JPH10509914A - Method for separating gaseous pollutants from heat treatment gas - Google Patents

Abstract

(57) [Summary] In order to convert gaseous pollutants into separable dust, a particulate absorber that is reactive with gaseous pollutants in the gas is introduced into the heat treatment gas. Through a contact reactor (10). The heat treatment gas is then passed to a dust separator (5). Most of the dust separated in the dust separator (5) is passed through a mixer (11) in which mixing and wetting is performed. Recirculates as an absorbent by being introduced into Quicklime is added as a new absorbent. The dust is such that the total residence time of the quicklime in the wet state in the mixer is sufficient for the quicklime and the water supplied to the mixer to react almost completely and for slaked lime to be formed from the quicklime. , Stay in the mixer for a long time and recycle many times.

Description

The present invention relates to a method for separating gaseous pollutants , such as sulfur dioxide, from a heat-treating gas, such as a flue gas. The heat treatment gas is used to convert gaseous pollutants into separable dust, so that a particulate absorber that is reactive with gaseous pollutants is introduced into the heat treatment gas in a wet state. The heat treatment gas is further passed through a dust separator in which dust is separated from the heat treatment gas and the purified gas is discharged, and a part of the dust separated in the dust separator is The mixture is passed through a mixer in which water is supplied for wetting, and a part of the dust is introduced into the heat treatment gas together with an additional portion of the new absorbent, thereby absorbing the absorbent. As to how to recirculate To. The above-described methods for separating gaseous contaminants from heat treatment gases are known, for example, from SE-8504675-3 and SE-8904106-5. According to these two documents, granular slaked lime (calcium hydroxide) is preferably used as the new absorbent. The absorber is mixed with dust separated from the heat treatment gas in a dust separator, and the mixture is supplied with water so as to be introduced wet to the flue gas in the contact reactor. Slaked lime is relatively expensive, and various attempts have been made to use relatively inexpensive quick lime (calcium oxide) in place of such slaked lime. In these attempts, a plant must be used to digest quicklime, ie, a reaction between calcium oxide and water to form calcium hydroxide, before the quicklime is introduced into the purification process. Such lime digestion plants are expensive, which means that the replacement of slaked lime with quicklime does not have the expected benefits. It is an object of the present invention to provide a method for separating gaseous contaminants from a heat treatment gas, which can replace slaked lime with quick lime without the need for an expensive and separate plant for digesting quick lime. With the goal. According to the invention, such an object is a method of the kind mentioned in the introduction, in which quicklime is added as a new absorbent and most of the dust separated in the dust separator is fed to the mixer. Is supplied and discharged as a substantially continuous stream, and this dust is generated by the total residence time of the quicklime in the mixer in the wet state when the quicklime and the water supplied to the mixer almost completely react with each other, and from the quicklime. This is achieved by a process characterized by staying in the mixer for an extended period of time and recirculating it multiple times so that the slaked lime has a sufficient time to form. The new absorbent in the form of quicklime is preferably fed to the mixer, but may be added to a portion of the dust that is separated in the dust separator and fed to the mixer. Quick lime may also be introduced directly into the flue gas in the contact reactor. An air flow is preferably provided to the mixer to fluidize the mixed dust and thereby facilitate mixing. Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings. Here, the drawing schematically shows a plant for purifying flue gas from a coal-fired main heating plant, provided with equipment for carrying out the method according to the invention. is there. The drawing shows a plant for purifying flue gas from a coal-fired main heating plant 1, said flue gas comprising dust such as fly ash and gaseous pollutants such as sulfur dioxide. I have. The preheating device 2 is arranged to transfer heat from the hot flue gas to the combustion air supplied to the main heating plant 1 by the fan 3 via the duct 2a. The hot flue gas is conveyed through a duct 4 to a dust separator 5. In the embodiment shown, the dust separator 5 is an electrostatic precipitator having three consecutive precipitator units that purify by passing flue gas. The flue gas thus purified is passed through a duct 6 to a flue gas fan 7, which passes through a duct 8 to a chimney 9 so that the flue gas is released to the atmosphere. Supply flue gas. The dust separator may also be a bag filter. The duct 4 has a vertical section forming a contact reactor 10. The mixer 11 is in communication with the lower part of the contact reactor 10. The mixer 11 introduces the particulate absorbent, which is reactive with the gaseous pollutants in the flue gas, into the lower part of the contact reactor 10 in a moist state. This absorber converts gaseous pollutants into dust that is separated in the dust collector 5. The dust particles separated in the dust collector 5 are collected in the hopper 12 of the dust collector unit. Most of the collected dust particles are recirculated in the system in a manner described in detail below. The remainder of the collected dust particles is transported outside, for example, by a worm conveyor, which will not be described in detail. The mixer 11 is of the type described in SE-9404104-3. Accordingly, the mixer 11 has an essentially double-bottom box shape. The upper one of these two bottoms is made of a tensioned polyester fluidizing cloth. Between the two bottoms, the granular absorbent in the mixer 11 is passed through the air supply pipe 15. A chamber 14 into which air for fluidization is introduced is provided. The mixer 11 is supplied with water through a water supply pipe 16 and a nozzle 17 arranged in the upper part of the mixer. The granular material to be mixed is supplied to the mixer 11 through two material inlets 18 and 19 at the inlet end of the mixer. The mixer 11 further comprises two parallel agitators (only one of which is shown in the drawing) used in cooperation, each having a horizontal shaft and a plurality of inclined elliptical plates attached thereto. And a mechanical mixing mechanism 20 composed of an agitator having The outlet end of the mixer 11 extends to the contact reactor 10 in order to continuously supply the well-mixed and wet absorbent from the outlet 21 to the contact reactor 10. A portion of the dust particles collected in the hopper 12 of the dust collector unit, which is recirculated in the system, is fed to the mixer 11 through an inlet 19. Granular quicklime (calcium oxide) is supplied to the mixer 11 through the inlet 18 for mixing with the dust particles supplied through the inlet 19. The mixture is wetted by the water supplied from the nozzle 17. The water supplied from the nozzle 17 is also for digesting quicklime supplied to the mixer 11. Due to the structure of the mechanical mixing mechanism 20 and the fluidization of the material particles supplied to the mixer 11, the mixer 11 is continuously supplied to the contact reactor 10 as an absorbent through the outlet 21 of the mixer 11. , Producing a uniformly moistened, homogeneous mixture. The residence time of the material particles in the mixer 11 is about 5 to 60 seconds, especially 10 to 20 seconds. The residence time of the material particles in the mixer 11 as described above (10 to 20 seconds), i.e. the time during which the lime particles are wet, allows the quicklime and the water added for digestion to react completely, And it is not enough time for slaked lime to be formed from quicklime. Such a reaction is relatively slow and takes several minutes. Now, the present invention will be described in more detail with reference to the following theoretical examples. In this example, points A, B and C in the drawing, namely in the duct 4 respectively in front of the mixer 11, in the reactor 10 behind the mixer 11 the inlet side of the dust collector 5, and in the dust collector 5 The conditions governing the position corresponding to the exit side of are shown. The dust at point A is mostly fly ash, whereas the dust at point B is fly ash and absorbent. At point D, 2930 kg of dust is discharged per unit time, of which 2,000 kg is fly ash. In this example, therefore, the particulate absorbent, including the quicklime to be digested, averages about 35% in the system before being discharged at point D. Thus, the total residence time of the absorbent in the mixer 11 sufficient to digest quicklime is 350 to 700 seconds, ie about 6 to 12 minutes. The total water consumption in the example described above is 3,366 (l / h), of which 152 (l / h) is required for lime digestion. When such an amount of water is consumed, the water content of the absorbent discharged from the mixer 11 is about 6%. However, the moisture content preferably varies between 2 and 15% depending on the composition of the mixture. In the above example, if the fly ash content of the flue gas at point A is 0, that is, the emission at point D is 930 (kg / h), by analogy with the above case, the circulation Number is about 110 The interval is between 1,100 and 2,200 seconds, or about 18-37 minutes.

