CA1296865C - Method and system for purifying exhaust gas - Google Patents
Method and system for purifying exhaust gasInfo
- Publication number
- CA1296865C CA1296865C CA000487508A CA487508A CA1296865C CA 1296865 C CA1296865 C CA 1296865C CA 000487508 A CA000487508 A CA 000487508A CA 487508 A CA487508 A CA 487508A CA 1296865 C CA1296865 C CA 1296865C
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- CA
- Canada
- Prior art keywords
- particles
- absorbent
- exhaust gas
- water vapor
- cao
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/46—Removing components of defined structure
- B01D53/68—Halogens or halogen compounds
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/46—Removing components of defined structure
- B01D53/48—Sulfur compounds
- B01D53/50—Sulfur oxides
- B01D53/508—Sulfur oxides by treating the gases with solids
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- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Health & Medical Sciences (AREA)
- Biomedical Technology (AREA)
- Environmental & Geological Engineering (AREA)
- Analytical Chemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Treating Waste Gases (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
According to the dry method of the invention for purifying an exhaust gas, fine particles of a Ca-type absorbent such as dolomite are sprayed into the exhaust gas, and the absorbent particles absorbing acid harmful substances from the gas are collected by a dust collector along with other particles in the exhaust gas. The collected particles are divided by a classifier into a portion of coarse particles chiefly containing fly ash and a portion of absorbent particles not larger than a predetermined size, and the latter portion only is supplied to a hydration reactor. The absorbent particles in the reactor are hydrated with superheated water vapor and thereby expanded, whereby an inactive shell formed on the surface of each particle by the absorption of the acid harmful substances is broken to reactivate the particles. The reactivated absorbent particles are supplied to the exhaust gas again.
The acid harmful substances are therefore removable efficiently with a smaller amount of absorbent.
According to the dry method of the invention for purifying an exhaust gas, fine particles of a Ca-type absorbent such as dolomite are sprayed into the exhaust gas, and the absorbent particles absorbing acid harmful substances from the gas are collected by a dust collector along with other particles in the exhaust gas. The collected particles are divided by a classifier into a portion of coarse particles chiefly containing fly ash and a portion of absorbent particles not larger than a predetermined size, and the latter portion only is supplied to a hydration reactor. The absorbent particles in the reactor are hydrated with superheated water vapor and thereby expanded, whereby an inactive shell formed on the surface of each particle by the absorption of the acid harmful substances is broken to reactivate the particles. The reactivated absorbent particles are supplied to the exhaust gas again.
The acid harmful substances are therefore removable efficiently with a smaller amount of absorbent.
Description
TITLE OF THE INVENTION
METHOD AND SYSTEM FOR PURIFYING EXHAUST GAS
:
FIELD OF THE INVENTION
The present invention relates to a dry method of and a system for purifying exhaust gases discharged from boilers, incinerators and furnaces.
BACKGROUND OF THE INVENTION
Hot exhaust gases discharged from boilers or incinerators for waste materials usually contain 10 to 2000 ppm of harmful acid substances such as sulfur oxides (SOx), hydrogen chloride (HCl), hydrogen fluoride (HF) and the like. It is required to remove these substance for pollution control.
Such harmful acid substances are removed from the exhaust gas generally by a wet method wherein a liquid or slurry containing an alkaline absorbent is brought into dlrect contact with the exhaust gas as~cooled to a low temperature to purify the gas. Although achieving a high removal efficiency, this method encounters difficulties in treating the resulting waste water~ involves the necessity of reheating the exhaust gas and has- the problem of high :
equipment and running costs.
In view of the above problem, research has been made on~various methods substituting the wet method. The methods heretofore proposed include, for example, a method of removing harmful substances by causing active carbon to adsorb such substances, and a semi-dry method of spraying a slaked lime slurry into the exhaust gas. Nevertheless, these methods fail to achieve high removal efficiencies.
Further investigations have been made of a dry method in which particles of Ca-type absorbent (such as quick lime, slaked lime, limestone, dolomite or the like) are dispersed directly into the interior of a hot furnace ; 10 or a flue for removing harmful substances, but the absorbent, which is of low reactivlty, renders the method unamenable to practice except in special cases where the environmental regulations are very slack. Stated more specifically, the reactivity of the absorbent with SO is as low as up to about 20~, and that with HCl is limited to 50~ or lower, in the dry method.
The low reactivity of Ca-type absorbents, for example of CaO particles, is attributable to the following reason.
The reaction of CaO particles with SO , HCl or HF occurs first on the surface of CaO particles, forming a thin shell of CaSO4, CaCl2 or CaF2 on the surface of the particles.
The reaction further proceeds with the diffusion of SOx, HCl or HF into the particles. However, the surface shell is compact and therefore prevents diffusion of SO or the like into the CaO partl~les to rapidly inhibit the reaction .:
Ç`s8Çi5 of Sx or the like with CaO.
The above problem itself can be overcome by the method disclosed in Ishihara et al. U.S. Patent 3,481,289.
With this method, CaO particles which are covered with a she]l of a compound such as CaSO4, CaC12, CaF2 or the like and thereby rendered chemically inactive are hydrated with water, and the expansion of CaO particles resulting from the hydration reaction breaks the surface shell to reactivate the particles. Consequently, reuse of the CaO particles thus reactivated achieves higher reactivity than when the original CaO particles are used without reactivation.
However, the method of Ishihara et al. wherein water is used for hydration is a wet method, so that the CaO particles resuLting from the hydration treatment contain a large amount of water and are in the form of agglomerates of increased sizes. Because these wet coarse partlcles can not be sprayed directly into an exhaust gas, it is required to prepare CaO particles of the order of several microns for use in a dry process,by subjecting the coarse particles to a treatment involving drying, pulverization and classification. This treatment has problems in respect of labor, time and cost. Furthermore, the wet hydration treatment itself also has the problem of requiring~ a prolonged period of time since a very high temperature is not usable for the reaction.
