JPS6135827A - Purification of exhaust gas by dry lime method - Google Patents

Abstract

PURPOSE:To reactivate deactivated lime particles, in a method for purifying exhaust gas by a dry lime process, by hydrating lime particles deactivated by the shells formed to the surfaces thereof by the chemical reaction with SOX, HCl or HF. CONSTITUTION:In a method for purifying exhaust gas by recovering and removing acidic harmful substances from exhaust gas by spraying fine lime particles in said exhaust gas, the particle group recovered by a dust collector 2 is guided to a classifier 4 to be classified into a coarse powder group with a large particle size containing fly ash and a fine powder group with a small particle size containing lime particles having shells comprising the compound with the aforementioned acidic org. harmful substances formed to the surfaces thereof and the lime particles of said fine powder group are hydrated in a hydration reactor 5 by steam and the shells of the lime particles are destructed and removed by volumetric expansion at the time of hydration reaction to prepare recycle lime particles wherein hydration reaction lime exposed to the surfaces thereof and said recycle particles are again supplied to exhaust gas.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for purifying exhaust gas using a dry lime method.

Structure of the conventional example and its problems High-temperature exhaust gas discharged from lime and heavy oil boilers contains sulfur oxides (SOx), HCl1. Acidic harmful substances such as HF are normally contained in a proportion of 10 to 1,000 ppm, and it is mandatory to remove these harmful substances as a pollution control measure.

BACKGROUND ART Conventionally, as a method for removing acidic harmful substances, a wet method has been commonly used in which exhaust gas is cleaned by bringing an absorption liquid or slurry containing an alkaline absorbent into direct contact with the exhaust gas at a lower temperature. However, while the wet method has a high removal rate of harmful substances, it is difficult to treat wastewater and requires reheating the exhaust gas.
There was a problem that equipment costs and operating costs were high.

In addition to the wet method, for example, the activated carbon adsorption method, in which harmful substances are adsorbed with activated carbon and then desorbed, and the semi-dry method, in which slaked lime slurry is sprayed into the exhaust gas, have been proposed, but both methods achieve high removal rates. I couldn't.

In addition, there is a dry method in which lime is directly dispersed in a high-temperature furnace or flue to remove acidic harmful substances, but the reaction rate of the absorbent lime (slaked lime, quicklime) with SOx is about 20%. Therefore, it was not put into practical use except in cases where environmental regulations were extremely lax.

In addition, the present inventor has previously proposed a method for improving the reaction rate of lime by reducing the particle size of lime and devising a method for dispersing lime particles into exhaust gas (Japanese Patent Application No. 72246/1989).
proposed that the reaction rate of lime with SOx was increased to 60% by Jin's method.

Next, lime particles become SOx in the high-temperature exhaust gas.

The process of reacting with acidic harmful substances such as HCl and HF and removing these harmful substances will be explained.

The lime particles mentioned in the present invention mean slaked lime whose main component is Ca(OB)g and quicklime whose main component is CaO, limestone whose main component is CaC01, Ca(OH
)2*Ca CO@ decomposes to produce CaO, so all of these are ultimately equivalent to CaO.

Ca (OH)2 →CaO+ HIO, Ca Ca2
-CaO+C02 This CaO reacts with SOx, HCII, HF, etc. as shown in the following formula to form CaS04, Ca(J2
, which generates CaF2, CaO+502 + 02-CaSO4CaO+so8
→CaSO4 CaO12HCA' -CaC1g +H2OCaO+
2HF 4 CaF2 + H2OCaO particles and SOx
The reaction with , HCl, and HF first occurs on the surface of the CaO particles, and as a result, CaSO4*C is deposited on the particle surface.
aC111! , producing a thin shell of CaF2. The subsequent reaction occurs when SOx, HCI, and HF diffuse into the interior of the particles, but the dense shells formed on the surfaces of CaO particles prevent SOx and the like from diffusing into the interior of the CaO particles. Therefore, S on the surface of CaO particles
When a shell of Ox, etc. is generated, the reaction between CaO and SOx, etc. decreases rapidly. Additionally, since the residence time of CaO particles in a furnace, boiler, or high-temperature flue is normally 2" to 4 seconds, the thickness of the shell formed on the surface of CaO particles within this residence time is approximately 11 chrome. It is thought that for this reason,
In practical terms, the reaction rate (utilization rate) of CaO particles with SOx was limited to 60%.As a way to increase the reaction rate of CaO particles, Mr. Ishihara et al.
, CaCA'l * Water is added to the CaO particles, which have become chemically inert due to being covered with a CaF4 compound, to hydrate them, and the volume expansion that occurs during the hydration reaction destroys the surface shell. Succeeded in obtaining chemical activity comparable to that of CaO particles whose surface is not covered with an inert shell (US Patent Registration No. 8481289 (1969)) @L'
However, the method invented by Mr. Ishihara et al. hydrates CaO particles with water, that is, it is a wet method, and the lime particles after treatment contain a lot of water, and each lime particle aggregates and becomes coarse. These wet coarse particles cannot be sprayed into the exhaust gas as they are, but in order to be applied to the dry process, they have to undergo processing steps such as drying, pulverization, and classification to form microscopic and custom-sized particles. The problem is that lime particles must be produced.

