WO1998047984A1

WO1998047984A1 - Thermochemical process for converting urban and special refuse into basic chemical products, and plant for implementing the process

Info

Publication number: WO1998047984A1
Application number: PCT/EP1998/002324
Authority: WO
Inventors: Gualtiero Valeri
Original assignee: Montevenda S.R.L.
Priority date: 1997-03-21
Filing date: 1998-04-20
Publication date: 1998-10-29
Also published as: EP1137742A1; AU7527698A; IT1297681B1; ITFE970004A0; ITFE970004A1

Abstract

A thermochemical process for converting urban and special refuse into basic chemical products, characterised by effecting, within several physically interconnected thermodynamic environments, on refuse to which inerting substance have been previously added: pyrolysis treatment at a temperature slowly increasing from 100 °C to at least about 700 °C, gasification treatment on the carbonaceous residues originating from the preceding pyrolysis treatment, at a temperature of between 700 °C and about 1100 °C, by means of oxygen mixed with steam and carbon dioxide, to obtain hydrogen, carbon monoxide and carbon dioxide, which are used for conditioning the pyrolysis treatment, and inerting treatment on the heavy metal compounds, halogenated compounds and aromatic compounds obtained from the preceding pyrolysis and gasification treatments, utilizing the heat evolved by said treatments, the added inerting substances, and the actual substances produced by the process.

Description

THERMOCHEMICAL PROCESS FOR CONVERTING URBAN AND SPECIAL REFUSE INTO BASIC CHEMICAL PRODUCTS, AND PLANT FOR IMPLEMENTING THE PROCESS

This invention relates to a thermochemical process for converting urban and special refuse into basic chemical products, and a plant for implementing the process

Thermal pyrolysis, gasification and inerting processes for refuse are known with the aim of obtaining marketable organic and inorganic compounds In a known pyrolysis process, known as the THERMOSELECT process (P Pollesei, "Energy recovery from waste - the application of gasification technologies" in "La chimica e I'lndustna" No. 5/1996 page 603 - E De Fraja, M Giugliano "Incineration - Energy recovery", Chapter III "Innovative plant proposals", in: Milan Polytechnic (Department of hydraulic, environmental and measurement engineering - Environmental section), ANDIS - National association of environmental sanitary engineering "Solid urban refuse engineering" - XLIII Updating course in environmental sanitary engineering - Milan 5-9 June 1995), this takes place indirectly, le not by burning a part of the material, but by merely heating it Gasification occurs at 2000°C separately from the pyrolysis, while inerting occurs by virtue of the temperature (between 1200 and 2000°C), hence with decomposition of the organic compounds and vitrification of the inorganic compounds

A drawback of this known process is the considerable heat dispersion due to heat exchange, the high plant cost due to the need to use materials able to withstand the high temperature, the high grinding cost of the solid product and the formation of glass crusts and plant erosion ln another known pyrolysis process, known as the KWU process (E. de Fraja - M. Giugliano), this takes place indirectly as in the preceding case, but there is no gasification, and inerting occurs by virtue of the temeprature, as in the preceding case, at about 1300°C. A drawback of this known process is the fact that it operates at high temperature with low efficiency.

A further known pyrolysis process is described in European Patent

0292987. In this, the pyrolysis takes place indirectly as in the Thermoselect process, gasification takes place separately from the pyrolysis and with the addition of combustible material additional to the refuse, inerting occurring by virtue of the temperature as in the Thermoselect process.

A drawback of this known process is that it requires the addition of combustible material.

Another known process, known as the PUROX process, of the Union Carbide Co. (C. Casci, L. Cassitto "Energy from urban and industrial refuse", CLUP, Milan, 1984 - P. Pollesei), implements only high temperature gasification with the use of pure oxygen, to produce only gas and solid slag.

A drawback of this known process is that it enables only gas and solid slag to be obtained, rather than liquid, oil or tar. Another known process, known as the GARRETT FLASH

PYROLYSTS SYSTEM (C. Casci, L. Cassitto, A. Paratella, A. Buso, "Refuse disposal" Regione Veneto - Giunta Regionale, 1983), carries out indirect pyrolysis at 500°C, to obtain only very viscous and corrosive fuel oil with a considerable tendency to polymerize. Another known pyrolysis process, known as the LANGUARD

MONSANTO ENVIROCHEM SYSTEM (C. Casci, L. Cassitto - P. Pollesei) uses only gasification with air feed It poses serious control problems for the emissions originating from the gas combustion.

