production. The total cost of hydrogen production by thermal decomposition of methane was estimated as 72 EUR/MWh while the total OPEX and CAPEX were calculated as 815 kEUR/annum and 4203 kEUR, respectively. Based on the sensitivity analysis, the product carbon value was found to have the most significant effect on the hydrogen production cost for the thermal decomposition of methane. The cost of natural gas significantly influenced the hydrogen production cost by steam methane reforming. Technically, one major constraint of hydrogen production by thermal decomposition of methane and steam reforming of methane is catalyst deactivation. Studies have shown that the catalyst costs usually account for a large proportion of the operating cost of thermochemical processes. The cost implications of catalyst deactivation were not evaluated in the work of Keipi et al. [51] which could have a significant effect on the operating cost and hence the cost of hydrogen production from both Sustainability 2020, 12, x FOR PEER REVIEW 5 of 19

product carbon value was found to have the most significant effect on the hydrogen production cost for the thermal decomposition of methane. The cost of natural gas significantly influenced the hydrogen production cost by steam methane reforming. Technically, one major constraint of hydrogen production by thermal decomposition of methane and steam reforming of methane is catalyst deactivation. Studies have shown that the catalyst costs usually account for a large proportion of the operating cost of thermochemical processes. The cost implications of catalyst Sustainability 2020, 12, 10148 5 of 18 deactivation were not evaluated in the work of Keipi et al. [51] which could have a significant effect on the operating cost and hence the cost of hydrogen production from both processes. Since thermal processes.decomposition Since of thermal methane decomposition requires high of thermal methane en requiresergy, the high type thermal of energy energy, source the typeutilize of energyfor the sourceprocess utilize could forplay the a significant process could role playin determining a significant the role economic in determining viability of the the economic process. viabilityKeipi et al. of the[52] process. investigated Keipi the et al. economic [52] investigated analysis the of economichydrogen analysis production of hydrogen by thermal production decomposition by thermal of decompositionmethane using offour methane different using types four of dienergyfferent sources. types of The energy energy sources. sources The which energy include sources using which the includeenergy from using methane the energy combustion, from methane electricity, combustion, energy electricity,carrier catalyst energy and carrier energy catalyst from regenerative and energy fromheat regenerativereactors were heat found reactors to significantly were found in tofluence significantly the CAPEX influence and the the CAPEX OPEX and of thethe OPEXthermal of thedecomposition thermal decomposition process. The process.thermal decomposition The thermal decomposition of methane using of methane energy usingcarrier energy catalysts carrier was catalystsfound to washave found the highest to have CAPEX the highest and OPEX. CAPEX This and can OPEX. be attributed This can to be the attributed high cost to of the the high catalysts cost ofand the the catalysts frequent and replacement the frequent of the replacement catalyst due of to the deactivation. catalyst due Several to deactivation. authors have Several reported authors that havethe cost reported of catalytic that the materials cost of catalyticplays a significant materials playsrole in a the significant economic role analysis in the economicof a thermo-catalytic analysis of amethane thermo-catalytic decomposition. methane Various decomposition. studies have Various shown studiesthat the have cost shownof hydrogen thatthe production cost of hydrogen from the productionthermal decomposition from the thermal of methane decomposition is strongly of depende methanent ison strongly the nature dependent of the catalyst on the (Figure nature 2). of The the catalystcost of hydrogen (Figure2). production The cost of hydrogenis generally production low for non-noble is generally metal low forcatalysts non-noble such metal as Ni, catalysts Fe, Mo, such and asso. Ni, The Fe, cost Mo, of and hydrogen so. The production cost of hydrogen was reported production as USD catalysts 100/mol>USD H2. 100The/mol economic H2. The analysis economic of analysisthermal decomposition of thermal decomposition of methane ofat methanetemperature at temperature <600 °C has

3000 ) 2 2500 2000 1500 1000 cost ($/molH 2

H 500 0 Ni/TiO2 Fe/MgO Fe-Ni-Co Ni/Al2O3 Fe/Al2O3 Fe/Al2O3 Ni-La/Al2O3 Fe-Mo/MgO Fe-Co/Al2O3 Fe-Mo/Al2O3 5%Pd-Ni/SBA-15 10%Pd-Ni/SBA-15 15%Pd-Ni/SBA-15 20%Pd-Ni/SBA-15 0.2%Pd-Ni/SBA-15 0.4%Pd-Ni/SBA-15 unsupported bulk Fe

Figure 2. Hydrogen cost as a function of the type of catalysts.

Various modificationsmodifications have have been been employed employed to to improve improve the the effi efficiencyciency of hydrogenof hydrogen production production by thermalby thermal decomposition decomposition process. process. Timmerberg Timmerberg et et al. al [.54 [54]] employed employed a a plasma plasma reactor reactorfor for thethe thermalthermal decomposition of methane to produce hydrogen andand carbon black. The economiceconomic analysis of thethe plasma-assistedplasma-assisted thermal thermal decomposition decomposition of methaneof methane shows shows that hydrogen that hydrogen production production cost was estimatedcost was as EUR 1.6/kg compared to the hydrogen production cost of EUR 2.2/kg for the conventional reactor used for methane decomposition. The lower hydrogen production cost obtained from the plasma-assisted methane decomposition can be attributed to the improved process conditions offered by the plasma reactor. The improved process condition could have reduced the operating cost of the plasma-assistant thermal methane decomposition for hydrogen and carbon black production. Similarly, the use of plasma to improve the pyrolysis of natural gas to hydrogen and carbon black has been reported by Sustainability 2020, 12, 10148 6 of 18

Labanca et al. [55]. The economic analysis of the plasma-assisted natural gas pyrolysis revealed a lower CAPEX, OPEX and lower hydrogen production cost compared to the conventional natural gas pyrolysis reported by Gaudernack et al. [56]. The plasma-assisted natural gas pyrolysis process was analyzed to have CAPEX, and OPEX of USD 0.674 million and 0.228 million, respectively. Which is lower compared to the conventional high temperature natural pyrolysis with CAPEX, and OPEX of USD 29.4 million and 14.3 million, respectively. It is obvious that the use of plasma reactor for the pyrolysis of natural gas to produce hydrogen and carbon black signiﬁcantly reduce the OPEX.

