Cross-Reference to Related Applications

This Patent Application claims priority from Italian Patent Application No . 102022000015384 filed on July 21 , 2022 , the entire disclosure of which is incorporated herein by reference .

Field of the Art

The present invention relates to a burner and an apparatus for the firing of ceramic articles .

In particular, the present invention is advantageously, but not exclusively, applied to the firing of ceramic articles to obtain tiles , to which the following description will make explicit reference without thereby losing generality .

Background of the Invention

The firing of ceramic articles to obtain tiles generally takes place in tunnel kilns , delimited by two opposite walls and a roof . Such kilns are usually heated by two sets of burners , each arranged on one side of the tunnel .

Typically, the burners , running on natural gas ( e . g . methane ) , are located on the side walls of the tunnel on several levels and face the opposite wall .

The firing cycle of ceramic articles is designed with great precision and involves : heating the ceramic articles starting from the kiln entrance , holding them inside the firing chamber at a predefined temperature and cool ing them in a controlled manner before reaching the kiln exit .

Usually, the ceramic articles are transported on a large conveyor consisting of a series of ceramic rollers . Consequently, it is important to ensure that the temperature inside the firing chamber is uni form throughout the width of the kiln .

For this purpose , di f ferent types of industrial burners have been developed, as well as di f ferent burner arrangements within complex apparatuses , in order to achieve an increasingly constant temperature within the firing chamber . Particularly in very wide tunnel kilns , there is generally an uneven temperature distribution in the di f ferent cross sections and there are local temperature peaks determined by the position of the burners .

In order to overcome the above-mentioned problems , various types of so-called "high-speed" burners have been developed, which feed combustion fumes ( and flame ) deep into the firing chamber in order to improve heat exchange inside it .

However, as previously mentioned, ceramic burners of the known type are essentially fuelled by fossil fuels (methane , LPG) , resulting in an anti-ecological exploitation of non-renewable resources . For this reason, various "environmentally sustainable" solutions are under consideration, such as the use of non- fossil fuels , including hydrogen .

Nowadays , however, the use of hydrogen is hampered by several factors . First of all , this fuel causes high temperature peaks , which generate an increase in NOx production even compared to fossil fuels . In addition, hydrogen usually generates a highly unstable flame , which results in an extremely high flashback compared to methane ( or LPG) , and consequently generates a highly retarded flame front (near the fuel feeding duct ) , which causes the burner to overheat and risks causing uncontrolled explosions with possible damage to the burners themselves and to the firing apparatus .

In an attempt to overcome these problems , the Applicant filed Italian patent application 102021000013535 for a burner equipped with an oxidi zer partitioning system . Such a burner, however, does not allow the prolonged use of a flameless firing mode since , especially when using pure hydrogen as fuel , after a certain amount of time , the flame tends to reappear, generating risky detonations inside the burner, thus making flame firing preferable in any case .

All these elements , among others , lead to temperature inhomogeneity within the kiln, which inevitably results in firing defects in the ceramic articles . In particular, the defects may be both dimensional and shape-related, such as lack of flatness . This results in increased waste , energy expenditure and emissions .

The obj ect of the present invention is to provide an apparatus , a burner and a method, which overcome , at least partially, the drawbacks of the prior art and are , at the same time , simple and economical to implement .

Summary

In accordance with the present invention, a burner, an apparatus and a method for firing ceramic articles are provided as claimed in the independent claims below, and preferably in any of the claims dependent directly or indirectly on the independent claims .

The claims describe preferred embodiments of the present invention forming an integral part of the present disclosure .

Brief Description of the Drawings

The present invention will now be described with reference to the accompanying drawings , showing some nonlimiting embodiments thereof , wherein :

- Figure 1 is a cross-sectional front view of a first embodiment of an apparatus according to the present invention;

- Figure 2 is a plan and schematic view of a section of a second embodiment of an apparatus according to the present invention;

- Figure 3 is a perspective and schematic view of part of the apparatus of Figure 1 comprising a burner according to the present invention; - Figure 4 is a cross-sectional front view of the part of Figure 3 ;

- Figure 5 is a plan and cross-sectional view of the part of Figure 3 ;

- Figure 6 is a perspective and schematic view of a part of the burner of Figure 3 ;

Figure 7 is a longitudinal and detailed cross- sectional view of the burner of Figure 6 ;

Figure 8 is a cross-sectional front view of a combustion head of the burner in figure 3 ;

- Figures 9 and 10 are two cross-sectional front views of part of the combustion head of Figure 8 ;

- Figure 11 shows a graph comparing the temperature of the burner ' s combustion products along its axis as a function of the distance from the kiln wall for a burner operating in flame mode and one operating in flameless mode ; and

- Figure 12 illustrates a graph showing a comparison of NOx production ( chamber temperature of 900 ° C ) as a function of fuel flow rate ( speci fically pure hydrogen) in a burner operating in flame mode and one operating in flameless mode .

Detailed Description

In Figure 1 , 1 denotes overall a burner for the firing of ceramic articles T according to a first aspect of the present invention .

The burner 1 can preferably, but not necessarily, be installed in an industrial kiln 2 , in particular a tunnel kiln, comprising a firing chamber 3 .

In particular, as illustrated in Figures 1 and 2 , the ceramic articles T are moved by a transport system 4 along a conveying path P .

More precisely, the ceramic articles T are any type of ceramic article requiring at least one firing cycle in a kiln .

In the non-limiting embodiment of Figures 1 and 2 , the transport system 4 comprises a conveyor belt , on which the unfired ceramic articles T to be fired are arranged, preferably in an orderly manner.

