The lower heating value (LHV) or higher heating value (HHV) of a gas is an important consideration when selecting a gas engine or CHP plant. 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. 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 vaporisation; in simple terms, as heat energy stored in the vapourised ‘state’ of water.
The total amount of heat liberated during the combustion of a unit of fuel, the HHV or HCV, includes the latent heat stored in the vapourised water. In some applications it is possible to condense this vapour back to its liquid state and ‘recover’ a proportion of this energy. However, engine exhaust temperatures are above that at which the water vapour would condense, and hence the steam ‘escapes’ with the exhaust gases carrying with it the stored energy.
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 suppliers will usually quote the HHV and it will be this measure that will be used when kWh unit charges are applied for the fuel. In the case of natural gas the ratio of HHV to LHV is approximately 1.108:1. Hence, when performing a cost benefit analysis for a CHP application, it is the HHV figure which should be used.
The LHV of a fuel determines the fuel flow rate required when going into the engine because the total quantity of energy input necessary for the engine to produce a specific output power is defined and fixed. Hence the gas flow rate has to be such in order to provide the required energy input.
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.