Study of the interaction between a Mn ore and alkali chlorides in chemical looping combustion

Abstract

Chemical looping combustion (CLC) is a novel technology for heat and power generation with inherent CO2 capture. Using biomass in CLC (bio-CLC), negative CO2 emissions can be attained. Biomass usually contains high content of alkalis (mainly K and Na) which can be problematic in the process, such as potential alkali-bed interaction, and this is the focus of current work. This work uses charcoal with and without the impregnation with alkali chlorides, KCl and NaCl. The results are compared to previous data from samples impregnated with K2CO3 and Na2CO3. A low-alkali braunite manganese ore is used as bed material to study the oxygen carrier interaction with the alkalis in cyclic experiments at 950 °C in a quartz batch fluidized-bed reactor. As compared to charcoal without alkali impregnation, the impregnation with KCl, NaCl, K2CO3, and Na2CO3 can improve the rate of gasification by a factor of 4, 3, 10, 8, respectively. 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 defluidization than Na2CO3 and NaCl. Further, indications of partial defluidization were earlier and more permanent with the carbonates than the chlorides. Partial agglomeration with soft agglomerates of the bed was observed, while hard agglomerations were never seen. Accumulation of K, Na, Si, and Ca was found in the agglomerates after cycles with K2CO3-charcoal and Na2CO3-charcoal, while little K and Na was detected in the bridges between particles after the KCl and NaCl cycles. A significant fraction of the alkali added was found in the oxygen carrier, with 80% or more being retained for the Na salts, and around 40% for the K salts. There was no clear difference between chlorides and carbonates with respect to retention. The fresh and used braunite have very similar reactivity with CH4 and H2, whereas some decrease in reactivity is noticed with CO.