GB2164031A

GB2164031A - Treatment of exhaust gas

Info

Publication number: GB2164031A
Application number: GB08516824A
Authority: GB
Inventors: Naoharu Shinoda; Atsushi Tatani; Masakazu Onizuka; Susumu Okino; Taku Shimizu
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1984-07-03
Filing date: 1985-07-03
Publication date: 1986-03-12
Also published as: JPS6115719A; DE3523820C2; GB2164031B; DK299585A; GB8516824D0; DE3523820A1; GB8516416D0; DK299585D0; DK167102B1

Abstract

A method for treating an exhaust gas containing SO2, HCl and HF is characterized by feeding, to an exhaust gas treating tower, a stoichiometric amount of at least one of a sodium compound and a potassium compound which will become NaCl and KCl in the exhaust gas treating tower, and a calcium compound as an absorbent for SO2; and blowing air into an absorbing solution which will be brought into contact with the exhaust gas. Manganese ions may be present in the absorbent solution.

Description

SPECIFICATION Treatment of exhaust gas The present invention relates to a method for treating an exhaust gas, and more particularly to a method for a wet treatment of an exhaust gas containing SO2, HCI and HF such as a coal combustion exhaust gas.

When desulfurization of smoke is carried out by a prevalently known wet coal process, the resultant exhaust gas will contain harmful components HCI and HF in addition to SOx sometimes. One example of the coal combustion exhaust gas contains components of about 1,000 ppm of SOx, about 60 ppm of HCI and about 40 ppm of HF.

If such an exhaust gas is treated in a wet exhaust gas treating tower by the use of CaCO3 which is an absorbent for SO2, the following reactions will occur: CaCO2 + SO, < CaSO3 + CO2 (1) CaCO2 + 2HCI o CaCI2 + CO2 + H20 (2) CaCO3 + 2HF# CaF2 + CO2 + H20 (3) In this case, however, the reaction of the formula (2) above will predominate, and the dissolution of CaCO3,will be prevented owing to the existence of Ca2 from CaCD2 which will be produced in accordance with the reaction of the formula (2).As a result, the reaction of the formula (1) will be hindered and an SO2 absorbing capacity will decline, and additionally, CaSO4-2H2O which will be produced along with a desulfurization reaction in the exhaust gas treating tower will adhere, as scale, to the wall surfaces of a used apparatus, with the result that the exhaust gas treating apparatus will be disturbed in its operation.

In order to overcome these inconveniences, a method is considered to be effective in which in view of an amount of HCI, a proper amount of sodium sulfate Na2SO4 or calcium sulfate K2504 is added to the above-mentioned reaction system and the following reactions are utilized therein to remove CaCI2 in the form of CaSO4#2H2O: Na2SO4 + CaCI2 + aq. < 2NaCI + CaSO4-2H2O (4) K2504 + CaCI2 + aq. < 2KCI + CaSO4 2H2O (5) Incidentally, the former reaction is generally well known as the reaction by which gypsum CaSO4 2H2O is prepared from an Na2SO4 solution by adding CaCI2 thereto.

The compounds Na2SO4 and K2504 which are necessary for the reactions for removing CaCI2 can be prepared only by feeding a basic salt of sodium and a basic salt of potassium to the wet exhaust gas treating tower. One example of reactions regarding Na2CO2 is as follows: Na2CO3 + SO2 < Na2SO3 + CO2 (6) Na2SO3 + 1/2 O2 < Na2SO4 (7) The above-mentioned compound HF will be converted into CaF2 which is shown in the formula (3) mentioned above, but the latter compound CaF2 cannot be stabilized as a solid having little solubility.

This CaF2 compound will cause an aluminum component contained in a dust in the exhaust gas to dissolve, and an interaction of the resultant aluminum ions and fluorine ions will inhibit a dissolution of limestone. For the purpose of eliminating such an inconvenience, the method in which a basic salt of sodium is used has been suggested in Japanese patent Provisional Publication No. 167023/1980.

It can thus be supposed from the foregoing that the addition of a basic sodium salt, an amount of which depends upon amounts of HCI and HF, is effective for the treatment of the exhaust gas containing SO2, HCI and HF in order to overcome the above-mentioned inconveniences due to CaCI2, aluminum ions and fluorine ions.

The present inventors have found in the course of experiments on this treating method that when the basic sodium or potassium salt is added to the reaction system in an amount relative to that of HCI and when air is blown into an absorbing solution in the exhaust gas treating tower, not only an effect of eliminating the aforesaid disadvantages but also a noticeable improvement in the dissolving reaction rate of CaCO2 can be obtained.

