Introduction

The estimation of calorific value is an important operation in the area of energy exploration which reveals the bound of potential energy in various fuels. One of the most essential parameters in understanding the efficiency and energy content in fuels is the calorific value, or heat of combustion. The main subject of this blog is the process by which we calculate the calorific values in all cases the energy generated through the combustion of fuels is quantified.

Before knowing about different methods to measure Calorific Value,it is necessary to know about what is a calorific value?

Defining Calorific Value

The calorific value refers to the measure of heat energy released when one kilogramme of a specific fuel is completely combusted. It is usually quantified in joules per kilogram (J/kg) or British thermal units per pound (BTU/lb). Calorific value therefore needs to be established in order to assess the suitability of different fuels used in various applications, ranging from industrial processes to electricity generating plants.

Constituent	Higher calorific value
C	8080 kcal/kg
H	34500 kcal/kg
S	2240 kcal/kg

If oxygen is also present, it combines with hydrogen to form `H_2O`.Thus, the hydrogen in 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.

$H_{2} + (\frac{1}{2}) \times O_{2} = H_{2} O$

1 8 9

that is 8 parts of oxygen combines with 1 part of hydrogen to form water or for every 8 parts of oxygen, 1 part of hydrogen gets fixed.If the fuel contains x mass of oxygen then

Fixed hydrogen = 18×X=Mass of oxygen in fuel8

Amount of hydrogen available for combustion = (H-`frac{o}{8}`)

Dulong's formula for calculating calorific value is given as

Gross calorific value (HCV) = `frac{1}{100}`[8080C + 34500(H-`frac{o}{8}`) + 2240S] kcal/kg

Here C, H, O and S are percentages of carbon, hydrogen, oxygen and sulphur in fuel.

Net calorific value (LCV) = (HCV-`frac{9H}{100}times587`) kcal/kg

(HCV – 0.09 H × 587) kcal/kg

(Latent heat of steam = 587 kcal/kg).

Units of calorific value and heat

Unit of calorific value

The units of calorific value for solid, liquid and gaseous fuels are given below.

System	Solid / Liquid fuels	Gaseous fuels
CGS	calories/g	cm3
MKS	kcal/kg	m3
BTU	BTU/lb	Btu/ft3

These units can be interconverted as follows

1 cal/g =1 kcal/kg = 1.8 BTU/lb

1 kcal = 0.1077 BTU/

f t^{3}

1 BTU/

f t^{3}

= 9.3 kcal/

m^{3}

Units of heat

1.Calorie:It is defined as the amount of heat required to raise the temperature of 1 g of water by 1 °C ( from 15 °C to 16 °C)

1 calorie = 4.185 Joules =

4.185 \times 10^{7}

ergs.

2.Kilocalorie:It is defined as the amount of heat required to raise the temperature of 1 kg of water by 1 °C (from 15 °C to 16 °C). 1 kcal = 1000 cal.

3.British Thermal Unit (BTU):It is defined as the amount of heat required to raise the temperature of 1 pound (lb) of water by 1 °F (from 60 °F to 61 °F)

1 BTU = 252 cal = 0.252 kcal = 1054.6 Joule =

1054.6 \times 10^{7}

ergs.

4.Centigrade Heat Unit (CHU):It is defined as the amount of heat required to raise the temperature of one pound of water by 1 °C (from 15 °C to 16 °C).

1 kcal = 3.968 BTU = 2.2 CHU

Gross and Net Calorific Value

Gross Calorific Value (GCV):It is also called higher calorific value (HCV) and is defined as the total amount of heat produced when a unit quantity (mass/volume) of fuel is burnt completely, and the products of combustion are cooled to room temperature.

Usually all fuels contain hydrogen. During combustion, the hydrogen present in the fuel is converted into steam. When the combustion products are cooled to room temperature, the steam gets condensed into water and heat that equals the latent heat of condensation of steam is evolved. This heat gets included in the measured heat, and so its value is high; hence, it is called higher calorific value.

Low Calorific Value (LCV):It is also termed as net calorific value (NCV) and is defined as the heat produced when a unit quantity (mass/volume) of a fuel is burnt completely and the hot combustion products are allowed to escape.

In actual practice, when a fuel is burnt water vapor escapes along with the hot combustion gases; hence, heat available is lesser than the gross calorific value. Therefore, this is called low calorific value or net calorific value.

Thus LCV = HCV – Latent heat of water vapour formed.

As 1 part by weight of hydrogen gives 9 parts by weight of water,

H2+ ½ O2 → H2o

LCV = HCV – Weight of hydrogen in unit mass/volume of fuel × 9 × latent heat of steam.

Basically there are two ways to measure the Calorific Value.

Bomb calorimeter
Boy's Gas calorimeter

Bomb calorimeter

Principle A known amount of fuel is burnt in excess of oxygen and the heat liberated is absorbed in a known amount of water. This heat liberated is measured by noting the change in temperature. Calorific value of the fuel is then calculated by applying the following principle:

Heat liberated by fuel = Heat absorbed by water and the calorimeter.

