6.4.1 Describes the Combustion Process in a Boiler or an Engine Cylinder

Combustion is a chemical reaction between fuel and oxygen.

In a diesel engine, during the compression stroke, the incoming fresh air is greatly compressed. When a gas is compressed, its temperature increases. At the end of the compression stroke, fuel is injected, thoroughly mixing with the air. Due to the increased temperature, this fuel gets rapidly vaporized and ignited automatically (a process known as compression-ignition).

For complete and efficient combustion, the correct quantities of fuel and air must be supplied to the combustion chamber. When the air supply is insufficient, the fuel is not completely burnt, and black exhaust gases and less power output will result.

6.4.1.1 Combustion Process in a Boiler

Boiler is used to produce steam for various marine applications such as heating and to drive machines in some vessels. In a boiler, water is heated to produce steam under pressure.

The combustion of a fuel oil in a boiler furnace takes place in a number of stages, which are described as follows:

1. Heating: The oil is first heated in fuel oil heaters. When the fuel is heated, the viscosity (Viscosity is fluid's resistance to flow) reduces and makes it easier to pump, filter, and finally to atomize.

2. Atomization: The heated oil is now passed through the burners where it is atomized; this process breaks it up into a fine spray of droplets which helps in better combustion process.

3. Air Supply: The air from the supply fan passes through the air register (a device which can control the air supply) to the swirling ring, which swirls the air to achieve better and even mixing of fuel and air.

   

Combustion air is supplied in two stages: primary air and secondary air.

• Primary air impacts directly onto the fuel cone issuing from the burner, assists the atomization and starts the combustion process.

• Secondary air supplies air to continue the combustion process further into the furnace.

6.4.2 Describes the chemical reaction in combustion as being between combustible materials such as hydrocarbon fuels and the oxygen contained in atmospheric air

Combustion is a chemical reaction between fuel and oxygen.

Combustion can occur when the following conditions are met:

• Fuel

• Heat

• Oxygen

• Chemical reaction

All flammable material has a "FLASH POINT" and an “AUTOIGNITION POINT”.

• The flash point of fuel is the lowest temperature at which sufficient vapors are given off in a momentary flash when an ignition source is applied near the surface.

• The autoignition temperature of a substance is the lowest temperature at which it will spontaneously ignite in normal atmosphere without an external source of ignition, such as a flame or spark.

Air

Air is a mixture of nitrogen and oxygen with a small amount of carbon dioxide and other gases.

Fuel

There are many different types of substances used as fuels. One large group of fuels are those obtained from crude oils, such as petrol, gas oil, diesel oil, and heavy oil.

All fuel oils are made up from mixtures of hydrocarbons. Hydrocarbons are compounds that consist of a mixture of carbon and hydrogen atoms.

6.4.3 States that, as a result of combustion, heat energy becomes available, enabling thermodynamic operations to be carried out

As a result of combustion, heat energy becomes available, enabling thermodynamic operations to be carried out. Thermodynamic operations will be studied separately under thermodynamics.

6.4.4 The calorific value (CV) of a substance is the amount of energy released during complete combustion

The heat energy released during the complete combustion of a unit mass of substance under specific conditions is known as calorific value (CV).

The units are: Mega joules per kilogram [MJ/kg].

6.4.5 The calorific valves for fuel are usually states with respect to unit mas in the case of solid and liquid fuels and unit volume in the case of gaseous fuels

The calorific valves for fuel are usually states with respect to unit mass in the case of solid and liquid fuels and unit volume in the case of gaseous fuels. This is the normal practice.

6.4.6 States that the main combustible elements in marine fuels are carbon, hydrogen and sulphur

The most important elements for combustion in fuel oils are:

• Carbon (C)

• Hydrogen (H)

• Sulphur (S)

6.4.7 States the appropriate calorific values of the elements given in the above objective

The calorific value of a fuel depends on its makeup. The constituents of fuel which release energy when they are burnt are Hydrogen, Carbon, and Sulphur.

Here are the calorific values for the main combustible elements in fuel:

• Hydrogen (H) releases the most energy: 144000 kJ/kg

• Carbon (C) releases 33700 kJ/kg

• Sulphur (S) is the lowest, only releasing 9750 kJ/kg

It should be evident, therefore, that the less sulphur, and the more hydrogen a fuel contains, the greater its calorific value will be.

6.4.8 State that sulphur is usually present in marine fuel

Sulphur is usually present in marine fuel.

For example, a typical marine heavy fuel oil may contain 85% carbon, 12% hydrogen, and 3% sulphur by mass.