METHOD AND DEVICE FOR SEPARATING GASEOUS POLLUTANTS

Abstract: In a method and a device for separating gaseous pollutants, such as hydrogen chloride and sulphur dioxide, from flue gases generated upon the combustion of fossil fuel, such as coal, in an atmospheric or pressurised fluidised bed (1) in the temperature range of 700-900 DEG C, an absorbent containing burnt lime and/or substances that, in this temperature range are converted to burnt lime, is added in excess to the bed in order to react with these gaseous pollutants in the form of sulphur dioxide and convert them to separable, particulate pollutants. The particles entrained by the flue gases from the bed are separated from these gases in a first dust separator (5). A partial amount of the particles separated from the flue gases and containing burnt lime is hereby not recycled to the bed (1) but instead supplied to a contact reactor (8), in which they are mixed with the flue gases in the temperature range of 90-200 DEG C, preferably 120-150 DEG C, in order to react with the remaining gaseous pollutants in the form of hydrogen chloride in the flue gases. Then, the resulting particulate pollutants are preferably separated in a second dust separator (9).

Description: METHOD AND DEVICE FOR SEPARATING GASEOUSES POLLUTANTS FIELD OF THE INVENTION This invention relates to a method and a device for separating gaseous pollutants, such as sulphur dioxide and hydrogen chloride, from flue gases generated upon the combustion of fossil fuel, such as coal, in an atmospheric or pressurised fluidised bed in the temperature range of 700-900°C. An absorbent containing burnt lime and/or substances that, in this temperature range, are converted to burnt lime is added in excess to the bed in order to react with these gaseous pollutants in the form of sulphur dioxide and convert them to separable, particulate pollutants. The particles entrained by the flue gases from the bed are hereby separated from the gases and then recycled to the bed. DESCRIPTION OF THE PRIOR ART The discharge of acidifying gases, such as hydrogen chloride and sulphur dioxide, from various combustion processes in, for instance, coal-fired power plants and waste incineration plants is an increasing problem. There exist a great number of different methods for reducing discharges of this type. Thus, it has been suggested that the fuel be cleaned, that measures be taken during the combustion proper, and/or that the flue gases generated upon combustion be cleaned. Certain pollutants, such as nitrogen oxides, can be resolved into harmless substances, such as nitrogen gas and water vapour, by a catalytic process in which ammonia is added as reducing agent. These pollutants may also be reduced to a certain extent by optimising the combustion process, thereby to prevent or render more difficult their formation. Unlike conventional waste incineration of e.g. household waste, the combustion of coal involves a fuel containing much more sulphur than chlorine, resulting in higher contents of sulphur dioxide than of hydrogen chloride in the generated flue gases. In coal firing, the molar ratio of chlorine to sulphur usually is in the range of 0.03-0.5, which is to be compared with a quotient of about 2 for waste incineration. However, these acidifying pollutants both pose an environmental problem In flue-gas cleaning in connection with coal combustion in FBC and PFBC plants for the separation of various acid pollutants, primarily sulphur-oxide compounds (SO 2 and SO 3 ), one usually adds an absorbent containing those substances that form solid and stable compounds with the pollutants. Since the sulphur-oxide compounds chiefly consist of sulphur dioxide, they will in the following be referred to as sulphur dioxide. The absorbent is added in the hearthcombustion chamber or in a special flue-gas-cleaning system. FBC and PFBC here refer to furnaces of the type atmos¬ pheric or pressurised fluidised bed. If coal is used as fuel in FBC and PFBC plants, limestone (CaCOj) in the form of a finely-ground or slightly coarser powder is usually added along with the coal already in the hearthcombustion chamber in order to bind the pollutants formed upon the combustion of coal, especially sulphur dioxide, hence preventing or reducing the discharge of these pollutants. With a normal excess of air, corresponding to an increased oxygen content of 4-8%, sulphur is in this manner bound as sulphate in accordance with the reaction CaCO 3 + SO 2 + -O 2 → CaSO 4 + CO ; This reaction results in a binding of sulphur of up to approximately 90% in the temperature range of 700-900°C, which is prevalent in fluid-bed combustion. However, the dosage of limestone in the hearthcombustion chamber requires a fairly large excess of limestone and, depending on the combustion temperature, the sulphur content of the coal, the aimed-at degree of sulphur cleaning and so forth, an excess of limestone of 50-200%, i.e. a Ca/S ratio of 1.5-3, is required. The excess of limestone is calcinated in the hearthcombustion chamber in accordance with the reaction CaCO j → CaO + CO, As to the chlorine content of the coal, which as indicated above is much lower than the sulphur content, the thermodynamics provide much poorer conditions for binding chlorine in the form of calcium chloride in the hearthcombustion chamber, since the calcium chloride formed at the temperatures (700-900°C) prevailing in the hearth is not stable but disintegrates in the presence of water vapour in accordance with the reaction CaCl 2 (s) + H 2 O (g) → CaO (s) + 2HC1 (g) Even though the binding of sulphur in the hearthcombustion chamber can thus be rendered efficient in order to meet the emission requirements placed on sulphur dioxide, the main part of the chlorine found in the coal is emitted, despite a con¬ siderable excess of limestone. As a rule, coal has a chlorine content in the range of 0.05-0.2% by weight, corresponding to a discharge of 50-200 mg of HCl/Nm 3 of flue gas, which often exceeds current emission standards. In Germany, for instance, there is a maximum limit of 100 mg of HCl/Nm 3 of flue gas for plants having a thermal effect exceeding 300 MW. In order to solve this environmental problem, one has suggested different methods for separating hydrogen chloride, as well as sulphur dioxide, from flue gases. DE 35 36 899, for instance, discloses a method for separating first sulphur dioxide and then hydrogen chloride from the flue gases generated upon the combustion of coal in an atmospheric or pressurised fluidised bed (Wirbelschichtfeuerung). Thus, sulphur dioxide is first separated by the addition of an additive, such as limestone, in a reaction channel arranged downstream from a first dust separator. Then, hydrogen chloride is separated by the addition of e.g. fresh limestone, the flue gases passing through a heat-recovery stage in which the additive reacts with hydrogen chloride. The hereby formed reaction products and the unreacted additive are then separated in a subsequent dust separator. Preferably, the reaction takes place in the temperature range of 400-500°C. At temperatures below 700°C, the limestone is not, however, calcinated, which has to be compensated for by a fairly large excess of limestone. Using, on the other hand, calcium hydroxide (Ca(OH) 2 ) as absorbent is more effective, but also much more expensive. Thus, the addition of fresh lime absorbent results in a costly cleaning process. DESCRIPTION OF THE PRESENT INVENTION TECHNICAL PROBLEM It is thus a problem to achieve satisfactory binding of chlorine to the ash in the hearthcombustion chamber upon the combustion of coal in an atmospheric or pressurised fluidised bed. Despite the fact that limestone is added in considerable excess to the bed, the main part of the chlorine found in the coal is emitted, since the calcium chloride formed is not stable but disintegrates in the presence of water vapour in the prevailing temperature range of 700-900°C, resulting in the formation of hydrogen-chloride gas. One object of this invention is, therefore, to provide a simple and inexpensive method for effective separation of gaseous pollutants, preferably hydrogen chloride, without resorting to the addition of a fresh and costly absorbent, such as calcium hydroxide or limestone, besides the absorbent added to the hearthcombustion chamber in order to bind sulphur dioxide. Another object of the invention is to provide a device of simple design, which is suitable for implementing the above method. THE SOLUTION According to the invention, the above problem concerning the achievement of satisfactory separation of gaseous pollutants, such as sulphur dioxide and hydrogen chloride, from flue gases generated upon the combustion of fossil fuel, such as coal, in an atmospheric or pressurised fluidised bed in the temperature range of 700-900°C, is solved by adding in excess to the bed an absorbent containing burnt lime and/or substances that, in this temperature range, are converted to burnt lime, with a view to achieving a reaction with these gaseous pollutants in the form of sulphur dioxide and a conversion thereof to separable, particulate pollutants. The particles entrained by the flue gases from the bed are separated from the gases and then recycled to the bed. This method is characterised in that a partial amount of the particles separated from the flue gases and containing burnt lime is not recycled to the bed but instead supplied to the flue gases in the temperature range of 90-200°C, preferably 120- 150°C, in order to react with the remaining gaseous pollutants in the form of hydrogen chloride in the flue gases, so as to convert these pollutants to particulate pollutants, whereupon the resulting particles are separated. The basic idea behind the invention thus is to utilise the fine-grained ash accompanying the combustion gases from the hearthcombustion chamber. The partial amount of fine-grained ash drawn off preferably makes up at least 1% of the separated particles and preferably contains 2-30% by weight of burnt lime, depending on the dosage of limestone, the ash content and composition of the coal, and so forth. The particles of this partial amount are supplied to the flue gases in finely-divided pulverulent form, the particle size preferably being in the range of 0-70 mm, and constitute a dust load preferably falling in the range of 0.5-10 g/Nm 3 of flue gas. In order to further improve the separation of hydrogen chloride, it is, in accordance with a special mode of implementation of the invention, possible to hydrate the lime-containing dust, thereby to convert burnt lime to calcium hydroxide (slaked lime), which considerably improves the HC1 separation, especially when the flue gases have a low relative humidity. During moistening, the following reaction takes place CaO + H,O → Ca(OH) 2 In this slaking reaction, the burnt lime is disintegrated, which is of special importance when the dust employed is fairly coarse, as is the case when use is made of cyclone dust. The resulting dust product has, after hydration, a water content of 5-20% by weight. It is essential that the water content should not be too high, but that the dust product should retain its pulverulent character. The present invention further provides a device which is suited for use in the implementation of the above method and which comprises a first dust separator in which the particles entrained from the bed are separated from the flue gases and then recycled to the bed. This device is characterised in that a partial amount of the particles separated from the flue gases and containing burnt lime is not recycled to the bed but instead supplied to a contact reactor, in which they are mixed with the flue gases in the temperature range of 90-200°C, preferably 120-150°C, in order to react with the remaining gaseous pollutants in the form of hydrogen chloride in the flue gases, whereupon the resulting particulate pollutants are separated in the first dust separator and/or in a second dust separator. When the particles of this partial amount are, in finely-divided pulverulent form, supplied to the contact reactor, which preferably is arranged upstream from the first dust separator, or arranged downstream from the first dust separator and upstream from the second dust separator, a large amount of the hydrogen-chloride gas found in the flue gases will react with the burnt lime forming part of the amount of particles supplied, thereby forming particulate and separable pollutants in the form of calcium chloride. The contact reactor achieves an even distribution of the supplied lime-containing dust product in the flue-gas channel upstream from the dust separator. Also, the contact reactor prolongs the contact time and enhances the possibilities of a reaction between the lime-containing dust and the hydrogen chloride, thereby enabling an effective bonding of hydrogen chloride. According to the invention, the contact reactor and the following dust separator operate in the temperature range of 90-200°C. Preferably, the first dust separator consists of a cyclone or several successive cyclones, while the second dust separator preferably consists of a bag filter or an electrostatic precipitator. The lime-containing particles fed to the contact reactor, which are supplied to the flue gases in the contact reactor in dispersed pulverulent form, are preferably hydrated in the contact reactor and/or in a separate moistening unit. DESCRIPTION OF A PROPOSED EMBODIMENT The invention will now be described in more detail with reference to the appended drawing, which schematically illustrates a plant for separating gaseous pollutants, such as hydrogen chloride and sulphur dioxide, as well as particulate pollutants, from flue gases generated upon the combustion of coal in a furnace of the type atmospheric or pressurised fluidised bed. In a furnace 1, coal 2 is, besides limestone 3, added to the fluidised bed in order to bind the pollutants formed upon the combustion of coal, primarily sulphur dioxide, resulting in an approximately 90% separation in form of solid stable compounds, such as calcium sulphate. The acidifying gaseous pollutants formed upon the combustion of coal, such as hydrogen chloride and remaining sulphur dioxide, are, along with particles entrained from the bed, conducted through a channel 4 to a first dust separator 5. This separator is connected to the furnace 1 and, in the embodiment illustrated, consists of a cyclone, in which the particles entrained by the flue gases from the combustion are separated in a first stage. The flue gases are here compelled to perform a movement of rotation, such that at least the larger particles collide with the walls and drop towards the narrow end of the cyclone under the action of the centri- fugal force. The flue gases then leave the cyclone and are conducted, via a channel 6, through an energy-recovery stage 7 in the form of a heat exchanger, while being cooled and conducted to a contact reactor which, in the embodiment illustrated, consists of an elongate tube reactor 8. A partial amount of the particles separated in the cyclone, which chiefly consist of burnt lime as absorbent, fly ash and calcium sulphate formed upon the combustion, is, in finely-divided pulverulent form supplied to the tube reactor 8 and there mixed with the flue gases, such that the lime-containing particles react with the gaseous pollutants, converting them to particulate, separable pollutants. The particulate pollutants formed upon the reaction, the unreacted absorbent particles and the fly ash remaining in the flue gases are then separated in a bag filter 9 arranged downstream from the tube reactor 8. The flue gases thus cleaned of particulate and gaseous pollutants are then conducted through a channel 10 to a flue-gas fan 11, which feeds the cleaned flue gases through a channel 12 to a chimney 13, where they are discharged into the surrounding atmosphere. The particles separated in the dust separator 9 are collected in hoppers 14 and 15 arranged at the bottom of the separator. These particles are, via a conduit, conducted to a storage silo (not shown), as indicated by the arrow 16. A partial amount of the separated, particulate dust accumulated in the cyclone 5 and containing burnt lime is drawn off to be used for separating the gaseous pollutants remaining in the flue gases, primarily hydrogen chloride. The remainder of the cyclone dust is recycled to the fluidised bed via a sluice 17 and a conduit, as indicated by the arrow 18. The partial amount of lime-containing dust drawn offis then conducted by a conveyor, as indicated by the arrow 19, to a moistening unit 20, in which the burnt lime in the dust is hydrated, resulting in the formation of fine-grained, pulverulent slaked lime (calcium hydroxide). The moistened lime-containing dust is thereafter conducted through a conduit, as indicated by the arrow 21, to the tube reactor 8, where the dust is dispersed in the flue gases with the aid of pressurised air and hence is effectively mixed with the flue gases from the heat exchanger 7 arranged upstream. The tube reactor 8 achieves an even distribution of the absorbent particles supplied, resulting in a prolonged contact time and enhanced possibilities of a reaction between these particles, i.e. the moistened dust containing burnt lime, and the gaseous pollutants found in the flue gases, preferably the hydrogen chloride. Absorption and reaction of the gaseous pollutants and the lime-containing dust in the flue gases chiefly take place in the tube reactor 8, but continue to a certain extent in the dust cake that the flue gases have to pass on their way through the bag filter 9. The tube reactor 8 and the following bag filter 9 operate in the temperature range of 90-200°C. SUMMARY It goes without saying that the invention is by no means restricted to the embodiment described above, but that it may be modified in many ways within the scope of the appended claims. For example, a partial amount of the particles separated in the second dust separator 9 may be supplied to the contact reactor 8, preferably after hydration and optionally in combination with a partial amount of the particles separated in the first dust separator 5. This can be done instead of using only a partial amount of the particles separated in the first dust separator 5. For example, the contact reactor 8 may be arranged upstream from the first dust separator 5, in which case use need only be made of this first dust separator, instead of be arranged downstream from the first dust separator 5 and upstream from the second dust separator 9. For example, the first dust separator 5 may consist of several successive cyclones, instead of but a single cyclone. For example, the second dust separator 9 may consist of an electrostatic precipitator instead of a bag filter. According to the invention, use is preferably made of a bag filter, since such a filter mostly results in lower emissions than an electrostatic precipitator, owing to the fact that the absorption and the reaction between the gaseous pollutants and the lime-containing dust may continue during the passage of the gases through the dust cake formed on the outside of the filter bags in operation. For example, the lime-containing particles of the separated partial amount may be hydrated in the contact reactor 8, instead of in a separate moistening unit 20. For example, the partial amount of separated particles may be supplied to the flue gases directly at the inlet to the second dust separator 9, in which case the latter serves as the only contact reactor, instead of be supplied to the tube reactor 8. For example, the energy-recovery stage 7 may consist of a steam or hot-water boiler. In this stage, also lime-containing dust can be drawn off and utilised, primarily for separating hydrogen chloride from the flue gases in the same manner as the cyclone dust, as has been described above in connection with the proposed embodiment. EXAMPLE In a combustion plant having an output of approximately 60 MW and being of the type pressurised fluidised bed, coal to which was added limestone in an excess of 100%, i.e. with a Ca/S ratio of 2, was burnt. The coal had a sulphur content of approximately 1% by weight and a chlorine content of approximately 0.12% by weight. The amount of flue gas was approximately 100,000 Nm 3 /h, and the first dust separator of the plant consisted of a cyclone. A complete analysis of the composition of the material revealed that, in % by weight, the bed ash chiefly consisted of 21.5% of unreacted limestone, 15.0% of burnt lime and 16.2% of calcium sulphate, while the cyclone dust contained 14.0% of burnt lime, 20.8% of calcium sulphate, non-analysable amounts of limestone, and a total of approximately 65% of ash (SiO 2 , Al 2 O , etc.) containing certain amounts of unbumt material. After the cyclone and the energy-recovery stage, the flue gases still contained approximately 180 mg of SO 2 /Nm 3 of flue gas and 120 mg of HCl/Nm 3 of flue gas (approximately 12 kg of HCl/h). In the test, about 20% of the cyclone dust, which in total was about 500 kg/h, was drawn off and supplied at the inlet to the second dust separator, which consisted of a bag filter. The additional dust load due to this addition thus was 100 kg/h per 100,000 Nm 3 /h of flue gas or an equivalent of 1 g/Nm 3 of flue gas. This dust contained 14% of burnt lime, which corresponds to 14 kg of CaO/h. At a temperature of 120°C in the flue gases and the bag filter, this resulted in a considerable reduction of the HC1 emission by approximately 70% to 36 mg of HCl/Nm 3 of flue gas, while the SO 2 emission was not noticeably influenced. In another test, about 30% of the separated cyclone dust was drawn off and hydrated in a dust moistener with approximately 15 kg of water/h. When this hydrated dust was added to the flue gases, the HC1 emission was reduced by approximately 90% to 12 mg/Nm 3 of flue gas. In this test, the SO 2 emission was reduced by approximately 20% to 140 mg/Nm 3 of flue gas.

Claims: CLAIMS 1. A method for separating gaseous pollutants, such as sulphur dioxide and hydrogen chloride, from flue gases generated upon the combustion of fossil fuel, such as coal, in an atmospheric or pressurised fluidised bed in the temperature range of 700-900°C, an absorbent containing burnt lime and/or substances that, in this temperature range, are converted to burnt lime being added in excess to the bed in order to react with these gaseous pollutants in the form of sulphur dioxide and convert them to separable, particulate pollutants, and particles entrained by the flue gases from the bed being separated from the gases and then recycled to the bed, characterised in that a partial amount of the particles separated from the flue gases and containing burnt lime is not recycled to the bed but instead supplied to the flue gases in the temperature range of 90-200°C, preferably 120-150°C, in order to react with the remaining gaseous pollutants in the form of hydrogen chloride in the flue gases, thereby to convert these pollutants to particulate pollutants, whereupon the resulting particles are separated. 2. A method as claimed in claim 1, characterised in that said partial amount makes up at least 1% of said separated particles and contains 2-30% by weight of burnt lime. 3. A method as claimed in claims 1 and 2, characterised in that the particles of said partial amount are supplied to the flue gases in finely-divided pulverulent form, the particle size being in the range of 0-70 mm and with a dust load being 0.5-10 g/Nm 3 of flue gas. 4. A method as claimed in claims 1-3, characterised in that the particles of said partial amount are hydrated before being supplied to the flue gases. 5. A device for implementing the method claimed in claim 1 to separate gaseous pollutants, such as sulphur dioxide and hydrogen chloride, from flue gases generated upon the combustion of fossil fuel, such as coal, in an atmospheric or pressurised fluidised bed (1) in the temperature range of 700-900°C, an absorbent containing burnt lime and/or substances that, in this temperature range, are converted to burnt lime being added in excess to the bed (1) in order to react with these pollutants in the form of sulphur dioxide and convert them to separable particulate pollutants, said device comprising a first dust separator (5), in which the particles entrained from the bed are separated from the flue gases and then recycled to the bed (1), characterised in that a partial amount of the particles separated from the flue gases and containing burnt lime is not recycled to the bed ( 1 ) but instead supplied to a contact reactor (8), in which they are mixed with the flue gases in the temperature range of 90-200°C, preferably 120-150°C, in order to react with the remaining gaseous pollutants in the form of hydrogen chloride in the flue gases, whereupon the resulting particulate pollutants are separated in the first dust separator (5) and/or in a second dust separator (9). 6. A device as claimed in claim 5, characterised in that the contact reactor (8) is arranged upstream from the first dust separator (5). 7. A device as claimed in claim 5, characterised in that the contact reactor (8) is arranged downstream from the first dust separator (5) and upstream from the second dust separator (9). 8. A device as claimed in claims 5-7, characterised in that the first dust separator (5) consists of one or more successive cyclones. 9. A device as claimed in claims 5-8, characterised in that the second dust separator (9) is a bag filter. 10. A device as claimed in claims 5-8, characterised in that the second dust separator (9) is an electrostatic precipitator. 11. A device as claimed in any one of the preceding claims, characterised in that the lime-containing particles of said partial amount supplied to the contact reactor (8) are hydrated in the contact reactor and/or in a separate moistening unit (20).

Inventors: BENGTSSON SUNE, BENGTSSON, SUNE
Applicants: BENGTSSON SUNE, FLAEKT AB, ABB FLAEKT AB, BENGTSSON, SUNE
Application year: 1995
Main classification: B01D53
IPC codes: B01D53