Combination of CO 2 electrochemical reduction and biomass gasification for producing methanol: A techno-economic assessment

Combining CO 2 electrochemical reduction (CO 2 R) and biomass gasification for producing methanol (CH 3 OH) is a promising option to increase the carbon efficiency, reduce total production cost ( TPC ), and realize the utilization of byproducts of CO 2 R system, but its viability has not been studied. In this work, systematic techno-economic assessments for the processes that combined CO 2 R to produce CO/syngas/CH 3 OH with biomass gasification were conducted and compared to stand-alone biomass gasification and CO 2 R processes, to identify the benefits and analyze the commercialization potential of different pathways under current and future conditions. The results demonstrated that the process that combined biomass gasification with CO 2 R to CO represents a viable pathway with a competitive TPC of 0.39 € /kg-CH 3 OH under the current condition. For all the combined cases, electricity usage for CO 2 R accounts for 36 – 76% of total operating cost, which plays a key role for TPC . Sensitivity analysis confirmed that the process that combined biomass gasification with CO 2 R to CO is sensitive to the price of electricity, while both CO 2 R performance and prices of stack and electricity are important for the processes that combined with CO 2 R to syngas/CH 3 OH.


Introduction
The continuous increase of fossil CO 2 emissions is the main cause of climate change.The challenge to reduce fossil CO 2 emissions demands the development of novel technologies, e.g., using renewable sources, such as biomass, to produce heat, power, fuels, and chemicals.Biomass gasification is an effective pathway to convert biomass into advanced products.Currently, biomass gasification for producing methanol (CH 3 OH) is widely studied [1,2], due to its potential as a transportation fuel and H 2 carrier, and as a solvent and feedstock for producing bulk chemicals.Using black liquor (BL) as the feedstock for biomass gasification is an interesting route that could reuse the waste BL from the pulp mill (PM), which has been demonstrated on a pilot scale with an accumulated operating time of 28,000 h at the 3 MW th pilot plant in Piteå, Sweden [3].For BL gasification to CH 3 OH production, the overall chain consists of a gasifier to produce raw syngas gas, and a gas cleaning part to remove acid gases (H 2 S/CO 2 ) before CH 3 OH synthesis.The removed CO 2 is directly emitted into the atmosphere, resulting in low carbon efficiency (~33 %) [4].
CO 2 conversion is a promising way to reduce CO 2 emissions and realize CO 2 utilization [5].Among different CO 2 conversion technologies, CO 2 electrochemical reduction (CO 2 R) is the most potential in development, as it can be operated under mild reaction conditions and integrated with renewable electricity generation [6].In addition, CO 2 R, which can be called "Power-to-X" technologies, plays a vital role in storing renewable energy into energy-dense carries for long-term application.The target products from CO 2 R can be CO, syngas (CO + H 2 ), and CH 3 OH, which are directly linked to CH 3 OH production from syngas.The performance of CO 2 R can be measured through three key parameters: Faradaic efficiency (FE), current density, and cell voltage (V cell ), which indicate selectivity, reaction rate, and energy usage, respectively.
Using ionic liquids (ILs) as electrolytes and promoters in CO 2 R has been intensively considered, and promising results have been obtained [7].For example, CO 2 R to CO showed the optimal performance with the highest FE (99.7 %) and current density (>180 mA/cm 2 ) in H-cell at the V cell of 3.29 V, where syngas with the targeted H 2 /CO ratio can also be obtained by adjusting the potential [8].Meanwhile, CO 2 R to CH 3 OH has been attracting great attention recently, and the work by Guo et al. [9] provided the best results so far within those using ILs in CO 2 R, where the FE and current density could reach 88.6 % and 67 mA/cm 2 , respectively, with the V cell of 3.28 V.All these research achievements open the door of Power-to-CH 3 OH pathways by using CO 2 R and providing the opportunity to combine with biomass gasification.When biomass gasification, e.g., BL gasification, is combined with CO 2 R, the separated CO 2 from bio-syngas can be used as the feedstock for CO 2 R, increasing the carbon efficiency of the BL gasification process and decreasing the operation cost of CO 2 R.Meanwhile, the byproduct of O 2 in CO 2 R could be served as the O 2 source for BL gasification [10].Therefore, combining CO 2 R to CO, syngas, or CH 3 OH with BL gasification will be an attractive way to reduce CO 2 emissions, produce valuable carbon-based products from CO 2 , enhance carbon efficiency, and may improve the profit of the overall process.
