Note: Descriptions are shown in the official language in which they were submitted.
~ ,~Z~67 4
This invention relates to a desulfurizing method for
removing H2S and/or SO2 from combustion exhaust gases of a com-
bustion furnace of the fluidized bed type specifically, by con-
tacting the combustion exhaust gases with a fluidized heating
medium, having a high desulfurizing efficiency.
In a fluidized bed combustion furnace, fuels burn
while in contact with the heating medium maintained in fluidized
state, thereby providing close contact between gasified fuels
and the heating medium and providing high combustion efficiency.
This type of combustion has many advantages in that it may use
low calorie or bulky fuels, the desulfurization of exhaust gases
is carried out in the dry state and it requires less installa-
tion cost.
Since it is now necessary to use fuels with high
sulfur content or to gasify solid fuels with high sulfur
content because of the recent increases in energy cost,
there is an inevitable increase of the sulfur concentration
in exhaust gases or product gases obtained in fluidized bed
heating furnaces. In addition, since city dust or sludge is
mainly composed of waste matter such as garbage and sewage
and contains a high concentration of sulfur, the exhaust gases
which are produced by burning such waste matter, also contain
a high level of sulfur, which leads to undesired atmospheric
pollution.
Under such circumstances, there is a need to develop a
desulfurizing method that can easily remove sulfurous ingred-
ients in the combustion exhaust gases at a low cost and with high
efficiency. No desulfurizing method used heretofore in fluidized
bed combustion furnaces is satisfactory because of the above
requirements.
A conventional desulfurizing method was reported by
Robert L. Gamble under the title "Operation of the Georgetown
University Fluidized Bed System" in the Proceedings of the
--1-- .i~
~22~674
Institute of Ene~gy's International Conference in London,
November 1980, "Fluidized Combustion: Systems and Applications".
This desulfurizing method uses limestone as a heating medium
and a desulfurizer for the fluidized bed combustion furnace.
Air is blown into the furnace from the bottom of the limestone
bed through air diffuser plates to fluidize the limestone and
to burn coal, and the combustion exhaust gas is contacted with
the fluidized limestone for desulfurization. Other desulfurizing
agents applicable as heating media and desulfurizers for the
above desulfurizing method include natural carbonate ores such
as limestone or dolomite which are mainly composed of CaO or
MgO, as well as industrial products such as ~ortland cement
clinker or a hardened hydration product of Portland cement mainly
composed of CaO. These products have inherent advantages and
disadvantages. Thepresent invention is intended to provide a
desulfurizing method having a highstability and a high desulfur-
izing efficiency for removing sulfurous ingredients from combust-
ion exhaust gases of a fluidized bed combustion furnace.
In order to accomplish the above purpose, the desul-
furizing method according to the present invention uses as theheating medium, a hardened material having a particle size of
0.7 to 2 mm which is prepared by mixing 10 to 70% by weight of
cement with limestone or dolomite and adding water to harden the
mixture. The heating medium in the fluidized bed is contacted
with the combustion exhaust gases to remove at least one of H2S
and SO2 contained in the combustion exhaust gases.
The desulfurizing method of the present invention uses
another heating medium wherein the above mixture has a particle
size of less than 0.3 mm, and 5 to 20 weight % of a material
having a particle size of 0.3 to 1.2 mm is added to the mixture,
~ '
~2216~4
The invention is illustrated by means of the
following drawings.
FIGURES lthrou-gh 9 are graphs showing the results
of test made with heating medium and a desulfurizing agent
according to this invention; more particularly
FIGURES 1, 3, 5 and 7 are graphs showing the results
of a test to determine the SO2 absorption;
FIGURES 2 and 4 are graphs showing the results of a
test to determine the H2S absorption;
FIGURES 6 and 3 are graphs showing the abrasion loss
of a heating medium and desulfurizing agent; and
FIGURE 9 is a graph showing the contents of pellets
whose particle size is 1.0 - 2.0 mm.
