A METHOD FOR INCINERATION OF REFUSE

Abstract: The emission of acid-forming gases and/or of dioxins from a refuse incineration plant is reduced and at least part of or substantially all the fly ash formed by the incineration of refuse is converted into slag, when the refuse incineration is carried out by introducing the refuse to be incinerated into the incinerator(s) of the incineration plant together with fly ash produced by incineration in the plant, which fly ash may optionally be supplemented with fly ash from other sources than the plant in question, and one or more emission-reducing agents, for example bases, whereupon this mixture is exposed to the incineration conditions prevalent in the incinerator(s). An incineration plant for carrying out the method comprises means for adding fly ash and an emission-reducing agent to the material to be incinerated.

Description: i A METHOD FOR INCINERATION OF REFUSE The present invention relates to a method for reducing the emission of acid-forming gases from a combustion plant with simultaneous conversion of at least part of or substantially all the fly ash formed by the combustion process in the combustion plant into slag, and a combustion plant system for use in the method. The disposal of the rapidly increasing amounts of refuse generated all over the world, and the environmental problems associated with this, is a theme which is the subject of comprehensive discussion and which is associated with extensive legislation in many countries. Refuse is conventionally disposed of by incineration, leading to the formation of both solid, liquid and gaseous combustion products of a more or less harmful nature, and these combustion products themselves also have to be disposed of. The combustion products which are at present considered most harmful and difficult to handle are fly ash, which is normally defined as the combustion residues present in the flue gas and which is collected in a filter system connected to the chimney of the combustion plant, and acidic compounds, for example sulphur oxides, nitrogen oxides, hydrogen fluoride and hydrogen chloride, present in the flue gas. Especially in connection with the production of heat and/or electricity in which coal combustion is employed, a large number of processes for the removal of sulphur oxides and other emissions from flue gases have been developed. These processes generally comprise bringing the flue gas into contact with lime, which reacts with the acidic compounds in the flue gas, whereby they are rendered harmless. Similar methods for the purification of flue gas from refuse combustion plants have not yet been developed to the same extent. The fly ash formed in connection with refuse combustion has proved difficult to dispose of in a satisfactory manner. This fly ash has chemical composition which is different from and is more alien to the environment than the fly ash formed in connection with coal combustion, and it is difficult to deposit in the environment, either directly or when used as a filling material in connection with, for example, land reclamation projects and in the cement and concrete industry, which is a frequently employed use of fly ash formed in connection with power plant operation, for example using coal combustion. Most types of combustion lead to the formation of a broad spectrum of combustion residues or by-products which may usually be classified within the categories slag, fly ash and flue gas, in which categories various salts and/or other chemical compounds or materials which may have been formed or modified during the combustion process, or which may have been unaffected by the combustion process, and which may be solid, liquid or gaseous, may be present. The nature of the by¬ products formed depends, of course, on the nature of the material subjected to combustion, as well as on the conditions under which the combustion has taken place. The by-products formed are typically removed from the combustion plant without any significant further treatment thereof (apart from, for example, cooling and filtering of the flue gas and cooling of the slag, with optional removal therefrom of iron and other magnetic materials at the same time) . Slag, which normally constitutes the largest proportion of the by-products formed during the combustion process, may, as described in the present specification, be used directly in many cases, for example in connection with road construction, whereas the use and disposal of the fly ash formed and of the environmentally harmful components of the flue gas, which comprise, inter alia, acid-forming gases, especially sulphur and nitrogen oxides, and hydrogen chloride and hydrogen fluoride, may be associated with considerable difficulties and thus costs. Generally, and in the present context, slag is defined as the solid combustion residue remaining after cooling, and it typically consists of mainly inorganic compounds, such as quartz, feldspar, calcium sulphate, and other inorganic compounds which are not decomposable under the combustion conditions prevailing in the kiln. In many cases the slag will contain heavy metals, for example lead, cadmium and mercury, which have been released from the combusted material in connection with the combustion process. Certain very resistant organic materials may also be present in the slag. This applies to, for example, tars, graphite, bitumen and other similar materials. The composition of the slag and the amount in which it is formed will naturally depend on the nature of the material which is subjected to combustion, i.e., the material subjected to the combustion process, and on the combustion conditions prevailing in the kiln. For example, household refuse, of which a large proportion consists of organic and easily decomposable components, but which also contains small amounts of inorganic materials which are often very difficult to decompose, such as glass and ceramic materials, metal-containing materials such as cans and the like, will normally result in relatively limited amounts of slag which mainly consists of the above-mentioned normal slag components, i.e. quartz, feldspar, anhydrite and other inorganic and organic components. Refuse of a more industrial type containing a larger proportion of inorganic, non-flammable or non-decomposable materials, for example building materials, iron-containing objects and the like, will typically result in larger amounts of slag which, apart from the components mentioned above, may also contain partially combusted building materials, etc. Slag formed in connection with combustion of fossil fuel, such as coal, for example in power plants, is most frequently basic, for example with a pH above 8, for example a pH in the interval 8.5- 10.5. Slag formed by combustion of refuse normally has a pH of at least 9, typically a pH of 10-11. Normally, as will be described below, the slag is removed continuously or discontinuously from the kiln for further treatment or for deposition. If the slag does not contain environmentally harmful components, such as heavy metals or other components which are relatively easily leached or otherwise released from the slag, possibly when it is exposed to water, such as rain water, or other leaching agents, it may immediately and largely without any further measures be used for filling purposes, for example in connection with road construction and land reclamation, or in the concrete industry, or may quite simply be deposited at suitable locations in the surrounding environment. As mentioned above, fly ash is another by-product formed in connection with combustion processes. The term "fly ash" generally and in the present context designates the solid, gas-borne combustion residue which is present in the smoke leaving the kiln. The smoke is generally filtered before being released into the atmosphere, and the solid components present in the smoke are retained by the filter employed. It is these solid components which are usually denoted fly ash. As is the case for slag, the nature and the amount of fly ash depend on the composition of the combusted material as well as on the prevailing combustion conditions, and they also depend on the filter employed, which may be of any suitable type. A filter which will be suitable for most flue gas purification purposes, is, for example, an electrostatic filter or a bag filter, for example a filter of the type which is described below in further detail. However, filtration of the smoke by means of one or more cyclones will also be suitable. In connection with combustion of fossil fuel such as coal, and other aromatic compounds, typically in connection with energy production, the fly ash formed and collected normally has a relatively basic character, for example having a pH of 11-12, although more acidic fly ash may also be found. The fly ash formed in connection with conventional refuse combustion is of a more varying nature and may thus typically be slightly acidic or slightly basic. When fly ash from refuse combustion plants is left under conditions where