CSVTU NOTES ONLINE: July 2009

UNIT – 2 (Mr. NILAY KASHYAP)

DEFINITION OF FUEL:-

Fuels can be defined as any combustible substance which during combustion gives large amount of heat which can be used economically for domestic and industrial purposes.

During combustion of a fuel the atom of carbon, hydrogen etc. combines with oxygen and liberates large amount of heat and also forms new compounds (like CO2, H2O etc.). These new compounds have less energy. The heat released during combustion is the difference in reactants (C, H, and O) and products (CO2, H2O) energy.

Fuel + O2 -----------> Products + Heat

(More heat)

CLASSIFICATION OF FUEL:-

Fuels can be classified –

On the basis of their Occurrence -

Natural or Primary fuel – Fuels which are found in nature as such are called Natural fuels.

Ex. – Wood, Coal, Peat, Petroleum etc.

Artificial or Secondary fuel – Fuels which are prepared form primary fuel are called secondary fuels.

Ex. – Coke, Petrol, Bio gas etc.

On the basis of Physical State of aggregation –

On this basis their physical state of aggregation fuels are classified in to three types - solid, liquid and gas.

Fuels

Primary Fuels

Secondary Fuels

Solid

Liquid

Gas

Liquid

Gas

Solid

Wood Crude Natural Charcoal Petrol Coal gas

Peat Oil Gas Coke Diesel Water gas

Lignite

Coal Tar Oil gas

Dung Kerosene

CHARACTERISTICS OF GOOD FUEL –

High Calorific Value: - A fuel should have high calorific value, since the amount of heat liberated and temperature attained thereby depends upon the caloric value.
Moderate Ignition Temperature :- A fuel should have Moderate Ignition Temperature .If a fuel has low ignition temperature it can causes fire hazards during storage and transport of fuel and if fuel have high ignition temperature then it is safe for storage and transport but their might be some difficulty during ignition.
Low Moisture Content: - The moisture if present in the fuel reduces its heating value. Hence, fuel should have low moisture content.
Low Non-Combustible Matter: - After combustion the non – combustible matter remains, generally in the form of ash or clinker. They reduce the heating value. Each % of non – combustible matter in fuel means a heat loss of about 1.5%. Hence, a fuel should have low non – combustible matter.

Moderate Rate of Combustion: - If the rate of combustion is low then the required high temperature may not be possible. If the rate of combustion is high then it gets out of control. So fuel must be burn with a Moderate Rate.
Harmless Combustion Products: - They should not pollute the atmosphere by emitting CO, SO2 & H2S.
Low Cost: - A good fuel should be readily available in bulk and at low cost.
Easy to Transport: - Fuel must be easily handled either solid , liquid or gas. fuels can easily be transported from one place to another.
Uniform Size: - In case of solid fuel, the size should be uniform so that combustion is regular.
Controllable Combustion: - So that combustion can be started or stopped when required.

COMBUSTION:-

Combustion is an exothermic chemical reaction. It is followed by the release of heat & light at a rapid rate, thus the temperature rises. In combustion of fuel the atoms of carbon, hydrogen etc. combine with oxygen with the simultaneous liberation of heat at a rapid rate.

This energy is liberated due to the formation of new compounds having less energy in them, thus the energy (heat) released during combustion process is the difference in energy of reactants and that of products generated.

Fuel + O2 Products + Heat

(more heat) (less heat)

Ex. – Combustion of carbon in presence of oxygen; liberates 97 Kcal of heat

C (s) + O2 (g) CO2 (g) + 97 Kcal

Ignition Temperature: - It is the minimum temperature at which a substance ignites and burns without further addition of heat from outside.

Calculation of Air Quantities: - To find out the amount of air required for the combustion of a unit quantity of a fuel, it is necessary to apply the following chemical principles –

Substance always combines in definite proportion. These proportions are determined by their molecular masses.

Mass Proportion

C (s) + O2 (g) CO2 (g)

12 + 32 44

This indicates that mass proportion of C and O2 and CO2 are (12:32:44). This means 12 gm of carbon requires 32 gm of oxygen and 44 gm of CO2 formed.

22.4L of any gas at 00C and 760 mm pressure has a mass equal to its 1 mol. Since the molar mass of O2 is 32 gm thus 22.4 L of O2 S. T. P. will have a mass of 32 gm.
Air contains 21% of oxygen by volume and 23% of oxygen by mass. Hence, the amount of oxygen required by the fuel, & the amount of air, can be calculated.

23 Kg of oxygen is supplied by 100 Kg of air

1 Kg of oxygen is supplied by 100 Kg of air = Kg.

21 m3 of oxygen is supplied by 100 m3 of air

1 m3 of oxygen is supplied by 100 m3 of air = m3 of air

28.94 g mol-1 is taken as molar mass of air.
Minimum oxygen required for combustion = Theoretical oxygen required – O2 present in fuel.
Minimum O2 required should be calculated on the basis of complete combustion. If the combustion products contain CO and O2, then excess O2 is found by subtracting the amount of O2 required to burn CO to CO2.
The mass of any gas can be converted to its volume by assuming that gas behaves ideally

PV = nRT

P = Pressure (atm) V = Volume (liters)

n = No. of moles T = Temp. (Kelvin)

Total amount of hydrogen is either present in the combined form (H2O) or free form. Combined form is not combustible. Rest of hydrogen is called available hydrogen, takes part in combustion.

2H2 + O2 2H2O + Heat

Mass- 4 32

Now 1 part of hydrogen combines 8 parts by mass of oxygen, so available oxygen.

Nitrogen, ash and CO2 present in fuel are non combustible matters. Hence they do not take oxygen during combustion process.
Total amount of oxygen consumed by the fuel will thus be given by the sum of the amount of oxygen required by the individual combustible constituents present in the fuel.

Since air has 23% by wt. of oxygen or 21% by volume of oxygen.

For complete combustion.

(i) Wt. = Net O2 gm.