The inventors of the present invention have made
various studies for developing a heating medium having a
sufficient heat stability and a high desulfurizing efficiency
that can be used to form the fluidized bed of a combustion
furr,ace and found that heading media and desulfurizing agents
used in conventional types of fluidized bed combustion
furnaces have the following advantages and disadvantages.
Limestone and dolomite are more readily collapsible
when exposed to high temperatures but are less effective for
desul~urization as the ore particle size thereof becomes
~2Z~674
larger, because water is included in the ores at the grain
boundary thereof and its content will then possibly be
greater, which may cause exploding phenomena when the ore
is heated to high temperature. Further, both limestone and
dolomite become porous when they are decarbonated at high
temperature. However, since the pore diameter formed by
decarbonation is small, the pores are blocked with products
that are the result of their reaction with SO2 or H2S,
which restricts the site of the reaction between So2 or
H2S and the desulfurizing agent only at or near the surface
area. Thus, the surface area of the desulfurizing agent
available for the reaction decreases as the particle size
increases, to thereby reduce the desulfurizing effect.
Turning now to the cement clinker, the latter is
a dense clinker consisting mainly of calcium silicate that
usually is sintered at high temperature above 1450C. Con-
sequently, although the cement clinker has a sufficient
strength as a heat medium, its desulfurizing performance
is much lower than limestone or dolomite since the reaction
of calcium silicate with SO2 or H2S at high temperature is
significantly lower than that of CaO or MgO.
On the other hand, the hardened portland cement
product is composed mainly of CaO-SiO2-H2O hydrate and Ca(OH)2,
which are substantially converted into calcium silicate and
CaO when heated above 500C. Although CaO is highly reactive
with SO2 or H2S, since it usually accounts f~r only about
15 - 25% of the hardened portland cement, its desulfurizing
performance is not satisfactory as a whole. However, its
stability at high temperature has been found to be excellent.
Based on the foregoing findings, various experiments
have been conducted and, as a result, it has been found that
, ~
.~J
~ 2Z~L674
a hardened product prepared by hardening a mixture of pul-
verized limestone or dolomite with cement and incorporating
water to the mixture, constitutes a product which has excel-
lent absorbing performance for SO2 or H2S because the har-
dened product has pores which are large enough to allow SO2
or H2S to easily penetrate thereinto. In addition, the pro-
duct has a great porosity, and its stability as heat medium
at high temperature can be maintained since the particles
of the limestone or dolomite are bonded to each other by
the calcium silicate layer of hydrated cement.
This invention is based on the foregoing findings
and the use of the hardened material as the heating medium for
the fluidized bed of a fluidized bed combustion furnace wherein
the hardened material is prepared by mixing cement and limestone
or dolomite followed by hardening with water.
The cement that can be used according to the present
in~ention includes, for example, various types of portland
cement such as ordinary portland cement, and rapid hardening
portland cement, mixed cement such as blast furnace cement,
silica cement, fly ash cement, etc.
The explanation for the presence or absence of
limestone and dolomite and the mixing ratio of limestone or
dolomite as well as the particle size of the mixture, will be
given hereinafter based on the following examples.
Example 1
Ordinary portland cement, limestone and dolomite,
each having the chemical composition described in Table 1
were mixed in each of the ratios described in Table 2. The
mixture was hardened by incorporating water thereto and was
cured at a curing temperature of 20 + 3C and a curing
~ 22~674
humidity (RE~/o) greater than 80% for seven days to prepare
hardened products of cement. The limestone and dolomite
used were pulverized to a particle size less than 0.5 mm.
