FR2568141A1 - PROCESS AND INSTALLATION FOR PURIFYING REJECTION GASES - Google Patents

Abstract

THE PROCESS ACCORDING TO THE INVENTION CONSISTS OF SPRAYING FINE PARTICLES OF A TYPE CA ABSORBENT IN THE WASTE GAS, IN PURIFYING AND COLLECTING THE ABSORBENT PARTICLES IN WHICH THE HARMFUL ACID SUBSTANCES CONTAINED IN THE REJECTED GAS ARE ABSORBED. AT THE SAME TIME AS THE OTHER PARTICLES CONTAINED THEREIN. IT IS CHARACTERIZED BY HYDRATING THE ABSORBENT PARTICLES COLLECTED WITH WATER VAPOR, IN ORDER TO EXPAND THESE PARTICLES AND BREAK THE INACTIVE ENVELOPE FORMED ON THEIR SURFACE, AND RECYCLE THE RESULTING ACTIVE ABSORBENT PARTICLES IN THE DISCHARGE GAS. THE INSTALLATION ACCORDING TO THE INVENTION ESSENTIALLY CONSISTS OF A GAS-PRODUCING APPARATUS 1, A DUST COLLECTOR 3 AND A HYDRATION REACTOR 9.

Description

Method and apparatus for purifying waste gases

  The invention relates to a dry process and a

  plant for purifying waste gases from boilers,

cookers and ovens.

  Hot exhaust gases from boilers or waste incinerators commonly contain 10 to 2000 ppm of

  harmful acids such as oxides of sulfur (SOx), hydrochloric acid

  as (HCl), hydrofluoric acid (HF) and the like. It is necessary

  to eliminate these substances to fight against pollution.

  The harmful acidic substances present in the off-gases are generally removed by a wet process in which a liquid or sludge containing an alkaline absorbent is brought into direct contact

  with the off-gas that is cooled to a low temperature, to purify

  proud of the gas. Although it achieves a high removal efficiency, this process faces difficulties in the treatment of the resulting waste water, involves the need to heat the reject gas and is tainted by the problem. Costs of fitting and

high functioning.

  In view of the problem mentioned above, research has been

  various processes that could replace the wet process. The methods heretofore proposed include, for example, a process for removing harmful substances by absorbing these substances with active carbon, and a semi-dry process for spraying slaked lime sludge in the feed gas. rejection. However, these methods do not make it possible to obtain high elimination yields. Further studies have been made of a dry process in which particles of a Ca-type absorbent (such as quicklime, slaked lime, limestone, dolomite or the like) are dispersed directly to the interior of a hot furnace or flue to remove harmful substances, but the absorbent, which is of low reactivity, renders the process impractical except in special cases in which environmental regulations are very flexible. More specifically, the reactivity of the absorbent vis-à-vis SO decreases to about 20% and that vis-à-vis HCl x

  is limited to 50% or less in the dry process.

  The low reactivity of the Ca-type absorbers, for example CaO particles, is attributable to the following reason. The reaction of the CaO particles with SOx, HCl or HF occurs first on the surface of the CaO particles, forming a thin envelope of CaSO4, CaCl2 or CaF2 on the surface of the particles. The reaction continues to unfold

  with the diffusion of SOx, HCl or HF in the particles. However, the

  The superficial lump is compact and thus prevents diffusion of SO x or the like into the CaO particles by rapidly inhibiting the reaction.

SOx or the like with CaO.

  The aforementioned problem can itself be overcome by the process described in US Pat. No. 3,481,289. With this method, the CaO particles which are covered with a shell of a compound

  such as CaSO4, CaCl2, CaF2 or the like and thereby rendered chemically inactive

  are diluted with water, and the expansion of the CaO particles resulting from the hydration reaction has the effect of breaking the envelope.

  superficial by reactivating the particles. As a result, reusability

  the CaO particles thus reactivated makes it possible to obtain reactivity

  higher than when using the initial CaO particles without reaction

vation.

