Advanced Design and Comparative Analysis of Methanol Production Routes from CO 2 and Renewable H 2 : via Syngas vs. Direct Hydrogenation Processes

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Introduction
CO 2 emission from the excessive use of fossil fuels is considered an acute and severe environmental problem.The Intergovernmental Panel on Climate Change (IPCC) emphasized the importance of limiting the global temperature rise to less than 1.5 °C to solve this issue [1].Therefore, many researchers have also focused on solving this problem by proposing different CO 2 utilization technologies, such as enhanced oil recovery [2], CO 2 -enhanced coalbed methanol recovery [3], and carbon capture and utilization (CCU) [4][5][6][7].CCU is one of the promising strategies as it produces high-value materials and chemicals, such as carbonates, alcohols, and hydrocarbons, while mitigating CO 2 emissions [8,9].A number of studies on CCU have been extensively reported in the literature across various research areas such as catalyst/material synthesis and development [10,11], reactor design and kinetic study [12,13], process integration and intensification [14,15], and technoeconomic and life-cycle assessments [16,17].In detail, Aresta et al. assessed the CO 2 conversion of potential biocatalysts and corresponding energy products [18].Rafiee et al. described various pathways, development progress, and process benchmarks on CO 2 capture and utilization [15].González-Garay et al. studied a molecule-to-planet level assessment for the green methanol production processes using CO 2 and renewable H 2 [19].Kim et al. integrated solar-based fuel processes with CO 2 , evaluated the environmental impacts through life-cycle assessment, and accordingly proposed a new framework for CO 2 utilization [20][21][22].Do et al. proposed 72 CO 2 -to-fuel pathways and evaluated their technical, economic, and environmental performances by utilizing process systems engineering-centric approaches, such as process synthesis, simulation, and optimization [23].As a practical project launched in 2017, the ALIGN-CCUS project has focused on optimizing the carbon capture, usage, and storage (CCUS) chain, specifically life cycle assessments, public acceptance, and exploring implementation options [24].Its findings inform future research, guide policy decisions, and support efforts toward achieving low-carbon industrial growth.With their concept, Moser et al. [25] conducted a comparative technoeconomic analysis on complete chains of dimethyl ether (DME) production, while Troy et al. [26] focused on the life cycle assessment of both DME and polyoxymethylene dimethyl ethers (OMEs).Albrecht et al. proposed a standardized methodology to assess the economic viability of sustainable fuels derived from the Fischer-Tropsch (FT) synthesis [27].Moreover, Von der Assen et al. developed a framework for conducting reliable environmental evaluations of CCU processes using life cycle assessment methods [28].
Of the various CO 2 -based products, methanol is widely used in energy industries because it serves as a raw material for other chemicals, as a fuel itself, or as a fuel additive (blended with gasoline) [29].Thus, researchers have focused on methanol-oriented studies, from catalyst development to process design and system analysis.Typically, methanol is produced using fossil fuels.It is synthesized using syngas (a mixture of CO and H 2 ), which is produced by converting raw materials (such as natural gas or coal) via stream reforming or gasification processes [30,31].However, the captured CO 2 has been used as a cofeed with natural gas in dry reforming [29] or with H 2 in the reverse water-gas shift for syngas intermediates [32][33][34] due to the increasing demand for methanol in the energy sectors and the global attention on CO 2 mitigation.Joo et al. proposed a new methanol process using CO 2 and H 2 as feeds, which is known as the CAMERE process ("CArbon dioxide hydrogenation to form methanol via a Reverse water-gas shift reaction") [35].In this process, CO 2 and H 2 are first converted into CO and H 2 O through the reverse water-gas shift reaction (RWGS: CO 2 + H 2 ↔ CO + H 2 O), and CO obtained from RWGS and additional H 2 are resynthesized to produce methanol via a methanol synthesis process In the paper, the unreacted gases in methanol synthesis are purged out of the reactor to maintain operating conditions and feed pressure.However, this proposed process scheme causes losses of valuable H 2 and the reemission of CO 2 into the atmosphere.Therefore, Kim et al. added a recycled stream from a methanol reactor to the RWGS reactor and thus modified the CAMERE process; this resulted in a higher methanol yield [21].Although these two papers have evaluated the economics involved in their proposed methanol processes using CO 2 and H 2 , there is a need to increase the compatibility of the CAMERE process compared to the state-of-the-art methanol production technologies, such as direct CO 2 hydrogenation.
In contrast to the CAMERE process that produces syngas as an intermediate, direct CO 2 hydrogenation, an up-to-date technology, produces methanol from CO 2 without producing syngas [36].Thus, direct hydrogenation has a simple process scheme due to the absence of syngas production and reduces the utility consumption required for reactor or separator operations.An et al. examined the Cu/Zn/Al/Zr catalyst activities and developed corresponding kinetic models suitable for direct hydrogenation [37].Kiss et al. [38,39] and Do and Kim [40] proposed new process schemes appropriate for direct CO 2 hydrogenation and conducted a technoeconomic analysis.In addition, Cho et al. developed a new knowledge-based assessment platform for early-stage screening of CO 2 direct hydrogenation catalysts based on the simulation results [41].
This study aims to propose new methanol synthesis routes from CO 2 , advanced syngas-to-methanol (AS2M), and direct CO 2 -to-methanol processes (DC2M), which exhibit improved performance compared to the existing routes.This study also comparatively analyzed technoeconomic and environmental performances of the two proposed routes using various evaluation criteria.A detailed explanation of the AS2M and DC2M processes can be found in Section 2, and the process modeling and simulation are discussed in Section 3. We evaluated and compared their technical, economic, and environmental performances with those of the original CAMERE process proposed by Joo et al. [35] and the modified CAMERE process proposed by Kim et al. [21].For this, carbon efficiency (CE), energy efficiency (EE), carbon reduction (CR), and unit production cost of methanol (UPC) were used as key performance indicators (Section 4).Moreover, a sensitivity analysis of utility sources and H 2 production technologies is performed in Section 5, and the paper is concluded in Section 6.

