Carbon Dioxide Utilization Electrochemical Conversion of CO2 â€“ Opportunities and Challenges
Research and Innovation, Position Paper 07 - 2011
DNV is a global provider of services for managing risk. Established in 1864, DNV is an independent foundation with the purpose of safeguarding life, property and the environment. DNV comprises 300 offices in 100 countries with 9,000 employees. Our vision is making a global impact for a safe and sustainable future.
Research and Innovation in
The objective of strategic research is to enable long term innovation and business growth through new knowledge and services in support of the overall strategy of DNV. Such research is carried out in selected areas that are believed to be of particular significance for DNV in the future. A Position Paper from DNV Research and Innovation is intended to highlight findings from our research programmes.
Contact details: Narasi Sridhar â€“ Narasi.Sridhar@DNV.Com Davion Hill â€“ Davion.M.Hill@DNV.Com
Summary Nature utilizes CO2 to produce myriad substances that are consumed by humans and animals. Some industrial processes aim to accelerate the utilization of CO2. There are essentially three pathways for utilizing CO2: conversion of CO2 into fuel, utilization of CO2 as a feedstock for chemicals, and non-conversion use of CO2. The various utilization technologies together have the potential to reduce CO2 emissions by at least 3.7 gigatons/year (Gt/y) (approximately 10 % of total current annual CO2 emissions), both directly and by reducing use of fossil fuels. However, much greater reductions are possible through wider adoption of these technologies. Biochemical or chemical conversion of CO2 to fuels using biomass is an attractive technology for converting large quantities of CO2 into readily usable chemicals. Should only 5 % of liquid fossil fuel be replaced by biomass-based liquid fuel, then, based on a range of lifecycle CO2 emissions, a reduction of approximately 0.4 Gt/y of CO2 would result. CO2 conversion to minerals and insertion into polymers may have the benefit of sequestering CO2 in relatively stable matrices. If 10 % of global building material demand was met by conversion of CO2 to stable minerals, then a potential reduction of 1.6 Gt/y of CO2 has been estimated. Chemical and electrochemical conversion of CO2 into value-added chemical feedstock and intermediates is attractive in terms of fossil fuel avoidance. It is estimated that the total CO2 emissions avoidance potential of this pathway is about 0.3 Gt/y. The non-conversion uses of CO2, such as enhanced oil recovery and solvent use, have the potential to consume about 1.4 Gt/y of CO2. There is no single, universally applicable pathway for CO2 utilization. Depending on the industry, location, and other constraints, one or more technologies may fit better than others. An approach that integrates different methods may be the most practical solution for many applications. In this report, we present a small-scale demonstration of an electrochemical technology for converting CO2 into formic acid and formate salts. The technology appears to be promising, but several factors must be addressed to ensure commercial viability.
Pathways for Utilization of CO2 CO2 can be utilized in three major pathways [1-3]: 1) as a storage medium for renewable energy, 2) as a feedstock for various chemicals, and 3) as a solvent or working fluid (Figure 1). The use of CO2 to convert solar energy into biomass and, from there, to various renewable fuels is now widely supported by industry and governments as a means to secure future energy supplies and to decrease net CO2 emissions to atmosphere. While the use of food crops, such as corn, as a source for biomass fuels will probably decrease in the future, second and third generation biofuels that
Figure 1. Different pathways for utilizing CO2.
are based on grasses and algae will increase in supply. It is expected that, by 2050, biomass-based sources will supply 200 – 500 exajoules per year or about 50 % of the world’s energy requirements . It is anticipated that about 5 % of the world’s liquid fuel usage may arise from biomass, with a net CO2 reduction ranging from 20 to over 100 %, in comparison with conventional fuels over their lifecycles .
It has been estimated that by 2035, the world will produce 15 Gt/y of CO2 from burning liquid fuels . Therefore, replacing about 5 % of liquid fuels with biofuel, and assuming a 50 % lifecycle reduction in CO2 emissions in comparison with petroleum-based fuel, has the potential to reduce CO2 emissions by 0.4 Gt/y.