According to some non-limiting and non-illustrated embodiments, the transport system 4 comprises a plurality of ceramic rollers (possibly also moving at different speeds to differentiate the firing of the articles) .

Advantageously but not limitatively, the burner 1 is configured to fire ceramic T-products in flameless mode.

As illustrated in Figures 1 to 6, the burner 1 comprises a mixing body 5, which in turn comprises a duct 6 to feed a fuel FL provided with a certain percentage of hydrogen (in particular greater than 50%, more precisely greater than 70%, preferably entirely hydrogen) , a duct 7 to feed an oxidizer, a spark device 8 to start a combustion and a flame detection device 9. In other words, the mixing body 5 is that part of the burner required to generate the mixture of air and gas which (following a spark to obtain a flame) will heat the firing chamber 3.

In particular, the fuel FL is a gaseous fuel.

Specifically, the fuel FL injected via the fuel feeding duct 6 is mainly hydrogen (possibly mixed with natural gas such as methane or LPG) , while the oxidizer OX injected via the oxidizer feeding duct 7 is essentially ambient air (with approximately, for example, 21% oxygen) . Preferably, but not limitatively, the oxidizer OX is pre-heated, in particular to a temperature above 100°C, in particular between 130°C and 300°C, more particularly between 150°C and 250°C.

The burner 1 further comprises a tubular discharge element 11, which is adapted to (configured to) be crossed by a fluid F flowing out of the mixing body 5 (formed by the mixture of fuel FL and oxidizer OX and/or a combustion thereof, if any) and is provided with an end 12 having an opening 13, within which at least a part of the mixing body 5 is inserted, and an end 14 opposite the end 12 and having an opening 15. In other words, the end 14 faces towards the firing chamber 3 .

According to some non-limiting embodiments , the mixing body 5 is coupled to the tubular discharge element 11 by means of fastening elements . Advantageously but not necessarily, as in the embodiment illustrated in Figures 4 and 5 , the fastening elements are bolts 16 .

In the non-limiting embodiment illustrated in Figures 3 , 4 and 5 , the mixing chamber 5 i s partly inserted into the discharge element 11 and partly arranged outside the kiln 2 . In particular, in the embodiment of Figures 4 and 5 , the discharge element 11 is inserted into a side wall 17 of the tunnel kiln 2 . More precisely, the discharge element 11 extends completely inside the side wall 17 . On the other hand, in other non-limiting embodiments , the discharge element 11 extends along the entire length of the side wall 17 , partially entering the firing chamber 3 of the kiln 2 .

Advantageously, the burner comprises an introduction element 18 , configured to inj ect fuel FL downstream of the tubular discharge element 11 , at the second end 14 or, as illustrated in the non-limiting embodiments of the appended figures , proj ecting from the second end 14 into the firing chamber 3 .

In detail , the introduction element 18 is configured to introduce fuel FL directly into the firing chamber 3 , i . e . not into the tubular discharge element 11 .

In particular, introduction element 18 is ( f luidically ) separate from the mixing body 5 .

In particular, the introduction element 18 is fluidically separate from the inside of the tubular discharge element 11 .

Advantageously but not limitatively, therefore , the fuel FL introduced into the firing chamber 3 does not come into contact with the oxidi zer OX inside the tubular discharge element 11 , but only exiting it , i . e . downstream of it . In the non-limiting embodiments of Figures 3 to 7 , the introduction element 18 comprises a tubular duct 19 extending at least from the end 12 to the end 14 , in particular from the opening 13 to the opening 15 of the tubular discharge element 11 . Speci fically, the introduction element 18 is configured to feed fuel FL directly into the firing chamber 3 , downstream of the tubular discharge element 11 .

According to some preferred but not limiting embodiments , the tubular element 18 crosses the tubular discharge element 11 longitudinally from side to side .

In some non-limiting cases , the burner 1 comprises only one introduction element 18 .

In other non-limiting cases , the burner 1 compri ses a plurality of introduction elements 18 .

Preferably, as illustrated in the non-limiting embodiments of Figures 4 to 6 also, the introduction element 18 extends into the tubular discharge element 11 . Thus , it is possible to replace a standard burner with the burner 1 without making any maj or changes to the kiln 2 .

In other, non-limiting and non-illustrated forms , the introduction element 18 extends outside the tubular discharge element 11 . In particular, the introduction element 18 is mounted/ inserted (by means of appropriate openings/holes and any insulating and/or supporting elements ) to/ from the side wall 17 of the kiln 2 and in any case is configured to feed fuel FL directly downstream of the tubular discharge element 11 . In this way, although not interchangeable with existing burners , the burner 1 can still operate in flameless mode by exploiting the introduction of fuel FL directly into the firing chamber 3 and thus avoiding the reformation of the flame front inside the tubular discharge element 11 .

Advantageously, but not limitatively, the burner 1 is configured to operate initially in flame mode , i . e . where a flame front is present ( as is conventionally the case with ceramic systems) , and once a predefined temperature is reached, i.e. a temperature greater than or equal to the auto-ignition temperature of the fuel-oxidizer mixture to be introduced into the kiln.

Advantageously but not limitatively, the introduction element 18 is configured to feed fuel FL into the firing chamber 3 alternatively to the fuel FL feeding duct 6. In other words, the introduction element 18 is configured to introduce fuel FL into the firing chamber 3 alternatively when the burner 1 is operating in flameless mode, i.e., without leading to the generation of a flame front (i.e., that limited region, usually less than one millimetre, in which combustion reactions take place in flame mode) .

