A Review of Industrial Waste Heat Recovery Technologies in China

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At present, China’s energy utilization still faces major issues such as low efficiency, poor economic returns, and significant pressure on the ecological environment. Reducing energy consumption and emissions and improving the overall efficiency of energy use are the fundamental ways to address China’s energy problems, and they are given top priority in development efforts. 　　Achieving the goals of energy conservation, emission reduction, and improved energy efficiency relies primarily on the industrial sector. Energy consumption in China’s industrial sector accounts for about 70% of the country’s total energy consumption, and the average energy intensity per unit of major industrial products is roughly 30% higher than the international advanced level. Apart from relatively backward production processes and an irrational industrial structure, the low utilization rate of industrial waste heat is a major cause of high energy consumption. In China, the energy utilization rate stands at only about 33%, which is roughly 10% lower than that in developed countries. At least 50% of industrial energy consumption ends up being wasted as waste heat in various forms. From another perspective, our country is rich in industrial waste heat resources, which are present in the production processes of various industrial sectors. Waste heat accounts for 17% to 67% of the total fuel consumption, with a recoverable rate of 60%; there is therefore significant room for improving the utilization of waste heat, offering great potential for energy savings. Industrial waste heat recovery is considered a type of “new energy,” and in recent years it has become an important aspect in advancing energy conservation and emission reduction efforts in our country. 　　Characteristics of industrial waste heat resources Waste heat resources are considered secondary energy sources; they are products resulting from the conversion of primary energy sources or combustible materials, or they represent the heat remaining after the heat released during fuel combustion is utilized in a certain industrial process. Based on temperature levels, industrial waste heat is generally classified into high-temperature waste heat above 600°C, medium-temperature waste heat between 300°C and 600°C, and low-temperature waste heat below 300°C. According to its source, industrial waste heat can also be divided into waste heat from flue gases, waste heat from cooling media, waste heat from exhaust gases and wastewater, heat generated by chemical reactions, waste heat from high-temperature products and slag, as well as waste heat from combustible exhaust gases and waste materials. 　　Waste heat resources have a wide range of sources, temperature levels, and forms of existence. From the perspective of utilization, waste heat resources generally share the following common characteristics: the instability of waste heat due to periodicity, intermittency, or fluctuations in the production process; poor properties of the waste heat medium, such as high dust content or the presence of corrosive substances in the flue gas; and limitations imposed on waste heat utilization devices by factors such as site conditions. 　　Therefore, the operating environment for systems or equipment used to utilize industrial waste heat is relatively harsh; such systems require a stable operating range, the ability to adapt to changing process requirements, high reliability of their components, and high initial investment costs. From an economic perspective, it is necessary to conduct an overall system design and layout in conjunction with the production process, so as to improve the efficiency of the equipment in the waste heat utilization system. 　　Industrial waste heat utilization technologies: Waste heat exists within a wide temperature range and can be carried in various forms of energy. Due to differences in the operating environment and production processes, as well as constraints imposed by site-specific conditions, there are a variety of equipment designs, such as air preheaters, furnace regenerative chambers, waste heat boilers, and low-temperature steam turbines. There are various ways to classify industrial waste heat recovery. Based on the characteristics of energy transfer or conversion during the utilization of waste heat resources, the current industrial waste heat utilization technologies in China can be divided into heat exchange technologies, thermodynamic conversion technologies, and waste heat-based cooling and heating technologies. 　　1. Heat exchange technology: Waste heat recovery should be prioritized for the equipment in this system or for this process flow, in order to minimize the number of energy conversion steps. The utilization of waste heat does not change the form of its energy; rather, it involves using heat exchange equipment to transfer this energy directly to the energy-consuming processes within a particular process, thereby reducing the consumption of primary energy. Such technical devices can be collectively referred to as heat exchange technologies. This represents the most direct and efficient economic method for recovering industrial waste heat. The corresponding devices include various types of heat exchangers, such as those with traditional designs as well as heat pipe exchangers, in addition to waste heat steam generators (waste heat boilers). 　　