Bio-diesel can be produced by using as raw materials both oils extracted from oil cultures (vegetable oil extraction process: soy, sunflower, rape, etc.) and oils that are recovered (regeneration process of vegetable oils) from alimentary uses through separate collection systems. The products that are obtained upstream of the extraction and the regeneration can be directly applied as a combustible or directed to the transesterification process to obtain bio-diesel [1-3, 30].

2.4.1 Vegetable oil extraction

The steps involved in the extraction of vegetable oils from oil cultures are:

1. cleaning (with electrovalent elements or magnets);

2. grinding;

3. heating and conditioning (80-90°C, humidity 7%);

4. mechanical extraction (hydraulic press or strew-shaped) or chemical (solvents);

5. purification for depuration or refining (neutralization of the free fatty acids) if the oils are to be used for bio-diesel production.

The seeds derived from oil cultures are first cleaned, using magnets or electrovalent elements to remove additional materials or collect gross residuals, and are then decorticated. In the following grinding step, there is an outburst of the oils from the cells. Heating and conditioning, in the temperature range 80-90°C and at a humidity of 7-10%, promote the lysis of the cells, the diffusion of the seeds’ fat material and the separation of the proteic components. Oil extraction from milled seeds can be done by mechanical or chemical techniques. Mechanical extraction uses a screw or hydraulic press, and it leaves an unextracted residual fat content equal to 5-12%. Chemical extraction, characterized by an unextracted residual fat content equal to 1% (with a seed-solvent ratio equal to 1:18, reaction environment temperature of 50°C and contact times equal to 2 hours for rape seeds and 1 hour for sunflower seeds), involves the use of organic solvents (such as trichloroethylene, hexane, carbon sulphur). Chemical extraction can be done in a discontinuous man­ner (batch), which is the preferred option for plants that treat at least 250-500 t/day. Chemical and thermochemical extraction can be integrated with each other. Despite a high investment cost, the yield is close to 100%. In this technique, the milled material is first subjected to mechanical extraction, which leaves a residual fat content of 20-24%, and, subsequently, chemical extraction is carried out. The yield of raw oil obtained from the extraction process is variable, from rape and sunflower seeds 36-38% of oil weight is extracted. If we want to convert the raw oil obtained into bio-diesel, it must be subjected to a purification step that can be performed depending on two modalities: purification and refining. The two proc­esses, which are preceded by a centrifugation step, are diverse in terms of the qualitative levels, which is higher in the refining step. Depuration is directed to the removal of impurities (waxes, resins, pigments and mucilages) present in the raw oil and it is carried out using sulphuric acid, salt water solutions or by percolation using absorbing grounds. Refining removes impurities working in salt solutions with sulphuric acid or citric acid. Furthermore, refining reduces the acidity of the raw oils for the neutralization, which can be done in a physical (at 240-260°C and in conditions of vacuum at -1 mbar) or chemical manner (working with sodium hydroxide at a temperature of 60-80°C and at atmospheric pressure). The best quality of the refined oil, compared to the depured oil, is the reduction of the acid­ity of the raw oils. At the end of the purification step, the vegetable oil yield is about 34.4%. The main by-product of the vegetable oil extraction is the proteic panel, which is used in zootechny as animal feeds [2, 14, 24].

In the wood industry three kinds of wastes are produced:

• blank wood wastes (sawdust, small chips, chips);

• treated wood wastes (residuals with glues and/or presence of paints);

• impregnated wood wastes (wood wastes impregnated with salt base preservatives).

Excluding the plants equipped with anti-pollution technology, for energy produc­tion purposes, it is possible to only use wood residuals and by-products which are not chemically treated (barking residuals, cut, pruning, etc.) or treated with prod­ucts that do not contain heavy metals or organic halogenated compounds (typical of wood treated with preservatives or other chemical substances) [2, 15].

The Italian furnishing industry of produces wooden wastes that are equal to 4.7 millions tons a year, of which 55% is not treated wood. Such a considerable residual quantity already has a market: it is applied for energy production purposes or as secondary raw materials for the production of pellets, panels or paper [2, 7, 16].

3.4.1 The cellulose and paper industry

From the paper industry, residuals appropriate for use as raw materials instead of energy residuals are obtained. Such residuals are mainly present as muds and they are generally produced from the water depuration process, both chemical-physical and biological.

In Italy, there is a lack of a strict regulation framework and, as has already happened in other European Community countries, it has resulted in the develop­ment of advanced forms of rubbish treatment. In Italy, in fact, only 25% of the energy recovery is obtained from the paper industry residuals against 50% in the European Union. Furthermore, the European Directive 2000/76 does not recognize all the paper production residuals as an adequate or clean fuel and this leads to the obligated disposal in dumps of residuals otherwise usable for the energy recovery [2, 17].

depuration □Muds — biol. dep.