In some preferred, non-limiting cases, such as those illustrated in Figures 4 to 7, in detail, as can be seen in Figure 5, the tubular duct 19 extends asymmetrically in relation to a longitudinal axis of symmetry AA of the burner 1. In particular, the tubular duct 19 extends at least partially parallel to an internal wall 20 of the tubular discharge element 11. In other words, the tubular duct 19 does not extend along the longitudinal axis of symmetry AA. The asymmetry in the arrangement of the tubular duct 19, in detail its offset arrangement towards the wall 20, makes it possible to reduce thermal stress, and thus preserve it, during operations in flame mode. In fact, as a result, the duct 19 is less immersed in the flame.

Advantageously but not limitatively, the tubular duct 19 follows the course of the inner wall 20, which, at the opening 15, has a narrowing 21. In particular, the tubular duct 19 comprises at least one straight portion 22 and one straight portion 23, connected to each other by a straight portion 24 of the tubular duct 19 itself. In detail, the straight portion 22 is arranged radially at a greater distance from the longitudinal axis of symmetry AA than the straight portion 23. In this way, the tubular duct 19 is only subj ected to greater stress on the straight portion 23 , which preferably passes through the opening 15 and protrudes into the firing chamber 3 . For this reason, advantageously but not limitatively, the curved portion 24 is placed close to , i . e . at a distance of less than 20 cm, in particular less than 15 cm, from the opening 15 .

Advantageously, but not necessarily, in order to allow for deep entry of the fuel FL into the firing chamber 3 during flameless mode , the tubular duct 19 has an internal ID diameter, i . e . a lumen, less than or equal to 10 mm, in particular less than or equal to 7 mm, more in particular less than or equal to 5 mm .

In the non-limiting embodiments of Figures 4 to 6 , the introduction element 18 comprises an end 25 proj ecting from the opening 15 for a distance greater than or equal to 5 mm, in particular greater than or equal to 10 mm, preferably between 10 and 20 mm . In this way, fuel FL can be introduced into the firing chamber 3 ( during flameless combustion) reducing the risk of combustion propagating back into the tubular discharge element 11 ( recreating the flame front and, especially in the case of hydrogen as fuel FL, generating dangerous detonations internal to the burner 1 ) .

In certain non-limiting cases , and as illustrated in the embodiment of Figure 5 , the burner 1 comprises a fuel FL supply system 26 connected to the fuel FL supply duct 6 and a fuel FL supply system 27 , separate from the fuel FL supply system 26 , connected to the introduction element 18 . In particular, the fuel FL supply system 26 and the fuel FL supply system 27 are selectively and/or independently operable from each other, e . g . by means of special solenoid valves 28 (preferably redundant for safety reasons ) .

Advantageously, but not necessarily, as visible in Figure 5 , the fuel FL supply system 26 and the fuel FL supply system 27 are both connected to di f ferent ducts 6 , 6 ' obtained on a breech 29 of the mixing body 5 . In other non-limiting and non-illustrated cases , the fuel FL supply system 26 and the fuel FL supply system 27 are connected to the same duct 6 on the breech 29 , which is provided with a diverter valve that feeds the fuel FL into the mixing body 5 or into the introduction element 18 depending on the operating mode , flame or flameless respectively, of the burner 1 .

The following describes some advantageous but not limiting features during the flame operating mode of the burner 1 . These characteristics are combined synergistically with the rest of the description to allow the use o f a 100% hydrogen or at least a high percentage hydrogen fuel .

Advantageously but not limitatively, and as illustrated in the non-limiting embodiment of Figures 4 to 8 , the mixing body 5 comprises both a fuel FL partitioning system FPS , which is configured to partition the fuel FL into a plurality of portions FL' , FL' ’ , FL' ’ ’ , and an oxidi zer OX partitioning system OPS , which is configured to subdivide the oxidi zer OX into a plurality of portions OX' , OX' ' , OX' ' ' ( see , for example , Figure 8 ) . The burner 1 is configured in such a way that the plurality of FL' , FL' ' , FL' ' ' and the plurality of portions OX' , OX' ' , OX' ' ' are conveyed in such a way that they mix with each other in at least two ( in particular three or more ) stages ( forming the respective mixtures M' , M' ' , M' ' ' ) . Thus , flame generation is contained and controlled when the fuel FL is mainly or entirely hydrogen .

Advantageously, but not necessarily, the oxidi zer partitioning system OPS comprises a combustion head 10 , which is arranged ( at least partially) inside the first tubular discharge element 11 ( through the opening 13 ) and comprises one or more combustion chambers 30 , 31 , each configured to contain a di f ferent stage ( or mixture Ml , M2 ) of flame combustion .

Advantageously but not necessarily, the fuel FL partitioning system FPS comprises an inj ection element 32 , which is configured to inj ect , during flame combustion, at least the maj ority FL' ’ ’ of the fuel FL downstream of the combustion head 10 ( inside the tubular discharge element 11 ) towards the end 14 , i . e . towards the firing chamber 3 . In this way, most of the flame develops away from the end 12 of burner 1 , while bringing the flame front closer to the firing chamber and reducing overheating o f the mixing body and the discharge element 11 .

In particular, the tubular discharge element 11 is configured to contain a primary stage F' of flame combustion .

According to a preferred but not limiting embodiment illustrated in Figures 4 to 10 , the inj ection element comprises a tubular duct 33 , in particular axial ( i . e . arranged at to the longitudinal axis AA of the burner 1 ) , which crosses from side to side the one or more combustion chambers 30 , 31 and so as to convey the portion FL' ’ ’ of fuel FL ( greater than 50% , preferably 70% to 80% ) downstream of the combustion head 10 advancing the maj ority of the flame towards the firing chamber 3 .

