UNIT-III: Calorific Value and solid fuels Dr. M. Venkatanarayana GITAM University, HTP Campus, Hyderabad
Calorific value: It is defined as the total quantity of heat liberated when a unit mass of a fuel is burnt completely.
Units of Calorific value:
The quantity of heat can be measured in the following units:
(i) Calorie: It is defined as the amount of heat required to raise the temperature of 1gm of water by 1oC 1 calorie = 4.184 Joules
(ii) (ii) Kilo Calorie: 1 k cal = 1000 cal
This is the unit of metric system and may be defined as the amount or quantity of heat is required to raise the temperature of one kilogram of water through degree centigrade.
(iii) British thermal unit: (B. T. U.) It is defined as the amount of heat required to raise the temperature of 1 pound of water through 1oF.
1 B.T.U. = 252 Cal = 0.252 k cal
(IV) Centigrade heat unit (C.H.U): It is defined as the amount of heat required to raise the temperature of 1 pound of water through 1oC.
1k cal = 3.968 B.T.U.
Gross and net calorific Value
Gross Calorific Value or HCV: It is the total amount of heat generated when a unit quantity of fuel is completely burnt in oxygen and the products of combustion are cooled down to the room temperature.
As the products of combustion are cooled down to room temperature, the steam gets condensed into water and latent heat is evolved. Thus in the determination of gross calorific value, the latent heat also gets included in the measured heat. Therefore, gross calorific value is also called the higher calorific value.
The calorific value which is determined by Bomb calorimeter gives the higher calorific value (HCV)
Net Calorific Value: It is defined as the net heat produced when a unit quantity of fuel is completely burnt and the products of combustion are allowed to escape.
The water vapour do not condense and escape with hot combustion gases. Hence, lesser amount than gross calorific value is available. It is also known as lower calorific value (LCV).
LCV=HCV-Latent heat of water vapours formed
Since 1 part by weight of hydrogen gives nine parts by weight of water i.e.
Therefore,
LCV=HCV-weight of hydrogen x 9 x latent heat of steam
= HCV-weight of hydrogen x 9 x 587
Determination of Calorific value
1. Determination of calorific value of solid and non volatile liquid fuels: It is determined by bomb calorimeter.
Principle: A known amount of the fuel is burnt in excess of oxygen and heat liberated is transferred to a known amount of water. The calorific value of the fuel is then determined by applying the principle of calorimetery i.e. Heat gained = Heat lost.
Construction: 1. It’s consists of a strong cylindrical stainless steel bomb in which the combustion of fuel is takes place. The bomb has a lid, which can be screwed to the body of bomb so as to make a perfect gas-tight seal. The lid is provided two stainless electrodes and an oxygen inlet valve.
2. To one of electrode, a small ring is attached. In this ring a nickel or stainless crucible can be supported.
3. The bomb is placed in a calorimeter, which is surrounded by air-jacket & water jacket to prevent heat losses due to radiation.
4. The calorimeter is provided with an electrically operated stirrer and Beckmann’s thermometer, which can read accurately temperature difference up to 1/100 th of a degree.
O
H
O
H
2
2
2
1
Working: 1. A known mass (0.5 to 1g) of the given fuel is taken in clean crucible. The crucible is then supported over the ring. A fine “
electrode.
2. The bomb is tightly screwed and bomb filled with oxygen
is then lowered into copper calorimeter containing a known mass of water.
3. The stirrer is worked and initial with 6V. battery and circuit completed.
4. The sample burns and heat is liberated. Uniform stirring of water is continued and the maximum temperature attained is recorded.
Calculations
Let weight of the fuel sample taken = x g
Weight of water in the calorimeter = W g
Water equivalent of the Calorimeter, stirrer, bomb, thermometer =
1. A known mass (0.5 to 1g) of the given fuel is taken in clean crucible. The crucible is then “Mg” wire, touching the fuel sample is then stretched
he bomb is tightly screwed and bomb filled with oxygen atmospheric pressure 25 to is then lowered into copper calorimeter containing a known mass of water.
initial temperature of the water is noted. The electrodes are connected with 6V. battery and circuit completed.
burns and heat is liberated. Uniform stirring of water is continued and the maximum temperature attained is recorded.
