1. History, current situation and classification of industrial furnaces
Industrial furnaces and furnaces, used for processes such as casting, roasting, sintering, melting and heating, have been present in China since the Shang Dynasty, with furnace temperatures capable of reaching 1200°C.
During the spring and autumn period, the development of furnace technology led to the production of cast iron.
In 1794, the world saw the introduction of a straight cylindrical dome for casting cast iron.
Then, in 1864, Martin of France built the first open gas-fired steelmaking furnace, based on the regenerative furnace principle developed by Siemens of Great Britain. By preheating the air and gas in the regenerative chamber, the furnace was able to reach the 1600°C temperature required for steel production.
By the 1920s, sufficient energy sources were available, leading to the widespread use of resistance furnaces, electric arc furnaces, and tubular induction furnaces in industry. At the same time, the introduction of mechanized and automatic kilns improved productivity and working conditions.
In the 1950s, coreless induction furnaces experienced rapid growth. Later, the electron beam furnace was invented, using beams of electrons to impact solid fuel to heat and melt high-melting point materials.
At present, China has approximately 130,000 industrial furnaces, mainly located in the metallurgical, building materials, machinery and chemical industries, which account for more than 85% of the total number of furnaces.
The annual energy consumption of these furnaces is 25% of national energy consumption, with approximately equal portions allocated to fuel furnaces and electric furnaces.
There are several important challenges facing industrial furnaces in China, including outdated combustion methods, high labor intensity, environmental pollution, excessive reburning, low thermal efficiency, and inadequate automatic monitoring and control systems.
Industrial furnaces can be classified based on process characteristics, working temperature, thermal operation characteristics and working system.
Common types of industrial furnaces and furnaces include smelting furnaces, melting furnaces, heating furnaces, petrochemical furnaces, heat treatment furnaces, sintering furnaces, chemical working furnaces, calcining furnaces and furnaces, drying furnaces and furnaces, kilns electric arc furnaces, induction furnaces for high-temperature smelting, coke ovens, incinerators and others.
Table of industrial furnace classification codes
| Code | Industrial Furnace Category | Code | Industrial Furnace Category |
| 010 | Smelting Furnace | 071 | Calcium Carbide Furnace |
| 011 | Blast oven | 072 | General calciner |
| 012 | Steel furnace and mixer | 073 | Fluidized Bed Furnace |
| 013 | Ferroalloy Smelting Furnace | 079 | Other chemical furnaces |
| 014 | Non-Ferrous Metal Smelting Furnace | 080 | Firing furnace |
| 020 | Melting Furnace | 081 | cement kiln |
| 021 | Steel Melting Furnace | 082 | lime kiln |
| 022 | Non-ferrous metal melting furnace | 083 | Refractory furnace |
| 023 | Non-metallic melting furnace and smelting furnace | 084 | Daily Ceramic Oven |
| 024 | Dome | 085 | Sanitary ceramic kiln construction |
| 030 | Heating furnace | 086 | Masonry |
| 031 | Steel Continuous Heating Furnace | 087 | Tang Porcelain Firing Kiln |
| 032 | Non-Ferrous Metal Heating Furnace | 088 | Other firing furnaces |
| 033 | Steel Intermittent Heating Furnace | 090 | Drying oven (oven) |
| 034 | Soaking well | 091 | Foundry Drying Furnace (Oven) |
| 035 | Non-Metallic Heating Furnace | 092 | Cement drying oven (oven) |
| 039 | Other heating and holding furnaces | 099 | Other drying ovens (ovens) |
| 040 | petrochemical furnace | 100 | Smoke burning furnace (furnace) |
| 041 | Tube oven | 110 | arc furnace |
| 042 | Contact reactor | 120 | Induction furnace (high temperature smelting) |
| 043 | Cracking Furnace | 130 | coke oven |
| 049 | Other petrochemical furnaces | 131 | Coal Coke Oven |
| 050 | Heat Treatment Furnace (<1000℃) | 132 | Oil Coke Oven |
| 051 | Steel Heat Treatment Furnace | 140 | Chu burning furnace |
| 052 | Non-Ferrous Metal Heat Treatment Furnace | 141 | Solid waste incinerator |
| 053 | Non-Metallic Heat Treatment Furnace | 142 | Alkali Recovery Furnace |
| 054 | Other heat treatment furnaces | 143 | Chu household stove |
| 060 | Sintering furnace (black metallurgy) | 144 | Chu medical waste burning furnace |
| 061 | sintering machine | 145 | Gas dream burner |
| 062 | Pelletizing shaft furnace, belt pelletizing | 149 | Other dream burners |
| 070 | Chemical working furnace | 190 | Other industrial ovens |
2. Energy saving status of industrial furnaces
The energy consumption of industrial furnaces is influenced by numerous factors, however, current primary methods for conserving energy include optimizing design, improving equipment, utilizing waste heat, and improving monitoring and control control. production management.
1. Thermal Test
In China, despite the presence of globally advanced technologies, many industrial furnaces have limitations. This is associated with high replacement costs and a significant increase in energy consumption. Therefore, scientific and technological innovation is crucial.
