Method for heavy metal enrichment of waste incineration fly ash based on front-end enhanced condensation and short-range enrichment

Abstract

The application discloses a method for heavy metal enrichment of waste incineration fly ash based on front-end enhanced condensation and short-range enrichment, which comprises the following steps: adding an alkali earth metal compound-containing material into a flue of a coal economizer to destroy a high-chlorine range of the front-end coal economizer, reduce the formation of heavy metal chlorides, and combine the regulation of the outlet flue gas temperature of the coal economizer and the flue gas flow rate in the flue of the coal economizer to make the fly ash in the coal economizer undergo multi-metal enhanced condensation and short-range enrichment, thereby reducing the migration of heavy metals to the terminal fly ash, rapidly enriching the heavy metals at the front end, avoiding the enrichment of the heavy metals and chlorides thereof in the terminal fly ash due to the migration of the heavy metals and chlorides thereof to the terminal, reducing the heavy metal pollution of the terminal fly ash, achieving the effect of detoxification of the heavy metals in the terminal fly ash, and achieving the purpose of treating solid waste and heavy metal pollution.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of solid waste heavy metal pollution treatment, and more particularly relates to a method for treating heavy metals in waste incineration fly ash based on front-end enhanced condensation and short-range enrichment. BACKGROUND

[0002] Fly ash is a hazardous solid waste rich in heavy metals produced by waste incineration. In the process of waste incineration, heavy metals in waste raw materials have different enrichment behaviors due to their different volatilities. Easily volatile heavy metals (such as Cd and Pb) and moderately volatile heavy metals (such as Zn and Cu) are directly volatilized into gaseous or particulate phases and enter subsequent processes with high-temperature flue gas for treatment. Difficult-to-volatile heavy metals (such as Cr and Ni) are mainly enriched in bottom slag through oxidation and melting. In the waste incineration flue gas, the content of Cl is about 40% or more, belonging to a high-chlorine atmosphere. The high-chlorine atmosphere promotes the volatilization and separation of heavy metals in fly ash, weakens / delays the condensation effect of heavy metals (especially easily volatile heavy metals and moderately volatile heavy metals) from the gas phase to the solid phase, and causes fly ash along the way to be contaminated by heavy metals, thereby increasing the amount and toxicity of terminal fly ash. As a hazardous waste, the terminal fly ash needs to be stabilized and solidified before further disposal due to its high heavy metal content. However, heavy metals in the terminal fly ash are difficult to remove by stabilization and solidification and remain in the fly ash, which still poses a great leaching pollution risk after landfill.

[0003] To remove heavy metals from terminal fly ash, reduce the leaching toxicity of heavy metals, and prevent heavy metal pollution, the existing technology usually takes terminal fly ash as the treatment object, creates a high-chlorine atmosphere by adding chlorides based on the principle that the boiling point of metal chlorides is lower than that of the corresponding metal oxides, and performs high-temperature heat treatment to make the heavy metals in the terminal fly ash volatilize and separate, thereby achieving the purpose of removing heavy metals from fly ash and preventing heavy metal pollution. For example, in patent CN108715519A, added calcium chloride and magnesium chloride are pressed into spherical particles with incineration fly ash, and then calcined to make the heavy metals in the fly ash react with the chlorination agent to generate easily volatile heavy metal chlorides. In patent CN119857716A, terminal fly ash is taken as the treatment object, and magnesium chloride or calcium chloride is added to make the chlorine for decomposing dioxins in the fly ash and the added chlorides react with the easily volatile and semi-volatile heavy metals in the fly ash to form gaseous heavy metal chlorides and remove the heavy metals, while the difficult-to-volatile heavy metals are solidified in the sintered material to reduce leaching and further prevent and control heavy metal pollution.

