The present invention relates to a gasification plant, i.e. a plant for producing combustible gas by pyrolysis and combustion of biomass, such as, for example, wood chips, that is wood reduced into chips, vegetable biomass, or general solid-state biomass, with a humidity rate preferably not higher than 20%.

They are known from the state of the art gasification plants comprising a reactor in which the biomass gasification occurs and which is typically divided into three zones:

- a first pyrolysis zone, in which the biomass introduced in the gasification apparatus is submitted to pyrolysis, that is a thermal decomposition which occurs exposing biomass at high temperatures, that is temperatures of a few hundred degrees Celsius, totally, or almost totally, oxygen-free.

- a second combustion zone in which the biomass is subjected to a combustion process under oxygen shortage conditions, that is, with an amount of oxygen lower than the stoichiometric amount required for a complete combustion of material, equal, for example to 40% of the stoichiometric amount of oxygen;

- a third reduction zone in which the combustible gas is produced, commonly referred to as syngas or synthetic gas, which contains carbon oxide, hydrogen, methane and carbon dioxide, with heating power comprised between 4 and 6 MJ/Nm ³ .

The gasification plants known from the state of the art, have the drawback that in the synthesis gas produced there is a high quantity of tars with high-molecular-weight, which must be removed, or converted into low-molecular-weight tars, to be able to use synthesis gas for supplying, for example, internal combustion engines.

This requires submitting the synthesis gas produced to a cracking process, for removing high-molecular- weight tars from gas.

This results in an increase in plant complexity and construction costs.

Another drawback of the known gasification plants relates to unloading solid residues of the biomass combustion, residues substantially composed of carbon powder having very small granulometry which causes easy obstructions in the unloading devices, with the need to interrupt the plant functioning to remove the aforesaid obstructions.

A further drawback is related to air infiltrations from the outside into the reactor which can cause alterations in the combustion conditions, with consequent alterations in the composition of the synthesis gas produced, with the risk that combustion can possibly extend to the pyrolysis zone, compromising the whole process of synthesis gas production. Another further drawback, regarding pressure reactors, is the risk of leakage of pyrolysis or synthesis gases to the outside.

The present invention is directed to provide a gasification plant in which a remedy for the above indicated drawbacks is found.

According to the present invention it is provided a gasification plant for producing synthesis gas according to claim 1.

Thanks to the invention it is provided a gasification plant in which it is possible to obtain a synthesis gas with a very low percentage of high-molecular-weight tars, in which it is possible to unload efficiently the solid residues of the biomass combustion, with no risk of obstructions that block the unloading of said solid residues and in which air delivery from the outside into the reactor is prevented in such an amount as to alter the combustion conditions and propagate combustion to the pyrolysis zone.

The invention will be better understood and implemented with reference to the following description and to the enclosed drawings, showing a non-limiting embodiment of a gasification plant according to the invention, wherein:

Figure 1 is a schematic view of a gasification plant according to the invention;

Figure 2 is a perspective view of the reactor of a gasification plant according to the invention, in which the biomass pyrolysis, the combustion of the pyrolysis products and the reduction of gases produced during combustion are carried out, in order to obtain the synthesis gas;

Figure 3 is an elevation view of the reactor illustrated in Figure 2;

Figure 4 is a section of Figure 3, along the line A- A in Figure 3;

Figure 5 is a section of Figure 3, along the line B-B in Figure 3;

Figure 6 is a perspective view of a detail of the gasification plant according to the invention;

Figure 7 in an elevation view of the detail of Figure 6 in a first operative position,

Figure 8 is a section view of the detail of Figures 6 and 7, in a second operative position; Figure 9 is a section view of a detail of Figure 7.

Referring to Figure 1, a gasification plant according to the invention comprises a reactor 2 in which the pyrolysis of the biomass to be gasified, the combustion of pyrolysis products and the reduction of the combustion products occur, for producing a combustible gas, commonly denominated as synthesis gas, or syngas. The reactor 2 comprises a shell 10, within which a reactor body 11 is housed (Figure 4), into which the biomass to be treated is delivered and inside which the chemical-physical processes conducting to the synthesis production occur.

The gasification plant 1 further comprises a supply tank 3, which contains the biomass with which the reactor 2 is supplied and a filter 4 in which filtering of synthesis gas produced occurs.

The biomass contained in the supply tank 3 is transported to the reactor 2 by means of a first conveying device 5, for example a first cochlea conveying device, which introduces the biomass into the reactor body 11 by means of a hopper 6, connected to a supply opening 7 of reactor 2, placed in an upper zone of the reactor body 11.

