Technical field of the Invention

The present inventive concept relates to an infrared emitter, also referred to as an IR-emitter, for drying an object, such as e.g. a paper web. The inventive concept also relates to a method for drying a paper web using such IR-emitter.

Background of the Invention

Infrared radiation, abbreviated IR, refers to electromagnetic radiation in the infrared spectra of light, i.e. with wavelengths between 700 nm and 1 mm. The IR-drying technology is widely spread in various technical and industrial fields such as e.g. the food industry and paper industry, and accepted as an efficient tool for drying, heating and curing of objects such as e.g. paper and board products. In the paper industry, IR-drying is regularly used for coating drying after coaters, for profiling of uneven moisture content across a paper sheet e.g. after a conventional drying section and before reel and pre-heating for incremental drying. Today, many IR-drying systems are electrical or are using fuel gas to generate infrared radiation.

IR-dryers or IR-emitters are sometimes used for complementary drying rather than a replacement of traditional drying methods. Thus, an important aspect is the space of which the IR-emitter occupies, it is preferable if the IR- emitter may be arranged for drying an object without substantial

rearrangement of existing tools and production components. In other applications, the space for a drying tool such as in IR-emitter is limited for other reasons.

Often an electrical IR-emitter is used for purposes where the space is limited due to e.g. their uncalled need for bulky gas supply means and gas exhaust means. A further advantage of using an electrical IR-emitter is the reduced risk of causing a fire by igniting the object to be dried, as compared to using a fuel gas IR-emitter where the flames of the latter may ignite the object. However, a problem with using electrical IR-emitters is the increased cost for electricity. A gas-fired IR-emitter on the other hand may be operated with various kinds of gases, such as bio-gas or natural gas, providing for a potentially cheaper operating cost. Thus, there is a need for gas IR-emitter providing for a compact design and where the risk of fire is reduced.

Summary of the Invention

An object of the present inventive concept is to alleviate the drawbacks of prior art. This and other objects, which will become apparent in the following description, are accomplished by an IR-emitter and a method for drying a paper web with an IR-emitter, as defined in the accompanying claims.

The present inventive concept is based on the realization that a gas- fired IR-emitter for drying an object and comprising a porous material for a flameless combustion of gas, together with the use of cooling tubes arranged between a burner of the IR-emitter and the object to be dried, provides for a compact IR-emitter which may be arranged close to the object with a reduced risk of fire.

According to at least one aspect of the present inventive concept, an infrared emitter for drying an object is provided. The infrared emitter comprises:

at least one gas fired burner;

a plurality of cooling tubes being transparent to infrared radiation and arranged between said object and said at least one gas fired burner;

said at least one gas fired burner comprising:

a combustion zone for combustion of gas to emit infrared radiation through said cooling tubes to said object,

a chamber for receiving said gas from a gas supply means,

a gas distribution plate arranged between said combustion zone and said chamber and comprising a plurality of apertures for transporting said gas from said chamber towards said combustion zone;

said combustion zone comprising a porous material of e.g. sintered silicone carbide, SSiC, allowing a flameless combustion of said gas. Flameless combustion is to be understood as the porous material being able to stabilize the combustion in such a way that the flame of the gas fired burner does not burn freely or at the surface of the porous material but is stabilized in the porous material. Another way to describe this stabilization of the combustion is that the flame is dispersed over the whole volume of the porous material leading to a homogeneous combustion zone without the typical shape of a free flame (flame color and so on).

Thus, a gas fired IR-emitter with a high radiation power and where the at least one gas fired burner are allowed to be arranged close to the object with a reduced risk of igniting said object is provided for. The combination of providing an IR-emitter with cooling tubes and the use of a porous material such as e.g. SSiC allowing a flameless combustion of the gas, results in an efficient drying of the object using a limited installation space.

The inventor has realized that, even though the IR-emitter utilizes a flameless combustion, the risk of igniting the object may be reduced since shielding and/or cooling by the cooling tubes may provide for a way to control the temperature of the object below its flame temperature. Furthermore, the temperature of the gas fired burner may be increased without reaching the flame temperature of the object as the temperature of the object, or close to the object, may be reduced by the cooling tubes. In other words, the cooling tubes provides for a way to control the temperature of, or close to, the object and reduce the risk of fire since the temperature of the object may be adjusted to be below its flame temperature. By cooling the heat irradiated from the IR-emitter, an increased temperature of the gas fired burner may be used and infrared radiation of a wavelength more suitable for drying the object may be provided for. Furthermore, the use of cooling tubes allows the at least one gas fired burner to be arranged closer to the object to be dried for the reasons stated above.

