A DRY METHOD FOR CLEANSING A GAS CONTAMINATED WITH ACID CONSTITUENTS

Abstract: The invention relates to a method and an arrangement for enabling a contaminated medium to be cleansed in a contact reactor (2) by introducing (11) thereinto a pulverulent absorption material capable of reacting with contaminants in the medium, the medium and the absorption material being caused to mix together in the contact reactor (2) over a period of time in which contaminants react with the material, therewith to absorb on and react with individual particles in the pulverulent absorption material. The method comprises a) removing (17,18) part of the absorption material reacted with contaminants downstream of the contact reactor (2); b) storing (22) this removed part of the absorption material for a pre-determined time duration in excess of 20 hours; c) causing the contaminants to diffuse into respective individual particles during this storage time, therewith to impart to the surfaces of the particles an increased ability to absorb further contaminants; and d) by returning (13) stored, previously reacted absorption material to the contact reactor (2) as a pulverulent absorption material capable of reacting with the contaminants con­ tained in the medium.

Description: Technical Field The present invention relates to a dry method for cleansing a contaminated gas as stated in the preamble of claim 1. Background Prior Art It is known that so-called "acid gases", i.e. gaseous media containing acid forming substances, such as sulphur dioxide, hydrogen chloride and the like, can be cleansed of their acid constituents, by washing such gases with water containing alkaline components. Although this method provides good results, however, it is encumbered with high investment costs and the apparatus required to carry out the method must be carefully maintained. It is also known to provide such apparatus with advanced water and sludge treatment auxiliaries, otherwise it would be immediately ascertained that an air contaminant has transformed to water contaminant. One reason for the high investment costs incurred is that the whole purification plant must be constructed for high-grade stainless steel, since acidiferous aqueous solutions are extremely aggressive. It is also known to employ so-called "semi-dry" purification systems or "fully dry" purification systems, in an attempt to eliminate the aforesaid drawbacks. In methods used in conjunction with such purification plants there is injected into the acidic gas a pulverulent absorbent which presents a large specific surface area and which is reactive with the acid components of the gas, e.g. a flue gas, which acid components as a result of this reaction adhere to and absorb on the discrete particles of the pulverulent absorbent. The absorbent material carrying the contaminants can then readily be separated from the gas by conventional methodology, with the aid of electrostatic separators and/or fabric filters. It is well known that these dry purification systems functioning by absorption afford many advantages compared with known wet separation methods, inter alia because the contaminated flue gases, is treated with a dry absorbent and at high temperatures. The use of a dry absorbent in hot flue gases has enabled standard constructional steel to be used in the construction of the plant apparatus required. This has greatly reduced construction costs. Maintenance costs, personnel costs and running costs are also lower when practising this latter type of purification methodology. It has been found, however, that the reaction between an acid gas component, for example gaseous hydrogen chloride, and a dry solid particle, for example slaked lime Ca(OH)₂, is not as effective as the reaction between hydrogen chloride gas and a basic water droplet. It has also been established that in a dry purification system reactivity can be enhanced by decreasing the particle size of the dry particles. This renders the absorbent more expensive however. The arrangements and methods described and illustrated in a) British Patent Specification No. 941 618, b) Swedish Patent Specification No. 422 890 and c) especially U.S. Patent Specification No. 3976 747 form part of the prior art. The British Specification 941 618 describes and illustrates a plant in which fresh reactant is mixed with varying amounts of previously used and thus reacted material. Although this latter material is said to be taken from a silo, the specification makes no mention of any specific length of time over which the reacted material shall be stored in the silo. The sole purpose of this storage silo is one of smoothing out variations in material flow due to fluctuations in operation, start and stop, and to disturbulances which result in losses of material. The Swedish Patent Specification No. 422 890 describes and illustrates a plant of principally similar operational features with regard to the storage and recycling of reactants which have previously undergone a reaction, similar to the aforesaid British Patent Specification. The U.S. Patent Specification 3976 747 describes and illustrates a method as mentioned in the preamble of claim 1. Disclosure of the Present Invention Technical Problem When reviewing the present state of this art as expressed in the foregoing it will be seen that one qualified technical problem resides in the provision of method which will enable a coarse-grain, and therewith less expensive pulverulent, reactive absorbent to be used to the same good effect as a fine-grain, and therewith more expensive absorbent. A further technical problem is one of enabling substantially the full absorbing capacity of the absorbent to