A Review of Catalysts for the Electroreduction of Carbon Dioxide to Produce Low-carbon Fuels

Introduction

Climate change, of which greenhouse gas emission is the main driver, is one of the near urgent challenges humanity is currently facing. Equally depicted in Figure 1, the atmospheric CO_two concentration has been rising rapidly since the showtime of the measurements in March 1958, with an average increase of approximately 2 ppm per year over the by decade (Tans, 2019). In 2016, the atmospheric CO₂ concentration stayed above the symbolic 400 ppm marker all year round for the showtime time, corresponding to a 30% increment compared to the pre-industrial (before circa 1750) levels of 270 ppm (Betts et al., 2016). The Paris Agreement under the Un Framework Convention on Climatic change adopted in 2015 illustrates the worldwide commitment to reduce greenhouse gas emissions to mitigate global warming, only this will crave an order-of-magnitude increment in public and private investments in inquiry and development between 2019 and 2030 (Rockström et al., 2017).

Figure 1. Concentrations of CO₂ in the atmosphere measured at Mauna Loa Observatory (Hawaii) since the offset of the measurements in March 1958 (316 ppm) (Tans, 2019).

Strategies to reduce CO₂ emissions can be divided in 4 categories that focus on either avoidance of CO_ii emissions or bounden the emitted CO₂ in a natural or non-natural sink. The start category is improving free energy efficiency, which currently provides the greatest return on investment and has already been successfully practical in many industrial contexts. Although this approach yet has potential, annual improvements of 1–2% will not be sufficient to meet the climate targets. The second category, using non- or depression-carbon energy sources, (e.g., solar, current of air, geothermal), is at big scale still challenging due to the fluctuating nature of the energy supply and the tiresome rate at which the electricity production is becoming more renewable. Carbon Capture and Sequestration (CCS), i.east., a series of technologies combining CO₂ capture from large point sources such as power plants, transportation to a storage site, and sequestration into a (natural) sink, is the tertiary category, but its potential is currently rather limited due to technical and economic hurdles (Spigarelli and Kawatra, 2013; Leung et al., 2014). The fourth category is Carbon Capture and Utilization (CCU), in which CO₂ is converted to (loftier-value) products. This category can be considered every bit a special example of the third category with the utilization part acting as a non-natural sink (Whipple and Kenis, 2010; Kuhl et al., 2014; Schouten et al., 2014).

CO₂ is a thermodynamically very stable molecule and thus a substantial input of energy combined with constructive reaction conditions and active catalysts are required for its conversion, c.f. Figure 2. To obtain the desired overall negative CO_ii balance, the energy required for its conversion should originate from non- or depression carbon free energy sources. Hence, the development of CO_ii conversion processes has focused on minimizing the required energy input by using the non or low-carbon energy sources in the most efficient way possible. According to a recent report (Voltachem, 2016), the development of new products through the awarding of innovative technologies powered by renewable energy is one of the main drivers for "electrification" of the chemical manufacture, i.e., replacing thermal and chemical free energy by electric energy. Other main drivers are economic benefits and improved sustainability through the reduction of feedstocks, past-products, waste, energy utilize, solvents, and CO₂.

Effigy 2. Standard Gibbs costless energy of formation (at 298 K), expressed in kJ mol⁻¹, of CO_ii and possible reduction products.

Among all the proposed methods for converting CO_ii, which accept as common advantage the ease of integration of non- or low carbon free energy sources, electrochemical methods are considered to exist the almost promising (Endrődi et al., 2017), as several advantages have been claimed compared to the other methods: 1) they can be conducted at ambient conditions (allowing for rapid changes in the product rate as the availability of the renewable energy changes), 2) past a conscientious selection of the electrocatalyst, electrolyte and operating conditions, it is possible to drive the electrochemical conversion of CO_two toward the desired products, 3) the chemic consumption can be minimized by recycling the electrolytes, 4) the reaction systems are compact, modular and hence scale-upward is relatively straightforward, and 5) the electrons are directly used for production formation. Still, there are articulate challenges for electrification, such equally the overall loftier price of electricity, the big investment costs, the often poor selectivities and depression conversions related to low reaction rates (resulting in large reactor volumes needed for a globe-scale plant), the technical and economic feasibility of turning plants on and off safely on brusk find, etc. This implies that at that place is a lot of skepticism whether electrification of the chemical industry is really feasible (Van Geem et al., 2019; Gani et al., 2020) or whether it is another hype similar the numerous ones that take been presented in the final ii decades (Banholzer, 2012; Banholzer and Jones, 2013).