[0004] However, the terminal fly ash has a large amount of heavy metal enrichment, and the cost of creating a high chlorine atmosphere for high-temperature heat treatment of the terminal fly ash with high heavy metal enrichment is high, and the return is low. However, the present application reverses the process, destroys the high-chlorine atmosphere in the front-end flue gas, reduces the formation of volatile heavy metals and medium-volatile heavy metal chlorides, thereby reducing the migration of heavy metals into the terminal fly ash, promoting the enrichment of heavy metals in the front-end fly ash of the economizer, reducing the content of heavy metals in the terminal captured fly ash, fundamentally preventing heavy metal pollution, achieving the effect of detoxification of heavy metals in the terminal fly ash, preventing heavy metal pollution, and allowing it to be directly landfilled or disposed of in other ways without treatment, thereby greatly saving the cost of solid waste treatment and heavy metal pollution prevention. SUMMARY

[0005] In view of the above defects or improvement needs of the prior art, the present application provides a method for short-range enrichment of heavy metals in waste incineration fly ash based on front-end enhanced condensation, which aims to destroy the high-chlorine atmosphere in the flue gas of the front-end economizer by adding an alkaline earth metal compound material, reduce the migration and enrichment of gaseous heavy metal chlorides with flue gas to the terminal, and at the same time regulate the outlet flue gas temperature of the economizer and the flue gas flow rate in the economizer, enhance the condensation and enrichment of heavy metals in the economizer section, significantly reduce the heavy metals in the terminal fly ash, achieve the effect of detoxification of heavy metals in the terminal fly ash, and allow it to be directly landfilled or disposed of in other ways without treatment, thereby solving the technical problem of the prior art that heavy metals migrate and enrich to the terminal, resulting in high heavy metal content in the terminal fly ash, large production, the need for stable solidification treatment before landfill disposal, and the risk of leaching pollution of heavy metals remaining in the fly ash after landfill.

[0006] To achieve the above-mentioned purpose, according to one aspect of the present application, a method for short-range enrichment of heavy metals in waste incineration fly ash based on front-end enhanced condensation is provided, which comprises the following steps:

[0007] (1) Destroy the high-chlorine range of the front-end economizer: according to the amount of waste feed, add an alkaline earth metal compound material to the flue gas of the economizer, so that the addition rate of the alkaline earth metal compound material is 30-120 kg / h; the main components of the alkaline earth metal compound material include CaO, Ca(OH)2 and / or MgO, Mg(OH)2, and the content of the main components is ≥60%;

[0008] (2) Enhance the condensation of heavy metals in the front-end economizer: reduce the outlet flue gas temperature of the economizer and the flue gas flow rate in the economizer, so that the outlet flue gas temperature of the economizer is reduced to 160-170℃, the condensation of heavy metals is enhanced, and at the same time the flue gas flow rate is regulated to be between 1-3 m / s, so that the fly ash particles carrying the condensed heavy metals are short-range enriched and quickly settled in the front-end of the economizer, and the front-end fly ash enriched with heavy metals is collected.

[0009] Preferably, the method, the average particle size of the material containing alkaline earth metal compound in step (1) is less than or equal to 45 microns.

[0010] Preferably, the method, the main component of the material containing alkaline earth metal compound in step (1) is CaO and Ca(OH)2, and the content of CaO and Ca(OH)2 is 60% to 90%.

[0011] Preferably, the method, the proportion of the material containing alkaline earth metal compound in step (1) with a particle size of less than or equal to 53 microns is greater than or equal to 90%.

[0012] Preferably, the method, the material containing alkaline earth metal compound in step (1) is dry lime powder, and the dry lime powder is added according to the following method:

[0013] If the garbage feeding amount is 700 to 800 t / d, the dry lime powder feeding rate in the economizer flue is adjusted to 30 to 60 kg / h;

[0014] If the garbage feeding amount is 800 to 900 t / d, the dry lime powder feeding rate in the economizer flue is adjusted to 60 to 90 kg / h;

[0015] If the garbage feeding amount is 900 to 1000 t / d, the dry lime powder feeding rate in the economizer flue is adjusted to 90 to 120 kg / h.

[0016] Preferably, the method, the heavy metal includes Pb, Cd, Cu, Zn, Cr and Ni.

[0017] Preferably, the method, the flue gas flow rate in the economizer flue is calculated according to the total of the primary air and the secondary air of the incinerator and the cross-sectional area of the economizer flue, and the flue gas flow rate in the economizer flue is controlled to be between 1 to 3 m / s by adjusting the primary air and the secondary air of the incinerator, while ensuring that the temperature of the incinerator is in the range of 850-1000℃.

[0018] Preferably, the method, the heat exchange efficiency is enhanced by increasing the frequency of cleaning the inner wall of the economizer flue and / or adjusting the supply amount of circulating water of the economizer, so that the flue gas temperature at the outlet of the economizer is reduced to 160 to 170℃.

