CA1266959A - Treatment of solids containing calcium salt of sulfuric oxyacid and method of purifying exhaust gas utilizing same - Google Patents
Treatment of solids containing calcium salt of sulfuric oxyacid and method of purifying exhaust gas utilizing sameInfo
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- CA1266959A CA1266959A CA000489530A CA489530A CA1266959A CA 1266959 A CA1266959 A CA 1266959A CA 000489530 A CA000489530 A CA 000489530A CA 489530 A CA489530 A CA 489530A CA 1266959 A CA1266959 A CA 1266959A
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- calcium
- particles
- gas stream
- reactor
- sulphide
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-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B17/00—Sulfur; Compounds thereof
- C01B17/42—Sulfides or polysulfides of magnesium, calcium, strontium, or barium
- C01B17/44—Sulfides or polysulfides of magnesium, calcium, strontium, or barium by reduction of sulfates
-
- 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
- B01D53/501—Sulfur oxides by treating the gases with a solution or a suspension of an alkali or earth-alkali or ammonium compound
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B17/00—Sulfur; Compounds thereof
- C01B17/16—Hydrogen sulfides
- C01B17/164—Preparation by reduction of oxidic sulfur compounds
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F11/00—Compounds of calcium, strontium, or barium
- C01F11/18—Carbonates
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- Organic Chemistry (AREA)
- Inorganic Chemistry (AREA)
- Engineering & Computer Science (AREA)
- General Chemical & Material Sciences (AREA)
- Analytical Chemistry (AREA)
- Health & Medical Sciences (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Life Sciences & Earth Sciences (AREA)
- Geology (AREA)
- Environmental & Geological Engineering (AREA)
- Biomedical Technology (AREA)
- Treating Waste Gases (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A method of treating solids containing the calcium salt of a sulfuric oxyacid comprising feeding to a reduction reactor a reducing gas stream containing hydrogen and/or carbon monoxide and having a first predetermined temperature and dry particles containing the calcium salt of sulfuric oxyacid and having a predetermined mean particle size to reduce the calcium salt to calcium sulfide, subse-quently cooling the gas stream obtained from the reduction reactor to a second temperature in a carbonation reactor to react the calcium sulfide with carbon dioxide and water vapor and produce calcium carbonate and hydrogen sulfide, guiding the resulting gas stream from the carbonation reactor into a dust collecting unit to collect solid particles, and further collecting the hydrogen sulfide from the gas stream flowing out from the dust collecting unit.
A method of treating solids containing the calcium salt of a sulfuric oxyacid comprising feeding to a reduction reactor a reducing gas stream containing hydrogen and/or carbon monoxide and having a first predetermined temperature and dry particles containing the calcium salt of sulfuric oxyacid and having a predetermined mean particle size to reduce the calcium salt to calcium sulfide, subse-quently cooling the gas stream obtained from the reduction reactor to a second temperature in a carbonation reactor to react the calcium sulfide with carbon dioxide and water vapor and produce calcium carbonate and hydrogen sulfide, guiding the resulting gas stream from the carbonation reactor into a dust collecting unit to collect solid particles, and further collecting the hydrogen sulfide from the gas stream flowing out from the dust collecting unit.
Description
~6~
TITLE OF THE INVENTION
TREATMENT OF SOLIDS CONTAINING CALCIUM
SALT OF SULFURIC OXYACID AND METHOD OF
PURIFYING EXHAUST GAS UTILIZING SAME
FIELD OF THE INVENTION
The present invention relates to a dry method of trea-ting solids containing the calcium salt of a sulfuric oxyacid such as gypsum, and to a process utili~ing -the method for purifying exhaust gases from boilers or furnaces such as incinerators by efficiently removing harmful acid subs-tances from the yas.
BACKGROUND OF THE INVENTION
A dry method is already proposed oi purifying the exhaust gas from a boiler or waste incinerator or like furnace by dispersing particles of a Ca-type absorbent (quick lime, slaked lime, limestone, dolomite or the l:ike) directly into the furnace or flue to cause the absorbent parti.cles to absorb harm-ful ac.id substarlces, such as sulfur oxicles (SOx), Erom the cJaS fo.r removal. With this method, the absorbent. partlcles a:re part.:Ly convertecl to cJypsum (CaSO~I) by the rcact:ion w:ith thc suleur oxides and collected by a clust collecto.r aloncJ with other solid particles as soot and clust. q'he soot and dust obtained, although containi.nc3 cJypsum, further contain fly ash and unreacted .
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.
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absorbent components and therefore are not usable for preparing gypsum board or cement for which gypsum is generally used. Fur-ther if the furnace is large-sized, the soot and dust, which are produced in large quantities, can not be discarded directly, since various problems will then result. Further if the gypsum obtained has a relatively high purity and is useful, problems will be encountered in treating the gypsum in the event of excess supply.
Aeeordingly, methods have been proposed whieh inelude one eomprising ealeining gypsum in a kiln in the presenee of an insufficien-t amount of air to obtain quick lime and a yas eontaining SO2 and reeovering sulfurie aeid from the SO2, and a me-thod eomprising eonverting gypsum to ealeium sulfide in a redueing atmosphere in a kiln with a limited supply of air, preparing an aqueous slurry ~rom the resulting ealeium sulEide aEter pulverization and passlng earbon dioxide through the slurry to eolleet hydrogen sulEide.
~ owever, the eonventional methocls not only require a larcJe-slzed apparatus hut also :invo;Lve uneven heat:ing by ~he kiln to render the reswlting c~uick lime or ea:lcium su:lE:lde less reactive ow:ing to aceelerated crystal-:li.zation ancl ea-lse trouble to the subsecluent treatment.
Moreover, the rnethocls are unsatisfaetory Erom the viewpoint oE savings in energy.
~5 SUMMARY OF THE INVENTION
, ~ .
An object of the present invention is to overcome the foregoing problems heretofore encount~red.
According to a first aspect of the invention, thare is provided a dry method of treating solids containing the calcium salt of a sulphuric oxyacid comprising feeding to a reduction reactor a reducing gas stream containing hydrogen and~or carbon monoxide and dry particles containing the calcium salt of sulphuric oxyacid and ha~ing a mean particle size of up to 10 microns, reducing the calcium salt to calcium sulphide at 700 to 900~C in the reduction reactor, feeding the gas stream entraining the produced calcium sulphide obtained ~rom the reduction reactor to a carbonation reactor, cooling the gas stream entraining the calcium sulphide to 300 to 500C in the carbonation reactor to react in a dry state the calcium sulphide with carbon dioxide and water vapour and produce calcium carbonate and hydrogen sulphide, yuiding the resulting gas stream ~rom the carbonation reactor into a dust collecting unit to collect solid particles, separating the hydrogen sulphide ~rom the gas stream ~lowing out from the dust collecting unit, then ~eeding back to the reduction reactor the remaining gas containing hydrogen, carbon monoxide and carbon dioxide as a carrier yas ~or Eeedi.ng the dry particle~ into the reductlon reactor.
The above method is not limited only to the treatment o~ gyp~um resultincJ ~rom the purif.ication o~ exhaust gases but is use~ul also eor solids containiny the calcium salt of any sul:Euric oxyacid capable of.giving calcium sulfide upon reduction.
