Lecture Notes on Fuels and Combustion | Thermodynamics

In this article we will discuss about:- 1. Introduction to Fuel 2. Chemical Equations (Combustion Equations) Involved in Combustion 3. Excess Air Supply for Combustion 4. Mass Fraction and Mole Fraction of Constituents.

Introduction to Fuel:

The fuel is a material which when once raised to its ignition temperature continues to burn if sufficient oxygen or air is available. The principal constituents of any fuel are carbon and hydrogen. The materials which evolve heat after burning, are called combustibles. Carbon and hydrogen are combustibles. Sulphur is also a combustible material.

When anything slowly combines chemically with oxygen, the process is called oxidation. When the same process occurs with a considerable swiftness and exotherm chemical reaction, it is called combustion; whereas such a process with almost instantaneous action is called detonation.

The mechanical engineers are interested in combustion. Chemists are interested in oxygenation while designers of arms, ammunition, missiles etc. are interested in the process of detonation.

Main priorities of fuels that in general interest to an engineer are:

1. Heat liberated by complete combustion i.e., the calorific value of the fuel

2. Combustion temperature

3. Air required for complete combustion and

4. Volume and composition of the products of combustion.

Combustion Temperature:

The combustion temperature indicates the degree of calorific intensity of the fuel. Quicker the rate of burning of a fuel, larger is the calorific intensity. The combustion temperature is the temperature of the burning fuel as measured on the products of combustion. If C_Pm is the mean specific heat in kJ/kg-K of the products, W_p kg is the mass of the products of combustion per kg fuel burnt, Q kJ the amount of heat released by combustion, t_a the temperature of atmosphere, then the combustion temperature t is given by the equation-

Q = m_pC_pm(t_c-t_a)

The amount of air supplied per kg fuel, as well as the completeness of combustion influence the combustion temperature of fuels.

Chemical Equations (Combustion Equations) Involved in Combustion:

The process of combustion of elements and compounds can be represented by the chemical equations. These equations are also called as combustion equations. The equations can be treated as those dealing with molar weights or molar volumes (kmol). These dealings can further be simplified by the fact, that equal volumes of different gases at same pressure and at any temperature sufficiently above the liquefying temperature of the respec­tive gas, always contain the same number of molecules. Thus the relative weights of equal volumes of gases will be the same as their molar weights. Thus, at constant pressure, in case of complete combustion on carbon, is repre­sented by-

C + O₂ = CO₂

So that, 1 kmol of carbon + 1 kmol of oxygen = 1 kmol of carbon dioxide

This equation represents molecules.

Similarly, (negligible volume of C) + (1 volume of O₂) = (1 vol of CO₂)

This equation represents volumes.

Again, (1 molar weight of carbon) + (1 molar weight of oxygen) = (1 molar weight of carbon dioxides)

This equation represents weights or masses.

Thus, (12 kg of carbon) + (32 kg of oxygen) = (44 kg of carbon dioxide) or (1 kg of carbon) + (8/3 kg of oxygen) = (11/3 kg of carbon dioxide)

From this, we see that 1 kg of carbon requires 8/3 = 2.667 kg of oxygen to procedure 11 /3 3.667 kg of carbon dioxide or negligible volume of carbon combines with 1 cu.m (m³) oxygen and produces 1 m³ of carbon dioxide. In these two cases, both total weight and total volume of the gases before combustion have remained the same even after combustion.

Let us now consider the case of hydrogen chemical or combustion equation is-

2H₂O + O₂ = 2H₂O

i.e., 2kmol + 1kmol = 2kmol

or 2 m³ + 1 m³ = 2 m³ by volume

or 4 kg+ 32 kg = 36 kg by weight

or 1 kg + 8 kg = 9 kg by weight

Thus, the total volume before combustion was (2+1=3 m³) and became 2 m³ after combustion. Thus contraction took place during combustion. According to the principle of conservation of mass, the total weight before and after combustion is unaltered. Similarly, in case of some other combustion process expansion takes place during the process.

The above type of equations can also be made to show the amount of heat liberated or absorbed during the process, depending upon whether the process is exothermic or endothermic Thus, for example-

C + O₂ = CO₂ + 406976 kJ

This means that complete combustion of 1 kmol (12 kg) of carbon produces 1 kmol (44 kg) of carbon dioxide, thereby liberating 406976 kJ of heat. Similarly,

2H₂ + O₂ = 2H₂O + 573619 kJ

means that two kmols (4 kg) of hydrogen liberate 573619 kJ heat during its complete combustion.

We will now, consider some more examples of combustions.