Problem Statements
2.1.Description of CO 2 to Methanol Production Process. Figure 1 shows four different process configurations for methanol production from CO 2 and H 2 .It is assumed that CO 2 is captured from flue gas using amine-based absorption, and H 2 is produced via water electrolysis using renewable electricity.Meanwhile, utilities (electricity and heat) required to operate unit processes are assumed to be generated from conventional sources.Figure 1(a) indicates the process scheme of the original CAMERE process (henceforth referred to as the o-CAMERE process) initially proposed by Joo et al. [35].As mentioned, CO 2 and H 2 are fed to the RWGS reactor, which converts them to both CO and H 2 O.The resulting stream consists of produced CO,unreacted H 2 , and CO 2 that are moved to a methanol synthesis (MS) reactor to produce methanol (mainly via CO hydrogenation).The raw methanol synthesized by the MS reactor is then fed to the methanol purification (MP) unit to obtain a high-purity methanol product (more than 99% purity).Figure 1(b) represents the modified CAMERE process developed by Kim et al. (henceforth referred to as the m-CAMERE process) [21].The m-CAMERE process has an analogous configuration compared to the o-CAMERE process, including RWGS, MS, and MP.The distinct difference between the two CAMERE processes is that in the m-CAMERE process, the unreacted gases at the MS reactor are separated and recycled back to the RWGS reactor to increase the yield of methanol.Although the two CAMEREs are rigorously simulated in this paper using a commercial process simulator, both processes are used only as references for evaluating the impacts of advanced processes, as shown in Figures 1(c) and 1(d).Figures 1(c) and 1(d) show the two advanced methanol processes proposed in this paper: AS2M and DC2M.The AS2M process is a modified CAMERE process, which includes CO pressure swing adsorption (CO PSA), i.e., selectively captures CO from the RWGS outlet gas (mixture CO 2 /CO/H 2 ).The captured CO and outsourced H 2 are fed into the MS process, while CO 2 and H 2 are recycled into the RWGS process to increase CO production.The H 2 flow was manipulated to achieve the optimal H 2 /CO ratio for methanol production.Thereby, high-purity methanol is fully obtained in the MS reactor with CO/H 2 feed, stating that additional MP process is not required.Meanwhile, the DC2M process directly converts CO 2 to methanol under a Cu-based catalyst in one-spot conversion [37,39].Figure 1(d) shows that CO 2 and H 2 are fed to a CO 2 direct MS (DMS) reactor.Compared to the MS reactor, the DMS performs a lower one-pass conversion to methanol due to the high stability and low reactivity of CO 2 .The unreacted CO 2 and H 2 are recycled to the DMS reactor to obtain a higher methanol yield, resulting in more extensive recycle streams and larger equipment sizes.The raw methanol is then purified in the MP process to achieve over 99% purity of methanol.
2.2.Evaluation Criteria.Four metrics such as carbon efficiency, energy efficiency, CO 2 reduction, and unit production cost were used to evaluate the technical, environmental, and economic feasibilities of the processes.These four metrics were also used to compare the advantages of the AS2M and DC2M processes against the o-CAMERE and m-CAMERE processes.
Carbon efficiency (CE) represents the ratio of carbon in the product to carbon in feeds.It evaluates the number of carbons converted to the final product, as expressed in the following: where C MeOH and C CO 2 denote the total number of carbons in methanol and CO 2 , respectively, calculated as shown in the following: where F MeOH and F CO 2 denote the flow rates of produced methanol and reacted CO 2 (kmol/h), and τ C MeOH and τ C CO 2 denote the amount of carbon element in methanol and CO 2 (kmol of carbon/kmol of compounds).Energy efficiency (EE) is calculated using the chemical energy of methanol and the total utility consumed in the process as forms of electricity or heat.As shown in Equation ( 4), the EE represents how efficiently the energy is used to convert feed (i.e., CO 2 ) to product (i.e., methanol).
where E MeOH denotes the chemical energy of methanol (MJ of methanol/yr), which is calculated as shown in Equation ( 5).E elect and E heat denote consumed utilities in the process as forms of electricity and heat (MJ of electricity or heat/yr), respectively.
where δ denotes the operability of the process, which is assumed to be 8,000 hours/year, π MeOH denotes the low heating value of methanol (i.e., 22.7 MJ/kg MeOH), and M MeOH denotes the molar weight of methanol (i.e., 32.04 kg/kmol).The CO 2 reduction (CR) effect may occur in the CO 2 utilization processes as CO 2 is used as a feedstock.Nevertheless, CO 2 can also be emitted during the process via direct emission and indirect emission.Direct emission refers to CO 2 emissions as a form of vent-out gas during the operation, while indirect emission indicates CO 2 emissions that are caused by utility production from conventional sources.Accordingly, the net CR of the methanol production process is defined in this paper to evaluate the cradleto-gate effect of CO 2 in methanol production, as shown in Equation ( 6).In detail, a positive CR states that the amount of CO 2 used as a feed is greater than the amount of CO 2 released during the process.In other words, CO 2 concentration is reduced by utilizing the given process.
where M CO 2 denotes the molar weight of CO 2 (i.e., 44.01 kg/ kmol).DE denotes the direct emission of CO 2 during the process (kg CO 2 /h), and IE represents the indirect emission of CO 2 , and it is calculated as follows: where e heat and e elect denote the CO 2 emission factors of heat and electricity (kg of CO 2 /MJ of heat or electricity), respectively, which are calculated based on energy mix.In this study, the unit production cost (UPC) of methanol is a key performance indicator to evaluate the economic capability of the methanol production processes.It is calculated using the total production cost (TPC) and the annual production rate of methanol, as expressed in the following equation: Therein, TPC ($/yr) is composed of capital cost (CC), operating cost (OC), and byproduct credit (BC), in consideration of capital charge factor (a) (Equation ( 9)).Here, CC denotes the equipment purchase costs obtained from the Aspen Process Economic Analyzer.OC denotes the expenses incurred during the process operation and is composed of fixed-operating cost (FC), which comprises labor cost, plant overhead, maintenance cost, and variable operating cost (VC), which comprises raw material cost and utility cost, as shown in Equation (10).The FC is usually equivalent to a fixed percentage of the CC, while the VC is calculated, as shown in Equation (11).
where θ k denotes a unit price of utility k, E k denotes the amount of utilities k ∈ K u (i.e., electricity and heat) consumed (MJ/yr), and A k denotes the amount of other utilities, except for electricity and heat k ∈ K \ K u , such as cooling water, adsorbent, absolvent, and ionized water.BC is a credit from selling O 2 (byproduct) produced through water electrolysis.As stated in Equation ( 12), it is calculated by multiplying the unit price of O 2 (θ O 2 ) and the amount of O 2 produced along with H 2 (F O 2 ).The capital charge factor (a) is used to amortize the capital cost into the annual cost, which is calculated based on economic plant lifetime (n) and interest rate (s), as shown in Equation (13).

