Investigation of different manganese ores as oxygen carriers in chemical-looping combustion (CLC) for solid fuels

Abstract

The chemical-looping combustion (CLC) process is a novel solution for efficient combustion with direct capture of carbon dioxide. The process uses a metal oxide as an oxygen carrier to transfer oxygen from an air to a fuel reactor, where the fuel reacts with the solid oxygen carrier. In this work, six different manganese ores are investigated as oxygen carriers for CLC application. The chemical-looping characteristics of the oxygen carriers were evaluated in a laboratory-scale fluidized-bed reactor in the temperature range of 900–970°C during alternating reducing and oxidizing conditions. Three of the manganese ores showed a small oxygen release in inert environment between 850 and 950°C. During reactivity tests, the gas yield with methane increased with the temperature and complete conversion of 50% CO in H2 was obtained for all of the ores. The rates of char gasification of two fuels, namely Mexican petroleum coke and Swedish wood char, were compared for the different manganese ores at 970°C and with 50% H2O in N2 as fluidizing gas. Ilmenite and a manufactured Mn-oxide oxygen carrier consisting of Mn3O4 and MgO-stabilized ZrO2 as support were also included for comparison. The char gasification rate and the gas conversion were higher with the manganese ores and the Mn-oxide oxygen carrier compared to ilmenite. However, the higher reactivity of the manganese ores with H2 and the ensuing decrease in H2 inhibition for manganese ores is not sufficient to explain their higher rate of char gasification. Surface analysis of partially gasified petcoke particles in the presence of manganese ores showed formation of cavities and channels as well as a uniform distribution of potassium and sodium elements. The rate of char gasification also increased with the concentration of potassium and sodium impurities in the manganese ores. Thus the results suggest that the increased rate of char conversion for manganese ores is due to alkali-catalyzed steam gasification. The increase in rate of char gasification, in combination with potentially low costs of these materials suggests that manganese ores could be interesting materials for CLC with solid fuels.