Operating method of a circulating fluidized bed boiler

Abstract

This invention provides a method for operating a circulating fluidized bed boiler that can effectively utilize bottom ash, thereby improving operational efficiency and reducing costs, and a method for producing fluidized sand. [Solution] This is a method for operating a circulating fluidized bed boiler 10, in which the bottom ash produced by burning wood-derived biomass fuel in another circulating fluidized bed boiler 10 is recovered and used as fluidized sand in the circulating fluidized bed boiler 10.

Description

Technical Field

[0001] The present invention relates to an operation method of a circulating fluidized bed boiler and a method for producing fluidized sand.

Background Art

[0002] A circulating fluidized bed (CFB) boiler is a boiler with high-efficiency combustion and flexibility to handle various fuels. The circulating fluidized bed boiler has the characteristic that it can utilize a wide variety of fuels in a ratio according to the user's requirements. It can not only use biomass, RDF, etc. alone, but also plan and operate the equipment with any co-firing ratio. The circulating fluidized bed boiler is a boiler that can burn a wide range of fuels such as low-grade coal including high-ash coal and high-moisture coal, petroleum coke, pulp sludge, wood chips, waste tires, RPF, and other industrial wastes in an environmentally friendly manner. In a circulating fluidized bed boiler, unburned components are recovered by a cyclone and returned to the furnace again, so that fuels with difficult combustion can also achieve high combustion efficiency. In a circulating fluidized bed boiler, the discharge of sulfur oxides (SOx) can be suppressed by injecting limestone into the furnace. In a circulating fluidized bed boiler, nitrogen oxides (NOx) are also suppressed by low-temperature and two-stage combustion, and dust can also meet the environmental law standards by installing a dust collection device.

[0003] In the following Patent Document 1, an example of a circulating fluidized bed boiler is disclosed. The circulating fluidized bed boiler of Patent Document 1 includes an external circulating fluidized bed furnace that supplies and burns fuel in a combustion chamber where the fluidizing material circulates, and a cyclone connected to the external circulating fluidized bed furnace that mainly separates the gas of combustion gas from the solid of the fluidizing material. Further, in this circulating fluidized bed boiler, at least a part of the cyclone introduction path that guides the solid-gas two-phase flow mainly composed of the fluidizing material and the combustion gas from the external circulating fluidized bed furnace to the cyclone is provided along the side wall of the external circulating fluidized bed furnace, and it is said that high efficiency is achieved.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] Japanese Patent Publication No. 2001-311503 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, there was room for improvement in terms of operational efficiency and cost reduction for conventional circulating fluidized bed boilers.

[0006] Therefore, the present invention aims to provide a method for operating a circulating fluidized bed boiler that can effectively utilize bottom ash, thereby improving operational efficiency and reducing costs, and a method for producing fluidized sand. [Means for solving the problem]

[0007] To solve the aforementioned problems, the present invention proposes the following means. A method for operating a circulating fluidized bed boiler according to one aspect of the present invention is a method for operating a circulating fluidized bed boiler, wherein bottom ash produced by burning wood-derived biomass fuel in another circulating fluidized bed boiler is recovered, and the bottom ash is used as fluidized sand in the circulating fluidized bed boiler.

[0008] A method for producing fluidized sand according to one aspect of the present invention involves burning wood-derived biomass fuel in a circulating fluidized bed boiler and recovering the resulting furnace bottom ash. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a method for operating a circulating fluidized bed boiler that can effectively utilize bottom ash, thereby improving operational efficiency and reducing costs, and a method for producing fluidized sand. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram of a circulating fluidized bed boiler used in an embodiment of the present invention. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described below. Figure 1 is a schematic diagram of the circulating fluidized bed boiler used in this embodiment. The circulating fluidized bed boiler 10 used in this embodiment includes a combustion furnace 11, a fuel supply unit 12, a fluidized sand tank 13, an air introduction unit 14, a circulation unit 15, a duct 16, a heat exchange unit 17, a dust collection unit 18, and the like.