[0019] Overall, compared with the prior art, the above technical solutions conceived by the present application can achieve the following beneficial effects:

[0020] The method for enriching heavy metals in waste incineration fly ash based on front-end enhanced condensation and short-range enrichment provided by the application can destroy the high-chlorine range of the front-end economizer by adding an alkaline earth metal compound-containing material into the economizer flue, reduce the formation of heavy metal chlorides, and combine the regulation of the outlet flue gas temperature of the economizer and the flue gas flow rate in the economizer flue to cause the multi-metal enhanced condensation and short-range enrichment of the fly ash in the economizer, thereby reducing the migration of heavy metals to the terminal fly ash. Since the method concentrates and enriches heavy metals in the front end of the economizer, the amount of terminal fly ash is reduced, the heavy metal content and leaching toxicity of the terminal fly ash are reduced, and the fly ash landfill disposal problem (landfill site capacity is tight, and the environmental risk of long-term leaching of heavy metals) is alleviated. In addition, the terminal fly ash itself does not have the value of heavy metal recycling, but after the heavy metals are concentrated and enriched in the front end, heavy metal resources can be recycled, which may make the front-end fly ash have the value of resource recycling. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a conventional process flow diagram of fly ash heavy metal enrichment in waste incineration;

[0022] Figure 2 is a schematic diagram of the method. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical scheme and advantages of the application clearer, the application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the application and do not limit the application. In addition, the technical features involved in each embodiment of the application described below can be combined with each other as long as they do not conflict with each other.

[0024] In the waste incineration flue gas deposition phase, the content of Cl element is about 40% or more, which belongs to a high-chlorine atmosphere. However, under the condition of a high-chlorine atmosphere, volatile and semi-volatile heavy metals form a multi-metal eutectic system in the flue gas, which causes the supercooling problem of heavy metals in fly ash (i.e., the delay of phase transfer of heavy metals from the gas phase to the solid phase), delays the condensation effect of heavy metals from the gas phase to the solid phase, and causes the volatile and semi-volatile heavy metals to be transported with the flue gas to an area where the flue gas temperature is lower than the melting point of the heavy metals, slowly condense, settle and deposit in the economizer and the downstream flue, and along the way, the fly ash is contaminated by heavy metals, resulting in the problems of large amount of terminal fly ash and high toxicity.

[0025] The present application provides a method for short-range enrichment of heavy metals in fly ash from waste incineration based on front-end enhanced condensation, which can quickly enrich heavy metals at the front end, avoid the migration of heavy metals and their chlorides to the terminal, and significantly reduce the amount and heavy metal content of the terminal fly ash, so that the terminal fly ash can be directly landfilled or disposed of in other ways without treatment.

[0026] The present application provides a method for short-range enrichment of heavy metals in fly ash from waste incineration based on front-end enhanced condensation, which can quickly enrich heavy metals at the front end, avoid the migration of heavy metals and their chlorides to the terminal, and significantly reduce the amount and heavy metal content of the terminal fly ash, so that the terminal fly ash can be directly landfilled or disposed of in other ways without treatment.

[0027] (1) Destroy the high-chlorine environment in the front-end economizer: according to the amount of waste feed, add an alkaline earth metal compound-containing material to the economizer flue, so that the addition rate of the alkaline earth metal compound-containing material is 30-120 kg / h; the main components of the alkaline earth metal compound-containing material include CaO, Ca(OH)2 and / or MgO, Mg(OH)2, the content of the main components is ≥60%, and the average particle size of the material is ≤45 μm.

[0028] By introducing alkaline earth metal compounds into the front-end economizer flue, they react with chlorides in the economizer flue, destroying the high-chlorine environment in the economizer flue, overcoming the supercooling effect of heavy metals, and thus enhancing the condensation effect of easily volatile and semi-volatile heavy metals from the gas phase to the solid phase; on the other hand, it reduces the formation of semi-volatile heavy metal chlorides, avoiding the migration of semi-volatile heavy metals in the form of gaseous metal chlorides to the terminal and enriching them in the terminal fly ash; the average particle size of the added material is ≤45 μm, which increases the proportion of small and medium-sized particles (within 100 μm) carrying heavy metals in the economizer section, promotes the condensation of heavy metals and their settlement in the front-end economizer, and thus enriches heavy metals in the front-end fly ash.