Modeling and Simulation
The rigorous process models of the four processes were developed using Aspen Plus V11 [42].The processes were simulated using stoichiometry and kinetics obtained from the literature [21,40].A Soave-Redlich-Kwong was chosen as a property method.To maximize methanol production, the operating conditions (temperature and pressure) for each unit process and the optimal H 2 flows were optimized by using the optimization tool built into Aspen Plus.By default, Aspen Plus assigns successive quadratic programming (SQP) algorithm to solve small or large-scale optimization problems [43].The mass and energy balance and sizing and costing data were obtained for further evaluation.Economic performance was evaluated by using the simulation results obtained by the Aspen Process Economic Analyzer.It is assumed that heat exchange networks are not considered in process simulation, which will be further improved in future work.[23] 5 International Journal of Energy Research (iii) Catalyst: Cu/Zn/Al/Zr [23,40] 1 Simplified PFDs, except for the PFD of H 2 electrolyzer and MP, are adapted from Ref [23]. 2 Information on the rate constant for each unit process can be found in relevant references.6 International Journal of Energy Research 3.2.Simulation Results.For all examined methanol production processes, CO 2 feedstock is assumed to be captured from the flue gas of the 30 MW power plant (flue gas flow rate: 2,959 kmol/h).The amine-based absorption using monoethanolamine (MEA) captures 386 kmol/h of CO 2 mixed with 16 kmol/h of H 2 O. Pure CO 2 (96% purity) was supplied to the methanol production processes.Meanwhile, H 2 is produced via water electrolysis, with the amount required depending on the optimal operating condition of the unit processes.Figure 2 presents the process flow diagram and major stream information (e.g., temperature, pressure, and mass flow) of the AS2M process.All captured CO 2 (386 kmol/h, stream #4) from the CO2R, H 2 produced by the ELZ (561 kmol/h, stream #7), and recycle stream from the COPSA (3,666 kmol/h, stream #12) are fed to the RWGS reactor.The RWGS reactor converts 369 kmol/h of CO 2 to CO, resulting in 53.6% CO 2 conversion (e.g., 369 kmol/h out of 687 kmol/h of CO 2 ).The outlet stream from the RWGS is fed to the COPSA, which captures 90% of CO from the mixture of CO 2 , CO, H 2 , and H 2 O (e.g., 367 kmol/h of CO out of 407 kmol/h).The filtered CO was supplied to the MS for methanol production (stream #13), while other gases were recycled back to the RWGS reactor (stream #11).In the MS reactor filled with Cu/ZnO/Al 2 O 3 catalysts, the captured CO, additional H 2 , and the inner recycle stream (mainly H 2 , stream #16) were converted into methanol under optimal operating conditions and H 2 /CO ratio (optimal ratio of H 2 /CO in methanol synthesis ≈2.0).The MS reactor produced 361 kmol/h of methanol from 479 kmol/h of CO, resulting in a 75.4% CO conversion.H 2 O and unreacted gas (a mixture of H 2 /CO) were removed from the flash tank to improve the purification of methanol; 360 kmol/h of methanol with over 99% purity was obtained (stream #17).
Figure 3 presents the flow diagram of the DC2M process with key stream information.Similar to the AS2M, all captured CO 2 (386 kmol/h, stream #4), produced H 2 (1,147 kmol/h, stream #6), and an inner recycle stream (total 8,316 kmol/h, stream #10) are fed to the DMS reactor.The DMS reactor with Cu/Zn/Al/Zr catalysts directly converts CO 2 and CO to yield 360 kmol/h of net methanol.As mentioned before, the DMS process has the advantages of simple configuration and easy operation, but its drawback includes low methanol yield.It only has 17% CO 2 one-pass conversion, which requires a large amount of recycling to obtain a higher overall process conversion.As a result, the DC2M process has five times larger recycle flows than the feed flows      Owing to the relatively better performances of RWGS, CO PSA, and MS, the AS2M process has a higher carbon efficiency than the DC2M process.As discussed in Section 3, the DC2M process has a large volume of recycle stream due to the low one-pass conversion of the CO 2 direct hydrogenation process, thus improving the yields of methanol.Figure 5 presents the energy consumption and energy efficiency of both processes.The AS2M process consumes a total of 4,973 TJ/yr of utility, while the DC2M process requires 4,602 TJ/yr of utility.Compared with the AS2M process, the DC2M process involves a simple process scheme and easy operation, owing to low utility consumption.Electricity is the main utility consumed in both processes (i.e., 80% in the AS2M process and 78% in the DC2M process).Both processes consume approximately 95% of total electricity to decompose water into H 2 and O 2 in an ELZ.In the AS2M process, less than 5% of total electricity (i.e., 200 TJ/yr) is used to synthesize methanol in RWGS and MS.Likewise, in the DC2M process, 5.4% of electricity (i.e., 195 TJ/yr) is used for methanol production.CO2R is the main contributor to heat consumption in both processes as the separation process of CO 2 and MEA solvent in a stripper requires a large amount of heat (see the simplified PFD of CO2R in Table 1).RWGS also uses 352 TJ/yr of heat in the AS2M process because its reactor operates at a high temperature (~540 °C).DMS reactor requires less heat (167 TJ/yr) than RWGS in the AS2M process due to its rela-tively mild operating temperature (c.f., RWGS: ~540 °C; DMS: ~225 °C).Such low heat consumption of DMS in the DC2M process is offset by the large heat consumption of MP (i.e., 247 TJ/yr), which is required to improve the purity of methanol.Consequently, the DC2M process uses more heat utility than the AS2M process.A total of 360 kmol/h and 356 kmol/h of methanol produced in the AS2M and DC2M processes correspond to 2,094 TJ/yr and 2,072 TJ/yr, respectively; this results in energy efficiencies of 42.1% and 45.0% in the AS2M and DC2M processes, respectively.Notably, the AS2M process produces more methanol than the DC2M process (i.e., AS2M: 360 kmol/h, DC2M: 356 kmol/hr, see Figure 4), although it has lower energy efficiency due to its high energy consumption.1, all unit processes, except for ELZ, use electricity and heat utilities obtained from conventional sources.Indirect CO 2 emissions from utility production and direct CO 2 emissions by ventout or purge streams should be included in the evaluation of the environmental impacts of both AS2M and DC2M processes.Indirect CO 2 emissions are calculated based on Equation (7), and the CO 2 emission factors of the utilities are estimated to be 0.115 kg of CO 2 /MJ of electricity and 0.062 kg of CO 2 /MJ of heat, respectively.Here, it is assumed that heat is generated only from natural gas, while electricity is followed by the energy mix (e.g., coal (30.4%), natural gas (33.8%), nuclear (19.7%), oil (0.6%), renewable (14.9%), and others (0.6%)) [3,46].International Journal of Energy Research Figure 6 shows the CO 2 emissions and CO 2 reductions of the AS2M and DC2M processes.The AS2M process emits a total of 90.0 kton of CO 2 /yr, while the DC2M process emits a total of 92.7 kton of CO 2 /yr.Heat consumption (AS2M: 68.1%; DC2M: 67.9% of the total emission), followed by electricity consumption (AS2M: 25.6%; DC2M: 24.3% of the total emission), and direct emissions (AS2M: 6.3%; DC2M: 7.8% of the total emission) are the main contributors to CO 2 emissions in both processes.Heat utility has a more negative impact on CO 2 emissions than electricity, even though both processes consume more electricity than heat (see Figure 5).This is because the electricity consumed in ELZ to decompose water is produced from renewable sources, which do not contribute to CO 2 emissions.Thus, the unit process that consumed more heat utilities emits more indirect CO 2 (such as CO2R, RWGS, and DMS).In addition, the DC2M process has larger direct CO 2 emissions than the AS2M process due to its large recycle flows (i.e., direction emission of AS2M: 5.7 kton/yr; DC2M: 7.3 kton/ yr).As a result, both AS2M and DC2M processes are estimated to have CO 2 reductions of 0.50 and 0.47 kg CO 2 /kg MeOH, respectively.These values indicate that, for instance, the AS2M process can reduce 0.50 kg of CO 2 by producing 1 kg of MeOH, mitigating the negative impact of CO 2 .

