Examination of oxygen uncoupling behaviour and reactivity towards methane for manganese silicate oxygen carriers in chemical-looping combustion

Cheap and environmental friendly Mn-Si oxygen carriers manufactured from Mn3O4 and SiO2 by spray-drying have been investigated with respect to properties for chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU). Fifteen oxygen carriers with SiO2 content varying from 2 wt% to 75 wt% were prepared and calcined at 1050 °C and 1150 °C. The ability of material to release O2 and their reactivity towards CH4 were examined in the temperature range 900-1100 °C. Particles with a SiO2 content of more than 45 wt% and calcined at 1150 °C showed limited CLOU behaviour and poor reactivity towards CH4 at all temperatures investigated. The rest of the materials had significant CLOU properties and provided high conversion of CH4 under the experimental conditions chosen. Increasing the temperature of operation enhanced the CLOU behaviour and reactivity towards CH4. At temperatures above 950 °C, the CH4 conversion was 90-100% for these materials. Crystalline phases identified by XRD in the oxidized samples with more than 45 wt% SiO2 and calcined at 1150 °C were mainly rhodonite MnSiO3. For materials with SiO2 content below 45 wt%, braunite Mn7SiO12 was detected as the main phase in most of the samples after oxidation. This indicates that braunite Mn7SiO12 is the main active phase for oxygen transfer in CLC and CLOU, which is supported by thermodynamic calculations. The reactivity of all of the materials were also studied with syngas (50% CO and 50% H2), showing complete gas conversion at 950 °C, except for materials with a SiO2 content of more than 45 wt% and calcined at 1150 °C. The mechanical integrity and attrition resistance of the oxygen carriers were examined in a jet-cup attrition rig, and although the attrition rates varied, some reactive material showed low rates of attrition, making them very promising oxygen carrier materials for applications related to CLC and CLOU. However, measures should probably be taken to improve the crushing strength to some extent.