The goal of this report is to explore whether the electrochemical conversion of CO₂ tin exist a viable culling product route of ethylene, which is the key building block of the chemic industry and representative for products with a reasonably high added value. First, a curt overview is given of the CO₂ reduction process and the performance trends with the focus on ethylene formation. Next, a techno-economic model is developed for a CO_two conversion plant integrated with CO₂ capture from a blast-furnace flue gas stream. With this model, the economic competitiveness of this alternative ethylene product route is compared against the current state of the art for ethylene production, i.e., naphtha-based steam cracking, under both electric current and future conditions. Finally, the CO₂ avoidance potential of the process is assessed based on a Life Cycle Analysis, adopting a cradle-to-gate boundary.

Methodology

Electrochemical Conversion of CO₂

The electrochemical reduction of CO₂ is a complex conversion consisting of multiple elementary proton-electron reactions leading to the (co-)germination of various products of which ethylene has the highest commercial value. Equally depicted in Figure 3, CO₂ is converted at the negatively charged cathode to primarily CO, methane, ethylene and formic acid, while H_iiO is oxidized into O₂ at the anode. The half-cell reactions for the electrochemical reduction of CO₂ and the corresponding formal reduction potential are summarized in Table 1 (Bard et al., 1985). For all possible reduction products, the reaction proceeds via CO equally intermediate species (Hori, 2008). In aqueous environment, the hydrogen evolution reaction competes with the reduction of CO₂. Aside from the employed electrocatalyst and electrode potential, the product distribution obtained from the electrochemical reduction of CO₂ depends on the choice of electrolyte, the electrolyte concentration, the concentration of dissolved CO₂ and the reaction conditions, i.east., pressure and temperature.

Figure 3. Schematic representation of the product of high-value chemicals past the electrochemical reduction of CO_ii making utilize of renewable energy sources.

Table 1. Half-prison cell reactions and the corresponding formal redox potential E ⁰ (Five) for the electrochemical reduction of CO_two. All potentials are referenced confronting the standard hydrogen electrode (Bard et al., 1985).

To evaluate the technological operation of this electrochemical process and enable a meaningful comparing between different electrocatalysts, several figures of merit are commonly used (Pander Three et al., 2017). Because the reduction of CO₂ has to overcome a kinetic energy barrier, the cell potential at which the redox reaction is experimentally observed (Due east), is college than the reversible jail cell potential (E ⁰) and the departure betwixt the two is denoted every bit the overpotential $\left(\eta \right)$ , c.f. Eq. 1.

Minimizing the overpotential of the desired electrochemical reaction minimizes the required energy input. The Faradaic efficiency $\left(FE\right)$ or electric current efficiency is a measure for the product selectivity of the reduction procedure for a given product, and is equal to the ratio of the charge used to generate a given product and the total charge passed during the electrolysis process (Eq. 2).

With $z$ the number of electrons transferred in the corresponding one-half reaction, $\mathrm{north}$ the number of moles of a certain product and $F$ Faraday'south constant. A 2nd efficiency indicator is the free energy efficiency (EE), c.f. Eq. 3, which is the ratio of the amount of energy in the products and the amount of free energy put into the system.

In general terms, the lower the overpotential and the college the Faradaic efficiency, the college the energy efficiency of the process. Finally, the current density $\left(i\right)$ , which is defined as the ratio of the current at a given jail cell potential (R′) and the active electrode surface area (A), determines together with the Faradaic efficiency the specific electrochemical reaction rate, c.f. Eq. four. The lower the current density, the college the electrode surface surface area required to obtain a sure reaction rate. Hence, this parameter significantly influences the price of the electrochemical reactor, and the sensitivity to Majuscule expenditure (CAPEX) and Operational expenditure (OPEX) has been investigated (Jhong et al., 2013).