Another known pyrolysis process, known as the ANDCO-TORRAX process of the Carborundum Co. (C. Casci, L. Cassitto - P Pollesei) is similar to the PUROX process, but uses a mixture of oxygen and preheated air

Another known pyrolysis process, conceived by Hamburg University (A. Paratella. A. Buso), involves only the pyrolysis of plastic materials and rubber, to obtain carbon black, oil and gas in a fluidized bed reactor.

A drawback of this known process is that it operates only on a precise type of refuse

Another known process, known as the PROCEDINE CORPORATION process (A Paratella, A. Buso), involves only direct pyrolysis, as in the Thermoselect process, at a pressure of 3.4-7 atm on molten plastic material in a fluidized bed Besides the drawbacks indicated for the Tehermoselect process, this has the further drawback of operating under pressure and hence with very costly technologies, which inter alia cannot be used for liquid and tar production.

Another known pyrolysis process, known as the HERBOLD process of Oscar Herbold of Mackeshem, Germany (A Paratella, A Buso) involves only the pyrolysis of tyres and other plastic materials, operating under a negative pressure of 40 mm of mercury

Another known process, known as the HERKO-KIENER process, involves only indirect pyrolysis, as in the Thermoselect process, of purification refuse and sludge Another known process, known as the FORNI INCENERITORI process, carries out partial combustion of the refuse with partial gasification in a deficiency of air The gaseous products (the off-gas) are then inerted separately by thermal decomposition in an excess of air and are discharged into the atmosphere The solid products (ash) are dumped without being inerted A drawback of this known process is that it discharges solid and gaseous products into the atmosphere and does not produce marketable products

In another know process, known as the WRS process (Waste Reclaim Loteπos - "L'indipendente" of 13 March 1996 page 28), pyrolysis takes place indirectly at a temperature of about 500°C there is no gasification and inerting is carried out separately from the pyrolysis

Drawbacks of this process include lower heat transfer efficiency of the indirect pyrolysis compared with direct pyrolysis, the impossibility of recovering the carbon dioxide evolved by the combustion, the need to use a large quantity of the pyrolysis products as fuel to obtain the heat for the pyrolysis itself, the impossibility, because of the low process temperature, of converting any aromatic, chlorinated and oxine compounds present in the refuse, and the possibility that the pyrolysis is not homogeneous because of the dimensions of the objects constituting the refuse, which is not ground Another known process used for refuse treatment in Art Oregon -

USA ("INF-INN" No 4, November 1996 " Energy, Environment and Innovation 1995, 8-9, 57) carries out pyrolysis on the refuse at 310-480°C, does not include gasification, and involves inerting separately from the pyrolysis

Drawbacks of this process consists of problems of lead and mercury emission into the atmosphere and, given the low pyrolysis temperature, the impossibihty of converting any aromatic, chlorinated and oxine compounds present in the refuse

Another known process, known as the DISMO process (EP-A2- 0536468) operates at high temperature (2200°C) and high pressure (100 bar), with the drawbacks of a short life of high cost of the refractory materials used An object of the invention is to eliminate all the drawbacks jointly or separtely encountered in known chemical conversion process for refuse, to obtain marketable organic and inorganic compounds from urban and special refuse, whether dangerous or non-dangerous This and further objects which will be apparent from the ensuing description are attained according to the invention by a thermochemical process for converting urban and special refuse into basic chemical products as described in claim 1

Two preferred embodiments and an executive modification of the present invention are further described hereinafter with reference to the accompanying drawings, on which Figure 1 is a schematic view of a first embodiment of a plant for implementing the process of the invention, Figure 2 is a horizontal section therethrough on the line ll-ll of Figure 1 , Figure 3 shows an executive modification thereof in the same view as Figure

1 , Figure 4 is a horizontal section therethrough on the line IV-IV of Figure 3, and Figure 5 is a schematic view of a different embodiment thereof

As stated, the process of the invention effects pyrolysis, gasification and inerting treatment on dangerous and non-dangerous urban and special refuse, preceded in the case of solid refuse by pretreatment for the purpose of

- eliminating metal and glass refuse,

- reducing the size of the refuse pieces to about 1 -3 cm, - reducing the water quantity present in the refuse by evaporation, and

- adding the inerting substances to the refuse

This pretreatment is not carried out on liquid refuse, which is fed directly to the process

The pyrolysis is implemented in accordance with the criteria of dry distillation of wood, le slowly and with temperature progression by gradually increasing the material temperature during treatment

