DE2056117A1 - Process for removing sulfur dioxide from exhaust gases - Google Patents

Description

Dipl.-Ing, H. G. Hebbel November 12, 1970 20561

Patent attorney 27. 6

PATENT APPLICATION

Applicant; The Babcock & Wilcox Company Corporation, 161 East 42nd Street, New York, N ₀ Y. 10017 U ₀ S, A ₀

¹¹ Procedure for removing sulfur dioxide from exhaust gases "

The invention relates to a method for removing sulfur dioxides from exhaust gases that have previously been dedusted and cooled.

Known to contain at Metallverhüttungsanlagen, chemical plants, power plants and the like ₀ exhaust gases resulting sulfur oxides, that is, harmful substances which cause in the open air damage. When fossil fuels containing sulfur are burned, the gaseous products also contain sulfur oxides, which, if released into the atmosphere, represent a nuisance to the environment.

So far, many attempts have been made to reduce the sulfur oxide content of smoke gases released into the atmosphere, and from a technical point of view there were many of them Attempts to be successful from a strictly economic point of view

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however, the removal of sulfur dioxide from the combustion products of fossil fuels cannot be justified. Under pressure from public opinion, the reduction or elimination of this form of air pollution is being pushed forward and public utilities in particular are being forced to reduce the pollutants of chimney emissions into the air ₀

The invention is accordingly based on the object of the degree of separation the procedures used in removing SO_ from flue gases to improve while converting the sulfur into a usable or salable form to overcome the economic disadvantage of such a system to a minimum and at the same time the air pollutants emitted into the atmosphere Reduce emissions.

According to the invention, this object is achieved with previously dedusted and cooled Dissolved exhaust gases in that the exhaust gases are washed in an aqueous absorption medium to remove the sulfur-containing To absorb gases. The essentially dusty and gaseous Sulfur-free flue gases are then released into the atmosphere. According to the invention, the absorption medium is an aqueous magnesium compound containing solution in which the SO «is absorbed and then magnesium sulphite and magnesium bisulphite are formed and in which the sulphite is then precipitated in crystal form from the solution by adding magnesium oxide pulp and / or magnesium hydroxide will. The magnesium sulphite crystals are then precipitated from the solution and the solution is returned to the absorption zone.

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out, and the deposited crystals are thermally treated to the decomposition into reactive MgO particles and gaseous SO bewirJken ₀ The MgO is recycled into the absorption zone, while the SO., which may be of high or low purity, then in a salable product, such as B. sulfuric acid, elemental sulfur or liquefied S0 "can be converted"

Although the invention is illustrated and described in application to a coal-fired industrial boiler as an example, it is not limited to such an application. It can also be used in connection with the combustion of other fossil fuels and in other types of plants described in the introduction. Sulphurous, liquid or even gaseous fuels can be burned in addition to solid fuels in the furnace described below and the heat generated by the combustion can also be used for purposes other than steam generation _{or the like}

A graphical example is shown in simplified form in the drawing and is described in more detail below

In the boiler system 10, coal dust is burned in a manner known from the prior art _o The hot combustion gases give off their heat in direct contact with the heating surfaces in order to generate steam and heat ₀ The flue gases exit the boiler system 10 through a channel 11 with a temperature which is, for example, in the range from 110 to 260 C, and are through

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a dust extractor 12 passed to remove the dust from the flue gases remove.

The deduster can, as it is known from the technology of power generation, be an electrostatic precipitator in which essentially all coarse solid particles are removed from the flue gases. Such a dust extractor is usually a dry dust extractor in which the separated dust is removed continuously or intermittently and discharged separately. But others can too Types of dry dust extractors such as B. Multiclone or the like can be used. Wet scrubbers for dedusting are also known, and In the drawing, such a device is shown as a separate system, although under certain circumstances a wet scrubber is one Form part of the absorption system 14 or can be connected to it.

In the illustrated practical embodiment, the dust-free gases exiting from the dust separator 12 to flow, through a channel 13 in a Sun-absorption device or zone 14 is a _0, the purified gases exit via the channel 15 in a chimney, and this in the atmosphere. The absorption device 14 is designed as a wet absorber in which the SO 2 -containing flue gases come into contact with a magnesium-containing absorption medium. The device 14 can be a series-connected Venturi type, as disclosed in US Pat. No. 3,273,961, in which a magnesium-containing solution absorbs SO "from the flue gases and forms magnesium bisulphite. Other types of gas / liquid bulk contact devices are also used,

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around the gaseous SO ₀ from the flue gases in a magnesium-containing one

Absorb solution.

In the absorption system disclosed in US Pat. No. 3,273,961, the temperature of the gas / liquid contact zones is maintained below 49 ° C. and under such conditions there are certain values of magnesium sulphite solubility in the absorption solution at specific temperatures. Subsequent experience has led to the use of higher temperatures in the contact zones at a certain ratio of the strength of the solution to the ratio of the magnesium bisulphite and magnesium sulphite contained therein. It has been found that 65 to 71 C is a convenient temperature range in the gas / liquid contact zones, although higher and lower «
degree of absorption can be applied <

zones, although higher and lower temperatures with high SO emissions

With the exit of the flue gases from the separator 12 in the temperature range of 110-260 ° C., it is expedient to cool these gases further before they enter the zones of gas / liquid contact absorption of the system 14. This can easily be done by passing the flue gases through a separate gas / liquid contact device which can be a venturi or a comparable device in which the gases are cooled by evaporation of water sprayed into the gases. Such spray contact with the gases can expediently be used to remove any residue of entrained solids present in the gas _o When the solids are removed from the gases, it becomes necessary to keep the flow of the cooling liquid contaminated by solids separated from the absorption medium ₀ This can be done in that

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an excess of water is passed into the flue gases, so that the gases reach their saturation temperature. The excess water that leaks from the contact cooler with the solids collected in it, can be discharged through a filter or a centrifuge.

The use of a scrubber to cool the gases and to reduce their dust content not only increases the SO_ absorption in the absorption zone 14, but also reduces the solids content in the absorption solution. Thus, the monosulphite crystals formed in the container 20 will be of great purity and will increase the quality of the MgO and SO2 gas flowing through the product line 35, as will be described below _{or the like}

The absorption medium containing magnesium consists of the primary components MgO, H_0 and SO «. The following reactions occur a possible precipitation depends on which constituent is excessively present in the solution.

(Ί) H ₂ O (excess) + SO ₂ * - H ₃ SO ₃ + H ₃ O

(2) H ₂ SO ₃ (excess) + MgO + HgO ^ Mg (HSO ₃ ^ + H ₂ SO ₃ + H ₃ O

(3) 2H "SO" + MgO ■ - • - Mg (HSOj ₉ (aq) + H ₉ O

(4) Mg (HSOg) ₂ (excess) + Η _£ 0 + MgO >> -Mg (HSOg) ₂ + 2MgSO ₃ + H ₃

(5) Mg (HSOg) ₂ + MgO + H ₂ O 2MgSO ₃ + 2H ₂ O

(6) MgSO ₃ + H ₂ O + SO ₂

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If MgO is added to the aqueous Mg (HSO) _ solution with a solid SO ^ concentration, the MgO reacts with Mg (HSO ₀ ) and forms MgSO according to reaction (4), as a result of which it increases

Mg ion concentration in the acid, If the amounts of MgO, S0_ and HJD are in solution equilibrium, a saturated solution of Mg (HSO) _ + MgSO «in water is obtained. Will the SO ^ concentration intensified, reaction (6) occurs, in which MgSO_ is converted into Mg (HSO) _ and there is no precipitation of MgSO or MgO. at

If the concentration of the dissolved SO is reduced in order to change the equilibrium ratio of MgO, SO «and H ^ O, the excess MgSO is precipitated out of the solution. An increase in the HO content only dilutes the solution, while a decrease in the HO content changes the equilibrium and tends to precipitate MgSO
ο

In contrast to that disclosed in the referenced U.S. Patent 3,273,961 System in which the aim is the precipitation of MgSO crystals

in the solution to prevent it from coming out of the SOrt absorber escapes and is later strengthened to a cellulose cooking liquor, in the present invention the solution is in a container 20 passed, whereby the addition of the MgO slurry and / or magnesium hydroxide precipitates the MgSO_ crystals.

The absorption medium with absorbed SO 2 flows from the device 14 through the pipe 18 to the container 20, where the solution with magnesia slurry which is supplied through the valved pipe 21. The addition of the pulp turns magnesium sulphite crystals precipitated when the solubility exceeds the saturation value of the sulphite in the solution. The crystals formed appear in the hydrous form and can in the chemical formula

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Mg / SCL · XH ₀ O, the value of X depending on the temperature of the solution. In general, the hydrous crystals appear in the form of MgSO- · 6H ₀ O, and when heated to about 9o C and higher, the crystal water becomes 3H _{o be} 0 and less.

The residence time in the container will affect the size of the precipitated crystals, but generally the crystal size will be on the order of 75-300 microns. After the crystal formation, the solution with the magnesium sulphite crystals contained therein is passed through the line 22 to a crystal separator 23, which can be in the form of a filter or a centrifuge. After exiting the separator 23, the solution is passed through the pipe to the container 17. Trace amounts of impurities can be periodically or continuously removed from the system ₀ The deposited Magnesiumsulphitkristalle be through the blowdown line 25 conveyed via a conveyor 28 to a reactor 30

Inside the reactor 30, the crystals are removed by the application of heat decomposed, so that MgO particles and S0_ gas are formed, which through the Channel 31 are passed into a cyclone separator 32, where entrained MgO solids are separated from the gaseous substances. It was found that the temperatures for the decomposition of the Magnesium sulphite crystals, so that a reactive MgO particle is formed, must be of the order of 760 to 990 C »

As already stated above, the sulphite crystals occur in the water-containing form, and while it is also possible to pass the crystals directly into the reactor 30, it is through this

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The incoming heat load is too high to remove the crystal water and disintegrate the crystals. Since even a slight heating to about 95 C can be used to remove the water of crystallization from MgSO ". To convert 6HJ0 into the form MgSO- · 3H «0, it is advisable to use this procedure before the crystals are fed into the reactor 30. It has been found, however, that further preheating of the crystals can lead to partial decomposition, during which magnesium and SO2 are formed. It arises z. B ₀ found that heating the crystals to 232 to 372 C significantly reduces or eliminates the water of crystallization. This would reduce the amount of heat required in the reactor, but would also initiate the decomposition of the magnesium sulphite.In the temperature range from 232 to 372 C, 5-15 % percent by weight of the crystals would be broken down, and the system must then be designed and arranged in such a way that the components for the Procedures are not lost.

It should be noted also that the reactor for the separation of the crystals in the desired products, d _o h. MgO in particulate form and gaseous SO- must be controlled so that the desired reaction is completed, but also so that the MgO is produced in a reactive form o It is already known in the art that, depending on how long a MgO particles to which temperature is exposed, a "dead-burned" or non-reactive particle shape can be formed o The temporal and temperature-related influence of a particle to be thermally treated depends on other factors as well as on the size of the particle with which it enters the reactor and on its residence time and the temperature inside the reactor. The theory shows that the decomposition rate is controlled as follows:

V ΔΗ / V 1

t . t \ 6k 3h τ α

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-ίο- 2058117

Included is: θ = Time tf = Furnace temperature ta = Decomposition temperature V.
O = Radius of the particle = Heat of decomposition P. = Particle density k = thermal product conductivity H = Surface heat transfer coefficient.

It is therefore important that the process steps in reactor 2O ₇ 23 and 30 are properly related to one another so that the overall process proceeds successfully. Experience shows that MgO parts can be exposed to temperatures of 1270 ° C. for a few seconds and still retain their reactivity. At temperatures of 815 C the exposure time can be a few hours without the MgO particles "burning to death" ¹¹ .

For the purposes of the invention it is very desirable that virtually all of the product MgO particles occur in a reactive form, so that they can be reused in the absorption system 14.

If the crystals are thermally broken down in a fluidized bed at, for example, 815 C, the residence time should be at this temperature Do not exceed five hours so that MgO particles are reactive be formed. At higher temperatures, the residence time

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A suitable fluidized bed construction with a appropriately short fluidized bed residence time is in the patent application U.S. no. 823 056 Other forms of reactor device disclosed can be used for the thermal decomposition of magnesium sulphite crystals, such as an electrically heated one Chamber for treating the crystals in suspension. Sometimes it is advisable not to thermally decompose the crystals in direct contact with gaseous combustion gases or non-condensable gases, since they decomposed thermally Crystals release MgO particles, SO_ gas and vapors the gaseous products emerging from the separator 32 through the pipe 33 leak, contain SO and vapors; the vapor can be removed by condensation in a condensation device 34. For this reason it is advisable to use non-condensable Avoid gases in the reactor, and if an additional gaseous medium is necessary for the correct use of the reactor is, such as a fluidized medium in a fluidized bed, this medium should be a condensable, such as steam or be separated S0_ gases that are returned from the product stream will,. Under such conditions, the product gas exiting the condenser through passage 35 would be predominant consist of SOrt gas. The one from the condenser Condensed steam or vapor steam exiting the pipe 36 can then also be used as a washing medium in the separator 23 or directly are passed to the container 17 (not shown).

The MgO solid particles from the reactor 30 can (as shown) be passed through a line 37 to the container 17, where they with Water can be mixed to help shape the material.,

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the wifd passed through the tube 16 into the absorbent system _c The required for the system makeup water and / or magnesium is also added to the needs of the system in the container 17 accordingly.

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Claims (1)

_τ 13 - 2 Whether hi 17 PATENT CLAIMS: 1. Process for removing sulfur dioxide from exhaust gases, which have previously been dedusted and cooled, characterized by the following features: a _o absorption of gaseous SO from the gases in a solution containing magnesium, so that magnesium sulphite and magnesium bisulphite are formed, b. Precipitation of hydrous magnesium sulphite crystals from the solution by exceeding the saturation limit of the 'solution _p c. Separation of the crystals from the solution and recycling of the magnesium-containing solution into the process stage of the absorption of gaseous SO ₉ , d. Thermal decomposition of the magnesium sulphite crystals to form reactive magnesium oxide particles and one containing sulfur dioxide Generating gas and separating the magnesium oxide particles from the gas containing sulfur dioxide, e. Conversion of the SO 2 -containing product gas into a preferred one Shape, 2. The method according to claim 1, characterized in that at least some of the MgO particles in the process stage for the absorption of gaseous S0_ to produce the additive to the solution that is used in the SO ^ gas absorption stage of the process. _{1O9826 / 0} g _A9 - 14 - 2056Ί Method according to claim I _7, characterized in that the gases are cooled to the dew point temperature by direct contact with a cooling liquid, 4. The method according to claim 1, characterized in that the water-containing magnesium crystals by Control can be brought to the order of 75 to 300 microns 5. The method according to claim 1, characterized in that that the hydrous magnesium sulphite crystals are heated to a temperature of the order of about 95.degree to reduce the crystal water in the crystals. 6. The method according to claim 1, characterized in that that the process stage of thermal decomposition is controlled with a suitable temperature and the residence time at a such a minimum is kept that with the practically complete decomposition of the crystals into particulate MgO, SO "gas and vapor is compatible. 7. The method according to claim 6, characterized in that that the exhaust vapor from the mixed gases emerging from the decomposition zone is condensed and a SO ^ product higher Quality leaves behind. 8. Device for removing sulfur dioxide from incineration of carbon and sulfur-containing substances 109826/0949 standing gases, which are dedusted and cooled, thereby characterized in that the cooled and solid-free gases in a device (20) with an S0 "absorption liquid which contains magnesium in solution and means to form hydrous magnesium sulphite crystals to precipitate from this solution by exceeding the saturation capacity of this solution, as well as means for the separation of the water-containing magnesium sulphite crystals from the solution and for the thermal decomposition of the water-containing ones Crystals with the proviso that reactive magnesium oxide particles and S0 "gases are released. 10 9 8 26 / 00A9 Blank page