1. Solid Fuels
Wood โPeat โLignite โBituminous coal โAnthracite
Carbon content, calorific value and hardness
Moisture content, C,O,N and S content, volatile matter
Coal
Coal is a highly carbonaceous matter that has been
formed as a result of alteration of vegetable matter
(e.g., plants) under certain favorable conditions.
โขIt mainly composed of C, H, N, and O besides
non-combustible inorganic matter.
2. Analysis of coal and its significance
The proximate analysis involves the determination of
moisture, volatile matter, ash, and fixed carbon.
โขThis gives quick and valuable information regarding
commercial classification and determination of
suitability for a particular industrial use.
The ultimate analysis involves the determination of
carbon, hydrogen, sulphur, nitrogen, oxygen and ash.
โขThe ultimate analysis is essential for calculating heat
balances in any process for which coal is employed as a
fuel.
3. Proximate analysis
i) Moisture: An air-dried coal sample is weighed into a dry
silica crucible and heated for about one hour at 1100 C in an
electric hot air-oven. The crucible is cooled first in air then
in a desiccator and then weighed.
% moisture = Loss in weight ร 100
Wt. of coal taken
โข Moisture in coal evaporates during the burning of coal
and it takes some of the liberated heat in the form of
latent heat of evaporation.
โข Moisture lowers the effective calorific value of coal.
โข Lesser the moisture content better is the quality of coal
4. ii) Volatile matter: The dried sample of coal left in the
crucible in step (i) is then covered with a lid and placed in a
muffle furnace, maintained at 950 0 C. The crucible is taken
out after 7 minutes of heating. It is cooled first in air then
in a desiccator and finally weighed.
% Volatile matter = Loss in weight ร 100
Wt. of coal taken
โข A high volatile matter content means that a high
proportion of fuel will distill over as vapour and a large
portion of which escapes un burnt. So, higher % of volatile
matter in coal is undesirable.
โข A high volatile matter containing coal burns with a long
flame, high smoke and has low calorific value.
โข Lesser the volatile matter, better is the rank of coal.
5. iii) Ash: The residual coal left in the crucible in step (ii) is
then heated without lid in a muffle furnace at 700-7500 C,
until a constant weight of residue is obtained.
% Ash = Wt. of ash left ร 100
Wt. of coal taken
Ash-forming constituents in coal are undesirable
for the following reasons:
โข The calorific value of the coal is decreased.
โข The removal and disposal of ash poses problems.
โข The ash deposited in the fire bars interferes with
circulation of air.
โข If the ash fuses to form a clinker on the fire bars, it
hinders air circulation and also promotes corrosion
of the fire bars.
6. Intrinsic ash:-
The mineral matter originally present in the vegetable
material from which the coal was formed, consists of
oxides of K, Mg, and Ca is called intrinsic ash.
Extrinsic ash:-
Non-essential mineral matters such as clay, gypsum or
other inorganic matters, give rise to extrinsic ash.
The intrinsic ash content rarely exceeds 1 to 2 % whereas
extrinsic ash content is relatively high.
7. iv) Fixed carbon: It is reported as the difference
between 100 and the sum of the percentages of
moisture, volatile matter and ash content of a
coal sample.
โข Higher the percentage of fixed carbon, greater is its
calorific value and better is the quality of coal.
โข Greater the percentage of fixed carbon, smaller is
the percentage of volatile matter. It is the fixed
carbon which burns in the solid state.
โข Information regarding the percentage of fixed
carbon helps in designing of the furnace and the
fire-box.
8. Significance:-
It also helps to;
(1) Rank coal
(2) Show the quantity of combustible and non-
combustible constituents
(3) Provides the basis for buying/selling
(4) Identify appropriate coal for different purposes.
9. Ultimate Analysis
i) Carbon and Hydrogen: An accurately weighed coal
sample (1-2g) is burnt in a current of oxygen in
combustion apparatus. As a result C and H of the coal are
converted into CO2 and H2O respectively. These are
absorbed respectively in KOH and CaCl2 tubes of known
weights. The increase in the weights of KOH and CaCl2
tubes corresponds to the amount of CO2 and H2O formed
respectively.
C + O2 โ CO2
12 44
2 KOH + CO2 โ K2CO3 + H2O
H2 + 1/2O2 โ H2O
2 1 8
CaCl2 + 7 H2O โ CaCl2 .7 H2O
10.
11. % Carbon = Increase in wt .of KOH tube ร 12 ร 100
Wt. of coal taken ร44
% Hydrogen = Increase in wt. of CaCl2 tube ร 2 ร100
Wt. of coal taken ร18
โข C and H in coal directly contribute towards the calorific
value of the coal.
