Evaluation of Different Oxygen Carriers for Chemical Looping Reforming of Toluene as Tar Model Compound in Biomass Gasification Gas: A Thermodynamic Analysis

A thermodynamic study on a toluene chemical looping reforming process with six metal oxides was conducted to evaluate the product distribution for selecting an appropriate oxygen carrier with thermodynamic favorability towards high syngas yield. The results show that a suitable operation temperature for most oxygen carriers is 900 °C considering syngas selectivity and solid C formation whether the toluene is fed alone or together with fuel gas. The syngas selectivity of all oxygen carriers decreases with the increasing equivalence ratio, but the decrease degrees are quite different due to their different thermodynamic natures. With the increasing amounts of H2 and CO, the syngas selectivity for various oxygen carriers correspondingly decreases. The addition of CO2 and H2O(g) benefits reducing the solid C formation, whereas the addition of CH4 leads to more solid C being produced. Under the simulated gasification gas atmosphere, a synergetic elimination of solid C and water–gas shift reactions are observed. In terms of syngas selectivity, Mn2O3 possesses the best performance, followed by CaFe2O4 and Fe2O3, but NiO and CuO exhibit the lowest performance. BaFe2O4 presents a high H2 selectivity but a very poor CO selectivity due to the formation of BaCO3, which has a high thermodynamic stability below 1200 °C. Nevertheless, Mn2O3 is more likely to form solid C than feeding toluene alone and has a lower melting point. Considering syngas selectivity, carbon deposit and melting point, CaFe2O4 exhibits the highest performance concerning the tar chemical looping.