Table 1
Chemical _ _
Composition Ig.Loss SiO2 A123 Fe2 3
Material
~_.__ _
Ordinary
Portland 0.6 22.2 5.03.3 65.1 1.4
10 Cement
Limestone 42.4 1.5 0.90.6 53.4 0.7
Dolomite 44.7 0.6 0.20.5 35.1 18.1
Table 2
Experiment ¦ Material Used twt%) Water Cement
No. Ratio (w/c)
, ~
1. (Comparative100 0 0 0.30
example)
2. (This invention) 33 66 0 0.65
3 (rhi~ ~verti~) 33 o 66 0.65
The hardened products prepared as indicated above
were dried at 110C for 24 hours and then pulverized to a
particle size of 0.59 - 1.19 mm. 2 g of each of the pulverized
hardened products thus prepared were loaded in a fixed bed and
an absorption test for SO2 or H2S was carried out by passing
gases containing SO2 or H2S through the fixed bed. For com-
parison, limestone and dolomite were separately pulverized to
the same particle size in order to prepare samples for the
test.
~LZ~L6~4
The conditions for the test are the following:
S2 Absorption Test
Gas composition : S02 700 ppm, 2 5%~ C2 12%,
balance ~2
Gas flow rate : 1 ~/min
Fixed bed cross-sectional area: 7.09 cm2
Fixed bed temperature: 850C
H2S Absorption Test
Gas composition : H2S S00 ppm, balance N2
Gas flow rate: 1 ~/min
Fixed bed cross-sectional area : 7.09 cm2
Fixed bed temperature : 850C
The results of the tests are shown in FIGURES 1 and
2, in which the desulfurizing ratio was calculated according
to the following formula:
S2 or H2S concentration - S0 or H S con-
Desulfurizing centration at
ratio (SO2 or = at inlet (ppm) exit (ppm) x 100
H2S) (%) S2 or H25 concentration at inlet
(ppm)
As can be seen from FIGURES 1 and 2, each desulfurizing
agent used in the embodiments of the present invention
(Experiment No. 2,31 has a much better absorption performance
for S02 or H2S than limestone, dolomite or a cement hardened
product (Experiment No. 1) which were used previously.
It is believed that such a remarkable high desul-
furizing performance of the desulfurizing agent according to
this invention is attributable to the fact that the S02 or H2S
component can penetrate into and be absorbed into the interior
of the desulfurizing agent due to its large porosity (about
30% before heating) and also due to the presence of coarse
pores between the particles since powdery particles of lime-
stone or dolomite are bonded to each other by hydrated cement.
~.,,
~2Z16~A
On the other hand, the particles of limestone or
dolomite have an extremely small porosity, usually less than
1% (before heating). It is believed that although the poro-
sity increases by the decarbonating effect resulting from
heating, the pores thus formed are fine and, consequently,
are blocked with CaS04 or CaS formed as the absorption takes
place. It will therefore be understood that SO2 or H2S
hinders the penetration inside the desulfurizing agent, thus
rapidy reducing the desulfurizing ratio of limestone or dolo-
mite as shown in FIGURE 1 and FIGURE 2.
The desulfurizing performance of the cement hardenedproduct (Experiment No. 1) is inferior because, it is believed
that the content of free CaO effective to provide a good
desulfurization product is insufficient. Undesired powder
formation or sintering was not observed after the desulfuri-
zing test for the samples of Experiment ~o. 1, 2, 3.
Example 2
The desulfurizing performance was measured in the
same manner as in Example 1 while varying the temperature of
the fixed bed. The desulfurizing agent and the procedures
for the test were the same as in Example 1. The results are
shown in FIGURES 3 and 4.
As shown in FIGURES 3 and 4, the desulfurizing
agent according to this invention has a much better desulfur-
izing performance than the prior art product (limestone,
dolomite) and a cement hardened product (Experiment ~o. 1)
within a temperature range between ~50 - 950C for the fixed
- bed. Again, undesirable pulverization or sintering was not
observed in the samples of Experiment No. 1, 2, 3 after the
desulfurizing test.
- ~3 -
: , .
3 2Z~674
Example 3
The llmestone used in Example 1 was pulverized to
a particle size less than 0.297 mm ana rapid hardening port-
land cement was mixed thereto in various mixing ratios as des-
cribed in Table 3. 1 Kg of the mixture was prepared for each
experiment.
Table 3
ExperimentMixina Ratio (wt%)
No.Rapid hardening Portland Cement Limestone
4 100 o
6 50 50
7 33 67
8 20 80
9 10 go
.
