In this article we will discuss about how to Calculate calorific value of fuels.
Meaning of Calorific Value:
The calorific value of a fuel is amount of heat liberated by its complete combustion. For solid and liquid fuels, calorific value is expressed in kJ/kg, whereas for gaseous fuels it is expressed as kJ/m3 where m3 is normal cubic metre measured at NTP conditions i.e., at 0°C temperature and 760 mm Hg barometric pressure (1.01325 bar). Sometimes, calorific value of gaseous fuels can also be expressed as kJ per cubic metre expressed at STP conditions. STP (standard temperature and pressure) conditions are taken as 15°C and 760 mm Hg barometric pressure (1.01325 bar).
A fuel consists of one or more combustible components like carbon, hydrogen, carbon monoxide, hydrocarbons, sulphur etc. Of the above mentioned combustibles, sulphur is not a desirable ingredient due to corrosive properties of SO2 formed by its combustion. The above mentioned combustibles liberate heat by their combustion, but to evaporate the water formed by combustion of hydrogen or hydrocarbons as well as to evaporate the moisture content of the fuel, its sensible heat from temperature to the saturation temperature at combustion pressure and its latent heat and superheat, if any, have to be supplied to bring it to the temperature of the products of combustion.
This heat is naturally taken from the heat of combustion of the fuel. Thus the total heat of combustion of the fuel is not available to do external work. The heat liberated by the combustion of the fuel, neglecting the heat required to evaporate the water is called the Higher Calorific Value of the fuel (HCV).
ADVERTISEMENTS:
Higher calorific value is the maximum heat energy liberated by the complete combustion of the fuel. This is also called as the gross calorific value of the fuel. If we subtract from the higher calorific value, an amount of heat required to evaporate the water formed, we get Lower Calorific Value (LCV) or Net Calorific Value (LCV) or Net Calorific Value of the fuel.
No definite agreement is to be found in the literature on fuel as to whether the lower calorific value shall be found simply by subtracting latent heat of steam or both the latent heat and sensible heat in cooling from 100°C, from the gross calorific value of the fuel; in the latter case it would be necessary to fix the temperature to which the products are finally reduced. In literature the net calorific value of the fuel is obtained by subtracting from higher calorific value the amount 2466 kJ/kg (latent heat of dry and saturated steam at STP (15°C) conditions) (amount of water formed because of the combustion of 1 kg fuel.)
Calorific values of solid and liquid fuels are found experimentally with the help of Bomb Calorimeter.
Theoretical Determination of Calorific Value (Provided Composition by Weight is known):
The calorific value of a fuel is the heat liberated from the combustion of 1 kg of the fuel. This value can be calculated from the analysis of the fuel if the calorific values of its constituents are known. Any oxygen contained in the fuel is assumed to be already combined with the hydrogen; the heat of this hydrogen is not, therefore, available for further combustion.
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The calorific values of the individual combustible in the fuel are given below:
Carbon C burns to CO2 → 33915 kJ/kg
Carbon C burns to CO2 → 10200 kJ/kg
Hydrogen H, burns to H2O → 1444515 kJ/kg
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Sulphur S burns to SO2 → 9630 kJ/kg
Goutel suggested the following formula from calculating the higher calorific value when the percentage proximate analysis of fuel is known. The formula is, cal. value = 343.3 x fixed carbon % + α x % volatile matter kJ/kg.
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The value of the factor depends on the percent amount of volatile matter on the dry ash free basis. The value of α decrease with the rise in volatile matter as can be seen from the table below-
Goutel formula is unreliable for fuels having high percentage in oxygen.
Experimental Determination of Calorific Value of Fuel:
An apparatus which is used for determining the calorific value of a fuel is known as a fuel calorimeter.
ADVERTISEMENTS:
The principle of all the calorimeters is the transference of heat of combustion of the given weight of fuel to water and the vessel. From the observed rise of temperature of the water and the container the calorific value of the fuel can be determined by equating the heat given out by the fuel to the heat taken by the water and the container. In order to know the heat taken by the container, water equivalent of the container should be known.
In this method of determining the calorific value of the fuel the following conditions should be satisfied:
(i) The combustion of the fuel must be complete
(ii) The heat must be transferred completely to the water
(iii) Cooling losses from the calorimeter must be corrected
(iv) The rise of temperature of water must be correctly determined because the mass of the fuel is very small in comparison with the quantity of the water heated.
Boy’s Gas Calorimeter:
This is the standard instrument used for measuring the calorific value of gas. It consists of two burners in which a known volume of gas is burnt.
The hot gases produced by combustion pass up the copper chimney. This is surrounded by a double coil of metal tubing through which a known weight of cooling water is circulating. After passing upwards the hot gas are deflected downwards through the space containing the inner coil. By the time the gases reach the top of this passage, practically the whole of their heat has been absorbed by the circulating water. The gases are then passed away into the atmosphere.
The cooling water enters the outer coil. After circulating through both coils it leaves at the water box. Its temperature is measured at inlet and outlet by thermometers at inlet and outlet by thermometers shown. These can be read to a fraction of a degree by the reading leases. For this purpose special thermometer giving least count of 0.01 can also be used.
The temperature of the exhaust gases is measured by another thermometer. Any water formed by the combustion of hydrogen is condensed on the coils and is collected at the base of the instrument. From here it is drained through the pipe and is collected in a measuring glass.
In order to increase the cooling surface of the coils, the outsides of the tubes are ribbed or finned as shown in the cross-sectional view. The outer cylinder of the instrument is lined with a felt jacket to prevent any leakage of heat. For measuring the volume of the gas consumed, a meter is used reading 1/100 cu. meter for one revolution of the hand.
