Treatment method for overtemperature ash leakage of boiler wall

Abstract

The invention discloses a treatment method for overtemperature ash leakage of a boiler wall. The treatment method comprises the following treatment steps: S1, a pretreatment process; s2, spraying a bottom layer; s3, staged composite curing; S4, surface layer spraying; and S5, the multi-layer expansion structure can be achieved through the process steps, thermal stress concentration can be relieved while micro cracks can be automatically closed, and the ash leakage occurrence rate is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of boiler furnace walls, and particularly to a method for treating over-temperature ash leakage of a boiler furnace wall. Background Art

[0002] As a core component of a power station boiler, the insulation structure of the boiler furnace wall has been facing the triple severe environmental challenges of high temperature, high pressure, and alternating thermal load for a long time. With the development of the power industry, traditional insulation technologies have been difficult to meet the reliability and economic requirements of modern units.

[0003] The surface curing of traditional boiler furnace walls relies on slow cross-linking at ambient temperature, with a slow curing speed, and there are problems such as surface drying shrinkage and cracking. Moreover, after the surface insulation layer is damaged, it cannot self-repair small gaps, and small gaps often gradually crack into large gaps, ultimately resulting in serious ash leakage from the boiler furnace wall, poor insulation efficiency of the insulation layer, and further causing poor efficiency of the entire boiler and a large increase in surface temperature, leading to potential safety hazards. To solve the above problems, the present invention proposes a brand-new method for treating over-temperature ash leakage of a boiler furnace wall. Summary of the Invention

[0004] The purpose of the present invention is to solve the deficiencies existing in the prior art, and to propose a method for treating over-temperature ash leakage of a boiler furnace wall.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A method for treating over-temperature ash leakage of a boiler furnace wall includes the following treatment steps:

[0007] S1, Pretreatment process: Remove the old insulation layer and accumulated ash, weld L-shaped anchor bolts, and coat a high-temperature resistant interface agent on the surface;

[0008] S2, Bottom layer spraying: Spray a 50-mm-thick nano-composite ceramic fiber layer using a high-pressure spraying device, and embed a Φ1.2-mm annealed stainless steel wire mesh to form a reinforcement layer;

[0009] S3, Staged composite curing: In the first stage, microwave curing uses 2.45 GHz microwave radiation, and in the second stage, infrared-assisted curing enhances the surface density;

[0010] S4, Surface layer spraying: Spray twice to a total thickness of 300 - 500 mm, cover the galvanized steel wire mesh and the aluminum outer protection plate, and embed self-healing microcapsules in the material;

[0011] S5, Multi-layer expansion structure: Set up a three-layer composite expansion layer in the areas of wall-piercing pipes, ceilings, and door holes.

[0012] Preferably, the self-repairing microcapsule comprises an inner core, an intermediate transition layer and an outer shell, the inner core is a mixture of polyimide resin and nano-alumina particles, the intermediate transition layer is a phosphate binder, and the outer shell is a silicon carbide nano-coating.

[0013] Preferably, the nano-composite ceramic fiber spraying material is composed of silicate non-metallic minerals, nano-crystals with a particle size of 10 to 50 nm, ceramic microbeads and a silica sol binder, wherein the mass of the nano-crystals accounts for 5% to 8%.

[0014] Preferably, the multi-layer expansion structure comprises an inner layer of high temperature resistant elastic ceramic fiber felt, a middle layer of corrugated stainless steel foil and an outer layer of aerogel composite felt, the inner layer is 10 mm thick and the middle layer is 0.1 mm thick.

[0015] Preferably, the expansion joint adopts a multi-stage stepped structure with a width of 8 to 12 mm, and elastic ceramic fiber strips are embedded in the joint to achieve dynamic compensation.

[0016] Preferably, the welding spacing of the L-shaped anchor nails in the pretreatment stage is dynamically adjusted according to the temperature gradient of the furnace wall, with a staggered arrangement of 200mm×200mm in the high temperature zone, 250mm×250mm in the medium temperature zone, and 300mm×300mm in the low temperature zone.

[0017] Preferably, during the microwave curing process, microwaves penetrate the thermal insulation layer to promote rapid cross-linking of the silica sol binder, and infrared curing enhances the density of the surface layer in the local high temperature area.

