The thermochemical conversion processes are [3, 36]:

• direct combustion,

• gasification and

• pyrolysis.

3.1 Direct combustion

Among the several processes for the thermochemical conversion of biomasses, direct combustion is, without doubt, the most ancient and mature technology. Despite this, many research studies are being continuously carried out with the aim of developing this technique further, making it more and more efficient and with lower environmental impact. The combustion process allows the transforma­tion of the chemical energy in the biomass into thermal energy, through a series of chemical-physical reactions. When a biomass enters the combustion room, first, it is subjected to drying; subsequently, as the temperature increases, pyrolysis, gasification and combustion processes occur. With appropriate combustibles/air profiles, the biomass decomposes and volatilizes, freeing a carbon residual (cin­ders), which is mainly made of mineral inert compounds. The end result of these processes is the production of heat which is recovered through heat exchangers in which the thermochemical energy is transferred to other vector fluids such as air or water. The quantity of thermal energy that is contained in the biomass is a function of type of the cinders and the humidity content and it is generally defined by the lower calorific power [4, 28].

The different combustion technologies used are [4, 28, 30]:

• Grate shaped (fixed or moving), fundamental element in addition to the thermal reaction, also for the removal of the cinders; fixed systems are generally used for combustors of small size. For industrialized plants, moving grates are used as

they facilitate the handling, the mixing of the combustible and the removal of its cinders; such grates can be of different types: horizontally or vertically vibrat­ing, belt, rotating, steps, rolls, etc., and, in some cases, they are cooled with air or water to allow a higher specific heat load.

biomass

secondary air

bed

Figure 17: Grate oven working scheme.

• In suspension, appropriate for powdery and light biomasses, such as rice husk, sawdust, wood dust and chaff, in which the biomass is fed in the upper part of the combustor where it burns and falls on the grate beneath, whose main function is to remove the cinders.

• Rotating drum, used for applications in which the combustible has thermo­physical characteristics, particularly, poor and high polluting load charac­teristics. During combustion, the biomass is continuously remixed by the low drum rotation and the direction of the combustion product can be either in the same direction or in the opposite direction to the biomass progress direction.

• Double stadium shaped, in which, first, gasification and material pyrolysis take place in one room and complete combustion of the gasified products takes place in another room, resulting in the transfer of a major portion of the energy to the operating fluid.

• Fluid bed, in which several kinds of biomass can be treated, including selected urban solid rejections even with a high humidity percentage (>40%). The combustion room is partially filled with inert material such as sand or alumina, which is fluidized from the primary combustion air to establish a ‘boiling bed’ or if there is higher air speed and material dragging, the so-called ‘recirculator bed’, which is recovered and re-fed into the combustion room. In addition to the inert material, even the material that allows to change the environment conditions in which the combustion takes place can be fed into the combustion room: in fact, if polluting combustibles with acidic compounds or containing low-flux cinders are present, limestone or dolomite can be used to neutralize the polluting acids and to avoid fusion of the cinders in the combustor’s operating conditions.

The use of the combustion devices facilitate the recovery of the maximum amount of the energy developed during the process. This recovery can take place in a direct manner through the device’s walls (stoves) or in an indirect manner through a vector fluid (boilers). The presence of the heat recovery sections is not only convenient from the energy and economic point of view, but it is also neces­sary to reduce the temperature of the fumes that are emitted from the combustion room (temperatures of 1200°C can be reached) as it is possible to bring down their temperature (to not higher than 300°C). The combustion devices show different constructive characteristics depending on their usage, whether it is meant for the civil, agricultural or industrial sector. The devices that are used in the civil sector (environment heating) include many models that are used commercially at both the national and the European level, and are classified as follows [28]:

• thermal wood kitchen, which are only used for monofamiliar purposes, both for heating environments and for cooking food, having a global yield of 70-75%;

• thermal wood chimneys, also meant for monofamiliar use, with water or air exchangers and which have a medium efficiency equal to 50%;

• wood little-medium power boilers (20-300 kWth), having a medium variable efficiency between 60% and 80%, which allow the heating of single habitable units or small residential plants; they are equipped with smaller-sized fixed grates and involve manual loading of the combustible, whereas for the higher

powers, there are loading hoppers, feed devices, fixed and moving grates, cinder and dust fellers before the fumes are discharged to the chimney evacuation sys­tems. The boilers for water heating are of the fume pipe type, in which the hot combustion gas passes through the tube bundle which is immersed in water to which the heat is transferred.

