Appendix - Ecoprofile methodology (cont'd)

A7. Fuels and energy

Fuels are naturally occurring materials which when burned in air or oxygen produce potentially useful thermal energy; that is, the reaction between the fuel and oxygen is exothermic. Although in principle it is possible to calculate the energy liberated in the reaction if the chemical composition is known, most commercial fuels are mixtures of hydrocarbons and their precise composition is seldom known with any certainty. As a consequence, the combustion energy is commonly expressed as an experimentally determined calorific value. Calorific value is defined as the heat energy evolved by the combustion of unit quantity of fuel. In practice two types of calorific value may be determined: gross calorific value and net calorific value.

Gross calorific value, or high heat value, is the heat evolved when all of the products of combustion are cooled to atmospheric temperature and pressure as in a bomb calorimeter. The gross calorific value will therefore include the latent heat of vaporization and the sensible heat of the water in the combustion products. The net calorific value, or low heat value, is the heat evolved when the products of combustion are cooled so that the water remains as a gas. It is therefore equal to the gross calorific value less the sensible heat and latent heat of vaporization of water. The magnitude of this deduction is 2.45 MJ/kg water condensed.

The gross calorific value therefore represents the maximum energy that can be derived from a fuel and is therefore a measure of the total energy that must be extracted from the earth. Most equipment burning hydrogen-containing materials is not operated at a temperature at which the water in the products will condense to liquid. By allowing the water to leave the system in the flue gases, corrosion within the plant can be reduced. This however, is a limitation of current technology and as a consequence many processes do not harness the total energy available from the fuel. Throughout this work gross calorific values have been used. The effect of this can be gauged from Table A1 which compares the gross and net calorific values of a number of different fuels.

Table 1.
Comparison of gross and net calorific values for selected fuels.