Entrained-Flow Gasifier
The entrained-flow gasifier is a downdraft type of direct gasifier, where the biomass feedstock, steam, and oxygen or air is introduced at the top of the gasifier. The basic design of this gasifier is shown in Figure 11.4. Within this gasifier, high temperature, pressure, extremely turbulent gas and fuel flow causes a rapid biomass to syngas conversion, achieving a high throughput. Relatively higher temperatures are involved in this type of reactor compared to other designs, and this may shorten the life of reactor system components. Also, it may be necessary to add fluxes or blend feedstock to achieve good slagging characteristics in entrained flow gasifiers.
The use of entrained flow gasifiers with biomass is a relatively new development, and there are reports on testing this type of reactor on wood, straw, and dried lignin [12]. Hernandez et al. have studied [13] the effect of the addition of steam to air as gasifying agent in biomass entrained flow gasification. The entrained flow gasifier can be seen as a promising technology due to its commercial availability, high efficiency and high potential for the production of biofuels and chemicals from biomass.
Several research groups have recently studied the performance of entrained flow gasifiers using different biomass forms such as wood powder [14], raw and torrefied bamboo [15], oil palm residue [16], and coir dust [17]. In one study Hernandez and coworkers used deal — coholized marc of grape as fuel in the entrained flow gasification. In these experiments they found that a higher temperature increases the CO and H2 content in the product gas for air gasification, whereas air-steam gasification leads to a boost in the H2 production at higher temperatures, as well as an increase in the CH4 content [18].
Some important features of entrained flow 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