An Introduction to Combustion processes

An Introduction to Combustion processes

Introduction

combustion to occur three things must be present: a fuel to be burned, a source of oxygen, and a source of heat. As a result of combustion, exhausts are created and heat is released. You can control or stop the combustion process by controlling the amount of the fuel available, the amount of oxygen available, or the source of heat

Fuel is expensive, and inefficient combustion processes are wasteful. Furthermore, inefficient combustion results in atmospheric pollution which can lead to prosecution. To understand what happens during combustion processes you need a little knowledge of chemistry. The main combustible elements which combine with oxygen are hydrogen, carbon and sulfur. Their atomic and molecular weights, together with that of nitrogen are listed in Table 1.

TABLE 1

You will recall that the atomic weight of an element tells you how much heavier its atoms are than an atom of hydrogen. The atoms of the gases hydrogen, oxygen and nitrogen combine together naturally in pairs to form molecules which is why they are listed in Table 1 as H₂,O₂ and N₂. Their molecular weights are thus twice their atomic weights.
During combustion, hydrogen in a fuel combines with oxygen in the air to form H₂O which eventually condenses into water. When completely burnt, carbon in the fuel combines with oxygen
to form CO₂, which is carbon dioxide. If there is insufficient oxygen, however, some of the carbon combines to form CO, which is carbon monoxide. This is a dangerous gas whose toxic effects
have been responsible for many deaths due to badly ventilated heating systems. The presence of carbon monoxide in exhaust gas
always indicates inefficient combustion. Sometimes however this is unavoidable. It is always present in motor vehicle exhaust gases but in much smaller quantities than previously, due to advances in engine management and exhaust systems.
Sulphur in fuel combines with oxygen to form SO₂, which is sulphur dioxide. This can also have undesirable effects. It combines with the water produced from the combustion of hydrogen,
to form sulphurous acid which is a corrosive pollutant. Oxygen for combustion comes from the atmosphere which is made up of about 23% oxygen and 77% nitrogen by mass. There are very small
amounts of other gases present but these can be neglected. Nitrogen in the air plays no part in the combustion process. It passes through the system and leaves with the exhaust gases. Its is an unwelcome passenger, since it carries heat energy away with it to the atmosphere but its presence is unavoidable. There might also be unburned oxygen present in the exhaust gases, which has the same effect.
The molecular weights of the products of combustion, H₂O, CO, CO₂ and SO₂, can be found by adding together the atomic weights of their component elements as shown in Table 2. Oxygen
and nitrogen are also included because of their likely presence in exhaust gases.

TABLE 2

Stoichiometric air to fuel ratio

The word stoichiometric means ‘chemically correct’ and the stoichiometric air to fuel ratio is that which will in theory provide a sufficient mass of oxygen for the complete combustion of a given
mass of fuel. In order to find the mass of oxygen required per kilogram of fuel, we need to know how much is needed to burn the hydrogen content, how much is needed to burn the carbon content and how much is needed to burn the sulphur content. These can then be added together to find the total amount of oxygen required per kilogram of fuel. Because we know that air is 23% oxygen and
77% nitrogen by mass, we can then calculate the total amount of air required. We now need to take a closer look at the chemical equations for the combustion of hydrogen, carbon and sulphur.

Complete combustion of hydrogen
The chemical equation for this process which results in the formation
of steam is

2 H₂ + O₂ = 2 H₂O

This indicates that two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water. The molecular weights on both sides of the equation must be equal, that is,
(2×2)+32=(2×18)
4+32= 36
Dividing both sides by 4 gives
1+8=9
This tells us that for the complete combustion of 1 kg of hydrogen, it requires 8 kg of oxygen, and produces 9 kg of steam.

Complete combustion of carbon
The chemical equation for this process which results in the formation of carbon dioxide is

C +O₂ = CO₂

This indicates that one atom of carbon combines with one molecule
of oxygen to form one molecule of carbon dioxide.
Molecular weights: 12+ 32 = 44
Dividing both sides by 12 gives
1+(32/12)=44/12

This cancels down to
1+8/3=11/3
This tells us that for the complete combustion of 1 kg of carbon, it requires 2 ²⁄₃ kg of oxygen, and produces 3 ²⁄₃ kg of carbon dioxide.
Incomplete combustion of carbon
If there is insufficient oxygen for complete combustion, carbon monoxide will be formed. The chemical equation for this process which results in the formation of carbon monoxide is

2 C+ O₂ = 2 CO

This indicates that two atoms of carbon combines with one molecule of oxygen to form two molecules of carbon monoxide.
Molecular weights: (2×12) + 32 = (2× 28 )
                                 224+32 = 56
Dividing both sides by 24 gives
1+ (32/24) = 56/24
This cancels down to
1+(4/3) = 7/3
This tells us that 1 kg of carbon will combine with 1¹⁄₃ kg of oxygen to produce 2¹⁄₃ kg of carbon monoxide.

Complete combustion of sulphur

The chemical equation for this process which results in the formation
of sulphur dioxide is

S + O₂ = SO₂

This indicates that one atom of sulphur combines with one molecule of oxygen to form one molecule of sulphur dioxide.
Molecular weights: 32 + 32 = 64
Dividing both sides by 32 gives
1+1=2

This tells us that for the complete combustion of 1 kg of sulphur, it requires 1 kg of oxygen, and produces 2 kg of sulphur dioxide

EXPLAINING 

IF 1 kg of a fuel is made up of H kg of hydrogen, C kg of carbon and S kg of sulphur. Using the above values, the total oxygen required to completely burn the kilogram of fuel will be

There may be oxygen of some form already present in the fuel. Let this be O kg per kilogram of fuel. We can assume that this will be used for combustion so that it requires a little less oxygen from the
air. The above formula can then be written as

Now, we will recall that air contains 23% of oxygen by mass and so the mass of air required per kilogram of fuel, which is the stoichiometric air to fuel ratio, will be given by

Sometimes, and particularly with coal-fired furnaces, it is found necessary to supply more air than is theoretically required. This is because of the difficulty in mixing the fuel and air together, even
when the coal has been pulverised. The percentage of excess air required varies with the type of boiler or furnace. Too much is a disadvantage as it carries heat away to the atmosphere.

Key Points
-The atomic weight of an element is a comparison of the weight of its atoms to those of hydrogen.
-The incomplete combustion of carbon results in the production of carbon monoxide.
-By mass, air contains 77% nitrogen and 23% oxygen.
-The complete combustion of a fuel generally requires more air than is theoretically required.