Economic Analysis.
The economic capability of the proposed two processes was evaluated using UPC, allowing a direct comparison of the economic viability of the different methanol production methods.In detail, the equipment cost of each unit such as the pump, heater, and distillation column is adopted from the Aspen Process Economic Analyzer  0.06 $/kg [49] International Journal of Energy Research to estimate investment cost, and operating cost is calculated by using the amount of consumed utility and utility prices, as shown in Table 2. Figure 7 presents the total production cost, which includes annualized capital cost (aCC), operating cost (OC), and O 2 credits, as well as the cost breakdown of the AS2M and DC2M processes.The TPC is $136.8M/yr for the AS2M process and $93.1 M/yr for the DC2M process.The AS2M process has a more complex configuration than the DC2M process, which results in higher aCC and OC.In both processes, OC is approximately five times larger than aCC.In both processes, ELZ is the main contributor to aCC and OC.This is because the ELZ has a relatively higher CC than the other unit processes and requires a significant amount of electricity to decompose water.This implies that the economic feasibility of both processes can be greatly improved by lowering the CC and improving the conversion efficiency of the ELZ.COPSA has a significantly higher OC than the aCC (almost 35 times) because it uses an expensive adsorbent (see Table 2).As a result, the UPCs of the AS2M and DC2M processes are predicted to be $1.48/kg MeOH and $1.02/kg MeOH, respectively.
Figure 8 shows a standardization of five indicators such as carbon efficiency, energy efficiency, CO 2 reduction, profit, and water savings; their mean is zero and their standard deviation is one.Water savings are calculated based on the amount of cooling water consumed in the process (excluding the water consumed by the ELZ to produce H 2 ).The water savings are the reciprocals of the amount of cooling water.Meanwhile, the methanol profit is estimated by calcu-lating the revenue from the methanol produced.The market price of methanol is assumed to be $0.47/kgMeOH [45].The detailed stream information of the o-CAMERE and m-CAMERE processes can be found in Figures S1 and S2 of the Supplementary Materials.In addition, the numerical results of the technical, economic, and environmental results of the two reference processes are also summarized in Table S1 in the Supplementary Materials.Compared to the conventional methanol processes, the AS2M and DC2M processes have a relatively higher carbon efficiency, energy efficiency, and methanol yield.The AS2M and DC2M processes reduce the net CO 2 level by effectively managing the involved processes.This result verified that the two proposed processes can provide environmental benefits.The economic profit of the AS2M process is less than that of m-CAMERE but more than that of o-CAMERE.As discussed in previous sections, the lower profit of the AS2M process is due to its complicated process scheme and tough operating conditions.On the other hand, the DC2M process is expected to have the largest profit among all four processes.Interestingly, the AS2M process does not have a MP unit, which requires a huge amount of water to purify methanol, and thus, it is expected to largely reduce water consumption compared to other processes.