□ Rejections □Muds

■ Not — dangerous

■ Dangerous

Biomasses are considered neutral energy sources in terms of greenhouse effects because their combustion does not result in an increase in the concentration of atmospheric carbon dioxide. In fact, given that the quantity of carbon dioxide emitted in the combustion phase is equivalent to that absorbed by the vegetables during their growth, the CO2 cycle is closed. But taking into account the entire life cycle of the combustibles that form the biomass does not result in a nil bal­ance of CO2. In fact, the production, working and transport steps often result in negative impacts on the environment that are determined by energy and mate­rial consumptions which are necessary to sustain the processes. Table 1 lists the carbon dioxide balance for the fuels obtained from the main biomass production spinnerets. The avoided emissions were estimated as a function of the substi­tuted fossil fuel (coal, diesel, methane) by taking into account the respective calorific powers [2, 3].

1 Biomass energy conversion

There are multiple methods for the conversion of biomass energy. Generally, the biomass is transformed into a more easily manageable form (solid, liquid or gaseous bio-fuel) in which it is used [1, 3, 5]. Before proceeding with the analysis of the main modalities of biomass energy conversion, it is important to note that conver­sion is only one aspect of the wider problem, which includes, on one hand, the present reality or the eventual future situation where the biomasses are produced and, on the other hand, the possible utilization of the deliverable energies. The resulting circuit is depicted as follows:

Production-Collection-Conversion-Utilization

This has to be studied in an optimal context that includes extensive co-ordinated initiatives and interventions with both public and the entrepreneurial support. The biomass energy conversion processes are divided into biochemical and thermochemical processes [3-5, 34, 35]:

• Biochemical conversion processes allow to exploit the energy obtained through chemical reactions, due to the action of enzymes, fungi and micro-organisms, that take place in the biomass under particular conditions. These processes are suitable for those biomasses in which the C/N ratio is lower than 30 and the humidity at the time of collection is higher than 30%. Water cultivations that are appropriate for biochemical conversion include the by-products of some cul­tures (leaves and beet stems, vegetable garden, potato, etc.), livestock sewage wastes and some working wastes (pot ale, vegetation water, etc.) as well as the heterogeneous stored biomasses in the controlled biomasses.

• Thermochemical processes are based on the action of heat which allows the chemical reactions that are necessary to transform the material into energy to take place. These processes are suitable for products such as cellulose and wooden residuals in which the C/N ratio has values higher than 30 and the humidity content does go beyond 30%. The most appropriate biomasses for

thermochemical conversion processes are wood and all its by-products (sawdust, shavings, etc.), the most common cultural by-products of the lignocellulosic kind (straw of cereals and orchards, vine pruning residuals, etc.) and working wastes (such as husk, chaff, hulls, hazels).

Of the several technologies available for energy conversion of biomasses, some of them can be considered to be at the development level to allow their utilization on an industrial scale, while others, on the contrary, need further experimentation to raise their efficiencies and reduce the energy conversion costs.

Usually, upstream, all the conversion processes require appropriate base material for pre-treatment, which can include water washing, drying with mechanical or thermal instruments (pressing), reduction into smaller dimensions, densification (pelletization and briquetting) and separation of the fibres (extraction with solvents). Even the final product, depending on its use, must be treated to separate them (e. g. from the substrate that does not react, catalysts, micro-organisms and solvents), purify them and concentrate them. Depending on the application, we resort to sedimentation, filtration, centrifugation, distillation, absorption and extraction with solvents [4].

The alimentary origin-exhausted vegetable oils that can be used for energy valoriza­tion are those that are obtained from industrial processes (from ovens and fryers) and from domestic users (frying oils and oil for food conservation). Before being transformed into bio-diesel, the oils of alimentary origin must be subjected to the regeneration process which involves the following steps: removal of the rough impu­rities through subsequent filtrations, neutralization and dehydration. The exhausted regenerated vegetable oils can be used in a similar manner as the oils obtained from dedicated cultures and from this point they follow the same spinneret [2].

Some wastes that are produced from the agro-alimentary industry, because of their organic lead and their high humidity content, are appropriate for the treat­ment, through anaerobic digestion (par. 2.1, Chapter 4). The main agro-alimentary industry wastes that can be applied to energy recovery through bio-methane, with a specific production of bio-gas between 0.25 and 0.35 m3/kg ds, are:

• the dairy sector wastes: serum, main cheese working waste with a high organic load; serum can be evaluated from the energy point of view through the anaero­bic digestion bio-gas production (in co-digestion with other substrates to avoid an excessive acidification);

• the compartment wastes from butchering (deriving from the meat production effluents for human feed, show high organic loads due to the presence of blood, fat and dung material, in addition to dejections);

• the working and fish conserving wastes;

• beverage industry wastes (particularly the high organic load coming from fruit juice, beer and distillates load wastes);

• the sacchariferous industry wastes (particularly coming from the molasses working which show effluents with a high organic substance content).

