The agricultural residuals include the set of by-products which are derived from the cultures cultivation, and they are generally for an alimentary purpose; otherwise, they are not usable or have alternative and marginal uses. The residuals that come from this compartment show physical and energy characteristics that, together with economic barriers (collection costs, low density for unit surface), do not make them easily applicable for energy production. For this purpose, the following can be applied:

• straws of autumn-winter cereals (soft wheat and hard wheat, barley, oats, rye);

• stocks, corncobs and maize sculls;

• rice straw;

• vine shoots of vine pruning;

• slash of orchards pruning;

• olive branches.

Table 2: Main chemical-physical cultural residual characteristics [2]. Agricultural subproduct Collection humidity (%) Medium production (t/ha) Report C/N Cinders (% in weight) LCP (kcal/kg ds) Soft wheat straw 14-20 3-6 120-130 7-10 4,100-4,200 Hard wheat 14-20 3-5 110-130 7-10 4,100-4,200 Autumn-winter 14-20 3-5.5 60-65 5-10 3,300-3,400 other cereals straw Rice straw 20-30 3-5 60-65 10-15 3,700-3,800 Maize stocks 40-60 4.5-6 40-60 5-7 4,000-4,300 Corncobs and vine 30-55 1.5-2.5 70-80 2-3 4,000-4,300 shoots Vines vine shoots 45-55 3-4 60-70 2-5 4,300-4,400 Olive branches 50-55 1-2.5 30-40 5-7 4,400-5,400 Fruit residuals 35-45 2-3 47-55 10-12 4,300-4,400

Despite the cultural residuals representing an energy source which is easily accessible, it is necessary to consider some limitations (low productivity for unit surface and chemical composition of the biomasses) which are linked to their exploitation: the quantities of agricultural residuals, which are available for a unit surface, are relatively modest and it can make the collection disadvanta­geous and inconvenient and also the removal and transport of the biomass to the thermal central; relative to the chemical composition of the agricultural residu­als it is necessary to underline that an elevated cinder content increases the danger of formation of wastes with a damage for the burnings and also increases the particulate emissions [2].

3.2.1 Dedicated cultures

The term ‘dedicated cultures’, or ‘energetic cultures’, refers to the cultures pre­pared with the aim of producing biomass destined for preparing electric and/or thermal energy.

There are three main dedicated cultures:

1. cultures from lignocelluloses,

2. oil cultures and

3. alcohol producing cultures.

Table 3 lists the main usable species and the bio-fuel obtainable from them.

Table 3: Usable for energy cultivation and their characterization species [2, 7]. Species Production cycle Intermediate product Transformed product Kenaf Herbaceous annual Fibre Chips Hemp Herbaceous annual Fibre Residual bundles Miscanthus Herbaceous annual Fibre Common reed Herbaceous long term Fibre Fibre sorghum Herbaceous annual Fibre Cardoon Herbaceous long term Fibre Measly Herbaceous long term Fibre Robinia Wooden long term Wood Broom Wooden long term Wood Eucalyptus Wooden long term Wood Willow Wooden long term Wood Poplar Wooden long term Wood Rape Herbaceous annual Oilseed Vegetable oil Sunflower Herbaceous annual Oilseed Soy Herbaceous annual Oilseed Ricin Herbaceous annual Oilseed Saf flower Herbaceous annual Oilseed Sugar reed Herbaceous annual Rhizome Sugars/alcohols Sugar sorghum Herbaceous annual Stem Topinambour Herbaceous long term Tubercle Maize Herbaceous annual Granel Wheat Herbaceous annual Prills

3.2.2.1 Lignocelluloses biomass cultures The lignocellulose cultures include the herbaceous or wooden species which are characterized by biomass produc­tion that is mainly composed of lignin and/or cellulose substances. The cultures are divided into three groups: annual herbaceous cultures, long-term herbaceous cultures and arboreal cultures [2, 7].

Annual herbaceous cultures They include herbaceous species which are charac­terized by a yearly life cycle. The most interesting ones are generally the sorghum, in addition to maize, kenaf, reed, etc. Such cultures do not occupy the ground permanently, allowing farming in rotation cycles. They can be also cultivated depending on the traditional set-aside kept in rest grounds [2, 7].

