System integration and optimization of copper-chlorine thermochemical cycle with various options for hydrogen production

Abstract

The Copper-Chlorine (Cu-Cl) thermochemical water splitting cycle is one of the most attractive alternative thermochemical cycles for clean hydrogen production due to its lower temperature requirement and better overall efficiency. CuCl electrolysis is considered a key process in the Cu-Cl cycle of hydrogen production where H2 gas is produced by oxidation of CuCl particles dissolved in concentrated HCl solution. A lower electrochemical cell voltage than water electrolysis is a significant advantage of CuCl electrolysis and makes this process attractive for hydrogen production. Nevertheless, an integration of both hydrolysis and electrolysis processes is one of the most important engineering challenges associated with the Cu-Cl cycle of hydrogen production. The kinetics of the hydrolysis reaction indicates the reversibility of this process. This requires H2O in excess of the stoichiometric quantity which significantly decreases the overall thermal efficiency of the Cu-Cl cycle. Moreover, the HCl concentration in the produced gas mixture of H2O and HCl in the hydrolysis reaction is in much lower concentration of the electrolysis reaction requirement for an effective electrolytic cell performance. In this PhD thesis, an integrated process model of the hydrolysis and electrolysis processes is simulated by introducing intermediate heat recovery steam generator (HRSG) and HCl-H2O separation process consisting of rectification and absorption columns. In the separation processes, the influence of operating parameters including reflux ratio, mole fraction of HCl in the feed stream, solvent flow rate and temperature, and column configuration variables, such as the location of feed stage and number of stages on the heat duty requirements and the composition of products are investigated and analyzed. It is shown that the amount of steam generated in the HRSG unit satisfies the extra steam requirement of the hydrolysis reaction up to 14 times more than its stoichiometric value and the separation process effectively provides HCl acid up to the concentration of 22 mol% for the electrolysis reaction. In order to achieve an effective integration of the electrolysis process with hydrolysis and decomposition reactions of the Cu-Cl cycle, a lab-scale CuCl electrolysis unit is designed, fabricated and tested. The influences of operational factors on the cell performance are then investigated. In the experiments, the effects of operating parameters, including HCl and CuCl concentrations, applied current density, temperature and solution flow rate on the cell potential and hydrogen production rate are experimentally investigated and analyzed. A fractional factorial design is performed, based on design of experiment methods, to find a correlation between cell voltage and operation factors. The present model predicts the effects of various operating variables on the cell voltage to provide new insight into an integration of the electrolysis process. A close agreement of the measured and theoretical hydrogen production rate confirms the accuracy of measurements and reliability of the experimental studies. An innovative integration of gasification process and Cu-Cl cycle, which can effectively contribute to hydrogen production with higher efficiency and lower environmental impact, is also studied and evaluated. In this study, the effects of using oxygen instead of air in the gasification process, where it is produced and supplied by the integrated Cu-Cl cycle is investigated. It is shown that using oxygen instead of air in the gasification process increases the gasification temperature and helps to eliminate NOx emissions. It is demonstrated that increasing the equivalence ratio (ER) from 0.1 to 0.4 improves the gasification exergy efficiency by about 10%. The influence of ER on the iv syngas composition is also studied. The gasification products rely on specific syngas compositions and could potentially provide a precursor to the combined cycle for power generation in an Integrated Gasification Combined Cycle (IGCC) power plant. The process model of a gasification process is simulated based on the industrial Texaco IGCC plant in which the heat of syngas cooling process is utilized to supply extra steam requirement of the hydrolysis reaction in the Cu-Cl cycle. The effects of steam recovery in the hydrolysis reaction on energy and exergy efficiencies of the Cu-Cl cycle are analyzed and discussed.