Heat of Combustion

The heat of combustion of a given substance, which is also called the energy value or calorific value, can be described as the amount of heat liberated when a given substance undergoes combustion. In general, the heat of combustion is recognized to be a synonym of calorific value, which can be described as the total amount of energy liberated when the mass of a substance that is given undergoes combustion completely in the presence of an adequate quantity of oxygen under the standard conditions for pressure and temperature. The heat of combustion of ethanol can also be calculated using its chemical formula.

Units of Heat of Combustion of a Substance

Generally, the hydrocarbons potential, which is used in fuels, is expressed in terms of calorific value, according to the combustion reaction, which they undergo with oxygen to produce carbon dioxide and water. It should note that the heat of combustion of a substance is expressed in terms of the units given below.

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

• Energy (either in joules or kilojoules) is liberated when one liter of the fuel undergoes complete combustion with the oxygen.

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

• It should also note that the heat of combustion of fuels can be calculated using a bomb calorimeter.

Determination of Heat of Combustion

The heat of combustion is often categorized into two types, as listed below.

• Higher Calorific Value - It is also called higher heating value and gross calorific value,

• Lower Calorific Value - It is also called lower heating value and net calorific value.

Let Us Discuss Determining Each Type of Heat of Combustion

Determination of Higher Calorific Value

The higher calorific value of a substance (which is often abbreviated to HCV) is determined by bringing back the total combustion products into the initial temperature (the reaction environment's temperature prior to the combustion occurs).

It should also note that this involves condensation of any vapour produced during the combustion reaction. It should also note that these measurements are performed under some standard conditions, where the temperature of the reaction environment is placed at 25℃.

The Gross Calorific Value (GCV) can be counted similarly to the thermodynamic combustion heat because the reaction enthalpy transition assumes a particular compound temperature both before and after the combustion phases, in a case, where the combustion water is condensed into a liquid.

It should also be noted that the gross calorific value is considered as the latent heat of water vaporization in the final combustion product. It is also quite useful to measure the heating values for fuels where it is possible for the condensation of the reaction products, in the case of a space heat gas-fired boiler, as an example.

Determination of Lower Calorific Value

The lower calorific value of a substance can also be known as the lower heating value and the fuel's net calorific value. It can be determined by subtracting the latent heat value of water vaporization formed in the reaction from the gross calorific value (which is the higher heating value). This method of expressing the heat of combustion includes the assumption that the water formed (if any) during the combustion reaction will be in the vapour form. Thus, the energy expended during the reaction for the conversion of water into water vapour (and, hence, not liberated as heat during the reaction) is considered during this quantity calculation.

The lower heating value (LHV) calculations assume that the combustion process's water portion is in the vapor phase at the end of the combustion reaction, compared to the higher heating value (HHV). Where HHV assumes that all the water present in the combustion process is in a liquid state after the combustion process takes place.

The lower calorific value (LCV) presumes there exists no latent heat recovery of water vaporization both in the fuel and the reaction materials. This helps compare fuels in such cases, either where the combustion product condensation is impossible or where the heat can not be used at temperatures below 150℃.

Determination of Measuring Heating Values

The higher heating value can be determined experimentally in a bomb calorimeter. The combustion of a stoichiometric mixture of oxidizer and fuel (for example, one mole of oxygen and two moles of hydrogen) in a steel container at 25°C can be initiated by an ignition device, and the reactions, which are allowed to complete. When oxygen and hydrogen react during combustion, there produces water vapor. Then, both the vessel and its contents are cooled to the original temperature of 25 °C, and the higher heating value is obtained as the heat released between identical initial temperature and final temperatures.

When there determines a lower heating value (LHV), cooling stops at 150 °C, and the reaction heat is recovered only partially, where the limit of 150 °C is based on the dew-point of acid gas.

It should be noted that a Higher Heating Value (HHV) can be calculated with the product of water being in liquid form. In contrast, a Lower Heating Value (LHV) can be calculated using the product of water being in vapor form.

Did You Know?

• Gross heating value (GHV) accounts for exhaust water, leaving as vapour, and includes liquid water in the fuel before combustion. And, this value is necessary for fuels such as coal or wood, which usually contain an amount of water prior to burning.