A Dual Circulating Fluidized Bed (dcfb) System for Chemical Looping Processes

A fluidized bed system combining two circulating fluidized bed reactors is proposed and investigated for chemical looping combustion using metal oxides as oxygen carriers. The global solids loop starts in one of the two reactors (primary reactor) where solids are entrained, separated from the gas in a cyclone and sent to the other reactor (secondary reactor) via a fluidized loop seal. From the secondary reactor, the solids are flowing back into the primary reactor via a second loop seal connecting the bottom regions of the two reactors. The secondary reactor features a circulation loop in itself (secondary reactor cyclone and loop seal) and may be optimized with respect to good gas-solid contact and low particle attrition. The global circulation rate can be effectively controlled by staged fluidization of the primary reactor. The direct hydraulic communication of the two circulating fluidized bed reactors via the lower loop seal allows stable solids distribution in the system. A 120 kW fuel power laboratory unit has been designed for chemical looping combustion of natural gas. The air reactor, where oxygen carrier particles are oxidized, is designed as the primary reactor. The fuel reactor, where the oxygen carrier serves to oxidize gaseous fuel, is designed as the secondary reactor. The reactor dimensions have been determined considering fast fluidization in the air reactor and turbulent fluidization in the fuel reactor. Experimental results with a NiO-based oxygen carrier show that fuel conversion increases with operating temperature. For the investigated system, the CO2 yield from CH4 is 0.94 at 950 °C (1223 K). It is expected that fuel conversion can be further improved by increasing the height of the riser reactors.