[0018] Preferably, a buffer layer is arranged between the galvanized steel wire mesh and the aluminum outer protective plate, and a bionic corrugated expansion joint is provided on the surface of the outer protective plate.

[0019] Preferably, the microcapsules are prepared by in-situ polymerization, the thickness of the silicon carbide nano-coating is 50 to 100 nm, and it has high temperature resistance and oxidation resistance.

[0020] Preferably, the nanocrystals are uniformly embedded in a silicate matrix by ultrasonic dispersion technology to form a three-dimensional network reinforcement structure.

[0021] The present invention has the following beneficial effects:

[0022] 1. The nano-composite ceramic fiber layer is combined with the gradient anchor nail arrangement to reduce the surface temperature of the furnace wall by more than 10%. At the same time, the stainless steel wire mesh reinforcement layer is used to improve the impact resistance. The microcapsule self-repairing technology can automatically close tiny cracks.

[0023] 2. The three - stage expansion layer design, elastic fiber felt + corrugated stainless steel foil + aerogel can increase the expansion absorption amount, thereby increasing the allowance during material deformation. Through the synergistic effect of stepped expansion joints and embedded elastic fiber strips, the concentration of thermal stress is dynamically relieved, reducing the ash leakage rate.

[0024] 3. Microwave curing realizes the synchronous cross - linking of internal binders, and infrared assistance precisely regulates the surface density, thus achieving uniformity during curing and reducing surface cracking caused by one side being completely cured while the other side is not fully cured during curing. Description of the Drawings

[0025] Figure 1 It is a flow chart of a method for treating over - temperature ash leakage in the boiler furnace wall proposed by the present invention.

[0026] Figure 2 It is a broken - line graph of the curing degree in 24 hours. Detailed Implementation Modes

[0027] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0028] A method for treating over - temperature ash leakage in the boiler furnace wall includes the following treatment steps:

[0029] S1. Pretreatment process: Remove the old insulation layer and accumulated ash, weld L - shaped anchor bolts, and coat the surface with a high - temperature - resistant interface agent;

[0030] S2. Bottom - layer spraying: Use a high - pressure spraying device to spray a 50 - mm - thick nano - composite ceramic fiber layer to form a flexible buffer layer; Reinforcement layer: Embed a Φ1.2 - mm annealed stainless - steel wire mesh to enhance the overall strength;

[0031] S3. Staged composite curing: The first - stage microwave curing: Use 2.45 - GHz microwave radiation to penetrate the inside of the insulation layer and promote the rapid cross - linking of the silica sol binder. The second - stage infrared - assisted curing applies infrared heating to local high - temperature areas to enhance the surface density and avoid cracking;

[0032] S4. Surface - layer spraying: Spray twice to a total thickness of 300 - 500 mm, cover with a galvanized steel wire mesh and an aluminum outer protection plate after trimming, and embed self - repairing micro - capsules in the surface - layer spraying material;

[0033] S5. Multi - layer expansion structure: Set up a three - layer composite expansion layer in the areas of through - wall pipes, ceilings and door holes, including: Inner layer: High - temperature - resistant elastic ceramic fiber felt, fitting the heated surface; Middle layer: Corrugated stainless - steel foil, installed through pre - compression and expanding to absorb the expansion amount after heating; Outer layer: Aerogel composite felt, filled into the bionic corrugated expansion joint.

