Gasification Theory - Biomass Gasification, Pyrolysis and Torrefaction - page 214

The carbon dioxide concentration in the rising gas increases rapidly in
the combustion zone, but once the oxygen is nearly depleted, the CO 2 enters
the gasification reaction ( Eq. (7.26) ) with char, resulting in a decline in CO 2
concentration in the gasification zone.

C 1 CO 2 - 2CO 1 172 kJ = mol

(7.26)

C 1 H 2 O - CO 1 H 2 1 131 kJ = mol

Sensible heat of the hot gas provides the heat for the two endothermic
gasification reactions R1 and R2 ( Table 7.2 )in Eq. (7.26) . These reactions
are responsible for most of the gasification products like hydrogen and car-
bon monoxide. Because of their endothermic nature, the temperature of the
gas reduces.
The zone above the gasification zone is for the pyrolysis of biomass. The
residual heat of the rising hot gas heats up the dry biomass, descending from
above. The biomass then decomposes (pyrolyzed) into noncondensable
gases, condensable gases, and char. Both gases move up while the solid char
descends with other solids.
The topmost zone dries the fresh biomass fed into it using the balance
enthalpy of the hot product gas coming from the bottom. This gas is a mix-
ture of gasification and pyrolysis products.
In a downdraft gasifier, biomass fed from the top descends, while air
injected meets with the pyrolysis product, releasing heat (Figure 7.5).
Thereafter, both product gas and solids (char and ash) move down in the
downdraft gasifier. Here, a part of the pyrolysis gas may burn above the gas-
ification zone. Thus, the thermal energy required for drying, pyrolysis, and
gasification is supplied by the combustion of pyrolysis gas. This phenome-
non is called flaming pyrolysis.
In downdraft gasifiers, the reaction regions are different from those for
updraft gasifiers. Here, steam and oxygen or air is fed into a lower section of
the gasifier ( Figure 7.5 ) but biomass is fed at the top. The pyrolysis and
combustion products flow downward. The hot gas then moves downward
over the remaining hot char, where gasification takes place. Such an arrange-
ment results in tar-free but low-energy-content gases.

7.3.6.2 Fluidized-Bed Reactor
Unlike other types of reactors, a fluidized-bed gasifier contains nonfuel gran-
ular solids (bed solids) that act as a heat carrier and mixer. In a bubbling flu-
idized bed, the fuel fed from either the top or the sides mixes relatively fast
over the whole body of the fluid bed ( Figure 7.6 ). The gasifying medium
(air, oxygen, steam, or their mixture) also serves as the fluidizing gas and so
is sent through the bottom of the reactor.