Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 255

Biomass

Drying

Pyrolysis

Air Air

Combustion

Gasification

Gas

FIGURE 8.4 Schematic of a throated-type downdraft gasifier.

Biomass

I Biomass fuel

II Flaming pyrolysis

Air nozzles Air

III Char combustion

IV Char gasification

Product gas

500 1000 1500
Temperature (K)

Ash

FIGURE 8.5 Schematic of the operation of a throatless downdraft gasifier. Temperature gradi-
ent along the height shown at the right.

Downdraft gasifiers work well with internal-combustion engines that
need cleaner gas. The engine suction draws air through the bed of fuel, and
gas is produced at the end. Low tar content (0.015 3 g/nm 3 ) in the product

gas is another motivation for their use with internal-combustion engines. A
downdraft gasifier requires a shorter time (20

30 min) to ignite and bring
the plant up to working temperature compared to the time required by an
updraft gasifier.
There are two principal types of downdraft gasifier: throatless and
throated. The throatless (or open-core) type is illustrated in Figure 8.5 .