SE504755C2 - Method and apparatus for separating gaseous pollutants, such as sulfur dioxide and hydrogen chloride, from flue gases formed during combustion in fluidized bed - Google Patents

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

504 755 10 15 20 25 30 35 part consists of sulfur dioxide, they will hereinafter be referred to as sulfur dioxide.

The addition takes place in the fireplace or in a specially adapted flue gas purification system.

FBC and PFBC here refer to combustion boilers of the atmospheric and pressurized fl uidised bed type, respectively.

If coal is used as a fuel in FBC and PFBC plants, limestone (CaCO3) is usually added in the form of a ground or slightly coarser powder next to the coal already in the fireplace to bind the pollutants formed during the combustion of the coal, especially sulfur dioxide. and thereby also prevent or reduce the emission of these pollutants. With a normal lu fi excess, which corresponds to an increased oxygen content of 4-8%, sulfur is bound in this way as sulphate according to the reaction: caco, + so, »få-o, _» caso, + co, This reaction gives up to about 90% binding of sulfur in the temperature range 700-900 ° C, which occurs during vid bed combustion. Limestone dosing in the fireplace, however, requires a relatively large excess of limestone and depending on the combustion temperature, carbon sulfur content, desired sulfur purification degree, etc., a limestone excess of 50 to 200% is required, ie a Ca / S ratio of 1.5 to 3. The excess of limestone is calcined in the fireplace according to the reaction: CaCOs -> CaO + CO 2 With regard to the chlorine content of carbon, which according to the above is significantly less than sulfur, the terrnodynarnics provide much worse conditions for binding of chlorine in the form of calcium chloride in the fireplace, because the formed calcium chloride fireplace temperatures (700-900 ° C) are not stable but decompose in the presence of water vapor according to: cac12 (s) + H2O (g) -> Cao (s) + zHc1 (g) Although binding of sulfur in the fireplace can thereby be made efficient In order to meet the emission requirements that apply to sulfur dioxide, despite the high excess of limestone, most of the chlorine found in the carbon is emitted. Usually the chlorine content of carbon is in a range of 0.05 to 0.2% by weight, corresponding to an emission of 50 to 200 mg HCl / Nm3 flue gas, which is higher than the current emission standards. In Germany, for example, a HCl requirement of a maximum of 100 mg HCl / Nm3 flue gas applies to plants larger than 300 MW thermal power.

To solve this environmental problem, various methods have been proposed for the separation of hydrogen chloride in addition to sulfur dioxide from flue gases.

For example, DE 35 36 899 describes a method for first separating sulfur dioxide and then hydrogen chloride from the flue gases formed during coal combustion in an atmospheric or pressurized flidised bed (Wirbelschichtfeuerung). In this case, sulfur dioxide is first separated by adding an additive, such as limestone, in a reaction channel located downstream of a first mare separator. Hydrogen chloride is then separated by the addition of, for example, fresh limestone, where the flue gases pass through a heat recovery step, in which the additive reacts with the hydrogen chloride. The reaction products formed and the unreacted additive are then separated in a subsequent dust separator. The reaction here preferably takes place in the temperature range 400-500 ° C. At temperatures below 700 ° C, however, the limestone is not calcined, which must be compensated by a relatively large excess of limestone. If, on the other hand, calcium hydroxide (Ca (OH) 2) is used as an absorbent, the effect will be better, but the cost will be significantly higher. The addition of fresh lime absorbent thus makes the purification process expensive in dri fi.

DESCRIPTION OF THE PRESENT INVENTION TECHNICAL PROBLEM It is thus a problem to achieve a satisfactory bonding of chlorine to the ash in the fireplace during coal combustion in an atmospheric or pressurized fl unidised bed.

Although limestone is added in large excess to the bed, most of the chlorine present in the carbon is emitted, as the calcium chloride formed is not stable but decomposes in the presence of water vapor in the temperature range 700-900 ° C, whereby hydrogen chloride gas is formed.

An object of the present invention is therefore to provide a simple and inexpensive process for efficient separation of gaseous impurities, preferably hydrogen chloride, without the addition of expensive, fresh absorbent, such as calcium hydroxide or limestone, in addition to the absorbent which is added to the fireplace to bind sulfur dioxide.

Another object of the present invention is to provide a device which is simple and suitable for carrying out the above-mentioned method.

