Page 278 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

254 Biomass Gasification, Pyrolysis and Torrefaction

The dry-ash technology has been used at SASOL in South Africa, the
3
world’s biggest gasification complex. SASOL produces 55 million nm /day
of syngas, which is used to produce 170,000 bbl/day of Fischer Tropsch
liquid fuel (Figure 11.3).

8.2.1.2 Slagging Gasifier
The British Gas/Lurgi consortium developed a moving-bed gasifier that
works on the same principle as the dry-ash gasifier, except a much higher
temperature (1500 1800 C) is used in the combustion zone to melt the ash

(hence its name slagging gasifier). Such a high temperature requires a lower
steam-to-fuel ratio (B0.58) than that used in dry-ash units (Probstein and
Hicks, 2006, p. 169).
Coal crushed to 5 80 mm is fed into the gasifier through a lock hopper
system (Minchener, 2005). The gasifier’s tolerance for coal fines is limited,
so briquetting is used in places where the coal carries too many of them.
Gasification agents, oxygen and steam, are introduced into the pressurized
(B3 MPa) gasifier vessel through sidewall-mounted tuyers (lances) at the ele-
vation where combustion and slag formation occur.
The coal introduced at the top gradually descends through several process
zones. The feed is first dried in the top zone and then devolatilized as it des-
cends. The descending coal is transformed into char and then passes into the
gasification (reaction) zone. Below this zone, any remaining carbon is oxi-
dized, and the ash content melts, forming slag. Slag is withdrawn from the
slag pool through an opening in the hearth plate at the bottom of the gasifier
vessel. The product gas leaves from the top, typically at 400 500 C

(Minchener, 2005).

8.2.2 Downdraft Gasifiers

A downdraft gasifier is a cocurrent reactor where air enters the gasifier at a
certain height below the top. The product gas flows downward (giving the
name downdraft) and leaves from lower section of the gasifier through a bed
of hot ash (Figures 8.4 and 8.5). Since it passes through the high-temperature
zone of hot ash, the tar in the product gas finds favorable conditions for
cracking (see Chapter 6). For this reason, a downdraft gasifier, of all types,
has the lowest tar production rate.
Air from a set of nozzles, set around the gasifier’s periphery, flows down-
ward and meets with pyrolyzed char particles, developing a combustion zone
(zone III shown schematically in Figure 8.5 and described in the discussion

of throatless downdraft gasifiers that follows) of about 1200 1400 C. Then
the gas descends further through the bed of hot char particles (zone IV), gasi-
fying them. The ash produced leaves with the gas, dropping off at the bottom
of the reactor.