Each mixture was pelletized using a small pan
pelletizer (610 mm in diameter, 170 mm in height) by spraying
water to prepare pellets of various particle sizes (0.59 mm -
20 3.36 mm). The pellets were cured at a temperature of 20C
+ 3C and a relative humidity higher than 80% for 24 hours and,
thereafter, dried at 105C for 24 hours. An SO2 absorption
test was carried out for the pellets in the size range of
1.19 mm - 1.68 mm from each of the test samples under the same
conditions as in Example 1. The results of the test are shown
in FIGURE 5. In addition, a fluidizing test was carried out
for pellets in the size range of 1.19 mm - 1.68 mm from each
of the test samples, and the abrasion loss in the medium due
to the fluidizing was measured. The results of the measurement
are shown in FIGURE 6.
The conditions for the fluidizing test are as
follows:
~ 2'~674
Fluidized bed temperature : 850C
Fluidizing gas : air
Fluidized bed cross-sectional area : 7.09 cm2
Bed thickness of the charged medium : 50 mm
Fluidizing velocity : 0.7 m/sec
Fluidizing time : 2 hours
As can be seen from FIGURE 5, the desulfurizing
performance of the desulfurizing agent is excellent if the
amount of cement in the mixture is less than 70 wt%, and it
is also apparent from FIGURE 6 that the abrasion loss of the
medium is increased due to the fact that fluidization is
carried out at high temperature, if the amount of cement is
less than 10 wt%. Accordingly, the preferred amount of cement
in the mixture of limestone or dolomite for producing the
desulfurizing agent according to this invention is between
10 - 70% by weight.
Example 4
The same absorption test for SO2 as in Example 3
and the measurement for the abrasion loss of the medium due
to the fluiciation at 850C were carried out on the sample
prepared in Example 3 (Experiment No. 8, cement : limestone
- 1 : 4) with respect to three types of pellets (0.71 - 1.00 mm,
1.00 - 2.00 mm and 2.00 - 3.36 mm) in the same manner as in
Example 3. A similar test was also carried out for samples
prepared separately from limestone and dolomite by pulverizing
and adjusting them to the same particle size for purpose of
comparison.
The fluidizing gas velocity for each of the samples
in the fluidized bed test is shown in Table 4. The results
of the test are shown in FIGURES 7 and 8.
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~2~ 74
Table 4
_
Sample
Particle Fluidizinq Gas Velocity (m~sec)
Size (mm) Experiment No. 7 Limestone Dolomite
I
q.71 - 1.00 0.4 ` 0.6 0.6
1.00 - 2.00 0.7 1.0 1.0
2.00 - 3.00 2.0 3.0 3.0
.
As will be apparent from FIGURE 7, the desulfurizing
agent has a satisfactory desulfurizing performance if the
particle size is less than 2 mm and, as will be seen from
FIGURE 8, the abrasion loss increases in the limestone and
dolomite as the particle size thereof increases. ~n the other
hand, the heating medium used in the emhodiments according to
the present invention shows less abras~on loss irrespective
of the particle size.
As will be apparent from the result of Example 4,
the preferred particle size for the desulfurizing agent is
less than 2 mm when it is used as the heat medium for the
fluidized bed and also for removing sulfur compounds such as
S2 or H2S in the gases. In addition, since the product accor-
ding to this invention has a high porosity, it can be fluidizedunder a lower fluidizing gas velocity than that of the prior
art product. On the other hand, a desulfurizing agent with
less than 0.7 mm particle size is not suitable as heating
medium since it has an extremely low fluidizing gas velocity
and tends to scatter readily. A process for producing the
desulfurizing agent according to this invention will now be
described.
The known techniques for industrially obtaining a
product of predetermined particle size generally include the
following steps:
~2~t74
(I) pulverizing the hardened product as in Example 1,
(II) pelletizing using the pelletizer as in Example 3
(III) extrusion molding the water-kneaded product.