Let weight of the fuel sample taken = x g
Weight of water in the calorimeter = W g
Water equivalent of the Calorimeter, stirrer, bomb, thermometer = w g
1. A known mass (0.5 to 1g) of the given fuel is taken in clean crucible. The crucible is then stretched a cross the
pressure 25 to 30. The bomb
of the water is noted. The electrodes are connected
Initial temperature of water = t1oC
Final temperature of water = t2oC
Higher or gross calorific value = C cal/g
Heat gained by water = W x Dt x specific heat of water
= W (t2-t1) x 1 cal
Heat gained by Calorimeter = w (t2-t1) cal
Heat liberated by the fuel = x C cal
Heat liberated by the fuel = Heat gained by water and calorimeter
x C = (W+w) (t2-t1) cal
C=(W+W)(t2-t1) cal/g
x
(a) Fuse wire correction: As Mg wire is used for ignition, the heat generated by burning of Mg wire is also included in the gross calorific value. Hence this amount of heat has to be subtracted from the total value.
(b) Acid Correction: During combustion, sulphur and nitrogen present in the fuel are oxidized to their corresponding acids under high pressure and temperature.
The corrections must be made for the heat liberated in the bomb by the formation of H2SO4 and HNO3. The amount of H2SO4 and HNO3 is analyzed by washings of the calorimeter.
For each ml of 0.1 N H2SO4 formed, 3.6 calories should be subtracted.
For each ml of 0.01 HNO3 formed, 1.43 calories must be subtracted.
(C) Cooling correction:As the temperature rises above the room temperature, the loss of heat does occur due to radiation, and the highest temperature recorded will be slightly less than that obtained. A temperature correction is therefore necessary to get the correct rise in temperature.
If the time taken for the water in the calorimeter to cool down from the maximum temperature attained, to the room temperature is x minutes and the rate of cooling is dt/min, then the cooling
correction = x ×××× dt. This should be added to the observed rise in temperature.
Therefore, Gross calorific value
C=(W+w)(t2-t1+Cooling correction)-[Acid+ fuse corrections] / Mass of the fuel
3
4
2
2
2
5
2
2
2
4
2
2
2
2
2
2
2
2
HNO
O
H
O
N
SO
H
O
H
O
SO
SO
O
S
→
+
+
→
+
Theoretical calculation of Calorific value of a Fuel:
The calorific value of a fuel can be calculated if the percentages of the constituent elements are known.
Substrate Calorific value
Carbon 8080
Hydrogen 34500
Sulphur 2240
If oxygen is also present, it combines with hydrogen to form H2O. Thus the hydrogen in the combined form is not available for combustion and is called fixed hydrogen.
Amount of hydrogen available for combustion = Total mass of hydrogen-hydrogen combined with oxygen.
1g 8g 9g
Fixed Hydrogen = Mass of oxygen in the fuel
Therefore, mass of hydrogen available for combustion = Total mass of hydrogen-1/8 mass of oxygen in fuel
=H-O/8
Dulong’s formula for calculating the calorific value is given as:
Gross calorific Value (HCV)
Net Calorific value (LCV)
Characteristics of Good Fuel:
(i) Suitability: The fuel selected should be most suitable for the process. E.g., coke made out of bituminous coal is most suitable for blast furnace.
(ii) It should posses high calorific value when a unit weight of fuel is under combustion (iii) It’s moisture content should be low so that it would have high heating value.
(iv) Fuel, on burning should not give out objectionable gases and harmful gases like co, SO2, NOx etc.
(v) It should not give non combustible matters like ash, clinkers. (vi) It should not give any offensive odour.
O H O
H2 2 2
2 1 → + kg kcal S O H
C ) 2,240 ] /
8 ( 500 , 34 8080 [ 100
1 + − +
(vii) A good fuel should be easily available with low cost
(viii) Combustion should be easily controllable i.e. combustion of fuel should easy to start or stop, when required.
(ix) Should not undergoes spontaneous combustion; spontaneous ignition can cause fire accidents.
(x) Storage cost in bulk should be low
(xi) Should burn in air with efficiency, without much smoke.
(xii) In case of solid fuel, the size should be uniform so that combustion is regular.
Classification of coal by Rank
Solid Fuels: Primary as well as secondary are widely used in domestic and industrial purposes.
e.g., wood, coal, charcoal and coke.
