The production of electrical energy represents the most interesting gas use modality. Currently, the vapour plants in which the direct combustion of the biomass takes place are the most widely used technology to produce electrical energy from biomass.

A possible use of the producer gas is the co-firing (co-combustion) with tradi­tional combustibles in vapour plants. In this way, there is a considerable saving of fossil fuel. Furthermore, the investment costs are low; resorting to co-firing requires only little modifications to the existing plants.

The most promising technology is represented by the combined gas-vapour cycles that are integrated with gasification (IGCC, integrated gasification com­bined cycle). In systems with powers of tens of megawatts, global earnings as high as up to 50% have been achieved. The plans proposed until now are essentially of three types:

• fluid bed atmospheric gasification with air and cleaning of the gas by humid cleaning;

• fluid bed pressurized gasification with air and hot cleaning of the gas;

• indirect heating atmospheric gasification with humid cleaning.

Commercially, the IGCC systems are not yet competitive, but they can become com­petitive in the short term, especially due to the possibility of realizing co-generation plants [2, 46, 48].

The limitations posed by the use of vegetable oils in some typologies engines, in particular those that require auto traction, due to the high viscosity of the combustible, can be overcome by the transformation of oils into bio-diesel, which is obtained by the esterification process (this will be analysed in detail in the par. 2.4.3, Chapter 4). Vegetable oils are mainly made of triglycerides and their viscosity (median of 40 mm2/s) is higher than that of fuels of fossil origin (diesel has a viscosity equal to 3 mm2/s). The transformation into bio-diesel is due to the conversion of the triglycerides into methylic esters which reduces the prod­uct’s viscosity. The mixture of methylic esters, called bio-diesel, has a viscosity of 5 mm2/s which is similar to that of diesel. Bio-diesel can be produced even from saturated cooking oils. These substances must be purified before the esterification process. The esterification process involves the use of alcohol, mainly methanol, and it yields glycerine as a by-product (which is re-sold to the chemical and phar­maceutical industry as a raw material) [1, 2]. Consequently, by using bio-diesel, on one hand, an improvement in the invested capitals is achieved (compared to that necessary to start the spinneret which aims to produce energy using vegetable oils); on the other hand, there is a noticeable increase in the possible use scenarios. From Table 12 we can see that bio-diesel shows some characteristics which are similar to diesel.

The substitution of diesel for auto traction in vehicles equipped with diesel engines and the feeding of boilers for electricity generation are the main uses of bio-diesel as an energy product. The recent interest shown by the European Com­mittee (Directive 2003/30 of the European Parliament and of the Council, 8 May 2003) in the development of the biocombustible spinneret and in view of the fact that it is already theoretically possible to feed diesel engines which are present in the market with a mixture of diesel and bio-diesel make the use of bio-diesel as a fuel for auto traction particularly interesting.

Directive 2003/30 of the European Parliament and of the Council, 8 May 2003, on the promotion of bio-fuels and other renewable fuels in the transport sector, stipulates that each member state should fix its target for bio-fuel usage relative to the bio-fuel quotation available in the market. Such targets must be based on the

reference levels as per the Directive: 2% of the total fuels (petrol and diesel) that are available in market by 31 December 2005 and 5.75% by 31 December 2010. Legislative Decree 120/2005 acknowledges Directive 2003/30 at a national level and for Italy it establishes the target of 1% by 31 December 2005 and 2.5% by 31 December 2010.

Currently, in Italy, the use of bio-diesel for auto traction is limited to the realiza­tion of mixtures with diesel only up to 5%. Studies conducted by German and Austrian experts demonstrated the possibility of using bio-diesel in mixtures up to 30% without making any modifications in the engine. To use pure bio-diesel, on the contrary, it is necessary to substitute the rubber rings with other compatible materials (copper, carbon steel, brass, fluorinated rubbers, etc.).

