A METHOD FOR ABSORBING GASEOUS COMPONENTS FROM ACIDIFYING FLUE GASES

Abstract: The invention relates to a method for absorbing the acidifying gaseous components of flue gases generated, for instance, when burning fossil fuels or waste materials, by supplying finely-ground burnt lime (CaO) to the system. The burnt lime is supplied (8) in powder form within a section in which the flue gases have a temperature of from 65 DEG C to 180 DEG C and a moisture content between 3 and 40 % by volume.

Description: TITLE OF THE INVENTION: A METHOD FOR ABSORBING GASEOUS COMPONENTS FROM ACIDIFYING FLUE GASES. TECHNICAL FIELD The present invention relates to a method for absorbing the acidifying components of acid flue gases generated, for instance, when burning fossil fuels or waste, such as domestic waste, by supplying to the acidifying flue gases finely-divided burnt lime (CaO). BACKGROUND PRIOR ART The combustion of the majority of fossil fuels engenders the production of acidifying gases which are harmful to both animate and inanimate matters. Various methods and apparatus for reducing the emission of these acid gas components have been proposed, The methods proposed primarily involve: 1) Cleansing the fuel used. 2) Binding the acid components concerned in conjunction with the combustion of the fuel. 3) Cleansing the gases produced by fuel combustion. The present invention relates to a method for cleansing combustion gases in accordance with 3) above. There are known to the art a number of methods by means of which, in accordance with 3) above, acid components can be removed from combustion gases en¬ gendered when burning fossil fuels or waste material, by establishing contact between the acid gas components and an absorbent charged to the gas mixture, such that this absorbent is able to react with and bind the acid components and therewith form one or more salts. Present day processes intended for this purpose can be classified under the following groups, with regard to the characteristic features of the processes; a) wet processes carried out in so-called scrubbers, b) semi-dry processes, so-called spray drying processes carried out in spray dryers, scrubbers, etc, and c) dry processes carried out in dry scrubbers. With regard to classification a) above, the feature most characteristic of the wet processes is one of bringing the gas mixture into contact with a large quantity of aqueous solution or aqueous suspension containing one or more alkaline components, wherein the ratio between the quantity of liquid supplied and the quantity of ' gas de¬ livered to the absorption stage, effected in a reaction chamber, is of such large magnitude that the enthalpy content of the gas can vaporize only a minor persentage of the water supplied. The flue gases departing from the absorption stage of this process are saturated with water vapour. The present state of the art includes . plants which operate in accordance with the aforesaid method and which are intended for separating sulphur dioxide from flue gases produced in coal-fired boilers . The advantage of the wet process is that the cost for chemicals are low, mai nly because inexpensive l imestone can be used for neutral izing purposes and because stoichiometric quantities wi l l suffice . With regard to classification b) above, the feature most characteristic of the spray drying process is one in which an absorbent is εlurried or dissolved in a limited amount of water which is smaller than the amount required to cool the gas down to saturation, and in which the aqueous solution or slurry is delivered to the gas in droplet form. The droplets, however, shall be small enough to vaporize in the reaction chamber. Dry residual products are then extracted from the gas in a conventional downstream dust separator. In this porcess normally wet-slaked lime in slurry form is sprayed into a reactor. This process also requires the chemicals used to be present in excessive quantities. Finally, in the case of the dry processes under classifi¬ cation c) above, the flue gases are brought into contact with a dry absorbent, which is normally blown in powder form into the reaction chamber and disperses therein. The absorbent normally used in this process is dry-slaked lime, i.e. Ca(OH) 2 , which is more expensive than burnt lime (CaO) . This higher cost is caused by the inclusion of the slak¬ ing process and by a weight increase due to the water content of the product. Practical experience has shown that although finely ground limestone will provide a certain degree of separa- tion, the extent to which the acid components are ex¬ tracted is often much too small. It is also recognized within the art that a moist absor- bent tends to bind the acid components of an acidified flue gas to a greater extent than a dry absorbent. Furthermore, a finely divided absorbent will bind the acid components of a flue gas to a greater extent than a coarsely divided absorbent. Acid components will also bind more readily to the surfaces of a porous absorbent than to the surfaces of a more dense absorbent. It is also recognized in the art that the flue gases must not be cooled to a temperature which is equal to or lies beneath an adiabetic saturation temperature at which moisture will precipitate onto the walls of the reaction chamber and passageways. Conse¬ quently, there shall be chosen a temperature which lies above this value, so that no moisture is precipitated. Finally, it is also recognized in the art that the use of larger quantities of absorbent and/or absorbents which possess