[0012] The combustion furnace 11 is a furnace that burns fuel in a fluidized bed formed of fluidized sand. The fuel supply unit 12 supplies fuel to the combustion furnace 11. The fluidized sand tank 13 contains fluidized sand received from the outside and supplies it to the combustion furnace 11. It is preferable to separate and remove large-diameter particles from the fluidized sand before it is contained in the fluidized sand tank 13. Large-diameter particles may be separated by, for example, a vibrating screen, or removed by crushing them to reduce their diameter. The fluidized sand contained in the fluidized sand tank 13 is then fed into the combustion furnace 11 using pneumatic conveying equipment or the like.

[0013] The air inlet 14 introduces air from multiple height positions on the bottom side of the combustion furnace 11, for example, and forms a fluidized bed with fluidized sand. The circulation unit 15 extracts the high-temperature mixture generated by combustion in the combustion furnace 11 along with the fluidized sand, separates the fluidized sand, and recirculates it back into the combustion furnace 11. The remaining mixture is sent to the duct 16. The duct 16 leads the remaining portion from the circulation section 15 to the outside of the system. This remaining portion includes, for example, products such as gases generated by combustion, as well as fly ash.

[0014] The heat exchange unit 17 generates steam using the heat produced in the combustion furnace 11 and is then superheated. In this embodiment, the heat exchange unit 17 is located in the middle of the duct 16, but the installation location of the heat exchange unit 17 is not limited to this. The dust collection unit 18 separates and recovers fly ash and other materials from the mixture after the fluidized sand has been separated and sufficient heat has been dissipated.

[0015] The combustion furnace 11 of this embodiment further includes an extraction section 21, a cooling section 22, a sorting section 23, and a recovery section 24. The extraction section 21 extracts the bottom ash, which is the main ash in the combustion furnace 11, from the bottom of the combustion furnace 11. The cooling section 22 dissipates heat from the extracted high-temperature bottom ash to cool it. The sorting section 23 separates and removes large-diameter objects from the extracted bottom ash and also removes foreign substances such as metals. The large-diameter objects may be separated, for example, by a vibrating screen or crushed to reduce their diameter.

[0016] The recovery section 24 is composed of, for example, a recovery tank, etc., and recovers and stores the bottom ash that has been sorted by the sorting section 23 and from which large-diameter objects and foreign substances have been removed. The bottom ash recovered by the recovery section 24 may be投入 back into the combustion furnace 11 and recycled as fluidized sand, or discharged outside the system.

[0017] Next, the operation method of the circulating fluidized bed boiler 10 of this embodiment will be described. In this embodiment, an example of using a plurality of circulating fluidized bed boilers as shown in FIG. 1 will be used for the description. Here, the plurality of circulating fluidized bed boilers 10 are roughly classified into two, and one circulating fluidized bed boiler 10A (hereinafter also referred to as the first circulating fluidized bed boiler 10A) and the other circulating fluidized bed boiler 10B (hereinafter also referred to as the second circulating fluidized bed boiler 10B) are operated with different fuels and operating conditions.

[0018] In this operation method, fluidized sand is produced by recovering the bottom ash generated in the second circulating fluidized bed boiler 10B. Then, the produced fluidized sand is used as the fluidized sand of the first circulating fluidized bed boiler 10A, and a fluidized bed is formed in the combustion furnace 11 of the first circulating fluidized bed boiler 10A to burn the fuel.

[0019] In the first circulating fluidized bed boiler 10A, for example, a fuel mainly composed of waste tires, waste plastics, or paper sludge is burned. Examples of such fuels include at least one of waste tires, waste plastics, paper sludge, and RPF (Refuse derived paper and plastics densified Fuel).

[0020] In the second circulating fluidized bed boiler 10B, a biomass fuel derived from wood is burned. The biomass fuel derived from wood is a fuel derived from wood, and examples include wood chips, wood pellets, forest residues (including bark), wooden waste materials in the construction industry, and palm residues (PKS).

