1.1 The forest and agro-forest behaviour

The forest residuals, resulting from the different kinds of forest-cultural interven­tion, are commonly indicated as forest biomasses. The interesting operations for the sample of forest biomass aimed at energy production purposes include forest — cultural interventions in woods which are controlled both by high fores (which is applied when the wood comprises plants that are allowed to grow until maturity) and by coppice (where the growth of the plant is interrupted with periodical cuts). In the first case, a typical operation is the sample of the lower sorting, generally left in the wood after the cut of the major forest sorting (truncate with a diameter bigger than 18 cm) for commercial uses. The wood derived from the inserted cut material (interventions which are applied to young fores or to replenishing fores to improve the stability and regulate the specific composition) represents a further source of supply. Another important forest biomass source is represented by the coppice woods: the Italian coppice, in fact, is mainly destined for the production of fuel biomass and agricultural use posts.

Another source of supply for wooden biomasses is represented by the materials of agro-forest origin that are derived from forestation activity in agricultural range biomass. In this case, the usable biomass sources for energy production purposes are derived from wooden cultivation commercially used residuals, to the linear formations uses (e. g. hedges, rows and little woods) as well as wood formations uses, which are dedicated to agricultural uses (in this last case we mainly refer to the poplar culture) [2, 7, 12]. The physical characteristics of the wooden biomasses which are relevant on the energy production front are the grade of humidity and the density which, with the material’s chemical composition, affect the calorific power of the wood.

The calorific power expresses the quantity of heat that is released during the complete combustion of the weight unit or in fuel volume. There are lower calorific power (LCP) and higher calorific power (HCP) fuels; depending on the hydrogen combustion that is eventually present in the fuel water, we consider the vapour or the liquid state. The difference between the two kinds of calorific power corresponds to the vaporization heat of water that is formed during combustion.

Practically, the LCP fuels are always of interest because the fumes are always discharged at a temperature where the water is present as vapour. It is expressed in MJ/kg and, for convenience, also in kcal/kg.

The humidity is a variable that assumes considerable importance because, in addi­tion to the combustion mechanisms, it influences the chemical characteristics of the wood and its specific weight. The quantity of water that is contained in the material varies as a function of many factors such as the species, age and the plant part that is considered (trunk, branches, etc.). The humidity expresses the quantity of water (free or linked) that is present in the wood; it is expressed as a percentage in terms of both the dry weight and the fresh wood weight; in the first case, we look at the water content as an absolute value and in relation to its anhydrous mass.

U(%) = [(M і — Ma)/Mi] x 100

where Mі is the exact wood mass and Ma is the mass of dry wood.

This method is the most frequently used method to determine the humidity [2, 8]. The most common wooden combustible quality indicator is represented by the density (mass for unit volume, measured in kg/m3). It is directly proportional the wooden calorific power. The density varies as a function of factors such as the seasonal conditions, the species (the most elevated in the broad-leaved species and in the conifers), the age, the considered part and the form of the wood government. The density can be calculated by considering the wood in the fresh state or in the dry state; in the first case, it generally varies from 360 to 810 kg/m3 [2, 8, 9]. The chemical composition of the wood is one of the main analytical characteristics for the forest and agro-forest biomass qualification on the energy production front. The main polymers which make up the wooden biomass are [2, 8]:

• lignin, which gives rigidity to the plant (reinforces the cellular wall), is present in percentages that vary from 20% to 30% of the dry weight and has a high calorific power (6,000 kcal/kg approximately).

• cellulose, which is the main wood component (constitutes 50% of the weight) and has a calorific weight of approximately 3,900 kcal/kg;

• hemicelluloses, which are present in the cellular wall of the plants, in the free spaces left from cellulose. It constitutes from 10% to 30% of the wood and it has a more contained calorific power.

Compared to its elementary composition, wood is almost entirely made of carbon (49-51%), oxygen (41-45%) and hydrogen (5-7%). It is also composed of, even if in reduced quantities, nitrogen (0.05-0.4), sulphur (0.01-0.05) and other mineral elements that make up the cinders (0.5-1.5%) [10, 11].

The quantity and especially the relationship among the elements that make the biomass are very important to verify its value as combustible. In particular, the relationship between hydrogen and carbon and between oxygen and carbon are
also really important, as well as the quantity of nitrogen and cinders; generally, a high carbon and hydrogen content determines a high calorific power, whereas elevated oxygen, nitrogen and cinders has an opposite effect [8].

Table 1: Main wooden biomass chemical-physical characteristics (ds, dry substance) [2].

Composition

Cellulose

Hemicellulose

Lignin

Physical and energetic characteristics

Humidity

Mass density

LCP (considering a humidity of 12-15%)

The biomass which comes from wood is sold in the market in very different coal sizes for shape and humidity grade. In some cases, it starts with the production of denser forms (pellets, briquettes, which are analysed in par. 4.1.3 and 4.1.4). The most common coal sizes are wooden stub-pipes (used in the rural or mountain environments) and chips (par. 4.1.2).

The wide availability of the source at a national level makes the exploitation of forest biomasses for energy production interesting. However, we have to face the logistic difficulties that are linked to biomass retrieval (e. g. the presence or not of an exploitable viability from the common collection and transport tools); especially in mountainous regions, in fact, the woods are not always easy to reach [2].