Waste heat recovery refers to the recovery and reuse of waste heat generated in industrial processes.The main technologies include heat exchange technology, thermal power conversion technology, waste heat refrigeration and heating technology.
At present, China's energy utilization still has the main problems of low utilization efficiency, poor economic benefits, and great pressure on the ecological environment. As an important part of the strategic planning of energy development, energy conservation and emission reduction, reducing energy consumption, and improving the comprehensive utilization rate of energy are the fundamental ways to solve China's energy problems, and are in a priority position for development.The goal of energy conservation, emission reduction and energy efficiency improvement mainly depends on the industrial sector.In China, which is in the middle and later stage of industrialization, industry is the main energy consuming field and also the main source of pollutants.The energy consumption of China's industrial sector accounts for about 70% of the total energy consumption of the country, and the unit energy consumption of major industrial products is about 30% higher than the international advanced level on average.In addition to the relatively backward production process and unreasonable industrial structure, the low utilization rate of industrial waste heat and insufficient comprehensive utilization of energy (energy) are important reasons for high energy consumption. The energy utilization rate in China is only about 33%, about 10% lower than that in developed countries, and at least 50% of industrial energy consumption is directly discarded in various forms of waste heat.
Therefore, from another perspective, China's industrial waste heat resources are abundant and widely exist in the production process of various industries. Waste heat resources account for 17%~67% of its total fuel consumption, of which the recoverability rate reaches 60%. There is great room for improving the utilization rate of waste heat, and there is huge potential for energy conservation. Industrial waste heat recovery is also considered as a "new energy",In recent years, it has become an important part of promoting energy conservation and emission reduction in China[1]。
Characteristics of waste heat resources
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Waste heat resource belongs to secondary energy, which is the product of conversion of primary energy or combustible materials, or the heat generated in the fuel combustion process after completing a process.According to the temperature grade, industrial waste heat is generally divided into three types: high temperature waste heat above 600 ℃, medium temperature waste heat of 300-600 ℃ and low temperature waste heat below 300 ℃;According to the source, industrial waste heat can be divided into: flue gas waste heat, cooling medium waste heat, waste steam waste water waste heat,Heat of chemical reaction, high temperature products and slag waste heat, as well as combustible waste gas and waste heat.
Specifically, the residual heat of flue gas is large, and the temperature distribution range is wide, accounting for more than 50% of the total industrial waste heat resources, which is widely distributed, such as metallurgy, chemical industry, building materials, machinery, power and other industries, various smelting furnaces, heating furnacesinternal-combustion engineAnd boiler exhaust, and the residual heat of flue gas of some industrial kilns is even up to 30%~60% of the fuel consumption of the furnace itself, which has great energy saving potential and is the main object of waste heat utilization.The waste heat of cooling medium refers to the waste heat taken away by air, water, oil and other cooling media in industrial production in order to protect high-temperature production equipment or meet the cooling requirements of process flow. Most of the waste heat belongs to medium and low temperature waste heat, which accounts for 20% of the total industrial waste heat resources.Waste water waste steam waste heat is a low-grade steam or condensate waste heat, accounting for 10%~16% of the total waste heat resources;Chemical reaction waste heat accounts for less than 10% of the total waste heat resources, mainly in the chemical industry;High temperature products and slag waste heat mainly refer to sensible heat of billet, coke, slag, etc., and sensible heat of oil and gas products in petrochemical industry;Combustible waste gas and waste heat refer to combustible components contained in exhaust gas, liquid and slag during production, such as blast furnace gas and converter gas in metallurgical industry.
Although waste heat resources have a wide range of sources, temperature ranges and various forms of existence, from the perspective of waste heat utilization, waste heat resources generally have the following common points: due to the cyclical, intermittent or production fluctuations in the process production process, the residual heat is unstable;The waste heat medium has poor properties, such as high dust content or corrosive substances in the smoke;The waste heat utilization device is limited by the inherent conditions of the site and the original production.Therefore, the operation environment of industrial waste heat resource utilization system or equipment is relatively harsh, which requires a wide and stable operation range to adapt to the changing production process requirements, high reliability of equipment components, and high initial investment costs. From the perspective of economy, it is necessary to carry out the overall design and layout of the system in combination with process production, and comprehensively utilize energy to improve the efficiency of waste heat utilization system equipment[1]。
Industrial waste heat utilization technology
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The waste heat temperature range is wide, and the forms of energy carriers are diverse. Due to the different environment and process flow and the inherent conditions of the site, the production and living needs, and the equipment types are diverse, such as air preheaters, kiln regenerators,Waste heat boiler, low temperature steam turbine, etc.Common industrial waste heat recovery and utilization methods have various classification methods. According to the characteristics of energy transmission or conversion during the utilization of waste heat resources, the current domestic industryWaste heat utilization technologyIt is divided into heat exchange technology, thermal power conversion technology, and waste heat refrigeration and heating technology.