It should be understood that the cooling tubes are transparent to infrared radiation in order to allow the infrared radiation emitted from the at least one gas fired burner to reach the object via the cooling tubes. The cooling tubes may be opaque to radiation of other wavelengths without affecting the function of the IR-emitter. The cooling tubes also reduces the risk of contamination of the at least one gas fired burner. Preferably, the cooling tubes cover an area corresponding to the whole of a surface of the at least one gas fired burner.

Furthermore, the flameless combustion of gases is emission free as a function of burner material used. Thus, the exhaust gases from the

combustion of the gas in the at least one gas fired burner comprise the residues of the burned gas together with unburned gases. Furthermore, the flameless combustion facilitated by the porous material (e.g. SSiC), allows for more complete combustion compared to combustion with a flame. A reason being that the gas is retained in the porous material for a longer time compared to combustion of gas with a flame.

The porous material in the combustion zone is preferably chosen to withstand high temperatures. According to at least one example

embodiment, the porous material is manufactured as a cellular structure. The cellular structure may stabilize the combustion in such a way that the flame of the gas fired burner does not burn freely or at the surface of the porous material but is stabilized in the porous material / cellular structure. The most suitable material known to the inventor today is sintered silicone carbide, SSiC. The porous material SSiC may be used for high temperatures, i.e. approximately 1350 °C - 1400 °C, and fulfils the requirements of thermo shock resistance in addition to having a long lifetime. The use of high burner temperature (i.e. approximately 1350 °C 1400 °C) provides for an improved power of the IR-emitter and a more suitable wavelength of the emitted infrared radiation for drying e.g. cellulose fibres.

According to at least one example embodiment, the infrared emitter comprises a gas distribution space arranged between said gas distribution plate and said combustion zone and adapted to receive said gas from said gas distribution plate and distribute said gas to said combustion zone.

Hereby, the gas may be distributed more uniformly compared to a gas fired burner without such a gas distribution space. Hence, the porous material in the combustion zone may receive the gas more uniformly and the location of a gas igniter inside the combustion zone may be chosen more freely. According to at least one example embodiment, said plurality of apertures of said gas distribution plate are adapted such that when a gas is supplied to the at least one gas fired burner, a gas pressure in the chamber is higher compared to a gas pressure in said gas distribution space, such that back flow of gas through said gas distribution plate is avoided.

Back flow of gas is undesired as it may harm the IR-emitter. The pressure difference between the gas distribution space and the chamber may be achieved by that the size and/or number of apertures are adapted accordingly. The size of the apertures may e.g. relate to the size of the opening of the aperture. A smaller size of apertures would entail a larger pressure drop over the gas distribution plate.

It should be noted that the apertures of the gas distribution plate are through apertures, i.e. gas may flow from the chamber and into the gas distribution space through the apertures in the gas distribution plate. The size of the aperture does not have to be uniform throughout the gas distribution plate but may be decreasing and/or increasing in size.

According to at least one example embodiment, said plurality of apertures comprises a first set of apertures and a second set of apertures, and wherein the second set of apertures are larger than the first set of apertures.

For example an opening of the second set of apertures may be larger than an opening of the first set of apertures. Hereby, the flow of gas through the gas distribution plate may be adjusted by varying the location of the first and second set of apertures.

According to at least one example embodiment, said second set of apertures are arranged closer to edges of the gas distribution plate compared to the first set of apertures.

Since the gas supply means often supply gas in the center of the gas fired burner with the result of that a majority of gas flows through the center of the gas distribution plate, an arrangement where the second set of apertures, being larger than the first set of apertures, are arranged closer to the edges of the gas distribution plate may counteract the uneven flow of gas through the gas distribution plate. Hence, the gas will be more uniformly distributed in the combustion zone throughout the porous material facilitating the ignition of the gas.

