Bubbling Fluidized-Bed (BFB) Gasifier
The bubbling fluidized-bed (BFB) gasifier is the simplest fluidized — bed gasifier, where biomass is normally fed in or above the fluidized bed. The bed material is fluidized by a gas (air or an oxygen steam mixture) entering the gasifier through nozzles distributed over the bottom of the reactor. Combustion of part of syngas and/ or the oxidation of char produced provide the energy required for heating the biomass and the endothermic gasification process. Gas velocity in this type of gasifier is typically around 1 m/s.
Bed material plays an important role in bubbling fluidized-bed and other fluidized-bed reactors, and sometimes catalytic materials like transition metal salts are added to the bed material to improve yields and product profile. These catalysts are added to the bed material to promote char gasification, water-gas-shift and steam reforming reactions, and reduce tar yield. There is a great interest in in-bed catalytic additives, as these catalysts can improve the quality of the syngas. Consequently, the use and need for more complex and expensive downstream cleaning methods can be simplified [9].
Natural minerals such as dolomite, limestone, olivine and iron ores, and synthetic minerals, Ni-supported olivine, Fe-supported olivine, alkali metal-based material, and even char can be used as catalytic materials. Char in the reactor can act as a tar cracking material as well, however, as char itself gets converted during the process, an external continuous supply of char into the gasifier is required [9]. In addition to this, sorbents such as limestone can retain sulfur compounds like H2S and COS further simplifying downstream gas cleaning steps. Even though several in-bed catalysts have shown good results for improving the quality of the synthesis gas, some of them are quite expensive and a number of these catalysts leave contaminated residues behind. Additionally, there have been erosion problems and loss of catalytic activity. For example, Ni-based catalysts are quickly deactivated due to carbon deposition and sulfur poisoning [9]. Therefore, development of good quality and inexpensive catalytic bed materials remains a high priority research area.
In one study where bed materials impregnated with different catalysts have been compared, inert quartzite was used as a reference case, olivine, dolomite as natural catalysts, and nickel-alumina as the artificial catalyst [10]. In this experiment gasifications were carried out on a bubbling fluidized bed and it was found that artificial catalyst has the highest effectiveness in enhancing the hydrogen yield as well as in tar reduction. A stable activity of the nickel-alumina catalyst was observed for the whole duration of the reaction, suggesting that no deactivation phenomena occurred due to coke deposition or morphological modifications of the particles [10].
There are a number of important features in bubbling fluidized — bed (BFB) design, which include:
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. Good gas-solid contact and mixing
7. In-bed catalytic processing possible
8. Production of syngas with moderate high heat value (HHV) but low tar levels and high particulates
9. Carbon loss with ash
10. High conversion efficiency
11. Suitable for large-scale capacities (up to 1MW or even higher)