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Examples of calcium salts of sulfuric oxyacids are, in addition to gypsum (calcium sul~ate), calcium sulfite (CaSo3), calcium thiosulfate (CaS203) and calcium polythionate ( CaSX6 ) -According to a second aspect of the invention, there is provided a dry method of purifying an exhaust gas containing sulphur oxides by spraying dry particles of a Ca-type absorbent into the exhaust gas to cause the absorbent to absorb the sulphur oxides and thereafter collecting by a dust collecting unit the absorbent particles along with other particles contained in the exhaust gas, the method being characterised by feeding to a reduction reactor the used absorbent particles collected by the dust collecting unit and having a mean particles size of up to 10 microns, and reducing gas stream containing hydrogen and/or carbon monoxide, reducing at 700 to 900C in the reduction reactor calcium sulphate contained in the used absorbent particles to calcium sulphide, feeding the gas stream entraining the produced calcium sulphide and obtained .~rom the reduction reactor to a carbonation reactor, cooling the gas stream entraining the calcium sulphide 300~ to 500~ in the carbonation reactor to react in a dry state the aalcium sulphide with carbon dioxide and water vapour and produce calcium caxbonate and hydrogen sulphide, guiding the re~ulting gas ~tream Prom the carbonation reactor into a second duct collecting unit to collect solid particles, and spraying the solid parti.cles again into the exhaust yas as regenerated absorbent particles, the hydrogen sulphide being separated from the gas stream flowing out from the second dust collecting (~, I
~2~Ç~9~3 unit, the remaining gas containing hydrogen, carbon monoxide and carbon dioxide then being fed back to the reduction reactor as a carrier gas for feeding the used absorbent particles into the reduction reactor.
The Ca-type absorbents to be used in the present method include limestone (CaC03), slaked lime [Ca(OH)2~, quick lime (CaO), dolomite (CaC03.MgC03~, slaked dolomite [Ca(OH)2.Mg(OH)2 or Ca(OH)2.MgO], calcined dolomite (CaO.MgO
or CaC03.MgO) and any material containing CaO or capable of ~orming CaO at a high temperature. Accordingly, the particulate Ca-type absorbent originally contains CaO or produces CaO through the following reactions at a high temperature.
Ca(OH)2 ~ CaO + H20 (1) CaC03 ~ CaO ~ C02 (2) CaO absorbs sulfur oxides (SOx) from the exhaust gas upon reacting therewith as represented by the ~ollowing equations.
CaO ~ 52 ~ ~2 ' CaSO~ (3) CaO -~ S03 -~ CaSO~ (4) The CaSO~ shells formed on the surfaces of absorberlt particles by the reaction~ (3) and (~) have a compact texture and lnhibit harmPul acid substances such as Sx from di~using into the particles, consQquently reducing the reactivity o~ the absorbent. However, the CaSO~ shells are converted to calcium carbonate (CaC03) by the treatment according to the first aspect described, whereby the particulate absorbent is regenerated. The regenerated absorbent, if sprayed into khe exhaust gas again, greatly reduces the amount of fresh absorbent to be supplied.
3~0 Of the Ca-type absorbents mentioned above, 1~
';
9~3 dolomlte, slaked dolomite and calci~ed dolomite have a large void volume, permit harmful acid substances to readily diffuse into the particles thereof and are therefore advan-tageous. Since the extraneous components such as MgO
do not participate directly in the regeneration reaetions, -the change of such components will no-t be described for the sake of simplified descrip-tion.
Exhaust gases, especially those from coal boilers and waste incinerators generally contain fly ash of large partiele sizes in addition to harmful acid substances. In sueh a case, it is desirable to elassify the par-tieles eolleeted by the first dust eollecting uni-t into a eoarse partiele portion primarily eontaining fly ash and a fine partiele portion primarily eontaining absorbent partieles not larger than a predetermined partiele size and thereafter regenerate the Eine partiele portion only. In the ease of Euel oil boi.:Lers, the amount of fly ash is lesser than that of absorbent partiel~s, so khat the elasslEieation step is not always r~c~uirecl.
Va~lous Eeatures and aclvantacJes of the pre.sent inventlorl w~ L be read:ily unclerstood Erom the embodlrnents to be cleser.ibed below with referenee to the aeeompanyincJ
drawlncJs.
~RI~EF DE.SCRIPT~ON OF T~IE DR~W:tNGS
F:L~. 1 is a flow ehart showing a system for ~2~
treating sollds containing the calcium salt of sulfuric oxyacid according to the invention; and Fig. 2 is a flow chart showing a system for purifying exhaust gases according to the inven-tion.
EMBODIMENTS
Indicated at 1 in Fig. 1 is a hopper containing fine particles of used ea-type absorbent (par-tly eontaining gypsum) or solids containing the caleium salt of ~ sulfurie oxyaeid, sueh as high-purity gypsum. The fine particulate material is sprayed from the hopper 1 lnto a reduetion reaetor 3 as entrained in a carrier gas by an ejector 2 which is an example of a dispersing nozæle. Connected to the reduetion reaetor 3 is a redueing gas generator ~, in whieh a hydrocarbon, sueh as asphal-t, is partially burned with oxygen and water vapor to produee a reducing gas having a high temperature and eontaining earbon monoxide and hydrogen. Within the reduetion reaetor 3, therefore, -the caso4 componerlt Oe the p~rtieulate material ls redueed to ealeium sulficle (CaS) aeeording to the following equation.
CaSO4 ~ ~2 ~ CaS ~ ~12 CaSO~ CO ~ CaS ~ ~CO2 (6) Providecl clownstream Oe the reaetor 3 is a earbonat:ion reaetor 5 equipped wlth a eooler 6 (whieh may be a heat exchanger or a deviee adapted to evaporate wa-ter on spraying). The gas s-trearrl from the reduetion reaetor 3 9~
is cooled in the carbonation reactor 5, whereby the CaS
is carbonated and further gives hydrogen sulfide according to -the following equation.
CaS + H2O + CO2 ~ CaCO3 + H2S (7) The gas stream flowing out from the reaetor 5 and containing fine earbonated particles and H2S is then subjeeted to solid-gas separation by a dust eollecting uni-t 7, and a fine solid partieulate portion eonsisting primarily of fine carbonated particles is collected by a ealeium carbonate hopper 8.
The gas stream from -the dust eolleeting unit 7 is sent to a hydrogen sulfide eolleetor 9, by whieh the H2S is eolleetecl. The eolleeted H2S may be eonver-ted to elemental sulfur by the Claus proeess or -to sulfurie aeid by a we-t lS proeess, or rnay be used as a material for ehemieal synthesis.
I'he yas stream separatecl:Erom the ~12S eontains hydrogerl, earbon monoxide and earbon dioxide as unreacted eomponents, so that l.t is a~vantacJeous to :reeyele the gas stream throucJh a feedbaek l.ine 10 equipped with a eornpressor l:L as a ea.rr:ier gas for Eeed:incJ :eine partieles frorn the hopper 1 to the recluet:Lon reaetor 3. The earr:ier cJas~ when eontaining :lmpur:it.Les (malnLy n:itrogen) at an exeess.ively h:igh eoneent.ratl.on, recluires use of reaetors 3, 5 of inereased eapaeity ancl adversely affee~ the Eorrnation of ealeiurn earbonate. Aeeo:r~lingly the earrier gas is partially purged ~8--sg suitably via a purge line 12.
The smaller the fine particles passing through the reactors 3, 5, -the better is the result, because the reduction o~ the particle size assures smooth diffusion of heat and the reactants into -the particles, consequently shortening the reaction time required and permitting use of reactors 3, 5 of reduced size. Further because of an improved heat transfer efficiency then achieved, the reduction reactor 3 assures uniform heating to inhibit crystallization of CaS, while smooth carbonation takes place in -the reac-tor 5, yiving highly reactive calcium carbonate. In view of the power consumption needed for pulveriza-tion, the fine particulate material to be used for practicing the present invention is up to 10 microns, preferably 1 to 3 microns, in rnean particle size.
The ejector 2 serving as a disperslng nozzle and cooperating with the carr.ier gas pressure-fed through the .~eedback line 10 acts to un:i:Eormly fl:i.sperse the pre-~ulverlzed particu:Late material from the hopper 1 into the .reducing gas with:Ln the reaetor 3, whereby thc~ CaSO~ componen~ can be reclueecl w:lth a high reactiv:Lty. In p:Lace Oe the Eine pa:rk:ieulate ma-terial wh1ch has been f.inely cl:Lv:ided, soli~s contain:ing the ealcium salt Oe sul:Euric oxyacid may he pul.ver:Lzed on site by a jet mill or the like before being fecl to the dispersing nozzle 2.
