FIELD OF THE INVENTION

The present invention relates to the field of furnaces, and particularly to furnaces with a high operating temperature used for example in the steel industry for reheating steels.

BACKGROUND OF THE INVENTION

Furnaces with high operating temperatures are extensively used in many fields. One such field is the reheating of steels, which requires a heating medium able to provide uniform high temperature heating of the steel, generally in the range of 1100-1300 °C. There are several electrical heating methods currently used in the steel industry. One method involves electrical resistance heating elements. A limitation of this method for the reheating of steels is that the temperature range mentioned above is near the operational limits for the electrical elements, resulting in limited operational lifetimes. Other electric heating methods used are induction heating and direct resistance heating with electrical currents. Both these methods can, however, present problems with non-uniform heating of the steel.

Heating methods involving a combustion process are currently the favored method for uniform high temperature heating of steel. Combustion processes generally involve fossil fuels generating high levels of air contaminants such as NOx and CO2, which later are released to the atmosphere through an exhaust pipe or chimney, leading to air contamination. Another problem with this type of combustion process is the use of fossil fuels, which today is threatened with great future restrictions and is in the process of being completely phased out.

To render the heating process more energy efficient, integrating heat exchangers in the combustion system is used for recovering heat from the hot exhaust gases. EP2927587 discloses such an oxyfuel combustion system, comprising a heat exchanger circuit and a heat transfer fluid which flows there through to cool the hot exhaust gases. Due to the heat exchanger circuit, heat exiting the furnace with the exhaust gas through the exhaust pipe can be recovered and used for preheating combustion components. Although this type of system provides a more efficient use of the fuels, it does not remedy the problem with emission of air contaminants. SUMMARY OF THE INVENTION

An object of the present invention is to overcome, or at least lessen the above mentioned problems. A particular object is to provide a high temperature furnace which eliminates or at least greatly reduces the emission of air contaminants.

To better address this concern, in a first aspect of the invention there is presented a high temperature furnace comprising a heating chamber, at least one first burner for heating the heating chamber, and a condensation device arranged in fluid connection with the heating chamber via an exhaust outlet. The first burner is arranged for providing a water vapor atmosphere in the heating chamber, and the condensation device is arranged for condensing an exhaust gas generated by the first burner.

Due to the water vapor atmosphere within the heating chamber, it is possible to condensate the exhaust gas from the first burner in the condensation device. Any contaminant substance would exit the process in the condensed water and therefore, there would be no need for a chimney and no emission of air contaminants would occur. The water vapor atmosphere provided in the chamber according to the present inventive concept can further contribute to a reduction of the oxide scale formation on the metal and thus the oxide scale losses, which generally occur during the combustion driven heating process, even at high temperatures.

The condensation device can recover both the sensible heat and the latent heat of condensation from the exhaust gas exiting the heating chamber. Water vapor can account for around 18 % of the total energy in a fuel, and condensation of the water vapor for use in low temperature applications by means of exhaust gas condensation can therefore increase the fuel efficiency of the overall process by an equal percentage. The hot water resulting from the process and exiting the

condensation device can be directed to and used for local heating systems, whereby the energy used for heating the high temperature furnace is reused. This has a positive effect on the environment and on the world economy, as it can contribute to a reduction of an overall energy consumption related to heating. In accordance with an embodiment of the high temperature furnace, the at least one first burner comprises a hydrogen-oxyfuel burner. Hydrogen can be produced from the decomposition of water using renewable electricity, and it can be used as a high quality fuel in industrial furnaces in the steel industry, providing a carbon-dioxide free combustion process. The resulting rest product from the combustion is water vapor, the use of which can eliminate the problem with emission of NOx elements formed at high temperatures. This advantage is because the excess water vapor or steam from the hydrogen-oxyfuel combustion can be condensed instead of exhausted in a chimney, avoiding thus the emission of NOx or other associated air pollutants, such as CO and CO ₂ , from the furnace to the atmosphere. The NOx gases that form from the combustion process would exit the process in the water condensed from the exhaust gases of the heating chamber. Depending on the size of the high temperature furnace, more than one first burner can be provided. Whereas a substantially small high temperature furnace can comprise only one first burner, a substantially large high temperature furnace can comprise up to 100 first burners, or more, for an effective heating of the heating chamber and the material therein.