For the agricultural sector, there are particularly interesting large room combustors and the moving grate combustors, which are equipped with straw bale feeding systems, tree pruning residuals, agro-industrial working residuals, etc. The com­bustors must be planned appropriately to ensure good working with biomasses, which are characterized by wide variations in their humidity levels. The most fre­quent applications, and in many cases the most economically viable, which are registered in this sector are the drying of agricultural products and the greenhouses and buildings for piggish and poultry cattle heating, in addition to the normal domestic heating.

The power of thermal devices is generally between 200 and 2,000 kWth. Even in this case, the heat exchanger is the fume pipe type which has described previously [28].

In the industrial sector, there are many biomass direct combustion agro-forest or agro-industrial application for the urban solid rejections (RSU) and the industrial wastes. These applications allow the production of heat that is used in the production cycle for the generation of electrical energy and cogeneration products (simultaneous production of electrical and thermal energy). These plants comprise the following sections:

• biomasses stocking, which can have dimensions that can guarantee the supply of combustible for some days or very long periods (also for some months), if biomasses of a seasonal nature are processed;

• additional pre-treatment, which consists of reducing the biomass sizes and humidity to the specific requirements of the combustion system;

• feed line which is equipped with appropriate flow controls;

• combustor with the characteristics described previously;

• energy recovery, through fume pipe systems if the vector fluid is low pressure air or hot water, water pipes are used if it is necessary to have water at overheated pressure or vapour, diathermal oil, and additionally the exchanger (not in high power plants).

If meant for electrical energy production plants, it is necessary to introduce addi­tional components such as the vapour turbine and linked to it the electro-generator, the vapour condensator, the degasser and several thermal recuperators for the opti­mization of the thermal cycle. To drive the vapour turbines, vapour should be gen­erated at medium-high pressure. The power of the plants that produce only thermal energy can vary from some hundreds of kilowatts to some tens of MWth: the limit of the larger sized biomass industrial plants with wood spinnerets or other types of biomasses with both technical and organizing managerial character. Even the number of yearly working hours is often a limiting factor for the economic invest­ment gain, when compared with traditional feed combustible plants, because they generally show low investment costs against high energy costs.

The construction of plants for electrical energy production and for cogeneration by combustion is more economically advantageous only when the biomass is available in large quantities that are ideally located geographically placed and time distributed because the biomass has a low energy density, 10 times lower than that of petrol. This can be achieved only after a considerable reduction in the transport and stocking incidence of the quantities that are necessary for a central working, whose typical power is generally in the range 3-10 MWe. To give an idea of the biomass requirements for this kind realization, the need for 1 kg of biomass to produce 1 kW h of electrical energy should be considered.

The main energy parameter that is applied to evaluate the plants is the global net yield, which is given by the percentage ratio of the energy available for external users and that introduced by the combustible in the energy production plant, which are expressed in the same unit measures, net necessary consumptions for the working of the same plant [28, 30].

1.2 Liquid state combustible biomasses

Before being introduced into the market, the lignocellulose biomasses are usually subjected to a transformation process to give them the necessary physical and energy characteristics for their use in the energy plants. Firewood (logs or stub pipes), chips, pellets and briquettes are the main commercial forms for this biomass category.

1.2.1 Firewood

It is sold in logs or stub pipes with variable coal sizes from 50 to 500 mm and humidity values of lower than 50%. This kind of fuel at the domestic level is used mainly in small hand-feed plants, and its use is less than the use of briquettes and pellets (dense forms). The wood boiler, in fact, also does not allow an automatic loading of fuel and shows a lower energy efficiency (50-60% against 75-90% for the chips boilers and wooden pellet) [1, 2].

1.2.2 Chips

To make the wooden and material composition homogeneous and appro­priate for the automatic feed in the energy plants, we can resort to the use of

chips, a mechanical operation that reduces the sortings into flakes of small dimensions that are called chips. Such an operation can be applied, with no differ­ence, to the wooden or herbaceous biomasses. The flakes can be obtained through a crushing-milling process that is essentially based on the percussion and the defibering. The chip is obtained with the cut action.