be used, and not solely the exposable surface portions of the separate particles constituting the absorbant. Another technical problem is one of providing a method which enable the full absorbing capacity of the absorbent to be utilized irrespective of prevailing particle sizes and particle distribution, by enabling utilized absorbent to be recycled repeatedly over a relatively long period of time. Another technical problem is one of providing a method which enables the absorbing capacity of the dry absorbent material to be utilized to an extent greater than 50%. Another qualified technical problem in this respect is one of providing means with which the absorption capacity of consumed absorbent material can be substantially restored without requiring large structural modification to existing plants. Preferably, in addition to what is mentioned above, the recycled, restored absorbent particles have an absorptive, reactive capacity corresponding to 40-80% of their previous absorption capacity. Solution The present invention relates to a method which enables a contaminated gas to be cleansed in a contact reactor, as stated in the preamble of claim 1. The method according to the invention comprises the steps stated in the characterizing part of claim 1. Suitable features, in addition to the features mentioned in claim 1, are stated in the sub claims. Advantages The advantages primarily afforded by the method according to the present invention lie predominantly in the fact that the absorbent can be more effectively used without needing to radically modify existing plant apparatus, while enabling an absorbent of large particle size to be selected which gives a lower operating cost. Brief Description of the Drawing An exemplary embodiment having features characteristic for carrying out the method according to the present invention will now be described in more detail with reference to the accompanying drawing in which: Figure 1 is a perspective view of a dry flue-gas purification plant or principally known construction; and Figure 2 illustrates schematically an arrangement for the method according to the invention constituting a complement to the plant illustrates in Figure 1. Description of a known Dry Flue-Gas Purification Plant Figure 1 illustrates a prior known plant for dry flue-gas purification. Flue gases leave the boilers of a central boiler station (not shown in Figure 1) at a temperature of about 180°C and are passed firstly through a particle-separating cyclone. The cyclone is not shown in Figure 1, but is arranged to separate heavy dust particles, therewith facilitating continued cleansing of the gases. The cyclone extracts about 50% of the dust particles present. The cyclone is referenced 31 in the Figure 2 illustration. Thus, flue gases which have partially been cleansed of solid dust particles arrive through a conduit 1 and are passed to two vertical contact reactors 2, 3, where the flue gases are mixed with dry powdered lime through spray devices not shown. This results in a chemical reaction between the external surfaces of the powdered lime and the acid components in the flue gas, primarily hydrogen chloride, sulphur dioxide. Flue gases containing solid contaminants emanating from dust particles generated during combustion, together with lime particles, exhibiting surfaces that have parts reacted with acid components in the contact reactors 2 and 3, are passed through a conduit 4 to an electrostatic precipitator 5. Various designs of electrostatic precipitators are known to the art. The dust is removed from the bottom of the precipitator, though a transport conduit 6 provided therein. The flue gases cleansed in the electrostatic precipitator 5 are transferred for fine-cleansing in a a textile bag filter 8 through a conduit 7. The hose or bag filter 8 comprises a plurality of textile bags, in a manner known per se. The bag filter 8 is primarily adapted to collect extremely fine dust in a last stage. The fine dust extracted in the bag filter 8 is removed from the filter through a discharge system or transport conduit 9. The thus cleansed flue gases are discharged through a flue stack 10 via a conduit 10a, with the aid of a flue-gas fan 10b. Description of a Preferred Embodiment As illustrated in Figure 2 the present invention relates to an arrangement for enabling a contaminated gas passing through the conduit 1 to be cleansed in a contact reactor 2 or 3, by introducing thereinto a pulverulent material capable of reacting with the contaminants present in the gas. The reactive pulverulent material is supplied through a conduit 11 from a silo 12 containing fresh absorbent 23, and through a conduit 13 extending from an intermediate storage silo 14 for previously used absorbent 22. It is known to transport pulverulent material with the aid of transport fans 15,16 and compressed air, and hence the apparatus and devices required herefor will not be described in detail. The gas and the absorbent are mixed with one another in the contact reactor 2, 3 over a period of time sufficient for impurities to react with the material, so that the contaminants, in the form of acid gases, are absorbed on and react with the discrete particles of the absorbent. The HCl contained in the flue gas reacts with part of the surface located Ca(OH)₂ molecules in a particle to form CaCl₂ while giving off water. The illustrated arrangement comprises a conduit 4 extending between the contact reactor 2 and the dust separator 5, and a conduit 17 which corresponds essentially to the conduit 6 of the Figure 1 embodiment, in which dust separator the whole, or at least a major part of the absorption material partly reacted with the contaminants