In accordance with an embodiment of the high temperature furnace, the at least one first burner comprises a plasma electric arc jet burner. Plasma electric arc jet burners are highly efficient for high temperature heating. By using steam as the gas for forming the plasma jet, a water vapor atmosphere can be formed in the furnace chamber. This water vapor atmosphere can, in turn, be condensed in the condensation device such that the need for a chimney, and thus emission of any air contaminant, is avoided.

In accordance with one embodiment of the high temperature furnace, in which the at least one first burner comprises a hydrogen-oxyfuel burner, the high temperature furnace further comprises at least one second burner comprising a plasma electric arc jet burner. Providing a combination of a hydrogen-oxyfuel burner and a plasma electric arc jet burner could increase the effectiveness of the process, as the plasma electric arc jet burner could provide booster power when sufficient hydrogen is not available for the process. Likewise, hydrogen combustion for the hydrogen-oxyfuel burner can provide booster power when sufficient electricity is not available during the heating process. According to one embodiment, more than one second burner is provided. The number of first and second burners is selected depending on the size and design of the high temperature furnace, of the

requirements on the heating process, such as for example number of combustion zones, and on the material to be heated.

Furthermore, using hydrogen-oxyfuel combustion together with plasma electric arc jet heating provides the possibility of maintaining a low oxygen partial pressure which minimizes the formation of oxide scales on metals, such as steel, which form scale at high temperatures. In an embodiment combining plasma electric arc jet heating with hydrogen-oxyfuel combustion, hydrogen-oxyfuel combustion is used for the entire heating process. In an embodiment of the high temperature furnace combining plasma electric arc jet heating with hydrogen-oxyfuel combustion, hydrogen-oxyfuel combustion is used for the final stages, or combustion zones, of the heating process, corresponding to those requiring the highest temperatures. Correspondingly, in an embodiment combining plasma electric arc jet heating with hydrogen-oxyfuel combustion, plasma electric arc jet heating is used for the entire heating process. In an embodiment of the high temperature furnace combining plasma electric arc jet heating with hydrogen-oxyfuel combustion, plasma electric arc jet heating is used in the combustion zones of the heating process corresponding to those requiring the highest temperatures.

In accordance with one embodiment of the high temperature furnace, the condensation device is a condensing heat exchanger. The condensing heat exchanger is selected to be one suitable for the temperature range of the furnace, for example a radiative heat exchanger or a convective heat exchanger. Such

condensing heat exchangers can process exhaust gas at high temperatures and condense these to water, such that no exhaust gas is exiting the process and thus, no emissions are released to the atmosphere. In one embodiment, a convective heat exchanger of a plate and frame design is used. In one embodiment, a convective heat exchanger of shell and tube design is used. The hot exhaust gases entering the condensation device from the heating chamber can flow counter-current, cross-flow or parallel with the cooling medium. It is also possible that the exhaust gas flow presents a combination of the aforementioned flow behaviors, such as, multiple cross-flow paths in a counter current cross-flow design. The cooling medium of the condensation device can be steam, air, water or other fluid cooler than the exhaust gases. Water can be used for the final condensing stage and it can further be heated up to form steam in the hottest zones of the condensing heat exchanger. The exhaust gases can be either in the tubes or in the shell when using a convective heat exchanger of shell and tube design. When using air as a cooling medium, the resulting heated air can subsequently be used for direct heating of buildings.

In accordance with one embodiment of the high temperature furnace, the condensation device is arranged at a relatively cold zone of the furnace. Such a cold zone of the furnace may be, for example, near a charging door, at a bottom wall of the furnace chamber or at a distance from the burners. In an embodiment comprising a heat exchanger as the condensation device, the heat exchanger is preferably arranged such that radiative heat from the furnace does not radiate directly on the condensation device. By arranging the condensation device to receive combustion gases in a cooler zone, for example, near the charging door of a continuous high temperature furnace, the combustion gases can be condensed in a more energy efficient process. In accordance with one embodiment of the high temperature furnace, the furnace further comprises at least one airlock charging chamber and, optionally, an additional airlock discharging chamber. The additional discharging chamber is optional since the charging chamber can be used for both charging and discharging the material. Providing such airlock chambers allows minimizing air infiltration to the heating chamber and to maintain a positive pressure therein.

