This represents the heat released during the complete combustion of a sample, determined by burning it in a controlled environment; it is expressed as the energy content per mass unit (kJ/kg, MJ/kg) and can be referred to the wet basis, the dry basis or the dry and ash-free, basis. It can be divided into: higher heating value (HHV) and lower heating value (LHV), depending on whether
water obtained as a product of combustion is considered in liquid phase or vapor. The difference between HHV and LHV is therefore the latent heat of condensation of the steam present in the combustion products. It has to be noted however that the water content to be considered in exhaust gases, when calculating LHV from HHV is only the one derived by the oxidation of hydrogen present in the sample; steam coming from moisture in the sample is not considered in the LHV therefore HHV should be always corrected on a dry sample.

The relations that describe these quantities are:

Higher Heating Value (HHV):

( ACwb MCwb

HHVwb = HHVdaJ 1 — [kJ/kgwb]

( — Cdb

HHVdb = HHVdaJ 1 — [kJ/kgwb]

Lower Heating Value (LHV):

( — Cwb MCwb

LHVdb = LHVdaJ 1 — [kJ/kgwb] (5.7)

/ — Cdb

LHVdb = LHVdaJ 1 — [kJ/kgwb] (5.8)

The HHV is measured in laboratory using a calorimeter. The LHV is obtained, knowing the hydrogen content of the sample (determined through elemental analysis with a CHN analyzer) through the following relation:

LHVdb = HHVdb — 206.0Hdb [kJ/kgwb] (5.9)

in which Hdb is the content of hydrogen expressed in mass and referred to the dry basis. The heating value is a fundamental parameter that gives indications on the energetic potential of the biofuel. In fact the higher the heating value the higher is the energy yield per unit of mass obtained by the conversion process.

At operative level it is better to consider the heating value on dry basis, because it represents the real energy yield of the fuel. Moisture content lowers the energy content of biomass because the evaporation consumes energy that could be used for the thermo-chemical conversion processes.

The heating value can be calculated also through formulas derived from correlations, for exam­ple the Channiwala Parikh formula that correlates HHV with the elemental analysis (Demirbas,

2004) :

HHV = 349.1 C + 1178.3 H + 100.5S — 103.4O — 15.1N — 21.1 Ash [kJ/kg] (5.10)

Other attempts have been made to derive a heating value from proximate analysis (Parikh etal.,

2005) .

5.1.3.2 Carbon, hydrogen and nitrogen content (EN15104, 2011)

The concentration of the three elements in the biomass sample is measured through the ultimate analysis of the sample, that is the combustion of the same in controlled atmosphere and the successive analysis of flue gases. The three concentrations obtained are expressed in % dafb. The carbon/nitrogen ratio can be used as an indicator to identify the most suitable conversion technique for the biomass. When the C/N ratio is higher than 30, the thermochemical conversion process could be adopted, while when C/N is lower than 30, the most suitable conversion processes are biochemical processes.