. This involves a combination of electrochemical reactions to generate the alkaline reactant and necessary mineralization reactions. Initial estimates suggest that even if 10 % of the worldâ€™s building materials were to be replaced by such a source, consumption of 1.6Gt/y CO2 would result .
In addition to generating biomass, CO2 can be converted via chemical and electrochemical processes to other energy storage chemicals, such as syngas, formic acid, methane, ethylene, methanol, and dimethyl ether (DME) . Although it is more efficient to use the electrical energy derived from renewable power sources directly, their variability poses a problem for many industries. Furthermore, the distribution infrastructure for hydrocarbon fuel is well established. Finally, chemicals such as formic acid may be a useful storage medium for hydrogen that could be used in fuel cells or burned directly.
CO2 can also be used in various processes without first converting it into other chemical forms. The injection of supercritical CO2 into depleted oil wells to enhance the further recovery of oil is well established. Indeed, this is presently the only commercially viable technology for carbon capture and storage (CCS). It has been estimated that CO2 injection can increase oil recovery from a depleting well by about 10 to 20 % of the original oil in place. Similarly, CO2 can be used to recover methane from unmined coal seams. It has been estimated that in the U.S. alone, 89 billion barrels of oil could technically be recovered using CO2, leading to a storage of 16 Gt of CO2 in the depleted oil reservoirs . The use of supercritical CO2 as a solvent in processing many chemicals (e.g., flavor extraction) is also well established. New uses of supercritical CO2 in chemical processing are emerging, and have the added benefit of reducing water usage. Supercritical CO2 is also being explored as a heat transfer fluid for some geothermal applications. These non-conversion methods of utilization constitute a significant fraction of the total CO2 emissions.
An alternative pathway is to convert CO2 into chemical feedstock. The entire portfolio of commodity chemicals are currently manufactured from a few primary building blocks or platform chemicals in the fossil-based chemical industry. CO2 can be used as a source material and, utilizing renewable energy sources and water, can be converted into a similar suite of building block chemicals. Insertion of CO2 into epoxides to manufacture various polymeric materials is an exciting technology as it not only utilizes CO2, but also avoids using fossil feedstock and creating CO2 emissions. It has been estimated that the various chemical conversion pathways can consume approximately 0.3 to 0.7 Gt/y of CO2 . Conversion of CO2 into inorganic minerals that may be used in building materials is being pursued by some companies
Although there are many pathways for CO2 utilization, this position paper details DNVâ€™s efforts in electrochemical reduction of CO2. The electrochemical method has several advantages: 1. Extensive research during the last several decades has yielded high selectivity, low cost, heterogeneous catalysts for CO2 electrochemical reduction to various useful products for aqueous reaction systems [11-27]. 2. Electrochemical conversion can be performed at room temperature and ambient pressure. 3. If the supporting electrolytes are fully recycled and the anode reactions can be performed using waste water, then the overall chemical consumption can be minimized to just water or wastewater. 4. A renewable source of electricity can be used to drive the process, including solar, wind, hydroelectric, geothermal, tidal, and thermoelectric processes. Therefore this method can also be used as a renewable electricity storage mechanism; it converts the electrical energy to chemical energy by producing fuels from CO2, such as methanol and formic acid. The stored energy can be released later for enduse by oxidization of the fuels through fuel cells or normal fuel-burning engines. 5. Electrochemical conversion can be augmented using light energy or solar thermal energy. 6. The electrochemical reaction system is modular and thus scale-up is relatively simple. 7. In general, the electrochemical systems have a compact design. Using metal or alloy electrodes/catalysts, various products can be produced by electrochemical reduction of CO2, including carbon monoxide (CO), formic acid (HCOOH),
oxalates (C2O4-), hydrocarbons (e.g., ethylene C2H4), and alcohols (e.g., methanol, CH3OH). DNV selected the Electrochemical Reduction of CO2 to Formate/ Formic Acid (ECFORM) as the process for comprehensive evaluation of the technical feasibility for CO2 utilization because commercialization of this process was considered to be most likely to be profitable. As mentioned previously, formic acid can be a useful storage medium for hydrogen that could be used in fuel cells or burned directly. As shown in Figure 4, the energy density of formic acid, via its use in a formic acid fuel cell, is quite attractive in comparison with other storage methods. The recoverable energy density that would be available via the combustion of methanol, ethylene, or methane, or the use of formic acid in fuel cells, is higher than conventional energy storage technologies, as shown in Figure 2. Note that the vertical axis is log scale.