~6~i9 ~ sually, the CaSO4 component contains crystal water and loses the water at a high temperature of lO0 to 200 C. If this dehyd.ration reaction occurs at the por-tion of the dispersing nozzle 2 which is exposed to a high temperature, it is likely that the nozzle 2 will become plugqed up or particles will agglomerate (to result in a reduced reactivity), leading to poor economy. Accordingly, it is desirable to calcine the particulate material to render the CaSO4 anhydrous before the ma-terial reaches the nozzle 2.
For the regeneration of the CaSO~ component to CaCO3, i-t is advantageous tha-t the reduclng gas concentration wi-thin the reactor 3, and the water vapor concentration and the carbon dioxide concentration within the reactor 5 be maintained at a high level. For this purpose, concentrated oxygen, especially concentrated oxygen having a concentration of at least 95~, :ls more preferable than air which contains large amounts of nitrogen and other impurities, as the oxygen source to be used ~or part:ial:ly bu.rnlng a hyd.rocarbon within the reducirlc3 gas generator ~.
The reduction reactiorlC of Equat:ions (5) and (6) take p~.ace at a tcmpe:raturc Oe not lower than 700 C. While these react:ions progress:iv~ly p:roceed :E:rom the surface Oe the part:Lcles inwarcl, the reaction velocity is dependent on the rate of diEfusion Oe H2 or CO from the surface of the 2S particle into the interior, so that the reac-tion is ~2~
accelera-ted with decreasing particle size. Because the rate of diffusion is the predominant factor, the rise of the reaction temperature to more than 900C is not very effective for the reduction of CaSO~ to CaS. Further temperatures over llaO C.promote crystallization of the CaS
produced, inhibiting the diffusion of H2 or CO into the particles and adversely affecting the subsequent carbonation, hece objectionable. Accordignly, i-t is advantageous that the ternperature within the reduction reactor 3 be adjusted to 700 to 900 C.
The carbonation reaction of Equation (7) i.s an equilibrium reaction, and it is required to maintain the tempera-ture at a level of up to 500 C for -the completion of the reaction, while it is known -that this carbonation reaction proceeds at a low velocity. It is therefore necessary to maintain the reacti.on temperature at the highest possib].e level within the range of up to 500 C for the promot:lon oE the reaction. ~lthough it was not elear whethe.r the carbonat:ion reaet:ion can be carr:iecl out wlthin a pract:LcalL,y usefu:L periocl Oe reclct:ion (partlc:Le retention) time, we have Eouncl that the clesi.recl.reactiv.i-ty can be achieved w:Lth a react.Lon time Oe about 3 to 20 seconds by us:i.n~ particJ.es oE reducecl mean particle size (especially 1 to 3 mlcrons) at a reaction temperature Oe 300 -to 500 C.
Ne~t, experiments will be described which were conducted to substantiate the advantages of the foregoing method of treatment.
EXPERIMENT I
Hollow cylindrical refractory bricks, 20 cm in inside diameter, were stt~ked up to a height of 12 m to build an experimental reaction column corresponding to the reactors 3 and 5 of Fig. 1 combined toge-ther. A combustion chamber (corresponding to the reducing gas generator ~) made of refractory bricks was provided on the top of the column. A
dispersin nozzle (corresponding to the ejector 2) was installed at the column top under the chamber. A water spray nozzle (corresponding to -the cooler 6) was attached to the column at a positlon 1.5 m below -the nozzle. Tempera-ture measuring thermocouples were inserted into the reaction column at a position 0.5 m below the nozzle and at a position 6 m below the water spray nozzle. The portion of -the reacti.on co~lumn above the water spray nozzle was externally coverecl with an electrlc heater, which was protected with heat insualt:Lnc3 brLcks and iron pipe from outside. The resuLt:Lng as.sernbly wa9 Eurther eoverecl with a heat lnsula-tor.
The other portion Oe the column WC15 protected only w:ith an iron p:i.pe. The water spray nozz:Le usecl was one called a supersonic nozz:Le wh:ich is capable of producing atomized water (fine water droplets). The outlet of the reaction column was provicled with a Jetclone Collec-tor (high-performance cyclone, product of Nippon Pneumatic MFG. Co., Ltd.) for trapping dust.
Town gas was burned in the combustion chamber to heat the reaction column to a specified temperature. An experiment was then conducted for 2 hours under -the following conditions. Firs-t, the town gas was replaced by carbon monoxide and hdyrogen. Carbon monoxide was fed to the chamber at a flow rate of 14 Nm3/h (N representing stanclard state), hydrogen at 6 Nm3/h, and air at 34.8 Nm /h for partial cornbustion (to maintain a prede-termined reaction temperature). On the o-ther hand, anhydrous gypsum (purity 99~) finely divided to a mean par-ticle size of 3 microns and serving as a solid containiny -the calcium salt of a sulfuric oxyacid was fed at a ra-te of 13.6 kg/h to -the dispersing nozzle at the column top, as entrained in a carrier gas composed of 50~ of carbon monoxide and 50~ of hydrogen and suppl:Led at a rate o~ 24.2 Nm3/h, whereby the finely d~vi.ded qypsum was di~persed into the hot combustion gas from t.he cornbust:ion chatrlber~ the carr:i.er qas belng accelerated to a Elow rate Oe 200 m/sec by khe nozzle. Atomized water was suppll~d at a rate of ~1.2 kq/h from the water spray nozzle. At th:is tlme, the reductlon reactor portion above the water nozzLe hacl a temperature of 820 C, whlle the carbonat:Lon reactor por-tion above the nozzle had a temperature o~ 3~0 C.
.., 6~9~
Dust and particles were collected through the cyclone at a rate of 9.5 ~g/h. When analyzed, the dust and particles were found to contain 92.4~ of CaCO3, 3.5~ of CaS,
TITLE OF THE INVENTION
TREATMENT OF SOLIDS CONTAINING CALCIUM
SALT OF SULFURIC OXYACID AND METHOD OF
PURIFYING EXHAUST GAS UTILIZING SAME
FIELD OF THE INVENTION
The present invention relates to a dry method of trea-ting solids containing the calcium salt of a sulfuric oxyacid such as gypsum, and to a process utili~ing -the method for purifying exhaust gases from boilers or furnaces such as incinerators by efficiently removing harmful acid subs-tances from the yas.
BACKGROUND OF THE INVENTION
A dry method is already proposed oi purifying the exhaust gas from a boiler or waste incinerator or like furnace by dispersing particles of a Ca-type absorbent (quick lime, slaked lime, limestone, dolomite or the l:ike) directly into the furnace or flue to cause the absorbent parti.cles to absorb harm-ful ac.id substarlces, such as sulfur oxicles (SOx), Erom the cJaS fo.r removal. With this method, the absorbent. partlcles a:re part.:Ly convertecl to cJypsum (CaSO~I) by the rcact:ion w:ith thc suleur oxides and collected by a clust collecto.r aloncJ with other solid particles as soot and clust. q'he soot and dust obtained, although containi.nc3 cJypsum, further contain fly ash and unreacted .
~l _ "
~ .
.
~6~
absorbent components and therefore are not usable for preparing gypsum board or cement for which gypsum is generally used. Fur-ther if the furnace is large-sized, the soot and dust, which are produced in large quantities, can not be discarded directly, since various problems will then result. Further if the gypsum obtained has a relatively high purity and is useful, problems will be encountered in treating the gypsum in the event of excess supply.
Aeeordingly, methods have been proposed whieh inelude one eomprising ealeining gypsum in a kiln in the presenee of an insufficien-t amount of air to obtain quick lime and a yas eontaining SO2 and reeovering sulfurie aeid from the SO2, and a me-thod eomprising eonverting gypsum to ealeium sulfide in a redueing atmosphere in a kiln with a limited supply of air, preparing an aqueous slurry ~rom the resulting ealeium sulEide aEter pulverization and passlng earbon dioxide through the slurry to eolleet hydrogen sulEide.