Sensitivity Analysis and Comparison
In this section, scenario-based sensitivity analysis was performed to analyze the economic and environmental 5.1.Utility Production Alternatives.As the baseline, the ELZ for H 2 production is assumed to be powered by electricity generated from renewable sources.The utilities consumed in the other subprocesses are assumed to be supplied from conventional sources.We analyzed the impact of various utility sources on the economic and environmental performances of both processes because the utility supply strategy is a critical factor in determining economic and environmental capabilities.
Figures 9(a) and 9(b) show the economic (orange spiral line) and environmental performances (green spiral line) of both AS2M and DC2M processes with a share of renewable utility, respectively.When conventional utilities (electricity and heat from fossil fuels) are fully used, even for water electrolysis, both processes can produce methanol at the lowest price (e.g., AS2M: $1.28/kg MeOH; DC2M: $0.81/kg MeOH).However, both processes indirectly emit a large amount of CO 2 during operation, resulting in a negative reduction effect (e.g., AS2M: −4.37 kg CO 2 /kg MeOH; DC2M: −3.80 kg CO 2 /kg MeOH).In contrast, the environmental capability of both processes can be improved by  12 International Journal of Energy Research replacing all utilities with renewable utilities (CO 2 reduction; AS2M: 1.39 kg CO 2 /kg MeOH; DC2M: 1.41 kg CO 2 /kg MeOH).However, it cannot be an economically favorable alternative compared to using conventional utilities (UPC; AS2M: $1.64/kg MeOH; DC2M: $1.14/kg MeOH).Given the trade-off between the economic and environmental performances of the processes depending on the types of utilities, it is desirable to identify the ratio of renewable utilities used for net-zero emission processes.In this context, it is identified that both processes can produce methanol without emitting CO 2 (from a cradle-to-gate perspective) by partially utilizing renewable utilities.In detail, if at least 69% and 66% of the total utilities consumed in the AS2M and DC2M processes, respectively, can be replaced by renewable electricity and heat, both processes will produce carbon-neutral methanol, which makes the processes to be economically and environmentally favorable.

