(686d) A Dual Circulating Fluidized Bed (DCFB) System for Chemical Looping Processes

For chemical looping combustion or chemical looping reforming processes with selective oxygen transport by the bed material, the following basic requirements can be stated:

a) High global solids circulation is required in order to provide enough oxygen in the fuel reactor and in order to keep the temperature difference between air reactor and fuel reactor low.

b) Excellent gas-solids contact is required in both reactors in order to obtain satisfactory gas conversion. This is especially true for the fuel reactor of chemical looping combustion systems where significant amounts of unconverted fuel will hardly be tolerable in applications.

c) Low particle attrition rates are appreciated especially if costly oxygen carriers are to be used.

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 as long as the lower loop seal is designed large enough to not significantly hinder solids flow. Imposing moderate pressure differences between the two reactors changes the theoretical solids levels in the system. This can be done by changing the backpressure from the exhaust gas lines and allows active control of the solids hold-up in each reactor.

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 (air reactor height: 4 m, fuel reactor height: 3 m, inner diameter both reactors: 0.15 m).

First, the fluid dynamics of the system are studied using a scaled cold flow model of the pilot plant. It is shown that the primary reactor defines the global solids circulation rate and that the fluid dynamic regime in the secondary reactor may be adjusted to the requirements of the chemical reactions without influencing the global solids loop.

The hot laboratory unit has been built and operated with both natural minerals and designed synthetic oxygen carrier particles. The total particle inventory is 60-80 kg. A cooling system using boiling water/steam in natural circulation at ambient pressure is used to optionally cool the air reactor and to cool the gas streams downstream of the reactor system. Experimental results show satisfactory fuel conversion rates in spite of the limited riser heights.