Bio-diesel, pure or as a mixture, can also be used in diesel burners by making only modest remedial interventions (regulation of the air/fuel ratio, modification of the atomization nozzles’ slant, etc.) [2, 14, 24].

The main advantages of bio-diesel compared to the use of the traditional diesel are [24]

• high number of cetanes (higher flammability in diesel cycles);

• high lubricant capacity;

• absence of sulphur;

• high percentage of oxygen (high stability of combustion, lower production of PM10, lower volatile organic residuals);

It is useful to remember that bio-diesel production is an option for the agricultural cultivation of the abandoned or rotated soils after intensive cultures. Figure 18 shows the historical trends of bio-diesel production in Europe.

The statistical almanac on bio-diesel production, edited by the European Bio-diesel Board (EBB), describes the highly positive bio-fuel production situation in 2005. Particularly in Europe, an increase in production equal to 65% of the production in the previous year was observed. This increment was a result of the involvement of 15 member states of the European Community, of whom Germany again affirmed itself as the leader in bio-diesel production with 1.6 millions tons produced in 2005. Although the scenario is positive, the target of 2% of the biocombustible production as per European Directive 2003/30 has not yet been achieved [26].

In this section, bio-ethanol is obtained from raw materials that are rich in starch (soft wheat and corn). In this case, it is necessary to treat the starch by a hydrolysis reaction to make the glucose contained in it fermentable.

Wheat and maize grains are crushed and dehydrated to obtain starch paste. Starch is then gelated directly at a temperature of 175°C and a pressure of 2 atm. The hydrolysis is generally carried out using an enzyme called amylase, which has the ability to free the glucose molecules that are present in the starch chains. The temperature at which hydrolysis is carried out should be maintained below 60°C

for a fermentable sugar yield of 80%. The remaining part of the starchy section is similar to the sacchariferous section [2, 30].

In the combustible production from the biomass, the agricultural compartment has and it will have a more and more relevant role. This compartment, in fact, gives a great number of materials that are applicable for energy production (residual prod­ucts that are derived both from other cultivations and from specialist cultivations that are dedicated to the production of combustible biomass materials). The main products of the agricultural sector are [2]:

• wooden cultural residuals, which come from vineyard and orchard management;

• composite nature cultural residuals which come from cereals and sowable cultivations;

• lignocelluloses from woody and herbal dedicated cultures biomasses;

• culture products of the oil culture (sows, etc.) for the production of vegetable oils and bio-diesel;

• the products of the alcohol-producing cultures (tubercles, prills, etc.) for the production of bio-ethanol

The process of pyrolysis consists of a thermochemical conversion that allows transforming the organic substance into final fuel products (solid, liquid, gase­ous). Pyrolysis takes place in the absence of oxidizing agents, or with a limited presence of these agents so that the oxidation reactions can be neglected. The heat required for the evolution of the process can be indirectly supplied through the reactors walls (transport of heat for convention and irradiation) or directly by recirculating a heating tool in the bed (heat transport for conduction) [44].

The products of pyrolysis, although they differ depending on the feed material, can include the following [44]:

• a fuel gas having a medium calorific power (13-21 MJ/N m3), mainly made of CO, CO2 (if oxygen is present in the basic material), H2 and light hydrocarbons (both saturated and unsaturated);

• a liquid product (obtainable from the condensation of the vapour phase) that is separated into two phases: an aqueous phase containing low molecular weight organic species that are soluble and a non-aqueous phase that is mainly made of organic molecules and oils with high molecular weight, called tar or bio-oil;

• a solid carbon product (char) and the cinders.

The most common modalities for execution of the pyrolysis process are [30, 44]:

• ‘Carbonization’, the most ancient and well-known pyrolysis process, which takes place at temperatures between 300°C and 500°C. From this process, only the solid fraction (vegetable carbon) is obtained and therefore the other fractions can be minimized.

• ‘Conventional pyrolysis’, which takes place at temperatures lower than 600°C, with moderate reaction times. From this process three fractions in about the same proportions are obtained.

