Evaluation of Manganese Minerals for Chemical Looping Combustion

The use of mineral materials as oxygen carriers for Chemical Looping Combustion (CLC) is an attractive option due to their low cost. This paper reports an experimental study of four manganese minerals as potential oxygen carriers focusing on the behavior in CLC as well as in Chemical Looping with Oxygen Uncoupling (CLOU). Experiments were carried out in a thermogravimetric apparatus (TGA) and a fluidized-bed reactor. Repeated tests with all the minerals showed no sufficient CLOU properties. Then, they can only be used in CLC applications involving the redox pairs Mn3O4/MnO and Fe2O3/Fe3O4. Oxygen transport capacity and reactivity to the main fuel gases (H-2, CO, and CH4) were determined in a TGA. Manganese minerals were also tested during 35-54 h in a fluidized-bed reactor to evaluate the evolution of reactivity to H-2, CO, and CH4 as well as their attrition rate and mechanical strength. The reactivity of the materials decreased in the first 10 cycles with CH4 and then became quite stable for the rest of the cycles performed with CH4, CO, and H-2. In comparison to previously tested Fe-based minerals, lower reactivity with CH4 was found for the manganese minerals. However, in terms of CO and H-2 combustion, their reactivity was adequately high. During the tests, agglomeration and defluidization were never found for any material. Mechanical crushing strength of the particles decreased with cycles, which led to the increase of attrition rate of some materials above acceptable levels. Nevertheless, materials with adequate crushing strength and low attrition were identified. Combining the reactivity and attrition resistance, the manganese minerals MnSA and MnGBHNE from South Africa and Gabon, respectively, can be suggested as promising materials for CLC with coal.