CA1200675A

CA1200675A - Method and apparatus for thermal decomposition of waste materials to stable end products

Info

Publication number: CA1200675A
Application number: CA000408598A
Authority: CA
Inventors: Jan Thornblom; Sven Santen
Original assignee: SKF Steel Engineering AB
Current assignee: SKF Steel Engineering AB
Priority date: 1982-01-18
Filing date: 1982-08-03
Publication date: 1986-02-18
Also published as: BE894276A; ES8305109A1; AU8663482A; IT8222451A0; IT1157294B; CH656636A5; AU545901B2; PH18313A; MX161949A; IL66144A0; YU145582A; IL66144A; FI70998C; ES514168A0; BR8205688A; FI70998B; YU44350B; ZA824787B; FI822340A0; FI822340L

Abstract

A B S T R A C T

The invention relates to a method and plant for converting waste material containing and/or comprising thermally decomposable chemical substances to stable end products, such as CO2, H20 and HC1. The waste material, in order to effect decomposition thereof, is subjected to a high temperature plasma gas generated in a plasma generator. The plant comprises a reaction chamber having a refractory lining, at least one plasma generator and means for the supply of waste material. A tuyére is arranged immediately in front of the plasma generator and the means for the supply of waste material is connected to the tuyére. Supply means for oxygen, is also connected to the tuyére.

Description

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THIS INVENTION relates to a method of converting waste material, containing an`d/or comprising thermally decomposable ~hemical substances, to stable end products, such as CO2, H20 and HC1, and to a plant for carrying out such a method.

It has already been proposed to burn waste material in a reaction furnace provided with a reaction hearth and a plurality of plasma burners arranged above the hearth, the plasma gas produced by the plasma burner being collected and directed in the ~orm of a jet towards the waste material in the hearth. The waste material is thus mechanically disintegrated, although not to particulate form, and remains in the hearth under the influence of the plasma gas. Stablè
end products thus obtained can be withdrawn in molten or gaseous form. The task of the plasma burners is to produce the requisite high temperatures. Considered as a whole, the reactions here can only be controlled to an extremely limited extent. Furthermore, the individual volume elements of the waste are not in a uniform thermodynamic environment.
All this means that a defined generation of stable end products cannot be assured by this known method.

The obJect of the present invention is to control the method described in the introduction in such a way that the reactions ,...

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can be totally controlled in order to ensure the desired genera-tion of stable end products. Another object of the invention is to provide a plant to enable the method according to the invention to be carried out simply and functionally.
According to the invention there is provided a method of converting waste material comprising thermally decomposable chemical substances to stable end products, including the following steps:
(a) generating a high temperature plasma gas in a plasma generator;
(b) subjecting the waste material to the plasma gas in a reaction zone heated to at least 2000C by the plasma gas to decompose the waste material; and (c) maintaining an oxygen potential in the reaction zone suffici-ent to continuously convert the decomposition products of the waste material to stable end products.
Also according -to the invention there is provided a plant suitable for carrying out the method according to the inven~ion, comprising a reaction chamber having a refractory lining, at least one plasma generator disposed for directing a plasma gas jet into the reaction chamber, means for supplying waste material to the reaction chamber, a tuyère arranged 12a)~;7~

immediately in ~ront of the plasma generator~ the waste material supply means being connected to the tuyère and including oxygen supply means also connected to the tuyere. The plasma gas jet from the plasma burner is directed into the reaction chamber and the gaseous reaction products can be removed from the reaction chamber.

~t is irnportant that the reaction temperature, reaction times and oxidation potential are carefully controlled in order to achieve a defined generation of stable end products. There is a relation between the reaction temperature and reaction time, the tlme required decreasing with increasing reaction temperature, and vice versa. According to one aspect of the invention a defined decomposition is primarily ensured by adjusting the reaction temperature and time at low oxidation potential. Adjustment of the reaction temperature may be achieved by suitable setting of the plasma burner. The reaction time may be controlled by arranging a pre reaction chamber between the tuyere for the supply of waste material and the main reaction chamber. Only after the defined decomposition is achieved is the reaction continued, again at a defined oxygen potential through the addition of oxygen, to giYe stable end products. The reaction time may be varied here by different manipulation of the flow path.
It is extremely advantageous if the waste material can ~L2~ L?6~5 be supplied in a finely divided form both during the decomposition stage and the subsequent continuance of the reaction to stable end products. This provides a large surface area and particularly good reaction tendency for the individual parts of the waste material. Added to which, both during decomposition and during the subsequent reaction stage, practically all these ~ndividual parts will be in the same thermodynamic environment with respect to pressure, temperature and reaction partners.
"~inely divided form" implies that the waste material is in such a form that it can be transported, i.e. it is in feedable form and can be injected into the tuyère.