• ‘Fast pyrolysis’, which takes place at temperatures between 500°C and 650°C, with brief reaction times. This process favours the production of a liquid fraction up to 70-80 % of the feed biomass weight.

• ‘Flash pyrolysis’, similar to fast pyrolysis but which take place at temperatures higher than 700°C and have reaction times that are lower than the former. This process allows the production of a liquid fraction up to 80% of the feed biomass weight, but with a composition variation that is more restricted than that of the fraction obtained by fast pyrolysis.

The main parameters that influence the process are [44]:

• temperature and pressure;

• speed of feed heating;

• dimensions and shape of the biomass to be treated;

• presence of additional catalysts;

• residence times of the solid phase and volatile phase in the reactor.

The products of pyrolysis can be used for the following purposes [44]:

• The gas: It can be burnt to give heat to the reactor involved in the pyrolysis or it can be applied as a fuel in turbo-gas or internal fuel engines.

• The tar: In most of cases, it is not directly applicable as a fuel because of its high viscosity and acidity due to the presence of oxygenated organic compounds. Before combustion it is necessary subject them to catalytic hydrogenation (upgrading) which involves, practically, the removal of the oxygen present. Recent studies have evaluated the possibility of using bio-oils for the production of H2 by catalytic reforming for application in combustible cells [30, 45];

• The aqueous solution: This fraction is derived from the pyrolysis of the feed’s humidity. It helps in the dissolution of the organic oxygenated species that orig­inate from the pyrolysis as organic acids, aldehydes, ketones, phenols, which are otherwise difficult to dispose.

• The char: These solid carbonaceous residuals can be used as fuel or find application in the chemical industry.

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Ethanol can be produced chemically starting from a fossil source or by fermenta­tion starting from biomasses. This second method leads to the so-called bio-ethanol production.

Bio-ethanol is a vegetable origin fuel which is obtained by the fermentation of alcohol from sugars and complex carbohydrates, such as starch, cellulose and hemi — cellulose. However, the raw materials for ethanol production can be derived from alcohol-producing dedicated cultures if they are sacchariferous (sugar beet, sugar

cane, sugar sorghum) or starchy (soft wheat and corn) as well as from lignocellulosic residuals obtained from the forest and agricultural workings. The last materials mentioned do not require specific workings, as in the case of dedicated cultures, which reduces the cost for the retrieval of the raw material; therefore, they represent the most interesting option from the economical point of view.

The bio-ethanol production spinneret comprises three sections (analysed in detail in par. 2.3, Chapter 4): sacchariferous, starchy and lignocellulose.

In addition to the biomasses described above, as raw materials for bio-ethanol, agricultural and food industries residuals and urban wastes can also be used, and based on their nature they can be included in one of the three categories of the bio-combustible production spinneret.

Bio-ethanol can be mixed with gasoline or, in some cases using appropriate expedients, it can be substituted as the feed in vehicles; this bio-combustible, in fact, shows chemical-physical characteristics that are similar to gasoline. Table 14 lists the main energy characteristics of bio-ethanol compared with those of gasoline [1, 2, 24, 29].

The country that stands out for the use of bio-ethanol is Brazil where, even in the 1970s, the engines were modified for the use of anhydrous bio-ethanol (with 5% of water residuals as a substitute for gasoline. This practice is today a reality in the South American country. The interventions needed to adapt engines for the use of anhydrous bio-ethanol as a substitute for gasoline include valves regulation and replacement of the components that can corrode. In USA and Canada, on the contrary, anhydrous bio-ethanol is used in mixtures with gasoline up to 10% in non-modified engines and up to 85% in modified engines. The feeding of these latter engines, called flexible fuel vehicles (FVV), can be realized either with gasoline and bio-ethanol mixtures or with gasoline only; in fact, they are equipped with the automatic regulation of the injection times and the ratio of air-fuel mixing. In European and American studies, the possibility of using bio-ethanol in mixtures up to 23.5% without changes to the motor has been emphasized.