[0029] (2) Enhance the condensation of heavy metals in the front-end economizer: reduce the outlet flue gas temperature of the economizer and the flue gas velocity in the economizer flue, so that the outlet flue gas temperature of the economizer is reduced to 160-170 ℃, the condensation of heavy metals is enhanced, and the flue gas velocity is controlled between 1-3 m / s, so that the fly ash particles carrying condensed heavy metals are quickly and short-range enriched and settled at the front end of the economizer, and the front-end fly ash enriched with heavy metals is collected; the heavy metals include Pb, Cd, Cu, Zn, Cr and Ni. The flue gas velocity in the economizer flue is calculated according to the total amount of primary and secondary air of the incinerator divided by the cross-sectional area of the economizer flue.

[0030] The flue gas temperature at the outlet of the economizer should not be lower than 160°C, otherwise the downstream pollution control facilities will not operate normally (e.g. the bag filter will be clogged if the flue gas temperature is too low), and the flue gas temperature at the outlet of the economizer should not be too high, because the heavy metal chlorides cannot be condensed from gas phase to solid phase at a high flue gas temperature. By adjusting the flue gas temperature and the flue gas flow rate at the outlet of the economizer, the heavy metals in the flue gas can be enriched in the short distance in the flue duct of the economizer, so as to avoid the pollution of the heavy metals to the fly ash along the way during the migration process to the terminal.

[0031] In some embodiments, the high-chlorine range of the front-end economizer is broken down in step (1), and a material containing alkaline earth metal compounds is added to the flue duct of the economizer, the main components of which are CaO and Ca(OH)2, the content of CaO and Ca(OH)2 being 60% to 90%, and the average particle size of the material being ≤45 μm, wherein the proportion of particles with a particle size of ≤53 μm is greater than or equal to 90%, and most of the particles are much smaller than 45 μm. The purpose of having a proportion of particles with a particle size of ≤53 μm of ≥90% is to avoid having too many large particles. For example, the material containing alkaline earth metal compounds is dry lime, which is added according to the following method:

[0032] If the garbage feed quantity is 700-800 t / d, the dry lime powder addition rate in the flue duct of the economizer is adjusted to 30-60 kg / h;

[0033] If the garbage feed quantity is 800-900 t / d, the dry lime powder addition rate in the flue duct of the economizer is adjusted to 60-90 kg / h;

[0034] If the garbage feed quantity is 900-1000 t / d, the dry lime powder addition rate in the flue duct of the economizer is adjusted to 90-120 kg / h.

[0035] In some embodiments, the condensation of heavy metals in the front-end economizer is strengthened in step (2), wherein the flue gas temperature at the outlet of the economizer is lowered according to the following method:

[0036] The flue gas temperature at the outlet of the economizer is lowered to 160-170°C by increasing the frequency of cleaning the accumulated ash on the inner wall of the flue duct of the economizer and / or adjusting the supply amount of circulating water of the economizer to enhance the heat exchange efficiency.

[0037] The flue gas flow rate is adjusted to 1-3 m / s according to the following method:

[0038] The flue gas flow rate in the flue duct of the economizer is calculated based on the total of the primary air quantity and the secondary air quantity of the incinerator divided by the cross-sectional area of the flue duct of the economizer, and the flue gas flow rate in the flue duct of the economizer is adjusted to be between 1-3 m / s while ensuring that the temperature of the incinerator is within the range of 850-1000°C.

[0039] The following is an example

[0040] A waste incineration power plant as an example, the conventional process flow diagram of waste from incinerator to terminal fly ash collection is shown in Figure 1 , the design of waste incinerator processing capacity is 900t / d, the daily waste feed is stable at about 700~900t / d. The incinerator primary air volume is about 60000~90000m 3 / h, the secondary air volume is about 10000~35000m 3 / h, the terminal chimney flue gas flow is about 120000~180000m 3 / h. The coal economizer flue has completed the transformation of dry powder feeding device, the feeding rate is about 30~120kg / h, the sprayed environmental protection material is quicklime. The cross-sectional area of the coal economizer flue is about 8.8m 2 , the flue temperature is about 190-230℃, the flue gas velocity is about 2.5~4.0m / s, the cleaning frequency of the flue inner wall ash is about 8~24h / once.