water is present, it has a tendency in many cases to assume a more basic character. In many cases the fly ash will have adsorbed and/or absorbed various gaseous compounds present in the flue gas. These compounds may, for example, be condensed salts, such as potassium or sodium chloride or sulphate, hydrogen chloride and sulphite-containing compounds, heavy metals and compounds (especially salts) containing these, for example lead chloride, cadmium chloride and the like, or other impurities. As may be understood implicitly from the designation "fly ash", it is a material which is very mobile and which may therefore be difficult to handle. Fly ash is generally a very finely divided particulate material with a low density, typically a density of less than 800- 1000 kg/m 3 , such as about 500 kg/m 3 . The fly ash is often in the form of particle agglomerates in which the size of the individual particles is typically of the order of a few hundred Angstrøm to 2 μm, and the agglomerates are typically of the order of up to 100 μm. In connection with most types of combustion processes, there may be great difficulties associated with disposal of the fly ash formed during the combustion, and this often makes great demands on storage facilities and the like. The fly ash formed is often collected in large outdoor piles before being further treated or deposited, and this may give rise to considerable difficulties since steps must be taken to "hold" the fly ash, i.e., to prevent the fly ash from being spread by the wind, etc. In many cases it has been found practical to sprinkle the fly ash with water and thereby obtain a moist, cohesive, sludge-like mass which is easier to handle. However, sprinkling of fly ash may cause the environmentally harmful substances to be leached from the fly ash, and sprinkling at low temperatures, at which the water freezes to ice, may also be problematical. As the fly ash in many cases contains environmentally harmful substances, as mentioned above, it cannot be deposited directly - the harmful substances must first be removed or rendered harmless. Alternatively, the fly ash may be converted into a form from which the harmful substances cannot escape. Because of the content in the fly ash of substances which are alien/harmful to the environment, it usually cannot be employed directly in the same manner as slag, i.e., for example, for road construction, land reclamation, etc., and a pre-treatment of the fly ash is necessary. The harmful substances may, for example, be removed by leaching with water or another* suitable leaching agent, but this is normally an extremely expensive process. Apart from the fly ash mentioned above, the flue gas also contains other gaseous compounds, for example sublimed salts, the composition and amount of which will naturally depend on the material combusted. Apart from water vapour, flue gas resulting from combustion of material with a high content of organic compounds will in most cases contain substantial amounts of harmless lower carbonaceous compounds, such as carbon dioxide. In many cases the flue gas also contains substantial amounts of environmentally harmful substances which it will be necessary to remove before the flue gas is emitted into the surrounding atmosphere. Such environmentally harmful substances are typically acid-forming gases, such as nitrogen oxides, hydrogen fluoride and various sulphur- and chlorine-containing compounds, for example sulphur dioxide and hydrogen chloride. As mentioned above, great efforts have been made in recent years to reduce the emission of such acidic components from combustion plants, which, as mentioned above, has been attempted by the use of various emission-reducing processes, including treatment with a base, such as lime. More specifically, the wet and dry processes, respectively, known to the worker skilled in the art, have been used particularly for removal of sulphur-containing compounds. GB 1,325,460 describes a method for reducing the emission of halogen compounds formed by incineration of halogen-containing plastic materials. This method comprises addition of a base, for example a hydroxide or a carbonate, to the refuse before It is incinerated. The base, which is preferably In solution, Is absorbed by paper and/or other water-absorbing components in the refuse and is thus distributed in the refuse. Base In powder form may optionally be added directly to the refuse. Fly ash is not mentioned in this connection. Recirculation of fly ash formed during power plant operation in connection with the production of cement clinker is described by K.K.K. Krøyer in Danish patent applications Nos. 5872/77 and 5873/77. The present invention now provides a cheap and effective method for disposing of fly ash and reducing the amount of environmentally harmful acid- orming compounds present in the flue gas. Furthermore, the formation of a large number of compounds normally formed in connection with the combustion process from the by-products arising during combustion, such as acid-forming gases and the like, is prevented or reduced. The invention relates to a method for operating a combustion plant, in which the emission of acid-forming gas or gases and/or of dioxins from the combustion plant is reduced, and at least part of or substantially all the fly ash formed by the combustion process in the combustion plant, which fly ash may optionally be supplemented with fly ash from sources other than the combustion plant in question, is converted into slag, wherein a mixture of fly ash, one or more emission-reducing agents and material to be combusted is made, and said mixture is introduced into the combustion kiln or kilns of the combustion plant and is then subjected to the combustion conditions prevailing in the kiln or kilns. It is presumed that the formation of dioxins may be avoided to a very large extent by using the method according to the present invention. As will be explained below, it is presumed that dioxins originate to a large extent from chlorine-containing compounds released from the material being combusted. As mentioned above, the fly ash is converted into slag by the method of the invention, the fly ash being incorporated into the slag formed during the combustion. As will appear from the following exam les, the composition of the slag is not affected to any substantial extent by the incorporation of the fly ash, and the slag may therefore be used in a manner similar to that for the slag normally formed by combustion. As mentioned above, use of the method of the present invention renders it possible at one and the same time to dispose of the fly ash formed in a given combustion process while achieving a considerable reduction in the emission of the acid-forming gases formed during the combustion. The method of the invention is suitably carried out in a closed system in which the fly ash collected by the filter is recirculated directly to the combustion plant without influence from the surrounding environment. During the recirculation, one or more emission-reducing agents is/are added to the fly ash under conditions which enable extensive mixing with the fly ash, and the fly ash containing the emission-reducing agen (s) is then mixed with the material to be combusted before the latter is introduced into the kiln. In this manner, the fly ash contributes to efficient distribution of the emission-reducing agent in the refuse to be combusted. It is, however, also presumed that it will be possible that the recirculation of fly ash and the delivery of the emission-reducing agent(s) to the chute(s) of the combustion plant or the feed device can take place separately so that mixing does not take place until the passage of the combined material to the kiln(s). In such cases the fly ash will also function as a distributing agent for the emission-reducing agen (s) in the combined material, as a consequence of the very mobile character of the fly ash. The emission-reducing agent or the emission-reducing agents normally consist(s) of one or more bases. The fly ash and the emission-reducing agent(s) are suitably introduced together with the material to be combusted at a point in the chute(s) or feed device conveying the refuse to the combustion kiln(s) at which a reduced pressure has been created, i.e., a pressure of the order of 1/10 atm. This is to ensure that the fly ash and the emission-reducing agent(s) are sucked down through the chute or feed device so as to substantially prevent any blowing of the fly ash, and particularly the emission-reducing agent(s) , upwards out of the chute or the feed device. A suitable reduced pressure will normally be established when the chute is filled with a suitable amount of refuse acting like a stopper in the chute. As will be explained in further detail below, it is important that the emission-reducing agent(s) is/are mixed with the material to be combusted before the latter is subjected to heating to any significant extent. This is due to the fact that in order to achieve a satisfactory reduction of the acidic emissions