WO2007035169A1

WO2007035169A1 - A method for the cleaning of flue gases and the treatment of ash from the combustion of refuse

Info

Publication number: WO2007035169A1
Application number: PCT/SE2006/050342
Authority: WO
Inventors: Margareta Lundberg
Original assignee: Metso Power Ab
Priority date: 2005-09-21
Filing date: 2006-09-19
Publication date: 2007-03-29
Also published as: SE0502099L; SE529103C2; EP1933971A1

Abstract

In a method for the cleaning of flue gases and the treatment of ash from a boiler with a fluidised bed for the combustion of solid household refuse, in which boiler the flue gases pass from the furnace (1) to a cyclone (5), after which active carbon (18) and slaked lime (19) can be added and the flue gases led through a filter (6), for example a fabric filter, bottom ash (11) from the furnace, cyclone ash (15) from the cyclone, and filter ash (16) from the filter, among other residues, leave the boiler during operation. In order to clean the cyclone ash (15) such that it satisfies the requirements for deposition into a normal landfill with a clear margin, the cyclone ash is leached with leaching fluid (26) during the formation of an ash sludge (21), which is fed to a fluid separator (23), for example a pressure filter, where the process fluid (26) is separated from the ash (25) that has been leached. It is preferable that the process fluid is recycled through a part of it being used for the leaching and for the formation of the ash sludge, and through another part (28) of it being injected into the flue gas flow after the cyclone (5) but before the addition, if any, of active carbon (18) and slaked lime (19).

Description

A METHOD FOR THE CLEANING OF FLUE GASES AND THE TREATMENT OF ASH FROM THE COMBUSTION OF REFUSE

DESCRIPTION

TECHNICAL AREA

The present invention concerns a method for the cleaning of flue gases from the combustion of refuse and the reduction of the volume of environmentally hazardous ash landfills, in which cleaning at least two fractions of ash are bled from the process of combustion and cleaning of flue gases, where the first ash fraction is bled closer to the furnace and has a first level of environmentally hazardous substances and the second ash fraction is bled later from the flue gas passage and has a second level of environmentally hazardous substances.

THE PRIOR ART

Modern boilers for the combustion of solid household refuse often have a fluidised bed or other type of furnace for the combustion, and the flue gases from the furnace of the boiler pass from the boiler and cooling surfaces of the boiler to equipment for the cleaning of flue gases, which may comprise a pre-separator, for example a cyclone, and it may also or alternatively comprise a filter, for example a fabric filter, for the final separation of solid components before the flue gases pass through a flue gas fan out through a chimney, see, for example, US 4 862 813. The cleaning of flue gases can in certain cases comprise several stages of flue gas cleaning using either dry, partially dry or wet cleaning technology in one or several stages in various combinations.

If the furnace is equipped with a fluidised bed, it is normal that larger quantities of ash are generated than the quantities generated by conventional grate boilers where the refuse lies on the bottom during the combustion without the intensive mixing that takes place in boilers with a fluidised bed.

It is appropriate that slaked lime and active carbon be added at the stage before the filter in order to neutralise acidic components in the flue gases and in order to reduce further the release of heavy metals, dioxms and organic pollutants, see, for example, US 5 443 022, US 5 220 111 and US 5 220 112

At least three flows of ash leave the combustion and the cleaning of flue gases. At least one bottom ash is obtained from the combustion in the case in which a flmdised bed is used, or slag in the case in which the combustion of refuse takes place on gratings It is possible subsequently to obtain a second flow of ash, known as "return flue ash" from the return flue of the boiler. The cleaning of flue gases commences after this, which often has a cyclone as its first stage, which separates cyclone ash. A second separation of ash then normally takes place with the aid of a suitable separator, for example a fabπc filter, form which filter ash is obtained.

