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.

Figure 34: Dual bed gasificator.

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.