Environmental Engineering Reference
In-Depth Information
Table 9.4
The Main Reactions Involved in Gasification
Solid-gas reactions
Gas-gas reactions
C + O
2
→
CO
(exothermic)
CO + H
2
O
→
CO
2
+ H
2
(exothermic)
C + O
2
→
CO
2
(exothermic)
CO + 3H
2
→
CH
4
+ H
2
O (exothermic)
C + 2H
2
→
CH
4
(exothermic)
C + H
2
O
↔
CO + H
2
+ 131 MJ/kmol:
Water
gas reaction
C + H
2
O
→
CO + H
2
(endothermic)
CO + H
2
O
↔
CO
2
+ H
2
-41 MJ/kmol:
Water
gas shift reaction
C + CO
2
→
2CO
(endothermic)
C + CO
2
↔
2CO + 172 MJ/kmol:
Boudouard
reaction
The calorific value of the syngas produced is highest (10-15 MJ/m
3
) in O
2
atmospheres and lowest (4-7 MJ/m
3
) in air (Arena, 2012). Unlike with
incineration, the oxygen-deficient atmosphere used in gasification
discourages the formation of any dioxins (Arena et al., 2010). At higher
temperatures (>1200°C), any toxic polyaromatic hydrocarbons (PAHs)
formed are cracked into safe gasses. The Texaco gasification process, for
instance, involves two steps. First, the plastic is thermally cracked into oil
and condensable gases at around 700°C. This mix is then passed through a
gasifier with air and steam at 1200-1500°C to obtain a mix of CO and H
2
(Syngas) which is then purified.
Some tar or char is invariably formed in the process, reducing the yield of
syngas. Using activated charcoal (Kim et al., 2011) or olivine (Mastellone
and Arena, 2008) in the feed can reduce the tar fraction. With
low-temperature (600-900°C) gasification (Namioka et al., 2011), some
liquid oils might also be produced and the residual ash/char may require
vitrification or other treatment before disposal. The residue in
high-temperaturegasificationisgenerallyaninertslagthatcanbelandfilled
without further processing. Any PVC in the feed stream can present a
problem because of the HCl produced on thermal degradation. This
dehydrochlorination product is generally absorbed on lime, and the
inorganic chloride formed is safely disposed of in a landfill.