(1) Shell-and-tube heat exchangers Industrial heat exchangers can be broadly classified into shell-and-tube heat exchangers, mixed-flow heat exchangers, and regenerative heat exchangers based on the principle of heat transfer. Among them, the partition-type and regenerative types are commonly used devices for industrial waste heat recovery. Mixed-type heat exchangers transfer heat by allowing hot and cold fluids to come into direct contact or mix with each other; examples of such devices include cooling towers, washing towers, and pressure condensers used in industrial production, but they are not frequently used in waste heat recovery. 　　Interwall heat exchangers mainly include tube-type, plate-type, and co-current heat exchangers. Although tube-type heat exchangers have a relatively low thermal efficiency, averaging only 26%–30%, and are inferior to other types in terms of compactness and metal consumption, they feature a robust structure, great adaptability, and a wide range of available materials, making them the most widely used heat exchange equipment for industrial waste heat recovery. 40% of the heat exchanger equipment in metallurgical enterprises are tubular heat exchangers, which can tolerate an inlet flue gas temperature of over 1,000°C, with an outlet flue gas temperature of around 600°C, resulting in an average temperature difference of about 300°C. 　　Plate heat exchangers include finned plate types, spiral plate types, and shell-and-plate heat exchangers. Compared with tube heat exchangers, their heat transfer coefficient is approximately twice that of shell-and-plate heat exchangers; they offer high heat transfer efficiency, a compact structure, and material savings. In the metallurgical industry, where there are many small and medium-sized enterprises, plate heat exchangers are commonly used to preheat the air required for combustion; the average heat recovery rate is 28%–35%, with the inlet flue gas temperature at around 700°C and the outlet temperature reaching 360°C. However, due to the greater limitations on operating temperature and pressure for plate heat exchangers compared to tube heat exchangers, their application range is restricted. 　　For the recovery of high-temperature flue gases from various industrial furnaces, recuperative heat exchangers are also commonly used. There are two main types: radiant and convective. These heat exchangers are widely applied; they are frequently used in soaking pits, heating furnaces, and similar equipment to recover waste heat from flue gases, preheat combustion air or fuel, and reduce both the volume of flue gas emissions and their discharge temperature. In common radiation convection heat exchangers, the inlet flue gas temperature can exceed 1,100°C, while the outlet flue gas temperature is also as high as 600°C. The air used for combustion can be heated to 400°C, resulting in an effective combustion effect; the temperature efficiency can reach over 40%, but the heat recovery rate is low, averaging between 26% and 35%. 　　(2) Regenerative heat exchanger The principle of regenerative heat exchange equipment is that hot and cold fluids flow alternately through the regenerative elements to exchange heat; it is a type of heat exchange device that operates intermittently and is suitable for recovering waste heat generated in intermittent processes. It is often used for heat exchange between high-temperature gas media, such as for heating air or materials. 　　Based on the difference in the heat storage medium and the form of thermal energy storage, regenerative heat exchange systems can be divided into sensible heat storage and phase change latent heat storage. Sensible heat storage has been applied for a long time; examples include simple heat exchange devices such as common rotary heat exchangers, and complex equipment like regenerative hot blast stoves in blast furnaces for ironmaking. Due to drawbacks such as low energy storage density, large size, and the inability to maintain a constant temperature during heat storage, sensible heat storage heat exchange devices have limitations in industrial waste heat recovery. Phase change latent heat energy storage and heat exchange devices utilize the inherent heat capacity and phase change latent heat of thermal storage materials to store and transfer energy. Their energy storage density is at least one order of magnitude higher than that of sensible heat energy storage devices. Therefore, when storing the same amount of heat, these devices require 30%–50% less volume compared to conventional thermal storage systems. 　　In addition, stable heat output, a basically constant temperature of the heat transfer medium, and stable operating conditions of the heat exchange system are other advantages of phase change latent heat energy storage heat exchange equipment. Phase change energy storage materials can be broadly classified into high-temperature and medium/low-temperature phase change materials based on their phase change temperatures. The former have high phase change temperatures and large latent heats of phase change; they are mainly composed of various inorganic salts and their mixtures, alkalis, metals and alloys, combined with ceramic or metal matrices. They are suitable for recovering high-temperature waste heat at temperatures ranging from 450 to 1,100°C and above, and are widely used. The latter primarily consist of crystalline hydrates or organic substances, making them appropriate for low-temperature waste heat recovery. 　　