The typologies of wood that are prepared for chipping are forest, agricultural and urban pruning residuals, slashes and lops or sawmill subproducts and wood coming from the brief rotation plants (SRF) [1, 2, 20].

The expense during chipping production energy, which is variable depending on the humidity of the biomass used, varies from 2 to 5 kWh/t, which corresponds to

less than 0.5% of the energy contained in the wood. Hard and dry wood chipping requires 18% more of energy than the just demolished humid wood working.

The geometry of the chips varies with the cut techniques adopted, as a function of the dimensions required by the type of the energy plant and, especially, of its power system. The chips usually show a variable length from 15 to 50 mm, a width that is equal to half of the length and a variable thickness from 1/5 to 1/10 of the length. A typical dimension is 40 x 20 x 3 mm. The homogeneity (obtained with chip screener adjusting) is the most important parameter for the chips that are destined for combustion, given that non-homogeneous chip dimensions result in annoying blockages of the plant’s feed systems.

The standard of the desired humidity is usually obtained prior to cumulus stock­ing, for an appropriate time: the maximum humidity that is accepted from the disposal of the chips combustion technologies is equal to 50%. It is also important, to avoid chips’ working problems, that the standard of the biomass humidity is limited between 25% and 59% [2].

The usable high quality chips in automated combustion systems do not have bark (or it contains only a minimum part) ensuring optimal combustion with a minimum cinder content, which is lower than 0.5%.

Similar to the analogy of the pellet, even for chips it is important to use pure wood. Impurities such as plastic or paints result in higher polluting emissions and cinder content. For this purpose, their use is generally restricted in biomass boilers that do not have a provision for waste gas purification. The chips are good for feed­ing all the types of biomass boilers with powers that vary from a few kilowatts

to tens of megawatts [1, 2]. The market has at its disposal different kinds of chips that are able to provide different quality and dimension wood (up to 30 cm diam­eter), with a working capacity that varies from a few tons to some tens of tons in an hour, both railcars and carried from agricultural tractors. The cut system can be disks (more diffused and used in the little power chips) or in rolls (that are availa­ble in very heavy and powerful versions). For the chips of herbaceous biomasses, cut-waders are often used, machines that work by directly cutting the trunks at the base, chipping them and pushing them on towing [2].

The anaerobic digestion process can be realized simultaneously using more typologies of substrates (co-substrates): in this case, we talk about co-digestion.

The applicable co-substrates can be effluents from intensive animal breeding, agricultural residuals, bio-solids, agro-alimentary industry rejections, etc.

Co-digestion involves the use of complete mixing reactors, where the substrate is usually diluted to obtain a dry substance concentration that is between 8% and 15%. The use of co-digestion cannot be justified for higher bio-gas yields, but it results in the possibility of obtaining an extra income from the plant’s administration [2, 37, 38].

Gasification is a thermochemical conversion process that transforms a solid com­bustible into a gaseous combustible which shows ease of complete combustion without the need for excess air, ease of turning on and ease of transport and clean­ing of the combustion. The disadvantage is the energy expense that is required for the gasification process. The gas obtained from the gasification of biomasses is called producer gas; it is composed of a mixture of carbon monoxide, hydro­gen, carbon dioxide, methane, hydrocarbons (ethylene, ethane), vapour, nitrogen (in air gasification); it also contains several pollutants such as cinder and char particles (agglomerates of a complex nature which are mainly composed of carbon), tar (complex mixture of condensable hydrocarbons) and oils. Producer gas can be obtained through partial combustion of the biomass (using air or oxygen) or through pyrolytic gasification (using vapour). Gasification in air results in a low calorific power gas (5.5-7.5 MJ/N m3), whereas by oxygen and vapour gasification a medium calorific power gas is obtained (11 MJ/N m3 and 10 MJ/N m3, respectively). For pyrolytic gasification (or indirect heating), an external heat supply is necessary.

All gasification processes involve, with different modalities depending on the technology applied, the following four steps: drying, pyrolysis, oxidation and reduction.

