Study on energy-saving thermal insulation effect of high-temperature steam pipelines in thermal power plants using nanoporous aerogel super insulation technology

Different silicon sources will have a large impact on the structure and properties of nanoporous SiO₂ aerogels [20]. Among the many silicon sources, silanol salts are currently the preferred materials for the preparation of SiO₂ aerogels due to their easy solubility in common organic solvents, the ability to obtain highly pure, highly dispersed, and homogeneous solutions, and the ease of achieving chemical compositional ratios, low reaction temperatures, and the avoidance of unwanted by-product generation. Among them, methyl orthosilicate (TMOS) has higher silicon content and a faster hydrolysis rate. In the early stage, it was mostly used to prepare aerogel from a TMOS silicon source, which has a narrower pore size and more homogeneous distribution. Still, the methanol generated from hydrolysis is more toxic. Ethyl orthosilicate (TEOS) is currently the most used silicon source. The preparation of aerogel performance is better. The process is more stable. Aerogels prepared with methyltriethoxysilane (MTES) and methyltrimethoxysilane (MTMS) as the silicon source, due to the relatively small number of alkoxyl groups (Si-OR) contained in the structure, only a small part of the network structure formed after hydrolysis is connected, and the aerogel obtained has a larger pore size and tends to have a certain degree of flexibility. The performance of SiO₂ aerogels prepared with water glass, polysiloxane, and rice husk as silica sources needs to be further improved.

Three different silica sources were used to prepare nanoporous SiO₂ aerogels in the preparation tests, and the observation of their peripheral structure samples can be concluded that the SiO₂ aerogel network skeleton structure obtained by using TEOS as the silica source is dense, and the pore size is smaller. The SiO₂ aerogel prepared by using MTMS or water glass as the silicon source has a looser structure, a less complete network structure, a low skeleton strength, a larger pore size, and an unfavorable pore size to inhibit heat conduction. Therefore, TEOS was selected as the silica source in this paper.

The role of catalysts is to promote the hydrolysis and condensation reactions of silica sources such as TEOS, which mainly include acid-catalyzed, base-catalyzed, and acid-base two-step catalysis. Different catalytic types have different mechanisms, and the obtained sol structures and properties are also different, as follows (using TEOS as the silicon source): 1)

Acid catalysis

The main types of acid catalysis used are HF, HCI, HNO₃, H₂SO₄, CH₃COOH, C₂H₂O₄, etc. [21]. Among them, HF is able to attack the silicon nucleus directly due to the smaller F^-radius, and the hydrolysis rate is faster, but HF is more corrosive.CH₃COOH and C2H₂O₄ are weak in acidity, which is easy to cause insufficient hydrolysis of TEOS. HNO₃ and H₂SO₄ are strong in acidity. Therefore, HCI was used as the catalyst in this paper.

Under HCI-catalyzed conditions, the mechanism of the TEOS hydrolysis reaction is as follows: H⁺ firstly attacks and protonates a -OR group in the TEOS molecule, which causes the electron cloud to shift to this -OR group so that the surface gap on the other side of the nucleus of the silicon atom increases and becomes electrophilic, and therefore the more electronegative CI^- is able to attack the silicon atom to make the TEOS undergo hydrolysis, and the specific reaction is as shown in Eq. (4) and Eq. (5). The specific reaction is shown in equation (4) and (5):

According to the electrophilic hydrolysis mechanism of TEOS, as the hydrolysis proceeds, part of Si-OR is replaced by Si-OH, which leads to less and less negative charge and more and more positive charge of the oxygen atoms of the central silicon atoms and Si-OR due to its electron-absorbing effect, while H⁺ also carries a positive charge, and repulsive effect between the same kind of charge makes it difficult for H⁺ to approach with the silicon atoms, which leads to the decrease of the reactivity and the slowing down of the hydrolysis rate. The further occurrence of Si-OH substitution reaction is more difficult. Due to less Si-OH available for polycondensation reaction, and when the intermolecular polycondensation reaction occurs, by the influence of spatial site resistance effect, hydrolysis and further polycondensation reactions are more difficult to occur. Therefore, under acidic conditions, the SiO₂ aerogel condensation products have a low cross-linking degree, easy to form a one-dimensional chain structure, and smaller pore size but a larger degree of contraction. 2)

Alkali catalyzed

Hydrolysis of TEOS under alkaline catalyst conditions is a nucleophilic reaction mechanism in which OH^- ions directly attack the nucleus of silicon atoms, as shown in Eq. (6). OH^- is negatively charged and has a small ionic radius, and the phase-absorption effect between the anisotropic charges makes it easy to approach the attacking group and the central silicon atom, and the reactivity of the molecule is significantly increased, with a faster hydrolysis rate:

During the NH₃H₂O-catalyzed process, the TEOS hydrolysis products condense at a fast rate, and the molecular weight of SiO₂ sol increases rapidly, which is macroscopically manifested by a rapid increase in viscosity. Moreover, the polymerization reaction proceeds in a multidimensional direction, forming a structure of short-chain cross-linking. As the polymerization reaction proceeded, the short chains were continuously crosslinked with each other, and the gel was finally formed. 3)