Procedure Weather condition and Selectivity

To enable a viable large-scale implementation of an electrochemical CO₂ reduction process, the development of an agile, selective, stable, and relatively low-cost electrocatalyst is a prerequisite. Over the last few years, many researchers focused on the exploration of different electrocatalysts with the aim of addressing the cardinal challenges of this electrochemical process (Hori et al., 1985; Hara et al., 1997; Hori, 2008; Rakowski Dubois and Dubois, 2009; Peterson and Nørskov, 2012; Schneider et al., 2012; Qiao et al., 2014; Kortlever et al., 2015; Mao and Hatton, 2015; Engelbrecht et al., 2016; Kortlever et al., 2016; Wu et al., 2016; Tao et al., 2017): 1) reduce the overpotential (or increase energy efficiency), ii) increase the selectivity (or Faradaic efficiency), 3) increase electric current density, and iv) expand catalyst lifetime (less than 100 h) with order of magnitude. While new studies reporting improved Faradaic efficiencies and lower overpotentials are consistently being published, the of import question remains what the operation of the reduction process should be to enable implementation of a viable large-scale industrial process. As a dominion of pollex, industrial reactors are typically operated at geometric current densities in a higher place 100 mA cm⁻² with at least 50% Faradaic efficiency for the required products, in order to minimize investment costs as much as possible (Oloman and Li, 2008). In Figure 4, an overview is given of the electrochemical performances for CO₂ reduction to ethylene from a selection of studies reported in the open literature published in the period from 1986 to 2017. The overpotential, which determines the free energy efficiency of the procedure, ranges from −0.8 to −2.iv V. While meaning progress has been made over the last few years, the functioning of state-of-the fine art technologies seems to be currently not yet at the level required for an economically viable large-calibration process, indicated past the light-green zone and applying the dominion of pollex specified higher up.

FIGURE 4. Summary of the electrochemical performance of CO₂ reduction to ethylene from a selection of studies reported in the open literature published in the period from 1986 to 2019: Faradaic efficiencies (A) and free energy efficiencies (B) equally a function of current density (mA cm^−two) (Hori et al., 1985; Kaneco et al., 1999; Ogura et al., 2004; Hori, 2008; Engelbrecht et al., 2016; Ma et al., 2016; Mistry et al., 2016; Wu et al., 2016; Ke et al., 2017; Peng et al., 2017; Gao et al., 2019).

Aside from activity and selectivity issues, rapid deactivation of the catalysts, which leads to a shift in the product distribution favoring the hydrogen evolution reaction, is also one of the chief challenges. In most cases, the goad lifetime is under 100 h (Qiao et al., 2014). The factors influencing the catalyst lifetime have so far not been analyzed in item, also because almost experiments are performed in a limited time span focusing on improving the initial catalyst performance. Although the verbal cause for catalyst deactivation is not ever articulate, several hypotheses have been suggested, including electrolyte trace impurity degradation, accumulation of adsorbed or insoluble reaction by-products and morphological changes of the catalyst. A lifetime in the society of magnitude of thousands of hours is required for a viable large-scale process. Otherwise, frequent regeneration of the electrodes should be foreseen in the process (Dominguez-Ramos et al., 2015; Martin et al., 2015).

It is clear that several technological breakthroughs are needed before the electrochemical reduction of CO₂ can go industrially feasible. A roadmap for the electrochemical reduction of CO₂ has recently been developed within European union'southward Energy program, proposing both brusque-term and long-term practical goals (Koper and Roldan, 2019). In the adjacent 5 years, pregnant progress should be made on the development of CO_two electrocatalysts and electrolyzers, operating at relevant current densities (>100 mA cm⁻²), with high Faradaic efficiency for high-value products such as ethylene, at lower overpotential (2.0–2.2 V), and skilful stability (>100 h). In the long term, the integration with downstream operations too every bit integration with upstream CO₂ capture should be considered.

Conceptual Process Design

In Effigy five, a conceptual process scheme for the electrocatalytic conversion of CO₂ is depicted. The first step of the procedure is the capture of CO₂ from the flue gas stream and send to the conversion unit. The CO₂ source for this study is considered to be blast furnace gas, which contains ∼22 mol% CO, ∼22 mol% CO_two, ∼5 mol% H_two, and ∼51 mol% Northward_ii. The CO₂ is captured via chemical absorption with monoethanolamine (MEA) as solvent. The blast furnace technology will go on to dominate steel product in the coming decade and the only fashion to substantially reduce the associated CO_ii emissions is to combine it with CCS or CCU options. In an cushion column the CO₂-containing gas stream is contacted with a solvent, afterwards which information technology is desorbed over again from the solvent in a stripper column. As MEA can undergo degradation and is besides lost via the gasses that are vented into the temper, a brand-up of this chemical is required. Chemic absorption with MEA results in a high CO₂ purity production stream (>98 wt%), with H₂O every bit the primary impurity, while traces of Northward₂ and MEA tin can also be present (Li et al., 2016). Gaseous impurities tin have an effect on the electrolysis process in different ways, i.e., they can act as 1) every bit diluents (e.g., Due north_two), 2) as reducible species (e.one thousand., O₂), and 3) as goad poisoning species (e.yard., NO₂, SO_ii, H₂S, organic gases) (Zhai et al., 2019). To enable industrial application, the influence of gaseous impurities on the electrolysis process needs to be better understood to avert catalyst degradation. The captured CO_two is combined with a possible recycle stream and sent to the reactor in which the electrochemical reduction takes place. At the cathode, CO_two is reduced resulting in the germination of the main products CO, ethylene, methane, hydrogen and formic acrid, while at the anode H₂O is oxidized into O_two. The global reaction for the production of ethylene is:

$2{\text{ CO}}_{\mathrm{ii}}+2{\text{H}}_2\text{O}\leftrightarrow {\text{C}}_2{\text{H}}_4+3{\text{O}}_2(\mathrm{five})$

The production stream that leaves the cathode compartment of the reactor is sent to a wink vessel. The obtained liquid stream contains mainly electrolyte and unconverted CO₂ and can be sent back to the reactor. In gild to avoid the aggregating of liquid byproducts (i.due east., formic acid), part of this stream is purged. The gas stream contains ethylene, unconverted CO₂, and significant amounts of other byproducts, such as CO and H₂. A gas purification department is required which serves 2 main goals: separation of the unconverted CO₂ for re-apply and purification of the desired products (in this example ethylene), and byproducts.

Effigy 5. Envisioned process scheme for the electrochemical conversion of a CO₂-containing flue gas stream to high-value chemicals.

Economic Assay

To appraise the actual economic feasibility of this alternative product route for ethylene, the CAPEX and OPEX accept been estimated. A grass-roots institute is considered, congenital in Northwestern Europe. Cost calculations are thus based on European prices. Price functions are introduced to approximate the CAPEX and OPEX for the dissimilar steps of the electrochemical conversion process, i.e., CO_two capture, CO_ii conversion and product separation and purification. Because of the large calibration of this industrial process, the feasibility study is washed for the replacement of a office of the installed product capacity based on fossil fuels, i.e., steam cracking of naphtha. This means it is assumed that the gaseous product stream, after CO₂ removal, is farther processed on the separation department of an existing steam corking facility. No costs accept thus been estimated for the product separation and purification steps, except for capturing and recycling of the unconverted CO₂. Using estimates from literature data for the specific capital cost and energy requirement, the CAPEX and OPEX are written as a function of four unlike parameters for the dissimilar steps. These parameters, which are related to both the process and to external factors, are: (carbon-based) production selectivity, (single-pass) conversion, CO₂ value, and electricity toll. All other parameters actualization in the cost functions are rewritten every bit a function of these 4 disquisitional parameters.

Capital Expenditure Estimation

The estimated CAPEX for the CO₂ capture plant is based on a specific capital toll of 70 €/metric ton CO₂/twelvemonth (Kuramochi et al., 2011). For the installed cost of the electrochemical reactors, a value of 66 million € is reported in literature, for the conversion of 100 metric ton CO_two per day (Oloman and Li, 2008). This value corresponds to 70 electrochemical menses reactors each with 100 cells of 0.five grand². No economies of scale are taken into business relationship for the electrochemical reduction of CO₂, due to the modular grapheme of the electrochemical cells. To improve the overall conversion of the electrochemical process, unconverted CO₂ is separated from the gaseous products and recycled back to the reactor. This is done via an boosted MEA absorption system, for which the same specific capital toll of 70 €/metric ton CO_ii/yr is taken (Kuramochi et al., 2011). Because the assumption is made that the gaseous production stream, after CO₂ removal, is further candy in the separation section of the existing steam keen facility, no costs are associated to further production separation and purification steps. The maintenance cost is estimated as ii.five% of the CAPEX.

Operational Expenditure Estimation

The primary energy contribution for the CO₂ capture procedure is the generation of steam required for the desorption procedure. This energy is provided making use of natural gas with an efficiency of 85% and a thermal energy price of eight.3 €/GJ (Eurostat (2019)). Mechanical energy required for the pumps and compressors, is causeless to be delivered past electromotors. The thermal and electrical energy requirements for the MEA assimilation organisation are taken from Kuramochi et al. (2011), i.e., three.2 and 0.50 GJ/metric ton CO_2, respectively. The cost of MEA losses is assumed to be 4.vi €/metric ton CO_ii (Karl et al., 2011). With respect to the OPEX of the electrochemical reactor, at that place are 2 main contributions: the usage of chemicals and the consumption of free energy (i.e., electricity). The free energy required for the conversion of CO_ii is amidst others determined by the selectivity or equivalently the Faradaic efficiency of the desired reaction, in this case the conversion to ethylene. For a Faradaic efficiency of 60%, approximately 20 MW h per metric ton of converted CO₂ is required. The specific energy consumption decreases with a factor two when considering the limiting example of a Faradaic efficiency of 100% (Agarwal et al., 2011). This confirms the importance of technological advancements regarding the energy efficiency of the procedure to enable large-scale industrial production. With respect to the cost of chemicals, the critical assumption is made that the electrolytes tin be fully recycled, which means that the main cost is included in the CAPEX, and in the OPEX only recycling costs need to be taken into account.

