Carbonaceous refuse to which the present invention relates comprise, but without limitation, solid urban refuse and the like (containing paper, cardboard, wood, plastics, kitchen residues, tyres etc), the residues deriving from sanitary uses (cellulose, cotton, textiles, plastics etc) sludge from paint bays, the discharges deriving from the production of polymers, biomasses, the residues from agricultural and industrial processes, paper mills, rubber works, and the residue of biological purification installations.

The said types of refuse contain compounds of polymeric type with carbon and hydrogen (such as cellulose, lignin, polythene, polypropylene, polyethylene terephthalate, butadiene-styrene copolymers, polystyrene and others).

As is known, these compounds like all organic carbon compounds, when subjected to high temperature decompose by carbonising and releasing carbon dioxide, water and various by-products. Likewise the mechanism of the reaction between red hot carbon and steam is known, which reaction was once widely used in town gas installations which produces a combustible gas, the so- called water gas, mainly containing carbon monoxide and hydrogen according to the following equation: C + H20 + heat = CO + H2.

For this reaction to take place it is necessary to introduce heat and, according to chemical principles, the conversion yield is influenced by the temperature and will be greater by adopting a high temperature. Both carbonisation and gasification of the carbon with water take place at high temperature, so that, if, in place of carbon, organic compounds of carbon are used directly, it will be possible to obtain the production of gas without any need to proceed through separate phases first of carbonisation and then of gasification.

The quantity of water necessary for gasification to carbon monoxide and hydrogen (water gas) depends on the different substances; and thus, on the composition of the refuse which can be extremely varied both in the case of industrial refuse but also as far as urban solid refuse is concerned, which varies in dependence on local uses, the type of urban area and the economic level.

For gasification of 1 kilogram of plastics material of parafinic type (for example polythene) to water gas about 0.57 kilograms of water is theoretically required, whilst for gasification of 1 kilogram of pure cellulose about 0.11 kilograms of water is necessary. Very generally, urban refuse of average composition, aster elimination of inert material (glass, metal and earth) has a composition in percentage terms by weight which, on the basis of the dry substance, can be about: 15% plastics, 55% cellulose, 10% inert and mineral salts, as well as a moisture content of about 20%.

To gasify 1 kilogram of refuse of this composition, the quantity of water theoretically necessary is about 0.25 kilograms and the optimum quantity, including the excess required in order to have a complete gasification, is about 0.3 kilograms.

Naturally, these data vary depending on the composition of the refuse. It must be observed that if the quantity of water in the reaction is insufficient the reaction will be incomplete; on the other hand, if too great an excess is adopted, as well as a consumption of energy to produce useless steam, there will be a further cooling of the reaction zone, thus looked at another way there would be a smaller gasification yield.

In conclusion, in the gasification of refuse, as generally occurs for any chemical reaction, in order to have a good energy yield it is desirable to have an optimum ratio between the reagents.

An object of the present invention is that of providing a process and apparatus which although intended for the treatment of refuse and therefore of heterogeneous materials of variable composition, makes it possible to effect control of the gasification reaction and its stoichiometry, which aspects are not achievable and which are not encountered in refuse gasification methods known to date.

The known methods for gasification of refuse comprise: a) the partial combustion of refuse in the absence of air (or oxygen) with respect to complete combustion, with or without the addition of water; b) thermal carbonisation followed by partial combustion of the above mentioned type; c) exposure to an electric arc with partial combustion due to the presence of air and possible uncontrolled introduction of steam or water.

All the above mentioned refuse gasification methods involve the disadvantages either in yield or in quality of gas obtained and in fact have not found significant application. In these methods the necessary energy is provided totally or partially (in electrical arc methods) by burning the refuse entirely or in part with the introduction of air (or oxygen) thus obtaining, for a given part of the carbon present, combustion with the formation of heat and carbon dioxide according to the reaction C + 02 = C02 + heat.

The calorific value of the gas thus obtained is low due to the presence of a high carbon dioxide content and can be used directly only with difficulty so that to overcome the disadvantage of such methods it is often necessary to raise the calorific value of the gas by having recourse to a subsequent passage over a bed of carbon coke maintained red hot with the introduction of energy from the outside for the purpose of reacting the carbon with carbon dioxide thus forming carbon monoxide according to the reaction: CO2 + C + heat = 2CO.

In all known methods the gasification process takes place in a progressive but not immediate manner, with the material remaining for some time in the gasification chamber and therefore with progressive elevation of the temperature of the material itself. This fact involves two separate disadvantages.

The first is that of having a low yield of carbon monoxide and hydrogen due to formation at pyrolytic temperatures between 180 and 400° C of large quantities of carbon dioxide.

The second lies in the formation, at the same pyrolytic temperatures, of organic acids and other substances of pyrolysis which distil with the gases (tars, pitches, phenols, cresols, creosote, acetic acid, pyroligneous acid etc,) which it is necessary to eliminate before being able to use the gas, especially in internal combustion engines.

In these situations the passage of the gas over a red hot coke bed is absolutely essential in order to avoid obtaining an unacceptable product, unacceptable that is in quality, calorific value and energy yield.