3.1.2. Other Emerging Technologies for Natural Gas Reforming The economic analysis of hydrogen production from various emerging thermochemical processes such as auto-thermal methane reforming, syngas chemical looping, chemical looping reforming in comparison with steam methane reforming has been reported by Khojasteh Salkuyeh et al. [57]. The economic analysis revealed that auto-thermal methane reforming has the lowest total capital cost compared to the syngas chemical looping, chemical looping reforming, and steam methane reforming. However, the integration of carbon capture facilities into each of the process significantly increased the total capital cost. The chemical looping reforming without carbon capture facilities was found to have the lowest minimum hydrogen selling price of USD 0.98 kg H2 which is competitive with the USD 1.07/kg H2 calculated for the steam methane reforming under the same scenario. The economic analysis of hydrogen production from auto-thermal reforming of methane from biogas sources has been reported by Montenegro et al. [58]. The hydrogen production cost of 2.5 EUR/kg was calculated for the process which was lower than that of the 5 EUR/kg H2 European targets. The economic analysis of natural gas conversion using oxidative coupling techniques on an industrial scale was reported by Godini et al. [58]. Unlike other studies, the fixed and operating costs of each of the sections of the oxidative coupling of methane process were performed. The compressors and the CO2 removal section accounted for the largest part of the capital investment cost. Besides, factors such as the types of utilities and the operating conditions each section of the plants were found to significantly influence the process of operating cost. The analysis revealed that the operating cost and the annual utility cost for the entire conversion process were estimated as EUR 254 and 228.6 million, respectively. Fei et al. [59] reported an economic analysis of bioconversion of natural gas to lactic acid using methanotrophic bacteria. A minimum selling lactic acid price of USD 5.83/kg, OPEX of USD 47.0 million, and CAPEX of USD 654.96 million were calculated for the production process. The lactic acid production unit was found to account for the highest part of the operating cost contribution. The sensitivity analysis revealed that each of the cost items was significantly influenced by the natural gas flow rate and lactic acid productivity. Additionally, the economic analysis of methane bio-conversion to various chemical products in a biorefinery concept has been reported by Cantera et al. [60]. The OPEX of the methane bioconversion was estimated as USD 1021/h and the total bioproduct values of USD 10,000/h. The reactor energy consumption accounted for a large proportion of the OPEX. Based on an extensive literature search, there are myriads of studies on economic analysis of various thermochemical processes used for natural gas conversion to value-added products. However, there is a dearth of literature on the economic analysis of natural gas conversion by biochemical and photocatalytic conversion. The lack of studies on economic analysis of the natural gas conversion by biochemical and photocatalytic could impede a comparative analysis of the whole process for economic viability.

3.2. Well Established Technologies

Steam Methane Reforming Steam methane reforming is a well-established process used for commercial production of hydrogen. However, due to constraints such as emissions of CO2 and high operating cost due to catalyst deactivation, research focus had centered on designing highly stable catalysts as well as incorporating carbon capture facilities to curb CO2 emission. Hence, various studies have investigated Sustainability 2020, 12, 10148 7 of 18 if several design modifications to the existing steam methane reforming process are economically viable. Recently, sorption enhances steam reforming with carbon capture has been reported as one of the strategies to overcome the challenges with the conventional steam methane reforming. To this effect, Diglio et al. [61] investigated the economic implications of hydrogen production by sorption-enhanced steam methane reforming with carbon capture. The cost of hydrogen production by the sorption-enhanced steam methane reforming was calculated to be 1.6 EUR per kg methane which is lower compared to that of steam methane reforming estimated as 117 EUR/MWh [51]. Unlike the work of Keipi et al. [51], the reaction network (which consists of eight packed bed reactors) of the sorption-enhanced steam methane reforming was found to account for about 70% of the total capital cost. Moreover, based on sensitivity analysis, the specific cost of the natural gas was found to significantly influenced the economic performance of the sorption-enhanced steam methane reforming which is consistent with the work of Keipi et al. [51]. The integration of the sorption-enhanced steam methane reforming with solid oxide fuel cells for power generation resulted in a better economic performance with a high daily revenue. The economic analysis of the co-utilization of carbon dioxide and methane in the steam reforming process for methanol synthesis has been reported by Zhang et al. [62]. Contrary to the work of Keipi et al. [51] and Diglio et al. [61], the authors employed economic evaluation indicators such as net present value (NPV), internal rate of return (IRR), and discounted payback period (DPBP) for measuring the economic viability of the methanol production process. Based on the NPV, IRR, and the DPBP values obtained for the process, the co-utilization of the carbon dioxide and methane for methanol production by steam reforming was found to be economically competitive than a scenario whereby only natural gas was utilized. However, the economic analysis of Zhang et al. [63] did not put into consideration the cost associated with the catalytic network which is a vital component of the production process as reported by Diglio et al. [62]. Similar to the work of Keipi et al. [51], Mondal and Ramesh Chandran [53] evaluated the economic performance of hydrogen production from steam methane reforming in comparison with thermal decomposition of methane. Based on the economic analysis, the CAPEX and the hydrogen production cost from the steam methane reforming were calculated as USD 291 million and 899/MT, respectively. While the CAPEX and the hydrogen production cost by the thermal decomposition of methane were calculated as USD 301 million and 948/MT, respectively. The higher cost of production calculated for the hydrogen by thermal decomposition of methane can be attributed to the high cost of electricity compared to the steam reforming process. The integration of various hydrogen production process could have a synergistic effect on the overall economic performance of the process. This has been demonstrated in the work of Gangadharan et al. [63] who performed an economic analysis of hydrogen production by combined carbon dioxide and steam methane reforming. The total capital investment cost of the combined dry methane reforming and steam methane reforming was found to be higher compared to the standalone steam methane reforming process which can be attributed to the cost of additional process equipment integrated with the steam methane reforming process. However, the study did not consider the other economic evaluation indicators such as the hydrogen production cost, and the operating costs. Moreover, the estimation of the payback period or return on investment could be advantageous in determining whether the combined process is viable compared to the standalone process. The choice of onsite hydrogen production by steam methane reforming for utilization by hydrogen fuel cell vehicle has been reported by Luk et al. [64]. The study revealed that onsite hydrogen production offers the benefits of a shorter payback period and a high rate of return on investment compared to the conventional means of hydrogen production. The cost associated with transporting and storing hydrogen production by the conventional steam methane reforming are eliminated using onsite production for direct utilization. The comparative analysis of the cost components of steam methane reforming, auto thermal methane reforming, syngas chemical looping and chemical looping reforming with or without carbon capture are depicted in Figure3. The total capital cost for each of the reforming processes is strongly dependent on whether carbon capture facilities are incorporated into the design. The reforming processes without carbon capture facilities are found to be less capital intensive compared Sustainability 2020, 12, x FOR PEER REVIEW 8 of 19 transporting and storing hydrogen production by the conventional steam methane reforming are eliminated using onsite production for direct utilization. The comparative analysis of the cost components of steam methane reforming, auto thermal methane reforming, syngas chemical looping and chemical looping reforming with or without carbon capture are depicted in Figure 3. The total capital cost for each of the reforming processes is strongly dependent on whether carbon capture facilities are incorporated into the design. The reforming processes without carbon capture facilities are found to be less capital intensive compared to those with carbon capture facilities [56]. The total capital costs for the steam methane reforming, auto-thermal reforming, syngas chemical looping and the chemical looping reforming with carbon capture were estimated as USD 831 million, 628 million, 885 million, and 638 million, respectively. The high capital cost of the syngas chemical looping could be attributed to the incorporation of the chemical looping gasification reactor. The total capital costs of the reforming processes without carbon were found to be less capital intensive. The total capital cost for steam methane reforming, auto-thermal reforming, syngas chemical looping and the chemical looping reforming without carbon capture was estimated as USD 241 million, 326 million, 813 million, and 570 million, respectively. Obviously, the economic analysis of the various technological pathways for natural gas conversion revealed that the estimated costs for each process differs. Technically, each of the natural gas conversion process has a unique configuration from the processing of the feedstocks to finished products. For instance, in the well-established steam reforming of natural gas, the reforming reaction occurs in the reformer, hence most of the CAPEX and OPEX costs revolve round the reformer. However, as a result of advances in research and development, the design modifications to the steam methane reforming whereby carbon capture, storage and utilization facilities in incorporated have been proposed to mitigate the effects of CO2 emissions. Although these additional facilities have the Sustainability 2020, 12, 10148 8 of 18 potential of reducing the CO2 emissions, nevertheless additional costs are inbuilt into the CAPEX and OPEX of the overall process. Besides, the analysis of each the process was based on different toscenarios. those with For carbonexample, capture Keipi facilities et al. [52] [56 performe]. The totald economic capital costs analysis for the of steamthermal methane decomposition reforming, of auto-thermalmethane based reforming, on four syngasscenarios chemical namely looping the scenario and the with chemical energy looping from reformingpartial combustion with carbon of capturemethane, were energy estimated from aselectricity, USD 831 energy million, from 628 million,carrier 885catalyst million, and and using 638 million,a regenerative respectively. heat Theexchanger high capital reactor. cost The of detailed the syngas analysis chemical of these looping scenarios could revealed be attributed that the to the economic incorporation performance of the chemicalof each of looping the processes gasification differs. reactor. Additionally, The total capital Mondal costs ofet theal. reforming[53] reported processes that withoutthe economic carbon wereperformance found to of be the less thermo-catalytic capital intensive. methane The total decomp capitalosition cost for varies steam depending methane reforming, on the conversion auto-thermal rate reforming,of the process. syngas The chemical economic looping analysis and of the various chemical technologies looping reforming used for without natural carbon gas conversion capture was to estimatedvaluable products as USD 241has million,been reported 326 million, in the813 literature, million, as and summarized 570 million, in respectively.Table 1.