In the non-limiting embodiments of Figures 4 to 10 , the tubular duct 33 has a substantially constant cross-section, preferably circular . In particular, the tubular duct has a first cross-section with an internal diameter ranging from 2 mm to 12 mm, speci fically from 4 mm to 10 mm . In this way, it is possible to guarantee a high speed that favours a reduction/control of flashback, which is usually problematic in the case of hydrogen, and also assists the rest of the burner to reach the speed necessary to introduce the flue gases F deep into the chamber 3 .

In some non-limiting cases , such as the one illustrated in Figures 4 to 10 , the tubular duct 33 comprises an end 34 connected to the fuel FL supply duct 6 and an end 34 ' within the tubular discharge element 11 towards the end 14 .

In certain non-limiting cases , the tubular duct 33 is configured so that one end 34 remains within the tubular discharge element 11 . In particular, the tubular duct 33 is configured to remain in the hal f of the tubular discharge element 11 furthest away from the firing chamber 3 ( i . e . from the end 14 ) , more speci fically the end 34 is located at ( in particular al igned with) the end of the combustion head 10 . More speci fically, the tubular duct 33 has a length of 40 mm to 150 mm, preferably 60 mm to 110 mm .

Advantageously, but not necessarily, as illustrated in the non-limiting embodiments of Figures 7 , 9 and 10 , the tubular duct 33 has one or more openings 35 for distributing the fuel FL at each combustion chamber 30 , 31 so as to inj ect into each of them at least one of the portions FL' , FL' ’ . In particular, the one or more distribution openings 35 are through-holes 36 that connect an internal zone of the tubular duct 33 to a combustion chamber 30 , 31 .

Advantageously, but not necessarily, the through holes 36 are radial holes , preferably ring-shaped, e . g . extending radially from the axis AA. Preferably, the holes 36 have a diameter of less than 5 mm, in particular 1 mm to 3 mm .

Advantageously, but not necessarily, the fuel FL feeding duct 6 comprises at least a narrow portion 37 of the type described in the Applicant ' s Italian patent application 102021000013535 .

Advantageously, but not necessarily, the burner 1 ( the mixing body 5 ) comprises the breech 29 ( in particular made of aluminium or cast iron and provided with the final part of the channels 6 and 7 for feeding the oxidi zer and fuel ) , which closes the burner 1 from the side opposite the firing chamber 3 . In particular, the breech 29 is of the type described in Italian patent application 102021000013535 by the same Applicant , except for the system 27 for fuel supply and thus for the duct 6 ' to which fuel introduction element 18 is connected in the flameless mode .

Advantageously, but not necessarily, and as illustrated in the non-limiting embodiments of Figures 1 to 4 , the burner 1 comprises a tubular discharge element 38 (illustrated, for example, with broken lines in Figure 4) extending from the end 14 of the element 11 in the opposite direction to the end 12, i.e. towards the (more precisely, the inside of the) firing chamber 3.

In some non-limiting cases, the burner 1 comprises a suction element 39 which is adapted to (configured to) carry at least part of the gases G present to the outside of the burner 1.

Preferably, the discharge element 38 and suction element 39 are of the type described in Italian patent application 102021000013535 by the same Applicant. In particular, together with the discharge element 11, the discharge element 38 and the suction element 39 form a combustion block of the type described in Italian patent application 102021000013535 by the same Applicant.

Advantageously and completely different from the standards used in the ceramic market, the combustion head 10 is a multistage combustion head, i.e. adapted to (configured to) split the flame formation into several stages. This makes it possible to use the so-called "air staging" technique.

Advantageously and profoundly different from the standards used in the ceramic market, the duct 33, together with openings 35, assists the combustion head 10 in dividing the flame into different stages, in particular by splitting the fuel FL. In this way, the so-called "fuel staging" technique can be used.

From the combination of the above techniques, it is possible to use fuel FL with a substantial percentage of hydrogen and at the same time increase the flame speed to over 160 m/s, in particular up to over 180 m/s, more precisely up to around 200 m/s. In fact, the term "high speed" means, specifically for burners, a flame speed of 150 m/s or more.

Advantageously but not necessarily, the combustion head 10 (with the tubular duct 33 inside it ) is mounted at least partially within the tubular discharge element 11 so as to be coaxial with it along the longitudinal axis of symmetry AA of the burner 1 .

As illustrated in the non-limiting embodiments of Figures 4 to 10 , advantageously, the multistage combustion head 10 comprises ( at least ) a combustion chamber 30 , which is adapted to ( configured to ) generate a first combustion phase of the flame ( in particular to generate the so-called flame "root" ) given by the combination of the portions FL' and OX' , and ( at least ) a combustion chamber 31 , communicating with the combustion chamber 30 and adapted to ( configured to ) generate a second combustion phase of the flame ( given by the combination of the portions FL' ’ and OX’ ’ ) exiting the combustion chamber 30 . In particular, the combustion chambers 30 and 31 are configured to convey a secondary portion F' ’ ( or secondary state ) of the flame within the tubular discharge element 11 towards the end 14 and in particular, through the suction element 39 towards the tubular discharge element 38 .

In the non-limiting embodiment of Figures 9 and 10 , in which two sections of the multistage combustion head 10 are shown in detail , the combustion chamber 30 comprises at least one inlet opening 40 and one outlet opening 41 (more precisely arranged on opposite sides of the combustion chamber 30 ) . The outlet opening 41 faces towards firing chamber 3 .

