Conversions of CO2 of 65.3% with yields of formic acid of 40% were obtained by CO2 hydrothermal reduction in the frame of the project AQUA-CO2NV in High Pressure Process of the University of Valladolid. The process consists of introducing CO2 in water at temperatures above 250 ºC and at pressures over the saturation pressure of water, in order to keep water as a liquid. Metals such as Zn, Fe, Al or Ni acts as reduction agents, and they oxidized in the process. Thus, AQUA-CO2NV process is similar to photosynthesis in which plants transform CO2 into organics, but to achieve CO2 transformation, AQUA-CO2NV process uses the energy of water at high pressures and temperatures instead of solar light. This process has the advantage of avoiding the use of gaseous hydrogen that implies technical and safety issues. In addition, it is favored by the supply of CO2 absorbed in a basic solution such as bicarbonate. The absorption of CO2 in basic solutions is a well-known technology developed at industrial scale and easy to use in power plants. Thus, hydrothermal CO2 reduction has advantage of being easily integrated with conventional thermal power plants.
The objective of AQUA-CO2NV project is to develop a continuous CO2 hydrothermal reduction process easily scalable and susceptible to be commercialized. Our goal in the first stage of the project is to evaluate chemical pathways and kinetics. Currently, we are studying the kinetics of transformation of CO2 into formic acid using Zn as a metal reducing agent:
Zn + CO2 + H2o –> HCOOH + ZnO
With experiments carried out in batch reactors, conversions of bicarbonate of 65.3% have been achieved with yields of formic acid up to 40%, at temperatures of 275ºC and residence times of 90 min. Other products found in the liquid phase are methanol, ethanol and acetic acid.
Daniel Román González – Project ENE2014-53459-R, “AQUA-CO2NV” funded by MINECO