METHOD FOR REDUCING WEAR ON NOZZLES OR OTHER SUPPLY MEANS

Abstract: In a method for reducing wear on nozzles or other supply means when finely dividing a slurry consisting of a particulate absorbent suspended in water, in a reactor (5) for separating gaseous pollutants from flue gas containing fly ash, said flue gas with a substantial portion of its fly ash contents is introduced into the reactor to which the finely divided slurry is supplied and in which it is mixed with the flue gas. The water is evaporated and resulting reaction products as well as unreacted absorbent and fly ash are separated as a dry powder from the flue gas in the reactor (5) and/or in a subsequent filter (13), whereupon a portion of this separated material is classified so as to form a first fraction of fine particles and a second fraction of coarse particles, only the first fraction being recycled to the reactor to be used again. Before said portion of the separated material is classified, it is suspended in a liquid.

Description: METHOD FOR REDUCING WEAR ON NOZZLES OR OTHER SUPPLY MEANS Field of the Invention The present invention relates to a method for reduc- ing wear on nozzles or other supply means when finely di¬ viding a slurry consisting of a particulate absorbent suspended in water, in a reactor for separating gaseous pollutants from flue gas containing fly ash. The flue gas, with a substantial portion of its fly ash contents, is supplied to the reactor. The finely divided slurry is mixed with the flue gas. The water is evaporated and re¬ sulting reaction products as well as unreacted absorbent and fly ash are separated as a dry powder from the flue gas in the reactor and/or in a subsequent filter, where- upon a portion of this separated material is classified so as to form a first fraction of fine particles and a second fraction of coarse particles, only said first fraction being recycled to the reactor to be used again. Description of the Prior Art Prior apparatus for cleaning flue gas from combustion of e.g. coal frequently comprise but dust separators for separating the fly ash of the flue gas. The gaseous pol¬ lutants of the flue gas, such as sulphuric dioxide and gaseous hydrogen chloride, are however usually not removed but are emitted to the atmosphere where they are a consid¬ erable strain on the environment. This strain on the environment can be reduced to a considerable extent if the apparatus for cleaning flue gas is supplemented with a contact reactor. In the contact reactor, the flue gas is mixed with an absorbent reacting with the gaseous pollutants and converting them into par¬ ticulate pollutants which can be separated in ordinary separators. When rebuilding the apparatus, the contact reactor therefore is in most cases positioned upstream of the existing separators so as not to necessitate an addi¬ tional separator downstream of the contact reactor. The absorbent which is supplied to the contact reac¬ tor cannot be fully utilised in a single passage through the system. To reduce the working expenses, the absorbent is usually recycled a number of times before removal. Un- reacted absorbent is separated together with reaction pro¬ ducts and fly ash and recycled to the contact reactor, for example in the form of a slurry which is finely divided when flowing out at a high velocity from compressed-air nozzles or rotary means. Since the fly ash includes rela- tively coarse, very hare particles, it will have an abra¬ sive effect on nozzles or equivalent supply means for fine division. This means that the nozzles or equivalent means must be replaced frequently, which is both expensive and time- consuming. Summary of the Invention Technical problem Fly ash, especially fly .ash from coal, includes a large amount of oxides which are used in many contexts as abrasives. Problems are therefore always involved in con¬ veying fly ash or material including fly ash by means of a flowing gas or liquid. The wear on the conveying devices highly depends on the velocity of the fly ash particles. The higher the ve¬ locity, the greater will the wear be. In bends and narrow passages and in stop valves, this wear may require repairs at short intervals. The object of the present invention therefore is to provide a simple and effective method for reducing the wear on the nozzles of a contact reactor or equivalent supply means. Solution of the problem The present invention reduces the problems with wear on nozzles or other supply means when finely dividing a slurry in a contact reactor where the absorbent is re- cycled, in that the portion of the fly ash having the greatest particles is separated before or in the recycling system proper. Since the heaviest particles have the