CA1181927A - Fluidized bed sulfur dioxide removal - Google Patents

Abstract

FLUIDIZED BED SULFUR DIOXIDE REMOVAL
ABSTRACT

Method of stripping SO2 (sulfur dioxide) from flue gases by passing the SO2 containing gas through to a bed of particles to fludize said particles and to form a fluidized bed, injecting a reaction chemical into the bed and reacting it with SO2 in the fluidized bed thereby to form particles of a combined product, combining the particles with other such particles in the fluidized bed to form bigger pellets. While said method is carried out, at least some of the pellets so formed are removed from the bed. In the preferred arrangement the reaction chemicals are injected into the bed in an aqueous medium and gas entering the bed is at a temperature which evaporates the aqueous medium to form a dry combined product.

Description

FIELD OE` THE INVENTXON
Present invention relclte~ to a fluidized hed ~ys-tem for removal of sulfur diox:Lde (Soz) (and fly ash) from flue gases. More speci~ically the present invention relates to an efficient fluidized bed system for recovery of SO2 from flue gases as a dry composition in particle form.
PRIOR ART
In the combustion process, particularly in the burning of coal, CO2 and f:Ly ash are generated from the sulphur content of -the coal and this SO2 unless removed from the ~lue gases passes up the s-tack and is dissipated into the atmosphere where it forms acid rain. It has recently been established that acid rain is significantly altering the pH of lakes and of the countryside even in 3 remote areas and that these changes in pH are very detri-metal to the ecology. In fact, in some regions the low-ered pH has resulted in extermination of all fish life.
The emission of fly ash from the stack has been deemed objectionable for other reasons.
Steps have been taken to reduce sulphur and par-ticulate emissions by removing the sulphur and fly ash from the flue gases before they are released to the atmo-sphere. Techniques for sulphur removal include wet scrub-bing wherein the SO2 containing gases are sprayed with a solution or slurry generally of an alkali material, e.g.
lime, limestone, magnesium oxide etc. or combinations thereof which react with the sulphur to form compounds (generally solid) such as calcium sulfite or magnesium sulphite which leaves the scrubber as a wet ~lurry and are ,~ ~ 2 - ~

'7 disposed o~.
I~ly clsh, when L~resellt is normilLy separated ~rom the flue gas in a bag house or electrostatic precipitation in what is ~enerally referred to as a prilnary particulate removal operation. This primary particulate relnoval operation is carried out upstream of the SO2 wet scrubbing removal system. Fly ash that may carry over into the scrubbincJ operation is washed Erom the gas and Eorms part of the wet s:ludge formed in the scrubber and is disposed of with the sludge.
Attempts have been made to produce a dry by-product by chemically combining the SO2 in the flue gas. Such devices may ta]ce the form of spray dryers wherein a solution or slurry of say lime is sprayed into the hot flue yas and the lime combines with the SO2 to form a mixture of CaSO3/CaSO4 while the heat from the flue gas evaporates the water and dries the mixture. This dry product is then captured, for example in a bag house or the like downstream of the spray drier and removed. The spray drying system permits elimination of the primary particulate removal operation and the bag house downstream of the spray drier also used to capture fly ash particles.
It is also well knownl particularly in the phar-maceutical industry, to utilize the fluidized bed as a drver and granulater in a process that is often called spray granulation. This process has been known for some time as a batch granulation process and ilas recently been made continuous utilizing a combination of spray drying and fluidized bed drying for particle formation and yrowth. With this process :Eeed liquor is atomized and sprayed into a .Eluidized klyer of already dri.ed or partially drled particles. The El.u:Ldizing medium being the drying air.
It is also well known in the Pulp and Paper industry particularly with the neutral sulphite process and more recently with the Kraf-t process, to burn residual li~uor in a fluidized bed thereby to convert -the inorgan-ics in the liquor into pelle-t.s composed primarily of 10 sodium sulphclte and sodium carbonate. Pellet growth occurs as the material. is oxidized at about the utectic temperature of the inorganic materials and the newly formed material is rendered adherent and is bound to the nuclei in the bed. Growth of the pellets in layers is sometimes referred to as onion type pellet growth.
BROAD DESCRIPTION OF THE INVENTION
The main object of the present invention is to provide an efficient system for removing sulphur dioxide ~rom industrial gas streams preferably at elevated temperatures, utilizing a spray of an aqueous slurry or solution of a reaction chemical into a bed of pellets fluld-ized by the gas stream containing the SO2 to be captured, forming dry pellets containing a combined product of the reaction chemical and SO2.
Broadly the present invention relates to a method of stripping SO2 from gases, said method comprising: providing a bed o$ pel.lets, passing SO2 containing gases through said bed to fluidize said pellets and to form a fluidized bed of said pellets, in~ecting a reaction chemical into the . 4 bed, reacting said SO2 with sa.id reaction chemical in said flu:idized bed thereby to form a combined product which com-bines in said flui.dized bed to .Eorm said pellets and d:i.s~
carding at least a portion of sa:Lcl pellets from said bed for disposal. Preferably said reaction chemical will be injected in an aqueous medium and heat from said gas will evaporate said medium. In -the event the SO2 containing gas also contains fly ash the pellets so formed will also contain fly ash.
BRIEF DESCRIPTION OF THE DRAWINGS
Fur-ther features, objects, and advantages will be evident following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which Figure 1 is a schematic illustration of the one form of the present invention, and Figure 2 is a graph obtained from the result of table 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in Figure 1 flue gases from the boiler 10 pass through the economizer section 12 and then in the illustrated arrangement by via line 14 directly lnto the fluidized bed unit 16.
The unit 16 is composed of a fluidized bed 18 of pellets, these pellets are generated and grow in the bed and generally will have the composition of a combined product formed by the reaction of the S32 in the flue gas with the reactant or absorbant chemicals added in an aqueous medium to the bed. Xf the flue gas contains fly ash the fly ash will also be substantially removed from the gas in the fluid bed and will be bound by the reaction product into the pellets be:inc~ Eorm~ld. water in the a~ eolls me-liuln is dried by the ~lue ~s alld 1e~ves with the gases exhaused froln the bed.
To generate the Fluidized bed 18 the ~lue gas ma~
be drawn by a blower 20 into the plenum charnber 22 at the bottom of the unit 16 and then through a su:itable grid or defuser pl.ate 24 and into the bed 18 to Llu;.dize same.
The term fluidized bed as used herein is inten~led to include conventional fluid beds, circulating fluid beds (fast moving .Eluid transport systems whereirl the pellets are carried in the gas stream, separated and returned to a point o~ introduction), spouting beds, etc.
A suitable heat exchanger 26 may be provided withi.n the bed to absorb heat entering the bed with the hot flue gases forming the fluidizing medium. If suffi-cient heat is available the exchanger 26 may be provided and the heat extrated from the bed via heat exchanger 26 and be used, for example, to preheat the boiler feed water or to preheat the combustion air~ In the illustrated arrangement heat may be extracted from the flue gas i.n the heat exchanger 28 or in the exchanger 26 or both and used to preheat the combustion air in line 30. The exchanger 28 may be used to fine tune the temperature of the flue gas entering the unit 16. Normally, unless the flue gas temperature is exceptionally high the exchanger 26 will be omitted.
The absorbent slurry (absorbent or reactent chemical in an aqueous medium) in the illustrated arrange-ment enters the system via a line 32 and is sprayed onto ~ ~ ~3~ '7 the top o~ the becl via .c.pr~y no7.æle 3~. rrhis spray is ~djus~ed to project the ~ rry into the 1~(1 L~3 wllerein it is intilnately mixed w~th alld ~l~nera]~y coats the p~lletx alld contacts the gases passirlg up through the be(~. Thi.s absorbent slurry may contain a dissolved or slurried alkali such as lime, magnesium oxide, dolomitic lime, limestone, soda ash, treated Ly ash, dolomite, etc.
; While the slurry is indicated as sprayed onto the top of the bed 18 via no~le 34 it may also be ~njected directly into the bed by no%zles located in ancl pre~erably acljacent the bottom of the bed 18 to acilitate contact o~
the SO2 with the reactant chemical in the slurry and en-sure that the water in the slurry or solution evaporates.
Xf desired, the reacti.ng chemical may be intro-lS duced dry to the bed but this increases the reaction timeand may limit the effectiveness of the process. The dry injection process will normally not be used if it is intended to capture fly ash as well as SO2.
It is preferred not to introduce the absorbent chemical in slurry form before diffuser plate 24 or in-let orifice depending on the type of fluidized bed employ~
ed as some difficulty may be encountered by plugging of the difruser plate or orifice.
The flue gases leaving the fluidized bed unit 16 pass through a pneumatic cyclone 3G where the small parti-cles or pellets entrained in the gas stream are removed.
These removed particles are carried via a line 38 and injected into the bed 18. The cleaned gas passes via line 40 to the induced draft an or blower 20 and may be exhausted to atmosphere via line ~. r ~ necessary a bag house may be prGvidecl downstrealll o~ the cyclone 36 to do the final cleanup.
Particles are generated in the bed 1~ by the reaction of the SO2 in the ~Lue gas with the reaction chemical, say :Lime, sprayed int:o the bed preferably in an aqueous medium, i.e., the SO2 reacts with the lime to form calcium sulphite and sulphate. I'he particles rnay grow to provide pellets of larger si~e b~ a variety of mechanisms. For example, particles or pellets may agglomerate with other particles or pellets to Eorrn pellets. Growth may also be achieved by the formation of further calcium sulphite and/or sulphate directly on the pellets or particles by coating the surfaces of the pellets with reaction chemical and reacting same in situ with SO2 in the flue gas, i.e., the absorbent slurry may wet the surface of pellets or particles in the bed and the reaction take place on the surface of the particles or pellets. The heat of the flue gas evaporates the water forming the medium which is carried as water vapour in the gas stream from the bed through the exhaust line 40 so that the bed 1~ is formed essentially of dry particles or pellets with substantially all the moisture entering the bed being evaporated and carried out with the stripped flue gases. The concentration of the scrubbing solution is such that the flue gases leaving the fluid bed are not saturated and to insure there is sufficient energy available in ~he flue gas to evaporate all the water.
Preferably the minimum concentration of scrubbing solution 3~'7 to meet ~h~ above constrclints will be llse(l to obtain maximulll s~rubbing e~ticiellcy.
Partic:les and pellets are bled from the bed 1~3 via Line 44 pre~erably screened via screens fi6 with the ~ines being returned to the bed 18 via line 48. I'he larger particles or pellets leave the system via line 60 and provide a dry by-product that may be used in any convenient manner.
In some cases, to minimi~e chemical consumption, i.e. to increase chemical utllization, it rnay be (^3esirable to bleed off a portion of the material in lines, 38, 44, 48 or 60 and utilize this material to form a portion of the absorbent slurry added in line 32.
In the arrangement illustrated the flue gas from the boiler passes directly via line 14 into the fluidiæed bed 18 and thus a significant portion of fly ash carried in the gas will be separated from the gas in the fluid bed and will tend to agglomerate into pellets and will be eventually removed via the lines 60 and 26~ Thus the arrangement illustrated provides a means for removal of both SO2 and fly ash from the system.
It will be apparent that the dry product bled from the beds permits the production of materials that may easily be handled for disposal or for transport to other locations for chemical recovery. For example, if sodium hydroxide is used as the absorbing chemical sodium sulfite or sodium sulEate will be produced and this material could be used as make up chemical in the pulp and paper indus--try.

g ExampLes A number of tests were carried out in the lahorcl-tory using a 12 inch diameter fluid bed having a grid wi-th about a 4% open area supporting an inert bed of about 3 inches initial static height. The fluidization of the bed particles which had a mean size (diameter) of about 800 microns resul-ted in a pressure drop across the bed of approxima-tely 2-4 inches of water. The -tests were all carried out using an absorbing solution of 10~ Na2CO, in water. From the results as shown in I'able 1, the absor~en-t utilization which is by definition (2).(1) was calculated as shown in the last column on the righ-t. From these results, the graph shown in Figure 1 was obtained. This graph illustrates the relationship between SOz removal efficiency (1) of Table 1 and the stoichiometric ratio that is the mole ratio of Na2/S (2 of Table 1) or 2,1 using a fluidized bed.
In all of the above tests, absorbent utilization of up to 96% could be achieved and dxy spent absorbent layers (Na2SO3:Na2SO4:Na2CO3) encapsulating the original bed pellets were consistently observed. The thickness of these layers due to the batch type experimental operations depended on thelength of the experimental run. In all cases the aases leaving the bed were not fully saturated.
In summary said Table illustrates that for an inlet gas temperature of 150-167C, a gas outlet temperature 87-117C, and for an inlet gas having Sb2 concentration of 825-1058 ppm the ~ removal efficiency was found to depend on the Na2/S stoichiometric ratio. From the results .~

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shown in Table 1 and as seer- :Erom Fig-lre 2 derived from Table 1 a subs-t~ntial removal o.F SO2 ~Jas, more than 90%
could theoritically ~e obt-di~led with cl supply of absorbent equivalen-t to abou-t 1.5 moles Na2 C03 per mole of S02 .
For sake of comparison, and to demonstrate the advantage o:E the present invention, an 8 foot tall spray drier with a mean gas residence time of about 4 seconds and a 10~ Ma2 C03 solution as the absorbing solution was used -to treat contaminated gas. It was found tha-t 72.6%
to 75.3~ SO2 removal eff:ic:iencies were obtained with about 1.6 to 1.9 Na2 to S mole ratios. The SO2 concentration of the injected gas stream, in all cases, was in the range of 1160 to 1175 ppm. At lower Na2 to S r~tios, say about 1.2 ratio, the efficiency was found significantly lower.
The test using the fluid bed were limited due -to equipment constraints that Na2 to S ratios above about 1 tusing concentration of Na2CO3 of 10~) could not be tried, however, it will be apparent that high efficiencies will be obtained with higher Na2 to S ratio up to a practical limit that can be easily found for any given set of cond.itions.
Modifications may be made without departir.g from the spirit of the invention as defined in the appended claims.

~.

'7 SUPPLE.~ENTARY DISCLOSURE
BROAD DESCRIPTION OE` THE INVENTION
The main objec-t of ~:he presel-lt inverlti.o~ is to provide an efficient system :or sulph~lr dioxide recovery from gas streams preferably a-t elevated temperatures utilizing a spray of an aqueous slurry or soluti.on of a reaction chemical into a bed o~ pellets Elui.di~ed by the gas stream containing the SO2 to be captured and forming clry pellets contain:Lng a combi.necl product of the reaction chemical and SO2, and exhausti.ng gases from said bed sub-stanti.ally free of SO2 at a temperature as low as possible but high enough to prevent the condensation of the moisture present in the gas stream by adjusting the concentration and feeding rate of said reaction chemical, and to discard a portion vf said pellets from said bed for disposal.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features, objects, and advantages will be evident following detail.ed description of the preferxed embodiments of the present invention taken in conjunction with the accompanying drawings in which:
Figure 3 is another graph illustrating that relation-ship between SO2 removal e:Eficient and the stoichiometric ratio using just a spray drying of SO~ without a fluidized bed, fox sake of comparison.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As indicated on page 5, line 27, if the flue gas contains fly ash the 1y ash will also be substantially removed from the g~s in the fluid bed and will be bound by the reaction product into the pellets being formed:

- SD 13 _ In this manner, water in the aqueous medium is not spray-dried by the flue gas but allows prolongation of the scrubbing time between said reaction chemical and ~lue gas.
Thereafter said water leaves with the yases exhausted frorn the bed.
Normally, the exchanger 26 will be omitted, unless the Elue gas temperature is much higher than said 400-500F (200-260C) as otherwise the evaporation oE
khe absorbing solution will not be able to briny about the exhaust gas temperature within the minimum required 40-l15C and preEerably 100-200F range (38-94 C).
In any event it is essential that the solution containing the reaction chemical at spray nozzle 34 be adjusted in concentration and Eeeding rate so that the gases leaving the ~luidized bed unit 16 preEerably be at a temperature in between 100 F and 200 F (38-94 C) or 1~-30F (5-l5C) hiyher than the temperature at which the condensation of the gas moisutre starts to occur.
That relatively low temperature of fluidiæation is required to provide sufficient time for the reaction chemical to efficiently scrub said flue gas, and to prolong said scxubbing by the corresponding prolongation oE the period required for drying out the liquid~ avoiding flashing out of liquid which produces gas-solid reaction absorption inhibiting the stripping out of So2 (when higher temperatures are used), ~hile yet at a temperature sufficiently high to avoid the unwanted dew-point phase situation. At this low temperature the energy used is just enough to dry. I'he energy not required may be ~ 113 3~'7 ~dvant~geous.l.y converted prior to fluidization to more useful ends, by means of sultab:le heat exchangers. Also in comparison to the prior a.rt, a shallow bed may be used and to that extent, tile pressure drop across the bed is reduced, all this adding to increasing the economi.c advantages over the prior art.
Example With reference to Table 1 on page lOa, we should add the following:
The gas res.idence time in the system was about 4 seconds and the gas residence time in the fluid becl WclS less than 0.2 seconds. Due to equipment constrai.n-ts the test using Na2 to S ratios above about 1 (using concentration of Na2CO3 of 10%) could not be tried. It is apparent that high efficiencies will be obtained with higher Na2 to S ratio up to a practical limit that can be easily found for any given set of conditions.
The result obtained for comparison as referred -to line 5 on page 12 are illustrated in Figure 3. It is easily seen that in the absence of a fluidized bed, in order to obtain an efficiency of about 70%, an absorbent consumpti.on equivalent to about 1.5 moles Na2 C03 per mole of SO2 is required as compared to about 0.8 - 1.0 as evidenced from the examples from Table 1 above in the presence of a fluid bed. The experimental conditions used to obtain Figure 3 were as follows:
Gas inlet temperature: 135 - 145C
Gas outlet temperature 45 ~ 90~C
Gas inlet S2 concentration: 1150 - 1250 ppm : .~
: ~ .

With this method, auxil:iar:ies scrubbing elements that are typically used in associat:ion wit:h ~luidi7.ecl bed are elrnina-ted.
Contrary to the prior art, the applicant's inven-tion enables the removal of SO2 from gas with a single device and wi-th minimal energy (]us-t enough to dry~ there-by saving energy cost and capital inventment.
Moclifications rnay be made without departiny from the spirit of the invention as defined in the appended claims.

Claims (5)

1. In a method of using a fluidized bed for stripping SO2 from gases, comprising providing a bed of pellets, passing SO2 containing gases through said bed to fluidize said pellets and to form a fluidized bed of said pellets, injecting a reaction chemical into said bed, reacting said SO2 with said chemical in said fluidized bed thereby to form a combined product, combining said combined product with previously formed combined product in said fluidized bed to form said pellet and bleeding at least some of said pellets from said bed for disposal at a temperature wherein said gas is leaving said bed at a temperature in the vicinity of 87°-117°C.

2. A method as defined in claim 1 wherein said gases are at above ambient temperature and wherein said reaction chemical is introduced to said bed in an aqueous medium, transferring heat from said gases to said bed, evaporating said aqueous medium, carrying said evaporated aqueous medium from said bed in a cooled stream of said gases leaving said bed.

3. A method as defined in claim 2 wherein said gases are fed at high temperatures and wherein heat from said bed is absorbed in a heat exchanger embedded in said bed.

4. A method as defined in claim 1, 2 or 3 wherein said gases are flue gases from a combustion of sulphur containing fuels and wherein said gases also contain fly ash, said process further comprising uniting said fly ash with said combined product in said bed to form said pellets.

5. A method as defined in claim 1, 2 or 3 wherein said gases are flue gases from a combustion of sulphur containing fuels.

SD-6 A method of using fluidized bed for stripping SO2 from gases, comprising providing a bed of pellets, passing SO2 containing gases through said bed to fluidize said pellets and to form a fluidized bed of said pellets, injecting a reaction chemical into said bed, reacting said SO2 with said chemical in said fluidized bed thereby to form a combined product, combining said combined product with previously formed combined product in said fluidized bed to form said pellets and exhausing gases from said bed substantially free of SO2 at a temperature as low as possible but high enough and not above 117°C, to prevent the condensation of the moisture present in the gas stream by adjusting the concentration and feeding rate of said reaction chemical, and discarding a portion of said pellets from said bed for disposal.
SD-7. A method as defined in claim SD-6 wherein the SO2 is removed solely by the steps as recited in claim SD-6, and wherein said reaction chemical is introduced to said bed in an aqueous medium, transferring heat from said gases to said bed, and to said aqueous medium to evaporate it, carrying said evaporated aqueous medium from said bed in a cooled stream of said gases leaving said bed, whereby at said temperature which is as low as possible, and as additional heat is necessary for evaporating said aqueous medium, the flashing out of liquid producing gas-solid reaction absorption is inhibited and thereby the SO2 removal from said SO2 containing gases enhanced.
SD-8. A method as defined in claim SD-7 further comprising absorbing heat from said bed with a heat exchanger embedded in said bed enhancing SO2 containing gases in order to help maintaining said temperature as low as possible.
SD-9. A method as defined in claim SD-6 further comprising absorbing heat from said bed with a heat exchanger embedded in said bed in order to help maintaining said temperature as low as possible.
SD-10. A method as defined in claim SD-6,7 or 8 wherein the temperature of said exhausting gases ranges between 100°F and 200°F (38-94°C).
SD-11. A method as defined in claim SD-6 or 7 wherein said gases are flue gases from a combustion of sulphur containing fuels and wherein said gases also contain fly ash, said process further comprising uniting said fly ash with said combined product in said bed to form said pellets.
SD-12. A method as defined in claim SD-6 or 7 wherein said gases are flue gases from a combustion of sulphur containing fuels.
SD-13. The method as defined in claim SD-6 wherein said chemical is sodium carbonate present in stoichiometric ratio of between 1 and 1.5.
SD-14. The method as defined in claim SD-6, 7 or 8 wherein auxiliary scrubbing elements that are typically used in association with fluidized bed are eliminated.
SD-15. The method as defined in claim SD-7 wherein said aqueous medium has a concentration of about 10%
reaction-chemical.