In this manner several mixtures of volatile pyrolysis products can be withdrawn, each withdrawal corresponding to a precise temperature range

The pyrolysis temperature vanes from about 100°C (with initial moisture evaporation followed by release of the water of composition) to about 700-800°C, at which practically only a carbonaceous residue remains

The method of gasification, which takes place in the same physical environment in which the pyrolysis takes place, accentuates the natural reducing characteristics of the pyrolysis environment In addition, flash effects, le very rapid reactions with short contact times between the reactants and the reaction products, are absolutely prevented

In this manner all the reactions can take place, including those of very slow kinetics compared with the other pyrolysis reactions, such as double bond saturation, oxine bond reduction, and oxygenated product decomposition with water and carbon dioxide evolution Hence, products low in bonding oxygen and low in unsaturation are obtained, especially with regard to oils and tars.

The pyrolysis is controlled via the following parameters:

- temperature, - pyrolysis kinetics,

- partial pressures of the reaction products.

The temperature is controlled by regulating the temperature of the gas produced in the gasification zone. The pyrolysis kinetics are controlled by the residence time of the material to be treated and by regulating the temperature and partial pressure of the compounds obtained by the pyrolysis. The partial pressures are controlled by regulating the removal flow rates of the volatile reaction products.

The carbonaceous residues originating form the preceding pyrolysis treatment are gasified at a temperature exceeding 700°C but less than 1100°C by gaseous oxygen mixed with saturated steam and gaseous carbon dioxide.

The main reactions which occur are the following:

1 ) C + O₂ -» CO₂ ΔH = -97000 Kcal/Kmole

2) 2C + O₂ (600-800°C) -» 2CO ΔH = -29400 Kcal/Kmole 3) C + CO₂ - 2CO ΔH = +38000 Kcal/Kmole

4) C + H₂O -> CO + H₂ ΔH = +28200 Kcal/Kmole in which C signifies "carbonaceous residue".

Reactions 1) and 2) are normal combustion reactions. Reaction 2) is however favoured over reaction 1 ) by suitable temperature control by controlling the reactions 3) and 4), to contribute, together with their reaction products, to accentuating the natural pyrolysis reduction environment. The heat produced by reactions 1 ) and 2) develops the temperature necessary for reactions 3) (Boudouard reaction) and 4) to take place.

The system is therefore self-sufficient thermally because the combustion of just part of the refuse produces the heat necessary, and which in total is more than sufficient, for the reaction to proceed.

Reactions 3) and 4) are highly endothermic. By virtue of these it is hence possible to:

- control the gasification temperature (ie reactions 1 ) and 2)),

- control the reducing state of the pyrolysis environment, - control the temperature of the pyrolysis process.

Reactions 3) and 4) are controlled by regulating the partial pressure of the carbon dioxide and superheated steam. This regulation is achieved by controlling the carbon dioxide and superheated steam flows, flows which can even be fixed at zero for one of the two gases or for both. Controlling the thermal equilibrium between reactions 1 ) and 2) and reactions 3) and 4) hence enables the gasification temperature to be controlled, and therefore also, by virtue of the gases produced by this latter, allowing control of the partial pressures of the pyrolysis products.

Further important reactions are the methane formation reactions: 5) C + 2H₂ -» CH₄ ΔH = -21000 Kcal/Kmole

6) CO + 3H₂ -» CH₄ + H₂O ΔH = -49300 Kcal/Kmole

These reactions do not disturb the pyrolysis environment as methane is a reducing gas, but are favoured by temperatures which are too low (400- 660°C), ie such as to oppose reactions 3) and 4) and to cool the last pyrolysis stage. They are therefore mostly to be avoided during the progress of the process. The gasification is carried out in the presence of pure industrial oxygen rather than air for the following reasons

- air consists of 4/5 nitrogen, which does not take part in any of the process reactions but instead removes a large part of the heat produced by the gasification and pyrolysis reactions, without conversion of the thermal energy into chemical energy This conversion of thermal energy into chemical energy is however scrupulously observed in the process of the invention Being at low temperature, the heat removed by the nitrogen would also be difficult to recover as thermal energy I would also have to be compensated by burning a larger quantity of material, with a consequent reduction in efficiency Finally, the dimensions of the industrial plant provided for implementing the process of the invention would have to be increased proportionally to the inert gas quantity fed into it The nitrogen would also have to be separated from the volatile products of the gasification and pyrolysis, resulting in further operating costs,

