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Carbon Dioxide Capture and Conversion

Carbon Dioxide Capture and Conversion

An Introduction to CO2 Capture and Conversion

Electrochemical devices that convert CO2 into fuels and valuable compounds have been undergoing extensive research for over a decade now. The research in this area has been driven by the desire to reduce reliance on fossil fuels and reduce greenhouse emissions. As you are probably aware, the majority of the world’s energy used for transportation, industrial, and residential uses are made from coal, petroleum, and natural gas.

 

An Increase in CO2 Emissions

As we are all aware, the consumption of fossil fuels has led to an increase in CO2 emissions. As a result, the global atmospheric CO2 concentration has increased by approximately 20% since 1980 (see Figure 1) [1]. The accumulation of CO2 increases the ability of the atmosphere to absorb and remit infrared radiation, which leads to planetary warming. This causes changes in weather patterns (i.e., floods, droughts, storms) and causes the polar ice caps to melt, which results in the sea level rising along with increased ocean acidity due to increased CO2 intake. Therefore, technologies that can decrease the atmospheric concentration of CO2 would be beneficial. There are several ways to decrease the CO2 footprint (1) use renewable or non-carbon-based energy, (2) CO2 capture and storage, and (3) CO2 conversion. Long-term, we will need to incorporate all three of these actions into our national and regional energy and climate change plans.

Figure 1. CO2 Emissions by Fuel Type [1].

 

Other Products Derived from Fossil Fuels

If we could significantly reduce our reliance on fossil fuels for energy, we would still have the issue of how to produce all the other products derived from fossil fuels. Most people do not realize that fossil fuels are also used for countless non-combustible activities to generate commodity chemicals. After the CO2 has been captured, it is used as a chemical feedstock to synthesize many types of C1, C2, C3 and long chain polymers. Actually, almost every object that we use on a daily basis has something to do with fossil fuels. Fossil fuels are used to create plastics, solvents, coatings, fertilizers, as well as many other items. Figure 2 shows an example of a coal gasification process used to create various chemicals used for common household products.

 

Figure 2. Eastman Chemical Gasification Process [2].

 

How Can We Capture CO2?

Many CO2 capture techniques are undergoing research and development. CO2 can be captured from power plants, cement factories, vehicle exhaust gas, sea water, and directly from the atmosphere. The most economical method of carbon capture is from a stationary industrial plant because flue gas streams have high CO2 concentrations (> 95%) and CO2 can be obtained for as low as $5 per ton [3]. The cost of seawater and ambient air extraction can be 20 – 50 times greater than extracting the CO2 from a flue gas stream, so there is a significant amount of R&D that still needs to be conducted to extract the CO2 from these sources [3].

 

How is CO2 Converted to Other Products?

Several approaches for CO2 conversion have been investigated, including electrochemical, solar, chemical, and biological. The electrochemical research and development processes that are currently under investigation have the potential to aid in the replacement of fossil fuels. Among these, electrochemical reactions are the most attractive due to their high energy efficiency and mild reaction conditions. However, these processes require a significant amount of energy input; therefore, they would need to be integrated with renewable energy systems. When used with a renewable energy system such as wind or solar, CO2 can be converted during times of excess electricity generation and then stored as chemical fuel. This stored chemical fuel could then be extracted using combustion or electrochemically through a fuel cell. A variety of carbon-based chemicals could also be derived in this manner.

 

Chemical Reactions

The overall chemical reaction that occurs in the electrochemical device is:

 

The overall chemical reaction can be broken into two chemical half reactions. The oxygen evolution reaction (OER) is:

 

The carbon dioxide reduction reaction (CO2RR) is:

Equation 3 is written to show that many carbon containing products are possible. The actual products obtained depends upon several factors, but the type of catalyst used is the largest determining factor.

As you can see from (3), energy is required in the form of electrical power to convert the energy stored in the chemical bonds of the compounds produced. In the CO2RR reaction, the electricity can be from any source, such as wind, solar, nuclear, or hydro. Both OER and CO2RR require catalysts to overcome their large kinetic barriers. For both of these reactions, excess power beyond what is thermodynamically required is needed. The discovery of more active catalysts will increase the energy efficency of producing chemicals and fuels electrochemically.

 

Thermodynamics of CO2 Reduction

In thermodynamics, Gibbs free energy can be used to calculate the maximum reversible work performed by a thermodynamic system. The Gibbs free energy represents the net energy cost for a system created at a constant temperature with a negligible volume, minus the energy from the environment due to heat transfer. This equation is valid at any constant temperature and pressure for most electrochemical systems. From the second law of thermodynamics, the maximum useful work (change in free energy) can be obtained when a “perfect” electrochemical cell is operating irreversibly is dependent upon temperature. Thus, Welec, the electrical power output is [4]:

  

where G is the Gibbs free energy, H is the heat content (enthalpy of formation), T is the absolute temperature, and S is entropy. Both reaction enthalpy and entropy are also dependent upon the temperature. The potential of a system to perform electrical work by a charge, Q (coulombs) through an electrical potential difference, E in volts is:

 

If the charge is assumed to be carried out by electrons:

where n is the number of moles of electrons transferred and F is the Faraday’s constant (96,485 coulombs per mole of electrons).

