Understanding the Interaction of Potassium Salts with an Ilmenite Oxygen Carrier Under Dry and Wet Conditions

Abstract

This study describes how potassium salts representative of those in bio ash affect the reactivity of the oxygen carrier ilmenite under moist and dry conditions. Ilmenite is a bench-mark oxygen carrier for chemical-looping combustion, a technique that can separate CO 2 from flue gases with minimal energy penalty. Different potassium salts were mixed with ilmenite to a concentration of 4 wt % potassium. The salts used were K 2 CO 3 , K 2 SO 4 , KCl, and KH 2 PO 4 . Experiments were performed at 850 °C under alternately oxidizing and reducing conditions in a dry atmosphere or in the presence of steam. Analyses of the oxygen carrier regarding changes in reactivity, structure, and composition followed the exposures. This study showed that salts such as K 2 CO 3 , K 2 SO 4 , and KCl increase the reactivity of the ilmenite. For the samples mixed with KCl, most of the salt was evaporated. KH 2 PO 4 decomposed into KPO 3 , forming layers around the ilmenite particles that lead to agglomeration. Additionally, the KPO 3 layer was more or less nonpermeable for CO and decreased the reactivity toward H 2 significantly in both dry and wet conditions. This decreased reactivity indicates that the concentration of phosphorus in biofuel may have a significant effect on oxygen carrier degradation. It was also observed that the presence of steam changed the chemistry drastically for the nonphosphorus-containing salts. Alkali salts may react with steam, forming volatile KOH that evaporates partly. KOH may also form K-titanates by reaction with the oxygen carrier, leading to segregation of iron and titanium phases in the ilmenite.

Effect of the Mass Conversion Degree of an Oxygen Carrier on Char Conversion and Its Implication for Chemical Looping Gasification

“…The same phenomenon was also seen inside the red circle. Ilmenite has been previously reported to be able to interact with potassium, where iron in reduced ilmenite migrated to the outer layer and reacted with potassium to form K titanate. − …”

Thermodynamic Analysis on the Fate of Ash Elements in Chemical Looping Combustion of Solid Fuels─Iron-Based Oxygen Carriers

“…19,28 Other studies have found that, with increased time of exposure, K migrates into the ilmenite particle core 27 and that, depending on the chemical form of K, it will react differently in the matrix. 29 Research conducted on the ash layer build-up and sulfur interaction when using ilmenite in OCAC applications shows that calcium and potassium are bound to sulfur both in and on the ilmenite particles. 27,30 Limited research around the chemistry of heavy metals in chemical looping processes is available.…”

“…It is not uncommon to combine experimental tests with thermodynamic predictions. 29,36,37 Most recent studies have used FactSage and associated databases to monitor the slag formation and predict agglomeration behavior in the CLC of coal. 38,39 It was concluded that parameters that affect the bed composition are the ash composition, the ratio of ash to OC, and the type of OC, which had a significant impact on the rate of agglomeration.…”

Effects of Temperature, Operation Mode, and Steam Concentration on Alkali Release in Chemical Looping Conversion of Biomass─Experimental Investigation in a 10 kWth Pilot

“…The majority of published research on the effects of alkalis in CLC has been carried out in lab-scale investigations and has been primarily focused on the interaction of alkali-containing fuel ash components with the oxygen carriers. 24 − 27 Two pilot-scale investigations in continuous CLC systems have looked at effects of alkalis on oxygen carriers, 28 , 29 but only a few studies have attempted to directly measure and determine alkali release to the gas phase and alkali retention in the bed material. 30 − 32 …”

“…In fluidized bed biomass boilers, alkalis that are retained in the bed are known to cause bed material agglomeration that can result in operational upsets and unplanned boiler shutdowns. , The release of alkali compounds in the conventional combustion, gasification, and pyrolysis context has been studied in full scale in biomass boilers − and gasifiers, − in pilot-scale reactors, , at the lab scale, − and through modeling work. , In CLC, the implications of alkali release from biomass fuel conversion are not yet well-explored. The majority of published research on the effects of alkalis in CLC has been carried out in lab-scale investigations and has been primarily focused on the interaction of alkali-containing fuel ash components with the oxygen carriers. − Two pilot-scale investigations in continuous CLC systems have looked at effects of alkalis on oxygen carriers, , but only a few studies have attempted to directly measure and determine alkali release to the gas phase and alkali retention in the bed material. − …”