────────────────────────────────────────────────── ─── Continuation of front page    (81) Designated countries EP (AT, BE, CH, DE, DK, ES, FR, GB, GR, IE, IT, LU, M C, NL, PT, SE), OA (BF, BJ, CF, CG , CI, CM, GA, GN, ML, MR, NE, SN, TD, TG), AP (KE, LS, MW, SD, SZ, U G), AL, AM, AT, AU, BB, BG, BR, B Y, CA, CH, CN, CZ, DE, DK, EE, ES , FI, GB, GE, HU, IS, JP, KE, KG, KP, KR, KZ, LK, LR, LT, LU, LV, M D, MG, MK, MN, MW, MX, NO, NZ, PL , PT, RO, RU, SD, SE, SG, SI, SK, TJ, TM, TT, UA, US, UZ, VN

Claims (1)

[Claims]   1. Gaseous pollutants such as sulfur dioxide from heat-treated gases such as flue gas The heat treatment gas converts gaseous contaminants into separable dust. The particulate absorbent, which is reactive with gaseous pollutants, Through the contact reactor (10) which is introduced into the heat treatment gas, wherein the heat treatment gas further comprises Inside, dust is separated from the heat treatment gas and the purified gas is discharged. Part of the dust passed through the dust separator (5), and separated in the dust separator (5), A mixer (1) in which mixing is performed and water is supplied for wetting. 1), and part of this dust is treated with heat treatment gas together with the addition of new absorbent. In the method of recirculating as absorbent by being introduced into Most of the dust added as a new absorbent and separated in the dust separator (5) Is supplied and discharged as a substantially continuous stream to the mixer (11). The total residence time of the quicklime in the wet state in the kisar (11) is The water supplied to the sir reacts almost completely, and slaked lime is formed from quicklime. To stay in the mixer for a long time, and A method characterized by recirculation across.   2. The new absorbent in the form of quicklime is supplied to the mixer (11). The method of claim 1, wherein:   3. The new absorbent in the form of quicklime is separated in the dust separator (5) and 2. The method according to claim 1, wherein the dust is added to a part of the dust supplied to the mixer. The method described.   4. A new absorbent in the form of quicklime is the flue gas in the contact reactor (10). The method according to claim 1, wherein the method is introduced directly into the device.   5. Air flow is supplied to the mixer (11) to fluidize the mixed dust 5. The method according to claim 1, wherein the method is performed.