.?s~65 SUMMARY OF THE INVENTION
An object of the present invention is to provide a dry method of purifying exhaust gases with a Ca-type absorbent and a system therefor which are free of the problems of the foregoing U.S. patent although making US8 of the advantages thereof.
According to a first aspect of the invention, there is provided a method of purifying an exhaust gas containing harmful acid substances by spraying fine particles of a Ca-type absorbent into the exhaust ga~ and collecting by means ofa dust collector the absorbent particles absorbing the harmful acid substances along with other particles contained in the exhaust gas, the method being characterized by separating the absorbent particles of size about 2 to 3 microns from said other particles, hydrating the collected absorbent particles with superheated water vapor having a temperature of about 150-300C in order to thereby expand the particles and break an inactive shell formed on the surface of the particles by the absorption of the harmful acid substances, and supplying the resulting activated absorbent articles to the exhaust gas again.
The Ca-type absorbents to be used in the present process include limestone (CaCO3), slaked Lime ~CatOH~2], quick lime (CaO), dolomite (CaC03.MgC03), slaked dolomite [Ca(OH)z.Mg(OH)2 or Ca(OH)2.MgO~, calcined dolomite (CaO.MgO
or CaCO3.MgO) and any material containing Cao or capable of forming CaO at a high temperature (of not lower than about 300C). Accordingly, the particla~e Ca-type , .
i5 absorbent originally contains CaO or produces CaO through the following reactions at a high temperature of at least about 300 C.
Ca(OH) ~ CaO + H O (1) CaCO3 ~ CaO + CO2 (2) CaO absorbs acid harmful substances (SOx, HCl, HF) from the exhaust gas upon reacting therewith as follows.
CaO + SO4 + 22~ CaSO4 (3) CaO + SO3 ~ CaSO4 (4) CaO + 2HCl ~ CaC12 + H2O (5) CaO + 2HF ~ CaF2 + H2O (6) The CaSO4, CaC12 and CaF2 resulting from the reactions (3) to (6) form an inactive shell over the surface of the absorbent particles. According to the present invention, however, CaO in the interior of the absorbent particles is hydrated with water vapor into Ca(OH)2, whereupon the particles expand to break the surface shell and become activated. Moreover, since the hydration reaction is conducted with use of water vapor through a dry process, the activated absorbent particles can be supplled directly to the exhaust gas again without necessltating any special subsequent treatment.
Exhaust gases often contain fly ash of large ~ particle sizes in additlon to harmful acidsubstances. In such a case, it is desirable to separate the particles .
~5-~B
. , .
~.~8~iS
collected by the dust collector into a portion of coarse particles chiefly containing fly ash and a portion of fine particles including predominantly absorbent particles not larger than a predetermined size and to hydrate the fine particle portion only with water vapor.
Preferably the water vapor has a temperature of 150 to 300 C, more preferably 200 to 250 C. If the temperature is below 150 C, the hydration reaction will not proceed rapidly, whereas at temperatures above 300 C, the reactio~ wherein Ca(OH)2 releases H~O proceeds in preferance to the hydration of CaO into Ca(OH)2. Thus, the hydration treat~ent can not be carried out as intended.
Of the Ca-type absorbents mentioned above, dolomite, slaked dolomite and calcined dolomite are higher in void volume and therefore have the advantage of readily permitting diffusion of acid harmful su~stances into the particles.
According to a second aspect of the invention, there is provided a system for purifying an exhaust gas produced by ; 20 a gas produced and container harmful~acid substances by spray-ing fine~particles of a Ca-type absorbent into the exhaust gas and collecting by a dust collector the absorbent particles ; absorbing the harmful acid substances along with other partic-les contained in the exhaust gas, the system being character-ized in that the system further comprises a classifier for dividing the particles collected by the dust collector into a portion of coarse particles chiefly containing fly ash and a :~
portion of fine particles predominantly including absorbent particles of about 2 to 3 microns in size, a hydration reactor for hydrating the fine absorbent particles ~lith superheated water vapor having a temperature of about 150-300C to thereby expand the particles and break an inactive shell formed on the surface of the particles by the absorption of the harmful acid substances and supplying the resulting activated absorbent particles to the exhaust gas again.
Various features and advantages of the present invention will be readily understood from the following description of an embodiment with reference to the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
The drawing is a diagram schematically showing the construction of an exhaust gas purifying system embodying the present inventlon.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With re~erence to the drawing, indicated at 1 is a coal boiler, fuel oil boiler, waste incinerator or like furnace which produces an exhaust gas of high temperature, or a flue connected to the furnace or gas producer.
Particles of a Ca-type absorhent having a predetermined mean particle size are supplied to the furnace or flue 1 via a line 2, and acid harmful substances, such as SO , HCl and HF,are absorbed by the absorbent particles from the exhaust gas. The gas is sent to a dust collector 3, by which various klnds of particles are removed. ThP gas is then released into the atmosphere through a line 4 and a chimney 5.
The particles collected by the dust collector 3 are fed through a line 6 to a classifier 7, in which they are separated into a portion of coarse particles chiefly containing fly ash and a portion of fine particles (2 to 3 microns in mean size) predominantly including absorbent particles which are inactivated on the surface by the absorption of acid harmful substances.
In the case of coal boilers and incinerators for waste ma-terials, the amount of fly ash is several times the amount of lnactivated absorbent particles, so that it is essential to use the classifier 7, while in the case of fuel oil boilers, the amount of fly ash is less than that of inactivated absorbent particles. The classification step is not always necessary in the latter case.
In the case of coil boilers, the fly ash is coarse and is about 15 microns in mean particle size even if fine, whereas the particulate inactivated absorbent is 2 to 3 microns in mean particle size. Accordingly, the absorbent can be effectively separated from fly ash when an air classifier is used as the classifier 7. Experiments conducted with use of an air classifier show that the collected particulate mixture can be separated into-a fly ash portion ; 25 contalning about 10% of inactlvated absorbent particles and :: : :
~ -8-a portion of inactivated absorbent particles containing about 10% of fly ash.
