Page 236 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 7 Gasification Theory 213

Fuel

Product gas

Drying zone

Dry fuel
Pyrolysis zone
= Char + volatiles
C+CO 2 = 2CO
C+H 2 O = CO+H 2
Gasification
CO+H 2 O = CO 2 +H 2
C+H 2 = CH 4

Combustion zone C+O 2 = CO 2
C+0.5O 2 = CO

Air/Steam 0 300 600 900 1200
Temperature (°C)

Ash
FIGURE 7.4 Stages of gasification in an updraft gasifier.

The gas then rises through a bed of descending fuel or ash in the gasifier
chamber.
The air (the gasifying medium), as it enters the bottom of the bed, meets
hot ash and unconverted chars descending from the top (Figure 7.4). The
temperature in the bottom layer well exceeds the ignition temperature of car-
bon, so the highly exothermic combustion reaction (Eq. (7.24)) takes place
in the presence of excess oxygen. The released heat heats the upward-
moving gas as well as the descending solids.
C 1 O 2 -CO 2 2 394 kJ=mol (7.24)

The combustion reaction (Eq. (7.24)), being very fast, rapidly consumes
most of the available oxygen. As the available oxygen is reduced further up,
the combustion reaction changes into partial combustion, releasing CO and a
moderate amount of heat.
C 1 1=2O 2 -CO 2 111 kJ=mol (7.25)
The hot gas, a mixture of CO, CO 2 , and steam (from the feed and the
gasifying medium), from the combustion zone moves further up into the gas-
ification zone, where char from the upper bed is gasified by Eq. (7.26).