[0034] Specifically, the microcapsule includes an inner core, an intermediate transition layer, and an outer shell. The inner core of the microcapsule is a polyimide resin and a curing agent such as nano-alumina particles; the intermediate transition layer of the microcapsule is a phosphate binder, which serves as the interface between the resin and the shell; the outer shell of the microcapsule is a silicon carbide nano-coating, which provides high temperature protection and mechanical strength. The nanocomposite ceramic fiber spray material is composed of silicate non-metallic minerals, nano-crystals, ceramic microbeads and silica sol binder, with a density of 160-180 kg / m 3 , thermal conductivity ≤0.10W / (m·K). In the multi-layer expansion structure, there are three composite expansion layers, the inner layer of high-temperature resistant elastic ceramic fiber felt is 10mm thick, and the middle layer: the thickness of the corrugated stainless steel foil is 0.1mm. In the dynamic compensation step, the expansion joint is a multi-level stepped structure with a width of 8 to 12mm. The elastic ceramic fiber strips embedded in the joints realize dynamic compensation through fiber strip compression-rebound during thermal expansion. The nano-crystal additives are uniformly embedded in the silicate matrix through nano-dispersion technology, with a particle size of 10 to 50nm and a mass proportion of 5% to 8%. In the pretreatment stage, the spacing of the welded L-shaped anchor nails is dynamically adjusted according to the temperature gradient of the furnace wall area: high temperature zone: the spacing of the anchor nails is 200mm×200mm staggered arrangement; medium temperature zone: the spacing of the anchor nails is 250mm×250mm staggered arrangement; low temperature zone: the spacing of the anchor nails is 300mm×300mm staggered arrangement. Furthermore, the aluminum outer guard plate is modular and detachable, thereby avoiding the need for overall disassembly and repair when a single point is severely damaged.

[0035] Embodiment 1:

[0036] All the above technical means are adopted, among which the anchor nail arrangement is: 200mm×200mm in high temperature zone, 250mm×250mm in medium temperature zone, 300mm×300mm in low temperature zone, bottom layer thickness: 50mm nano-composite ceramic fiber layer (density 175kg / m 3 , thermal conductivity 0.095W / (m·K)), microcapsule parameters: polyimide resin + nano-alumina core, silicon carbide shell, expansion joint structure: three-level stepped 8mm joint width, pre-embedded elastic fiber strips.

[0037] Embodiment 2:

[0038] The above technical means are used, but no microcapsules are added as a gap filling aid.

[0039] Embodiment three:

[0040] Adopt the commonly used process steps in the market:

[0041] S1. Base treatment: remove oxide scale and apply high temperature adhesive

[0042] S2, Anchoring System: Weld L-shaped stainless steel nails (spacing 250 mm)

[0043] S3, Filling Layer: Inner layer: 50 mm thick aluminum silicate fiber blanket (density 140 kg / m 3 ) Outer layer: 150 mm thick high-aluminum refractory castable (Al2O3 ≥ 45%). Expansion joint: Fill the annular gap with elastic ceramic fiber rope.

[0044] S4, Surface Spraying: Spray outside the filling layer until the total thickness reaches 300 - 500 mm. Self-healing microcapsules are embedded in the surface spraying material.

[0045] S5, Laying the Outer Protective Layer: 0.8 mm color steel plate + sealant

[0046] It should be noted that in the comparative examples, two common methods for treating overheating and ash leakage of boiler furnace walls in the market are adopted. Specifically, refer to Table 1. In terms of thermal conductivity efficiency, in Example 1 and Example 2: These two examples use a nano-composite ceramic fiber spraying material, which is composed of silicate non-metallic minerals, nano-crystals, ceramic microspheres, and a silica sol binder. The nano-crystals are evenly embedded in the silicate matrix through nano-dispersion technology. This special material composition and structure contribute to reducing heat conduction. The small-size effect and quantum effect of the nano-materials can effectively scatter phonons, reduce heat flow transfer, and thus lower the thermal conductivity coefficient. In contrast, in Comparative Example 1, ordinary ceramic fiber spraying is used, and in Comparative Example 2, an aluminum silicate fiber blanket and high-aluminum refractory castable are adopted. These traditional materials are not conducive to suppressing heat conduction in terms of microstructure and composition, so the thermal conductivity coefficient is relatively high;

[0047] The thermal conductivity coefficient of Example 1 is lower than that of Example 2 because the density setting of the nano-composite ceramic fiber layer in Example 1 is more conducive to forming a good heat insulation structure, and the microcapsule technology contained also reduces heat transfer. In Example 3: Although the microcapsule technology is also used, the bottom layer adopts a traditional aluminum silicate fiber blanket and high-aluminum refractory castable filling layer. The thermal conductivity of this part of the material itself is not as good as that of the nano-composite ceramic fiber material, so the thermal conductivity coefficient is higher than that of Example 1 and Example 2 but lower than that of the two comparative examples, indicating that other technical means such as microcapsules also play a role in reducing the thermal conductivity coefficient to a certain extent.