The method according to the invention can be developed further in several respects. For instance, some of the oxygen required f~r stabilizing the decomposition products can be mixed with the carrier gas and/or plasma gas. In particular, the oxygen can be mixed in a heated state with the carrier or plasma gas with its decomposition products.
If extremely high temperatures are required, the oxygen can be introduced in the form of a plasma gas flow with a temperature of between 2000 and 4000C. The oxygen can be supplied in the form of air and/or in the form of oxygen-enriched air or in the form of technically pure oxygen. However, water can also be used as an oxygen . .

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carrier since water in the plasma gas dissociates tooxygen and hydrogen due to the high temperature.

Within the scope of the invention waste material in feedable form can be supplied completely or partially to the plasma gas downstream of the plasma burner. In the case of waste material such as dioxides, PCB, oil-polluted earth, and so on, reproducible results are obtained by working with reaction tlmes in the order of milliseconds, and the carrier gas or the plasma gas formed is suitably subjected to turbulence or guided in a suitable circuit in the plasma burner and in the reaction chamber. The gas with the stable end products may be cooled as or after it leaves the reaction chamber.

According to a preferred embodiment of the invention the jets from the plasma generator are suitably direc~ed towards a gas-permeable coke filler arranged in the combustion chamber, the waste material in feedable form and/ or its reaction products being fed into the reaction chamber with the plasma jet.

According to a preferred embodiment of the invention, the reaction chamber is provided with a coke filling consisting of eoarse pieces of coke, it being advisable to locate ,, .

2(~4~675 the reaction chamber in a shaft furnace with a blast furnace top ~or the supply of coke and a lower slag outlet. This enables the consumed coke to be replaced continuously via the blast furnace top, as is normal in shaft furnaces.
Naturally,khe gaseous reaction products extracted are generally subjected to a subsequent treatment, cooling andfor dust filtration, for instance.

It is further important that the reactions required for convertiag the waste material to stable end products be performed under well da`Pined thermodynamic conditions, i.e. at a specific temperature, specific pressure and specific reaction potentials, especially with respect to the oxygen potential. There should be a certain excess of oxygen, for instance, until the reactions have progressed to the stablè end products, but at the same time, the formation of interferirg chemical compounds must be prevented.
It has now surprisingly been found that this problem can be solved by means of the invention, since the coke filling in the combustion chamber quickly takes care of the excess oxygen. The coke filling can also be used to provide a reducing atmosphere for the reactions.

The coke filling stabilizes the conversion reactions.
The plasma gas flow may be adjusted wlth respect to the ~U~67~

temperature and composition in accordance with the existingoperating conditions and thus to the waste material in question. The waste material can be mixed, for instance, in finely divided form in a carrier gas flow which is converted into the plasma gas flow in the burner, its oxygen potential being insufficient for combustion of the waste material or the formation of decomposition products of the waste material, so that the waste material is first decomposed in the plasma gas and thereafter treated further by the addition of oxygen. However, oxygen may even be introduced into the carrier gas. The decomposition can take place at a temperature of between 2000C and 4000C, and even after that the h~igh temperatures are still available. Due to certain circumstances, it may be advisable according to the invention to arrange a pre-reaction chamber before the reaction chamber, in the form of a turbulence chamber, for instance~ in which the oxygen is supplied.

The invention will now be described, by way of an example, with reference to the accompanying drawing showinK a sectional s.ide view of a plant for the decomposition of waste material according to the inventionO

The plant illustrated in the drawing is intended for converting waste material containing and/or consisting of -7~

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~z~ s thermally decomposable chemical substances. Specifically, -it may apply to the combustion of plastic materials. The desired stable end products comprise CO 2~ H 2 and HC1, for instance. The plant principally comprises a combustion or reaction ch~mber 1 with a refractory lining 2, at least one plasma burner 3 and means 4 for supplying waste material to the reaction chamber 1. The plasma burner 3 is preferably of the type which utilizes two cylindrical electrodes with an intermediate annular gap through which the plasma gas enters. The plasma gas is heated in the electric arc generated across the annular gap between the electrodesO

Plasma gas is supplied through an inlet pipe 12 and the plasma gas jet 5 leaving the burner 3 enters the reaction chamber 1. The gaseous reaction products formed flow up in the reaction chamber 1 and out through a gas outlet 11. The reaction chamber 1 is provided with a coke filling 6 of a type permitting gas to percolate therethrough.
The plasma æas jet 5 feeds the waste material and/or reaction products of the waste material into the reaction chamber 1. In the e~ample shownt the coke filling 6 consists of a column of coarse pieces of coke. Within the area where the plasma gas jet 5 enters, a burnt-out cavity 7 is produced during the process, which constitutes the reaction ~one where conversion to stable end products takes place.

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According to a preferred embodiment of the invention, the reaction chamber l, in this example, is a shaft furnace having a blast furnace top 8 for the supply of coke, and a slag outlet 9 at the bottom. A pre-reaction chamber 10, in the form of a turbulence chamber, is arranged in front of the reaction chamber.

The coke is fed into the reaction chamber 1 in particulate form through a blast furnace top in which the coke is deflected towards the sides of the chamber as shown in the drawing. The limiting surface of the coke in the upper part of the chamber therafore forms a conical crater in accordance with the natural bosh angle of the material, i.e. with a thickness decreasing upwardly, the layer of material wil]. cover the inner limiting surface of the chamber.
The distribution of the particulate material thus obtained at the upper part of the chamber promotes a central gas flow inside the coke filling 6 and out through the gas outlet 11 at the same time considerably reducing the thermal qtress on the blast furnace top and the chamber lining 2. Furthermore, a substantially constant flow of gas i9 achieved inside the entire reaction chamber 1, which is of great importance for achieving uniform thermodynamic conditions for the material involved in the reaction processes.

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The waste material is thus fed through supply means 4 into the tuyère which is arranged immediately in front and downstream of the plasma generator. In the embodiment shown, the tuyère is integral with the pre-reaction chamber 10. Oxygen can be supplied both ahead of and behind the pre-reaction chamber 10, as shown at 13 in the drawing, for instance.

The advantages of the invention are that the reaction can be carried out under very good control and that a generation of stable end products can thus be ensured.
The method according to the invention is suitable for widely differing types of waste material containing or consisting of thermally decomposable chemical substances and also waste material which is non-combustible or difficult to burn. The fact that the process can be performed with ~imple equipment, thus-ensuring reliable functioning, is a particular advantage.

E~ample 1 In a test run according to the invention in a plant as shown in the drawing 37 kg of a 10% solution of pentachlorophenol in an organic solvent was degraded. In the experiment air was used as plasma gas and the temperature of the gas leaving ,...

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) the plasma burner was regulated to about 2500C. After heating the experimental apparatus to an operating temperature, i.e. about 2000C, the pentachlorophenol solution was fed into the tuyere at a rate of 1.3 kg/min.
The plasma generator power was regulated to 460 ~W.
Compres,sed air is used as plasma gas and the plasma gas flow was 1.8 m3~n)~min. 1.2 m3(n) oxygen gas was added per minute in the tuyére in front of the plasma burner. Decomposition of !I`o !~ntachlorophenol occurs when it is exposed to the high temperature of the plasma gas and complete decomposition is achieved in the hot coke grid in the cavity 7 in front of the tuyere. Immediately after decomposition, and primarily in the cavity 7 formed in the coke filling in f`ront oP
the tuyère, all the carbon released as well as a ~mall quantity of the hydrogen released, react with the oxygen in the plasmà gas and the oxygen gas supplied. The gas leaving the coke shaft via the outlet 11, which is still at a temperature of about 1900C 9 iS quenched and washed in a caustic soda solution to bind the chlorine and any hydrocarbon. The gas leaving the wash consists of a mixture of carbon monoxide, hydrogen and nitrogen with about 4%
carbon dioxidè. Analysis was unable to indicate pentachloro-phenol either in the washing solution or in the exhaust gasc The total quantity of gas leaving the shaft was measured .
to be 8 m3(n)/min. Analysis of the washed gas indicated ~Z01~36~S

36% C0~ 4% C02 and 42% hydrogen gas, the remainder mainly being nitrogen gas. The total consumption of coke during the experiment was about 2.5 kg and a certain amount of slag cauld be found in the bottom part of the furnaceO
The quantity of chlorine bound in the washing liquid was 2.45 kg.

Example 2 In a test run in accordance with the invention sand impreænated with transformer oil containing chlorinated hydrocarbon was degraded. The total mass of the sample was 60 kg and it contained 6.Z kg oil with 2% (about 125 g) chlorinated hydrocarbon. Durlng the experiment air was used as plasma gas and the temperature of the gas leaving the plasma burner was regulated to about 2500C. The polluted sand was mixed with 55 kg of quicklime (to adjust the melting point and buoyancy of the slag formed) and it was injected with the aid of air as carrier gas into the plasma gas at its exit f`rom the burner. The reactants were carried into the reaction shaft by the plasma gas. The shaft contained a coke f`illing.
The coke was in particulate form (40 - 60 mm). Prior to the e~periment, the reaction chamber was heated to an operating temperature of about 2000C. The feed rate was 2 kg/min and the quantity of carrier gas was 0.6 m3(n)/min~

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The plasma burner power was regulated to 540 kW and theplasma quantity was 1.8 m3(n)/min. The transformer oil and chlorinated hydrocarbons were decomposed into carbon (soot), hydrogen and chlorine, which immediately reacted with the oxygen in the air to form carbon monoxide and a small quantity o~ water vapour. At the same time, the sand turned into slag due to the in~luence of the qu.icklime, giving CaO . SiO2 slag which was removed at 9 from the lower part of the shaft. The gas, comprising C02, H2, H20 and C12/HC1, leaving the sha~t was quenched and washed in a caustic soda solution. Analysis was unable to indicate chlorinated hydrogen in either the washing solution, the exhaust gas or the slag formed. The quantity of ch]orine absorbed by the washing solution was 77 g and analysis of the washed gas gave 28% CO, 4% CO2 and 7% H2, the remainder pr.imarily being N2. The amount of coke consumed during the experiment was 4.1.kg and the amount of slag ~ormed was 117 kg.

The above examples constitute only preferred embodi.ments.
The method proposed according to the invention can also be used ~or the destruction of many other materials. The material to be destroyed may be a liquid, a gas or a solid.

Examples of liquids are organic solvents, dioxines, biocides, and so on, as well as excess solvent from industrial manufacturing processes.

Solids may be pentachlorophenol, polluted sand and earkh, etc.

Gasses may be freons, chemical and biological warfare gases, etc.

According to the invention the starting material should be brought into a feedable form. Solids, for example, may be dissolved, suspended or crushed.

Solids to be fed in with the help of a carrier gas should be disintegrated to a particle size of less than 2 mm.
The blow-in pressure should exceed 2 bar.

When suspended in a liquid, the particle size should be less than 0.25 mm. In view of the risk of poisolllng, suspensions or solutions are to be preferred, since these can be prepared in closed systems. It is more diff`icult to prevent spreading with mechanical disintegration.

Irrespective of whether the supply is in a gaseous or liquid state, the blow-in velocity should preferably exceed 5 m/second ~,.. .

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and preferably be between 40 and 100 m/second. This alsoapplies to liquids. Blowing in should preferably be performed in the tuyère in front of the plasma burner.

If the waste material is in a gaseous state, it is preferably fed through the plasma burner. It can of course also be divided so that only part is fed through the plasma generator with the plasma gas while the rest is fed into the plasma gas downstream of the generator or directly-i~to the reaction zone. The plasma gas used according to the invention should preferably consist of a gas ~ith a suitable oxygen content for the process. Alternatively extra oxygen may be added, such as the addition of oxy~en to the tuyère or in the reaction zone.

The starting temperature of the plasma gas from the burner should be at least 2000C and it should preferably have an ener~y content sufficient to raise the temperature in t~ chamber above 2000C.

The plasma gas may consist of air or gas recirculated during the process, for example.

As to localisation of the cavity 9 i.e. the reaction zone in the shaft, this is formed in front o~ the plasma generator ~,.., ~

?675 during the reaction. However, the cavity is not permanent.
It soon collapses and is then reformed and so on. In principle the cavity comprises the spaces between the particles in the coke filling, which spaces are enlarged as the reaction progresses.

Oxygen may be added in any form, such as water, water vapour, etc.

However, the filling may also contain dolomite or similar substances, such as chalk, to bind sulphur.

The carbonaceous material is preferably coke in particulate form, suitably larger than 20 mm, preferably between 40 and 60 mm.

The material should preferably remain in the actual cavity for some milliseconds and in the remaining column of coke for between about 1 and 5 seconds.

However, these periods may be regulated in a variety of ways, to suit the requirements of a speciric reaction, by suitable adjustment of the feed rate 7 f`or example.

If desired, for technical reasons, the temperature of` the gas in the upper part of the shaft may be reduced to about i: `

~20t~675 1000~C by supplying water.

The gas flowing out of the shaft is suitably quenched toambient temperature.

If necessary, a suitable sla~-former ~ay be added.

The invention is obviously not limited to the embodiments described above, but can be varied in many ways within the scope of the following claims.

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Claims

CLAIMS:

1. A method of converting waste material comprising thermally decomposable chemical substances to stable end products, including the following steps:-(a) generating a high temperature plasma gas in a plasma generator;
(b) subjecting the waste material to the plasma gas in a reaction zone heated to at least 2000°C by the plasma gas to decompose the waste material; and (c) maintaining an oxygen potential in the reaction zone sufficient to continuously convert the decomposition products of the waste material to stable end products.

2. A method according to Claim 1, wherein the waste material is injected into the plasma gas downstream of the plasma generator.

3. A method according to Claim 1, wherein the waste material is injected into the plasma gas upstream of the plasma generator.

4. A method according to Claim 1, wherein the waste material is injected directly into the reaction zone.

5. A method according to Claim 1, wherein oxygen is added to the plasma gas upstream of the plasma generator.

6. A method according to Claim 1 wherein oxygen is added to the plasma gas downstream of the plasma generator.

7. A method according to Claim 1 wherein oxygen is introduced directly into the reaction zone.

8. A method according to Claim 1, wherein the waste material, in so far as it exists in gaseous form, is at least partially mixed with the plasma gas upstream of the plasma generator.

9. A method according to Claim 1 wherein waste material present in solid form is converted into a feedable form by bringing it into solution or suspension.

10. A method according to Claim 1 wherein the waste material is disintegrated to a particle size of not greater than about 2 mm and injected into the plasma gas by means of a carrier gas.

11. A method according to Claim l, wherein the waste material is introduced in the form of a liquid, which liquid contains suspended particles of waste material having a maximum particle size of about 0.25 mm.

12. A method according to Claim 1, wherein the waste material is injected into the plasma gas under a pressure which exceeds 2 bar.

13. A method according to Claim l, wherein the waste material is introduced at a velocity which exceeds 5 m/second.

14. A method according to Claim 13 wherein the feed velocity of the waste material is between about 40 m/second and about 100 m/second.

15. A method according to Claim l, wherein the plasma gas with the waste material or the waste material decomposition products are introduced into a pre-reaction chamber located between the plasma generator and the reaction zone and subjected to strong turbulent motion in the pre-reaction chamber.

16. A method according to Claim l, wherein the plasma gas comprises air or another oxygen containing gas suitable for conversion of the decomposition products to stable end products.

17. A method according to Claim 1, wherein a jet of plasma gas is directed towards a gas-permeable filling of particulate material in a reaction chamber so that a cavity is formed in the filling and which cavity acts as the reaction zone.

18. A method according to Claim 17, wherein the filling comprises a carbonaceous material.

19. A method according to Claim 18 wherein the carbonaceous material comprises coke.

20. A method according to Claim 17, wherein the filling comprises a sulphur binding material.

21. A method according to Claim 20 wherein the sulphur binding material comprises dolomite.

22. A method according to Claim 17 wherein the period during which the reactants remain in the cavity is in the order of milliseconds and wherein the period spent in the remainder of the filling is about l to 5 seconds.

23. A method according to Claim 17, wherein the gas leaving the reaction chamber is rapidly cooled in a caustic soda solution to bind any chlorine or hydrogen chloride present.

24. A plant suitable for carrying out the method according to Claim 1, comprising a reaction chamber having a refractory lining, at least one plasma generator disposed for directing a plasma gas jet into the reaction chamber, means for supplying waste material to the reaction chamber, a tuyère arranged immediately in front of the plasma generator, the waste material supply means being connected to the tuyère and including oxygen supply means also connected to the tuyère.

25. A plant according to Claim 24, wherein the reaction chamber is located in a shaft furnace and wherein the reaction chamber is provided with a gas-permeable filling of a particulate material.

26. A plant according to Claim 25, wherein the shaft furnace is provided with a blast furnace top for the provision of the filling and a slag outlet at its bottom.

27. A plant according to Claim 25, wherein the filling comprises a carbonaceous material.

28. A plant according to Claim 27 wherein the carbonaceous material comprises coke.

29. A plant according to Claim 25, wherein the area of entry for the plasma gas into the filling comprises a cavity burned in the filling and which cavity constitutes a reaction zone.

30. A plant according to Claim 24, wherein a pre-reaction chamber is provided between the tuyère and the reaction chamber to increase the decomposition time of the waste material, and wherein a second tuyere is arranged downstream of the pre-reaction chamber for the further addition of oxygen.

31. A plant according to Claim 30 wherein the pre-reactlon chamber comprises a turbulence chamber.

32. A plant according to Claim 24, wherein the plasma generator comprises two concentric cyllndrical electrodes with an annular gap between the electrodes and means for supplying plasma gas through the annular gap.