Acid-base two-step catalysis

Under neutral conditions, the nucleophilic attack ability of H₂O is very weak. TEOS is difficult to hydrolyze and can only rely on the trace protons in it to catalyze the reaction. Under acidic conditions, the rate of hydrolysis reaction is greater than the rate of polycondensation reaction, and the polycondensation reaction of silicate monomer forms a multi-branched, weakly cross-linked polymer-like gel. Under alkaline conditions, the rate of polycondensation is greater than the rate of hydrolysis, and silicate monomers polycondense form a gel network composed of relatively dense gel particles. The former network is prone to contraction, and the latter gel particles are too linear. In order to regulate the difference between the relative rates of hydrolysis and polycondensation reaction, the sol-gel can be prepared by the two-step method of acid-alkali so that the sol-gel process can be carried out under the catalytic conditions of strong acid and weak alkali, respectively, which is conducive to the control of the network structure, and to the preparation of the structurally homogeneous gels. In this thesis, the acid-alkali two-step method was used to formulate SiO₂ aerogels.

The newly formed gel is poorly structured and needs to be aged. Aging of the gel serves two purposes: first, it allows the catalyst to evaporate. The second is to increase further cross-linking of the gel and strengthen the gel skeleton to prevent cracking during subsequent drying. The more thorough the aging process, the less shrinkage will occur during gel drying. Aging can be regarded as a continuation of the gelation process; condensation and coarsening will occur during the aging process, and the Si-OH and Si-OCH₂CH₃ groups that are still present in the gel will continue to condense to form Si-O-Si, the gel volume will shrink, and the network will gradually become coarser. However, if the aging time is too long, the gel will produce excessive volume contraction. Generally, the aging time is selected as 1~2 days.

Thermal properties were analyzed based on the thermogravimetric change curve (TG) differential scanning calorimetry (DSC) curve produced by the specimen with temperature to analyze the physical and chemical reactions as well as the phase changes occurring inside the specimen under the condition of increasing temperature. The TG-DSC test was done by taking 15 mg of dried SiO₂ aerogel specimen, and the curve obtained is shown in Fig. 2. Comparison of the TG and DSC curves shows that there are two distinct weight loss steps in the TG curve. Between room temperature and 268°C, there is no appearance of heat absorption and exothermic peaks on the DSC curve, corresponding to the absence of weight loss on the TG curve, which is mainly due to the fact that water and alcohols present in the aerogel in a physically adsorbed state were basically displaced by n-alkanol during the azeotropic distillation and drying process. There was a clear exothermic peak at 330.98 °C, which was mainly due to the oxidation of organic groups and residual solvents in the aerogel during the forging process. Another obvious exothermic peak appeared at 531.25 °C, which was due to the oxidation of the unoxidized hydrophobic groups, as evidenced by the weak absorption peak of the hydrophobic group Si-OC₄H₉ at and after forging at 300 °C in the IR spectral analysis. After about 800°C, the TG curves tend to be parallel, and no obvious exothermic peaks appear in the DSC curves. This belongs to the viscous flow densification stage of the network skeleton when the weight loss of the specimen is very small. The plasticity of the nanoskeleton increased viscous flow occurred, the gel gradually tended to be dense, and the total weight loss of the sample reached 16.09%. It can be determined that the organic groups and residual impurities can be basically removed from the aerogel after forging at 550 °C, the quality can be further stabilized, and the strength of the skeleton can be strengthened after forging. Accordingly, the SiO₂ aerogels were subjected to forging treatment at 300 °C, 550 °C, 850 °C, 1000 °C and 2 h each.

Figure 2.

TG and DSC curves of SiO₂ xerogel

In order to understand the chemical bonding mode of SiO₂ aerogel, the specimens were analyzed by red-over-sea absorption spectroscopy after calcination at different temperatures, and the test results are shown in Fig. 3. The absorption peaks of 61 cm^-1, 400 cm^-1 and 665 cm^-1 in the figure are the characteristic absorption peaks of SiO₂, which correspond to the antisymmetric stretching vibration, symmetric stretching vibration and bending vibration of Si-O-Si bond, respectively. After the azeotropic distillation and drying process, its infrared spectrum showed absorption peaks representing C-H vibration at 798~956 cm^-1 and 2368~2584 cm^-1, while the absorption peaks corresponding to hydroxyl group at 2941 cm^-1 and 542 cm^-1, whose peak intensities were very weak, indicating that the hydroxyl group on the surface of SiO₂ particles had been replaced by the butoxy group of n-butanol in the majority, which resulted in the formation of Si-OC₄H₉. The residual hydroxyl group on the surface of SiO₂ aerogel is the main source of hydrophilicity, and the lower the residual amount, the better the hydrophobicity of SiO₂ aerogel. After the hydrophobic modification process, the contact angle of the solid-liquid interface between the surface of the solid particles of the SiO₂ aerogel and the solvent in the gel can be changed, which can reduce the effect of the high surface tension solvent on the pore structure of the gel during the drying process. In addition, the azeotropic distillation drying process replaces the water in the wet gel with n-butanol, which has low surface tension and greatly reduces the degree of pore structure collapse caused by the capillary phenomenon during the drying process. It can be seen that this process can simultaneously achieve the purpose of hydrophobicity modification on the particle surface and sol replacement in the gel, which creates favorable conditions to ensure the formation and stabilization of the nanopore network structure.