Drying involves the evaporation of the water content, introduced in the reactor, in the biomass. Pyrolysis is the decomposition of the biomass at high temperatures without exposure to oxygen: the products of pyrolysis are gas (containing tar in the vapour state, in addition to substances such as methane, hydrogen, carbon monoxide, carbon dioxide and hydrocarbons with a few carbon atoms) and char. In the oxidation phase, the exothermic reactions, which provide the heat required for the reduction reactions (endothermic), take place from which the constituents of the producer gas originate. There are multiple technologies by which it is possible to realize the gasification and which are mainly distinguished by the manner in which the bio­mass is brought into contact with the gasifying agent. We can distinguish two main reactor categories: fixed bed (updraft, downdraft, crossdraft) and fluid bed (boiling and circulating). Most of the gasificators in use are of the downdraft type [2, 46].

The birth of the wood pellet can be traced back to 1973 when, in Idaho (USA), an engineer created a new kind of wooden fuel. In the beginning, it was created for an industrial use, but rapidly its use spread to the domestic boilers market. By the term pellet we mean a densified bio-combustible, normally cylindrical-shaped (usually with a variable diameter between 5 and 8 mm and length of 10-20 mm), that is obtained by pressuring the pulverized biomass with or without the help of pressing bidding. At present, the pellet is exclusively obtained from the wooden biomass, but it is necessary to underline the existence of studies that aim to develop the pellet spinneret from herbaceous cultures or from a mixture of these cultures with wooden biomasses. However, the different quality of the starting biomass, especially in terms of calorific power and cinder content, leads to unavoidably dif­ferent final qualities of the pellet, for different purposes. To avoid these variations, we should make use of the pellet spinneret production technology for the two different biomass typologies [2, 20].

Through a frame, the pellet making process can be divided into the following phases: drying, cutting, pelletization, cooling, separation, storage/bagging. In some cases (if the typology of the biomass requires), before the drying phase there are also the phases of roughing, grinding and deferrization. If the working material is rough-shaped (logs or slashes), grinding should be done first, using rotor knives the biomass is reduced to flakes.

Before entering the grinding mill, the raw material is subjected to the magnetic action, which separates from the raw material iron elements (ironing) whose pres­ence could damage the extruder. The most well-known technologies of pelletiza­tion do not allow the raw material pressing, if this has a high humidity. After the primary crushing, it is necessary for the material to be dried (usually through a rotary dryer). In this way, the biomass reaches the appropriate humidity grade and allows the lignin that is present in the raw material to be the binding material. At the end of the drying phase, the material reaches a maximum humidity of 10% which allows proceeding with the following treatment steps.

In the second step of grinding, or before the grinding step (if the starting mate­rial in the production cycle already has small dimensions: shavings, sawdust, wooden or herbaceous chips, etc.), the material is crushed (usually through a ham­mer mill) to reduce and level the width down to 3 mm. Such dimensions allow to obtain a standard characteristic for the product. After the material crushing, it passes to the conditioning step, where it is prepared to enter the pelletization spin­neret. This phase can also include the embedding of bidding or additive agents. Usually, this conditioning operation is realized using dry water vapour to frizz the wooden fibres and to induce a partial gelling of the biomass.

The pelletization that results from compression comprises perforated, cylindrical and flat forms (matrix) through whose holes the conditioned bio­mass is pushed at high pressure (up to 200 atm) using roll systems. The pellet is formed due to the transformations that happen during the passage of the fibre through the extrusion holes, when the temperature is up to 90°C. At these temperatures, there is a fluidization of the lignin that comes out from the cellular structure; this allows the fibres to stick to each other. Appropriate blades cut, at the desired length, the compressed material and the surface bakelized that overflows from the matrix holes. The extruded and cut material then passes to the following cooling step (realized through ventilation plants), where the product undergoes hardening. Subsequently, in the separation sec­tion, the whole pellet is deleted and it is re-entered in the extrusion system. At the end of the cycle, the pellet is stored in silos or bagged (storage/bagging steps) [2]. The mechanical energy spent for the pellet production is equal to the 2% of the final product’s energy content. As a consequence, pellets are considerably better than the fossil energy sources, for which 10-12% of their energy content is used for refining [1]. The cost of pellet production varies between 0.05 and 0.16 €/kg based on the applied technology and on the biomass used. The wholesale price is equal to 0.11-0.21 €/kg, whereas the detail price is 0.21-0.30 €/kg [20]. The national pellet production, which was estimated at around 240,000 tons in the 2004/2005 season, shows a strong growing trend [22].