When a system transforms from the initial state to the final state, the decrease in Gibbs free energy equals the work done by the system to its surroundings. Therefore, the spontaneity of a given reaction can be measured by the change of Gibbs energy (ΔG), specifically, negative change in Gibbs energy is a sign of spontaneity. Gibbs energy is a function of temperature and pressure, making it difficult to precisely control the spontaneity. Combining the last three equations allows us to calculate the maximum reversible voltage provided by the cell; therefore, ΔG can be precisely controlled by applied potential, as shown in equation 7 [4].

where n is the number of electrons transferred in the electrochemical reaction, F is Faraday’s constant and Ecell is the cell potential. In this way, electrochemistry allows direct control of a reaction’s ΔG, and thus, its spontaneity. Calculated from Gibbs energy, the standard potentials for CO2 reduction to commonly reported electrochemical products are listed in Table 1. The thermodynamic cell potential helps us to estimate the minimum energy requirement for a given product, and thus, given a cost of energy, provides a method to estimate the economic viability of a product.

 

Table 1. Equilibrium Potentials of Example CO2RR Half Reactions [5].

CO2 Half Reaction

Equilibrium Potential

-0.10

0.03

0.08

 

Carbon dioxide reduction can also be achieved using a photoelectrochemical device, where catalysts are coupled for the reaction directly to the surface of the semiconductor device. This allows the coupling of catalysts for the reaction directly to the surface of the semiconductor. This approach requires finding a semiconductor with the desired properties to generate the power required to reduce the CO2. The photoelectrochemical devices have similar requirements as the electrochemical devices; therefore, catalysts can be created to further develop both types of systems.

 

Is CO2 Reduction Cost-Competitive?

Like every new technology, the feasibility of CO2 utilization will depend largely upon the cost. The largest cost for CO2 reduction will be for the price of electricity. Therefore, one of the ways to decrease cost is to find a better catalyst to increase the efficiency and reduce the cost. Scale is also an important factor when researching CO2 electrochemical systems. In order to meet chemical product demands, the electrochemical device must have a high production rate. Fortunately, there are already electrolytic processes for refining aluminum and for the chlor-alkali process, so examples of these large-scale electrochemical systems demonstrates that an electrolytic process is capable of meeting large-scale demands.

 

What is the best product to make using CO2 Reduction?

When starting CO2RR research, a common question is how to determine which product to make. The best method of determining this is to conduct market research to find out which products are more likely to have a higher profit margin than others. Table 2 shows the electricity cost of producing one metric ton of each product assuming a 300 mV potential for CO2RR and OER. It compares the cost of electricity to the market price, which is an estimate of the upper limit of possible profit. Also, when performing market research, you can find products that are not easily created from petroleum-based products. Examples include formic acid, methanol, ethylene glycol, and 1-propanol. There are also certain products, such as methane and ethylene, that are easily and cheaply produced from fossil fuels, so it would not make sense to conduct research on electrochemical devices and catalysts that can create those types of systems. However, changes in electricity or fossil fuel prices or government regulations could easily change the price of CO2 derived products.

 

Table 2. Simple cost estimate for one metric ton of each CO2RR product considered.

Product

Market Price ($/MT)

Annual World Production (MT)

Electricity Cost ($/MT)

Market Electricity Cost ($)

Methanol

3333

44443000

605

2728

Ethanol

1004

38870000

814

191

Ethylene

1289

98863000

1344

-55

 

Conclusion

Electrochemical conversion of CO2 to fuels and chemicals is a promising approach to reduce fossil fuel usage. There is a signifigant amount of research and development needed for carbon capture and catalysts, but there is enormous potential for technologies to reduce CO2 or prevent further CO2 pollution. With improved carbon capture and better catalysts, the process could solve energy storage problems for intermittent electricity sources such as wind and solar as well as a source for carbon-based chemicals.

 

References

[1] Roser, M., & Ritchie, H. (n.d.). Co2 Emissions By Fuel. Retrieved March 10, 2021, from https://ourworldindata.org/emissions-by-fuel

[2] Spiegel, C.S. (2006). Opportunities for Coal-based Products: Clean Coal and Coal Processing Technologies, 1st Ed. Norwalk, CT: BCC Research.

[3] Metz, B., Davidson, O., Coninck, H. D., Loos, M., & Meyer, L. (2005). Carbon Dioxide Capture and Storage. Cambridge: University Press.

[4] Spiegel, C.S. (2008). PEM Fuel Cell Modeling and Simulation Using MATLAB, 1st Ed. New York: Elsevier Science.

[5] Mistry, H., Varela, A. S., Bonifacio, C. S., Zegkinoglou, I., Sinev, I., Choi, Y., . . . Cuenya, B. R. (2016). Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene. Nature Communications, 7(1). doi:10.1038/ncomms12123

 

 

 

 

 

 

 

 

Dr. Colleen Spiegel Posted by Dr. Colleen Spiegel

Dr. Colleen Spiegel is a mathematical modeling and technical writing consultant (President of SEMSCIO) and Professor holding a Ph.D. and an MSc degree in Engineering. She has seventeen years of experience in engineering, statistics, data science, research & technical writing work for many companies as a consultant, employee, and independent business owner. She is the author of ‘Designing and Building Fuel Cells’ (McGraw-Hill, 2007) and ‘PEM Fuel Cell Modeling and Simulation Using MATLAB’ (Elsevier Science, 2008). She previously owned Clean Fuel Cell Energy, LLC, which was a fuel cell organization that served scientists, engineers, and professors world-wide.

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