HK1057601B - Method for removing organic pollutants in fly ash - Google Patents

Description

Method for removing organic pollutants in fly ash

Technical Field

The present invention relates to a method for minimizing the concentration of toxic organic pollutants in fly ash of combustion plants, in particular of waste incinerator units, wherein at least part of the fly ash produced in the incinerator is returned to the combustion process.

Background

Toxic organic pollutants in fly ash, in particular polychlorinated dibenzodioxins (PCDD), polychlorinated dibenzofurans (PCDF), PCDD and PCDF precursor compounds, i.e. chlorobenzene, polychlorinated biphenyls (PCBs) and other compounds with similar structure or action. These organic pollutants are often described in the literature and emission standards and quantified by the internationally common toxicity equivalent expressed in nanograms per kilogram of fly ash (ng I-TEQ/kg). I-TEQ is the equivalent toxicity (international toxicity equivalent) relative to the total amount of the bulk organic contaminants of Seveso dioxin (2, 3, 7, 8-tetrachlorodibenzodioxin).

It is known from EP 0862019 a1 to return at least part of the fly ash produced by a combustion plant to the high temperature region of the combustion plant to induce vitrification and sintering of the fly ash. The product obtained in this way can be added again to the grate ash (grate ash) or used separately. Thus, the amount of residual fly ash can be reduced. The fly ash is removed by cleaning the boiler or stripping it from the filter unit and then, when the grate firing system is used, the fly ash is again fed into the furnace above the combustion layer. This method does not take into account the presence of toxic compounds such as dioxins and their precursors.

It is known from DE 3320466C 3 to return fly ash to the combustion chamber of a combustion plant. The fly ash is chemically treated outside the combustion chamber before being returned in order to reduce pollutants. In this way, fly ash of a low pollutant component is returned and mixed into the bottom ash in the high temperature process.

Disclosure of Invention

It is an object of the present invention to provide a method for removing organic pollutants from fly ash, wherein the return of fly ash to the combustion process is regulated such that the largest possible fraction of precursors or other organic pollutants is destroyed, thus minimizing the amount of toxic organic compounds leaving the combustion plant with the fly ash.

In order to achieve the above object, the invention is characterized in that the fly ash is returned as a function of the specific combustion conditions in which organic pollutants, such as PCDD or PCDF, or precursor compounds (precursors of PCDD or PCDF) are produced at high levels.

In summary, the present invention provides several aspects as follows:

1. a method for removing organic pollutants from fly ash of a combustion plant, wherein at least a part of the fly ash produced by an incinerator is returned to the combustion process at intervals, characterized in that the return of fly ash takes place when at least one of the following combustion conditions is fulfilled:

when the concentration of carbon monoxide became 100mg/m³Or higher;

when the oxygen content in the waste incineration plant is less than 5% by volume O₂When the current is over;

when the excess air rate in the waste incineration plant falls below 1.4;

when the combustor temperature of the burner is less than 800 ℃ as measured at a height of about 6-10 meters above the primary combustion zone; or

When the I-TEQ threshold becomes higher than 0.1-5ng I-TEQ/m³A predetermined value in between;

wherein the concentration of carbon monoxide, oxygen content, excess air rate, I-TEQ threshold are measured in the flue gas.

2. The method for removing organic pollutants from fly ash according to aspect 1, characterized in that said measurement in flue gas is performed by on-line analysis of flue gas.

3. The method for removing organic contaminants from fly ash according to aspect 2, wherein the return of the fly ash occurs within a period of 10 minutes to 6 hours after the combustion conditions are found.

4. A method for removing organic pollutants from fly ash as claimed in any of aspects 2 to 3, characterized in that the duration of the return of the fly ash is specified as a function of the level of the measured quantity determined according to aspect 1.

5. A method for removing organic pollutants from fly ash as described in any of aspects 1 to 3, characterized in that the fly ash in the main temperature zone is returned to the combustion plant.

6. The method for removing organic pollutants from fly ash according to aspect 5, wherein the fly ash is returned to a combustion layer in the main combustion zone when the grate system is used in the combustion plant.

7. The method for removing organic contaminants from fly ash according to any one of aspects 1 to 3, wherein the return of the fly ash is performed during or after the boiler cleaning.

8. The method for removing organic contaminants from fly ash according to aspect 7, wherein the boiler cleaning is performed by tapping, brushing or soot blowing.

9. The method for removing organic contaminants from fly ash according to aspect 6, characterized in that fly ash accumulated in the flue gas treatment unit on the downstream side of the vapor generation unit is returned together with the fly ash returned according to aspect 6.