Heat exchange technology
Waste heat recovery should be used preferentially for the system equipment or the process flow to reduce the primary energy consumption and minimize the number of energy conversionsair preheater, recuperators, heaters and other heat exchangers recover waste heat to heat combustion supporting air, fuel (gas), materials or workpieces, improve furnace performance and thermal efficiency, reduce fuel consumption and reduce flue gas emissions;Or generate steam hot water from high-temperature flue gas through waste heat boiler or vaporization cooler for process flow.The utilization of waste heat by this kind of technical equipment does not change the form of waste heat energy, but directly transfers the waste heat energy to the energy consumption process of its own process through heat exchange equipment to reduce the primary energy consumption, which can be collectively referred to as heat exchange technology. This is the most direct and efficient economic method to recover industrial waste heat. The corresponding equipment is various heat exchangers,There are not only traditional heat exchangers and heat pipe heat exchangers of various structures, but also waste heat steam generators (waste heat boilers).
Intermediate wall heat exchanger
Industrial heat exchangers are basically divided intoIntermediate wall heat exchanger, hybrid heat exchanger and regenerative heat exchanger.Its middle wall type and regenerative type are common equipment for industrial waste heat recovery,Hybrid heat exchangerIt relies on direct contact or mixing of cold and hot fluids to transfer heat, such as cooling tower, washing tower, air pressure condenser, etc. in industrial production, which is not common in waste heat recovery.The inter wall heat exchanger mainly includes tube type, plate type and co flow heat exchanger. Although the heat efficiency of tube type heat exchanger is relatively low, averaging 26%~30%, and its compactness and metal consumables are inferior to other types of heat exchanger, it has the characteristics of solid structure, large flexibility and wide range of materials, and is the most widely used heat exchange equipment in industrial waste heat recovery.40% of the heat exchangers in metallurgical enterprises are tubular heat exchangers. The allowable inlet flue gas temperature is more than 1000 ℃, the outlet flue gas temperature is about 600 ℃, and the average temperature difference is about 300 ℃.
Plate heat exchanger includes fin plate, spiral plate, plate shell heat exchanger, etc. Compared with tubular heat exchanger, its heat transfer coefficient is about twice that of tubular heat exchanger, with high heat transfer efficiency, compact structure, material saving, etc.In the metallurgical industry, the joint and small and medium-sized enterprises mostly use plate heat exchangers to preheat combustion supporting air. The average heat recovery rate is 28%~35%, the inlet flue gas temperature is about 700 ℃, and the outlet temperature is 360 ℃.However, the application range of plate heat exchanger is limited due to the greater restrictions of temperature and pressure than that of tubular heat exchanger.For high-temperature flue gas of various industrial furnaces, block hole heat exchangers, air coolers and co flow heat exchangers are also often used.Among them, the same flow heat exchanger belongs to the gas gas heat exchanger, which mainly includes radiation type and convection type. It is widely used in soaking pits, heating furnaces and other equipment to recover the waste heat of flue gas, preheat combustion supporting air or fuel, and reduce the amount of flue gas and flue gas emission temperature.The inlet flue gas temperature of common radiation co flow heat exchanger can reach more than 1100 ℃, and the outlet flue gas temperature can also reach 600 ℃, which can heat the combustion supporting air to 400 ℃, with good combustion supporting effect;The temperature efficiency can reach more than 40%, but the heat recovery rate is low, averaging 26%~35%.
regenerative heat exchanger
Regenerative heat exchange equipment is a kind of heat exchange equipment with cold and hot fluid alternately flowing through the heat storage element for heat exchange. It belongs to intermittent operation heat exchange equipment, suitable for recovering waste heat resources discharged intermittently, and is mostly used for heat exchange between high-temperature gas media, such as heated air or materials.According to the difference of heat storage medium and heat storage form, the heat storage heat exchange system can be divided into sensible heat storage and phase change latent heat storage.Sensible heat energy storage system has been used in industry for a long time, simple heat exchange equipment such as common rotary heat exchanger;Complex equipment such as ironmaking blast furnaceRegenerative hot blast stoveRegenerator of glass furnace.Sensible heat storage heat exchange equipment has limitations in industrial waste heat recovery due to its low energy storage density, large size, and inability to keep the temperature of heat storage constant.
