Industrial Furnaces: Classification, Energy Conservation and Pollution

1. History, current situation and classification of industrial furnaces

As the thermal equipment for smelting, roasting, sintering, melting, heating and other processes of materials or workpieces, industrial furnaces and kilns have appeared in China as early as the Shang Dynasty, and the furnace temperature can reach 1200 ℃.

In the Spring and Autumn Period, with the development of furnace temperature technology, cast iron appeared at that time.

In 1794, a straight cylindrical cupola for smelting cast iron appeared in the world.

Later, in 1864, Martin (France) built the first steel making open hearth furnace heated by gas fuel on the basis of the regenerative furnace principle of Siemens (Britain).

By preheating the air and gas in the regenerative chamber, he made the furnace temperature reach the temperature above 1600 ℃ required for steelmaking.

By the 1920s, the power supply was gradually sufficient, and various resistance furnaces, electric arc furnaces and cored induction furnaces were used widely in industry.

At the same time, the appearance of mechanized and automatic furnace has improved the productivity of the furnace and the working conditions to a certain extent.

In the 1950s, coreless induction furnaces developed rapidly.

Later, electron beam furnace appeared, which used electron beam to impact solid fuel to strengthen surface heating and melt high melting point materials.

At present, there are about 130000 industrial furnaces in China, mainly distributed in metallurgy, building materials, machinery and chemical industry, accounting for more than 85% of the total number of furnaces.

The annual total energy consumption is 25% of the national total energy consumption, and the proportion of fuel furnace and electric furnace is equivalent.

The main problems of industrial furnaces in China are relatively primitive combustion mode, high labor intensity, environmental pollution, high reburning consumption, low furnace thermal efficiency, and poor automatic monitoring and control means.

Industrial furnaces can be classified mainly according to process characteristics, working temperature, thermal operation characteristics and working system.

The commonly used furnaces and kilns in industry mainly fall into the following categories: smelting furnace, melting furnace, heating furnace, petrochemical furnace, heat treatment furnace, sintering furnace, chemical working furnace, calcining furnace, calcining kiln, drying furnace (kiln), calcining furnace (kiln), electric arc furnace, induction furnace (high-temperature smelting), coke oven, incinerator and other industrial furnaces.

Classification code table of industrial furnaces

CodeIndustrial furnace categoryCodeIndustrial furnace category
010Smelting furnace071Calcium carbide furnace
011Blast furnace072General calciner
012Steelmaking furnace and mixer073Fluidized bed furnace
013Ferroalloy smelting furnace079Other chemical furnaces
014Non ferrous metal smelting furnace080Firing kiln
020Melting furnace081Cement kiln
021Steel melting furnace082Lime kiln
022Non ferrous metal melting furnace083Refractory furnace
023Non metal melting furnace and smelting furnace084Daily ceramic kiln
024Cupola085Building sanitary ceramic kiln
030Heating furnace086Brickyard
031Steel continuous heating furnace087Tang porcelain firing kiln
032Non ferrous metal heating furnace088Other firing kilns
033Steel intermittent heating furnace090Drying furnace (kiln)
034Soaking pit091Casting drying furnace (kiln)
035Non metal heating furnace092Cement drying furnace (kiln)
039Other heating and holding furnaces099Other drying furnaces (kilns)
040Petrochemical furnace100Smoke burning furnace (kiln)
041Tubular furnace110Arc furnace
042Contact reactor120Induction furnace (high temperature smelting)
043Cracking furnace130Coke oven
049Other petrochemical furnaces131Coal coke oven
050Heat treatment furnace (<1000 ℃)132Oil coke oven
051Steel heat treatment furnace140Chu burning furnace
052Non ferrous metal heat treatment furnace141Solid waste incinerator
053Non metal heat treatment furnace142Alkali recovery furnace
054Other heat treatment furnaces143Chu household stove
060Sintering furnace (black metallurgy)144Hospital waste Chu burning furnace
061sintering machine145Gas dream burner
062Pellet shaft furnace, belt pelletizing149Other Dream Burners
070Chemical working furnace190Other industrial furnaces

2. Energy saving status of industrial furnaces

The energy consumption of industrial furnaces is affected by many factors, but the main measures for energy conservation at present are generally inseparable from optimization design, improvement of equipment, recovery of waste heat, strengthening detection control and production management.