The portion of fine particles including the inactivated absorbent and separated off by the classifier 7 is led via a line 8 to a hydration reactor 9, to which superheated water vapor having a temperature of 200 to 250 C is supplied through a line 10. The inactivated absorbent particles are dispersed through the superheated water vapor, whereby the interor CaO component is instantaneously hydrated to expand. Consequently, the inactive shell (of CaSO4, CaC12, CaF2, etc.) covering each absorbent particle breaks to expose the interior unreacted portion.
The hydration reactor 7 may be one utilizing a jet mill through whicn superheated water vapor is passed at a high speed. In this case, the pulverizing action of the jet mill per se and the expansion due to the hydration reaction coact to readily rupture the inactive shell over the surface of the absorbent particles. Another advantage is available in this case in that the activated absorbent particles obtained can be rapidly supplied to and dispersed into the exhaust gas. Irrespective of whether the jet mill lS used, a gas having a high temperature and containing water vapor is usable in place of superheated water vapor.
The activated absorbent particles are supplied again to the furnace or flue 1 via a return line 11, along -9_ ~ . .
::
.,. , - ~, ~ ': -' , '' . :
~' s~5 with a fresh portion of absorbent particles supplied through the line 2. Thus, the same process as above is repeated.
To prevent an excess of absorbent particles from circulating through the system, the inactivated absorbent particles run off from the classifier 7 may be partly purged through a purge line 12.
With the method of the present invention, the inactive shell is broken or ruptured by the volume expansion resulting from the hydration of CaO into Ca(OH)2, so that the Ca-type absorbent must contain CaO when the hydration treatment is carried out. On the other hand, the harmful acid substances are removed by the Ca-type absorbent in two temperature ranges: i.e. in a high-temperature range of 800 to 1200 C in which SO is absorbed, and a low-temperature range of 150 to 400 C where HCl and HF are absorbed. In the high-temperature range, both Ca(OH)2 and CaO are thermally decomposed to give CaO, so that all the aforementioned Ca-type absorbents are usable. In the low-temperatu~e range, however, CaCO3 is not thermally decomposable, so that the Ca-type absorbents to be used are limited to those containing CaO and Ca(OH)2. At 150 to 300 C within the low-temperature range, Ca(OH)2 is not thermally decomposable either, while CaO will be converted to Ca(OH)2. Accordingly the present invention is to be practiced at temperatures of above 300 C.
;5 Exhaust gas purifying experiments were conducted using a coal burning test furnace which produced an exhaust gas at a rate of 2000 Nm /hour (N standing for standard state), as will be stated below.
Experiments 1 and 2 were conducted under the following conditions.
SO concentration of exhaust gas: 740 ppm (at the outlet of bag filter ?
Temperature at absorbent inlet: 1070 C.
Absorbent retention time in effective temperature range (800 to 1050 C) for desulfurization reaction:
3 seconds.
Experiment _ Limestone (CaCO3) particles, 1.8 microns in mean lS size, were fed as a Ca-type absorbent at a rate of 13.5 kg/h, as entrained in 30 Nm /h of air. The absorbent was sprayed into the exhaust gas through a nozzle on the furnace wall, at a speed of 200 to 300 m/sec along with the air.
Consequently the gas treated had an Sx concentration of 73 ppm at the outlet of the bag filter used as a dust collector~ The treatment achieved a desulfurization efficiency of 90~. The CaCO3/SO mole ratio was 2Ø
When the experiment was continued for 10 hours, 347 kg of particles were collected from the bag filter.
s The partlcles were classified by an air classifier to obtain 220 Icg of a portion of coarse particles, 20 microns in mean particle size, and 127 kg of a portion of fine particles, 2.2 microns in mean size. When chemically analyzed, the fine particle portion was found to contain 57.0% of CaSO4, 25.8 of CaO, 5.1~ of CaCO3, and 12.1% of SiO2, A12O3, MgO and other compounds combined.
Experiment 2 With the supply of fresh limestone particles interrupted, the fine particle portion recovered by Experiment 1 was introduced into a jet mill along with lD0 kg/h of water vapor superheated to 450 C and having a pressure of 6 kg/cm2G (cooling to the foregoing temperature range on expansion when flowing out from the jet mill), whereby the particulate limestone in the portion was reactivated. The reactivated particulate limestone was sprayed into the furnace through the nozzle on the furnace at a rate of 25.8 ~cg/h and speed of 200 to 300 m/sec, with the efective components (CaO and CaCO3) maintained at a CaO/SOx mole ratio of 2Ø
The treatment resulted in an Sx concentration of 75 ppm at the outlet of the bag filter as a dust collector and achleved~a desulfurization efflclency of 89~.
; When the~experiment was continued-for 4 hours, 25~; 209 kg of particles were collected from the bag filter.
.
;: `~
:
8~S
Using the classifier, the particles were classified to obtain 89 kg of a portion of coarse particles having a mean size of 20 microns and 120 kg of a portion of fine particles having a mean size of 2.3 microns. When chemically analyzed, S the fine particle portion was found to contain 67.6% of CaSO4, 11.0% of CaO, 2.2~ of CaCO3 and 19.2% of SiO2, A12O3, MgO and other compounds combined.
Experiments 3 and 4 were conducted under the following conditions.
SO concentration of exhaust gas: 730 ppm (at bag filter outlet).
Temperature at absorbent inlet: 1050 C.
~bsorbent retention time in effective temperature range (800 to 1050 C) for desulfurization reaction:
METHOD AND SYSTEM FOR PURIFYING EXHAUST GAS
:
FIELD OF THE INVENTION
The present invention relates to a dry method of and a system for purifying exhaust gases discharged from boilers, incinerators and furnaces.
