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The heat of combustion(Hc0) is theenergyreleased asheatwhen a compound undergoes

completecombustionwithoxygenunderstandard conditions

The heating value or energy value of asubstance, usually afuelorfood(seefood energy), is the amount

ofheatreleased during the combustion of a specified amount of it. The energy value is a characteristic for each

substance. It is measured in units ofenergyper unit of the substance, usuallymass, such as:

kJ/kg,kJ/mol,kcal/kg,Btu/m. Heating value is commonly determined by use of abomb calorimeter.

Higher heating value

The quantity known as higher heating value (HHV) (or gross energyor upper heating valueor gross calorific

valueor higher calorific valueHCV) is determined by bringing all the products of combustion back to the original pre-

combustion temperature, and in particular condensing any vapor produced. Such measurements often use a temperature

of 25 C. This is the same as the thermodynamic heat of combustion since theenthalpychange for the reaction assumes

a common temperature of the compounds before and after combustion, in which case the water produced by combustion

is liquid.

The higher heating value takes into account thelatent heat of vaporizationofwaterin the combustion products, and is

useful in calculating heating values for fuels wherecondensationof the reaction products is practical (e.g., in a gas-fired

boiler used for space heat). In other words, HHV assumes all the water component is in liquid state at the end of

combustion (in product of combustion).

Lower heating value

The quantity known as lower heating value (LHV) (net calorific valueor lower calorific value- LCV) is determined by

subtracting theheat of vaporizationof the water vapor from the higher heating value. This treats any H2O formed as a

vapor. The energy required to vaporize the water therefore is not realized as heat.

LHV calculations assume that the water component of a combustion process is in vapor state at the end of combustion,

as opposed to thehigher heating value(HHV) (a.k.a. gross calorific valueor gross CV) which assumes all of the water in

a combustion process is in a liquid state after a combustion process.

The LHV assumes that thelatent heat of vaporizationofwaterin the fuel and the reaction products is not recovered. It is

useful in comparing fuels where condensation of the combustion products is impractical, or heat at a temperature below

150 C cannot be put to use.

Gross heating value

Gross heating value (see AR) accounts for water in the exhaust leaving as vapor, and includes liquid water in thefuel prior to combustion. This value is important for fuels likewoodorcoal, which will usually contain some amount of

water prior to burning.

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