Economizer for Boiler Tail Flue Gas

The core characteristics of waste heat boilers in the industrial sector are their adaptability and high tolerance to operating conditions. Waste heat from industrial production (such as steel, chemical, cement, and non-ferrous metal smelting) comes from complex sources, with large temperature fluctuations (ranging from low-temperature flue gas at 200°C to high-temperature slag at over 1000°C). Flue gas often contains dust and corrosive gases (such as sulfides and nitrogen oxides). Therefore, these boilers require targeted designs. For example, waste heat boilers for sintering machines in the steel industry often utilize membrane-type wall structures to enhance wear resistance. The chemical industry, on the other hand, focuses on anti-corrosion coatings and flue gas purification and pretreatment devices to prevent acid dew point corrosion. Furthermore, industrial waste heat utilization must be integrated with production processes. Boilers offer flexible capacity (ranging from a few tons to hundreds of tons of evaporation capacity) and are often designed as skid-mounted or modular systems to facilitate integration with existing production lines, prioritizing the plant's own steam needs or power generation needs, thereby reducing industrial energy consumption.

Waste heat boilers in the power sector, centered around "efficient coordination and stable output," are primarily used in combined cycle gas turbine (CCGT) systems or desulfurization and denitrification systems in coal-fired power plants. The high-temperature flue gas (approximately 500-600°C) emitted by gas turbines is relatively clean. Boilers often utilize a "triple-pressure reheat" or "multi-pressure" design to maximize flue gas heat, generating steam with varying parameters to drive steam turbines for power generation. Overall, combined cycle efficiency can reach over 60%. Furthermore, the power sector places extremely high demands on boiler operational stability, requiring rapid start-up and shutdown, and variable load regulation to match grid load fluctuations. Furthermore, strict environmental standards are adhered to, with flue gas temperatures kept below 100°C. Some also incorporate low-pressure economizers to further recover waste heat and improve overall power plant efficiency.

Waste heat boilers in this sector emphasize pollution resistance and environmental compatibility. In waste incineration, flue gas contains corrosive components such as chlorine and sulfur, as well as fly ash. Therefore, boilers must be constructed of corrosion-resistant materials (such as ND steel) and designed with specialized heating surface structures (such as spiral finned tubes) to reduce ash accumulation and corrosion. High-efficiency dust removal and deacidification devices must also be installed to ensure flue gas emissions meet standards. In the renewable energy sector (such as biomass power generation and photovoltaic/solar thermal power generation), waste heat often comes from biomass combustion flue gas (which contains alkali metals and is prone to coking) or low-temperature waste heat from solar thermal systems. Boiler furnace designs must be optimized to suit the fuel characteristics. For example, biomass waste heat boilers can incorporate decoking devices, while photovoltaic/solar thermal power generation boilers focus on efficient recovery of low-temperature waste heat (such as low-parameter steam designs adapted for the Organic Rankine Cycle (ORC)). These boilers balance environmental protection with the specificities of renewable energy utilization, helping to achieve the dual goals of "waste resource utilization" and "clean energy complementation."

The most popular type recovers waste heat from flue gas discharged from the tail flue of boilers (e.g., coal-fired, gas-fired, and biomass-fired boilers). Flue gas temperatures typically range from 120-400°C, and the heating medium is boiler feed water. The core goal is to reduce flue gas temperatures and improve boiler thermal efficiency.The design must adapt to boiler flue gas volume and temperature fluctuations. It is typically arranged in series with an air preheater (with an economizer placed after the air preheater to recover cooler flue gas). The structure is typically a serpentine or spiral finned tube type. Materials used include carbon steel, ND steel, or heat-resistant steel, depending on the flue gas composition. Boiler tail flue gas economizers are widely used in coal-fired, oil-fired, gas-fired, and waste heat boilers. When the high-temperature flue gas generated by combustion enters the tail duct, it still contains a significant amount of usable heat. The economizer transfers heat from the flue gas to the cold feed water entering the boiler through tube bundles or finned tubes, thereby reducing flue gas temperature, increasing feed water temperature, reducing the heat load of fuel combustion, and improving boiler efficiency. Boiler exhaust economizers are essential energy-saving devices in boiler systems. By effectively recovering waste heat from exhaust gases, preheating boiler feed water, and reducing fuel consumption, they not only improve boiler thermal efficiency but also reduce environmental pollutant emissions, making them a key tool for achieving energy conservation, consumption reduction, and green production.

Recovering waste heat from exhaust gases from industrial kilns (such as ceramic kilns, glass kilns, and metallurgical kilns) involves significant flue gas temperature variations (ceramic kiln exhaust temperatures can reach 500-600°C, glass kilns 400-500°C), and flue gas can contain dust (e.g., clay dust in ceramic kilns) and corrosive media (e.g., sulfides in metallurgical kilns). The module design requires enhanced "high-temperature resistance and anti-clogging" features: High-temperature-resistant materials (e.g., 12Cr1MoVG, Incoloy 800H) should be used, tube gaps should be increased to prevent dust clogging, and high-pressure steam sootblowing devices should be installed. In addition to boiler feed water, the heated medium can also be used to heat process water (e.g., water for pulping in ceramic factories), achieving "cascaded utilization of waste heat."

