6.5.1 States That the Elements Carbon and Hydrogen Combine Chemically with Oxygen during Combustion to Form the Gaseous Products Carbon Dioxide and Water Vapour

Fuel contains hydrocarbons, which are compounds of hydrogen and carbon. When the fuel burns, its hydrocarbons react with oxygen.

• The hydrogen atoms combine with oxygen to make water vapour, H2O

  

The carbon atoms combine with oxygen to make carbon dioxide, CO2

6.5.2 Explains the Part Played by Nitrogen in the Combustion Process

99% of engine intake air is comprised of nitrogen (N2) and oxygen (O2). NOx is formed during the combustion process. During combustion, a large amount of heat energy is released. At these elevated flame or combustion temperatures, nitrogen is no longer inactive and reacts with oxygen to form nitric oxide (NO) and nitrogen dioxide (NO2).

6.5.3 States that, to ensure that the combustion process is as complete as possible, excess air is normally supplied.

In theory, to have the most efficient combustion in any combustion process, the quantity of fuel and air would be in a perfect ratio to provide perfect combustion with no unused fuel or air. This type of theoretical perfect combustion is called stoichiometric combustion. In practice, additional air beyond the theoretical “perfect ratio” needs to be added to the combustion process—this is referred to as “excess air.”

6.5.4 States That Excess of Air Must Be Kept to a Minimum, Consistent with Good Combustion

6.5.4.1 Boiler burner

Excess air must be kept minimum, consistent with good combustion.

 

Increasing excess air reduces efficiency of the boiler due to the cooling effect. Further, the flame also would be unstable due to excess air. Optimizing excess air usage can be one of the simplest ways to achieve significant fuel savings.

6.5.5 States That either the Percentage of Carbon Dioxide or the Percentage of Oxygen in Exhaust Gas Should be Continuously Recorded

Diesel emissions include pollutants that can have adverse health and environmental effects. Most of these pollutants originate due to incomplete combustion. Common pollutants include unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and Sulphur oxides (SOx). Therefore, the percentage of CO2 or O2 in Exhaust Gas Should be Continuously Recorded in order to check the effective combustion. Analysis of exhaust gas from combustion engines can help evaluate engine performance and diagnose problems.

6.5.6 States That Although Excess Air is Supplied, There May be Some Incomplete Combustion of Carbon-to-Carbon Monoxide (CO)

Although theoretically enough oxygen is supplied to burn all the carbon-to-carbon dioxide, this is not totally achieved in reality. There will be local areas of incorrect fuel/air ratios within the cylinder where the carbon is not completely oxidized to CO2. Instead, it is partially burnt to form carbon monoxide.

2C + O2 = 2CO

6.5.7 State that in practice products of combustion are normally a gaseous mixture of carbon dioxide, Sulphur dioxide, water-vapour, possibly carbon monoxide and ash, possibly containing sodium and vanadium.

Products of combustion in practice are normally a gaseous mixture which contains:

• Carbon dioxide 

• Sulphur dioxide 

• Water-vapour 

• Nitrogen oxides 

• Possibly carbon monoxide in case of incomplete combustion and ash

Vanadium is a naturally occurring element in marine fuel and fuel when delivered often contains a small amount of sodium. Exhaust gas possibly contains these two elements as well.

6.5.8 State that poor combustion creates the smoke, which pollutes the atmosphere and wastes fuel and reduces the efficiency of the engine or boiler.

The dark color of the smoke is caused by suspended fine carbon particles due to incomplete combustion taking place. In marine diesel engines, poor combustion is basically due to two main reasons: the first one is insufficient air supply, and the second one is incorrect fuel injection. Poor combustion wastes fuel and reduces the plant's overall efficiency. These various pollutants can pollute the air as well as contribute to acid rain.

6.5.9 State that the production of smoke may lead to prosecution.

With stringent environmental regulations and health concerns, the production of smoke may lead to prosecution. Many ports are against ships emitting thick black smoke. Such situations can lead to unwanted delay and result in the company having to pay extra charges to the port authorities. Engineers are constantly under pressure to eliminate the causes producing such unwanted black smoke when the ship is at port.

6.5.10 The proportion of CO2 or O2 in exhaust gases provides an indication of combustion efficiency.

Combustion efficiency is a measure of how effective energy in the fuel is converted into useful energy. Since combustion converts the carbon in the fuel to CO2, the content of CO2 in a flue gas is an important indication of combustion efficiency. High CO levels indicate incomplete oxidation. Excess air is supplied to ensure enough oxygen reacts with the fuel, and this excess is measured in the flue as a percentage of O2. High oxygen levels reduce the efficiency of the plant, making O2 a good indicator for tuning efficiency.

6.5.11 Instrument available to indicate and record the percentage of CO2 and O2 in exhaust gas.

Electronic instruments have been developed to analyze combustion routinely for tune-ups, maintenance, and emissions monitoring. They remove a sample from the stack or flue with a vacuum pump and then analyze the sample using gas sensors. The monitors indicate the percentage of CO2, O2, and some other gases in exhaust gases. In modern boilers, fixed electronic instruments show the condition of flue gas to do adjustments in real-time.

6.5.12 States the Ranges of Percentages of CO2 Which Indicate:

6.5.12.1 Good combustion

CO2 percentage will be between 12% - 14% for good combustion.

6.5.12.2 Poor combustion

CO2 percentage will be between 6% - 10% for poor combustion.

6.5.12.3 Bad combustion

CO2 percentage will be between 3% - 5% for bad combustion.

6.5.13 Explains the Importance of Atomization When It is required To Mix a Liquid Fuel with Air Prior to Combustion

Atomization is the process of breaking down the liquid fuel into minute droplets with the help of a fuel injector. Fuel oil is pressurized through tiny holes, which splits the fuel into small droplets. When the fuel is atomized, the surface area which contacts the air increases, which helps in rapid absorption of heat and ensures the fuel is thoroughly mixed with air prior to combustion.

6.5.14 State that actual air/fuel ratio, allowing for normal excess air in:

Air–fuel ratio (AFR) is the mass ratio of air to fuel present in a combustion process. If exactly enough air is provided, the ratio is known as the stoichiometric mixture.

6.5.14.1 The furnace of a steam boiler

In practice, burning conditions are never ideal, so more air than the theoretical requirement (excess air) must be supplied. Boilers normally run about 10 to 20 percent excess air.

6.5.14.2 The cylinder of the diesel engine

To ensure complete combustion, diesel engine combustion chambers are supplied with excess air, which increases the amount of oxygen available. In a diesel engine, typically up to about 3.5 times the minimum required air is supplied.

6.5.15 State that if Sulphur dioxide contacts a low temperature surface, sulphuryl acid will be produced – will cause corrosion.

When the Sulphur in fuel burns, Sulphur oxides are produced. These mix with water present in the air or produced by combustion of hydrogen to form sulphuric acid. If this acid deposits on metal surfaces, it will cause corrosion. The acid deposition occurs if the metal surface temperature drops below the dew point, which is the temperature at which the vapour reaches saturation.