Utilization of latent heat storage and heat exchange equipmentHeat storage materialThe inherent heat capacity and phase change latent heat store and transfer energy, with an energy storage density at least one order of magnitude higher than that of sensible heat storage equipment. Therefore, under the condition of storing the same heat, the volume of phase change latent heat storage and heat exchange equipment is 30%~50% less than that of traditional heat storage equipment.In addition, stable heat output, basically constant temperature of heat exchange medium and stable operation of heat exchange system are another advantage of this kind of latent heat storage and heat exchange equipment with phase change.Phase change energy storage materials can be roughly divided into high temperature according to their phase change temperaturePhase change materialAnd medium and low temperature phase change materials. The former has high phase change temperature and large phase change latent heat. It is mainly made of some inorganic salts and their mixtures, alkalis, metals and alloys, oxides, etc., and ceramic matrix or metal matrix composite. It is suitable for high-temperature waste heat recovery at 450~1100 ℃ and above, and is widely used;The latter is mainly crystalline hydrates or organics, suitable for low-temperature waste heat recovery.
Heat exchange equipment based on heat pipe
The heat pipe is an efficient heat conduction element, which transfers heat through the phase change process of the evaporation and condensation of working medium in the fully enclosed vacuum tube and the secondary wall heat exchange. It is a phase change energy storage and heat exchange device that combines heat storage and heat exchange devices.The heat pipe has excellent thermal conductivity, its heat transfer coefficient is nearly one order of magnitude higher than that of traditional metal heat exchanger, and it also has a series of advantages such as good isothermal property, controllable temperature, strong heat transfer capacity, freely changeable heat transfer area on both cold and hot sides, long-distance heat transfer, and no external auxiliary power equipment.The working temperature of heat pipe is divided into low temperature (- 200~+50 ℃), normal temperature (50~250 ℃), medium temperature (250~600 ℃), and high temperature (>600 ℃) heat pipes. The corresponding pipes and working fluids need to be selected according to the different operating temperatures.The carbon steel water gravity heat pipe has simple structure, low price, convenient manufacture and easy popularization, which makes this kind of heat pipe widely used.In practical application, the temperature of heat pipe used for industrial waste heat recovery is between 50 and 400 ℃, which is used for heat recovery or waste steam recovery of drying furnace, curing furnace and drying furnace, as well as for boiler or furnaceair preheater。
Waste heat boiler
useSteam generatorThat is, waste heat recovery by waste heat boiler is an important means to improve energy utilization rate. Nearly 80% of flue gas waste heat in metallurgical industry is recovered by waste heat boiler, with remarkable energy-saving effect.Waste heat boilerThere is no combustion process in the. Essentially, it is just a gas water/steam heat exchanger. It can use the waste heat of high-temperature flue gas, chemical reaction, combustible gas and high-temperature products to produce high-pressure, medium pressure or low-pressure steam or hot water for process flow or heat supply into the pipe network.At the same time, the waste heat boiler is an important equipment in the low-temperature steam turbine power generation system, providing steam working medium for steam turbine and other power machinery.In practical application, the majority of waste heat boilers use 350~1000 ℃ high-temperature flue gas. Compared with the operating temperature of coal-fired boilers, they are low-temperature boilers with low efficiency.Due to waste heatDust content of flue gasLarge, contains more corrosive substances, and is more likely to cause problems such as ash deposition, corrosion and wear of the boiler. Therefore, preventing ash deposition and wear is the key to the design of waste heat boiler.The straight through furnace type, large volume cavity radiation cooling chamber, sealed furnace wall, dust removal chamber, and a large number of rapping soot blowing devices are all structural considerations of waste heat boiler to solve the problems of ash deposition and wear.In addition, due to the space limitation of the process production site, the HRSG dispersedly installs the heat exchange parts in various parts of the process flow, rather than assembling them as a whole like ordinary boilers.
