Fuels - Part VII

2. Calorific value of fuels
Heat energy is measured in units of joules or calories (1calorie = 4.18 joules). The heat generated by fuels when they burn in joules or calories measures quality of fuels. All fuels do not burn efficiently. Thus there are fuels that produce more heat than the others are. This can be distinguished in terms of number of joules or calories that they generate on burning.

The amount of energy generated when 1 unit mass of fuel is burnt completely is known as the calorific value of the fuel. The word calorific is used, not “joulific” because of the use of the word calorific has been in use for a very long time. When 1 gram of charcoal is burnt, it produces 33 kilo joules. Thus the calorific value of charcoal is 33kJ/g. Sometimes instead of calorific value, another term kilowatt per kilogram (KWh/kg) is used. The table below gives the calorific values of some of the common fuels used for domestic and industrial use.

Later on in the chapter we will see how calorific value is measured. But from the table it is easy to understand that comparison between fuels can be done when their properties as fuels is standardized in terms of calorific values.

Hydrogen as fuel : Hydrogen gas has the highest calorific value in the table given above. Thus hydrogen is the best fuel but since its transport and handling is difficult, hydrogen is used as fuel where it is absolutely necessary. Otherwise hydrogen gas as a source of common fuel is not used in domestic or in industrial situations. Hydrogen gas jet along with oxygen gas is used for producing a very hot flame, which is used for cutting metals in industries.

Methane or butane as fuel :  Both methane and butane produce good amounts of heat. They are ideal for use as domestic fuels. Since methane has higher percentage of hydrogen than butane, its calorific value is more. Methane (CH₄) has 25% hydrogen and butane (C₄H₁₀) has 17% hydrogen.

Wood as fuel :  Wood has been traditionally used as fuel. The main content of wood is cellulose (C₆H₁₀O₅)_n. The presence of oxygen in a fuel, helps oxidation but does not contribute to heat or its calorific value. In fact it is seen that if a substance contains oxygen, it will produce less heat energy per unit weight when the substance burns. In wood therefore, the percentage of carbon and wood is quite less. This gives wood quite a less calorific value.

How to measure calorific value of a fuel :
The method by which calorific value of substances is measured is called a calorimeter. The fuel whose calorific value is to be measured, is first weighed. Let its mass be g grams. Let m grams of water be heated by this fuel. Measure the temperature of the water before and after the fuel is burnt completely. Let t be the rise in temperature of m grams of water when g grams of fuel is burnt completely.

Heat produced = Q = m x s x t

m =  mass of water in grams

s   =   specific heat of water = 4.2.J/gm x °C

t   =  rise in temperature of the water.

Thus Q amount of heat is generated by g amount of fuel. The calorific value is given by the following equation.

Calorific value =    Q / g    joules per gram. (for value in kilo joules, divide by 1000)

The figure above shows an arrangement where a candle’s calorific value is being measured. Heat the water for a while and note the rise in temperature.

Let       W1 = initial weight of the candle

            W2 = final weight of the candle

            W1 - W2 = weight of wax burnt to heat the water.

            T1 = initial temperature of water

            T2 = final temperature of the water

            T2 - T1 = rise in temperature of water.

            m = mass of water

            s = specific heat of water in J/gm/°C

                                         heat produced               m x s x (T2 - T1)
Calorific value of wax =      =         =     J/gm
                                       mass of wax burnt                (W1 - W2)

There are many sources of error in the simple experiment described here.  The table below shows how the errors can be minimized.

Type of error	How to minimize the error
Heat lost to the surrounding is neglected	Enclose the calorimeter in an insulating box
Heat lost by the beaker is neglected	Determine the thermal capacity of the beaker and include it in the equation
Incomplete or inefficient combustion of fuel in air	The fuel should be burnt in oxygen atmosphere and not air.

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