Large-Scale Recycling of Carbon: Carbon Dioxide as Source of Carbon

Carbon dioxide can be used as building block for chemicals or source of carbon for energy products (fuels). The latter have a market that is over fifteen times that of chemicals and are, thus, of greater interest for the large-scale implementation of the CCU strategy (capture and utilization of CO2). The drawback is that while the chemical use of CO2 may not require neither an energy input nor hydrogen, the conversion of CO2 into fuels requires both energy and hydrogen that cannot be derived from fossil-C. The solution is the use of C-free primary energy sources for powering the reduction process and water as source of hydrogen or, even better, of protons and electrons. Noteworthy, the reduction of CO2 may either use H2 or, alternatively, may be implemented avoiding the production and use of molecular hydrogen, mimicking nature that uses the proton-coupled to electron transfer-PCET strategy. In this chapter, the putative technologies that may promote a carbon cyclic economy (CCE) are discussed together with their readiness (on the short- to long-term) and degree of innovation. Technical barriers to the scale-up of electrolyzers for H2 production are discussed, considering the existing options: AWE, PME, HTWE. Routes to avoiding H2 production (either the electrolysis of CO2 in water or the photochemical- or photo-electro-chemical co-processing of H2O and CO2) are analyzed. The direct use of solar energy is considered both at ambient temperature (co-processing of CO2 and H2O under solar irradiation), or at high temperature (processes based on the use of solar power concentrators-SPC). The analysis of the TRL of the various technologies concludes the discussion. A deep technological innovation that may improve the efficiency in the production and use of e-fuels or solar fuels is necessary for a real reduction of the impact of the chemical and energy sectors on climate change. Whether the substitution of H2 for C-based fuels is a real benefit for climate is questioned.