- the partial pressure of the gasification oxygen is controlled by the presence of the carbon dioxide and saturated steam which participate in reactions 3) and 4) The use of air would hence exclude the use of carbon dioxide and saturated steam (one of the fundamentals of the process of the invention), as the oxygen partial pressure would be excessively reduced, with consequent inhibition of the gasification reactions,

- before taking part in the gasification, the pure industrial oxygen completes the elimination of any carbonaceous residues present in the solid pyrolysis products, in accordance with reaction 1 ) This process is also made possible by the low rate of all the reactions, and by the precise plant details Liquid refuse is preferably injected directly into the gasification zone or in any event into a high temperature zone, in order to evaporate the water present and to undergo pyrolysis at that temperature

All the pyrolysis and gasification reactions take place under slight overpressure, sufficient only to compensate the gas pressure drop and to hence allow its removal In this respect, any pressure increase would, in accordance with chemical equilibrium laws, inhibit reactions 3) and 4) and reduce the volatility of the heave compounds (oils), which would therefore tend to remain in the reaction environment, to undergo further cracking Inerting is carried out on the compounds originating from the preceding pyrolysis and gasification, ie heavy metal compounds, halogenated compounds and aromatic compounds

The heavy metals are inerted by feeding the refuse with powder formed from materials rich in pure or combined silica, suche as calcium silicate, silica, clay etc

That silicate characteristics of low melting point is utilized, and hence its ability to form the liquid phase at the gasification temperature This is true with the exception of aluminium silicate, which melts beyond 1800°C Aluminium oxides are not however soluble to any extent in water and are therefore not subject to dispersion by being washed away, nor to dispersion into the atmosphere as the plants with which the process of the invention is implemented do not discharge any gas or dust Silicate vitrification is prevented

The halogenated compounds are inerted by feeding the refuse with calcium carbonate or similar compounds such as sodium carbonate, or the usual products obtained from the process as these are rich in free oxides The calcium carbonate reacts at the initial process temperature with the hydrogen halide acids released in higher temperature stages in accordance with the reaction

7) 2HX + CaCO₃ -» CaX₂ + H₂O + CO₂ The excess calcium carbonate enables this reaction to take place at all the temperatures involved in the pyrolysis, until the termination of this latter when at about 800°C the calcium carbonate has decomposed completely into calcium oxide in accordance with the reaction

8) CaCO₃ (600-800°C) -» CaO + CO₂ At this temperature the calcium oxide reacts with the hydrogen halide acids to salify them in accordance with the reaction

9) CaO + 2HX (600-800 ) - CaX₂ + H₂O

There are no calcium, magnesium, sodium or potassium halides damaging to the environment The alkyl and aromatic halides of higher molecular weight are inerted by calcium oxide, as these form at a temperature comparable to that at which reaction 8) occurs The schematic inerting reaction is hence the following

10) CaO + 2XA -» CaX₂ +A + H₂O

In their turn the halogenated compounds which form in this manner tend to occur as radicals rather than as stable compounds, and to be inerted as such

The aromatic compounds are inerted in a different manner, depending on their type It is well known that aromatic compounds form up to a temperature of about 700-800°C, equal to the maximum pyrolysis temperature of the invention This temperature also corresponds to the formation of the aromatic compounds of higher molecular weight (condensed-nucleus arenes and polynuclear arenes) by condensation of the lighter aromatic compounds

However this is the temperature of formation of the heaviest pyrolysis fraction, corresponding to the formation of tars and heavy oils This fraction is not cracked, but is withdrawn as such (as in fact happens for all the other fractions)

It is therefore apparent that the aromatic compounds are partly withdrawn as such together with the heavy oils and tars, and are partly bonded to them In the first case inerting takes place by incorporation The high oil and tar viscosity, the low vapour pressure of the aromatic compounds which is similar to that of the oils and tars which incorporate them, and the mutual solvent effect due to the chemical compatibility of the oils and tars with the aromatic compounds prevent these escaping from the mass In the second case inerting takes place by the formation of molecular bonds

From the aforegoing it is apparent that the process of the invention is not a succession of three reaction groups (pyrolysis, gasification and inerting), but a combination of these reaction groups In this respect it is founded on the meraction of each of these reaction groups with the other two, these interactions taking place as follows

By their nature the pyrolysis reactions take place in a reducing environment, as the pyrolysis gives off the following gasses carbon monoxide up to about 300°C (together with an abundant quantity of carbon dioxide), hydrogen, methane ethylene and further carbon monoxide (but to a lessen extent) beyond 300°C and up to about 700°C In the process of the invention the reducing atmosphere is accentuated by the presence of the reducing gases produced by the gasification, expressly to favour hydrogen development and control the reducing capacity of the pyrolysis environment and favour the production of hydrogen and of slightly oxygenated and slightly unsaturated products.