The present invention has now been accomplished on the basis of the above-mentioned knowledge and is directed to a method for treating an exhaust gas containing SO2, HCI and possibly HF which is characterized by feeding, to an exhaust gas treating tower, a stoichiometric amount of at least one of a sodium compound and a potassium compound which will become NaCI and KCI in the exhaust gas treating tower, and a calcium compound as an absorbent for SO2; and blowing air into an absorbing solution which will be brought into contact with the exhaust gas. The invention can be applied to an exhaust gas which does not contain HF.

Now, the present invention will be described in detail as follows: If the sodium or potassium compound is not added to the system in an amount corresponding to that of HCI, the dissolved CaCI2 compound will be present therein as mentioned above, and consequently the absorbing capacity of the absorbent CaCO2 will become poor. In other words, the production of CaCI2 will cause the amount of dissolved Ca2 to increase and the latter compound will diminish the solubility of CaSO4.2H2O (gypsum) produced along with the desulfurization and the oxidation reaction, which fact will accelerate the growth of gypsum scale. Further, since in the absorbing solution containing CaCI2, a partial pressure of SO2 will become higher, its SO2 absorbing performance will deteriorate.

That is to say, in the absorbing solution containing CaCI2, the concentration of the calcium ions will increase, and in consequence the solubility of 5022 will fall, as understood from the dissolution equilibrium formula (Can') (SO32-' = Ksp. A decrease in the concentration of 5022 which is the dissolution component of an SO2 gas means a deterioration in the solubility of the SO, gas. In other words, in such an absorbing solution, the concentration of 5022 produced by absorbing the SO2 gas will reach a saturation level immediately, and thus the partial pressure of SO2 will easily be higher.Inversely, if the dissolved chloride is present owing to the addition of NaCI or KCI, a saturation concentration of 5022 will become higher, in other words, a dissolving power of the absorbing solution will become greater, so that the partial pressure of SO2 in the absorbing solution will be maintained at a lower level.

The blow of air into the absorbing solution in the exhaust gas treating tower was experimentally carried out, whereby interesting results were obtained. That is to say, when the amount of air to be blown into the absorbing solution was increased, a conversion rate of the absorbent CaCO2 and an absorption rate of SO2 were improved.

In order to confirm whether or not the effect of the air blow was brought about with the aid of the addition of the sodium compound or the potassium compound which was accomplished simultaneously with the air blow, experiments were carried out varying amounts of air to be blown into the absorbing solution under conditions that neither sodium compound nor potassium compound was added thereto and that Cl- in the absorbing solution was allowed to exist as CaCI2. According to the results of the experiments, the increase in the amount of air permits improving the conversion rate of CaCO2 and the absorption rate of SO2, but a calcium utilization rate was relatively lower as compared with the case where Cl- was present as NaCI or KCI.

Therefore, it is definite that Cl- in the absorbing solution is required to be present as NaCI or KCI.

The reason why the conversion rate of CaCO2 and the absorption rate of SO2 can be improved by blowing air into the absorbing solution is not understood clearly yet but can be presumed as follows: That is to say, when air is blown into the absorbing solution, HS03- which is a weak acid will be converted into HSO4- of a strong acid, and thus the dissolution reactivity of CaCO2 will be easy to recover, even though aluminum and fluorine ions are dissolvingly present in the absorbing solution. As a result, the absorption rate of SO2 as well as the reactivity of CaCO2 will also be improved.

The sequential reactions are as follows: (Reactions in the exhaust gas treating tower) SO2 (gas) + H20 < H2502 (8) H2502 < H + HSO3- (9) (Reactions in the absorbing solution tank) HSO3- + 1/2 02 (gas) e HSO4- (10) H+ + HSO4- + CaCO2 (solid) + aq.

CaSO4.2H20 + CO2 1i (11) (Reaction in the absorbing solution tank without aerating) 2HSO3- + Ca2+ + CaCO2 (solid) < 2CaSO3 1/2 H20 + CO2 12 (12) As be apparent from the above, by detecting an amount of HCI in the exhaust gas; feeding, to the exhaust gas treating tower, a stoichiometric amount of at least one of a sodium compound and a potassium compound which will become NaCI and KCI in the exhaust gas treating tower, together with a calcium compound as an absorbent for SO2; and blowing air into the absorbing solution which will be brought into contact with the exhaust gas, there can be solved inconveniences such as the deterioration in the SO2 absorption performance due to HCI and HF, and scale trouble caused by the deposition of CaSO4-2H2O and the diminution of the reactivity of the absorbent for SO2, and additionally, the conversion rate of the absorbent for SO2 and the absorption rate of SO, can also be heightened effectively.