[0048] Specifically, in terms of the highest surface temperature, in Example 1 and Example 2, they are only 38.9°C and 42.1°C respectively. Due to the low thermal conductivity, the heat transfer from the furnace interior to the furnace wall surface is effectively blocked, resulting in a relatively low furnace wall surface temperature. In addition, the nano-composite ceramic fiber layer sprayed on the bottom layer forms a flexible buffer layer, which can better adapt to thermal expansion and contraction, reduce heat leakage caused by thermal stress concentration, and further lower the surface temperature. In contrast, the thermal conductivities of Comparative Example 1 and Comparative Example 2 are high, and heat is more likely to be transferred to the furnace wall surface, leading to an increase in the surface temperature.

[0049] Example 3: Although the surface temperature is higher than that of Example 1 and Example 2, it is lower than that of the comparative examples. This is because the presence of microcapsules repairs the tiny cracks in the material to a certain extent, reducing the heat leakage channels. At the same time, structures such as its filling layer and outer protection layer also play a certain heat insulation role. However, the overall heat insulation effect is not as good as that of Example 1 and Example 2 with a nano-composite ceramic fiber bottom layer.

[0050] Specifically, in terms of the ash leakage amount, in Example 1: it is 0.03 kg / m 2 , because it adopts a self-healing microcapsule technology. When tiny cracks appear in the furnace wall, the microcapsules rupture and release repair materials such as polyimide resin to fill the cracks and prevent dust leakage. At the same time, the multi-layer expansion structure can effectively absorb the thermal expansion amount, reduce the cracks generated by thermal expansion and contraction, and further reduce the possibility of ash leakage. In addition, the anchor bolts dynamically adjust the spacing according to the temperature gradient in the furnace wall area, making the insulation layer fit more tightly with the furnace wall and enhancing the overall sealing performance.

[0051] Example 2: it is 0.11 kg / m 2 Although it does not use the microcapsule technology, the presence of the nano-composite ceramic fiber layer and the multi-layer expansion structure makes the overall structural stability of the furnace wall better, which can reduce the generation of cracks to a certain extent, so the ash leakage amount is lower than that of the comparative examples. However, due to the lack of the self-healing function of the microcapsules, the ash leakage amount is higher than that of Example 1.

[0052] Example 3: it is 0.35 kg / m 2 , although it uses the microcapsule technology and reduces the ash leakage to a certain extent, due to the use of traditional materials for the bottom layer and filling layer, the sealing and crack resistance of the overall structure are not as good as those of Example 1 and Example 2. Therefore, the ash leakage amount is higher than these two examples but lower than that of Comparative Example 1 and Comparative Example 2: The traditional construction process and materials are prone to generating cracks when dealing with thermal expansion and contraction, and there is no effective crack repair mechanism and perfect expansion compensation structure, resulting in dust being easily leaked from the cracks, so the ash leakage amount is large.

[0053] In summary, the embodiments perform better than the comparative examples in terms of thermal conductivity, maximum surface temperature, ash leakage, etc. This is mainly because advanced technologies and processes such as nano-composite ceramic fiber materials, self-healing microcapsule technology, multi-layer expansion structure, and reasonable arrangement of anchor bolts are adopted. The combined effect of these factors improves the heat insulation performance, structural stability, and sealing performance of the boiler furnace wall.

[0054] Table 1: Comparison table of thermal conductivity and ash leakage

[0055]

[0056] Specifically, referring to Table 2; in terms of compressive strength, Φ1.2mm annealed stainless steel wire mesh is embedded in the bottom layer spraying as a reinforcement layer in these two embodiments. The stainless steel wire mesh has high strength and toughness, can evenly disperse the pressure, and enhance the compressive capacity of the overall structure. At the same time, the nano-composite ceramic fiber layer itself also has a certain strength, and after the silica sol binder is subjected to staged composite curing, the fibers can be closely combined, further improving the compressive performance of the material; the compressive strength of Example 1 is slightly higher than that of Example 2 because the overall process of Example 1 is more perfect, the cooperation between steps is better, making the structure denser, thus improving the compressive strength.

