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| Fig. 1: A photovoltaic cell provides electrical energy to a CO2 reactor containing a Au nanoparticle catalyst used in reducing CO2. [6] (Courtesy of the U.S. Department of Energy) |
The oxidation of hydrocarbons to generate electricity contributes the most to greenhouse gas emissions such as carbon dioxide (CO2). [1] The effects of CO2, a greenhouse gas, on the environment aren't left unnoticed by the scientific community. Climate scientists have shown that atmospheric CO2 concentrations have been increasing over the years, and have attributed global warming to greenhouse gasses and their increase in concentration. Furthermore, climate scientist also show in a 2012 global CO2 emissions report that 40% of global emissions are linked to the generation of electricity and heat. This electricity is consumed in order to maintain energy demand set by population size, and by industrial sectors within each country. Some of the top consumers of energy are China, the United States, India, Russian Federation, Japan, Germany, Korea, Canada, Islamic Republic of Iran, and Saudi Arabia. [1,2]
In response to an increase demand in energy and emissions, energy policies and environmental protection agencies have emerged throughout the years. In conjunction, global collaboration on technology to improve the efficiency of existing technology, and develop low-carbon, carbon sequestration, and carbon reduction technologies have also been emerging. The basis of these technologies is to lower emissions by reducing CO2 to useful compounds. [2]
The conversion of CO2 to a higher value product is a chemically unfavorable reaction that won't start unless the energy barrier is lowered and outside energy is provided to start the reaction. In order to chemically reduce CO2, catalysts are utilized to lower the activation energy and make the conversion reaction kinetically favorable. An efficient catalyst is usually in a heterogeneous (multiphase) catalyst that favors CO2 binding and dissociation. If a catalyst is utilized, it lowers the required input energy and triggers a cascade of reduction reactions. The first catalytic step, the dissociation of CO2 to CO is the first kinetic limitation in CO2 reduction that catalysts help overcome. The activation energy for the non-catalytic conversion of CO to CO2 is -40 Kcal/mole at a temperature of 700°C. [3] However, if a catalyst such as Pt is present, the activation energy for creating CO2 is greater than -20 Kcal/mole at a temperature of 100°C. [3] The difference in temperatures and energy required to dissociate CO2 with and without a catalyst highlight how essential catalysts and supplementary energy are in lowering the activation barrier needed to dissociate CO2.
Current CO2 technologies utilize CO2 emitted during various stages production stages in power plants (i.e. SMR, natural gas cleanup, ethylene cracking). In order to determine which technologies are effective in capturing and utilizing CO2, various technologies will be compared in order to determine the strengths and weaknesses each technology possess. The technologies under investigation are electrochemical, thermochemical, and biochemical technologies.
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| Fig. 2: Different types of existing CSP technologies that concentrate solar energy onto a receiver. (Source: Wikimedia Commons) |
There currently exists technology to electrochemically convert CO2 into syngas which is a building block for a variety of synthetic fuels. However, this technology is currently primitive in many fields. The first is the input energy being expended to catalyze the electrochemical reduction of CO2 (ERC). In order for this technology to be justified as carbon-neutral, the input energy must not be derived from fossil-fuels. [1] In addition, if an abundant source of non-fossil fuel derived energy is found, finding a source of water to provide hydrogen is an additional burden. [1] Although water may seem abundant, places that produce or contain the most stranded gas (i.e. Africa, China) may lack access to a supply of pure water, and thus have smaller profit margins. [4] In spite of the tradeoffs, existing companies and researchers are striving to improve existing technologies.
If existing ERC technologies were to improve either by developing a catalyst with improved structure, composition, and kinetic properties, companies would be able to synthesize the world's capacity of commodity chemicals (~120 Mt/yr.), reduce the carbon impact associated with traditionally producing these chemicals, and remove 7-10% of any excess CO2 present. [1] The end result would be a chemical market that delivers carbon-neutral or carbon-negative chemicals at a large scale. [5] However, this status can only be reached if carbon-neutral source of energy (i.e. photovoltaics, windmills) is used to overcome the activation energy barrier. [6] (See Fig. 1)
Low conversion efficiencies associated with CO2 reduction and utilization of catalytic materials have limited electrochemical technologies. [7] In order to eliminate catalytic constraints, technologies have incorporated solar cavity-receiver reactors to utilize the sun's abundant source of thermal energy to facilitate the dissociation of CO2 and H2O. [7] This technology differs from electrochemical technologies since this technology relies on thermal energy to increase dissociation, while the dissociative force driving CO2 reaction depends on catalytic efficiencies and electricity input in electrochemical technologies.