Figure 1 diagrammatically shows one embodiment of the present invention; Figure 2 shows experimental data obtained in an example of the present invention in the form of a graph and indicates variations in concentrations of SO2 contained in a purified gas 6 with respect to flow rates of air to be- blown into an absorbing solution, in the case that Na2CO2 is added in an only sto ichiom-etic amount based on an amount of HCI and the case that no Na2CO2 is added; ; Figure-3 shows experimental data obtained in the example of the present invention in the form of a graph and indicates variations in concentrations of CaCO2 in the absorbing solution with respect to flow rates of air to be blown into the absorbing solution, in the case that Na2CO2 is added in an only stoichiometric amount based on an amount of HCI and the case that no Na2CO2 is added; and Figure 4 shows experimental data obtained in the example of the present invention in the form of a graph and indicates interrelations of concentrations of SO, in the purified gas 6 and concentrations of CaCO2 in the absorbing solution with respect to concentrations of Mn ions in the absorbing solution up to a level of about 400 mail, in the case that Na2CO2 is added in an only stoichiometric amount based on an amount of HCI and the case that a predetermined amount of air is blown into the absorbing solution.

By the use of an apparatus shown in Figure 1, a method of the present invention was put into practice.

In Figure 1, an exhaust gas 1 from a coal-fired boiler was guided to an exhaust gas treating tower 2 through a denitrating device, an electric dust collector and a heat exchanger (they were omitted in the drawing ).

At an inlet of the exhaust gas treating tower 2, a detector 3 was provided, and this element 3 detected that the exhaust gas 1 containing about 1,000 ppm of SO2, about 60 ppm of HCI and about 40 ppm of HF was introduced thereinto at a flow rate of about 4,000 Nm3/h.

The exhaust gas treating tower 2 was packed with grids, and an absorbing solution was sprayed from the top of the tower 2 at 60 m3/h via an absorbing solution circulating pump 4. At this time, SO2, HCI and HF in the exhaust gas were absorbed by the absorbing solution, and the exhaust gas was discharged as a purified gas 6 from the tower 2 through a mist eliminator 5.

According to detection results, the purified gas 6 contained about 100 ppm of SO?, but contents of HCI and HF were less than 1 ppm which was a detection lower limit value.

In view of an absorption amount of SO2, CaCO2 was fed to the tower 2 through a line 7 at about 17 kg/ h, and simultaneously Na2CO3 was fed thereto through a line 8 at a flow rate of not less than 0.52 kg/h which corresponded to a stoichiometric amount based on an absorption amount of HCI.

Into a tank 9 in a lower portion of the exhaust gas treating tower 2, air was blowr at about 20 m3N/h in order to oxidize a sulfite, which had been produced by absorbing SO2, to a sulfate.

The absorbing solution in the tank 9 was in the state of a suspension containing crystalline CaSO4-2H2O and some CaCO2 powder, and water balance was then adjusted by replenishing the absorbing solution with water so that a concentration of the slurry might become about 18% by weight.

Afterward, in order to discharge crystalline CaSO4-2H2O (gypsum) from the system by balancing the absorption of SO2, a part of the absorbing solution was delivered out to a separator 12 throughout a pump 11. In the separator 12, the produced gypsum was recovered as a by-product, and a part of the resultant filtrate was discharged therefrom through a line 14 and the remaining filtrate was forwarded back to the exhaust gas treating tower 2.

While the operation was continued in a steady state, chlorine ions were present in a concentration of about 280 mmol/l in the absorbing solution, but a concentration of sodium ions was never less than 280 mmol/l which was an equivalent of the chlorine ions. Further, according to detection results, a concentration of fluorine ions present in the absorbing solution did not exceed 5 mmol/l, and judging from the fact that the concentration of the fluorine ions was about 190 mmol/l (estimated value) at the time when all of the fluorine ions was dissolved therein, it could be understood that some fluorine ions were discharged from the system in the form of a solid of Cay2.

When Na2CO2 was fed in an amount of less than an absorption amount of HCI, an absorption performance of SO2 and a pH value of the absorbing solution were lowered, and the lowered SO2 absorption performance did not recover, though a feed rate of the absorbent CaCO2 for SO2 was increased. Moreover, when the feed of Na2CO2 was stopped, an absorption rate of SO2 and a conversion rate of CaCO2 were diminished remarkably, with the result that gypsum scale appeared extraordinarily in the exhaust gas treating tower.

As a sodium or potassium compound to be used in the present invention in addition to Na2CO2, any one is acceptable so long as it can produce NaCI or KCI by the reaction with HCI, and easily available chemical agents are now employed generally.

In the above-mentioned example, air was experimentally blown into the absorbing solution through an air nozzle 10 while an air amount was gradually increased, and obtained results were very interesting.

That is to say, as the amount of air to be blown into the absorbing solution was getting greater and greater, the conversion rate of CaCO2 and the absorption rate of SO2 were improved more and more.

