Oxygen Carrier and Alkali Interaction in Chemical Looping Combustion: Case Study Using a Braunite Mn Ore and Charcoal Impregnated with K2CO3 or Na2CO3

Alkali is a problematic component in biomass and may create various operation issues in normal combustion as well as chemical looping combustion using biomass fuels (bio-CLC). To investigate the interaction of alkali with an oxygen carrier, a methodology has been developed where alkali salts are added with impregnated charcoal particles. This work studies the effect of K2CO3 and Na2CO3 on the fluidization/agglomeration behavior and reactivity as well as the interaction of a braunite manganese ore oxygen carrier with K and Na in a batch fluidized bed reactor. Charcoal impregnated with K2CO3 (K-charcoal) and charcoal impregnated with Na2CO3 (Na-charcoal) were used as solid fuels in the reduction step of the simulated CLC cycles. CH4 and syngas (50% CO + 50% H2) were periodically used to evaluate the reactivity of braunite before and after solid fuel experiments. In total, more than 50 cycles were performed for both K-charcoal series and Na-charcoal series tests, while some additional cycles with nonimpregnated charcoal were conducted and considered as a reference. Partial agglomeration and partial defluidization were found after cycles with K-charcoal and Na-charcoal, and the use of K-charcoal tends to lead to the partial agglomeration/defluidization faster than the use of Na-charcoal. K, Na, Si, and Ca were found at a higher concentration on the surface of the agglomerated particles and can be assumed to be responsible for the partial agglomeration. The partial agglomeration with K-charcoal happened likely as a result of surface melting of the braunite particles, whereas the formation of the low-melting-point Na-Si-Ca system could be responsible for agglomeration in the Na-charcoal experiments. The concentration of K and Na in the braunite bed was found to increase during cycles with the alkali charcoals. In total, the added masses of K and Na were 0.8 and 1.2% of the bed, and around 40 and 80% of added K and Na were found, respectively, in the used oxygen carrier particles. Although partial agglomeration and accumulation were observed in the presence of these alkalis, the reactivity of used braunite was scarcely changed in comparison to the fresh sample.

Automated summary

Alkali is a problematic component in biomass combustion and chemical looping combustion using biomass fuels. This study investigated the interaction between alkali and an oxygen carrier by adding alkali salts to impregnated charcoal particles. The experiments showed that using alkali-impregnated charcoal led to partial agglomeration and defluidization, with the use of K-charcoal resulting in faster agglomeration/defluidization compared to Na-charcoal. Partial agglomeration could be due to surface melting of the braunite particles, while the formation of a low-melting-point Na-Si-Ca system could be responsible for agglomeration in Na-charcoal experiments. The concentration of alkali in the braunite bed increased with the use of alkali charcoals, but the reactivity of the used braunite was hardly affected.