(3) Heat exchange equipment based on heat pipes A heat pipe is an efficient heat conduction element that transfers heat through the phase change processes of evaporation and condensation of a working fluid within a completely sealed vacuum tube, as well as through secondary wall heat exchange; it represents a phase-change energy storage heat exchange device that combines heat storage and heat exchange functions in one. Heat pipes have excellent thermal conductivity; their heat transfer coefficient is nearly an order of magnitude higher than that of traditional metal heat exchangers. They also possess a number of advantages, including good isothermality, the ability to control temperature, strong heat transfer capacity, the possibility of adjusting the heat transfer area on both the hot and cold sides, the capability for heat transfer over long distances, and the absence of the need for any external auxiliary power equipment. For heat pipe operation, it is necessary to select the appropriate tube material and working fluid based on the different operating temperatures. Among them, the carbon steel-water gravity heat pipe features a simple structure, low cost, easy manufacturing, and ease of widespread adoption, which has led to its extensive use. In practical applications, the operating temperature range of heat pipes is between 50 and 400°C. They are used for heat recovery or waste steam recovery in drying ovens, assimilation furnaces, and curing ovens, as well as in air preheaters for boilers or furnaces. 　　(4) Waste heat boilers: The use of steam generators, that is, waste heat boilers, is an important means of recovering waste heat in order to improve energy efficiency. Nearly 80% of the waste heat from flue gas in the metallurgical industry is recovered through waste heat boilers, resulting in significant energy savings. 　　In a waste heat boiler, no combustion takes place; instead, the waste heat from hot flue gases, chemical reactions, combustible gases, and high-temperature products is utilized to generate steam or hot water, which is then used in industrial processes or supplied through heating networks. At the same time, the waste heat boiler is an important device in low-temperature steam turbine power generation systems, providing steam as a working medium for power machinery such as steam turbines. 　　In practical applications, waste heat boilers that utilize the waste heat from flue gases at temperatures of 350–1,000°C are commonly used; compared to the operating temperatures of coal-fired boilers, these are considered low-temperature boilers with lower efficiency. Due to the high dust content and abundance of corrosive substances in the waste heat flue gas, it is more likely to cause problems such as ash deposition, corrosion, and wear in boilers. Therefore, preventing ash deposition and wear is crucial in the design of waste heat boilers. The straight-through furnace design, the large-volume cavity for radiation cooling, the sealed furnace walls, the dust removal chamber, and the numerous rapping and soot-blowing devices are all structural features adopted by waste heat boilers to address issues related to ash accumulation and wear. Furthermore, due to space constraints at the production site, the waste heat boiler has its heat exchange components installed in various parts of the process flow, rather than being assembled as a single unit like ordinary boilers. 　　In the past decade, with the advancement of energy conservation and emission reduction efforts, major domestic designers and manufacturers of waste heat boilers have seen rapid development. Waste heat boilers are increasingly trending toward larger sizes and higher operating parameters; examples include waste heat boilers in the non-ferrous metallurgy industry with an evaporation capacity of 50 tons per hour and an operating pressure of 4.2 MPa, as well as dry quenching waste heat boilers in the iron and steel industry with an evaporation capacity of 100 tons per hour and an operating pressure of 12.5 MPa. Furthermore, to further improve the heat transfer efficiency and thermal utilization of boilers, as well as to reduce problems such as ash accumulation and wear, improvements and innovations in aspects such as boiler circulation methods, heating surface structures, flue gas flow paths within the boiler, and ash cleaning methods constitute the main focus of technological advancements in waste heat boilers. 　　2. Heat-to-work conversion technology: Heat exchange technology recovers waste heat in the form of thermal energy by reducing its temperature level; it represents a form of suboptimal utilization and is unable to meet the power demands of industrial processes or those within a company. In addition, the recovery of abundant medium- and low-temperature waste heat resources through heat exchange technology yields insignificant benefits. Therefore, utilizing thermodynamic conversion technology to improve the quality of waste heat is another important technique for recovering industrial waste heat. 　　