BACKGROUND OF THE INVENTION
Hot exhaust gases discharged from boilers or incinerators for waste materials usually contain 10 to 2000 ppm of harmful acid substances such as sulfur oxides (SOx), hydrogen chloride (HCl), hydrogen fluoride (HF) and the like. It is required to remove these substance for pollution control.
Such harmful acid substances are removed from the exhaust gas generally by a wet method wherein a liquid or slurry containing an alkaline absorbent is brought into dlrect contact with the exhaust gas as~cooled to a low temperature to purify the gas. Although achieving a high removal efficiency, this method encounters difficulties in treating the resulting waste water~ involves the necessity of reheating the exhaust gas and has- the problem of high :
equipment and running costs.
In view of the above problem, research has been made on~various methods substituting the wet method. The methods heretofore proposed include, for example, a method of removing harmful substances by causing active carbon to adsorb such substances, and a semi-dry method of spraying a slaked lime slurry into the exhaust gas. Nevertheless, these methods fail to achieve high removal efficiencies.
Further investigations have been made of a dry method in which particles of Ca-type absorbent (such as quick lime, slaked lime, limestone, dolomite or the like) are dispersed directly into the interior of a hot furnace ; 10 or a flue for removing harmful substances, but the absorbent, which is of low reactivlty, renders the method unamenable to practice except in special cases where the environmental regulations are very slack. Stated more specifically, the reactivity of the absorbent with SO is as low as up to about 20~, and that with HCl is limited to 50~ or lower, in the dry method.
The low reactivity of Ca-type absorbents, for example of CaO particles, is attributable to the following reason.
The reaction of CaO particles with SO , HCl or HF occurs first on the surface of CaO particles, forming a thin shell of CaSO4, CaCl2 or CaF2 on the surface of the particles.
The reaction further proceeds with the diffusion of SOx, HCl or HF into the particles. However, the surface shell is compact and therefore prevents diffusion of SO or the like into the CaO partl~les to rapidly inhibit the reaction .:
Ç`s8Çi5 of Sx or the like with CaO.
The above problem itself can be overcome by the method disclosed in Ishihara et al. U.S. Patent 3,481,289.
With this method, CaO particles which are covered with a she]l of a compound such as CaSO4, CaC12, CaF2 or the like and thereby rendered chemically inactive are hydrated with water, and the expansion of CaO particles resulting from the hydration reaction breaks the surface shell to reactivate the particles. Consequently, reuse of the CaO particles thus reactivated achieves higher reactivity than when the original CaO particles are used without reactivation.
However, the method of Ishihara et al. wherein water is used for hydration is a wet method, so that the CaO particles resuLting from the hydration treatment contain a large amount of water and are in the form of agglomerates of increased sizes. Because these wet coarse partlcles can not be sprayed directly into an exhaust gas, it is required to prepare CaO particles of the order of several microns for use in a dry process,by subjecting the coarse particles to a treatment involving drying, pulverization and classification. This treatment has problems in respect of labor, time and cost. Furthermore, the wet hydration treatment itself also has the problem of requiring~ a prolonged period of time since a very high temperature is not usable for the reaction.
.?s~65 SUMMARY OF THE INVENTION
An object of the present invention is to provide a dry method of purifying exhaust gases with a Ca-type absorbent and a system therefor which are free of the problems of the foregoing U.S. patent although making US8 of the advantages thereof.
According to a first aspect of the invention, there is provided a method of purifying an exhaust gas containing harmful acid substances by spraying fine particles of a Ca-type absorbent into the exhaust ga~ and collecting by means ofa dust collector the absorbent particles absorbing the harmful acid substances along with other particles contained in the exhaust gas, the method being characterized by separating the absorbent particles of size about 2 to 3 microns from said other particles, hydrating the collected absorbent particles with superheated water vapor having a temperature of about 150-300C in order to thereby expand the particles and break an inactive shell formed on the surface of the particles by the absorption of the harmful acid substances, and supplying the resulting activated absorbent articles to the exhaust gas again.
The Ca-type absorbents to be used in the present process include limestone (CaCO3), slaked Lime ~CatOH~2], quick lime (CaO), dolomite (CaC03.MgC03), slaked dolomite [Ca(OH)z.Mg(OH)2 or Ca(OH)2.MgO~, calcined dolomite (CaO.MgO
or CaCO3.MgO) and any material containing Cao or capable of forming CaO at a high temperature (of not lower than about 300C). Accordingly, the particla~e Ca-type , .
i5 absorbent originally contains CaO or produces CaO through the following reactions at a high temperature of at least about 300 C.
Ca(OH) ~ CaO + H O (1) CaCO3 ~ CaO + CO2 (2) CaO absorbs acid harmful substances (SOx, HCl, HF) from the exhaust gas upon reacting therewith as follows.
CaO + SO4 + 22~ CaSO4 (3) CaO + SO3 ~ CaSO4 (4) CaO + 2HCl ~ CaC12 + H2O (5) CaO + 2HF ~ CaF2 + H2O (6) The CaSO4, CaC12 and CaF2 resulting from the reactions (3) to (6) form an inactive shell over the surface of the absorbent particles. According to the present invention, however, CaO in the interior of the absorbent particles is hydrated with water vapor into Ca(OH)2, whereupon the particles expand to break the surface shell and become activated. Moreover, since the hydration reaction is conducted with use of water vapor through a dry process, the activated absorbent particles can be supplled directly to the exhaust gas again without necessltating any special subsequent treatment.
Exhaust gases often contain fly ash of large ~ particle sizes in additlon to harmful acidsubstances. In such a case, it is desirable to separate the particles .
~5-~B
. , .
~.~8~iS
collected by the dust collector into a portion of coarse particles chiefly containing fly ash and a portion of fine particles including predominantly absorbent particles not larger than a predetermined size and to hydrate the fine particle portion only with water vapor.
Preferably the water vapor has a temperature of 150 to 300 C, more preferably 200 to 250 C. If the temperature is below 150 C, the hydration reaction will not proceed rapidly, whereas at temperatures above 300 C, the reactio~ wherein Ca(OH)2 releases H~O proceeds in preferance to the hydration of CaO into Ca(OH)2. Thus, the hydration treat~ent can not be carried out as intended.
