Aerobic digestion involves the metabolizing of the organic substances by micro­organisms, whose development is controlled by the presence of oxygen. These bacteria convert the complex substances into other simpler substances, freeing CO2 and H2O and result in high substrate heating (the layer under the biomass that is not yet digested by bacteria), which is proportional to their metabolic activity.

The heat produced can be transferred outside through a fluid exchanger. It is a process that is particularly suitable for liquid manures and industrial wastewater purification plants. In Europe, it is used (especially in Germany where strict rules were imposed for the destruction of pathogenic substances) in the thermophilic aerobic self-heated digestion process (auto-heated thermophilic aerobic digestion) for the treatment of the wastewater; more recently, this technology has also been used in Canada and the United States [3, 4, 35].

1 Introduction

The term biomass encompasses a large number of materials of an extremely heterogeneous nature. We can state that everything that has an organic matrix is a biomass. Plastics and fossil materials have been excluded, even though they belong to the family of carbon compounds, because they do not have anything in common with the characterization of the organic materials discussed here. In scientific terms, the word biomass includes every kind of material of biological origin; it is so linked to carbon chemistry which directly or indirectly derives from the chlorophyllian photosynthesis.

The biomass is the most sophisticated storage of solar energy. In fact, through the photosynthesis process vegetables are able to convert the radiant energy into chemical energy and to stock it as complex molecules with high energy content. For this reason, the biomass is considered renewable and unexhaustive, if appro­priately used as a resource; that is, if the use tax of the same does not exceed the regeneration capacity of the vegetable forms.

The biomass is also an energy source that considers as neutral the aim of the greenhouse gas emissions increment. In fact, vegetables, through photosynthesis, contribute to the subtraction of atmospheric carbon oxide and carbon fixation in the textures (a total of 2 x 1011 tons of carbon are fixed in a year, with an energy content of the order of 70 x 103 MTep, which is equivalent to ten times the world’s energy requirements).

The quantity of carbon oxide released during the decomposition of biomasses, if it happens both naturally and through energy conversion processes (even if it is through combustion), is equivalent to that absorbed during the growth of the same biomass.

There is no contribution to the increase of the CO2 level in the atmosphere. There­fore, in this case the improvement in the quotation of the energy produced using bio­masses, rather than fossil fuels, can contribute to the reduction of the CO2 that is emitted into the atmosphere. For this reason, the use of biomass for energy applications is considered one of the priorities of the post-Kyoto development policies [2, 4-6].

Legislative Decree no. 387 of 29 December 2003 defines biomass as ‘the biodegradable part of the products, wastes and coming from agriculture residuals

(comprehending vegetable and animal substances) from the forestry and from the connected industries, as well as the biodegradable part of the urban and industrial wastes.’

There are other terms which are associated with the term biomass, and they are now commonly used in the renewable energy sector, such as ‘bio-fuel’, which generally means ‘every organic substance that is different from petrol, natural gas, carbon or from their derivatives, and which is usable as combustible’, and ‘bio-energy”, which represents the energy produced from biomasses [7].

A fluid bed is a suspension of solid particles in an ascendant current of gas. The gas is introduced at pressure from the bottom of the reactor, whereas the parti­cles enter from above. When the solid remains in suspension, we talk about the fluidization condition that is reached for a determinate speed of the gas in which the fluid bed, made of a solid phase and a gaseous phase, acts as a liquid. The application of the fluid-bed technology to gasification ensures very good mixing between the biomass (reduced into small particles) and the aerating agent, improv­ing the reaction speed. We can also introduce inert fluidizing material (silica sand, alumina, refractory oxides) in the bed with the aim of equalizing the temperature so as to facilitate the heat transfer between the particles and the combustible.

The fluid-bed reactors, in contrast to fixed-bed applications, are characterized by a uniform temperature in the reactor (typically 800-850°C). Using the fluid — bed technology, we obtain a tar content in the producer gas that is intermediate between that obtained from the updraft gasificator and the downdraft gasificator.

The gas exiting from the reactor shows a higher content of solid particles (char, cinder, sand) [2].

The several kinds of lignocellulose biomass (arboreal, perennial herbaceous, annual herbaceous) that have different applications. The wooden biomasses are applied to the selling of logs or wood trunks, as well as to chipping and pellet or briquette transformation.

In the first case, the use is limited to small domestic heating plants, whereas chip­ping is necessary for the use of the biomass in large heat generation industrial plants. The briquettes optimally substitute the firewood. The low quality pellet, not suitable for the feed in small plants, can be efficiently used in industrial plants [2, 13].