from the combustion plant, it is necessary to render the acid-forming components harmless at the moment of their formation. It is thus preferred that the mixing takes place before decomposition of the refuse begins, i.e., before the refuse is subjected to temperatures at which harmful substances are released from the refuse. In connection with refuse containing large amounts of PVC and/or other chlorine-containing compounds, it is important that the mixing takes place before the refuse is subjected to temperatures higher than about 150°C, as chlorine may be released from the material already at this temperature. In connection with reduction of the emission of nitrogen oxides such as N0 χ , it will, however, often be necessary to add the emission- reducing agent(s) to the flue gas in the last part of, or downstream of, the combustion kiln(s) in order to obtain any effect. Suitable agents for the reduction of nitrogen oxide emission are, for example, ammonia-containing compounds, for example, NH3 or urea. The method according to the invention is applicable in connection with many different types of combustion processes, for example combustion of coal for, for example, heat/electricity production, or combustion in connection with industrial production, for example in connection with the production of certain types of fibres , such as mineral fibres. Combustion plants for which the present method has been found to be particularly applicable are plants for the combustion of industrial and/or household refuse, from which the heat produced may optionally be exploited for industrial heating and/or heating of dwellings and/or for the production of electricity, or a fuel-burning combustion plant producing heat for industrial heating and/or heating of dwellings and/or for the production of electricity, or a combination thereof. As mentioned above, the process parameters to be used for efficient combustion of different types of material will in most cases have to be adapted to the type and amount of material to be combusted. In connection with conventional refuse combustion taking place, for example, in a plant such as that described in further detail below, the refuse is normally subjected to temperatures of at least 875°C, such as at least about 950°C, for a suitable period of time to achieve the desired degree of decomposition of the refuse, often a substantially complete decomposition thereof. The heating of the refuse may, as described below, take place in stages, optionally using one or more kilns and/or kiln sections in series, whereupon the combustion proper takes place. Heating and combustion may, however, also take place in one and the same kiln. The type and amount of the emission-reducing agents to be used for a given combustion process will in most cases, as mentioned above, depend on the type and amount of the emissions to be reduced, and thus of the type and amount of the material to be combusted. As the emissions which it is desired to reduce have an acidic or acid- forming nature, the emission-reducing agent(s) preferably comprise(s) one or more representatives of the following classes of inorganic bases: oxides, hydroxides, carbonates or hydrogen carbonates of an alkali metal or alkali metals or an alkaline earth metal or alkaline earth metals. Further, ammonia and urea are presumed, as mentioned above, to be useful for reducing the emission of nitrogen oxides. The alkaline metal or metals is/are preferably Na and/or -K, and the alkaline earth metal or metals is/are preferably one or more of the following: Mg, Ca, Sr or Ba. The choice of emission-reducing agent(s) will, apart from the above- mentioned parameters, often be based on economic considerations. In this context the presently preferred bases comprise Ca 0H 2 > CaO and CaCθ3 as well as NaOH, or mixtures thereof. As will be described below, Ca-containing bases are especially useful for reducing sulphur-containing emissions, while NaOH is useful for chlorine- containing emissions. In the case of reduction of the emissions of several different components it may be advantageous to use a mixture of several emission-reducing agents. The amount of fly ash incorporated in the material to be combusted can vary within wide limits. Normally it Is preferred that the amount of fly ash used in the mixture of fly ash, emission-reducing agent(s) and material to be combusted constitutes about 1-10% by weight. In certain cases it may, however, be advantageous to incorporate larger amounts of fly ash, for example fly ash in an amount of up to about 20% by weight or 50% by weight of the combined material fed to the kiln, or even in an amount approaching 100% by weight, i.e., practically all the material being fed to the kiln consists of fly ash. In the latter case, the fly ash sinters substantially with itself upon being subjected to the high temperatures applying in the kiln, and the fly ash is thereby converted into a form which is easier to handle. As mentioned above, the fly ash may be the fly ash formed by combustion in the combustion plant in question, or fly ash formed by another and independent combustion process, and/or mixtures of these types of fly ash. It is also possible to add clay and/or sand together with fly ash so as to possibly thereby dilute the content of heavy metals in the slag. Furthermore, it is possible to add asbestos-containing material to the refuse with a view to having the asbestos converted into part of the slag. The fly ash may further contain certain energy-rich combustion residues, such as coal dust or the like, which may enable more efficient combustion of the recirculated or added fly ash. If desired, energy-rich components may be added to the fly ash when the latter is recirculated and/or fed to the combustion plant, in order to obtain a more efficient combustion. Such energy-rich components may, for example, be various types of fossil fuel, such as oil or coal. In most cases the amount of fly ash fed to the kiln will be adapted to the circumstances in question, including the amount of fly ash to be disposed of, the type and capacity of the combustion plant, the slag, etc. In all cases, however, it is preferred that the fly ash constitutes an amount of at least about 1% by weight of the combined material fed to the kiln, since a smaller amount is presumed not to be sufficient to achieve a satisfactory distribution of the emission- reducing agents in the material. An amount of fly ash of 1-5% by weight, such as 2-4% by weight, and particularly 2-3% by weight, based on the weight of the combined material fed to the kiln, is presumed to be a suitable amount in connection with combustion in a conventional refuse combustion plant. As mentioned above, the amount of emission-reducing agent(s) added to the fly ash depends on the amount of the acid-forming gases which is to be reduced, and thus on the refuse to be combusted. The amount of emission-reducing agent(s) added to the mixture will usually be based on rough estimates based on average emissions from the plant in question. Since the various acid-forming components have different requirements with regard to the amount of the emission-reducing agen (s) to be added to the refuse, as they contain different numbers of acidic groups, it is convenient to specify the amount of base presumed to be necessary to reduce a particular composite emission of acid-forming gases in the form of acid equivalents. It is presumed that in most cases it will be advantageous to incorporate the emission-reducing agent(s) in the mixture of fly ash, emission-reducing agent(s) and material to be combusted in an amount of at least 50 acid equivalents per ton of material to be combusted, such as 50-1000 acid equivalents per ton of material to be combusted. An amount exceeding 1000 acid equivalents per ton will normally exceed the required amount for most types of fuel and refuse and will therefore not be reasonable to vise, since at least part of the excess amount will be retained together with the fly ash in the filter used and be recirculated together with the fly ash, and thus accumulate in the kiln. Amounts smaller than 50 acid equivalents per ton will for most types of fuel and refuse be insufficient to reduce the emission of the acid-forming gases to the desired degree, and may, f f urthermore, be difficult to distribute in a satisfactory manner in the material to be combusted. In the case of very thorough mixing of the in-going components, for example by means of a refuse grinder, it will be possible to obtain a more homogeneous distribution of the emission-reducing agent(s) in the mixture, and it will therefore be possible to use smaller amounts of such an agent or agents. For most purposes it is presumed that the emission-reducing agent(s) is/are incorporated in the mixture of fly ash, emission-reducing agent(s) and material to be combusted in an amount of about 50-500, particularly 50-300 and preferably 150-250 acid equivalents per ton of material to be combusted. The above amounts are specified on the basis of stoichiometric calculations