METHOD FOR CLEANING FLUE GAS FORMED ON REFUSE INCINERATION

Abstract: For cleaning flue gas formed on the incineration of refuse, for example domestic refuse, and containing particulate impurities; gaseous organic impurities, such as aromatic chlorine compounds; gaseous inorganic and acidifying impurities, e.g. hydrogen chloride, sulphur dioxide, and nitrogen oxides; and gaseous trace elements, such as heavy metals, the gas is cleaned of its particulate impurities in a separation step (52). Then, the flue gas is cleaned of its gaseous inorganic and acidifying impurities in at least one gas washing step (54a, 54b). Before the separation step (52), the flue gas is brought into contact with one or more substances reacting with the gaseous trace elements and supplied in the form of a dry powder or fine liquid droplets, and/or an adsorption material reacting with the gaseous organic impurities, for transforming the trace elements and the last-mentioned impurities into particulate impurities which are separated from the flue gas in the above-mentioned separation step. The particulate impurities separated in the separation step (2; 22; 52) are heated to such a high temperature that substantially all the organic impurities are decomposed, and that most of the trace elements are evaporated, while the remaining particulate impurities are transformed into vitreous sand by melting.

Description: METHOD FOR CLEANING FLUE GAS FORMED ON REFUSE INCINERATION Technical area The present invention relates to a method for cleaning flue gas formed on incineration of refuse, for example domestic refuse, and containing particulate impurities, such as fly ash; gaseous organic impurities, inter alia aromatic chlorine compounds; gaseous trace ele¬ ments, for instance heavy metals; and acidifying impurities, such as nitrogen oxides, hydrogen chloride, and sulphur dioxide. The flue gas is cleaned in at least two steps. First, the parti¬ culate impurities are separated in a separation step, and later, the acidifying impurities are removed from the flue gas in a gas washing step. However, before the separation step the flue gas is brought into contact with one or more substances reacting with the gaseous trace elements and supplied in the form of a dry powder or fine liquid droplets, and/or an adsorption material reacting with the gaseous organic impurities, for transforming the trace elements and/or the last-mentioned impurities into particulate impurities which are separated from the flue gas in the above-mentioned separation step. Background of the invention In the cleaning of flue gas, solid, liquid or gaseous impurities are separated from the flue gas in order to reduce the negative effect of the gas on the environment. The separation of solid particles and droplets is, for instance, carried out with the aid of inertial forces or electrostatic fields, or by utilising the screening effect of barrier filters. Gaseous impurities are separated by being absorbed by or dissolved in additives, or by being adsorbed to solid surfaces. Other possible methods include converting the gaseous impurities into liquid or particle form by means of additives or by cooling, or by decomposing the impurities into harmless components. Frequently, several methods are combined. In one of the most common methods for cleaning flue gas formed on refuse incineration, particles are separated in an electrostatic precipitator, whereupon lime in the form of particles or a suspension in water is added, and the resulting reaction products, together with unreacted lime, is separated in a textile barrier filter (see, for example, US 4,975,366). DE 3320466 and DE 3324 133 disclose another example of a method combining several cleaning techniques. In this method, the main portion of the particulate impurities, containing most of the heavy metals, is first separated from the flue gas, whereupon hydrogen chloride and mercury are separated in a wet washing step in which the washing liquid is acid, i.e. has a low pH value. Finally, sulphur oxides and nitrogen oxides are washed away in a wet washing step in which the washing liquid is alkaline, i.e. has a high pH value. Technical problems As a rule, flue gas cleaning implies removing from the flue gas an element injurious to the environment. However, the environmental hazard frequently remains, since the contamination only is transformed from the gaseous phase to the liquid or solid phase and instead pollutes the soil, lakes, water courses, or ground water. Regrettably, this frequently means that considerable amounts of unusable waste products have to be kept in safe storage for all time. Therefore, the problem is to provide methods which reduce, as far as possible, the total strain on the environment, and methods which result in small amounts of waste products while permitting a high degree of reuse of the substances in the refuse, which accompany