Although step (I) can produce a desulfurizing
agent having a large porosity and a high desulfurizing per-
formance since a great amount of water can be used for kneading,
it requires much energy in the drying, pulverizing and sieving
of the hardened product, as well as giving a product with an
extremely poor yield. Step (III) requires great installation
cost and, in addition, it needs a large amount of ]cneading
water for the extrusion molding, which tends to result in agg-
lomeration of the molding product immediately after the molding
to thereby reduce the yield.
On the other handj step (II, using the pelletizer,
presents no such problems as steps (I) and (III). In view of
this, the present inventors have made a study of a process for
industrially producing the desulfurizing agent at a reduced
cost by using the pelletizer and, as a result, have found that
pellets of small diameter between 0.7 mm - 2.0 mm having a
high desulfurizing performance can be produced in each case and
with a preferred stability by adding as the seed material
5 ~ 15% by weight of particles whose particle size is between
- 0.3 mm - 1.2 mm into the starting powdery material.
The process will now be explained with reference
to the following Examples.
Example 5
The same dolomite as used in Example 1 was pulver-
ized to a particle size of less than 88 ~u and was blended with
blast furnace cement (type A) so as to contain 10% by weight
of cement. Silica sand was sieved separately into various
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~2216~74
particle sizes as described in Table 5 and was mixed in an
amOunt of 10% by weight with a mixture of dolomite and cement.
There was prepared 2 Kg of each mixture. Then each mixture
was pelletized using the same small pan pelletizer as in
Example 1 while spraying water. The pelletizing conditions
are the following:
Small dish type pelletizer
pan diameter : 610 mm
pan height : 170 mm
pan inclined angle : 55
pan rotating speed : 30 rpm
starting material feeding rate : 2Q g/min
After curing the pelletized products at a tempera-
ture of 20C + 3C and at a relative humidity higher than 80%
for 3 days, they were dried at 110C for 24 hours and then
sieved to determine the contents of pellets having a particle
size between 0.70 mm - 2.00 mm. The results are shown in
Table 5.
- 13 -
~2Z~6~74
,~ ~
~D -0- ~ .
,i
~1 ~ ~
~1-
,,
~ ~. , 0
U~
,~ o _
1 ~
,, o
o ~
~ o
~ 0 ~â ~0~
.~ ~ ~. ~ ~
~ U~ ~0~
~1 ~ N r-l O
X O ~ r(
h ~ I
-- 14 --
~221674
As can be seen from Table 5, the preferred particle
size of the silica sand to be added as seed material for the
pellets is between 0.3 mm - 1.19 mm. If the particle size is
smaller than 0.3 mm, no substantial effect is obtained in the
seed material. If it is larger than 1.19 mm, the number of
seeds per unit weight is decreased, therefore a greater amount
of silica sand must be added. Generally, the presence of the
silica sand has so far been considered detrimental because it
does not contribute to the desulfurization. On the contrary,
it reduces the desulfurizing performance or the like. How-
ever, it has now been confirmed that the desulfurizing per-
formance and the heat medium stability of the desulfurizing
agent (Experiment ~os. 11 - 13) according to this invention
are not reduced even by the addition of silica sand. This
result has been obtained by the adsorption test for SO2 in
a fixed bed at 850C and the fluidizing test at 850C for
the desulfurizing agent obtained by this Example. As the
result of the absorption test for SO2 with respect to the
pellets whose size is between 1.00 - 2.00 mm under the same
conditions as in Example 4, a 100% desulfurizing ratio was
maintained for 73 min.
E mple 6
Limestone used in Example 1 was pulverized and
sieved to provide two kinds of pulverizates whose particle
sizes are less than 0.3 mm and between 0.3 - 1.19 mm, res-
pectively. Fly ash cement (type A) was added to and mixed
with the pulverizates of particle size less than 0.3 mm so
as to obtain a mixture comprising 33% by weight of cement.
