Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Patent
1997-01-27
1998-06-02
Straub, Gary P.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
55225, 55233, 55240, 422168, 422176, 422227, B01D 5350, B01J 1000
Patent
active
057595054
DESCRIPTION:
BRIEF SUMMARY
This application is a national stage filing under 35USC371 of PCT/SE95/00404 filed Apr. 13, 1995.
FIELD OF THE INVENTION
This invention relates to a method and a device for removing sulphur dioxide from a gas, such as a flue gas, by means of an aqueous suspension of an absorbent selected from lime and limestone.
BACKGROUND OF THE INVENTION
Gaseous sulphur dioxide is 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, for instance flue gases from oil-fired power stations, 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 the 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 more sparingly-soluble alkali, such as lime.
This invention concerns a calcium-based system, in which limestone or lime is used 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 countercurrently 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. It should further be emphasised that washing towers of the above type are large constructions, the difference in height between the storage tank for the absorbent suspension and the spray nozzles being about 20-40 m. Also, the pressure in the spray nozzles corresponds to a liquid column of about 10 m. This means that considerable amounts of energy are required for lifting and recycling the absorbent suspension to the nozzles and for feeding the suspension countercurrently to the flue gas.
EP 0 162 536 and DE 34 10 109 may be mentioned as instances of background art as regards the removal of sulphur dioxide from process gases on the basis of countercurrent co
REFERENCES:
patent: 1565366 (1925-12-01), Howard
patent: 4239515 (1980-12-01), Yanagioka et al.
patent: 4263021 (1981-04-01), Downs et al.
patent: 5246471 (1993-09-01), Bhat et al.
Bengtsson Sune
Johansson Lars-Erik
Lindberg Leif
Maripuu Mati
Nolin Kjell
ABB Flakt Industri AB
Straub Gary P.
Vanoy Timothy C.
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