Total capital cost ($Million) Chemical looping gasification reactors CO2 liquefaction Air separation unit Boiler & turbines Pressure swing adsorption CO2 removal Water gas shift Natural gas reformer Capital cost ($Million - 2016 U.S. dollars) Hydrogen Electricity Products (MW, LHV) CO2 captured (tonne/tonne H2) Oxygen consumption from air separation unit… Natural gas consumption (tonne/tonne H2) 0 1000 2000 3000 4000 5000 6000

SMR No CC SMR CC ATR No CC ATR CC SCL No CC SCL CC CLR No CC CLR CC

Figure 3. Cost distributions of hydrogen production from various reforming process with or without carbon capture (CC) (adapted from [57]).

Obviously, the economic analysis of the various technological pathways for natural gas conversion revealed that the estimated costs for each process differs. Technically, each of the natural gas conversion process has a unique configuration from the processing of the feedstocks to finished products. For instance, in the well-established steam reforming of natural gas, the reforming reaction occurs in the reformer, hence most of the CAPEX and OPEX costs revolve round the reformer. However, as a result of advances in research and development, the design modifications to the steam methane reforming whereby carbon capture, storage and utilization facilities in incorporated have been proposed to mitigate the effects of CO2 emissions. Although these additional facilities have the potential of reducing the CO2 emissions, nevertheless additional costs are inbuilt into the CAPEX and OPEX of the overall process. Besides, the analysis of each the process was based on different scenarios. For example, Keipi et al. [52] performed economic analysis of thermal decomposition of methane based on four scenarios namely the scenario with energy from partial combustion of methane, energy from electricity, energy from carrier catalyst and using a regenerative heat exchanger reactor. The detailed analysis of these scenarios revealed that the economic performance of each of the processes differs. Additionally, Mondal et al. [53] reported that the economic performance of the thermo-catalytic methane decomposition varies depending on the conversion rate of the process. The economic analysis of various technologies used for natural gas conversion to valuable products has been reported in the literature, as summarized in Table1. Sustainability 2020, 12, 10148 9 of 18

Table 1. Summary of literature on the economic analysis and environmental impact of various natural gas conversion technologies.

Process Output Technology Economic Analysis Environmental Impact Nature of Technology References (Products) Plasma-assisted Hydrogen production cost EUR 1.6 H2, carbon black N.R Emerging Methane decomposition (USD 1.89)/kg H2 [54] Methane decomposition using Hydrogen production cost EUR 2.2 H2, carbon black N.R Emerging molten reactor (USD 2.6)/kg H2 CAPEX: USD 0.674 million, OPEX: Plasma pyrolysis of natural gas H2, carbon black USD 0.228 million, hydrogen N.R Emerging [55] production cost: USD 14.36/kg CAPEX: USD 17.18 million/Nm2 Steam reforming of natural gas 2 H2, CO2 H2, OPEX: USD 4.28 million/Nm N.R Well Established without CO2 capture H2, CAPEX: USD 31.1 million/Nm2 Steam reforming of natural gas with 2 [56] H2 H2, OPEX: USD 5 million/Nm N.R Emerging CO2 capture H2 CAPEX: USD 29.4 million/Nm2 High temperature Pyrolysis of H , carbon black H , OPEX: USD 14.3 million/Nm2 N.R Emerging natural gas 2 2 H2

H2 production cost: EUR 72 (USD 90)/MWh Methane thermal decomposition H , carbon 40 kgCO /MWhH Emerging 2 CAPEX: EUR 4203 (USD 5258) 2 2 OPEX: EUR 815 (USD 1020)

H2 production cost: EUR 117 (USD 146)/MWh Steam methane reforming H , CO 293 kgCO /MWhH Well Established [51] 2 2 CAPEX: EUR 10,705 (USD 13,392) 2 2 OPEX: EUR 776 (USD 971)

H2 production cost: EUR 57.5 (USD 71.9)/MWh Steam methane reforming with CCS H , CO 133 kgCO /MWh H Emerging 2 2 CAPEX: EUR 162,000 (USD 202,662) 2 2 OPEX: EUR 97,662 (USD 122,175) Sustainability 2020, 12, 10148 10 of 18

Table 1. Cont.

