Potassium migration and transformation during the deep reduction of oxygen carrier (OC) by char in coal-direct chemical looping hydrogen generation using potassium-modified Fe2O3/Al2O3 OC

•The loss mechanism of K2CO3 during the deep reduction of oxygen carrier by char.•The occurrences mode of potassium in char and oxygen carrier residues.•K content variations and distributions in char and oxygen carrier residues.•The transformation of K2CO3 to K-Fe-O composites could avoid the loss of K2CO3. Coal-direct chemical looping hydrogen generation (CLHG) is an innovative technology for high-purity hydrogen production and in situ CO2 capture. The rate-limiting process of the technology is the deep reduction of OC by char, and the addition of K2CO3 to the OC could significantly enhance the reaction rate. Unfortunately, the transformation and migration of K in the reaction may cause the loss of K2CO3. In this work, the K content variations and distributions in the reduction process were investigated in a fixed-bed reactor at 900 °C. The K species in char and OC residues were determined. The results show that K loaded on OC could migrate to the char, and with the reaction prolonged the K in char residues could partially transfer back. After the reduction process, the K in OC and char residues, and the volatile K were 90%, 5.2% and 4.8%, respectively. The possible K species migrating between OC and char were K2CO3. K2CO3 in OC could transform to K-Fe-O composites, which could avoid the K2CO3 loss. The possible K-Fe-O composites were K2Fe22O34, K2Fe10O16 in the initial stage and transformed to KFeO2 finally. A small part of KFeO2 could be regenerated to K2CO3 by char. The K species in char residues were in the form of K-aluminosilicates, K2CO3 and organic-K during the reduction process. After the reduction process, about 3.5% of K in char residues was water-soluble and could be recovered. 1.7% of K was water-insoluble and difficult to be regenerated.