  However, the process according to the aforementioned US patent, in which water is used for hydration is a wet process, 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 size. Since these moist coarse particles can not be sprayed directly into a reject gas, it is necessary to prepare CaO particles of the order of several.

  microns intended for use in a dry process, in

  placing the coarse particles in a treatment including drying, spraying and classification. This treatment poses problems with regard to manpower, time and cost. In addition, wet treatment based on hydration also poses a problem.

  itself a problem because it requires a prolonged period of time because it does not

  do not use a very high temperature for the reaction.

  The object of the invention is to provide a process for purifying waste gases, by dry process with a Ca-type absorbent, and an installation for carrying out this process, which are free from the problems affecting the process according to the invention. aforementioned US patent, while

  taking advantage of the benefits of it.

  According to a first characteristic of the invention, there is provided a method for purifying a waste gas containing harmful acid substances, by spraying fine particles of an absorbent of

  type Ca in the off-gas and collection, by means of a dust collector

  absorbent particles in which the substances are absorbed.

  at the same time as the other particles contained in the off-gas, which process is characterized in that

  with water vapor the absorbent particles collected, in order to

  void the expansion of these particles and the breaking of the inactive envelope formed on their surface by the absorption of the harmful acid substances, and the resulting activated absorbent particles are recycled into the gas

rejection.

  Ca absorbers intended for use in the

  of the invention include limestone (CaCO 3), slaked lime [Ca (OH) 2], quicklime (CaO), dolomite (CaCO 3 MgCO 3), extinguished dolomite

2 3

  [Ca (OH) 2.Mg (OH) 2 or Ca (OH) 2.MgO], calcined dolomite (CaO.MgO or CaCO3.

  MgO) and any substance containing CaO or capable of forming CaO at an elevated temperature (not less than about 300 C). As a result, the particulate Ca-type absorbent contains CaO or produces CaO via the following reactions at elevated temperature.

at least about 300 C.

  Ca (OH) 2 --- CaO + H 2 O (1) CaCO 3 -CaO + CO 2 (2) CaO absorbs the harmful acidic substances (SOx, HCl, HF) present in

  the reject gas by reacting with it as follows.

1

  CaO + SO4 + 2 02-- CaSO4 (3) CaO + SO3 --- CaSO4 (4) CaO + 2HCl --CaCl + H2O (5)

2 2

  CaO + 2HF --- CaF2 + H20 (6) CaSO4, CaCl2 and CaF2 substances resulting from reactions (3)

  (6) form an inactive envelope on the surface of the absorbing particles.

  bant. According to the invention, the CaO present inside the particles

  however, the absorbent is hydrated with water vapor of Ca (OH) 2, after which the particles expand by breaking the surface envelope and become activated. Furthermore, since the hydration reaction is carried out using water vapor in a dry process, the activated absorbent particles can be recycled directly into the process.

  the reject gas without requiring any particular subsequent treatment.

  link. Discharge gases often contain fly ash of large grain size in addition to harmful acidic substances. In this

  In this case, it is desirable to separate the particles collected by the

  dust in a fraction of coarse particles containing mainly

  usually fly ash and a fraction of fine particulate matter

  predominantly absorbing the absorbent particles of at least a maximum dimension to a predetermined size, and hydrating only the fraction

  fine particles with water vapor.

  Preferably, the water vapor has a temperature of 150 to 300 C,

  and preferably from 200 to 250 ° C. If the temperature is below

  below 150 ° C., the hydration reaction does not proceed rapidly, whereas at temperatures above 300 ° C., the reaction in which Ca (OH) 2 releases H 2 O preferably proceeds with the hydration of CaO at Ca ( OH) 2. Thus, the hydration treatment can not be performed as intended. Of the Ca-type absorbers mentioned above, dolomite, dolomite extinguished and calcined dolomite have a higher pore volume and hence have the advantage of easily allowing the diffusion of

  harmful acid substances in particles.

  According to a second characteristic of the invention, there is provided an installation for the purification of a product discharge gas.

  gas-producing equipment containing harmful acids and

  ss, by spraying fine particles of a Ca-type absorbent into the exhaust gas and collecting, through a dust collector,

  absorbent particles having absorbed the harmful acidic substances,

  jointing with the other particles contained in the off-gas, this installation being characterized in that it further comprises a reactor

  moisture to hydrate with water vapor the absorbing particles

  bant collected to dilate these particles and break the inactive envelope formed on their surface by the absorption of harmful acidic substances, and to recycle the resulting activated absorbent particles

in the reject gas.