Techno-economic assessment of the technologies is usually needed for industrial-scale adaptation of any process, which can be divided into two options briefly depending on the development status.In the first option, the technologies have been conducted at a large-scale, and a combination of experimental results, simulation, and process evaluation is the common way to conduct process analysis.For example, technoeconomic assessments have been carried out on the feasibility of the stand-alone biomass gasification based on our previous work [11,12].For the second option, i.e., the new technologies, the data from the labscale is often used to preliminarily evaluate the profitability of the technology in the current state, to understand the relationships between process parameters and viability, and offer direction for R&D, where sensitivity analysis on the performance parameters is usually combined to consider the uncertainty and future trend.For example, to assess the stand-alone CO 2 R, techno-economic analysis has been carried out based on the lab-scale research, and sensitivity analysis on the key performance parameters (current density, FE, V cell , etc.) was conducted [13][14][15][16].Currently, even though the assessments have been carried out on the feasibility of stand-alone biomass gasification and CO 2 R, there is a lack of comparative analysis at the industrial level for these combined processes from the economic aspect, and the techno-economic feasibility, the key economic drivers, and the challenges along these combined processes have not been studied.
In addition, the comparison with the commercial CH 3 OH production is also important to evaluate the significance and the reference value of the studied work.Currently, the commercial CH 3 OH production is mainly based on fossil feedstocks, such as coal and natural gas.Detailed information relating to the state-of-the-art catalyst development and reactor design is reviewed by Sarvestani et al. [17] and Leonzio et al. [18].Briefly, syngas and CO 2 hydrogenation are the two primary approaches for CH 3 OH synthesis.The copper-based catalysts showed excellent performance for CH 3 OH synthesis.Most CH 3 OH facilities use a two-stage distillation process to achieve the required CH 3 OH purity (>99.85 wt% based on the standards given by the International CH 3 OH Producers and Consumers Association) [19].However, since the main focus of this work was to study how the combination of CO 2 R and biomass gasification would impact the process, the CH 3 OH price could be directly used as the reference for comparing with the fossil-based CH 3 OH production.
In this work, comprehensive and quantitative techno-economic assessments were performed to fill in the research gap and analyze the potential for scaling-up the combined processes, as well as provide guide directions for cutting the production cost of CH 3 OH and making it competitive with the fossil CH 3 OH.In addition, stand-alone BL gasification and CO 2 R processes were evaluated for comparison.Depending on the target products (CO, syngas, or CH 3 OH) and combining the original BL gasification process to produce CH 3 OH, in total, four cases were investigated, i.e., (i) combining BL gasification with CO 2 R to CO to produce CH 3 OH and the coproduct CO (Case 1); (ii) combining BL gasification with CO 2 R to CO followed by the hydrothermal CH 3 OH production to produce CH 3 OH (Case 2); (iii) combining BL gasification with CO 2 R to syngas followed by the hydrothermal CH 3 OH production to produce CH 3 OH (Case 3); (iv) combining BL gasification with CO 2 R to CH 3 OH to produce CH 3 OH (Case 4).Total production costs (TPCs) and carbon efficiency were calculated to identify the feasibility for each case.Sensitivity analyses were conducted to identify key performance parameters and the commercialization achievability of the combined processes in the near future, considering the rapid improvement of CO 2 R performance and technology development.