[0021] In a so-called biomass-only-fired CFB such as the second circulating fluidized bed boiler 10B, in order to prevent the generation of clinkers due to the concentration of Na and K derived from the fuel, a large amount of bottom ash was extracted, treated as industrial waste without reuse. However, when the inventor checked the components of the bottom ash, it was found that a large amount of silica sand effective as a fluidizing material was contained. On the other hand, in a CFB such as the first circulating fluidized bed boiler 10A that uses, for example, waste tires as the main fuel, the concentration of Na and K in the bottom ash is relatively low. Therefore, the inventor thought that it might be acceptable to use the bottom ash of the second circulating fluidized bed boiler 10B as the fluidizing sand, which would increase the introduction of Na and K. Therefore, in the operation method according to this embodiment, the bottom ash of the so-called biomass-only-fired CFB (the second circulating fluidized bed boiler 10B), which was conventionally treated as industrial waste, is reused as the fluidizing material of a CFB (the first circulating fluidized bed boiler 10A) that uses, for example, waste tires as the main fuel.

[0022] Here, the bottom ash of the second circulating fluidized bed boiler 10B generated by burning the biomass fuel derived from wood contains a large amount of silica sand. Moreover, the content of impurities that are not useful for combustion or inhibit combustion is small. Since silica sand acts as a heat-resistant heat transfer medium, it flows at high speed together with the fuel in the circulating fluidized bed boiler 10, allowing the fuel to burn uniformly and over a wide area within the boiler 10, thus enabling efficient fuel combustion. For this reason, furnace ash generated in the second circulating fluidized bed boiler 10B and containing a large amount of silica sand can be suitably used, for example, as fluidized sand for the first circulating fluidized bed boiler 10A.

[0023] While not particularly limited, the SiO2 (silica) content of the furnace ash used for fluidized sand is preferably 40% by mass or more. A SiO2 content of 40% by mass or more ensures good thermal efficiency when forming a fluidized bed, allowing for effective use as fluidized sand. However, a SiO2 content of less than 40% by mass is also acceptable.

[0024] Furthermore, the bottom ash used in this fluidized sand preferably has an Al2O3 content of 7% by mass or less, or a CaO content of 15% by mass or less, and is particularly preferably both. Using such bottom ash can suppress a decrease in the melting temperature of the ash and the formation of clinker, and can also maintain good thermal efficiency. However, the Al2O3 content may exceed 7% by mass, and the CaO content may exceed 15% by mass.

[0025] On the other hand, the ash produced from burning wood-derived biomass fuel contains sodium and potassium. As the content of these components increases, it lowers the melting temperature of the ash, making it easier for deposits called clinker to form. However, the fuels burned in the first circulating fluidized bed boiler 10A, such as waste tires, waste plastics, paper mill sludge, and RPF, are highly flammable. The ash produced by burning these fuels does not easily increase in sodium and potassium concentrations, and these effects are less likely to occur during combustion.

[0026] Therefore, in the operating method of this embodiment, the bottom ash generated in the second circulating fluidized bed boiler 10B can be suitably used as the fluidized sand in the first circulating fluidized bed boiler 10A. This suppresses the formation of clinker, and the predetermined bottom ash recovered as described above can be used as the fluidized sand in the circulating fluidized bed boiler, thereby enabling effective utilization of the bottom ash.

[0027] The bottom ash generated in the second circulating fluidized bed boiler 10B, that is, the bottom ash produced by burning wood-derived biomass fuel, is collected in the recovery unit 24. In this embodiment, the bottom ash from the recovery unit 24 of the second circulating fluidized bed boiler 10B is, for example, transported as appropriate and stored in the fluidized sand tank 13 of the first circulating fluidized bed boiler 10A, and supplied as fluidized sand to the combustion furnace 11 of the circulating fluidized bed boiler 10A.