Then, 2.5 - 25% by weight of pulverizates whose particle size
30 are between 0.3 mm - 1.19 mm were added to and mixed with the
- 15 -
,,
~LZ~67~
mixture of limestone and cement to give samples of 2 Kg
each. The mixture was pelletized following the same prose-
dure as in Example 5. The pelletizing products were tightly
sealed in a plastic bag, cured inside a room for one day and
thereafter dried in the same room for three days while being
left as it was in the form of a thin layer. Then the pro-
ducts were sieved and the percentage of pellets whose particle
size is between 0.7 - 2.0 mm with respect to the total pellets
was measured. FIGURE 9 shows the results of this measurement.
As will be apparent from FIGURE 9, it has been found that
pellets having a particle size between 0.7 - 2.0 mm could be
produced in a large amount and at a high yield when the addi-
tional amount of coarse particles whose particle size is
between 0.3 mm - 1.19 mm is between 5 - 20% by weight. ~t
has also been confirmed that the desulfurizing performance
and the heat medium stability of the desulfurizing agent
according to this invention are not reduced even when the
pelletization was conducted by adding the coarse particles to
the starting material when carrying out the S02 absorption
test in the fixed bed at 850C and the fluidizing test at
850C. As a result of the S02 absorption test carried out
with pellets whose particle size is between 1.00 mm - 2.00 mm
under the same conditions as in Example 4, a 100% desulfurizing
yield could be maintained for 77 min.
As specifically explained in the foregoing Examples,
the hardened product consisting of a mixture comprising lime-
stone powder or dolomite powder and cement, according to this
invention, can effectively remove SO2 or H2S in exhaust gases.
Moreover, the product exhibits a much better desulfurizing
performance as compared with prior art limestone or dolomite
when it is used as a heat medium and desulfurizer in a
fluidized bed heating furnace.
- 16 -
~XZ~674
Although a mixture of limestone powder and dolomite
powder or magnesite powder can be used as the starting material
to produce the desulfurizing agent according to the invention,
magnesite is not recommended because it is expensive and is
not advantageous.
The cement used as the binder for the desulfurizing
agent according to this invention includes portland cement,
mixed cement and high alumina cement. However, the use of
high alumina cement as the starting material is not recommended
due to its high cost. The preferred mixing ratio of cement
in the mixture of limestone or dolomite powder and cement is
between 10 - 70% by weight as shown in Example 3. If the
cement content is less than 10% by weight, the abrasion loss
when the product is used as heat medium, is undesirably high.
If the cement content is greater than 70% by weight,
the desulfurizing performance is much reduced.
The desulfurizing agent according to this invention,
when used simultaneously as a heating medium and desulfurizer
for a fluidized bed heating furnace, exhibits an excellent
desulfurizing effect if it has a particle size less than 2 mm.
Although the desulfurizing effect can be further increased as
the particle size of the desulfurizing agent is decreased,
the fluidizing gas velocity is decreased markedly to easily
scatter the desulfurizing agent out of the system if the par-
ticle size is less than 0.7 mm. In addition, it is difficult
to produce at a low cost a desulfurizer whose particle size
is less than 0.7 mm. Accordingly, the preferred particle
size for the desulfurizing agent actually lies between 0.7 mm
and 2 mm.
According to this invention, a suitable method for
industrially producing the desulfurizing agent with a good
- 17 -
~221674
yield and at a reduced cost involves a rolling pelletization
as shown in Examples 5 and 6, wherein 5 - 10% by weight of
coarse particles whose particle size is between 0.3 mm - 1.2 mm
are added and mixed as the seed material with the starting
material followed by pelletization under rolling while spray- -
ing water on it in a rolling type pelletizer.
The coarse particles which can be used as the seed
material include limestone or dolomite as well as silica sand,
cement clinker dust, fly ash and concrete fine sand. If the
particle size of the seed material is less than 0.3 mm, an
insufficient seed effect is obtained and, on the contrary, if
it is larger than 1.2 mm, the seed material has to be used in
large amounts and reduces the desulfurizing performance.