Process Output Technology Economic Analysis Environmental Impact Nature of Technology References (Products) Thermal decomposition of methane CAPEX: 12,646 (* kEUR) (14,037 kUSD) using energy from partial H , carbon 0.55 tCO /MWh Emerging 2 OPEX: 32,229 (* kEUR) (35,774 kUSD) 2 e combustion of methane Thermal decomposition of methane CAPEX: 8225 (* kEUR) (9130 kUSD) H , carbon 0.83 tCO /MWh Emerging using energy from electricity 2 OPEX: 32,274 (* kEUR) (35,824.14 kS) 2 e Catalytic decomposition of Thermal [52] CAPEX: 28,354 (kEUR) (31,473 kUSD) methane using an energy H , carbon 0.48 tCO /MWh Emerging 2 OPEX: 36,884 (kEUR) (40,941 kUSD) 2 e carrier catalyst Thermal decomposition of methane CAPEX: 10,139 (kEUR) (11,254 kUSD) using a regenerative heat H , carbon 0.85 tCO /MWh Emerging 2 OPEX: 32,445 (kEUR) (36,014 kUSD) 2 e exchanger reactor 2 Steam methane reforming H2, CO2 N.R 1.8 kgCO2/Nm H2 Well-Established 2 Autothermal reforming H2, CO2 N.R 0.4 kgCO2/Nm H2 Emerging [65] 2 Autocatalytic decomposition H2, carbon N.R 0.2 kgCO2/Nm H2 Emerging CAPEX: 1036.21 MUSD, Production cost: 222.35 USD/Mt Steam methane reforming with Methanol NPV (i = 12%): 483.41 N.R Emerging [62] CO utilization c 2 IRR: 17.15 DPBP: 9.52 Combined dry reforming and steam CAPEX: USD 21 Million Potential environmental Syngas (H2 + CO) 3 Emerging reforming of methane OPEX: USD 5231.82/h impact: 2.83 10− PEI/h × [63] Standalone Steam CAPEX: USD 18 Million Potential environmental H , CO Well Established methane reforming 2 2 OPEX: USD 5308.72/h impact: 2.83 10 4 PEI/h × − Thermocatalytic methane Total capital cost: USD 301 million Reported as low H , carbon Emerging decomposition with 38% conversion 2 Production cost: USD 948/MT carbon footprint Thermocatalytic methane Total capital cost: USD 291 million Reported as moderate H , carbon Emerging [53] decomposition with 50% conversion 2 Production cost: USD 899/MT carbon footprint Total capital cost: USD 326 million Reported as high Steam methane reforming H , CO Well Established 2 2 Production cost: USD 873/MT carbon footprint Sustainability 2020, 12, 10148 11 of 18

Table 1. Cont.

Process Output Technology Economic Analysis Environmental Impact Nature of Technology References (Products) Thermal decomposition using H , carbon N.R 0.25 kg CO /Nm2 H Emerging carbonaceous catalyst 2 2 2 Thermal decomposition using 2 carbonaceous catalyst H2, carbon N.R 0.16 kg CO2/Nm H2 Emerging (96% conversion) [66] Thermal decomposition using 2 carbonaceous catalyst H2, carbon N.R 0.22 kg CO2/Nm H2 Emerging (78% conversion) Steam reforming with carbon H N.R 0.32 kg CO /Nm2 H Emerging capture and utilization 2 2 2 CAPEX: EUR 6.31 million Sorption enhanced Steam reforming H , CO N.R Emerging 2 2 Cost of production: EUR 1.6/kg

Steam reforming of methane H2, CO2 Minimum H2 price: USD 1.08/kg 10 kg/kg H2 Well Established

Autothermal reforming H2, CO2 Minimum H2 price: USD 1.0/kg 9 kg/kg H2 Emerging [57] Syngas Chemical looping H2, CO2 Minimum H2 price: USD 1.38/kg 13 kg/kg H2 Emerging

Chemical looping reforming H2, CO2 Minimum H2 price: USD 0.8/kg 8.6 kg/kg H2 Emerging Minimum selling price: USD 5.83/kg Bioconversion Lactic acid OPEX: USD 47.0 MM 8.89 kg CO2eq Emerging [59] CAPEX: USD 654.96MM Ectoine, Bioplastics, OPEX: USD 1021/h Bioconversion Biofuel, Extracellular 4807 kg CO /h Emerging [60] Bioproducts values: USD 10,0000/h 2 polysaccharides N.R implies not reported, * k implies 1000. Sustainability 2020, 12, 10148 12 of 18

4. Environmental Impact of Natural Gas Conversion Technologies The various types of natural gas conversion technologies have been adjudged to have great potentials of producing important energy carriers such as hydrogen and other value-added chemicals and chemical intermediates. However, these emerging technologies employed for natural gas conversion have been associated with various types of environmental impact based on the life cycle assessment studies. The environmental impact and global warming potential of combined dry methane reforming and steam methane reforming has been reported by Gangadharan et al. [63]. The authors employed the waste reduction algorithm to estimate the potential environmental impact and global warming potential. The analysis revealed that the combined dry and steam methane reforming has a lower global warming potential compared to the standalone steam methane reforming. This can be attributed to the utilization of carbon dioxide as a feedstock for the combined process whereas, in the standalone steam methane reforming process, carbon dioxide is emitted. However, the total potential environmental impact of the combined conversion process which is a measure of the human toxicity, ecological toxicity, regional atmospheric, global warming potential, and ozone depletion potential was found to be higher compared to the standalone steam methane reforming process. The environmental feasibility and greenhouse gas emissions reduction of thermal and autocatalytic decomposition of methane in comparison with steam methane reforming with or without carbon capture have been investigated by Dufour et al. [67]. Using the life cycle assessment approach, the authors analyzed the environmental impact of the various technological scenarios. The analysis revealed that the steam methane reforming without carbon capture has the highest CO2 emissions whereas the autocatalytic methane conversion with 100% conversion recorded the lowest CO2 emission. However, the steam methane reforming with carbon captured was found to significantly reduced the CO2 emissions by over 60%. This is an indication that appropriate technological modification to an existing conversion process with high CO2 emissions can significantly reduce the global warming potential of the process. Moreover, for a catalytic conversion process, an optimized catalytic design can also mitigate the amount of CO2 emission from the process. This was demonstrated in the work of Dufour et al. [68] who employed investigated the environmental impact of methane thermal decomposition using a carbonaceous catalyst. The analysis revealed that the methane decomposition with 96% conversion using the carbonaceous catalyst have the lowest CO2 emissions compared to steam methane reforming with carbon capture and other scenarios. With further modifications on the thermal decomposition of methane, the CO2 emissions from the process can be reduced [69]. Dufour et al. [66] reported the substitution of direct heating using natural gas as fuel for thermal decomposition of methane with a solar concentrator and found that there was over 25% reduction in CO2 emissions. Comparing the greenhouse gas emissions of hydrogen production by thermal methane decompositions with auto thermal methane reforming and steam methane reforming, Dufour et al. [65] found that the hydrogen production by auto-thermal methane reforming has the lowest CO2 emissions. However, with design modifications such as integrating the steam methane reforming with carbon capture and methane auto-maintained decomposition, the CO2 emissions were significantly reduced compared to the unmodified hydrogen production processes. Kothari et al. [70] compared the environmental impact of hydrogen production by steam methane reforming with CO conversion and partial oxidation reforming and found that a lower CO2 emission was obtained from the steam methane reforming with CO conversion compared to the partial oxidation reforming. The environmental impact of bioconversion of natural gas to lactic acid using methanotrophic bacteria has been reported by Fei et al. [59]. Based on the life cycle assessment, the natural gas bioconversion process was found to have some potential environmental impacts. The global warming potential was calculated as 5.83 kgCO2eq with negligible effects of acidification, ozone depletion carcinogenic and respiratory effects. In a similar study, the environmental impact of methane bioconversion to various chemical products was reported by Cantera et al. [60]. The bioconversion process was reported to have 4807 kgCO2/h with CO2 emitted from energy consumption accounted for 3157 kg/h. Despite there being several studies on the environmental impact of various thermochemical conversion processes used for natural gas, there is a dearth of literature on the environmental impact of Sustainability 2020, 12, 10148 13 of 18 photocatalytic and bioconversion of natural gas. Therefore, more research attention should be focused on the environmental impact of the photocatalytic and bioconversion of natural gas for a comparative analysis with that of thermochemical conversion processes.

