Methods for incinerating sewage sludge

Abstract

To provide a sewage sludge combusting method which suppresses clogging of a gas duct due to combustion ashes in a combustion furnace of sewage sludges, and which enables an appropriate combusting process on the sewage sludges.SOLUTION: A sewage sludge combusting method includes the steps for: adding, to sewage sludges, an additive that contains at least one or both of aluminum and silicon; and combusting the sewage sludges to which the additive is added in a combustion furnace 1. The additive contains at least one kind selected from polyaluminum chloride, silica sand, kaolin, and water purification dirt. Moreover, by adding the additive to the sewage sludges, an index value is adjusted to be equal to or greater than 1.0, and the Seger value is also adjusted to be higher than a value prior to the addition.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for incinerating sewage sludge.

Background Art

[0002] In recent years, in the incineration treatment of sewage sludge, there have been cases where the flue between the incinerator and the air preheater is blocked by incineration ash. In addition, due to the advanced treatment of water treatment, the phosphorus content in sewage sludge has increased, and the incineration ash has a lower melting point, which further increases the possibility of the flue being blocked (see Non-Patent Document 1 below).

[0003] Regarding the melting mechanism of incineration ash, various investigations have been carried out, but it is considered that the influence of the composition of incineration ash is great. Also, in combined sewer systems, it has been found that the index value and melting point of incineration ash increase after rainfall compared to sunny days (see Non-Patent Document 2 below). This is considered to be due to an increase in the inorganic components in the sludge accompanying rainfall. Furthermore, it is considered to be caused by containing a large amount of high melting point substances such as aluminum (Al), silicon (Si), iron (Fe), etc. derived from the inflowing sediment.

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] This invention was proposed in view of the above conventional circumstances, and aims to provide a method for incinerating sewage sludge that suppresses the blockage of the flue by incineration ash during the incineration treatment of sewage sludge. [Means for solving the problem]

[0006] To achieve the above objective, the present invention provides the following means. [1] Sewage sludge contains at least aluminum and silicon and include Water purification sludge and additives and The process of adding, The aforementioned water purification sludge and The process includes the step of incinerating the sewage sludge to which the additive has been added in an incinerator. fruit, By adding the water treatment soil and the additive to the sewage sludge, the index value shown in the following formula (1) is adjusted to be greater than 1.0. Furthermore, the Seger value shown in formula (2) below is adjusted to be higher than that before the addition. Methods for incinerating sewage sludge.

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[0007] As described above, the present invention makes it possible to provide a method for incinerating sewage sludge that suppresses blockage of the flue by incineration ash during the incineration treatment of sewage sludge. [Brief explanation of the drawing]

[0008] [Figure 1]It is a block diagram showing the configuration of an incineration system for sewage sludge according to an embodiment of the present invention. [Figure 2] It is a graph showing the relationship between the index value and the Zeigel value of the incineration ash of the incinerator without flue blockage and the incineration ash of the incinerator with flue blockage. [Figure 3] (A) Microscopic photograph (1000 times) of incineration ash of an incinerator without flue blockage, (B) Microscopic photograph (1000 times) of incineration ash of an incinerator with flue blockage. [Figure 4] It is a diagram showing a method for calculating the area shrinkage rate. [Figure 5] It is a diagram showing a method for calculating the crushing strength. [Figure 6] It is a graph showing the measurement results of the area shrinkage rate and the crushing strength due to the addition of Al in Samples 1 to 3. [Figure 7] It is a graph showing the measurement results of the area shrinkage rate and the crushing strength due to the addition of Si in Samples 1 to 3. [Figure 8] It is a graph showing the measurement results of the area shrinkage rate and the crushing strength due to the addition of kaolin in Samples 1 to 3. [Figure 9] It is a graph showing the measurement results of the area shrinkage rate and the crushing strength due to the addition of water purification generating soil in Samples 1 to 3.

Embodiments for Carrying Out the Invention

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. (Incineration System for Sewage Sludge) First, as an embodiment of the present invention, for example, an incineration system 100 for sewage sludge shown in FIG. 1 will be described. Note that FIG. 1 is a block diagram showing the configuration of the incineration system 100.

[0010] As shown in FIG. 1, the incineration system 100 of this embodiment includes an incinerator 1, an air preheater 2, a heat exchanger 3, a dust collector 4, and a flue gas treatment tower 5.

[0011] Incinerator 1 is designed to incinerate sewage sludge (dewatered cake), fuel, and air by supplying them into the furnace and burning (heating) the sewage sludge inside the furnace. Incinerator 1 is connected to air preheater 2 via flue 6.

[0012] The air preheater 2 is used to preheat the air supplied to the incinerator 1 by heat exchange with combustion exhaust gas (soot) discharged from the incinerator 1 through the flue 6. The air preheated in the air preheater 2 is supplied to the incinerator 1 under pressure by the blower 7. The air preheater 2 is connected to the heat exchanger 3 via the flue 8.

[0013] The heat exchanger 3 is designed to prevent white smoke from being released into the atmosphere by exchanging heat between the combustion exhaust gas discharged through the flue 8 and the air supplied via the blower 9. This heat exchanger warms and dilutes the combustion exhaust gas, thereby diluting the water vapor in the combustion exhaust gas and preventing white smoke from being released into the atmosphere. The heat exchanger 3 is connected to the outlet of the flue gas treatment tower 5 via the flue 15.

[0014] The dust collector 4 is for capturing and removing soot and other particles contained in the combustion exhaust gas discharged through the flue 10. The dust collector 4 is connected to the flue gas treatment tower 5 via the flue 11. The combustion exhaust gas from which soot and other particles have been removed is pumped to the flue gas treatment tower 5 under pressure by a blower 12 installed in the flue 11.

[0015] The flue gas treatment tower 5 circulates circulating water within the tower using a pump 13 to remove acidic gases contained in the combustion exhaust gas before releasing the combustion exhaust gas into the atmosphere. In addition, the flue gas treatment tower 5 neutralizes the circulating water by supplying caustic soda (NaOH) into the tower using a pump 14. The circulating water is supplied as filtered water from the top of the flue gas treatment tower 5 and discharged as wastewater from the bottom of the flue gas treatment tower 5.

[0016] (Incineration methods for sewage sludge) Next, a method for incinerating sewage sludge using the above-described incineration system 100 will be explained. A method for incinerating sewage sludge to which the present invention is applied is characterized by comprising the steps of adding an additive containing at least one or both of aluminum (Al) and silicon (Si) to sewage sludge, and incinerating the sewage sludge to which the additive has been added in an incinerator 1.

[0017] The additives can contain at least one or both of Al and Si, and among these, at least one selected from polyaluminum chloride (PAC), silica sand, kaolin, and water purification soil can be suitably used.

[0018] In the process of adding additives to sewage sludge, the blockage suppression index value shown in the following formula (1) (hereinafter referred to as the "index value") becomes 1.0. Larger than It is preferable to adjust it to achieve this.

[0019]

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[0020] In formula (1) above, Fe2O3[%] represents the iron(III) oxide analysis value of the incinerated ash, Al2O3[%] represents the aluminum oxide analysis value of the incinerated ash, CaO[%] represents the calcium oxide analysis value of the incinerated ash, MgO[%] represents the magnesium oxide analysis value of the incinerated ash, P2O5[%] represents the phosphorus pentoxide analysis value of the incinerated ash, and M(i)[g / mol] represents the molecular weight of compound i.

[0021] The index value represented by the above formula (1) is an index for determining the possibility of blockage of flue 6 by incinerated ash, and this index value is 1.0 below If this is the case, it will be judged as "there is a risk of obstruction."

[0022] In this embodiment, the analytical values ​​of each component were calculated from a sample obtained by heating a dehydrated cake at 600°C and ashing it to remove organic matter. In addition, in this embodiment, the index value was calculated by analyzing the composition of the sample using an X-ray fluorescence spectrometer (XRF) (measured in terms of oxides) and then substituting it into the above formula (1).

[0023] Furthermore, in the process of adding additives to sewage sludge, it is preferable to adjust the sewage sludge so that the Seger value shown in formula (2) below becomes higher than that before the additive was added.

[0024]

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[0025] In equation (2) above, RO and R2O are basic oxides (calcium oxide (CaO), potassium oxide (K2O), sodium oxide (Na2O), etc.), and a, b, x, and y represent the molar ratios of each substance. Divide the entire equation so that a + b = 1. Let x + y (the sum of the molar ratios of neutral oxides and acidic oxides) be the "Seger value".

[0026] The Seger value, represented by equation (2) above, is calculated using the Seger formula, which is commonly used in the ceramics industry. It serves as an indicator of the amount of wood ash or lime (basic oxide) added to melt high-melting-point alumina (aluminum oxide (Al2O3): neutral oxide) and silicon (silicon dioxide (SiO2): acidic oxide). Furthermore, a larger x+y value (Seger value) indicates that the material is less soluble.

[0027] In this embodiment, the composition of the sample was analyzed using an X-ray fluorescence (XRF) spectrometer (measured in terms of oxides). The molar ratios of each substance were then calculated by dividing the analytical values ​​(%) of basic oxides, aluminum oxide, and silicon dioxide by their molecular weights. Furthermore, the x+y value (Seger value) was calculated by dividing the total molar ratio of all substances by the total molar ratio of basic oxides, such that a+b equals 1.

[0028] In the sewage sludge incineration method of this embodiment, by adding the above-mentioned additive to the sewage sludge, the melting of the incinerated ash when the sewage sludge to which the additive has been added is incinerated in the incinerator 1 is suppressed. This makes it possible to suppress the blockage of the flue 6 by the incinerated ash. [Examples]

[0029] The effects of the present invention will be made clearer by the following examples. However, the present invention is not limited to the following examples and can be modified as appropriate without altering its essence.

[0030] (1. Regarding the relationship between index values ​​and Seger values) In this embodiment, first, the incinerated ash from an incinerator where there was no flue blockage and the incinerated ash from an incinerator where the flue blockage occurred were analyzed, and the index value shown in formula (1) and the Seger value shown in formula (2) were calculated. The relationship between these index values ​​and Seger values ​​is shown in the graph in Figure 2.

[0031] As shown in Figure 2, the incinerated ash from incinerators where the flue was blocked was found to be concentrated in regions with relatively low index values ​​and Seger values ​​compared to the incinerated ash from incinerators where the flue was not blocked.

[0032] Based on the above, it can be concluded that Al and Si are significantly involved in raising the melting point of incinerated ash, and that the addition of both substances can be expected to suppress flue blockage.

[0033] Therefore, in this example, we conducted experiments involving the addition of both Al and Si to sewage sludge and heating, and also verified the selection of substances containing both substances and their inhibitory effects.

[0034] (2. Experiment Overview) In this experiment, incinerated ash from an incinerator with no flue blockage and incinerated ash from an incinerator where the flue blockage occurred were observed using an electron microscope. The micrographs are shown in Figures 3(A) and (B). Figure 3(A) is a micrograph (1000x magnification) of the incinerated ash from the incinerator with no flue blockage, and Figure 3(B) is a micrograph (1000x magnification) of the incinerated ash from the incinerator where the flue blockage occurred.

[0035] In the incinerator ash from an incinerator where the flue was not blocked, as shown in Figure 3(A), fine particles can be observed, whereas in the incinerator ash from an incinerator where the flue was blocked, as shown in Figure 3(B), it can be seen that the ash was partially melted and sintered.

[0036] Previous experiments have shown that, as shown in Table 1 below, the dissolved blockage material increases in strength and shrinks. On the other hand, there are also reports that the weakening of the blockage material in the incinerator (decrease in apparent specific gravity and strength) leads to improved peelability (see Non-Patent Literature 3: "Study of Clinker Weakening Agents for Coal Boilers," Chubu Electric Power Co., Inc. Technology Development News, March 2014 (No. 150)).

[0037] [Table 1]

[0038] Therefore, in this experiment, as an indicator of the effect of suppressing flue blockage, the percentage of shrinkage due to heating (area shrinkage rate) shown in Figure 4 and the crush strength of the heated sample shown in Figure 5 were calculated as follows, from the perspective of reducing sample shrinkage and hardening. In this experiment, the effect of reducing sample shrinkage and hardening is referred to as the "suppression effect".

[0039] This experiment was conducted in the following three stages. (1) Confirmation of the inhibitory effect by adding Al and Si. (2) Selection of substances containing high concentrations of Al and Si. (3) Confirmation of the inhibitory effect of the selected substance.

[0040] Furthermore, the experimental conditions for additives and heating for each sample are as follows. Experimental sample: Ashed dewatered sludge (dewatered sludge strongly heated at 600°C) Sample 1: Index value 0.96, Seger value 0.79 Sample 2: Index value 0.99, Seger value 0.88 Sample 3: Index value 1.26, Seger value 0.99 Additive ratio: No additives, 5wt%, 10wt%, 20wt%, 30wt%

[0041] Heating conditions: Each additive was added to the sample, packed into a platinum container with an outer diameter of Φ5 mm, and heated at 900°C for 1 hour. The area shrinkage rate was then calculated. The incinerator's set temperature was 850°C, but sufficient strength was not achieved, so 900°C was used as the heating temperature.

[0042] Furthermore, the load F[N] at which the heated sample completely collapses and the crushing strength σ[N / mm] are also determined. 2 The force was measured using a force gauge (IMADA, DST-500N).

[0043] (3. Experimental Results) (3-1-1) Confirmation of the inhibitory effect by adding Al For each sample (1-3), dried PAC (containing 90.5% Al) was added as an additive, and then each sample (1-3) was heated. The area shrinkage rate and crushing strength were measured for each additive rate. The results are summarized in Figure 6. Note that the graph in Figure 6 shows relative values ​​with the measurement value for the sample without additive set to 100%.

[0044] As shown in Figure 6, samples 1-3 with added PAC showed decreased area shrinkage and crushing strength compared to samples 1-3 without PAC. Furthermore, when 5 wt% of PAC was added, the area shrinkage decreased by 13-17% and the crushing strength decreased by more than 60% compared to the unadded samples. On the other hand, when more than 30 wt% of PAC was added, all samples 1-3 were close to powder, and there was almost no increase in strength due to heating.

[0045] (3-1-2) Confirmation of the inhibitory effect by adding Si For each of the samples 1-3, crushed silica sand (containing 89.7% Si) was added as an additive, and then each sample 1-3 was heated. The area shrinkage rate and crushing strength were measured for each additive rate. The results are summarized in Figure 7. Note that the graph in Figure 7 shows relative values ​​with the measurement value for the sample without additive set to 100%.

[0046] As shown in Figure 7, samples 1-3 with added silica sand showed reduced area shrinkage and crushing strength compared to samples 1-3 without added silica sand. Furthermore, when 5 wt% silica sand was added, the reduction in area shrinkage was limited to approximately 6.5-12% compared to the unadded sample, and the reduction in crushing strength was at most 36%. On the other hand, at all addition rates, the inhibitory effect was lower compared to the case with added Al(PAC).

[0047] (3-2) Selection of substances containing high concentrations of Al and Si Next, kaolin (containing 18.5% Al and 77.7% Si) was selected as a substance rich in both Al and Si. Kaolin is a clayey ore known as a material for pottery and is sold as a reagent. Kaolin is a type of mineral, and its chemical composition is Al4Si4O 10 (OH)8 is a type of clay mineral, also known as tahryite.

