There are two different types of heating value, which are
the lower heating value and the higher heating value . Higher heating value
(HHV) is calculated with the product of water being in liquid form while lower
heating value (LHV) is calculated with the product of water being in vapor
form.
By definition the higher heating value is equal to the lower
heating value with the addition of the heat of vaporization of the water
content in the fuel.
Lower and higher heating values
where
●
M is percent Moisture,
●
H is percent Hydrogen,
● Y is percent Oxygen (from an ultimate analysis which
determines the amount of carbon, hydrogen, oxygen, nitrogen and sulphur as
received (i.e. includes Total Moisture (TM)).
The lower heating value (LHV) or higher
heating value (HHV) of a gas is an important consideration when selecting a
gas engine . Gas engines efficiency is typically quoted based upon the LHV of
the gas.
Whenever a hydrocarbon fuel is
burned one product of combustion is water. The quantity of water produced is
dependent upon the amount of hydrogen in the fuel(For this lean burn engine
have high thermal efficiency). Due to
high combustion temperatures, this water takes the form of steam which stores a
small fraction of the energy released during combustion as the latent heat of
vaporization; in simple terms, as heat energy stored in the vaporized ‘state’
of water.
The amount of heat available from a
fuel after the latent heat of vaporisation, the LHV or LCV, is deducted from
the HHV, and it is this, that is available when the fuel is burned in an
engine. The energy input into a gas engine should be defined using the LHV of
the fuel.
Fuel LHV is normally quoted using
units of kWh/Nm3 and therefore, if the energy input to the engine is
known, the gas flow rate in m3/hr can easily be calculated.
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