Of the Ca-type absorbents mentioned above, dolomite, slaked dolomite and calcined dolomite are higher in void volume and therefore have the advantage of readily permitting diffusion of acid harmful su~stances into the particles.
According to a second aspect of the invention, there is provided a system for purifying an exhaust gas produced by ; 20 a gas produced and container harmful~acid substances by spray-ing fine~particles of a Ca-type absorbent into the exhaust gas and collecting by a dust collector the absorbent particles ; absorbing the harmful acid substances along with other partic-les contained in the exhaust gas, the system being character-ized in that the system further comprises a classifier for dividing the particles collected by the dust collector into a portion of coarse particles chiefly containing fly ash and a :~
portion of fine particles predominantly including absorbent particles of about 2 to 3 microns in size, a hydration reactor for hydrating the fine absorbent particles ~lith superheated water vapor having a temperature of about 150-300C to thereby expand the particles and break an inactive shell formed on the surface of the particles by the absorption of the harmful acid substances and supplying the resulting activated absorbent particles to the exhaust gas again.
Various features and advantages of the present invention will be readily understood from the following description of an embodiment with reference to the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
The drawing is a diagram schematically showing the construction of an exhaust gas purifying system embodying the present inventlon.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With re~erence to the drawing, indicated at 1 is a coal boiler, fuel oil boiler, waste incinerator or like furnace which produces an exhaust gas of high temperature, or a flue connected to the furnace or gas producer.
Particles of a Ca-type absorhent having a predetermined mean particle size are supplied to the furnace or flue 1 via a line 2, and acid harmful substances, such as SO , HCl and HF,are absorbed by the absorbent particles from the exhaust gas. The gas is sent to a dust collector 3, by which various klnds of particles are removed. ThP gas is then released into the atmosphere through a line 4 and a chimney 5.
The particles collected by the dust collector 3 are fed through a line 6 to a classifier 7, in which they are separated into a portion of coarse particles chiefly containing fly ash and a portion of fine particles (2 to 3 microns in mean size) predominantly including absorbent particles which are inactivated on the surface by the absorption of acid harmful substances.
In the case of coal boilers and incinerators for waste ma-terials, the amount of fly ash is several times the amount of lnactivated absorbent particles, so that it is essential to use the classifier 7, while in the case of fuel oil boilers, the amount of fly ash is less than that of inactivated absorbent particles. The classification step is not always necessary in the latter case.
In the case of coil boilers, the fly ash is coarse and is about 15 microns in mean particle size even if fine, whereas the particulate inactivated absorbent is 2 to 3 microns in mean particle size. Accordingly, the absorbent can be effectively separated from fly ash when an air classifier is used as the classifier 7. Experiments conducted with use of an air classifier show that the collected particulate mixture can be separated into-a fly ash portion ; 25 contalning about 10% of inactlvated absorbent particles and :: : :
~ -8-a portion of inactivated absorbent particles containing about 10% of fly ash.
The portion of fine particles including the inactivated absorbent and separated off by the classifier 7 is led via a line 8 to a hydration reactor 9, to which superheated water vapor having a temperature of 200 to 250 C is supplied through a line 10. The inactivated absorbent particles are dispersed through the superheated water vapor, whereby the interor CaO component is instantaneously hydrated to expand. Consequently, the inactive shell (of CaSO4, CaC12, CaF2, etc.) covering each absorbent particle breaks to expose the interior unreacted portion.
The hydration reactor 7 may be one utilizing a jet mill through whicn superheated water vapor is passed at a high speed. In this case, the pulverizing action of the jet mill per se and the expansion due to the hydration reaction coact to readily rupture the inactive shell over the surface of the absorbent particles. Another advantage is available in this case in that the activated absorbent particles obtained can be rapidly supplied to and dispersed into the exhaust gas. Irrespective of whether the jet mill lS used, a gas having a high temperature and containing water vapor is usable in place of superheated water vapor.
The activated absorbent particles are supplied again to the furnace or flue 1 via a return line 11, along -9_ ~ . .
::
.,. , - ~, ~ ': -' , '' . :
~' s~5 with a fresh portion of absorbent particles supplied through the line 2. Thus, the same process as above is repeated.
To prevent an excess of absorbent particles from circulating through the system, the inactivated absorbent particles run off from the classifier 7 may be partly purged through a purge line 12.
With the method of the present invention, the inactive shell is broken or ruptured by the volume expansion resulting from the hydration of CaO into Ca(OH)2, so that the Ca-type absorbent must contain CaO when the hydration treatment is carried out. On the other hand, the harmful acid substances are removed by the Ca-type absorbent in two temperature ranges: i.e. in a high-temperature range of 800 to 1200 C in which SO is absorbed, and a low-temperature range of 150 to 400 C where HCl and HF are absorbed. In the high-temperature range, both Ca(OH)2 and CaO are thermally decomposed to give CaO, so that all the aforementioned Ca-type absorbents are usable. In the low-temperatu~e range, however, CaCO3 is not thermally decomposable, so that the Ca-type absorbents to be used are limited to those containing CaO and Ca(OH)2. At 150 to 300 C within the low-temperature range, Ca(OH)2 is not thermally decomposable either, while CaO will be converted to Ca(OH)2. Accordingly the present invention is to be practiced at temperatures of above 300 C.
;5 Exhaust gas purifying experiments were conducted using a coal burning test furnace which produced an exhaust gas at a rate of 2000 Nm /hour (N standing for standard state), as will be stated below.
Experiments 1 and 2 were conducted under the following conditions.
SO concentration of exhaust gas: 740 ppm (at the outlet of bag filter ?
Temperature at absorbent inlet: 1070 C.
Absorbent retention time in effective temperature range (800 to 1050 C) for desulfurization reaction:
3 seconds.