Classified by working fluid, thermodynamic conversion technologies can be divided into the traditional steam turbine power generation technology using water as the working fluid, and the power generation technology using low-boiling-point organic working fluids. Due to the significantly different properties of the working fluids, the corresponding waste heat recovery systems and their components also have distinct characteristics. The current main application involves using water as the working fluid, with a waste heat boiler combined with a steam turbine or expander to form a low-temperature steam turbine power generation system. 　　The recoverable waste heat resources for power generation using low-temperature steam turbines are mainly medium- to high-temperature flue gases with temperatures above 350°C; examples include flue gases from furnaces in building materials industries such as glass and cement production, or flue gases that have been cooled to 400–600°C after one round of utilization. The power output per unit ranges from several megawatts to dozens of megawatts. There are various forms of waste heat power generation, including waste heat recovery from oxygen converters and sintering processes in the steel industry, waste heat recovery from coke dry quenching in the coking industry, and low-temperature waste heat power generation in the cement industry. However, considering the temperature range of waste heat resources, this technology belongs to the category of medium- to high-temperature waste heat power generation technologies. 　　Furthermore, a large amount of steam recovered from the process flow through waste heat boilers or heat exchangers consists, to a significant extent, of low-pressure saturated steam or hot water at around 1 MPa; much of this excess steam is often released. Currently, for the utilization of such low-pressure saturated steam for power generation, screw expansion engine technology is primarily employed. This technology has the following characteristics: it can utilize various heat source working fluids as power sources; it is suitable for superheated steam, saturated steam, and two-phase steam-liquid mixtures, as well as flue gas, contaminated hot water, and hot liquids. It features a simple and compact structure, automatic speed regulation, long service life, and minimal vibration. The flow velocity inside the machine is low; apart from leakage losses, other energy losses are minimal, resulting in high efficiency. Thanks to its dual-rotor non-contact design, it generates a shear effect during operation, thereby providing self-cleaning and scale removal capabilities. 　　Screw expansion engines belong to the category of positive-displacement expansion engines. Due to limitations in their expansion capacity, the heat sources that can directly drive these engines are steam with a pressure of 0.15–3.0 MPa and a temperature below 300°C, or hot water with a pressure of over 0.8 MPa and a temperature above 170°C. Because of their structural characteristics, the power output of individual screw expansion engines is limited, usually being below 1,000 kilowatts; they are primarily used in applications where the amount of waste heat is relatively small. 　　3. Cooling and heating technologies (1) Waste heat cooling technology Compared with traditional compression-based refrigeration units, absorption or adsorption refrigeration systems can utilize inexpensive energy sources and low-grade heat, thereby avoiding electricity consumption and addressing the issue of insufficient power supply. They use natural refrigerants that do not contain chlorofluorocarbons that damage the ozone layer, offering significant energy-saving benefits and environmental advantages; as a result, they were widely adopted by the end of the 20th century. 　　The thermodynamic cycle characteristics of absorption and adsorption refrigeration technologies are very similar; both follow a cycle process of “generation (desorption)—condensation—evaporation—absorption (adsorption)”. However, in absorption refrigeration, the absorbent is a liquid with good fluidity, while the refrigerants used include ammonia-water and lithium bromide aqueous solution. The generation and absorption processes occur in a generator and an absorber, respectively. In adsorption refrigeration, the adsorbent is generally a solid medium, and the adsorption modes can be either physical or chemical. Commonly used working pairs include molecular sieve–water and calcium chloride–ammonia; the desorption and adsorption processes take place in an adsorber. 　　Absorption refrigeration systems that use lithium bromide aqueous solutions as the working fluid are the most widely used. They can utilize low-temperature heat sources in the range of 80–250°C. However, since water is used as the refrigerant, the temperature of the chilled fluid can only reach 0°C or above 5°C. Such systems are primarily used for air conditioning or industrial cooling water applications. The energy efficiency ratio varies greatly depending on the thermal properties of the refrigerant and the design of the thermodynamic cycle; in practical applications, the energy efficiency ratio usually does not exceed 2, which is much lower than that of compression-based refrigeration systems. However, such units can utilize low-grade heat sources such as low-temperature industrial waste heat, solar energy, and geothermal energy, without consuming high-quality electrical power, thus holding certain advantages in the utilization of industrial waste heat. Absorption-type waste heat refrigeration units feature high cooling efficiency and are suitable for the recovery of large amounts of waste heat. Their cooling capacity can range from several dozen kilowatts to several megawatts. These units have been widely used in China, with mature technology and a wide range of available specifications and models. 　　