made on the basis of the average emissions of acid-forming gases given in the examples below, these emissions being derived from a typical refuse combustion plant. In cases in which the refuse itself does not give rise to release of significant amounts of acid-forming gases, it will in most cases still be necessary to add base to the fly ash in order to reduce the acid-forming emissions which may be due to the sublimed salts (for example, chlorides and sulphates) which may be present in the fly ash, and which are returned with it to the kiln. So far, althrough without any careful investigation having been made, it has been noted that refuse combustion plants emit more hydrogen chloride (which leads to the formation of hydrochloric acid) than sulphur dioxide (leading to the formation of sulphuric acid) into the atmosphere. The chemistry on which the formation of these acid- forming emissions is based is discussed further in the following: Substantial sources of SO2 formation in refuse incineration are, apart from oxidation of sulphur-containing organic materials, decomposition of alkali metal sulphates and of any trivalent metal sulphates, such as ferric and aluminium sulphate, which may be present. It is known that sodium and potassium sulphate are substantially completely decomposed at red heat (about 600 C C) with release of SO2, and that, e.g., ferric sulphate decomposes already at 480°C. Sulphates of most divalent metals are first decomposed at temperatures above red heat; thus, appreciable decomposition of, e.g., calcium sulphate does not take place until about 1100°C, a temperature which is at the upper limit of what is normally found in refuse incineration plants. Substantial sources of HC1 formation in refuse incineration are, apart from oxidation of chorine-containing organic materials (especially PVC) , the reaction between SO2 and chlorides in the presence of oxygen and water (water vapour) . An example of the latter reaction is the so-called "Hargreaves reaction" : 2 NaCl + S0 2 + 1/2 0 2 + H 2 0 → Na 2 S0 4 + 2 HC1 The above reaction will take place under the conditions under which refuse incineration takes place, i.e. , at the temperatures to which the refuse is subjected In the incineration plant. It appears from the above that performing the method according to the present invention using the preferred calcium-containing bases, which in the case of sulphur dioxide will immediately lead to the formation of calcium sulphite which will react with oxygen present to form calcium sulphate ("anhydrite"), must be presumed to lead to elimination of at least part of one of the sources of the formation of HC1, viz. S0 2 . Furthermore, the present invention is presumed to have a beneficial effect on the emission of dioxins from combustion plants which combust materials with a content of chlorine-containing compounds (as used herein, the term "dioxins" refers to chlorinated dibenzo-p- dioxins and chlorinated dibenzofurans) . This is explained further below: Large-scale combustion of material with the on average highest content of chlorine-containing compounds usually takes place in plants adapted thereto for incineration of household, commercial and industrial refuse. In the Western world, household refuse typically contains about 0.3-1% by weight of chlorine; for example, "HC1- Emlssionen aus der Mύllverbrennung und PVC" (Verband Kunststofferzeugende Industri e.V. , Frankfurt, 1986) refers to a chlorine content in West German household refuse of about 0.7% by weight, of which about 40% is derived from PVC (chlorine in the form of organically bound chlorine) , while the remaining part of the chlorine content primarily derives from refuse components, such as food scraps (chlorine primarily in the form of chloride) and paper (chlorine In the form of chloride and organic chlorine compounds from bleaching of the paper pulp and of chalk in coated paper) . In certain countries (for example in Denmark) experiments are being made with recycling programs, leading to the removal of, amongst other things, food scraps and paper from especially household refuse, whereby PVC then becomes the absolutely dominant source of chlorine in household refuse. The composition of commercial and industrial refuse may, of course, vary a lot, depending on its sources, but the content of organically bound chlorine, for example in the form of PVC packaging material, PVC-coated cable ends and leftovers from processing of PVC articles/goods, will often be considerable. It is believed that dioxin formation in connection with refuse incineration results from hydrogen chloride formed during the incineration process reacting, in the kiln itself and/or in subsequent cooler areas on the flue gas' way out of the plant, with flue gas-borne organic compounds and/or carbon particles with the formation of, inter alia, dioxins. It is further believed that a metal content, especially a copper content, if any, in the flue gas-borne fly ash has a catalytic effect on dioxin formation, whereby the higher temperature otherwise necessary for such a synthesis is lowered to about 250-350°C. It is thus very important to prevent the release of hydrogen chloride in the kiln and/or in subsequent cooler areas on the flue gas' way out of the plant. Incineration of PVC and other materials containing organically bound chlorine leads to the formation of hydrogen chloride, just as the so-called "Hargreaves reaction" mentioned above in the present specification will be able to contribute to release of further hydrogen chloride. Since performance of the method according to the present invention will to a large extent lead to release of the content of the organically bound chlorine in the refuse with conversion into chloride before the incineration process proper, and to conversion of the sulphur dioxide formed during the incineration process, sulphur dioxide being a necessary component in the "Hargreaves reaction" , substantially at the moment of its formation into sulphite and then into sulphate, the performance of the method according to the present invention using the preferred calcium- containing bases must thus be presumed to bring about a reduction in dioxin formation. It is important that satisfactory mixing of the fly ash, the emission-reducing agent(s) and the material to be combusted is ' achieved before subjecting the latter to temperatures at which the acid-forming gases begin to be formed. By the time the acid-forming gases begin to be formed, the emission-reducing agent(s) must be distributed in the material to be combusted in such a manner that adequate possibility of reaction between the acid-forming gases and the emission-reducing agent(s) is provided. Adequate mixing of fly ash, emission-reducing agent(s) and material to be combusted may, for example, be achieved by forming the mixture of fly ash, emission-reducing agent(s) and material to be combusted in a continuous or discontinuous process by grinding together or comminuting in connection with a mixing of the components of the mixture and/or by the mixing which takes place naturally when the components of the mixture are fed to the combustion kiln(s) through the chute(s) or feed devlce(s) of the combustion plant, since the passage through the chute or feed device of the combustion plant may be sufficiently turbulent to achieve adequate mixing of the components of the mixture. It may also be advantageous to add further emission- educing agent(s) to the mixture. This/these agen (s) may be of the same type as the emission-reducing agent(s) already present in the mixture of fly ash, emission-reducing agent(s) and material to be combusted, but may also be different from this/these agent(s). The further emission-reducing agent(s) may suitably be added to the mixture in the chute or feed device and/or in the combustion kiln(s) and/or the further emission-reducing agent(s) is/are added to the combusted material In a last part of the kiln(s) or after the material has left the kiln(s) . The form in which the emission-reducing agent(s) is/are incorporated in the mixture of fly ash, emission-reducing agent(s) and material to be combusted may vary according to what is considered most suitable for the combustion process in question. The emission-reducing agen (s) incorporated into the mixture of fly ash, emission-reducing agent(s) and material to be combusted, and/or added to the mixture in the chute or the feed device, in the combustion kiln(s) , to the partially combusted material in a last part of the combustion klln(s) and/or after the material has left the combustion kiln(s) , is/are in the form of a solid, preferably a finely divided particulate solid, or in the form of an aqueous dispersion or solution. The form in which the emission-reducing agent(s) is employed will in most cases depend on the use in question. The combustion conditions to which the mixture of fly ash, emission- reducing agent(s) and material to be combusted is subjected, will, of course, vary within wide limits, depending on the type of combustion in question. The