In certain preferred non-limiting cases , the burner 1 includes additional fuel FL distribution openings 42 (particularly of the portion FL' ) connecting the fuel FL feeding duct 6 to the combustion chamber 30 . In particular, the further openings 42 for feeding fuel FL comprise axial through-holes 43 , preferably arranged in a crown ( along mutually parallel directions ) around the longitudinal axis of symmetry AA of the burner 1 . More particularly, the further openings 42 are made on the inlet opening 40 , which is adapted to ( configured to ) communicate with the duct 6 for feeding the fuel FL and to receive a volumetric flow rate , more precisely variable , of said fuel FL . Pre ferably, the holes 43 have a diameter of less than 5 mm, in particular 1 mm to 3 mm .

Advantageously, but not necessarily, upstream of the combustion chamber 30 , the mixing body comprises a first distribution chamber 44 , which is configured, during the operating mode with the burner flame 1 , to inj ect part of the fuel FL passing through it via the further openings 42 and the remaining part into the tubular duct 33 via the end 34 .

In the non-limiting embodiment of Figures 4 to 10 , the burner 1 comprises a combustion chamber 31 arranged downstream of the combustion chamber 30 and provided with an inlet opening 45 and an outlet opening 46 opposite each other . The inlet opening 45 is configured to communicate with the outlet opening 41 and to receive the oxidi zer- fuel mixture M' . In particular, the outlet opening 46 faces towards the firing chamber 3 .

In the non-limiting embodiment of Figures 6 to 10 , the combustion head 10 comprises a crown 47 configured to regulate the inlet of the oxidi zer OX into the tubular discharge element 11 that does not pass through the combustion chambers 30 and 31 . In particular, the crown 47 extends from the edge of the outlet opening 46 towards (up to ) the inner wall of the tubular discharge element 11 .

Advantageously but not necessarily, the crown 47 comprises slots 48 ( or any other type of opening) configured to convey a part OX'" of the oxidi zer into the tubular discharge element 11 downstream of the combustion chambers 30 and 31 . Thus , together with the oxidi zer- fuel mixture M' ’ and the main portion FL' ’ ’ of the fuel , a mixture M' ’ ’ is generated at the outlet of the tubular discharge element 11 , possibly through the suction element 39 towards the tubular discharge element 38 . In particular, the primary flame F' of the burner 1 is generated .

Preferably but not limitatively, the tubular duct 19 passes through the crown 47 , in particular through one of the slots 48 .

Advantageously, but not necessarily, the diameter of the narrowing 21 is less than 30 mm, in particular less than or equal to 25 mm . In detail , the diameter of the narrowing 21 is comprised from 5 mm ( in particular 10 mm; more in particular 20 mm) to 60 mm ( in particular 40 mm; more in particular 30 mm) . This feature also allows the fluid F exiting the burner 1 to be accelerated, counteracts flashback and thus better manages combustion with very hydrogen-rich fuel FL mixtures .

According to a preferred but not limiting embodiment , as illustrated in Figures 4 - 8 , the flame detection device 9 comprises a UV detection probe 49 . In particular, the UV probe 49 is arranged along the longitudinal axis AA of the burner at the edge of the breech 29 , i . e . at the edge of the mixing body 5 .

Advantageously, but not necessarily, the flame detection device 9 (more precisely the UV detection probe 49 ) is configured to receive a UV beam (ultraviolet radiation) from the flame passing through the tubular discharge element 11 . In use , the UV detection probe 49 provides data on the state of the flame generated by the burner, through which the flow rate of fuel FL and/or oxidi zer OX can be adj usted accordingly . Furthermore , once the flameless mode is activated as described below, the UV probe 49 is disabled because it is no longer able to detect any flame , the flame front being diluted inside the firing chamber 3 of the kiln .

In accordance with a second aspect of the present invention, an industrial apparatus 50 is provided for the firing of ceramic articles T , in particular as described above .

According to some non-limiting embodiments , the ceramic articles T are , once fired, tiles . In particular, the ceramic articles T are unfired at the entrance to the apparatus 50 and fired at the exit .

The industrial apparatus 50 comprises the kiln 2 ( described above ) , in particular a tunnel kiln, having at least one side wall 17 delimiting the firing chamber 3 and having a surface 51 inside the f iring chamber 3 and a surface 52 outside the firing chamber 3 .

The industrial apparatus 50 further comprises the above-described transport system 4 , in particular hori zontal , which is configured to move the plurality of ceramic articles T along the conveying path P within the firing chamber 3 ( from the inlet to the outlet of the firing chamber 3 ) .

Advantageously but not limitatively, the apparatus 50 comprises a (hydrogen) burner 1 as described above .

Advantageously, the apparatus 50 comprises a hydrogen feeding system S configured to inj ect hydrogen or a mixture comprising hydrogen to the fuel FL supply systems 26 and 27 . In particular, the hydrogen feeding system S is configured to selectively ( i . e . , mutually exclusively) inj ect hydrogen or a mixture comprising hydrogen into the fuel feeding duct 6 or the introduction element 18 .

Speci fically, during the flameless operating mode , all the fuel FL passes through the introduction system 18 , while preferably all the oxidi zer OX pas ses through the discharge element 11 before being mixed into a mixture M* directly inside the firing chamber 3 , without the generation of a locali zed, hence flameless , flame front .

According to the non-limiting embodiment of figure 2 , the apparatus 50 comprises a plurality of burners 1 arranged in series along a direction DD parallel to the conveying path P . In particular, the burners 1 are arranged on several levels inside at least one of the walls 17 of the kiln 2 .

In the non-limiting embodiments of Figures 1 to 4 , the burner 1 is coupled via the fastening elements 16 to the wall 17 of the ki ln 2 . In particular, the discharge element 11 is inserted into the wall 17 . Thus , since the burner in accordance with the present invention essentially retains the dimensions of a ceramic kiln burner, replacement is facilitated .

