**3.3 Entrained flow gasifiers**

gasifying agent gets maximum at this position due to the decreasing crosssectional area of the orifice. As a result of this structure, the combustion increased sharply at the throttle while the amount of feeding agents is still. Downdraft gasifiers have higher conversion yield than that of their updraft models [41]. Syngas from downdraft gasifiers have much less tar and

*(a) an updraft gasifier converting rice husk to syngas, (b) the inside wall of the top opening is clogged with a*

incomplete decomposed substances because they have to pass the combustion zone before exit with the syngas. However, downdraft gasifiers cannot be scaled up easily due to difficulties in controlling the movement of solid fuels through the throttle. Another difficulty in designing and fabricating downdraft gasifiers is "bridging problems" for feedstock with low densities [42]. The downward flow of the solid fuel is dictated by gravity while the pyrolysis zone is right above the narrow throat. The melting and adhesivity of lignin in biomass, as well as the local condensation of volatile substances, also facilitate the formation of stiff domes above the device throat, blocking the coming feedstock. It was observed that a rice husk downdraft gasifier kept stop working within some minutes of operation due to this problem and it was not an easy job to remove the bridging dome of "melting" rice husk inside the

• Crossdraft gasifiers (**Figure 3c**): as an intermediate between downdraft and updraft design, crossdraft gasifiers has the simplest design when biomass is fed from the top, gasifying agent from the rear side, and syngas is withdrawn from the other rear side of the reactor. Thanks to this arrangement, the pyrolysis zone is separated from reduction zone, where syngas is obtained, and between them is the combustion zone to reduce tar and soot. Bridging problem is not a

Fluidization is an advance technique for solid fuel combustion. It is also applied for gasification. Inert materials (sand, dolomite, crushed stone, etc.) are employed to hold fluidization. The gasifying agents enter the reactor from the bottom upward to the top at velocities of 1–3 m/s through the biomass + inert material bed. Gasification reactions occur inside the bed then the resulted gases drag the particle before going up like "bubbling". This technique provides the mixture a uniformity for heat

equipment (**Figure 5**).

**Figure 4.**

*thick layer of condensed tar and soot.*

*Biotechnological Applications of Biomass*

**3.2 Fluidized bed gasifiers**

**554**

concern in this case, and scaling up is feasible.

Entrained flow gasifiers are applied for biomass with small particle sizes so that the specific contact area with gasifying agents is large enough for suitable reaction rate. Simply described as illustrated in **Figure 6a**, the solid and the gas agents are fed co-currently to the reactor in the same downward direction. The agent surrounds the solid particles and react to convert the biomass to syngas. At the end of the falling routine to the bottom of the reactor of the feed, only ash and slag are expected to be remained solid collected in cyclone systems while syngas is passing through. The operation is carried out at high temperature and in high pressure. The

**Figure 6.** *(a) Entrained flow gasifier, (b) rotary drum gasifier.*

extremely turbulent flow of the aerosol mixture causes rapid conversion and allows high throughput [46].

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