Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Patent
1996-09-30
1997-12-02
Straub, Gary P.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
42324303, 42324308, 42324407, B01D 5350, B01D 5380
Patent
active
056933010
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT International application Ser. No. PCT/SE95/00203 filed 27 Feb. 1995.
FIELD OF THE INVENTION
The present invention relates to a method for removing sulphur dioxide from a gas, such as a flue gas, by means of an aqueous suspension of an absorbent.
BACKGROUND OF THE INVENTION
Sulphur dioxide is a gas formed upon the oxidation of sulphur-containing material, such as refuse, coal, oil, natural gas and peat. Even though this invention is especially concerned with the removal of sulphur dioxide from flue gases generated by the oxidation (combustion) of such material, it is by no means restricted thereto, but relates to the cleaning of sulphur-dioxide-containing gases in general. Such cleaning is previously known and is generally based on sulphur dioxide being absorbed in an aqueous washing liquid. At present, use is chiefly made of three different systems for cleaning sulphur-dioxide-containing flue gases, namely calcium-based systems, sodium-based systems and indirect calcium-based systems. In the calcium-based systems, limestone (CaCO.sub.3) and lime (CaO, Ca(OH).sub.2) are used as alkali, whereas in the sodium-based systems, sodium hydroxide (NaOH) or soda (Na.sub.2 CO.sub.3) is used as alkali. In the indirect calcium-based systems, a readily-soluble alkali, such as NaOH, is used for primarily absorbing the sulphur dioxide in a gas washer. Having absorbed sulphur dioxide, the washing liquid is regenerated outside the gas washer by means of a sparingly-soluble alkali, such as lime.
The present invention relates to the first-mentioned system and uses limestone or lime as alkali.
Prior-art calcium-based systems of this type include a washing tower, in most cases a spray tower, into which flue gas is introduced and treated concurrently with an injected suspension of fine-grained limestone or lime as absorbent. When contacted with the finely-divided absorbent suspension, the sulphur dioxide is absorbed to form, after oxidation, gypsum (CaSO.sub.4.2H.sub.2 O) with the absorbent. The flue gas thus rid of sulphur dioxide then leaves the spray tower. In order to bring about the required oxidation and gypsum formation, oxygen-containing gas, such as air, is injected into the absorbent suspension, usually in the tank employed as storage means and for recycling of the absorbent suspension. As a rule, large suspension flows are recycled through the spray tower, and the storage tank contains a very large volume of absorbent suspension. As an example, it may be mentioned that the suspension volume in installations for flue-gas cleaning in power stations may exceed 1000 m.sup.3, and volumes of up to 6000 m.sup.3 occur. Such large volumes are seen as necessary in order to efficiently use the oxygen-containing gas supplied, since the solubility of oxygen increases with the depth at which it is injected. Furthermore, a considerable volume of absorbent suspension is regarded as necessary in order that the absorbent supplied should have a sufficient holding time to be dissolved. Also, a long holding time is needed if the precipitated gypsum crystals are to grow and become easy to filter off. The holding time of the recycled suspension typically is in the range of 6-12 min, while the holding time of the formed gypsum often is about 20-30 h.
Apart from spray towers of the above type, the prior art encompasses the use of packed towers and plate columns (tray-type towers) for removing sulphur dioxide from flue gases. Also in these types of tower, the final oxidation to gypsum occurs in a separate tank having a considerable volume, as stated above.
In jet bubbling reactors of the type marketed by Chiyoda Chemical Engineering and Construction Company, Ltd., all reactions occur in the same tank but in separate zones. Sulphur-dioxide-containing gas is injected from above and downwards in a jet bubbling zone. Then the gas rises upwards through the jet bubbling zone and leaves the jet bubbling reactor. Below the jet bubbling zone there is a reaction zone which is provided with an agitator and to which
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patent: 4853195 (1989-08-01), Lento
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Kohl et al. "Gas Purification" 4th ed. Gulf Publishing Co. Houston, Texas U S A (1985, no month) pp. 319-320.
ABB Flakt AB
Straub Gary P.
Vanoy Timothy C.
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