Reduction Kinetics of Cu-, Ni-, and Fe-Based Oxygen Carriers Using Syngas (CO + H2) for Chemical-Looping Combustion

The reactivity of three Cu-, Fe-, and Ni-based oxygen carriers to be used in a chemical-looping combustion (CLC) system using syngas as fuel has been analyzed. The oxygen carriers exhibited high reactivity during reduction with fuel gases present in syngas (H2 and CO), with average values in the range 8−30% min-1. No effect of the gas products (H2O, CO2) on the reduction reaction rate was detected. The kinetic parameters of reaction with H2 and CO have been determined by thermogravimetric analysis. The grain model with spherical or platelike geometry in the grain was used for the kinetic determination, in which the chemical reaction controlled the global reaction rate. The activation energies determined for these reactions were low, with values ranging from 14 to 33 kJ mol-1. The reaction order depended on the reacting gas, and values from 0.5 to 1 were found. Moreover, the reactivity of the oxygen carriers when both H2 and CO are simultaneously present in the reacting gases has been analyzed, both at atmospheric and pressurized conditions. For the Cu- and Fe-based oxygen carriers, the reaction rate of the oxygen carrier with syngas corresponded to the addition of the reaction rates for the individual fuel gases, H2 and CO. For the Ni-based oxygen carrier, the reaction rate was that corresponding to the fuel gas that reacted faster with the oxygen carrier at the reacting conditions (fuel concentration, temperature, and pressure). The consequences of the behavior of the reaction of syngas and the water-gas shift (WGS) equilibrium on the design of the fuel reactor of a CLC system have been analyzed. A preliminary estimation of the solids inventory for the use of syngas in the fuel reactor of a CLC system gave values in the range of 19−34 kg MW-1 when the WGS equilibrium was considered to be instantaneous.