1. Thermal test

In China, compared with the advanced technologies in the world, some industrial furnaces have many shortcomings.

At the same time, in addition to the high replacement cost, energy consumption has increased to a large extent.

Therefore, scientific and technological innovation is very important.

Scientific testing methods are indispensable for energy-saving technical transformation, comprehensive understanding of the thermal process of industrial furnaces, analysis and diagnosis of the “illness” of heating furnaces, and finding out its “cause”.

Among the current thermal testing methods, the thermal balance test is recognized.

Through the thermal measurement of the industrial furnace, the thermal efficiency of the heating furnace is further improved, the unit consumption is reduced, and various parameters of the economic and technical performance indicators of the heating furnace operation are obtained.

The operating conditions of the heating furnace are analyzed, and the working conditions of the heating furnace are adjusted in time to make it reach the optimal operating state, so as to find out the effective way and direction of energy conservation.

This is the main objective of thermal testing.

However, there are some problems in the use of thermal testing methods, such as the complexity of testing, and the simulation of stable production conditions is easy to be inaccurate, which makes the testing to a certain extent have a large gap with the actual.

Therefore, the future development of testing technology will become the research direction of some experts and scholars.

2. Furnace structure, furnace building materials and combustion technology

After the test, we have a preliminary understanding of the furnace, which also provides a basis for technical transformation.

When designing the furnace, the new energy-saving furnace that meets the production process requirements shall be adopted as far as possible.

In practice, furnace type, material, seal, heat transfer (combustion) process, temperature distribution, etc. are usually considered.

According to relevant data, there are mainly the following energy-saving measures:

(1) The use of a circular furnace instead of a box furnace can strengthen the effect of uniform heat transfer of the furnace to the workpiece, reduce the heat dissipation of the furnace wall, and form a heat exchange system in the furnace to conduct heat exchange between the heating elements, furnace lining and workpiece.

By using reasonable furnace space and increasing the inner wall area of the furnace without increasing the furnace space volume, the heat exchange of the furnace can be improved by increasing the heat exchange area to improve the heat efficiency.

(2) Fan shall be installed in the furnace to strengthen convection heat transfer in the furnace. Especially for small heating furnaces, high speed airflow can destroy the bottom layer of the furnace gas boundary that is stagnant on the surface of the workpiece and hinders heat transfer, thus shortening the heating time and accelerating the increase of the workpiece temperature.

(3) Furnace body sealing, including the sealing of various leading out components, furnace shell, furnace door, etc. in the furnace.

If the furnace body is not sealed tightly, it will cause fire running and leakage everywhere, resulting in a large amount of energy waste, equipment burning, bad environment and other conditions.

Therefore, the furnace body sealing directly affects the quality of workpieces and energy consumption.

At the same time, sealing is also the key to atmosphere control in the furnace.

The appearance of refractory fiber products creates conditions for solving the furnace body sealing and realizes the soft sealing.

(4) The heating furnace with refractory castable as a whole has the advantages of high strength, integrity, good air tightness and long service life.

(5) New furnace materials are used to optimize the furnace lining structure.

On the premise of ensuring the structural strength and heat resistance of the furnace, the furnace lining should try to improve the insulation capacity and reduce heat storage.

Simply increasing the lining thickness to reduce the furnace wall temperature will not only increase the lining heat storage and cost, but also reduce the effective utilization of the furnace bottom area.

The refractory fiber and rock wool are selected as the insulation layer, and the light brick is used as the lining of the furnace body to reduce the heat storage loss of the furnace body, enhance the heat insulation of the furnace, and reduce the heat dissipation loss of the furnace wall.

(6) Applying high temperature and high radiation coating on the inner wall of the furnace wall to strengthen the radiation heat transfer in the furnace is conducive to the full use of heat energy. Its energy-saving effect is 3%~5%, which is an advanced energy-saving method in the near future.

(7) Different burners are used according to different working conditions.