Alternatively or additionally, more apertures are arranged closer to the edge compare to near the center of the distribution plate. Thus, according to at least one example embodiment, the plurality of apertures are

heterogeneously arranged in said gas distribution plate, such that apertures arranged near edges of the gas distribution plate are closer to each other compared to apertures arranged near a center of the gas distribution plate.

According to at least one example embodiment, the infrared emitter comprises a gas exhaust arrangement for receiving exhaust gases from the at least one gas fired burner, said gas exhaust arrangement comprising a filter for receiving said exhaust gases and being heated by said exhaust gases in order to emit infrared radiation to said object.

Hereby, the exhaust gases are utilized by heating up a filter which may emit infrared radiation similar to the gas fired burners.

According to at least one example embodiment, the infrared emitter comprises a gas exhaust blower device for receiving exhaust gases from the at least one gas fired burner, and comprising a blower for blowing the exhaust gas to said object, and a duct for transporting said exhaust gases from said at least one gas fired burner to said blower.

Thus, the exhaust gases are used for drying the object by being blown towards the object. This may also facilitate the break-up of boundary layer on the surface of the object, which boundary layer may prohibit effective penetration of the infrared radiation.

According to at least one example embodiment, said plurality of cooling tubes is hollow for transporting a cooling medium such as e.g. air or a cooling fluid.

Hereby, the temperature of the object, or the temperature near a surface of the object, may be even further reduces and/or better controlled.

Furthermore, when the object to be dried is a paper web, the at least one gas fired burner may emit infrared radiation at a wavelength which is better absorbed by the paper and cellulose fiber in the paper web. According to at least one example embodiment, said combustion zone is operated at a temperature of approximately 1350 °C - 1400 °C for emitting infrared radiation with a peak wavelength in range of 1 .5 μιτι - 1 .75 μιτι.

Infrared radiation with a peak wavelength in such range, may be effectively absorbed by the paper and cellulose fiber in the paper web.

It should be noted that the emitted infrared radiation is distributed over a large range of wavelengths with a peak wavelength in the range of 1 .5 μιτι - 1 .75 μιτι. Thus, by operating the combustion zone at a temperature of approximately 1350 °C - 1400 °C, infrared radiation at wavelengths e.g. from 0.5 μιτι to over 4 μιτι will be emitted from the combustion zone, but the peak wavelength, i.e. a majority of the radiation, will be in the range of 1 .5 μιτι - 1 .75 μιτι.

According to at least one example embodiment, the infrared emitter comprises a frame for supporting said gas distribution plate and housing said chamber.

According to at least a second aspect of the present inventive concept, a method for drying a web of paper and/or pulp comprising cellulose material by operating an infrared emitter according to the first aspect of the inventive concept is provided for.

Effects and features of this second aspect of the present inventive concept are largely analogous to those described above in connection with the first aspect of the inventive concept. Embodiments mentioned in relation to the first aspect of the present inventive concept are largely compatible with the second aspect of the inventive concept.

According to at least one example embodiment, the method comprises the step of arranging the emitter in such a way that at least one gas fired burner cover essentially a full width of the paper and/or pulp web.

Alternatively, a plurality of emitter comprising at least one gas fired burner may be used to cover the full width of the paper and/or pulp web.

It should be noted that the terms gas exhaust and exhaust gases (or simply exhaust) have the same meaning, all relating to the residues of the burned gases together with the unburned gases remaining after the gas has been combusted in the at least one gas fired burner. It should be noted that throughout the application the term infrared is sometimes abbreviated with the term IR, hence for example, the infrared emitter is the same as the IR-emitter. Furthermore, the terms gas IR-emitter and IR-emitter are used interchangeably throughout the application.

Brief description of the drawings

The present inventive concept will now be described in more detail, with reference to the illustrative and non-limiting appended drawings showing example embodiments of the inventive concept, wherein:

Fig. 1 is a schematic side view illustrating an exemplary embodiment of the inventive concept;

Fig. 2 illustrates in cross section an IR-emitter according to at least one example embodiment;

Fig. 3 is a perspective view of a detail of the IR-emitter according to at least one example embodiment.

Detailed description of the drawings

In the following description, the present inventive concept is described with reference to an IR-emitter for drying an object, such as e.g. a paper web. Furthermore, the inventive concept is described with reference to a method for drying a paper web using such IR-emitter.

