FR2732008A1 - PROCESS FOR TREATING LIQUID SLAG WHICH OCCURS DURING THE COMBUSTION OR GASIFICATION OF WASTE - Google Patents

Abstract

In a process for the thermal treatment of waste containing combustible constituents, liquid slag is formed, directly or during the combustion or gasification of a gaseous effluent. At the same time as the liquid slag is formed a hot flue gas with a temperature between 1200 and 1800 deg.C. The hot gas is cooled and dedusted, and chlorides and sulfur compounds are removed at least in part by means of at least one washing liquid. A mixture of dehydrated solid matter forms which contains chlorides and sulfur compounds. The liquid slag is mixed in a mixing zone with the dehydrated solid mixture which contains chlorides and sulfur compounds, at temperatures between 1200 and 1800 deg.C. Liquid iron oxide slag which has a CaO: SiO2 molar ratio of 0.4 to 1.2 and an FeO content of 5 to 20% by weight is removed from the mixing zone, which is then carried out. cools and solidifies. This slag is environmentally neutral.

Description

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  The invention relates to a process for the thermal treatment of waste containing combustible constituents, liquid slag and a hot smoke gas created at a temperature. between 1200 and 1800 C, directly during the heat treatment or during combustion or even during the gasification of a gaseous effluent formed during the heat treatment and containing combustible constituents, the hot smoke gas being cooled and dedusted and chlorides and sulfur compounds being removed at least in part from the flue gas by direct contact with at least one washing liquid and a suspension, which contains chlorides and sulfur compounds and which are dehydrated to a mixture of solid matter,

being formed.

  i 5 It is described in DE-A42 35 412 and DE-A-44 12 004 the thermal treatment of waste in a circulating fluidized bed, a hot smoke gas, which is burned or gasified forming liquid slag, being form. During combustion, we ensure a supply of hyper-stoichiometric oxygen, while, during gasification, we operate with an almost hypo-stoichiometric supply of oxygen. In the process according to DE-A-44 24 583 and 44 24 707, the waste is charged in a slag bath, in a reactor, it is also possible in this case to operate with an oxygen supply which can be a little hypo-stoichiometric and which can go so far as to be

  stoichiometric, or also with a hyper- oxygen supply

  stoichiometric. The slag bath consists of 50 to 100% by weight of waste residues and a hot smoke gas is formed which is removed from the

slag bath reactor.

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  In the mentioned gasification or waste combustion processes, the hot flue gas contains chlorine compounds, fluorine compounds and sulfur compounds. During the purification of the flue gas, these substances appear in dry form, and also during the necessary purification in the wet, in suspensions or in sludge. The invention aims to fix the polluting substances which are formed during the purification of the gas in a liquid slag also formed. It must be formed from the slag which, after solidification, is so environmentally neutral that it can be reused or also landfilled without difficulty. This is achieved according to the invention, in the process described at the beginning of this specification, in that in a mixing zone, the liquid slag is mixed at temperatures of 1200 to 1500 ° C. with the dehydrated mixture of solid substances containing chlorides and sulfur-containing compounds and which are removed from the mixing zone of the liquid slag with an iron oxide content which has a CaO: SiO2 molar ratio of 0.4 to 1.2 and an FeO content of 5 to% by weight, which is cooled and solidified. Attempts to elute slag having these criteria have given that it can be considered practically as

  environment neutral.

  Preferably, the heat treatment of the waste is carried out by gasification or by combustion in a fluidized bed reactor and the gaseous effluent which is formed therein is burned or gasified in a decomposition reactor at temperatures between 1200 and 1800 C. with

  formation of liquid slag and hot smoke gas.

  Preferably, the waste is put for heat treatment in a slag bath, the slag consisting of 50 to 100% by weight of waste residues and having temperatures between 1200 and 1800 C, and the gas is removed hot smoke formed above the slag bath at temperatures between 1200 and 1800 C. Preferably, it is blown into the liquid slag, in the zone of

  mixture, combustible gas and gas with 02 content.

  Preferably, the cooled flue gas is passed first through a wet wash to remove chlorides and fluorides and then through a gas wash to remove sulfur compounds using a

washing solution.

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  Waste suitable for the process is, for example, municipal or industrial waste or also sludge from waste. If the iron content

  waste is too small, iron content waste can be added.

  Likewise, the calcium or SiO2 content can be adjusted in particular during the formation of the liquid slag.

