High-pressure boiler tubes are used in various pipelines in thermal power plants.

The Application Regulations of High-Pressure Boiler Tubes in Power Plants

Seamless steel tubes, specifically designed for boilers, are crucial raw materials in the production of power plants. High-tech standards for high-pressure boiler tubes come in numerous specifications and entail significant manufacturing complexity. They are primarily used in the production of various components, including reheaters, boiler economizers, heating surfaces, reheaters, and connecting pipelines for steam and water connections.

Principles of Power Plant Boilers:

Steam Circulation System: Freshwater is sent from the condenser deaerator to the boiler economizer, heated by waste heat recovery from the flue gas, and then sent to the reheater. The reheater heats the water, which is then directed through a network of cylinders, downcomers, and the boiler drum, forming a heating system. The generated steam, after separation from water in the separators, sends the downcomer water to the reheater. Subsequently, the steam from the separators is directed back to the reheater for further heating before being introduced to the heating surfaces. It is heated from the hottest part of the furnace to the designed steam parameters and is then used to generate power in the steam turbine generator. After performing work, the low-temperature steam returns to the heating furnace, is heated in the economizer, and is again heated to the defined steam parameters before being sent to the steam turbine generator for further work (in some units, for further efficiency improvement, the low-temperature steam after the second work is heated in the reheater and sent to the steam turbine generator for a third work cycle). The steam, after performing work, is condensed into water in the condenser, then sent back to the boiler economizer to complete the cycle.

Coal Powder Combustion: Raw coal from coal storage equipment is sent to a coal mill for pulverization. After heating in an air preheater, it is mixed with powdered coal and burned in the furnace using natural gas burners. The ash from the burned coal is collected in a hopper below the furnace and discharged using ash removal equipment.

Looking at the development trend of power plant boilers, as power plants progress from subcritical to supercritical and ultra-supercritical, the single-unit capacity also increases from 125,000 kW, 200 million kW, and 300 million kW to 600 million kW, 800 million kW, and 1 billion kW. Due to the increasing demand for energy efficiency and environmental protection, the requirements for high-pressure boiler tube materials are becoming more stringent. The development trend of thermal power indicates that thermal power units use materials at high temperatures, thus determining the increase in steam parameters and efficiency of thermal power units.

Actively raising steam parameters in thermal power units can bring significant economic benefits, saving a considerable amount of fuel and reducing CO2 emissions. Boilers with main steam inlet pressures exceeding 22.0 MPa are known as supercritical working pressure heaters, while those with main steam inlet pressures above 28 MPa or main steam inlet temperatures exceeding 580°C are defined as ultra-supercritical units. Studies have shown that raising the main parameters from the main parameter (3.5 Cpa, 435°C) to 12%-15% savings (9 MPa, 535°C), and then further raising to extremely high main parameters and intermediate reheating (13.5 Cpa, 535°C/535°C), can save 8% of materials. Elevating it to ultra-supercritical main parameters and single intermediate reheating (24 MPa, 550°C/550°C) can save an additional 8%, with a relative efficiency increase of 4.5%. Changing to double intermediate reheating, i.e., 30 MPa, 600°C/600°C/600°C, 600°C/600°C/600°C, and then raising it to 30 MPa, 30 MPa, 600°C/600°C, can also increase efficiency by 2%. Therefore, the increase in steam parameters for power plant boilers still has significant development potential. In the European AD700 and UK DOE/OCDO projects, the main task is to address the challenges of materials at high temperatures, up to 700 degrees Celsius.

Currently, research on new heat-resistant materials mainly takes place in Japan, Europe, and the UK. In the late 1950s to the 1960s, the development trend in the UK for ultra-supercritical main parameter thermal power units regressed to subcritical main parameters due to difficulties in obtaining high-temperature tube materials. However, the application of 91 steel in the 1990s allowed for the development of ultra-supercritical main parameter thermal power units. More recently, the use of new nickel-based high-temperature alloy materials has enabled the production of thermal power units with excellent ultra-supercritical main parameters at 700°C/720°C. In China's "Twelfth Five-Year Plan" for the power industry, it is also clearly stated that ultra-supercritical main parameter units should be constructed. Table 2 shows the proportion of new materials required for 1000 MW ultra-supercritical units in China.

Power plants have stringent requirements for high-pressure boiler tubes. It is generally stipulated that seamless steel tubes must have sufficient compressive strength with high ultimate strength and a reasonable safety factor. They should also exhibit good ductility to ensure resistance to ductile failure when subjected to external loads. Additionally, they should possess excellent properties for production, processing, and use, including hot and cold production characteristics, forming characteristics, and electric welding properties. The material used for high-temperature components must have excellent high-temperature properties, including sufficient stress relaxation compressive strength, long-term compressive strength, long-term plastic deformation, and reliable high-temperature structural integrity and oxidation resistance, among others. Under ultra-high-pressure operating conditions, the materials for high-temperature components in power plant boilers must have outstanding high-temperature compressive strength and excellent oxidation resistance. These new materials occupy a significant share in boiler seamless steel tubes for ultra-supercritical units. Table 4 shows the proportion of new components required for 1000 MW ultra-ultra-supercritical units in China.