Moreover the endothermic gasification reactions 3) and 4) control the exothermic pyrolysis reactions taking place at a temperature greater than

270°C. In this respect, in order for reactions 3) and 4) to progress, being endothermic they use the heat of the gasification reactions 1 ) and 2) and the heat of the exothermic pyrolysis reactions.

The process therefore comprises the following enthalpic zones:

- up to about 270°C: endothermic pyrolysis reactions;

- from about 270°C to about 700-800°C: exothermic pyrolysis reactions;

- beyond about 700-800°C: gasification zone with endothermic and exothermic reactions.

Reactions 3) and 4) consequently control reactions 1 ) and 2) and the pyrolysis reactions.

Hence by controlling the thermal equilibrium between reactions 1 ), 2),

3) and 4), it is hence possible to also control the temperature of the exothermic pyrolysis reactions. These temperatures are regulated by controlling the inlet partial pressure of the threee gases, namely pure industrial oxygen, carbon dioxide and saturated steam.

This temperature control is essential for orientating the process towards the production of determined compounds. It is also possible to control the entire process by controlling the partial pressures of all the gaseous fractions. This control is achieved by facilitating or preventing the withdrawal of determined fractions compared with others, it is also achieved by regulating the partial pressures of the reducing gasification gases (carbon monoxide and hydrogen) by the gasification reactions 3) and 4) Controlling the gasification reactions 3) and 4) and the exothermic pyrolysis reactions by the gasification reactions 1 ) and 2) is fundamental in controlling the progress of inerting In this respect the gasification temperature must not generally be forced beyond 1100°C, to prevent vitrification of the process solid residue consisting entirely of inorganic materials Moreover the inerting reactions can take place only at the temperature at which the pyrolysis and gasification reactions occur

The pyrolysis, gasification and inerting reactions are hence combined in the process of the invention into a precise system which has its own characteristics regarding the pyrolysis, gasification and inerting, each considered independently of the others

From the aforegoing it is clear that compared with the prior art the process of the invention introduces numerous advantages, and in particular

- the pyrolysis rate, and hence the rate of all the gasification and inerting reactions, is maintained low by progressive controlled temperature increase This is in contrast to previous processes, which operate at constant temperature and with high temperature gradients This therefore prevents flash processes occurring, which result in excessively oxygenated and unsaturated compounds in the pyrolysis products,

- the pyrolysis is effected with progressive temperature, to obtain a number of product mixtures (ie one mixture for each temperature range), rather than a single mixture or a single pyrolysis product, as in the case of previous processes,

- the products obtained are the following hydrogen, carbon monoxide, carbon dioxide (which is partly reused in the process for the gasification reaction 3)), methane, ethane and traces of other light gaseous hydrocarbons, pyrohgneous acid, ie an aqueous mixture of acetic acid acetone, methanol and other light oxygenated compounds, heave oils, tars, mineralixed solids,

- inerting takes place by adding silicates or inorganic carbonates, and incorporating aromatic compounds in tars and heavy oils, and is effected at the corresponding pyrolysis temperatures, the aromatic compounds being withdrawn together with the tars and heavy oils to which they are bonded or into which they are incorporated The heavy metals combine with silicates in the gasification stage, without vitrifying All this enables non-pollutant products to be obtained, in contrast to known processes,

- the carbonaceous residue is entirely gasified by scrubbing it at high temperature (such as to enable combustion to take place) with unmixed industrial oxygen Hence there is no very pollutant carbonaceous residue present, in contrast to other processes, - a gasifying mixture is used composed of carbon dioxide, saturated steam and pure industrial oxygen, by varying the partial pressures of the three gases the gasification temperature and hence the temperatures of the pyrolysis reactions (ie the temperature gradient of the entire process) can be controlled In contrast to the other processes, varying the partial pressures of the three gases hence enables temperature to be controlled, together with the reducing component of the pyrolysis environment, - pure industrial oxygen is used instead of air, the inert gases such as nitrogen being replaced by carbon dioxide Hence in this manner, in contrast to the other processes, a gas (carbon dioxide) is made available having a double purpose, namely for diluting the oxygen (and hence controlling the partial pressure of this latter), and for participating in the gasification reactions (Boudouard reaction),