the flue gas formed on refuse incineration. It is an advanced technical problem not only to separate the aromatic chlorine compounds, such as dioxins, formed on the incine- ration, but also to render these totally innocuous by decomposing them into harmless substances. Furthermore, it is an advanced technical problem to create possi¬ bilities for an efficient separation of heavy metals in one separation step which is primarily intended for fly ash, as well as to recover, in the same method and in a simple manner, the main portion of the heavy metals from the fly ash and to convert the remainder into a highly stable waste product which, without any risks whatsoever, can be kept in safe storage. Finally, it is also an advanced technical problem to create such conditions that acid inorganic impurities can be washed from flue gas formed on refuse incineration, and that a substantially closed liquid flow can be obtained and all the resulting by-products recycled or re- used. Description of the invention Solution To efficiently reduce the strain on the environment caused by the incineration of refuse, the present invention provides a method for cleaning flue gas in several steps. First, the flue gas which contain both particulate and gaseous impurities is brought into contact with one or more substances reacting with the gaseous trace elements and/or gaseous organic impurities, such that the gaseous impurities are transformed into particles. The addi¬ tives may be suspended or dissolved in a liquid, e.g. water, but they may also be dry particles. They may also be activated carbon or zeolites adapted to bind impurities on their surface by adsorption. The impurities transformed in this fashion may be mercury, lead or aromatic chlorine compounds, such as chlorobenzenes. Then, the fly ash and the impurities transformed into particles are separated in a dust separator which preferably consists of a textile barrier filter which has excellent separation ability, also for very fine-grained particles. Suitably, the flue gas cleaned of particulate impurities in this fashion is then conducted to a device where nitrogen oxides are reduced and decomposed into nitrogen and water vapour, and thus are rendered completely harmless. From this device, the gas is conducted to a gas washing device, preferably comprising two part steps, where the gas is brought into contact with a liquid which absorbs the acidifying impurities. In the first part washing step, the liquid has a low pH value, which means that the main portion of the hydrogen chloride is separated, while the sulphur dioxide remains unchanged. In the second part step, the washing liquid is neutral or basic, entailing a highly efficient separation of the sulphur dioxide. Sodium hydroxide is a suitable absorbent. The flue gas thus cleaned can be discharged into the atmosphere without having any real negative effects on the environment. According to the method according to the present invention, the fly ash and other particles separated in the dust separator are heated to such a high temperature that all the organic component parts are decomposed, so that most of the separated heavy metals depart in gaseous state and the ash melts into vitreous sand. The gas resulting from this process is cooled very rapidly, such that there is not time for any new formation of dioxins. Suitably, the cooling is carried out by an admixture of cold air, whereupon the heavy metals are separated in one or more steps so as to be recovered. As need be, this is done by use of additives admixed to the departing gas before or after the cooling, optionally in several steps. The main contaminant of the gas departing from the heated ash is, after these steps, hydrogen chloride. Therefore, the gas is recycled to the flue gas from the incinerator, before the first separation step. The hydrogen chloride will be separated in the previously described first washing step. Only vitreous sand and reusable residues with a high concentration of heavy metals remain after the particle cleaning step. The washing liquid used in the first part washing step is enriched with sodium chloride and can be efficiently cleaned by electrolysis, whereby products which can be recycled to the process and/or be used in the chemical industry are obtained. The washing liquid used in the second part washing step is mainly enriched with sodium sulphate. After evaporation, a solid residue, Glauber's salt which has a market in the chemical industry, is obtained. Advantages The present invention provides a method which makes it possible to clean the flue gas formed on the incineration of refuse, e.g. domestic refuse, at moderate costs and with a very slight strain on the environ¬ ment. The main portion of the impurities normally discharged into the atmosphere on this kind of incineration are separated in such a manner that they can be reused. The formed dioxins are