Hydrogen Production
Alternatives.H 2 obtained from ELZ is the most critical factor in determining the economic feasibility of the proposed processes.H 2 can be produced from natural gas, coal, and biomass via different technologies (e.g., reforming or gasification) at a low price, but these processes have negative CO 2 emission.In this section, the economics and environmental impacts of the AS2M and DC2M processes, which are coupled with different H 2 production technologies (e.g., coal gasification, biomass gasification, and steam methane reforming), were evaluated.Figure 10 represents the UPC and CO 2 reduction of the AS2M and DC2M processes integrated with different H 2 production technologies, such as steam methane reforming with/without carbon capture (SMR, SMR+CCS), coal gasification with/without carbon capture (CG, CG+CCS), and biomass gasification with/without carbon capture (BG, BG +CCS).The parameters for conventional technologies are adopted from previous studies [50,51].
Compared to the renewable ELZs, the AS2M and DC2M processes using conventional H 2 production technologies produce cheaper methanol (A7: $1.48/kg, D7: $1.02/kg).Among alternatives, SMR is the cheapest among all conventional H 2 technologies, while BG+CCS is the most expensive in both the AS2M and DC2M processes.However, SMR reduces the CO 2 reduction impact, whereas BG+CCS has the largest CO 2 reduction effect than other H 2 production technologies.CG gives the worst CO 2 reduction effect compared to the other conventional technologies.Interestingly, the additions of CCS to the SMR, CG, and BG increase the total costs, although they largely improve the CO 2 reduction effect of conventional technologies.For example, the CO 2 reduction effect of the BG was improved by around five times by adding CCS.The production cost of methanol from the AS2M process with conventional H 2 technologies is still higher than the market price (c.f., MeOH market price: $0.23-$0.83/kgMeOH [52]), except for the cases using SMR, due to the complex process scheme and tight operation conditions of the AS2M process.In contrast, using conventional H 2 technologies improves the economic performance of the DC2M process.In particular, the 13 International Journal of Energy Research DC2M process, coupled with SMR, produces the cheapest methanol (i.e., $0.41/kg), which is competitive with the market price of methanol.