In the non-limiting embodiment of Figure 1 , the burners

1 are oriented in a direction DP transverse ( in particular, perpendicular ) to the direction DD ( and thus to the conveying path P ) .

Advantageously, but not necessari ly, the tubular element 11 of the burner 1 is installed in such a way that it passes through, at least partially ( in particular totally and transversely) , one of the side walls 17 of the kiln 2 . In this way, the flame produced by the burner 1 will flow directly to the inside of the firing chamber 3 of the kiln

2 .

In particular, the axis AA is perpendicular to the conveying path P . More speci fically, the axis AA is also perpendicular to the side wall 17 of the industrial tunnel kiln 2 .

According to certain non-limiting and not illustrated embodiments , the discharge element 11 of the burner 1 is installed so as to partially protrude into the firing chamber

3 .

Included by reference to earlier patents .

Advantageously, but not necessarily, the apparatus 50 ( or each burner 1 ) comprises at least one electronic control unit 53 configured to control the burner 1 so as to switch from the flame heating configuration of the firing chamber 3 to the flameless firing configuration .

In particular, the electronic control unit 53 is configured to control the burner 1 in such a way that it switches from flame to flameless mode when a predefined temperature TV is reached . More precisely, the predefined temperature TV is higher than the auto-ignition temperature of the fuel mixture M* ( e . g . above 800 ° C ) . In this way, the mixture M* generated inside the firing chamber 3 will be without a flame front , with all the advantages this entails , described below .

By means of such control of the feeding to burner 1 of the oxidi zer OX and the fuel FL, together with the presence of the introduction element 18 which inj ects the fuel directly into the firing chamber 3 , it is possible to achieve flameless combustion even with fuels such as hydrogen which tend to reform the flame front in a sudden and undesirable manner inside the burner i f the flameless mode is prolonged over time even at low capacities (where the flame impulse , by reducing its intensity, determines the establishment of conditions suitable for the formation of the flame front ) . In particular, the electronic control unit 53 is configured to , once a predef ined temperature TV is reached in the firing chamber 3 , extinguish the flame by decreasing or interrupting the feeding of fuel FL, and possibly (but not limitatively ) oxidi zer OX, into the fuel feeding duct 6 and oxidi zer feeding duct 7 , respectively . Once the flame is extinguished, the control unit 53 is configured to feed fuel FL into the introduction element 18 ( and restore the supply of oxidi zer OX in case it was interrupted) allowing the burner 1 to generate the mixture M* directly inside the firing chamber 3 and to fire the ceramic articles T in flameless mode . Preferably but not limitatively, the control unit 53 is also configured to selectively inhibit flame control (via the detection device 9 ) during the flameless operating mode .

By using flameless combustion, i . e . combustion that takes advantage of the fact that the temperature inside the kiln is higher than the fuel ' s auto-ignition temperature , it is in fact possible to drastically reduce the NOx emissions normally generated in the combustion of hydrogen-rich mixtures ( and in general by combustions with high flame peaks ) , thus allowing the use of an environmentally sustainable fuel with low emissions . In particular, the alternating combination of flame and non- flame modes compensates for the extreme ignition and propagation ( flashback) of hydrogen .

In particular, but not limitatively, the electronic control unit 53 i s configured to control the apparatus 50 in such a way that it only fires the tiles T in the flameless configuration .

Advantageously, but not necessarily, and as illustrated in the non-limiting embodiment of Figure 1 , the apparatus 50 comprises at least two temperature control devices 54 , in particular at least two thermocouples 55 with double filaments , arranged in at least two di f ferent " signi ficant" points of the kiln 2 . These two points are such that it can be ensured that at every point in the firing chamber the temperature is suf ficiently higher than the auto-ignition temperature of the fuel mixture ( thus allowing for reliable flameless firing) .

Advantageously, but not necessarily, should the temperature detected by the two thermocouples 55 fall below the auto-ignition temperature , the flame is triggered and ignited again, i . e . the electronic control unit 53 immediately resets the burner 1 to flame mode .

According to a further aspect of the present invention, a method is provided for firing ceramic articles conveyed into a tunnel kiln .

The method comprises at least one step of supplying a burner 1 as described above with a fuel FL comprising at least 20% hydrogen, more particularly more than 50% , more particularly more than 70% , preferably all hydrogen . Such fuel mixtures are usable in flame operating mode due to the special burner geometry described above , in particular due to the multi-stage combustion head 10 in combination with the inj ection element 32 . Furthermore , the additional geometries described above synergistically lead to the important technical e f fect of reducing the environmental impact by allowing the use of a hydrogen-rich fuel mixture and lowering NOx, respectively .

In some non-limiting cases , the fuel FL comprises more than 90% hydrogen . In particular, the fuel is 100% hydrogen .

The method further comprises the step of simultaneously supplying the burner 1 with oxidi zer OX and sparking ( igniting) the flame (via the spark device 8 ) that extends at least partially into the burner 1 and the firing chamber 3 of the kiln 2 .

Once the flame ignition is complete , the method provides for controlling the flame in feedback by means of the detection device 9 , in detail until the predefined temperature TV is reached inside the firing chamber 3 .

Advantageously, but not necessarily, the method comprises the further steps of , once the firing chamber 3 of the kiln 2 has reached the predefined temperature TV ( in particular higher than the auto-ignition temperature of the fuel FL ) , extinguishing the flame by reducing ( or interrupting) the feeding of the fuel FL and possibly the oxidi zer OX ; preferably disabling the aforementioned flame feedback control ; and introducing fuel FL into the introduction element 18 ( i . e . into the tubular duct 19 , possibly restoring the feeding of the oxidi zer OX ) , generating the mixture M* directly inside the firing chamber 3 determining flameless combustion which fires the ceramic articles T .

