DE3235559A1 - Process for the removal of sulphur oxides from flue gas - Google Patents

Abstract

In a process for the dry removal of sulphur oxides and, if required, other gaseous pollutants from flue gases, a circulating fluidised bed is employed, which contains in particular calcium oxide, calcium hydroxide and/or calcium carbonate as sorbent. In order to achieve a high degree of removal of sulphur oxides and, if required, other pollutants even with large variations in concentration with simultaneously low expenditure on the control aspect, the operating conditions are adjusted in such a manner that the mixing temperature of flue gas and sorbent is at most 150 DEG C, the gas velocity in the fluidised bed reactor is 1 to 10 m/sec (reported as empty tube velocity), the mean suspension density in the fluidised bed reactor is 0.1 to 100 kg/m<3>, preferably 0.2 to 2 kg/m<3>, the mean particle size of the sorbent is 1 to 300 mu m and the amount of sorbent circulated per hour is at least 5 times, preferably 20 to 150 times, the amount of sorbent located in the shaft of the fluidised bed reactor.

Description

Process for removing sulfur oxides from flue gas

The invention relates to a method for dry removal of Sulfur oxides and possibly other gaseous pollutants from flue gases by means of a sorbent guided in a circulating fluidized bed Sodium, potassium, calcium and / or magnesium as cation and oxide, hydroxide and / or Carbonate as an anion.

When burning fossil fuels, flue gases are produced, the - depending on the sulfur content of the starting materials - considerable sulfur oxide contents, in particular, levels of sulfur dioxide may have. Even the steadily increasing Number of waste incinerators produces exhaust gases in addition to sulfur sludge from substances that react with the pollutants are used (Ullmann's Encyklopadie der Techn. Chemie, 3rd Edition, Volume 2/2 (1968>, page 419).

Other processes work on the principle of the so-called dry Gas scrubbing. Thereby the gases are filled with the impurities by a static pouring reacting solids such as activated carbon or lignite coke. the Flue gas cleaning can also be done with the help of a so-called moving layer, in which the solids are increasingly loaded during the downward movement in the reactor and will eventually be discharged. An amount of fresh material corresponding to the discharge is given to the reactor in the upper area (Meier zu Köcker "assessment and prospects of processes for flue gas desulfurization "; V.G.B. Kraftwerkstechnik 53 (1973), page 516 ff.).

In another known method, for example, sulfur oxides are used removed from gases by pneumatically introducing adsorbents into them, the so obtained gas / solid dispersion pneumatically through a reaction section and then passes through a residence zone and then separates the solid from the gas. According to certain In the event of regeneration measures, a partial flow of solids is returned to the gas cleaning system (U.S. Patent 3,485,014).

Finally, there is a device for the dry removal of pollutants known from flue gases, which in the exhaust direction behind the combustion area in a boiler area with a flue gas temperature of 700 to 900 0C. she has a fluidized bed that completely fills the flue gas cross-section and / or a circulating fluidized bed, e.g.

charged with calcium and / or magnesium carbonate as an absorbent is on (DE-OS 30 09 366). In this case, the inflow base of the fluidized bed is expediently chilled.

The main disadvantages of the wet cleaning process are that the accumulating sulfites and sulfates, possibly also Chlorides and Fluoride-containing sludge is difficult to landfill and that the cleaned Flue gases have to be reheated. The well-known dry cleaning methods with stationary or moving bed have disadvantages insofar as because of the coarseness of the absorbents, the binding capacity for those contained in the flue gas Impurities are only used very imperfectly and because of the comparatively low permissible gas velocity as well as the large amounts of gas to be cleaned considerable reactor dimensions are required.

The main disadvantages of the process according to US Pat. No. 3,485,014 are in the requirement of a division of the gas flow to be cleaned and a precise one Dosage of the absorbent in a suitable, especially wear-resistant To have to make device. Another disadvantage is that for a sufficient distance the impurities inadequately long residence time of the gases in the reaction zone or the considerable height that would otherwise be required.

The removal of pollutants from flue gases from 700 to 900 OC by means of the fluidized bed technology causes difficulties insofar as the Rust of the fluidized bed made of extremely heat-resistant material or else must be provided with a cooling system in a procedurally complex manner. In addition, the freshly applied cold sorbent removes palpable sorbents from the flue gas Heat that is then no longer available for steam generation. Farther additional measures are required to reduce the temperature of the smoke gases when they escape Usually hotter from the combustion chamber and usually hotter when exiting the waste heat boiler are colder, to a value in the range of 700 to 900 0C. It requires an economically practically unjustifiable conversion or new building of an already existing one Waste heat systems, whereby special constructions are required.