Thermochemical technologies operating under realistic industrial operating conditions that incorporate metal oxides as a surface sties for CO2 and H2O dissociation have been proven effective. In this technology, the CO2 and H2O are dissociated to form syngas, a mixture of CO and H2. This mixture is then further processed to liquid fuels such as Methanol, Diesel, and Kerosene via the Fischer-Tropsch process. [8] Although current thermochemical conversion technologies report up to 36% efficiencies, more progress within the thermochemical conversion industry must be made in order to come up to speed to existing concentrated solar panel (CSP) technology (see Fig. 2), and to transition from the laboratory scale to the pilot-plant scale. [9]
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| Fig. 3: Algal open pond. (Source: Wikimedia Commons) |
Algae are photosynthetic organisms that have been proven to be useful in the development of biofuel. [10] Current biochemical technology utilizes genetically engineered algae to convert CO2 into energy-dense liquid fuels. [11] These algae are engineered to be highly prolific, withstand high temperatures, overproduce carbon rich compounds, and become tolerant of certain environmental conditions. [12] If these engineered algae were grown in optimum conditions they would capture up to 90% of the CO2 in comparison to other existing carbon-capture technologies. [11] Although these efficiencies are high and do not require any catalyst, the infrastructure that's required to maintain algal carbon-capture systems is troubling. (See Fig. 3)
In order to effectively capture 80% of carbon emissions produced from 200-megawatt-hour (MWh) natural gas-fired power plants, an algal pond of 3600 acres is needed, while a pond of 7000 acres is required to capture CO2 emissions from a 200-MWh coal-burning power plant during the day. [11][13] This poses a constraint to power plants striving towards reducing stranded gas emissions, since this requires the power facility to be next to a pond (depending on land availability) and for the power plant to divert its CO2 emissions to the pond, under the expectation that climate and sunlight are at optimum conditions for the algal pond. [11][13]
As the population size of the world increase, energy companies combust fossil fuels in order to meet the energy demand of the world's population. The combustion of fossil-fuels emits CO2 in the process which has been linked to global warming. In order to reduce CO2 emissions, researchers have developed technologies aiming at capturing and recycling CO2 into building blocks for commodity chemicals or synthetic fuels. These technologies can be grouped up into 3 different categories: electrochemical, thermochemical, and biochemical conversions. Electrochemical conversions are efficient in the sense that they are capable of reducing CO2 present if a non-hydrocarbon based energy source and novel catalyst are provided to catalyze the reaction. Although catalysts are incorporated to reduce CO2 emissions and drive reactions, more research and development must be conducted in order to produce industrial-scaled ERC systems. Thermochemical conversion technologies are effective in driving unfavorable stoichiometric reactions such as CO2 and H2O dissociation forward and producing syngas in the absence of a catalyst through solar thermal energy. However, the thermochemical conversion of CO2 becomes disadvantageous if sun's thermal energy is intermittent. Lastly, although the biochemical conversion of CO2 using photosynthetic organisms (i.e. algae) has proven itself to be an efficient manner of utilizing CO2 to produce fuel- precursor molecules, there are many biological constraints. Although these algal systems are genetically modified to become more tolerant to their surroundings or effectively utilize CO2, their efficiency is highly dependent on the amount of sunlight present, metabolic activity, and the environmental carrying capacity. All technologies contain strengths and weaknesses, but the determining factor for all technologies is highly dependent on the researcher or company's available resources. In order for these technologies to be fully improved and incorporated into existing energy infrastructure, the existing government must take action in creating an environment that fosters scientific creativity amongst researchers and motivates companies to reduce their carbon footprint for financial benefits.
© Joel Dominguez. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Aresta, Carbon Dioxide Recovery and Utilization (Springer, 2003)
[2] "CO2 Emissions From Fuel Combustion Highlights 2014," International Energy Agency, 2014.
[3] R. M. Heck, R. J. Farrauto and S. T. Gulati, Catalytic Air Pollution Control: Commercial Technology, 3rd Ed. (Wiley, 2009), pp. 5-6.
[4] M. Godec, "Acquisition and Development of Selected Cost Data for Saline Storage and Enhanced Oil Recovery (EOR) Operations," U.S. National Energy Technology Laboratory. DOE/NETL-2014/1658, May 2014.
[5] M. LaMonica, "A Cheaper Route to Making Chemicals from CO2," Technology Review, 12 Mar 14.
[6] D. R. Kauffman et al., "Efficient Electrochemical CO2 Conversion Powered by Renewable Energy," ACS Appl. Mater. Interfaces 7, 15626 (2015).
[7] W. C. Chueh et al., "High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria," Science 330, 1197 (2010).
[8] C. Chien-Hua and P. Howard, "Syngas Production by Thermochemical Conversion of CO2 and H20 Using a High-Temperature Heat Pipe Based Reactor," Proc. SPIE 8469, Solar Hydrogen and Nanotechnology VII, 846900, 5 Oct 12.
[9] A. Meier and A. Steinfeld, "Solar Thermochemical Production of Fuels," Adv. Sci. Tech. 74, 303 (2010).
[10] Y. Chisti, "Biodiesel from Microalgae," Biotechnol. Adv. 25, 294 (2007).
[11] R. Sayre, "Microalgae: The Potential for Carbon Capture," Bioscience 60, 722 (2010).
[12] A. A. Snow and V. H. Smith, "Genetically Engineered Algae for Biofuels: A Key Role for Ecologists," BioScience 62, 765 (2012).
[13] H. Herzog and D. Golomb, "Carbon Capture and Storage from Fossil Fuel Use," Encyclopedia of Energy, Volume 1, Article Number NRGY 00422 (Elsevier, 2004).
