New energy-saving technologies for power plants: Recovery and reuse of waste heat from low-temperature flue gas in boilers

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The recovery and utilization of waste heat from power plant boilers should follow the following principles: based on the amount and quality of the waste heat in the flue gases as well as the requirements of the users, effort should be made to achieve an optimal match in terms of energy levels. Under conditions that comply with thermodynamic principles and technical-economic considerations, appropriate systems and equipment for recovering waste heat from power plant boilers should be selected in order to maximize the utilization of this waste heat. http://p1.pstatp.com/large/28960002da49b13fc6ea（1）The total energy system principle: The total energy system involves matching energy sources based on the level of thermal energy available in the flue gas, and making use of this energy in a hierarchical manner; it also ensures proper coordination and conversion between various forms of energy such as kinetic energy, thermal energy, potential energy, and internal energy. Not only must objectives and factors such as thermodynamics, economics, and environmental protection be taken into account, but the energy efficiency of the equipment system also needs to be comprehensively evaluated in order to achieve the optimal level of energy utilization. The commonly seen total energy systems are widely applied, such as combined cycles, cogeneration, tri-generation, multiple combined cycle systems, and total energy plant systems. (2) The thermal losses in systems for maximizing heat energy utilization are mainly caused by irreversible processes; therefore, reducing these irreversible losses is the fundamental principle in the process of recovering waste heat from boiler flue gas, that is, to maximize the utilization of the waste heat from the flue gas. Energy supply must be provided in the right quantity and quality based on actual needs. On one hand, it is necessary to reduce the temperature difference during heat transfer and avoid irreversible losses such as throttling and friction; on the other hand, the quantity of energy supplied must meet the requirements of the heat users, while its quality must correspond to the desired standard of thermal energy. It is worth noting that waste heat utilization is aimed at saving energy consumption and improving energy efficiency, and it should not be pursued merely for the sake of recovery. Before selecting a scheme for recovering waste heat from flue gas in a power plant boiler, it is necessary to conduct a thorough investigation to determine whether the system itself has potential for improvement. It is also important to assess whether an increase in the system’s thermal efficiency as a result of waste heat recovery will still ensure its normal operation, and whether the installation of specialized waste heat recovery devices for power plant boilers can help reduce pollution emissions. Additionally, it is essential to evaluate the technical and economic viability of such a solution. Only by taking all of the above issues into comprehensive consideration can waste heat utilization be carried out effectively. New energy-saving technologies for power plants: Low-temperature flue gas waste heat recovery heat exchangers. These heat exchangers for recovering waste heat from boiler flue gas are made of imported fluoroplastic materials, which enables them to overcome the acid dew point corrosion problem that is difficult to address with conventional heat pipes; as a result, they hold a significant advantage in industries such as chemicals, metallurgy, and steel manufacturing. Main features of the low-temperature flue gas waste heat recovery heat exchanger: http://p1.pstatp.com/large/2a38000256d667bf3432 (1) No special requirements regarding flue gas composition, wall temperature, or acid dew point; it does not suffer from corrosion and has strong versatility ； (2) It can recover flue gas waste heat at 180 – 75°C, which is much higher than the 160 – 120°C flue gas waste heat that can be recovered by a \"low-temperature economizer\" ； (3) Long service life of 8–10 years, with a short payback period of 1–2 years ； (4) The material has self-cleaning properties, resists sticking, and is easy to clean ； (5) The pipe will not develop stress cracks, and there is no risk of leakage. For power plants with flue gas temperatures around 180°C, it is possible to recover up to the heat contained in the flue gas between 180°C and 75°C ； For power plants with flue gas temperatures around 120°C, it is possible to recover up to the heat from the flue gas ranging from 120°C to 75°C. That is, fluoroplastic heat exchangers can be used to recover the waste heat from flue gas; the temperature range within which this is possible is 180°C to 75°C. This approach effectively