~ owever, the eonventional methocls not only require a larcJe-slzed apparatus hut also :invo;Lve uneven heat:ing by ~he kiln to render the reswlting c~uick lime or ea:lcium su:lE:lde less reactive ow:ing to aceelerated crystal-:li.zation ancl ea-lse trouble to the subsecluent treatment.
Moreover, the rnethocls are unsatisfaetory Erom the viewpoint oE savings in energy.
~5 SUMMARY OF THE INVENTION
, ~ .
An object of the present invention is to overcome the foregoing problems heretofore encount~red.
According to a first aspect of the invention, thare is provided a dry method of treating solids containing the calcium salt of a sulphuric oxyacid comprising feeding to a reduction reactor a reducing gas stream containing hydrogen and~or carbon monoxide and dry particles containing the calcium salt of sulphuric oxyacid and ha~ing a mean particle size of up to 10 microns, reducing the calcium salt to calcium sulphide at 700 to 900~C in the reduction reactor, feeding the gas stream entraining the produced calcium sulphide obtained ~rom the reduction reactor to a carbonation reactor, cooling the gas stream entraining the calcium sulphide to 300 to 500C in the carbonation reactor to react in a dry state the calcium sulphide with carbon dioxide and water vapour and produce calcium carbonate and hydrogen sulphide, yuiding the resulting gas stream ~rom the carbonation reactor into a dust collecting unit to collect solid particles, separating the hydrogen sulphide ~rom the gas stream ~lowing out from the dust collecting unit, then ~eeding back to the reduction reactor the remaining gas containing hydrogen, carbon monoxide and carbon dioxide as a carrier yas ~or Eeedi.ng the dry particle~ into the reductlon reactor.
The above method is not limited only to the treatment o~ gyp~um resultincJ ~rom the purif.ication o~ exhaust gases but is use~ul also eor solids containiny the calcium salt of any sul:Euric oxyacid capable of.giving calcium sulfide upon reduction.
~1 t~
.
. .
Examples of calcium salts of sulfuric oxyacids are, in addition to gypsum (calcium sul~ate), calcium sulfite (CaSo3), calcium thiosulfate (CaS203) and calcium polythionate ( CaSX6 ) -According to a second aspect of the invention, there is provided a dry method of purifying an exhaust gas containing sulphur oxides by spraying dry particles of a Ca-type absorbent into the exhaust gas to cause the absorbent to absorb the sulphur oxides and thereafter collecting by a dust collecting unit the absorbent particles along with other particles contained in the exhaust gas, the method being characterised by feeding to a reduction reactor the used absorbent particles collected by the dust collecting unit and having a mean particles size of up to 10 microns, and reducing gas stream containing hydrogen and/or carbon monoxide, reducing at 700 to 900C in the reduction reactor calcium sulphate contained in the used absorbent particles to calcium sulphide, feeding the gas stream entraining the produced calcium sulphide and obtained .~rom the reduction reactor to a carbonation reactor, cooling the gas stream entraining the calcium sulphide 300~ to 500~ in the carbonation reactor to react in a dry state the aalcium sulphide with carbon dioxide and water vapour and produce calcium caxbonate and hydrogen sulphide, guiding the re~ulting gas ~tream Prom the carbonation reactor into a second duct collecting unit to collect solid particles, and spraying the solid parti.cles again into the exhaust yas as regenerated absorbent particles, the hydrogen sulphide being separated from the gas stream flowing out from the second dust collecting (~, I
~2~Ç~9~3 unit, the remaining gas containing hydrogen, carbon monoxide and carbon dioxide then being fed back to the reduction reactor as a carrier gas for feeding the used absorbent particles into the reduction reactor.
The Ca-type absorbents to be used in the present method include limestone (CaC03), slaked lime [Ca(OH)2~, quick lime (CaO), dolomite (CaC03.MgC03~, slaked dolomite [Ca(OH)2.Mg(OH)2 or Ca(OH)2.MgO], calcined dolomite (CaO.MgO
or CaC03.MgO) and any material containing CaO or capable of ~orming CaO at a high temperature. Accordingly, the particulate Ca-type absorbent originally contains CaO or produces CaO through the following reactions at a high temperature.
Ca(OH)2 ~ CaO + H20 (1) CaC03 ~ CaO ~ C02 (2) CaO absorbs sulfur oxides (SOx) from the exhaust gas upon reacting therewith as represented by the ~ollowing equations.
CaO ~ 52 ~ ~2 ' CaSO~ (3) CaO -~ S03 -~ CaSO~ (4) The CaSO~ shells formed on the surfaces of absorberlt particles by the reaction~ (3) and (~) have a compact texture and lnhibit harmPul acid substances such as Sx from di~using into the particles, consQquently reducing the reactivity o~ the absorbent. However, the CaSO~ shells are converted to calcium carbonate (CaC03) by the treatment according to the first aspect described, whereby the particulate absorbent is regenerated. The regenerated absorbent, if sprayed into khe exhaust gas again, greatly reduces the amount of fresh absorbent to be supplied.
3~0 Of the Ca-type absorbents mentioned above, 1~
';
9~3 dolomlte, slaked dolomite and calci~ed dolomite have a large void volume, permit harmful acid substances to readily diffuse into the particles thereof and are therefore advan-tageous. Since the extraneous components such as MgO
do not participate directly in the regeneration reaetions, -the change of such components will no-t be described for the sake of simplified descrip-tion.
Exhaust gases, especially those from coal boilers and waste incinerators generally contain fly ash of large partiele sizes in addition to harmful acid substances. In sueh a case, it is desirable to elassify the par-tieles eolleeted by the first dust eollecting uni-t into a eoarse partiele portion primarily eontaining fly ash and a fine partiele portion primarily eontaining absorbent partieles not larger than a predetermined partiele size and thereafter regenerate the Eine partiele portion only. In the ease of Euel oil boi.:Lers, the amount of fly ash is lesser than that of absorbent partiel~s, so khat the elasslEieation step is not always r~c~uirecl.
Va~lous Eeatures and aclvantacJes of the pre.sent inventlorl w~ L be read:ily unclerstood Erom the embodlrnents to be cleser.ibed below with referenee to the aeeompanyincJ
drawlncJs.
~RI~EF DE.SCRIPT~ON OF T~IE DR~W:tNGS
F:L~. 1 is a flow ehart showing a system for ~2~
treating sollds containing the calcium salt of sulfuric oxyacid according to the invention; and Fig. 2 is a flow chart showing a system for purifying exhaust gases according to the inven-tion.
EMBODIMENTS
Indicated at 1 in Fig. 1 is a hopper containing fine particles of used ea-type absorbent (par-tly eontaining gypsum) or solids containing the caleium salt of ~ sulfurie oxyaeid, sueh as high-purity gypsum. The fine particulate material is sprayed from the hopper 1 lnto a reduetion reaetor 3 as entrained in a carrier gas by an ejector 2 which is an example of a dispersing nozæle. Connected to the reduetion reaetor 3 is a redueing gas generator ~, in whieh a hydrocarbon, sueh as asphal-t, is partially burned with oxygen and water vapor to produee a reducing gas having a high temperature and eontaining earbon monoxide and hydrogen. Within the reduetion reaetor 3, therefore, -the caso4 componerlt Oe the p~rtieulate material ls redueed to ealeium sulficle (CaS) aeeording to the following equation.
CaSO4 ~ ~2 ~ CaS ~ ~12 CaSO~ CO ~ CaS ~ ~CO2 (6) Providecl clownstream Oe the reaetor 3 is a earbonat:ion reaetor 5 equipped wlth a eooler 6 (whieh may be a heat exchanger or a deviee adapted to evaporate wa-ter on spraying). The gas s-trearrl from the reduetion reaetor 3 9~
is cooled in the carbonation reactor 5, whereby the CaS
is carbonated and further gives hydrogen sulfide according to -the following equation.