The combustion solid residues and the synthesis gas produced in reactor 3 are transported from reactor 2 to the filter 4 by means of a second conveying device 8, for example a second cochlea conveying device actuated by a motor 20, or by pneumatic transport.

The reactor body 11 substantially comprises four zones: a first pyrolysis zone 12, placed in the upper part of the reactor body 11, into which biomass to be treated is delivered and in which biomass undergoes a pyrolysis treatment. In this first zone 12, a level sensor 23 is provided, which can consist of, for example, a touching device, to verify whether the level of the biomass introduced in said first zone 12 reaches and exceeds a pre-established maximum value and thus to reduce the biomass supply.

A second combustion zone 13, placed in an intermediate part of the reactor body 11, in which combustion of products of the pyrolysis treatment is carried out.

A third reduction zone 14, placed in a lower part of the reactor body 11, where a reduction reaction of the gases produced by the combustion and the so called "cracking" of tars produced during the biomass combustion, that is the conversion of heavy hydrocarbons into light hydrocarbons, occur.

A fourth unloading zone 15 where solid products of the combustion of biomass, consisting substantially in powder and carbon granules, and the synthesis gas produced are unloaded from reactor 2.

The second combustion zone 13 can end up with a truncated cone element 16, that narrows downwards to an outlet port 48 having a first diameter D, and facilitating the transit of the combustion solid residual material from the second zone 13 to the third zone 14.

In the second zone 13 an air flow is delivered for implementing the combustion of material coming from the biomass which has already undergone the pyrolysis treatment in the first zone 12 of the reactor body 11. The amount of air delivery is lower than the stoichiometric amount required for a complete combustion of products of the pyrolysis treatment, such as to carry out an incomplete combustion under oxygen shortage conditions. The amount of air delivered can be equal, for example, to 40% of the stoichiometric amount required for a complete combustion.

In the second zone 13 at least one ignition device 49 is arranged, for example consisting in an electric cartridge resistance, useful for triggering combustion of the biomass when the reactor 2 is started. In the illustrated embodiment, two ignition devices 49 are provided. However, any number of ignition devices 49 can be provided, without departing from the scope of the present invention.

Air is delivered into the second zone 13 through a plurality of air injectors 22, which lead to said second zone 13 and which are supplied by means of a fan (not visible in the drawings). Injectors 22 are distributed at a substantially constant angular pitch along the periphery of the second zone 13 and are tilted in relation to a direction perpendicular to the outer surface 51 of the second zone 13, with a tilt angle comprised, for example, between 10° and 20°, so that to create a vortex motion of the air in said zone 13 to promote an improved mixing of air with the material to be submitted to combustion, in order to obtain a greater combustion uniformity.

The third reduction zone 14 has a first truncated cone portion 17, enlarging downwards until a second diameter Dl, followed by a cylindrical portion 18 with diameter Dl.

The Applicant has experimentally determined that suitably selecting the ratio between diameters D and Dl and between volume V of the second combustion zone 13 and volume VI of the third reduction zone 14 it is possible to greatly reduce the percentage of tars in the gas produced, for example until few tens of part per million, which allows to obtain a gas which can be directly used without further treatment for supplying, for example, an internal combustion motor.

The Applicant has determined that the above indicated result can be advantageously obtained with a ratio Dl/D > 2.5 and with a ratio Vl/V > 3.5.

In order to avoid an undesired delivery of air from the outside which might compromise the pyrolysis process in the first zone 12 of the reactor body 11, alter the combustion process of the second zone 13 of the reactor body 11 and the reduction process in the third zone 14 of the reactor body 11, the reactor body 11 is made of a very low porosity refractory material, which dramatically limits air infiltrations, and therefore, oxygen from the outside and emission of vapours, gases and fumes from the reactor body 11.

By very low porosity refractory material it is meant a refractory material which has a closed porosity percentage not lower than 85% and pores with average diameter not higher than 60 microns.

Such a type of material can be, for example, the silicon carbide -based material sold with the trade name "Iker" from Italker S.r.l. company, which comprises among its constituents 60-80% of silicon carbide, 10-30% of silica and 10-20% of alumina.

The reactor body 11 is externally coated, at the first zone 12, second zone 13 and third zone 14 with a thermally insulating material in order to minimize heat dispersion from the reactor body 11 towards the outside, thereby improving the performances of the gasification apparatus according to the invention. The external surface of the fourth unloading zone 15 of the reactor body 11 is not coated on the contrary with said thermally insulating material.

Around the fourth unloading zone 15 of the reactor body 11 a heat exchange device 32 is arranged, in which the air supplying injectors 22 is preheated. The heat exchange device 32 is provided with a delivery conduit 33, through which air to be preheated can be delivered into the further heat exchange device 32.

