Calorific Value
The calorific value of a fuel is the amount of heat it generates on its complete combustion. For solid and liquid fuels, we express it in unit kJ/kg.
Furthermore, for the gaseous fuels, we express it as kJ/m3. Here m3 is a normal cubic meter that we measure at NTP conditions. Also, we can express the calorific value of gaseous fuels as kJ per cubic meter which we express at STP conditions.
The Science Behind the Calorific Value
Fuel consists of one or more combustible elements or compounds like carbon, hydrogen, carbon monoxide, hydrocarbons, Sulphur, etc.
Moreover, these combustibles generate heat by their combustion. Naturally, we take this heat from the heat of combustion of the fuel.
But the total heat of combustion of the fuel is not available to do external work. in addition, the heats that combustion of the fuel produces neglect the heat that it requires to evaporate the water which we call the Higher Calorific Value of the fuel (HCV).
However, hcv is the maximum heat energy that the complete combustion releases on the burning of the fuel. We refer to it as the gcv of the fuel.
We can subtract from the hcv, an amount of heat required to evaporate the water formed, we can easily find Lower Calorific Value (LCV). Besides, we also know it as Net Calorific Value (LCV) i.e. Net Calorific Value of the fuel.
There is no definite agreement in the literature is available on fuel. Whether to lower calorific value can be found simply by subtracting latent heat of steam or both the latent heat and sensible heat in cooling from 100°C from the gcv.
However, in literature, the ncv of the fuel is obtained by subtracting from the hcv the amount 2466 kJ/kg. Also, this is the latent heat of dry and saturated steam at STP conditions and 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.
Procedure to Calculate Calorific Value
It is the measurement of the heat value or amount of energy released and is either measured in gcv or ncv.
Gross calorific value (GCV) that is Higher Heating Value (HCV) is the amount of heat that complete combustion releases by burning a unit of fuel. It assumes all water vapour produced during the combustion process is fully condensed.
Net Calorific Value (NCV) means lower heating value (LHV) i.e. lower calorific value (LCV) is determined by the subtraction of heat of vaporization of the water vapour from the higher heating value.
In addition, It assumes that water vapour leaves with combustion products without full condensation. Furthermore, we shall purchase Fuel on the basis of NCV.
For the conversion of GCV to NCV, we can use the formula given as follows:-
GCV = NCV + 584 (9 x H2%+M%)/100)
Where,
GCV, Kcal/Kg
NCV, Kcal/Kg
H2%, Hydrogen Percentage by Weight in the fuel
M% is the moisture percentage by weight in the fuel 584. Latent heat corresponding to the partial pressure of water vapour in Kcal/kg.
Calculation of GCV of fuel
For calculation of GCV of fuel, we need to do an analysis of fuel and get the constituent of fuel on a weight basis. Based on the enthalpy of formation we can estimate the calorific value of the fuel.
C+ O2 —- CO2 + 8137.5Kcal/kg of carbon
H2 + ½ O22—-H20 + 28905 Kcal/Kg of hydrogen
S +O2—SO2+ 2181 Kcal/kg of Sulphur
Suppose a fuel contains carbon, hydrogen, Nitrogen, Sulphur, Oxygen as 86.5%, 12%, 0.9%, 0.2%, 0.4% respectively,
To calculate GCV of fuel we use the formula as follows:-
GCV of Fuel
= QC*(%C/100)+ QS*(%S/100) + QH*2*(%H/2) -[2*(%O/32)]/100
GCV of fuel
= 8137.5*(86.5/100)+ 2181*(0.2/100) + 28905*2*(12/2)- [2*(0.4/32)]/100
=7038.9 + 4.3 + 3454 = 10497.2 Kcal/Kg
NCV of Fuel = GCV – 584 [(9 * H2%+M%)/100]
=10430 – 584[(9*12+0)/100]
= 10497.2- 630.7 = 9866.5 Kcal/kg
Solved Questions for you
Q. How do we determine the calorific value of food?
Ans. To determine the number of kcals in a given food we directly measure the energy that it produces. Place the food in a sealed container i.e. a bomb calorimeter and surround it by water. Also, burn the food was complete. Then it results in the rise of temperature. Further, this will give the calorific value of food.
I was looking for this answer, “As all the other are mild steel means they have high resistance while stainless steel is a hard steel that has low resistance.” stainless steel has low resistance?
Low resistance to what type of load, impact or tensile load. With change in low type, property of steel may very. And, also, Will stainless steel have high yield strength?