Figure 2: Kenaf, reed and fibre sorghum.

Herbaceous long-term cultures The number of herbaceous long-term exploit­able species for the production of lignocellulose biomasses is really large. The most important ones are the common reed, the Miscanthus, and the measly. Such cultures, against a considerable impact on the organization of the farm holding (they occupy the ground for 10-15 years) and a high system cost, permit the pro­duction of a considerable quantity of biomass for more years and at low addi­tional costs (compared to the annual species). They also require few fertilizers and parasiticides [2, 7].

Figure 3: Common reed, measly and miscanthus.

Arboreal cultures The energy producing wooden cultivations comprise species selected for their high yield of biomass and for their capacity of rapid growth after a cut. Usually such cultures show brief coppicing turns (2-3 years) and a high density of plants (6,000-14,000 plants/ha). In this case, we speak about short rotation forestry (SRF). Generally, in SRF, specific clones are appropri­ately selected to be used and the coppicing of the plants, annual or two-year, is completely mechanized using appropriate wood-chipping machines (see par. 4.1.2). The most interesting brief turn arboreal cultivable cultures are willow, poplar, rocinia, eucalyptus, broom (shrub) [2, 7].

Table 4 lists the physical and energy characteristics of the main vegetable species.

Figure 4: Willow, eucalyptus and poplar.

Table 4: Productive and energetic parameters of the biomass from dedicated cultures [2]. Fresh substance production (t/ha year) Collection medium humidity (%) Dry substance production (t/ha year) LCP (kcal/kg of ds) Fibre sorghum 50-100 25-40 20-30 4,000-4,050 Kenaf 70-100 25-35 10-20 3,700-3,900 Miscanthus 40-70 35-45 15-30 4,200-4,250 Common reed 45-110 35-40 15-35 3,950-4,150 Measly 25-60 35-45 10-25 4,100-4,200 Poplar 20-30 50 10-15 4,100-4,200

So, the biomasses of forest origin and the agricultural residuals, the lignocellu — lose biomasses from dedicated cultures can be used as fuels in modern plants for heating and more rarely for the combined production of thermal and electrical energy in cogeneration plants. For feeding automatic plants with SRF products and long-term herbaceous cultivations, such as common reed and Miscanthus, it is always preferred to proceed with chipping (par. 4.1.2) of the collected material to make it homogeneous in terms of dimensions. The remaining cultures are more appropriate for briquettes densification (par. 4.1.4) and pellets (par. 4.1.3), but these markets does not exist yet because, at present, wood is the the exclusive raw material for their production.

The choice of the more indicated species must be the result of an attentive eval­uation of determinate factors. In fact, although the biomasses of herbaceous origin, coming from long-term cultures, show lower costs of production compared with

Figure 5: Rape, sunflower and soy.

that coming from wooden culture biomasses, a series of obstacles limit their usage in the production of heat and electricity: the lower efficiency during the combus­tion, the lower specific weight, the lower calorific power for unit weight and the higher cinder content and other undesired compounds such as potassium (K), phosphorus (P), sulphur (S), which are corrosive, or sulphur (S), nitrogen (N) and chlorine (Cl), which are polluting [2, 13].

3.2.2.2 Oil cultures The oil and alcohol-producing cultures, compared to the cultures just discussed, do not directly provide fuel, but instead the raw material from which fuel is obtained by chemical and biochemical transformations.

Many species both arboreal (coconut palm) and herbaceous (sunflower, rape and soy) belong to the oil cultures and they are characterized by a high oil content of the seeds. Sunflower and rape have an intermediate oil content of 48% (with a top of 55%) and 41% (with a top of 50%), respectively. Soy seeds show lower concentrations (18%, with a top of 21%) and they are less appropriate than sunflower and rape for energy production.

The raw oils which are obtained from the oil cultures show an elevated LCP (median 9,400 kcal/kg), so they can be applied as bio-fuels, as a substitute for diesel, for the production of thermal, electric and cogeneration energy. Their conversion into bio-diesel also allows their use for auto traction [1, 2, 7, 14].

