A METHOD AND AN APPARATUS FOR THERMAL DECOMPOSITION OF STABLE COMPOUNDS

Abstract: A method and an apparatus for thermal decomposition of stable substances, preferably chemical hazardeous waste. The intention is to achieve so high a decomposition temperature that the quantity of remaining chemical traces after the decomposition has been reduced to an absolute minimum. This is achieved by giving the waste the necessary decomposition temperature in a plasma generated in a plasma burner (12). Part of or the whole quantity of the waste is brought into the carrier gas of the plasma. Alternatively part of or the whole quantity of the waste can be supplied to the plasma and mixed with its high temperature carrier gas. The carrier gas of the plasma can be given a temperature of 3000-4000`C or under certain conditions still higher.

Description: A method and an apparatus for thermal decomposition of stable compounds This invention concerns a method and an apparatus for ther- 5 mal decomposition of stable compounds, preferably chemical waste. The waste may be either in solid, liquid or gaseous form. The typical feature of the new method is that the waste is 10 given the necessary decomposition temperature in a plasma generated in a plasma burner. At this operation the charact¬ eristic properties of the plasma to create extremely high temperatures in the carrier gas under highly varying redox- conditions is used. 15 These properties are extremely advantageous in connection with the decomposition of the main part of so called hazard- eous waste. » 20 That group of chemical substances that is named hazardeous waste consists of product mixtures, for instance PC3 as well as more well-defined compounds like pentachlor phenol. The danger of that group of chemicals is related to the toxicity and stability of the included compounds. The com- 25 pounds might easily migrate into the ecological system in an uncontrolled way at unpermitted discharges or at leakage from inferior decomposition processes. At thermal decomposition - i.e. at a strong heating under 0 oxidative decomposition - it is predominately the thermal stability of the compounds that is determining the efficien¬ cy of the decomposition. Mainly four primary process para¬ meters'must be mastered in order to receive an acceptable result: 5 1) The temperature must be sufficiently high - norm¬ ally over 1100 C - during a sufficiently long time in order OMPI_ to make a decomposition of the various compounds possible. For the very toxic dioxines is mentioned the minimum temperature of 1200°C during approximately 20 sees. 2) The reaction time must almost always be linked to the temperature, in the way that a higher temperature can compensate for a shorter reaction time. 3) The oxidation potential must be sufficiently high 0 to promote the decomposition to stable final products as CO-, H_0 and HCl in order to prevent a pyrolysis of the different chemicals due to the lack of oxygen. According to the risks of chlorine formation at a high oxidation potential it may often be more suitable to choose a high -' temperature and a reasonable oxidation potential. 4) The neutralization capability for formed hydro¬ chloric acid is a factor of strength in order to suppress the formation of chlorine, that has a tendency to promote * 0 the reformation of toxic compounds at a temperature decrease. To evaluate the changes of to-day existing technology to bring about that decomposition efficiency required by en¬ vironmental control authorities it is advisable to analyze its ability to give acceptable values regarding for instance the four mentioned process parameters. Now used technique can handle one or two of the critical parameters. In large rotary kilns, as for instance in the cement industry, it is thus possible to generate tempera¬ tures above 1200 C during approximately 5 sees. The rapid cooling properties from this temperature level are however not so good and risks for the synthesis of toxic compounds can of that reason not be excluded. Thermo-stable ^ compounds like dioxines can hardly be treated in this type of kiln. Another fact that must be regarded is the necessary react¬ ion time. In many situations mentioned the expression -fTSI c / * - OMPI residence time or in the best case mean residence time. Saying for instance that the actual process residence time is sufficient for a certain requirement may often be something very irrelevant. In most cases a residence time distribution of a certain shape is at hand and for the actual situation also a shortest residence time of some volume element of the flow. If for instance 1 per cent of the gas volume has a residence time shorter than the critic¬ al reaction time to bring about for instance a decomposition of 99.99 per cent, it is of very little value with a mean residence time 100 or even 1000 per cent in excess of the critical one; the integral decomposition will only be slight¬ ly over 99 per cent and not 99.99 per centI This fact is pointing -out the necessity to be able to work independently of the temperature in order to easily assimil¬ ate the critical reaction time and its process related guaranteed minimum reaction time of the system. » Finally can be said that existing techniques can not bring about a necessary temperature/residence time profile for actual decomposition requirements. The used plasma technique according to the invention is eliminating all the above mentioned weaknesses. The plasma burner or the plasma generator in which the passing gas - the carrier gas - can be heated to 3000-4000 °C or higher if want¬ ed, can depending of the properties of the waste and/or other process requirements be placed at different locations in the process. One solution can be to evaporate or disperse the waste into the carrier gas initially in a separate unit and using for this purpose well-known techniques. After that the carrier gas with its dispersed content of waste is introduced into the plasma generator and is overheated to a suitable tempera¬ ture without any combustion by the presence of a sufficient amount of air, oxygen enriched air or pure oxygen, after which the amount of oxygen necessary for the complete oxidative decomposition is added in the most suitable form immediately after the plasma flame in a separate reaction chamber. According to the process requirements different temperature patterns can be received. Alternatively it is also possible to add all the needed combustion air or oxygen already in the plasma. Another process variety can be the combustion of the waste by utilizing for instance an advanced flash technique or even with the help of some other less efficient technique in a separate unit and just using the plasma generator as a super¬ heater to reach the necessary temperature/reaction time of the actual decomposition. In connection with the treatment of solid waste the technique to inject the material together with a minimum portion of carrier gas ^ immediately after the plasma burner and into a special reaction chamber seems to be the most suitable pro¬ cess alternative. In all cases it is suitable to keep the combustion gases at a high temperature after the plasma burner and the connected reaction chamber during 1 - 10 sees, to ensure the decomposi¬ tion. This operation can preferably be performed in a com¬ bustion chamber formed as a brick-lined shaft and which can either be empty or contain some arrangement of a heat resist¬ ant filling in order to influence the flow pattern with the ultimate intention to achieve a true minimum residence time (reaction time) . With the knowledge of for instance the ability of certain metal compounds to accelerate the decomposition of organic compounds, suitable so called, decomposition catalysts can be mixed into the heat resistant filling. If - and this is frequently occuring - the treated waste is containing OMPI chlorine or other halogens as for instance luorine in so called freones, it may be very essential to bind the chlorine as for instance calcium chloride as much as possible. This is achieved in a simple way by adding calcium hydrate, lime or lime stone or other active neutralizing agents at a suitable process step. This energy consuming operation can in connection with a plasma process be placed within the process in a more free way than in connection with other techniques not being capable of generating the same energy density. Also if the problem to counteract the secondary formation of hazardeous organic substances at the cooling of the combustion gases is not primarely linked to the method of decomposition of the main waste by generating a high temperature, it is suitable to design an integrated -pro¬ cess with plasma superheating as the key operation. The rapid cooling of the combustion gases can be perform¬ ed either with a tubular cooler or in a scrubber aside from the decomposition unit. The degree of rapid cooling will determine the process design. The developed method according to the invention to decom¬ pose primarely so called hazardeous waste can be performed in a furnace that for instance can be designed in the manner as presented in the attached drawing. Also if the decomposition of very stable compounds that the invention is very well dopted to, is occuring at varying degrees in the whole temperature zone, the main portion of the decomposition may preferably be located to a reaction chamber (11) , into which primarely the very hot plasma gene¬ rated flame (12) emerges. Depending of the properties of the waste, part of or the whole amount of it can be added to the O PI reaction chamber through feed pipes (13) from the waste con¬ tainer (14) or in case of an eventual necessary pretreat ent over the corresponding equipment (15) . An efficient manner of preparing the waste for either the overheating/pyrolysis or for the combustion in the plasma unit and/or the reaction chamber is to evaporate it in an unit (16) in which the working part (17) can be either an atomizer or an efficient burner, both of them receiving their atomizing resp. combustion air from for instance a compressor (18) . Also if the far most number of compounds of the hazardeous waste have been decomposed to a sufficient degree by the very high temperature of the reaction chamber, due to the toxicity of the trace amounts it may be wise to further expose the exhaust gases to a high temperature - so high that a reformation of toxic substances will not occur. In order to achieve a fairly closed residence tinte distri¬ bution of the exhaust gases, it can be suitable to design the final combustion chamber (21) as an insulated shaft that preferably also is arranged with a heat resistant filling of high permeability. Outside the specific decomposition unit, comprising of the plasma unit (12, 16) , the reaction chamber (11) and the shaft (21) j the combustion gases shall be conducted into a device for rapid cooling. Much indicates the use of an efficient so called quasi-dry scrubber (31) with a neutraliz¬ ing slurry (32) as a coolant will be the best alternative to cool the exhaust gases enough rapidly .to avoid the reforma¬ tion of toxic or generally un-wanted substances. The neutralization product that has trapped the major part of the hydrochloric acid, hydrofluoric acid or other acidic products formed at the combustion, can partly be recirculat¬ ed after bleeding away a certain quantity. This regulation like the addition of make-up water and the slurry preparation are trivial operations and are marked according to their location in the flow sheet with the unit 34. As mentioned earlier in the presentation, the neutralizing agent can eventually be added already in the reaction chamber or immediately after it. It is primarely the temperature and the melting properties of the neutraliza¬ tion product that are governing this operation per- formance. OMPI

Claims: CLAIMS • 1. A method for thermal decomposition of stable sub¬ stances, preferably so called hazardeous waste, characteriz- ed in that the waste is given the necessary decomposition temperature in a plasma generated in a plasma burner (12) . 2. A method according to claim 1, characterized in that part of or the whole quantity of the waste is brought into the carrier gas of the plasma. 3. A method according to claims 1 or 2, characterized in that part of or the whole quantity of the waste is intro¬ duced into the plasma and mixed with its high temperature carrier gas. 4. A method accrding to claim 3, characterized in that the waste is preheated to give a consistency suitable for the decomposition before it is introduced into the plasma. 5. A method according to claim 4, characterized in that the pretreatment comprises a thermal treatment re¬ sulting in a partial decomposition. 6. A method according to any of claims 1 to 5, characterized in that part of the carrier gas of the plasma burner comprise . the recirculated process exhaust gas. 7. A method according to any of claims 1 to 6, characterized in that a neutralization agent, for instance calcined lime or calcium hydrate is added to neutralize hydrogen halogenids, usually hydrochloric acid formed in the process. 8. A method according to any of claims 1 to 7, characterized in that a combustion chamber (21) with the OMPI- ability to maintain the necessary temperature is used in order to guarantee a high decomposition degree by controlling the very important residence time distribut¬ ion of the process exhaust gases. 9. A method according to claim 8, characterized in that a part of or the whole combustion chamber (21) is occupied by a heat resistant filling in order to improve the residence time distribution of the exhaust gases. 10. A method according to any of claims 8 or 9, characterized in that a decomposition catalyzing substance or a mixture of several similar substances is applied on a heat resistant carrier in the combustion chamber (21) . 11. A method according to any of claims 8 to 10, characterized in that the exhaust gases to avoid the format¬ ion of toxic or generally un-wanted organic substances are rapidly cooled after the combustion chamber (21) . 12. A method according, to any of claims 8 to 11, characterized in that the content of hydrogen halogenids in the exhaust gases is reduced to a planned level by neu¬ tralizing with a suitable neutralization agent, preferably in connection with the rapid cooling. 13. An apparatus for thermal decomposition of stable substances, preferably so called hazardeous waste according to the method of claim 1, characterized in that it comprises a plasma burner (12) for the generation of a plasma with a reaction chamber for the mentioned decomposition. 14. An apparatus according to claim 13, characterized in that a reaction chamber is placed in close connection to the plasma burner. OMP • 15. An- apparatus according to claim 13, characterized in that a reaction chamber (11) is arranged to receive the process exhaust gases, before these are introduced into a combustion chamber (21) located after the reaction chamber. 5 16. An apparatus according to claim 15, characterized in that feeding devices (13) for waste are connected to the reaction chamber (11) . 0 17. An apparatus according to any of claims 14 to 16, characterized in that a pretreatment unit (16) with an atomizing unit or a burner (17) is connected to the plasma burner (12) . 1 18. An apparatus according to claim 17, characterized in that a further pretreatment unit (15) is connected in a shunt between a waste container (14) and the mentioned feed¬ ing device (13) . • ° 19. An apparatus according to any of claims 15 to 18, characterized in that the reaction chamber (11) ends in the brick-lined combustion chamber (21) shaped as a shaft. 20. An apparatus according to any of claims 15 to 19, 2 characterized in that the combustion chamber (21) is completely or partly filled with bodies of a heat resistant material with or without an application of decomposition catalyzing substances. 30 21. An apparatus according to any of claims 15 to 20, characterized in that the combustion chamber (21) ends into a primary cooling chamber (31) equipped with a feed device (32) for a coolant. 35 22. An apparatus according to claim 21, characterized in that a feed device (32) for a neutralizing agent is connected * to the cooling chamber (31) . OMPI 23. An apparatus according to any of claims 21 or 22, characterized in that the coolin chamber (311 is connected to a treatment unit (34) . O PI

Inventors: BJOERKLUND L, FAELDT I
Applicants: BJOERKLUND L, FAELDT I
Application year: 1981
Main classification: F23G7
IPC codes: C10B19, C10B53, F23G5, F23G7, F27B1