Manganese ores as oxygen carriers for chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU)

Manganese-based oxygen carriers can be used in chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU). This paper expands on a previous investigation into manganese ores as oxygen carriers for CLC. The paper presents reactivity data for eight ores with widely different compositions. Most importantly, the ores contained significant quantities of Fe, Si, and Ca, with Fe/Mn and Si/Mn molar ratios that varied between 0.1-4.5 and 0.07-0.48, respectively. Pre-treatment of the ores consisted of heat treatment in air at 950 °C for 24 h followed by sieving to obtain a particle size fraction of 125-180 μm. Reactivity towards methane and synthesis gas (50 vol% CO in H2) and the oxygen uncoupling behaviour at 900, 950, and 1000 °C was investigated in a fluidized batch reactor. Material characterization of the ores included an attrition test, crushing strength, and BET surface area measurement. The oxygen carriers had a CO2 yield of 31-80% for syngas with 2 g of oxygen carriers using 450 ml/min of syngas. The performance for all ores as a function of number of cycles was stable. By using a pseudo-first-order rate constant, apparent activation energies were calculated at ω = 0.999 and 0.99 for methane and syngas, respectively, and the apparent activation energies varied in a wide range, from 110 to 328 kJ/mol for CH4 and 10 to 168 kJ/mol for CO. The attrition resistance of the particles was measured using a jet cup attrition rig. The findings indicate that a higher reactivity is associated with higher attrition, however, there is no direct correlation between the two.