Construction A simple sketch of the bomb calorimeter is shown in the Figure 1.

Figure 1

It consists of the following parts:

Stainless Steel Bomb:It consists of a long cylindrical container made up of stainless steel. It has a lid that is made air tight with the help of screws. The lid is provided with two holes for electrodes and has an oxygen inlet valve. A small ring is attached to one of the electrodes. This ring acts as a support for nickel or stainless steel crucible in which the fuel is burnt. Magnesium wire touching the fuel sample extends across the electrodes. The steel bomb is lined inside with platinum to resist corrosive action of HNO3 and H2 SO4 vapors formed because of burning of fuel and is designed to
withstand high pressure (25–50 atm).
Copper Calorimeter:The bomb is placed in a copper calorimeter containing a known amount of water. The calorimeter is provided with an electrical stirrer and a Beckmann thermometer that can read accurate temperature difference of up to 1/100th of a degree.
Air Jacket and Water Jacket:The copper calorimeter is surrounded by an air jacket and a water jacket to prevent loss of heat owing to radiation.

Working A known amount of fuel (0.5–1 g) is taken in a clean crucible supported over the ring. A fine magnesium wire, touching the fuel sample, is then stretched across the electrodes. About 10 mL of distilled water is introduced into the bomb to absorb vapors of sulphuric acid and nitric acid formed during combustion, and the lid of the bomb is tightly screwed. The bomb is filled with oxygen at 25 atmospheric pressure and placed in the copper calorimeter containing a known weight of water. The stirrer is started and the initial temperature of water is noted. The electrodes are then connected to a 6-volt battery to complete the circuit. The sample burns and heat is liberated. This heat is absorbed by water. Maximum temperature shown by the thermometer is recorded. Time taken to cool the water in the calorimeter from maximum temperature to room temperature is also noted. The gross calorific value of the fuel is calculated as follows.

Calculations

Let

Weight of fuel sample taken = x g

Weight of water in the calorimeter = W g

Water equivalent of calorimeter, stirrer, thermometer, bomb etc = Wg

Initial temperature of water in the calorimeter = `t_1` ºC

Final temperature of water in the calorimeter = `t_2` ºC

Higher calorific value of fuel= H calorie / g

Heat liberated by burning of fuel = x × H

Heat gained by water = W × ∆T × specific heat of water = W (`t_2` - `t_1`) × 1 cal

Heat gained by calorimeter = w (`t_2` - `t_1`)

Total heat gained = W (`t_2` - `t_1`) + w (`t_2`- `t_1`)

= (W + w) (`t_2` - `t_1`)

But

Heat liberated by the fuel = Heat absorbed by water and calorimeter.

x × H = (W + w) (`t_2` - `t_1`)

H= `frac{(W+w)(t_2-t_1)}x` cal/g (or kcal/kg)

Net (lower) calorific value

LCV = HCV – 0.09 H × 587 cal/g or kcal/kg

(Latent heat of condensation of steam = 587 kcal/kg).

Corrections

The following corrections are applied to get more accurate results.

1.Fuse Wire Correction:The gross calorific value calculated above includes the heat liberated by the ignition of Mg fuse wire; hence, this amount of heat has to be subtracted from the total value.

2.Acid Correction:During combustion, sulphur and nitrogen
present in the fuel get oxidised to `H_2SO_4` and `HNO_3`.

S + `O_2` `rightarrow` `SO_2`

2`SO_2` + `O_2` + 2`H_2O` `rightarrow` 2`H_2SO_4` ∆H = – 144000 cal

2`N_2` + 5`O_2` + 2`H_2O` `rightarrow` 4`HNO_3` ∆H = – 57160 cal

Hence, the formation of acids is exothermic and this should be subtracted from the obtained value of GCV.

3.Cooling Correction:Heating and cooling are simultaneous processes. As the temperature rises above the room temperature, the loss of heat occurs due to radiation and the highest temperature recorded will be slightly less than that obtained if there was no heat loss. A temperature correction (cooling correction) is therefore necessary to get the correct rise in temperature.

If the time taken for the water in the calorimeter to cool from maximum temperature attained to room temperature is ‘x’ minutes and the rate of cooling is dt/min, then the cooling correction is x × dt and this is to be added to the rise in temperature.

HCV of fuel (H) =

(W+w)×(t2-t1+cooling correction)-(Acid+fuse wire correction)×Mass of the fuel(x)=Calorific Value of Gaseous Fuels

Junker’s Gas Calorimeter: It is used for measuring the calorific value of gaseous and volatile liquid fuels.

Principle: A known volume of gas is burnt at known pressure in a small enclosed combustion chamber. The heat liberated is absorbed by water flowing at constant rate through the water jacket. By knowing the initial and final temperatures of water, the quantity of water and weight of water condensed, the calorific value can be determined.