[0028] In this embodiment, when the bottom ash is collected in the recovery unit 24 or when the fluidized sand is stored in the fluidized sand tank 13, it is preferable to separate and remove large-diameter particles from the bottom ash. Although not particularly limited, in this embodiment it is preferable to separate and remove large-diameter particles with a particle size of 3000 μm or more. By removing large-diameter particles, the fluidity when used as fluidized sand in the circulating fluidized bed boiler 10 can be improved, and as a result, the combustion efficiency of the circulating fluidized bed boiler 10 can be improved. However, the fluidized sand may contain particles (large-diameter particles) with a particle size of 3000 μm or more.

[0029] According to the operating method of the circulating fluidized bed boiler 10 described above, the bottom ash produced by burning wood-derived biomass fuel in the second circulating fluidized bed boiler 10B is used as fluidized sand in the circulating fluidized bed boiler 10A that is under operation. Therefore, the amount of bottom ash generated in the second circulating fluidized bed boiler 10B that is disposed of as waste can be reduced or eliminated. As a result, the operating method of this embodiment allows for the effective utilization of bottom ash, leading to improved operational efficiency and cost reduction.

[0030] Furthermore, according to the method for producing fluidized sand, the fluidized sand for the first circulating fluidized bed boiler 10A is produced by recovering the bottom ash generated from burning wood-derived biomass fuel in the second circulating fluidized bed boiler 10B. Therefore, it is easy to produce fluidized sand for the circulating fluidized bed boiler 10.

[0031] It should be noted that the technical scope of the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention. For example, in the above embodiment, multiple circulating fluidized bed boilers as shown in Figure 1 were used, but the structure of the circulating fluidized bed boiler is not particularly limited.

[0032] Furthermore, in the above embodiment, an example was described in which waste tires, waste plastics, or papermaking sludge were used as fuel for the first circulating fluidized bed boiler 10A. However, it is also possible to use woody biomass fuels such as wood chips, wood pellets, forest residues (including bark), construction-related woody waste, and palm residues (PKS) as part or all of the fuel for the first circulating fluidized bed boiler 10A in the above embodiment.

[0033] In that case, it is preferable to reduce or remove the sodium and potassium content by bringing the furnace ash into contact with an aqueous solution such as water, or by washing the furnace ash with an aqueous solution.

[0034] Furthermore, although the above embodiment mainly describes an example in which only the bottom ash produced by burning wood-derived biomass fuel in the second circulating fluidized bed boiler 10B is used as the fluidized sand in the first circulating fluidized bed boiler 10A, it is not particularly limited. In the embodiment, the fluidized sand in the first circulating fluidized bed boiler 10A can also be a mixture of fluidized sand made from bottom ash produced by burning wood-derived biomass fuel (bottom ash from the second circulating fluidized bed boiler 10B) and other fluidized sand different from this fluidized sand (supplementary sand). [Examples]

[0035] The following describes embodiments of the present invention.

[0036] [Examples 1-3] Long-term combustion tests were conducted using two independently installed circulating fluidized bed boilers 10A and 10B. The two circulating fluidized bed boilers 10A and 10B had the configuration shown in Figure 1. In the combustion tests, a constant amount of fuel was continuously burned in the circulating fluidized bed boilers 10A and 10B. Three combustion tests were conducted, with combustion periods of 21 days (Example 1), 46 days (Example 2), and 41 days (Example 3). The combustion conditions for each of the circulating fluidized bed boilers 10A and 10B were kept as constant as possible, although there were fluctuations during the combustion period.

[0037] The first circulating fluidized bed boiler 10A burned fuel containing 40% by mass of waste tires, 30% by mass of wood chips, and 30% by mass of papermaking sludge. The combustion conditions were such that the furnace bottom temperature was 850-900°C. The fluidized sand in this circulating fluidized bed boiler 10A was made from furnace bottom ash generated from the combustion of the second circulating fluidized bed boiler 10B. The second circulating fluidized bed boiler 10B burned wood chips as fuel. The combustion conditions were such that the furnace bottom temperature was 770-820°C.

