Assessment of the improvement of chemical looping combustion of coal by using a manganese ore as oxygen carrier

Finding suitable low-cost materials to be used as oxygen carrier in Chemical Looping Combustion (CLC) of coal is a key issue to achieve the CO2 capture at low economic cost. Recently, a Mn-ore from Gabon has been identified as an alternative to the state of the art of oxygen carriers based on minerals or wastes with high iron content. This Mn-ore showed a high reactivity and a long particle lifetime during batch characterization to be considered as a suitable oxygen carrier. To evaluate the potential of this material in CLC, this work analyses the behaviour of the Mn-ore during continuous combustion of a bituminous coal in a 0.5 kWth CLC unit. The CLC process was evaluated and the effect of the main operating variables - such as fluidizing medium, oxygen carrier circulation rate, temperature, and solids inventory in the fuel reactor - on the combustion efficiency and CO2 capture was investigated. A direct relation between the char conversion rate and the CO2 capture is given, being mainly affected by the mean residence time of solids and temperature in the fuel reactor. The use of a carbon separation system with separation efficiency above 90% would be required to achieve CO2 capture rates higher than 95%. Total oxygen demand values as low as 4.5% were found when optimal operating conditions were selected, mainly being related to oxygen carrier to fuel ration higher than phi > 3. At these conditions, the Mn-ore material showed similar combustion efficiencies than other Fe-based low-cost materials previously tested, but with higher CO2 capture rates.