โข Higher the percentage of C and H, better is the quality
of the coal and higher is its calorific value.
ii) Nitrogen: Determined by digesting a known quantity
(1g) of powdered air-dried coal sample in a kjeldhalโs flask
with conc. H2SO4 and HgSO4 in the presence of K2SO4 as a
catalyst. After the solution becomes clear, it is treated with
excess of NaOH.
12.
13. The liberated ammonia is distilled into a known volume of
standard acid solution. The volume of unused acid is then
determined by back titration with standard NaOH solution.
From the volume of acid used by ammonia liberated, the
percentage of nitrogen is calculated.
% Nitrogen = Wt. of nitrogen ร 100
Wt. of coal taken
Where,
Wt. of nitrogen = Vol. of acid used ร Nacid ร 14
1000
Thus, % Nitrogen = Vol. of acid used ร Nacid ร1.4
Wt. of coal taken
14. โข Nitrogen in the coal does not contribute any useful value to
the coal.
โข Since it is generally present in small quantities (~ 1%) its
presence is not of much significance.
โข A good quality coal should have very little nitrogen content.
iii)Sulphur: A known amount of coal sample is burnt
completely in a bomb calorimeter. Sulphur present in
coal is oxidized to sulphates. The ash left after
combustion from the bomb calorimeter is extracted
with dil. HCl. The acid extract is then treated with
barium chloride solution to precipitate sulphate as
barium sulphate. The precipitate is filtered, washed,
ignited and weighed.
15. % Sulphur = Wt. of BaSO4 obtained ร 32 ร 100
Wt. of coal taken ร 233
โข Sulphur containing coal is not suitable for the
preparation of metallurgical coke as it adversely
affects the properties of the metal.
โข Oxides of sulphur pollutes the environment and leads
to corrosion.
(iv) Ash: The ash content of coal sample is determined as
described under proximate analysis.
(v) Oxygen:
% Oxygen = 100 โ % (C + H + N + S + Ash)
โข The lower the oxygen content, the more is the maturity
of coal and greater is its calorific value
16. A sample of coal was analysed as follows. Exactly 2.5 g
was weighted into a silica crucible. After heating for 1
hr. at 110oC, the residue weighed 2.415g. The crucible
next was covered with a vented lid & strongly heated
for exactly seven minutes at 950ยฑ20oC. The residual
weighed 1.528g. The crucible was then heated without
cover, until a constant weight was obtained. The last
residue was found to weigh 0.245g. Calculate
percentage of moisture, voltaile matter, ash & fixed
carbon in the sample.
17. Solution
Mass of moisture in coal sample = 2.500-2.415 = 0.085 g
Mass of volatile matter = 2.415-1.528 = 0.887 g
Mass of ash = 0.245 g
Percent of moisture = =3.40%
Percent of volatile matter = =35.48%
Percent of ash = = 9.80%
Percent of fixed carbon = 100 โ (3.40+35.48+9.80)
= 51.32%
500
.
2
100
085
.
0 x
500
.
2
100
887
.
0 x
500
.
2
100
245
.
0 x
18. 4. 0.5 g of a sample of coal was used in a bomb
calorimeter for the determination of calorific value.
Calorific value of coal was found to be 8,600 cal/g.
The ash formed in the bomb calorimeter was
extracted with acid and the acid extract was heated
with barium nitrate solution and a precipitate of
barium sulphate was obtained. The precipitate was
filtered, dried and weighed. The weight of precipitate
was found to be 0.05 g. Calculate the percentage of
sulphur in the coal sample.
19. Solution.
Percentage of sulphur in the coal sample
=
=
= 1.3734%
omb
letakeninb
ofcoalsamp
xWt
x
obtainedx
ppt
ofBaSO
Wt
.
233
100
32
.
. 4
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x
x
gx
5
.
0
233
100
32
05
.
0
20. 5. 1.0g of a sample of coal was used for nitrogen
estimation by kjelldahl method. The evolved
ammonia was collected in 25 mL N/10 sulphuric acid.
To neutralize excess acid, 15 mL of 0.1 N sodium
hydroxide was required. Determine the percentage of
nitrogen in the given sample of coal.
21. Solution.
15 mL of 0.1 N NaOH solution = 15 mL of 0.1 N
H2SO4
Vol. of H2SO4 used to neutralize the ammonia evolved
= 25 mL of 0.1 N โ 15 mL of 0.1 N
= 10 mL of 0.1 N
Percentage N =
= = 1.4%
taken
sample
coal
of
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xNormality
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