A Solar Thermochemical Fuel Production System Integrated With Fossil Fuel Heat Recuperation

Significance Statement

Thermochemical cycling is a technology of converting solar energy and storing it using synthesized chemical fuels by utilizing concentrated solar heat alone to drive endothermic chemical reactions. Thermochemical cycle involves two steps, these are thermochemical cycling of ceria CeO2 and splitting of H2O or CO2 to H2 or CO. In ceria-H2O and CO­2 splitting processes, oxygen is first released when using solar energy to reduce metal oxide at a high temperature TRE. The reduced metal oxide is re-oxidized with water or CO2 at a low temperature TOX yielding H2 or CO.

Researchers led by Professor Yong Hao of the Institute of Engineering Thermophysics at the Chinese Academy of Science with his students Hui Kong and Hongsheng Wang carried out a research on solar thermochemical with a view of integrating it with fossil fuel to end the intermittency of solar energy and also produce a cleaner energy source. To avoid using fossil fuel, the researchers used thermochemical process that involves concentrating the solar heat. The study is published in Applied Thermal Engineering.

Ceria-based thermochemical cycling delivers a stable fuel production at temperature of about 16000C due to the absence of phase change of ceria and relatively low tendency of sintering of ceria microstructure under typical conditions. The team also cited the inability of the practical solar-driven thermochemical cycling to meet up to the theoretical proven solar-to-fuel efficiency of solar driven thermochemical cycling.

The scientists reported that high efficiency of heat recovery is easier to achieve with gases than with solids. This led the team to isothermal thermochemical cycling which has the possibility of making simple highly efficient solar thermochemical cycling reactor that convert H2O or CO2 to fuel. In improving the efficiency of isothermal thermochemical cycling, the research team suggested effective heat recovery and energy cost reduction. In separation of H2 from H2O, it involves no additional energy due to condensation while that of separation of CO from CO2 requires extra energy to obtain pure CO.

Methane was integrated to generate a fossil fuel heat, the combination of methane with the mixture of CO to H2 and CO2. The composition of the gas is given in term of ratio CH/CO2. The reaction of carbon dioxide reforming CDR occurs easily due to the partial pressure of CO2 downstream the isothermal reactor. The authors reported that syngas efficiency of newly proposed system for both CO2 and H2O splitting are higher than the efficiency of isothermal thermochemical cycling without heat recovery. The rational use of waste from T0 to TH after 17000C, the solar-to-syngas efficiency of isothermal CO2 splitting system integrated with methane begins to decrease. The specific CO2 emission CO2 and H2O co-splitting isothermal thermochemical cycling process integrated with methane, shows increase in H2O/CO2 ratio with corresponding increase in CO2 emission.

When solar thermochemical polygeneration system was integrated with methane, the scientists observed simultaneous power and methanol production which can also eliminate self-generated power plant. Polygeneration is found to consist of four parts, they are isothermal splitting of CO2 and H2O, methane reforming, methanol synthesis and combined cycle for power generation.

This study showed heat and unreacted gas considered to be waste can now be recovered into useful energy. Isothermal thermochemical cycling has shown potential to produce high quality syngas and lower the temperature of isothermal oxidation products to 600-8500C where heat recuperation technology is matured.

Journal Reference

Hui Kong1,2, Yong Hao2, Hongsheng Wang1,2, A solar thermochemical fuel production system integrated with fossil fuel heat recuperation, Applied Thermal Engineering 108 (2016) 958–966

Show Affiliations
  1. University of Chinese Academy of Sciences, No. 19A Yuquan Rd, Beijing 100049, PR China
  2. Institute of Engineering Thermophysics, Chinese Academy of Sciences, 11 Beisihuanxi Rd., Beijing 100190, PR China

 

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