Figure 2. Products created from electrochemical CO2 conversion processes have significantly more energy density than other energy storage technologies.
Formic acid and carbon monoxide require little energy for their respective market value, as shown in Figure 4. Methanol is another attractive fuel, but requires more electrical energy than formic acid for its production from CO2. Ethylene and methane require significantly more energy input, and the methane market price is constrained by natural gas prices.
The current world market demand for formic acid and formate salts is quite low (several million metric tons). The traditional uses of formic acid have been in the leather tanning industry and animal feed markets. However, new uses, in terms of hydrogen storage and fuel cells, are being developed by BASF and others, making this an attractive chemical. Formate salts are used in oil well completion and in de-icing of airport runways. Larger volumes and somewhat lower prices may expand these, and other, applications.
Figure 3. Formic acid and carbon monoxide have higher value from the energy required for their creation than conventional fuels such as methanol or methane.
Both formic acid and carbon monoxide sell for near $1,200 per ton of product and require approximately 2500 kWh/ton for their production via electrochemical CO2 conversion. These prices are likely to decrease as their production volume increases, and their usage may also increase as their price decreases. Other products, such as methane, require nearly 40,000 kWh/ton for conversion, and would only achieve $200-$300 per ton on the market. Carbon monoxide is difficult to store and transport, and therefore formic acid is a more practical and desirable product.
The ECFORM Process A schematic diagram of the ECFORM process is shown in Figure 4. It consists of two electrodes, the cathode (negative electrode) and the anode (positive electrode), across which an electrical voltage is applied. The two electrodes are placed in two different chambers, separated from each other by an ion exchange membrane. This prevents bulk mixing of the solutions flowing in each of the chambers, while simultaneously allowing ions to move across the membrane and maintain electrical continuity. A suitable electrolyte is introduced into the cathode chamber along with CO2. The electrolyte comes into contact with the cathode, and the dissolved CO2 is electrochemically reduced to the desired products. This electrical circuit is completed by the complementary oxidation reaction occurring in the anode chamber. In ECFORM, tin or proprietary tin-based alloys are used as the cathodes that convert CO2 to formate salts. Small concentrations of byproducts (hydrogen and CO) are also produced at the cathode. An oxygen evolution reaction takes place at the anode.
Faraday efficiency (FE). The FE denotes the percentage of the total current used for the desired product (i.e., the selectivity). The calculations in Figure 5 include additional energy consumed by auxiliary components, such as pumps.
An important metric of the process is the energy consumption, which is determined by the number of electrons (n) involved in reducing 1 molecule of CO2 to products, cell voltage, and the current efficiency, also called
An economically viable electrochemical technology requires optimization of four key parameters (Figure 6): high current densities, high FE, low specific electricity consumption, and long electrode lifetime. The minimum
As shown in Figure 5, the reduction of CO2 to formate/ formic acid and to carbon monoxide, respectively, appears to be the best option for practical development for at least two reasons. First, both reactions involve the participation of only two electrons, and therefore the electrical power consumption is the lowest. Secondly, the high FE of CO and formate/formic acid reactions have been achieved on affordable metal cathodes, further minimizing the energy consumption and cost. The next promising reaction may be the production of methanol. Although this involves 6 electrons for each molecule of methanol formed, the low over potentials on the catalysts reduce the cell potential to nearly half of that for other electrochemical processes. Thus, relatively lower specific energy consumption can also be achieved.