In order to stabilise heavy metals such as lead and cadmium in the fly ash against leachmg when the ash has been placed in a landfill, it is suggested in US 5 220 111 that the fly ash, which has initially been subjected to treatment with slaked lime m a scrubber, is calcined in the presence of an oxygen-containing gas stream at a temperature 375 ⁰C < T < 800 ⁰C during a peπod 170 s < t < 5 h, whereby the heavy metal level in the leachate from the landfill site is to he below the limit that has been determined by the Environmental Protection Agency (EPA) in the USA. The same result is obtained, according to US 5 220 112, through the fly ash being mixed a calcium-containing compound such that the weight ration between calcium and ash lies between 0 04:1 and 0 5 1, after which calcining is carried out at 475 ⁰C < T < 650 ⁰C for a peπod of 0.5 h < t < 3 h. Requirements for ash quality and the management of different ashes at landfills have successively increased for ashes produced during the combustion of refuse. The EU directive 2000/76/EC contains rules for sampling and the minimisation of ash flows during the combustion of refuse. Furthermore, a decision taken by the EU Council of Ministers (2003/33/EG in Swedish, 2003/33/EC in English) contains cπteπa and methods for assessment when receiving refuse at refuse landfills. This implies, in general, for ashes from refuse combustion in flmdised beds that the bottom ash can be placed into a normal landfill or used as fill mateπal, while cyclone ash lies at the limit of what can be managed at a normal landfill, and filter ash must be sent to a landfill for hazardous waste. The requirements are valid not only for the actual contents of environmentally hazardous substances, they are valid also for the leachability of these substances m the landfill, since it is desired to limit the quantities of the vaπous environmentally hazardous substances that can be leached out and enter surface water or ground water. Thus, the environmentally hazardous substances are to be separated from, or stabilised in, the ash that is placed into the landfill.

The difference in cost between deposition into a normal landfill and into a landfill for hazardous water amounts to SEK 500-1000 per tonne in Sweden in 2005, and thus there is an economic incentive, in addition to the aspects of environmental care, for the cleaning of cyclone ash such that it satisfies with a clear margin the requirements for deposition into a normal landfill.

BRIEF DESCRIPTION OF THE INVENTION

The principal aim of the current invention is thus to use a method of the type described in the introduction to clean the first ash fraction such that it satisfies the requirements for a normal landfill by a clear margin, not only with respect to reducing the amounts of the environmentally hazardous substances but also with respect to reduced leaching of the residues of environmentally hazardous substances that remain in the ash fraction. This is achieved through the first ash fraction being leached with a leaching fluid in order to bind a fraction of the environmentally hazardous substances that are present in the first ash fraction in this leaching fluid, and at least a part of this leaching fluid, with its content of environmentally hazardous substances leached from the first ash fraction, is reintroduced into the flue gas passage before the bleeding position for the second ash fraction, whereby a part of the environmentally hazardous substances that are present in the first ash fraction bind to the second ash fraction, whereby the level of environmentally hazardous substances in the first ash fraction is reduced while the level of environmentally hazardous substances in the second ash fraction increases, with the result that the total volume of ash that is bled from the combustion and that requires further treatment or deposition is reduced. Such a leaching process is relatively simple and does not involve significant cost. It can be carried out either batchwise or continuously.

It is appropriate that at least 50%, preferably at least 75%, of at least one of the environmentally hazardous substances in the first ash fraction is leached from this ash fraction. It is also appropriate that at least 50% of at least one of the environmentally hazardous substances in the first ash fraction is leached out from this ash fraction and is bled through the second ash fraction. The first ash fraction is in this way cleaned such that it safely satisfies the requirements for it to be placed into a normal landfill, while the second ash fraction, which constitutes hazardous waste in any case, will obtain an increased level of leachable pollutants.

According to one preferred embodiment in which the flue gases come from a boiler with a fluidised bed for the combustion of refuse, in which boiler flue gases pass from the furnace to a cyclone, after which active carbon and slaked lime are added and the flue gases are led through a filter, whereby bottom ash from the furnace, cyclone ash from the cyclone, and filter ash from the filter leave the boiler while it is in use, and whereby cyclone ash constitutes the first ash fraction and the filter ash constitutes the second ash fraction, the first ash fraction is mixed, according to the invention, with the leaching fluid and is leached, after which the mixture is fed to a fluid separator, where process fluid is separated from the leached ash.

It is therefore appropriate that the boiler is constructed in such a manner that the fluidised bed is a bubbling or circulating fluidised bed, and that the flue gases pass from the furnace into a withdrawal chute, turn in a return flue and pass onwards into superheaters, pass heating surfaces and through an economiser to the cyclone, and that the return flue ash is separated from the flow of gas in the return flue. Ash that separates spontaneously from the flue gases, by, for example, the flue gases turning in a return flue, are normally denoted "boiler ash" by convention, while the ash that is separated by special equipment (cyclones, electrical filters, etc.) is denoted "ash from pre-separators". This return flue ash is added to the cyclone ash and they are leached together, such that both of these ash fractions can be leached and obtain a lower content of, for example, heavy metals.