To achieve energy-saving technical transformation, scientific testing methods are essential. These methods help you gain a comprehensive understanding of the thermal process of industrial furnaces, analyze and diagnose any problems, and determine the root cause.
Among the available thermal testing methods, thermal equilibrium testing is widely recognized. Measures the thermal efficiency of the industrial furnace, leading to improved thermal efficiency, reduced unit consumption and the determination of various indicators of economic and technical performance of the furnace operation.
By analyzing the operating conditions of the heating furnace, its working conditions can be adjusted to achieve the ideal operating state, thereby finding effective ways and directions for energy conservation. This is the main purpose of thermal testing.
However, there are certain challenges associated with thermal testing methods, such as the complexity of testing and the difficulty in accurately simulating stable production conditions, which can result in a large gap between test results and actual performance.
Therefore, the future development of testing technology will be a research focus for experts and academics.
2. Furnace structure, furnace construction materials and combustion technology
After carrying out the test, we now have a preliminary understanding of the oven, which serves as the basis for its technical transformation.
When designing the furnace, it is advisable to adopt a new furnace that saves energy and meets the requirements of the production process as much as possible.
During the design process, factors such as furnace type, material, seal, heat transfer process (combustion), and temperature distribution are typically taken into consideration.
According to available data, the main energy saving measures are as follows:
(1) Using a circular furnace instead of a box furnace can improve uniform heat transfer to the workpiece, reduce heat dissipation from the furnace wall, and create a heat exchange system within the furnace to facilitate heat exchange between heating elements, oven lining and workpiece.
By optimizing the furnace space and increasing the inner wall area, the heat exchange efficiency can be improved by increasing the heat exchange area.
(2) Installing a fan in the oven can improve convective heat transfer. This is especially important for small heating furnaces, as the high-speed airflow can break through the stagnant boundary layer of furnace gas on the surface of the workpiece and shorten the heating time, accelerating the temperature rise of the workpiece .
(3) The sealing of the oven body is crucial, including the sealing of various components leaving the oven, the oven shell and the oven door.
If the furnace body is not sealed properly, fire and leakage may occur, resulting in significant energy waste, equipment damage, and unfavorable environmental conditions.
Therefore, the quality of parts and energy consumption are directly affected by the sealing of the furnace body. Furthermore, sealing is also crucial for controlling the atmosphere inside the oven.
The appearance of refractory fiber products has created opportunities to solve the problem of sealing the furnace body and achieved smooth sealing.
(4) The refractory concrete heating furnace as a whole has high strength, integrity, good tightness and a long service life.
(5) New furnace materials are used to optimize the structure of the furnace lining.
While ensuring the structural strength and heat resistance of the furnace, the furnace lining should aim to improve insulation capacity and reduce heat storage. Simply increasing the lining thickness to lower the furnace wall temperature will result in greater lining heat storage, higher costs, and reduced utilization of the lower furnace area.
The insulation layer is composed of refractory fiber and rock wool, and the furnace body lining is made of lightweight brick, which reduces heat storage loss, improves thermal insulation and decreases heat dissipation loss of the oven wall.
(6) The application of high-temperature and high-radiation coating on the inner wall of the furnace increases the radiative heat transfer in the furnace, promoting the full utilization of thermal energy. This energy saving method has an effect of 3% to 5% and is considered an advanced energy saving method for the near future.
(7) Different burners are used according to different working conditions, such as flame regulating burners, flat flame burners, high speed nozzles, self-preheating burners, low nitrogen oxide burners and regenerative burners recently developed, providing a range of advanced burners suitable for gas and diesel use.
Correct use of advanced, efficient burners can often result in energy savings of more than 5%. Flat flame burners are more suitable for heating furnaces, while high-speed burners are suitable for various heat treatment furnaces and heating furnaces.
Self-preheating burners, which combine burners, heat exchangers and smoke exhaust devices, are suitable for various industrial furnaces such as heating, melting and heat treatment.
(8) The selection of energy-efficient combustion devices, combined with efficient fans, oil pumps, valves, thermal detection and automatic control systems, can significantly improve energy savings depending on the type of fuel.
Conventional energy-saving combustion technologies include high-temperature air combustion, oxygen-enriched combustion, heavy oil emulsification, oxygen-enriched pulverized coal injection for blast furnace, and magnetization treatment of common furnace fuel.
Of these, high-temperature air combustion and oxygen-enriched combustion are the most widely used.
High-temperature air combustion technology was developed in the 1990s and allows air preheating to reach 95% of the flue gas temperature through regenerative flue gas recovery. This results in a uniform furnace temperature of ≤±5℃ and a combustion thermal efficiency of 80%.
This technology has several advantages, such as high efficiency, energy saving, environmental protection, low pollution, stable combustion, large combustion area, wide fuel adaptability, easy combustion control, reduced equipment investment, longer furnace life and easy operation.
However, there are still some challenges to be faced, such as optimizing control and regulation systems, improving the relationship between thermal parameters and design structure, gas and regenerator quality, and the useful life of the regenerator and of the regenerative heating furnace.