At present, in Europe, the presence of anhydrous bio-ethanol in gasoline in concentrations up to 5% is allowed. It is also important to underline that the properties of bio-ethanol increases the engine efficiency and reduces the fuel combustion [1, 2, 24].

Instead of bio-ethanol it is possible to use ethyl tert-butly ether (ETBE), obtained by the reaction between ethanol and isobutylene in the presence of appropriate catalysts, which finds use as an antiknock with a high octane number. ETBE can be used in place of benzene (carcinogen) and methyl tert-butyl ether (MTBE; highly polluting, especially in subterranean waters) compared to which it has a lower environmental and human health impact. Furthermore, ETBE, if it is used in mixtures with gasoline at 15% gives an octane number equal to 110, which is higher than the octane number of 95-98 that is typical of traditional antiknocks [2, 30]. Some stud­ies have also demonstrated the possibility of using bio-ethanol in mixtures with diesel: up to mixtures of 15%, without any modification to the diesel engine.

In this section, ethanol is obtained from raw cellulosic materials or from materials with high content of cellulose and hemicelluloses. Even though it is not possible to register the industrial scale production of cellulosic origin at the worldwide level, the possibility of using lignocellulosic residuals in this manner has initiated many research and development activities, particularly in the United States.

The main components of the lignocellulosic biomass are cellulose and hemicel — luloses, and being made of fermentable sugars, they can be used to obtain ethanol. One of the critical points that characterize this spinneret section is the physical separation of cellulose and hemicelluloses from lignin. This can be achieved by chemical-physical (the most well known of which is called steam explosion, which applies saturated vapours at high pressure), chemical (with acids) and mechanical (with press systems) treatments. Subsequently, the cellulose and hemi­celluloses are subjected to hydrolysis, which can be carried out in two ways: chemical or enzymatic.

The acid chemical hydrolysis can take place in only one step or in two different steps. In the first case, hydrolysis is carried out using concentrated (at a 77% con­centration) sulphuric acid (H2SO4) which is added to the cellulose material in a ratio of 1.25:1 and at a temperature of 50°C. In the second case, dilute sulphuric acid is used: first, the hemicelluloses are attacked by H2SO4 at a concentration of

0. 4% and at a temperature of 215°C. In enzymatic hydrolysis, the cleavage of the cellulose and hemicelluloses chains is achieved using enzymes that are called cellulases, which have been discovered in the micro-organism Trichoderma reesei and have also been subsequently identified in many other microbic groups.

Enzymatic hydrolysis is preferred to chemical hydrolysis. The hydrolysis of cellulose yields glucose molecules, which is an easily fermentable six-carbon atom sugar; the hydrolysis of hemicelluloses gives five-carbon atom sugars that are ethanol fer­mented with more difficulty. The total yield of bio-ethanol in the cellulosic section is still a matter of high concern, especially with regard to hemicelluloses.

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.

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.

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].

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].

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.

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].

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

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.

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).

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

1 Reduction of emissions into the atmosphere

The higher environmental benefit due to the use of biomasses for energy production purposes is connected with the substitution of fossil sources with renewable sources. This is rendered both by the reduction in the use of these sources (that are exhausted because of their nature) and by the decrease in the polluting emissions that are produced from the combustion. The combustion of petroliferous products emits into the atmosphere carbon dioxide that was stored in the vegetable textiles billions of years ago, with an immediate reflex action in terms of an increase in the concentration of greenhouse gases and other pollutants such as nitrogen oxide, sulphur, etc. The combustion of biomasses also generates polluting emissions but they are less dangerous and in smaller quantities com­pared with the emissions produced by the combustion of fossil fuels. The use of biomasses for energy production purposes represents an instrument to cushion the climate changes and, more generally, to reduce the environmental impacts that are linked to the use of fossil sources.

