GHV or HHV,LHV or UHV

Q. Define different heat related terms such as GHV or HHV, LHV or UHV.

A. Let's discuss the full form of various acronyms stated above.
     GHV-- gross heat value
     HHV-- higher heating value
     LHV-- lower heating value
     UHV-- ultimate heat value

As far as GHV and HHV are concerned, they are two terms for the same topic.Let's discuss GHV and LHV

(1) GHV or HHV:The higher heating value is experimentally determined in a bomb calorimeter. The combustion of a stoichiometry mixture of fuel and oxidizer in a steel container at 25° is initiated by an ignition device and the reactions allowed to complete. When hydrogen and oxygen react during combustion, water vapor is produced. The vessel and its contents are then cooled to the original 25°C and the higher heating value is determined as the heat released between identical initial and final temperatures.

One point is to be noted here that water is considered as fuel in GHV, contrary to LHV.

(2) LHV or UHV: When the lower heating value (LHV) is determined, cooling is stopped at 150°C and the reaction heat is only partially recovered. The limit of 150°C is an arbitrary choice.



When these terms apply to coal, we consider intrinsic elements of coal--which are capable of producing heat. There are basically three terms responsible for heat-- fixed carbon(C),sulphur(S),hydrogen(H). Apart from those, coal contains water,nitrogen,phosphorus,etc.A high carbon containing coal has approximately same value for UHV and LHV. As carbon contents reduce, UHV value becomes greater than LHV's.

We can calculate UHV or LHV with some constraints as follows:
UHV(Kcals/kg)= 8900 - 138 ( A(ash %)+ M(moisture %) )

Constraints are that if the non coking coal contains less than 2% moisture and less than 19% volatile matter,from the value calculated by the above formula,deduct 150 Kcal/kg for every 1% VM(volatile matter) below 19% and that gives the actual UHV for coal.


A common method of relating HHV to LHV is:

HHV or GCV  = LHV + h_v x (n_H2O,out/n_fuel,in)

where h_v is the heat of vaporization of water, n_H2O,out is the moles of water vaporized and n_fuel,in is the number of moles of fuel combusted.