  If the waste is treated in the slag bath reactor, it is formed from both liquid slag and a hot smoke gas at temperatures between 1200 and 1800 C. This smoke gas is cooled and dusted off and , in order to remove chlorides, fluorides and sulfur-containing compounds, io subjected to at least one wet washing. A mixture with a content of polluting substances is thus formed which is fixed in the slag. To this end, the liquid slag is continuously or discontinuously removed from the slag bath reactor and it is loaded into a mixing zone to which the mixture of solids with pollutant content from the purification is also supplied. smoke gas. It is then ensured, in the manner already mentioned, that it is formed from the liquid slag which has a CaO: SiO2 molar ratio of 0.4 to 1.2 and an FeO content of 5 to 20% by weight. Preferably, the CaO: SiO2 molar ratio is between 0.4 and 0.8 and the FeO content is at least 10% by weight. Generally, ashes from gasification or refuse combustion contain relatively a lot of SiO2, so that the basicity is usually much less than 0.4. In the slag from such ash, the pollutants from the purification of the flue gas are only insufficiently fixed. The FeO content should not be too large, especially because otherwise the fixation of polluting substances is also made bad and because the aggressiveness of the slag vis-à-vis a masonry

refractory increases.

  Other possibilities of carrying out the process are explained with the aid of the drawing. The drawing is a flow diagram of the process, the waste

  being heat treated in a circulating fluidized bed.

  It is sent to a reactor (1) with a fluidized bed, by the pipe (2), of waste, and by the pipe (3) of gas with 02 content, that is to say air, air enriched with oxygen or technically pure oxygen. If necessary, gas with oxygen content can also be sent to the reactor (1) through the secondary inlet (3a). At temperatures between

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  750 and 1000 C, the waste burns or is gasified in the reactor (1) and a gaseous effluent with a solids content is sent by the sleeve (5) to the cyclone (6). Separate solids are returned at least

  partially through the conduit (7) to the lower region of the reactor (1).

  We remove ash from the sleeve (8) which leaves from the grid (9) and which goes down to the duct (10). In a screening plant (11), coarse grain is separated, which is evacuated via the conduit (12), from fines, which are sent via the conduit (13) to an optionally grinding installation (14).

  necessary which can also be additionally coupled to a magnetic separation.

  Fine-grained ash, which still has a carbon content, is supplied on the transport path (15) to a decomposition reactor (16). The dusty gaseous effluent which leaves the cyclone (6) is also sent to this decomposition reactor via the conduit (17). It is also possible to derive part of the solid substances from the conduit (17) to the conduit (7a) and to pass these

  solid substances also in the reactor (16).

  Gas with oxygen content, in particular air enriched with oxygen or technically pure oxygen, is sent to the decomposition reactor (16) through the pipe (19). It is possible to dose the supply of oxygen to the reactor (16) so as to comply either with gasification conditions, that is to say a hypo-stoichiometric operation, or with combustion conditions for a supply of O2 hyper-stoichiometric. In each case, temperatures between 1200 and 1800 C are ensured in the reactor (16), so that it is formed from liquid slag. Fuel can be sent to the reactor (16) via the line (20)

  additional and / or aggregates which are desired in the slag formed.

  These aggregates can be, for example, calcium or iron compounds.

  A hot smoke gas is formed in the decomposition reactor (16) which contains dust and polluting substances, in particular chlorides, fluorides and sulfur compounds. This hot smoke gas is sent first through the conduit (22) to cool it in a recovery boiler (23) in which also part of the dust is separated. The cooled flue gas is then sent to continue its purification in different types of wet washes. In the drawing, there is first shown a wash comprising water in the column (24), into which the gas enters through the conduit (25). Fresh water is brought

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  in the conduit (26), mixed with a liquid, which comes from the conduit (27) and which circulates in a closed circuit, and projected into the column (24). Washing with water is used in known manner in particular for removing chlorides and fluorides from the flue gas. A pollutant-containing liquid which forms at the bottom of the column (24) is drawn off through the pipe (28), part of which is returned through the pipe (27). The remaining liquid reaches a dewatering installation (30). It is first formed there in a known manner which is not illustrated a thickened sludge which is then dehydrated for example in a filter press. Dehydrated solid substances are discharged through the line (31). Unlike what is shown in the drawing, washing with

  water can also be arranged in several stages.

  The smoke gas, which leaves the washing column (24) via the conduit (34), then reaches the desulfurization column (35). A washing solution, which contains for example calcium hydroxide, is sprayed on it. Spent washing solution is partly returned through the pipe (36), fresh washing solution comes from the pipe (37) and purified gas is drawn off through the pipe (39). The washing solution drawn off from the bottom of the column (35) through the conduit (38) partly reaches the dewatering installation (40), dehydrated solid substances also forming in this case in the conduit (41). Unlike what is shown in the drawing, desulfurization can also

take place over several floors.

  In the mixing tank (44), liquid slag is produced which, cooled and solidified, is environmentally neutral. In this tank, the liquid slag which forms in the decomposition reactor (16) is sent through the conduit (18) and, in addition to this, the dust which forms

  in the recovery boiler (23) and which is brought through the conduit (29).

  In addition, the dehydrated solid substances from the conduits (31) and (41) are sent to the tank (44). The tank (44) can be heated in a known manner by

  which is not shown, in order to maintain a bath (45) of liquid slag.