[0057] Example 3: Although the microcapsule technology is adopted, the bottom layer uses traditional aluminosilicate fiber blanket and high-alumina refractory casting material filling layer. The compressive performance of this traditional structure is inferior to that of the structure combined with nano-composite ceramic fiber and stainless steel wire mesh, so the compressive strength is lower than that of Example 1 and Example 2, but higher than that of Comparative Example 1.

[0058] Comparative Example 1: Ordinary ceramic fiber spraying is used. The structure of this material itself is relatively loose and lacks an effective reinforcement structure, so the compressive strength is low.

[0059] Comparative Example 2: Although the partial compressive strength reaches 2.25 MPa, the overall structure is a composite structure of aluminosilicate fiber blanket and casting material. The compressive performance of the fiber blanket is relatively weak, which may affect the compressive performance of the overall structure. And from the comprehensive performance, it is inferior to the embodiments in other aspects.

[0060] Specifically, looking at Examples 1 and 2 in terms of expansion compensation: They adopt a multi-layer expansion structure, and three layers of composite expansion layers are set in the areas of wall-piercing pipes, ceilings, and door holes. The inner high-temperature resistant elastic ceramic fiber felt has good elasticity and can deform to a certain extent when heated; the middle corrugated stainless steel foil is installed through pre-compression and can expand after heating to absorb a large amount of expansion; the outer aerogel composite felt is filled in the bionic corrugated expansion joint and can also assist in absorbing expansion. The cooperation of this multi-layer structure greatly enhances the expansion compensation ability.

[0061] The expansion compensation amount of Example 1 is slightly higher than that of Example 2. This is because for the microcapsules in Example 1, the cracked surface was repaired in a timely manner, enabling better adaptation to thermal expansion. Example 3: Although there is also a design of expansion joints, compared with the multi-layer expansion structures of Example 1 and Example 2, its structure is relatively simple and the expansion compensation ability is limited. Therefore, the expansion compensation amount is lower than these two examples but higher than that of the comparative example.

[0062] Comparative Example 1 and Comparative Example 2: The expansion joint in Comparative Example 1 is a simple straight joint of 15 mm filled with rock wool strips, and Comparative Example 2 is an annular gap filled with elastic ceramic fiber ropes. These traditional expansion joint designs have less deformation ability and compensation effect than the multi-layer expansion structure when dealing with thermal expansion. Therefore, the expansion compensation amount is relatively small.

[0063] Specifically, from the perspective of self-healing, Example 1: The self-healing microcapsule technology is used. The inner core of the microcapsule contains polyimide resin and curing agent. When cracks appear in the furnace wall, the microcapsules rupture, and the resin and curing agent react rapidly to fill the cracks. Moreover, its overall structure is relatively dense, and the crack propagation is relatively slow, which is conducive to the timely action of the microcapsules. Therefore, the self-healing response time is relatively short.

[0064] Example 2, Comparative Example 1 and Comparative Example 2: Since no microcapsules are added as auxiliary for gap filling, they do not have self-healing ability.

[0065] Example 3: Although the microcapsule technology is adopted, the bottom layer uses traditional materials, and the structure is relatively complex. The generation and expansion of cracks may be more complex, resulting in an increase in the difficulty for the microcapsules to play a role. Therefore, the self-healing response time is longer than that of Example 1.

[0066] In summary, the examples perform better than the comparative examples in terms of compressive strength, expansion compensation amount, self-healing response time, etc. This is mainly because advanced materials, reasonable reinforcement structures, multi-layer expansion structures, and self-healing microcapsule technology are adopted. These factors jointly improve the performance of the boiler furnace wall.

[0067] Table 2; Tensile Strength and Expansion Compensation Comparison Table

[0068]

[0069] Specifically, refer to Table 3. Figure 2 , From the comparison of the overall curing speed, Examples 1 and 2 have significant advantages: The curing degrees of Examples 1 and 2 at each time node far exceed those of Comparative Examples 1 and 2. At 24 hours, the curing degree of Example 2 reaches 98%, while that of Comparative Example 2 is only 76%. This is because of the formulation containing nano-catalytic particles or high-active resins. The high specific surface area of the nano-particles can greatly reduce the activation energy of the curing reaction and accelerate molecular cross-linking, resulting in a much faster curing speed than that of Comparative Examples 1 and 2.

