Iron-Based Coal Direct Chemical Looping Combustion Process: 200-h Continuous Operation of a 25-kW th Subpilot Unit

The coal direct chemical looping (CDCL) combustion process using an iron-based oxygen carrier has been developed and demonstrated in a 25-kW(th) subpilot unit. The CDCL subpilot unit is the first chemical looping demonstration unit with a circulating moving bed for the solid fuel conversions. To date, the CDCL subpilot unit at OSU has been operated for more than 550 h. The feasibility of the subpilot unit with various types of solid fuels including sub-bituminous coal and lignite coal has been tested. This article discusses the operational experience of a successful 200-h integrated, continuous demonstration with sub-bituminous coal and lignite coal. Throughout the 200-h continuous operation, the CDCL subpilot unit showed steady behavior in terms of solid circulation, coal handling, and oxygen carrier reactivity and recyclability. Tests with both coals confirmed more than 90% coal conversion with 99.5 vol % purity of CO2 achieved in the reducer. The sound design of the reducer allowed for nearly full coal conversion with a high purity of CO2, eliminating the need for additional downstream fuel polishing and separation units. The combustor gas contained lean oxygen concentrations with minute amounts of carbonaceous gases (CO2, CO, and CH4) detected. The combustor gas analysis implied the proper regeneration of iron-based oxygen carriers, good gas sealing between the reducer and combustor, and no indication of unconverted carbon carry-over. Moreover, the fates of coal pollutants such as NOx and SOx that are commonly observed in the conventional coal combustion process were also investigated during the subpilot unit operation. The NOx analysis showed that the CDCL process is capable of significantly reducing NOx emissions by avoiding thermal NO formation. The sulfur analysis indicated SO2 generation in both reducer and combustor, agreeing with the sulfur chemistry in the CDCL scheme.