GB2163968A - Method for the treatment of exhaust gases - Google Patents

Abstract

A method for the wet treatment of combustion exhaust gases comprising SO2 and HCl, the method comprising detecting an amount of HCl in the exhaust gas and feeding, to an exhaust gas treating tower, a sodium salt in an amount corresponding to at least the detected amount of HCl so that all the HCl is able to be converted into NaCl in the exhaust gas treating tower, along with CaCO3 or Ca(OH)2 used as an SO2 absorbent.

Description

SPECIFICATION Method for the treatment of exhaust gases This invention relates to a wet treatment of exhaust gases comprising SO2 and HCI such as exhaust gases from coal firing, heavy oil combustion exhaust gases and the like.

Flue gas desulfurization apparatus using wet lime and gypsum techniques are known in which limestone or slaked lime is used as an absorbent to remove SO2 from exhaust gases and collect the resulting gypsum as a by-product. Such apparatus have been widely utilized for the treatment of exhaust gases from oilfired boilers. As is well known, an increasing number of boilers have recently made use of coal in Japan and, in some instances, heavy oil is also used for these purposes. However, exhaust gases generated by combustion of coal or heavy oil may contain larger amounts of dust, SO2, HCI, HF and the like than exhaust gases from oil firing.Especially, when exhaust gases comprising SO2 and HCI in large amounts are treated with an absorbent such as CaCO3 or Ca)OH)2, SO2 reacts with CaCO3 or Ca(OH)2 to produce calcium sulfite and at the same time, HCI reacts with CaCO3 or Ca(OH)2 to produce CaCI2 which has high solubility.

Dissolution of CaCI2 results in an increase of Ca2+ ions, leading to reduction in amount of dissolution of calcium sulfite, calcium sulfate (produced by oxidation of calcium sulfite with oxygen in the exhaust gas), calcium carbonate and calcium hydroxide. More particularly, as Ca2+ ions which are common ions for these compounds increase, anions of SO32#, 504% CO32- and OH- decrease, which in turn brings about an increase of the partial pressure of SO2 in an absorption slurry or a decrease in concentration of a dissolved absorbent.

As a consequence, the desulfurization rate lowers with an attendant disadvantage that because the solubility of calcium sulfate decreases, gypsum scales are apt to deposit.

HCI in the exhaust gas is collected as CaCI2 by reaction with CaCO3 or Ca(OH)2 used as the absorbent.

While SO2 is absorbed, oxidized, removed by filtration in the form of secondarily produced gypsum crystals and withdrawn to outside, CaCI2 having great solubility is entrained in the filtrate after separation of the gypsum crystals. Almost all of the filtrate is circulated and used as a liquid for adjustment of the absorbent,# with the result that CaCI2 is accumulated in the slurry being circulated in the absorption tower, promoting the above disadvantage still more.

We have made studies on a method of preventing the adverse influence of CaCI2 and, as a result, found that when HCI in exhaust gases is fixed as NaCI, the disadvantages can be overcome. The present invention is accomplished based on the above finding. According to the invention, there is provided a method for the treatment of exhaust gases comprising SO2 and HCI which comprises detecting an amount of HCI in an exhaust gas, and feeding, to an exhaust gas treating tower, a sodium salt capable of producing NaCI by reaction with CaCI2, e.g. Na2SO4, Na2SO3, NaOH, Na2CO3, NaHCO3, NaHSO3 and the like, in an amount corresponding to the amount of HCI and CaCO3 or Ca(OH)2 used as an absorbent for S02.

By the method described above, all chlorides in a circulation slurry can be present as NaCI, so that deterioration of desulfurization performance and promotion in growth of gypsum scale can be suitably prevented for reasons described hereinafter. In carrying out the method of the invention, amounts of HCI and SO2 in an exhaust gas are detected and a sodium salt is added to the circulation slurry so that a molar concentration [ Na+ ] of sodium ions in the slurry being circulated in the absorption tower has an interrelation with a molar concentration [ Cl-i of chlorine ions according to the following equation (1) [Na+] a [ Cl- ] (1) and CaCI2 is all converted into NaCI according to the following formula (2) CaCI2 + Na2X# -, 2NaCI + CaX (2) in which X represents an anion.

Wet methods of treating exhaust gases in which calcium and sodium compounds are used in combination so as to absorb SO2 therewith are known, for example, in Japanese Patent Nos. 894725 and 903276 and Japanese Laid-open Patent Application Nos. 53-129167, 55-124530, 56-65615 and 51-97597. However, there has never been known a method in which a sodium salt is fed in an amount corresponding to an amount of HCI, which is one of important features of the present invention, while absorbing SO2 with CaCO3 or Ca(OH)2.

In Japanese Laid-open Patent Application No. 53-17565, there is described a method of treating exhaust gases comprising SOP and HCI in which a magnesium compound is fed in an amount corresponding to an amount of HCI. Subsequent studies revealed that this method involved the disadvantage in that the magnesium compound reacted with HCI and the resulting MgCl2 did not remain dissolved stably. This is because if Ca(OH)2 or CaCO3 used as an absorbent for SO2 remained in excess, the pH of the slurry being circulated in the absorption tower, would become high, with the attendant disadvantage that Mg Cl2 would decompose into Mg(OH)2 in the form of a precipitate with formation of soluble CaCI2.In addition, when the pH of the slurry reaches about 8 under conditions where small amounts of various cations dissolved in the circulation slurry coexist, part of Mg2+ ions start to precipitate, so that even with a slurry in which Ca(OH)2 or CaCO3 are not left in so large amounts, MgCl2 unfavorably decomposes.

In the practice of the invention, HCI is converted into stably dissolved NaCI and thus the above disadvantages can be overcome.

Objects, features and advantages of the present invention will become apparent from the following detailed description with reference to the accompanying drawing.