Experiment _ Limestone (CaCO3) particles, 1.8 microns in mean lS size, were fed as a Ca-type absorbent at a rate of 13.5 kg/h, as entrained in 30 Nm /h of air. The absorbent was sprayed into the exhaust gas through a nozzle on the furnace wall, at a speed of 200 to 300 m/sec along with the air.
Consequently the gas treated had an Sx concentration of 73 ppm at the outlet of the bag filter used as a dust collector~ The treatment achieved a desulfurization efficiency of 90~. The CaCO3/SO mole ratio was 2Ø
When the experiment was continued for 10 hours, 347 kg of particles were collected from the bag filter.
s The partlcles were classified by an air classifier to obtain 220 Icg of a portion of coarse particles, 20 microns in mean particle size, and 127 kg of a portion of fine particles, 2.2 microns in mean size. When chemically analyzed, the fine particle portion was found to contain 57.0% of CaSO4, 25.8 of CaO, 5.1~ of CaCO3, and 12.1% of SiO2, A12O3, MgO and other compounds combined.
Experiment 2 With the supply of fresh limestone particles interrupted, the fine particle portion recovered by Experiment 1 was introduced into a jet mill along with lD0 kg/h of water vapor superheated to 450 C and having a pressure of 6 kg/cm2G (cooling to the foregoing temperature range on expansion when flowing out from the jet mill), whereby the particulate limestone in the portion was reactivated. The reactivated particulate limestone was sprayed into the furnace through the nozzle on the furnace at a rate of 25.8 ~cg/h and speed of 200 to 300 m/sec, with the efective components (CaO and CaCO3) maintained at a CaO/SOx mole ratio of 2Ø
The treatment resulted in an Sx concentration of 75 ppm at the outlet of the bag filter as a dust collector and achleved~a desulfurization efflclency of 89~.
; When the~experiment was continued-for 4 hours, 25~; 209 kg of particles were collected from the bag filter.
.
;: `~
:
8~S
Using the classifier, the particles were classified to obtain 89 kg of a portion of coarse particles having a mean size of 20 microns and 120 kg of a portion of fine particles having a mean size of 2.3 microns. When chemically analyzed, S the fine particle portion was found to contain 67.6% of CaSO4, 11.0% of CaO, 2.2~ of CaCO3 and 19.2% of SiO2, A12O3, MgO and other compounds combined.
Experiments 3 and 4 were conducted under the following conditions.
SO concentration of exhaust gas: 730 ppm (at bag filter outlet).
Temperature at absorbent inlet: 1050 C.
~bsorbent retention time in effective temperature range (800 to 1050 C) for desulfurization reaction:
3 seconds.
Experiment 3 -Dolomite particles serving as a Ca-type absorbent, having a mean particle size of 1.7 microns and composed of 16.3% of MgO, 35.3% of CaO, 45.5% of CO2 and 2.1% of SiO2 were sprayed into the exhaust gas at a rate of 21.Q kg/h and speed of 150 to 250 m/sec through an ejector and the nozzle. The absorbent was supplied along with 45 ~m3/h or alr .
The treatment resulted in an SO concentration of 65 ppm at the bag filter outlet ~(corresponding to 91%
., ~
~ :
, . . ,~ . ,, . : .
in desulfurization). The CaO/SO mole ratio was 2Ø
The experiment was continued for 10 hours under the above conditions, whereby 385 kg of particles were collected from the bag filter serving as a dust collector.
The particles were treated by the air classifier to obtain 219 kg of a portion of coarse particles having a mean size of 20 microns and 166 kg of a portion of fine particles having a mean size of 2.0 microns. When chemically analyzed, the fine particle portion was found to contain 40.7% of CaO, 18.5% of MgO, 25.7g6 of SO3, 0.7g6 of CO2 and 14.4g~ of SiO2, A12O3 and the like combined.
Experiment 4 With the supply of fresh dolomite particles interrupted, the fine particle portion recovered by Experiment 3 was introduced into a jet mill along with 110 kg/h of water vapor superheated to 400 C and having a pressure of 6 kg/cm G, whereby the particulate dolomite in the portion was reactivated. The reactivated particulate dolomite was sprayed into the furnace through the nozzle at a rate of 30.0 kg/h and speed of 150 to 250 m/sec, with the effective components maintained at a CaO/SO mole ratio of 1.85.
The treatment resulted in an SO concentration of 90 ppm at the bag filter outlet,~thus achieving a desulfuri-25 ~ zation efficiency of 88%.
~,,,`. :
s The experiment was continued for 4 hours under the above conditions to collect 228 kg oE particles from the bag filter.
The particles were treated by the air classifier to obtain ~7 kg of a portion of coarse particles having a mean particle size of 20 microns and 140 kg of a portion of fine particles 2.0 microns in mean size. When chemically analyzed, the fine particle portion was found to contain 32.5% of CaO, 14.3% of MgO, 31.6~ of SO3, 0.5% of CO2 and 10 21.1~ of SiO2, A12O3, etc. combined.
' ., ,.. .. . .
Experiment 3 -Dolomite particles serving as a Ca-type absorbent, having a mean particle size of 1.7 microns and composed of 16.3% of MgO, 35.3% of CaO, 45.5% of CO2 and 2.1% of SiO2 were sprayed into the exhaust gas at a rate of 21.Q kg/h and speed of 150 to 250 m/sec through an ejector and the nozzle. The absorbent was supplied along with 45 ~m3/h or alr .
The treatment resulted in an SO concentration of 65 ppm at the bag filter outlet ~(corresponding to 91%
., ~
~ :
, . . ,~ . ,, . : .
in desulfurization). The CaO/SO mole ratio was 2Ø
The experiment was continued for 10 hours under the above conditions, whereby 385 kg of particles were collected from the bag filter serving as a dust collector.