  Other features and advantages of the invention will emerge

  clearly from the description of a mode of implementation that will be given

  Hereinafter by way of example, in no way limiting with reference to the accompanying drawing in which: - the single figure shows a schematic diagram showing the structure of a plant for purifying a waste gas compliant

to the invention.

  With reference to the drawing, it has been indicated in 1 a boiler with

  good, an oil fired boiler, a waste incinerator or an analogous furnace that produces an off-gas at a high temperature, or a connected flue

  in the oven or gas producing apparatus. We send in the oven or car-

  1, through a conduit 2, particles of a Ca-type absorbent having a predetermined average particle size, the harmful acidic substances, such as SOx, HC1 and HF being thus absorbed by the particles of absorbent to from the exhaust gas. The gas is sent to a dust collector 3, whereby different kinds of particles are removed. The gas is then released into the atmosphere by

a conduit 4 and a chimney5. -

  The ducts are sent through the particles

  collected by the dust collector 3 in a classifier 7 in which they are separated into a fraction of coarse particles mainly containing fly ash and a fraction of fine particles (average size of 2 to 3 microns) predominantly comprising absorbent which are inactivated on the surface by absorption

harmful acid substances.

  In the case of coal boilers and waste incinerators, the amount of fly ash is several times the amount of the inactivated absorbent particles, so it is essential to use the classifier 7, whereas in the case of boilers to

  oil, the amount of fly ash is less than that of

  inactivated absorbent cells. The classification stage is not always

necessary in the latter case.

  In the case of coal boilers, fly ash is coarse particles and in the case of fine particles, the average particle size is about 15 microns, while the average particle size of the particulate-inactivated absorbent is 2 to 5 microns. 3 microns. As a result, the absorbent can be effectively separated from the fly ash when

  uses an air classifier as a classification device 7.

  Experiments carried out using an air classifier show that the mixture of particles collected can be separated into a fraction

  of fly ash containing approximately 10% of inactive absorbent particles

  and a fraction of inactivated absorbent particles containing

10% fly ash.

  The fraction of fine particles comprising the absorbent

  inactivated and separated by the classifier 7, via a

  8, in a hydration reactor 9 in which superheated steam is passed through line 10, having a temperature of 200 to 250 C. The inactivated absorbent particles are dispersed in the water vapor. Overheating, which has the effect of hydrating the inner CaO component and causing it to expand. As a result, the inactive envelope (of CaSO 4, CaCl 2, CaF 2, and others) covering each particle of absorbent breaks up, revealing the inner portion having

not reacted.

  The hydration reactor 7 may be a reactor using a

  projection mill in which superheated steam is passed through the

  fairy at a high speed. In this case, the spraying action of the grinding mill itself and the expansion due to the hydration reaction exert a joint action which has the effect of easily breaking the inactive envelope on the surface of the absorbent particles. Another advantage is offered in this case because the activated absorbent particles obtained

  can be quickly sent and dispersed in the reject gas. India-

  in addition to the possible use of the jet mill, a gas having a high temperature and containing water vapor may be used

  instead of superheated steam.

  The activated absorbent particles are recycled to the furnace or flue 1 through a return line 11, together with a fresh portion of absorbent particles delivered through line 2. Thus, the same process as here above is repeated. To prevent an excess of absorbent particles from circulating in the system, the inactivated absorbent particles flowing in

  from the classifier 7, via a drain line 12.

  With the process of the invention, the inactive envelope is broken or broken by the increase in volume resulting from the CaO hydration of Ca (HO) 2, so that the Ca-type absorbent can contain CaO when the hydration treatment is performed. On the other hand, the harmful acidic substances are removed by the Ca-type absorbent in two temperature ranges: namely a temperature range. from 800 to 1200 ° C in which SO is absorbed, and a low temperature range x from 150 to 400 ° C in which HCl and HF are absorbed. In the interval of

  high temperatures, Ca (OH) 2 and CaO are both thermally decomposed

  supplying CaO, so that all Ca-type absorbers pre-

  cited are usable. In the low temperature range, however, CaCO3 is not thermally decomposable, so that the Ca-type absorbents that are intended to be used are limited to those containing CaO and Ca (OH) 2. At temperatures of 150 to 300 ° C, in the range of low temperatures, Ca (OH) 2 is also not thermally decomposable, while CaO is converted to Ca (OH) 2. As a result, the invention

  must be carried out at temperatures above 300 C.