Herbaceous biomasses are appropriate for the production of chips or pellets which, because of the lower quality, are not suitable for use in small plants, but they find better application in large dimensions equipped with appropriate expedients to optimize com­bustion plants. A possible future scenario is the mixing of the herbaceous and wooden biomass to exploit, in a synergic way, the peculiarities of the two combustibles [2].

Alcohol fermentation is a biotechnological process which allows the production of bio-ethanol from simple carbohydrates (glucose, sucrose, mannose) and from long chain polysaccharides (starch, cellulose, hemicelluloses). Therefore, the raw materials necessary for obtaining this biocombustible can be derived from alcohol producing dedicated cultures both sacchariferous (sugar beet, sugar sorghum, etc.) and starchy (soft wheat and corn) as well as from lignocellulose residuals. The production spinneret for bio-ethanol is articulated in three sections: saccharifer — ous, starchy and lignocellulosic.

The definition of biomass which is taken from Directive 2001/77/CE, and acknowl­edged at a national level from the Legislative Decree no. 387 of 29 December 2003, reunites a wide class of materials of vegetable and animal origin that also includes rubbish. More simply, the biomasses which are appropriate for energy transformation, if it happens directly using the biomass or prior to changing of the same in a solid, liquid or gaseous fuel, can be divided based on their source into the following compartments [2]:

• forest and agro-forest compartment: forest-cultural or agro-forest activities and operations residuals, use of the coppice, etc.

• agricultural compartment: farming residuals which come from the agricultural activities and the dedicated lignocelluloses species cultures, oil bearing plants for the extraction of oils and their transformation into bio-diesel, alcohol-producing plants for bio-ethanol production;

• zoo technique compartment: livestock sewage wastes for the production of biogas;

• industrial compartment: coming from wood or wood product industries and paper industries, as well as agricultural and food industry residuals;

• urban rubbish: maintenance of the public green and urban solid rubbish maintenance operations.

The term biomass groups materials that differ from each other in terms of chemical and physical characteristics. They can have multiple uses on the energy production front. Generally, as we will see hereafter, it is possible to group trans­formation processes into different categories: the processes of biochemical conversion, which allows the gain of energy through chemical reactions due to the presence of enzymes, fungi and other micro-organisms that form, in particular conditions where the biomass is held; the processes of thermochemical conversion has, as it basis, the heat action that allows the development of chemical reactions which are necessary to transform matter into energy. The factors that favour the choice of one of the two processes are the carbon/nitrogen (C/N) ratio and the grade of humidity at the time of collection: when the C/N ratio is lower than 30 and the humidity content exceeds 30%, biochemical processes are generally used; on the contrary, thermochemical processes are more suitable [2-5].

fluid boiling bed gasificators, the height of the bed is limited (1-2 m) and the speed of the gas is 0.8-2 m/s (minimum speed necessary to keep the solid phase in suspension). Over the bed, there is a region where only the gaseous phase is present. Inside the bed the gas bubbles are formed whose movement on the surface resembles the phenomenon of a liquid that is boiling; this provokes an internal agitation that leads to further mixing of the phases. These reactors show higher temperatures compared with the temperatures observed in the fluid circulated bed reactors, which results in a lower tar content in the producer gas but also presents a bigger danger of cinder fusion.

The main characteristics of this type of reactor are:

• high reaction speed;

• better temperature control than the fixed-bed reactors;

• high solid particle content in the producer gas;

• low tar content in the producer gas;

• maximum loads of the order 10-15 t/h of dry biomass;

• flexibility in terms of the granulometry of the biomass feed;

• for equal dimensions, the fluid-bed gasificators have higher powers than the fixed-bed gasificators;

• ease of turning on and switching off;

• catalysts can be added to the bed for the cracking of the tars;

• loss of carbon in the cinder.

The fluid boiling bed atmospheric gasificators are appropriate for different types of biomass and for applications with medium and small powers up to 25 MWth [2, 46, 48].

3.2.2.1 Fluid circulating bed reactors (CFB, circulating fluidized bed) The

reactor has heights reaching 8 m and it has a limited diameter. Given that the gas speed is high (>4 m/s), the solid particles (char and sand) are dragged until they go out of the main column, to be, then, separated from the gas through a cyclone and introduced again at the bottom of the reactor.

Starting from the bottom, the bed shows three different areas:

• dense phase: it is characterized by a high density and by the formation of gas bubbles;

• intermediate phase: unstable area with regions of different densities;

• dilute phase: the mixing of the solid in the gas is homogeneous and the density is low.

The main characteristics of the circulating fluidized bed gasificators that differentiate them from the boiling fluid bed gasificators are:

• for low powers, they involve higher costs compared with the boiling fluid bed gasificators;

• difficulty in the realization of cracking of the tars inside the bed;

• utilization for biomass loads that are higher than 15 t/h.