[0038] In each combustion test, the bottom ash generated in the second circulating fluidized bed boiler 10B was recovered and used instead of being returned to the combustion furnace 11. Fluidized sand was produced from the recovered bottom ash by removing large-diameter particles of 3000 μm or more using a vibrating screen. The fluidized sand was fed into the fluidized sand tank 13 of the first circulating fluidized bed boiler 10A and used at a rate of 1 ton / day.

[0039] A portion of the bottom ash recovered from the second circulating fluidized bed boiler 10B was extracted, and its particle size distribution was measured before the removal of large-diameter particles. The bottom ash had the particle size distribution shown in Table 1. Furthermore, a component analysis of a portion of the extracted bottom ash yielded the results shown in Table 2. The total percentages in Table 2 were less than 100% by mass, and the remaining portion consisted of components not listed in the table. As is clear from Tables 1 and 2, the bottom ash recovered from the second circulating fluidized bed boiler 10B contained large-diameter particles of 3000 μm or more. Furthermore, the bottom ash contained a large amount of silica sand components such as silicon dioxide, as well as components that cause clinker formation, such as sodium and potassium.

[0040] [Table 1]

[0041] [Table 2]

[0042] In the combustion tests of Examples 1 to 3, fluidized sand produced by separating and removing large-diameter objects of 3000 μm or more from the bottom ash of the second circulating fluidized bed boiler 10B was used, and no problems such as clogging occurred when it was used in the first circulating fluidized bed boiler 10A. Furthermore, in the combustion tests of Examples 1 to 3, no operational problems such as clogging due to clinker formation or signs of abnormalities such as a drop in furnace bottom temperature were observed in the first circulating fluidized bed boiler 10A. After the completion of each test period, an inspection of the boiler interior revealed no increase in clinker formation or ash accumulation in the heat exchange section, etc. Furthermore, in Examples 1 and 2, when the concentrations of sodium and potassium in the bottom ash of the first circulating fluidized bed boiler 10A were measured during operation, it was confirmed that these concentrations were similar to those of the blank material (bottom ash of the second circulating fluidized bed boiler 10B) before it was used as fluidized sand, and it was determined that there was no concentration of sodium and potassium.

[0043] (Note) The above embodiment can be understood, for example, as follows: <1> One aspect of the present invention relates to a method for operating a circulating fluidized bed boiler, which involves recovering the bottom ash produced by burning wood-derived biomass fuel in another circulating fluidized bed boiler and using the bottom ash as fluidized sand in the circulating fluidized bed boiler.

[0044] This circulating fluidized bed boiler operation method uses the bottom ash from another circulating fluidized bed boiler as the fluidized sand for the boiler under operation. The bottom ash produced by burning wood-derived biomass fuel in another circulating fluidized bed boiler contains a large amount of silica sand and few impurities. Therefore, it can be suitably used as the fluidized sand for the circulating fluidized bed boiler.

[0045] Furthermore, by recovering the bottom ash generated in other circulating fluidized bed boilers and using it as fluidized sand, the amount of bottom ash that is disposed of as waste can be reduced or eliminated. As a result, according to the operating method of the circulating fluidized bed boiler of the present invention, it is possible to effectively utilize the bottom ash and improve operational efficiency and reduce costs.

[0046] <2> the above <1> In the operating method for the circulating fluidized bed boiler described herein, the circulating fluidized bed boiler is preferably one that uses the fluidized sand to burn at least one of the following fuels: waste tires, waste plastics, papermaking sludge, and RPF.

[0047] The bottom ash produced by burning wood-derived biomass fuel in other circulating fluidized bed boilers contains sodium and potassium. An increase in these elements lowers the melting temperature of the ash, making it easier for deposits called clinker to form. However, waste tires, waste plastics, paper mill sludge, and RPF are easily combustible fuels, and the concentrations of sodium and potassium do not increase easily, so these effects are less likely to occur when burned. As a result, the operating method of the present invention suppresses the generation of deposits, and the predetermined bottom ash recovered as described above can be used as fluidized sand in the circulating fluidized bed boiler, thus enabling effective utilization of the bottom ash.