Oxygen-enriched combustion technology involves the use of gases with oxygen concentrations greater than 21% in combustion. The objective is to develop burners suitable for industrial ovens.
This technology has several benefits, such as reducing furnace exhaust heat loss, increasing flame temperature, extending furnace life, increasing production, reducing equipment size, improving production cleanliness and facilitating recovery, comprehensive utilization and storage of CO2 and SO2.
However, the increase in oxygen content in oxygen-enriched combustion also leads to a sharp increase in temperature and an increase in NOx, which limits its adoption in various fields. When designing an industrial furnace to use oxygen-enriched air for combustion, it is important to avoid irregular temperature fields in the furnace.
3. Waste heat recovery and utilization
Waste heat can be categorized into seven types: high-temperature waste gas, cooling medium, waste steam and water, high-temperature products and slag, chemical reactions, combustible waste gas, and high-pressure fluid waste pressure.
According to research, the total waste heat produced by various industries ranges from 17% to 67% of total fuel consumption, with 60% coming from recyclable waste thermal resources.
Combustion gases are responsible for removing 30% to 70% of the total heat supplied by fuel furnaces.
As such, the recovery and utilization of waste heat in flue gases is crucial to energy conservation. This can be achieved through:
(1) Installation of preheaters to preheat the air and fuel supporting combustion using flue gases.
(2) Installation of waste heat boilers to generate hot water or steam for production or domestic purposes.
(3) Use flue gas as a heat source for low-temperature furnaces or preheat cold parts or furnace loads.
In China, preheaters for preheated air have been used in industrial furnaces since the 1950s, mainly in the form of tubular, radiant cylindrical and cast iron block heat exchangers. However, the exchange efficiency is low.
In the 1980s, domestic heat exchangers such as jet type, jet radiation type and double table type were successively developed to solve the issue of waste heat recovery in medium and low temperature applications. These developments have resulted in significant improvements in the recovery of waste heat from combustion gases at temperatures below 100°C.
However, at high temperatures, the limitations of heat exchanger materials, including low service life, high maintenance requirements and high costs, still pose challenges to the promotion and use of these systems.
At the beginning of the 21st century, China developed a ceramic heat exchanger, which has the same production process as kiln furniture. The main application properties of the materials used are thermal conductivity and oxidation resistance. This heat exchanger works by placing it close to the smoke outlet, where temperatures are high, without exposing it to cold air or requiring protection against high temperatures.
When the furnace temperature is between 1250-1450°C, the chimney outlet temperature should be 1000-1300°C. The ceramic heat exchanger is capable of recovering waste heat up to 450-750°C. The recovered hot air is then sent back to the furnace to be mixed with fuel gas for combustion, resulting in a 35% to 55% reduction in energy use and a corresponding reduction in production costs.
The ceramic heat exchanger has proven to be a valuable solution in cases where metallic heat exchangers are limited by corrosion and high temperature resistance. Its advantages include good thermal conductivity, high temperature resistance, good oxidation resistance, thermal shock resistance, long service life, low maintenance requirements, reliable performance and simple operation.
Ceramic heat exchangers are widely used in a variety of industries, including metallurgy, non-ferrous, refractory, chemical and building materials, to recover waste heat from high-temperature flue gases. Other types of high-efficiency heat exchangers that have become popular in China include foil heat exchangers, various jet heat exchangers, insertion tube heat exchangers, cyclone tube heat exchangers, tube heat exchangers fried dough twist, various combined heat exchangers, gas tube heat exchangers and heat exchangers with heat storage.
The regenerative heat exchanger is expected to be the technical development trend for the future, as it can lead to energy savings of more than 30% when the exhaust gas emission temperature after heat utilization is below 200°C .
The superconducting heat pipe is the main heat conduction component of waste heat recovery systems and offers advantages over traditional heat exchangers. The heat exchange efficiency of heat pipe waste heat recovery systems can reach more than 98%, which is unattainable with traditional heat exchangers. Additionally, these systems are smaller, only 1/3 the size of common heat exchangers.
4. Thermal detection and control
Currently, industrial furnaces in our country consume a large amount of energy and generate significant waste. The problem of excessive excess air coefficient is also common.
This is largely due to outdated regulatory methods, the high labor intensity of workers and the difficulty in maintaining ideal combustion conditions.
Improving the level of thermal sensing and control can therefore result in significant energy savings.
The development direction of industrial furnace automatic control is toward advanced automatic control technology, particularly microcomputer control systems.
By implementing an automatic control system, energy can be saved through efficient and precise coordination and control of relevant systems, such as precise control of key heating furnace process variables, temperature cascade control and fuel flow , control of the proportion of fuel and combustion air and control of oxygen content in combustion gases.
3. Conclusion
In conclusion, the furnace industry has ample room for growth and improvement in the areas of energy efficiency, thermal sensing techniques and waste heat utilization in the coming years.
Although the traditional equilibrium method will likely remain the dominant method for thermal detection for now, it is important to continually seek out and develop new methods.
Furthermore, transitioning from fossil fuels to clean, renewable energy sources as a primary energy source for industrial furnaces and reducing emissions will become a crucial area of research in the future.