5. Effect of Operating Parameters on the CO2 Emissions and Process Economics Study has shown that process parameters such as the flowrate of the reactants, and the reaction temperature usually influence the amount of CO2 emissions and the process economics [71]. A sensitivity analysis reported by Hamid et al. [71] revealed that the increase in the methane flowrate during steam methane reforming resulted in the increase in the CAPEX, OPEX and the CO2 emissions as shown in Figure4. The increase in the methane flowrates results in a corresponding increase in the conversion of the methane to gaseous products such as hydrogen, CO, and CO2. As the natural gas conversion progresses with time on stream, there could be challenges of catalysts deactivation by methane cracking, CO2 disproportionation and sintering thereby resulting in the periodic replacements of the catalysts. The periodic replacement of catalysts during the natural gas conversion will certainly influence the CAPEX and OPEX of the process. Additionally, in the same study by Hamid et al. [71], increasing the water contents during steam reforming of natural gas, was reported to have a significant influence on the OPEX per hydrogen production as shown in Figure5. Since water is needed to produce steam needed for the reforming process, the amount of steam produced is depended on the amount of water available. As the water contents increases, more steam is generated for the natural gas conversion process, thereby driving the reaction towards more products. As more products are formed the lifeSustainability span 2020 of, the 12, x FOR reforming PEER REVIEW catalysts are shortened with eventuality of replacements.13 of 19

101.11 865.50 CAPEX (Euro/year) OPEX (Euro/year) 101.10 865.00 101.09 101.08 864.50 101.07 864.00 101.06 101.05 863.50 101.04 863.00 CAPEX (Euro/year) OPEX (Euro/year) 101.03 862.50 101.02 101.01 862.00 0.625 0.675 0.725 0.775 0.85 0.925 Methane flowrate (MMSCFD)

(a) 555

550

545

540

535 emission (Tonne/hr) 2

CO 530

525 0.625 0.675 0.725 0.775 0.85 0.925 Methane flowrate (MMSCFD)

(b)

Figure 4. EﬀFigureect of 4. methane Effect of ﬂowratemethane flowrate on the on (a) the economics (a) economics of natural of natural gas gas conversion conversion by by steam steam reforming reforming and (b) CO2 emission from natural gas conversion by steam reforming. and (b) CO2 emission from natural gas conversion by steam reforming. Sustainability 2020, 12, 10148 14 of 18 Sustainability 2020, 12, x FOR PEER REVIEW 14 of 19

2.65 ) 2

2.60

2.55

2.50

2.45

2.40 production (MMEuro/Tonne H 2 2.35

OPEX/H 2.30 20 25 30 35 40 45 Water content (%)

FigureFigure 5.5. EEffectﬀect ofof waterwater contentcontent on the OPEX of natural gas conversion conversion process process by by steam steam reforming. reforming.

6.6. ConclusionsConclusions andand ResearchResearch OutlookOutlook NaturalNatural gas gas as as a fossila fossil fuel fuel resource resource can can be applied be applied directly directly in electricity in electricity generation, generation, domestic domestic heating , andheating, as starting and as materialsstarting materials to produce to produce valuable valuable chemicals chemicals and chemical and chemical intermediates. intermediates. Besides Besides being usedbeing directly used directly for various for various applications, applications, natural natural gas can gas be convertedcan be converted to various to valuablevarious valuable chemical productschemical and products chemical and intermediates. chemical Steamintermedia reformingtes. isSteam a well-established reforming thermochemicalis a well-established process usedthermochemical for converting process natural used gas for to hydrogenconverting and na syngas.tural gas The to steamhydrogen reforming and syngas. process The is however steam constraintreforming with process challenges is however such asconstraint catalyst with deactivation, challenges the such high as thermal catalyst energy deactivation, required the for steamhigh generationthermal energy and to required initiate the for reaction steam generation and the emissions and to initiate of CO2 .the Research reaction attention and the has emissions focused of on CO other2. emergingResearch thermochemicalattention has focused processes on such other as theemerging CO2 reforming thermochemical of methane, processes partial oxidation such as reforming,the CO2 auto-thermalreforming of reforming, methane, andpartial photocatalytic oxidation reformin reformingg, toauto-thermal overcome the reforming, challenges and associated photocatalytic with the steamreforming reforming. to overcome Although the echallengesfforts have associated been made with to tackle the steam some ofreforming. these challenges, Although these efforts emerging have processesbeen made are to yet tackle to be some fully of developed these challenges, into commercial-scale these emerging production. processes are Besides yet to thebe fully thermochemical developed process,into commercial-scale other processes production. such as thebiochemical Besides the andthermo thechemical photocatalytic process, processes other processes are currently such receiving as the researchbiochemical attention. and the To photocatalytic ascertain the economicprocesses are viability currently and receiving environmental research impact attention. of these To ascertain emerging technologiesthe economic in viability comparison and environmental with the well-established impact of these steam emerging reforming technologies process, in several comparison researchers with havethe well-established conducted an economic steam reforming analysis and process, environmental several researchers impacts on have various conducted natural gasan conversioneconomic technologies.analysis and environmental Based on several impacts studies on on various the economic natural analysis,gas conversion there is technologies. a consensus thatBased the on emerging several technologiesstudies on the are economic yet to be analysis, economically there is competitivea consensus that with the the emerging well-established technologies steam are reforming.yet to be However,economically there competitive is a huge environmentalwith the well-establishe benefit ofd thesesteam emerging reforming. technologies However, there in terms is a of huge CO 2 emissionsenvironmental and the benefit total environmentalof these emerging index. technologies The study revealedin terms that of thereCO2 emissions is a wide researchand the gap total on theenvironmental economic analysis index. andThe environmentalstudy revealed impactthat there of theis a conversionwide research of naturalgap on gasthe economic by the biochemical analysis andand photocatalytic environmental processes. impact of Therefore,the conversion the economic of natural feasibilities gas by the and biochemical the environmental and photocatalytic impacts of theprocesses. biochemical Therefore, and photocatalytic the economic feasibilities processes cannot and the be environmental compared with impacts the steam of the reforming biochemical process. and Thephotocatalytic study also revealedprocesses thatcannot technical be compared modifications with the to thesteam original reforming design process. of the thermochemicalThe study also revealed that technical modifications to the original design of the thermochemical processes processes significantly reduced CO2 emissions. More research attention needs to be focused on comparativesignificantly analysisreduced ofCO the2 emissions. thermochemical, More research biochemical, attention and photocatalyticneeds to be focused conversions on comparative process to determineanalysis of their the economicthermochemical, feasibility biochemical, and environmental and photocatalytic impact. conversions process to determine their economic feasibility and environmental impact. Author Contributions: F.O.A. and B.V.A. conceptualized the idea and wrote the draft manuscript. S.I.M. and N.M.Author reviewed Contributions: the draft F.O.A. manuscript and B.V.A. for technicality. conceptualized S.I.M. the acquired idea and the wrote funding the draft for the ma research.nuscript. AllS.I.M. authors and haveN.M. read reviewed and agreed the draft to the manuscript published for version technicality. of the manuscript.S.I.M. acquired the funding for the research. All authors have read and agreed to the published version of the manuscript. Sustainability 2020, 12, 10148 15 of 18

