|Fig. 1: Schematic of a carbon recycling using solar conversion on-site at a power plant. (Courtesy of the U.S. Department of Energy)|
We have been harvesting the energy stored in hydrocarbons through combustion for centuries now, emitting carbon dioxide in the process. The common consensus is that there is an overabundance of the product of this reaction, greenhouse gases in the atmosphere, and dwindling resource of reactant, naturally available hydrocarbons. Beyond the environmental consideration of carbon dioxide emission, several states including California are moving to a cap and trade carbon system where reducing emissions can have significant financial benefits for large scale emitters like power plants.
The simplest method to solve both of these issues is to reverse this combustion reaction and convert the carbon dioxide back into hydrocarbons. Of course this direction of the reaction requires a tremendous input of energy, which must come from somewhere other than burning hydrocarbons. Preferably this energy comes from a renewable source, like solar power, and thus the reverse combustion reaction can store the power of the sun into the C-H bonds of hydrocarbons and also reduce the global budget of carbon dioxide. The ideal setup proposed to maximize the positive effect of reverse combustion is retrofitting already existing hydrocarbon-fueled power plants that emit carbon dioxide. In this setup, effluent carbon dioxide from the exhaust would be captured and sent to another reactor, it would use electric energy from sunlight along with water (or hydrogen gas) and catalysts to drive the reverse combustion reaction. Finally the product hydrocarbons would be sent back to the plant to completely close the loop on the carbon cycle. The overall name for this process is "chemical carbon mitigation".
Many challenges are associated in making this concept become a reality. Uphill reactions such as this one require not only enough energy to drive the reaction but also specialized catalysts that are specific to the reactants and limit the amount of back or side reactions that can occur.
A typical coal-fired power plant emits ~2 lbm-CO2/kwh of electricity produced.  Current technology exists that can capture ~90% of the emitted CO2 using an amine-based solvent solution to absorb the CO2 in the flue gas with a stripper. There is ultimately a barrier to higher efficiencies with this approach since the energy that is used to capture and transport the CO2 comes from burning more fossil fuels which emits more CO2.  Therefore with the 90% collection, the plant will still emit .29 lbm-CO2/kwh.  The power plant would decrease overall efficiency since more hydrocarbons would have to be burned in order to produce the same amount of power output, but the reduction in CO2 emissions and recycling of carbon to avoid purchasing of upstream hydrocarbons could save more money in the long run.
There are two different approaches that are currently being investigated to convert the carbon dioxide into hydrocarbons using energy from the sun. In one approach, a solar- thermal cell is used to concentrate the sun's rays into a reaction cell where the temperature can be elevated high enough to drive the reverse combustion reaction.  In the other approach, a semiconductor material is used as one electrode in an electrochemical cell. The energy from the sun's photons is absorbed by the semiconducting material which creates the potential difference to drive the reverse combustion reaction.  In this approach the required temperature for dissociating the gas molecules is much lower. Both of these approaches can be fitted into the retrofitted facility process shown in Fig. 1.
The length of the hydrocarbon chain that forms from this reaction is determined by the type of catalyst used and the reactor process conditions. The current technology can make methane using equation (1), where x=1, with an efficiency of 10.2% with reference to the amount of incident solar energy compared to the change in enthalpy of formation of the methane produced.  The reaction above is a multi-step process in which both the CO2 and H2O must be split into their constituent atoms in order to reorganize into hydrocarbon products . One current approach is to split these molecules using a Zn/ZnO electrode that uses solar power to provide the driving force. 
|Solar Energy + x CO2 + (x+1) H2 O → CxH2x+2 + (1.5x + 0.5) O2||(1)|
Overall, more energy is being put into converting the carbon dioxide into hydrocarbons than can be extracted from the product fuel. However, the inputs to this reaction are nearly costless to the power plant: waste carbon dioxide, water and sunlight. The economic feasibility of this process will ultimately depend on the cost of raw hydrocarbon resources balanced against the cost of the capital equipment for the reverse combustion reaction and the potential savings from reduced carbon dioxide emissions. Private companies such as Liquid Light as well as national laboratory projects such as Sunshine to Petrol (S2P) are trying to make this become a reality on a large scale. [3,4] While this does nothing to reduce carbon dioxide that is already in the atmosphere or reduce emissions from downstream burning of fossil fuels, such as transportation, this process can be added to the existing infrastructure of energy producing plants and significantly reduce the future greenhouse emissions from this major player.
© Michael Maas. 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.
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