Page 276 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

252 Biomass Gasification, Pyrolysis and Torrefaction

TABLE 8.2 Characteristics of Fixed-Bed Gasifiers
Fuel (wood) Updraft Downdraft Crossdraft
Moisture wet basis (%) 60 max 25 max 10 20
Dry-ash basis (%) 25 max 6 max 0.5 1.0
Ash-melting temperature ( C) .1000 .1250

Size (mm) 5 100 20 100 5 20
Application range (MW) 2 30 1 2
Gas exit temperature ( C) 200 400 700 1250

3
Tar (g/N m ) 30 150 0.015 3.0 0.01 0.1
3
Gas LHV (MJ/N m ) 5 6 4.5 5.0 4.0 4.5
Hot-gas efficiency (%) 90 95 85 90 75 90
Turn-down ratio ( ) 5 10 3 4 2 3
2
Hearth load (MW/m ) ,2.8
Source: Adapted from Knoef (2005), p. 26.

8.2.1 Updraft Gasifiers

An updraft gasifier is one of the oldest and simplest of all designs. Here, the
gasification medium (air, oxygen, or steam) travels upward while the bed of
fuel moves downward, and thus the gas and solids are in countercurrent
mode. The product gas leaves from near the top of the gasifier as shown in
Figure 8.3. The gasifying medium enters the bed through a grate or a distrib-
utor, where it meets with the hot bed of ash. The ash drops through the grate,
which is often made moving (rotating or reciprocating), especially in large
units to facilitate ash discharge. Chapter 7 describes this process in more
detail.
Updraft gasifiers are suitable for high-ash (up to 25%), high-moisture (up
to 60%) biomass. They are also suitable for low-volatile fuels such as char-
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coal. Tar production is very high (30 150 g/nm ) in an updraft gasifier,
which makes it unsuitable for high-volatility fuels. On the other hand, as a
countercurrent unit, an updraft gasifier utilizes combustion heat very effec-
tively and achieves high cold-gas efficiency (Section 8.11.1). Updraft is
more suitable for direct firing, where the gas produced is burnt in a furnace
or boiler with no cleaning or cooling required. Here, the tar produced does
not have to be cleaned.
Updraft gasifiers find commercial use in small units like improvised
cooking stoves in villages and in large units like South African Synthetic