Solid Fuels 3 (Properties and Testing of Coal)

1. Solid Fuels
Properties and Testing of Coal

2. Properties and Testing of Coal
• Proximate analysis of coal
• Ultimate Analysis
• Determination of Calorific Value
• Density
• Hardness
• Mechanical strength
• Gray King Assay
• Swelling Index

3. Testing of Coal
Ultimate Analysis
Determination of Carbon and Hydrogen
• A known amount of coal is burnt in dry oxygen
• C and H are converted into CO2 and H2O respectively
• The products of combustion are passed over weighed
tubes of anhydrous CaCl2 and KOH
• The increase in the weight of CaCl2 tube represents the
weight of water formed
• The increase in the weight of KOH tube represents the
weight of CO2 formed

4. Testing of Coal
Ultimate Analysis
Determination of Carbon and Hydrogen
• X=weight of coal sample
• Y=increase in the weight of CaCl2 tube
• Z=increase in the weight of KOH tube
• % carbon in coal=
• %Hydrogen in Coal =
100
44
12









Z
X
100
18
2









Z
Y

5. Testing of Coal
Ultimate Analysis
Determination of Sulphur
• A known quantity of coal is burnt in bomb calorimeter
in oxygen
• The residue ash is treated with dilute hydrochloric
acid
• Acid extract is treated with barium chloride solution to
precipitate the sulphate as barium sulphate
• The precipitate is filtered washed dried and weighed
• The %age of sulphur is computed from the weight of
BaSO4

6. Testing of Coal
Ultimate Analysis
Determination of Nitrogen
• The Kjeldahl–Gunning macro method is the one most
widely used for determining nitrogen (ASTM D-3179)
• By this method, any nitrogen present in the sample is
converted into ammonium salts by a hot mixture of
concentrated sulfuric acid and potassium sulfate
• sodium or potassium hydroxide is added to alkaline
the mixture
• ammonia is expelled which is absorbed into a sulfuric
acid solution

7. Determination of Calorific Value
• Determination of Calorific Value by Bomb Calorimeter
Already discussed
• If a coal does not have a measured calorific value, it is
possible to make a close estimation of the calorific value
(CV) by means of various formulas.
• One of the most popular formula is
Modified Dulong Formula for G.C.V.
G.C.V. = kg
kcal
S
N
O
H
C /
2220
8
)
1
(
34500
8080
100
1











C, H, O, N and S are the percentages of carbon, hydrogen, oxygen, nitrogen
and sulphur

8. Coalification
• The formation of coal from a variety of plant
materials via biochemical and geochemical
processes is called coalification. The nature
of the constituents in coal is related to the
degree of coalification, the measurement of
which is termed rank.

9. Density of Coal
Density is an important parameter, and it reflects the nature and structure of a
material. The density depends on the closeness of the molecular structure and the
molecular arrangement and there is also a relationship between density and degree
of coalification. The density can also be used for structural analysis of coal, using
statistical methods. The coal density is the coal mass per unit volume. Coal volume
has different meanings in different situations, because of the inhomogeneity of
coal, so coal density has various definitions.
The true relative density (TRD) of coal refers to the coal mass per unit volume,
excluding the pores in the coal. It is an important indicator for calculating the
average mass of a coal seam and in coal quality research. The TRD can be
determined in aqueous media using a pycnomete. When different substances (for
example, helium, methanol, water, n-hexane, and benzene) are used as the
replacement substances for determining coal density, the values obtained vary.
Usually, the result obtained using helium as the replacement substance is taken as
the TRD (also known as the helium density). The diameters of the smallest pores in
coal are about 0.5–1 nm, whereas the diameter of the helium molecule is 0.178 nm;
therefore helium can completely penetrate the porous structure of coal. The
general ranges of the TRDs of various types of peat, lignite, bituminous coal, and
anthracite are about 0.72, 0.8–1.35, 1.25–1.50, and 1.36–1.80 g cm−3, respectively.