Another disadvantage which has generally arisen in gasification of refuse in the presence of air (or oxygen) lies in the formation of pollutant substances which are difficult to eliminate (dioxins, furans, etc). Dioxins belong to a family of 75 similar compounds consisting of 2 benzene rings joined by means of 2 oxygen atoms and having from 1 to 8 chlorine atoms bound to the benzene rings. These compounds are very stable, hardly biodegradable, resistant to even very high temperatures and are therefore the cause of environmental pollution.

Although the mechanism of formation is complex it can be summarised by saying that the dioxins form in the presence of oxygen and chlorate or chlorinated compounds in reactions conducted at modest temperatures.

Nitrogen in the air, normally present in the above-mentioned gasification systems, also involves problems due to the formation of nitrogen oxides.

In view of the above-mentioned disadvantages, another objective of the present invention is that of providing a process and apparatus which makes it possible to obtain a gasification product comprising a combustible gas free from unwanted polluting compounds and substances.

Moreover, in all known gasification methods in which reaction with water is adopted, this is introduced as liquid water or steam in an uncontrolled manner with respect to the effective requirements. Consequently, the conversion yield is poor or if the excess of steam is too great this involves a reduction in the reaction temperature and therefore of the yield, or if the steam is insufficient, then the reaction is not completed and this therefore also involves a fall in yield.

A further objective of the invention is therefore that of providing a process and apparatus which makes it possible to achieve an improved gasification yield. The above-mentioned objects are achieved by a gasification process and associated apparatus as defined in the following claims.

In particular, the process and the apparatus which form the subject of the present invention, by the adoption of particular operating and constructional conditions, makes it possible conveniently to gasify refuse having a carbonaceous matrix by obtaining: a) optimum gasification in the substantial or complete absence of air, b) limited formation of carbon dioxide, c) no formation of dioxins, furans and nitrogen oxides due to the substantial absence of oxygen and nitrogen and to the very high temperature at which the plasma produced by the electric arc is maintained (higher than 4500°C), d) destruction of dioxins and other organochlorine compounds which are difficult to eliminate (for example polychlorobiphenyls) possibly present in the refuse, thanks to the very high gasification temperature, e) optimum, constant ratio between the reagents, that is to say between the carbonaceous products and the water, f) gasification at the effective temperature of the electric arc (greater than 4500°C) in that the necessary water is already in the vaporised state at the moment of reaction and therefore no thermal energy has to be removed from the plasma within the electric arc for its evaporation, g) immediate gasification without delay of the material in the gasification chamber, therefore without formation and distillation of pitch or tarry substances, h) production of a gas which is easily purifiable by simple washing and drying, usable in internal combustion engines, i) no need to treat the gas produced on red hot coke beds to raise the calorific value or to eliminate organic decomposition substances, j) high overall energy yield.

The process and apparatus according to the invention will now be further described hereinafter with reference to the attached drawings provided purely by way of non-limitative example, in which; -Figure 1 is a schematic representation of the gasification plant and process, and -Figure 2 is a schematic representation of a detail of the plant.

With reference to the attached Figures 1 and 2 the process takes place as follows.

The refuse is broken up in a grinder 1 to dimensions of about 50mm and then supplied to a rotary classifier 2 for separation of heavy components (glass, metals, glazed material, rubble, etc) which are passed along a conveyer belt 3 and caused to pass under a magnetic separator belt 4 where separation of ferrous materials takes place which are then forwarded on for recovery, whilst the heavy fraction is discharged.

The main flow of refuse is supplied to a high speed mill 5 where the material is broken down to a fine size (preferably to average dimensions of about 5mm) and simultaneously is heated to a temperature of about 50°C. Then, via a conveyor system 6, the material is transferred to an empty silo of a group of three silos 7,8 and 9, in which three alternative phases take place, namely filling, preparation of the charge and discharge.

Supposing that at a certain time the silo 7 is empty, ready for the filling phase, that the silo 8 is in the charge preparation phase and that the silo 9 is in the discharge phase, the material from the mill 5 is then delivered to the silo 7; when the silo becomes full the flow of material is diverted to the silo 8 and the silo 7 enters the charge preparation phase.

The charge preparation phase starts with homogenisation of the material by blowing air from below for about one hour. After homogenisation a sample is drawn off and its moisture content is determined and elementary analysis performed (carbon, oxygen and hydrogen). By means of the analytical results the quantity of water necessary to obtain the best gasification yield of carbon dioxide and hydrogen is determined.

In the silo 7 the calculated quantity of water is introduced in the form of steam, which remains in the material in the form of condensate, whilst the temperature rises to about 90-95°C. When the silo 9 becomes empty the silo 7 will enter the discharge phase. At the same time the silo 8 will have finished the filling phase and will enter the charge preparation phase and so on alternately.

When one of the three silos enters the discharge phase the material is transferred progressively by a mechanised system 10 to gasification apparatus 11.

With reference to Figure 2, the gasification apparatus comprises a hopper 20 provided in its lower part with an extraction device 21 combined with a compression device 22, both operated by a drive unit 23.