The particles were treated by the air classifier to obtain 219 kg of a portion of coarse particles having a mean size of 20 microns and 166 kg of a portion of fine particles having a mean size of 2.0 microns. When chemically analyzed, the fine particle portion was found to contain 40.7% of CaO, 18.5% of MgO, 25.7g6 of SO3, 0.7g6 of CO2 and 14.4g~ of SiO2, A12O3 and the like combined.
Experiment 4 With the supply of fresh dolomite particles interrupted, the fine particle portion recovered by Experiment 3 was introduced into a jet mill along with 110 kg/h of water vapor superheated to 400 C and having a pressure of 6 kg/cm G, whereby the particulate dolomite in the portion was reactivated. The reactivated particulate dolomite was sprayed into the furnace through the nozzle at a rate of 30.0 kg/h and speed of 150 to 250 m/sec, with the effective components maintained at a CaO/SO mole ratio of 1.85.
The treatment resulted in an SO concentration of 90 ppm at the bag filter outlet,~thus achieving a desulfuri-25 ~ zation efficiency of 88%.
~,,,`. :
s The experiment was continued for 4 hours under the above conditions to collect 228 kg oE particles from the bag filter.
The particles were treated by the air classifier to obtain ~7 kg of a portion of coarse particles having a mean particle size of 20 microns and 140 kg of a portion of fine particles 2.0 microns in mean size. When chemically analyzed, the fine particle portion was found to contain 32.5% of CaO, 14.3% of MgO, 31.6~ of SO3, 0.5% of CO2 and 10 21.1~ of SiO2, A12O3, etc. combined.
' ., ,.. .. . .
Claims (7)
1. A method of purifying an exhaust gas containing harmful acid substances by spraying fine particles of a Ca-type absorbent into the exhaust gas and collecting by means of a dust collector the absorbent particles absorbing the harmful acid substances along with other particles contained in the exhaust gas, the method being characterized by separating the absorbent particles of size about 2 to 3 microns from said other particles, hydrating the collected absorbent particles with superheated water vapor having a temperature of about 150-300°C in order to thereby expand the particles and break an inactive shell formed on the surface of the particles by the absorption of the harmful acid substances, and supplying the resulting activated absorbent articles to the exhaust gas again.
2. A method as defined in claim 1 wherein the superheated water vapor has a temperature of 200 to 250°C.
3. A method as defined in claim 1 wherein the absorbent particles having the inactive shell are pulverized by a jet mill simultaneously with the hydration with water vapor.
4. A method as defined in claim 1 wherein the Ca-type absorbent comprises one or the combination of at least two of limestone, slaked lime, quick lime, dolomite, slaked dolomite and calcined dolomite.
5. A system for purifying an exhaust gas produced by a gas produced and container harmful acid substances by spraying fine particles of a Ca-type absorbent into the exhaust gas and collecting by a dust collector the absorbent particles absorbing the harmful acid substances along with other particles contained in the exhaust gas, the system being characterized in that the system further comprises a classifier for dividing the particles collected by the dust collector into a portion of coarse particles chiefly containing fly ash and a portion of fine particles predominantly including absorbent particles of about 2 to 3 microns in size, a hydration reactor for hydrating the fine absorbent particles with superheated water vapor having a temperature of about 150-300°C to thereby expand the particles and break an inactive shell formed on the surface of the particles by the absorption of the harmful acid substances and supplying the resulting activated absorbent particles to the exhaust gas again.
6. A system as defined in claim 5 wherein the classifier is an air classifier.
7. A system as defined in claim 5 wherein the hydration reactor is provided with a jet mill.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP15816784A JPS6135827A (en) | 1984-07-27 | 1984-07-27 | Purification of exhaust gas by dry lime method |
| JP158167/1984 | 1984-07-27 | ||
| JP59277996A JPS61157328A (en) | 1984-12-28 | 1984-12-28 | Purification of exhaust gas by dry dolomite method |
| JP277996/1984 | 1984-12-28 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1296865C true CA1296865C (en) | 1992-03-10 |
Family
ID=26485378
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000487508A Expired - Fee Related CA1296865C (en) | 1984-07-27 | 1985-07-25 | Method and system for purifying exhaust gas |
Country Status (7)
| Country | Link |
|---|---|
| KR (1) | KR920003768B1 (en) |
| BE (1) | BE902935A (en) |
| CA (1) | CA1296865C (en) |
| DE (1) | DE3526857A1 (en) |
| FR (1) | FR2568141B1 (en) |
| GB (1) | GB2162162B (en) |
| IT (1) | IT1182791B (en) |
Families Citing this family (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS61178022A (en) * | 1985-02-05 | 1986-08-09 | Mitsubishi Heavy Ind Ltd | Simultaneous treatment of so2, so3 and dust |
| DE3610408A1 (en) * | 1986-03-27 | 1987-10-01 | Hoelter Heinz | Two-stage semi-dry desulphurisation process |
| DE3635027A1 (en) * | 1986-10-15 | 1988-04-28 | Steinmueller Gmbh L & C | DIRECT DESOLFURATION PROCESS WITH FLUE DUST RETURN |
