Method for desulfurizing exhaust smoke

Abstract: A method for desulfurizing an exhaust gas containing SO₂ and NO X in accordance with a wet lime process, the aforesaid desulfurizing method being characterized by comprising the steps of drawing a part of an absorbing slurry, subjecting a filtrate obtained by the solid-liquid separation of the drawn absorbing slurry to an electrolysis treatment in the presence of chlorine ions, and then refeeding the filtrate as a make-up water to an absorbing tower.

Description: The present invention relates to an improved method for desulfurizing an exhaust smoke. In desulfurizing an exhaust smoke containing sulfur dioxide (SO 2 ) and nitrogen oxides such as NO and NO 2 (hereinafter referred to as "NO x ") in accordance with a wet lime process, SO 2 is absorbed by an absorbing slurry containing lime as shown in the formula (1) SO 2 + CaCO 3 → CaSO 3 + CO 2 to produce calcium sulfite, but a part of NO x is simultaneously absorbed by the absorbing slurry. However, absorbed SO 2 and NO x are partially reacted in the absorbing slurry to produce nitrogen-sulfur compounds (hereinafter referred to as "N-S compounds"). These N-S compounds hinder an oxidation reaction for producing gypsum from calcium sulfite, and this oxidation reaction can be represented by the formula (2) CaSO 3 + ½02 → CaSO 4 and it is an important reaction in the wet lime process. As a result of the above-mentioned reaction, unoxidized sulfite ions remain, and the dissolution of an absorbent (lime stone) is hindered owing to these sulfite ions and the partial pressure of SO 2 rises, so that the performance of the desulfurization deteriorates. Therefore, in a conventional method for desulfurizing the exhaust smoke containing SO 2 and NO x in accordance with the wet lime process, the oxidation hindrance attributable to these N-S compounds has been relieved by adding an excessive amount of the absorbent or using an absorbing device having a large absorbing capacity. The document DE-A- 2306805 discloses a process in which instead of limestone, an alkaline hydroxide (e.g. calciumhydroxide) and an alkaline hypochlorite are used in a wet desulphuration process, whereby the alkaline hypochlorite is produced by electrolysis of an alkaline chloride solution. The alkaline hypochlorite may be NaOCl, and in this case NaOH is the alkaline hydroxide. The document WO-A-8901820 discloses a wet type desulphuration method for waste gas which is only concerned with adjusting the ratio of chloride ion concentration of the aqueous phase of the washing suspension to the chloride ion concentration of the emitted raw gas, to a value of about 10.000. This is done because, for a residual oxygen content in the raw gas of at least 2 vol %, the oxydation of SO 2 of the raw gases to form SO 4 2- in the washing tower occurs without additional oxydation air inside the washing tower. DE-A-3540973 (Hölter) discloses a wet process employing iron chelate salts to separate SO 2 and NO x from the raw gas. An object of the present invention is to provide a method for desulfurizing an exhaust smoke by which a bad influence attributable to N-S compounds present in an absorbing slurry can be excluded, the oxidation of calcium sulfite can be accelerated, and the high performance of desulfurization can be maintained. The present invention is directed to a method for desulfurizing an exhaust gas containing SO 2 and NO x in accordance with a wet lime process, the aforesaid desulfurizing method being characterized by comprising the steps of drawing a part of an absorbing slurry, subjecting a filtrate obtained by the solid-liquid separation of the drawn absorbing slurry to an electrolysis treatment in the presence of chlorine ions, and then refeeding the treated filtrate as a make-up water to an absorbing tower. JP-A-2211217 published 22.08.90 discloses a wet desulfurization method of exhaust gas, in which chlorine compounds in the absorption slurry are removed from the system by electrolytic decomposition into chlorine gas and hydrochloric acid gas, so that the rate of dissolution of lime stone, which is an ingredient for the absorbent, does not down. In the present invention chlorine compounds are added from the outside, as will be seen in the following, if chlorine ions are not present in the slurry, in order to decompose N-S compounds which causes degraded desulphurization performance. The present inventors have first investigated the N-S compounds produced in the method for desulfurizing the exhaust smoke containing SO 2 and NO x in accordance with the wet lime process, and in consequence, it has been elucidated that the following three substances are the main components of the N-S compounds: hydroxyamine monosulfonate HONHSO 3 - hydroxyamine disulfonate HON(SO 3 ) 2 2- and hydroxyamine trisulfonate ON(SO 3 ) 3 3- . Furthermore, the technique for decomposing these compounds has been researched, and as a result, it has been found that if these compounds are electrolyzed in the presence of chlorine ions, the decomposition reaction of these compounds can be remarkably accelerated as shown in Fig. 2. As described above, according to the present invention, a bad influence on a desulfurization performance attributable to SO 2 and NO x present in the exhaust gas can be excluded, whereby the desulfurization performance can be maintained at a high level. Fig. 1 is a schematic view illustrating an example of the present invention; and Fig. 2 is a graph showing a decomposition removal effect of N-S compounds by electrolysis in the presence of chlorine, the aforesaid N-S compounds being produced during carrying out a method for desulfurizing an exhaust smoke in accordance with a wet lime process. Now, the present invention will be described in more detail in reference to an example, but it should not be limited to this example. Example One example of the present invention will be described in reference to Fig. 1. An exhaust gas coming at 200 m 3 N/hour from a small-sized fine powder coal-fired device (not shown) was collected, and after the removal of dust, the exhaust gas was treated through a method for desulfurizing the exhaust smoke of the present invention by the use of an apparatus shown in Fig. 1. The exhaust gas to be treated had an SO 2 concentration of 3000 ppm and an NO x concentration of 1000 ppm. The exhaust gas to be treated is led into an absorbing tower 2 through a line 1, and SO 2 and NO x are partially removed from the gas therein and then discharged through a line 3. A slurry tank 4 for storing an absorbing slurry therein is provided under the absorbing tower 2, and the bottom portion of the slurry tank 4 is fed with air for oxidation through a line 6. The absorbing slurry is circulated to the upper portion of the absorbing tower 2 from the slurry tank 4 by means of an absorbing liquid pump 5. A part of the absorbing slurry is drawn through a line 7 and fed to a centrifugal separator 8. High-purity gypsum 9 is separated from the absorbing slurry by the centrifugal separator 8, and the resultant filtrate is fed to an electrolytic tank 11 through a line 10. Afterward, a chloride is fed through a line 14 to the electrolytic tank 11 so that the concentration of chlorine ions in the filtrate may be 0.1 mol/liter. However, in the case of a usual exhaust gas generated from a fine powder coal, hydrogen chloride is present in the gas, and most of the same is absorbed by the absorbing slurry simultaneously with desulfurization. As a result, hydrogen chloride exists in the form of chlorine ions in the absorbing slurry. In such a case, the addition of the chloride is not always necessary. In the electrolytic tank 11, a pair of electrodes 12 having a clearance of 1 cm is disposed, and DC electric power is fed thereto from a power source 13 so that a current density may be 0.26 A/cm 2 . At this time, the flow rate of the filtrate which flows through the line 10 is about 25 liter/hour. The filtrate drawn through a line 15 from the electrolytic tank 11 is delivered to a lime stone slurry preparation tank 16. The lime stone slurry preparation tank 16 is fed with a lime stone powder through a line 17, and a lime stone slurry having a predetermined concentration is prepared in this tank 16. Afterward, a certain amount of the lime stone slurry is fed to slurry tank 4 by means of a pump 18. Under the above-mentioned conditions, a steady operation was carried out. In this case, the concentration of SO 2 in an outlet gas was 120 ppm and the stable operation could be achieved, and the pH of the absorbing slurry in the slurry tank 4 was 5.6. Furthermore, according to analysis, the concentration of sulfite ions in the absorbing slurry was a detection limit (about 1 mmol/liter) or less, which means that the oxidation reaction of sulfurous acid proceeded sufficiently. In addition, any N-S compounds were not detected in the absorbing slurry. Comparative Example Operation was carried out by the use of the same devices and the same gas conditions as in the example except that the feed of DC electric power from a power source 13 was stopped. In this comparative example, after the start of the operation, N-S compounds were gradually accumulated in an absorbing slurry, with the result that sulfite ions also increased. In the steady state, concentrations of the N-S compounds and the sulfite ions were 25 mmol/liter and 4.0 mmol/liter, respectively. As a result, the pH of the absorbing slurry was 5.1, and the SO 2 concentration of an outlet gas was 320 ppm, which was much higher than in the example.