[0070] Furthermore, the 24-hour curing degree of Example 1 is lower than that of Example 2 because, to ensure the initial stability of the system, the components on the surface of the microcapsules hardly react with the curing material in the early stage of curing, which slows down the reaction rate to a certain extent.

[0071] Table 3: Comparison table of 24-hour curing degree.

[0072]

[0073]

[0074] As described above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes should be covered by the protection scope of the present invention.

Claims

1. A method for treating over-temperature ash leakage in a boiler furnace wall, characterized in that The following steps are involved: S1. Pretreatment process: remove the old insulation layer and dust accumulation, weld L-shaped anchor nails, and apply high temperature resistant interface agent on the surface; S2. Bottom layer spraying: Use high-pressure spraying equipment to spray a 50mm thick nano-composite ceramic fiber layer, and embed a Φ1.2mm annealed stainless steel wire mesh to form a reinforcement layer; S3, staged composite curing: the first stage microwave curing uses 2.45GHz microwave radiation, and the second stage infrared assisted curing enhances the surface density; S4, surface spraying: spray twice to a total thickness of 300-500mm, covering the galvanized steel mesh and aluminum outer guard plate, and embedding self-repairing microcapsules in the material; S5. Multi-layer expansion structure: three layers of composite expansion layers are set in the wall pipe, ceiling and door hole areas.

2. The method for treating superheated temperature and ash leakage of a boiler furnace wall according to claim 1, characterized in that: The self-repairing microcapsule comprises an inner core, an intermediate transition layer and an outer shell, wherein the inner core is a mixture of polyimide resin and nano-alumina particles, the intermediate transition layer is a phosphate binder, and the outer shell is a silicon carbide nano-coating.

3. The method for treating over-temperature ash leakage of a boiler furnace wall according to claim 1, characterized in that: The nano-composite ceramic fiber spray material is composed of silicate non-metallic minerals, nano-crystals with a particle size of 10 to 50 nm, ceramic microbeads and silica sol binder, of which the mass of the nano-crystals accounts for 5% to 8%.

4. A method for treating over-temperature ash leakage in a boiler furnace wall according to claim 1, characterized in that: The multi-layer expansion structure comprises an inner layer of high temperature resistant elastic ceramic fiber felt, a middle layer of corrugated stainless steel foil and an outer layer of aerogel composite felt, the inner layer is 10 mm thick and the middle layer is 0.1 mm thick.

5. The method for treating over-temperature ash leakage of a boiler furnace wall according to claim 1, characterized in that: The expansion joint adopts a multi-stage stepped structure with a width of 8 to 12 mm, and elastic ceramic fiber strips are pre-embedded in the joint to realize dynamic compensation.

6. The method for treating over-temperature ash leakage of a boiler furnace wall according to claim 1, characterized in that: The welding spacing of the L-shaped anchor nails in the pretreatment step is dynamically adjusted according to the temperature gradient of the furnace wall, with a staggered arrangement of 200mm×200mm in the high temperature zone, 250mm×250mm in the medium temperature zone, and 300mm×300mm in the low temperature zone.

7. A method for treating over-temperature ash leakage of a boiler furnace wall according to claim 1, characterized in that: During the microwave curing process, microwaves penetrate the thermal insulation layer to promote rapid cross-linking of the silica sol binder, and infrared curing enhances the density of the surface layer in the local high temperature area.

8. The method for treating over-temperature ash leakage of a boiler furnace wall according to claim 1, characterized in that: A buffer layer is arranged between the galvanized steel wire mesh and the aluminum outer protective plate, and a bionic corrugated expansion joint is arranged on the surface of the outer protective plate.

9. A method for treating over-temperature ash leakage of a boiler furnace wall according to claim 2, characterized in that: The microcapsule is prepared by an in-situ polymerization method, the thickness of the silicon carbide nano-coating is 50 to 100 nm, and the microcapsule has high temperature resistance and oxidation resistance.

10. The method for treating over-temperature ash leakage of a boiler furnace wall according to claim 3, characterized in that: The nano-electric crystals are uniformly embedded in a silicate matrix through ultrasonic dispersion technology to form a three-dimensional network reinforcement structure.

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