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, or after adjusting the filtrate to pH 3-4, mixing a hydrochlorite with the filtrate, 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 (SO₂) and nitrogen oxides such as NO and No₂ (hereinafter referred to as "NO x ") in accordance with a wet lime process, SO₂ is absorbed by an absorbing slurry containing lime as shown in the formula (1) SO₂ + CaCO₃ --> CaSO₃ + CO₂ (1) to produce calcium sulfite, but a part of NO x is simultaneously absorbed by the absorbing slurry. However, absorbed SO₂ 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) CaSO₃ + ½02 --> CaSO₄ (2) and it is an important reaction in the wet lime process. As a result of the above-mentioned reaction, unoxidised sulfite ions remain, and the dissolution of an absorbent (lime stone) is hindered owing to these sulfite ions and the partial pressure of SO₂ rises, so that the performance of the desulfurization deteriorates. Therefore, in a conventional method for desulfurizing the exhaust smoke containing SO₂ 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. 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 SO₂ and NO x in accordance with a wet lime process, the aforesaid desulfurizing method being characterized by comprising the steps of drawing off a part of an absorbing slurry containing lime from an absorbing tower, adjusting the filtrate obtained by the solid-liquid separation of the drawn absorbing slurry to pH 3-4, mixing a hypochlorite with the filtrate, and then refeeding the mixture as a make-up water to the absorbing tower. It is known that similar problems have been resolved by the following patents: DE-3507690 A1 relates to a method for separating NO from exhaust gases by oxydation, producing NO₂, and using NaClO₂ as oxidation compound. DE-3230352 A1 discloses a method for separating SO₂ , HCl, HF and preferably NO x from exhaust gases, using sodium hypochlorite in a first stage, and then an alkaline washing water, in a second stage, for absorbing NO₂. In contrast, the object of the present invention is to use a hypochlorite to decompose N-S compounds. The present inventors have first investigated the N-S compounds produced in the method for desulfurizing the exhaust smoke containing SO₂ 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 HONHSO₃⁻ hydroxyamine disulfonate HON(SO₃)₂²⁻ and hydroxyamine trisultonate ON(SO₃)₃³⁻ . Furthermore, the technique for decomposing these compounds has been researched, and as a result, it has been found that if the pH of these compounds is adjusted to a level of 3 to 4 and a hypochlorite is then added thereto, the decomposition reaction is remarkably accelerated as shown in Fig. 2, but if the pH is in excess of 4, the decomposition rate noticeably deteriorates. Particularly, it can be easily presumed that the lower the pH value is, the higher the decomposition rate is. However, in the method for desulfurizing the exhaust smoke in accordance with the wet lime process, the consumption of lime stone which is one of the raw materials is inconveniently increased in order to lower the pH of the absorbing slurry of the absorbing slurry filtrate. Thus, the present inventors have arrived at a conclusion that in the decomposition of the N-S compounds, a pH range of from 3 to 4 is desirable. In addition, it has been selected that the hypochlorite is added to the solution containing no absorbent which is alkaline on the downstream side of a solid-liquid separation step and on the upstream side of an absorbent preparation step. As described above, according to the present invention, a bad influence on a desulfurization performance attributable to SO₂ 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 the present invention; and Fig. 2 is a graph showing a decomposition removal effect of the N-S compounds by the addition of a hypochlorite. Now, the present invention will be described in more detail in reference to an example thereof, 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 m³N/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 SO₂ concentration of 3.000 ppm and an NO x concentration of 1.000 ppm. The exhaust gas to be treated is led into an absorbing tower 2 through a line 1, and SO₂ 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 19 through a line 10. The pH of the filtrate in electrolytic tank 19 is adjusted to a predetermined range of 3-4 by adjusting an adjusting valve 21 having a pH regulator 20 to control the amount of an acid to be fed through a line 22. On the other hand, sodium hypochlorite which is a decomposer for N-S compounds is fed at 0.15 mol/liter to the electrolytic tank 19 through a line 23. 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 19 is delivered to a lime stone slurry preparation tank 16. The lime stone slurry preparation tank 16 is fed with a line stone powder through a line 17, and a lime stone slurry having a predetermined concentration is prepared. 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 SO₂ 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. The pH of the absorbing slurry in the electrolytic tank 19 was adjusted to 3.5. 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 except that the feed of sodium hypochlorite through a line 23 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 SO₂ concentration of an outlet gas was 320 ppm, which was much higher than in the example.

Claims: A method for desulfurizing an exhaust gas (1) containing SO₂ and NO x in accordance with a wet lime process, said desulfurizing method being characterized by comprising the steps of drawing off (7) a part of the absorbing slurry containing lime from an absorbing tower (4), adjusting (20, 21, 22) the filtrate obtained by the solid-liquid separation of the drawn absorbing slurry (7) to pH 3-4, mixing (23) a hypochlorite with the filtrate, and then refeeding (15) the mixture as a make-up water to the absorbing tower (4).

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