Industrial waste heat refers to the waste heat energy generated during industrial production processes. Traditionally, this waste heat is released into the atmosphere through cooling devices, resulting in significant energy waste and environmental pollution. Therefore, the recovery and utilization of industrial waste heat has crucial economic and environmental significance.
What is Industrial Waste Heat Recovery?
Industrial waste heat recovery technology is a method that converts waste heat generated during industrial production into usable energy. This technology is of great significance in improving energy efficiency, reducing energy consumption, and mitigating environmental pollution.
The working principle of waste heat recovery equipment is based on the fundamental laws of heat transfer and conversion. In industrial production, such as metallurgy, chemical industry, and power industry, a large amount of high-temperature waste gas, wastewater, and heat carried by the high-temperature products themselves are generated in the smelting, reaction, and power generation processes. Waste heat recovery equipment uses specific heat exchange devices to transfer waste heat to other media that require heat, thereby achieving heat transfer and reuse. For example, in high-temperature waste heat recovery, a waste heat boiler (WHR boiler) is used to introduce high-temperature flue gas into the boiler’s heating surfaces (such as economizers, evaporators, and superheater tube bundles). As the flue gas flows through the heat exchange tube bundles, it transfers heat to the water inside the tubes, causing the water to gradually absorb heat, vaporize, and form steam. Some systems can further superheat the steam, thereby converting the heat energy originally emitted with the waste gas into usable high-quality steam for power generation or industrial process steam.
Main Types of Technologies for Industrial Waste Heat Recovery
WHR Boilers
WHRB boiler working process recovers waste heat through steam generators, a key step in improving energy efficiency. In the metallurgical industry, nearly 80% of flue gas waste heat is recovered through waste heat boilers, resulting in significant energy savings. Essentially, this type of boiler is a gas-water/steam heat exchanger that utilizes multiple sources of waste heat, including high-temperature flue gas waste heat and chemical reaction waste heat, to produce high-pressure, medium-pressure, or low-pressure steam or hot water for process control or heating.
In recent years, with the deepening of energy conservation and emission reduction efforts, waste heat boiler design and manufacturing enterprises have experienced rapid development. To adapt to the adjustment and growth of industrial production capacity, waste heat boilers are developing towards larger scale and higher parameters. For example, the non-ferrous metallurgical industry has already seen waste heat boilers with an evaporation capacity of 50t/h and an operating pressure of 2MPa, while the dry quenching waste heat boilers in the iron and steel metallurgical industry have an evaporation capacity as high as 100t/h and an operating pressure of 5MPa. Meanwhile, to further improve the heat transfer effect and heat utilization rate of boilers and reduce problems such as ash accumulation and wear, manufacturers of waste heat boiler recovery equipment, such as Ruichang, have been taking the lead in exploring and innovating technologies such as boiler circulation mode, heating surface structure, flue gas flow channel design, and ash removal method in recent years.
Heat Exchanger
1. Traditional Heat Exchangers
Traditional heat exchangers mainly include tubular heat exchangers, plate heat exchangers, and parallel-flow heat exchangers.
Tubular heat exchangers feature a robust structure and excellent high-temperature resistance. Nearly 40% of heat recovery equipment used in metallurgical industries adopts tubular heat exchangers. The inlet flue gas temperature can reach up to 1000°C, while a temperature difference of around 600°C can still be maintained at the outlet. The average heat recovery efficiency can reach approximately 30%.
Plate heat exchangers offer heat transfer coefficients nearly twice those of shell-and-tube heat exchangers. They are compact and lightweight, making them suitable for small and medium-sized enterprises to preheat combustion air. Their heat recovery efficiency is typically around 28%–35%.
Parallel-flow heat exchangers can divide the flue gas into two streams: one passing through the radiation section and the other through the convection section. The outlet air temperature can be heated up to 400°C, although the overall heat recovery efficiency generally remains between 26% and 35%.
2. Regenerative Heat Exchangers
Regenerative heat exchangers mainly rely on equipment such as rotary regenerators and blast furnace hot stoves for sensible heat storage. However, this type of system generally features low energy storage density and large equipment size. In contrast, phase-change latent heat storage systems use materials such as molten salts and metal alloys as thermal storage media, increasing energy storage density by one to two orders of magnitude while reducing system volume by 30%–50%. They also provide a more stable output temperature and are particularly suitable for high-temperature applications ranging from 450°C to 1100°C.
3. Direct-Contact Heat Exchangers
Direct-contact heat exchangers allow hot and cold fluids to come into direct contact and are commonly used together with equipment such as cooling towers, scrubbers, and barometric condensers. These systems offer high heat transfer efficiency; however, they are more likely to generate wastewater and scaling issues, which means additional water treatment systems are usually required.
Organic Rankine Cycle (ORC) Power Generation System
The Organic Rankine Cycle (ORC) is a technology that utilizes low-boiling-point organic refrigerants to generate electricity from waste heat. It is primarily used to recover low- and medium-temperature industrial waste heat and directly convert it into electrical energy.
The core principle of the ORC system is similar to that of the traditional steam Rankine cycle, but the difference lies in its use of low-boiling-point organic refrigerants such as pentane or R245fa, enabling the system to achieve phase change processes at lower temperatures. During operation, the low-temperature waste heat (typically 80–300°C) first enters the evaporator and heats the organic refrigerant, causing it to vaporize into high-pressure steam. This high-pressure steam then drives an expander or turbine to perform work and power a generator, completing the energy conversion. The refrigerant, after performing work, is cooled and liquefied in the condenser and then returned to the evaporator by a circulating pump, thus achieving a closed-loop waste heat power generation process.
Heat Pump Waste Heat Recovery System
A heat pump waste heat recovery system is an energy-saving technology that “upgrades” low-grade heat energy for utilization. Its core idea is to consume a small amount of high-quality energy (usually electricity, but can also be combined with a small amount of high-temperature heat sources) to raise the temperature of waste heat, which was originally too low to be directly usable, to a higher usable temperature level, thereby achieving secondary utilization of heat energy and energy conservation.
Is a WHR Boiler the Most Common Solution?
Yes — especially in high-temperature industrial waste heat recovery applications.
Among various industrial waste heat recovery technologies, the WHR boiler (Waste Heat Recovery Boiler) is one of the most widely used and technologically mature solutions. It is particularly suitable for recovering heat from high-temperature flue gases generated by industrial processes such as cement kilns, steel sintering plants, and chemical furnaces.
WHR boilers dominate the industrial sector primarily due to the following key factors:
First, WHR boilers can directly handle high-temperature flue gas ranging from 300 to 1000°C. This temperature range precisely covers the primary waste-heat sources in most heavy industrial emissions, making it widely applicable. Second, WHR boilers can directly convert flue gas heat into steam through heating surfaces (economizer, evaporator, superheater, etc.), achieving efficient energy recovery and reuse. This “flue gas → steam” path is simple and efficient.
This technology has been validated in long-term industrial use, showing a mature system structure and high operational stability, making it suitable for continuous, large-scale industrial production environments. More importantly, WHR boilers can not only be used for heating independently but can also be combined with steam turbine power generation systems to form waste heat power generation systems, thereby further enhancing energy utilization value and economic benefits.
In the industrial waste heat recovery system, WHR boilers mainly undertake the core conversion task of high-temperature waste heat to steam energy, which is currently the most mature and widely used technology route in the high-temperature industrial field, while other technologies are more used for supplementary applications in medium and low temperatures or specific scenarios.
