Page 277 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 8 Design of Biomass Gasifiers 253

Biomass

Gas

Drying
Pyrolysis

Gasification
Oxidation Grate

Steam plus air
Ash

FIGURE 8.3 Schematic of an updraft gasifier.

Oils (SASOL) for production of gasoline from coal. The following is a brief
description of two important large-scale commercial updraft gasifier
technologies.

8.2.1.1 Dry-Ash Gasifier
Lurgi, a process development company, developed a pressurized dry-ash
updraft gasifier. It is called dry ash because the ash produced is not molten.
One that produces molten ash is called a slagging gasifier.

Though the peak temperature (in the combustion zone) is 1200 C, the

maximum gasification temperature is 700 900 C. The reactor pressure is in
the neighborhood of 3 MPa, and the residence time of coal in the gasifier is
between 30 and 60 min (Ebasco Services Inc., 1981). The gasification
medium is a mixture of steam and oxygen, steam and air, or steam and
oxygen-enriched air. It uses a relatively high steam/fuel carbon ratio (B1.5).
The coal is first screened to between 3 and 40 mm (Probstein and Hicks,
2006, p. 162) and then fed into a lock hopper. The gasifying agent moves
upward in the gasifier while the solids descend. The reactor is a double-
walled pressure vessel. Between the two walls lies water that quickly boils
into steam under pressure, utilizing the heat loss from the reactor. As the
coal travels down the reactor, it undergoes drying, devolatilization, gasifica-
tion, and combustion. Typical residence time in the gasifier is about an hour
(Probstein and Hicks, 2006, p. 162). In a dry-ash gasifier, the temperature is
lower than the melting point of ash, so the solid residue dries and is removed
from the reactor by a rotating grate.