In the past decade, with the promotion of energy conservation and emission reductionWaste heat boilerThe design and manufacturing enterprises have accelerated their development. In order to adapt to the capacity adjustment and growth in the industrial field, waste heat boilers are developing in the direction of large-scale and high parameters, such as the evaporation capacity of 50 t/h and working pressure of 42 MPa waste heat boiler, or the evaporation capacity of iron and steel metallurgy industry reaches 100t/h, and the working pressure is 125 MPa CDQ waste heat boiler.In addition, further improving the heat transfer effect and heat utilization rate of the boiler, reducing problems such as ash deposition and wear, and reforming and innovating the boiler circulation mode, heating surface structure, flue gas flow channel in the boiler and ash removal mode are the contents of the technical progress of the waste heat boiler.
Thermal power conversion technology
Heat exchange technology recovers waste heat resources in the form of heat energy by reducing temperature grade, which is a kind of degraded utilization and cannot meet the demand of process flow or power consumption inside and outside the enterprise.In addition, if the heat exchange technology is used to recover a large number of medium and low temperature waste heat resources, the economy is poor or the recovered heat cannot be used in this process flow, so the benefits are not significant.Therefore, using thermal power conversion technology to improve the grade of waste heat is another important technology for recovering industrial waste heat.According to the classification of working medium, thermal power conversion technology can be divided into traditional steam turbine power generation technology with water as working medium andLow boiling point working mediumPower generation technology based on organic working medium.Due to the significant difference of working medium characteristics, the corresponding waste heat recovery system and equipment composition also have their own characteristics.
At present, the main industrial applications take water as the working mediumWaste heat boiler+Low temperature steam turbine power generation system composed of steam turbine or expander.Compared with the technical parameters of conventional thermal power generation, low temperature steam turbine generator sets use low temperature, low parameters and low power waste heat, which is often referred to as low temperature waste heat steam turbine power generation technology in the industry. The new dry process cement kiln low temperature waste heat power generation technology is a typical low temperature steam turbine power generation technology with medium and low temperature parameters.The available waste heat resources for low temperature steam turbine power generation are mainly medium and high temperature flue gas above 350 ℃, such as sintering furnace flue gas, glass, cement and other building materials industry furnace flue gas, or flue gas cooled to 400~600 ℃ after one-time utilization. The single unit power ranges from several megawatts to dozens of megawatts, such as oxygen converter waste heat power generation in the steel industry, sintering waste heat power generation,There are various forms of waste heat power generation such as dry quenching waste heat power generation in coking industry, low temperature waste heat power generation in cement industry, and waste heat power generation of building materials such as glass and ceramic bricks.However, from the temperature range of waste heat resources, the medium and high temperature waste heat utilized by this technology belongs to the medium and high temperature waste heat power generation technology.In addition, a large amount of steam is recovered from the process flow through the waste heat boiler or heat exchanger, of which the low-pressure saturated steam (about 1 MPa) or hot water accounts for a large proportion. In addition to being used for production and living, a large amount of surplus is often released.
At present, the power generation or driving technology using such low-pressure saturated steam is mainly the screw expansion power machine technology.This technology has the following characteristics:
A variety of heat source working fluids can be used as the power source, which is suitable for superheated steam, saturated steam, vapor liquid two-phase mixture, flue gas, dirty hot water, hot liquids, etc;
Simple and compact structure, can automatically adjust the speed, long life, small vibration;
The flow rate in the machine is low, with less energy loss and high efficiency except for leakage loss;
With the non-contact characteristics of double rotors, the shear effect formed during operation has the self-cleaning function and self descaling ability.
The screw expansion power machine belongs to the volumetric expansion machine. Due to the limitation of expansion capacity, the heat source application range of the direct driven screw expansion power machine is 015 ～ 3. 0 MPa steam or pressure 0For hot water above 8MPa and above 170 ℃, due to the structural characteristics, the power of single screw expansion power machine is limited, most of which are below 1000 kW, and mainly used for occasions with small waste heat scale.