The heat exchange device 32 is connected to injectors 22 by means of respective connecting conduits 34.

In order to pre-heat air heat coming from the fourth unloading zone 15 of the reactor body 11 is used, which is transmitted to the heat exchange device 32 as the external surface of said fourth unloading zone 15, as previously specified, is not provided with an insulating material external coating.

Air can be delivered into the heat exchange device 32 by means of a fan device (not illustrated), or creating a depression inside the reactor body 11, suctioning air through the discharge conduit 19. The depression created within the reactor body 11 is transmitted to the heat exchange device 32 through connecting conduits 34, thus determining air suction from the outside via the delivery conduit 33.

In the fourth unloading zone 15 an unloading device 38 is arranged of solid residue materials of the combustion of biomass and synthesis gas produced in reactor 2.

The unloading device 38 is composed of an internally hollow body 39, the inside of which communicates with the discharge conduit 19. The body 39 has a truncated cone or truncated pyramid shape on a side surface of which slits 40 are made through which solid residues and synthesis gas are sent into the discharge conduit 19.

The body 39 can be advantageously made of the same material as the reactor body 11 is made, or of a metal material.

If the body 39 has a truncated cone shape, slits 40 are made along generating lines of the truncated cone and are distributed with a constant angular pitch. If, otherwise, the body 39 has a truncated pyramid shape, slits 40 are made along corners of body 39.

The body 39 is fixed to a shaft 41 that can be driven to rotate, by means of a motor 42, about a longitudinal axis B of the reactor body 11. The shaft 41 is mounted in a support 43 with an airtight coupling, to avoid an undesired delivery, not controllable, of air in the reactor body 11.

The peculiar structure of the unloading device 38 resulted to be particularly suitable for carrying out the unloading of the solid products of the biomass combustion with no risk of obstructions that may block the unloading of said solid products.

The combustion solid residues and the synthesis gas produced in reactor 3 are unloaded from the fourth zone 15 of reactor 2, by means of the unloading device 38, into a discharge conduit 19 communicating with the second conveying device 8, by means of which the combustion solid residues and the synthesis gas are conveyed from the reactor 2 to the filter 4.

Discharging into the discharge conduit 19 the synthesis gas produced in reactor 3 together with the combustion solid residues has the advantage that solid residues, while being conveyed by the conveyor 8 to the filter 4 stay in contact with the synthesis gas and depurate it from possible tar residues present therein, before being separated from the synthesis gas in filter 4. This precaution allows to minimize tar content in the synthesis gas, thus obtaining a synthesis gas with a remarkably high purity degree.

The second conveyor device 8 is inserted inside a further heat exchange device 21 through which the synthesis gas exiting from reactor 2 is cooled by an air flow generated in the further heat exchange device 21 by means of a fan (not visible in the drawings). This fan can also be used to generate a depression in the discharge conduit 19 of the reactor body 11, in order to deliver air into the heat exchange device 32.

Hot air exiting from the heat exchange device 21 can be used for different purposes, for example to reduce the percentage of biomass humidity, before it is reinserted into reactor

2. The supply tank 3 is provided with a supply valve 9, which will be described below in more detail. The supply valve 9 is mounted in an airtight manner in a delivery opening of the supply tank 3 (not visible in the drawings). The supply tank 3 is made so that, when the supply valve 9 is in the closed position air cannot enter in an uncontrolled way inside the tank and, therefrom into the reactor body 11.

For the same reason, the first conveying device 5 is made airtight and it is airtightly coupled with the supply tank 3 and with the hopper 6. The latter is connected airtightly to the supply opening 7 of the reactor body 11.

Air-tightness is required to avoid significant amounts of air to penetrate in an uncontrolled manner from the outside inside the first zone 12 of the reactor body 11 causing a biomass combustion in said first zone 12 with the risk of bursts and flash-backs and fumes in the first conveying device 5 and in the supply tank 3.

It is true that, when the supply valve 9 opens to allow the delivery of biomass into the supply tank 3, a certain amount of air enters the supply tank 3 together with the biomass and can reach the first zone 12 of the reactor body 11, however they are small amounts of air, not sufficient to trigger a combustion of the biomass in said first zone 12.

The second conveying device 8 also is made airtight and is airtightly connected with the discharge conduit 19, to avoid an uncontrolled inlet of air that might trigger a combustion of the synthesis gas produced.