- vitrification of the solid residue is prevented, by operating at a temperature lower than 1100°C In this manner, in contrast to the other processes, because of the type of inerting used, the residue does not contain pollutant substances and does not undergo hardening which would prevent its grinding Problems connected with incrustation and erosion of the equipment by the molten vitreous mass are also avoided,

- in contrast to other processes, the operating temperature is below 1100°C, the surplus of thermal energy is converted into chemical energy (producing in particular hydrogen and carbon monoxide) It is hence possible to achieve considerable saving in plant costs, as low-cost refractory materials can be used Chemical products such as hydrogen and carbon monoxide can also be made available instead of thermal energy, this being the energy form at highest entropy level, - the absence of nitrogen due to the use of oxygen instead of air prevents dilution of pyrolysis products in such gas in contrast to other processes, this results in a reduction in plant dimensions and operating costs, in addition to considerable simplification in gas separation plant,

- in contrast to other processes, there is operating flexibility in that production can be orientated towards those pyrolysis and gasification products of greatest interest, by controlling the respective temperatures, - 17 -

- in contrast to other processes, the process of the invention offers two systems for using the gases obtained, namely the use of such gases as basic chemical products, and the combustion of such gases, after carbon dioxide separation, for energy production (for example electricity); - in contrast to other processes, all the products obtained from the process of the invention can be marketed, ie no product has to be disposed.

Depending on the use for which they are intended, the products obtained from the process may need further chemical, physical and/or chemico-physical processing, such as the following: - Gaseous products: desulphuration, compression, separation, mixing, storage, combustion, feeding into gas pipelines.

- Liquid products: desulphuration, distillation, cracking, reforming, reduction, oxidation, mixing, storage, combustion, feeding into liquid pipelines.

- Solid products: washing with or without recovery of any substances washed out, grinding, sintering, separation, mixing, storage, transport.

- All or part of the products: separation and mixing of the gaseous, liquid and solid products obtained from the process or from subsequent processing, either together or with other products extraneous to the process.

To implement the aforedescribed process the invention provides three different plant arrangements, namely two relative to a single-stage reactor

(see Figures 1 -4) and one relative to a multi-stage fluidized bed (see Figure

5).

With reference to Figures 1 and 2, the plant comprises a single-stage reactor consisting of a vertical cylinder from four to six times as high as its width. It is of metal construction with an internal refractory lining. A feed pipe 2 for pretreated solid refuse, provided with a loading hopper 3, is connected to the top of the reactor 1.

An injector 7 for the liquid refuse to be treated is connected to the bottom of the reactor 1. The reactor interior is provided with suitable semi-circular steel baffles

4 and grids 5 which are mutually superposed and are inclined downwards at an angle equal to the angle of friction of the mateπal to be treated.

The baffles 4 prevent material compacting caused by its weight. The facilitate mixing, to as a result favour contact between the material and hot gas. They enable the pyrolysis products to be withdrawn through lateral offtakes 6 suitably distributed along the reactor height, below the grids 5 and baffles 4.

The grids 5 perform the same function as the baffles 4, by supporting the material under treatment, preventing its compaction and favouring its remixing. They also distribute the hot gas through the mass, including non- condensable gas re-injected into the furnace.

At the side offtakes 6 for gas and pyrolysis product withdrawal there are provided labyrinth or other separation systems which prevent dust and material particles from being entrained. Mineralized solid products are withdrawn from the bottom 8 of the reactor 1 via ordinary extraction systems.

Three headers for oxygen 9, for carbon dioxide 10 and for steam 11 are connected into the base of the reactor 1. A fourth header 12 is also provided for nitrogen, used as purge gas when the reactor is at rest. One of the three headers 9, 10 and 11 can also be used as the nitrogen header. The gases injected into the bottom of the reactor operate and control the gasification of the completely pyrolyzed material. The pyrolysis stage is sustained by the hot gas from the gasification zone and the condensable products which are superheated and re-injected into the reactor. The gas ascends through the reactor, within which a temperature varying from about 100°C at the top to about 1100°C at the bottom is maintained, while the refuse mass under treatment descends through the reactor to encounter the gas in countercurrent.

The reactor 1 is provided with a coaxial shaft 13 to which arm 14 are fixed, which by rotation prevent any formation of material bridges and facilitate material fall.

