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].