The test is carried out after the gas has been burning under uniform conditions for 45 minutes, the rate of gas flow is obtained by timing the rotating hand of the gas meter with a stop watch. The cooling water is collected in a measuring vessel during the test.
The cold water supply to the coils should not be less than 5°C, below the room temperature. The barometer reading, gas temperature and room temperature should be taken during the test. The thermometers should be read at regular intervals whilst the test is in progress and the average values obtained from these readings.
Let V = Volume of the gas burnt during the test, reduced to NTP
mw = Weight or mass of cooling water used during the test
t1 = Average inlet water temperature
t2 = Average outlet water temperature
w = Weight or mass of condensed water collected
hfg = Latent heat of steam at its partial pressure
= 2466 kJ/kg (assumed)
Generally mass flow rate of cooling water is adjusted such that the gas exhaust temperature is same as that of atmosphere.
Bomb Calorimeter:
One of the best instruments for measuring the calorific value of powdered and liquid fuels is the bomb calorimeter. The fuel is burnt in a strong steel chamber, known as a bomb which is immersed in a known mass of water. The fuel is placed in a crucible inside the bomb which is filled with oxygen under a pressure of 25-30 atmospheres. It is then electrically ignited by a platinum or magnesium wire. The heat liberated by the rise in temperature of water surrounding the bomb and then calorific value of the fuel is determined.
The advantages of using such a procedure for the determination of calorific value are as given below:
(a) Due to high pressure of oxygen supplied, large excess oxygen is provided and therefore the combustion is complete.
(b) All heat liberated will be given to the surrounding water as the combustion is at constant volume.
(c) The addition of oxygen and very small amount of water (generally 10 cc) in the bomb do not affect the combustion.
(d) The test procedure and the arrangement employed favour the accurate computation of temperature loss correction (cooling correction).
The calorimeter consists of a stainless steel vessel called the bomb which is placed in a calorimeter vessel. The bomb is also made of monel metal. The calorimeter is placed in a double walled chromium plated jacket vessel containing water. The calorimeter is closed on top with ebonite cover.
This is because of the reduction of radiation losses to the surroundings. An electrically driven stirrer is provided to agitate water in the vessel. Mercury in glass thermometer having accuracy not less than 0.01°C is used to measure the temperature of water surrounding the bomb. A thermometer called Beckman’s thermometer is used for this purpose.
Generally silica-crucible is used to hold the fuel. For filling the bomb with oxygen, necessary arrangement is provided such as copper tubing, a pressure gauge, control valve on the oxygen cylinder are provided. For test 4 or 12 volt battery is required.
About 1 gm of the fuel is weighed in the crucible. Then 10 cc of water is taken in the bomb. When solid fuel as coal is used, then it is ground to a fine powder and then in a hand press this powder is made into a pallet and while preparing the pallet, a cotton thread will be embedded and connected to the two electrodes. The crucible is kept in a ring connected to the two electrodes. The lid or a screwed cover is then tightened.
Then bomb is connected to an oxygen cylinder through a pressure gauge and copper tubing. Oxygen at 25-30 bar pressure is filled in the bomb. Then measured quantity of water is taken such that water will not come to the electric wire leads, (generally this quantity may be 1500 cc or 2500 cc). Motor of the electrodes are connected to the main power supply and the electrodes are connected to the battery through a tap-key.
After this arrangement is done, the set-up is ready for the experiment or test:
(1) Preliminary Period:
Start the stirrer so that temperature of water in calorimeter will be same and uniform throughout. Take the readings after every one minute till the temperature becomes uniform for 2-3 consecutive readings. This generally requires 5 minutes.
(2) Main Period:
Once the temperature in the calorimeter becomes uniform, then the fuel is ignited by pressing the tap-key so that current will flow through the fuse wire and it will glow. A cotton thread is connected to the wire so that fuel pallet will be ignited giving out heat. Continue to take readings of temperature at the interval of 1 minute till the maximum temperature is reached.
(3) After Period:
After the maximum temperature is reached, the temperature will start decreasing. Continue to take temperature readings at the interval of one minute. Take 10-15 readings, and then stop the test.
Take the bomb out of the calorimeter and release the pressure by opening the valve. After removing the lid of the bomb, measure the quantity of water in the bomb. The difference in this water will be the weight of H2O formed because of the combustion of hydrogen in the fuel. This weight is very much essential to measure or determine the lower calorific value of the fuel. Ash content of the fuel is also obtained after weighing the crucible.
Observation Table:
1. Weight of empty crucible — W1gm
2. Weight of crucible + fuel — W2gm
3. Weight of the fuel taken — (W2-W1)gm
4. Weight of water in the bomb — 10 gm (generally)
5. Weight of water after test — 10 + x gm
6. Weight of moisture condensed — x gm
7. Weight of water in the calorimeter — mw gm
8. Weight of water equivalent of calorimeter — me gm
9. Weight of fuse wire — mfw gm
10. Calorific value of fuse-wire — (C.V.)FW
11. Weight of crucible + ash
12. Weight of ash — W4 = W3 – W1 gm
13. Weight of ash.
14. Observed temperature rise during main period — Δt = (Tn – T0)
Calculations for Calorific Value of Fuel:
Total heat liberated = Total heat absorbed
Heat released by fuel + Heat released by fuse wire + Heat released by cotton thread = Total heat absorbed by (Water + Bomb + Calorimeter)
For liquid fuels, the same procedure is followed except of making a pallet of a coal powder. One end of the cotton thread will be connected to the fuse wire and the other end is dipped in the liquid fuel.