Calorific value Archives

What is meant by the calorific value of a fuel?

Fuels are substances that can normally burn readily in air or oxygen to produce large quantity of heat.

Examples of some common fuels are petroleum, natural gas and coal. These are called fossil fuels.

Different types of fuels have different heats of combustion. Heat of combustion is expressed in kilojoules per mole.

In our daily life, it is not very practical to measure the quantity of a fuel in mole. We normally measure the quantity of a fuel by its mass.

The calorific value or fuel value of a fuel is the amount of heat energy given out when one gram of the fuel is completely burnt in excess of oxygen.

The unit for fuel value is kJ g^-1.

Fuel value is also known as heat value.

A fuel with a high fuel value releases a lot of heat per gram when it burns.

Different types of fuels have different fuel values. Some fuels have very high fuel values but have low density, thus occupy a bigger volume. Others have high density but have low fuel values.

Some fuels can cause environmental pollution. There are fuels with high fuel values but are not very economical, not easily available or non-renewable.

The learning of thermochemistry helps us in the selection of a suitable fuel for a specific purpose.

The table below shows the fuel values of some food and common fuels.

Substance	Fuel value (kJ g^-1)
Apple	2
Egg	6
Bread	11
Wood	20
Coal	30
Butter	34
Gasoline	34
Kerosene	37
Natural gas	50
Hydrogen	143

The selection of a fuel to be used in industries depends on the following factors:

Fuel value of the fuel

Cost of the fuel

Availability and sources of the fuel

Effect of the fuel on the environment

Technology used to harness the energy from the fuel

Storage space needed to store the fuel

Fuels with the following qualities are normally chosen:

High fuel value

Cheap

Easily available

Do not pollute the environment

Easily harnessed

Require less storage space

Calorific value example: The heat of combustion of ethanol is -1376 kj mol^-1. Calculate the fuel value of ethanol.
[Relative atomic mass: H, 1; C, 12; O, 16]
Solution:

People also ask

What are the Different Types of Fuels and Their Characteristics

Fuels

Most of the substances used for burning are fuels. Substances that produce heat and light energy on burning are called fuels. Some commonly used fuels are wood, coal, cow dung cakes, kerosene, LPG, petrol, and diesel. We use fuels for cooking, heating, in automobiles, and for the generation of electricity. Based on their physical state, fuels can be classified as solid, liquid, or gaseous.

Types of fuels

Solid fuels	Liquid fuels	Gaseous fuels
Fuels that exist in solid state at room temperature are called solid fuels. Wood, charcoal, cow dung cakes, agricultural waste, and coal are examples of solid fuels.	Fuels that exist in liquid state at room temperature are called liquid fuels. Kerosene, petrol, diesel, and fuel oil are some examples of liquid fuels.	Fuels that exist in gaseous state at room temperature are called gaseous fuels. Petroleum gas, natural gas, and biogas are examples of gaseous fuels.

Characteristics of Fuels

Not all combustible substances can be used as fuels for all applications. This is because although all fuels produce heat and light on burning, some produce more energy than others. Besides the amount of energy produced, the choice of fuel, whether for domestic or industrial use, also depends upon its cost, efficiency, availability, and the extent of pollution it causes. Two important characteristics of a fuel are its calorific value and efficiency.

Calorific value of a fuel is defined as the amount of heat produced in kilo joules when one gram of a fuel is completely burned. The unit for representing calorific value is kilo joule (kJ). Calorific value is expressed in kJ/g. In simple terms, the higher the calorific value of a fuel, the more heat it produces when burned. Thus, the higher the calorific value of a fuel, the better fuel it is. Calorific values of some fuels are given in Table.

Fuel	Calorific value (kJ/g)
1. Wood pellets	17
2. Coal	25-33
3. Charcoal	35
4. Cow dung cakes	7
5. Coke	33
6. Kerosene	48
7. Petrol	47
8. Biogas	35-40
9. LPG	50
10. Methane	55
11. Hydrogen	150

When a fuel is burnt, some of the energy produced is given off as waste heat, which cannot be used for cooking or other purposes. This is what affects the efficiency of a fuel. Till date, no fuel-burning device is known to exhibit 100% efficiency.
The following are some of the important characteristics of an ideal fuel.

It should have a high calorific value.
Its ignition temperature should be low but well above the room temperature. If the ignition temperature is too low, the fuel will catch fire very easily (which could be dangerous) and if it is very high, the fuel has to be heated for a long time before it can catch fire.
It should have a moderate rate of combustion and should release heat in a controlled manner.
It should be fairly cheap and easily available. A fuel may have a very high calorific value but if it is expensive and not easily available, it cannot be used on a day-to-day basis.
It should be safe to handle, store, and transport.
It should not cause pollution on burning.

Let us discuss some fuels based on these characteristics.

Hydrogen as fuel: Hydrogen is considered the best fuel as it has the highest calorific value. Hence, it is considered the best fuel. However, as it is highly inflammable, there are difficulties associated with its transport, storage, and handling. It is, therefore, used as a fuel only where it is absolutely necessary (e.g., as rocket fuel).

Methane and LPG as fuels: Both methane and LPG have fairly high calorific values. They burn with a smokeless fire and, therefore, do not cause pollution. They are ideal for use as domestic fuels.

Petrol and diesel as fuels: Both petrol and diesel are mainly used in automobiles. Their main disadvantage is their limited availability. Another disadvantage is that their combustion releases harmful gases into the atmosphere.