The materials which are burnt to. Produce heat energy are known as fuels. Examples of fuels are: wood, Coal, Cooking gas (LPG), Kerosene, Diesel and Petrol. In our day life, the most important source of energy are fuels. Fuels are, in fact, the concentrated store- house of energy. This energy is released in the form of heat when the fuels are burnt. Since a variety of fuels are available to us, we should have some means of deciding which fuel is the best for our purpose.
Before we describe some of the characteristics (or properties) of fuels which will help us choose the best possible fuel for a particular use, we should know the meaning of two terms 'calorific' value of a fuel and 'ignition temperature' of a fuel.
These are discussed below:
All the fuel produce heat energy on burning. Different fuels produce different amounts of heat on burning. Some fuels produce more heat whereas other produce less heat. The usefulness of a fuel is measured in terms of its calorific value.
Higher the calorific value, better the fuel will be. The amount heat produced by burning a unit mass of the fuel completely is known as its calorific value. The unit of mass usally taken for measuring the calorific value of a fuel is "gram". So, we can also say that "That amount of heat produced by burning 1 gram of a fuel completely is called it's calorific value".
For example, when one gram of a carbon fuel (like charcoal) is burned completely, it produces about 3300 joules of heat, so the calorific value of charcoal is 33000 joules pet gram or 33000 J/g. Since joule is a very small unit of heat energy, so the calorific value is usually expressed as kilojoules per gram (kJ/g).
Thus, the calorific value of charcoal becomes 33 kilojoules per gram which is written in short form as 33 kJ/g. Thus, the common unit of measuring calorific value is kilojoules per gram (kj/g). Please note that the 'calorific' value of fuel represents the heat value' of the fuel.
Actually, the term " calorific value' comes from the fact that the earlier unit of measurement of heat was calorie. These days, although we mostly use joule ad the unit of heat but the term calorific value continues to be used as such. If we look up the table of calorific value, we will find that hydrogen gas has the highest calorific value of 150 kilojoules per gram. Thus, because of its High calorific value, hydrogen is an extremely good fuel. Most of the common fuels are the compounds of hydrogen and carbon called 'hydrocarbon'. Since hydrogen has the highest calorific value therefore, a fuel containing higher percentage of hydrogen will have a higher calorific value than another fuel which has a lower percentage of hydrogen in it.
For example, L.P.G has higher percentage hydrogen than coal, so LPG has a higher calorific value than coal. We will now describe the ignition temperature of a fuel.
We know that a fuel has to burnt to obtain heat energy. Now, before a fuel can catch fire and burn, it must be heated to a certain minimum temperature. The minimum temperature to which a fuel must be heated so that catch fire and start burning, is known its ignition temperature. When we little heat to cooking gas coming out of gas burner so that it gets stove, we actually supply a little heat to cooking gas coming out of gas burner so that it gets heated to its ignition temperature and start burning. No fuel burn unless it is heated to its ignition temperature.
