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.

Figure 19: Rotating drum oven working scheme.

• 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.

Figure 20: Fluid boiling bed.

Figure 21: Fluid recirculated bed.

Flue gas Figure 22: Fluid boiling bed.

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

Figure 23: Fluid recirculated bed.

Figure 24: Wood thermal kitchen.

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.

Figure 25: Wood burning stoves.

Figure 26: Little power for the combustion of wood log boiler.

Figure 27: Little-medium power chips or pellet powered boiler.

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].

Figure 28: Biomass combustion plant.