[0041] Example 1 based on front-end enhanced condensation and short-range enrichment of waste incineration fly ash heavy metals

[0042] The real-time operating conditions of waste incineration power plant A are as follows:

[0043] The waste feed quantity is about 722t / d, the primary air volume of the incinerator, the secondary air volume is 75891m 3 / h, 13475m 3 / h, the terminal chimney flue gas flow is about 127206m 3 / h.

[0044] The flue temperature of the coal economizer is 196℃, the flue gas velocity is about 2.8m / s (obtained by dividing the total of the primary air volume and the secondary air volume (89366m 3 / h) by the cross-sectional area of the coal economizer flue (8.8m 2 )), the cleaning frequency of the flue inner wall ash is about 8h / once (the daily working frequency of the incineration plant).

[0045] According to the method, the process flow diagram of waste from incinerator to terminal fly ash collection is shown in Figure 2 , for waste incineration power plant A, the specific treatment according to the method is as follows:

[0046] (1) Destroy the high chlorine range of the front-end coal economizer: the waste feed quantity of waste incineration power plant A is 700~800t / d, and the dry powder feeding rate of quicklime in the coal economizer flue is adjusted to about 30~60kg / h.

[0047] (2) Strengthen the condensation of heavy metals in the front-end economizer: adjust the cleaning frequency of the economizer flue inner wall ash to 4h / time, moderately adjust the circulating water supply of the economizer to improve the heat exchange efficiency, and finally adjust the economizer outlet flue gas temperature to be in the range of 160~170℃; the economizer outlet flue gas temperature should not be lower than 160℃, otherwise it will cause the abnormal operation of the pollution control facilities downstream of the economizer (such as the bag filter will be caked if the flue gas temperature is too low);

[0048] At the same time, adjust the primary air volume of the incinerator to be in the range of 30000~60000m 3 / h, adjust the secondary air volume to be in the range of 3000~10000m 3 / h, and the end chimney flue gas flow is about 50000-100000m 3 / h (generally, the end chimney flue gas flow is about 1-1.5 times of the total of the primary air volume and the secondary air volume), so that the flue gas flow rate in the economizer flue is reduced to about 1.0~2.2m / s (calculated by the total of the primary air volume and the secondary air volume (33000-90000m 3 / h) divided by the cross-sectional area of the economizer flue (8.8m 2 )).

[0049] The primary air volume and the secondary air volume of the incinerator can also be adjusted according to the operation experience, while the temperature of the incinerator should be ensured to be in the range of 850-1000℃.

[0050] Example 2

[0051] The real-time operation conditions of the household waste incineration power plant B are as follows:

[0052] The waste feed amount is about 889t / d, the primary air volume and the secondary air volume of the incinerator are 89805m 3 / h and 30836m 3 / h respectively, and the end chimney flue gas flow is 167206m 3 / h. The flue gas temperature in the economizer flue is 217℃, the flue gas flow rate is about 3.8m / s, and the cleaning frequency of the flue inner wall ash is about 8h / time.

[0053] For the household waste incineration power plant B, the specific treatment according to the method is as follows:

[0054] (1) The waste feed amount of the household waste incineration power plant B is 800~900t / d, and the feeding rate of dry lime powder in the economizer flue is adjusted to be 60~90kg / h; the waste amount entering the furnace is increased, and the material addition amount is correspondingly increased.

[0055] (2) Strengthen the condensation of heavy metals in the front-end economizer: adjust the cleaning frequency of the inner wall of the economizer flue to about 2h / time (increase the material addition amount, and increase the cleaning frequency accordingly), adjust the circulating water supply amount of the economizer to enhance the heat exchange efficiency, and finally adjust the flue gas temperature at the outlet of the economizer to be in the range of about 160~170℃.

[0056] At the same time, adjust the primary air volume of the incinerator to be in the range of 50000~70000m 3 / h, adjust the secondary air volume to be in the range of 3000~20000m 3 / h, and the flue gas flow rate at the end of the chimney is about 80000-135000m 3 / h, so that the flue gas flow rate in the economizer flue is reduced to be in the range of about 1.5~2.8m / s.