temperatures at which the combustion process takes place will, however, typically be 900-1200 β C, particularly 950- 1100°C. The invention also relates to a plant suitable for performing the method described above. The plant may be characterized as a combustion plant for incineration of commercial, industrial and/or household refuse, from which the heat produced may optionally be exploited for industrial heating and/or heating of dwellings and/or for the production of electricity, or a combustion plant producing heat for industrial heating and/or heating of dwellings and/or for the production of electricity by combustion of fossil fuel, which plant is characterized by comprising means for adding fly ash and an emission-reducing agent in liquid or solid form to the material to be combusted. The means are suitably adapted to add the fly ash and the emission-reducing agent to material to be combusted in a chute or other feed device arranged upstream of the combustion kiln(s) of the plant. In many cases it is convenient that the plant comprises means for adding one or more further emission-reducing agent(s) to the mixture of fly ash, emission-reducing agent and material to be combusted, in the chute or feed device and/or in the combustion kiln(s) and/or to the wholly or partially combusted material in a last part of the combustion kiln(s) or downstream of the combustion kiln(s) . The plant will normally comprise a flue gas treatment system or systems for separating fly ash from the flue gas from the combustion kiln(s), and means for recirculating the separated fly ash to the means for adding fly ash. The flue gas treatment system(s) comprise(s) a filtration system preferably comprising one or more filters, preferably a filter of the bag filter type or the electrostatic filter type. Various useful types of combustion plants will be discussed below; the present invention is, however, not to be regarded as being limited to these. A suitable system in which the method according to the present Invention may be performed i.e. , a system in which there may be brought about a reduction of the emission of acid-forming gases from a combustion plant with simultaneous conversion into slag of part of or substantially all the fly ash formed by the combustion process in the combustion plant, which fly ash may optionally be supplemented with fly ash from sources other than the combustion plant in question, comprises one or more flue gas treatment systems, one or more fly ash transport systems and one or more feed devices for the emission-reducing agent(s). The flue gas treatment system or systems suitably comprise(s) a filtration system. The type of filtration system is not critical for the method according to the present invention, although the filter(s) used must, however, be able to retain, suitably, substantially the flue gas-borne particles in the flue gas. The filtration system may comprise one or more filters, according to the combustion process in question. Types of filter which have been found to be particularly suitable in connection with flue gas filtration are filters of the bag filter type or the electrostatic filter type. Suitable fly ash transport systems for use in connection with the present invention suitably comprise one or more pipes or pipe systems equipped with transporting means which contribute to transportation of the fly ash from the filtration system(s) back to the chute(s) or other feed device(s) of the kiln(s) or from an external fly ash source. The fly ash recirculation or transport systems may suitably be mechanically or pneumatically driven. The type of feed device which will be suitable for the introduction of the emission-reducing agent(s) will in many cases depend on the form of the emission-reducing agent(s) . In cases where the emission- reducing agent(s) is/are to be incorporated in the form of a powder, the feed device(s) used suitably comprise(s) a pneumatic system and/or a screw conveyor system. hen the emission-reducing agent(s) is/are in the form of a suspension or solution, examples of suitable feed devices are pump- driven, base-resistant systems. In certain - and many - cases, as mentioned above, it may be suitable that the addition of the emission-reducing agent(s) to the fly ash and/or to the mixture of fly ash, emission-reducing agent(s) and material to be combusted takes place substantially continuously, for example in conjunction with continuous addition of fly ash, such as is shown in the accompanying drawing. In other cases it may be most suitable that the emission-reducing agent(s) is/are added discontinuously to the fly ash and/or the mixture of fly ash, emission-reducing agent(s) and material to be combusted, for example by mixing the emission-reducing agent(s) into the fly ash at uniform or varying intervals. The invention will now be further illustrated with reference to the drawing, in which Fig. 1 is an example of a refuse incineration plant system according to the present invention, in which the method according to the present invention may suitably be carried out, and Fig. 2 shows relevant parts of the plant used for fly ash recirculation and base supply in the full scale experiments described in the examples given below. Fig. 1 Illustrates a refuse incineration plant system for use according to the present invention. Fly ash produced during refuse incineration is collected in an electrostatic filter 1, which may be an electrostatic filter of any suitable type. From the electrostatic filter 1, the fly ash is passed through one or more fly ash transporting pipes 2 (represented in Fig. 1 by two fly ash transporting pipes 2) to a screw conveyer 3, through which the fly ash is transported to a transfer container 4, in which the fly ash is collected. From the transfer container 4, the fly ash Is passed by means of a stream of dry air, generated by a compressor 5 and an air drying system 6, through a fly ash transporting conduit 7 to a transfer container 8 located near a chute 13 through which refuse to be incinerated is passed to a rotary kiln 20. The transfer container 8 is equipped with a stirrer 9 for stirring the fly ash In the transfer container 8. From the transfer container 8, the fly ash is passed via a metering screw 10 to a feed screw 11 which feeds the fly ash to the chute 13. The feed screw 11 is provided with a hollow axle through which liquid from an optionally present liquid container 12 may be metered. The liquid In the liquid container 12 may be a solution or slurry of a base for use in the method of the invention. Base in solid form, for example powder form, for use according to the present invention is stored in a transportable silo 14 which may be exchanged as required, for example in connection with refilling or changing to another type of base. The base in the transportable silo 14 is transported via. a base transporting conduit 17 to a transfer container 18 by means of a stream of dry air generated by a compressor 16 and an air drying system 15. The base In the transfer container 18 is passed to the refuse chute 13 by means of a feed screw 19. The fly ash, optionally mixed with base, is introduced into the chute 13 at a location where the temperature is about 20°C, and where a reduced pressure has been established as explained above, so as to ensure that fly ash and/or base does not move upwards in the chute 13. Refuse to be combusted Is transported to the chute by means of a grab 21. The refuse, in which fly ash and base are mixed, is transported, by means of a system of grids 50 where the refuse is gradually heated to combustion temperature, to the rotary kiln 20 in which it is subjected to the final combustion. From the rotary kiln 20, slag is removed via a slag outlet 51, and flue gas is passed via a flue gas baffle means 52 and a boiler 53, in which the flue gas partially gives off heat, to the electrostatic filter 1. The flue gas remaining after the treatment in the electrostatic filter 1 is passed to a chimney 55 via a suction blower 54. Fig. 2 illustrates the fly ash recirculation and base supply system used in the full scale experiments described in the examples given below. Fly ash collected in the electrostatic filter of the plant, which filter is shown in Fig. 1, is passed via fly ash transporting pipes 2a to a screw conveyer 3a in which the fly ash is mixed with base. In the case of base in solid form, the base is supplied from an airtight silo 30 by means an an optionally insulated and heated metering screw 31 to a feed screw 32 which may be insulated and/or heated. The base in the airtight silo 30 and in the metering screw 31 and the feed screw 32 is kept under an N 2 atmosphere, N being supplied via N pipes 34 from N pressure cylinders 33. The base is mixed with the fly ash in the screw conveyer 3a. The screw conveyer 3a presses the mixture of fly ash and base against a non-return flap 22 biassed by a spring 23. The mixture passes a flap valve 25 and Is passed on through a compartment sluice 26 to a transporting conduit 29. The mixture of fly ash and base is passed through the transporting conduit 29 by means of compressed air supplied via a compressed air pipe 28, the compressed air being provided by means of a compressor 27. The mixture of fly ash and base is passed, via the transporting conduit 29, to the refuse chute 13 which is also shown in Fig. 1. The process may be controlled by means of a control panel 35. The space between the non-return flap 22 and the flap valve 25 Is ventilated by means of a ventilation pipe 24. 