Since the ash sediments easily during the leaching, which may be carried out either batchwise or continuously in tanks or columns, it is appropriate to maintain the ash suspended in the leaching fluid by stirring, whereby the leaching is accelerated, the ash particles become porous, they obtain a more irregular shape and become smaller, such that the fraction of larger particles is reduced and the fraction of smaller particles increases. The leached ash is preferably formed to a bed in the fluid separator, from which bed leaching fluid is removed by replacement washing with a washing fluid, preferably water. This may take place, for example, batchwise in a pressure filter, or continuously in a centrifuge with the opportunity to wash the filter cake.

It is appropriate that the ash bed that has been washed in a pressure filter is compacted for the removal of the principal part of the washing fluid, preferably to a dryness level that is greater than 80%. The dryness level in this way increases and the weight of the leached and washed ash that is to be placed into a normal landfill is reduced.

It is appropriate that the fluid removed from the ash bed is led to a process fluid tank. The process water that is separated in the fluid separator contains salts and heavy metals, and it must therefore be cleaned. There are normally no fluid cleaning facilities that can deal with this type of contaminated fluid in refuse combustion plants of the current type. It is therefore appropriate, in order to reduce the release of process water, that a part of the process fluid is recycled to be used in the wash and the formation of the ash suspension. It is also appropriate that another part of the process fluid is recycled and injected into the gas flow after the cyclone, but before the addition of active carbon and slaked lime, whereby the process fluid that has been injected is converted to vapour, and salts and heavy metals are separated out in the filter. To describe it simply, the contaminants have been transferred from the return flue ash (if present) and the cyclone ash to the filter ash, and in this way it has become possible to reduce the deposition costs for the return flue ash and cyclone ash. The filter ash has become somewhat more highly contaminated, but this is going to end up in a landfill for hazardous waste in any case, and this means that the deposition costs for the filter ash are unchanged.

By recirculating process water in the ash leaching process and adding only a small amount of fresh water or other suitable washing fluid, the amount of process fluid is maintained at a low level and a considerable energy loss when the fluid is converted to vapour is avoided. Injection of the process fluid into the gas flow after the cyclone does, however, reduce the temperature of the flue gases, and this temperature reduction can be of the order of 25-50 ⁰C at the volumes of process fluid that it is required to deal with. Thus, the temperature of the flue gases on input to the cyclone should be raised somewhat, in order not to obtain temperatures at the input to the fabπc filter that are too low, appropriately such that the temperature at input to the fabπc filter with the preceding fluid injection will be essentially unchanged from the temperature that was obtained without the preceding fluid injection The process fluid is taken care of in this manner in a simple way, and the cleaning of the flue gases will become somewhat more efficient. The separation of HCl and SO₂ in the filter increases, namely, with increasing moisture levels in the flue gases

BRIEF DESCRIPTION OF THE ATTACHED DRAWING

The invention will hereafter be descπbed in more detail with reference to preferred embodiments and the attached drawing.

The drawing is a simplified sketch showing the principles of a boiler with flue gases cleaning for the combustion of refuse in a fluidised bed with a block diagram of the subsequent flue gas cleaning and the treatment of ash according to one preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The boiler shown in the drawing is of a conventional design and it compπses a furnace 1 for the combustion of solid refuse in a fluidised bed The flue gases pass from the furnace 1 into an withdrawal chute 2, which constitutes the input to a return flue 3, and they pass onwards through a rear chute 4 out of the boiler. The flue gases pass in the return flue 3 and the rear draw 4 boiler components that are not shown in the drawing such as superheaters, heating surfaces, and economisers, where they are cooled to approximately 150 ⁰C The flue gases subsequently pass through gas cleaning equipment, which compπses a cyclone 5 and a filter 6, before they are allowed to escape through a chimney 7 with the aid of a flue gas fan, not shown in the drawing The filter 6 is normally constituted by a conventional fabπc filter, but also other types of filter may be used. The cleaning of the flue gases compπses in the embodiment shown also the addition of active carbon, denoted by the arrow 18, and slaked lime, denoted by the aπow 19, which is carried out either directly in the flue gas channel or in a reactor, which is, in pπnciple, an expanded part of the flue gas channel. The present invention concerns a method for the cleaning of flue gases from the combustion of refuse, and for a reduction in the volume of environmentally hazardous depositions of ash, in which cleaning at least two fractions 15, 16 of ash are bled from the process, where the first ash fraction 15 is bled more closely to the furnace 1 and has a first content of environmentally hazardous substances and the second ash fraction 16 is bled later in the flue gas passage and has a second content of environmentally hazardous substances.