CaS + H2O + CO2 ~ CaCO3 + H2S (7) The gas stream flowing out from the reaetor 5 and containing fine earbonated particles and H2S is then subjeeted to solid-gas separation by a dust eollecting uni-t 7, and a fine solid partieulate portion eonsisting primarily of fine carbonated particles is collected by a ealeium carbonate hopper 8.
The gas stream from -the dust eolleeting unit 7 is sent to a hydrogen sulfide eolleetor 9, by whieh the H2S is eolleetecl. The eolleeted H2S may be eonver-ted to elemental sulfur by the Claus proeess or -to sulfurie aeid by a we-t lS proeess, or rnay be used as a material for ehemieal synthesis.
I'he yas stream separatecl:Erom the ~12S eontains hydrogerl, earbon monoxide and earbon dioxide as unreacted eomponents, so that l.t is a~vantacJeous to :reeyele the gas stream throucJh a feedbaek l.ine 10 equipped with a eornpressor l:L as a ea.rr:ier gas for Eeed:incJ :eine partieles frorn the hopper 1 to the recluet:Lon reaetor 3. The earr:ier cJas~ when eontaining :lmpur:it.Les (malnLy n:itrogen) at an exeess.ively h:igh eoneent.ratl.on, recluires use of reaetors 3, 5 of inereased eapaeity ancl adversely affee~ the Eorrnation of ealeiurn earbonate. Aeeo:r~lingly the earrier gas is partially purged ~8--sg suitably via a purge line 12.
The smaller the fine particles passing through the reactors 3, 5, -the better is the result, because the reduction o~ the particle size assures smooth diffusion of heat and the reactants into -the particles, consequently shortening the reaction time required and permitting use of reactors 3, 5 of reduced size. Further because of an improved heat transfer efficiency then achieved, the reduction reactor 3 assures uniform heating to inhibit crystallization of CaS, while smooth carbonation takes place in -the reac-tor 5, yiving highly reactive calcium carbonate. In view of the power consumption needed for pulveriza-tion, the fine particulate material to be used for practicing the present invention is up to 10 microns, preferably 1 to 3 microns, in rnean particle size.
The ejector 2 serving as a disperslng nozzle and cooperating with the carr.ier gas pressure-fed through the .~eedback line 10 acts to un:i:Eormly fl:i.sperse the pre-~ulverlzed particu:Late material from the hopper 1 into the .reducing gas with:Ln the reaetor 3, whereby thc~ CaSO~ componen~ can be reclueecl w:lth a high reactiv:Lty. In p:Lace Oe the Eine pa:rk:ieulate ma-terial wh1ch has been f.inely cl:Lv:ided, soli~s contain:ing the ealcium salt Oe sul:Euric oxyacid may he pul.ver:Lzed on site by a jet mill or the like before being fecl to the dispersing nozzle 2.
~6~i9 ~ sually, the CaSO4 component contains crystal water and loses the water at a high temperature of lO0 to 200 C. If this dehyd.ration reaction occurs at the por-tion of the dispersing nozzle 2 which is exposed to a high temperature, it is likely that the nozzle 2 will become plugqed up or particles will agglomerate (to result in a reduced reactivity), leading to poor economy. Accordingly, it is desirable to calcine the particulate material to render the CaSO4 anhydrous before the ma-terial reaches the nozzle 2.
For the regeneration of the CaSO~ component to CaCO3, i-t is advantageous tha-t the reduclng gas concentration wi-thin the reactor 3, and the water vapor concentration and the carbon dioxide concentration within the reactor 5 be maintained at a high level. For this purpose, concentrated oxygen, especially concentrated oxygen having a concentration of at least 95~, :ls more preferable than air which contains large amounts of nitrogen and other impurities, as the oxygen source to be used ~or part:ial:ly bu.rnlng a hyd.rocarbon within the reducirlc3 gas generator ~.
The reduction reactiorlC of Equat:ions (5) and (6) take p~.ace at a tcmpe:raturc Oe not lower than 700 C. While these react:ions progress:iv~ly p:roceed :E:rom the surface Oe the part:Lcles inwarcl, the reaction velocity is dependent on the rate of diEfusion Oe H2 or CO from the surface of the 2S particle into the interior, so that the reac-tion is ~2~
accelera-ted with decreasing particle size. Because the rate of diffusion is the predominant factor, the rise of the reaction temperature to more than 900C is not very effective for the reduction of CaSO~ to CaS. Further temperatures over llaO C.promote crystallization of the CaS
produced, inhibiting the diffusion of H2 or CO into the particles and adversely affecting the subsequent carbonation, hece objectionable. Accordignly, i-t is advantageous that the ternperature within the reduction reactor 3 be adjusted to 700 to 900 C.
The carbonation reaction of Equation (7) i.s an equilibrium reaction, and it is required to maintain the tempera-ture at a level of up to 500 C for -the completion of the reaction, while it is known -that this carbonation reaction proceeds at a low velocity. It is therefore necessary to maintain the reacti.on temperature at the highest possib].e level within the range of up to 500 C for the promot:lon oE the reaction. ~lthough it was not elear whethe.r the carbonat:ion reaet:ion can be carr:iecl out wlthin a pract:LcalL,y usefu:L periocl Oe reclct:ion (partlc:Le retention) time, we have Eouncl that the clesi.recl.reactiv.i-ty can be achieved w:Lth a react.Lon time Oe about 3 to 20 seconds by us:i.n~ particJ.es oE reducecl mean particle size (especially 1 to 3 mlcrons) at a reaction temperature Oe 300 -to 500 C.
Ne~t, experiments will be described which were conducted to substantiate the advantages of the foregoing method of treatment.
EXPERIMENT I
Hollow cylindrical refractory bricks, 20 cm in inside diameter, were stt~ked up to a height of 12 m to build an experimental reaction column corresponding to the reactors 3 and 5 of Fig. 1 combined toge-ther. A combustion chamber (corresponding to the reducing gas generator ~) made of refractory bricks was provided on the top of the column. A
dispersin nozzle (corresponding to the ejector 2) was installed at the column top under the chamber. A water spray nozzle (corresponding to -the cooler 6) was attached to the column at a positlon 1.5 m below -the nozzle. Tempera-ture measuring thermocouples were inserted into the reaction column at a position 0.5 m below the nozzle and at a position 6 m below the water spray nozzle. The portion of -the reacti.on co~lumn above the water spray nozzle was externally coverecl with an electrlc heater, which was protected with heat insualt:Lnc3 brLcks and iron pipe from outside. The resuLt:Lng as.sernbly wa9 Eurther eoverecl with a heat lnsula-tor.
The other portion Oe the column WC15 protected only w:ith an iron p:i.pe. The water spray nozz:Le usecl was one called a supersonic nozz:Le wh:ich is capable of producing atomized water (fine water droplets). The outlet of the reaction column was provicled with a Jetclone Collec-tor (high-performance cyclone, product of Nippon Pneumatic MFG. Co., Ltd.) for trapping dust.
Town gas was burned in the combustion chamber to heat the reaction column to a specified temperature. An experiment was then conducted for 2 hours under -the following conditions. Firs-t, the town gas was replaced by carbon monoxide and hdyrogen. Carbon monoxide was fed to the chamber at a flow rate of 14 Nm3/h (N representing stanclard state), hydrogen at 6 Nm3/h, and air at 34.8 Nm /h for partial cornbustion (to maintain a prede-termined reaction temperature). On the o-ther hand, anhydrous gypsum (purity 99~) finely divided to a mean par-ticle size of 3 microns and serving as a solid containiny -the calcium salt of a sulfuric oxyacid was fed at a ra-te of 13.6 kg/h to -the dispersing nozzle at the column top, as entrained in a carrier gas composed of 50~ of carbon monoxide and 50~ of hydrogen and suppl:Led at a rate o~ 24.2 Nm3/h, whereby the finely d~vi.ded qypsum was di~persed into the hot combustion gas from t.he cornbust:ion chatrlber~ the carr:i.er qas belng accelerated to a Elow rate Oe 200 m/sec by khe nozzle. Atomized water was suppll~d at a rate of ~1.2 kq/h from the water spray nozzle. At th:is tlme, the reductlon reactor portion above the water nozzLe hacl a temperature of 820 C, whlle the carbonat:Lon reactor por-tion above the nozzle had a temperature o~ 3~0 C.
