Effect of co-precipitation and impregnation on K-decorated Fe2O3/Al2O3 oxygen carrier in Chemical Looping Combustion of bituminous coal

Abstract

Chemical Looping Combustion (CLC) of coal is an innovative combustion technology for CO2 inherent capture, which uses oxygen carrier (OC) to transfer lattice oxygen to coal. However, coal gasification is the rate-limiting process for CLC of coal. Although carbon conversion and gasification rate are substantially improved with oxygen carriers in the presence of alkali additives, alkali loss in oxygen carrier is still a serious problem in the process of CLC of coal. The present work focuses on the OC–potassium interaction for two preparation methods of K-decorated OC. Different contents of catalyst (K2CO3) were added into the preparation of Fe-based oxygen carrier by both co-precipitation and impregnation. And the effect of K-decorated methods on oxygen carriers was investigated in a fluidized bed reactor. For co-precipitated oxygen carriers, CO2 gas yields (fCO2) were higher and CO gas yields (fCO) were lower than the ones for impregnated oxygen carriers. Total carbon conversions for co-precipitated oxygen carriers were also higher than those for impregnated oxygen carriers, and a shorter time of fast reaction stage always corresponded to co-precipitated oxygen carrier. 9 redox cycles were also conducted to investigate the stability of oxygen carrier reactivity and the potassium loss with cycle number. It was concluded that the reactivity of co-precipitated oxygen carriers was more stable during 9 redox cycles. Especially for K10-cp (oxygen carriers of 10% potassium content by co-precipitation), fCO2 increased slightly and fCO changed very little with cycle number. According to morphological features of oxygen carriers, co-precipitation was superior to impregnation in preventing oxygen carrier particle sintering. On the other hand, although potassium contents of all K-decorated oxygen carriers decreased with cycle number, the potassium loss for co-precipitated oxygen carriers was smaller. K10-cp performed the best characteristic in avoiding potassium loss: the potassium content decreased from 8.47% (fresh oxygen carrier) to 7.79% (after 9 cycles). X-ray diffractometer (XRD) analysis showed that the potassium ferrite, K2Fe22O34, was presented in K-decorated oxygen carriers. Based on the peak intensity ratio of IK2Fe22O34/IFe2O3, the higher the content of K2Fe22O34 in K-decorated Fe2O3/Al2O3 oxygen carrier was, the larger the conversion of CO to CO2 was, and the shorter the time of fast reaction stage was. Therefore, it was inferred that K2Fe22O34 acted as a support with catalytic effect. In addition, the content of K2Fe22O34 was higher in co-precipitated oxygen carriers than that in impregnation oxygen carriers. It can be used to explain that why a better reactivity was always found for co-precipitated oxygen carriers.