Batch fluidized bed study of the interaction between alkali impurities and braunite oxygen carrier in chemical looping combustion

Other conference contribution, 2022

Chemical looping combustion (CLC) is a novel technology for heat and power generation with low-penalty CO2 capture. Using biomass in CLC (bio-CLC), negative CO2 emission can be attained. Alkali (mainly K and Na) in biomass can be problematic in bio-CLC, as it can interact with the oxygen carrier bed. The current work used charcoal and four charcoal samples impregnated with K2CO3, Na2CO3, KCl and NaCl, respectively, to study alkali interaction with a low-alkali braunite manganese ore oxygen carrier. The experiments were successfully carried out at 950°C in a quartz batch fluidized-bed reactor. For each alkali-fuel sample, more than 30 cycles of redox were performed. Using the solid fuel impregnated with K2CO3, Na2CO3, KCl and NaCl, char gasification was improved by a factor of 10, 8, 4 and 3 as compared to the non-impregnated fuel. Partial-defluidization of the braunite particles was found with all the alkali-fuels, although the extent differed, e.g. K2CO3 and KCl resulted in earlier onset of partial defluidization than Na2CO3 and NaCl. Hard agglomeration was never observed, while soft partial agglomeration was seen. Accumulation of K, Si, and Ca in agglomerates and particle boundary was found after cycles with K2CO3- and KCl-charcoal, while Na, Si and Ca was found after the Na2CO3- and NaCl-charcoal cycles. The mechanism of agglomeration formation seems different for these alkali-charcoals. For K2CO3- and KCl-charcoal, it seems the potassium reacted with Fe and Mn in the braunite, forming low-melting point components and thus led to agglomeration. In the case of Na2CO3- and NaCl-charcoal, direct reaction with the braunite was not seen, and it seems as if other reactive species combined were formed, which acted as a binder between particles to form agglomerates. In addition, after cycles with the K2CO3- and Na2CO3- charcoals, 80% K and 40% Na were retained in the oxygen carrier particles. After the use with all the alkalis, the braunite reactivity with CH4, CO and H2 was similar to the fresh particles. It is clear that alkali species could react with the braunite oxygen carriers, and this could affect reactivity and fluidization tendency in the long run. Still, only soft agglomerates and partial defluidization were found, which may not be the case in a real CLC system operating at higher fluidizing velocities.