.., 6~9~
Dust and particles were collected through the cyclone at a rate of 9.5 ~g/h. When analyzed, the dust and particles were found to contain 92.4~ of CaCO3, 3.5~ of CaS,
2.7~ of CaSO4 and 1.4% of others. On the other hand, a gas stream having a temperature of 340 C and containing 2.4%
of H2S was collected from an upper portion of the cyclone.
EXPERIMENT Il The procedure of EXPERIMENT I was repeated wi-th the exception of the following.
*Solid containing calcium salt of sulfuric oxyacid:
Used Ca-type absorbent (anhydrous gypsum conten-t 43.7~) *Mean particle size: 2 microns *Rate of supply of particles: 20 kg/h *Flow rate of carrier gas: 25 Nm /h *Speed of carrier gas from dispersing nozzle: 300 m/sec *Rate of supply Oe atomlzecl water: 20.0 kg/h *Temperature of reduction reaction: 810 C
*Temperature of carbonation reaction: 335 C
Consecluently, dust ancl partlc:Les were collected Erom the cyLone at a rate oE 21..l kg/h. When analyzec:l, the clust and partLc.les were Eound to contaln 67.7'k Oe CaCO3, 0.7~ of CaS, 0.4~ Oe CaSO4 and 31.2~ Oe others. On the other hancl, t~ gas ~tream containln~ 1.5~ of H2S ancl having a l:emperature Oe 335 C was collected erom the ~pper portion oE the cyclone.
~2~ 3 Eig. 2 shows an exhaust gas purifying system including the treatment circuit of Fig. 1. The system is an exma~le as used for a the~moelectr1c power plant of 1000 MW class specifically designed for burning coal. With reference to the drawing, indicated at 13 is a coal burning boiler, in which coal is burned with air sup?lied by a blower 14 through an air heater 15 and which produces a combustion exhaust gas con-taining Ely ash and harmful acid substances. Fine particles of a Ca-type absorbent, for exampl.e, finely divided limestone (CaCO3) having a mean particle size of 1 -to 3 microns is sprayed in-to the top of -the boiler 13 via a feed line 16. Consequently, the particulate absorbent is retained at a -temperature of 900 to 1100 C for about 2 to abou-t 3 seconds, whereby CaCO3 is thermally decoraposed instantaneously -to give highly reac-ti.ve CaO according to Equation (2). The CaO .Eurther undergoes the reactlons of Equations (3) and (~), absorbing sulfur oxides and :forrning calcium sul:eate (CaSO~L) on the surface oE the absorb~nt part;icle~. The reactivity oE the CaO is about ~0 to about 50~, ancl unr0acted CaO remains as it is in th~ :interlo:r oE th~ pa:rticles. The heat evolved .erorn the r~actiorls is uki:Llzed by the bo:Lle.r 13.
The exhaust gas Erom the boiler 13 i5 passed through a denitrating unit L7, which removes nitrogen oxi.des Erom the gas. Via the air heater 15, the gas then.enters a ~6~9~
a dust collecting unit 18, in which the used particulate absorbent and soot and dust containing rly ash are removed from the gas stream. The exhaust gas thus purified is -then released to the atmosphere through a chimney 19.
According to the invention, desulfurization is conducted at a high temperature of 900 to 1100 C, so that the exhaust gas has a reduced acid dew-point temperature. Consequently, the exhaust gas as released from the chimney 19 to the atmosphere has a temperature of about 100 C as reduced from the conventional ternpera-ture level of 140 to 150 C. This means that an increased amount of heat can be utilized by the boiler 13. Before reaching the dust collectlng unit 18, the particulate absorbent absorbs hydrogen ~luoride (HF) and hydogen chloride (EICl) from the exhaust gas, in addition to the sulfur oxlcles, but since the CaF2 and CaC12 consequen-tly formed do no-t partlcipate in th~ subsequent treatment Oe the absorbent, these halides will not be cleserlbed.
rrhe soot, dust and particles colleeted by the unit 18 are led through a hopper 20 to an air elassi~ier 21, by which they are separated :into a coarse particle portion primaril~ contai.nincJ ely ash of 12 to 15 microns i.n rnean particle size, ancI a ~.ine particle portion c~Iiefly contain:Lng the usecl absorbent. The coarse particle portion separated o~f is s-tored in a container 22. The Eine particle portion is led through a hopper 1 into a reyenerating 9~
treatment circuit.
The regeneration circuit shown in Fig. 2, like the one shown in Fig. 1, includes an ejector 2, reduction reactor 3, reducing gas generator 4, carbonation reac~or 5 having a cooler in the ~orm o~ a heat exchanger, dust collecting unit 7, calcium carbonate hopper 8, hydrogen sulfide collector 9, feedback line 10 having a compressor ll, and purge line 12. The dust collecting unit 7 comprises cyclones 23 and a bag filter 2~ in cornbination. The hydrogen sulfide collector 9 comprises a cooler 25 having a cooling water circulation pump 26, a hydrogen sulfide absorption column 27 and a stripper 28. The stripper 28 is connected to a hydrogen sulfide container 29.
The CaSO4 component contained in the used absorben-t ~ed to the reduction reactor 3 through the e]ector 2 is reduced to CaS by the reactions of Equations (5) and (6) in the hot reducing gas Erom the generator ~. The CaS further - reacts with carbon dioxide and water vapor in the carbonation reactor 5 accordiny to Equakion (7), forming CaCO3.
Consequentl~, the absorbent is regenerated, and H2S is produced. ~uring -the carbonation reaction, the unreacted component ~CaO) in~ide the ~bsorbent particles also undergoes the following reaction and is carbonated.
CaO ~ CO2 ~ CaCO3 (~) The reduction reac-tion and the carbonation reaction ~' ~
~2~9~
described above are each exothermic, and the heat of reaction is recovered by the heat exchanger 6.
The gas stream flowing out from the carbonation reac-tor S and containing the regenerated absorbent, ~2S and other unreacted gas components is subjected to solid-gas separation by the dust collecting unit 7. The particulate absorber separa-ted off is fed back to the boiler 13 via the calcium carbonate hopper 8, rotary valve 30, ejector 31 and feed line 16. Since some of the absorbent particles are los-t when classified by the air classifier 21, the feed line is replenished with a fresh absorber (which need not always contain CaCO3 but may contain CaO or Ca(OH)2) from an absorbent feeder 32 via a hopper 33, rotary valve 3~ and ejector 35 to compensate for the loss. Indicated at 36 is a feeding compressor connected to the feed line 16.
The gas stream flowing out from the dus-t collecting unit 7 is cooled to about ~0 C by the cooler 25 and then subjected to an amine washing step by the absorption column 27 and the stripper 28, whereby H2S only is separated off for collect,ion. The collected H2S is stored in the container 29 and is t.reated, Eor example, by the Claus process at a ~uitable t.Lme. ~ndicated at 37 and 38 are elemental sulfur and water vapor resulting Erom the Claus process.
The gas stream flowing out from the hydrogen suli~e collector 9 is returned to the redu~tion reactor 3 by the compressor 11 via the feedback line 10. The gas stream is partly released to the outside via the purge line 12.
If it is attempted to reduce the amount of absorbent particles to be lost at the air classifier 21, the particulate S material to be sent to the absorbent regenera~ion circuit downstream thereof will contain an increased amount of impurities (fly ash, etc.), giving an increased load to -the regeneration circuit. Accordingly -the ratio of separation to be conclucted by the classifier 21 needs to be set in view of the balance between the amount of loss of absorbent and the load on the regeneration circuit.