  Rejection gas purification experiments were conducted using a coal-fired test furnace which produced a reject gas at a rate of 2000 Nm3 / hr (N meaning normal conditions),

as will be described below.

  Experiments 1 and 2 are carried out under the following conditions.

  SOx concentration of the exhaust gas: 74Q ppm (at the exit of the x

filter bag).

  Temperature at the inlet of the absorbent: 1070 C.

  Absorber residence time in the effective temperature range (800 to 1050 C) for the desulfurization reaction:

3 seconds.

  EXPERIMENT 1 Calcium particulate (CaCO 3) feed with a mean particle size of 1.8 microns was carried out as a Ca-type sorbent at a rate of 13.5 kg / h, entrained in 30 Nm 3 / h air. The absorbent is sprayed into the off-gas via a nozzle provided on the furnace wall at a rate of 200 to 300 m / second together with air. As a result, the treated gas has a SOx concentration of 73 ppm x at the outlet of the filter bag used as a dust collector. The treatment reaches a desulfurization efficiency of 90%. The molar ratio

CaCO3 / SOx is 2.0.

  When we continue the experiment for 10 hours, we collect

  347 kg of particles from the filter bag.

  The particles are classified by an air classifier and 220 kg of a fraction of coarse particles, with an average particle size of 20 microns, and 127 kg of a fraction of fine particles, with an average particle size of 2.2 microns. On chemical analysis, it is found that the fraction of fine particles contains 57.0% CaSO 4, 25.8% CaO,

  5.1% CaCO3 and 12.1% SiO2, Al2O3, MgO and other compounds combined.

  Experiment 2 The supply of fresh limestone particles being interrupted, the fraction of fine particles recovered is introduced into a jet mill.

  in Experiment 1, together with 100 kg / h of superheated steam

  at 450 ° C and having a pressure of about 6.9 bar (6 kg / cm 2 g) (cooling in the aforementioned temperature range by expansion by flowing out of the mill projection), which has for effect of reactivating the limestone into particles of the fraction. The reactivated limestone is pulverized in the furnace through the nozzle provided therein

  at a flow rate of 25.8 kg / h and a speed of 200 to 300 m / second, the

  effective treatments (CaO and CaCO 3) are maintained at a CaO / SOx

of 2.0.

  The treatment provides a 75 ppm SOx concentration at the outlet of the filter bag used as a dust collector and reaches a

  desulfurization yield of 89%.

  When we continue the experiment for 4 hours, we collect

  209 kg of particles from the filter bag. Using the classifica-

  The particles are classified and 89 kg of a coarse particle fraction having an average particle size of 20 microns and 120 kg of a fraction of fine particles having a particle size of 2.3 microns are obtained. On chemical analysis, it is found that the fraction of fine particles contains 67.6% CaSO 4, 11.0% CaO, 2.2% CaCO 3 and 19.2% CaCO 3.

  SiO2, Al2O3, MgO and other compounds combined.

  Experiments 3 and 4 are carried out under the following conditions: SO concentration of the reject gas: 730 ppm (at the outlet of the x filter bag)

  Temperature at the inlet of the absorbent: 1050 C.

  Absorber residence time in the effective temperature range (800 to 1050 C) for the desulfurization reaction:

3 seconds.

  Experiment 3 Ca-type absorbent dolomite particles having an average particle size of 1.7 micron and composed of 16.3% MgO, 35.3 / CaO, 45.5% CO 2 and , 1% SiO2 to a discharge gas spray at a rate of 21.0 kg / h and a speed of 150 to 250 m / s through an ejector and the nozzle. Absorbent feeding is performed

at the same time as 45 Nm3 / h of air.

  Treatment provides an SO concentration of 65 ppm x

  filter bag outlet (corresponding to 91% desulfurization). Rap-

CaO / SO molar port is 2.0.