Economic Analysis

For the example written report, an almanac ethylene production of ten⁵ metric ton is considered, which corresponds to approximately 5–20% of a typical ethylene production site based on fossil feedstocks. At 100% conversion and selectivity, the electrocatalytic conversion of CO₂ would crave 3.14 × ten⁵ metric ton of CO₂ per year, which corresponds to virtually 5–10% of the CO₂ emissions of that aforementioned ethylene production site. Equally a base case, the production of polymer-grade ethylene based on a fossil feedstock, i.e., steam peachy of naphtha, is taken, as this is and volition remain the predominant process for the production of olefins in the coming decades (Amghizar et al., 2017). For this process, a yield of loftier-value chemicals (HVC) of 55 wt% is causeless, with an ethylene and propylene yield of, respectively, 30 and 15 wt% (after hydrogenation of acetylene, methyl acetylene and propadiene). The detailed effluent composition obtained from Zimmermann and Walzl (2000) can exist found in the Supplementary Cloth.

The estimated CAPEX and OPEX for the furnace section of a naphtha-based steam cracker located in Europe are equal to respectively 500 and 225 €/metric ton ethylene (Brown, 2019, Personal Advice). This corresponds to an energy requirement of approximately ix GJ/metric ton HVC or 16 GJ/metric ton ethylene for the furnace section, in agreement with the value reported by Ren et al. (2008). Process upsets, technical issues, and turnarounds, are accounted for via the plant annual uptime, which is equal to viii,440 h per year or 96.3%. An overview of the chief techno economical assumptions can be found in the Supplementary Cloth.

The revenues for the electrochemical process result from the sale of ethylene. Note that high-purity O₂ (90–95 wt%) can be considered as a valuable byproduct from the electrolysis process, simply is it not taken into account in the product revenues. If possible, O₂ will be used in nearby chemical plants to avert the high-cost send needs. For the conventional steam cracking process the sale of other important products such as propylene, butadiene and BTX is likewise considered. The prices determined for March 2018, are summarized in Table 2, i.e., ethylene at 1,050 €/metric ton, propylene at 840 €/metric ton and naphtha at 500 €/metric ton. The gross margin is calculated every bit the divergence betwixt the revenue from the sale of products and the feedstock price.

TABLE 2. Applied pricing (March 2018) level for the major materials in this study (ICIS Pricing Database; Platts Global Ethylene Price Index; Platts Global Propylene Price Alphabetize).

5 hypothetical cases were considered to appraise the economic potential of this alternative ethylene product route compared to the base example, i.e., steam cracking of naphtha. These cases, with different values for the four disquisitional parameters as shown in c.f. Table 3, are:

(i) Reference example: State-of-the-fine art electrolyzer performance based on Ogura et al. (2004) with a product selectivity of 70% and conversion of l%, CO₂ value of −30 €/metric ton and an industrial electricity price of 35 €/MW h (Haegel et al., 2017).

(2) High selectivity: Selectivity of 100% and conversion of l%, CO₂ value of −30 €/metric ton and an industrial electricity price of 35 €/MW h.

(3) Loftier conversion: Selectivity of lxx% and conversion of 100%, CO₂ value of −30 €/metric ton and an industrial electricity price of 35 €/MW h.

(4) High CO₂ cost: Selectivity of 70% and conversion of l%, CO₂ value of −100 €/metric ton and an industrial electricity price of 35 €/MW h.

(5) Free electricity: Selectivity of 70% and conversion of 50%, CO_ii value of −thirty €/metric ton and zero toll electricity.

Tabular array 3. Values for the three input parameters, i.e., production selectivity (%), conversion (%), CO_ii value (€/metric ton) and electricity price (€/MW h) for the 5 different cases.

