Performance of Combined Manganese–Silicon Oxygen Carriers and Effects of Including Titanium

Combined oxides of manganese and silicon have earlier been identified as suitable oxygen carriers for chemical-looping combustion. In this study, one pure manganese-silicon oxide and one similar material with titanium included in the formulation have been examined as oxygen carriers. Experiments studying the oxygen release and the reactivity with syngas, methane and wood char have been carried out in a bench-scale circulating chemical-looping combustor and in a batch fluidized-bed reactor in the temperature range 800-1050°C. Both oxygen carriers released oxygen in inert atmosphere and the concentration of oxygen released increased with temperature. The conversion of syngas and methane also increased with temperature for both materials and in both experimental setups. The reactivity with devolatilized wood char showed that the rate of oxygen uncoupling increased with temperature. However, it could be concluded that the main fuel conversion mechanism was CLC and not CLOU for these materials. The inclusion of titanium in the manganese-silicon combined oxide significantly affected the physical properties of the oxygen carrier particles. The MnSi particles could only be operated for 7 h in the bench-scale circulating chemical-looping combustor before the circulation was disrupted due to the large fines formation. The MnSiTi particles were operated for 24 h in the circulating unit without any circulation disruption. It was concluded that it is possible to greatly decrease the attrition rate of the particles by including titanium in the formulation. However, the inclusion of titanium lowered the reactivity with fuel. As the thermodynamic properties are very similar for the two oxide systems, the reduced reactivity is most probably an effect of the lower porosity of MnSiTi. This emphasizes the importance of optimizing the physical structure of the oxygen carrier particles. The physical structure of the particles was found to be greatly affected by the inclusion of titanium, giving, for example, a higher resistance to attrition. The physical structure of the particles is important for the fuel conversion as well, as it will likely have implications on the internal diffusion in the particles.