Chemical Looping Co-combustion of Sewage Sludge and Zhundong Coal with Natural Hematite as the Oxygen Carrier

Abstract

Co-combustion of sewage sludge (SS) and Zhundong (ZD) coal can be an attractive way of disposing SS and using ZD coal. Chemical looping combustion (CLC) technology is an alternative solution applied to this co-combustion with capturing carbon dioxide and minimizing gaseous pollutants, especially NOx. The performance of chemical looping co-combustion of SS and ZD coal (SZ) was investigated in a 1 kWth continuous CLC reactor using natural hematite as the oxygen carrier in this work. The influences of the temperature in the fuel reactor (800–930 °C) were studied. The carbon conversion efficiency, carbonaceous gas conversion efficiency, and carbon capture efficiency for SZ increased with the increase of the FR temperature. In comparison to dewatered SS, SZ could obtain higher carbonaceous gas conversion efficiency among the temperature range. Although SZ obtained lower carbon conversion efficiency and carbon capture efficiency, both efficiencies of SZ reached an appropriate value at a high temperature and were higher than these for single ZD coal. The carbonaceous gas conversion efficiency and carbon conversion efficiency of SZ reached 91.3 and 86.5%, respectively, at 930 °C. After 5 h of operation, the reacted oxygen carrier showed similar reactivity compared to the fresh oxygen carrier, indicating that the hematite oxygen carrier possessed good long-term reactivity during the co-combustion process. Besides, although ZD coal had a higher content of alkali metal, sodium, there was no melting on the hematite and ash agglomeration occurred during continuous long time operation, which could be ascribed to the reduction of the sodium content in the ash and the generation of alkali metal compounds with high-temperature melting points, sodium aluminosilicate and sodium pyrophosphate aluminum.