Results and Discussion

Economic Evaluation

In Effigy half-dozen, the main energy input and material streams considered in the economic analysis are shown. Note that for the reference instance, the loftier CO_two cost case and the complimentary electricity case, these values are identical, as the only deviation between these iii cases is caused by a change in CO₂ and electricity price. For a production selectivity of 70%, a larger flow of CO₂ is required to obtain the desired ethylene production chapters (10^v metric ton per twelvemonth), i.e., 4.48 × 10^v metric ton CO₂ per year compared to 3.xiv × 10⁵ metric ton CO_two per twelvemonth in case of a product selectivity of 100%. This lower product selectivity leads to larger energy requirements for the CO₂ capture step. The thermal energy required in the CO₂ capture stride is equal to approximately 87% of the full energy need. The thermal energy input in the CO_ii conversion stride is used in the recycle loop for the separation of unconverted CO_two from the gaseous product. In the case of consummate conversion of CO₂, at that place is no demand for this separation stride and hence the thermal energy input becomes negligible. The free energy consumption for the CO_two conversion stride is dominated by the electricity need. Taking into account that the plant runs for 8,440 h in one year (or 96% of the time), this corresponds to a continuous power requirement of 367 MW. If the production selectivity decreases to 70%, the required electric ability increases to 751 MW.

FIGURE vi. Black box representations of the electrochemical reduction of CO₂ with the electrolyzer performance, i.eastward., conversion (C) and selectivity (Due south), energy input and material streams that have been considered in the economical assay for the (A) reference, high CO₂ cost and free electricity cases, (B) the high selectivity case, and (C) the high conversion example.

Figure 7 summarizes the CAPEX, OPEX and gross margins, i.e., the deviation betwixt revenues and feedstock price, for the five studied cases and the base of operations instance, i.due east., naphtha-based steam cracking. It tin exist seen that currently the main disadvantage of the electrochemical process is the loftier CAPEX of the electrochemical reactor, which is a issue of expensive electrode materials combined with limited economies of scale due to the modular character of the electrochemical cells. Nevertheless, when considering the gross margin, the electrochemical process looks promising compared to the conventional steam cracking road, due to the lower feedstock cost. Hence, hereafter R&D efforts should aim to develop highly active, selective, stable and low-cost electrocatalysts in order to subtract the reactor CAPEX. For each case, the CAPEX, OPEX, and gross margin of the different steps, i.eastward., CO_ii capture, CO₂ conversion, and product separation, can be found in the Supplementary Fabric.

Figure 7. Capital expenditure, Operational expenditure, and gross margin (in 10^six €) for the naphtha-based steam cracker, the reference electrochemical case, the high selectivity example, the loftier conversion case, the high CO_ii cost case and the gratuitous electricity case.

Sensitivity Assay

A global sensitivity analysis has been conducted in order to investigate the influence of the dominant parameters, i.e., selectivity, conversion, CO₂ cost, and electricity price, on the economics of the electrochemical process. The results for the sensitivity of the CAPEX, OPEX, and gross margin with respect to the reference electrochemical case are shown in Figure eight. As expected the selectivity to ethylene is the parameter with the largest influence on the CAPEX and the OPEX of the process. For a lower selectivity, more free energy is lost in byproduct germination. More than energy is thus required to obtain a desired production capacity of ethylene. A higher electricity consumption likewise increases the total electrolyzer area, resulting in a higher electrolyzer CAPEX. The conversion, which determines the required capacity of the recycle loop, has only a limited influence. This is too the example for the price of CO₂, which is encouraging every bit information technology reduces the dependence of the process on a gene that is mainly adamant by macro-economical and political factors ("CO_two tax" vs. capture costs). The electricity price is i of the principal parameters influencing the economic feasibility of the process. Some people argue whether or non it would exist beneficial to operate an electrochemical process merely when the electricity price is below a sure threshold value, i.e., operate the product plant in a flexible manner co-ordinate to the energy market. The mild operating atmospheric condition, i.east., ambient temperature, and the modular character of a earth-calibration plant would let rapid changes in production rate. However, due to the extremely loftier CAPEX of the electrochemical process, we believe that it would exist more beneficial to run the process continuously with a fluctuating energy price, rather than operating information technology equally a discontinuous process. This is primarily motivated by the payback time and for the chemic manufacture this is typically in the order of a decade for large investments (Anderson and Fennell, 2013). Turning the installation on and off would result in an unacceptably loftier payback time. Besides, the applied feasibility of ramping upwards or down such a big calibration product unit of measurement should be considered and there is not a lot of published work on the start-up and shutdown of electrochemical reactors (Rousar et al., 1986; Bisang 1997).

FIGURE 8. Sensitivity assay of the (A) Upper-case letter expenditure (CAPEX), (B) Operational expenditure (OPEX), and (C) gross margin for a positive (green) or negative (scarlet) modify of the model parameters (selectivity, conversion, CO₂ price and electricity price) for the reference electrochemical case.

