Gas and coal (and biomass)-fired power stations can respond to changes in demand more readily than many other sources of electricity production, hence the importance of retaining them as an option in the energy mix (Boot-Handford et al., 2014).
Various technologies for the capture of CO2 from fossil fuelfired power plants are available. The four different capture methods are post-combustion, pre-combustion, oxy-combustion, and chemical looping combustion. Each technology has its own advantages and disadvantages and is at different stages of development (Spigarelli and Komar Kawatra, 2013). Recently, molten carbonates are emerging as challenging electrolytes for CO2 capture (Cassir et al., 2012). In effect, molten carbonate fuel cells (MCFCs) can be used as CO2 separator and concentrator while producing electricity from hydrocarbons fuels. According to calculations and some tests, using MCFC for carbon capture, the resulting SPECCA (Specific, Primary Energy Consumption for CO2 avoided), is about 0.5 MJ kg CO2, which is well below the value obtained with conventional post-combustion removal based on amines (Suguira et al., 2003; Cassir et al., 2012).
A Large Panel of Transformation Media
In 2010, 12 carbon dioxide valorization recovery methods were identified by the Alcimed Company, whose activity lies at the interface of science and marketing (Figure 1).
CO2 can be valorized with three main routes. The “No transformation” route will not be described in this review as it is out of the scope of this article. With a “Chemical CO2 conversion” route, it is technically possible to use CO2 as a carbon source for the synthesis of commodity products, from simple CO to liquid fuels and high-molecular-weight polymers (Hu et al., 2013). The electrochemical deposition route will be more detailed being our principal focus.
Biologic Transformation
Microalgae
Microalgae, due to their photosynthetic activity, could be used to capture and valorize the CO2. Microalgae have the ability of fixing CO2 directly from waste streams such as flue gas as well as using nitrogen from the gas as a nutrient. Before microalgae can be converted into fuels, the biomass content has to be harvested and dried. The conversion can be carried out through thermochemical or biochemical conversion. The products synthesized depend on the algae or bacteria variety chosen and also on the operating conditions. Pharmaceutical industry already uses this route to produce food supplements (spirulina, β-carotene, etc.). Biofuels (biodiesel precursors, ethanol) are emerging. The US start-up SOLIX[1] has initiated commercialization of biofuel from CO2 proceeding from combustion smokes.
Biocatalysis
It is the use of biochemical agents, as enzymes, to stimulate a chemical reaction and, thus, obtain products by using less drastic operating conditions than in heterogeneous or homogenous catalysis. Inspired by nature, a large body of research has been devoted to new materials catalyzing CO2 functionalization in either electrochemical or chemical ways. In 2009, the US start-up Carbon Science[2] has produced liquid fuels thanks to a prototype. The demonstration prototype uses Carbon Science innovative biocatalytic process to break down CO2 and water and then combines carbon and hydrogen to form ethanol, a liquid fuel. Enzymes from microorganisms transform CO2 and H2O to methane, methanol, and butanol. CO2 can be captured by a biologic agent and then electroconverted to bicarbonate and formate (Karthikeyan et al., 2014).
Chemical Transformation
Energetic Value Product
Photonic energetic power source route
In the photonic energetic power source route, carbon dioxide reacts with photogenerated electrons in order to create an anion radical as intermediate species (Hu et al., 2013). Photoelectrochemical reduction of CO2 requires photoelectrocatalysts combining a