For example, flame regulating burner, flat flame burner, high-speed nozzle, self preheating burner, low nitrogen oxide burner and recently developed regenerative burner provide a variety of advanced burners to adapt to the use of gas and diesel.

The correct use of efficient advanced burners can generally save more than 5% of energy.

Flat flame burners are most suitable for use in heating furnaces.

High-speed burners are suitable for various heat treatment furnaces and heating furnaces. Self-preheating burners are combustion devices that combine burners, heat exchangers, and smoke exhaust devices, and are suitable for various industrial furnaces such as heating, melting, and heat treatment.

(8) According to the type of fuel, it is also an effective energy-saving measure to select energy-saving combustion devices with good performance and matching fans, oil pumps, valves and thermal detection and automatic control systems to ensure good combustion conditions and control and regulation functions.

In terms of combustion technology, conventional energy-saving combustion technologies include: high-temperature air combustion technology, oxygen-enriched combustion technology, heavy oil emulsification technology, oxygen-enriched pulverized coal injection technology for blast furnace, magnetization treatment technology for ordinary furnace fuel before it is put into the furnace, etc.

Among them, high-temperature air combustion technology and oxygen enriched combustion technology are widely used.

High-temperature air combustion technology is a combustion technology developed in the 1990s.

High-temperature air combustion technology can make the air preheating temperature reach 95% of the flue gas temperature through regenerative flue gas recovery, the furnace temperature uniformity is ≤± 5 ℃, and the combustion thermal efficiency can reach 80%.

This technology has many advantages, such as high efficiency and energy saving, environmental protection, low pollution, good combustion stability, large combustion area, wide fuel adaptability, easy combustion control, reduced equipment investment, longer furnace life, and convenient operation.

However, there are still some problems in high temperature air combustion, such as the quantitative relationship between the thermal parameters and the design structure, the optimization of the control system and the regulation system, the relationship between the gas quality and the regenerator, the improvement of the life of the regenerator and the life of the regenerative heating furnace, which need to be further explored.

The technology of using gases with oxygen concentration higher than 21% to participate in combustion is called oxygen enriched combustion technology.

The technology of oxygen enriched combustion is mainly to develop burners suitable for industrial furnaces.

The oxygen enriched combustion supporting technology has the advantages of reducing the heat loss of the furnace exhaust, increasing the flame temperature, extending the life of the furnace, increasing the output of the furnace, reducing the size of the equipment, cleaning the production, and facilitating the recovery, comprehensive utilization and storage of CO2 and SO2.

However, the increase of oxygen content in oxygen enriched combustion leads to a sharp rise in temperature and increases NOx, which is one of the factors seriously restricting the oxygen enriched combustion technology from entering more fields.

In addition, when the industrial furnace is designed to use oxygen enriched air for combustion support, the uneven temperature field in the furnace should be avoided.

3. Recovery and utilization of waste heat

Waste heat includes seven types: waste heat of high-temperature waste gas, waste heat of cooling medium, waste heat of waste steam and waste water, waste heat of high-temperature products and slag, waste heat of chemical reaction, waste heat of combustible waste gas and waste, and residual pressure of high-pressure fluid.

According to the survey, the total waste heat resources of various industries account for 17%~67% of the total fuel consumption, and the recyclable waste heat resources account for about 60% of the total waste heat resources.

The heat taken away by the flue gas accounts for 30%~70% of the total heat supply of the fuel furnace.

Therefore, the recovery and utilization of the waste heat in the flue gas will be another key point of energy conservation. Generally, the waste heat utilization of flue gas includes:

(1) Preheaters are installed to preheat combustion supporting air and fuel with flue gas.

(2) Waste heat boilers are installed to generate hot water or steam for production or domestic use.

(3) The flue gas is used as the heat source of the low-temperature furnace or to preheat the cold workpiece or furnace charge.

In China, preheaters of preheated air have been used in industrial furnaces since the 1950s, mainly in the form of tubular, cylindrical radiant and cast iron block heat exchangers, but the exchange efficiency is low.

In the 1980s, domestic heat exchangers such as jet type, jet radiation type and double table type were developed successively to mainly solve the waste heat recovery of medium and low temperature.