Figure 2 shows a variation in the concentrations of SO2 contained in the purified gas 6 during the stepwise aeration over a change in a flow rate of air to be blown into the absorbing solution from 0 to about 120 m3N/h. In other words, Figure 2 sets forth data regarding the experiment in which the sodium compound was fed in an only stoichiometric amount corresponding to an amount of HCI. Further, in Figure 2, results in the case that no sodium compound was fed are also shown for comparison.

Additionally, in a similar experiment, there were sought variations of concentrations of the remaining CaCO2 in the absorbing solution with respect to the blowing amount of air, and obtained results are shown #in Figure 3, where the existence of the sodium compound is employed as a parameter, as in Figure 2. From the results of Figures 2 and 3, it is definite that as the amount of air to be blown into the absorbing solution is increased, the SO2 absorption performance and the CaCO2 conversion rate are correspondingly improved.

This performance improvement by the air blow can be obtained also in the experiment in which any sodium compound was not added, but if the SO2 absorption rate and the CaCO2 conversion rate are required up to the same level as in the case where the sodium compound was added, the blowing amount of air must be increased, which fact means a large consumption of a blowing power and thus a rise of operating costs.

Further, when no sodium compound was added, gypsum scale remarkably appeared on inside walls of the exhaust gas treating tower and the absorbing solution tank.

According to an experiment in which the amount of Mn ions in the absorbing solution was gradually increased by adding MnSO4 thereto in the above-mentioned example, it was found from the results that the SO, absorption rate and the CaCO3 conversion rate were improved more and more by increasing the concentration of the Mn ions in the absorbing solution on condition that the flow rate of air to be blown into the absorbing solution was constant In Figure 4, there are shown a variation in concentrations of SO2 in the purified gas 6 during adding MnSO4 stepwise to the absorbing solution in order to heighten the concentrations of the Mn ions from 0 to a level of about 400 mg/l, and a variation in concentrations of remaining CaCO3 in the absorbing solution.

As be apparent from the above, it could be confirmed that the SO, absorption performance and the absorbent conversion rate will be improved in treating the exhaust gas containing HF, HCI and SO2, if a sodium or potassium compound is fed to the absorbing solution in an amount corresponding to that of HCI, if a calcium compound is fed as an absorbent in an amount corresponding to that of SO2, and if air is blown into the absorbing solution to oxidize a sulfite therein to a sulfate.

In the example, the flow rate of air to be blown was within the range of 5 to 110 m2N/h, i.e., about 0.125 to 2.75% based on the flow rate of the exhaust gas on condition that the flow rate of the exhaust gas was 4,000 m3N/h and a concentration of SO2 at the inlet was 1,000 ppm. However, when the concentration of SO2 at the inlet is higher, the amount of the sulfite to be oxidized will increase, and thus the flow rate of air is to be adjusted in view of the amount of the sulfite.

The adjustment of the flow rate of blowing air in the present example can be most conveniently accomplished by successively measuring a concentration of the sulfite in the absorbing solution and blowing air so as to maintain the concentration of the sulfite at 10 mmol/l or less by the use of suitable means. That is to say, in the experiments in which the concentration of SO2 in the purified gas was within the range of 50 ppm or less in Figures 2 and 4, the concentration of the sulfite in the absorbing solution was 10 mmol/l or less. Therefore, it is desirable that the flow rate of blowing air is determined by detecting the concentration of the sulfite in the absorbing solution and increasing the flow rate of air so that the concentration the sulfite may be 10 mmol/l or less. Examples of effective manganese compounds to be added include MnSO4, MnOOH, MnO2 and MnCI2. It was found that manganese was effective in the present invention when in such a concentration as to bring about the oxidation-reduction reaction in the absorbing solution, and thus with regard to an anion of the manganese compound, any particular limitation is not necessary.

Claims

1. A method for treating an exhaust gas containing SO2, HCI and optionally HF which is characterized by feeding, to an exhaust gas treating tower, a stoichiometric amount of at least one of a sodium compound and a potassium compound which will become NaCI and KCI in said exhaust gas treating tower, and a calcium compound as an absorbent for SO2; and blowing air into an absorbing solution which will be brought into contact with said exhaust gas.

2. The method for treating an exhaust gas according to claim 1, wherein manganese ions are caused to coexist in a concentration of 400 mg/l or less in said absorbing solution.

3. The method for treating an exhaust gas according to claim 1 or 2, wherein a concentration of a sulfite in said absorbing solution is detected, and the flow rate of air is adjusted so that the concentration of the sulfite is 10 mmol/l or less.

4. A method of treating an exhaust gas containing SO2 HCI and HF, substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.