Defluidization of ilmenite, an oxygen carrier for chemical looping combustion – laboratory experiments with potassium salts

Use of biomass in Chemical Looping Combustion (CLC) and subsequent CO2 capture ideally leads to negative CO2 emissions (Bio-CLC).  In Bio-CLC ash-forming matter of biomass, mainly consisting of potassium, calcium, sulfur, phosphorus, and chlorine, may interact with the oxygen carrier, causing agglomeration, and thus inhibit the chemical looping process. The detailed/exact mechanisms behind these effects are not properly understood. Ilmenite, an iron-titanium mineral, can be used as an oxygen carrier in CLC.In this study the interactions between ilmenite and potassium compounds, typical for biofuel ashes, in a labscale furnace were investigated. Mixtures of ilmenite with different potassium compounds were thermally treated in an oxidizing environment at 850 and 950 °C. The interactions between potassium compounds and the carrier material were studied using DTA, XRD, and SEM-EDS. Thermodynamic equilibrium calculations were carried out to verify the reactions.  Finally, results from the laboratory tests were used to explain the behavior of ilmenite in the presence of salts in a lab-scale fluidized bed conversion. The bed agglomeration mechanisms depend on the potassium salt: dihydrogen phosphate or chloride glued the particles, whereas carbonate reacted with the bed particles.