Coals are mainly classified on the basis of their degree of coalification from the parent material, wood. When wood is converted into coal, there is gradual increase in the concentration of carbon and decrease in the percentage of oxygen and nitrogen.
Coal is given a ranking depending upon the carbon content of the coal from wood to anthracite
Wood: Wood has been used as a fuel from ancient times. Due to large scale deforestation, wood is no longer used except in forest areas where wood is available at a low cost.
Wood when freshly cut contains 25-50% moisture.
Normally it is used in air dried condition with 10-15 percent moisture content.
The calorific value of air dried wood is about 3500-4500 kcal/kg.
When wood burns, the ash content is low but the oxygen content is very high. This makes even dry wood a fuel of low calorific value.
Wood charcoal is obtained by destructive distillation of wood.
The major use of wood charcoal is for producing activated carbon.
”For more information please see Jain & Jain text book page no. 63-64”
Analysis of Coal
Coal is analysed in two ways:
1. Proximate analysis
2. Ultimate analysis
The results of analysis are generally reported in the following ways:
As received basis
Air dried basis
Moisture free basis (oven dried)
Moisture and ash free basis
Proximate Analysis
The data varies with the procedure adopted and hence it is called proximate analysis.
Proximate analysis of coal determines the moisture, ash, volatile matter and fixed carbon of coal.
1. Moisture Content: Air dried moisture is determined by heating a known amount of coal to 105-110 oC in an electric hot air oven for about one hour. After one hour, it is taken out from the oven and cooled in a dessicator and weighed.
Percentage of moisture= Loss in weight × 100
Wight of coal taken
2. Volatile Matter: consists of a complex mixture of gaseous and liquid products resulting from the thermal decomposition of the coal.
For efficient use of fuel, the outgoing combustible gases has to be burnt by supplying secondary air.
High volatile matter content is desirable in coal gas manufacture because volatile matter in a coal denotes the proportion of the coal which will be converted into gas and tar products by heat.
3. Ash: Coal contains inorganic mineral substances which are converted into ash by chemical reactions during the combustion of coal.
Ash usually consists of silica, alumina, iron oxide and small quantities of lime, magnesia etc.
Ash content is determined by heating the residue left after the removal of volatile matter at 700 ±±±± 50oC for ½ an hour without covering
Percentage of ash = Weight of the residue left ×100
Weight of coal taken
Ash can be classified as intrinsic ash and extrinsic ash.
The mineral matter originally present in vegetable matter from which the coal was formed is called intrinsic ash. It consists of oxides of Na, K, Mg, Ca and Si.
The mineral matter like clay, gypsum, dirt which gets mixed up during mining and handling of coal constitute the extrinsic ash which remains as a residue after the combustion. E.g., CaSO4, CaCO3, Fe2O3 etc.
4. Fixed Carbon: Fixed carbon content increases from lignite to anthracite. Higher the percentage of fixed carbon greater is its calorific value and better is the quality of coal.
The percentage of fixed carbon is given by:
Percentage of fixed carbon = 100-[% of moisture+volatile matter+ash]
Significance
Higher the percentage of fixed carbon, greater its calorific value
The percentage of fixed carbon helps in designing the furnace and shape of the fire-box because it is the fixed carbon that burns in the solid state
It is carried out to ascertain the composition of coal.
Ultimate analysis includes the estimation of carbon, hydrogen, sulphur, nitrogen and oxygen.
1. Carbon and Hydrogen: A known amount of coal is taken in a combustion tube and is burnt in excess of pure oxygen.
. Estimation of carbon and hydrogen
44 g of CO2 contain = 12 g of carbon Y g of CO2 contain = 12/44 × y
18 g of water contain = 2 g of hydrogen
Z g of water contain
Significance:
Calorific value of a fuel is directly related to its carbon content.
A higher percentage of carbon reduces the size of the combustion chamber
O
H
O
H
CO
O
C
2
2
2
1
2
2
2
→
+
→
+
O
H
CaCl
O
H
CaCl
O
H
CO
K
CO
KOH
2
7
.
2
2
7
2
2
3
2
2
2
→
+
+
→
+
taken
coal
of
weight
100
44
12
carbon
of
Percentage
=
×
y
×
hydrogen
of
zg
18
2
×
=
taken
coal
of
weight
100
18
2
hydrogen
of
Nitrogen: Nitrogen present in coal sample can be estimated by Kjeldahl’s method.
