Module 8. Fuels

Lesson 19

CLASSIFICATION AND COMBUSTION CHEMISTRY OF FUELS

19.1  Introduction

Fuels may be chemical or nuclear. Here, we are mainly concerned with chemical fuel which reacts with oxygen in air if the conditions are favorable and gives a lot of heat energy. A chemical fuel is a substance existing in any form of solid, liquid or gas. The heat energy liberated by complete burning of a unit mass or volume of fuel is known as its calorific value. In the burning of fuel, the chemical energy of fuel is converted into heat energy. Beneath the earth and sea, the plant and fossils have been converted to fuels while lying under earth’s heat and pressure for millions of years. Thus chemical fuels are also called as fossil fuels. A chemical fuel mainly contains carbon and hydrogen as its main constituents, however some other elements like sulphur, lead, nitrogen, ash etc may be present but in small proportions. Therefore chemical fuels are also named as hydrocarbon fuels. Examples of chemical fuels are coal, petroleum, natural gas etc. These are still the major source of energy to human beings. The other sources of energy are nuclear fuels which are not in the scope of this e-course.

19.2  Classification of Fuels

The chemicals fuels may be classified mainly on the basis of phase in which they exist normally as

1)         Solid Fuels

2)         Liquid Fuels

3)         Gaseous Fuels

Each of these fuels may further be classified as

1.      Naturally available fuels:  Fuels which are found naturally beneath the earth surface

2.      Transformed fuels: Fuels which are made or produced by processing of natural fuels

19.2.1  Solid fuels

A solid fuel is mainly wood and coal. The wood is naturally available by cutting of trees. However, coal may be naturally available or it may be prepared artificially from wood or natural coal. Depending on this, coal may be classified as.

19.2.1.1  Naturally available coal

1)      Peat:-It is the first stage of formation of coal deep below the surface of earth. It contains a large amount of moisture content upto 35%. So, it has to be dried before use. Its average calorific value is 23000 kJ/kg.

2)      Lignite:-It comes in the next stage of formation of coal after peat. It also has a large moisture content may be upto 40%. On drying, it becomes brittle and converted to flakes. It is considered superior to Peat. Its calorific value is approximately 25000 kJ/kg.

3)      Bituminous Coal:-It is a superior and naturally available coal containing very less moisture content but high carbon content. Its average calorific value is 33500 kJ/kg. It is further of two types: Caking Bituminous coal and Non-Caking Bituminous coal. Caking bituminous is a soft coal but swells and form pasty mass on burning. So burning is difficult to take place. It is useful for producing coal gas. Non-Caking variety is superior in burning and produces less smoke. It is mostly used in boilers for making steam and so is called Steam Coke.

4)      Anthracite Coal:-It is the final stage of coal formation and contains more than 90% carbon. So it is also most superior quality of naturally available coal. Its burning is smokeless and has a calorific value of approximately 36000 kJ/kg.

19.2.1.2  Artificially prepared coal

1)      Wood charcoal:-It is prepared by heating of wood with limited supply of air at a temperature nearly 300°C. It is used in metallurgical Industries.

2)      Coke:-It is produced by heating of naturally available coal continuously for two days in the absence of air. This process is known as carbonization of coal as the carbon content increases by removing of moisture and other gases. Due to high carbon content, it has a high calorific value. If coal is prepared on heating at a temperature of 500-700oC, it remains soft and called as Soft Cake. It is used in domestic applications. If coke is prepared at a higher temperature i.e. 900oC to 1100oC, it is called as hard coke. It is used in Cupola furnace for producing Cast Iron.

3)      Briquetted coal:-It is produced by mixing of finely ground coal with a binding material like coal tar, clay etc and then moulding it under pressure. It has advantage that it does not brake while handling and so prevents loss of fuel.

4)      Pulverized coal:-The name pulverized is given to coal which is crushed to fine powder by pulverizing machines. Generally low graded coal with high contents of ash is pulverized before using in cement and metallurgical Industries.

5)      Petroleum coke:- It is the residual of crude oil left after separating the other useful fractions like petrol, diesel etc in distillation and refinement processes.