In accordance with an embodiment of the high temperature furnace, the furnace further comprises an electronic control system for controlling the at least one first and second burners, and the pressure within the heating chamber. The electronic control system is arranged to control parameters such as fuel and oxygen flow rates, allowing to control the process temperature, and thereby provide a proper furnace temperature. In accordance with an embodiment of the high temperature furnace, the furnace further comprises a water hydrolysis system for producing hydrogen and oxygen to the first burner. Hydrolysis of water provides an environmental friendly process of producing hydrogen and oxygen gas. Such a decomposition of water can be made using renewable electricity. Other methods providing a continuous supply of fuel to the furnace are also conceivable within the concept of the present invention.

In accordance with an embodiment of the high temperature furnace, the water vapor atmosphere is selected preferably within the range of 90 to 100 %. A slight level of excess oxygen is preferable in the combustion gases to allow a complete combustion of all exhaust gases generated by the combustion of the burner, which in turn leads to an elimination or at least a great reduction of any contaminating exhaust gas released to the air. In accordance with an embodiment of the high temperature furnace

comprising at least one plasma electric arc jet burner, steam is utilized to form the plasma jet.

According to a second aspect of the present invention, a high temperature furnace as disclosed herein is used for reheating steels. The use of a high

temperature furnace as herein disclosed for the reheating of steels provides an energy efficient and environmental friendly process, as nearly no emissions of air contaminants are released to the atmosphere. Instead, any air contaminant resulting from the burner of the furnace is condensed and removed from the process in the condensed water. The hot condensed water exiting the condensation device can, for example, be provided to a local heating system for heating buildings in the nearby area of the high temperature furnace. It may also be collected in a tank where it can be stored for a later distribution and use for heating. According to a third aspect of the present invention, there is provided a method for high temperature heating without emission of air contaminants comprising the steps of providing a high temperature furnace as disclosed herein, heating the heating chamber of the high temperature furnace by means of the at least one first burner, and providing a water vapor atmosphere in the heating chamber. The method further comprises receiving an exhaust gas in the form of water vapor from the heating chamber in the condensation device of the high temperature furnace, and condensing the exhaust gas to water in the condensation device, whereby the emission of exhaust gas to the air is avoided. The hot condensed water exiting the condensation device can be directed to a heating system and used for heating therein. The method thereby provides an energy effective solution for a high temperature heating furnace, which is particularly environmental friendly as no emission of air contaminants is generated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail and with reference to the appended drawings in which:

Fig. 1 is a schematic view of an embodiment of a high temperature furnace according to an aspect of the invention; and

Fig. 2 is a schematic view of a second embodiment of a high temperature furnace according to an aspect of the invention.

DESCRIPTION OF EMBODIMENTS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and to fully convey the scope of the invention to the skilled addressee. Like reference characters refer to like elements throughout.

Fig. 1 shows an embodiment of a high temperature furnace 100 according to the present invention. The high temperature furnace 100 comprises a heating chamber 1. The heating chamber 1 is adapted to be able to receive a material 101 , such as for example steel, which is to be thermally treated, for example heated, at a high temperature. The heating chamber 1 comprises a first end 2 and an opposite second end 3, arranged at a distance from the first end 2. The heating chamber 1 further comprises opposing bottom and top walls 4,5, and opposing front and back walls, all extending between the first end 2 and the second 3 end, providing a closed heating chamber 1. The dimensions of the heating chamber 1 may vary depending on the application for which the heating chamber 1 is used, and the material to be treated therein. The heating chamber 1 is typically arranged to be gas tight and comprises at least one sealable door which can be used for both charging and discharging the material to be treated. The sealable charging and discharging door enables a controlled atmosphere of the chamber. In the shown embodiment, the heating chamber 1 comprises a sealable charging and discharging door, not shown in Fig.1 , arranged at either the front wall or at the second end 3, respectively, of the heating chamber 1. Providing an air lock for charging and discharging the chamber at either position relative to the heating chamber is also conceivable within the present inventive concept. A person skilled in the art realizes that the heating chamber 1 can comprise one or more heating zones, or combustion zones, for the different stages of heating a material in the high temperature furnace 100. As an example only, a large high temperature furnace can comprise from 2 to 5 combustion zones.

