This gasifier type is also called open top, or stratified throatless. Here, the top is exposed to the atmosphere, and there is no constriction in the gasifier vessel because the walls are vertical. Figure 6.5 shows that a throatless design allows unrestricted movement of the biomass down the gasifier, which is not possible in the throated type shown in Figure 6.4. The absence of a throat avoids bridging or channeling. Open-core is another throatless design, but here air is not added from the middle as in other types of downdraft gasifiers. Air is drawn into the gasifier from the top by the suction created downstream of the gasifier. Such gasifiers are suitable for finer fuels—for example, lighter biomass such as rice husk.

The following are some of the shortcomings of a downdraft gasifier:

• It operates best on pelletized fuel instead of fine light biomass.

• The moisture in the fuel must not exceed 25%.

• A large amount of ash and dust remains in the product gas.

• As a result of its high exit temperature, it has a lower gasification temperature.