1. Laboratory scale experiments (a) General: Samples of fly ash, slag and slag cooling water (i.e., the water used for cooling the slag when it leaves the kiln) was collected from a large Danish incineration plant used for incineration of the combustible part of household refuse (daily refuse), commercial and industrial refuse. The plant as a whole comprises 3 kilns, of which the newest is a so-called rotary kiln; it is from this kiln that the samples are derived. The composition of the refuse delivered for incineration is naturally somewhat variable from day to day, and this is expected to be reflected to some extent in the composition of the fly ash, the slag and thereby the slag cooling water. The samples taken from all these materials are believed to be representative. The raw slag containes a number of non-crushable "foreign bodies", such as beer bottle tops, nails and the like. These were removed before the various treatments of the slag which are described in the following sections. (b) Chemical analyses of fly ash, slag and slag cooling water: The analyses were carried out by atomic absorption spectrophotometry, gravimetry, automatic analysis and titration. Furthermore, the pH of aqueous slurries of fly ash and slag, and of the slag cooling water itself, was measured. The results are shown in Table 1.1. TABLE 1.1 Fly ash Slag Cooling water % by weight % by weight ppm Si0 2 39.5 67.5 4.5 A1 2 0 3 12.9 8.4 7.9 Fe 2 0 3 1.9 1.8 2.7 MgO 2.6 1.9 0.2 CaO 12.4 8.6 1379 Na 2 0 7.6 4.9 8033 κ 2 o 5.6 2.0 2362 ci- 7.8 0.7 7100 PO 4 - - - 0.15 0.12 N0 3 - 10 so 3 * 107 Cd 0.018 Pb 0.567 0.026 0.58 Hg * CaS0 3 0.57 pH after slurrying 7.14 11.51 8.29 after 1 hour 9.32 11.36 8.51 after 2 hours 10.68 11.18 8.42 after 3 hours 10.76 11.19 8.39 after 22 hours 10.43 11.21 8.41 * not measured The specification of the main components as oxides is purely conventional and not an expression of how the substances are present. The X-ray analyses mentioned below (see section (f)) thus show that a large amount of KCl and NaCl is present in the fly ash. However, it appears that fly ash and especially slag contain large amounts of silicon dioxide/silicates. (c) Experiments concerning the influence of mixing fly ash with slag on the fireproofness of the slag. The determination of "hemisphere point": For these experiments, samples prepared in the following manner were used: Sample (i) : A portion of raw slag was crushed manually in a mortar until its consistency was qualitatively like that of flour. Sample (ii) : A portion of slag was sieved to remove particles >2 mm. The fraction with a grain size of <2 mm was crushed in the same manner as for Sample (i) . Sample (iii) : A metered portion of raw slag ' which had been crushed in the same manner as for Sample (I) was mixed in the mortar with an amount of fly ash corresponding to 10% (w/w) of the amount of slag. The hemisphere point (the melting point) was determined for dry- pressed 3x3x3 mm cubes of each of the samples (I) , (ii) and (iii) . The samples were heated (10°C/min.) on a Leitz heating microscope. The results are shown in Table 1.2. TABLE 1.2 Temp./C c Sample (i) Sample (ii) Sample (iii) Beginning of rounding 1122 1178 1134 Beginning of blistering 1143 1180 1139 • Rounding 1163 1199 1154 Hemisphere point 1199 1215 1201 The main conclusion is that the slag fraction <2 ram is somewhat more fireproof than raw slag as a whole. The addition of 10% (w/w) of fly ash led to very small changes in the fireproofnesss of the slag. (d) Sintering experiments with slag, slag + ash and ash: The experiments were carried out with cylindrical dry-pressed sample • bodies (diameter 46 mm) prepared from crushed slag + 6% of water, crushed slag + fly ash + 6% of water, and ash + 6% of water (the crushing method used here is described further , below) . The experimental conditions were the following: The sample was placed in a tube oven which was heated slowly (10°C/min.) to about 1020°C. The maximum temperature was maintained for 2 minutes. The diameter of the sample was measured continuously during the experiment by means of a laser dilatometer. The results are shown in Table 1.3. TABLE 1.3 Sintering/ β C Max./°C Change in diameter/% A. Crushed slag L )704 2 >952 1020 -2.5 B. Crushed slag + 10% ash L )693 2 >980 1027 -0.9 C. Slag <2 mm 1)673 2)942 1027 -4.4 D. Slag <2mm + 10% ash - - 1018 +1.2 E. Crushed slag - - 1018 0 F. Crushed slag - 987 1044 -1.0 G. Crushed slag + 2% ash 1003 +1.9 H. Crushed slag + 2% ash 1035 1044 +1.1 I. Ash 602 1020 -0.2 • ' First sintering step 2 ' Second sintering step The first experiments (A+B) were carried out with slag which had been crushed in an agate ring crusher to about fly ash fineness. They showed that the first small sintering step at about 700°C takes place at a slightly lower temperature when 10% (w/w) of fly ash is mixed into the slag, but the sintering temperature proper is raised from 952°C to 980°C while the shrinkage is reduced from 2.5 to 0.9% despite a slightly higher maximum temperatures On the assumption that the coarse part of the slag might not have time to react with the ash, experiments (C+D) were carried out with a slag fraction sieved off and having a grain size of <2 mm. This fraction was crushed as before, and showed weak sintering at 673°C and sintering proper at 942°C. After 2 minutes at 1027°C and cooling, the total shrinkage was 4.4%. Mixing in of 10% (w/w) of fly ash in the crushed slag fraction of <2 mm led to no sintering below 1018°C and a total expansion of 1.2%. The pure ash showed (experiment I) weak sintering at 602°C, but after 2 minutes at 1020°C and cooling, the total sintering was 0.2%. From about 960 β C strong sublimation took place from the pure ash, presumably mainly of chlorides, which rendered measurement with the dilatometer impossible. For the last batch of sintering experiments (E-H) , manually crushed slag was used, i.e., having a grain size between fly ash and the slag fraction of <2 mm. With a maximum temperature of 1018°C no sintering was observed. A repetition of the experiment, but with a maximum temperature of 1044°C, showed sintering at 987°C, the total sintering being 1%. Addition of 2% (w/w) of fly ash led to a sintering at 1035°C, but not more pronounced than that 2 minutes at 1044°C and cooling resulted in a total expansion of 1.1%. The varying results for the last batch of sintering experiments are an expression of lack of homogeneity in the starting slag as a consequence of its relatively coarse grain size. The conclusion is that the addition of fly ash will lead to a slightly higher sintering temperature in the incineration process in the incineration plant. Apart from the grain size, the results will depend heavily on inhomogeneity in the slag and variations in the composition of the refuse. (e) Extraction of the sintered samples with water; chemical analyses of the extracts: The products from sintering experiments A-I (see section (d)) were extracted with water with stirring for 24 hours. The extracts were analyzed as described for fly ash, slag and slag cooling water (see section (b)). The results are shown in Table 1.4. TABLE 1.4 A B C D E F G H I pH 7.4 7.4 9.1 8.4 7.6 ** 7.7 7.7 7.7 Cd - - - . . . . . . Pb - - . . . . . . Cl" 0.05* - ** - ** 0.19 - 3.37 P0 4 " " 0.0007 ** 0.007 - ** N0 3 " ** N0 2 ' * Content in percent ** Not measured The conclusion is that sintered material formed from slag to which up to 10% (w/w) of fly ash has been added does not differ appreciably from sintered slag with regard to leaching of Inorganic components. (f) X-ray studies: Powder diffractograms were recorded for the materials mentioned in Table 1.5. With the exception of the fly ash, all materials were finely crushed (to flour consistency) before recording the diffractograms. An automatically registering powder dlffractometer coupled to a pen recorder was used. In the table 1 x means that the component in question is identified with certainty, whereas 6 x means that the highest peak fills the full paper width at the sensitivity at which the analysis was made, which corresponds to it being a main component. TABLE 1.5 Alkali Low quartz Anhydrite Feldspar NaCl KCl sulfates Other Fly ash xx xx xxxx XXXX many uncertain Slag, crushed xxxx X X X? X? CaC0 3 Slag <2mm xxxxxx X X CaCO 3 Sintered samples Slag xxxx X XX Ca 3 P0 4 Slag + 10% of fly ash xxxx X XX Ca 3 P0 4 Slag <2m XXX X XX Ca 3 PO 4 Slag <2mm + 10% fly ash XXXXX X Fly ash XXXX X Condensate from cold kiln tube XXXXX XXX X? anall content of uniden¬ tified phases The X-ray analyses show that the slag consists predominantly of quartz and