In particular, the method involves feeding fuel FL by the introduction element 18 directly into the firing chamber 3 . This reduces the risk of explos ion of the burner 1 due to possible re-ignition of the flame inside it , especially in the case of a very hydrogen-rich fuel FL.

In particular, the method involves switching from flame mode to flameless mode when the predefined temperature TV is reached. More precisely, the predefined temperature TV is higher than the auto-ignition temperature of the fuel mixture .

Advantageously, but not necessarily, the method involves firing the tiles T only after the flameless configuration has been reached. In other words, the burner 1 is supplied with oxidizer OX (via the discharge element 11) and fuel FL (via the injection element 18) , all without sparking the flame, by de-energizing and energizing (closing and opening) a solenoid valve 28 (in particular, two solenoid valves 28 in series in accordance with current regulations) of the supply system 27. At the same time, an air solenoid valve (not shown) is also momentarily closed.

In particular, the excitation steps of the solenoid valves 28 (i.e., feeding and interruption of oxidizer) are preferably carried out after the electronic control unit 53 has extinguished the flame in the burner 1 and inhibited the spark electrode 8 and the UV flame detection probe 49 (the flame front no longer being localized in the burner 1 but diluted in the chamber 3 of the kiln) . More in particular, by means of the above-mentioned solenoid valves, the feeding of oxidizer OX and fuel FL is controlled digitally (ON/OFF) , i.e. by switching from maximum flow to zero and vice versa. This prevents the formation of a stable and anchored flame front inside the burner 1. In detail, this effect is due to the fact that a very high pulse is given to the flame feeding, such that the flame front is not formed in the burner and is therefore diluted directly in the firing chamber 3.

In other, non-limiting cases, in accordance with the same principle explained above, the method involves, via the control unit 53, maintaining the supply of oxidizer OX and interrupting and subsequently supplying the duct 6 and the duct 19 respectively. In this way, it is possible to maintain a constant pressure state inside the chamber 3 of the kiln 2, without swinging the flue gas chimney draught. A possible re-ignition of the flame front inside the burner 1 is also prevented to a greater extent.

In the flameless phase, it is substantially diluted directly in the chamber 3 of the kiln 2 with the combustion products already present in the chamber 3 with a lower oxygen content than in the oxidizer OX (e.g. up to 14%, or even up to 4%, or even up to 2%, to make the flame front more inert) . In other words, in this way, the oxidizer OX and the fuel FL flowing separately from burner 1 into the firing chamber 3 form the mixture M*, which is oxidized inside the chamber 3 itself .

In this way, temperature peaks (which are among the main causes of NOx production) can be avoided compared to traditional flame-only solutions. This in turn results in a lower thermal load on the components of the burner 1 (e.g. on the combustion head 10, on the tubular ducts 19 and/or 33, on the mixing body 5, on the combustion block, on the fuel and oxidizer tubes, etc.) . At the same time, a strong reduction in heat loss caused by the burner is therefore achieved, thus improving the efficiency of the kiln 2. In addition, in the absence of flame, the burner 1 will be quieter, thus also reducing the noise pollution produced by it .

In this respect, the aforementioned effects are evident from figure 11, which illustrates a graph comparing the temperature (y-axis) inside the kiln 2 as a function of the distance (x-axis) from the wall 17 of the kiln 2 for a burner 1 operating in flame mode FW and one operating in flameless mode FLS, respectively. The reduction of the initial temperature peak is clear, as is the maintenance of a higher temperature at greater depth inside the firing chamber 3.

Consistently, Figure 12 illustrates a graph showing a comparison of NOx production ( y-axis ) as a function of the flow rate (x-axis ) of fuel (hydrogen) in a burner 1 operating in flame mode FW and one operating in flameless mode FLS . Again, the advantages in terms of emissions , regardless of the power of the burner 1 , are obvious .

Advantageously, but not necessarily, the power of the burner is less than 100 KW, in particular less than 70 KW, preferably less than 50 KW .

In use , the spark device 8 ( in particular the spark electrode ) generates a spark which, together with the fuel FL entering from the duct 6 and the oxidi zer OX entering from the duct 7 , determines the generation of the flame . In particular, the part OX' of the oxidi zer and FL' of the fuel generate the mixture M' inside the combustion chamber 30 , which defines a first stage of the flame and continues to the combustion chamber 31 , inside which the mixture M' , the portion FL' ’ of the fuel and the part OX’ ’ of the oxidi zer form the mixture M' ’ , which defines a second stage of the flame . The mixture M' ’ inside the tubular discharge element 11 , in which, by mixing with the part OX'" of the oxidiser OX and with the portion FL' ’ ’ of the fuel FL exiting the end 34 ' forms the fluid F ( and the primary flame F' ) . Thus , the mixing body 5 generates an at least partially combusted mixture , i . e . a flame , whose fluids F flow through the tubular discharge element 11 , which introduces the flame into the combustion chamber 3 .

The products of combustion emitted by the burner 1 are not fully combusted on their first passage through the discharge element 11 , but combustion is increased ( completed) by the continuous recirculation of gases G (present inside the firing chamber 3 ) through the high speed of the fluid F and possibly by the presence of the suction element 39 . In other words , the burner 1 generates , by means of the spark device 8 , a primary combustion on the gases introduced by the ducts 6 and 7 ( fuel and oxidi zer ) and a secondary combustion thereof , exploiting the recirculated gases G from inside the firing chamber 3 which are not completely combusted ( in which residual oxygen is present ) sucked in by the speed of the fluid F and the suction element 39 . Speci fically, primary combustion takes place inside the discharge element 11 and secondary combustion takes place inside the firing chamber 3 .