Figure 4. A schematic representation of the ECFORM process to convert CO2 to formate/formic acid.
Figure 5. Specific energy consumption vs. Faradaic efficiency (FE) for various products. The thicker lines with data points indicate experimental results achieved in various studies.
values for each parameter in a commercially viable electrochemical process are also included in Figure 6, along with target areas for improvements. In addition, there are other important requirements, such as high onepass conversion rate and continuous operation.
In general, higher current densities result in lower FE and shorter lifetimes because of competing reactions. With longer run times, FE tends to decrease (catalyst/cathode degradation) and cell voltage increase, both of which result in greater power consumption. DNV has developed novel cathode and anode catalysts that reduce the total cell voltage by almost 1 volt [26, 27]. Additionally, DNV has designed a reactor that reduces the resistive losses
Figure 6. The relationship among the key parameters in ERC processes.
by another 2 volts, thus resulting in an overall decrease in the total cell voltage by about 60 %, compared with the data published in the literature . Furthermore, the long-term performance of the cathode catalyst has been increased by at least 20 times over that reported in the literature. This has mainly been achieved through improvements in the electrochemical cell design and operational parameters. Fundamental studies performed by The Ohio State University, in collaboration with DNV, have improved our understanding of the cathode catalyst degradation mechanisms. This will enable further advances in catalyst life. DNV has also identified less corrosive electrolytes that will reduce both capital and operational expenditure.
ECFORM Reactor Demonstration Testing of a filter-press type, bench-scale reactor indicated a set of conditions for most favorable selectivity and reactivity for formate production. Figure 7 shows an example in which, under optimum pressure and flow rate control, the selectivity (FE) of a High Surface Area (HSA) cathode is kept constant over a range of applied potentials for one day. Since large electrodes have a tendency to display potential variation, this analysis indicates that slight changes in electrode potential will not affect the productivity of ECFORM, once process parameters are controlled. Long-term stable performance of HSA electrodes was determined by periodic measurement of reactivity (current density) and selectivity (FE from formate product measurement in catholyte samples) under constant optimum operation. The results in Figure 8 indicate stable performance over 4 days, with no appreciable damage or degradation of the tin electrodeposited carbon electrode. This is a significant improvement over results reported in the literature. These results suggest that electrochemical
conversion of CO2 may be a commercially viable technology in the future. A semi-pilot size reactor with a superficial area of 600 cm2 (capable of reducing approximately 1 Kg/d of CO2) was built and assembled, with other process components and instruments, into a solar-powered trailer to demonstrate the operation of the process using completely renewable power (Figure 9). The demonstration reactor serves several purposes. Firstly, it showcases the capability of the ECFORM process to utilize renewable energy, such as solar, to convert CO2 into a commercially useful product. Secondly, the reactor system can be used to test and improve the process, in terms of the hydrodynamics, heterogeneity of the surfaces, and effects on selectivity, automation, and controls, safety, and the overall efficiency of the system. Finally, the demonstration reactor provides a useful means by which process and value chain analysis models can be validated. The reactor has been modeled using a model-based flow sheet simulator, gPROMS, and this model will also be used for scale-up assessments.
Figure 7. Near constant FE over a potential range (-1.4 to -2.3 Vsce) of a HSA cathode under optimum operating conditions.
Figure 8. HSA electrodes displayed constant reactivity and selectivity over 4 days.
Figure 9. Demonstration reactor assembled in a solar-powered trailer.