Furthermore, it is not recommended to add less than 5% by
weight of seed material since the number of seeds to be formed
becomes insufficient and pellets which are larger than 2.0 mm
in particle size are easily produced. On the other hand, the
addition of more than 20% by weight seed material is not recom-
mended, since this increases the number of pellets whose
particle size is less than 0.7 mm which reduces the desulfur-
izing performance.
Limestone or dolomite powder of smaller particle
size gives better pelletizing property. If the particle size
is too coarse, the pelletizing property is markedly worsened,
the strength of the pellets immediately after the pelletiza-
tion is decreased and the pellets are liable to become powdery.
A powder who particle size is less than 0.3 mm is usually pre-
ferred. The pelletized product, when cured in a humid chamber
at ambient temperature for more than one day , develops suf-
ficient strength to be used as a heat medium. Although,after curing the pelletized products can be used as such as
- 18 -
~22167~
de~ulfurizing agent, it is preferable to use them afterdrying.
While there are various kinds of rolling type
pelletizers, the rotational drum or inclined dish type pelle-
tizer is preferably used for-the production of the desulfuri-
zing agent according to this invention.
Example 7
The same limestone as used in Example i was pulver-
ized to prepare finely pulverized products (11% of 88,um
residue) and coarser particles whose parti~le size in between
0.3 mm - 1.19 mm. Then, ordinary portland cement was blended
so that the mixing ratio of finely pulverized limestone :
coarse particles : cement is 68 : 12 : 20 by weight. The
mixing operation was carried out in a nauta mixer. There was
produced two tons of the mixture.
The mixture ,was pelletized in a rotary drum type
pelletizer (600 mm in diameter and 3000 mm in length) while
spraying water. After curing the pellets in a hopper for
three day~, they were taken out and dried in a rotary drier
using hot air streams at 150 - 250C.~ Sieving gave pellets
whose particle size are between 0.71 mm - 2.00 mm with 95%
~ield. The thus prepared pellets were used as the heating
-~ medium for the fluidized bed of a fluidized bed combustion
furnace in which desulfurizing tests were carried out.
Desulfurizing Test 1
~ The desulfurizing agent according to this invention
; (0.71 mm - 2.00 mm) was charged to a bed thickness of about
600 mm in a fluidized bed combustion furnace (~ = 450 mm) and
oil cokes (sulfur content : 4.9%) were burnt. The furnace
was operated to give a fluidized bed temperature of 800 - 850C
30 while feeding the oil cokes at 8.0 kg/h. The ratio of air in
-- 19 --
~ ,
'~' '`
~XZ~674
the exhaust gas was 1.10 - 1.25. The desulfurizing ratio was
greater than 93% at Ca/S = ~.5 molar ratio based on the amount
of the desulfurizing agent supplied and the analytical result
of the sulfur content in the exhaust gas. The scattered
amount of desulfurizing agent was about 1.8% of the charged
amount based on the result of a chemical analysis of the
scattered dust. As the result of a similar test carried out
with limestone particles whose particle size is 0.7 mm -
2.38 mm, the ratio of limestone required for attaining more
than 93% desulfurizing ratio was Ca/S = 5.5 molar ratio and
the scattered amount of the desulfurizing agent was about
6.5% of the charged amount.
Fluidized Bed - Test 2
The combustion furnace was the same one as in the
fluidized bed - test 1. It was used for gasifying oil cokes
(sulfur content : 4.9%) at an air ratio in the exhaust gas
between 0.8 - 0.9. The desulfurizing agent was charged in
the furnace so that the bed thickness of the heat medium was
about 600 mm. The temperature of the fluidized bed was con-
20 trolled to 800 - 850C. The analytical result for the pre-
sence of H2S in the exhaust gas shows that a desulfurizing
ratio of more than 90% could be attained at Ca/S = 2.0 molar
ratio by using the desulfurizing agent according to this in-
vention when the scattered amount thereof was 2.1%. When
using limestone particles in the same particle size, a
desulfurizing rate of more than 90% was attained at Ca/S = 6
molar ratio and the scattered amount was 6.2%.