Process description
All the combined pathways were based on BL gasification to CH 3 OH, which has been developed sufficiently, and some plants have been operated at pilot and commercial levels [1,3,20].In this work, the process studied by Carvalho et al. [12] was selected because the detailed capital and operating costs have been provided.More specifically, as shown in Fig. 1, i.e., the parts with grey color, BL was provided from a PM plant with a production capacity of 3334 ton-BL/day (dry mass basis).A pressurized, oxygen-blown, entrained-flow gasifier was used for the BL gasification at 1050 ℃.Raw syngas left from the bottom of the gasifier was cooled to 40 ℃, and then half of the raw syngas was sent to the water gas shift (WGS) unit after reheating to adjust H 2 /CO ratio to 1.95.The shifted syngas and the other half of the raw syngas were sent to the acid gas removal (AGR) unit before entering the methanol synthesis (MSY) unit.Crude CH 3 OH was generated at 30 ℃ and transferred into the methanol distillation (MDL) unit to upgrade the crude CH 3 OH to the grade AA CH 3 OH (>99.85 wt%).The removed acid gases flowed into a CO 2 /H 2 S separation (CHS) unit to separate CO 2 from the acid gases.
The separated CO 2 from BL gasification flowed into a CO 2 electrolysis system for conversion (utilization).The overall process for the CO 2 electrolysis system can be divided into two subprocesses, i.e., CO 2 R (green part in Fig. 1) and product separation (blue part in Fig. 1).The detailed introduction of the CO 2 R process is shown in S1.As CO 2 R exists only at the bench scale, the published results were ranked and selected according to their current density and FE as a measure of process productivity.For product separation, relevant technologies have been verified at a pilot scale or commercialized, and thus, the data for economic analysis was directly extracted from the previous publications [21,22] and presented in S2.
Based on BL gasification to CH 3 OH, CO 2 R was combined with different scenarios (Cases 1-4).The specific process description for each case was described in S3.
For comparison, the stand-alone processes of both BL gasification (Cases B-1 and B-2) and CO 2 R (Cases C-1, C-2, and C-3) for CH 3 OH synthesis were also created, as shown in S3.For the stand-alone BL gasification processes, we assumed CO 2 was directly released into the air (Case B-1) or transported and stored (Case B-2).It should be mentioned that, for the stand-alone CO 2 R processes, CO 2 was assumed to be from industrial gas streams, and the average cost of 50 €/ton extracted from the literature [23,24] was taken in this work.

Economic analysis
TPC is an important parameter to evaluate the economic feasibility of a process, which can be calculated based on the annual capital cost (ACC) and the total operating cost (TOC).ACC is thereafter converted from the total capital cost (TCC) according to Eq. (1) [14].
where i and n stand for the interest rate and lifetime, respectively.Capital cost.TCC includes equipment cost, balance of the plant (BoP), installation cost, and indirect cost.The calculation of TCC for BL gasification was based on the actual estimations and those from the literature, and detailed information can be found in the work reported by Carvalho et al. [12] The cost estimation to the price level of 2022 was escalated by using the Chemical Engineering Plant Cost Index (CEPCI).For the units in the CO 2 R chain other than the CO 2 R itself, for example, product separation units, the factorial method was used to calculate the capital cost.Details of how the calculation was performed are demonstrated in S5.
CO 2 R is a new process and exists only at the bench scale.Therefore, the cost estimation of the electrolyzer was based on the alkaline water electrolyzer, since the CO 2 R electrolyzer is analogous to water electrolysis in many ways, which has been widely used by others for the cost calculation of a CO 2 R unit [13,15].The required area of electrolyzer (AE) was calculated based on Eq. ( 2) [14].
where I and j represent the total current and current density, respectively.m, M, z, and FE are the production rate, the molecule weight, the number of electrons needed, and the Faradaic efficiency for producing the target product, respectively.F is the Faradic's constant.
Operating cost.Variable and fixed costs are two parts of TOC.Variable cost includes energy usage and materials consumption.The operating costs for BL gasification [12] and product separation units in the CO 2 R chain [21,22] were derived from the literature.For CO 2 R, electricity is the only energy driver, which can be calculated based on the power needed (P) and electricity price, according to Eq. (3) [10].Fixed costs include expenses for operation and maintenance, assumed to be 3.2 % of the direct cost.The key components for calculating TCC and TOC of the CO 2 R process are shown in Table S11.

Carbon efficiency
Carbon efficiency reflects the carbon emissions performance of each process, and it relates to the carbon loss caused by the feedstock selected, reaction selectivity, and the maximum theoretical production yields.The carbon efficiency was calculated according to [4]: Carbon in product Carbon in feed × 100 (4)

Parameters
The detailed information about the parameters used for the calculation is shown in Table 1 and S6 of Supporting Information.