The high temperature furnace 100 further comprises a first burner 10. The first burner 10 is arranged for heating the heating chamber. The first burner 10 is, in the exemplifying embodiment of Fig. 1 , arranged at the first end 2 of the heating chamber 1 , from where it provides heating to high temperatures of the closed heating chamber 1 and, thus, any material arranged therein. It is however possible to arrange the first burner 10 at other locations of the heating chamber 1 such to provide an effective heating of the material therein. It is furthermore possible to provide a high

temperature furnace comprising a plurality of first burners, distributed at different locations of the heating chamber for an effective heating of the material. According to an embodiment, the number of first burners of a large high temperature furnace can be from 5 to 100.

The first burner 10 is further arranged for providing a water vapor atmosphere in the heating chamber 1. In the exemplifying embodiment of Fig. 1 , the first burner 10 comprises a hydrogen-oxyfuel burner 10. Flydrogen and oxygen is provided in separate gas pipelines 11 , 12, respectively, to the hydrogen-oxyfuel burner 10. The input oxygen and hydrogen for the hydrogen-oxyfuel burner 10 is set in amounts such to generate a water vapor or steam furnace atmosphere as the rest product of the combustion. This enables a condensation of the rest product, i.e. the water vapor or steam, instead of emission of the air contaminant normally associated with fossil fuel combustion. The hydrogen and oxygen provided to the hydrogen-oxyfuel burner 10 can be generated by a hydrolysis system, which optionally can be integrated with the high temperature furnace 100.

Using pure hydrogen, as opposed to a hydrocarbon fuel, further eliminates the risk for forming any poisonous carbon monoxide (CO) when controlling

understoichiometric combustion. An understoichiometric furnace atmosphere would be expected to burn when the furnace door or doors are opened. However, in the high temperature furnace 100 provided with the hydrogen-oxyfuel burner 10, only water vapor would be formed when the gases ignite. The fact that

understoichiometric combustion is possible facilitates keeping out unwanted oxygen from entering the high temperature furnace 100, allowing the furnace to operate more efficiently. Operating at near stoichiometric combustion can further increase the flame temperature, reduce the oxide scale losses and increase the process yield. When e.g. steel is heated in a high temperature furnace, oxide scale losses can be as large as about 0.2%-1 % of the steel input, depending on the steel alloy, steel dimensions, furnace temperature, holding time, type of fuel, excess oxygen, etc. The cost associated with oxide scale losses are sometimes of about the same magnitude as the cost for the fuel needed to fire the furnace. A reduction in the scale losses could thus help to compensate for a higher cost involved when using hydrogen as a fuel.

In another embodiment of the high temperature furnace, the first burner comprises a plasma electric arc jet burner. The use of a plasma electric arc jet burner as the first burner is possible when steam or water vapor is used as the gas forming the plasma jet. The skilled person understands that it is possible, within the concept of the present invention, to provide a plurality of plasma electric arc jet burners for the different combustion zones of the heating chamber 1.