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.
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SUPPLEMENTARY DISCLOSURE
FIGURE 10 is a section view showing the structure
of a fluidized bed combustion furnace used in the method for
desulfurizing combustion gases according to the invention.
With reference to FIGURE l0 it will be seen that
the fluidized bed combustion furnace 10 comprises an external
steel casing 1, inside of which there is an internal fire
brick lining 2. The bottom of the furnace is made of a heat
resistant perforated plate 3 on which a fluidized bed of
heating medium is formed and which is connected on its lower
side to a blower 4 for blowing air into the vessel. The
furnace also comprises an ignition device 5 which is used
to ignite solid fuels such as coke, coal, etc., fed at fuel
supply 7. Finally, the furnace comprises a combustion gas
outlet 8 and a heat exchange water pipe ~.
Test 1 described in the principal disclosure was
carried out in the combustion furnace illustrated in FIGURE
l0. The desulfurizing agent is made of pellets 6 having the
particle.size mentioned above (0.71 mm - 2.00 mm). The
pellets 6 are charged in the furnace to a bed thickness of
approximately 600 mm to constitute a layer of heating
medium deposited on the heat resistant perforated plate 3
of the fluidized bed combustion furnace. The treatment is
thereafter carried out as follows.
Oil coke was supplied fromthe fuel supply 7 onto the
layer of heating medium (pellets) and then ignited with the
ignition device 5.
After the oil coke was ignited, the blower 4 was
operated to blow air through the heat resistant perforated
plate to fluidize the pellets.
. -
~
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The quantity of oil coke and the air flow were
adjusted so that the operating temperature of the fluidized
bed was 800 to 850 degrees C as mentioned in the principal
disclosure. Oil coke was fed to the fluidized bed at a rate
of 8.0 kg/hour and pellets were supplied to the fluidized bed
from a feed system which is not shown. During the operation,
the ratio of air in the exhaust gas was 1.1 to 1.25 as
mentioned. The desulfurizing ratio is as mentioned in test 1.
,The scaltered amount of desulfurizing agent and the ratio of
limestone are also as indicated in test 1.
Test 2 was carried out in the fluidized bed
combustion furnace 10 which was supplied with the above
pellets to a thickness of approximately 600 mm on the
heat resistant perforated plate 3. Oil coke (sulfur content:
4.9%) was supplied onto the pellets and burnt at
an air ratio in the combustion exhaust gas of 0.8 to
0.9, and the furnace was operated at a fluidized bed
temperature of 800 to 850 degrees C and an oil coke feed
rate of 8.0 Kg/hour, all as mentioned in the principal
disclosure.
The analytical result for the presence of H2O in
the exhaust gas and the desulfurizing rate are as shown in
test 2.
Thus, by using the above pellets as a heating
medium for the fluidized bed of a fluidized bed combustion
furnace, the desulfurizing effects for H2S and SO2 are
notably improved and the scattered quantity of desulfurizing
agent (heating medium) is substantially reduced as compared
with a conventional desulfurizing method for combustion gases
wherein limestone is used as the heating medium. As will be
~22~674
obvious from the above experimental examples, similar
advantages can be obtained not only with the above described
pellets but also with a hardened material which is prepared
by mixing 10 to 70% by weight of cement with limestone or
dolomite and hardening in the presence of water.
When a gaseous fuel is used in place of the solid
fuels for the fluidized bed combustion furnace in the above
embodiments, the gaseous fuel can be blown into the fluidized
bed. A fuel oil can also be blown into the fluidized bed
without atomization. Pulverized coal can be similarly used
as a fuel for the fluidized bed combustion furnace.
The heat resistant perforated plate can be a bar
grid wherein air is blown into the heating medium through
slits between bars.
Similar effects can be obtained using a heating
medium which is prepared by adding the above described mixture
whose particle size is less than 0.3 mm with 5 to 20~ by weight
of a seed material whose particle size is 0.3 to 1.2 mm and
pelletizing the final mixture.
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