Four flows of ash, shown in the embodiment with arrows, are withdrawn from the boiler and the equipment for flue gas cleaning, namely bottom ash 11, return flue ash 13, cyclone ash 15 and filter ash 16. It has previously been normal for the bottom ash 11 and the return flue ash 13 to be mixed and collected in a silo in order to be placed into a normal landfill or to be used as fill material. The cyclone ash 15, which constitutes the said first ash fraction, and the filter ask 16, which constitutes the said second ash fraction, have been withdrawn either mixed or separately, and they have been so contaminated that it has been necessary under the stricter rules for the handling of ash to take these to a landfill for hazardous waste, which has involved high costs. Thus, it is desirable that the cyclone ash 15 be cleaned such that it satisfies the requirements to be placed into a normal landfill with a clear margin.

Table 1 below shows typical compositions of the ashes that are obtained during the combustion of solid household refuse.

Table 1

Bottom ash Return flue ash Cyclone ash Filter ash

Moisture % 0.2 0.2 0.2 0.375

S % 0.245 0.445 0.735 0.815

C % 0.2 0.2 1.05 3.3

CO₃ % 0.025 0.17 0.94 1.415

C uncombusted % 0.2 0.2 0.2 1.85

Cl % 0.09 0.22 1.495 14.7

Si % 31.15 30.15 17.6 3.585

Na % 6.95 3.69 3.455 2.175

Ca % 6.59 7.385 15.7 31.8

Al % 4.16 6.71 10.945 2.2

Fe % 2.93 2.475 2.72 0.675

K % 1.865 1.85 1.675 0.545

Mg % 0.94 0.795 1.615 0.81

Ti % 0.36 0.68 1.115 0.645

P % 0.175 0.41 0.905 0.39

Ba % 0.16 0.165 0.345 0.13

Mn % 0.07 0.085 0.21 0.065

Cu mg/kg 3125 5310 5310 7540

V mg/kg 30 35 54.5 26

Cr mg/kg 500 475 350 200

Co mg/kg 74 27.5 41 22.5

Ni mg/kg 207.5 230 150 53.5

Zn mg/kg 3375 3890 7310 4885

Pb mg/kg 1215 1025 1405 4470

Cd mg/kg 2 3.85 10.5 49

As mg/kg 69 47 30 41.5

Mo mg/kg 21 35 21.5 17.5

Hg mg/kg 0.02 0.025 0.075 6.32

According to the invention, a first ash fraction 15 is leached in 20 with a leaching fluid 26 in order to bind a fraction of the environmentally hazardous substances that are present in the first ash fraction 15 in this leaching fluid, and at least a portion 28 of this leaching fluid with its content of environmentally hazardous substances leached from the first ash fraction 15 is reintroduced into the flue gas passage before the bleeding position 6 of the second ash fraction 16, by which means a part of the environmentally hazardous substances that are present in the first ash fraction 15 bind to the second ash fraction 16, whereby the level of environmentally hazardous substances in the first ash fraction 15 is reduced while the level of environmentally hazardous substances in the second ash fraction 16 increases, with the result that the total volume of ash that is bled from the combustion and that requires more stringent handling or deposition is reduced.

This can take place, as is shown in the drawing, through cyclone ash 15 being mixed with leaching fluid and leached, after which the mixture is fed to a fluid separator in which the process fluid is separated from the leached ash. The leaching is carried out in the block that is denoted in Figure 1 by 20. The block 20 can comprise, for example, one or more leaching tanks, not shown in the drawing, or it may comprise columns for batchwise or continuous leaching process. Ash sediments easily, such that the ash particles after a sort period will become surrounded by stationary fluid, and it is for this reason appropriate to carry out the leaching duπng stirring of the mixture of ash and leaching fluid in order to accelerate the leaching process. The stirring may be achieved in many ways, as one skilled m the arts will be aware. It is advantageous, for example, to use propeller stirrers with essentially plane blades located obliquely in tanks, while it is possible to allow the leaching fluid in a column to ascend through a sedimentmg bed of ashes.

