JP6004512B2 - Low temperature corrosion prevention method for waste heat boiler of waste treatment facility - Google Patents

Description

The present invention relates to a method of waste treatment facilities waste heat boiler in the stop of heat recovery from the combustion exhaust gas generated at, it prevents cold corrosion due to the chlorine content in the ash in the waste heat boiler.

  In a facility that burns and melts waste such as general waste and industrial waste, combustible gas generated is burned in the combustion chamber, and the high-temperature combustion exhaust gas discharged from the combustion chamber is guided to the waste heat boiler for heat recovery. (See Patent Document 1).

  FIG. 3 is a system diagram showing the entire exhaust gas treatment facility of the waste melting treatment facility. In FIG. 3, an exhaust gas treatment facility is connected to the waste melting furnace 21, and a combustion chamber 22, a waste heat boiler 23, an exhaust gas temperature controller 24, a dust collector 25, an induction blower 26, and a chimney 27 are sequentially arranged in the exhaust gas treatment facility. Has been placed.

  The combustible exhaust gas generated by the thermal decomposition of the waste in the waste melting furnace 21 is sent to the combustion chamber 22 for combustion. The high-temperature combustion exhaust gas generated by the combustion in the combustion chamber 22 is sent to the waste heat boiler 23 for heat recovery. The exhaust gas from the waste heat boiler 23 is cooled by the exhaust gas temperature controller 24 to adjust the exhaust gas temperature, and is introduced into the dust collector 25. In the exhaust gas introduced into the dust collector 25, a reaction aid such as slaked lime is blown in order to remove hydrogen chloride in the exhaust gas, and is collected in the dust collector 25. The exhaust gas after the dust collection discharged from the dust collector 25 is attracted by the induction blower 26 and discharged from the chimney 27.

  In general, when processing waste, the exhaust gas generated from the waste treatment facility contains chlorine in the waste, so when the exhaust gas is burned in the combustion chamber, hydrogen chloride gas is generated in the combustion exhaust gas. .

  Usually, in order to remove hydrogen chloride from combustion exhaust gas, as shown in Fig. 2, waste heat is recovered from the combustion exhaust gas, the temperature is lowered, and then an alkali agent such as slaked lime is added to collect calcium chloride together with ash. The method of removing with a vessel is taken.

  However, when recovering waste heat from a combustion exhaust gas containing hydrogen chloride before removing hydrogen chloride with a dust collector, ash containing chlorine (CL) gradually adheres to the waste heat boiler. To do. And, when the waste heat boiler is stopped, it is affected by moisture in the atmosphere, so that adhering ash absorbs moisture and low-temperature corrosion occurs, which requires periodic repairs.

  In Patent Document 1, hydrogen chloride in combustion exhaust gas obtained by burning combustible exhaust gas generated from a waste melting furnace is removed, and corrosion is prevented when heat is recovered from the combustion exhaust gas with a waste heat boiler. In the waste melting treatment apparatus provided with a device for burning the generated gas from the waste melting furnace in the combustion chamber and recovering waste heat from the combustion exhaust gas in order to recover the steam of the wastewater and perform high-efficiency power generation, A collection device for collecting the combustible ash from the gas generated in the waste melting furnace, and a device for kneading and granulating the combustible ash by adding an alkaline agent such as slaked lime are provided. A waste melting furnace-generated gas processing apparatus that burns in a secondary combustion furnace is disclosed.

  However, in the technique according to Patent Document 1, since an alkali agent is added to combustible ash and kneaded and granulated, it is supplied to the secondary combustion furnace in a large particle size. The removal rate was not sufficient.

JP-A-8-11001

  The waste heat boiler has a water-cooled wall structure. In heat recovery boilers, when the waste heat boiler is suspended during periodic inspections or annual inspections, the heat transfer tubes become below the dew point temperature, and the attached ash adhering to the heat transfer tubes Pipe thinning proceeds due to low-temperature corrosion caused by the chloride component (CL) in the attached ash due to moisture absorption and deliquescence.

Adhesive ash of the waste heat boiler has a chlorine component of about 10% by mass, and during operation, the form of metal (M) chloride (MCL ₂ , for example, PbCL ₂ , NaCL, ZnCL _2, etc.) Exists.

In this state, when the waste heat boiler is cooled during regular maintenance, it reacts with moisture in the atmosphere and corrodes the heat transfer tube in the following manner.
MCL ₂ + 2H ₂ O → 2HCL + M (OH) ₂
With this HCL, the steel (Fe) heat transfer tube is
_{2HCL + Fe → FeCL 2 + H} 2
According to the chemical formula, low temperature corrosion proceeds.

  If a tube group such as a superheater and an evaporator in the waste heat boiler is corroded, enormous labor and cost are required for replacement and repair.

Therefore, the present invention is a waste heat boiler that recovers heat from combustion exhaust gas in a waste treatment facility, and is a waste that suppresses low temperature corrosion caused by chloride components in the attached ash in a low temperature state of a heat transfer tube when the waste heat boiler is stopped. A method for preventing corrosion of a thermal boiler is provided.