Fig. 1 is a side view illustrating an IR-emitter 1 for drying an object 2, here illustrated as a web 2 or a paper web 2, according to at least one example embodiment of the inventive concept. The IR-emitter 1 comprises at least on gas fired burner 10 for combustion of gas provided by a gas supply means 3, a plurality of cooling tubes 20 arranged between the paper web 2 and the gas fired burner 10, and a gas exhaust arrangement 40 for receiving exhaust gases from the gas fired burner 10. The gas exhaust arrangement 40 is arranged next to the gas fired burner 10 such that the cooling tubes 20 is arranged between the gas exhaust arrangement 40 and the paper web 2.

The gas supplied to the gas fired burner 10 by the gas supply means 3 are combusted inside the gas fired burner 10 to emit infrared radiation to the paper web 2. By adjusting the temperature of the gas fired burner 1 , the wavelength of the infrared radiation may be appropriately controlled for the drying of the paper web 2. Paper and cellulose fibers comprised in the paper web 2, will not absorb any significant amounts of infrared energy at

wavelengths shorter than 1 .35 μιτι. Energy between 1 .35 μιτι and 2.0 μιτι is weakly absorbed where radiation penetrates deep into the paper web 2 and heats its interior efficiently. At wavelengths above 2.0 μιτι, absorption in the paper web 2 is strong, hence the surface of the web 2 may be heated up fast causing overheating and fiber raise. Hereby, when using an IR emitter emitting wavelengths with a peak wavelength of or above 2.0 μιτι, water inside the paper web 2 may be trapped by the overheated surface. According to at least one embodiment of the present inventive concept, the peak wavelength for the IR-emitter 1 is between 1 .5 μιτι and 1 .75 μιτι which is a suitable wavelength for drying of paper and cellulose fiber as the radiation penetrates deep into the paper web 2 and thereby avoiding overheating of the surface, and the trapping of water, of the paper web 2.

According to at least one embodiment of the present inventive concept, the combustion of gas of the gas fired burner 10 results in that the

temperature of the gas fired burner 10 is approximately 1350 °C - 1400 °C. Hereby, infrared radiation may be emitted at a wavelength which penetrates into the paper web 2 and thereby provides for an effective and uniform drying which is beneficial for a coating layer of the paper web 2.

The cooling tubes 20 should be transparent to the infrared radiation in order for the infrared radiation emitted from the gas fired burner 10 to reach the object 2, through the cooling tubes 20. The cooling tubes 20 allow the IR- emitter 1 to operate close to the object 2 since the cooling tubes provides for a way to control the temperature of the object, or close to the object, below its flame temperature. This improves the drying due to an increased intensity. The cooling tubes 20 also protect a surface of the gas fired burner 10 from being contaminated. The cooling tubes 20 may be hollow to allow for a cooling fluid, such as e.g. water or air, to flow through the tubes 20. However, the cooling fluid is preferably transparent to infrared radiation.

The function of the IR-emitter 1 will now be described in further detail. Gas is supplied by the gas supply means 3 to the gas fired burner 10 alternatively together with combustion air from a combustion air supply means 5. Inside the gas fired burner 10, the gas is ignited with an igniter such as e.g. an electrical igniter, a spark or an ignition flame whereby combustion of the gas occurs. The combustion of the gas causes an increase in temperature inside the gas fired burner 10 whereby the gas fired burner 10 emits infrared radiation towards to the object 2 via and through the cooling tubes 20 for drying of the object 2. The exhaust gases from the combustion of gas inside the gas fired burner 10 subsequently flows into the gas exhaust arrangement 40 whereby a filter 42 receives the exhaust gases. Heat in the exhaust gases are transferred to the filter 42 in the gas exhaust arrangement 40 whereby the filter is heated to a temperature in order to emit infrared radiation through the cooling tubes 20 and to the object 2.

As illustrated in fig . , the gas exhaust arrangement 40 is connected to a gas exhaust means 4 for transporting the gas exhaust from the IR-emitter 1 after the filter 42 has been heated by the exhaust gases. The gas exhaust may be recirculated with the combustion air and/or the gas from the gas supply means 3 in order to preheat the combustion air and/or the gas.

Alternatively, or additionally, the gas exhaust may be heat exchanged with the combustion air and or the gas, i.e. heating, but not mixing with, the

combustion air and/or the gas.