Recover waste heat from exhaust gas from chemical and petrochemical process equipment, such as the flue gas from refinery atmospheric and vacuum unit heaters and coal-to-methanol synthesis towers. Flue gas temperatures range from 250-400°C and contain flammable, explosive, or corrosive media such as H₂S, Cl⁻, and CO. The module must meet the requirements of explosion-proof, corrosion-resistant, and adaptable to variable operating conditions. It utilizes a sealed manifold (to prevent combustible gas leakage) made of 316L or nickel-based alloy. It also integrates temperature, pressure, and combustible gas concentration monitoring sensors. When process load fluctuates (e.g., when a refinery's load increases from 80% to 100%), it can adjust the feedwater flow to maintain stable heat exchange and prevent overheating or overpressure.

Spiral finned tubes are highly efficient heat exchange components formed by machining or wrapping spiral fins around the outer surface of a base tube (usually a round metal tube). They are widely used in boilers, heat exchangers, air conditioners, refrigeration equipment, and other applications requiring enhanced heat transfer. Their core design concept is to increase the heat transfer area, thereby improving the heat exchange efficiency between the fluid (usually gas or liquid outside the tube) and the medium inside the tube. Spiral finned tubes are widely used in boilers, petrochemicals, metallurgy, power generation, and waste heat recovery systems, and are a core component of industrial heat exchange equipment. The spiral fins significantly increase the tube's surface area, enhancing heat transfer capacity on the flue gas or fluid side. While maintaining the same heat transfer capacity, they can significantly reduce the heat exchanger's volume. The fins and base tube can be manufactured using high-frequency welding, integral extrusion, or coiling processes, resulting in a tight bond, excellent heat transfer performance, and strong durability. Fin thicknesses range from 0.8 to 2 mm, providing high mechanical strength and the ability to withstand flue gas erosion and prolonged high-temperature operation. Corrosion-resistant materials such as stainless steel and alloy steel can be selected as needed to accommodate acidic or high-humidity operating conditions. Improves the thermal efficiency of boilers and waste heat recovery units, reducing fuel consumption and operating costs. This technology offers significant energy savings in power plants and industrial waste heat utilization. Fin height, pitch, and thickness can be flexibly customized to accommodate varying flow rates, temperatures, and media conditions.

Heat-tubes are components that utilize phase changes (evaporation and condensation) of a working fluid to achieve efficient heat transfer. They boast strong heat transfer capabilities, minimal temperature gradients, and a compact structure, making them widely used in energy, chemical, electronics, aerospace, and other fields. In power plant boiler systems, heat pipes, as highly efficient heat transfer components, leverage their "superconductivity" and flexible structural design to effectively address the pain points of traditional heat exchange equipment in waste heat recovery, low-temperature corrosion protection, and heating surface optimization. They have become a key technology for improving boiler thermal efficiency and reducing energy consumption. Shell and tube materials include carbon steel, stainless steel, copper, and aluminum alloy, depending on operating conditions and media requirements.Working media include water, ammonia, methanol, and sodium-potassium alloys, adapting to various temperature ranges (low, medium, and high temperature heat transfer). Heat pipes are typically evacuated and filled with a small amount of working fluid. Heat is absorbed at one end of the heat pipe, and the evaporated fluid is rapidly transferred to the other end, releasing heat and condensing back, thus forming a phase change heat transfer cycle. They can transfer large amounts of heat even with a small temperature gradient, and their thermal conductivity is over ten times higher than that of metals like copper and aluminum. Heat pipes provide uniform temperature distribution along the axial direction, effectively preventing thermal stress in equipment caused by large temperature differences. Heat can be transferred over a long pipe, achieving "long-distance heat transfer" in space. Flexible choices of shell and tube materials and working fluid materials are available to meet the needs of high-temperature, low-temperature, corrosive, or special environments.

The H-type finned tube is a highly efficient, enhanced heat transfer element. Named for its H-shaped fin cross-section, it is also commonly referred to as a "double-finned tube" or "flat finned tube." Its unique structural design improves heat transfer efficiency while effectively addressing the dust accumulation and high resistance issues of traditional finned tubes. It is widely used in power plant boilers, waste heat boilers, industrial heat exchangers, and other applications where dust-laden flue gas is present or low-resistance operation is required. H-type fins divide the flue gas flow, enhancing flue gas turbulence, reducing air film heat transfer resistance, and improving the overall heat transfer coefficient. Compared to traditional spiral-finned tubes, the heating area can be increased by 1.5–2 times. The H-type fins offer low resistance and strong resistance to dust accumulation, creating parallel channels between the fins for low flue gas flow resistance. Their unique geometric structure slows dust deposition, provides self-cleaning capabilities, and reduces the risk of clogging. The fins are welded to the base tube using high-frequency welding for a secure, non-detachable bond. Optional materials such as ND steel and stainless steel are wear-resistant, sulfur-resistant, and chloride-ion-resistant, extending service life. The tube wall thickness is ≥3mm, capable of withstanding high pressures and temperatures, making it widely used for waste heat recovery from high-temperature flue gas. The moderate fin thickness (0.3–0.4mm) ensures heat transfer efficiency while maintaining excellent mechanical strength. Its operating life is significantly longer than traditional finned tubes under harsh operating conditions.