A large number of operational issues confront a biomass gasifier. Universal to all gasifier types are problems related to biomass handling and feeding. Bridging of biomass over the exit of a hopper is common for plants that use low-shape-factor (flaky) biomass such as leaves and rice husk. This problem is discussed in more detail in Chapter 8.

Fixed-Bed Gasifier

Charcoal particles become porous and finer during their time in the gasification zone. Thus, in a downdraft gasifier, when fine charcoal drops into the ash pit, the product gas can easily carry the particles as dust. Escaping particles can be a source of carbon loss, and they often plug downstream equipment.

The movement of solids in any layer of a moving-bed gasifier should be equal to the feed rate of the fuel at the top. Even with that balance, if the fuel is dry, the pyrolysis zone may, in an updraft gasifier, travel upward faster, thus consuming the layer of fresh fuel above and leading to premature pyrolysis. The gas lost in this way may result in lower gasification efficiency.

On the other hand, if the fuel is moist, its pyrolysis may be delayed. This may move the pyrolysis zone downward. In the extreme case, the cooler pyroly­sis zone may sink sufficiently to extinguish the gasification and combustion reaction. Clearly, a proper balance of rates of fuel flow and air flow is required for stabilization of each of these zones in respective places.