Combustion performance and sodium absorption of ZhunDong coal in a CLC process with hematite oxygen carrier

Chemical looping combustion (CLC) is considered to be a superior method for converting coal into energy with less pollution. Zhundong coal (ZD) is readily available in China but is characterized by high sodium content. This paper shows that, with the presence of silica and alumina in the hematite, CLC with hematite as oxygen carrier (OC) is an attractive alternative technology for ZD conversion without severe slagging and fouling problems. In this work, a series of experiments on the CLC with ZD were conducted in a fluidized bed reactor. At first, the combustion performance of ZD in CLC was compared to those of three other types of Chinese coal. CLC of ZD was a superior high-sodium coal conversion technology. High content of sodium in ZD gave rise to an increase of carbon conversion and accelerated the carbon conversion process. In addition, the effect of three gasification agents, i.e. nitrogen, carbon dioxide or steam, on sodium absorption in CLC with hematite OC was investigated. The absorption reaction between hematite and sodium was more effective in CLC of ZD under steam atmosphere as compared to nitrogen or carbon dioxide. At last, 20 redox cycles were also conducted to investigate the stability of hematite OC reactivity and sodium transfer with cycle number. Sodium content in hematite OC increased during the initial 12 cycles, resulting in improved porous properties. However, excessive sodium content resulted in sintering on the surface of hematite OC. The porous properties started to deteriorate sharply after 20 cycles. (C) 2015 Elsevier Ltd. All rights reserved.