TPCs and carbon efficiencies of combined processes
The TPCs and carbon efficiencies of combined processes were calculated and compared with stand-alone BL gasification and CO 2 R processes under base conditions.For all the combined processes, carbon efficiencies are higher than 81.7 %.In terms of TPCs, BL gasification combined with CO 2 R to CO (Case 1) exhibited the lowest TPC of 0.39 €/kg-CH 3 OH owing to the extra income for selling CO with a market price of 0.5 €/kg-CO [14], as reflected by the great contribution of CO profit.In addition, Case 2 showed a lower TPC of 0.61 €/kg-CH 3 OH compared to Cases 3 and 4 due to its much better CO 2 R performance.In addition, it was found that the operating cost is the primary driver of the high TPCs of all combined processes.This is because the CO 2 R part of the combined processes could cause high electricity usage as two and six electrons are required to convert CO 2 to CO and CH 3 OH, respectively [10].By comparing Cases 2-4 and stand-alone CO 2 R processes (Cases C-1, C-2, and C-3), the TPCs of combined processes are much lower than those of their corresponding stand-alone CO 2 R processes, and especially, the stand-alone CO 2 R process with higher TPC displayed a much starker drop after combination.For example, the TPC of Case C-1 dropped by 0.11 €/kg-CH 3 OH, while Case C-2 dropped by 0.45 €/kg-CH 3 OH after the combination.By comparing Cases 2-4 and Cases B-1 and B-2, much higher carbon efficiencies were obtained after the combination.The TPC of Case 2 is equal to the stand-alone BL gasification process combined with CCS (Case B-2); however, Cases 3 and 4 exhibit higher TPCs than the stand-alone BL gasification.From the above comparison, it can be suggested that the cost of CO 2 R chain provides a significant impact on the high TPCs of Cases 2-4.Therefore, it is crucial to lower the CO 2 R chain costs in order to make Cases 2-4 commercially viable in the future.
Even though Cases 3-4 showed higher TPCs than the stand-alone BL gasification processes under current conditions, combining BL gasification and CO 2 R increased the carbon efficiency.Also, the rapid improvement of the CO 2 R performance in recent years offers the combined processes a high potential for a TPC reduction.Therefore, in the following part, the capital and operating costs of four combined cases were investigated to verify the key economic drivers and provide research directions to reduce TPCs.
As shown in Fig. 3A, the capital costs of the CO 2 R chain in Cases 1 and 2 are relatively low, less than 25 % of TCCs.This is attributed to the high current density and FE for producing CO via CO 2 R, resulting in a small electrolyzer area and capital cost.In Cases 3 and 4, the cost of CO 2 R, including stack cost, BoP, and indirect cost, is the main part of the capital cost due to their low current density, which takes the proportions of 74 % and 65 % of TCCs for Cases 3 and 4, respectively.
In terms of operating cost exhibited in Fig. 3B, the cost of electricity accounts for 36-76 % of TOCs for CO 2 R, which is the main part for each combined process.Case 1 has the lowest operating cost due to the lowest electricity usage, which is the main reason for the lowest TPC.In Case 2, the cost of H 2 also plays an important role in the operating cost, making TPC in Case 2 much higher compared to that in Case 1.The high electricity usage in Case 3 is due to the large electricity consumption by HER for the H 2 generation.Case 4 showed higher electricity usage compared to Case 3, which is because a reduction of CO 2 to CH 3 OH needs six electrons while only two electrons are required to produce CO.From the results demonstrated in Figs. 2 and 3, it can be seen that the operating cost associated with the electricity of CO 2 R is significant for TPCs of Cases 1 and 2, while both capital and electricity costs of CO 2 R play an important role for TPCs of Cases 3 and 4, further highlighting the great importance of reducing the cost of CO 2 R chain to improve the feasibility of combined process from an economic standpoint.

Sensitivity analysis and viability of combined processes in the future
As shown in Fig. 3 and according to the above discussion, the CO 2 R performance, the prices of stack and electricity, and the price of H 2 (depending on the H 2 -sources) affect capital and operating costs of CO 2 R chain and thus TPC, which were further studied in this section.The influence of the different parameters on TPC was investigated by changing one studied parameter, while the other parameters were maintained at the base scenario.