Value Chain Analyses Analysis of the CO2 utilization processes can be conducted in terms of cradle-to-grave CO2 emission (lifecycle analysis or LCA) or a source-to-gate analysis, in which the boundaries start with the source of CO2 and end with the product that is delivered by a given process. The latter analysis, referred to here as value chain analysis (VCA) is convenient for understanding the net CO2 emitted in a given utilization process, since the product delivered is no different from that made by utilizing fossil fuel. Most importantly (and unlike LCA), VCA also computes the net present value of the process. Thus, VCA provides an opportunity for comparing any new process with conventional processes, as well as indicating future developmental work that could be targeted in an economically meaningful way. The VCA model that we developed for the ECFORM process can be readily modified for analysis of other CO2 utilization processes. Most CO2 utilization processes require mixed gas collection from the emissionsâ€™ source. If the exhaust source contains additional gases (such as nitrogen, sulfur, or nitrogen oxides), some additional purification or capture of the CO2 will be needed. The delivery of the mixed exhaust gas to the capture stage, and the capture process itself, requires inputs of energy and/or consumables, and these must be included in the total VCA. Once the purified CO2 has been diverted to the conversion process, this delivery may also require further energy inputs. Finally, the conversion process itself will have energy and consumables inputs. The entire value chain can be compared with direct emissions (with or without fines), carbon capture and storage (CCS), or with conventional processes for manufacturing the same product. Multiple scenarios can be computed, and these can include carbon taxes (if any), energy costs, consumables, and the value of the final product, such that the total impact of these factors on the
profitability and net present value of the investment into the CO2 conversion process can be assessed.
Figure 10. The CO2 value chain is modular and applicable to multiple processes
Emissionsâ€™ source and gas delivery DNV has analyzed emissionsâ€™ scenarios and sources, ranging from the size and scale of a coal-fired power plant to point sources within a petrochemical refinery. The electrochemical CO2 conversion process has been tested via the model and demonstrates the greatest probability of profitability occurs when the following conditions are met: - the CO2 is delivered in pure or mostly-pure form; - process heat or other renewable energy forms are available; - process volumes are manageable (<100 tons per day); - electrolyte consumables are significantly reduced or completely eliminated; - opportunities for other energy management scenarios are available.
Figure 12. Reaction pathways that minimize consumables become more dominated by energy, which must be effectively managed.
Figure 11. Reaction pathways that are heavily dependent on consumables drive the profitability of the reaction in the negative direction, more so than by the energy costs.
When the above conditions are met, the profitability, energy balance, and carbon balance of the CO2 conversion process become most sensitive to the parameters of the conversion process itself. CO2 Separation, Capture, and Delivery Because of the conditions described previously, the separation, capture, and delivery of CO2 to the conversion process are considered separately and independently from the CO2 conversion process. The availability of already captured or purified CO2 will affect the profitability of the process. The energy penalty for a coal plant capturing CO2 (not including transport and storage) ranges from 0.2 to 0.35 MWh/t of CO2 captured. This represents about 5 to 10 % of the energy required for conversion on a kWh/ton basis, as ECFORM requires approximately 2.5 to 4 MWh/ ton of converted CO2. The difference between ECFORM
and the CCS process is that whereas ECFORM produces a useful product, CCS does not. Therefore ECFORM is an energy conversion process. There are other possible CO2 separation and sequestration technologies that could lower these energy requirements. CO2 Conversion Based on different chemical reaction routes, the profitability of the CO2 conversion process depends not only on the value of the final products, but also on the energy and consumables that are required to support the electrochemical reaction. As shown in Figure 11, if the process requires additional chemicals, such as sodium hydroxide and hydrochloric acid, to support the reactions, then the net present value of the reaction is largely negatively driven by these consumables, on top of the already substantial energy demands.
Figure 13. Value-added process improvements decrease the energy costs of the ECFORM process.
However, if the use of consumable chemicals is decreased, for example through the use of electrolyte recovery processes and the utilization of alkaline waste water, then energy demands dominate the overall process economics (Figure 12). While the energy costs are increased, the reaction is more sustainable if renewable energy is used for the process. As is shown in Figure 13, the profit margins can be increased as energy costs are reduced. There is potential for additional revenues in utilizing the load leveling needs of the electric grid. These opportunities are called responsive ancillary services. For example, if energyintensive processes such as ECFORM are used to regulate voltage from a wind energy facility, the processes gain additional revenues while being renewably powered.