Funding: The research was financially supported by Universiti Tenaga Nasional through BOLD2025 Grant (No. 10436494/B/2019008). Acknowledgments: B.V.A., S.I.M., and S.M. acknowledge the financial support of Universiti Tenaga Nasional, Malaysia. Conflicts of Interest: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations/Symbols

ATR Auto-thermal methane reforming CAPEX Capital Expenditure CCS Carbon Capture and Storage CLR Chemical Looping reforming CO2 Carbon dioxide DPBP Discounted payback period OPEX Operating Expenditure H2 Hydrogen IRR Internal rate of return Kg Kilogram SMR Steam methane reforming SCL Syngas chemical looping NPV Net proﬁt value MMSCFD Million standard cubic feet per day TcF Trillion cubic feet EUR Euro USD US dollar

References

1. Leung, G.C.K. Natural Gas as a Clean Fuel. In Handbook Clean Energy Systems; American Cancer Society: Atlanta, GA, USA, 2015; pp. 1–15. [CrossRef] 2. Steele, B.C.H. Running on natural gas. Nature 1999, 400, 619–621. [CrossRef] 3. Faramawy, S.; Zaki, T.; Sakr, A.A.-E. Natural gas origin, composition, and processing: A review. J. Nat. Gas. Sci. Eng. 2016, 34, 34–54. [CrossRef] 4. Fadiran, G.; Adebusuyi, A.T.; Fadiran, D. Natural gas consumption and economic growth: Evidence from selected natural gas vehicle markets in Europe. Energy 2019, 169, 467–477. [CrossRef] 5. Powell, J.B. Natural gas utilization: Current status and opportunities. Catal. Today 2020. [CrossRef] 6. Khan, M.I. Policy options for the sustainable development of natural gas as transportation fuel. Energy Policy 2017, 110, 126–136. [CrossRef] 7. Ogden, J.; Jaﬀe, A.M.; Scheitrum, D.; McDonald, Z.; Miller, M. Natural gas as a bridge to hydrogen transportation fuel: Insights from the literature. Energy Policy 2018, 115, 317–329. [CrossRef] 8. Walker, S.B.; Lanen, D.; van Mukherjee, U.; Fowler, M. Greenhouse gas emissions reductions from applications of Power-to-Gas in power generation. Sustain. Energy Technol. Assess. 2017, 20, 25–32. [CrossRef] 9. Thiruvengadam, A.; Besch, M.; Padmanaban, V.; Pradhan, S.; Demirgok, B. Natural gas vehicles in heavy-duty transportation-A review. Energy Policy 2018, 122, 253–259. [CrossRef] 10. Gilbert, A.Q.; Sovacool, B.K. Benchmarking natural gas and coal-ﬁred electricity generation in the United States. Energy 2017, 134, 622–628. [CrossRef] 11. International Energy Agency. World Energy Outlook 2016; International Energy Agency: Paris, France, 2016. [CrossRef] 12. International Energy Agency. World Energy Outlook 2019; International Energy Agency: Paris, France, 2019. [CrossRef] 13. Carmona, M.; Feria, J.; Golpe, A.A.; Iglesias, J. Energy consumption in the US reconsidered. Evidence across sources and economic sectors. Renew. Sustain. Energy Rev. 2017, 77, 1055–1068. [CrossRef] 14. Ayodele, B.V.; Hossain, S.S.; Lam, S.S.; Osazuwa, O.U.; Khan, M.R.; Cheng, C.K. Syngas production from

CO2 reforming of methane over neodymium sesquioxide supported cobalt catalyst. J. Nat. Gas Sci. Eng. 2016, 34, 873–885. [CrossRef] Sustainability 2020, 12, 10148 16 of 18

15. Ayodele, B.V.; Bin Mohd Yassin, M.Y.; Naim, R.; Abdullah, S. Hydrogen production by thermo-catalytic

conversion of methane over lanthanum strontium cobalt ferrite (LSCF) and αAl2O3 supported Ni catalysts. J. Energy Inst. 2019, 92, 892–903. [CrossRef]

16. Ayodele, B.V.; Khan, M.R.; Cheng, C.K. Greenhouse gases mitigation by CO2 reforming of methane to hydrogen-rich syngas using praseodymium oxide supported cobalt catalyst. Clean Technol. Environ. Policy 2016, 1–13. [CrossRef] 17. LeValley, T.L.; Richard, A.R.; Fan, M. The progress in water gas shift and steam reforming hydrogen production technologies—A review. Int. J. Hydrogen Energy 2014, 39, 16983–17000. [CrossRef] 18. Ayodele, B.V.; Khan, M.R.; Lam, S.S.; Cheng, C.K. Production of CO-rich hydrogen from methane dry reforming over lanthania-supported cobalt catalyst: Kinetic and mechanistic studies. Int. J. Hydrogen Energy 2016, 41, 4603–4615. [CrossRef] 19. Dagle, R.A.; Dagle, V.; Bearden, M.D.; Holladay, J.D.; Krause, T.R.; Ahmed, S. An Overview of Natural Gas Conversion Technologies for Co-Production of Hydrogen and Value-Added Solid Carbon Products; (No PNNL-26726, ANL-17/11); Pacific Northwest Natl Lab(PNNL): Richland, WA USA; Argonne Natl Lab(ANL): DuPage, IL, USA, 2017; Volume 65. [CrossRef] 20. Gharibi, M.; Zangeneh, F.T.; Yaripour, F.; Sahebdelfar, S. Nanocatalysts for conversion of natural gas to liquid fuels and petrochemical feedstocks. Appl. Catal. A Gen. 2012, 443, 8–26. [CrossRef] 21. Gür, T.M. Comprehensive review of methane conversion in solid oxide fuel cells: Prospects for efficient electricity generation from natural gas. Prog. Energy Combust. Sci. 2016, 54, 1–64. [CrossRef] 22. Pirker, G.; Wimmer, A. Sustainable power generation with large gas engines. Energy Convers. Manag. 2017, 149, 1048–1065. [CrossRef] 23. Alipour-Dehkordi, A.; Khademi, M.H. Use of a micro-porous membrane multi-tubular fixed-bed reactor

