CA1221674A - Desulfurizing method for combustion exhaust gases - Google Patents

Abstract

ABSTRACT
In the desulfurizing method for removing sulfurous ingredients from combustion exhaust gases by contacting the combustion exhaust gases with a desulfurizing agent as heating medium in the form of fluidized bed of a fluidized bed combustion furnace, successful desulfurizing results can be obtained by using as the desulfurizing agent a hardened material whose particle size is 0.7 to 2 mm, prepared by mixing 10 to 70% by weight of cement with limestone or dolomite and hardening by adding water. The improved method shows high desulfurizing efficiency and stability.

Description

~ ,~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 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.

~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 ~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.

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~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 ,, ~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.

~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 ~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 ~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 ~ , '~' '`

~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%.

.

122~674 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.
. -~

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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Claims (3)

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. A process of preparing a desulfurizing agent which comprises providing a mixture of cement and limestone or cement and dolomite whose particle size is less than 0.3 mm, adding 5 - 20% by weight of seed material having a particle size between 0.3 mm and 1.2 mm to said mixture, pelletizing the resulting mixture by adding water, and har-dening.

2. A desulfurizing method for removing at least one of H2S and SO2 from combustion exhaust gases comprising contacting said combustion exhaust gases with a desulfuriz-ing heating agent in a fluidized bed, said heating agent comprising a hardened material whose particle size is between 0.7 and 2 mm prepared by mixing 10 to 70% by weight of cement with limestone or dolomite as the balance and hardening the resulting mixture by adding water.

3. A desulfurizing method as claimed in claim 2, wherein said mixture, before hardening, has a particle size of less than 0.3 mm, and about 5 to 20% by weight of a seed material whose particle size is between 0.3 and 1.2 mm is added, and the resulting mixture is hardened by adding water.