Table 5: Oil culture yields in seeds, raw oil and bio-diesel. Oil culture Seeds yield (t/ha) Raw oil yield (t/ha) Bio-diesel yield (t/ha) Rape 2.7 1 0.9 Sunflower 3 1.1 1

3.2.2.3 Alcohol-producing cultures The cultures that are named alcohol — producing are those that produce biomass with a high fermentable carbohydrate content that can be applied, through a fermentation process, to the production

Figure 6: Beet, maize and wheat.

of bio-ethanol production. This bio-ethanol can in turn be used as bio-fuel, as a substitute for gasoline or explosion-proof compounds (e. g. methyl tert-butly ether (MTBE)).

The raw material that is used at the start of the production line for bio-ethanol can comprise simple sugars, such as sucrose and glucose, or complex sugars (starch), which are obtained, respectively, from dedicated sucrose cultures (sugar beet and sugar sorghum are not the most appropriate for the Italian conditions) or from amylaceous cultures (soft wheat in southern Italy and maize in northern Italy). The simple sugar content of the sucrose cultures is high: the fermentable sugar extract in beet is about 20% of the collected dry biomass, in sorghum it is 18%. The amylaceous cultures contain the starch as grains and the glucose residu­als that it is composed of can be hydrolysed and, subsequently, fermented into bio-ethanol: soft wheat has a starch content equal to 70%, for maize it is 78%.

Table 6: Alcohol-producing culture bio-ethanol yields.

Culture	Bio-ethanol yield (t/ha)
Sugar beet	5.5
Sugar sorghum	4.5
Soft wheat	2.5
Maize	3-6

3.3 The zoo technique compartment

The farming wastes produced are termed zootechnical dejections, whereas we speak of dejections only when we refer to the physiological subproducts of the animals (faeces and urine). The composition of the zootechnical dejections varies depending on the origin (cattle/piggish, poultry) and on the farming modality and management. In particular, the water supply (or, on the contrary, the dry substance content) is important for choosing the most appropriate treatment/disposal modalities.

Sewages are the most appropriate for the energy exploitation through anaerobic digestion of zootechnical dejections (par. 2.1, Chapter 4), because they show a dry substance content that is lower than 10-12%.

0 5 10 15 20 25 30 35 40

> 1 ■________ ■ ■ ■________ і________ » ■

|« Solid dejections

Figure 7: Classification of zootechnical dejections as a function of the dry sub­stance content.

The energy content of the zootechnical sewages is in direct relation with the organic substance content. In fact, it is the organic substance which, through a fermentable or anaerobic digestion process, results in the formation of the bio-gas, a high calorific power fuel.

As evident from the data in reported Table 7, this is the case of the piggish or bovine liquid manures, which are characterized by a high organic substance level (or volatile solids).

Table 7: Bovine and piggish liquid manure yields in bio-gas. Dry substance Organic substance (volatile solids — vs; % Bio-gas yield Material (ds; %) on the ds) (N m3/kg vs) Piggish sewage 3-8 70-80 0.25-0.5 Bovine sewage 5-12 75-85 0.2-0.3

In addition to the quantity of organic substance, it is important to consider the quality of the material; these aspects can, in fact, affect the bio-gas yield and methane content.

The main factors are:

• Composition of the material: It affects the speed of degradation that, in decreasing order, can be schematized as: lignin-cellulose-fats-proteins — carbohydrates. The speed of degradation of a bovine liquid manure, with a higher cellulose material content, is quicker than that of a piggish liquid manure, which is richer in fats (substance that favours higher bio-gas yields).

• Presence of essential elements: Micro-nutrients such as sodium (Na+), potassi­um (K+), calcium (Ca2+), magnesium (Mg2+), ammonia (NH4+) and sulphur (S2-), if they are in excess can provoke toxicity. Concentrations even higher than 1 mg/l of heavy metals such as copper (Cu2+), nickel (Ni2+), chromium (Cr3+), zinc (Zn2+) and lead (Pb2+) can be harmful. Other substances that are capable of blocking the digestion are the cleaning and chemical compounds of synthesis [2].

3.4 Industrial activities