Purpose of the Invention In order to solve the above problems, the present invention provides SOx on the surface.
- Lime particles that have become chemically inert due to the formation of shells through chemical reactions with HCl e HF, etc. are hydrated with water vapor, and the hydration reaction creates lime particles in which unreacted lime is exposed. The object of the present invention is to provide a method for purifying exhaust gas using a dry lime method in which the reaction rate of lime is improved by resupplying the particles as recycled lime particles into the exhaust gas.

Structure of the Invention In order to achieve the above object, the present invention provides an exhaust gas purification method that collects and removes acidic harmful substances by spraying fine particles of lime into exhaust gas containing acidic harmful substances and causing the acidic harmful substances and lime to react. In the method, the particles collected by the dust collector are led to a classifier to separate large-diameter coarse particles containing fly ash and lime particles on the surface of which a shell consisting of a compound with the acidic harmful substance is formed. It is classified into a small diameter fine powder group containing
The lime particles of the fine powder group are hydrated with steam, and the volume expansion of the lime during this hydration reaction destroys and removes the shells on the surface of the lime particles, exposing unreacted lime on the surface.

The regenerated lime particles thus obtained are configured to be re-supplied into the exhaust gas. That is, the present invention uses a classifier to separate lime particles with a shell made of a compound with an acidic harmful substance on the surface from the particles in the exhaust gas, and then hydrates the lime particles with steam to destroy the surface shell. to expose unreacted lime and reactivate the lime particles. By reusing the reactivated lime particles as an acid gas absorbent, the reaction rate of lime can be improved to 70 to 80%.

Embodiment and Function An embodiment of the present invention will be described below with reference to the drawings. The drawing shows the flow of the exhaust gas purification method using the dry lime method according to the present invention. Among them, SOx in exhaust gas, HCI,
Lime particles react with acidic harmful substances such as HF. (2)
is a dust collector for collecting particle groups from the exhaust gas containing various particle groups sent from the furnace (1), and the exhaust gas from which the particle groups have been removed by this dust collector (2) is sent to the chimney (3). ) etc. into the atmosphere.

(4) is a classifier that classifies the particles collected by the dust collector (2) into large-diameter coarse particles and small-diameter fine particles; (5) is a hydration reactor; Among the small-diameter fine particles classified by the classifier (4), lime particles with unreacted portions remaining are hydrated with high-temperature superheated steam.

Next, each process of the purification method according to the present invention will be explained.

High-temperature exhaust gas containing acidic harmful substances such as SOx, HCI, HF, etc. comes into contact with lime particles in the furnace (high-temperature flue) (l), and CaO and SOx, HCI, HF, etc. combine to form CaSO4゜CaC4go CaF2, etc. is generated. After that, the high-temperature exhaust gas is led to the dust collector (2), and the particle groups are collected from the gas.The zero particle group is collected by the classifier (
In 3), it is classified into a large-diameter coarse powder group containing fly ash and a small-diameter fine powder group. This fine powder group includes lime particles with shells formed on the surface made of compounds with acidic harmful substances and unreacted lime remaining inside. In the case of heavy oil-fired boilers, the amount of fly ash is small compared to the amount of unreacted lime, so it is not a problem, but in coal boilers and garbage incinerators, the amount of fly ash is many times the amount of unreacted lime. It is preferable to handle the fly ash and unreacted lime separately.

In the case of coal boilers, fly ash is coarse and has an average particle size of about 16 microns even when fine, while unreacted lime has an average particle size of 2 to 8 microns, so these are separated using a wind classifier. It is possible. Experiments have shown that it is possible to separate fly ash containing about 10% unreacted lime particles and unreacted lime particles containing about 10% fly ash. However, when the amount of fly ash generated is small, such as in a heavy oil-fired boiler, the classification step is not necessarily necessary.

The exhaust gas from which the particles have been removed is released into the atmosphere from the chimney (3).

The fine powder group containing lime particles containing unreacted lime that has been classified and separated by the classifier (4) is led to the hydration reactor (5).

Hydration reaction! (6), the fine powder group is 150℃~8OO℃
Unreacted lime is instantaneously hydrated by being dispersed in superheated steam at a temperature range of 250°C, preferably WOO°C to 250°C. Then, Ca5o4t CaC1t e on the surface
Lime particles with shells of CaF2, etc., break their shells due to the volume expansion of unreacted lime inside them during hydration, so lime particles with unreacted lime exposed on the surface (regenerated lime particles) ).

Regarding the hydration treatment of lime particles, it is possible to treat them in high-speed superheated steam using a device such as a jet mill, and in the case of lime particles, the pulverizing action of the jet mill and the volume expansion due to the hydration reaction are considered. In combination, the shells on the surface of the lime particles are easily destroyed, and the lime particles with unreacted lime exposed on the surface (in the exhaust gas) can be quickly dispersed. Note that for hydration, high-temperature gas containing steam can be used instead of superheated steam.