19.2.2  Liquid fuels

These are the fuels which are found in liquid state and easier to handle and use as compared to solid fuels. The naturally available liquid fuel is petroleum which is not used directly as it is mixture of a number of useful substances. Thus before using, it is refined by boiling and condensation process in refineries distilleries to separate into a number of useful substances like petrol, kerosene, fuel oils, lubricating oils, coal tar etc. Out of these some are used as fuels, but some like lubricating oil, coal tar are used in other applications, From our subject point of view the following liquid fuels are important:

1)      Petrol or gasoline: - It is the most light and most volatile liquid fraction of petroleum fuel. It is distilled at a lower temperature upto 200°C by boiling of petroleum, Thereafter cracking process is used to prepare light petrol. Petrol is used as a fuel in all S.I. Engines.

2)      Kerosene:-It is a heavier fraction and less volatile fuel and distilled in boiling range upto 300°C. It is used in Jet engines and also in domestic stoves and ovens.

3)      Diesel Oils: - Diesel oils are the fuels which distillate on further higher temperature upto 370. Diesel oils lie between Kerosene and lubricating oils while separating from crude petroleum. These are the fuels which are commercially used in all C.I. engines, Diesel Gensets, boilers etc.

4)      Fuel Oils:-Fuel oil is similar to Diesel in specific gravity and distillation range, but their composition varies in a wide range. Rather fuel oils are used for industrial purpose. Diesel is also a kind of fuel oil.

19.2.3  Gaseous fuels

These are also naturally available or artificially prepared. The natural gas is available under the earth’s surface near petroleum fields. It is a mixture of methane, ethane and other like gases. Gaseous fuels are needed to be compressed for storing in containers and also for effective use. They are difficult to handle and require large heavy containers but their advantage is that they readily catch fire and free from impurities. So combustion is complete and also pollution is less.

The gaseous fuels, which are artificially prepared from other fuels, are categorized as:

1)      Coal gas:- It is obtained by carbonization of coal as discussed earlier in solid fuels. It consists mainly of hydrogen and some hydrocarbons. It’s calorific value is approximately 25000 kJ/m3.

2)      Liquified petroleum gas: - It is prepared from natural gas by separating lighter hydrocarbons i.e. Propane and butane. It is liquified and stored in cylinders under high pressure. Only under high pressure, a gas can be liquified at atmospheric temperature. Due to this a large quantity of gas occupies less space in cylinder. LPG is commonly used as cooking gas and also in gas engines and other Industrial processes.

3)      Producer gas:-It is obtained by partial combustion of coal in the presence of air and steam blast. It is used in glass melting process.

Some other types of fuel gases are also produced by carrying out different types of heating process over coal.

19.3  Calorific Values of Fuels

The calorific value of the fuel is defined as the energy liberated by the complete combustion or oxidation of a unit mass or volume of a fuel. It is expressed in kJ/kg for solid and liquid fuels and kJ/m3 for gaseous fuels. Fuels which contain hydrogen have two types of calorific values as higher calorific value and lower calorific value.

 19.3.1  Lower calorific value

It is the heat liberated per kg of fuel on its complete combustion after deducting the heat necessary to convert the water formed from hydrogen into steam, i.e. it is the actual heat liberated per kg of fuel and which can be further used.

19.3.2  Higher calorific value

It is the quantity of heat indicated by a constant volume calorimeter in which the steam formed in burning process is condensed and the heat of vapour is recovered.

The lower or net calorific value of fuel is obtained by subtracting latent heat of water vapour from higher or gross calorific values.

i.e.        LCV= HCV – 2465 mw

            where mw = mass of water vapour formed per kg of fuel.

            2465 kJ/kg is latent heat of water at temperature of 15oC.

 19.4  Combustion Chemistry of Different Constituents of Fuel

19.4.1  Carbon

It is the major constituent of a chemical fuel.

For complete combustion of carbon, the chemical reaction is as given below:

C + O2­ = CO2 +Heat generated

  or 1 mole of ‘C’+ 1 mole of ‘O2’=1 mole of CO2

  or       12 + 32=44    

                    ∵  Molecular weight of C = 12

                                              Molecular weight of O2 = 32

                                            Molecular weight of CO2 = 44                           

Thus for complete combustion, one kg of carbon requires  kg of O2 and produce  kg of CO2

    For incomplete combustion of Carbon, the chemical reaction is given as:

    2C + O2 =2CO

    or 2 mole of ‘C’+ 2 mole of ‘O2’=2 mole of ‘CO’

    or 24 + 32 = 2 × 28 = 56

    or 1 kg of C +  kg of O2 =  kg of CO2

Thus one kg of carbon requires  kg of Oxygen to convert to  kg of carbon-monoxide gas.