Remarkable results have been achieved in the waste heat recovery of flue gas below 100 ℃, and the heat exchange efficiency has been improved.

However, at high temperature, the material of heat exchanger is still limited, the service life is low, the maintenance workload is large or the solid cost is expensive, which affects the promotion and use.

At the beginning of the 21st century, a ceramic heat exchanger was developed in China.

Its production process is basically the same as that of kiln furniture.

Thermal conductivity and oxidation resistance are the main application properties of materials.

Its principle is to place the ceramic heat exchanger near the flue outlet, where the temperature is high, without mixing cold air and high temperature protection.

When the furnace temperature is 1250-1450 ℃, the temperature of the flue outlet should be 1000-1300 ℃.

The ceramic heat exchanger can recover waste heat up to 450-750 ℃.

The returned hot air is sent to the furnace to form a mixture of gas and fuel gas for combustion, which can save 35% – 55% of energy, thus directly reducing production costs, Increase economic benefits.

Ceramic heat exchanger has been well developed under the use limitation of metal heat exchanger, because it has better solved the problems of corrosion resistance and high temperature resistance, and has become the best heat exchanger for recovering high temperature waste heat.

After many years of production practice, it shows that the ceramic heat exchanger has a good effect.

Its main advantages are: good thermal conductivity, high temperature strength, good oxidation resistance and thermal shock resistance.

Long service life, small maintenance, reliable and stable performance, simple operation.

It is the best device for recovering waste heat of high-temperature flue gas at present.

At present, ceramic heat exchangers can be used in major thermal kilns in metallurgy, non-ferrous, refractory, chemical, building materials and other industries.

The heat exchanger is the most effective and widely used one to recover the waste heat of flue gas.

High efficiency heat exchangers developed and popularized in China in recent years include sheet heat exchangers, various jet heat exchangers, various insert tube heat exchangers, cyclone tube heat exchangers, fried dough twist tube heat exchangers, various combined heat exchangers, gas tube heat exchangers and heat storage heat exchangers.

Regenerative heat exchanger is the technical development trend in the future.

The exhaust gas emission temperature after other heat utilization is below 200 ℃, and the energy-saving benefit can reach more than 30%.

Superconducting heat pipe is the main heat conduction component of waste heat recovery device, which is essentially different from ordinary heat exchanger.

The heat exchange efficiency of the heat pipe waste heat recovery device can reach more than 98%, which is impossible for any ordinary heat exchanger.

The heat pipe waste heat recovery device is small, only 1/3 of the ordinary heat exchanger.

4. Thermal detection and control

At present, the energy consumption of industrial furnaces in our country is large and the waste is serious, and the problem of excessive air excess coefficient generally exists.

This is mainly due to the backward regulation means, the labor intensity of workers is high, and it is difficult to ensure the ideal combustion conditions.

Therefore, it has great energy saving potential to improve the level of thermal detection and control.

Advanced automatic control technology, especially microcomputer control system, has become the development direction of industrial furnace automatic control.

By setting up an automatic control system, energy can be saved through timely and accurate coordination and control of relevant systems.

Such as the quantitative control of the main process variables of the heating furnace, the cascade control of furnace temperature and fuel flow, the ratio control of fuel and combustion air, and the oxygen content control of flue gas.

3. Conclusion

To sum up, in the next few years, there is still much room for the development of the furnace in terms of energy conservation, thermal detection methods, and waste heat recovery.

The objective and practical thermal detection method will continue to use the balance method for a period of time in the future, but it is necessary to carry out rapid exploration of new methods.

At the same time, how to quickly replace fossil fuel with clean and renewable energy to become the main energy source of industrial furnaces and reduce pollutant emissions will become a research topic in the future.

How useful was this post?

/ 5. Vote count:

No votes so far! Be the first to rate this post.

Expert Help and Customized Price Quotes

Need a price quote or have questions? Contact us and let us know your detailed requirements. Our experts will provide you with personalized assistance and a competitive price quote.

About The Author

Leave a Comment

Your email address will not be published. Required fields are marked *