The supply valve 9 comprises a valve body 24 comprising a first truncated cone shaped portion 25 with the major base addressed upwards, serving as a hopper into which the biomass to be charged into the tank 3 is delivered by means of a conveying device, not visible in the drawings. The body 24 further comprises a second externally cylinder portion 26, which is connected airtightly with a supply opening (not visible in the drawings) of the supply tank 3 and it is provided with an inner pass-through hole 36 having truncated cone shape with the minor base addressed upwards.

The valve 9 further comprises a shutter device 27 which is actuated to move along a longitudinal axis A of the valve body 24 by means of a linear actuator 28.

The shutter device 27 comprises a first portion 29, connected with a manoeuvring arm 37, in turn connected with the linear actuator 28 which moves the manoeuvring arm 37 and, consequently the shutter device 27 along said longitudinal axis A. The shutter device 27 further comprises a second portion 30 having such shape and dimensions as to be able to couple by shape-coupling with the surface of said inner hole 36, to take the supply valve 9 into the closed position.

The surface of the inner hole 36 is provided by at least a seal gasket 31, preferably a plurality of seal gaskets, to guarantee that the geometric coupling between the shutter 27 and the inner hole 36 is airtight.

The valve 9 is associated to a level sensor 35 which is used to detect the level of the biomass inside the tank 3, to adjust the biomass supply to the tank 3.

The reactor 2 is provided with a safety device 44 configured for absorbing and venting possible overpressures which might occur within the reactor 2.

The safety device 44 can be composed of a so-called hydraulic guard, comprising a tank 45, within which an inert liquid is contained, i.e. a liquid which cannot react with gas substances coming from the inside of the reactor, for example water. The tank 45 is connected with the reactor 2 via a tube 46 which, for example, can be connected with the hopper 6 through which the biomass is introduced in the reactor 2.

The tank 45 is provided with a lid 48 which is connected with the tank 45 so that it can move in a direction parallel to a side wall 47 of the tank 45, so that between the tank 45 and the lid 48 a port is created through which a possible overpressure occurring within the reactor 2 is vented. The tank 45 can be provided with a level sensor configured to indicate whether the level of the liquid in the tank 45 lowers below a predefined minimum level and whether some liquid must be delivered in the tank 45, to return the level of the liquid above said minimum level.

Instead of the hydraulic guard a rupture disk (not illustrated) can be provided, placed on a flange of the shell 10 of the reactor 2, which the hopper 6 is fixed to.

The reactor 2 is further provided with a plurality of temperature sensors 50, distributed over the first 12, second 13, third 14 and fourth 15 zone of the reactor body 11, through which it is possible to monitor the temperature in the aforesaid zones and to verify the correct functioning of the reactor 2.

Hereinafter the functioning of the gasification plant 1 according to the invention is disclosed.

Before starting the reactor 2 it is necessary to fill with carbon powder the second conveying device 8 and the reactor body 11 up until a level near to the outlet port 48 of the truncated cone element 16. The remaining volume of the reactor body 11, i.e. the pyrolysis zone 12 and the combustion zone 13 can be filled with the biomass to be treated.

Once the filling of the reactor body 11 is terminated, the at least one ignition device 49 is activated to start the combustion of the biomass in the second combustion zone 13 of the reactor body 11 and air is delivered into the second combustion zone 13 through injectors 22, in an amount lower than the stochiometric amount required for a complete combustion of the biomass, for example equal to about 40% of said stoichiometric quantity.

Heat produced by the biomass combustion in the second combustion zone 13 starts and supplies the pyrolysis treatment of the biomass in the first pyrolysis zone 12.

In full operation, the biomass introduced into the reactor 2 undergoes a first treatment of pyrolysis in the first pyrolysis zone 12, then it passes into the second combustion zone 13 in which it is subjected to incomplete combustion under oxygen- shortage conditions.

Biomass combustion products, substantially consisting in carbon powder, synthesis gas and tars pass into the third reduction zone 14 where a reduction reaction of gases produced by combustion and the so called "cracking" of tars produced during biomass combustion, that is the conversion of heavy hydrocarbons into light hydrocarbons, occur.

Finally, biomass combustion products arrive in the fourth unloading zone 15, wherefrom they are discharged through the discharge conduit 19 through the unloading device 38. Through the discharge conduit 19 the combustion products arrive to the second conveying device 8 which conveys them to the filter 4, in which the synthesis gas and the carbon powder are separated and sent to their respective stock tanks. Alternatively, the synthesis gas can be directly sent to the supply of an internal combustion motor.

Cooling of the conveying device 8 through the heat exchanger 21 allows to obtain condensation of possible tars contained in the synthesis gas, so that they are absorbed by the carbon powder, so as to obtain a synthesis gas substantially free from tars.