The DCFB concept
The introduction of the DCFB concept is based on two basic statements regarding chemical looping processes:
Bubbling fluidized bed (BFB) fuel reactors, as they have been proposed, suffer from the risk of gas bypass through the bubble phase. Significant slip of unconverted fuel, however, is hardly tolerable in CLC. The gas slip can be minimized by low fluidization numbers in the BFB and sufficient bed height. However, this will result in relatively large fuel reactor bed cross section areas and high solids inventories. In the practically particle-free freeboard of BFBs, no relevant reactions can be expected for CLC because the oxygen donating solids are missing. A turbulent or fast fluidization regime in the fuel reactor, on the other hand, allows gas-solids contact over the whole height of the reactor and potentially allows operation with lower solids inventories which is especially relevant at increased plant capacities. Another important feature of the DCFB concept is the inherent stabilization of solids hold-up obtained by the direct hydraulic link between the two CFB systems, i.e. the loop seal connection in the bottom region of the risers. Only the air reactor entrainment is responsible for the global solids circulation between the two reactors while the fuel reactor operation can be optimized with respect to maximum fuel conversion.
The main design parameters are shown in Table 1. The air reactor is operated as a fast fluidized bed, fluidized with cold or preheated air and transfers the solids via a cyclone separator and a loop seal to the fuel reactor. The fuel reactor is fluidized with a gaseous fuel (CH4, H2, CO) and operates close to turbulent regime. The entrained solids are separated in a cyclone separator and transferred back into the system. The oxygen carrier particles are flowing back into the air reactor through a loop seal connection, linking the bottom regions of the two reactors. The depleted air stream from the air reactor and the flue gas stream from the fuel reactor are cooled and analyzed to evaluate the conversion of the fuel as well as the leakages of the loop seals. The solids circulation rate between air and fuel reactor is controlled by a staged air injection in the air reactor.
Tobias Pröll Dr. techn. M.Sc