for tri-reforming of methane to syngas: CO2,H2O or O2 side-feeding. Int. J. Hydrogen Energy 2019, 44, 32066–32079. [CrossRef] 24. Kathe, M.; Fryer, C.; Sandvik, P.; Kong, F.; Zhang, Y.; Empﬁeld, A.; Fan, L.S. Modularization strategy for

syngas generation in chemical looping methane reforming systems with CO2 as feedstock. AIChE J. 2017, 63, 3343–3360. [CrossRef] 25. Kim, S.; Crandall, B.S.; Lance, M.J.; Cordonnier, N.; Lauterbach, J.; Sasmaz, E. Activity and stability of NiCe@SiO2 multi–yolk–shell nanotube catalyst for tri-reforming of methane. Appl. Catal. B Environ. 2019, 259, 118037. [CrossRef] 26. Tahir, M.; Tahir, B.; Zakaria, Z.Y.; Muhammad, A. Enhanced photocatalytic carbon dioxide reforming of

methane to fuels over nickel and montmorillonite supported TiO2 nanocomposite under UV-light using monolith photoreactor. J. Clean. Prod. 2019, 213, 451–461. [CrossRef]

27. Haﬁzi, A.; Rahimpour, M.R.; Hassanajili, S. Calcium promoted Fe/Al2O3 oxygen carrier for hydrogen production via cyclic chemical looping steam methane reforming process. Int. J. Hydrogen Energy 2015, 40, 16159–16168. [CrossRef] 28. Sastre, D.; Serrano, D.P.; Pizarro, P.; Coronado, J.M. Chemical insights on the activity of La1-xSrxFeO3

perovskites for chemical looping reforming of methane coupled with CO2-splitting. J. CO2 Util. 2019, 31, 16–26. [CrossRef] 29. Aboosadi, Z.A.; Yadecoury, M.F. Thermally Intensiﬁcation of Steam Reforming Process by Use of Methane Tri-Reforming: A Review. Int. J. Chem. React. Eng. 2019, 17.[CrossRef] 30. Angeli, S.D.; Monteleone, G.; Giaconia, A.; Lemonidou, A.A. State-of-the-art catalysts for CH4 steam reforming at low temperature. Int. J. Hydrogen Energy 2014, 39, 1979–1997. [CrossRef] 31. Nikolaidis, P.; Poullikkas, A. A comparative overview of hydrogen production processes. Renew. Sustain. Energy Rev. 2017, 67, 597–611. [CrossRef]

32. Sidik, S.M.; Triwahyono, S.; Jalil, A.A.; Majid, Z.A.; Salamun, N.; Talib, N.B.; Abdullah, T.A. CO2 reforming of CH4 over Ni-Co/MSN for syngas production: Role of Co as a binder and optimization using RSM. Chem. Eng. J. 2016, 295, 1–10. [CrossRef] 33. Aman, D.; Radwan, D.; Ebaid, M.; Mikhail, S.; van Steen, E. Comparing nickel and cobalt perovskites for steam reforming of glycerol. Mol. Catal. 2018, 452, 60–67. [CrossRef] 34. Sadykov, V.A.; Gubanova, E.L.; Sazonova, N.N.; Pokrovskaya, S.A.; Chumakova, N.A.; Mezentseva, N.V.; Bobin, A.S.; Gulyaev, R.V.; Ishchenko, A.V.; Krieger, T.A.; et al. Dry reforming of methane over Pt/PrCeZrO catalyst: Kinetic and mechanistic features by transient studies and their modeling. Catal. Today 2011, 171, 140–149. [CrossRef] Sustainability 2020, 12, 10148 17 of 18

35. Gokon, N.; Yamawaki, Y.; Nakazawa, D.; Kodama, T. Kinetics of methane reforming over Ru/γ-Al2O3-catalyzed metallic foam at 650–900 ◦C for solar receiver-absorbers. Int. J. Hydrogen Energy 2011, 36, 203–215. [CrossRef] 36. Abdullah, B.; Abd Ghani, N.A.; Vo, D.V.N. Recent advances in dry reforming of methane over Ni-based catalysts. J. Clean. Prod. 2017, 162, 170–185. [CrossRef] 37. Abdulrasheed, A.; Jalil, A.A.; Gambo, Y.; Ibrahim, M.; Hambali, H.U.; Shahul Hamid, M.Y. A review on catalyst development for dry reforming of methane to syngas: Recent advances. Renew. Sustain. Energy Rev. 2019, 108, 175–193. [CrossRef] 38. Rategarpanah, A.; Meshkani, F.; Wang, Y.; Arandiyan, H.; Rezaei, M. Thermocatalytic conversion of methane to highly pure hydrogen over Ni–Cu/MgO Al O catalysts: Inﬂuence of noble metals (Pt and Pd) on the · 2 3 catalytic activity and stability. Energy Convers. Manag. 2018, 166, 268–280. [CrossRef] 39. Kumar, R.; Kumar, K.; Choudary, N.V.; Pant, K.K. Eﬀect of support materials on the performance of Ni-based catalysts in tri-reforming of methane. Fuel Process. Technol. 2019, 186, 40–52. [CrossRef]

40. Kang, D.; Lim, H.S.; Lee, J.W. Mesoporous Fe2O3–CeO2–Al2O3 Oxygen Carrier for Chemical Looping Dry Reforming with Subsequent Water Splitting. Ind. Eng. Chem. Res. 2020, 59, 15912–15920. [CrossRef] 41. Chuayboon, S.; Abanades, S.; Rodat, S. Solar chemical looping reforming of methane combined with

isothermal H2O/CO2 splitting using ceria oxygen carrier for syngas production. J. Energy Chem. 2020, 41, 60–72. [CrossRef] 42. Wang, Y.; Zheng, Y.; Wang, Y.; Li, K.; Wang, Y.; Jiang, L.; Zhu, X.; Wei, Y.; Wang, H. Syngas production

modiﬁed by oxygen vacancies over CeO2-ZrO2-CuO oxygen carrier via chemical looping reforming of methane. Appl. Surf. Sci. 2019, 481, 151–160. [CrossRef] 43. Henard, C.A.; Smith, H.; Dowe, N.; Kalyuzhnaya, M.G.; Pienkos, P.T.; Guarnieri, M.T. Bioconversion of methane to lactate by an obligate methanotrophic bacterium. Sci. Rep. 2016, 6, 21585. [CrossRef] 44. Shoji, S.; Peng, X.; Yamaguchi, A.; Watanabe, R.; Fukuhara, C.; Cho, Y.; Yamamoto, T.; Matsumura, S.; Yu, M.W.; Ishii, S.; et al. Photocatalytic uphill conversion of natural gas beyond the limitation of thermal reaction systems. Nat. Catal. 2020, 3, 148–153. [CrossRef] 45. Aznar-Sánchez, J.A.; Belmonte-Ureña, L.J.; Velasco-Muñoz, J.F.; Manzano-Agugliaro, F. Economic analysis of sustainable water use: A review of worldwide research. J. Clean. Prod. 2018, 198, 1120–1132. [CrossRef]