The chemical conversion processes include [3, 36]:

• anaerobic digestion;

• aerobic digestion;

• alcohol fermentation;

• extraction from oils and bio-diesel production.

2.1 Anaerobic digestion

The fermentation of methane, also known as anaerobic digestion, is a complex natural process that involves bio-degradation of the organic substance in the absence of oxygen (anaerobiosis), resulting in the formation of bio-gas.

As we can see from Fig. 2, the anaerobic digestion process comprises three sequential steps involving different bacterial groups that act in series. In the first step (hydrolysis), the hydrolytic bacterium breaks the complex organic compounds (carbohydrates, proteins and fats) into simpler substances. Subse­quently (in the fermentation step), these simpler substances are transformed, first, into organic acids, through acid-genesis reactions, and then into acetate, carbon dioxide and hydrogen, through vinegar-genesis processes. In the last step, the most important step (methane genesis), the methanogenic bacterium transforms the products that are formed in the previous step into methane (CH4) and carbon dioxide (CO2), the main constituents of bio-gas. Therefore, the organic component is degraded releasing the chemical energy it contains in the form of bio-gas.

As evident from the above description, bio-gas production depends on the coordinated and sequential action of all the microbic groups involved. To achieve this goal, it is essential that the reaction environment is the result of a compromise between the requirements of each individual group involved, by a strict control of the process parameters [2, 31]. The anaerobic digestion processes can be classified based on the mass fraction of the dry substance to be digested: if it is lower than 10%, the process is called wet digestion; if it is between 10% and 20%, the process is called semi-dry digestion, and for values higher than 20%, it is called dry digestion [31, 32].

Depending on the temperature range in which the process takes place, anaerobic digestion is called:

• psychrophilous: if the process temperature is kept below 20°C; the systems that work in such conditions are also called ‘cold’;

• mesophily: if the temperature is between 20°C and 40°C;

• thermophile: if the temperature of the process is between 50°C and 65°C.

The processes described above can be reproduced in confined environments such as an anaerobic methane digester (for the digestion of liquid manures with a high organic load) or in a controlled dump (for the digestion of the organic component of solid rejections) [2, 31, 32].

The reaction that is used for the synthesis of bio-diesel is known as transesterification, a process in which oils react with methanol, in the presence of a catalyst, to form methyl ester (bio-diesel) and raw glycerine, as a secondary product. In other words, the reaction induces the breakdown of the triglycerides molecules that make up the vegetable fats to obtain the methyl ester of the fatty acids mixture. The main result of this process is the reduction in the viscosity of the starting oils, making them compatible with certain energy uses and particularly for use of the bio-diesel as a auto traction fuel.

triglyceride glycerol

The simplified representation of the entire process is as follows:

1000 kg of refined oil + 100 kg of methanol = 1000 kg of bio-diesel

+ 100 kg of glycerine

There are several plant solutions for the realization of bio-diesel from vegetable oils, where the medium yield of conversion is equal to 98%. The factors that affect the choice of the technology to be adopted are the quantity to be treated, the perio­dicity with which the raw materials are available and the quantity of oils at the plant entrance [2, 3].

There are three main technologies that are distinguished in terms of the process temperature and pressure [2, 14]:

• Environment temperature plant: The process takes place at environmental tem­perature and atmospheric pressure, using sodium or potassium hydroxide as the catalyst. It is appropriate for batch treatment (discontinuous), with a bio-diesel production capacity of up to 3,000 t/year. The time of reaction is 8 hours.

• Medium-high temperature plant: The process takes place at atmospheric pres­sure and at a temperature of 70°C, using sodium or potassium hydroxide as the
catalyst. It is appropriate for continuous or batch treatment, with a production capacity of up to 25,000 t/year. The reaction time is 1 hour.

• High temperature and pressure plant: The transesterification takes place at a pressure of 50 MPa and a temperature of 200°C. This technology, involving higher installation and management costs, is justified for production capacity higher than 25,000 t/year, both in continuous and in batch treatments. This technique, com­pared to the other two techniques, allows the treatment of high acidity oils (up to 4%) because it uses phosphoric acid for acid catalysis.

All the technological solutions described above involve the recovery of excess methanol for the vacuum distillation (stripping) and its reuse at the start of the plant.

The main by-product of the transesterification process is a glycerol that has a high economic value in pharmaceutical and cosmetic applications.

