Selection of anti-corrosion solutions for desulfurization chimneys

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In the current design standards for chimneys in the ** and power industries, there are relevant provisions regarding the anti-corrosion design of chimneys after flue gas desulfurization; however, there is a lack of specific, actionable guidelines for the chemical anti-corrosion design of old chimneys that have been modified for desulfurization purposes. Therefore, there are a wide variety of anti-corrosion materials used in chimney renovation projects in China at present, which can be roughly classified into the following categories: high-performance heat and acid-resistant coatings, acid-resistant rigid armor, integral acid-resistant castables for lining, flexible adhesive lightweight insulating blocks (such as Pergo bricks, domestic foam glass bricks, lightweight vitrified ceramic bricks), and the use of corrosion-resistant steel inner cylinders (titanium-steel composite plate inner cylinders), among others. High-performance heat and acid-resistant coating 1: Modified polyurea 1) Chemical principle of spray-applied modified polyurea. Modified polyurea materials are two-component polymer compounds; their spraying process belongs to the category of fast-reacting spraying systems. The raw material mixture does not contain catalysts, allowing for rapid curing. It can be sprayed and shaped on any curved, inclined, or vertical surface, making the process simple and easy to implement. 2) Material properties: Modified polyurea possesses good physical properties, such as resistance to chemicals, tensile strength, low-temperature flexibility, outdoor aging resistance, adhesion, wear resistance, and impact resistance. 3) Good construction properties: When applying modified polyurea, specialized spraying equipment is required to heat the raw materials to a certain temperature at the nozzle before spraying them; it cures rapidly, allowing the desired thickness to be achieved in a single application, thus overcoming the disadvantages of multi-layer application. This material is insensitive to moisture and humidity, and its application is not affected by the environment or humidity levels. Construction can be carried out throughout the year under dry surface conditions. It has good durability and temperature resistance. In the atmospheric environment, modified polyurea exhibits good aging resistance, with a durability of up to 15 years. This technology is widely used in the petrochemical industry, mainly for the corrosion protection of facilities such as pipelines, storage tanks, and liquid storage ponds; the corrosion protection temperature generally does not exceed 50 °C. 2 Glass flake composite materials 1) Corrosion resistance mechanism of glass flakes Glass flake coatings are made using vinyl ester resin as the main material, to which glass flakes and other materials are added. After application, in glass flake coatings, the flat glass flakes arrange themselves in parallel and overlapping layers within the resin continuous phase, thereby forming a dense impermeable layer structure. After curing, the glass flake coating can effectively reduce the penetration of water vapor and corrosive chemical agents due to the high cross-linking density of the vinyl ester resin; the unique structure of the glass flakes further enhances the effect of preventing or reducing permeation. 2) Characteristics of glass flake coatings: Strong adhesion: Glass flake coatings exhibit high bonding strength to concrete or carbon steel substrates, with a bonding strength of not less than 2.0 Mpa. Wear resistance: The glass flake coating has a high hardness after curing, 2 to 3 times higher than that of ordinary alkyd paints, resulting in good wear resistance. In the event of external mechanical damage, the damage to the glass flake coating is localized, with little tendency for spread, making it easy to repair. 3) Good construction properties: Glass flake coatings have a high solid content, allowing for the creation of thick coats in a single application; they can also be prepared on-site and cure at room temperature. 4) Applications of glass flake materials: Currently, glass flake materials are widely used in the absorption towers of flue gas desulfurization units as well as in the flues for purified flue gas. In China, they are also employed as anti-corrosion isolation layers inside exhaust (steel) ducts. However, in the anti-corrosion renovation of single-tube reinforced concrete chimneys, there are few cases in China where glass flake composite materials are used. Coatings such as 3 OM, Guster, and Savijin come in various types on the market today, each with its own specific properties. However, in terms of heat resistance, aging resistance, and adhesion, it is difficult to provide evidence of their reliability for long-term use in environments exposed to dilute sulfuric acid, flue gas, and temperature fluctuations; as a result, damage occurs frequently, which hinders their widespread use in the anti-corrosion of desulfurization chimneys. 3.1 Acid-resistant rigid armor: Anti-corrosive concrete or potassium-water modified acid-resistant mortar is sprayed inside the cylinder. Short anchor bars are welded inside the acid-resistant steel flue, a wire mesh is then attached, and finally 50–70 mm thick heat- and acid-resistant concrete or potassium water-modified acid-resistant mortar is sprayed on to form a rigid armor. Such a corrosion-resistant coating will impose a significant load on steel smoke stacks, whether they are stand-alone or suspended, and the wall thickness of the steel smoke stacks will increase considerably. Such anti-corrosion layers are relatively brittle; when their continuous length exceeds 100 meters, it far exceeds the maximum allowable temperature segmentation length for concrete-based materials. Cracks are inevitable as temperature and humidity levels change, and these cracks become weak points (or leakage points) in the chimney’s anti-corrosion protection. 3.2 Monolithic acid-resistant castables for linings: Compared with other anti-corrosion systems, monolithic acid-resistant castables for linings are more difficult to apply, and it is hard to ensure the quality of the installation; as a result, their use is quite limited. A power plant in the country adopted integral acid-resistant castable for chimney lining for the first time, but some problems arose during construction and operation. The designers identified the following reasons: 1) Comprehensive factors such as the skill level of the operators during construction, the control of the setting time and consistency of the castable, as well as the material mix ratio, have a significant impact on the construction quality. 2) Acid-resistant castables have only the standards set by the manufacturers themselves; there are no industry standards. Construction must be carried out in accordance with the manufacturer’s instructions, while acceptance checks are to be done based on relevant industry standards, which **increases the difficulties in construction management and quality control. 3) It is relatively sensitive to changes in environmental conditions during construction. 4. Flexible adhesives for bonding lightweight insulating blocks: Flexible adhesives are generally made of silicone rubber, and are typically composed of a base polymer, cross-linking agents, catalysts, fillers, and additives. The product is packaged in a sealed tube; it is extruded for use, and upon contact with air it self-vulcanizes into an elastomer, making it convenient to use. According to the data provided by the manufacturer, silicone rubber can be used over a long period of time in temperature ranges from –40 to +180°C. It possesses excellent electrical insulation properties and chemical stability, is resistant to water, ozone, and weathering, and exhibits good adhesion to various metal and non-metal materials. Lightweight insulating blocks can be classified into the following types: BINGAODE glass bricks, domestically produced foam glass bricks, and domestically produced lightweight vitrified ceramic bricks. 4.1 BINGUODE glass bricks: The main material used in the lining of BINGUODE glass bricks is a special grade of low-expansion borosilicate glass. This glass can resist corrosion from various acids other than hydrofluoric acid; it is also resistant to gas and condensed water penetration. Additionally, it features low weight and excellent thermal insulation properties. Bingode glass bricks are bonded to the inner lining surface using a special adhesive, and the gaps between the glass bricks are filled with this adhesive material to prevent smoke from corroding the structure of the flue. The corrosion resistance and durability of the bonding material are key to the corrosion protection of the lining; its aging and cracking will directly affect the corrosion resistance of the entire lining system. It is reported that the Pergo membrane on the substrate retains its elasticity even after 20 years of use in various smoke environments, enabling it to effectively isolate the substrate from high temperatures and corrosive gases. However, this material is expensive, making it difficult to promote it domestically.