Figure 3.

IR spectra of SiO₂ xerogel at different calcining temperatures

From the infrared absorption spectra of SiO₂ aerogels after different calcination temperatures, it can be seen that the intensity of the vibrational absorption peaks of hydrophobic Si-OCH at 798-956 cm^-1 and 2368-2584 cm^-1 decreased significantly after calcination at 300 °C, whereas the peak intensities of the O-H telescoping vibrational peaks at 2941 cm^-1 and Si-OH bending quasi-vibrational peaks at 542 cm^-1 increased. Combined with the exothermic peak at 330.98 °C in the TG-DSC curve and the mass change, it proves that the hydrophobic genes burn and decompose into hydroxyl groups after calcination at 300 °C. After calcination at this temperature, the residual organic solvent and water in the gel were removed, and the rich nanopore three-dimensional network structure inside the SiO₂ aerogel was formed, which made it show great activity. With the increase of calcination temperature, the anti-symmetric stretching vibration peak of Si-O-Si at 582 cm^-1 was gradually shifted to the direction of high wave, i.e., blueshift phenomenon, indicating that the condensation reaction led to the continuous expansion and strengthening of the silica-oxygen network. The peak strength of the hydroxyl absorption peaks weakened, and the three peaks almost completely disappeared at 1000 °C, indicating that various types of hydroxyl groups and adsorbed water on the particle surface can be removed at high temperature, and at this time, the aerogel is composed entirely of the network formed by the Si-O-Si bonds.

Meanwhile, the antisymmetric stretching vibration peak at 582 cm^-1 exists in the form of a single peak, indicating that the Si-O-Si bond is a short chain structure. This is mainly due to the fact that ammonia was used as an alkaline catalyst to accelerate the gel formation speed in this experiment during the sol-gel formation process. When the gel process reacts slower, the Si-O-Si bond in the form of a long chain is formed, and the bond shows a bimodal structure on the infrared absorption spectrum.

In order to determine the phase change of the aerogel during the calcination process, the specimens after calcination at different temperatures were analyzed by X-ray diffraction (XRD), and the results are shown in Figure 4. As can be seen from the figure, the SiO₂ aerogel is in an amorphous state without any phase change from room temperature to 1000°C and is suppressed in an amorphous structure. This indicates that during the calcination process, the specimen can only increase the coherence of the network structure through the spatial rearrangement of the SiO₂ cluster particles and increase the densification but cannot change its phase composition.

Figure 4.

XRD patterns of SiO₂ xerogel at different calcining temperatures

A thermal power plant high-temperature steam pipe types, including the use of traditional aluminum silicate cotton pipe and nanopore aerogel super insulation technology composite pipe, the two types of pipe as a comparative object, and at the same time in the same environment for testing, you can understand the two in the actual application of the functional performance. Test environment: ambient temperature of 22 °C, medium temperature of 65 °C. The two types of pipe insulation basic requirements for the insulation layer design temperature is not greater than 40 °C.

The traditional aluminum silicate cotton (M1) material and nano air gel (M2) are selected for comparative analysis. Set the steam temperature in the steam pipe to 500 °C. The pipe’s outer diameter is 0.5 m. Calculate the heat loss of the two materials under different thicknesses of the insulation layer. The results are shown in Table 1. From the table, it can be seen that in the same insulation effect requirements, nanoporous aerogel’s required insulation layer thickness is significantly smaller than aluminum silicate cotton, and the heat loss is significantly reduced.

The thickness of the nanoporous aerogel insulation layer is optimized and calculated with the goal of minimizing heat loss. Change the steam temperature in the steam pipe and the external ambient temperature and other parameters to get the optimized insulation layer thickness under different working conditions. The results are shown in Table 2. The data in the table show that with the increase in steam temperature and the decrease in ambient temperature, the thickness of the nanoporous aerogel insulation layer needs to be increased appropriately in order to ensure lower heat loss.

Table 3 shows the results of the economic benefit calculation of nanopore aerogel superinsulation technology applied to energy-saving thermal insulation of high-temperature steam pipes in thermal power plants. C1-C12 in the table indicates the heat saved by 1m pipe, the annual heat saved by 1m pipe, the annual saving benefit of 1m pipe, the length of pipe, the annual saving benefit of pipe, the total annual energy-saving benefit, the thermal insulation cost of nanopore aerogel material, thermal insulation cost of aluminum silicate cotton, pipe payback period calculation (shortest), piping payback period calculation (longest), piping 10-year energy-saving benefits, DN400, DN300 and DN250 are three kinds of high-temperature steam pipes used in thermal power plants. As can be seen from the table, the project investment estimate of 1,694,800 yuan, the use of nanopore aerogel super insulation technology to save steam costs 851,400 yuan per year (according to reduce heat loss conservative estimates, not taking into account the reduction in steam production after the steam temperature is raised), is expected to 0.98 ~ 1.99 years to recover the investment.