The reactor shown in Figures 3 and 4 is an executive modification of the reactor already described, and carries the same reference numerals for equivalent parts. It differs form this by comprising, instead of the semicircular grids and baffles, horizontal circular plates 15 alternately open at their centre and at their periphery to enable the material under treatment, urged by fins 16 applied to the arms 14, to descend downwards following a substantially labyrinth path.

The multi-stage reactor (see Figure 5) consists of a system of equal- sided or elongate cylindrical bodies 21 , constructionally similar to each other but not necessarily identical, in a number preferably between four and eight, each forming one stage.

Each stage 21 is loaded upperly, by a suitable device 22, with solid material originating from the preceding stage (the first stage is loaded with refuse via an entry hopper 23. The solid material leaving each stage passes into the next stage. Ash leaves the last stage, in a manner similar to that which occurs at the base of the single-stage reactor of the preceding examples

Liquid mateπal is fed into a stage 21 at high temperature, via an injector 29. The loaded material is maintained fluidized within each stage by injected superheated non-condesable gas produced by a stage 21 or by the gasifying mixture of oxygen 24, carbon dioxide 25 and steam 26. The non- condensable gas sustains the pyrolysis, while the mixture of gasifying gases sustains the gasification, as happens in the single-stage reactor. Each stage 21 of the multi-stage reactor operates at uniform temperature throughout its volume, given the considerable turbulence of the fluidized mass, but at a temperature higher than that of the preceding stage and less than that of the next stage.

The pyrolysis and gasification gases are withdrawn from each stage via a suitable device in accordance with the known technology of fluidized bed systems (for example a cyclone 27 or another separator situated within the cylindrical body 21 ). The solid fractions are also withdrawn from each stage, via traditional separator devices 28. The technology is hence similar to that of ordinary fluidized bed reactors. A gasification mixture of oxygen, carbon dioxide and steam is injected into the last stage. In addition an injector 30 for nitrogen is provided in at least one stage but preferably in all the stages.

In order to completely remove organic sustance traces and carbon from the ash and to recover the residual heat contained in it, the solid material leaving the main gasification stage is passed into a further stage 21 ' in which it is brought into contact with pure industrial oxygen. Although from the plant viewpoint the arrangement shown in Figure 5 differs from the arrangements shown in Figures 1-4, from the process viewpoint the three arrangements are similar, in that the zones of different and increasing temperature, which in the single-stage reactor are combined into a single body, are separated into a sequence of separate bodies in the multistage reactor. There is a precise correspondence between each section of a single-stage reactor and each stage of the multi-stage fluidized bed reactor.

Claims

C L A I M S

1. A thermochemical process for converting urban and special refuse into basic chemical products, characterised by effecting, within several physically interconnected thermodynamic environments, on refuse to which inerting substance have been previously added:

- pyrolysis treatment at a temperature slowly increasing from 100┬░C to at least about 700┬░C,

- gasification treatment on the carbonaceous residues originating from the preceding pyrolysis treatment, at a temperature of between 700┬░C and about 1100┬░C, by means of oxygen mixed with steam and carbon dioxide, to obtain hydrogen, carbon monoxide and carbon dioxide, which are used for conditioning the pyrolysis treatment, and

- inerting treatment on the heavy metal compounds, halogenated compounds and aromatic compounds obtained from the preceding pyrolysis and gasification treatments, utilizing the heat evolved by said treatments, the added inerting substances, and the actual substances produced by the process.

2. A process as claimed in claim 1 , characterised by withdrawing, during pyrolysis treatment, different mixtures of volatile products corresponding to different temperature ranges.

3. A process as claimed in claim 1 , characterised by controlling the pyrolysis process by regulating the quantity of heat developed by the gasification reactions.

4. A process as claimed in claim 3, characterised by controlling the quantity of heat developed by the gasification reactions by regulating the ratio of exothermic gasification reactions to endothermic reactions on the basis of the oxygen, steam and carbon dioxide partial pressures, which in their turn are regulated by the flow rates of these compounds entering the process.

5. A process as claimed in claim 1 , characterised by controlling the pyrolysis kinetics by acting on the residence times of the mate╧Çals to be treated within the respective thermodynamic environments.

6. A process as claimed in claim 1 , characterised by controlling the pyrolysis treatment by acting on the partial pressures of the reaction products.

7. A process as claimed in claim 6, characterised by regulating the partial pressures of the reaction products by acting on the removal flow rates of the volatile reaction products.

8. A process as claimed in claim 1 , characterised by effecting inerting treatment on heavy metals by adding silica-rich substances to the refuse to be treated.

