Heat recovery boiler TPP: operating principle and efficiency 

Operating principle of a thermal power plant waste heat boiler: from thermodynamics to actual output

A waste heat boiler at a thermal power plant (TPP) functions as a heat exchanger that converts the energy of hot flue gases into high-pressure steam without the use of additional fuel. In our operating practice, we note that the key efficiency parameter is not just the inlet temperature, but the stability of the gas flow and minimizing aerodynamic drag, which directly affects the operation of the main gas turbine. Modern units achieve a coefficient of performance (efficiency) in the range of 82–86%, however, these figures are only valid if the heating surfaces are in ideal condition and free of contamination.

Many engineers mistakenly believe that a recovery boiler is a passive piece of equipment that operates on its own. The reality is that this is a complex system that requires precise balancing between steam pressure, gas temperature and feedwater flow. One of our customers in Siberia experienced a 4% drop in power generation due to miscalculation of the sulfuric acid dew point, which led to economizer corrosion after just 18 months of operation. This article examines in detail the physical processes, design features and economic risks associated with the implementation of such systems, based on real monitoring data, and not on theoretical calculations from textbooks.

Physics of the process: how heat turns into electricity

The basic principle of operation is based on convective heat exchange. Flue gases exiting the gas turbine at a temperature of 450°C to 600°C enter the boiler flue. Inside this channel there are packages of pipes through which water circulates. The temperature difference between the gas and the working medium creates a heat flow that heats the water to the state of saturated steam, and then superheats it. It is critical to understand that the process occurs in three stages: water heating in the economizer, evaporation in the evaporating surfaces, and superheating in the superheater.

Unlike once-through boilers, where water passes through the system once, drum waste heat boilers use multiple circulations. Water is supplied to the drum, from where it flows by gravity or force (using pumps) into the lowering pipes, and then into the rising screen pipes, where boiling occurs. The resulting steam-water mixture is returned to the drum for separation. Clean steam goes further into the turbine, and water again participates in the cycle. This method provides high reliability and the ability to absorb water hammer, which is critical for variable operating modes of thermal power plants.

The efficiency of heat transfer directly depends on the gas flow rate. If the speed is too low (less than 8 m/s), soot and dust begin to settle on the pipes, creating an insulating layer that reduces heat transfer by 15–20%. If the speed is too high (more than 15 m/s), there is a risk of pipe erosion by abrasive particles contained in the gas. We recommend maintaining an optimal range of 10–12 m/s, which is supported by long-term testing data at facilities over 100 MW.

Particular attention should be paid to the temperature chart. The minimum flue gas temperature is limited by the dew point of water vapor and sulfuric acid. It is dangerous to drop below 140–150°C due to the risk of acid corrosion of the heating tail surfaces. However, modern designs seek to reduce this temperature to 110–120°C through the use of corrosion-resistant alloy materials or the application of special coatings, which allows an additional 3–5% of thermal energy to be extracted.

Design types and their impact on plant efficiency

The choice of waste heat boiler design determines not only its cost, but also the maneuverability of the entire power unit. Two main types dominate the market: horizontal and vertical layouts, as well as natural and forced circulation systems. Horizontal boilers are more often found in projects where the height of the building is limited, while vertical boilers make it possible to place heating surfaces more compactly and make it easier to clean them from ash due to gravitational drainage.

Systems with natural circulation do not require powerful circulation pumps, which reduces the station’s own electricity consumption (own needs). This increases net yield. However, such systems have high inertia: with a sharp change in the gas turbine load, the water level in the drum can fluctuate, requiring complex automatic control. In our practice, we have observed cases when, during an emergency load shedding, natural circulation stopped, leading to local overheating of pipes.

Forced circulation, implemented through direct-flow circuits or circuits with recirculation pumps, allows the use of pipes of smaller diameter and the creation of more compact units. Such boilers respond more quickly to changes in the operating mode of the gas turbine, which makes them preferable for peak and semi-peak operating modes of thermal power plants. The disadvantage is the dependence on the reliability of the pumping equipment: failure of the circulation pump requires immediate shutdown of the unit.