4.3.1 Bio-gas

Bio-gas is a combustible with high calorific power (4,500-6,500 kcal/N m3 depending on the chemical composition of the gas) that is obtained by anaerobic digestion (see par. 2.1, Chapter 4) of an organic substance. The main components of bio-gas are methane (CH4) and carbon dioxide (CO2); other substances with a lesser percentage are carbon monoxide (CO), nitrogen (N2), hydrogen (H2) and hydrogen sulphide (H2S).

Before being used for energy production, the bio-gas must be subjected to appropriate treatments (see Table 17) which, by raising the percentage of methane to the detriment of other gases, improves the calorific power.

Table 16: Energy equivalence between 1 m3 of bio-gas and the main combustibles.

Gasoline 0.8 l

Methane 0.7 m3

Ethylic alcohol 1.3 l

Wood coal 1.4 kg

Wood 2.7 kg

In fact, it is the concentration of methane in the mixture that determines the final calorific power of the gas: the higher the concentration of methane, the higher is the LCP; the presence of carbon dioxide, nitrogen and water has a contrary effect. The treatment to which the bio-gas is subjected should reduce the percentage of corrosive agents, such as hydrogen sulphide, which can damage the utilization plants. The choice of the treatment, or treatments, to which the bio-gas is subjected depends on its initial characteristics and on the final calculated utilization [1, 2, 31].

Table 18: Biomasses and anaerobic digestion organic wastes and their bio-gas yield (m3 per volatile solid ton) [32].

m3 bio-gas/t SV

Animal dejections (piggish, bovine, birds and rabbit) 200-500

Cultural residuals (straw, beet collar, etc.) 350-400

Agroindustrial organic wastes (serum, vegetable wastes, 400-800

yeast, muds, distillery effluents, beerhouse, cellar, etc.)

Abattoir wastes (fats, stomach and intestinal content, 550-1,000

flotation sludge, etc.)

Depuration muds 250-350

Urban wastes organic fraction 400-600

Energy cultures (maize, sugar sorghum, etc.) 550-750

Currently, the main uses of bio-gas are relative to the thermal and/or electrical energy production. In detail, it is possible to produce [2, 32]:

• heat, as hot water, vapour or air, with a medium energy efficiency of 80-85%;

• electricity, generally in engines with vapour or gas turbines, for plants with a high capacity whose medium yield is 30-35%;

• combined production of heat and electricity (cogeneration) in endothermic engines with total medium yields of 80-85% (medium thermal yield: 50%, medium electric yield: 35%), which is currently the most used solution.

Other emerging applications are [2, 32]:

• fuel production for vehicles;

• cold production (three-generation), for example, with absorbing machines;

• the use in industrial ovens as a primary or auxiliary combustible.

In Europe, since 1990 we have witnessed a continuous growth in bio-gas pro­duction from 2,304 tons in 1999 to 3,219 tons in 2003. The leading country in this sector continues to be England, with more than 15,000 technicians in the sector in 2003 and a production of raw bio-gas which is equal to 1,151 tons; then there is Germany which has declared 2,000 installations for bio-gas production that is equal to 685 tons [33].

Currently, Europe can count on [32]:

• 1,600 operative digestion tanks for the stabilization of depuration sludge;

• 400 bio-gas plants for the effluent industrial waters with high organic load;

• 450 plants that work on the recovery of bio-gas from the urban rubbish dumps;

• more than 2,500 plants that work on effluents from intensive animal breeding, particularly in Germany (>2,000), Italy, Austria, Denmark and Sweden;

• 130 plants for anaerobic digestion; each of them processes more than 2,500 tons in a year of urban rejections and or industrial organic residuals with organic content.

In Italy, on the contrary, there are [32]:

• 120 digestion tanks for the stabilization of sludge and purification of effluent urban waters;

• few experiences (seven plants) of anaerobic digestion of urban rejections with organic content;

• several bio-gas plants in the agro-industry;

• more than 100 bio-gas plants for effluents from intensive animal breeding.