Figure 14. The difference between sales price and operational cost for ECFORM process (red numbers) under different scenarios (only energy costs are included â€“ consumable costs are considered to be negligible).
Additionally, revenues from carbon credits or avoidance of carbon tax may also aid in profitability. In this analysis, a carbon credit revenue of up to $50/ton does not alter the profitability substantially, but the combined revenues from carbon credits and energy management reduce the energy costs by 15 %. Four Scenarios for CO2 Conversion Four possible scenarios are envisioned for assessing the profitability of an electrochemical conversion process. This assessment does not consider capital expenditures or the time value of money. Also, the cost of consumables is considered to be negligible in comparison with energy costs. Finally, it is assumed that the formic acid resulting from the electrochemical process does not need further concentration, for example through distillation or
evaporation. The X-axis represents the price of the product made in the ECFORM process. This depends on many factors, including the volume manufactured and market demands. The Y-axis represents the CO2 price, either through a trading scheme or a tax. The numbers represent different values of profitability (assumed as a simple difference between expected value of price minus cost) for the different scenarios. The cost is calculated assuming an energy consumption of 3859 kW/ton formic acid. Electricity prices are assumed to range from $0.07/ kWh to $0.15/kWh, with a peak frequency at $0.10/kWh. The energy cost therefore ranges from $270 to $578 per ton of formic acid, with an average of $420 per ton of formic acid. For example, if the price of formic acid is assumed to be $1220/ton and the price of CO2 is $200/ ton of CO2 (1 ton of formic acid reduces CO2 by almost 1 ton), then operational profit is $1200+$200-$420 = $980 per ton. Our analyses indicate that the simple margins (price minus the cost of manufacturing) are beneficial for this process under the scenarios considered.
The future CO2 utilization is being increasingly recognized as a method by which global CO2 emissions can be reduced in an economical manner. This is especially true for industries, such as refineries, which cannot implement CCS economically. Considerable research is being conducted in many directions to further the economic viability of processes that utilize CO2. Biomass conversion to fuels is perhaps the most intensively pursued route, not only to mitigate CO2 emissions, but also to secure alternative fuel supply. Conversion of cellulosic biomass into alcohols and algae into biodiesel or other hydrocarbon fuel is predicted to become extensively adopted in the coming decade. Lifecycle assessments of these fuel sources demonstrate considerable reductions in CO2 emissions compared with petroleum fuels. However, their present economic viability is dependent on government subsidies. Several companies are pursuing thermochemical conversion of CO2 into chemical feedstock or polymers. Research and development are currently focused towards reducing the temperature of conversion, increasing catalyst life, and decreasing the use of consumables. Conversion of CO2 into minerals has advanced significantly, with at least one company claiming commercial viability for large-scale deployment. Carbon policies that impose a significant increase in carbon prices are necessary to sustain these efforts until they can become economically viable. Electrochemical and photoelectrochemical conversion routes will come to the fore in the next decade. Current research is yielding catalysts with long-term performance characteristics and low energy use, but significant technical advances are still needed for large-scale use. Electrochemical conversion promises to be deployable in
many systems, because of its low footprint, its scalability, its fungible use of electricity, and its ability to produce many end products. The combination of the electrochemical process with grid-based ancillary services can make these processes economically viable, even without a carbon tax. DNV will continue its efforts in improving the ECFORM technology, particularly making it more robust and economically viable, and explore opportunities for customizing CO2 utilization methods for industrial applications. All these technologies will rely on efficient carbon capture, as many industrial sources produce dispersed and dilute effluents containing CO2. Just as integrated biorefineries have come to characterize the use of multiple technologies to make an array of products from biomass, multiple technologies for utilizing CO2 in interconnected systems, tailored to a given application, may be the path ahead for future sustainable management of CO2.
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