Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 310

8.12.2 Operational Considerations
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 12.
8.12.2.1 Fixed-Bed Gasifier
Charcoal particles become porous and finer during their residence in the gas-
ification 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.
Under steady state, the rate of drying, pyrolysis, or gasification at any
layer must be equal to the feed into the section. Otherwise, the conversion
zone will move either up or down in a moving-bed gasifier. Thus, dry fuel is
fed into an updraft gasifier designed for wet fuel, the pyrolysis zone may
travel upward faster, thus consuming the layer of fresh fuel above and lead-
ing to premature pyrolysis. The gas lost in this way may result in lower gasi-
fication efficiency.
On the other hand, if the fuel is more wet than designed, 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.
8.12.2.2 Fluidized-Bed Gasifier
The start-up of a fluidized-bed gasifier is similar to the start-up of a fluidized
bed combustor. The inert bed materials are preheated either by an overbed
burner or by burning gas in the bed. Once the bed reaches the ignition temper-
ature of the fuel, the feed is started. Combustion in bed is allowed to raise the
temperature. Once it reaches the required temperature, the air/oxidizer-to-fuel
ratio is slowly adjusted to switch from combustion to gasification mode.
One major problem with fluidized-bed gasifiers is the entrainment
(escape) of fine char with the product gas. The superficial velocity in a fluid-
ized bed is often sufficiently high to transport small and light char particles,
contributing to major carbon loss. A tall freeboard can reduce the problem,
but that has a cost penalty. Instead, most fluidized-bed gasifiers use a
cyclone and a recycle system to return the entrained char particles back to
the gasifier.