effectively destroyed, and the remaining fly ash chiefly contains harmless oxides. Any trace elements and heavy metals that may remain in the fly ash are entrapped in fused vitrified oxides and have excellent resistance against leaching. Previous methods (see, for example, DE 33 20466 and DE 33 24 133) do not provide a satisfactory overall solution, although a method for recovering part of the heavy metals is disclosed. Description of the embodiments The invention will be described in more detail below, reference being had to the accompanying drawings. Figs. 1-3 illustrate schematic embodiments of the method according to the present invention. The embodiment shown in Fig. 1 comprises a pretreatment step 1, a separation step 2, and a gas washing step 3 downstream of a conven¬ tional incineration plant (not shown) for burning refuse, e.g. domestic refuse. The flue gas formed on the incineration and containing parti¬ culate impurities, so-called fly ash; gaseous organic impurities, such as aromatic chlorine compounds; gaseous acidifying impurities, e.g. nitrogen oxides, hydrogen chloride, and sulphur dioxide; and gaseous heavy metals, is first introduced into the pretreatment step 1 via a conduit 4. In this step which comprises for example a contact reactor of the type disclosed in European Patent Application 86850134-7, the flue gas is brought into contact with one or more metal sulphides, preferably sodium sulphide, introduced into the pretreatment step in the form of a dry powder or fine liquid droplets, via a conduit 5. The metal sulphides will react with the gaseous heavy metals of the flue gas which are thus transformed into particles. In order to transform also the aromatic chlorine compounds of the flue gas into separable particles, activated carbon or zeolite serving as adsorption material for these chlorine compounds are introduced into the pretreatment step 1 via a conduit 6. Together with unreacted metal sulphides, unreacted adsorption ma¬ terial, and the fly ash of the flue gas, the particles formed in the 5 pretreatment step 1 are carried along by the flue gas, via a conduit 7, to the separation step 2 which may be a textile barrier filter. The heavier of the above-mentioned particulate impurities are not carried along by the flue gas, but fall to the bottom of the contact reactor, from where they are removed, via a conduit 8, to a storage container (not shown). In the separation step 2, the particulate impurities in the flue gas are separated, such that the flue gas, after the separation step, chiefly contains acidifying impurities, i.e. nitrogen oxides, hydrogen chloride, and sulphur dioxide. The particulate impurities separated in the separation step are then conducted, via a conduit 9, to the above- mentioned storage container. From the separation step, the flue gas is taken, via a conduit 10, to the gas washing step 3 which may be in the form of a wet scrubber. In the gas washing step, sodium hydroxide in the form of an aqueous solution is admixed to the flue gas, via a conduit 11. The sodium hy¬ droxide will react with the hydrogen chloride and the sulphur dioxide of the flue gas while forming sodium chloride, as well as sodium sulphite and sodium sulphate. Since these compounds are water-soluble, the aqueous solution leaving the gas washing step 3 via a conduit 12 will contain sodium chloride, sodium sulphite and sodium sulphate in dissolved form. The flue gas cleaned of particulate and gaseous impuri¬ ties is conducted, via a conduit 13, to a chimney (not shown) to be discharged into the atmosphere. The conduit 12 terminates in a neutralisation step 14 in which finely ground limestone or calcium hydroxide in the form of a dry lime powder or lime particles dissolved or suspended in water is admixed to the aqueous solution via a conduit 15. Thus, the sodium sulphite and sodium sulphate of the aqueous solution are precipitated as calcium sulphite and calcium sulphate which are separated from the aqueous solution in a subsequent filtration step 16 which may comprise a conventional water filter. From the filtration step, the calcium sulphite and calcium sulphate are conducted, via a conduit 17, to a storage container (not shown) and may, for example, be used for the production of gypsum. The aqueous solution now mainly containing sodium chloride is conducted from the filtration step, via a conduit 18, to a purification plant (not shown), for example an electrolysis plant, and is thereafter recycled to the gas washing step 3. The embodiment shown in Fig. 2 comprises, like the embodiment shown in Fig. 1, a pretreatment step 21, a separation step 22, a gas washing step 23, a neutralisation step 24, and a filtration step 25, as well as a number of conduits 26-39 for the supply or removal of flue gas, reaction material, reaction products, etc. Furthermore, this embodiment comprises a reduction step 40 adapted to reduce the