Another important design solution is the presence of a bypass pipe (bypass) for flue gases. It allows gases to be directed past the boiler directly into the chimney. This is necessary to start the gas turbine without loading the steam circuit, as well as to service the boiler without stopping the entire station. Having a quality bypass valve with fast response time (less than 30 seconds) is a mandatory requirement of modern design specifications.

Key performance parameters and methods for their optimization

When we talk about the efficiency of a waste heat boiler, we evaluate a set of indicators, not just efficiency. The primary metric value is the heat utilization factor (HUR), which shows how much of the flue gas enthalpy was usefully used. For modern three-circuit circuits (high, medium and low pressure) this figure reaches 90–92%. Dual-circuit circuits usually show results in the region of 85–88%.

The pinch-point temperature is the minimum temperature difference between the flue gases and the boiling water in the evaporator. The optimal value is 8–12°C. Reducing this difference increases the heating surface and the cost of the boiler, but increases steam production. An increase above 15°C leads to energy loss. Engineers have to balance capital expenditure (CAPEX) with operating benefit (OPEX). In one of the projects in Kazakhstan, reducing the pinch-point from 15°C to 10°C paid for itself in 2.5 years due to additional sales of electricity.

The aerodynamic resistance of the gas path is another critical parameter. Every millibar of pressure drop along the path of the gases requires additional power from the gas turbine to pump that volume, reducing its own efficiency. Modern finned tubes allow increasing the heat transfer area with a minimal increase in resistance. Using tubes with fin spacing of 2-3mm and fin height of 1.5-2mm has become the industry standard to balance these parameters.

Feedwater quality plays a critical role in long-term efficiency. Even trace amounts of oxygen or hardness lead to scale formation inside the pipes. A layer of scale just 1 mm thick can reduce heat transfer by 10% and cause overheating of the pipe metal to the point of rupture. Chemical water treatment systems must ensure an oxygen content of less than 5 μg/dm³ and a total hardness close to zero. Regular monitoring of the chemical composition of water is not a formality, but a necessity for the survival of the equipment.

Common operating errors and real failure cases

In our practice, the most common reason for decreased efficiency is contamination of heating surfaces. Many operators ignore the recommendations for regular purge and cleaning, assuming that the gas fuel is clean. However, even when natural gas is burned, the air contains dust that accumulates over the years. We have recorded cases where, after 5 years of operation without proper cleaning, boiler performance dropped by 7–9% solely due to a layer of dust on the finned tubes.

Another serious problem is low-temperature corrosion. When operating at partial loads, the flue gas temperature decreases. If it falls below the sulfuric acid dew point (which depends on the sulfur content of the fuel), an aggressive solution condenses on the walls of the economizer. In one case at a thermal power plant in the Volga region, this led to through corrosion of economizer pipes within 3 years. The solution was to install hot water recirculation at the economizer inlet to maintain the wall temperature above the critical level.

Incorrect adjustment of the water level regulators in the drum leads to the “carrying away” of moisture into the superheater. Drops of water entering superheated steam cause water hammer and erosion of steam turbine blades. This is a catastrophic failure requiring major repairs to the turbine. We strongly recommend installing two- or three-pulse level control systems that take into account not only the drum level, but also the steam and feedwater flow (“three elements”).

It is also worth noting the problem of thermal stress due to frequent starts and stops. Rapid heating of thick-walled elements (drum, collectors) creates uneven expansion of the metal, leading to cracks. The GOST R 55682 standard and ASME international standards regulate the rate of temperature rise (usually no more than 1–2°C per minute for thick-walled elements). Violation of these regulations in order to speed up the process will reduce the service life of the equipment significantly.

Integration with modern automation and control systems

A modern waste heat boiler cannot operate efficiently without an advanced distributed control system (DCS). Automation must analyze hundreds of parameters in real time: pipe metal temperature, pressure at various points, vibration of pumps, composition of flue gases. The introduction of predictive analytics allows you to predict failures before they occur. For example, analyzing the trend of increasing wall temperature of a particular pipe can signal the onset of coking a week before the critical state.

Continuous Emissions Monitoring Systems (CEMS) integrate with boiler controls to meet environmental regulations. Although the recovery boiler itself does not burn fuel, it is part of the overall circuit, and any changes in the operating mode of the gas turbine affect the composition of the gases. Synchronization of the bypass valves and water supply regulators must occur in a fraction of a second to avoid water hammer.

Digital twins are becoming the new standard for large thermal power plants. They allow you to simulate various operating scenarios and optimize modes without risk to real equipment. We have seen examples where the use of a digital twin was able to find the optimal opening angle of control valves, increasing steam production by 1.5% without any physical modifications to the equipment.

An important aspect is the cybersecurity of control systems. Connecting industrial controllers to a corporate network opens the door to potential attacks. Perimeter protection, network segmentation and regular software updates are mandatory requirements when upgrading automation systems at existing stations.

Economic justification and payback period of projects

Installing or upgrading a waste heat boiler is an investment project that requires a clear calculation of ROI (return on investment). The main source of income is the sale of additional electricity generated by the steam turbine. With the cost of electricity being 4–5 rubles per kWh (or the equivalent in foreign currency) and an additional generation of 50 MW, the annual revenue is a significant amount. The payback period for modern projects varies from 3 to 6 years, depending on the tariffs and operating mode of the station.

Don't forget about the reliability factor. A waste heat boiler failure often leads to a shutdown of the entire combined cycle, since the gas turbine cannot operate for long with the bypass closed or without heat removal. Downtime of a 400 MW power unit costs the company millions of rubles per day. Therefore, investments in high-quality materials and reliable automation pay off not only in direct production, but also in preventing losses from downtime.

Maintenance costs also play a role. Boilers with a horizontal layout may require less construction costs, but more surface cleaning costs. Vertical boilers are more expensive to install, but easier to operate. The selection should be based on the full life cycle (LCC), not just the initial purchase price. A cheap boiler can become expensive after 5 years due to frequent repairs and low efficiency.

State programs to support energy efficiency and modernization of thermal power plants can significantly improve the economics of the project. Participation in CSA programs (capacity supply agreements) or obtaining green certificates (where applicable) adds additional profitability. An analysis of the current legislative framework and forecasts for the development of the electricity market is mandatory before starting design.

Prospects for the development of heat recovery technologies

The future of waste heat boilers is associated with increasing steam parameters. The transition to supercritical parameters (pressure above 22.1 MPa) makes it possible to increase the efficiency of the Combined Steam Gas Unit (CCP) cycle to 60–62%. This requires the use of new heat-resistant steels and alloys capable of operating at temperatures above 600°C. Research in this area is being actively conducted by the world's leading manufacturers of energy equipment.

Integration with thermal energy storage systems is another promising trend. Excess heat generated during off-peak hours can be stored in molten salts or large volumes of water to be used during peak hours. This turns thermal power plants from a base station into a flexible source capable of smoothing out consumption peaks and compensating for the instability of renewable energy sources.

The use of hybrid steam turbine condenser cooling systems can reduce water consumption and increase efficiency in hot climates. Dry cooling towers in combination with wet ones provide flexibility in choosing the operating mode depending on the availability of water resources and ambient temperature. This is especially true for water-stressed regions such as Central Asia or the Middle East.

The development of additive technologies (3D printing) opens up the possibility of creating complex internal structures of pipes and manifolds optimized for a specific flow profile. This can reduce equipment weight and improve heat transfer. Although the widespread use of such technologies in the energy sector is a matter of the future, pilot projects are already demonstrating the potential of the method.

The role of specialized production in ensuring reliability

Theoretical calculations and advanced technologies are meaningless without high-quality material implementation. This is where the competence of the equipment manufacturer comes to the fore. CompanyWuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd.specializes in the development and production of highly efficient heat exchange solutions, including waste heat boilers, which become the heart of modern thermal power plants. Our experience shows that the durability of the unit directly depends on the choice of materials and manufacturing precision.