The extraction device 21 is typically a helical screw auger; the compression device 22 comprises a cylindrical casing 22a heated externally by means of a steam jacket 24, provided with a port 25 for the introduction of steam; the cylindrical casing 22a is further provided with a nozzle 26 for the direct injection of steam into the interior of the compression device 22.

Within the compression device 22 turns a helical screw 27 provided with a shaft of variable section and a frusto-conical head 28 which operates in association with a final frusto- conical restriction carried by the casing, in such a way as to cause a progressive compression and extrusion of the refuse with the elimination of air and superheating of the water present in the refuse itself. In the illustrated example the variable-section shaft with the frusto-conical head 28 constitutes an extension of the helical screw auger 21.

It will be understood, however, that the extraction device 21 of the hopper 20 and the compression device 22 can be constituted by separate and different units. It is likewise understood that the compression can be achieved in any other convenient manner to achieve de-aeration of the refuse and heating thereof under pressure whereby to bring the water contained in it to a superheated state.

The steam under pressure, injected by means of nozzle 26, penetrates into the refuse and upon condensing eliminates the air contained in the interstices, which flows out back towards the hopper 20 together with part of the injected steam. The constriction in the terminal part of the device subjects the refuse to extrusion, at a high pressure which brings the temperature beyond that of the boiling point of water at atmospheric pressure (that is to say above 100°C) so that the water in the refuse becomes superheated.

The thus-compressed refuse is extruded and passes into a decompression and conveyor device 29 which comprises a vertical duct 30 which opens into the gasification chamber 33 of the gasifier 11 and within the interior of which turns a shaft 31 having blades and driven by a drive unit 32. As the mass of compressed refuse passes into the decompression device 29 and is conveyed toward the gasification chamber 33, both maintained at a lower pressure (typically the pressure is maintained at a level slightly above atmosphere) the superheated water evaporates and the refuse expands forming small pieces which fall into the gasification chamber 33 directly into a zone 34 across which an electric arc is struck with the formation of plasma at a temperature higher than 4500°C.

The decompression device 29 is provided with a jacket 35 in which circulates the steam introduced via the port 25, which is then discharged to the outside via an outlet 36. The steam in the jacket 35 serves to protect the refuse supply duct from overheating by radiation and at the same time avoids cooling of the refuse in the expansion phase. The gasification chamber 33 is maintained at a pressure slightly greater than atmospheric pressure by adjustment of the counter pressure in a recovery device 12 (Figure 1). This is made in several parts with steel sheets clad internally with crushed refractory materials having a high softening index (greater than 2500°C). The gasification chamber 33 is preferably an hourglass shape with a constriction in the central part in which the electrodes 37 are housed. The electrodes 37, of which there are at least two, but typically three, are positioned angularly separated by 120° and connected to the three phases of a three-phase alternating current supply at about 400 volts, 50-60Hz, adopted in this case as functional, but this is not limiting. The power required by the electrodes depends on the dimensions and capacity of the installation. For a capacity of lOOOkg/h of supplied refuse, the necessary electrical power corresponds by way of example to about lOOOKw.

The gasification chamber 33, for such capacity, has by way of non-limitative example the following dimensions: height 3 meters, maximum internal diameter 1.5 meter, internal diameter at the point of maximum constriction 0.5 meters.

Upon starting up the gasification process the electrodes are brought together until they touch and then automatically separated as soon as the arc is struck and the discharge current is stable. By means of an adjustment device which acts on the electrode movement mechanisms the separation between them is continuously controlled in such a way as to maintain the arc constant and avoid reduction in power or extinction of the arc.

As previously indicated, the refuse does not remain in the gasification cnamber 33 but is immediately transformed as soon as it comes into contact with the electrical discharge. In this way the pyrolytic phenomena which lead to the reduction of the yield and formation of unwanted substances are avoided.

Since no energy is required at the electrical discharge for the formation of the steam necessary for the reaction the temperature at the region of the arc remains very high (about 4500°C) with almost instantaneous gasification of the refuse.

After gasification residual salts and vitrified slag falls to the bottom of the chamber and are extracted by means of a screw device 38 with a hydraulic guard seal device to prevent the escape of gas, whilst the gases which form and have a temperature of about 300°C are extracted from the outlet 39 situated in the upper part of the gasification chamber 33 and sent to the recovery device 12. Here they are cooled down to about 130°C, heating the water which will then be utilised as supply to a steam generator 18.

The gases are then fed through a washing column 13 in which flows water with added caustic soda, where the gases cool to about 20°C and are purified of water and mineral acids constituted principally by HC1 and S. The gases are then supplied to a gasometer 14 and then pass through a system of gravel filters 15 which serve as security against flame flash back and where residual moisture is eliminated; then they are supplied to engines 16 coupled to alternators 17 to produce all the electrical energy necessary for the requirements o the installation, as well as an excess which depends on the composition of the refuse to be gasified.

By way of example, for urban solids with average characteristics, the excess energy is about C. 7kWh per kg of treated refuse. The heat of the exhaust gases from the engines is recovered by directing the gases through the steam generator 18 before being released to atmosphere. In the case of breakdown in the gas utilisation system, a torch 19 automatically acts to burn the gases.