The pellet changing allows obtaining a product characterized by high energy density. It is also easily transportable: in terms of motion, in fact, it acts simi­lar to fluids. This allows a high grade of automation of the tools and the com­bustion plants. This pellet property is due to the particular from, dimension and homogeneity of its elements that can be sent to the combustion oven through simple mechanical instruments (transport foils, scrolls or suction systems), with important advantages such as automatic control, dosing and continuous feed [1, 2].

Compared to the non-densified biomasses (sawdust, chips and slashes, etc.),

pellets show some advantages that make it more valuable in the market [2]:

1. High apparent density (bulk density, bio-fuel volume for unit mass): vari­able between 650 and 780 kg/m3. The pellet density is seven times higher than the density of sawdust and chips and this optimizes its transport and storage.

2. Low humidity content: low hydric content improves the combustion yield and contributes to reduced transportation costs. Furthermore, during the storage phase, the combustible does not show risk of undergoing fermentative phenomena.

3. High calorific power for unit weight: it depends on the composition and the structure of the biomass applied for pellet making. The wood pellet has a LCP of about 4,000 kcal/kg (high energetic weight among the bio-fuels).

4. Homogeneity of the material has both physical and qualitative characteris­tics points of view: The first, together with the small dimensions of the pellet, allows to easily move the product through scrolls, transport foils or wheel suction systems and the possibility to use them in automotive boilers, whereas the second allows a better regulation of the combustion and a better control of the emissions.

The management of rubbish and particularly the final rubbish disposal in controlled dumps with a high level of compaction of the material allow the establishment of anaerobiosis conditions that are necessary for bio-gas production after the decom­position of the organic content which is present in the rubbish. The collection of the gas produced, which happens through appropriate cargo systems, is necessary not only for the exploitation of energy but also for safety reasons (methane, in addition to being harmful to humans and vegetation by its greenhouse gas effect, is explosive in confined environments).

The collector system comprises a series of vertical shafts from which horizontal cracked piping spiders originate. The collection and removal of the gas are controlled by the pressure to which it is subjected inside the body of the dump.

Biogas production phases

Biogas comp, (volume)

Time (not in scale)

These gasificators represent the most tested gasification technology. They show the highest limited dimensions and low reaction speed, although their use is lim­ited to the smaller powers. The feed material must have uniform granulometry and a low fine particle content, to avoid overloads and allow ‘empty space’ which is enough for the passage of gas through the bed.

3.2.1.1 The updraft or counter-current gasificators The reactor is made of steel cylinders coated inside with refractory material. In the upper part of the reactor, there is the biomass feed and the exit for the producer gas, whereas in the lower part there is a grid that functions as a support for the solid material bed. The grid allows the passage of both air that enters from the bottom and cinders that fall down and collect at the bottom. This gasificator typology has the following advantages:

• constructive and working simplicity;

• high combustion capacity of char, whose final residual is minimal;

• optimal thermal exchange between the opposite currents of biomass and the producer gas, which results in low exit temperature of the producer gas and therefore a high thermal efficiency;

• efficient drying of the combustible due to the internal thermal exchange; this allows the use of combustibles with high humidity levels (up to 60%).

The fundamental limitation of this technology is the high tar content in the producer gas. The tars mainly originate during the pyrolysis, and in this type of gasificator, the pyrolysis gas, containing tars, combines with the producer gas

without being burnt before The tars can pose considerable problems in the producer gas feed plants; in fact, they condense easily and provoke overloads. This is very important if the gas is used in a boiler, whereas in case of use in turbines or engines, an accurate cleaning of the gas is necessary. These gasificators are characterized by a maximum load of 4 t/h of dry biomass [2, 46].

3.2.1.2 Downdraft or equi-current gasificators In the downdraft gasificators, the current of producer gas is descending and so it is concordant with the current of the solid combustible. The gas exits the reactor from the bottom. Generally, they have a V-shaped throat above which the oxidation zone is located. The purpose is to create a compact zone at high temperature where the pyrolysis gas is generated and to realize the cracking of the tars (decomposition into lighter products); the air is allowed to directly enter this area through a central feed pipe or through nozzles that are placed on the groove walls.