[0048] (3-2-1) Confirmation of the inhibitory effect by adding kaolin Kaolin was added as an additive to each of the three samples (1-3), and then each sample was heated. The area shrinkage rate and crushing strength were measured for each additive rate. Figure 8 shows a graph summarizing these measurement results. Note that the graph in Figure 8 shows relative values ​​with the measurement value for the sample without additive set to 100%.

[0049] As shown in Figure 8, the inhibitory effect was not as significant as when Al(PAC) was added. On the other hand, the inhibitory effect of kaolin was almost superior to that of Si(silica sand) when Si(silica sand) was added.

[0050] The results above confirm that kaolin has a certain effect in suppressing blockage, but since the unit price of kaolin is approximately 400,000 to 1,000,000 yen / ton, there are challenges in terms of cost.

[0051] (3-3) Applicability of water treatment sludge Therefore, we investigated the applicability of water treatment sludge, which is industrial waste, as an additive rich in both Al and Si.

[0052] Water treatment sludge refers to the sediment from raw water that has been removed by coagulation and sedimentation, one of the treatment processes at a water treatment plant, as well as the chemicals (coagulants) used in the treatment, which are then dewatered.

[0053] In this experiment, the sludge generated from the water treatment plant was dried and crushed, and then its inorganic components were measured. The results showed that it contained 36.4% Al and 34.6% Si, both of which were high concentrations. Therefore, the sludge generated from the water treatment plant was added to samples 1-3 to confirm its inhibitory effect.

[0054] (3-3-1) Confirmation of the inhibitory effect by adding kaolin For each of the three samples (1-3), water purification sludge was added as an additive, and then each sample was heated. The area shrinkage rate and crushing strength were measured for each additive rate. The results are summarized in Figure 9. Note that the graph in Figure 9 shows relative values ​​with the measurement value for the sample without additive set to 100%.

[0055] As shown in Figure 9, the area shrinkage rate was similar to that when silica sand and kaolin were added, but the crushing strength decreased by about 40% with the addition of 5 wt% or more.

[0056] On the other hand, when water treatment-generated soil is added to the sludge fed into the dewatering machine, it was found that adding 0.03 wt% of water treatment-generated soil to sludge with a solid content of 3% and inorganic content of 20% had an effect in suppressing flue blockage.

[0057] (4. Summary) This experiment confirmed that both Al and Si have an inhibitory effect on shrinkage and hardening of ashed dewatered sludge. In particular, the reduction in crushing strength was significant when Al was at 5 wt%, reaching a maximum of over 60%.

[0058] Furthermore, a high inhibitory effect was confirmed with the addition of kaolin and water treatment sludge, which are rich in both Al and Si. In particular, with the addition of water treatment sludge, a reduction in crushing strength of more than 40% was confirmed with an addition of 5 wt% or more.

[0059] This experiment revealed the potential of adding wastewater treatment sludge to sewage sludge as a measure to suppress flue blockage in incinerators. This is expected to contribute to the effective utilization of wastewater treatment sludge, which is mostly disposed of in landfills. [Explanation of Symbols]

[0060] 100... Incineration system 1... Incinerator 2... Air preheater 3... Heat exchanger 4... Dust collector 5... Flue treatment tower 6... Flue 7... Blower 8... Flue 9... Blower 10... Flue 11... Flue 12... Blower 13... Pump 14... Pump

Claims

1. A process of adding water purification soil containing at least aluminum and silicon, and additives to sewage sludge, The process includes incinerating the water purification soil and the sewage sludge to which the additives have been added in an incinerator. By adding the water treatment soil and the additive to the sewage sludge, the index value shown in the following formula (1) is adjusted to be greater than 1.

0. Furthermore, the Seger value shown in formula (2) below is adjusted to be higher than that before the addition. Methods for incinerating sewage sludge. [Math 1] [Math 2]

2. Add to the sewage sludge at least 0.03 wt% of dried and crushed soil generated from water treatment, The incineration method according to claim 1.

3. The aforementioned additive includes at least one selected from polyaluminum chloride, silica sand, and kaolin. A method for incinerating sewage sludge according to claim 1 or 2.

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