DFT study of the reaction mechanism of CuO–char in chemical-looping combustion

Abstract

Understanding the reaction mechanism of CuO and char is crucial for developing chemical looping combustion of carbon-containing solid fuels. Density functional theory is used to study the chemical looping combustion mechanism of CuO. The three reaction processes of Cu₄O₄ cluster oxygen release, O₂-char, and Cu₄O₄ cluster-char are described in detail. The energies of paths are compared and verify that the solid–solid direct reaction mechanism between CuO and char is more favorable than the oxygen uncoupling mechanism, and this proves the results of previous experiments. The best possible reaction path for the solid–solid direct reaction is proposed. The agglomerated dioxygen and char form unstable intermediates; this increases the energy span of the reaction path and hinders the oxidation process of char. Cu can easily dissociate O₂ into singlet oxygen and transfer this oxygen to char in a lower energy single oxygen transfer mode to form a CO₂ precursor. This may be the essence of the catalytic effect of Cu in the chemical looping combustion reaction. The solid–solid direct reaction mechanism and the catalysis of metals should receive more attention in chemical looping combustion.