19.4.2 Carbon monoxide

For combustion of Carbon-Monoxide, the chemical reaction is as:

     2CO + O2 = 2CO2 + Heat

     2 mole of ‘CO’ + 1 mole of ‘O2’ = 2 mole of ‘CO’

     or 56 + 32 = 88

     or 1 Kg of CO +  Kg of O2 =  kg of CO2

  Thus one kg of CO requires  Kg of oxygen to produce  kg of ‘CO2’.

19.4.3  Sulphur

Sulphur is an undesirable constituent of fuel found in small proportions.

Its chemical reaction is given as:

      S + O2­ = SO2 +Heat generated

     or 1 mole of ‘S’ + 1 mole of ‘O2’=1 mole of ‘SO2’

     or 32 + 32= 64

     or 1 kg of S + 1 Kg of O2=2 kg of SO2

    i.e. One kg of sulphur requires equal weight of oxygen to produce 2 kg of sulphur-dioxide       

      gas, which is a poisonous gas.

19.4.4  Hydrogen

It is also found in fuel. Its chemical reaction is given as:

      2H2 + O2­ = 2H2O +Heat generated

      or 2 mole of H2 + 1 mole of O2=2 mole of H2O

      or 4 + 32=36

      or 1 kg of H2 + 8 kg of O2 = 9 kg of Water

      Thus one kg of hydrogen requires 8 kg of O2 to produce 9 kg of steam.

19.4.5  Methane

Methane or marsh gas is a hydrocarbon and one of the main constituent in gaseous fuels. Its

       Combustion is given by chemical reaction as:

             CH4 + 2O2 = CO2 + 2H2­O + Heat

        or 1 mole of CH4 + 2 mole of O2 = 1 mole of CO2 + 2 mole of H2O

        or 16 + 2 × 32 = 44 + 2 × 18

        or 16 + 64 = 44 + 36

        or One kg of CH4 + 4 Kg of O2= kg of CO2 kg of Steam

Thus one kg of methane requires 4 kg of oxygen to completely burn into kg of CO2 kg of Steam.

19.4.6  Ethane

Ethane is also one of the main constituent of gaseous fuels. Its combustion is given by chemical reaction.

       C2H2 + 3O2 = 2CO2 + 2H2­O

     or    1 mole of C2H2 + 3 mole of ‘O2’ = 2 mole of ‘CO2’ + 2 mole of H2O

     or    28 + 3 × 32 = 2 × 44 + 2 × 18

     or 1 kg of C2H2  kg of O2 =  kg of CO2 kg of Steam

In this way based on the chemical formulae of the constituents of a fuel and its chemical reaction with oxygen, we can estimate the products formed and also the exact mass or weight of oxygen required to completely burn the fuel. Also the analysis of the products of combustion i.e. flue gases formed can be done.

19.5  To convert volumetric composition into composition by weight of a mixture of gases

1)      Multiply the volume of each constituent by its molecular weight.

2)      Add all their weights and divide each weight by the total of all to find composition by weight.

To convert the weight compositions into volumetric composition reverse the procedure

For Engineering Calculations

 Air composition by weight is taken as 77% N2 and 23% of O2.

 Air composition by volume is taken as 79% N2 and 21% of O2.

19.6  Stoichiometric Quantity

Stoichiometric quantity of air is that quantity which is required for complete combustion of 1 kg of fuel without any oxygen left. Therefore, it is the chemically correct quantity of air required for complete combustion of 1 kg of fuel.

19.7  Excess Quantity of Air

It is the air supplied in excess than the stoichiometric quantity. It is always practically required for complete combustion because whole of air supplied for combustion purposes does not come in contact with the fuel completely and so portion of fuel may left unburnt. But disadvantage of excess air is that it exercises a cooling effect. Amount of excess air supplied varies with the type of fuel and firing conditions.