Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23G—CREMATION FURNACES; CONSUMING WASTE PRODUCTS BY COMBUSTION
  • F23G5/00—Incineration of waste; Incinerator constructions; Details, accessories or control therefor
  • F23G5/006—General arrangement of incineration plant, e.g. flow sheets
• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
  • C10L10/00—Use of additives to fuels or fires for particular purposes
  • C10L10/02—Use of additives to fuels or fires for particular purposes for reducing smoke development
• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
  • C10L9/00—Treating solid fuels to improve their combustion
  • C10L9/10—Treating solid fuels to improve their combustion by using additives
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23G—CREMATION FURNACES; CONSUMING WASTE PRODUCTS BY COMBUSTION
  • F23G2202/00—Combustion
  • F23G2202/10—Combustion in two or more stages
  • F23G2202/106—Combustion in two or more stages with recirculation of unburned solid or gaseous matter into combustion chamber
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23G—CREMATION FURNACES; CONSUMING WASTE PRODUCTS BY COMBUSTION
  • F23G2209/00—Specific waste
  • F23G2209/30—Solid combustion residues, e.g. bottom or flyash
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E20/00—Combustion technologies with mitigation potential
  • Y02E20/12—Heat utilisation in combustion or incineration of waste

Definitions

• 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.
• 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.
• 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.
• 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.
• 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 byproducts 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.
• 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.
• 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.
• 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 by combustion of refuse normally has a pH of at least 9, typically a pH of 10-11.
• 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.
• 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.
• 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.
• fly ash formed and collected 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.
• 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.
• fly ash is generally a very finely divided particulate material with a low density, typically a density of less than 8001000 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 ⁇ ngstr ⁇ m to 2 ⁇ m, and the agglomerates are typically of the order of up to 100 ⁇ m.
• fly ash formed 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.
• steps must be taken to "hold” the fly ash, i.e., to prevent the fly ash from being spread by the wind, etc.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the present invention now provides a cheap and effective method for disposing of fly ash and reducing the amount of environmentally harmful acid-forming 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.
• 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.
• 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.
• 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.
• 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.
• 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 agent(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.
• fly ash will also function as a distributing agent for the emission-reducing agent(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.
• a reduced pressure i.e., a pressure of the order of 1/10 atm.
• 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.
• Suitable agents for the reduction of nitrogen oxide emission are, for example, ammonia-containing compounds, for example, NH 3 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.
• 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.
• the refuse 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.
• 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 presently preferred bases comprise Ca(OH) 2 , CaO and CaCO 3 as well as NaOH, or mixtures thereof.
• Ca-containing bases are especially useful for reducing sulphur-containing emissions, while NaOH is useful for chlorinecontaining emissions.
• NaOH is useful for chlorinecontaining emissions.
• 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.
• 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.
• 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.
• energy-rich components may, for example, be various types of fossil fuel, such as oil or coal.
• 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 emissionreducing 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.
• 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 agent(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.
• 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, furthermore, be difficult to distribute in a satisfactory manner in the material to be combusted.
• 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.
• 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.
• Substantial sources of SO 2 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 600oC) with release of SO 2 , 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 HCl formation in refuse incineration are, apart from oxidation of chorine-containing organic materials (especially PVC), the reaction between SO 2 and chlorides in the presence of oxygen and water (water vapour).
• An example of the latter reaction is the so-called “Hargreaves reaction” :
• dioxins refers to chlorinated dibenzo-p-dioxins and chlorinated dibenzofurans. This is explained further below:
• 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 calciumcontaining bases must thus be presumed to bring about a reduction in dioxin formation.
• the emission-reducing agent(s) and the material to be combusted are 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.
• 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 device(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.
• This/these agent(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 agent(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 kiln(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, emissionreducing 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 900-1200oC, particularly 9501100°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.
• 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.
• filters preferably a filter of the bag filter type or the electrostatic filter type.
• 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).
• the feed device(s) used suitably comprise (s) a pneumatic system and/or a screw conveyor system.
• emission-reducing agent(s) is/are in the form of a suspension or solution
• suitable feed devices are pumpdriven, base-resistant systems.
• 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.
• 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.
• 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
• 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.
• 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.
• 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.
• 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 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.
• 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.
• 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
• a screw conveyer 3a in which the fly ash is mixed with base.
• 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 2 being supplied via N 2 pipes 34 from N 2 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.
• 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.
• 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 drypressed 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.
• 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
• 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.
• the experimental setup was a 2 m long aluminium silicate tube
• O 2 A Hartmann and Braun paramagnetic O 2 -meter, type MAGNOS 3K.
• SO 2 , CO 2 and CO 3 individual Beckmann infrared photometers, type 864.
• NO ⁇ A Beckmann photoluminescense meter, type 951A.
• HCl By absorption in standard NaOH solution. Cl- and SO 4 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 H 2 SO 3 /H 2 SO 4 ) was determined.
• 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.
• 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.

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• 1988
  • 1988-03-21 DK DK153888A patent/DK153888A/da not_active Application Discontinuation
• 1989
  • 1989-03-21 EP EP89904016A patent/EP0406287A1/de not_active Withdrawn
  • 1989-03-21 WO PCT/DK1989/000063 patent/WO1989009253A1/en not_active Application Discontinuation
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