Energy Considerations

If the CO₂ reduction procedure results in an overall negative net CO₂ balance, it tin be considered a viable carbon recycling technology. This means that the corporeality of CO_two emitted during the consummate process needs to be lower than the amount of CO₂ converted. For naphtha-based steam cracking, the CO₂ emissions are approximately one metric ton CO₂ per metric ton ethylene (Ren et al., 2006). These emissions are the consequence of fuel combustion and utilities, both of which apply fossil fuel. The main contributor is the furnace with over 90% of the CO₂ emissions (Amghizar et al., 2020). Because the electrochemical reduction of CO₂ to ethylene requires a meaning amount of electrical power, information technology is clear that the electricity needs to come from depression-carbon energy sources, such as wind and solar energy, to obtain an overall negative CO_ii balance. Because of their intermittent nature, both solar and wind energy have a reduced capacity factor, which is non accounted for in the presented assay. We assume that access to green electricity is continuous and steady country functioning is possible. The CO₂ emissions of the culling ethylene production route are based on a Life Cycle Analysis, adopting a cradle-to-gate boundary, i.e., usage and end-of-life handling are not included (von der Assen et al., 2013). In this analysis, the CO₂ feedstock is considered as a regular feedstock with its own product emissions. The emission intensities for Northwestern Europe, expressed in kg CO₂ equivalents per MW h for natural gas, solar and wind are equal to respectively 490, 48, and 12 kg CO₂eq/MW h (Schlömer et al., 2015). In Figure 9, the CO₂ emissions per metric ton of ethylene produced are compared between the base case, i.eastward., steam cracking of naphtha, and the electrochemical reduction of CO₂ using gray electricity from a natural gas ability institute, and green electricity from both solar and wind free energy. The overall CO_two balance for the electrochemical road is based on an ideal electrolyzer, i.e., operating at a conversion and selectivity of 100%. The production of 1 metric ton of ethylene requires 3.xiv metric ton of CO_ii as carbon feed, while the electrolyzer uses approximately x MW h per metric ton CO₂. The thermal energy demand for the CO₂ capture step, i.due east., three.2 GJ/metric ton CO₂ amounts to 0.56 metric ton CO₂ per metric ton ethylene. The CO₂ emissions related to the conversion step are equal to i.50 and 0.37 metric ton CO₂ per metric ton ethylene, using respectively solar and wind free energy. The minor deviation in emission intensity betwixt these two low-carbon energy sources leads to a significant difference in CO₂ emissions per metric ton CO₂ due to the large electricity demand. From Figure nine, it can exist seen that this alternative product route of ethylene tin potentially lead to a reduction of CO₂ emissions. Note that the CO_ii emissions related to the production separation and purification steps are not taken into account in this calculation.

Figure nine. CO₂ emissions per metric ton of ethylene produced: Comparison between steam cracking (SC) of naphtha and the electrochemical reduction (ECR) of CO₂ using electricity from natural gas (NG), solar and current of air free energy. For the ECR process, both the CO₂ used equally feedstock too as the CO_two emitted during the complete process is considered.

As stated earlier, it is clear that the electroreduction of CO₂ needs to exist powered by green electricity in club to obtain an overall negative internet CO₂ balance. The fact that reducing CO₂ emissions through CCU processes will only be possible if the electricity (and in some cases the thermal energy) inputs are from renewable sources has been included in earlier studies (Bennett et al., 2014; Jouny et al., 2018; Spurgeon and Kumar, 2018; Mohsin et al., 2020). Ane of the challenges for electrolyzers powered by renewable free energy is the operation at strongly fluctuating power inputs and with frequent interruptions due to low input (Mergel et al., 2013). The dynamic behavior of the electrochemical reactor equally well as the downstream organisation components, (due east.grand., electrolyte circuit, gas separator) needs to be analyzed such that load changes do non present whatsoever problems over a large ability input range. For water electrolysis, systems have been developed which allow for a large fractional load range (v–100%) and can adapt extreme overloads. Operating the electrolyzer at a very low level (i.e., at x% of peak load) avoids the need to shut down the chemical plant completely and the associated energy losses during kickoff-upwards (Brauns and Turek, 2020). In instance the electrolyzer has been designed for constant conditions, the occurring ability fluctuations can exist damped by additional energy storage devices, which are charged when excess renewable energy is bachelor. Overall, it tin can be concluded that more theoretical and experimental piece of work is needed to better sympathize the dynamic behavior of CO_two electrolyzers powered by intermittent renewable energy.

