fuel, mist, boiler burners, burn, chemical reaction, carbon, hydrogen, sulphur,
sulfur, oxygen, exhaust gas, low temperature corrosion, dewpoint
Get Hot on Combustion
By: Thomas Yoon
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 theoretical oxygen multiplied by 100/23 will give the
theoretical 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 sulfur 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
Folks, get hot!
Until next time…
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