Brief Description of Drawings

The method for removing organic pollutants in fly ash according to the present invention will be described with reference to the examples shown in the drawings.

FIG. 1 is a flow chart schematically illustrating the method for removing organic pollutants from fly ash according to the present invention.

Detailed Description

According to the method for removing organic pollutants from fly ash of the present invention, fly ash is returned when the amount of precursor compounds or other toxic organic pollutants to be minimized as much as possible is increased due to specific combustion conditions. This is an important feature of the present invention because the precursor compounds on the contact surfaces, which are on the downstream side of the vapour generating unit (boiler) and which are maintained at a certain temperature, for example 200-400 c, stick there and are converted into dioxins and/or furans, especially in the presence of copper, soot and chlorine. The conversion reaction can take place within minutes to hours, the exact time depending on the prevailing temperature conditions and concentrations of substances, such as copper, chlorine and soot as catalysts and reaction components.

It is advantageous to take the return of fly ash as a function of a measured quantity affected by the combustion process, said measured quantity being measured in the flue gas of the combustion plant.

More simply, the concentration of carbon monoxide or oxygen in the flue gas, the concentration of excess air produced by combustion, or the temperature within the combustion chamber may be measured.

In modern waste incinerator units, the carbon monoxide concentration in conventional combustion operations is about 5-20mg/m³However, 100mg/m³Or higher CO concentrations will be considered a particular one in the present inventionThe combustion conditions will be adjusted actively.

Furthermore, it is advantageous to use the oxygen content in the flue gas as one of the measured quantities, which is for the oxygen content in the waste incinerator unit to be reduced to less than 5% by volume of O₂Or when the excess air rate is measured to fall below 1.4. In the same way, the temperature in the combustion chamber of the combustion plant can be used as one of the measurements when the temperature drops below 800 ℃ measured at a height of about 6 to 10 meters in the main combustion zone.

Advantageously, as a further embodiment of the invention, the return of the fly ash can be taken as a function of the organic pollutants measured in the flue gases of the combustion plant, in particular PSDD and/or PSDF or their precursors.

Advantageously, the measured quantity is determined by on-line analysis of the flue gas.

In particular, the return of fly ash is taken as a function of a predetermined I-TEQ threshold. The threshold values defining a particular combustion condition may be set to: 0.1-5ng I-TEQ/m in flue gas³In the meantime.

This measurement preferentially covers gaseous and particle-bound organic pollutants and is carried out in the untreated gas above the boiler or on the upstream side of the flue gas treatment unit. Suitable analytical methods for this measurement are described in the literature, for example in Resonance Enhanced Multiple Photon ionization and Time of Flight Mass Spectrometry (REMPI-TOFMS), which make as direct an on-line analysis as possible of, for example, chlorobenzene. Empirical values indicate that chlorobenzene correlates very well with I-TEQ in flue gas. Accordingly, such an online analytical instrument may also be referred to as a TEQ sensor. However, according to the invention, sensors for other molecules or other types of molecules can be used if their signal is related to the characteristics of the content of toxic organic pollutants in the flue gas.

In another preferred embodiment of the invention, the return of fly ash occurs at a particular stage after the particular combustion conditions are found. The empirical values play a very important role here. For example, the return of fly ash occurs over a period of 10 minutes to 6 hours after the specific combustion conditions are found. The duration of the fly ash return can be determined as a function of the level of the measured quantity. The measured quantity is influenced here by the combustion process and is the concentration of carbon monoxide or oxygen in the flue gas, the concentration of excess gases resulting from the combustion, the temperature of the combustion chamber or the concentration of organic pollutants, in particular PCDD and/or PCDF or their precursors, in the flue gas.

If an analytical instrument is used to assist in the rapid determination of the measured value (as in the case for the above described measurement techniques and quantities) in order to determine a particular combustion condition, then in another preferred embodiment of the invention it is useful to indicate the duration of the return of fly ash as a function of the level of the measured value. Accordingly, if the predefined domain is significantly exceeded, the return duration will be longer than if it were slightly exceeded.

In order to ensure reliable disposal or decomposition of toxic organic pollutants or precursors, in another preferred embodiment of the invention the fly ash in the main temperature zone is returned to the combustion plant.

When using a grate system in a combustion plant, fly ash can advantageously be returned to the combustion layer of the main combustion zone.

In another preferred embodiment, the above-mentioned disadvantageous consequences, that the fly ash sticks to the boiler tubes during this operation and the precursors contained in the fly ash react with them to form dioxins, can be avoided if the return of the fly ash takes place after specific combustion conditions are found or during or after the boiler cleaning process. In this way not only fly ash but also the resulting aggregated ash is introduced back into the process.

