Fuel combustion is a fundamental process in mechanical production, especially in Kenya’s manufacturing and energy sectors where efficient fuel use directly impacts operational costs and environmental compliance. Understanding the chemical composition of fuels and their combustion characteristics enables mechanical production technicians to optimize machinery performance and reduce harmful emissions. This chapter equips technicians with the knowledge to analyze fuel elements, write balanced combustion equations, and perform essential calculations for solid and gaseous fuels commonly used in Kenya’s industrial settings.
A fuel is a substance that burns in air or oxygen and releases heat. The gases used in oxy-fuel welding and cutting (acetylene, propane), the charcoal, diesel and petrol used in workshops, and the coal or wood burned in boilers are all fuels. Every fuel is made of a few chemical elements, and the way those elements join in compounds decides how much heat the fuel gives, how much air it needs and which gases leave the flame. Before combustion can be calculated, the terms element, compound, atom, molecule, atomic mass and molecular mass must be clear. Combustion is the rapid chemical reaction of a fuel with oxygen that gives heat and light. Stoichiometry is the calculation of the exact amounts of fuel, oxygen and products in a reaction from the balanced equation.
Each term below is followed by a simple example from fuels used in a welding workshop.
An element is a pure substance that cannot be split into simpler substances by chemical means. It is made of one kind of atom only. Each element has a name and a symbol of one or two letters. The elements that matter in fuels are carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulphur (S). Carbon and hydrogen are the main combustible elements. Sulphur also burns, but it makes harmful gases and is not wanted.
Example: charcoal is mostly the element carbon, and the hydrogen gas that is sometimes used as a fuel is the element hydrogen.
A compound is a pure substance made of two or more different elements joined chemically in a fixed proportion. A compound has properties different from those of its elements, and it can be split into its elements only by a chemical process. Most fuels are compounds of carbon and hydrogen, called hydrocarbons.
Examples:
A mixture is different. Air is a mixture of nitrogen, oxygen and other gases that are not chemically joined, and a fuel such as diesel is a mixture of many compounds.
An atom is the smallest part of an element that still has the properties of that element. An atom has a small nucleus at the centre and electrons moving round it in shells.
Examples: hydrogen has 1 proton and 1 electron. Carbon has 6 protons, 6 neutrons and 6 electrons. Oxygen has 8 protons, 8 neutrons and 8 electrons. The electrons in the outer shell take part in chemical reactions, and these outer electrons decide how an atom joins to others.
A molecule is a group of two or more atoms joined chemically. It is the smallest part of a substance that can exist on its own and still has the chemical properties of the substance. A molecule of an element has atoms of the same kind (for example O₂, N₂ and H₂, each with two atoms), and a molecule of a compound has atoms of different kinds (for example CO₂, H₂O and CH₄).
The formula of a molecule shows how many atoms of each element it has. In C₃H₈, the small numbers show 3 carbon atoms and 8 hydrogen atoms. The sketches below show how the atoms are joined by bonds.
H H - O - H O = C = O
| (water) (carbon dioxide)
H -- C -- H
| H - C ≡ C - H
H (acetylene)
(methane)
The atomic mass of an element is the mass of one of its atoms compared with a standard. The standard is one twelfth of the mass of an atom of carbon with 12 particles in its nucleus, so carbon is exactly 12. It has no unit when written this way, and it is also given in kg per kmol.
For combustion calculations whole numbers are used: C = 12, H = 1, O = 16, N = 14 and S = 32. The more exact values are 12.011, 1.008, 15.999, 14.007 and 32.06. The whole numbers give results accurate to within about 1 % for fuel calculations.
Example: an atom of oxygen has an atomic mass of 16, so it is 16 ÷ 12 = 1.33 times the mass of an atom of carbon.
The molecular mass of a substance is the sum of the atomic masses of all the atoms in one molecule. It is also the mass in kilograms of one kmol of the substance (kg/kmol). It is used to find how many kilograms of a gas are needed or formed in a reaction.
Examples (using C = 12, H = 1, O = 16, N = 14):
| Substance | Formula | Calculation | Molecular mass |
|---|---|---|---|
| Hydrogen | H₂ | 2 × 1 | 2 |
| Oxygen | O₂ | 2 × 16 | 32 |
| Nitrogen | N₂ | 2 × 14 | 28 |
| Water | H₂O | 2 × 1 + 16 | 18 |
| Carbon monoxide | CO | 12 + 16 | 28 |
| Carbon dioxide | CO₂ | 12 + 2 × 16 | 44 |
| Methane | CH₄ | 12 + 4 × 1 | 16 |
| Acetylene | C₂H₂ | 2 × 12 + 2 × 1 | 26 |
| Propane | C₃H₈ | 3 × 12 + 8 × 1 | 44 |
| Butane | C₄H₁₀ | 4 × 12 + 10 × 1 | 58 |
Using the molecular mass to find the composition of a fuel by mass:
Using the atomic and molecular masses in a combustion equation:
The elements found in fuels, with their symbols and masses, are best kept in a table, and the atoms are sketched with the nucleus at the centre and the electrons in circles (shells) around it.
| Element | Symbol | Atomic number | Atomic mass (whole number) | Electrons in shells | Role in fuels |
|---|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 | Combustible. Gives much heat per kg and forms water |
| Carbon | C | 6 | 12 | 2, 4 | Combustible. Main element of most fuels. Forms CO₂ or CO |
| Nitrogen | N | 7 | 14 | 2, 5 | Does not burn. Present in air and in some fuels |
| Oxygen | O | 8 | 16 | 2, 6 | Supports burning. Present in air and in some fuels |
| Sulphur | S | 16 | 32 | 2, 8, 6 | Combustible but unwanted. Forms SO₂, which makes acids that corrode metals |
To sketch an atom:
To sketch a molecule, write the symbols of the atoms and join them with lines. One line is a single bond, two lines a double bond and three lines a triple bond. Each carbon atom has four bonds, each hydrogen atom one and each oxygen atom two, as in the sketches of methane, water, carbon dioxide and acetylene in the previous section.
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Create a free accountThis chapter began by clarifying fundamental terms related to fuels, such as elements, compounds, atoms, molecules, atomic mass, and molecular mass, along with the representation of elements and their symbols. It then progressed to the formulation and balancing of combustion equations for both solid and gaseous fuels, essential for understanding fuel behavior during burning. The discussion on conversion analysis covered methods to determine the theoretical minimum air volume for complete combustion and how to convert between volumetric and mass analyses of fuel composition. Calculations of the mass of carbon present in flue gases provided insight into the carbon content released during combustion. The chapter further explained how to calculate the mass of dry flue gases per kilogram of fuel burnt by comparing carbon masses in the fuel and exhaust gases. It also addressed the determination of excess air supplied by measuring unused oxygen in flue gases and the total air supplied during combustion. Finally, the use of the Orsat apparatus for flue gas analysis was described, including its components, operation, and practical application in monitoring combustion efficiency.