Engineering Chemistry S Datta

FUELS AND COMBUSTION 401
Reaction Zones in a Gas Producer
(a) Ash zone. The fuel bed in a normal producer is on metallic grate. The air-steam
blast is preheated by the ash zone which also protects the grate from intense heat.
(b) Oxidation zone. This is just next to the ash zone. The oxygen is consumed within
75 to 100 mm of the bed, which constitutes the zone. CO 2
forms at the expense of O 2
. Its
concentration reaches a maximum at the top when CO begins to appear. The temperature at
this zone is about 1100°C.
C + O 2
⎯⎯→ CO 2
+ 97 kcal
C + 1 2 O 2
⎯⎯→ CO + 29.5 kcal
(c) Reduction zone.
CO 2
+ C ⎯⎯→ 2CO – 36 kcal
C + H 2
O ⎯⎯→ CO 2
+ H 2
– 29 kcal
C + 2H 2
O ⎯⎯→ CO 2
+ 2H 2
– 19 kcal
As long as O 2
is in excess, CO 2
is formed, which then undergoes reduction into CO and
the reaction is endothermic and is called Bondonard reaction. Its equilibrium constant greatly
increases with rise in temperature. Other reactions are also endothermic, where red hot coke,
combines with steam to liberate H 2
and from oxides of C. The temperature is around 1000°C
(d) The uppermost layer of the bed is the drying and carbonisation zone. The water vapor and
the volatile matter are added to the escaping gas which results in production of enriched gases.
The topmost zone serves to preheat the precarbonised fuels coming in. The temperature of this
zone is from 400°C-800°C. The average composition of the producer gas is CO: 22-30%, H 2
:
8-12%, N 2
: 52-55%, CO 2
: 3%.
Producer gas is used for heating the furnaces for the production of steel and glass, muffle
furnaces for coal gas production, and in metallurgical operations.
For the production of producer gas, only air can be used, steam used along with air is
advantageous since (i) it raises the content of combustible components (CO + C 2
) compared to
noncombustible thereby increasing calorific value, (ii) the endothermic reaction of steam and
C prevents localised overheating of a fuel bed and leads to diminished clinker formation.
Water Gas
Water gas is a gaseous fuel of medium calorific value (2800 kcal/m 3 ) generated by gasifying
solid incandescent source of carbon in superheated steam. The equipment is known as water
gas generator and is more or less similar to that shown in Fig. 18.12 where the following
reaction takes place which is endothermic
C + H 2
O ⎯⎯→ CO + H 2
– 29 kcal
Cup and
cone feeder
Coke
Water gas
outlet
Red hot coke
at 900°C–1000°C
Grate
Steam
Air supply
supply
Ash
Refractory
outlet
brick lining
Ash
Fig. 18.13 Water gas production.