Solar fuels via two-step thermochemical redox cycles for power and fuel production
- Autores: Md. Azhar Uddin
- Directores de la Tesis: Jordi Llorca Piqué (dir. tes.), Massimo Santarelli (codir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2019
- Idioma: español
- Materias:
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- Resumen
With the issue of the rise of anthropogenic CO2, global warming and rise of the primary energy demand, strong measures for the energy transition and the diversification with renewables and existing fossil-based infrastructure are required. Also, carbon capture and utilization of CO2 would also be needed. In that sense, thermochemical redox cycles gain particular interest to produce synthetic fuels, which can be used for energy generation and production of chemicals. In a two-step redox cycles, metal oxides acts as oxygen carriers and undergo looping between two reactors. In the reduction reactor, metal oxide is reduced with release of oxygen (solar-thermal) or produces syngas (for fuel reduction) whereas, in oxidation, CO2/H2O splits for form syngas when in contact with the metal oxide. Ceria being readily available at large scale and due to its nature of undergoing reduction non-stoichiometrically at low temperature makes it a good candidate.
In the present thesis, a detailed investigation of thermochemical dissociation of CO2 and H2O considering solar thermal and fuel reduction with a focus on non-structured reactors is carried out.
For the solar-driven cycle, an assessment of counter-current flow moving bed reactors for reduction and oxidation is performed and a chemical looping (CL) unit is added to a 100 MW power plant. With an operating temperature of 1600oC and 10-7 bar pressure, a maximum power output of 12.9 MW with solar to electricity efficiency of 25.4% is calculated. This additional power would bring down the efficiency loss due to carbon capture from 11.3 to 6%. Even though a considerable efficiency is obtained on very optimistic operating conditions, it still requires a huge solar field. Economics revealed that with a carbon tax of $40/tone of CO2 the levelized cost of electricity (LCOE) achieved is 17.8 times higher than the existing market price (without carbon capture). If a higher carbon tax of 80$/MWh is considered that it would still be 6.28 times higher for a plant with a carbon tax.
As an alternative, methane-driven CL unit is integrated into a power plant to access the overall system efficiency and amount of efficiency regain after carbon capture. Since there exists no solid-state kinetic model in the literature for methane driven CO2/H2O splitting cycle, an experimental investigation was performed which revealed that an Avrami-Erofe’ev (AE3) model fit best to both oxidation and reduction, with activation energies of 283 kJ/mol and 59.7 kJ/mol, respectively. A comparative assessment was performed to investigate the influence of kinetics. A CL unit based on thermodynamics and kinetics (with moving bed reactors) were tested in a power plant. A drop of 20% in the efficiency of the CL unit was observed when the kinetic-based CL unit is considered. However, due to thermal balance within the system, a similar thermal efficiency of the overall plant was achieved as 50.9%. However, when the thermodynamic-based CL unit layout is considered there exists an excess heat which predicts the possibility of improving the efficiency. An economic assessment revealed a specific overnight capital cost of 2455$/kW, a levelized cost of CO2 savings of 96.25 $/tonneCO2, and a LCOE of 128.01 $/MWh. However, with a carbon tax of 6 $/tonneCO2, the LCOE would drop below 50 $/MWh.
The methane-driven CL unit is later integrated as an add-on unit to a polygeneration plant that produces electricity and dimethyl ether. The results showed that the plant can produce 103 MWe and 2.15 kg/s of DME with energy and exergy efficiency of 50% and 44%, respectively. The capital investment required for the plantis about $534 million. With the carbon tax of $40/tonne of CO2, a current DME price of $18/GJ and an electricity price of $50/MWh would be achieved.
Overall, the integration of the CL unit as an add-on unit to the power plant is more suitable than polygeneration with respect to the existing market price.