The present invention relates to a method for preventing low-temperature corrosion of a heat transfer tube during a pause of a waste heat boiler that recovers heat from combustion exhaust gas of a waste treatment facility, and in the waste heat boiler, an alkaline chemical is added to the waste heat boiler at the time of startup and before shutdown. Sprayed evenly at the time of falling, the amount of alkaline chemicals blown is obtained by multiplying the amount of ash scattered into the waste heat boiler by the composition ratio of the chlorine component in the attached ash, and in the waste heat boiler at a low temperature when the waste heat boiler is at rest The alkaline agent is kept in contact with the chloride component in the adhering ash.

The said structure WHEREIN: Spraying of an alkaline chemical | medical agent is performed intermittently or continuously during a driving | operation of a waste heat boiler, It is characterized by the above-mentioned.

  In the present invention, by spraying an alkaline chemical in a waste heat boiler, (1) the chlorine concentration of the attached ash adhering to the waste heat boiler during operation can be reduced and made uniform. (2) By blowing the chemicals into contact with the adhering ash and the alkaline chemical, the adhering ash adhering to the waste heat boiler and the alkaline chemical are brought into contact and neutralized in the low temperature state when the waste heat boiler is stopped. This can be prevented, and replacement and repair of the tube group in the waste heat boiler can be reduced.

It is a schematic diagram showing an example of a waste heat boiler to which the present invention is applied. (A) is a boiler heat-transfer-tube adhesion ash sectional drawing at the time of intermittent blowing, (b) is a boiler heat-transfer-tube adhesion ash cross section in case the slaked lime of a prior art is not injected in a waste heat boiler. It is a distribution diagram showing the whole exhaust gas treatment equipment of a waste melting treatment equipment.

The present invention will be described with reference to the drawings. Examples of slaked lime powder are shown below, but as alkaline chemicals, oxides and carbonates of alkaline earth metals such as calcium (Ca) and magnesium (Mg) in addition to slaked lime (Ca (OH) ₂ ) are suitable. ing. Specifically, a compound or mixture containing CaO, Ca (OH) ₂ , CaCO ₃ , MgO, Mg (OH) ₂ , MgCO ₃ or the like is suitable. Further, NaOH, NaCO ₃ , sodium bicarbonate (NH (CO ₃ ) ₄ ), or the like can be used. Moreover, you may spray an alkaline chemical | medical agent not only as a powder but as a slurry.

  In FIG. 1, a waste heat boiler 2 is connected to the rear stage of a combustion chamber 1 that combusts combustible exhaust gas from a waste melting furnace, and the combustion exhaust gas discharged from the combustion chamber 1 is introduced into the waste heat boiler 2 to generate heat. Heat is recovered by exchange.

  The waste heat boiler 2 includes a first pass part 4, a second pass part 5 in which a large number of heat transfer tubes 3 are arranged in the vertical direction, a third pass part 6 and a fourth pass part 7 above the partition wall. Combustion exhaust gas having lower portions alternately communicated and introduced from the combustion chamber 1 to the first pass portion 4 of the waste heat boiler 2 flows into the second pass portion from the lower portion of the first pass portion, and the second pass portion. Flows into the third path portion from above, and the fourth path portion is exhausted from the lower portion of the third path portion. Exhaust gas from the fourth pass section is discharged from the chimney through an exhaust gas temperature controller, a dust collector, and an induction fan as in the conventional exhaust gas treatment facility shown in FIG.

  In the first to fourth pass portions 4 to 7, slaked lime is blown into the waste heat boiler 2 in order to reduce and equalize the chlorine concentration in the attached ash in the waste heat boiler 2 and suppress low temperature corrosion due to the attached ash. For this purpose, a slaked lime supply pipe 8 is connected.

  The slaked lime inlet 9 into the waste heat boiler is provided at a position where the slaked lime is uniformly sprayed, such as the upper part of the first pass part 4, the lower part of the second pass part 5, and the lower part of the fourth pass part 7. For the spraying of slaked lime, the slaked lime powder has a particle size that can be conveyed at the gas flow rate in the combustion boiler. For example, highly reactive slaked lime is used.

  The chemicals used to remove hydrogen chloride gas are generally JIS special products for industrial slaked lime, but currently there are special chemicals for acidic gases (hydrogen chloride, sulfur oxides) called highly reactive slaked lime. Distributed and sold. High-reactivity slaked lime with increased hydrogen chloride reactivity increases the specific surface area by increasing the pore volume, improves the reactivity, and the particle size is equal to or greater than that of conventional products. Table 1 shows the physical characteristics of highly reactive slaked lime and industrial slaked lime JIS special products.

  In addition, the 1st pass part-the 4th pass part of a combustion boiler may differ in the number of passes by the composition of a combustion boiler, and a heat exchanger tube group may be arranged horizontally.