[0057] Collect the terminal fly ash after the household garbage incineration power plant A / B is treated according to the existing method (real-time operating condition) and the terminal fly ash after the household garbage incineration power plant A / B is treated according to the method in Example 1 and Example 2, respectively, detect the total amount of heavy metals in each terminal fly ash and the leaching toxicity of the terminal fly ash after chelation, and the results are shown in the following table.

[0058] Table 1 Total amount of heavy metals and leaching toxicity of terminal fly ash treated by different methods

[0059]

[0060] As can be seen from the results in Table 1, compared with the existing traditional treatment method, the terminal fly ash after stabilization and solidification treatment may not necessarily meet the landfill standard by 100%. The method enriches heavy metals in the front-end fly ash, directly reduces the total amount of heavy metals in the terminal fly ash, makes the content of heavy metals in the collected terminal fly ash low, especially the content of heavy metals Pb and Cd in the terminal fly ash is significantly reduced, the leaching toxicity of heavy metals in the fly ash is lower than the heavy metal leaching limit value, so that the terminal fly ash can be directly landfilled or disposed by other means without treatment, thereby reducing heavy metal pollution.

[0061] Those skilled in the art will readily understand that the above description is only the preferred embodiment of the present application, and is not intended to limit the present application, and any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for enriching heavy metals in waste incineration fly ash based on front-end enhanced condensation and short-range enrichment, characterized in that, Includes the following steps: (1) Disrupting the high-chlorine atmosphere of the front-end economizer: Add materials containing alkaline earth metal compounds into the economizer flue according to the amount of waste fed, so that the addition rate of the alkaline earth metal compound materials is 30~120kg / h; the main components of the alkaline earth metal compound materials are one or more of CaO, Ca(OH)2, MgO, and Mg(OH)2, and the content of the main components is ≥60%; (2) Enhance the condensation of heavy metals in the front-end economizer: reduce the outlet flue gas temperature of the economizer and the flue gas velocity in the economizer flue, so that the outlet flue gas temperature of the economizer is reduced to the range of 160~170℃, enhance the condensation of heavy metals, and at the same time control the flue gas velocity between 1~3m / s, so that the fly ash particles carrying the condensed heavy metals are enriched in the front end of the economizer in a short distance, settle quickly, and the front fly ash enriched with heavy metals is collected.

2. The method as described in claim 1, characterized in that, The material containing alkaline earth metal compounds in step (1) has an average particle size ≤45μm.

3. The method as described in claim 2, characterized in that, The material containing alkaline earth metal compounds mentioned in step (1) mainly consists of CaO and Ca(OH)2, with the content of CaO and Ca(OH)2 being 60%~90%.

4. The method as described in claim 3, characterized in that, The proportion of alkaline earth metal compound-containing materials in step (1) with a particle size ≤53μm is greater than or equal to 90%.

5. The method as described in claim 4, characterized in that, The material containing alkaline earth metal compounds in step (1) is quicklime powder, which is added according to the following method: If the garbage feed rate is 700~800t / d, adjust the quicklime dry powder feeding rate in the economizer flue to 30~60kg / h; If the garbage feed rate is 800-900 t / d, adjust the quicklime dry powder feeding rate in the economizer flue to 60-90 kg / h; If the waste feed rate is 900~1000t / d, adjust the quicklime dry powder feeding rate in the economizer flue to 90~120kg / h.

6. The method according to any one of claims 1 to 5, characterized in that, The front-end fly ash is enriched with heavy metals including Pb, Cd, Cu, Zn, Cr, and Ni.

7. The method as described in claim 6, characterized in that, Step (2) Calculate the flue gas velocity in the economizer flue by dividing the sum of the primary and secondary air volumes of the incinerator by the cross-sectional area of ​​the economizer flue. Adjust the primary and secondary air volumes of the incinerator to keep the flue gas velocity in the economizer flue between 1 and 3 m / s, while ensuring that the incinerator temperature is within the range of 850-1000℃.

8. The method as described in claim 7, characterized in that, Step (2) By increasing the frequency of cleaning the ash buildup on the inner wall of the economizer flue and / or adjusting the circulating water supply of the economizer, the heat exchange efficiency is enhanced, and the outlet flue gas temperature of the economizer is reduced to 160~170℃.

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