nitrogen oxides in the flue gas and positioned between the separation step 22 and the gas washing step 23. The reduction step comprises a selective catalytic reactor, for example of the type described in SE 8700117-8. The reducing agent is ammonia which is supplied to the reduction step via a conduit 41. Thus, the nitrogen oxides are transformed into nitrogen gas and water vapour presenting no danger to the environment. The embodiment shown in Fig. 3, like that shown in Fig. 2, has a pretreatment step 51, a separation step 52, a reduction step 53, and a gas washing step 54a, 54b, as well as a number of conduits 55-69 for the supply and removal of flue gas, reaction material, reaction products, etc. Unlike the previously described embodiments, this embodiment does not comprise conducting the particulate impurities removed from the pretreatment step and separation step to a storage container, but to a heating step 70. As described above, these particulate impurities are fly ash, the aromatic chlorine compounds, such as dioxins, and the gaseous heavy metals which have been trans¬ formed in the pretreatment step to particles, and the unreacted metal sulphides and activated carbon added to this step. The heating step may comprise an electrically heated melting furnace in which the particulate impurities are heated to such a high temperature (about 1300°C) that the aromatic chlorine compounds are decomposed and transformed into carbon dioxide, water vapour and hydrogen chloride; that most of the separated heavy metals depart in gaseous state; and that the fly ash melts into vitreous sand. The above-mentioned, unreacted metal sulphides are decomposed, and their metals depart in gaseous state, while the sulphur is transformed into sulphur dioxide. The activated carbon is oxidised to carbon dioxide. The vitreous sand is removed from the heating step 70 via a conduit 71 and collected in a container (not shown) and may, for example, be used as road construction material. The gas formed on the heating is conducted, via a conduit 72, to a cooling step 73 where it is rapidly cooled to about 200°C or lower by the admixture of cold air so as to prevent any new formation of dioxins. By this cooling most of the heavy metals, in particular lead, cadmium, zinc and antimony, pass on from the gaseous phase into the solid phase and may then be se- parated from the gas in a subsequent separation step 74. Thus, the dust separated from the gas in the separation step 74 has a high content of reusable heavy metals and is removed from said step 74 and collected in a container (not shown) via a conduit 75. Then, the gas is conducted, via a conduit 76, to an aftertreatment step 77 in which it is brought into contact with metal sulphides for the transformation into particles of the heavy metals, mostly mercury, which still are in the gaseous phase. This step may comprise a textile barrier filter, such as a bag filter, coated with metal sulphides. The heavy metal sulphides removed from the gas in this step are conducted, via a conduit 78, to a storage container (not shown). After these treatment steps, the gas thus chiefly contains gaseous and acidifying impurities, in particular hydrogen chloride, and is therefore recycled, via a conduit 79, to the flue gas upstream of the separator step 52. Another important difference between the embodiments in Figs. 1 and 2 and that in Fig. 3, is that the gas washing step in Fig. 3 comprises two part steps 54a and 54b. In the first part step 54a, the flue gas is washed with an acid washing liquid containing sodium hydroxide and having a pH below 2, which entails that the main portion of the hydrogen chloride of the flue gas is separated in the form of sodium chloride. The sulphur dioxide, which is not affected by the first part step, is instead separated in the second part step 54b where the flue gas is washed with a neutral or basic washing liquid also containing sodium hydroxide. Thus, the sulphur dioxide is mainly transformed into sodium sulphate. The sodium chloride solution formed in the part step 54a is conducted, via the conduit 65, to an electrolysis plant 80 where it is transformed into sodium hydroxide and chlorine gas by means of electro- lysis. The sodium hydroxide is recycled to the washing steps via a conduit 81, while the chlorine gas is conducted, via a conduit 82, to a storage container (not shown). The gas may, for instance, be reused in the paper or plastic industry. The sodfum sulphate solution formed in the part step 54b is transformed, in an evaporation step 83, into Glauber's salt and water vapour which are removed from this step via conduits 84 and 85, re¬ spectively. Glauber's salt may, for example, be used in the chemical industry. It goes without saying that the invention is not restricted to the embodiments described above and may be modified in various ways within the scope of the appended claims.

