Researchers at the National Center for Scientific Research (CNRS), France, are studying a solar thermochemical process for the recycling and upgrading of CO2 emissions for the production of synthetic fuels. In a paper published online in the ACS journal Energy & Fuels, they describe results from a series of proof-of-concept experiments.
Their approach consists of splitting carbon dioxide to form carbon monoxide and oxygen in two distinct steps. Zn- and SnO-rich nanopowders were first synthesized in a high-temperature solar chemical reactor via the thermal dissociation of ZnO or SnO2; the produced nanoparticles then react efficiently with CO2, which generates CO and the initial metal oxide that can be recycled.
The concentrated solar energy provides the requisite high temperature process heat. The metal oxides (ZnO/Zn and SnO2/Sn), although reacting in each individual reaction, are not consumed in the overall chemical-looping process because of its recycling, and thus, it can be considered as a catalyst for the CO2-splitting reaction.
This conversion offers the following advantages: (1) production of CO from a waste flue gas with a higher energy potential than CO2, (2) recycling of CO2 streams emitted from polluting industries, thus avoiding CO2 emissions, (3) storage of solar energy into chemical fuel in an amount equal to the enthalpy change of the reaction (ΔH° = 283 kJ/mol), (4) synthesis of either hydrogen (from water-gas shift, CO + H2O → CO2 + H2) or even liquid fuels, such as methanol, when CO is combined with H2, enabling H2 storage, and (5) production of various types of synthetic fuels, including Fischer-Tropsch chemicals, from only CO2, H2O, and solar energy input, thus achieving a solar-driven reverse combustion and saving fossil fuels.—Abanades and Chambon
This reactor was operated in a controlled atmosphere (N2 or Ar flow) at a reduced pressure (about 20 kPa, 0.2 bar) and a reaction temperature of about 1600 °C.
They found that the produced nanopowders are more reactive with CO2 than standard commercial powders. Zn can be oxidized by CO2 from 360 °C with both high reaction rates and final chemical conversions of greater than 90%. The CO2 dissociation with SnO requires higher temperatures (about 800 °C), and reaction rates are lower than for Zn. They also found that the influence of the amount of CO2 was also significant, because the reaction rates increased with the CO2 mole fraction.
A series of proof-of-concept experiments on CO2 splitting were performed in a fixed-bed reactor laden with solar produced Zn powder. The almost complete chemical conversions obtained in a few seconds suggest that this common reactor technology could be suitable for implementing the solid/gas reaction at a large scale.—Abanades and Chambon
Stéphane Abanades and Marc Chambon (2010)CO2 Dissociation and Upgrading from Two-Step Solar Thermochemical Processes Based on ZnO/Zn and SnO2/SnO Redox Pairs. Energy Fuels, Article ASAP doi: 10.1021/ef101092u