Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Patent
1995-10-11
1997-04-29
Straub, Gary P.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
42324301, 42324407, 423240S, 422168, 422170, 422172, B01D 5348, B01D 5368
Patent
active
056246480
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method and an apparatus for cleaning a flue-gas stream containing hydrogen chloride and sulphur dioxide.
Ever higher demands have been placed on flue-gas cleaning in recent years, such that flue-gas cleaning today includes not only the separation of dust, such as fly ash, by means of e.g. an electrostatic precipitator, but also the separation of gaseous impurities, such as sulphur dioxide and hydrogen chloride, found e.g. in the flue gases generated in refuse incineration. Sulphur dioxide can be separated by wet cleaning, for instance in a scrubber where the flue-gas stream is contacted with an alkaline aqueous solution that absorbs sulphur dioxide. Also hydrogen chloride can be removed by wet cleaning in a scrubber where the flue-gas stream is contacted with an aqueous solution that absorbs hydrogen chloride. The separation of hydrogen chloride and sulphur dioxide is often performed in a combined wet cleaner, in which case hydrogen chloride is absorbed in a first stage and sulphur dioxide is absorbed in a second stage. The flue gases leaving such a combined wet cleaner are substantially rid of solid particles, hydrogen chloride and sulphur dioxide and have hitherto been regarded as sufficiently clean to be let out into the atmosphere. As environmental standards are raised, inter alia as regards the removal of nitrogen oxides (NOx), such flue gases have to be further cleaned and any remaining residues of heavy metals and sulphur dioxide should be removed to the greatest possible extent. Thus, when removing nitrogen oxides catalytically from flue-gases, it is a considerable advantage if the incoming flue gases at the most contain 5 mg of sulphur dioxide and 0.1 mg of sulphur trioxide per Nm.sup.3.
In order to achieve such additional cleaning of the flue gases after the conventional wet cleaning, lime is injected into the flue gases, preferably in combination with active carbon, after the wet cleaning so as to react with any remaining impurities, whereupon the resulting product is collected on a filter. The lime injected into the flue-gas stream usually is in powder form but may also be a slurry of lime in water. Such a lime slurry is finely divided when injected into the flue-gas stream, and the water in the slurry is evaporated when contacted with the hot flue-gas stream. The resulting lime particles react with the impurities in the flue-gas stream, preferably sulphur dioxide, and are then collected on a filter. Use can be made of a bag filter in which filter bags of textile material are arranged in a filter chamber. The injected lime, preferably combined with active carbon, then deposit on the filter bags, and the cleaned flue gas leaves the bag filter. The lime-containing deposit on the filter bags may react with further impurities in the flue gases, such as sulphur dioxide, before being eventually removed from the filter bags and collected at the bottom of the filter chamber to be discharged as waste.
An instance of the above prior-art technique is described in Warner Bulletin, Feb. 1, 1993, "Retrofitting Waste Incineration Plant--Below Detectability Limits".
Furthermore, it is known, e.g. from Hans T. Karlsson, Jonas Klingspor, Marira Linn e and Ingemar Bjerle, "Activated Wet-Dry Scrubbing of SO.sub.2 ", Journal of the Air Pollution Control Association, Vol. 33, No. 1, January 1983, pp 23-28, that the separation of sulphur dioxide by means of lime in a filter, as described above, is considerably promoted by the presence of calcium chloride (CaCl.sub.2). In the absence of calcium chloride, the flue gases have to be contacted with a greater amount of lime, i.e. more lime has to be injected, involving an increase in costs. Naturally, the increase in lime consumption means that the bag filter yields a greater amount of spent lime to be discharged as waste, which is a disadvantage. In order to remedy this disadvantage, it is desirable that the sulphur-dioxide separation by means of lime in the filter takes place in the presence of calcium chloride.
The object of the present inven
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"Retrofitting Waste Incineration Plant--Below Detectability Limits", Warner Bulletin, Feb. 1, 1993. P. 19 by Gunter Jungmann.
"Activated Wet-Dry Scrubbing of SO.sub.2 ", Hans T. Karlsson et al., Journal of Air Pollution Control Association, vol. 33, No. 1, Jan. 1983, pp. 23-28.
ABB Flakt AB
Straub Gary P.
Vanoy Timothy C.
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