10. The apparent relative density (ARD) of coal is the coal mass per unit volume,
including the pores in the coal. This parameter is necessary for calculating coal
reserves and in the transportation, crushing, and combustion of coal. The ARD can
be determined using the wax-coating method. The coal porosity can be calculated
using the TRD and ARD of the coal:
The bulk density (BD) of coal is the ratio of the total mass of coal grains filling a
container using the free-stacking method to the vessel volume. The BD is used
when estimating the mass of a coal pile or calculating the coal capacity of a coke
oven.
For the same coal sample, the value of the TRD of the coal is highest, followed
by that of the ARD, and the value of BD is lowest. The densities of minerals are
significantly higher than that of organic matter, so the content and composition of
the minerals in coal has a significant influence on the coal density. In the study of
coal structure, it is usually necessary to eliminate the impact of minerals. The
density must be corrected roughly as follows:
for every 1 % increase in coal ash, the coal density will increase by 0.01 %.

11. Hardness
Coal hardness reflects the coal’s ability to withstand external mechanical actions.
The representation and determination of coal hardness differ depending on the
applied mechanical force.
The scratch hardness (Mohs hardness) is the relative hardness determined by
scratching the coal surface with 10 types of standard mineral. The scratch
hardness of coal is usually between 1 and 4. Coal hardness is related to
coalification. Lignite, which has a low coalification degree, and coking coal, with
medium coalification, have the lowest scratch hardnesses of 2–2.5, whereas
anthracite has the highest scratch hardness that is close to 4.
Mechanical Strength
The mechanical strength of coal refers to its capacity to resist external
forces and is related to physical properties of coal such as shatter indices
and grind ability index.

12. The shatter indices of coal can be determined using the drop method. The method is to
let coal lumps of size 60–100 mm fall freely from a point 2 m above a steel plate, sieve
them with a sieve of 25 mm, and repeat the process for coal samples of size greater
than 25 mm. After repeating the process three times, the mass of coal samples larger
than 25 mm is determined, and the percentage with respect to the mass of the original
coal samples is taken as the shatter indices of the coal. The grading standards for
determining the mechanical strength of coal using the drop test are shown in Table.
Table : Grading standards for mechanical strength of coal

13. Hardgrove Grindability Index (short HGI) is a measure for the grindability of
coal or (HGI) is a measure of coal’s resistance to crushing.
Grindability is indicated using the unit °H, e.g. "40°H" or "55°H". The smaller
the HGI, the harder and less grindable is the coal.
Grindability is an important factor for the design of a coal mill. As grindability
depends on many unknown factors, HGI is determined empirically using a
sample mill.
HGI is determined through a multi-step procedure:
1. A 50-gram sample of prepared coal that is uniform in size is placed inside
a grinding unit
2. The unit undergoes a standard number of revolutions under a specified
pressure
3. Steel balls within the unit crush the coal sample
4. The coal fines are sorted and the quantity of coal less than a specified size
is recorded and converted into a Hardgrove Grindability Index (HGI) value
5. Typical resulting HGI values lie between 30 (increased resistance to
pulverization) and 100 (more easily pulverized).

14. The hard groove grind ability index of coal is then
calculated using the following formula.
HGI = 13 + 6.93w
Where,
w = weight of the test sample passing through 75 micron
sieve and retained on 300 micron sieve after grinding in
the HGi machine.

15. Solubility
• Organic solvents are capable of dissolving coal in part, the amount dissolved
depending upon the solvent and varying from about 0.1 percent with cold
benzene to 47.3 per cent with boiling quinoline (C9H7N).
• The most complete solution is assured in any solvent by acting upon finely
divided coal at the boiling-point of the solvent or under pressure. Solubility in
solvents has been made use of in separating constituents from coal which
confer special properties such as caking power upon the coal. The two most
commonly used have been pyridine at 118°C and benzene under pressure at
275°C.
• The solubility of coal in pyridine varies with the type of coal from 7.5 to 42 per
cent and with benzene under pressure from 3.3 to 21 percent. The effect of
other solvents also varies with the type of coal. The more important of these
are: acetone up to 3 percent, aniline up to 12 per cent, and phenol up to 35
per cent, tetralin up to 45 per cent, alcohol, carbon disulphide and ether all
less than 1 per cent.

