Calorific Value

C, O and H can be broken down with the release of energy (heat) to form CO2, H2O and other by-products. The calorific value or energy content is a measure for the heat that can be produced and is measured in MJ/kg. The two major breakdown processes are:

C + O! CO2 + 32.8 MJ/kg (C)

2H2 + O2 ! 2H2O + 142.2 MJ/kg (H)

The amount of the released energy depends on the ratio of C, H and O in the biomass used and the MC, as part of the released energy will be consumed for the vaporization of present water. Commonly it is differentiated between gross and net calorific value or higher heating value (HHV) and lower heating value (LHV). The gross CV is the energy content of the sample without any moisture and therefore the maximum value, while the net value is measured in a moist sample, as it would be used for conversion.

Typical gross calorific values for different types of biomass that might be used for energy conversion are given in Table 8.5. It can be seen that the values do not differ much for woody biomass — they are generally between 19 and 20 MJ/kg. This suggests that other factors, such as ash content, are more important when deciding which biomass should be used for energy conversion.

The energy content is typically determined with a bomb calorimeter, which is a closed system in which small temperature increases after combustion in a pressurized oxygen atmosphere can be determined with high sensitivity. The calorific value is defined as the amount of thermal energy per weight unit produced by the total combustion of the sample in MJ/kg.

About 0.5 g of biomass are combusted in a pressurized oxygen atmosphere with a pressure of about 3,000 kPa. The measured temperature increase of the vessel is very small, requiring very sensitive temperature sensors in the calorimeter and good calibration before each measurement. Calibration is generally performed with benzoic acid.