If carbon burns in insufficient supply of oxygen, the product of combustion will be carbon monoxide instead of carbon dioxide, and the process will be represented by the chemical equation-

The carbon monoxide produced during the above combustion is a fuel and therefore it should not be allowed to escape otherwise it would be a loss. It is highly poisonous and has no smell; therefore it is dangerous if it escapes.

Also, combustion of carbon monoxide-

Sulphur is also one of the combustibles. It is of very minor importance in contributing to the calorific value of a fuel because only small quantities are present and its calorific value is very low being 9260 kJ. It has got injurious effects because it forms, on burning, sulphuric acid which corrodes the metal and therefore fuel having higher percentage of sulphur is undesirable particularly for steam raising purposes.

When sulphur burns with oxygen, it produces sulphur dioxide. The combustion of sulphur can be represented by a chemical equation-

S + O₂ = SO₂

Sulphur + Oxygen = Sulphur dioxide

i.e., 32 kg+ 32 kg = 64 kg

1 kg + 1 kg = 2kg

Sulphur + Oxygen = Sulphur dioxide

Minimum Quantity of Air Required for the Complete Combustion of One Kg of Solid or Liquid Fuel:

An adequate supply of oxygen is essential for complete combustion. If the combustion is complete, then and then only maximum heat is available from the given fuel. The theoretically exact amount of oxygen required can be calculated with the help of various chemical or combustion equations.

These calculations will give us the minimum or theoretical quantity of oxygen if we know the ultimate analysis of the fuel.

The oxygen required for combustion is taken from the atmospheric air although in some cases a certain amount of oxygen is a constituent of the fuel. Air is a mixture of oxygen, nitrogen, a very amount of carbon dioxide and very small traces of rare gases such as neon, argon, krypton etc. For all practical purposes we assume that air is made up of 23% by weight of oxygen and the remaining 77% by weight of nitrogen. If considered by volume, air contains 21% of oxygen and 79% of nitrogen.

Once we know the amount of oxygen necessary for complete combustion of 1 kg of fuel, we can determine the weight of air necessary for complete combustion of 1 kg fuel. This minimum or theoretical amount of air required for complete combustion of 1 kg, 1 kg fuel is also known as stoichiometric air.

Let C, H and S be the weights of carbon, hydrogen and sulphur contained in 1 kg of fuel

Excess Air Supply for Combustion:

In actual practice we supply air more than the theoretical minimum amount in order to ensure the complete combustion of the fuel because all of the air supplied does not come into intimate contact with the particles of the fuel. Weight of air supplied over and above the theoretical air supplied, is called excess air. Generally this is given as a percentage of the theoretical air.

A large amount of excess air has a cooling effect on the process of combustion and represents a loss, and in order to avoid this cooling effect air is preheated before it enters the furnace of a boiler. With natural draught system the excess air is more in comparison with the artificial draught system. The excess air may approach 100% but the modern practice is to use 20-25% or even 50%.

When the Lancashire type boiler uses hand firing with natural draught system 10 to 12% CO₂, in the flue gases would be considered good practice. With mechanical stokers and artificial draught it would be quite reasonable to expect 12 to 15% CO₂.

In case of I.C. engines all the air taken in during the induction or suction stroke will not come in contact with the fuel particles. Therefore excess air is supplied and it should be reduced to minimum to get more specific output.

If the supply of air is less than the minimum required then the mixture is rich and if the supply of air is more than 30% in excess of the theoretical minimum, the mixture is known as lean or weak mixture.

Mass Fraction and Mole Fraction of Constituents:

Let A, B, and C be three gases in a mixture. Let each of these gases having a mass of m_a, m_b and m_c. Then,

m = Total mass of the mixture considered

Then, mass fraction of the mass of the constituent is defined as the ratio of the mass of the constituent to that of the mixture and is denoted by m_f.

Mole is a unit used to express a quantity of gas whose mass is equal to its molecular weight in magnitude. Thus mass is given in kg, the one mole is represented by K-mol. Similarly, when the molecular weight is expressed in gm or pound, mole will be represented by gram-mole or g mole and pound-mole or p.mol. Thus one g mol of O₂ is 32 gm and p.mol, if H₂O is 18.0156 pounds and so on. The number of moles n, is obtained by dividing the mass m, in grams or pound-mass or kilograms, by the molecular weight M.

Thus in a mixture of gases A, B and C if n_a, n_b and n_c are the moles, then the total number of moles is n_a + n_b + n_c = s sag.

Then, mole fraction of a component in a gas mixture is defined as the ratio of number of moles of the component to the total number of moles of the mixture.