THE SOLUTION According to the present invention, the above-mentioned problem of achieving a satisfactory separation of gaseous pollutants, such as sulfur dioxide and hydrogen chloride, fi than in the combustion of fossil fuels, such as coal, in atmospheric or pressurized fl unidised bed in the temperature range of 700-900 ° adding in excess an absorbent containing quicklime and / or substances which in this temperature range are converted to quicklime to the bed for reaction with and conversion of these gaseous pollutants in the form of sulfur dioxide to separable particulate pollutants, the particulate gases brought from the bed are separated from the flue gases and then returned to the bed, characterized in that a subset of the burnt lime-containing particles separated from the flue gases is not returned to the bed but is fed to the flue gases in the temperature range 90-200 ° C, preferably 120-150 ° C, for reaction with residual gases in the flue gases gaseous impurities in the form of hydrogen chloride for the conversion of these into particulate pollutants, whereupon the particles formed are separated.

The basic idea according to the invention is thus to utilize the granular ash that accompanies the combustion gases from the fireplace. The subset of the fi n-grained ash extracted here preferably constitutes at least 1% of the said separated particles and preferably contains 2-30% by weight of burnt lime, depending on the limestone dosage, the ash content and composition of the carbon, etc. the constituent particles are supplied to the flue gases in a distributed powder form with a particle size preferably in the range 0-70 μm and constitute a mare load preferably in the range 0, 5-10 g / Nm 3 flue gas.

In order to further improve the separation of hydrogen chloride, it is possible according to a special embodiment of the invention to hydrate the calcareous substance, for the conversion of burnt lime to calcium hydroxide (slaked lime), which especially at low relative fi 504 755 5 in the flue gases significantly improves the HCl separation. During the humidification the reaction takes place: CaO + H 2 O -> Ca (OH) 2 In this quenching reaction, the burnt lime decomposes, which is especially important when the stone is relatively coarse, which is the case when using cyclone dust. The mare product thus formed contains, after hydration, a water content of 5-20% by weight. It is essential that the water content does not become too high, but that the mare product retains its powder character.

The present invention also provides a device suitable for carrying out the above method, comprising a first stone separator, in which the particles entrained from the bed are separated from the flue gases and then returned to the beds, and which are characterized in that a subset of the particles containing burnt lime separated from the flue gases are not returned to the bed but are fed to a contact reactor, in which they are mixed with the flue gases in the temperature range 90-200 ° C, preferably 120-150 ° C, for reaction with gaseous residues in the flue gases impurities in the form of hydrogen chloride, whether the particulate pollutants formed are separated in the first mare separator and / or in a second dust separator.

When the particles contained in said subset are supplied in a distributed powder form to the contact reactor, which is preferably located upstream of the first muffler separator, or downstream of the first and upstream of the second muffler separator, a large part of the hydrogen chloride-sensitive hydrogen chloride gas in the flue gases the constituent calcined lime and form particulate, separable impurities in the form of calcium chloride. In this case, the contact reactor achieves an even distribution of the added calcareous dust product in the flue gas duct upstream of the dust separator. This also increases the contact time and reaction possibilities between the calcareous mare and the hydrogen chloride and thus enables an efficient binding of the hydrogen chloride. The contact reactor and the subsequent dust collector operate according to the present invention in the temperature range 90 to 200 ° C.

A first mare separator preferably consists of one or more cyclones arranged one after the other, while a second mare separator preferably consists of a hose filter or an electrostatic mare separator. The calcareous particles fed to the contact reactor, which are fed to the flue gases in the contact reactor in dispersed powder form, are preferably hydrated in the contact reactor and / or in a separate humidification unit.

DESCRIPTION OF THE PROPOSED EMBODIMENT The invention will now be described in more detail in the following with reference to the accompanying drawing, which schematically shows a plant for the separation of both gaseous pollutants, such as hydrogen chloride and sulfur dioxide, and particulate pollutants from flue gas combustion pollutants. type atmospheric or pressurized fl unidised bed.

In a combustion boiler 1, carbon 2 is added next to limestone 3 to the unidised bed, in order to bind the impurities formed during the combustion of the carbon, in particular sulfur dioxide, whereby an approximately 90% separation in the form of solid stable compounds, such as potassium sulphate, is achieved. . The acidifying gaseous pollutants formed during the coal combustion, such as hydrogen chloride and residual sulfur dioxide, and particles entrained from the bed are led, via a channel 4, into a first mare separator 5 connected to the combustion chamber, which in the embodiment shown consists of a cyclone, in which the particles entrained with the flue gases from the combustion are separated in a first step.

