Page 280 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

256 Biomass Gasification, Pyrolysis and Torrefaction

Reactions in different zones and at different temperatures are plotted on the
right. The throated (or constricted) type is shown in Figure 8.4.

8.2.2.1 Throatless Gasifier
This gasifier type is also called open top or stratified throatless. Here, the
top is exposed to the atmosphere, and there is no constriction in the gasifier
vessel because the walls are vertical. Figure 8.5 shows that a throatless
design allows unrestricted movement of the biomass down the gasifier,
which is not possible in the throated type shown in Figure 8.4. The absence
of a throat avoids bridging or channeling. Open core is another throatless
design, but here air is not added from the middle as in other types of down-
draft gasifiers. Air is drawn into the gasifier from the top by the suction
created downstream of the gasifier. Such gasifiers are suitable for finer or
lighter fuels. Rice husk is an example of such biomass.
The followings are some of the shortcomings of a downdraft gasifier:
1. It operates best on pelletized fuel instead of fine light biomass.
2. The moisture in the fuel must not exceed 25%.
3. A large amount of ash and dust remain in the product gas.
4. As a result of its high exit temperature, it has a lower gasification
efficiency.

Operating Principle
Because an open top, or a throatless, gasifier is simple in construction, it is
used to describe the gasification process in the downdraft gasifier
(Figure 8.5). The throatless process can be divided into four zones (Reed and
Das, 1988, p. 39). The first, or uppermost, zone receives raw fuel from the
top that is dried in the air drawn through the first zone. The second zone
receives heat from the third zone principally by thermal conduction.
During its journey through the first zone, the biomass heats up (zone I in

Figure 8.5). Above 350 C, it undergoes pyrolysis, breaking down into char-
coal, noncondensable gases (CO, H 2 ,CH 4 ,CO 2 , and H 2 O), and tar vapors
(condensable gases). The pyrolysis product in zone II receives only a limited
supply of air from below and burns in a fuel-rich flame. This is called flam-
ing pyrolysis. Most of the tar and char produced burn in zone III, where they
generate heat for pyrolysis and subsequent endothermic gasification reactions
(Reed and Das, 1988, p. 28).
Zone III contains ash and pyrolyzed char produced in zone II. While
passing over the char, hot gases containing CO 2 and H 2 O undergo steam gas-
ification and Boudouard reactions, producing CO and H 2 . The temperature
of the downflowing gas reduces modestly, owing to the endothermic gasifi-

cation reactions, but it is still above 700 C.
The bottommost layer (zone IV) consists of hot ash and/or unreacted
charcoal, which crack any unconverted tar in this layer. Figure 8.5 shows the