Construction

It consists of the following parts:

Bunsen Burner: It is used for the combustion of gaseous fuel.It is clamped at the bottom and can be pulled out or pushed into the chamber during combustion.
Gasometer: It measures the volume of the gas burning per unit time. It is attached with a manometer fitted with a thermometer to record the pressure and temperature of the gas before burning.
Pressure Governor: It regulates the supply of a gaseous fuel at constant pressure.
Gas Calorimeter: It consists of a vertical cylindrical combustion chamber where combustion of gaseous fuel is carried out.The combustion chamber is surrounded by an annular water space where water is made to circulate. Loss of heat by radiation and convection is prevented by an outer jacket, which is chromium-plated. Moreover,the outer jacket contains air that is a very good heat insulator.There are openings at appropriate places where thermometers are placed for measuring the temperature of the inlet and outlet water.

Working: A known volume of gas is burnt at a constant rate in a combustion chamber in the presence of excess air. All the heat produced is absorbed by water circulating in the annular space around the combustion chamber.

Observations

The volume of gaseous fuel burnt at a given temperature and pressure
in a certain time = V`m^3`
Weight of water circulated through the coils in time t = W g
Temperature of inlet water = `t_1` ºC
Temperature of outlet water = `t_2` ºC
Weight of steam condensed in time t in a graduated cylinder = m kg.

Let GCV of the fuel = H

Heat produced by the combustion of fuel = V × H

Heat absorbed by circulating water = W (`t_2` - `t_1`)

Assuming no loss of heat,

V × H = W (`t_2` - `t_1`)

HCV or GCV

H = `frac{W(t_2-t_1)}V` kcal/`m^3`

Weight of steam condensed in a certain time t by the combustion of
V`m^3` of the fuel = m kg

Mass of `H_2O` condensed per `m^3` of the fuel = m/V kg

Latent heat of steam per `m^3` of the fuel
= `frac{mtimes587}V` Kcal,

therefore, NCV or LCV = [H - `frac{mtimes587}V`] Kcal/`m^3`

Boy’s Gas Calorimeter

Like Junker’s calorimeter, the Boy’s gas calorimeter is also used to find the calorific value of gaseous and volatile liquid fuels. It consists of the following parts.

Gas Burner:Gas burner is used for the combustion of a known volume of gas at a known pressure. The volume of the gas burnt is measured with the help of a gasometer and the pressure of the gas is monitored using pressure governor.
Combustion Chamber:The combustion chamber or chimney has copper tubes coiled inside and outside the combustion chamber. Water circulates in these coils. It enters from the top of the outer coil,passes through the outer coils, moves to the bottom of the chimney and then moves upwards through the inner coil and exits from the top.
Thermometers:Two thermometers t1 and t2 measure the temperatures of the incoming and outgoing water.
A graduated beaker is placed at the bottom to collect the condensed steam produced during combustion.

Working

The working is similar to Junker’s calorimeter. The water is circulated and the fuel is burnt to provide an initial warming period of 15 minutes. When the calorimeter is warmed, the rate of flow of the gas is adjusted and it is burnt inside the calorimeter. The heat produced by the combustion is absorbed by water circulating in the copper tubes. The rise in temperature, volume of gas burnt, volume of water circulated in the coils in time t and the mass of steam condensed help in finding the calorific value of the given fuel sample (for observation and calculation refer Junker’s calorimeter).

Applications and Important of Calorific Value

1. Fuel Quality Assessment

The calorific value further determination is especially important in the evaluation of the quality of different fuels. It gives the opportunity to compare energy content and efficiency enabling the choice of the best fuel for particular purposes.

2. Energy Production

Industries largely depend on correct calorific value assessments to precisely optimize combustion processes that are vital in energy generation. This guarantees maximum energy harvest and efficiency in power generation.

3. Environmental Impact

To evaluate the environmental impact of different fuels one has to familiarize with their calorific value. Higher calorific value fuels normally provide a higher energy output with lower emissions per unit of energy produced and this way promote cleaner and greener energy production.

4. Research and Development

Estimation of calorific value is one of the basic stages of a new fuel and energy products researches and developments. It offers an understanding of possible deployments, and how operate about emerging technologies.

Challenges and Considerations

The calorific value determination is a strong aid but impurities are, combustion completeness, and fuel composition variability should be taken into account as they may affect the measurements.

Conclusion

The assessment of calorific value is a voyage into the place of energy potential, giving essential information about the productivity and the appropriateness of various fuels for a horde of undertakings. Scientists and engineers utilize methods such as the bomb calorimetry, and the adiabatic flame calorimetry; hence, this produces the key to maximizing combustion processes and developing energy production into a future where energy utilization will be both efficient and green.

Know about different methods to measure Calorific Value,application and importance of it