Fig. 2 illustrates an IR-emitter 101 for drying an object 102 according to at least one example embodiment of the inventive concept, where the IR- emitter 101 comprises a first gas fired burner 1 10a, a second gas fired burner 1 10b, a gas exhaust arrangement 140 and cooling tubes 120. As illustrated in fig. 2 the two gas fired burners 1 10a, 1 10b and the gas exhaust arrangement 140 are arranged side by side.

The two gas fired burners 1 10a, 1 10b may be identical, as shown in fig. 2, but may also be different. For example, one of the gas fired burners 1 10a, 1 10b may be designed differently compared to the other gas fired burner 1 10a, 1 10b regarding number and/or design of chambers, distribution plates, combustion material etc. For simplicity, the structure and function of the gas fired burners 1 10a, 1 10b will be described in singulars in relation to fig. 2, as both gas fired burners 1 10a, 1 10b in fig. 2 are identical (however, reference numerals for each structure will be mentioned).

As illustrated in fig. 2, the gas fired burner 1 10a, 1 10b comprises a combustion zone 1 12a, 1 12b for combustion of gas to emit infrared radiation, a chamber 1 14a, 1 14b for receiving gas from a gas supply means 103a, 103b, and a gas distribution plate 1 16a, 1 16b arranged between the combustion zone 1 12a, 1 12b and the chamber 1 14a, 1 14b.The gas distribution plate 1 16a, 1 16b comprises a plurality of apertures 1 17 (seen in fig. 3) for transporting gas from the chamber 1 14a, 1 14b to the combustion zone 1 12a, 1 12b.

As seen in fig. 2, the gas fired burner 1 10a, 1 10b comprises a gas distribution space 1 18a, 1 18b arranged between the gas distribution plate 1 16a, 1 16b and the combustion zone 1 12a, 1 12b. The gas distribution space 1 18a, 1 18b receives gas from the distribution plate 1 16a, 1 16b and distributes the gas to the combustion zone 1 12a, 1 12b. Preferably, the gas distribution space 1 18a, 1 18b allows a more uniform distribution of gas to the combustion zone 1 12a, 1 12b compared to a gas fired burner without such gas distribution space 1 18a, 1 18b. The more uniform distribution of gas by the gas

distribution space 1 18a, 1 18b may e.g. be achieved by arranging the gas distribution space 1 18a, 1 18b large enough for the gas to be spread throughout the space 1 18a, 1 18b. For example, a distance d (shown in fig. 3) between the distribution plate 1 16a, 1 16b and the combustion zone 1 12a, 1 12b may be e.g. between 2 - 5 mm, or between 3 - 4 mm.

According to at least one example embodiment, the plurality of apertures of apertures 1 17 of the gas distribution plate 1 16a, 1 16b are adapted such that when a gas is supplied to the gas fired burner 1 10a, 1 10b, a gas pressure in the chamber 1 14a, 1 14b is higher compared to a gas pressure in the gas distribution space 1 18a, 1 18b. Hereby, back flow of gas through the gas distribution plate 1 16a, 1 16b may be prevented. This may be achieved by adapting the size or the number of the apertures 1 17. In other words, the pressure drop over the distribution plate 1 16a, 1 16b will prevent for a back flow of gas. The combustion zone 1 12a, 1 12b comprises a porous material, such as e.g. sintered silicone carbide SSiC, which allows a flameless combustion of gas. According to at least one example embodiment, the porous material is manufactured as a cellular structure, which stabilizes the combustion. Hence, the flame does not burn freely or at the surface but is stabilized in the porous material / cellular structure. The porous material SSiC may be used for high temperatures, i.e. approximately 1350 °C - 1400 °C, and fulfils the

requirements of thermo shock resistance in addition to having a long lifetime. The use of high burner temperature (i.e. approximately 1350 °C - 1400 °C) provides for an improved power of the IR-emitter and a more suitable wavelength of the emitted infrared radiation.