The urban solid wastes (USW) that, defined as biomass, can be considered as renew­able energy sources include all the green biodegradable fractions, which can be divided into those made of lignocellulose wastes component and an humid organic component. The residuals coming from the garden management and public or private boulevards of the habited centres belong to the lignocellulose wastes. Although the urban rubbish is generally used for composing, it can be applied after an appropriate conditioning for heat and/or electricity production through combustion.

The organic part with a higher humidity content can, however, be applied to the bio-gas through anaerobic digestion production. Fermentation in anaerobic condi­tions processes also happen in the dumps with a high level of material compaction. In these cases, through appropriate catchment systems (we talk about controlled dumps), it is possible that the cargo and the storage of bio-gas can be applied for energy purposes (Fig. 9) [2].

The engineering typologies of the anaerobic digestion systems that are currently available vary from extremely simple systems that are mainly applied to the live­stock sewage wastes on a business scale to those that are more sophisticated and use high technology and are applied for industrial effluent treatment.

2.1.1.1 Simplified plants Due to its constructive and managerial simplicity, this plant typology finds many possibilities for application in the zoo technical sector. These plants, in fact, only comprise a storage basin (containing the material to be digested), often pre-existing, equipped with an appropriate gasometric cover. The simpler systems are the ‘cold’ systems (psychrophiles) that have variable yields depending on the season and elevated permanence times (around 60 days). The annual bio-gas production for a swine liquid manure is about 25 m3/100 kg live weight. The systems that are equipped with heating, on the contrary, obtained from the bio-gas produced, allow to work in a mesophily regime and to obtain higher, more constant yields during the year, with more reduced retention times (median of 20 days). In this case, the annual production of bio-gas from swine liquid manure will be around 32 m3/100 kg live weight [2, 32].

The function of the gasometric cover is to retain and store the bio-gas that is formed; it can be dome shaped or have a floating shape [2]:

• Simple dome cover: It is not pressurized and is made of flexible canvas material that is anchored on the basin’s perimeter. The gas, being at very low pressure, is extracted and sent to its place of use through a blower.

• Double or triple membrane dome cover: This type of cover comprises two or three superimposed membrane layers which are fixed at the edge of the basin (see Fig. 3). In this case, the draining of the bio-gas is achieved by overpressure valves that are regulated by sensors.

• Floating cover: These are membranes which are equipped with a ballast system that is realized using flexible pipes filled with water to grant the bio-gas storage pressure (see Fig. 3).

The necessity to store a larger quantities of bio-gas than that which can be stored using a normal gasometric cover can be satisfied by using external gasometers (spherical shaped and made of two or three adjustable volume membranes).

cover device

bioeas

hot

water

heating system

Figure 5: Scheme of a simplified bio-gas plant with heating.

Depending on a census taken at the end of 2004, more than 100 bio-gas plants were present in Italy; of these, 70 are simplified and low cost plants that have been realized by superimposing a plastic material cover over an effluent from intensive animal breeding in a storage basin [32].

We will now discuss two typologies of digestors which are more complex than the simplified plants discussed above: mixed reactors and ‘plug flow’ reactors.

2.1.1.2 Mixed reactors The mixed reactor is the more classic digestion typol­ogy. They are silo-shaped and are built in armoured concrete or steel. These reac­tors, working in the thermophile or mesophily regime, are equipped with a heating system that comprises a heat exchanger and they are insulated at the perimeter. Mechanical agitators at a low rotation regime allow mixing of the material to be digested. Depending on the number and the position of the agitators, the reactor can be completely or partly mixed. The gas produced by the anaerobic digestion process is retained by a gasometric dome placed on the top of the reactor and mainly made of a polymeric sheet that is protected by a steel cover or armoured concrete. This typology of the reactor allows treating liquid manures with a dry substance content that is lower than 10%, with medium permanence times, which are between 15 and 35 days depending on the composition of the substrate and the process temperature.

2.1.1.3 ‘Plug flow’ reactors These reactors, equipped with a heating system, agitators and gasometers, allow the horizontal scroll of the liquid manure. They are only used on a small scale, because of technical and economic constraints that limit their volume to a maximum of 300-400 m3. These systems, which are appro­priate to treat liquid manures with a dry substance content of up to 13%, allow to obtain bio-gas yields that are higher than those obtained with mixed reactors, at equal temperatures.

biogas

liquid manure

cl-*

■ ‘ ■ ‘ exit

agitator

Figure 6: Complete mixing reactor.

In addition to the two reactor versions analysed, which are more commonly applied, the market also offers other typologies of digestors that are more sophis­ticated and use high technology, which are particularly suitable for industrial efflu­ent treatment at high organic load: reactors for contact, anaerobic filters and upflow anaerobic sludge blanket (UASB) reactors.