Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
2000-11-15
2003-01-21
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S24000R, C423S244070, C423S640000
Reexamination Certificate
active
06508994
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for cleaning flue gases with slaked lime.
BACKGROUND ART
In different kinds of combustion, such as combustion of fossil fuels, waste etc, flue gases are generated, which contain pollutants which are removed before the flue gases are emitted into the atmosphere. The pollutants can be solid, such as fly ash, or gaseous, such as sulphur dioxide or hydrogen chloride. The solid pollutants can be removed by means of a filter, such as an electrostatic filter or fabric filter, whereas the gaseous pollutants are removed by absorption using an absorption agent. In wet cleaning of flue gases, gaseous pollutants are removed by means of an absorbing liquid containing an absorbing agent. Gaseous pollutants, for instance acid substances as sulphur dioxide and hydrogen chloride, can also be removed by contacting the flue gases with a particulate absorbent material, which usually is slaked lime (calcium hydroxide). The contact between the flue gases and the absorbent material can suitably occur after the flue gases have left the boiler. The present invention relates to the latter so-called dry sorption technique.
In the dry sorption technique, particulate slaked lime conditioned with water is dispersed in the flow of flue gas, on the one hand the hot flue gases being cooled by the water (usually from about 150° C. to about 70-80°C.) and, on the other hand, gaseous pollutants, such as sulphur dioxide, being absorbed by the particulate slaked lime. The particulate material is then removed from the flue gases by, for instance, filtration before the flue gases are emitted into the atmosphere. The separated particulate material, which below is referred to as powdered lime, is collected. A certain part of the collected powdered lime can be re-used in the flue gas cleaning by recirculation and addition of a small amount of unused, slaked lime. The remainder of the collected powdered lime is removed, for instance, by controlled dumping.
The slaked lime used in flue gas cleaning by the dry sorption technique is either purchased completed from a supplier of slaked lime or produced on site. Conventional production of slaked lime on site is performed in such manner that burnt lime is supplied from a storage to a dry-slaking apparatus, in which the burnt lime is slaked by adding an exactly dosed amount of water. After slaking, the slaked lime is supplied to a silo for storage. When slaked lime for flue gas cleaning is needed, it is fed from the silo through lines to a mixer, in which the slaked lime is mixed with recirculated powdered lime and conditioned with water before the mixture is contacted with the flue gases.
This conventional technique suffers from a number of drawbacks. Thus, the amount of water in the slaking of the lime must be dosed very exactly since adding more water than required for the slaking, which leaves as water vapour in the exothermal slaking reaction, causes aggregation into lumps. Such aggregation into lumps may result in blocking of the slaking apparatus and undesirable lumps in the storage silo and may interfere with or prevent feeding of slaked lime from the silo to the flue gas cleaning. A further drawback is that slaked lime has a stronger tendency to form dust and is more bulky than burnt lime. The same amount of slaked lime takes up almost twice the volume of burnt lime, which makes the slaked lime more expensive to handle and transport.
To enable handling and transporting of the slaked lime without any aggregating problems, the water content must be carefully controlled so that the slaked lime has a water content of 0-2% by weight. Such a low water content in turn causes a further problem, viz. that the slaked lime obtains a low surface area. It has in fact been established that the surface area of the slaked lime is essentially directly proportional to the water content of the slaked lime. Thus, slaked lime with a water content of 0-2% by weight has a surface area of about 15-18 m
2
/g, while slaked lime with a water content of 10% by weight has a surface area of about 23 m
2
/g and slaked lime with a water content of 15% by weight has a surface area of slightly more than 30 m
2
/g. Since the reactivity of the slaked lime in flue gas cleaning increases with an increasing surface area, it will be appreciated that a low water content and, resulting therefrom, a low surface area of the slaked lime result in a drawback.
One more drawback of the prior art is that the storage of the slaked lime makes it age and, inter alia, form carbonate by absorbing carbon dioxide from the atmosphere.
It will be appreciated that it would be desirable and would involve an improvement in the field if, in flue gas cleaning, it would be possible to use slaked lime with a high water content and high surface area without simultaneous aggregating problems. If also the other difficulties in storage and transport of slaked lime could be reduced or obviated, this would mean further advantages.
SUMMARY OF THE INVENTION
It has been discovered in the invention that the above difficulties in prior-art technique can be obviated by producing the dry-slaked lime in situ, i.e. in immediate connection with the flue gas cleaning, with a water content of at least 5% by weight.
According to the invention, a method for cleaning flue gases with particulate slaked lime is provided, said method being characterised in that burnt lime is first dry-slaked with water to a moisture content of at least 5% by weight, and that subsequently the slaked lime is contacted with the flue gases within 1 min of the slaking.
According to the invention, also an apparatus for cleaning of flue gases with particulate slaked lime is provided, said apparatus comprising a feeding end for feeding burnt lime and adding water, and a discharge end for discharging slaked lime, and mixing means between the feeding end and the discharge end, said apparatus being characterised in that between the feeding end and the discharge end it comprises a first section and a second section which are separated by a first overflow edge, the first section being arranged at the feeding end and comprising at least one mixing means, which is rotatable about a horizontal shaft extending in the transverse direction of the apparatus, for mixing and feeding of burnt lime and water in the longitudinal direction of the apparatus from the feeding end towards the overflow edge, and the second section being arranged at the discharge end and comprising at least one mixing means, which is rotatable about a horizontal shaft extending in the transverse direction of the apparatus, for mixing of material fed over the first overflow edge, and mixing of the material in the longitudinal direction of the apparatus towards a second overflow edge arranged at the discharge end.
Further characteristic features and advantages of the invention are evident from the following description and the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
It is preferred that the slaked lime in the invention has a water content of 5-30% by weight, more preferably 10-20% by weight. Even if water contents above 20% by weight are usable, they may cause a risk of aggregating, and water contents above 50% by weight are avoided in the invention for this reason.
The high surface area and the porosity of the slaked lime according to the invention imply that it can contain more water than conventional slaked lime and remains free flowing, i.e. the high surface area reduces the aggregating problems.
A distinguishing feature of the invention is that the slaked lime is produced in situ, i.e. in immediate connection with its use for flue gas cleaning. This condition is expressed in the invention in such manner that the slaked lime is to be contacted with the flue gas within 1 min of the slaking. Preferably, the lime is contacted with the flue gases within 30 s of the slaking. By the slaked lime being produced in immediate connection with the flue gas cleaning, the need for a storage silo and transport lines and thus also the pos
Åhman Stefan
Bringfors Nils
ABB Fläkt AB
Birch & Stewart Kolasch & Birch, LLP
Silverman Stanley S.
Vanoy Timothy C
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