Amity University Chhattisgarh Raipur
Engineering Chemistry HSC 3102
Module-II Fuel and Combustion
Carbonization: Manufacture of Coke from Coal: -
Coal versus coke in metallurgical processes:
(1) Coal does not possess as much strength and porosity as coke.
(2) By coking, much of undesirable sulphur is removed.
(3) Coke burns with short flame, due to expulsion of much of its volatile matter
during carbonization.
Because of these reasons, "coke is preferred to coal for metallurgical purposes",
particularly in blast furnaces. On the other hand, coal burns with a long flame,
which is suitable only for reverberatory furnaces.
Coke is the solid, lustrous and porous mass left in the distilling pot after destructive
distillation of coal. The process of converting coal into coke is called
carbonization. In this process, coal is heated in absence of air, to a sufficiently
high temperature, under which volatile are driven off leaving a solid, lustrous,
dense, porous mass known as coke.
TYPES OF CARBONIZATION OF COAL
These are two types of carbonizations of coal:
(1) Low-temperature carbonization: In this process, the heating of coal is
carried out at 500 - 700°C. The yield of coke is about 75-80% and it contains
about 5-15% volatile matter. It is not mechanically strong, so it cannot be
used as a metallurgical coke. However, it burns easily giving practically a
smokeless, hot and radiant fire. Hence, it is suitable for domestic purposes.
The byproduct gas produced (about 130-150 m3/tonne) by this process is
richer in heating value (about 6,500-9,500 kcal/m3) and is, therefore, a more
valuable gaseous fuel.
DEMERITS OF LTC
Coke is not mechanically strong.
It pollutes the air
Inconsistent quality of soft coke
Co-product cannot be utilized.
Heat liberated by combustion is not used
(2) High-temperature carbonization is carried out at 900 - 1,200°C with the
object of producing coke of the right porosity, hardness, purity, strength,
etc., so that it can be used in metallurgy. Nearly all the volatile matter of
coal is driven off and the yield of coke is about 65-75%, containing only 1-
3% volatile matter. The by-product gas produced is high in volume (about
300-390 m3/tonne), but its calorific value is low (about 5,400-6,000
kcal/m3).
PHYSICO CHEMICAL CHANGES DURING CARBONISATION OF COAL: -
Raw coal(400°C) → Plastic layer (900- 1200°C) → Semi coke High temperature
coke
MANUFACTURE OF METALLURGICAL COKE
The coke, for metallurgical purposes, is obtained by the following processes:
Otto Hoffman's by-product oven:
In order to: (1) increase the thermal efficiency of the carbonization process, and (ii)
recover valuable by-product (like coal gas, ammonia, benzol oil, tar, etc.), Otto
Hoffman developed modern byproduct coke oven which, unlike beehive oven, is
heated externally by a portion of coal gas produced during the process itself or by
producer gas or by blast furnace gas.
Moreover, the heating is done on the basis of "regenerative system of heat economy", i.e.,
utilizing the waste flue gases for heating the checker work of bricks.
The by-product coke oven consists of number of narrow silica chambers (each about 10 to 12 m
long, 3 to 4 m high and 0.40 to 0.45 m wide) erected side-by-side with vertical flues in-between
them to form a sort of battery. Each chamber is provided with a charging hole at the top, a gas
off-take and a refractory-lined cast iron door at each ends for discharging coke.
A charge consisting of finely crushed coal is introduced through the charging holes at the top of
chambers, which are then closed tightly at both ends to prevent any access of air. The coke ovens
are heated to 1,200°C by burning gaseous fuel (like producer gas) and usually employing a
regenerative principle to achieve as economical heating as possible. The flue gases produced
during combustion, before escaping to chimney, pass on their sensible heat to one of the two sets
of checker brick-work, until this brick-work has been raised to a temperature of about 1,000°C.
The flow of heating gases is then reversed and the inlet gases are passed through the heated
checker brick-work, which thus serves to preheat the inlet gases. The flue gases are then allowed
to pass through the second set of checker bricks to heat it. This checker-work then serves to
preheat the inlet gases. Thus, this cycle goes on. The heating is actually continued, till the
evolution of volatile matter ceases completely. Carbonization of a charge of coal takes about
between 11 to 18 hours.
When carbonization is completed, a massive ram pushes the red hot coke into a truck. It is
subsequently quenched by a water spray ('wet quenching'). In place of wet quenching, "dry
quenching" offers advantages, because the coke produced is more strong, dense, graphitized and
non-reactive. In this method, the red hot coke is placed in a chamber and cooled by passing inert
gases from boilers (like nitrogen). The heated inert gases are then circulated to boilers, where
they generate steam. The coke produced by 'dry quenching' is cheaper, drier and contains lesser
dust than 'wet-quenched' coke.
Recovery of by-products : The gas coming out from the oven is known as "coke oven gas" and
is mainly composed of ammonia, H2S, naphthalene, benzene, tar, moisture, etc.
(i)Recovery of tar: The gas is first passed through a tower in which liquor ammonia is sprayed.
Here dust and tar get collected in a tank below, which is heated by steam coils to recover back
ammonia sprayed. The ammonia is used again.
(ii) Recovery of ammonia: The gases from the chamber are then passed through a tower in
which water is sprayed. Here ammonia goes into solution as NH4OH.
(iii) Recovery of naphthalene: The gases are then passed through another tower in which water
at very low temperature is sprayed. Here naphthalene gets condensed.
(iv) Recovery of benzene: The gases are then sprayed with petroleum, when benzene and its
homologues are removed.
(v) Recovery of H2S: The gases are then passed through a purifier, packed with moist Fe2O3.
Here H2S is retained.
Fe2O3 + 3 H2S → Fe2O3 + 3 H2O
After some time, when all Fe2S3 is changed into Fe2O3, the purifier is exposed to atmosphere,
when Fe2O3 is regenerated,
Fe2S3 + 4 O2 → 2 FeO + 3 SO2
4FeO + O2 2 → Fe2O3