higher absorption tendencies - and therewith more expensive absorbents - increases the extent to which such acid gases are cleansed, and that there is a desire to maintain the amount of absorbent used per unit of time at a low level and to use an inexpensive absorbent to achieve a predetermined level of cleansing. The present invention can be considered to relate to the "dry process". In the case of contact reactors of the aforesaid kind in which the contaminated medium is a flow of flue gas, it is known when practicing the dry process, and also when practicing the spray drying process, to ensure that the process of actually cleansing the gas is effected at a temperature which lies as close as possible to the dew point of the gas. In this regard, the measures taken to lower the temperature of the process create conditions which enable the dust or contaminants to be moistened slightly, therewith to render the absorption of and the chemical reaction with the acid contaminants or compo¬ nents more effective. It has also been established that the extraction of gas- eous heavy metals and several organic compo¬ nents is improved when the gas cleansing process is effected at a low working temperature. In contact reactors which operate in accordance with the dry process, it is also known to use a slaked lime absorbent which is spread as a dust throughout the reactor chamber of the reactor and is binding chemically the acid contaminants present in the chamber, when these acid contaminants come into contact with the lime particles. It will be obvious that the chemical reaction taking place will -increase in number with lime particles which exhibit an increasing acitve surface area. An effective reaction surface of about 15 m /gram is estimated for slaked lime. SUMMARY OF THE INVENTION TECHNICAL PROBLEMS With regard to the prior state of the art as described above it will be seen that in order to be effective, the process of gas cleansing with the use of powdered ab¬ sorbent should be effected rapidly and effectively be¬ tween the contaminant, such as sulphur dioxide, hydrogen chloride and/or hydrogen fluoride, and the powdered ab¬ sorbent used, and it will also be seen that a technical problem resides in the provision of conditions which enable an inexpensive absorbent to be imparted a reaction tendency which is equivalent to that of the more expen¬ sive slaked lime previously used. One technical problem in this regard is to recognize that finely-ground burnt lime can actually be slaked in a gas flow which contains sufficient moisture for this purpose. A further technical problem resides in the ability of ad¬ justing the temperature and moisture conditions in a flow of flue gas, so that finely-ground burnt lime introduced in mist form into the flue gas is slaked effectively. Another technical problem in this regard is one of real¬ izing that when applying suitable temperature and mois¬ ture values, burnt lime will be slaked so rapidly in a gas flow that slaking of the lime can be effected during passage of the gas from the boiler to a downstream ab- sorption reactor. It will also be seen that when carrying out the dry cleansing process a technical problem resides in the pro¬ vision of conditions which will enable a flue gas temper- ature suitable for the supply of finely-ground burnt lime to be selected, while at the same time adapting the moisture content of the flue gases to the desired reac¬ tion in the absorption reactor. SOLUTION The present invention relates to a method for extracting by absorption the acid components of acidifying flue gases, e.g. the gases generated when combusting fossil fuels or waste materials, by introducing finely-ground burnt lime into the acidifying flue gases. In accordance with the invention, the burnt lime is in¬ troduced in powder form within a section in which the temperature of the flue gases lies within the range of 65 C to 180 C, and the moisture content of said gases lies between 3 and 40 % by volume. In accordance with one embodiment of the invention, when the moisture content of the flue gas generated by the combustion process is too low, water is added to the gas in an amount which is sufficient to cool the gas to a temperature of from 65 C to 180 C and which will achieve a gas moisture-content of from 3 to 40 % by volume, while observing the moisture generated during the actual com¬ bustion process. In accordance with a further embodiment of the invention, the flue gases are caused to pass through a reactor which may be of very simple construction and which may consist of for instance, a short channel or passageway located be¬ tween a cooler and a dust separator. The solid particles, the salts formed by the absorption process, and the un-reacted materia.1 present in the flue gases are extracted in a dust separator, for instance an electrostatic dust precipitator, a fabric filter, a bed filter, etc. According to a particularly preferred embodiment of the invention, the burnt lime is slaked by the water present in a first process stage such as to form an extremely fine powdered lime hydrate, and the acid gas components are caused to react with ■ the lime hydrate powder in a subsequent absorption stage. Finally, it is proposed that chlorine-containing sub¬ stances may be added, so as to form hygroscopic calcium chloride and therewith enhance the reaction between lime and sulphur dioxide. ADVANTAGES Those primary advantages afforded by a method according to this invention reside in the provision of conditions which enable an inexpensive burnt lime to