Conclusions
In this work, we proposed new methanol synthesis processes that use waste CO 2 and renewable H 2 as feedstock.We analyzed the technoeconomic and environmental capabilities of the proposed processes by developing a process model using a commercial simulator.Specifically, two distinct processes, i.e., AS2M and DC2M, were proposed and compared to conventional CO 2 -to-MeOH processes.The AS2M process produces methanol from syngas (a mixture of CO and H 2 ) after converting CO 2 to CO via reverse water-gas shift and separating CO from the mixture through pressure swing adsorption, while the DC2M process directly synthesizes methanol from CO 2 using the direct hydrogenation process.The optimal process configuration and operating conditions of the two processes were optimized for maximizing methanol production.The technical and economic capabilities of the two novel processes were examined using four evaluation criteria: carbon efficiency, energy efficiency, CO 2 reduction, and unit production cost.Our findings showed that both the AS2M and DC2M processes could achieve better technical and environmental performances compared to the existing methanol synthesis processes using CO 2 .The H 2 production cost was found to be the most important factor in determining the economic feasibility of the processes.The AS2M process had the highest methanol yields for the given amount of CO 2 feed, while the DC2M process had the lowest unit production cost at $1.02/kg because of its simple process design and easy operating conditions.In this study, we also identified that the proposed two processes can have economic competitiveness depending on the H 2 production technologies.Capital cost ($/yr) OC:

