Coal direct chemical looping combustion process: Design and operation of a 25-kWth sub-pilot unit

The Coal-Direct Chemical Looping (CDCL) process using iron-based oxygen carriers has been developed as a coal conversion process with in situ CO2 capture. The CDCL system cycles oxygen carriers between two reactors, a reducer reactor and a combustor reactor, in order to convert coal for electric power generation. The reducer reactor features a unique design of a gas–solid counter-current moving bed configuration to achieve the reduction of Fe2O3 particles to a mixture of Fe and FeO while converting the coal into CO2 and steam. The combustor reactor is a fluidized bed that oxidizes the reduced particles back to Fe2O3 with air. The oxidation of iron is an exothermic reaction and the heat can be recovered for electricity generation. In the riser, the particles are pneumatically transported back to the reducer. An integrated 25-kWth CDCL sub-pilot plant has been constructed and demonstrated, which is the first integrated chemical looping demonstration unit for the direct conversion of solid fuel with a circulating moving bed system. The design and operation experience of the 25 kWth CDCL sub-pilot unit are reported in this paper. Specifically, the design criteria and operation conditions of the CDCL reactor system are first discussed which is followed by the construction of the sub-pilot unit. The tests with metallurgical coke and sub-bituminous coal resulted in 81% and 97% carbon conversions, respectively. Both tests yielded CO2 purity greater than 99%, indicating the complete oxidation of volatile gases in the moving bed reducer. The gas analyses from the combustor outlet demonstrated a proper regeneration of oxygen carriers. A low CO2 concentration in the combustor also confirms that there was no unconverted carbon transfer to the combustor and hence eliminated the need for an additional carbon separation device such as a carbon stripper. The demonstration results from the sub-pilot system substantiate the process concept of the CDCL, which is capable of processing coal continuously with reactor in situ CO2 capture and the cyclic usage of oxygen carriers.