solves the problem of acid dew point corrosion caused by flue gas, resulting in no corrosion. Under full-load operation, the lifespan of the heat exchange tubes is 15 years; the equipment investment pays off within 1–2 years, and the payback period for the investment is around 10 years, giving it high investment value. Comparison between traditional flue gas heat pipe waste heat recovery and fluoroplastic flue gas waste heat recovery. Gas and coal-fired boilers are increasingly being used as distributed heat sources; the flue gas emitted from these boilers contains a large amount of heat, leading to resource waste and environmental pollution. Under normal conditions, the flue gas temperature of ordinary natural gas boilers and low-temperature coal-fired boilers is generally between 120 and 200°C; this flue gas contains 8% to 15% sensible heat and 11% latent heat of vaporization. Currently, the commonly used types in the market are traditional flue gas heat pipe waste heat recovery systems and fluoroplastic flue gas waste heat recovery heat exchangers. Both have their own advantages. When used for the recovery of waste heat from flue gas, heat pipes have several significant disadvantages: 1. They are unable to recover waste heat in the presence of acid dew points in the flue gas, and their corrosion resistance is very low; they suffer significant corrosion under various types of coal- and gas-fired flue gases, which results in a shorter lifespan for the equipment. http://p1.pstatp.com/large/2a3b0000d15a8a5c7e332. Due to the high hardness and low flexibility of their material properties, heat pipes are prone to accumulating dust or scale over time, which reduces their heat transfer efficiency and shortens the lifespan of the equipment. 3. Metal heat pipe flue gas waste heat recovery exchangers suffer from severe low-temperature corrosion and dust wear, which lead to defects such as tube rupture; as a result, their lifespan is very short – ranging from one to two years at most, and less than half a year in some cases. http://p3.pstatp.com/large/2899000479fca4b989c6 The above-mentioned problems associated with heat pipe-based flue gas waste heat recovery exchangers have been significantly addressed by the new fluoroplastic-based flue gas waste heat recovery exchanger technology. This is also one of the main reasons why our country introduces advanced foreign technologies – to thoroughly improve the current shortcomings in the recovery of waste heat from flue gases, by adopting advanced equipment to foster industrial development and ensure energy efficiency and environmental protection. 1. The greatest advantage of fluoroplastic flue gas waste heat recovery heat exchangers is their resistance to acid dew point corrosion, allowing the flue gas temperature to be recovered up to 75°C. http://p1.pstatp.com/large/28960002cb31a213d8132. The imported fluoroplastic tube bundles used in this fluoroplastic flue gas waste heat recovery heat exchanger equipment feature good surface smoothness and high flexibility; as a result, dust accumulation and scaling do not occur easily even under slight vibrations. It facilitates the achievement of very good heat exchange performance over the long term. http://p1.pstatp.com/large/2a380002529b62c114d83. The theoretical service life of fluoroplastic tube bundles is up to 15 years, and the actual service life of such flue gas waste heat recovery heat exchangers can reach 10 years, resulting in a very long payback period. The two main constraints in the customization of low-temperature flue gas waste heat recovery systems for power plant boilers are as follows: First, the space required by such recovery devices. Since power plant boiler systems occupy a large amount of space, it is necessary to determine in the initial design phase exactly how much space will be needed for these recovery devices, so that the equipment can be designed and installed appropriately within the available space. Especially for customers with specific space constraints, it is necessary to provide information about the site in advance so that designers can create an appropriate design; only in this way can the waste heat recovery unit for low-temperature boiler exhaust gases meet their requirements. Second, regarding the utilization methods of boiler flue gas waste heat recovery units, these units can be arranged in either horizontal or vertical flue configurations. Depending on the method of waste heat recovery, they can be used to heat condensate water or other industrial fluids; it is primarily the customer who determines their own requirements and intended uses. Therefore, large and medium-sized enterprises should make use of flue gas waste heat recovery equipment to effectively save energy and reduce resource waste; such equipment has high reuse value and can effectively promote the development of China’s environmental protection industry.