The heating value is the energy content of the fuel (sometimes also called calorific value). It is determined by complete combustion of a dried sample in a calorimeter. The heat produced by condensation of water formed by the combustion is measured as well. The measured heat production is corrected for the moisture content in the original sample. This is referred to as the upper heating value (or gross calorific value) and specified as MJ/kg a.r. (as received).
Actually water from evaporation of moisture and from combustion of hydrogen in the fuel is often not condensed. In this case the heating value is lower, i.e. the upper heating value has to be corrected for the loss of condensation heat. This is referred to as the lower heating value (or net calorific value) and is specified as MJ/kg a.r. as well.
The heating value, whether upper or lower, can be specified on an as received (a.r.), dry basis (d.b.) or dry and ash free (d.a.f.) basis. Thus, when comparing heating values it is very important to specify the basis.
The moisture content of the fuel is determined by oven drying a sample at a specified temperature, normally 105°C, for several hours until constant weight. The intention is to evaporate bulk water only, not chemically bound water
The ash content of a solid biomass fuel is determined by combusting a sample at a specified temperature, normally 550°C, in an oven using air as atmosphere. Due to the relatively low temperature, the process takes several hours until constant weight. The intention is to convert organic matter only, leaving all inorganic elements behind.