highest kinetic energy and the highest momentum, they cause substantially more wear than the smaller particles of the same composition. The coarse fly ash particles can be sorted out in several different ways and according to a number of physical principles. Gravitation, inertial forces and electric or magnetic forces etc. may be utilised. The sorting out can take place before or in the contact reactor or in the recycling part of the system. A single cyclone or screen-type separator mounted before the reactor can separate the very biggest particles already before they reach the reactor. A suitably designed reactor can, if adapted to use inertia effects or gravita¬ tion, also function as a separator. A certain fractiona- tion of the fly ash portion in the recycled material can be effected if that separated in the reactor is not re¬ cycled at all, but only that separated in a subsequent filter. However, the work done in the flue gas ducts usually involves considerable expense and necessitates a long shut down during assembly. The present invention therefore sug¬ gests that coarse fly ash particles be separated in the recycling process itself. This can be effected in essentially the same manner as in the flue gas duct, but at least brings the advantage that the flows to be treated are substantially smaller. One condition is however that the coarse fly ash particles significantly differ from particles containing incom¬ pletely reacted absorbent. Otherwise, also useful material is separated in an unfavourable fashion. The fly ash usually has such a size distribution that a large portion is coarser than the absorbent, preferably particles of slaked lime, which is suspended in water to be introduced into contact reactors of the type involved in this application. Thus there is a theoretical possibil¬ ity of separating a portion of the fly ash without a great portion of the absorbent being also removed. In practice, the slaked lime is however composed of agglomerates of particles. These agglomerates can be of the same size as the fly ash particles that are desired to be separated. Common dynamic separators will therefore not always yield satisfactory results. Therefore, the present invention suggests that the separation be performed in liquid phase. This is suitably performed in that the dry pulverulent material intended for recycling is suspended in a liquid, such as water, so that any agglomerates formed from the absorbent are decomposed, whereupon the resulting slurry is conducted through a device to be divided into one fraction of coarse particles and one fraction of fine particles. This divi¬ sion can be effected by e.g. electric or magnetic forces under certain conditions, but is preferably performed by means of a dynamic separator, such as a hydrocyclone. Description of a proposed embodiment The invention will now be described in more detail with reference to the accompanying drawing which shows an apparatus for cleaning flue gas from a coal-fired central boiler plant, said apparatus comprising a suitable device for carrying out the method according to the present invention. The flue gas formed during combustion of coal in the central boiler plant 1 shown in the drawing is conducted to an air preheater 2. This is adapted to transfer heat from the hot flue gas to combustion air which via a duct 2a is supplied to the central boiler plant by means of a fan 3. Subsequently, the flue gas is conducted, without prior separation of its fly ash contents, via a duct 4 to a contact reactor 5 where it is mixed with a particulate absorbent, preferably slaked lime, said absorbent being reactive with the gaseous pollutants of the flue gas, such as sulphur dioxide and gaseous hydrogen chloride. The ab¬ sorbent is supplied in the form of an aqueous slurry via a conduit 7 to nozzle assemblies 6 mounted in the upper part of the reactor. The preparation of the slurry and the composition thereof will be described in greater detail later on in the specification. The nozzle assemblies can be designed for example as disclosed in European Patents 82110320.7 and 85850112.5. In the nozzle assemblies, the slurry is finely divided by means of air which is supplied to the nozzle assemblies via a conduit 8 at a pressure above the pres¬ sure of the slurry. In the nozzle assemblies, air flows at a high velocity through the slurry and decomposes this so that it leaves the nozzles of the nozzle assemblies in the form of finely divided jets of mist 9. These jets of mist are sprayed into the interior of the contact reactor 5 so that effective mixing of the flue gas and the slurry is achieved. The water of the slurry is- evaporated while contacting the flue gas, and the gaseous pollutants of