The circulating fluid bed atmospheric gasificators are appropriate for a huge variety of biomasses, with powers that vary from few MWth up to 100 MWth. This technology is more appropriate for large-scale applications [2, 46, 48].

3.2.2.2 Dual bend gasificators for pyrolytic gasification In this case, the gasi­fication does not take place through partial oxidation but through indirect heating of the biomass (pyrolytic gasification). The plant is composed of two fluid-bed reactors: a circulating fluidized bed gasificator and a combustor (boiling or recir­culated fluidized bed). In the gasificator, the heat required for the decomposition is obtained from the freewheeling sand in the plant which is heated in the combustor. Vapour is used as the fluidized gas. The producer gas that exits from the gasificator drags the sand particles and char that are separated by a cyclone and carried to the combustor, where the char is burned. The heat generated is absorbed from the sand that is dragged out of the combustor by the waste gas. A second cyclone provides for the exhausted gas sand, allowing its reintroduction into the gasificator where it transfers the absorbed heat to the biomass.

The process is particularly complex and the huge dimensions makes the realiza­tion of the plant particularly difficult, because of the high investment costs required. The main advantage of this technology the use of vapour which allows producing a medium calorific power gas without the use of oxygen. Given that a part of the char must be used for the combustion, there is low carbon conversion in the gas. In fact, this technology shows a high tar content in the producer gas [2]

3.2.2.3 Pressurized fluid bed gasificators When the producer gas is applied as a combustible in gas turbine plants, it should enter the combustor at high pres­sures (10-25 bar). If the gasification takes place in an atmospheric reactor, the gas must be cooled and compressed, spending energy. One solution to this problem is represented by the pressurized fluid bed gasificator which allows obtaining high pressure gas directly.

The use of the pressurized fluid bed gasificators has the following advantages [2]:

• low internal energy consumption (because the gas notneed not be compressed);

• at high pressures, the tendency of the cinder to sinter is reduced;

• more compact dimensions compared with the atmospheric gasificators;

• low danger of condensation because the gas case is not cooled before use.

The disadvantages are:

• difficult to feed the biomass into the reactor;

• high investment costs;

• hot cleaning gas devices that are expensive and still in the development phase.

4.2.1 Vegetable oils

From sunflower seeds and rape seeds, oils are extracted which can, actually, be used without the need of further bio-fuels; therefore, they can be applied to the energy range just as combustibles of liquid fossil origin. Table 11 lists the main energy characteristics of combustible oils and compares them with the characteristics of gasoline.

The current energy applications of vegetable oils are relative to the use in diesel engines. Their use in energy production plants as a substitute for diesel poses some problems: the burners must be partially changed to cover the higher viscosity of the vegetable oils compared to diesel.

The high viscosity also excludes, at present, the application of vegetable oils for auto traction. In fact, in this case important changes would be necessary to the plans of engines [2, 14]. Vegetable oils are composed of 78% carbon, 12% hydrogen and 10% oxygen.

Oils are organic compounds, so they are highly biodegradable; these characteristics can represent an obstacle in their use as a combustible because they can undergo oxidation and polymerization in the storage reservoirs. For this reason, oils must be applied within 12 months from their production. Keeping in mind that the viscosity of the oils gradually increases as the temperature decreases (until solidification), such combustibles are not appropriate for use at temperatures below 5°C [1].

The sacchariferous section is aimed at the energy conversion of the sugars that are obtained from sugarcane, sugar sorghum and sugar beet. From a technological point of view, the energy spinneret is articulated in the following phases: extraction of the sugars from the vegetable textiles, their fermentation and ethanol distillation. Generally, the fermentation is induced by the yeast Saccharomyces cerevisiae. It is used in bioreactors that reproduce the ideal conditions which favour its anaero­bic metabolism (i. e. in presence of low oxygen concentrations), at a temperature between 5°C and 25°C and with a variable pH between 4.8 and 5.

The separation of ethanol from the mixture that is obtained at the end of the fermentation phase (whose duration is around 72 hours) is achieved by distillation, exploiting the components present in the mixture. From the distillation, ethanol is obtained in concentrations equal to 95% of weight and with a residual water content of 5% [2, 30].

To obtain ethanol concentrations close to 100%, it is possible to resort, but with an increase of the process costs, to fractional distillation (concentration of ethanol equal to 99% of weight) or to the separation by pervapouration (concentration of ethanol equal to the 97% of weight). Fractional distillation involves the addition of benzene in the starting mixture.

The separation by pervapouration involves transporting the mixture that is obtained downstream of the fermentation in the vapour phase to filter it with appropriate selective hydrophile membranes.

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].