feldspar, while alkali metal chlorides and sulphates have evaporated to a large extent and are to be found in the fly ash, which, however, also contains substantial amounts of quartz. The results for the sintered samples (cf. section (d)) show that no dramatic mineralogical changes take place as a result of the addition of fly ash. Aanalysis of condensate from the cold part of the oven which was used for the sintering experiments shows that at least part of the content of alkali metal chlorides and sulphates in the fly ash evaporates when the fly ash is added to the slag and heating is carried out again. (g) Experiments concerning neutralization of HCl from PVC by means of NaOH: For these experiments, lens-shaped PVC granules with a diameter of about 5 mm and a centre thickness of about 2 mm were initially used. Later, some of the experiments were repeated using PVC in powder form, without observing any notable difference in the results. Samples of PVC, PVC + fly ash and PVC + fly ash + NaOH (see Table 1.6) were heated to various temperatures in the interval of about 550-950°C under an N atmosphere. The HCl released was retained in standard NaOH solution, and the amount of HCl was determined by back-titration. The use of an N2 atmosphere was merely to avoid oxidation of carbon in the PVC, since the CO2 formed thereby would render back- itration difficult. The experimental setup was a 2 m long aluminium silicate tube (internal diameter 20 mm) , the middle section (39 cm) of which was placed in an Heraeus oven. The sample (in an aluminium silicate boat) was placed In the middle of the heated part of the tube. A weak* stream of N2 from a pressurized cylinder was passed through the tube, and the sample was heated (about 10°C/min.) to the maximum temperature in question. The oven was then turned off. During the entire heating phase and until the sample had cooled to about 100°C, the escaping gases were passed through a series of washing bottles containing measured portions of standard NaOH. The cooling phase lasted for about 4-6 hours. The results which all derive from experiments with granulated PVC, are shown in Table 1.6. TABLE 1.6 HCl release Test Atm , Conditions % of PVC Comments I N, 0.350 g of PVC - 600 C 32.3 II 0.502 g of PVC -*> 931°C 30.0 III 0.3767 g of PVC + 1.4009 g of ash 721°C 28.1 IV 0.3301 g of PVC + 0.9564 g of NaOH (Powd.) + 1.9128 g of ash -*> 586°C A little NaOH blown with 2 over into washing bottle V 0.5143 g of PVC + 0.1602 σ of NaOH(powd.) + 0.3475 g of ash -> - 600 C 18.5 VI 0.5108 g of PVC + 0.2524 g of NaOHftabl.) + 1.5047 g of ash → 600 C 12 As can be seen from the table, the preliminary conclusion from these experiments must be that it is possible to obtain an even considerable reduction in HCl release in the incineration of PVC in the presence of base (in this case NaOH) and fly ash. 2. Full scale experiments (a) General: The full scale experiments concern the rotary kiln described in section 1(a), which has a capacity of an average of 7 tons of refuse per hour. Full scale experiments were carried out concerning the effect of addition of fly ash and base (NaOH) on flue gas emissions and on the composition of the fly ash, the slag and the slag cooling water. During a test period of 20 days, emission measurements were made and samples of fly ash, slag and slag cooling water were taken at regular intervals. The last part of the experiment was carried out with addition of calcium carbonate as the base, while on the last day of the experiment sodium hydroxide was also added, although not, however, through the established feed channel (see Figs. 1 and 2), but discontinuously by pouring 25 kg of NaOH every 15 minutes and 50 kg of CaC0 3 every 30 minutes into the chute with the refuse (see Figs. 1 and 2). (b) Emission measurements: The flue gas was examined for its content of HCl and SO2; furthermore, the emission of O2, CO and N0 χ was monitored routinely throughout the entire test period. Samples were taken from the duct slanting upwards in the kiln room immediately upstream of the electrostatic filter (see Fig. 1). The following instruments/methods were used for the determinations; 0 : A Hartmann and Braun paramagnetic 0 -meter, type MAGNOS 3 T. SO2, CO2 and CO: 3 individual Beckmann infrared photometers, type 864. N0 χ : A Beckmann photoluminescense meter, type 951A. HCl: By absorption In standard NaOH solution. Cl ~ and SO^ 2- were determined in all samples, and for some of the samples the excess NaOH was back-titrated with HCl, whereby the total acid content (i.e. , content of HCl and H2S0 3 /H2S0 4 ) was determined. The results of the measurements concerning SO2- and HCl emissions converted to standard conditions are shown in Table 2.1. TABLE 2.1 Day of Measurement Addition S0 2 mg/Nm 3 /10%0 2 HClmg/Nm 3 /10%0 2 1 Fly ash- 590 474 8 " + NaOH 2 ) 787 202 9 It tl 2) 449 621 10 11 It 2) 806 507 14 It tl 2) 449 546 15 It It 2) 356 534 17 " + CaC0 3 3 ) 367 730 20 NaOH 4 ) 494 378 Mean value 500 536 Standard deviation 138 192 L ) 370 kg/hour 2 200 kg of fly ash/hour + 100 kg of NaOH/hour 3 ) 200 kg of fly ash/hour + 100 kg of CaC0 3 /hour 4 200 kg of fly ash/hour + (100 kg of CaC0 3 + 100 kg of NaOH) (see 2(a) above) A measurement of the SO2 emission was made immediately before the start of the experiment, which, converted to standard conditions, gave a value of 514 mg of Sθ2/ π /10%θ2. No corresponding measurement of the HCl emission was made immediately before the start of the experiment, but during a period of 2.5 years immediately prior to the experiments described, a series of 10 measurements of both SO2 and HCl emissions from the same kiln had been made; the following mean values were found for the 10 measurements: mg/Nm /10%02 standard deviation S0 2 310 173 HCl 902 304 It is thus apparent from Table 2.1 and the above that the mean value for SO2 emission during the experiment is substantially unchanged in relation to the measurement immediately before the experiment, but considerably above the mean value of the 10 measurements carried out previously. In contrast, the mean value for HCl emission during the experiment is only about half of the level found in the previous measurements. (c) Fly ash analyses: The fly ash analyses were carried out as described for the laboratory scale experiments. During the test period of 20 days, a total of 8 samples of fly ash were taken. Furthermore, one sample of fly ash was taken before the start of the experiment and 2 samples (on different days) after the end of the experiment. The mean values for the analysis results are given in Table 2.2. The amount of fly ash produced per ton of refuse during the experiment did not differ substantially from the amount of fly ash produced per ton of refuse before the start of the experiment. TABLE 2.2 Mean of 3 analyses before and after the Mean of 8 analyses test period in the test period % by weight standard % by weight standard deviation deviation Si0 2 45.93 7.19 32.16 6.11 Ti0 2 0.94 0.08 1.11 0.15 A1 2 0 3 12.07 0.74 12.1 1.3 Fe 2 0 3 3.15 1.40 3.45 1.75 MgO 2.33 0.25 1.88 0.80 CaO 11.97 1.12 13.14 1.76 Na 2 0 6.47 1.33 6.84 1.64 K 2 0 7.4 2.30 7.44 0.99 Ignition loss 2.9 0 2.30 0.49 Zn 1.54 0.36 2.00 0.42 Cd 0.021 0.09 0.12 0.22 Cu 0.09 0.05 0.20 0.13 Pb 0.45 0.13 1.00 0.31 S0 3 3.19 0.78 4.42 0.52 Cl 3.66 2.15 7.01 3.07 (d) Slag analyses: The analyses were carried out as described for the laboratory scale experiments. The sampling was carried out as described for fly ash (under (c)). The mean values for the analysis results are shown in Table 2.3. TABLE 2 . 3 Mean of 3 analyses before and after the Mean of 8 analyses test period in the test period % by weight % by weight Si0 2 63.5 56.6 Ti0 2 0.5 0.6 A1 2 0 3 8.8 9.9 Fe 2 0 3 5.5 7.4 MgO 1.5 1.4 CaO 8.0 11.2 Na 2 0 5.6 6.2 κ 2 o 3.2 2.3 Ignition loss 5.0 4.7 Zn 0.19 0.35 Cd 0 0.001 Cu 0.11 0.235 Pb 0.095 0.185 Cl 0.70 0.66 S0 3 0.40 0.36 It is apparent from the table that the average composition of the slag during the experiment does not differ substantially from the average composition of- the slag in the control samples..The conclusion is that performing the method of the invention does not reduce the applicability of the slag for construction work and the like. (e) Slag cooling water analyses: The analyses were carried out as described for the laboratory scale experiments. The sampling was carried out as described for fly ash (under (c) above) . The mean values of the analysis results are shown in Table 2.4. TABEL 2 .4 Mean of 3 analyses before and after the Mean of 8 analyses test period in the test period pp ppm Si 2.1 9.1 Ti 0.02 0.1 Al 5.1 305 Fe 1.3 1.5 Mg 10.6 0.1 Ca 1468 464 Na 3861 7783 K 1918 3807 Zn 0.025 2.0 Cd 0.132 0.062 Cu 0.138 2.38 Pb 0.068 2.05 PH. 11.8 Aug. 11.4 8.3 Dec. It is apparent from the table that the content of the heavy metals zinc, copper and lead is increased during the experiment in relation to the control samples. If necessary, an unacceptably high content of heavy metals, if any, in the slag cooling water could be reduced by, for example, precipitation.