The function of the IR-emitter in fig. 2 will now be described in more detail. Gas is supplied by the gas supply means 103a, 103b to the gas fired burner 10 by supplying gas to the chamber 1 14a, 1 14b, alternatively together with combustion air from a combustion air supply means as shown in fig. 1 . The gas is then transported through the apertures 1 17 in the gas distribution plate 1 16a, 1 16b to the gas distribution space 1 18a, 1 18b as previously described. From the gas distribution space 1 18a, 188b, the gas flows into the combustion zone 1 12a, 1 12b and the porous material inside the combustion zone 1 12a, 1 12b. The ignition of the gas is preferably carried out by an ignition means such as e.g. an electrical igniter or an ignition flame.

Alternatively, the first gas fired burner 1 10a is fed with gas, alternatively together with combustion air, as described above whereby the gas inside the first gas fired burner 1 10a is ignited. After the ignition of the first gas fired burner 1 10a, gas, alternatively with the addition of combustion air, are fed to the second gas fired burner 1 10b as described above. However, the ignition of the gas in the second gas fired burner 1 10b may not require ignition means such as an electrical ignition means or an ignition flame, but may be ignited by contact with the combustion zone/ignited gas of the first gas fired burner 1 10a.

Ignition of the gas causes combustion of gas in the combustion zone 1 12a, 1 12b. The combustion of the gas causes an increase in temperature inside the porous material of the gas fired burner 1 10a, 1 10b, whereby the porous material emits infrared radiation towards to the object 102 via and through the cooling tubes 120 for drying of the object 2. Similar to fig. 1 , the exhaust gases from the combustion of gas inside the gas fired burner 1 10a, 1 10b subsequently flows into the gas exhaust arrangement 140 whereby a filter 142 receives the exhaust gases. Heat in the exhaust gases are transferred to the filter 142 in the gas exhaust arrangement 140 whereby the filter is heated to a temperature in order to emit infrared radiation through the cooling tubes 120 and to the object 102.

The IR-emitter 1 , 101 is preferably comprised of modular units, which may be arranged to cover the whole object 2, 102 to be dried, such as the full width of the paper web 2, 102. For example, when drying a paper web 2, 102, the dimensions of an IR-emitter 1 , 101 may be 800 mm in machine direction (i.e. along the length of the paper web) and 150 mm in cross direction (i.e. along the width of the paper web). The dimensions of the gas fired burner 10, 1 10a, 1 10b may be 200 mm in machine direction and 150 mm in cross direction. The dimension of the gas exhaust arrangement 40, 140 may be 200 mm in machine direction and 150 mm in cross direction. Several IR-emitters 1 , 101 may be arranged side by side in a frame. The frame may then cover the full width of the paper machine. Furthermore, more than one frame can be installed in sequence, along the machine direction, or in other places along the paper web 2, 102.

Fig. 3 is a perspective view of a gas distribution plate 1 16a, 1 16b as described in relation to fig. 2. The gas distribution plate 1 16a, 1 16b comprises a plurality of apertures 1 17 for transporting gas from the chamber 1 14a, 1 14b towards the combustion zone 1 12a, 122b via the gas distribution space 1 18a, 1 18b. As illustrated in fig. 3, the gas distribution plate 1 16a, 1 16b has a rectangular shape and the apertures 1 17 are encompassed by a distribution plate frame 1 19. The gas distribution space 1 18a, 1 18b is defined by an inner edge of the distribution plate frame 1 19a and a distance d between the exit of the apertures 1 17 and the upper edge 1 19b of the distribution plate frame. Distance d may e.g. be between 2 - 5 mm, or between 3 - 4 mm. The shape of the combustion zone and the porous material in the combustion zone is preferably similar to that of the gas distribution plate, i.e. in this example rectangular.

The arrangement of apertures in fig. 3 is homogenous and all apertures 1 17 in fig. 3 are equally sized. However, according to at least one example embodiment, the apertures 1 17 may be heterogeneously arranged over the gas distribution plate 1 16a, 1 16b and/or be arranged with different sizes.

It should be noted that the IR-emitter may utilize co-combustion of the gas with other gases such as biogas and/or hydrogen. Hereby, the environmental impact of the operation of the IR-emitter may be decreased.

It should be noted that the present description by no means limits the scope of the present inventive concept, which is equally applicable to drying of an object other than a paper web. For example, the object may be a product in the food production industry or another object suitable for drying with infrared radiation. The IR-emitter may comprise one or more gas fired burner and additionally other burners such as electrical burners. The IR- emitter may comprise more than one gas exhaust arrangement.