16. Softening or Melting-point.
• When a coking coal is heated it passes through a series of stages,
softening, swelling, setting and shrinking.
• The softening stage may be due to the liquefaction of the soluble fractions
and is described variously as softening, melting or fusion of the coal.
• The temperature of inception varies with the type of coal and increases with
the maturity of the coal within the range 320° to 420°C.
• As would be expected, the actual temperature varies considerably with the
rate of heating of the coal; slow heating might give a value of 350°C, while
heating at the rate of one degree per minute might give 420°C.
• It is clear, however, that the temperature bears a relation to that of the rapid
evolution of gas and that both are of importance in coking practice in
deciding the optimum conditions for the treatment of coal in general and coal
blends in particular.

17. Gray-King Assay
In this method the coal (20 g passing a 72 B.S. sieve) is carbonized under standard
conditions to 600°C. Crushed coal is filled in a silica tube (length : 150 mm diameter
23 mm ) which is heated in an electrical furnace with 5 K /min upto 600 C or 900 C
under controlled conditions. The resulting coke taken out and examined visually and
letter A to G is assigned. If the residue appears strongly caking ( exceeding type G),
the test must be repeated diluting the sample with electrode carbon( non-caking).
The addition of electrode carbon is carried out in 5% point steps. The test is
repeated until standard type G is obtained and termed G1 to G10 respectively, the
subscript numbers indicating the number of grams of carbon necessary in the 20-g
blend. If the carbonized residue is pulverulent with no sign of coherence the coal is
termed ‘non-caking type A’. Coal which gives a hard, compact, non-fissured coke of
the same volume as the original coal is termed G, while the intermediate letters
designate coals whose coke-friability decreases within this range.

19. Swelling Index/Number of Coal
• It denotes the caking capacity of Coal. Caking power is
the ability to form a fused coke when coal is heated out
of contact with air.
• The crucible swelling index is determined by heating 1
gm of coal in a special crucible to 820 C under
standardized conditions for 2.5 minutes or until no
observable volatile matter is evolved
• The profile of the coke produced is compared with
series of standards

20. Swelling Index/Number of Coal
• No. Less than 2.5: very weak caking properties or non-
caking. Coal is suitable for steam raising but unsuitable
for carbonisation
• No. 3 - 3.5: weak to moderate caking power. Suitable for
all combustion purposes. Marginally suitable for
carbonisation
• No. 4 - 6.5: coals of moderate caking power. These are
suitable for combustion but may be strongly caking for
some forms of mechanical stokers. They are suitable for
gas-work and second grade metallurgical coke

21. Swelling Index/Number of Coal
• No. 7- 9: Strongly caking coals. These are too strongly
caking to be suitable for combustion. They are best for
metallurgical cokes

24. Example
The analysis of the coal in boiler is
C : 81% , H2: 4.5 % , O2 : 8 % and remainder is
incombustible
The dry flue gas analysis is CO2: 8.3 %, CO: 1.4 % O2 :
10 % N2: 80.3 %
Determine
(a) The weight of air supplied per kg of coal
(b) The percentage of excess air

25. Solution
Volume% Mol. weight Proportional
weight
Analysis by
weight
Carbon /kg
of
constituent
Weight of
Carbon/kg dry
flue gas
CO2
CO
O2
N2
8.3
1.4
10.0
80.3
44
28
32
28
365.2
39.2
320.2
2249
0.1228
0.0132
0.1076
0.756
12/44
12/28
0.0335
0.00566
Total 100 2973 0.03916

26. Solution
Weight of dry flue gas per kg of coal= 0.81/0.03916
=20.68 kg
Water formed = 0.045 x 9 = 0.405 kg per kg of coal
Incombustibles = 1 -0.81 -0.045 -0.08 = 0.065 kg / kg of coal
Air supplied per kg of coal = ?
Air + Coal = Dry Flue gas + Water + Incombustibles
Air = 20.15 kg per kg of coal
Combustion
Air ?
+ Coal 1 kg
Dry Flue Gas =20.68 kg
H2O = 0.405 kg
Incombustibles=0.065 kg
Theoretical air = 11.6 C + 34.8(H2 – O2/8)=10.6175
%age excess air = (20.15 – 10.6175)/10.6175 =?