Calorific value is defined as the amount of calories generated when a unit amount of substance is completely oxidized and is determined using the bomb calorimeter. The calorific value of biomass represents gross calorific value (HG), which contains the latent heat of water vaporization. When the latent heat of water vaporization is not included, the calorific value is called net calorific value (HN).
The relationship between gross calorific value and net calorific value is expressed by the following equation:
Where “w” is the moisture content and “h” is the hydrogen content under a constant moisture base.
The commonly used units for calorific value are MJ/kg and kcal/kg. Following is the unit conversion among the two units: 1 MJ/kg = 239 kcal/kg
NEED FOR NET CALORIFIC VALUE
Calorific values as determined with the bomb calorimeter represent the heat produced by a unit weight of biomass when completely oxidized, when the products of the combustion are cooled to room temperature. This value is not realized in practice because the products of combustion are not cooled to room temperature before being discharged to waste.
Sensible heat is lost in the hot waste products. Apart from this, further heat loss occurs in practice as the latent heat of steam in the hot waste gases. Water is present as such because moisture in the air-dried biomass and a further amount are formed by the combustion of the hydrogen combined with carbon in the biomass. In the bomb calorimeter, the moisture is first evaporated and then condensed to liquid water. Similarly, the water formed as steam by combustion is condensed to liquid water; the latent heat of condensation of the steam is recovered.
In industrial practice water from both sources is discharged as steam, so that both latent heat and sensible heat are lost. It is therefore useful to distinguish the calorific value as determined with the bomb calorimeter by calling it the gross calorific value.