Claims: CLAIMS 1. A method for cleaning flue gas formed on incineration of refuse, for example domestic refuse, and containing particulate i - purities; gaseous organic impurities, such as aromatic chlorine com¬ pounds; gaseous inorganic and acidifying impurities, e.g. hydrogen chloride, sulphur dioxide, and nitrogen oxides; and gaseous trace elements, such as heavy metals, the flue gas being cleaned of its particulate impurities in a separation step (2; 22; 52), whereupon it 10 is cleaned of its gaseous inorganic and acidifying impurities in at least one gas washing step (3; 23; 54a, 54b), the flue gas before the separation step (2; 22; 52) being brought into contact with one or more substances reacting with the gaseous trace elements and supplied in the form of a dry powder or fine liquid droplets, and/or an adsorption 15 material reacting with the gaseous organic impurities, for transforming the trace elements and/or the last-mentioned impurities into particulate impurities which are separated from the flue gas in the above-mentioned separation step, c h a r a c t e r i s e d in that the particulate impurities separated in the separation step (2; 2022; 52) are heated to such a high temperature that substantially all the organic impurities are decomposed, and that most of the trace ele¬ ments are evaporated, while the remaining particulate impurities are transformed into vitreous sand by melting. 2. A method as claimed in claim 1, c h a r a c t e r i s e d in 25 that the substance or substances reacting with the trace elements are one or more metal sulphides, preferably sodium sulphide. 3. A method as claimed in claim 1 or 2, c h a r a c t e r ¬ i s e d in that the adsorption material is activated carbon or zeo¬ lite. 30 4. A method as claimed in any one of claims 1-3, c h a r a c ¬ t e i s e d in that the heating is carried out to a temperature above 1100°C, preferably above 1300°C. 5. A method as claimed in claim 4, c h a a c t e i s e d in that the gas formed on said heating is rapidly cooled to a temperature 35 below 300°C, preferably 200°C or lower, by supplying cold air. 6. A method as claimed in claim 5, c h a r a c t e r i s e d in that one or more of the substances reacting with said gaseous trace elements are admixed to said gas before and/or after the cooling for the transformation of the trace elements into particulate impurities. 7. A method as claimed in claim 6, c h a r a c t e r i s e d in that said gas, after a further separation of particulate impurities, is admixed to the flue gas generated by the refuse incineration. 8. A method as claimed in claim 5 or 6, c h a r a c t e r ¬ i s e d in that the trace elements are separated in a number of selective steps. 9. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the flue gas, between the separation step (2; 22; 52) and the gas washing step (3; 23; 54a, 54b), is con¬ ducted through a selective catalytic reactor (40; 53) for the reduction of nitrogen oxides. 10. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the gas washing step comprises two part steps (54a, 54b), an acid washing liquid preferably having a pH below 2 being conducted through the first part step, and a neutral or basic washing liquid being conducted through the second part step. 11. A method as claimed in claim 10, c h a r a c t e r i s e d in that the washing liquid from the first gas washing part step (54a) is cleaned by electrolysis. 12. A method as claimed in claim 10, c h a r a c t e r i s e d in that the washing liquid from the second gas washing part step (54b) is cleaned by evaporation or precipitation.

Inventors: CARLSSON KURT, CARLSSON, KURT
Applicants: FLAEKT AB, FLAEKT AKTIEBOLAG
Application year: 1990
Main classification: B09B9
IPC codes: B01D53, B09B3