In the preferred embodiment shown in the drawing, the return flue ash 13 is added to the cyclone ash 15 and they are leached together in the block 20 in order to make it possible to leach both of these fractions in the same treatment step.

The mixture of ash and leaching fluid denoted by 21 is then fed out to a fluid separator block 23, this is carried out in the embodiment shown via a buffer tank 22, which evens out the flows through the leaching block 20 and through the fluid separator block 23. Process fluid is separated in the block 23 from the leached ash. It will be obvious to one skilled in the arts that several different conventional fluid separators can be used. It is preferable that these are of such a type that the leached ash is formed to a bed in the fluid separator, from which bed leaching fluid is removed by replacement washing with water or another suitable fluid, which is denoted in the drawing with reference number 24. This may take place, for example, batchwise in a pressure filter, or continuously in a centrifuge with the possibility of washing the filter cake. The basic design of a pressure filter is made clear by, for example, US 3 342 123, but this design has been improved in recent years in order to control more accurately the formation of the filter cake. See, for example, WO 03/057344.

It is appropriate that the bed of ashes that has been washed in a pressure filter is compressed in order to remove the major part of the leaching fluid, preferably to a dryness level that exceeds 80%. The dryness level increases in this manner and the weight of the leached and washed ash that is to be deposited in a landfill is reduced. The leached and washed ash that emerges from the fluid separator block 23 is denoted in the drawing with the reference number 25. The leaching fluid that has been separated in the block 23 is denoted by reference number 26. This fluid is led to a process fluid tank 27, from which a part of the fluid is recirculated for use as leaching fluid in the leaching block 20. The volume of process fluid is maintained in this manner at a low value, and only a small part of fresh water or other suitable washing fluid is added to the system. The consumption of fresh water amounted in trials to less than 1 m³ per tonne of dry filter cake.

The process fluid contains salts and heavy metals, denoted here as "environmentally hazardous substances", and must therefore be cleaned. There are normally no fluid cleaning facilities that can deal with this type of contaminated fluid in plants for the combustion refuse of solid household waste with dry or partially dry cleaning of flue gases. It would therefore be desirable, in order to obviate the necessity of installing water cleaning equipment, to be able to get rid of this fluid in an alternative manner. According to one preferred embodiment of the invention, therefore, another part of the process fluid, here denoted with the reference number 28, is recycled and injected into the gas flow after the cyclone 5. It is preferable that the injection take place before the addition of active carbon 18 and slaked lime 19, if these additives are added. The process fluid that has been injected will then be converted to vapour, and salts and heavy metals can be easily separated out in the filter 6. To describe it simply, the contaminants have been transferred from the return flue ash 13 (if present) and the cyclone ash 15 to the filter ash 16, and in this way it has become possible to reduce the deposition costs for the return flue ash 13 and cyclone ash 15. The filter ash 16 has become somewhat more highly contaminated, but this is going to end up in a landfill for hazardous waste in any case, and this means that the deposition costs for the filter ash are unchanged.

By recirculating process water 26 in the ash leaching 20 and adding only a small amount of fresh water or other suitable washing fluid 24, the amount of process fluid is maintained at a low level and a considerable energy loss when the fluid is converted to vapour is avoided. Injection of process fluid 28 into the gas flow after the cyclone 5 does, however, reduce the temperature of the flue gases, and this temperature reduction can be of the order of 25-50 ⁰C at the volumes of process fluid that it is required to deal with. Thus, the temperature of the flue gases on input to the cyclone 5 should be raised somewhat, in order not to obtain temperatures at the input to the fabric filter 6 that are too low, appropriately such that the temperature at input to the fabric filter 6 with the preceding fluid injection will be essentially unchanged from the temperature that was obtained without the preceding fluid injection.

A further alternative may be to have an auxiliary burner after the addition of the process fluid. It is also possible to use leaching fluid that is advantageous for the cleaning of the flue gases from a purely chemical point of view, or to add such a fluid to the process fluid. The process fluid 28 is taken care of in this manner in a simple way, and the cleaning of the flue gases will become somewhat more efficient. The separation of HCl and SO₂ in the filter 6 increases, namely, with increasing moisture levels in the flue gases.

TYPICAL OPERATING CONDITIONS

During the operation of a refuse combustion boiler with a boiler power of 20 MW, it is possible to combust approximately 7,100 kg of refuse per hour. Four different fractions of ash are obtained from this combustion: bottom ash, return flue ash, cyclone ash, and filter ash. Essentially equal amounts of these are obtained.

Approximately 450 kg/h of each of the fractions bottom ash, return flue ash, cyclone ash, and filter ash are produced. The volume of process fluid, in this case leaching fluid, that is injected into the flue gas passage before the final cleaning in the filter stage amounts to approximately 450-900 kg/h.

The volume of flue gases generated that passes the final filter stage amounts to approximately 9.8 m³/s of moist gas. It is possible by leaching of the return flue ash and the cyclone ash to leach an ash weight of approximately 450 + 450 kg/h to such a degree that this ash can be deposited without high deposition fees. The weight of filter ash is increased marginally, by a few percent. Given deposition fees for hazardous waste (in Sweden in 2005) of SEK 500-1000 per tonne, a saving of approximately SEK 675 SEK per hour is achieved

(900 kg/h x 750 SEK/ton), which amounts to SEK 16,200 SEK per 24-hour period, and just over SEK 4.8 million per year (300 operating days). Thus the invention ensures large savings and a reduction in the amount of hazardous waste to approximately one third.

Test operation that has been carried out shows that:

1. The cyclone ash and the return flue ash undergo physical and chemical changes during the leaching.

2. Large particles of ash are broken down when mixed with the aqueous leaching fluid, and a fraction of smaller particles increases. 3. The internal surface area of the particles increases significantly through the formation of pores. 4. Most soluble salts are leached out within 5 minutes in a tank with stirring at room temperature, and the leaching fluid that is recycled from the fluid separator has a high pH (i.e. it is strongly basic). 5. Filtering is a suitable method for separating solid substances from the fluid after the leaching step, which is characterised by low filtering resistance in the region from

9.6 x 10⁸ to 4.3 x 10⁹ m/kg.

6. The mother fluid (the leaching fluid) that is retained in the filter cake can be efficiently washed out by replacement washing using a washing fluid volume that is twice the volume of mother fluid.

7. The levels of those substances that in the raw, unleached ash lay over the limiting values determined by the EU for deposition of the ash into normal landfills were reduced in the leached and washed ash to acceptable levels. It has been possible to show in samples that the level of Cl is reduced well over 90% by approximately 92% and the leachability of Cl in the ash fraction after leaching has been reduced by

90-91%. The level of As was not significantly affected, but the amount of As remaining as a residue in the ash fraction after leaching has reduced its leachability by more than 90%. 8. The formation of dust during the transport of ash to the leaching process should be avoided, and the presence of ammonium salts and metallic aluminium in the ash means that it is necessary to take into consideration that ammonia and hydrogen gas may be released from the mixture of ash and leaching fluid.

It is obvious for one skilled in the arts that even though the method according to the invention has been described in detail, numerous modifications of the method are possible within the framework of the attached patent claims.

The invention can be modified in numerous ways within the framework of the patent claims. The different leaching processes and the leaching fluid used can be tailored for the different environmentally hazardous substances that are to be leached from the early ash fractions and that are subsequently to be reintroduced into the flue gas passage in order to be bound to later ash fractions. The leaching can also take place in more than one step, where the first step can be adapted or optimised in order to leach out certain of the environmentally hazardous substances, while other environmentally hazardous substances can be leached out from the ash fraction in other, subsequent steps. Thus, several different leaching fluids can be used, tailored according to the substances that are to be either leached out or stabilised, or both, in the ash fraction after leaching. Thus, it is also possible to use acidic leaching fluids for certain of the environmentally hazardous substances. Various other combinations of equipment for cleaning flue gases can be used. Wet gas cleaning technology can, for example, be used in a flue gas cleaning step between a cyclone and a fabric filter, where fluid from the wet flue gas cleaning step is partially or fully used in order to leach the ash from the cyclone or return flue. The leaching fluid can subsequently be inserted into the flue gas passage between the wet flue gas cleaning step and the fabric filter.

The leaching fluid from ash after leaching can also be inserted into the flue gas passage before a wet flue gas cleaning stage, where the environmentally hazardous substances are bled into the fluid flow from the wet flue gas cleaning stage. A further alternative can be to use the leaching fluid that is obtained as a wet medium in a subsequent wet flue gas cleaning step.