  The experiment is continued for 10 hours under the aforementioned conditions, which makes it possible to collect 385 kg of particles from the filter bag used as a dust collector. The particles are treated by the air classifier and 219 kg of a fraction of coarse particles having an average particle size of 20 microns and 166 kg of a particle are obtained.

  fraction of fine particles having an average particle size of 2 .ANG. microns.

  On the chemical analysis, it is found that the fraction of fine particles contains 7% CaO, 18.5% MgO, 25.7% SO 3, 0.7% CO 2 and 14.4%

SiO 2, Al 2 O 3 and the like combined.

Experience 4

  The supply of fresh dolomite particles being interrupted

  The fines fraction recovered in Experiment 3 was introduced into a jet mill at the same time as 110 kg / h of superheated steam at 400 ° C. and having a pressure of about 6.9 bar (6 psig). kg / cm gauge), which has the effect of reactivating the dolomite particles of the fraction. The reactivated particulate dolomite is sprayed into the furnace through the nozzle at a rate of 30.0 kg / hr and a rate of 150 to 250 m / sec, the effective components being maintained at a molar ratio

CaO / Sx of 1.85.

  x The treatment provides a 90 ppm SOx concentration at the outlet of the filter bag, thus achieving a desulfurization efficiency

88%.

  The experiment is continued for 4 hours in the conditions

  above and 228 kg of particles are collected from the filter bag.

  The particles are treated by the air classifier and 87 kg of a fraction of coarse particles having a mean particle size are obtained.

  20 microns and 140 kg of a fraction of fine particles having a granulo-

  average measurement of 2.0 microns. In chemical analysis, we find that the fraction

  Fine particles contain 32.5% CaO, 14.3% MgO, 31.6%

  SO 3, 0.5% CO 2 and 21.1% SiO 2, Al 2 O 3 and the like, combined.

1 1

Claims (9)

  1. Process for purifying a reject gas containing   harmful acids, by spraying fine particles of an absorbent   Ca-type binder in the exhaust gas and collecting, by means of a dust collector, absorbent particles in which the harmful acid substances are absorbed, together with the other particles contained in the exhaust gas, this characterized in that the collected absorbent particles are hydrated with water vapor in order to cause the expansion of these particles and the breaking of the inactive envelope formed on their surface by the absorption of the susbstances harmful acids, and the resulting activated absorbent particles are recycled in the reject gas.   2. Method according to claim 1, characterized in that the particles collected by the dust collector are divided into a fraction of coarse particles containing mainly fly ash and a fraction of fine particles predominantly comprising particles of absorbent dusts. a maximum value dimension equal to a predetermined value, and only the fraction of particles is hydrated fine with water vapor.   3. Method according to claim 1, characterized in that the water vapor is superheated water vapor having a temperature of   at 300 ° C., preferably from 200 ° to 250 ° C.   4. Method according to claim 1, characterized in that the absorbent particles comprising the inactive envelope are subjected to spraying by means of a jet mill simultaneously with hydration. with water vapor.   5. Method according to claim 1, characterized in that the Ca-type absorbent comprises one or the combination of at least two of the following substances including limestone, slaked lime, lime   live, dolomite, dolomite, calcined dolomite.   6. Installation for the purification of a waste gas produced by a gas producing apparatus (1) and containing harmful acidic substances, by spraying fine particles of a Ca-type absorber in the exhaust gas and collecting, by a dust collector (3), particles of absorbent having absorbed the harmful acidic substances, together with the other particles contained in the exhaust gas, this installation being characterized in that it further comprises a   hydration reactor (9) for hydrating with water vapor the   absorbent particles collected, in order to dilate these particles and to break the inactive envelope formed on their surface by the absorption of the substances   harmful acids, and to recycle the activated absorbent particles resulting from in the exhaust gas.   7. Installation according to claim 6, characterized in that a classifier (7) is provided between the dust collector (3) and the hydration reactor (9) for dividing the particles collected by the dust collector into a fraction of coarse particles containing mainly fly ash and a fine particle fraction predominantly comprising absorbent particles of a size of value at most equal to a predetermined value and for sending   only the fraction of fine particles in the hydration reactor.   8. Installation according to claim 7, characterized in that   the classifier (7) is an air classifier.   9. Installation according to claim 6, characterized in that   the hydration reactor (9) is provided with a projection mill.