Мы производим оборудование из широкого спектра материалов: от углеродистой и нержавеющей стали до экзотических сплавов, таких как титан, морская латунь C46400, медно-никелевые сплавы C70600 и никелевый сплав N06625. Такая гибкость позволяет нам предлагать индивидуальные решения для самых агрессивных сред, будь то высокие давления в нефтепереработке или коррозионная активность в системах опреснения морской воды. Наша продукция, сертифицированная по строгим международным стандартам ASME и PED, гарантирует устойчивость к высоким температурам и давлению, что критически важно для достижения заявленных 85–90% КПД.

Особое внимание мы уделяем производству комплектующих, определяющих эффективность теплообмена: гофрированных трубных пучков из нержавеющей стали 316, трубных решеток из стали 321 и других узлов. Использование наших воздушных охладителей и кожухотрубных теплообменников в смежных циклах позволяет создать единую, сбалансированную систему энергосбережения. Для заказчиков по всему миру мы предоставляем не просто оборудование, а стабильное решение, проверенное в условиях реальной эксплуатации в нефтегазовой, химической и энергетической отраслях.

Часто задаваемые вопросы

Какова типичная температура уходящих газов после котла-утилизатора?

Оптимальная температура уходящих газов составляет 110–140°C. Снижение ниже 110°C возможно только при использовании специальных коррозионностойких материалов и риске выпадения кислотного конденсата. Повышение выше 150°C свидетельствует о загрязнении поверхностей нагрева или неэффективной схеме теплообмена, что ведет к прямым потерям энергии. Мы рекомендуем регулярно мониторить этот параметр как основной индикатор здоровья котла.

Можно ли эксплуатировать котел-утилизатор без байпасной трубы?

Эксплуатация без байпаса крайне не рекомендуется и часто невозможна по условиям безопасности газовой турбины. Байпас необходим для запуска турбины без нагрузки на паровой контур, для предотвращения переохлаждения котла при аварийных остановках паровой турбины и для проведения ремонтов без остановки всего блока. Отсутствие байпаса резко снижает маневренность станции и увеличивает риски аварийных ситуаций.

Как часто нужно проводить очистку поверхностей нагрева?

Периодичность очистки зависит от качества топлива и воздуха, но обычно составляет 1 раз в 1–2 года для газовых турбин. При признаках роста температуры уходящих газов или падения выработки пара очистку следует проводить внепланово. Использование систем онлайн-мониторинга загрязнения позволяет планировать очистки точно в момент необходимости, избегая лишних остановов или работы с низким КПД.

В чем разница между одноразовым и многоконтурным котлом?

Одноконтурный котел генерирует пар только одного давления (обычно высокого), что проще и дешевле, но менее эффективно. Многоконтурные котлы (двух- или трехконтурные) генерируют пар нескольких давлений, используя тепло газов более полно: сначала для высокого давления, затем для среднего и низкого. Это увеличивает выработку электроэнергии на 15–25% по сравнению с одноконтурной схемой при тех же дымовых газах.

Какие материалы используются для труб котла-утилизатора?

Для участков с высокой температурой применяются легированные стали (например, 12Х1МФ, 15Х1М1Ф) или аустенитные стали. Для хвостовых поверхностей (экономайзер), где возможна кислотная коррозия, используют нержавеющие стали или углеродистые стали с защитными покрытиями. Выбор материала диктуется температурой газов и химическим составом конденсата. Неправильный выбор материала ведет к быстрому разрушению труб.

Понимание принципов работы и тонкостей эксплуатации котла-утилизатора — это фундамент надежной и прибыльной работы вашей ТЭС. Не позволяйте оборудованию работать вхолостую из-за незнания нюансов или отсутствия качественного сервиса.Contact us today, чтобы получить консультацию по оптимизации вашего энергообъекта, аудиту текущего состояния оборудования или подбору современных решений для повышения эффективности. Мы готовы поделиться опытом успешных проектов и помочь вам избежать ошибок, которые стоят миллионов.

Для получения более подробной технической документации и примеров реализованных проектов посетите наш разделпромышленные котлы-утилизаторы, где представлены спецификации и кейсы внедрения.