The influence of CO 2 R performance
For CO 2 R performance, the current density, V cell , and FE are three important parameters.Therefore, a sensitivity analysis was conducted based on these parameters.
Increasing current density will reduce the stack area of CO 2 R, and thus results in lower capital cost and TPC.Some work has reported that the current densities for producing target products were already higher than 300 mA/cm 2 in flow cell [26,27].Even though the current density in H-cell is lower than in flow cell, there is a huge potential for H-cell to reach a current density of 300 mA/cm 2 , considering the rapid development of CO 2 R performance.Since Case 3 shows the lowest current density of 30 mA/cm 2 , in this work, the sensitivity of current density was studied within the range of 30-300 mA/cm 2 .As shown in Fig. 4A, when the current density is higher than 180 mA/cm 2 , a further increase in the current density only leads to a slight decrease in TPC.The primary reason is that the operating cost dominates TPC at high current densities.Since the current densities in Cases 1 and 2 have already reached 182.2 mA/cm 2 , a further increase of the current density is not a proper strategy to reduce TPC.
Currently, a much lower V cell of 2.2 V for CO 2 R to HCOOH has been achieved (theoretical V cell , E 0 , towards HCOOH is 1.48 V) [28], which suggests that there is still room to reduce V cell .In this work, to study the influence of V cell on TPC, we assumed that the V cell for producing CO (E 0 = 1.34 V) and CH 3 OH (E 0 = 1.21 V) has the ability to reduce to 2.0 V, which corresponds to a V cell of 1.45 V for producing syngas.As demonstrated in Fig. 4B, a reduction of V cell decreased the total electricity usage and TPC.The case with a higher percentage of electricity for CO 2 R in TOC is more sensitive to V cell .This is because V cell is related to energy efficiency, which directly influences electricity usage and operating cost.For example, TPC was reduced by 0.053 €/kg-CH 3 OH in Fig. 2. TPCs and carbon efficiencies of different cases.FE influences TPC by influencing the stack area and total electricity usage.A quite high FE of 99.7 % was reached in Cases 1 and 2, while in Case 3, a fixed FE is needed to keep the H 2 /CO ratio constant.Therefore, we only investigated the influence of FE on TPC for Case 4 in the range of 85-100 % by referring to the quite high FE for producing CO (99.7 %) (Fig. 4C).The higher the FE, the lower the electricity-waste on the byproduct, and the lower the total current requirement, leading to a lower investment cost for CO 2 R and a lower total electricity usage.Since investment cost and electricity dominate the capital and operating costs of CO 2 R, the increase of FE finally reduces TPC.

Influence of stack and electricity prices as well as H 2 source
Stack and electricity prices influence TPC by directly affecting the total capital and operating costs.The stack price was assumed to be in the range of 0.5-2 times (587-2348 €/m 2 ) of the price under the current condition according to the reported articles [14,22].TPCs decrease linearly with reducing stack cost in different cases, and the decrease of TPCs in Cases 3 and 4 are more obvious than in Cases 1 and 2 (Fig. 5A).The reason for this is that the relatively low current densities of Cases 3 and 4 make the capital costs for CO 2 R playing a much larger role for the TOC of the combined process.Electricity prices are quite different in different European countries, and thus, the location would influence the selection of promising pathways.In addition, the energy structure and the international situation would also influence the electricity price.Taking all these things into account, a wide electricity price ranging from 0.02 to 0.20 €/kWh was covered.As displayed in Fig. 5B, electricity price has a significant influence on TPC.The change of electricity of 0.03 €/kWh leads to a change in TPC of 0.15-0.37€/kg-CH 3 OH.In addition, Case 1 showed the lowest TPC when the electricity was lower than 0.08 €/kWh, while Case 2 became more promising with the lowest TPC when the electricity price was higher than 0.08 €/kWh, confirming that the electricity price plays an important role on selecting potential process.For example, Case 2 would become more potential in Belgium, Poland, Italy, etc., since the electricity prices are higher than 0.1 €/kWh in these countries [29].
The H 2 costs and their corresponding carbon footprint vary widely, depending on the production methods.The commonly used H 2 production technologies, i.e., steam methane reforming (SMR) and coal gasification, produced a large amount of CO 2 emissions.Integrating H 2 production with CCS is thus a suitable option to promote low-carbon hydrogen production.In addition, water electrolysis for H 2 production by using electricity generated from renewable energy could realize zerocarbon emissions.In this work, the TPCs of Case 2 by using the H 2 produced from 'coal gasification + CCS', 'SMR + CCS', and water electrolysis (alkaline electrolysis (AE) and polymer electrolyte membrane (PEM) electrolysis) with current market H 2 prices and the prices in 2030, were calculated and compared as shown in Fig. 5C.The lowest TPCs of 0.61 and 0.58 €/kg-CH 3 OH were obtained by using (SRM + CCS) for H 2 production under both current and future H 2 prices, while H 2 from PEM has the highest TPC of 0.90 €/kg-CH 3 OH.Even though the AE method is expensive currently, it will be promising in the future considering the great decrease of TPCs from 0.73 to 0.60 €/kg-CH 3 OH.
Based on sensitivity analysis, electricity price is the key parameter influencing the TPC of Case 1. Case 2 is more sensitive to electricity and   H 2 price compared to the parameters linked to the CO 2 R performance, and thus research should focus on reducing the price of electricity and H 2 with increasing renewable energy usage for electricity generation and developing more economical blue and green H 2 production technologies.In terms of Cases 3 and 4, the influence of CO 2 R performance is significantly increased compared with Cases 1 and 2. Therefore, for these two cases, simultaneously improving CO 2 R performance as well as reducing stack and electricity prices are important to make these processes economically feasible in the future.

