Heat of Combustion

What is Heat of Combustion?

The heat of combustion of a substance, also known as the calorific value or the energy value, can be defined as the amount of heat liberated when a given amount of the substance undergoes combustion. Usually, heat of combustion is considered to be a synonym of calorific value, which can be defined as the total amount of energy liberated when a given mass of a substance undergoes complete combustion in the presence of (an adequate quantity of) oxygen under standard conditions for temperature and pressure. Usually, the potential of hydrocarbons that are used in fuels are expressed in terms of calorific value, based on the combustion reaction they undergo with oxygen to form water and carbon dioxide. It can be noted that the heat of combustion of a substance can be expressed in terms of the following units:

  • Energy (in joules or kilojoules) liberated when one mole of the fuel undergoes complete combustion with oxygen.
  • Energy (in joules or kilojoules) liberated when one gram or kilogram of the fuel undergoes complete combustion with oxygen.
  • Energy (in joules or kilojoules) liberated when one litre of the fuel undergoes complete combustion with oxygen.

It can also be noted that the heat of combustion of fuels can be calculated with the help of a bomb calorimeter.

How can the Heat of Combustion be Determined?

Heat of combustion is often categorized into the following two types:

  • Higher calorific value, also known as gross calorific value and higher heating value
  • Lower calorific value, also known as net calorific value and lower heating value

The determination of each of these types of heat of combustion is discussed in this subsection.

Determination of Higher Calorific Value

The higher calorific value of a substance (often abbreviated to HCV) can be determined by bringing back all the combustion products into the initial temperature (the temperature of the reaction environment before the combustion took place). It is important to note that this also involves the condensation of any vapour that is produced during the combustion reaction. It can also be noted that these measurements are known to be performed under certain standard conditions in which the temperature of the reaction environment is kept at 25 degrees celsius.

The gross calorific value can be considered to be similar to the thermodynamic combustion heat as the reaction enthalpy transition assumes a specific compound temperature before and after the combustion, in which case the combustion water is condensed into a liquid. It can also be noted that the gross calorific value takes into account the latent heat of vaporization of the water in the final combustion product. It is quite useful for measuring heating values for fuels where condensation of the reaction products is possible, as is the case in a space heat gas-fired boiler, for example.

Determination of Lower Calorific Value

The lower calorific value of a substance, also referred to as the net calorific value and the lower heating value of the fuel, can be determined by subtracting the value of the latent heat of vaporization of the water formed in the reaction from the value of the gross calorific value (the higher heating value). This form of expressing the heat of combustion involves the assumption that any water formed during the combustion reaction is in the form of a vapour. Therefore, the energy that was expended during the reaction to convert the water into water vapour (and which was,therefore, not liberated as heat during the reaction) is considered during the calculation of this quantity.

The lower heating value (often abbreviated to LHV) calculations assume that the water portion of the combustion process is in the vapor phase at the end of the combustion reaction, as compared to the higher heating value (abbreviated to HHV) which assumes that all the water in the combustion process is in liquid state after the combustion process.

The lower calorific value (abbreviated to LCV) presumes that there is no recovery of the latent heat of water vaporization in the fuel and the reaction materials. This is useful in comparing fuels in the cases where the condensation of the combustion products is impossible, or where heat can not be used at temperatures below 150 degrees celsius.

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Frequently Asked Questions on Heat of Combustion

What is the importance of heat of combustion?

The concept of the heat of combustion of a fuel is extremely important when it comes to the fuels that are used in internal combustion engines. The fuels that are used in internal combustion engines are often selected based on their calorific values (a value that denotes the heat of combustion of the fuel). The greater the heat of combustion of the fuel, the greater the amount of power that can be produced from it inside the internal combustion engine.

What is the SI unit of calorific value?

Calorific value reflects the amount of energy emitted when a fuel or substance undergoes a complete combustion process. This term is usually used in order to indicate the energy capacity of a substance. The SI unit of calorific value is Joules per kilogram or kilojoules per kilogram.

What is meant by lower calorific value (LCV) and higher calorific value (HCV)?

HCV, or the higher calorific value, is a term that denotes the amount of heat or energy emitted from a fuel burning when its by-products are concentrated to recover some of the energy generated from the burning. In comparison, the term LCV, or lower calorific value, denotes the amount of energy emitted from a fuel’s combustion when its by-products escape freely from the reaction environment.

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