With a special problem, namely the removal of hydrogen fluoride from waste gases, especially waste gases from aluminum electrolysis, is it is known, the hydrogen fluoride-containing gases as a fluidizing gas in a To conduct fluidized bed reactor and thereby the fluidizing gas velocity in such a way adjust that one is on the fluidized bed reactor, one downstream Cyclone separator and a return line circulating fluidized bed flowing can train (DE-OS 20 56 096). As solids to form the circulating Fluidized bed Aluininiuinoxid and / or sodium aluminate are mentioned, which after sufficient high load can be returned to the fused-salt electrolysis.

The essence of the aforementioned process is not just purification special exhaust gases, but in terms of its objective, in particular, that to obtain fluorine contained in the exhaust gas in a form that allows it to be converted into the To lead back fused-salt electrolysis and thereby reduce the fluorine requirement. In the case of flue gas cleaning, however, the recovery of sulfur oxides is in usually not intended.

Instead, the loaded sorbent is used e.g. for building purposes or used as backfill material, possibly also discarded.

The object of the invention is to provide a method that the Does not have disadvantages of the known gas cleaning process, easy to carry out and can use cheaper absorbents.

The object is achieved by the method of the aforementioned Kind is designed according to the invention in such a way that a) by a corresponding Cooling of the flue gas to the mixed temperature of flue gas and sorbent max 150 Or b) the gas velocity in the fluidized bed reactor to 1 to 10 m / sec (given as empty pipe speed), c) the mean suspension density in the fluidized bed reactor to 0.1 to 100 kg / m³, d) the mean particle size of the Sorbent to 1 to 300 / µm and e) the amount of the hourly sorbent circulation at least 5 times that in the shaft of the fluidized bed reactor Adjusts the amount of sorbent.

The principle of the expanded fluidized bed used in the invention is characterized by the fact that - in contrast to the ~ classical "fluidized bed, in which a dense phase through a clear density jump from the one above Gas space is separated - distribution states without a defined boundary layer exist. There is a leap in density between the dense phase and the dust space above it not available, instead the solids concentration increases within the reactor from bottom to top.

The main sorbents are calcium oxide and calcium carbonate, calcium hydroxide is particularly advantageous because of its particularly high sorption capacity.

The sorbents can be in solid form, but also in the form of aqueous Solutions or suspensions with a solids content of approx. 5 to 20% by weight are supplied will. The introduction of the sorbent in the aqueous phase is preferred because it is generally associated with improved sorption.

The sorbent capacity of the sorbents increases as the mixing temperature falls for flue gas and sorbents. A further development of the method according to the invention therefore provides for the mixing temperature by appropriate cooling of the flue gas from Set the flue gas and sorbent to a value below 90 OC.

Provided that the flue gases to be cleaned contain significant nitrogen oxide contents have, it is advisable to use known agents such as Alkali hydroxide, carbonate, bicarbonate and / or zeolite, or also hydrogen peroxide or add ammonia.

A preferred embodiment of the invention consists in setting the fluidized state in the fluidized bed in such a way that, using the definition of the Froude and Archimedes key figures, the following ranges result: respectively.

whereby and are.

Here: u mean the relative gas velocity in m / sec Ar die Archimedes number For the Froude number the density of the gas in kg / in k k is the density of the Solid particle in kg / in3 dk is the diameter of the spherical particle in m the kinematic viscosity in m2 / sec g the gravitational constant in / sec2.

The cooling of the flue gases to the appropriate temperature can in principle already in the existing waste heat boiler downstream of the incineration plant take place so that they can be entered directly into the fluidized bed reactor.

However, if hotter flue gases are produced, additional cooling is required, e.g. by means of heat exchangers or water injection, which are inserted into the gas flow in front of the Fluidized bed reactor or especially take place in the fluidized bed reactor itself can, required.

The gas velocity to be set should be in relation to the mean Particle size, with smaller particle sizes in the lower range and with larger ones Particle sizes in the upper range are.

The suspension density to be set in the fluidized bed reactor can vary within wide limits. However, it is particularly advantageous to use suspension densities to be selected in the lower area, because then the pressure loss when the flue gas passes through is particularly low due to the fluidized bed. An advantageous embodiment of the The invention therefore provides for the middle suspension-3.

Set density in the fluidized bed reactor to 0.2 to 2 kg / m.

To ensure the highest possible loading of the sorbent with those in the flue gas contained impurities as well as an optimal solid / gas mixture to achieve, provides a further advantageous embodiment of the invention, which Amount of sorbent circulating hourly to 20 to 150 times the im Adjust the amount of sorbent located in the shaft of the fluidized bed reactor.

In particular, the sorbent loading can also be improved, if one in further development of the invention Procedure laden Sorbent activated, e.g. by grinding and again in the circulating fluidized bed returns. For example, the grinding process makes new ones capable of sorption Surface created.

The one used to remove sulfur oxides or other pollutants Sorbent can be used at any point from the fluidized bed reactor, separator and return line formed circulation system are abandoned.

In particular, if the use of carbonate sorbents, how lime and / or dolomite is intended provides an advantageous embodiment of the invention, at least part of the sorbent in the hot flue gas stream to be entered within the waste heat boiler area. This has the merit of being through the sensible heat of the flue gas at least partially splits off carbon dioxide is achieved, creating a particularly active sorbent. The area of the waste heat boiler, which has a temperature of 600-1100 DC, is for this Purpose particularly suitable.

A further development of the method according to the invention, in particular in the event that the sorbent has an average particle size below 50 µm has, consists in the sorption with the simultaneous presence of a support bed forming solid with an average particle size of 100 to 500 mm. Suitable materials for this are e.g. calcium oxide, calcium hydroxide, calcium carbonate, Magnesium oxide, magnesium carbonate, dolomite and / or sand.

It must be taken into account that the solids forming the support bed not necessarily have to be able to remove sulfur oxides or other substances present in the flue gas To bind impurities whose presence essentially serves to reduce the residence time of the fine-grained sorbent within one pass through the fluidized bed reactor to increase. About that addition is due to the comparatively rough Grain spectrum of the support bed material a considerable relative movement between both types of solid present. It also causes the presence of a support bed a further increased solid / gas mixing.

In order to improve the sorbent capacity of the sorbent, it is advantageous, the water vapor partial pressure of the gas in the fluidized bed reactor accordingly 5 to 15, in particular 8 to 12,% by volume of water vapor. Under certain The flue gases meet these requirements, e.g. when burning lignite Already meet the condition. If the water vapor partial pressure is below this, can it can be brought into the area mentioned by the introduction of water or steam.

The water used to make the sorbent suspension is used as the water to consider.

The fluidized bed reactor used for flue gas cleaning can be of rectangular, square or circular cross-section. A nozzle grate can be used as a gas distributor are provided.

Especially with large reactor cross-sections and high gas throughputs however, it is advantageous to design the lower area of the fluidized bed actuator to be conical and to introduce the flue gas through a venturi nozzle. The latter training is because of the particularly low pressure loss and the lack of susceptibility to it Soiling and wear and tear are an advantage.

The sorbent is introduced into the fluidized bed reactor in the usual way, most expediently via one or more lances, e.g. by pneumatic Blowing in. As a result of the good cross-mixing that occurs with circulating fluidized beds a comparatively small number of entry lances is sufficient.

Dry cleaning can be carried out at largely any pressure, e.g. up to about 25 bar. An overpressure becomes in particular must then be provided if the combustion process is carried out under excess pressure will. In the case of sorption under pressure, it must be taken into account that the pressure increases the gas velocity in the fluidized bed reactor in the direction of the lower mentioned Speed is to be set. In general, however, the flue gas cleaning is used at a pressure of approx. 1 bar.

The circulating fluidized bed can be produced using a fluidized bed reactor, a cyclone separator and one in the lower area of the fluidized bed reactor opening return line are formed. In this case it becomes the cyclone separator leaving gas stream of fine cleaning, 2. B. by means of an electrostatic precipitator, subjected.

It is particularly useful to stop the separation of the gases the fluidized bed reactor discharged solids in a directly downstream Make electrostatic precipitators.

This also reduces the pressure loss of the gas. the Using a multi-field electrostatic precipitator also gives the option of using To fractionate the solids discharged from the gases according to the grain size and, for example eject the fine fraction with a correspondingly high load and the coarser or the coarser fractions - if necessary after reactivation - to form the circulating fluidized bed returned to the fluidized bed reactor.

With the help of the method according to the invention, all of the flue gas a combustion process can be cleaned. Depending on the respective According to official requirements, it is also possible to clean a partial flow of the flue gas and then - mixed with the non-cleaned partial flow - into the Guide fireplace. Finally, dry cleaning can also be combined with wet cleaning processes or spray absorption processes can be carried out. This parallel connection has the advantage that reheating, which is practically indispensable in wet cleaning processes of the purified gases to avoid falling below the dew point or a chimney plume can be omitted.

Flue gases in the context of the present invention are exhaust gases from combustion systems based on fossil fuels, from waste incineration plants, from plants for sludge incineration and the like., each with an excess of oxygen or a stoichiometric oxygen supply operate.

The advantages of the method according to the invention are that it is without or only insignificant change in the existing heat exchanger parts behind existing ones Incinerators that can be switched to each other if necessary Form of gas cleaning can be combined with gas cleaning following gas treatments can be omitted and that - on the unit of the area of the fluidized bed reactor related - very high flue gas throughputs are possible. As a result of the high in the circulating Fluidized bed circulating amount of sorbent, which has a considerable buffering effect exercises, the process is suitable, even with strong fluctuations in the sulfur dioxide content of the flue gas to bring about a safe gas cleaning without major control engineering effort.

Furthermore, in addition to sulfur oxides, the process according to the invention other gaseous pollutants, especially NOx, chlorine and / or hydrogen fluoride as well as aerosols, e.g.

in the form of heavy metal compounds, bound to a high degree.

Finally, a decisive advantage of the invention is that that of the circulation of the sorbent via fluidized bed reactor, separator and return line, the solids brought in with the flue gas are also recorded, so that their general ability to produce sulfur oxides and other gaseous or to bind aerosol-like pollutants, is also used.

The invention is explained in more detail and by way of example with reference to the figures and examples described.

1 shows a schematic representation of the circulating fluidized bed; FIGS. 2 to 4 illustrate different possibilities for the circuit of the invention Procedure.

The flue gas to be cleaned is via the conical and Lower part 1 provided with a Venturi nozzle is entered into the fluidized bed reactor 2. Via line 3 and possibly 4, sorbent or support bed is formed Solid supplied. The solid / gas suspension formed leaves the fluidized bed reactor 2 via line 4 and enters the doubtful electrostatic precipitator 5, in which the festival substance is deposited. The coarse fraction collects in the dust bunker 6 and is Recirculated into the fluidized bed reactor 2 via line ~ -7.

The fine fraction occurring in the dust bunker 8 is discharged via line 9. The cleaned flue gas reaches the chimney via line IQ.

Fig. 2 shows the dry cleaning of the entire boiler system K exiting flue gas stream. If necessary, the flue gas can initially pass through a filter Fl can be run to separate dust. It then goes to dry cleaning TR, which works with a directly downstream electrostatic precipitator F2. Dry cleaning TR and electrostatic precipitator F2 correspond to the illustration in Figure 1. Furthermore, a Device RA for reactivating the sorbent separated in the electrostatic precipitator F2 indicated. It is the discharge of the separated electrostatic precipitator contaminated material with VM, the entry of fresh sorbent with SRM and the discharge of loaded sorbent is referred to as BSM.

Fig. 3 illustrates the dry cleaning of a partial flow of the Flue gas. In addition to the device parts according to FIG one By-pass line L provided with which a more or less large partial flow to the Dry cleaning is shown around. The electrostatic precipitator Fl can control the entire flue gas flow otherwise an electrostatic precipitator must be installed in line L.

Fig. 4 shows a combined dry and wet cleaning.

Here, the flue gas flow is partly via the dry cleaning according to TR and F2 and partly via wet cleaning according to NR. Again, there are options a reactivation with RA indicated. The reactivated sorbent can thereby both the dry cleaning TR and the wet cleaning NR are fed. By the interrupted line L is indicated that also in this embodiment By-pass routing of flue gas can be made.

Example 1 A flue gas from a hard coal furnace was to be cleaned, which occurred in an amount of 160,000 m³ / h (in the standard state). The exhaust gas contained (based on the standard condition) 2500 mg / m³ SO2, 200 mg / m³ RCl and 3000 mg / m³ ash The fluidized bed reactor used had one in the cylindrical area Diameter of 3.5 m and a height of 14 m.

The flue gas was via the venturi-like device 1 fed to the fluidized bed reactor 2. Via line 3, quartz sand was with a mean particle size of 200 µm in an amount of 0.5 kg / h and over conduction 4 Calcium hydroxide with an average particle size of 5 µm in an amount of 444 kg / h metered in.

The gas velocity in the fluidized bed reactor 2 was 6 m / sec (indicated as superficial velocity), the mean suspension density approx. 0.4 kg / m³, the Mixing temperature of flue gas and that in the circulating fluidized bed Solid 65 OC.

The exiting at the top of the fluidized bed reactor 2 via line 4 Solid / gas suspension, which has a suspension density of 400 gJm3 (under normal conditions) then got into the two-field electrostatic precipitator 5. In the dust bunker 6 fell 62.82 t of solids per hour, all of which via line 7 into the lower area of the fluidized bed reactor 2 were returned to. About the dust bunker 8 took place the discharge of a total of 1180 kg of solids per hour. This essentially consisted from a mixture of calcium chloride, calcium sulfate and unreacted calcium hydroxide.

The exhaust gas discharged via line 10 contained - to normal related to -625 mg / m³ SO2, <10 mg / m³ HCl and 20 mg / m³ dust That means with one stoichiometric supply of calcium hydroxide based on the SO2 content in the flue gas (based on sulfur oxide) of 1: 1 the degree of sulfur dioxide removal was 75 %.

The supply of calcium hydroxide was doubled, i.e. 888 kg / h given via line 4, an exhaust gas was obtained which 100 mg / m³ SO2, < 10mg / m3 HCl and 20 mg / m3 dust contained. The degree of sulfur dioxide removal was 96%.

The HCl content was practically nonexistent in both test campaigns more detectable. In this case, the solid accumulation in the dust bunker 6 was 62.37 t / h and the one in the dust bunker 8 1630 kg / h.

Example 2 Example 1 was changed to be over conduction 4 the calcium hydroxide in the form of an aqueous suspension with a solid content of 15 wt .-% was abandoned. This changed the gas velocity and the mean suspension density in the fluidized bed reactor is only insignificant. the Mixing temperature of flue gas and that in the circulating fluidized bed Solid was 80 ° C. Negligible.

With a calcium hydroxide dosage corresponding to a stoichiometric one A ratio of 1: 1 was an exhaust gas with - based on the standard state -625 mg / m³ SO2, <10 mg / m³ HCl and 3 20 mg / m dust with a dosage corresponding to a stoichiometric Ratio of 2: 1 with 3 175 mg / m SO2, <10 mg / m³ HCl and 20 mg / m³ dust. The degree of sulfur dioxide removal was thus 75% or 93 t. The salary of HCl could no longer be determined analytically in both cases. The solids balances were essentially unchanged.

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Claims (10)

Claims 1. Process for the dry removal of sulfur oxides and optionally other gaseous pollutants from serving as fluidizing gas Flue gases by means of a sorbent guided in a circulating fluidized bed with sodium, potassium, calcium and / or magnesium as cation and oxide, hydroxide and / or Carbonate as anion, characterized in that a) by appropriate cooling of the flue gas, the mixed temperature of flue gas and sorbent to 0 max 150 c, b) the gas velocity in the fluidized bed reactor to 1 to 10 m / sec (given as superficial velocity), c) the mean suspension density in the fluidized bed reactor to 0.1 to 100 kg / in3, d) the mean particle size of the sorbent to 1 up to 300 #um and e) the amount of the hourly sorbent circulation at least 5 times the amount of sorbent in the shaft of the fluidized bed reactor adjusts. 2. The method according to claim 1, characterized in that the Sorbent in the aqueous phase, preferably with a solids content of 5 up to 20% by weight. 3. The method according to claim 1 or 2, characterized in that the mixed temperature of the flue gas is achieved by cooling the flue gas accordingly and adjusts sorbent below 90 DC. 4. The method according to claim 1, 2 or 3, characterized in that the circulating fluidized bed with regard to the numbers of Archimedes and Froude according to the conditions or operates. 5. The method according to claim 1, 2, 3 or 4, characterized in that that the average suspension density in the fluidized bed reactor is 0.2 to 2 kg / m3 adjusts. 6. The method according to one or more of claims 1 to 5 ,, thereby characterized in that one increases the amount of sorbent circulating every hour 20 to 150 times the amount of sorbent in the shaft of the fluidized bed reactor adjusts. 7. The method according to one or more of claims 1 to 6, characterized characterized in that a partial flow of the sorbent laden sorbent is circulating Removed from the fluidized bed and activated, e.g. by grinding, and again in the circulating Recirculated fluidized bed. 8. The method according to one or more of claims l to 7, characterized characterized in that at least part of the sorbent is in the hot Entries flue gas flow within the waste heat boiler area. 9. The method according to one or more of claims 1 to 8, characterized characterized in that one sorption at the same time At # 'esness a solid forming a support bed with an average particle size of 100 to 500 / µm. 10. The method according to one or more of claims 1 to 9, characterized characterized in that the water vapor partial pressure in the fluidized bed reactor is correspondingly 5 to 15, in particular 8 to 12,% by volume of water vapor is set.