There are many types of refrigerant pairs for adsorption refrigerators, including physical adsorption pairs, chemical adsorption pairs, and composite adsorption pairs. They can operate over a wide range of heat source temperatures, and do not require solution pumps or distillation units. Additionally, they avoid issues such as refrigerator contamination, salt solution crystallization, and corrosion of metals. Adsorption refrigeration systems have a simple structure, produce no noise and cause no pollution; they can be used in environments with vibrations and shocks, such as in cars and ships. However, their cooling efficiency is relatively low – the coefficient of performance of commonly used such systems is usually below 0.7. Due to manufacturing limitations, their cooling capacity is limited, generally to several hundred kilowatts or less; they are more suitable for the recovery of low-grade waste heat or for use in cogeneration systems. 　　(2) Heat pump technology: In industrial production, there is a large amount of waste heat at temperatures slightly above ambient temperature (30–60°C), such as industrial slag washing water and oil field wastewater. Although the temperature is low, the amount of residual heat is significant; heat pump technology is often used to recover such residual heat resources. 　　A heat pump uses a portion of high-quality energy (electric energy, mechanical energy, or high-temperature thermal energy) as a cost, and through the thermodynamic cycle of a refrigeration machine, it \"pumps\" the heat from low-temperature heat sources to high-temperature heat carriers such as hot water at temperatures above 50°C. This enables meeting the demand for hot water in industries, agriculture, commerce, for processes like distillation and concentration, drying, and heating, as well as for heating buildings. Currently, the coefficient of performance for heat pumps ranges from 3 to 5; in other words, for every 1 kilowatt of electrical energy consumed, 3 to 5 kilowatts of heat can be generated. Under certain conditions, this is a cost-effective technology for utilizing the waste heat from wastewater whose temperature is slightly higher than the ambient temperature. 　　Currently, most of the heat pumps being developed and produced are compression-type heat pumps. Medium-sized heat pumps are under development, while large-sized heat pumps remain nonexistent. Among compressive heat pumps, water-source heat pump technology is the most widely used; it can be employed for recovering waste heat from the circulating water in thermal power plants or nuclear power plants, as well as from industries such as printing and dyeing and pharmaceuticals. For example, power plants use circulating water as the heat source water, and heat pump units are employed to raise the temperature of the boiler feed water, increasing it from 15°C to 50°C. This reduces the amount of coal required by the boilers, thereby achieving energy savings and reduced consumption. 　　In summary, there are numerous types of technologies and equipment for waste heat utilization; however, each has its own applicable conditions. Therefore, the appropriate method of waste heat utilization should be selected based on the temperature and quantity of industrial waste heat, as well as production conditions, process flows, and internal and external energy requirements. 　　Low-temperature industrial waste heat power generation technology based on organic media 1. Low-temperature organic Rankine cycle For the large amounts of low-temperature waste heat below 200°C that cannot be effectively recovered using steam or hot water flash systems, the economically viable organic Rankine cycle waste heat power generation technology is more suitable. 　　Low-temperature industrial waste heat power generation technology based on organic media belongs to thermodynamic conversion technology. The organic Rankine cycle is a Rankine cycle that uses low-boiling-point organic substances as the working fluid. It operates on the same thermodynamic principles as conventional steam power generation systems, with the difference being the working fluid; as a result, the system is simpler and more compact. This power generation method has significant advantages for the utilization of waste heat in low-temperature ranges. The waste heat stream does not come into direct contact with the working fluid; organic working fluids have a low specific volume, resulting in smaller pipe sizes and turbine flow areas. This allows for higher power output per unit volume, making it highly suitable for the recovery of waste heat at low temperatures. 　　