Claims: CLAIMS 1. A method for operating a combustion plant, characterized in that the emission of acid-forming gas or gases and/or of dioxins from the combustion plant is reduced, and at least part of or substantially all the fly ash formed by the combustion process in the combustion plant, which fly ash may optionally be supplemented with fly ash from sources other than the combustion plant in question, is converted into slag, wherein a mixture of fly ash, one or more emission-reducing agents and material to be combusted is made, and said mixture is introduced into the combustion kiln or kilns of the combustion plant and is then subjected to the combustion conditions prevailing in the kiln or kilns. 2. A method according to claim 1, characterized in that the emission-reducing agent or the emission- reducing agents consist(s) of one or more bases. 3. A method according to claim 1 or 2, characterized in that the combustion plant is a plant for the combustion of commercial, industrial and/or household refuse, from which the heat which is produced may optionally be exploited for industrial heating and/or heating of dwellings and/or for the production of electricity, or a combustion plant producing heat for industrial heating and/or heating of dwellings and/or for the production of electricity by combustion of fossil fuel, or a combination thereof. 4. A method according to any of claims 1-3, characterized in that the mixing of fly ash, one or more emission- reducing agents and material to be combusted takes place in a chute or other feed device arranged upstream of the combustion kiln or kilns of the plant. 5. A method according to any of claims 1-4, characterized in that one or more further emission-reducing agent(s) is/are added to the mixture in the chute or the feed device and/or in the combustion kiln or kilns and/or that one or more further emission-reducing agents is/are added to the wholly or partially combusted material in a last part of the kiln or kilns or after the material has left the kiln or kilns. 6. A method according to any of claims 1-5, characterized in that the emission-reducing agent or agents consist(s) of one or more representatives of the following classes of inorganic bases: oxides, hydroxides, carbonates or hydrogen carbonates of one or more alkali metals or one or more alkaline earth metals, or ammonia or urea. 7. A method according to claim 6, characterized in that the alkali metal or the alkali metals is/are Na and/or K, and that the alkaline earth metal or the alkaline earth metals is/are one or more of the following: Mg, Ca, Sr or Ba. 8. A method according to any of the preceding claims, characterized in that the fly ash is incorporated into the mixture in an amount corresponding to 1-100% by weight, such as 1-50% by weight, more preferably 1-20% by weight, for example 1-10% by weight, especially 1-5% by weight, such as 2-3% by weight. 9. A method according to any of the preceding claims, characterized In that the emission-reducing agent or agents is/are incorporated into the mixture in an amount of at least 50 acid equivalents per ton of material to be combusted, such as 50-1000, preferably 50-500, particularly 50-300 and especially 150-250 acid equivalents per ton of material to be combusted. 10. A method according to any of the preceding claims, characterized in that the mixture of fly ash, emission-reducing agent or agents and material to be combusted is formed in a continuous or discontinuous process by grinding together or otherwise comminuting simultaneously with mixing together of the components of the mixture and/or by the mixing which takes place naturally when the components of the mixture are fed to the combustion kiln or kilns through the chute or chutes or feed device or devices of the combustion plant. 11. A method according to any of the preceding claims, characterized in that the emission-reducing agent or agents is/are in the form of a solid, preferably a finely divided particulate solid, or in the form of an aqueous dispersion or solution. 12. A method according to any of the preceding claims, characterized in that the combustion conditions prevailing in the combustion kiln or kilns comprise a temperature in the range of 875- 1250°C, preferably 900-1200°C, particularly 950-1100°C. 13. A combustion plant for combustion of commercial, industrial and/or household refuse, from which the heat produced may optionally be exploited for industrial heating and/or heating of dwellings and/or for the production of electricity, or a combustion plant producing heat for industrial heating and/or heating of dwellings and/or for the production of electricity by combustion of fossil fuel, characterized in comprising means for adding fly ash and an emission-reducing agent in liquid or solid form to the material to be combusted. 14. A combustion plant according to claim 13, characterized in that said means are adapted to add the fly ash and the emission-reducing material to material to be combusted, in a chute or other feed device arranged upstream of the combustion kiln or kilns of the plant. 15. A combustion plant according to claim 13 or 14, characterized in comprising means for adding one or more further emission-reducing agents to the mixture of fly ash, emission-reducing agent and material to be combusted, in the chute or feed device and/or in the combustion kiln or kilns and/or to the wholly or* partially combusted material in a last part of the combustion kiln or kilns or downstream of the combustion kiln or kilns. 16. A combustion plant according to any of claims 13-15, characterized in comprising a flue gas treatment system or flue gas treatment systems which separate(s) fly ash from the flue gas from the combustion kiln or kilns, and means for recirculating the separated fly ash to the means for adding fly ash. 17. A combustion plant according to claim 16, characterized in that the flue gas treatment system or flue gas treatment systems comprise(s) a filtration system preferably comprising one or more filters, preferably a filter of the bag filter type or the electrostatic filter type.

Inventors: HOLBEK KJELD, HOLBEK, KJELD
Applicants: FLS MILJOE A S, FLS MILJOE A/S
Application year: 1989
Main classification: C10L10
IPC codes: B09B3, C10L9, C10L10, F23G5