be used in¬ stead of slaked lime, by adjusting the temperature range and moisture content of the flue gases to values at which the burnt lime is slaked, so as to improve the absorption of the acid components of an acidifying flue gas gener- ated when burning fossil fuels and waste materials. The primary characteristic features of the inventive method are set forth in the characterizing clause of the following claim 1. BRIEF DESCRIPTION OF THE DRAWING An exemplifying embodiment of the inventive method for extracting gaseous acid components from acidifying flue gases will now be described in more detail with the aid of the accompanying drawing, the single figure of which is a highly simplified illustration of a plant in which the inventive method may be carried out. DESCRIPTION OF A PREFERRED EMBODIMENT The illustrated plant includes a combustion furnace 1 for burning waste material, the arrow 2 indicating the supply of combustible waste into the furnace and the arrow 3 indicating the supply of combustion air. The acidifying flue gases generated by the combustion process pass from the furnace 1 to a cooler 5, through a conduit 4. The cooler 5 may have the form of a heat exchanger but is preferably a cooling tower or some similar apparatus. Water is supplied to the cooler 5 through a pipe 5a and sprayed into the flue gases. The thus cooled flue gases pass to a chemical reactor 7 through a further conduit 6, into which finely- ground burnt lime is introduced, as indicated by an arrow 8. Reaction products in the form of salts formed in the re¬ actor 7, together with the solids and un-reacted substances present, pass with the flue gases through a conduit to a dust separator 10, which may be an electrostatic dust separator, a fabric filter or some corresponding apparatus, in which the particles are removed. The cleansed gases exit from the plant through a conduit 11 to a smoke stack, not shown. The material extracted from the flue gas in the dust separator 10 is passed through a conduit 12 to a unit 13, from which material is either recycled back to the chem¬ ical reactor 1 , through a conduit 14, or is removed for dumping, through a conduit 15. In accordance with the invention, powdered burnt lime is introduced through the conduit 8 within a section in which the flue gases have a temperature within the range of 65 C to 180°C and a moisture content of between 3 and 40 % by volume. These conditions can be achieved either by cooling the flue gases with water in the cooler 5 or, when the moisture content of the gases generated by combustion is sufficient for the intended purpose, by cooling the flue gases in the cooler 5 without the aid of water. With regard to the particular reaction which is strived for in accordance with the present invention by the in¬ troduction of finely-ground burnt lime through the con- duit 8 at the aforesaid flue gas temperature and parti¬ cularly within the aforesaid moisture-content range, the burnt lime will be slaked in a first reaction stage (either totally or substantially in the passageway sec¬ tion 6a) in accordance with the formula CaO + H 2 0 = Ca(OH) 2 therewith to form a very fine hydrated-lime powder that possesses a tendency to react with gaseous acid compo¬ nents. These fine reactive particles which exhibit a large spe- cific surface area are then passed to a second stage (an absorption stage), where they react with the acid gas components, such as HC1 , SOg and HF. The second reaction stage may be effected within the re¬ actor 7 , and in accordance with one or more of the re¬ actions Ca(OH) 2 + 2HC1 = CaCl 2 + 2H 2 0 Ca(0H) 2 + S0 2 = CaS0 3 iH 2 0 + 2 H 2 0 Ca(OH) 2 + 2HF = CaF 2 + 2H 2 0 The addition of a chlorine-containing substance improves the extraction of sulphur dioxide (S0 ? ), through the resultant formation of hygroscopic calcium chlorides (CaCl 2 ) . It will be understood that the invention is not restrict¬ ed to the illustrated and described example, and that modifications can be made within the inventive scope defined in the following claims.

Claims: ' CLAIMS 1. A method for absorbing acidifying components from acidifying flue gases generated, for example, when burn- ing fossil fuels or waste material by supplying finely- ground burnt lime (CaO) to the acidifying flue gases, characterized by introducing the burnt lime in powder form within a section in which the flue gases have a temperature of from 65 C to 180 C and a moisture content of from 3 to 40% by volume- and by extracting the solids, the salts formed by said absorption, and the un-reacted material present in the flue gases in a dust separator, e.g. an electrostatic dust separator, a fabric filter a bed filter or like device. 2. A method according to claim 1, characterized by cooling the flue gases to a temperature of from 65°C to 180 C by adding water to an extent which will produce a moisture content of between 3 and 40 % by volume. 3. A method according to claim 1 or claim 2, charactericed by slaking t burnt lime in a first stage (CaO + H-0 = Ca(OH) 2 ) to form a very fine hydrated lime powder, and by reacting the acid gaseous components, such as HCl, SO, HF, with the outer surfaces of the powdered hydrated lime in a subse¬ quent absorption stage. 4. A Method according to claim 1 , 2 or 3 , characterized by adding a chlorine-containing substance to form hygro¬ scopic calcium chloride in order to improve the extrac¬ tion of sulphur dioxide.

Inventors: CARLSSON KURT, CARLSSON, KURT
Applicants: FLAEKT AB
Application year: 1988
Main classification: B01D53
IPC codes: B01D53