The exhaust gas purifying system shown in Fig. 2 has the following advantages.
(a) Since used Ca-type absorbent is recycled for reuse upon regeneration, the exhaust gas can be puriEied eeficiently w,ith use of a smaller amount oE absorbent without entalling the troublesome problem oE by-proclucts.
(b) RecyclincJ oE th~ absorbent wh:ich has been pulver:izecl once serves to reduc~ the powcr eor pulver:iY.ation.
(c) The process erom the desuleur:i~ation with the absorbent through the re~erl~ration Oe the abso~bent (the ~eri0s of steps up to the dust collec-ting unit 7) :is practlced cont:inuously in a clry state and is there~ore low in hea-t loss. ~urther the heat developed from the desulfurization and regeneration i.s effectively used by 9~
the boiler 13 and heat exchanger 6 to achieve an improved energy efficiency. The present method is free of the problem of effluent treatment which is encountered with the wet or semi-wet process and which could cause secondary pollution.
(d) The boiler 13 and dust collecting unit 18, which serve also as desulfurizing units, render the system compact.
(e) Because the desulfurization with the absorbent is conducted at a high temperature of 900 to 1100 C, SO3 which is more harmful than SO2 and diffucult to remove at lower temperatures (SO3 suspends in the exhause gas in the form of stable minute crystals at low temperatures) can be removed effectively.
r~
.. ,~ .
of H2S was collected from an upper portion of the cyclone.
EXPERIMENT Il The procedure of EXPERIMENT I was repeated wi-th the exception of the following.
*Solid containing calcium salt of sulfuric oxyacid:
Used Ca-type absorbent (anhydrous gypsum conten-t 43.7~) *Mean particle size: 2 microns *Rate of supply of particles: 20 kg/h *Flow rate of carrier gas: 25 Nm /h *Speed of carrier gas from dispersing nozzle: 300 m/sec *Rate of supply Oe atomlzecl water: 20.0 kg/h *Temperature of reduction reaction: 810 C
*Temperature of carbonation reaction: 335 C
Consecluently, dust ancl partlc:Les were collected Erom the cyLone at a rate oE 21..l kg/h. When analyzec:l, the clust and partLc.les were Eound to contaln 67.7'k Oe CaCO3, 0.7~ of CaS, 0.4~ Oe CaSO4 and 31.2~ Oe others. On the other hancl, t~ gas ~tream containln~ 1.5~ of H2S ancl having a l:emperature Oe 335 C was collected erom the ~pper portion oE the cyclone.
~2~ 3 Eig. 2 shows an exhaust gas purifying system including the treatment circuit of Fig. 1. The system is an exma~le as used for a the~moelectr1c power plant of 1000 MW class specifically designed for burning coal. With reference to the drawing, indicated at 13 is a coal burning boiler, in which coal is burned with air sup?lied by a blower 14 through an air heater 15 and which produces a combustion exhaust gas con-taining Ely ash and harmful acid substances. Fine particles of a Ca-type absorbent, for exampl.e, finely divided limestone (CaCO3) having a mean particle size of 1 -to 3 microns is sprayed in-to the top of -the boiler 13 via a feed line 16. Consequently, the particulate absorbent is retained at a -temperature of 900 to 1100 C for about 2 to abou-t 3 seconds, whereby CaCO3 is thermally decoraposed instantaneously -to give highly reac-ti.ve CaO according to Equation (2). The CaO .Eurther undergoes the reactlons of Equations (3) and (~), absorbing sulfur oxides and :forrning calcium sul:eate (CaSO~L) on the surface oE the absorb~nt part;icle~. The reactivity oE the CaO is about ~0 to about 50~, ancl unr0acted CaO remains as it is in th~ :interlo:r oE th~ pa:rticles. The heat evolved .erorn the r~actiorls is uki:Llzed by the bo:Lle.r 13.
The exhaust gas Erom the boiler 13 i5 passed through a denitrating unit L7, which removes nitrogen oxi.des Erom the gas. Via the air heater 15, the gas then.enters a ~6~9~
a dust collecting unit 18, in which the used particulate absorbent and soot and dust containing rly ash are removed from the gas stream. The exhaust gas thus purified is -then released to the atmosphere through a chimney 19.
According to the invention, desulfurization is conducted at a high temperature of 900 to 1100 C, so that the exhaust gas has a reduced acid dew-point temperature. Consequently, the exhaust gas as released from the chimney 19 to the atmosphere has a temperature of about 100 C as reduced from the conventional ternpera-ture level of 140 to 150 C. This means that an increased amount of heat can be utilized by the boiler 13. Before reaching the dust collectlng unit 18, the particulate absorbent absorbs hydrogen ~luoride (HF) and hydogen chloride (EICl) from the exhaust gas, in addition to the sulfur oxlcles, but since the CaF2 and CaC12 consequen-tly formed do no-t partlcipate in th~ subsequent treatment Oe the absorbent, these halides will not be cleserlbed.
rrhe soot, dust and particles colleeted by the unit 18 are led through a hopper 20 to an air elassi~ier 21, by which they are separated :into a coarse particle portion primaril~ contai.nincJ ely ash of 12 to 15 microns i.n rnean particle size, ancI a ~.ine particle portion c~Iiefly contain:Lng the usecl absorbent. The coarse particle portion separated o~f is s-tored in a container 22. The Eine particle portion is led through a hopper 1 into a reyenerating 9~
treatment circuit.
The regeneration circuit shown in Fig. 2, like the one shown in Fig. 1, includes an ejector 2, reduction reactor 3, reducing gas generator 4, carbonation reac~or 5 having a cooler in the ~orm o~ a heat exchanger, dust collecting unit 7, calcium carbonate hopper 8, hydrogen sulfide collector 9, feedback line 10 having a compressor ll, and purge line 12. The dust collecting unit 7 comprises cyclones 23 and a bag filter 2~ in cornbination. The hydrogen sulfide collector 9 comprises a cooler 25 having a cooling water circulation pump 26, a hydrogen sulfide absorption column 27 and a stripper 28. The stripper 28 is connected to a hydrogen sulfide container 29.
The CaSO4 component contained in the used absorben-t ~ed to the reduction reactor 3 through the e]ector 2 is reduced to CaS by the reactions of Equations (5) and (6) in the hot reducing gas Erom the generator ~. The CaS further - reacts with carbon dioxide and water vapor in the carbonation reactor 5 accordiny to Equakion (7), forming CaCO3.
Consequentl~, the absorbent is regenerated, and H2S is produced. ~uring -the carbonation reaction, the unreacted component ~CaO) in~ide the ~bsorbent particles also undergoes the following reaction and is carbonated.
CaO ~ CO2 ~ CaCO3 (~) The reduction reac-tion and the carbonation reaction ~' ~
~2~9~
described above are each exothermic, and the heat of reaction is recovered by the heat exchanger 6.
The gas stream flowing out from the carbonation reac-tor S and containing the regenerated absorbent, ~2S and other unreacted gas components is subjected to solid-gas separation by the dust collecting unit 7. The particulate absorber separa-ted off is fed back to the boiler 13 via the calcium carbonate hopper 8, rotary valve 30, ejector 31 and feed line 16. Since some of the absorbent particles are los-t when classified by the air classifier 21, the feed line is replenished with a fresh absorber (which need not always contain CaCO3 but may contain CaO or Ca(OH)2) from an absorbent feeder 32 via a hopper 33, rotary valve 3~ and ejector 35 to compensate for the loss. Indicated at 36 is a feeding compressor connected to the feed line 16.
The gas stream flowing out from the dus-t collecting unit 7 is cooled to about ~0 C by the cooler 25 and then subjected to an amine washing step by the absorption column 27 and the stripper 28, whereby H2S only is separated off for collect,ion. The collected H2S is stored in the container 29 and is t.reated, Eor example, by the Claus process at a ~uitable t.Lme. ~ndicated at 37 and 38 are elemental sulfur and water vapor resulting Erom the Claus process.
The gas stream flowing out from the hydrogen suli~e collector 9 is returned to the redu~tion reactor 3 by the compressor 11 via the feedback line 10. The gas stream is partly released to the outside via the purge line 12.
If it is attempted to reduce the amount of absorbent particles to be lost at the air classifier 21, the particulate S material to be sent to the absorbent regenera~ion circuit downstream thereof will contain an increased amount of impurities (fly ash, etc.), giving an increased load to -the regeneration circuit. Accordingly -the ratio of separation to be conclucted by the classifier 21 needs to be set in view of the balance between the amount of loss of absorbent and the load on the regeneration circuit.
The exhaust gas purifying system shown in Fig. 2 has the following advantages.
(a) Since used Ca-type absorbent is recycled for reuse upon regeneration, the exhaust gas can be puriEied eeficiently w,ith use of a smaller amount oE absorbent without entalling the troublesome problem oE by-proclucts.
(b) RecyclincJ oE th~ absorbent wh:ich has been pulver:izecl once serves to reduc~ the powcr eor pulver:iY.ation.
(c) The process erom the desuleur:i~ation with the absorbent through the re~erl~ration Oe the abso~bent (the ~eri0s of steps up to the dust collec-ting unit 7) :is practlced cont:inuously in a clry state and is there~ore low in hea-t loss. ~urther the heat developed from the desulfurization and regeneration i.s effectively used by 9~
the boiler 13 and heat exchanger 6 to achieve an improved energy efficiency. The present method is free of the problem of effluent treatment which is encountered with the wet or semi-wet process and which could cause secondary pollution.
(d) The boiler 13 and dust collecting unit 18, which serve also as desulfurizing units, render the system compact.
(e) Because the desulfurization with the absorbent is conducted at a high temperature of 900 to 1100 C, SO3 which is more harmful than SO2 and diffucult to remove at lower temperatures (SO3 suspends in the exhause gas in the form of stable minute crystals at low temperatures) can be removed effectively.
r~
.. ,~ .
Claims (7)
1. A dry method of treating solids containing the calcium salt of a sulphuric oxyacid comprising feeding to a reduction reactor a reducing gas stream containing hydrogen and/or carbon monoxide and dry particles containing the calcium salt of sulphuric oxyacid and having a mean particle size of up to 10 microns, reducing the calcium salt to calcium sulphide at 700° to 900°C in the reduction reactor, feeding the gas stream entraining the produced calcium sulphide obtained from the reduction reactor to a carbonation reactor, cooling the gas stream entraining the calcium sulphide to 300° to 500°C in the carbonation reactor to react in a dry state the calcium sulphide with carbon dioxide and water vapour and produce calcium carbonate and hydrogen sulphide, guiding the resulting gas stream from the carbonation reactor into a dust collecting unit to collect solid particles, separating the hydrogen sulphide from the gas stream flowing out from the dust collecting unit, then feeding back to the reduction reactor the remaining gas containing hydrogen, carbon monoxide and carbon dioxide as a carrier gas for feeding the dry particles into the reduction reactor.
2. A dry method as defined in claim 1 wherein, the calcium salt of sulphuric oxyacid comprises one or more of calcium sulphate, calcium sulphite, calcium thiosulphate and calcium polythionate.
3. A dry method as defined in claim 1 wherein, the reducing gas stream is obtained by partially burning a hydrocarbon with water vapour and concentrated oxygen.
4. A dry method as defined in claim 1 wherein, the dry particles have a mean particle size of 1 to 3 microns.
5. A dry method of purifying an exhaust gas containing sulphur oxides by spraying dry particles of a Ca-type absorbent into the exhaust gas to cause the absorbent to absorb the sulphur oxides and thereafter collecting by a dust collecting unit the absorbent particles along with other particles contained in the exhaust gas, the method being characterised by feeding to a reduction reactor the used absorbent particles collected by the dust collecting unit and having a mean particles size of up to 10 microns, and reducing gas stream containing hydrogen and/or carbon monoxide, reducing at 700° to 900°C in the reduction reactor calcium sulphate contained in the used absorbent particles to calcium sulphide, feeding the gas stream entraining the produced calcium sulphide and obtained from the reduction reactor to a carbonation reactor, cooling the gas stream entraining the calcium sulphide 300° to 500° in the carbonation reactor to react in a dry state the calcium sulphide with carbon dioxide and water vapour and produce calcium carbonate and hydrogen sulphide, guiding the resulting gas stream from the carbonation reactor into a second duct collecting unit to collect solid particles, and spraying the solid particle again into the exhaust gas as regenerated absorbent particles, the hydrogen sulphide being separated from the gas stream flowing out from the second dust collecting unit, the remaining gas containiing hydrogen, carbon monoxide and carbon dioxide then being fed back to the reduction reactor as a carrier gas for feeding the used absorbent particles into the reduction reactor.
6. A dry method as defined in claim 5 wherein, the particles collected by the first duct collecting unit are classified into a coarse particle portion primarily containing fly ash and a fine particle portion primarily containing the used absorbent particles not larger than a predetermined particle size, and the fine particle portion only is fed to the reduction reactor.
7. A dry method as defined in claim 5 wherein, the Ca-type absorbent comprises one or more of limestone, slaked lime, dolomite, slaked dolomite and calcined dolomite.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP59183579A JPS6161615A (en) | 1984-08-31 | 1984-08-31 | Regenerative desulfurization method of stack gas by dry lime process |
| JP59-183579 | 1984-08-31 | ||
| JP59183578A JPH0244767B2 (en) | 1984-08-31 | 1984-08-31 | SETSUKONOSHORIHOHO |
| JP59-183578 | 1984-08-31 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1266959A true CA1266959A (en) | 1990-03-27 |
Family
ID=26501957
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000489530A Expired - Fee Related CA1266959A (en) | 1984-08-31 | 1985-08-28 | Treatment of solids containing calcium salt of sulfuric oxyacid and method of purifying exhaust gas utilizing same |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1266959A (en) |
| DE (1) | DE3530421A1 (en) |
| FR (1) | FR2569679B1 (en) |
| GB (1) | GB2163739B (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN1141168C (en) * | 1998-04-01 | 2004-03-10 | 富兰克林·S·查默斯 | Method of processing sulfur-containing materials derived from flue gas desulfurization or other sources |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR1420350A (en) * | 1964-01-15 | 1965-12-03 | Improvements to processes for desulphurizing gases | |
| GB1241575A (en) * | 1967-08-23 | 1971-08-04 | Elcor Chem Corp | Process for converting gypsum to calcium sulphide and apparatus therefor |
| US3574530A (en) * | 1967-10-02 | 1971-04-13 | Pullman Inc | Method of removing sulfur dioxide from waste gases |
| BR8300949A (en) * | 1983-01-13 | 1984-10-02 | Fundacao Inst Tecnologico Do E | PROCESS FOR OBTAINING SULFUR AND CALCIUM CARBONATE FROM GYPSITE |
-
1985
- 1985-08-21 GB GB08520975A patent/GB2163739B/en not_active Expired
- 1985-08-26 DE DE19853530421 patent/DE3530421A1/en active Granted
- 1985-08-28 CA CA000489530A patent/CA1266959A/en not_active Expired - Fee Related
- 1985-08-30 FR FR8512963A patent/FR2569679B1/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB2163739B (en) | 1988-05-18 |
| DE3530421C2 (en) | 1991-02-14 |
| DE3530421A1 (en) | 1986-03-13 |
| FR2569679B1 (en) | 1988-10-14 |
| FR2569679A1 (en) | 1986-03-07 |
| GB2163739A (en) | 1986-03-05 |
| GB8520975D0 (en) | 1985-09-25 |
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