The boiler is cleaned by tapping, brushing or soot blowing.

It is recommended to return fly ash accumulated in the flue gas treatment unit on the downstream side of the steam generation unit together with the returned fly ash as described above. This method is carried out when specific combustion conditions are found.

If specific combustion conditions are observed, it is possible according to the invention to return the fly ash recovered in the filter on the downstream side of the steam generating unit (boiler).

Naturally, though, the operators of combustion plants have always eliminated certain combustion conditions again as quickly as possible by suitable methods, which cannot be carried out automatically by, for example, incineration control.

If no interruption of the incineration process occurs, no specific combustion conditions are observed and boiler cleaning is carried out at normal frequency. In this regard, the interval between the two washing stages is generally set to about 4 hours. The fly ash produced is thus treated by normal treatment procedures.

Next, an embodiment of the method according to the invention will be briefly described and illustrated with reference to the flow chart of fig. 1.

In this embodiment, incineration is performed (step 100), and an on-line measurement of gases such as toxic organic pollutants or carbon monoxide is performed (step 102). Alternatively, the typical temperature of the flue gas is determined (same step 102). Then, it is judged whether or not the specific combustion condition of the returned fly ash is satisfied (step 104). If specific combustion conditions are observed in this process by an increase in the concentration of organic pollutants or carbon monoxide or by a significant deviation of the temperature from the set point, the boiler may be cleaned by tapping, brushing or soot blowing (step 106) at such a concentration increase or temperature deviation. The fly ash obtained by cleaning is then returned to the combustion plant (incinerator, combustion chamber) (step 108). If no faults occur, i.e., specific combustion conditions are not met, normal boiler cleaning is performed at regular intervals (step 110). The resulting fly ash is discharged to an off-process treatment (steps 112 and 114).

From the foregoing it is apparent that there has been provided, in accordance with the present invention, a method for removing organic pollutants from fly ash, wherein the return of the fly ash to the combustion process is regulated so that the largest possible fraction of precursors or other organic pollutants is destroyed, thereby minimizing the amount of toxic organic compounds that leave the combustion plant with the fly ash.

Although the present invention has been described with reference to the embodiments, the present invention is not limited to the descriptions, and all changes, variations and additions which can be easily made by those skilled in the art are included in the technical scope of the present invention.

Claims (9)

1. A method for removing organic pollutants from fly ash of a combustion plant, wherein at least a part of the fly ash produced by an incinerator is returned to the combustion process at intervals, characterized in that the return of fly ash takes place when at least one of the following combustion conditions is fulfilled:

when the concentration of carbon monoxide became 100mg/m³Or higher;

when the oxygen content in the waste incineration plant is less than 5% by volume O₂When the current is over;

when the excess air rate in the waste incineration plant falls below 1.4;

when the combustor temperature of the burner is less than 800 ℃ as measured at a height of about 6-10 meters above the primary combustion zone; or

When the I-TEQ threshold becomes higher than 0.1-5ng I-TEQ/m³A predetermined value in between;

wherein the concentration of carbon monoxide, oxygen content, excess air rate, I-TEQ threshold are measured in the flue gas.

2. The method for removing organic pollutants from fly ash of claim 1, wherein said measurement in the flue gas is performed by on-line analysis of the flue gas.

3. A method for the removal of organic pollutants from fly ash as claimed in claim 2, wherein the return of the fly ash occurs within a period of 10 minutes to 6 hours after the combustion conditions are found.

4. A method for the removal of organic pollutants from fly ash as claimed in any of claims 2 to 3, characterized in that the duration of the return of the fly ash is specified as a function of the level of the measured quantity determined according to claim 1.

5. A method for removing organic pollutants from fly ash as claimed in any of claims 1 to 3, characterized in that the fly ash in the main temperature zone is returned to the combustion plant.

6. The method for removing organic pollutants from fly ash as claimed in claim 5, wherein the fly ash is returned to a combustion zone in the main combustion zone when the grate system is used in the combustion plant.

7. A method for removing organic pollutants from fly ash as claimed in any of claims 1 to 3, characterized in that the return of the fly ash is carried out during or after boiler cleaning.

8. The method for removing organic contaminants from fly ash of claim 7, wherein the boiler cleaning is performed by tapping, brushing or soot blowing.

9. A method according to claim 6 for removing organic pollutants from fly ash, characterized in that fly ash collected in a flue gas treatment unit downstream of the steam generation unit is returned together with the fly ash returned according to claim 6.