Refrigeration and heating technology
Waste heat refrigeration technology
Compared with traditional compression typerefrigeration unitIn contrast, absorption or adsorption refrigeration systems can use cheap energy and low-grade heat energy to avoid power consumption and solve the shortage of power supply;The natural refrigerant is used, which does not contain CFC substances that damage the ozone layer. It has remarkable power saving ability and environmental protection benefits, and has been widely promoted and applied at the end of the 20th century.Absorption andAdsorption refrigerationThe thermodynamic cycle characteristics of the technology are very similar, all follow the cycle process of "generation (analysis) - condensation - evaporation - absorption (adsorption)", but the absorption material of absorption refrigeration is liquid with good fluidity, and the refrigerant is ammonia waterLithium bromide aqueous solutionThe generation and absorption process is realized by the generator and absorber;The adsorbent for adsorption refrigeration is generally a solid medium, and the adsorption methods are divided into physical adsorption and chemical adsorption. Molecular sieve water, calcium chloride ammonia and other working pairs are often used. The analysis and adsorption process are realized through the adsorber.
withLithium bromide aqueous solutionAbsorption refrigeration system with working medium is the most widely used. Generally, the low temperature heat source in the range of 80~250 ℃ can be used. However, because water is used as refrigerant, only the refrigerant temperature of 0 ℃ or above can be produced, which is mostly used for air conditioning or industrial chilled water. Its coefficient of performance COP varies greatly due to the different thermal properties of the refrigerant and the cycle mode of the thermodynamic system,The COP of the units actually used is no more than 2, which is far lower than that of the compression refrigeration system. However, such units can use low-grade heat energy such as low-temperature industrial waste heat, solar energy and geothermal energy, without consuming high-quality electric energy, and have certain advantages in the utilization of industrial waste heat.Absorption waste heatrefrigeration unitHigh refrigeration efficiency, suitable for large-scale heat recovery. The refrigeration capacity can range from dozens of kilowatts to several megawatts. It has been widely used in China, with mature technology and complete product specifications and types.There are many kinds of refrigerant pairs for adsorption refrigerators, including physical adsorption refrigerant pairs, chemical adsorption refrigerant pairs and composite adsorption refrigerant pairs. The applicable heat source temperature range is wide, and the heat source as low as 50 ℃ can be used. Moreover, there is no need for solution pumps or distillation devices, and there are no problems such as refrigerator pollution, salt solution crystallization and corrosion to metals.Adsorption refrigerationThe system is simple in structure, noiseless and pollution-free, and can be used in bumpy occasions, such as automobiles and ships. However, the refrigeration efficiency is relatively low. The performance coefficient of commonly used refrigeration systems is mostly below 0.7, which is limited by the manufacturing process. The refrigeration capacity is small, generally below several hundred kilowatts. It is more suitable for the use of small heat waste heat recovery, or for combined cooling, heating and power generation systems.
Heat pump technology
There is a large amount of waste heat (30~60 ℃) slightly higher than the ambient temperature in industrial production, such as industrial slag flushing water, cooling wastewater, thermal power plant circulating water, oilfield wastewater, low-temperature flue gas, water vapor, etc. The temperature is very low, but the residual heat is large. (Water source) heat pump technology is often used to recover such waste heat resources.The heat pump is compensated by consuming a part of high-quality energy (electrical energy, mechanical energy or high-temperature heat energy). Through the thermodynamic cycle of the refrigerator, the heat from the low-temperature heat source is "pumped" to the high-temperature heat medium, such as hot water at 50 ℃ or above, which can meet the demand for hot water for distillation, concentration, drying, heating or building heating of industry, agriculture and commerce.
At present, the heat supply coefficient of the heat pump unit is between 3 and 5, that is, it consumes 1 kW of electric energy and can produce 3 to 5 kW of heat. Under certain conditions and environments, it is an economically feasible technology to use waste heat slightly higher than the ambient temperature.At present, compression heat pumps are mostly developed and produced. Medium heat pumps are being developed, while large heat pumps are still blank.Among compression heat pumps, water source heat pump technology is the most widely used, which can be used for waste heat recovery in thermal power plants/nuclear power plants, printing and dyeing, tire manufacturing, oil fields, pharmaceuticals and other industries.For example, the power plant uses circulating water or process produced hot water as the heat source water, and improves the grade of boiler feed water through heat pump units, so that the original boiler feed water is increased from 15 ℃ (20 ℃, 25 ℃) to 50 ℃, reducing the demand of boiler for coal, and achieving the purpose of energy conservation and consumption reduction.