The fly ash in the waste heat boiler can be assumed by the balance of the collected ash, and the components of adhering ash can be grasped by analysis, so the alkaline component: slaked lime Ca (OH) ₂ is blown into the furnace. The neutralization of the adhering ash is performed by this, and the corrosion of the water pipe is suppressed.

MCL ₂ + 2H ₂ O → 2HCL + 10M (OH) ₂ (acidification)
2HCL + Ca (OH) ₂ → CaCL ₂ + H ₂ O
The amount of slaked lime blown is equivalent to the CL component depending on the composition ratio of the CL component in the attached ash.

The amount of ash scattered in the waste heat boiler: 345 kg / h and the CL component amount is 10 mass% The amount of CL is 345 × 0.1 = 34.5 kg / h (958 mol / h).
Therefore, the necessary amount of slaked lime is 958/2 = 479 mol / h.
As the Ca, the blowing amount is 479 × 20 g / mol = 9.6 kg / h.

  In the slaked lime blowing method, slaked lime equivalent to the CL content in the fly ash to the waste heat boiler is blown continuously or intermittently into the waste heat boiler. The timing of blowing in is from the period before the end of the waste heat boiler to the start-up period. Suppresses low temperature corrosion by neutralizing by contacting with ash.

  In addition, FIG. 2B shows a cross section of the ash attached to the boiler heat transfer tube when the slaked lime of the prior art is not blown into the waste heat boiler. When slaked lime was not blown into the waste heat boiler, it was found that the hydrogen chloride concentration was higher at the boiler heat transfer tube interface than at the surface of the adhering ash and the hydrogen chloride concentration was higher (hydrogen chloride concentration at the boiler heat transfer tube interface). = 17.69%, hydrogen chloride concentration on the surface of the attached ash = 8.00%). From this knowledge, it was found that blowing slaked lime into the waste heat boiler is effective not only at the time of falling but also at the time of starting up and during operation.

  In Fig. 2 (a), when slaked lime is blown at the time of start-up and at the time of fall, and when slaked lime is blown intermittently during operation, normal adhering ash (= adhered ash layer) and slaked lime layer adhering to the boiler heat exchanger (= Layer adhering at the time of rising and falling) and diluted ash (layer in which slaked lime is mixed with normal adhering ash during operation) is formed.

Assuming that the heat recovery boiler is suspended, an exposure test was conducted on steel plates with ash attached.
1. Test material Apply real ash on steel plate, heat and cool, and observe surface condition.
Ash ... A: Waste heat boiler adhering ash B: A mixture of slaked lime with A (addition rate 20wt%)
Steel plate: Carbon steel (32 □ × 4.5t)
Sample components and pH are as shown in Table 2.

2. Test Procedure Repeat heating / wetting exposure.
(1) Set a steel plate on which A and B ash are deposited in a heating furnace, and heat and hold at 500 ° C. and 260 ° C. for 2 days. (2) After heating, leave the sample in an atmosphere of 60 to 70% humidity for 1 week ( 3) Set the sample again in the heating furnace and keep it heated at 500 ° C and 260 ° C for 2 days. (4) After heating, leave the sample in an atmosphere of 60-70% humidity for 1 week.

3. Test results Corrosion was remarkable in A ash (boiler adhesion ash), whereas corrosion was suppressed in B ash (slaked lime mixed ash).

  Table 3 shows the case where slaked lime is blown into the first to fourth pass portions of the waste heat boiler.

  The slaked lime is blown in for 24 hours at the start-up, every 24 hours during the operation for 24 hours, and the slaked lime is blown in for 24 hours before the waste heat boiler is cooled down. There wasn't.

1: Combustion chamber 2: Waste heat boiler 3: Heat transfer tube 4: 1st pass part 5: 2nd pass part 6: 3rd pass part 7: 4th pass part 8: Slaked lime supply piping 9: Inlet 21: Waste Melting furnace 22: Combustion chamber 23: Waste heat boiler 24: Exhaust gas temperature controller 25: Dust collector 26: Induction fan 27: Chimney

Claims (2)

In the method of preventing low-temperature corrosion of heat transfer tubes when a waste heat boiler that recovers heat from the flue gas of a waste treatment facility is at rest,
Alkaline chemicals are sprayed evenly into the waste heat boiler when the waste heat boiler is started up and before it is shut down, and the amount of alkaline chemicals blown into the amount of ash scattered into the waste heat boiler Multiplied by
Prevention of low-temperature corrosion of waste heat boilers in waste treatment facilities, wherein the alkaline chemicals are held in contact with chloride components in the adhering ash in the waste heat boiler in a low temperature state when the waste heat boiler is at rest Method.   2. The method for preventing low temperature corrosion of a waste heat boiler of a waste treatment facility according to claim 1, wherein spraying of the alkaline chemical is intermittently or continuously performed during operation of the waste heat boiler.

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