TPC in future scenarios
As a new technology, the performance of CO 2 R has improved significantly in recent years and will be further improved in the near future.Depending on the targeted products, some processes have achieved sufficiently desirable performance, but not all of them.This makes it important to include an optimistic performance for each near-future scenario, in order to fairly evaluate their potential and provide a guideline for the researchers who are developing CO 2 R. As mentioned in the previous text above, electricity is the only energy driver of CO 2 R, and the electricity price is predicted to significantly decrease with increasing renewable electricity generation [31].Stack cost is one of the main parts of the capital cost of CO 2 R, and the price of stack is supposed to be reduced to half in 15 years based on the prediction released by US DOE H2A project for water electrolysis [32].Therefore, to understand the commercialization potential of combined processes in the future, in this part, two optimistic scenarios were evaluated, assuming as technology-ready in 2028 (Opt-1) and 2035 (Opt-2), respectively.
The detailed assumptions of the major parameters are shown in Table S14.Briefly, the electricity prices were assumed to be reduced to 0.04 and 0.03 €/kWh in 2028 and 2035, respectively [31].The stack prices would reduce by a quarter and half by referring to the report for water electrolysis [32].The assumptions of CO 2 R performance of two optimistic scenarios are based on the research development in recent years and the prediction released from the literature related to CO 2 R [13,14] to ensure the reliability of the used parameters and results.In addition, in Case 2, a predicted H 2 price of 1.7 €/kg-H 2 in 2030 by using (SMR + CCS) method for H 2 production, was used for the calculation of future scenarios [30].The market price of 0.50 €/kg-CH 3 OH was used as a reference.
As shown in Fig. S4, in Case 1, the TPC of the base scenario is 0.38  CH 3 OH, respectively.These results confirm that Case 2 is more readily economically feasible, where all combined cases show commercialization potential with technology development in the future.

Further discussion
In order to make the combined process competitive compared with commercial CH 3 OH synthesis routes, a high Technology Readiness Level (TRL), i.e., TRL ≥ 6 is required to make this combined process to be deployed at scale.BL gasification chain for CH 3 OH synthesis has the potential to be adopted at industrial-scale in the near future.However, the research on CO 2 R mainly exists on the lab-scale with a TRL of 3 or 4. The low TRL indicates higher uncertainty during scale-up, and is thus the limiting factor of the combined pathways to commercialization.In fact, the practical application of CO 2 R technology can be greatly delayed if the efforts only focus on fundamental developments without investigation on process optimization and scale-up.Fortunately, if we compare the situation with ten years ago, the future for CO 2 R process is promising, and CO 2 R will progress to higher TRLs sooner, considering the great CO 2 R performance improvement, increased research on the design and optimization for reaction cells to reduce mass transfer and kinetics limitations [33,34], and growing awareness on combining CO 2 R with upstream processes to improve economic feasibility.
In addition to the economic benefits, the climate benefits of the novel pathways are also concerns of governments and businesses.To evaluate how the CO 2 R process influences the combined processes from an environmental perspective, we summarized the previous work related to the environmental assessment of Power-to-CH 3 OH processes for reference since no work focuses on combined processes.Guzmán et al. [35] compared the greenhouse gas (GHG) emissions of CO 2 R with the thermocatalytic conversion of CO 2 to CH 3 OH, and found that these two pathways emit almost equal GHG emissions in the most optimistic scenarios.Adnan et al. [22] compared the environmental feasibility of (i) CO 2 R to CO followed by the hydrogenation of CO to CH 3 OH and (ii) direct CO 2 R to CH 3 OH pathways.The results indicate that Power-to-CH 3 OH pathways show higher GHG emissions compared to the conventional CH 3 OH under the current condition, but all the Power-to-CH 3 OH pathways have climate benefits when low-carbon electricity (e. g., nuclear, wind, and solar) is fully utilized.The work from Wyndorps et al. [36] exhibited that the pathway of CO 2 R to CO followed by the hydrogenation of CO to CH 3 OH has already satisfied the minimum requirements to achieve climate benefits, while direct CO 2 R to CH 3 OH shows a development gap.Our previous work [10] found that the electricity-related CO 2 emission for CO 2 R is the main part of the specific CO 2 emission.Increasing FE and reducing V cell are two ways to reduce the total electricity usage by increasing the energy efficiency, which indicates the great importance of improving the CO 2 R performance to increase the climate benefits of the combined pathways.These studies indicate that CO 2 R to CO followed by conventional CH 3 OH synthesis from CO and H 2 is more climate feasible than direct CO 2 R to CH 3 OH under current conditions.In addition, improving CO 2 R performance and increasing the use of renewable energy for electricity generation are two important approaches to achieving desirable climate benefits.

Conclusion
In this work, techno-economic viability of CH 3 OH production via combined BL gasification and CO 2 R processes was assessed and compared with stand-alone BL gasification and CO 2 R processes.It was revealed that combined cases showed lower TPCs compared with the stand-alone CO 2 R process.Even though Cases 3 and 4 have higher TPCs than stand-alone BL gasification, the increased carbon efficiency and the great potential of improving CO 2 R performance make the combined process economically promising.For different combined processes, Case 1 is economically feasible with TPC of 0.38 €/kg-CH 3 OH under the current condition.While none of Case 2-4 and stand-alone systems is viable currently.Based on the results from sensitivity analysis, Case 2 is more sensitive to electricity and H 2 price compared with CO 2 R performance.The high TPCs of Cases 3 and 4 under the base scenario are due to their relatively low CO 2 R performance, which leads to high stack area and total electricity usage; this is the main reason that TPCs are sensitive to both CO 2 R performance and the prices of stack and electricity.All combined processes will be economically profitable with the improvement of CO 2 R performance and cost-reducing technology development, indicating the great potential of combined processes in the near future.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 1 .
Fig. 1.Block diagram of the four combined processes.(A) Case 1: BL gasification combined with CO 2 R to CO, (B) Case 2: BL gasification combined with CO 2 R to CO followed by hydrothermal CH 3 OH production, (C) Case 3: BL gasification combined with CO 2 R to syngas followed by hydrothermal CH 3 OH production, and (D) Case 4: BL gasification combined with CO 2 R to CH 3 OH.

F
. Li et al.Case 4, while only by 0.016 €/kg-CH 3 OH in Case 2 when V cell was reduced to 0.3 V.

Fig. 4 .
Fig. 4. The influence of (A) current density, (B) V cell , and (C) FE on TPCs of different combined cases.

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.Li et al.

Table 1
Parameters for CO 2 R.
[a] According to the market value of CdS (99.995% trace metal basis) powder supplied by Sigma-Aldrich.[b]According to the market value of Sn (99.999% trace metal basis) and CuO (99.99% trace metal basis) powder supplied by Sigma-Aldrich.F.Li et al.
€/kg-CH 3 OH, which means this case is profitable as it is today, and thus a TPC analysis in the future scenario is unnecessary.It was found that TPCs of Cases 2-4 decreased significantly under optimistic scenarios.Case 2 is economically feasible under two optimistic scenarios with TPCs of 0.49 and 0.42 €/kg-CH 3 OH, respectively.While Cases 3 and 4 are profitable under an Opt-2 scenario with TPCs of 0.37 and 0.38 €/kg-