46. Jouny, M.; Luc, W.; Jiao, F. General Techno-Economic Analysis of CO2 Electrolysis Systems. Ind. Eng. Chem. Res. 2018, 57, 2165–2177. [CrossRef] 47. Towler, G.; Sinnott, R.A.Y. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design; Elsevier: Amsterdam, the Netherlands, 2013. [CrossRef] 48. Nikookaran, N.; Karakostas, G.; Todd, T.D. Combining Capital and Operating Expenditure Costs in Vehicular Roadside Unit Placement. IEEE Trans. Veh. Technol. 2017, 66, 7317–7331. [CrossRef] 49. Qian, J.X.; Chen, T.W.; Enakonda, L.R.; Liu, D.B.; Basset, J.M.; Zhou, L. Methane decomposition to pure hydrogen and carbon nano materials: State-of-the-art and future perspectives. Int. J. Hydrogen Energy 2020, 45, 15721–15743. [CrossRef] 50. Mueller-Langer, F.; Tzimas, E.; Kaltschmitt, M.; Peteves, S. Techno-economic assessment of hydrogen production processes for the hydrogen economy for the short and medium term. Int. J. Hydrogen Energy 2007, 32, 3797–3810. [CrossRef] 51. Keipi, T.; Tolvanen, H.; Konttinen, J. Economic analysis of hydrogen production by methane thermal decomposition: Comparison to competing technologies. Energy Convers. Manag. 2018, 159, 264–273. [CrossRef] 52. Keipi, T.; Hankalin, V.; Nummelin, J.; Raiko, R. Techno-economic analysis of four concepts for thermal

decomposition of methane: Reduction of CO2 emissions in natural gas combustion. Energy Convers. Manag. 2016, 110, 1–12. [CrossRef] 53. Mondal, K.C.; Ramesh Chandran, S. Evaluation of the economic impact of hydrogen production by methane decomposition with steam reforming of methane process. Int. J. Hydrogen Energy 2014, 39, 9670–9674. [CrossRef] 54. Timmerberg, S.; Kaltschmitt, M.; Finkbeiner, M. Hydrogen and hydrogen-derived fuels through methane decomposition of natural gas—GHG emissions and costs. Energy Convers. Manag. X 2020, 7, 100043. [CrossRef]

55. Labanca, A.; Feugeas, J.; Miranda, P. Economic Analysis of CO2-Free Hydrogen Production from Natural Gas by Plasma Pyrolysis; Rio Oil & Gas Expo and Conference: Rio de Janeiro, Brazil, 2006; pp. 1–19. Sustainability 2020, 12, 10148 18 of 18

56. Gaudernack, B.; Lynum, S. Hydrogen from natural gas without release of CO2 to the atmosphere. Int. J. Hydrogen Energy 1998, 23, 1087–1093. [CrossRef] 57. Khojasteh Salkuyeh, Y.; Saville, B.A.; MacLean, H.L. Techno-economic analysis and life cycle assessment of hydrogen production from natural gas using current and emerging technologies. Int. J. Hydrogen Energy 2017, 42, 18894–18909. [CrossRef] 58. Godini, H.R.; Xiao, S.; Jašo, S.; Stünkel, S.; Salerno, D.; Son, N.X.; Song, S.; Wozny, G. Techno-economic analysis of integrating the methane oxidative coupling and methane reforming processes. Fuel Process Technol. 2013, 106, 684–694. [CrossRef] 59. Fei, Q.; Liang, B.; Tao, L.; Tan, E.C.; Gonzalez, R.; Henard, C.; Guarnieri, M. Biological valorization of natural gas for the production of lactic acid: Techno-economic analysis and life cycle assessment. Biochem. Eng. J. 2020, 158, 107500. [CrossRef] 60. Cantera, S.; Muñoz, R.; Lebrero, R.; López, J.C.; Rodríguez, Y.; García-Encina, P.A. Technologies for the bioconversion of methane into more valuable products. Curr. Opin. Biotechnol. 2018, 50, 128–135. [CrossRef] 61. Diglio, G.; Hanak, D.P.; Bareschino, P.; Mancusi, E.; Pepe, F.; Montagnaro, F.; Manovic, V. Techno-economic analysis of sorption-enhanced steam methane reforming in a ﬁxed bed reactor network integrated with fuel cell. J. Power Sources 2017, 364, 41–51. [CrossRef] 62. Zhang, C.; Jun, K.W.; Gao, R.; Kwak, G.; Park, H.G. Carbon dioxide utilization in a gas-to-methanol process

combined with CO2/Steam-mixed reforming: Techno-economic analysis. Fuel 2017, 190, 303–311. [CrossRef] 63. Gangadharan, P.; Kanchi, K.C.; Lou, H.H. Evaluation of the economic and environmental impact of combining dry reforming with steam reforming of methane. Chem. Eng. Res. Des. 2012, 90, 1956–1968. [CrossRef] 64. Luk, H.T.; Lei, H.M.; Ng, W.Y.; Ju, Y.; Lam, K.F. Techno-economic analysis of distributed hydrogen production from natural gas. Chin. J. Chem. Eng. 2012, 20, 489–496. [CrossRef] 65. Dufour, J.; Serrano, D.P.; Gálvez, J.L.; Moreno, J.; González, A. Hydrogen production from fossil fuels: Life cycle assessment of technologies with low greenhouse gas emissions. Energy Fuels 2011, 25, 2194–2202. [CrossRef] 66. Dufour, J.; Serrano, D.P.; Gálvez, J.L.; González, A.; Soria, E.; Fierro, J.L.G. Life cycle assessment of alternatives for hydrogen production from renewable and fossil sources. Int. J. Hydrogen Energy 2012, 37, 1173–1183. [CrossRef] 67. Dufour, J.; Serrano, D.P.; Gálvez, J.L.; Moreno, J.; García, C. Life cycle assessment of processes for hydrogen production. Environmental feasibility and reduction of greenhouse gases emissions. Int. J. Hydrogen Energy 2009, 34, 1370–1376. [CrossRef] 68. Dufour, J.; Gálvez, J.L.; Serrano, D.P.; Moreno, J.; Martínez, G. Life cycle assessment of hydrogen production by methane decomposition using carbonaceous catalysts. Int. J. Hydrogen Energy 2010, 35, 1205–1212. [CrossRef] 69. Srilatha, K.; Bhagawan, D.; Kumar, S.S.; Himabindu, V. Sustainable fuel production by thermocatalytic decomposition of methane—A review. S. Afr. J. Chem. Eng. 2017, 24, 156–167. [CrossRef] 70. Kothari, R.; Buddhi, D.; Sawhney, R.L. Comparison of environmental and economic aspects of various hydrogen production methods. Renew. Sustain. Energy 2008, 12, 553–563. [CrossRef] 71. Hamid, U.; Rauf, A.; Ahmed, U.; Selim Arif Sher Shah, M.; Ahmad, N. Techno-economic assessment of process integration models for boosting hydrogen production potential from coal and natural gas feedstocks. Fuel 2020, 266, 117111. [CrossRef]

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