can be removed from the gas stream and passed to a dust feeder 18. The dust feeder 18 feeds, in a manner known per se, the dust received thereby to the aforementioned intermediate storage silo 14 or to a dumping silo 20, via a switching device 19. The material 25 transferred to the dumping silo 20 is removed therefrom for dumping, by means of a transport device 21. The transport device 21 may advantageously incorporate dust-moistening means, preferably water-sprinkling means. Dust is not transported to the dumping silo 20 until a high level allotted to the intermediate storage silo is exceeded. That part of the used absorption material 22 passed to the intermediate storage silo 14 is arranged to be stored therein for a given period of time. This storage period shall be of sufficient duration to enable the contaminants to diffuse into the discrete particles, thereby enhancing the ability of the particles to absorb additional contaminants. The intermediate storage silo 14 shall be of a size which enables the acid components in the contaminant to diffuse into the discrete particles and migrate towards the centres thereof during the time period in which the reacted particulate absorbent is stored in said silo, thereby to form fresh reactive surfaces. Subsequent to a desired intermediate storage time, the dust is metered and distributed into the flue gas, in which the newly regenerated surfaces of the dust absorb acid components. Longer periods are normally required for larger particle sizes. In accordance with the invention the recycled regenerated particles shall preferably have an absorption capacity corresponding to 40-80% of their previous absorption capacity. By absorption capacity is meant the maximum absorption ability of a particle should it be exposed in the contact reactor to a maximum quantity of contaminants or an above-maximum quantity. It this absorption capacity is set to 50%, this means that the particle surfaces are so large and so formed that 50% of the maximum number of atoms possible in the absorption process are able to absorb contaminants. It has been found that when the average particle size of the absorption material is slightly greater than 10 µm and comprises slaked lime, and the acid constituents of the polluted gas are in the form of hydrogen chloride, the used absorbent should be stored in the silo or container 14 for a period at least 20 hours at a temperature below 100°C. Practical tests and theoretical calculations indicate that the storage time should lie between 20-40 hours, and in all events not longer than 50 hours, provided that the temperature is not far beneath 100°C. Diffusability falls off at lower temperatures and the aforesaid storage or stay times can be considered to apply for ingoing particle temperatures of between 40°C and 100°C. The temperature falls but slightly over a storage period of 20-40 hours. During the time the particles are held in the silo, the chloride ions diffuse inwardly into the particles and the hydroxide ions migrate outwardly in a corresponding manner. If the absorption material is utilized in the reactor to 20% of its absorption capacity, the surfaces of the particles will be regenerated to 80% of their original activity upon the termination of a full diffusion process. When the particles are stored for from 20-40 hours, they can be regenerated to 40-80%, depending upon temperature and "particle size".

Claims: 1. A dry method for cleansing a gas contaminated with acid constituents in a contact reactor, by introducing thereinto a pulverulent absorption material capable of reacting with said contaminants carried by said gas, and in which the gas and the absorption material are mixed over a period of time in which said contaminants react with said material, so that said contaminants are absorbed by discrete particles of said pulverulent absorption material, utilizing the steps of: a) removing downstream of said contact reactor absorption material, which has reacted with said contaminants, and storing said removed material, b) charging said stored absorption material, which has previously reacted with said contaminants, back to the contact reactor as a pulverulent absorption material, characterized in that the acid constituents include hydrogen chloride and in that said removed material is stored over a given length of time in excess of 20 hours and at a temperature causing the contaminants to diffuse inwardly into the discrete particles during this storage period, so as to impart to the surfaces of the particles an increased ability to absorb further contaminants, 2. A method according to Claim 1, characterized by charging to the contact reactor a mixture of a previously unused pulverulent absorption material and said stored absorption material which has previously partly been reacted with contaminants. 3. A method according to Claim 2, characterized in that the mixing ratio between the unused absorption material and the stored absorption material lies in the range of 0.2 - 0.5. 4. A method according to claim 1, characterized in that the pre-determined storage time is selected in dependence on the particle shape and particle size of the pulverulent absorptin material, and is shorter than 100 hours, prefereably shorter than 50 hours. 5. A method according to Claim 4, characterized in that the storage time in respect of large particle sizes is longer than that for smaller particle sizes. 6. A method according to Claim 4, characterized in that for particle sizes of about 10 um the storage temperature is between 40°C and 100°C.

Inventors: CARLSSON, KURT
Applicants: FLAEKT AKTIEBOLAG
Application year: 1985
Main classification: B01D53
IPC codes: B01D53