Lime particles with unreacted lime exposed on the surface are led into the furnace (high-temperature flue) (1) as cycle lime particles together with freshly supplied lime particles, and the S in the high-temperature exhaust gas is
After being used to collect acidic hazardous substances such as Ox, HCA', and HF, they are collected in a dust collector (2) along with fly ash, etc.
, and classified by the classifier (4) as described above, or
Or it will be discarded.

Exhaust gas purification using the dry lime method is usually carried out at temperatures between 800℃ and 1
It is carried out in two temperature ranges: 20G'C or 150℃ to 400℃, but in the case of the temperature range of 800 @ OA to 1200℃, limestone, slaked lime, and quicklime are used as lime, and these limes are acidic harmful substances. It is thought that at the time of the reaction, all of the lime is converted into quicklime by thermal decomposition. 150" In the temperature range from 0 to 400 degrees Celsius, slaked lime and quicklime are used as lime; below 800 degrees Celsius, it is thought to react with acidic harmful substances mainly in the form of slaked lime, and above 800 degrees Celsius, mainly in the form of quicklime. In the present invention, the exhaust gas is purified using superheated water in a temperature range of 800° C. where slaked lime turns into quicklime.

Using a coal combustion test furnace, we conducted an experiment on exhaust gas purification using the dry coal method according to the flow shown in the drawing. The exhaust gas conditions in the test furnace used in the experiment are as follows. - SOx concentration in exhaust gas: 740 ppm (standard for bag filter output) Temperature at lime injection section: 1070"C Residence time in effective temperature range of desulfurization reaction (800°C to 1050°C) = 8 seconds [Experiment 1] Above Lime particles ls, s kg/h with an average particle size of 1.8 microns are conveyed in the exhaust gas under the conditions with air at 5 oNrz/h, and transported at a speed of 200 to 800 m/h from an injection port provided in the furnace wall. It was injected with.

As a result, the SOx concentration at the bag filter outlet of the dust collector was 78 ppm, and the desulfurization rate was 90%.
The molar ratio of SOx is 2. It was Q.

In addition, the amount of soot and dust collected from the bag filter during the 1O experiment was 847 kg.

This was classified using an air classifier to obtain 220 kg of coarse powder with an average particle size of 20 microns and 127 kg of fine powder with an average particle size of 2.2 microns. As a result of chemical analysis, the fine powder group showed Ca
SO4, CaO, and CaC01l are 570% and 2, respectively.
5.8%, 5.1%, S i 02 @ A40B
, MgO and others were 12.1%.

[Experiment 2] The fine powder collected in Experiment 1 was injected into the furnace as a desulfurizing agent into the exhaust gas under the same conditions as Experiment 1. The supply rate of the fine powder group was set at 25.8 kg/h so that the Cab/SOx molar ratio of the active ingredients (CaO, CaC06) was 2.0.

25, gkg/h of fine powder heated to 450℃ at a pressure of 6k
A jet 'i pL4 is guided along with 1iteOkg/h of superheated steam of g/CdG to reactivate lime particles and at a rate of 200 to 800 m/sic from the injection port provided on the furnace wall.
was injected into the furnace at a speed of

As a result, the SOx concentration at the bag filter outlet of the dust collector was 75 ppm, and the desulfurization rate was 89%.

The amount of soot and dust collected from the bag filter during the 4-hour experiment was 209 kg. This was separated into a coarse powder group with an average particle size of 20 microns and a coarse powder group with an average particle size of 2.8
120 kg of micron fine powder was obtained. Chemical analysis of the obtained fine powder group revealed that Ca50.4, CaOe C
aC0g is 67.6%, 11.0%, and 2.2%, respectively.
So, S J Os eA40B -MgO and others are 19
．． It was 2%.

Experiments 1 and 2 revealed that when unreacted lime particles are treated with superheated steam, a new desulfurization ability comparable to that of lime particles can be obtained.

As described in detail, the present invention has the advantage that acidic harmful substances contained in exhaust gas can be efficiently removed with a small amount of lime consumption.

[Brief explanation of drawings]

The drawings are diagrams showing the flow of the exhaust gas purification method using the dry lime method according to the present invention. (1) Furnace (high temperature flue), (2) Dust collector, (4) Classifier, (5) Hydration reactor

Claims (1)

[Claims] 1. Particle groups collected by a dust collector in an exhaust gas purification method that collects and removes acidic harmful substances by spraying lime particles into exhaust gas containing acidic harmful substances and causing the acidic harmful substances and lime to react. is introduced into a classifier and classified into a large-diameter coarse powder group containing fly ash and a small-diameter fine powder group containing lime particles on the surface of which a shell consisting of a compound with the acidic harmful substance is formed. Lime particles in the fine powder group are hydrated with water vapor, and the volumetric expansion of the lime during this hydration reaction destroys and removes the shells on the surface of the lime particles to create recycled lime particles with unreacted lime exposed on the surface. A method for purifying exhaust gas by a dry lime method, characterized in that lime particles are resupplied into the exhaust gas.