Once the predef ined temperature TV is reached, the control unit 53 shuts of f the flame and demotes the supply of fuel FL to the introduction element 18 , which introduces the fuel into the firing chamber 3 to generate the aforementioned mixture M* . In general , to achieve good flameless combustion, it is convenient to dilute the mixture M* as much as possible . In particular, the burner 1 is configured to keep the ratio between the quantity of recirculated flue gas and the sum of the quantities of oxidi zer OX and fuel FL introduced greater than 3 . Under these conditions , the reformation of the flame front can be success fully avoided and thus flameless mode can be continued in steady state .

It is therefore apparent that , by using an apparatus 50 or a set of burners 1 in accordance with the present invention, a greater uni formity of temperature along the width of the firing chamber 3 of the kiln 2 is obtained . In particular, the temperature in proximity to the wall 3 is signi ficantly increased due to the turbulence generated by the element 39 ( due to the additional speed allowed by the multi-stage combustion head 10 ) of the suction and the contribution of the radiation provided by the introduction element 18 in proximity to said wall 3 . In addition, the temperature in the centre of the kiln is increased with respect to the traditional case due to the use of the introduction element 18 , which allows the combustion block 38 to reach great depths inside the kiln 2 . Therefore , the flame coming out of said discharge element 14 is emitted deeper than with conventional solutions.

It is important to note that the temperature peak in proximity to the outlet of the burner 1 is also (at least partially) smoothed out.

Although the above-described invention makes particular reference to a very precise embodiment, it is not to be considered limited to that embodiment, all those variants, modifications or simplifications covered by the appended claims falling within its scope, such as for example a different geometry of the combustion head 10, of the introduction element 18 of the injection element 33, of the chambers 30 and 31, of the combustion block 38 and in particular of the suction element 39, a different method of suction of the gases G near the inner surface 51 of the side wall 17, a different arrangement of the burners 1 within the apparatus 50 (both in terms of position and alignment) , a different transport system 4, etc.

The apparatus and burner described above bring about numerous advantages.

First of all, the construction and assembly of the burner 1 is simplified with respect to solutions of the prior art comprising more components and thus, in addition to the complexity of assembly, also heavier and bulkier. In addition, the burner 1, given its geometry and penetration into the firing chamber 3, can safely be installed as a replacement (as an improvement) for a standard architecture.

Further advantages of the present invention, reside in the decrease of dispersions, in the increase of combustion (the recirculation obtained, of at least 50% of the combustion products of the burner, allows the use of regulations with reduction of the oxidizer, exploiting the residual oxygen present in the recirculated gases G) and of the uniformity of the temperature inside the firing chamber 3, determine, by the apparatus 50 and the burner 1 in accordance with the present invention, the need for a smaller amount of gas (particularly useful in the case of difficult to handle fuels such as hydrogen) to be introduced into the burner 1 in order to maintain a certain temperature, with respect to solutions of the prior art.

In addition, the use of a multi-stage combustion head 10 in combination with the injection element 32 makes it possible to reduce temperature peaks in flame mode, which are the main reason for the creation of nitrogen oxides. Therefore, the present invention results in a reduction of nitrogen oxides (NOx) , particularly below 50 ppm using natural gas .

The use of a multi-stage combustion head 10, in combination with the fuel injection element 18, allows the burner (operating with pure hydrogen, i.e. 100% hydrogen fuel, in flameless mode) to reduce its NOx emissions below those produced by a common high-speed burner operating with natural gas. In this regard, the NOx emissions produced by the burner 1 operating with pure hydrogen in flameless mode (grey continuous curve FSL) can be seen in the diagram of Figure 12.

These NOx emissions are, in the best combination of the diameter of the narrowing 21 and the diameter of the introduction element 18, less than 40 ppm at a chamber temperature of 900°C and oxygen of 2-3%.

The increase in oxygen in the kiln chamber results in a greater presence of nitrogen (N2) , which causes a gradual increase in NOx emissions (dash-dot curve FSL' ) , especially at low burner capacities (8-10 kW) where the settings are generally more in excess of oxidizer air (resulting in a greater presence of N2) .

In this particular case, the NOx emissions are still significantly lower than common high-speed burner architectures operating in flame mode with methane gas as fuel .

In addition, it was noted that increasing the diameter of the narrowing 21 and/or increasing the diameter of the fuel introduction element 18 results in a lower flame pulse in the kiln chamber with a consequent lower dilution of the flame with the fumes present . This results in a noticeable increase in NOx formation ( dashed curve FLS ' ' ' ) . The NOx emissions , even in this case , are still far lower than those of common high-speed burners operating in flame mode with methane gas as fuel .

In addition, the synergistic ef fect between the multistage combustion head 10 , the inj ection element 32 and the introduction element 18 allows a flame speed of approximately 200 m/ s to be achieved and to evenly dilute combustion in the flameless mode .

In addition, flameless combustion dilutes temperatures ( i . e . decreases peaks by increasing the median) and increases the convective exchange coef ficient with the ceramic articles T . For this reason, compared to a traditional architecture and with the same power, the present invention makes it possible to heat the material more without "attacking" it with temperature peaks at the flames and oxidi zing the organic substances contained in the ceramic articles T in a more uni form manner, thus preventing the appearance of a darker colouring on the inner portion of a sectioned article . This also partially inhibits the risk of ceramic articles T exploding in a preheating zone of the kiln 2 , e . g . when articles with an excessive moisture content are fired .

Finally, thanks to the special design of the introduction element , the mixture M* without a flame front is generated downstream of the discharge element 11 , reducing the problems connected with hydrogen backfire and detonations in the event of flame front reformation .