Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 292

gasifier, and for a throatless or stratified downdraft gasifier, it is based on
the gasifier cross-sectional area. The actual velocity of gas is, however, sig-
nificantly higher than the designed space velocity because much of the flow
passage is occupied by fuel particles. The velocity is higher in the throat also
because of the higher temperature there. Table 8.7 gives some characteristic
values of these parameters.
In a downdraft gasifier, the gasification air is injected by a number of noz-
zles from the periphery (refer to Figure 8.6 ). The total nozzle area is typically
7

4% of the throat area. The number of nozzles should be an odd number so
that the jet from one nozzle does not hit a jet from the opposite side, leaving a
dead space in between. To ensure adequate penetration of nozzle air into the
hearth, the diameter of a downdraft gasifier is generally limited to 1.5 m. This
naturally restricts the size and capacity of a downdraft gasifier.
Table 8.8 lists typical sizes for the Imbert-type downdraft gasifier and shows
the relation between throat size and air nozzle diameter and the unit output.

8.8.2 Fluidized-Bed Gasifiers
No established design method for sizing a fluidized-bed gasifier is available
in the literature because, though nearly a century old, this type is still

TABLE 8.7 Maximum Values of Hearth Load Based on Throat Area for

Downdraft Gasifiers

D air
entry
(m)

Superficial
Velocity at Throat
(m/s)

Hearth
Load a
(MW/m 2 )

Gasifier
Type

D throat
(m)

Plant Medium

Gengas Imbert Air 0.15 0.3 2.5 15

Biomass
Corp.

Imbert Air 0.3 0.61 0.95 5.7

SERI Throatless Air 0.15 0.28 1.67

Buck
Rogers

Throatless Air 0.61 0.23 1.35

Buck
Rogers

Throatless Air 0.61 0.13 0.788

Syngas Throatless Air 0.76 1.71 10.28

Syngas Throatless Oxygen 0.76 1.07 12.84

SERI Throatless Oxygen 0.15 0.24 1.42

a Based on throat area.
Source: Data compiled from Reed and Das (1988), p. 36.