The first burner 10 and the pressure within the heating chamber 1 , is controlled by means of an electronic control system, not shown in Fig. 1. Referring to Fig 2, there is provided a high temperature furnace 100

comprising a second burner 20. The first burner 10 is therein arranged at the first end

2 of the heating chamber 1 and the second burner 20 is arranged at the second end

3 of the heating chamber 1. It is, however, conceivable to provide both burners 10, 20 at the same end of the heating chamber 1 , or at any for the process suitable location, such as along the top wall 5. The skilled person would understand that it is also possible to provide the first burner 10 at the second end 3 of the heating chamber 1 , and to provide the second burner 20 at the first end 2 of the heating chamber 1. In the exemplifying embodiment of Fig. 2, the first burner 10 comprises a hydrogen-oxyfuel burner, and the second burner 20 comprises a plasma electric arc jet burner. The plasma electric arc jet burner 20 is provided with a pipe line 21 , through which the gas forming the plasma jet enters the burner. The gas forming the plasma jet is, in this embodiment, steam or water vapor. Referring back to Fig. 1 , a condensation device 30 is further arranged in fluid connection with the heating chamber 1 via an exhaust outlet 31. In the exemplifying embodiment of Fig. 1 , the exhaust outlet 31 is arranged near the first end 2 of the heating chamber 1 , having a top end arranged adjacent to the bottom wall 4, and a bottom end arranged adjacent to the condensation device 30. The exhaust outlet 31 further comprises side walls extending from its top end to its bottom end, providing a medium through which a gas can flow. A person skilled in the art understands that the exhaust outlet 31 can be arranged at any other, for the purpose suitable position of the heating chamber 1 , such as for example inside the bottom wall 4 of the heating chamber 1. Generally, the exhaust outlet 31 is arranged at a relatively cold zone of the heating chamber 1.

The condensation device 30 is arranged for condensing an exhaust gas generated by the first burner 10. It is thus arranged for receiving an exhaust gas from the heating chamber 1. The condensation device 30 is selected such that it is suitable for the temperature range of the furnace, and can, for example, be either one of a radiative heat exchanger or a convective heat exchanger. According to the embodiment of Fig. 1 , the condensation device 30 is arranged near the first end 2 of the heating chamber 1 at an offset location thereof. From this offset location, radiative heat from the heating chamber 1 does not reach the condensation device 30 directly. The condensation device 30 is preferably arranged near an end of the heating chamber at which charging of the high temperature furnace 100 occurs, or at a relatively cold location of the high temperature furnace 100, chosen preferably based on the gas flow circulation from the first burner 10 through the heating chamber 1. It is also possible, within the concept of the present invention, to provide the condensation device 30 in a wall of the heating chamber 1 , or at any other location of the high temperature furnace, preferably where it is protected from the radiative heat of the heating chamber 1. The exhaust gas will exit the process as primarily water vapor or steam which is condensed by means of the condensation device 30. The hot condensed water can be used, for example, for a water hydrolysis system or for local district heating. An example of a local district heating use is the heating of radiators in the proximity of the furnace, which might be required during the cold season of the year. When heating a material to a high temperature by means of the high temperature furnace 100 as previously described, the material 101 is placed in the heating chamber 1 of the high temperature furnace 100. The heating chamber 1 , and thus the material 101 , is heated by means of the at least one first burner 10 to the desired temperature. It is also possible that additional heating is provided by means of at least one second burner 20. The first burner generates an exhaust gas which is in the form of water vapor, whereby a water vapor atmosphere is provided in the heating chamber 1. The exhaust gas is removed from the heating chamber 1 and received in the condensation device 30 via the exhaust outlet 31 of the high temperature furnace 100. In the condensation device 30, the exhaust gas is condensed into water which subsequently is removed from the condensation device 30, whereby the need for a chimney for the emission of exhaust gas to the air is avoided.

The skilled person realizes that a number of modifications of the embodiments described herein are possible without departing from the scope of the invention, which is defined in the appended claims. For example, the condensation devices 30 herein described are to be taken as examples only, and a condensation device 30 different from those herein described may also be used, provided that a complete condensation of water vapor exhaust gas is conceivable. The condensation device can for example comprise the outer shell of the high temperature furnace given that it may, when providing a sufficient cooling, condense the water vapor generated in the heating chamber, The skilled person also realizes that the hot condensed water can be used in any way suitable for the process. It can for example be used within the high temperature furnace provided with a hydrolysis system, as a source of heating. It can also be removed from the high temperature furnace and used for heating an external process.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope.