DE2736488B2 - Process for removing sulfur oxides from flue gases - Google Patents

Description

The invention relates to a method for removing sulfur oxides from flue gases, in which the Flue gas is contacted in a scrubbing tower with dilute sulfuric acid as scrubbing liquid and that in sulfur dioxide enriched in sulfuric acid using an iron compound as a catalyst is oxidized.

Such methods are used to remove the flue gases that are in particular leaving power plants from the entrained ones To free sulfur oxides. Due to the continuously tightened environmental protection regulations increasingly higher demands are placed on such processes

In a known method the freed from fly ash, flue gases are cooled in a scrubber by injection of fresh water and recirculated wash liquor to 50 to 8O ⁰ C. Then they are treated in a washing tower with dilute sulfuric acid in countercurrent. The flue gas, which has been cleaned of the sulfur oxides, must then be ^{heated to over 100 °} C. in order to regain a sufficiently large buoyancy. It is then released into the atmosphere via a comb. The scrubbing liquid enriched with the sulfur oxides (SO2 and SO3) is pressed into an oxidation column via an injector through which the scrubbing liquid is mixed with air. A dissolved catalyst in the form of an iron compound is added to the washing liquid to accelerate the oxidation. Sulfuric acid is formed. The exhaust air from the oxidation column is fed to the washing tower. The sulfuric acid produced is partly returned to the washing tower and partly used to obtain gypsum in a crystallizer.

In other known methods, the sulfur oxides contained in the flue gas at temperatures of about 45O ⁰ C to sulfur trioxide are converted by the flue gas is guided via a V2Os contact. The sulfur trioxide produced in this way is then dissolved in the countercurrent sulfuric acid. Here, too, the end product is sulfuric acid. If the flue gases are at temperatures of 100 to 150 ^° C., they must be heated ^{to the corresponding reaction temperature of 450 ° C.} This is e.g. B. necessary for the flue gases that leave the power plant operation;

The invention is based on the object of a method operated with dilute sulfuric acid for Remove sulfur oxides from flue gases in such a way that the flue gases cool down superfluous and to find a catalyst that allows easy handling

The object is achieved according to the invention in that iron (II) sulfate is used as the catalyst and the catalyst is already added to the washing liquid and that in a partial stream from the Sulfuric acid drawn off from the washing tower is deposited electrolytically and metallic iron this metallic iron is returned to the washing liquid via a release device. With the Procedure according to the invention, it is possible to remove all of the pollutants still present in the flue gases to capture and separate from the smoke gases. This is a significant reduction in environmental pollution, especially of coal-fired power plants. In particular, there are no the wastewater polluting substances are separated. The generated Products are further processed into marketable or landfillable end products.

The dust load of the flue gas is reduced according to the invention or a return to the The washing tower is prevented from having the partial flow to remove the fly ash through a filter system to be led.

For a better understanding of the invention, an embodiment based on a for Implementation of the process suitable system, which is shown in the single drawing, explained

At 1, the flue gases are introduced into the scrubbing tower 2, which contain numerous pollutants and also fly dust. In the washing tower 2, the already washed flue gases are cleaned with the aid of the fully demineralized water fed in via line 3. At the same time, the fully demineralized water is used to compensate for water losses. A mixture of dilute sulfuric acid and iron (II) sulfate is fed into the washing tower 2 via the line labeled 4. At 5, the washed flue gases leave the washing tower 2 and are released into the atmosphere via the chimney. The washing liquid consisting of dilute sulfuric acid and iron (II) sulfate is drawn off at 6 and fed to the filter system 8 in a partial flow. The washing liquid contains H ₂ SO ₄ , H (NO) SO ₄ , Fe (NO) SO ₄ as well as iron (II) sulfates and Fe (II) chlorides and dust in aqueous solution.

A partial flow of the scrubbing liquid withdrawn at 6 flows via line 7 to the filter system 8, from which the flue dust is discharged via line 9a after appropriate separation. The filter system 8 is supplied with washing water via the line 10. In the filter system 8, two filters are preferably provided in parallel. When cleaning a filter, the inlet in line 7 is closed, the acid adhering to the filter cake drips off and is pumped into circulation line 13. The filter cake is then washed with fresh water from line 10 until the drain 9 is neutral. The washing water is collected in the template 11.
The partial flow of the withdrawn via line 7

Washing liquid is himer the filter system 8 via the Line 14 of the electrolysis system 15 is supplied. At the cathode 16, metallic iron is produced, which after Redissolving as Fe-H sulfate via line 17 in line 4 and thus back to i ^ s washing tower to be led. The iron concentration in the substream in line 14 is approx. 250 to 500 mg Fe / l. the Partial flow rate (l / h) in line 14 is directly dependent on the SOr entry into the washing solution. The one at the Cathode amount of Fe to be deposited hourly is therefore 250 to 500 mg Fe / h.

The amount of Fe to be separated results from the multiplication of the iron concentration in the partial flow and the Partial flow rate in l / h

/ mgFe J_ _ mgFe

Vl h "b

_{The Cl 2} produced at the anode is fed to a washing reservoir 20 via line 19. In these, the Cl ₂ reacts with the water coming from the line 21 to form a hypochlorite solution, which reaches the additional cooling tower water treatment system, not shown, via the line 22

Sulfuric acid, which is free of iron and is ^{heated to approx. 130 to 140 °} C. via the evaporator 23 and to approx. 70% H ₂ SO _{4, emerges from the electrolysis system 15} is concentrated, which can be withdrawn via line 24 as a marketable end product. At this Concentration, in addition to water vapor, hydrochloric acid, traces of HF and that at this temperature from the Nitrosyl compound released NO expelled in gaseous form. These substances get through the pipes 25 to a cooler 26, hydrochloric acid with small proportions of HF being formed via line 27 is deducted. The remaining NO is put together

ίο passed with the remaining steam over a coal bed 28 and converted to N ₂ and CO ₂ , which can be drawn off via line 29. The small amount of HF-containing hydrochloric acid flowing out of the cooler cannot be used as a regeneration column in the Drawn demineralization system or for decarbonization in the cooling tower additional water treatment (not shown), wherever they go via the Line 30 is discharged. Attempts carried out in the system described have shown that the aforementioned Almost one hundred percent of the pollutants are separated from the flue gases. There are no input raw materials, such as lime, sulfuric acid, coke, Sand or other expensive additives required. as The end product is in particular technical grade sulfuric acid of 70 to 76%, which is readily salable is. Fully demineralized water is only fed into the inlet side of the washing tower.

1 sheet of drawings

Claims (2)

Patent claims: 1. Process for removing sulfur oxides from flue gases, in which the flue gas in one Washing tower is contacted with dilute sulfuric acid as washing liquid and that in the Sulfuric acid enriched sulfur dioxide using an iron compound as a catalyst is oxidized, characterized in that that iron (II) sulfate is used as the catalyst and the catalyst is already the washing liquid is added and that in a partial stream of the sulfuric acid withdrawn from the washing tower Electrolytically deposited metallic iron and this metallic iron via a Releasing device is returned to the washing liquid. 2. The method according to claim 1, characterized in that that the partial flow is passed through a filter system to remove airborne dust.