9. A process as claimed in claim 8, characterised by adding silicates to the refuse to be treated.

10. A process as claimed in claim 9, characterised by adding calcium silicate to the refuse to be treated.

11. A process as claimed in claim 8, characterised by adding silica to the refuse to be treated.

12. A process as claimed in claim 8, characterised by adding glass to the refuse to be treated.

13. A process as claimed in claim 8, characterised by adding clay to the refuse to be treated.

14. A process as claimed in claim 1 , characterised by effecting inerting treatment on halogenated compounds by adding alkaline and/or alkaline-earth metal carbonates to the refuse to be treated.

15. A process as claimed in claim 14, characterised by adding calcium carbonate to the refuse to be treated.

16. A process as claimed in claim 14, characterised by adding sodium carbonate to the refuse to be treated.

17. A process as claimed in claim 1 , characterised by effecting inerting treatmente on halogenated compounds by adding, to the refuse to be treated, solid products obtained from the process itself.

18. A process as claimed in claim 17, characterised by effecting inerting treatment on halogenated compounds by adding products rich in free oxides to the refuse to be treated.

19. A process as claimed in claim 17, characterised by adding free oxides to the refuse to be treated.

20. A process as claimed in claim 1 , characterised by effecting inerting treatment on alkyl and aromatic halides by the effect of calcium oxide.

21. A process as claimed in claim 1 , characterised by effecting inerting treatment on alkyl and aromatic halides by the effect of calcium carbonate.

22. A process as claimed in claim 1 , characterised by effecting one or more of the treatments under slight overpressure substantially corresponding to the gas pressure drop during the process.

23. A process as claimed in claim 1 to be implement in a fluidized bed reactor, characterised by maintaining the be fluidized by the effect of gas originating from the process.

24. A process as claimed in claim 1 to be implemented in a fluidized bed reactor, characterised by maintaining the bed fluidized by injecting oxygen and/or steam and/or carbon dioxide of external origin.

25. A plant for implementing the process claimed in one or more of claims 1 to 24, comprising a reactor (1 ,21 ), means (2,7,23,29) for loading the reactor with the refuse to be treated, and means (6,8,27,28) for removing from the reactor the solid and gaseous products obtained, characte╧Çsed by providing within the reactor (1 ,21 ):

- different physically interconnected thermodynamic environments,

- solid refuse loading means (2,23) separate from the liquid refuse loading means (7,29),

- injectors (9,10,11 ) for oxygen, carbon dioxide and steam.

26. A plant as claimed in claim 25, characterised by comprising a reactor (1 ) consisting of a cylindrical body within which steel baffles (4) and/or grids are arranged inclined downwards by an angle substantially equal to the angle of friction of the refuse to be treated, the injectors (9,10,11 ) being positioned in the bottom of said cylindrical body.

27. A plant as claimed in claim 26, characterised by comprising an injector (12) for nitrogen in the bottom of the cylindrical body (1 ).

28. A plant as claimed in claim 26, characterised in that the cylindrical body comprises in its lateral wall, in a position below the grids (5) and/or the baffles (4), lateral offtakes (6) for withdrawing condensable and non-condensable gaseous products.

29. A plant as claimed in claim 26, characterised in that the cylindrical body (1 ) is provided with a rotary coaxial shaft (13) to which arms (14) are fixed.

30. A plant as claimed in claim 26, characterised in that separator members are provided at the lateral gas and pyrolysis product withdrawal offtakes.

31. A plant as claimed in claim 30, characterised in that the separators are of labyrinth type.

32. A plant as claimed in claim 23, characterised by comprising a reactor (1 ) consisting of a cylindrical body, within which there are arranged overlying circular plates (15) which are open alternately in their central region and in their peripheral region.

33. A plant as claimed in claims 27 and 30, characterised in that each arm (14) is provided lowerly with fins (16) acting on the underlying plate (15).

34. A plant as claimed in claim 25, characterised by consisting of a reactor comprising several fluid bed stages (21 ), each of which operates at a temperature higher than that of the preceding stage but lower than that of the next stage.

35. A plant as claimed in claim 34, characte╧Çsed by comprinsing a nitrogen injector (30) in at least one stage.

36. A plant as claimed in claim 34, characterised in that each stage (21 ) comprises at least one offtake for withdrawing condensable and non- condensable products.

37. A plant as claimed in claim 34, characterised in that separator members (27,28) are provided at the lateral gas and pyrolysis process withdrawal offtakes.

38. A plant as claimed in claim 37, characterised in that the separators (27,28) are of cyclone type.