Circulating Fluidized-Bed (CFB) Gasifier
In the circulating fluidized-bed (CFB) gasifier, air/O2 and H2O mixture is entered from the bottom while the biomass is added from the side of the gasifier similar to BFB, but higher gas velocities are used in this reactor compared to the BFB reactor. At higher gas velocities, part of the bed material gets entrained with the fuel and gets circulated inside the reactor. Representative gas velocity in the circulating fluidized-bed gasifiers are between 3 and 10 m/s. The entrained bed material, which is not completely converted fuel particles or char, are removed from the synthesis gas produced by a cyclone — type separation device. These particles are normally returned back to the bottom of the gasifier as shown in the CFB reactor plan in Figure 11.3.
Some of the most important features in circulating fluidized-bed (CFB) design are:
1. High fuel flexibility in terms of both size and type
2. Flexibility of operation at loads lower than design load
3. Ease of operation
4. Low feedstock inventory
5. Good temperature control and high reaction rates
6. In-bed catalytic processing possible
7. Production of syngas with moderate tar levels but high particulates
8. High carbon conversion
9. Good gas-solid contact and mixing
10. Suitable for large-scale capacities (up to 1MW or even higher)
11. High conversion efficiency
12. Very good scale-up potential