The main advantage of this type of gasificator is the low tar content in the producer gas. The limitations are:

• high content of solid particles in the producer gas, because the pyrolysis gas passes through the oxidation zone where it collects cinders and dust;

• the presence of grooves can pose overload problems;

• the humidity of the biomass must be lower than 35% because the internal drying is less efficient compared to the updraft gasificators;

• the relatively high temperature of the gas at the exit which reduces the thermal efficiency.

This type of gasificator, characterized by a maximum load of 500 kg/h of dry biomass, is applied in small-scale applications up to 1.5 MWth [2, 46, 48].

3.2.1.3 Crossdraft gasificators The working of crossdraft gasificators is similar to the other two gasificators, but in this case the combustible is injected from above, the oxidant enters transversally and the producer exits laterally.

The disadvantage of crossdraft gasificators that results in it not being used is the reduced capacity of tar conversion [46, 48].

Similar to pelletization, the briquette also represents a particularly interesting technology because by reducing the material density it allows to concentrate high energy reserves in a contained volume. The briquette, in fact, is a bio-fuel that has a parallelepiped or cylindrical shape; it is obtained by compressing some pulverized biomass with or without the help of pressing additives. Dur­ing the process of production, the wood is desiccated so that humidity is not higher than 10%. Briquettes can be stored easily and their calorific power is

18.5 MJ/kg [1, 2].

A complete line of briquetting is usually composed of subsequent steps that change the raw biomass, with variable humidity and pressing characteristics, into a standardized briquette that is ready to be sold in the market.

The line comprises steps that are very similar to those for pelletization, but with a relatively simpler technology. In principle, there is a biomass pre-treatment which is followed by compaction and by briquette changing.

Crushing, drying and biomass heating, which are necessary to reach the optimal characteristics of granulometry and water content, form the pre-treatment phase.

Through a feed system, which comprises holding under pressure, or by a con­veyor belt pipe, the pre-treated biomass is brought to the briquette phase, where it is compacted and transformed into briquettes. We can have low-, medium — and high-pressure briquetting systems depending on the pressure exerted. In the first two cases, the biomass is mixed with a binding substance, whereas the high — pressure systems work on the biomass as well as the biding effect obtained after the high pressures are exerted.

The most important high-pressure briquette technologies are screw briquetting and the piston briquetting (mechanic oleodynamic). In screw briquetting, the bio­mass is continuously extruded because of the rotation of one or more screws without the end inside a cone room (heated during the process). During mechanic piston briquetting, the compaction of the biomass is achieved through an alternative piston which is actuated by an electrical engine, whereas during the oleodynamic circuit piston process, there are two pistons which are actuated by the holding pressure of a closed-circuit oil which compress the material in orthogonal directions.

The most used system, which allows the treatment of the biomass with higher humidity and a better control of the applied pressure, is piston briquetting.

The final transformation of the product which is obtained from the briquetting phase includes the steps of the eventual briquette cut (only for screw systems where the material continuously exits), eventual cooling (for screw systems with heating), packing and final stocking of the briquettes [2].

The densification of the biomass in briquettes has the same advantages and disadvantages as the transformation into pellets: in fact, after the briquetting proc­ess we obtain an improvement in the physical biomass characteristics (density, homogeneity, etc.), a reduction in the volume, a reduction in the storage and trans­port costs, and an improvement in the behaviour during combustion. At the same time, the briquetting process, just as the pelletization process, needs, as we have seen, a preventive material conditioning, in particular the biomass drying. The briquettes can be used in the place of firewood and coal, by adjusting some opera­tive parameters such as the primary and secondary air distribution; relative to the two fuels, in fact, the briquettes require a higher quantity of secondary air and a lower quantity of primary air.

The briquettes with a higher thermal capacity (retain heat for a longer period of time and keep the temperature inside the oven high to allow easy combustion of the newly entering combustible) are a ‘better’ combustible than uncompressed wood.

This type of combustible is used most frequently in both domestic (they do not generate sparks which makes them appropriate for the chimney) and industrial applications [1, 2].