Claims

1. A method for the cleaning of flue gases from the combustion of refuse and for the reduction in the volume of environmentally hazardous ash landfills, in which cleaning at least two fractions (15, 16) of ash are bled in stages from the process of combustion, cooling, and flue gas cleaning, where the first ash fraction (15) is bled closer to the furnace (1) and has a first level of environmentally hazardous substances and the second ash fraction (16) is bled later from the flue gas passage and has a second level of environmentally hazardous substances, c h a r a c t e r i s e d i n that the first ash fraction (15) is leached (20) with a leaching fluid (26) in order to bind in this leaching fluid a fraction of the environmentally hazardous substances that are present in the first ash fraction (15), where at least a part of this leaching fluid (28), with its content of environmentally hazardous substances leached from the first ash fraction (15), is reintroduced into the flue gas passage before the bleeding position (6) of the second ash fraction (16), whereby a part of the environmentally hazardous substances present in the first ash fraction (15) are bound to the second ash fraction (16), whereby the level of environmentally hazardous substances is reduced in the first ash fraction (15) while the level of environmentally hazardous substances in the second ash fraction (16) increases.

2. The method according to claim 1, c h a r a c t e r i s e d i n that at least 50%, preferably at least 75%, of at least one of the environmentally hazardous substances in the first ash fraction (15) is leached out from this ash fraction, and in that at least one of the remaining environmentally hazardous substances in the first ash fraction is stabilised during the leaching process for reduction of the teachability of these environmentally hazardous substances in this ash fraction after the leaching.

3. The method according to claim 1, c h a r a c t e r i s e d i n that at least 50% of at least one of the environmentally hazardous substances in the first ash fraction (15) is leached from this ash fraction and bled via the second ash fraction (16).

4. The method according to any one of claims 1-3, where the flue gases come from a boiler with a fluidised bed for the combustion of refuse, particularly solid refuse, in which boiler flue gases pass from the furnace (1) to a cyclone (5), after which active carbon (18) and slaked lime (19) are added and the flue gases are led through a filter (6), whereby bottom ash (11) from the furnace (1), cyclone ash (15) from the cyclone (5) and filter ash (16) from the filter (6) are removed from the boiler during operation, and whereby the cyclone ash (15) constitutes the first ash fraction and the filter ash (16) constitutes the second ash fraction, characterised in that the cyclone ash

(15) is mixed with leaching fluid (26) and leached (in 20), after which the mixture (21) is fed to a fluid separator (23), where process fluid (26) is separated from the leached ash (25).

5. The method according to claim 4, whereby the fluidised bed is a bubbling fluidised bed and the flue gases pass from the furnace (1) into a withdrawal chute (2), return through a return flue (3) and pass onwards into superheaters, past heating surfaces and through an economiser to the cyclone (5), and whereby return flue ash (13) is separated from the flow of gases in the return flue (3), characterised in that the return flue ash (13) and the cyclone ash (15) are brought together and leached together.

6. The method according to any one of claims 1-5, characterised in that the leaching (in 20) is carried out under stirring in order to maintain the ash in suspension.

7. The method according to any one of claims 4-5, characterised in that the ash after leaching is formed to a bed in the fluid separator (23), from which bed leaching fluid (26) is removed by replacement washing with washing fluid (24), preferably water.

8. The method according to claim 7, characterised in that the washed bed of ashes is compacted in order to remove the principal part of the washing fluid to a dryness level greater than 50%, preferably to a dryness level greater than 80%.

9. The method according to claim 7 or 8, characterised in that the fluid (26) removed from the bed of ashes is passed to a process fluid tank (27).

10. The method according to any one of claims 1-9, characterised in that a part of the leaching fluid (26) after separation from the ashes after leaching is recycled for renewed use during the leaching (in 20).

11. The method according to any one of claims 1-10, characterised in that return of the leaching fluid (28) into the flue gas flow takes place before a position for the addition of active carbon (18) and slaked lime (19) between the bleeding positions (5 and 6) for the first ash fraction and the second ash fraction.

12. The method according to claim 11, characterised in that the temperature of the flue gases at input to the bleeding position (5) for the first ash fraction (15) is raised in order to compensate for the cooling effect of the injected leaching fluid (28), such that the temperature of the flue gases on input to the bleeding position (6) for the second ash fraction (16) remains essentially unchanged.