Figure 2 :
Figure 2: Process flow diagram and major stream information of the AS2M process.
Figure 4  presents the carbon flows and carbon efficiency of both processes.The carbon flows can be broken down into the major components: CO 2 , CO, methanol (MeOH), and direct emissions.As shown in Figure4(a), 360 kmol/h of methanol is produced in the AS2M process using 386 kmol/h of CO 2 captured from the flue gas, indicating a carbon efficiency of 93.2%.On the other

Figure 3 :
Figure 3: Process flow diagram and major stream information of the DC2M process.

Figure 5 :
Figure 5: Energy consumption and energy efficiency of AS2M and DC2M processes.

CO 2 Figure 6 :
Figure 6: CO 2 emissions and reduction in the AS2M and DC2M processes.

Figure 7 :
Figure 7: Cost contributions and unit production cost of AS2M and DC2M processes.

Figure 8 :Figure 9 :
Figure 8: Holistic representation of sustainability of the four processes (larger values indicate better solutions in each dimension).1Watersavings = 1/the amount of cooling water.2Methanol profit = revenuecost /yield of methanol = market price of methanol * yield of methanolcost /yield of methanol .

Figure 10 :
Figure 10: UPC and CO 2 reduction change with different hydrogen production technologies.

Table 1 :
Unit process specification and operating conditions.
3.1.UnitProcessModeling.The four main methanol production processes are composed of multiple unit processes, including conversion and separation processes.There are a total of seven unit processes such as CO 2 recovery (CO2R), CO pressure swing adsorption (COPSA), MP, Electrolyzer (ELZ), RWGS, MS, and DMS.Specifications, including simplified PFD, chemical equations, kinetics, and operating conditions, for each unit process are summarized in Table1.

Table 2 :
General and economic parameters for economic evaluation.
Indices/Sets/Subsets i ∈ I: Compounds {CO, CO 2 , H 2 , H 2 O, MeOH} k ∈ K: Utilities {Ec, Ht, Cw, Iw, Ad, Ab} Operability of the process (h/yr) τ C MeOH : Amount of carbon element in methanol (kmol of carbon/kmol of methanol) τ C CO 2 : Amount of carbon element in CO 2 (kmol of carbon/kmol of CO 2 ) π MeOH : Low heating value of methanol (MJ/kg MeOH) M MeOH : Molar weight of methanol (kg/kmol) M CO 2 : Molar weight of CO 2 (kg/kmol) e heat : CO 2 emission factor of heat utility (kg of CO 2 /MJ of heat) e elect : CO 2 emission factor of electricity utility (kg of CO 2 /MJ of electricity) θ k : Unit price of utility k ($/MJ or $/kg) θ O 2 : Selling price of oxygen ($/kg of O 2 ).MeOH : Total number of carbons in methanol (kmol of carbon/h) C CO 2 : Total number of carbons in CO 2 (kmol of carbon/ h) F MeOH : Mole flow of methanol (kmol of methanol/h) F CO 2 : Mole flow of CO 2 (kmol of CO 2 /h) F O 2 : Mole flow of oxygen (kmol of CO 2 /h) E MeOH : Chemical energy of methanol produced in the process (MJ/yr) E k : Amount of utilities k ∈ K u consumed in the process (MJ/yr) A k : Amount of utilities k ∈ K \ K u consumed in the