the flue gas simultaneously react with the absorbent and are transformed into particulate pollutants. The heavier particles of these particulate pollu¬ tants, unreacted absorbent and the fly ash contents of the flue gas drop into dust hoppers 10 formed in the bottom of the contact reactor. The remaining particles are conducted by the flue gas via a duct 12 to an electrostatic preci- pitator 13. In this, these particles are separated from the flue gas which is led through a duct 14 to a flue gas fan 15 which, through a duct 16, feeds the flue gas cleaned of particulate and gaseous pollutants to a chimney 17 for emission into the atmosphere. The particles separated in the electrostatic precipi- tator 13 are collected in dust hoppers 18 formed in the bottom thereof. These particles are conveyed by means of air through a conduit 19 to a container 20 in which they are mixed, by means of an agitator 22, with water supplied through a conduit 21. As a result, any agglomerates formed from the particles of the absorbent are decomposed, see the introductory part of the specification, whereupon the resulting slurry is passed through a conduit 23 to a hydrocyclone 24. In the hydrocyclone, the water-borne particles are classified so that the coarse particles, mainly fly ash particles, are thrown against the walls of the hydro¬ cyclone and drop onto the funnel-shaped bottom thereof. From here, these particles are conveyed together with the part of the water of the slurry that is collected on the bottom of the hydrocyclone through a conduit 25 to a mixer 26. In the mixer, the coarse particles of the slurry are mixed with the particles which are collected in the dust hoppers 10 and supplied to the mixer through a conduit 27. By mixing the last-mentioned particles which mainly con¬ sist of heavy fly ash particles, with the water-borne coarse particles of the slurry, a sufficiently moist resi¬ dual product is obtained, without necessitating the use of a special humidifier. The residual product then is con¬ veyed to a storage bin 28 through a conduit 29. On the other hand, the fine particles of the slurry accompany the major part of the water of the slurry out of the central part of the hydrocyclone and from there through a conduit 30 to a container 31 where it is mixed, by means of an agitator 33, with a fresh absorbent, preferably slaked lime, supplied through a conduit 32. Subsequently, this mixture is supplied to the contact re¬ actor 5 through the conduit 7, see ne beginning of the description of the figures. The invention is of course not restricted to the embodiment described above but can be modified in various ways within the scope of the appended claims. For example, the division of the particles into one fraction of fine particles and one fraction of coarse particles can be provided by means of electric or magnetic forces instead of by means of dynamic separators, such as cyclones.

Claims: CLAIMS 1. Method for reducing wear on nozzles or other sup- ply means when finely dividing a slurry consisting of a particulate absorbent suspended in water, in a reactor (5) for separating gaseous pollutants from flue gas containing fly ash, said flue gas with a substantial portion of its fly ash contents being introduced into said reactor to which the the finely divided slurry is supplied and in which it is mixed with the flue gas, the water being evaporated and resulting reaction products as well as unreacted absorbent and fly ash being separated as a dry powder from the flue gas in said reactor and/or in a subsequent filter (13), whereupon a portion of this separated material is classified so as to form a first fraction of fine particles and a second fraction of coarse particles, only the first fraction being recycled to said reactor (5) to be used again, c h a r a c t e r i s - e d in that said portion of the separated material is suspended in a liquid before being classified. 2. Method as claimed in claim 1, c h a r a c t e r ¬ i s e d in that said liquid is water. 3. Method as claimed in claim 1 or 2, c h a r a c - t e r i s e d in that said first fraction is mixed with fresh absorbent before being recycled to said reactor (5). 4. Method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the classifi¬ cation is carried out by means of a dynamic separator (24). 5. Method as claimed in claim 4, c h a r a c t e r ¬ i s e d in that: said dynamic separator is a hydrocyclone (24). 6. Method as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that said classification is carried out by means of electric or magnetic forces .

Inventors: LINDAU LEIF, LINDAU, LEIF
Applicants: FLAEKT AB, ABB FLAEKT AKTIEBOLAG
Application year: 1990
Main classification: B01D53
IPC codes: B01D53