Instead of considering the amount of CO₂ emitted or avoided per metric ton ethylene, one can also look at the amount of CO₂ avoided per energy unit of green electricity. In other words, what is the nigh efficient mode to reduce CO_two emissions with 1 MW h of green electricity? With an electrical energy of 1 MW h, approximately 0.032 metric ton ethylene can be produced, assuming again an ideal electrolyzer operation. This ethylene formation converts 0.ten metric ton of CO₂, but also leads to 0.030 metric ton of CO₂ emitted using air current ability as renewable free energy source. The production of 0.032 metric ton ethylene via the traditional steam cracking route results in approximately 0.058 metric ton of CO_ii emitted. Thus, by producing this ethylene via the alternative electrochemical route instead of the fossil-fuel based procedure, there is a potential of lowering the emitted CO₂ by 0.xiii metric ton per MW h. Using the emission intensities, 1 MW h of electricity generated from coal and natural gas, leads to respectively 0.82 and 0.49 metric ton CO₂ emitted per MW h (Schlömer et al., 2015). This implies that from an energy point of view, information technology is more benign to utilize 1 MW h dark-green electricity to replace 1 MW h gray electricity than use information technology to convert CO₂ electrochemically in ethylene.

A key performance indicator oft used to compare different CCU processes is the cost of CO₂ avoided, i.e., the toll to avert the emission of one metric ton of CO₂ relative to a reference instance. The production price for fossil ethylene considering a establish located in Europe, is dominated by the feedstock cost and equal to approximately 700 € per metric ton of ethylene. For CO₂-based ethylene, the product cost is equal to 1,950 € per metric ton of ethylene, bold again an platonic electrolyzer, and an electricity toll of 35 €/MW h. This value is in agreement with the study of Jouny et al. (2018), in which they study a product cost of approximately 2000 € per metric ton of ethylene, for their "optimistic case," using the same electricity price of 35 €/MW h and a faradaic efficiency of ninety%. The internet CO_ii emissions for the CO₂ reduction process using solar and wind energy are equal to respectively −ane.08 and −two.20 metric ton CO₂ per metric ton ethylene. Based on these values, the CO_ii abstention price amounts to 602 and 391 € per metric ton CO₂ avoided. Annotation that these values are significantly higher than the recent prices of the CO₂ allowances envisioned by the European Emission Trading Scheme, i.east., 28 €/metric ton CO_ii in 2030 and 43 €/metric ton CO₂.

Conclusions

Electrochemical conversion of CO₂ to ethylene could exist of interest to the chemical manufacture, but several breakthroughs are needed to make this competitive with the electric current state of the fine art under current marketplace atmospheric condition. Without a substantial subtract of the electricity price and large capacity increases in renewable electricity production (to become a reliable provider at continuous low prices), this culling ethylene production route seems infeasible for the chemic industry. Due to high capital costs for the electrochemical technology, it makes no sense to run these installations only in times when renewable power would be abundantly available and hence inexpensive. Turning large scale chemical processes "on" and "off" is today economically unfavorable, non even when bold that safety would exist guaranteed and an instantaneous close down would be feasible. When combined with green electricity, east.m., wind and solar, the electrochemical reduction of CO₂ tin can atomic number 82 to a negative overall CO₂ balance. Still, from an free energy point of view, using green electricity to supervene upon gray electricity, has a larger CO_two avoidance potential, compared to using information technology for the electrochemical production of ethylene.

Data Availability Statement

The raw data supporting the conclusions of this commodity will be made available past the authors, without undue reservation.

Author Contributions

CP: conceptualization, formal analysis, investigation, writing—original draft, visualization, writing—review and editing. MR: conceptualization, information curation, writing—review and editing. Thousand-FR: writing—review and editing. KG: conceptualization, supervision, writing—review and editing, and funding conquering.

Conflict of Interest

Writer MR was employed by the visitor Dow Benelux BV.

The remaining authors declare that the enquiry was conducted in the absence of any commercial or financial relationships that could be construed as a potential disharmonize of interest.

Funding

CP acknowledges fiscal support from a doctoral fellowship from the Enquiry Foundation—Flanders (FWO). The research leading to these results has also received funding from the European Research Council under the Eu's Horizon 2020 enquiry and innovation program (ERC Grant Agreement No. 818607).

Acknowledgments

Kees Biesheuvel, Ronald Wevers, Hanne Schatteman, Hannes Saeid Bakhsh, and Thomas Vandeputte are thanked for their valuable contributions.

Supplementary Material

The Supplementary Material for this commodity can be found online at: https://world wide web.frontiersin.org/articles/10.3389/fenrg.2020.557466/full#supplementary-material

Abbreviations

CCS, carbon capture and sequestration; CCU, carbon capture and utilization; FE, Faradaic efficiency; EE, energy efficiency; CAPEX, uppercase expenditure; OPEX, operational expenditure; MEA, monoethanolamine; HVC, high-value chemical; BTX, benzene, toluene, xylene

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