The selection of organic working fluids is an important aspect in the waste heat power generation technology using organic Rankine cycles. Typically, the working fluid is required to have good power generation performance and good heat transfer properties; appropriate thermophysical properties such as critical parameters and boiling point at normal pressure; good chemical stability, no tendency to decompose, low corrosivity and toxicity, environmental friendliness, and low flammability and explosiveness; as well as being cost-effective, readily available in large quantities, and inexpensive. However, in practical applications, it is difficult for a working fluid to meet all of the above conditions simultaneously. Moreover, as international requirements regarding the environmental friendliness of organic working fluids increase, the available options continue to evolve; therefore, a comprehensive consideration based on the type of heat source and its temperature level is necessary. 　　In the equipment of organic Rankine cycle power generation systems, the design and manufacture of heat exchangers, pumps, and pipeline valves can draw on the standards used in heat exchange equipment in the chemical and refrigeration industries. The generators are part of a series of products; only the selection and design of turbine expanders, along with sealing technologies, require special consideration and non-standard design approaches. Commonly used turbine expanders include multi-stage axial flow turbines, which are suitable for applications with high temperatures, large fluid flows, significant total enthalpy drops, and high capacity. However, their internal efficiency is relatively low, and their design is more complex. Radial flow turbines have a relatively higher internal efficiency, are compact in structure, and are easier to manufacture; yet their capacity per unit is smaller. They are widely used abroad in waste heat utilization scenarios where the amount of waste heat to be recovered is modest. 　　2. Kalina cycle – A pure working-fluid organic Rankine cycle; due to the isothermal evaporation process of the working fluid, which does not match well with an actual variable-temperature low-temperature heat source, the average temperature difference for heat transfer is large, resulting in significant irreversible losses. The Kalina cycle is a cycle system that uses an ammonia mixture as the working fluid. The simplest thermodynamic cycle is the single-stage distillation cycle: an ammonia solution of a certain concentration is pressurized by a water pump and heated in a preheater, after which it enters a waste heat boiler to evaporate and form superheated ammonia vapor that drives a turbine to generate power. A complex distillation and cooling subsystem is then used to address the issue of condensing the ammonia mixture, allowing the exhaust steam from the turbine to be converted back into a working fluid solution of a certain concentration, which subsequently returns to the feed water pump, thus completing one cycle. 　　In the Kalina cycle, the working fluid evaporates isobarically and at varying temperatures during the evaporation process, which reduces the irreversibility of the heat absorption process by the working fluid. Moreover, since the basic working fluid contains low levels of ammonia during the condensation process, this overcomes the drawback of high condensation losses in organic Rankine cycles using mixed working fluids. Theoretical analyses suggest that the Kalina cycle offers performance that is more than 15% better than that of a conventional organic Rankine cycle using pure working fluids. However, in practical operation, due to factors such as the complexity of the evaporation process of ammonia-water mixed working fluids, the Kalina cycle does not exhibit particularly high performance. 　　Studies have shown that in the recovery of medium and low-temperature waste heat, depending on the type of waste heat involved, both the Kalina cycle and the Rankine cycle have their own advantages in this regard. For waste heat sources with constant temperature and flow rate, which are discharged at a specific temperature for use in production processes after waste heat recovery, the organic Rankine cycle-based low-temperature waste heat recovery system offers greater advantages. 　　Conclusion At present, the adoption rate of technologies for utilizing medium and high-temperature waste heat in China is not high, while the use of low-temperature waste heat has been largely abandoned due to immature technology. Therefore, to advance efforts in industrial energy conservation and emission reduction, it is necessary, on the one hand, to further promote the use of technologies for utilizing medium- and high-temperature waste heat, especially to improve the waste heat utilization rate in small and medium-sized enterprises, as well as to optimize the entire process system along with the corresponding waste heat utilization technologies. On the other hand, from a technological development perspective, the low-temperature organic Rankine cycle technology represents an economical and effective solution for utilizing low-temperature industrial waste heat, geothermal energy, and solar energy. However, this technology is not yet mastered in China; therefore, intensifying research on low-temperature waste heat power generation technologies such as the organic Rankine cycle and actively promoting their engineering applications will play a crucial role in improving the utilization rate of low-grade waste heat.