[0048] <3> the above <1> or <2> In the operating method of the circulating fluidized bed boiler described above, it is preferable to obtain the fluidized sand by separating and removing large-diameter particles with a particle size of 3000 μm or more from the bottom ash of the furnace. By separating and removing large-diameter particles from the furnace bottom ash in this way, the fluidity of the ash can be improved when used as fluidized sand in a circulating fluidized bed boiler, and as a result, the combustion efficiency of the circulating fluidized bed boiler can be improved.

[0049] <4> the above <1> or <3> In the operating method of a circulating fluidized bed boiler described in any of the above, it is preferable to use the bottom ash with an SiO2 content of 40% by mass or more as the fluidized sand. As such, a high SiO2 content results in good thermal efficiency, making it effective for use as fluidized sand.

[0050] <5> the above <1> or <4> In the operating method of a circulating fluidized bed boiler described in any of the above, it is preferable to use the bottom ash having an Al2O3 content of 7% by mass or less as the fluidized sand. <6> Also, the above <1> or <5> In the operating method of a circulating fluidized bed boiler described in any of the above, it is preferable to use the bottom ash having a CaO content of 15% by mass or less as the fluidized sand. By keeping the Al2O3 and CaO content low, the melting temperature of the ash can be reduced, and the formation of clinker can be suppressed, resulting in good thermal efficiency and effective use as fluidized sand.

[0051] <7> A method for producing fluidized sand according to one aspect of the present invention involves operating a circulating fluidized bed boiler 10 by burning wood-derived biomass fuel in a circulating fluidized bed boiler and recovering the resulting bottom ash. By burning wood-derived biomass fuel in a circulating fluidized bed boiler and recovering the resulting bottom ash, fluidized sand for circulating fluidized bed boilers can be easily produced.

[0052] <8> the above <7> In the method for producing fluidized sand described above, it is preferable to separate and remove large-diameter particles with a particle size of 3000 μm or more from the recovered furnace bottom ash. In this way, it is possible to produce fluidized sand that is more likely to improve fluidity in a circulating fluidized bed boiler. [Explanation of symbols]

[0053] 10, 10A, 10B circulating fluidized bed boiler 11 Combustion furnace 12 Fuel supply section 13 Fluidized sand tank 14 Air intake 15 Circulation section 16 ducts 17 Heat exchange section 18 Dust collection unit 21 Extraction section 22 Cooling section 23 Sorting Department 24. Recovery Section

Claims

1. A method for operating a circulating fluidized bed boiler, A method for operating a circulating fluidized bed boiler, comprising recovering the bottom ash produced by burning wood-derived biomass fuel in another circulating fluidized bed boiler and using the bottom ash as fluidized sand in the circulating fluidized bed boiler.

2. The method for operating a circulating fluidized bed boiler according to claim 1, wherein the circulating fluidized bed boiler burns at least one fuel from among waste tires, waste plastics, papermaking sludge, and RPF using the fluidized sand.

3. A method for operating a circulating fluidized bed boiler according to claim 1, wherein fluidized sand is obtained by separating and removing large-diameter particles with a particle size of 3,000 μm or more from the bottom ash of the furnace.

4. SiO ２ The method for operating a circulating fluidized bed boiler according to claim 1, wherein the bottom ash having a content of 40% by mass or more is used as the fluidized sand.

5. Al ２ O ３ The method for operating a circulating fluidized bed boiler according to claim 1, wherein the bottom ash having a content of 7% by mass or less is used as the fluidized sand.

6. The method for operating a circulating fluidized bed boiler according to claim 1, wherein the bottom ash having a CaO content of 15% by mass or less is used as the fluidized sand.

7. Wood-derived biomass fuel is burned in a circulating fluidized bed boiler, and the resulting bottom ash is collected. A method for producing fluidized sand.

8. A method for producing fluidized sand according to claim 7, wherein large-diameter particles with a particle size of 3,000 μm or more are separated and removed from the recovered furnace bottom ash.

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