  The slag is moved into the bath to be mixed, which can be done by sending gas through the lance (46). The gas may for example be an inert gas (for example nitrogen) or else combustible gas is sent together with oxygen, the combustion also maintaining the slag bath (45) at the desired temperature of between 1200 and 1500 C and most often at most 1400 C. It is withdrawn from the tank (44) by the conduit (48) of

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  the liquid slag which is cooled in a manner known per se which is not shown, for example by sending it to a water bath. Gaseous effluent leaves the tank (44) through the conduit (47) and can be mixed with

  which is not shown in the flue gas from the duct (22).

  The slag from the conduit (48) has a CaO: SiO2 molar ratio of 0.4 to 1.2 and preferably at most 0.8. Its FeO content is between 5 and 20% by weight, and preferably close to at least 10% by weight. The slag usually contains 0.5 to 2% by weight of chlorine, 1 to 3% by weight of fluorine and 0.5 to 3% by weight of sulfur. In order to detect whether the slag can be reused or also landfilled without problems, the cooled, granulated slag is subjected to an elution test. 100 g of slag granulate are stirred in 1 l of distilled water contained in a glass beaker with a capacity of 2 l, at a temperature of 20 C, for 24 hours. The water then separated, filtered usually has the following data: - electrical conductivity: 50 to 500 mS (10,000 mS being accepted according to the German industrial standard), - pH value: 8 to 10 (5.5 to 13 being allowed according to German industrial standard), - fluorine content: 0.08 to 0.6 mg / I (5 mg / I being accepted according to German industrial standard), - residue of solid substance after boiling of water: <1 g (3g being

  admitted at most according to German industrial standard).

Example

  In an installation as shown in the drawing, but without the duct (7a), 7500 kg of gas per hour of sorted household waste is gasified. The features of the example have been partially calculated. The garbage has the following composition: C 25.8% by weight H 3.3% by weight O 15.1% by weight S 0.2% by weight Cl 0.5% by weight F 0.1% by weight ash SiO2 content 26.0% by weight moisture 29.0% by weight

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  By magnetic separation together with the grinding installation (14), the iron content of the ash is adjusted so as to obtain an FeO content of the slag from the conduit (48) of 12% by weight. Gasification in the fluidized bed reactor (1) is carried out at 900 ° C., the temperature in the decomposition reactor (16) is approximately 1450 C. It is introduced into the reactor (16), via the conduit (20) , 195 kg / h of CaO, technically pure oxygen is sent via line (19). We send a detergent of Ca (OH) 2 to the

column (35).

  We supply to the tank (44), through the conduit (18), 2150 kg / h of slag at 1400 io C, it arrives through the conduit (29) 50 kg / h of dust, we send through the conduit (31) dust and 68 kg / h of chlorides and, via line (41) 80 kg / h of sulfur compounds. One sends by the lance (46) 100 m3 under normal conditions of gas coming from the conduit (39) and 20 m3 of oxygen under the normal conditions, so that it is established in the gaseous effluent of the conduit (47 ) a CO / CO2 ratio of 2. Under these conditions, the formation of metallic iron

in the tank (44) is prevented.

  The cooled slag, which comes from the conduit (48) has the following characteristic data: molar ratio CaO: SiO2 = 0.6 FeO content 12.0% by weight CL content 1.7% by weight F content 0.5 % by weight S content 0.7% by weight The elution test described above gives an electrical conductivity of the filtered water of 230 mS and a solid residue after boiling of

0.6 g water.

Claims (4)

  1. A method of heat treatment of waste containing combustible constituents, liquid slag and a hot smoke gas created at a temperature between 1200 and 1800 C, directly during heat treatment or during combustion or during gasification of a gaseous effluent formed during heat treatment and containing combustible constituents, the hot smoke gas being cooled and dusted off and chlorides and sulfur compounds being eliminated at least in part from the smoke gas by direct contact with at least a washing liquid and a suspension, which contains chlorides and sulfur compounds and which are dehydrated to a mixture of solid material, being formed, characterized in that, in a mixing zone, mixing is carried out at temperatures of 1200 at 1500 C the liquid slag in the dehydrated mixture of solid substances containing chlorides and sulfur compounds and which is removed from e the mixing zone of the liquid slag with an iron oxide content which has a CaO: SiO2 molar ratio of 0.4 to 1.2 and an FeO content of 5 to   % by weight, which is cooled and solidified.   3. Method according to claim 1, characterized in that the thermal treatment of the waste is carried out by gasification or by combustion in a fluidized bed reactor and the gaseous effluent which is formed therein is burned or gasified in a decomposition reactor. temperatures between 1200   and 1800 C, with formation of liquid slag and hot smoke gas.   4. Method according to claim 1 or one of claims   following, characterized in that the liquid slag is blown into the   mixing zone, combustible gas and gas with 02 content. 9 2732008   5. Method according to claim 1 or one of claims   following, characterized in that the cooled flue gas is passed first in a wet washing to remove chlorides and fluorides and then in a gas washing to remove sulfur compounds by means of a solution of washing.