Energy in the form of heat is obtained when fuel is burnt in air. The release of this heat energy can be slow or can be very rapid.
When fuel oil is sprayed as a fine mist in the boiler burners, it is able to burn at a relatively slow rate. When fuel is sprayed into the cylinders of diesel engines, the fuel burns in such a rapid rate that explosions occur. Fortunately, these explosions are protected from persons as these engines are called internal combustion engines.
Whatever type of combustion, it is a chemical reaction between carbon, hydrogen, sulphur and oxygen.
C + O2 = CO2
2CO + O2 = 2CO
2H2 + O2 = 2H2O
S + O2 = SO2
2S + 3O2 = 2SO3
Air consists of 77% Nitrogen and 23% Oxygen by mass. For a particular design of combustion air, the theoritical oxygen multiplied by 100/23 will give the theoritical air required.
How do you measure a good combustion. The percentage of Oxygen or Carbon Dioxide will tell us whether the combustion is good or not good.
The lower the Oxygen content in the exhaust gas, the better the combustion. It means that the Oxygen has been fully utilized for burning. It also means that the fuel air ratio is set properly. Too much excess air is no good because the heat generated will be lost through the exhaust trunking.
Boilers are able to achieve a good combustion. Oxygen content percentage of up to 5% or lower can be achieved.
Internal combustion engines have a lot of excess air because mixing of the combustible mixture is a challenge for them. Furthermore, the combustion is meant to provide the power to drive the pistons.
The burning of sulphur in the fuel is a problem for combustion equipment. This is because the byproducts of combustion will create sulphur dioxide and sulphur trioxide. These will react with the water, also a byproduct of combustion of Hydrogen to form sulphuric acid and sulphurous acid.
SO3 + H2O = H2SO4
SO2 + H2O = H2SO3
2H2SO3 + O2 = H2SO4
However, the effects of corrosion, called low temperature corrosion can be avoided by keeping the temperature above the dewpoint. That means to keep the exhaust temperature high so that water droplets will not form on the exhaust ducts.
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