In the drawing, a sole figure is a flowchart of a pilot plant used to effect a test for examples and comparative example.

The data of examples according to the method of the invention and comparative example is obtained using a pilot plant shown in the sole figure.

In the figure, an exhaust gas 1 from coal firing is passed to a flue 2 having a SO2 and HCI concentration detector 3. By the detector3 are determined amounts of SO2 and HCI being charged into an absorption tower 4. A sodium compound from a line 5 is added to the absorption tower 4 in an amount corresponding to the amount of HCI and CaCO3 or Ca(OH)2 absorbent is added from a line 6 to the absorption tower 4 in an amount corresponding to the amount of SO2. A slurry in a tank 7 for the absorption tower is circulated through the absorption tower by means of a circulation pump 8. In order to control a concentration of the slurry, makeup water may be added from a line 9.

The calcium and sodium compounds being added for absorption of SO2 and HCI are subjected to reaction in the absorption tower tank 7 to form crystals of calcium sulfite and calcium sulfate. On the other hand, NaCI is produced as a dissolved component From the standpoint of material balance, the slurry is withdrawn from the tank 7 through a pump 10 to a separator 11. In the separator 11 the crystals of the calcium compound are withdrawn from a line 12 and the resulting filtrate is fed through a line 13 to an absorbent tank 14,to which CaCo3 or Ca(OH)2 serving as an SO2 absorbent is fed from a line 15to prepare an absorbent slurry, followed by passing from a pump 16through the line 6 to the absorption tower 4.

On the other hand, a purified gas 17 is discharged from the absorption tower 4 and released into the air through a de-mister (not shown) or a gas heater (not shown).

Comparative example This comparative example illustrates a prior art method in which the line 5 was closed in order not to feed a sodium compound. Test conditions are indicated in Table 1 below.

TABLE 1 Pilot plant test conditions Absorbent Cacao3 Flow rate of a gas being treated 2,000 Nm3/H (on dry basis) Concentration of SO2 in a gas at 900 ppm (on dry basis) inlet of absorption tower Concentration of HCI in a gas at 40 ppm (on dry basis) inlet of absorption tower Exhaust gas source combustion exhaust gas of finely divided coal Liquid-gas ratio in absorption tower 17.7 I/Nm3 The slurry composition in the absorption towertank in the stationary state and the composition of an - exhaust gas at the outlet of the absorption tower are, respectively, indicated in Tables 2 and 3.

TABLE 2 Slurry composition in the tank of the absorption tower CaSO4.2H2O 0.80 mol/l CaSO3.1/2H2O 0.10 molIl CaCO3 0.20 molll CaCI2 0.10 mol/l pH 5.5 TABLE 3 Gas composition at the outlet of the absorption tower Concentration of SO2 at the outlet of absorption tower 100 ppm Concentration of HCI at the outlet of absorption tower below 1 ppm After operation of the test plant over 280 hours, it was found that gypsum scale deposited on and locally clogged the filler in the absorption tower.

Example 1 According to the method of the invention, the line 5 was opened, through which NaOH was fed as a sodium compound to the absorption tower in an amount corresponding to an amount of HCI being passed into the tower, by which Na+ and Cm~ ions dissolved in the tank were controlled to satisfy the equation (1).

The test conditions other than the feed of NaOH were the same as those indicated in Table 1. The slurry composition in the tank in the stationary state and the gas composition at the outlet of the absorption tower are, respectively, indicated in Tables 4 and 5.

TABLE 4 Slurry composition in tank of absorption tower CaSO4.2H2O 1.0 molll CaSO3.1/2H2O 0.05 mol/l CaC03 0.05 mol/l NaCI 0.20 mol/l pH 5.5 TABLE 5 Gas composition at the outlet of the absorption tower Concentration of SO2 at the outlet of absorption tower 45 ppm Concentration of HCI at the outlet of absorption tower below 1 ppm As compared with the comparative example where CaCI2 is dissolved, better results are obtained: desulfurization performance is improved; the reactivity of CaC03 absorbent becomes high; and the remaining concentration of CaC03 is reduced.

At the time when the test plant was continuously operated over 300 hours, no growth of gypsum scale was found, which was significantly different from the case of Comparative Example.

Example 2 The general procedure of Example 1 was repeated using Na2CO3 instead of NaOH. Na+ and Cl- ions dissolved in the slurry of the tank were used to satisfy the equivalence relation in the form of NaCI with its concentration being the same as in Example 1. Thus, the disadvantage of Comparative Example 1 where Cl ions were dissolved a CaCI2 could be overcome.

Example 3 The general procedure of Example 1 was repeated using, instead of NaOH, Na2SO3, NaHC03, and Na2SO4, NaHSO3. As a result, it was found that the disadvantage involved in Comparative Example wherein CaCI2 was dissolved could be overcome. Cm~ and Na+ ions dissolved in the tank slurry satisfied the expression of [ Na# ] ~ [ Cl- ] with results similar to the results of Example 1.

Claims (2)

1. A method for the wet treatment of a combustion exhaust gas comprising SO2 and HCI, the method comprising detecting an amount of HCI in the exhaust gas, and feeding, to a circulating slurry in an exhaust gastreating tower, (a) a sodium salt in an amount corresponding to at least the detected amount of HCl so -that all the HCI is able to be converted into NaCI in the exhaust gas treating tower and (b) CaC03 or Ca(OH)2 serving as an absorbent for SO2.

2. The method according to Claim 1, wherein the sodium salt is added to the slurry in the absorptio#n tower in an amount sufficient to satisfy the following equation [ Na+ ] B # [ Cl-i in which [ Na+l represents a molar concentration of sodium ions, and [ Cl- ] represents a molar concentration of chlorine ions.