| JPH0729028B2 (en) * | 1986-10-31 | 1995-04-05 | バブコツク日立株式会社 | Denitration treatment method |
| FI76931B (en) * | 1986-12-12 | 1988-09-30 | Imatran Voima Oy | FOERFARANDE FOER RENING AV ROEKGASER. |
| JP2686292B2 (en) * | 1988-10-24 | 1997-12-08 | 北海道電力株式会社 | Method for producing lime-gypsum-coal ash hydrated cured body |
| AT394660B (en) * | 1989-07-28 | 1992-05-25 | Staudinger Gernot | METHOD FOR REMOVAL OR REDUCTION OF GASEOUS POLLUTANTS AND DEVICE FOR CARRYING OUT THIS PROCESS |
| SE465955B (en) * | 1989-08-22 | 1991-11-25 | Flaekt Ab | PROVIDED TO REDUCE NOZZLE OF THE NOZZLE OR OTHER SUPPLY ORGANIZATIONS BY FINDING A SLURRY OF A PARTICULAR ABSORBENT |
| JPH06198126A (en) * | 1992-09-16 | 1994-07-19 | Hokkaido Electric Power Co Inc:The | Method for treating exhaust gas |
| DE19607026A1 (en) * | 1996-02-24 | 1997-08-28 | Abb Research Ltd | Flue gas cleaning process for municipal refuse incineration plants |
| AT403664B (en) * | 1996-06-05 | 1998-04-27 | Austrian Energy & Environment | Process for the sorption of pollutants, preferably from waste gases from biomass gasification |
| KR20020075977A (en) * | 2001-03-26 | 2002-10-09 | 김재곤 | manufacturing method of boil oyster |
| JP6217845B2 (en) | 2014-04-15 | 2017-10-25 | 株式会社Ihi | Desulfurization apparatus and exhaust gas treatment system using the same |
| CN104826434B (en) * | 2015-04-29 | 2017-01-18 | 李森 | Nanocluster-plasma haze dust removal method and apparatus thereof |
Family Cites Families (17)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR1420350A (en) * | 1964-01-15 | 1965-12-03 | Improvements to processes for desulphurizing gases | |
| BE759979A (en) * | 1965-02-10 | 1971-06-07 | Grillo Werke Ag | PROCESS FOR ELIMINATION OF SULFUR IN RESIDUAL GASES |
| GB1229415A (en) * | 1967-07-19 | 1971-04-21 | ||
| US3574530A (en) * | 1967-10-02 | 1971-04-13 | Pullman Inc | Method of removing sulfur dioxide from waste gases |
| SE418152B (en) * | 1974-06-12 | 1981-05-11 | Ceskoslovenska Akademie Ved | SET FOR EXHAUSTABILITY OF GASES, IN PARTICULAR NITROGEN AND SULFUR OXIDES, WITH THE HELP OF CARBONATES |
| DE2615828A1 (en) * | 1976-04-10 | 1977-10-13 | Heinz Hoelter | Gas purificn. by addn. of absorbent - in series of stages with sepn. and recycling of absorbent after each stage |
| DE2539500B2 (en) * | 1975-09-05 | 1980-06-19 | Heinz Ing.(Grad.) 4390 Gladbeck Hoelter | Process for separating dust and gaseous pollutants from hot exhaust gases and device for carrying out the process |
| DE2934109A1 (en) * | 1979-08-23 | 1981-03-26 | L. & C. Steinmüller GmbH, 51643 Gummersbach | Dry treatment of exhaust gases in purificn. plants for boiler-rooms - incorporates grinding ashes from hearth and blowing into purificn. stages, economising in absorption agent |
| GB2068919A (en) * | 1980-02-12 | 1981-08-19 | Conoco Inc | Method for regenerating calcium sulfoxy compounds produced in the combustion of carbonaceous fuels in fluidized beds |
| BR8103078A (en) * | 1980-05-24 | 1982-02-09 | Hoelter H | PROCESS AND DEVICE FOR THE DISPOSAL OF Sulfurous Anhydride AND OTHER HARMFUL SUBSTANCES OF SMOKE GAS |
| DE3117601A1 (en) * | 1981-05-05 | 1982-12-16 | Hölter, Heinz, Dipl.-Ing., 4390 Gladbeck | Dry desulphurisation process with classifying action of the end product |
| US4387078A (en) * | 1981-07-22 | 1983-06-07 | Lin Ping Wha | Process for effecting removal of sulfur oxide gases from stack gases |
| US4454102A (en) * | 1981-09-18 | 1984-06-12 | Flakt Aktiebolag | Method of purifying flue gases from sulphur dioxide |
| AT372876B (en) * | 1981-11-19 | 1983-11-25 | Oesterr Draukraftwerke | METHOD AND DEVICE FOR THE DESCULATION OF FLUE GAS DESULFURING COAL BURNERS AFTER THE DRY ADDITIVE METHOD |
| US4442078A (en) * | 1982-07-07 | 1984-04-10 | The United States Of America As Represented By The United States Department Of Energy | Method of removing hydrogen sulfide from gases utilizing a zinc oxide sorbent and regenerating the sorbent |
| DE3232080C2 (en) * | 1982-08-28 | 1986-10-16 | Rheinisch-Westfälisches Elektrizitätswerk AG, 4300 Essen | Process for the dry removal of sulfur dioxide and other pollutants from flue gases |
| DE3235341A1 (en) * | 1982-09-24 | 1984-03-29 | Deutsche Babcock Anlagen Ag, 4200 Oberhausen | METHOD FOR PURIFYING EXHAUST GASES |
-
1985
- 1985-07-19 BE BE2/60749A patent/BE902935A/en not_active IP Right Cessation
- 1985-07-23 GB GB08518521A patent/GB2162162B/en not_active Expired
- 1985-07-25 CA CA000487508A patent/CA1296865C/en not_active Expired - Fee Related
- 1985-07-25 IT IT48410/85A patent/IT1182791B/en active
- 1985-07-26 FR FR858511479A patent/FR2568141B1/en not_active Expired - Fee Related
- 1985-07-26 KR KR1019850005357A patent/KR920003768B1/en not_active IP Right Cessation
- 1985-07-26 DE DE19853526857 patent/DE3526857A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| KR920003768B1 (en) | 1992-05-14 |
| IT1182791B (en) | 1987-10-05 |
| BE902935A (en) | 1985-11-18 |
| DE3526857C2 (en) | 1993-01-28 |
| GB8518521D0 (en) | 1985-08-29 |
| FR2568141B1 (en) | 1990-03-23 |
| GB2162162B (en) | 1988-07-13 |
| IT8548410A0 (en) | 1985-07-25 |
| GB2162162A (en) | 1986-01-29 |
| KR860000886A (en) | 1986-02-20 |
| DE3526857A1 (en) | 1986-02-13 |
| FR2568141A1 (en) | 1986-01-31 |
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