Engineering Chemistry S Datta

386 ENGINEERING CHEMISTRY
(ii) Nitrogen: About 1 g of accurately weighed coal sample taken in a Kjeldahl’s flask
along with conc. H 2
SO 4
, K 2
SO 4
and heated. Then it is treated with excess of
KOH and the liberated NH 3
is absorbed in known excess of standard acid solution.
The excess acid is back-titrated with standard NaOH solution. From the volume
of acid consumed N content is calculated as follows:
Percentage of N =
Volume of acid consumed × Normality × 1.4
.
Wt. of coal taken
(iii) Sulfur: While determining the calorific value of a coal sample in a bomb
calorimeter, the S in the coal is converted to sulfate. Finally the washings
containing sulfate is treated with dil. HCl and BaCl 2
solution, which precipitates
BaSO 4
which is filtered in a sintered glass crucible, washed with water and heated
to a constant weight.
Percentage of S =
Wt. of BaSO4 × 32 × 100
.
Wt. of coal sample in bomb × 233
(iv) Ash content: Ash content is determined similar to proximate analysis.
(v) Oxygen content = 100 – % of (C + H + S + N + ash)
Significance: Higher percentage of C and H increases the calorific value of coal and
hence better is the coal. Higher the percentage of O, lower is the calorific value and lower is
the coking power. Also, O when combined with H in the coal, H available for combustion becomes
unavailable. S, although contributes to calorific value, is undesirable due to its polluting
properties as it forms SO 2
on combustion.
Coke
Carbonisation or coking bituminous coal leads to the formation of coke. Coke obtained
from coal with high volatile matter forms swelling coke which is soft coke, and from a mixture
of high and low volatile coking bituminous coal (non-swelling) in coke ovens hard coke is
obtained. Both soft and hard cokes are obtained by high temperature carbonisation. A smokeless
fuel or semicoke is obtained from low temperature carbonisation.
Coke and Coal
• Coke possesses much strength and porosity compared to coal.
• By coking the undesirable sulfur content of coal is removed from coke and due to lower
volatile matter content of coke it burns with a short flame. All these properties of coke
make it suitable for metallurgical processes compared to coal.
Carbonisation of Coal
Depending on the operation temperature there are mainly two types of carbonisation
processes, namely,
(i) Low temperature carbonisation (LTC) and (ii) High temperature carbonisation (HTC).
Coarsely powdered coal taken in a closed retort and heated out of contact with air leads to the
breakdown of coal with the formation of water, ammonia, other volatile matters, gases and
coke. This process is called the carbonisation of coal or coking of coal.