Knowing the elemental analysis and higher heating value of the biomass, the lower heating value can be determined. It is usually 10 to 15% lower as compared to the higher heating value. The lower heating value can be linked with the higher heating value by the following expression.
λ
and W
f
are the latent heat of vaporization of water and weight fraction of water formed during combustion process. The lower heating values of selected biomass species are given in Table 1.2. HCV = LCV +
λ
W
f
+ expansion work The heating value of a biomass per unit weight is a function of the moisture content of the biomass. For a wet biomass available heat per unit weight of biomass is reduced and also a part of heat is required to vaporize the water present in the biomass as moisture.
1.3.2 Proximate Analysis
Proximate analysis provides information on the combustion characteristics of biomass. It is a measure of fixed carbon (FC), volatile matter (VM), Ash (A) and Moisture (M) in the biomass material and expressed as percent.
The term volatile matter and fixed carbon
8does not have clear definitions. The volatile matter of any substance in a broad sense is the fraction that is driven off by heating the sample to a specific time and temperature. The total amount of volatile matter and its composition is the function of heating rate as well as the final temperature. The volatile matter is an important parameter because it characterises the expected contamination of the raw gas with condensable vapours in any gasifier or pyrolysis equipment