Claims: A method for desulfurizing an exhaust gas containing SO 2 and NO x in accordance with a wet lime process wherein SO 2 is absorbed by an absorbing slurry containing lime according to SO 2 + CaCO 3 → CaSO 3 + CO 2 , said desulfurizing method being characterized by comprising the steps of drawing off (7) a part of an absorbing slurry from an absorbing tower (4), subjecting a filtrate obtained by the solid-liquid separation of the drawn absorbing slurry to an electrolysis treatment (11, 12, 13) in the presence of chlorine ions, and then refeeding (15) the treated filtrate as a make-up water to the absorbing tower (4).

Inventors: INOUE KENJI, OKINO SUSUMU, ONIZUKA MASAKAZU, TAKASHINA TORU, UKAWA NACHIKO, INOUE, KENJI, OKINO, SUSUMU, ONIZUKA, MASAKAZU, TAKASHINA, TORU, UKAWA NAOHIKO, INOUE, KENJI, HIROSHIMA TECHNICAL INSTITUTE, OKINO, SUSUMU, HIROSHIMA TECHNICAL INSTITUTE, ONIZUKA, MASAKAZU, HIROSHIMA TECHNICAL INSTITUTE, TAKASHINA, TORU, HIROSHIMA TECHNICAL INSTITUTE, UKAWA NACHIKO, C/O HIROSHIMA TECHNICAL INSTITUTE
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1991
Main classification: B01D53
IPC codes: B01D53