Note: Descriptions are shown in the official language in which they were submitted.
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
-- 3
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
5 -
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
-- 6
~ ~ ~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
-- 7
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
-SD 14 -
~ 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
SD 15-
: .~
: ~ .
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.
- SD 16 -