The contents are then transferred to a round bottomed flask and solution is heated with excess of KOH.
The ammonia gas thus liberated is absorbed in a known volume of a standard solution of acid used.
Estimation of nitrogen by Kjeldahl’s method
The unused acid is then determined by titrating with NaOH. From the volume of acid used by NH3
liberated, the percentage of nitrogen can be calculated
Percentage of N = Volume of acid used × Normality of acid ×1.4
---
Weight of coal taken
Sulphur:
Is determined from the washings obtained from the known mass of coal, used in a bomb calorimeter for
determination of a calorific value. During this determination, S is converted into sulphate. The washings
are treated with barium chloride solution, when barium sulphate is precipitated. This precipitate is
filtered, washed to constant weight.
% S = Weight of BaSO4 obtained × 32 ×100
---
Weight of coal sample taken in bomb ×233
Ash: Determination is carried out as in proximate analysis
4
2
)
4
(
Heat
4
2
Nitrogen
+
H
SO
→NH
SO
Oxygen: It is obtained by difference.
Percentage of O = 100-Percentage of (C + H + S + N+ Ash)
Manufacture of Metallurgical Coke:
(i) Beehive Oven
(ii) Otto Hoffmann oven
(i) Beehive oven: A beehive oven is a fire-brick chamber having a dome-shaped structure.
The dimensions of a typical oven are 4m and 2.5m high. The roof is provided with a hole
for charging the coal from the top. Another hole, the discharging hole is provided in the
circumference of the lower part of the wall. A number of ovens are built in a row with
common walls between neighbouring ovens.
Beehive coke oven
Working: Coal is charged through the top opening and leveled evenly to produce a layer, about 0.6 m
deep. Some air is supplied in and the coal ignited, resulting removal of volatile matter and moisture.
To make slow carbonization the supply of air is diminished gradually.
For complete carbonization it will take about 3-4 days. When the carbonization is complete, the hot
coke is quenched with water and collected.
As the oven is hot then next batch of carbonization process will be carried out easily which
economical too.
Demerits of Beehive ovens:
The demerits are
• No recovery of byproducts, which are useful chemicals and are allowed to escape.
• Lower coke yield due to partial combustion
• Lack of flexibility of operation
(ii) Otto-Hoffmann’s oven or By-product Oven:
The beehive ovens have been replaced by chamber ovens which works on regenerative
principle of heat economy. All the valuable products are recovered from the outgoing flue
gases.
Construction: It consists of no. of narrow rectangular chambers made of silica bricks.
Working: Coal is charged into the chamber.
The coke ovens are heated to 1200oC by burning gaseous fuels.
The process of carbonization takes place layer by layer in the coal charge.
As the coal adjacent to the oven walls gets heated, a plastic zone is formed which moves away from
the walls towards the central zone.
As the coal is converted into coke, there is decrease in volume. This is because of the removal of
volatile matter in the form of tar and gas at about 500oC. At further high temperature, the plastic
Recovery of byproducts: The gases and vapours evolved on carbonization in coke ovens are not
allowed to mix with the combustion and are collected separately.The coke oven gas is treated
separately for the recovery of the valuable byproducts.
Coke-Oven gas treatment plant
(i) Recovery of Tar: The gas from the coke ovens is passed through a tower in which liquor
ammonia is sprayed.Tar and dust get collected in a tank. The tank is provided with a heating
coils to recover back ammonia.
(ii) Recovery of ammonia: The gases are then passed through a tower where water is sprayed to
recover ammonia. The ammonia can also be recovered by dissolving it in H2SO4 to form
(NH4)2SO4, which is then used as a fertilizer.
(iii) Recovery of Naphthalene: The gases are passed through a cooling tower, where water at a
low temperature is sprayed.The gas is scrubbed with water until its temp. reduces
(iv) Recovery of Benzol: The gases are then introduced into a light oil or benzol scrubber, where
benzene along with its homologue is removed and is collected at the bottom.
(v) Recovery of H2S and other S compounds: are removed from the coke oven gas after the light
oil has been separated out.
The SO2 obtained can be used for the manufacturing of sulphuric acid, which can be used to
absorb NH3 from the coal gas