The flue gases are forced here to a rotating motion, whereby at least the larger particles, by the action of the centrifugal force, collide with the walls and fall down towards the tip of the cyclone. The flue gases then proceed from the cyclone and are led via a channel 6 through an energy recovery stage 7, in the form of a heat exchanger while they are cooled and introduced into a contact reactor, which in the embodiment shown consists of an elongate tubular reactor 8. A subset The particles separated from the cyclone, which consist mainly of burnt lime as absorbent, ash gas and calcium sulphate formed during combustion, are fed to tube reactor 8 in a distributed powder form and mixed with the flue gases therein, the calcareous particles react with and convert the gaseous particles to gaseous particles. , separable pollutants. The particulate pollutants formed during the reaction, the unreacted absorbent particles and the remaining flue gases of the flue gases are separated here in a hose filter 9 located downstream of the tubular reactor 8. The flue gases now purified from particulate and gaseous pollutants via a duct 10 are fed to the flue gases. a flue gas fan 11. This feeds via a duct 12 the purified flue gases to a chimney 13 for emissions into the atmosphere.

The particles which are separated in the dust separator 9 are collected in the bottom formed pieces 14 and 15. These particles are conveyed via a line, as indicated by the arrow 16, to a storage silo (not shown).

A subset of the separated particulate matter accumulated in cyclone 5 containing burnt lime is taken out to be used for separating the gaseous impurities remaining in the flue gases, primarily hydrogen chloride. The remainder of the cyclone dust is returned to the flidised bed via a lock 17 and a conduit, as indicated by the arrow 18. The extracted subset of calcareous solid is now led by means of a transport device, as indicated by the arrow 19, to a humidification unit 20, in which the burnt lime in the stone is hydrated, whereby unacceptable, powdered slaked lime (calcium hydroxide) is formed. The moistened calcareous dust is then passed on in a line, as indicated by the arrow 21 to the tube reactor 8, in which this rock is dispersed in the flue gases by means of pressurized lu fi and thereby effectively mixed with the flue gases from the upstream heat exchanger 7. The tube reactor 8 provides an even distribution of the supplied absorbent particles, which leads to an increased contact time and reaction possibility between these particles, ie the affected stone containing burnt lime, and the gaseous impurities present in the flue gases, preferably the hydrogen chloride. Absorption and reaction of the gaseous pollutants and the calcareous rock contained in the flue gases takes place mainly in the tubular reactor 8, but continues to some extent in a dust cake formed during operation on the outside of the filter hoses, which the flue gases must pass through their passage through the hose filter. The subsequent hose filter 9 operates in the temperature range 90-200 ° C.

SUMMARY The invention is of course not limited to the embodiment described above, but can be modified in various ways within the scope of the following claims.

For example, a subset of the particles separated from the second muffler 9 can be fed to the contact reactor 8, preferably after hydration, optionally in combination with a subset of the particles separated in the first muffler 5, 504 755 10 15 20 25 30 35 instead of to use only a subset of the particles separated in the first mare separator 5.

For example, the contact reactor 8 may be located upstream of the first mare separator 5, in which case a first dust separator is sufficient, instead of downstream of the first 5 and upstream of the second mare separator 9.

For example, the first dust separator 5 may consist of one or two cyclones arranged one after the other, instead of a single cyclone.

For example, the second dust collector 9 may be an electrostatic dust collector, instead of a hose filter. According to the present invention, a hose filter is preferable, as it produces lower emissions than an electrostatic precipitator due to the fact that the absorption and reaction between the gaseous contaminants and the calcareous dust can continue during the passage of the dust cake formed under the outside of the tube. .

For example, the lime particles contained in the separated aliquot can be hydrated in the contact reactor 8, instead of in a separate humidification unit 20.

For example, the subset of the separated particles can be fed to the flue gases directly in the inlet to the second mare separator 9, the mare separator 9 then serving as the only contact reactor, instead of supplying these particles to the tube reactor 8.

For example, the energy recovery step 7 may be a steam or hot water parma. In this step, calcareous mare också can also be extracted and used to primarily separate the hydrogen chloride of the flue gases in the same way as the cyclone mare as described above in the proposed embodiment.

EXAMPLE At an incineration plant of approx. 60 MW refueling type of pressurized fl unidised bed, coal was fired with the addition of limestone 100% excess, ie with a Ca / S ratio of 2. The carbon had a sulfur and chlorine content of approx. 1 and 0, respectively. 12% by weight.

The amount of flue gas was about 100 000 Nm3 / h and the first dust collector in this plant consisted of a cyclone. In a complete analysis of the material composition, the following, in% by weight, proportions were found in the bed ash and the cyclone stone: the bed ash consisted mainly of 21.5% unreacted limestone, 105.0 burnt lime and 16.2% calcium sulphate. while the cyclone ash contained 14.0% burnt lime, 20.8% calcium sulphate, non-analyzable proportions of limestone, and a total of about 65% ash (SiO 2, Al 2 O 3, etc.) with certain proportions of unburned material. After the cyclone and the energy recovery step, the flue gases still contained about 180 mg SO2 / Nm3 flue gas and 120 mg HCl / Nm3 flue gas (about 12 kg HCl / h).

In the experiment, about 20% of the cyclone mare was taken out, which in total was about 500 kg / h, and was fed in the inlet to the other mare separator, which consisted of a hose filter. The extra dust load due to this additive thus became 100 kg / h of approximately 100,000 Nm3 / h flue gas or the corresponding 1g / Nm3 flue gas. This stone contained 14% burnt lime, which corresponds to 14 kg CaO / h. At a temperature of 120 ° C in the flue gases and in the hose filter, a significant reduction of the HCl emission was obtained in this way by about 70% to 36 mg HCl / Nm3 flue gas, while the SO 2 emission did not change significantly. In another experiment, about 30% of the separated cyclone dust was taken out and hydrated in a mare humidifier with about 15 kg of water / h. When this hydrated substance was added to the flue gases, a reduction of about 90% of the HCl emission to 12 mg / Nm3 of flue gas was obtained. The SO 2 emission was reduced in this experiment by about 20% down to about 140 mg / Nm3 flue gas.

Claims (11)

504 755 10 15 20 30 10 PATENT CLAIMS A process for separating gaseous pollutants, such as sulfur dioxide and hydrogen chloride, from the combustion of fossil fuels, such as coal, in an atmospheric or pressurized fl -idised bed in the temperature range 700-900 ° C, whereby an absorbent containing burnt lime and / or substances which in this temperature range is converted to burnt lime is added in excess to the bed for reaction with and conversion of these gaseous pollutants in the form of sulfur dioxide to separable, particulate pollutants, whereby particles carried with the flue gases from the bed are separated from the flue gases, where they are recycled to the bed, characterized in that a subset of the particles containing burnt lime separated from the flue gases are not returned to the bed but are fed to the flue gases in the temperature range 90-200 ° C, preferably 120-150 ° C, for reaction with gaseous residues remaining in the flue gases impurities in the form of hydrogen chloride for their conversion into particulate matter serious impurities, after which the formed particles are separated. 2. A method according to claim 1, characterized in that said subset constitutes at least 1% of said separated particles and contains 2-30% by weight of burnt lime. 3. A method according to claims 1-2, characterized in that the particles included in said subset are supplied to the flue gases in a distributed powder form with a particle size in the range 0-70 μm and with a stone load of 0.5-10 g / Nm 3 of flue gas. 4. A method according to claims 1-3, characterized in that the particles included in said subset are hydrated before they are supplied to the flue gases. Device for carrying out the process according to claim 1 for separating gaseous pollutants, such as sulfur dioxide and hydrogen chloride, from flue gases formed during the combustion of fossil fuels, such as coal, in an atmospheric or pressurized bed (1) in the temperature range 700-900 ° C , wherein an absorbent containing quicklime and / or substances which in this temperature range is converted to quicklime is added to the bed for reaction with and conversion of these impurities in the form of sulfur dioxide to separable, particulate pollutants, comprising a first mop separator (5), in which the particles entrained from the bed are separated from the flue gases and then returned to the bed (1), characterized in that a subset of the burnt lime containing the particles separated from the flue gases is not returned to the bed (1) but is fed. to a contact reactor (8), in which they are mixed with the flue gases in the temperature range 90-200 ° C, preferably 1 20-150 ° C, for reaction with gaseous pollutants remaining in the flue gases in the form of hydrogen chloride, after which the particulate pollutants formed are separated in the first dust separator (5) and / or in a second dust separator (9). Device according to claim 5, characterized in that the contact reactor (8) is located upstream of the first dust separator (5). Device according to claim 5, characterized in that the contact reactor (8) is located downstream of the first (5) and upstream of the second dust separator (9). Device according to claims 5-7, characterized in that the first support separator (5) consists of one or more cyclones arranged one after the other. Device according to claims 5-8, characterized in that the second mare separator (9) consists of a hose filter. Device according to claims 5-8, characterized in that the second dust separator (9) consists of an electrostatic dust separator. Device according to any one of the preceding claims, characterized in that the calcareous particles supplied to said contact reactor (8) in said subset are hydrated in the contact reactor and / or in a separate humidification unit (20).