Burner Assembly with Enhanced Flexibility

Abstract

The invention relates to a burner assembly with enhanced flexibility, comprising a plurality of fuel rods that can be mounted in and removed from a fuel rod passage and that are connected to a fuel supply device by a flexible line, wherein each flexible line can be individually opened and closed.

Description

The present invention relates to a burner assembly with enhanced flexibility and in particular such a dual-fuel burner and/or a burner with staged combustion.

It is well-known practice to use burners, and notably burners with staged combustion, for the heating of a load in a furnace, such as a melting furnace.

Regular maintenance of the burner is essential for it to operate correctly. The maintenance of a burner usually includes the inspection of the state of the injectors or fuel rods, their cleaning or their replacement.

Such maintenance normally requires taking the burner out of service for the duration of the maintenance procedure.

In the case of a furnace furnished with a single burner, taking the burner out of service corresponds to stopping the heating of the furnace.

In the case of a furnace furnished with several burners, compensating for the loss of power due to taking one of the burners out of service for its maintenance by a corresponding increase in the power of the other burners of the furnace can in theory be envisaged. However, such a procedure causes a change in the thermal profile of the furnace while the burner is out of service.

It is evident that controlling the range of temperatures in an industrial furnace is essential for the performance of the furnace and for the quality of the product coming out of the furnace.

The result of the foregoing is that, because the maintenance of a burner causes it to be taken out of service, the operation of maintaining the burner or burners of an industrial furnace has a temporary negative impact on the performance of the furnace and/or the quality of the product. Moreover, the maintenance of the burner or burners is essential for the safety of the furnace and its longer-term productivity.

There is therefore a need to be able at least to limit the downtime of a burner in an industrial furnace when it is being maintained.

Another important problem which the operators of industrial furnaces have to face is the instability of the price of the fuels, such as natural gas.

In response, the operators of industrial furnaces wish to be able to take out intermittent fuel-supply contracts. This results in a lower cost of fuel; but, on the other hand, the distributors can temporarily stop the supply. The clients are warned in advance of the stoppage of supply and the contracts are negotiated so that the shorter the notification time, the lower the negotiated fuel cost.

The operators of an industrial furnace must then be capable of changing the type of fuel, for example switching natural gas with liquid fuel, in a very short time.

Switching a burner from a first fuel to a second fuel usually requires the fuel injectors to be replaced and therefore normally the burner to be taken out of service. This operation may take several hours in the case of cold reagents, but it may be spread over several days in the case of preheated reagents.

For the reasons indicated above with respect to the maintenance of the burners, there is a need to be able to limit the downtime of a burner in an industrial furnace when there is such a change from one fuel to another.

The present invention proposes a burner assembly which makes it possible to maintain the fuel rods without having to stop the burner, and even without having to reduce the power of the burner.

The present invention also proposes a burner assembly which allows the switch from a first fuel to a second fuel or to a combination of several fuels and vice versa without having to stop the burner, and even without having to reduce the power of the burner.

The present invention relates more particularly to a burner assembly comprising:

• • a burner unit,
  • n fuel rods or fuel injectors called first-fuel rods, where n>1,
  • an oxidant-supply device and
  • a first fuel-supply device.

According to the invention, the burner unit has an inlet face and an outlet face, at least one oxidant passageway between the inlet face and the outlet face and at least one fuel-rod passageway between the inlet face and the outlet face. Said burner unit is such that m fuel rods can be simultaneously mounted in the burner unit through the at least one fuel-rod passageway, where m>1.

Each of the first-fuel rods is capable of being mounted in the or in one of the fuel-rod passageways, and of being removed from said fuel-rod passageway via the inlet face of the burner unit. For the maintenance of said rods, it is specifically important to be able to insert and remove the rods without the rods, the burner unit and in particular the fuel-rod passageway(s) being damaged thereby.

The oxidant-supply device is capable of transporting a flow of oxidant from a source of oxidant to the burner unit for its injection through the at least one oxidant passageway into a combustion zone situated downstream of the outlet face.

Similarly, the first fuel-supply device is capable of transporting a flow of a first fuel from a first-fuel source to the burner unit for its injection into the combustion zone through one or the fuel-rod passageway(s). This first fuel-supply device comprises a first supply line, a first flow meter, a first distributor and n first flexible lines connecting the first distributor to the n first-fuel rods. The first supply line is more particularly capable of transporting said flow of the first fuel from the first-fuel source to the first distributor. The first flow meter is capable of regulating the rate of flow of the first fuel from the first-fuel source to the first distributor and the first distributor is capable of dividing the flow of first fuel into n subsidiary flows on the n first flexible lines.

The flexible lines make it much easier to insert the fuel rods into the fuel-rod passageway(s) and to remove the fuel rods from said passageways.

The burner assembly according to the present invention is characterized notably in that the first fuel supply device comprises one or more valves making it possible to close or open the n first flexible lines one by one so that, when x of the n first flexible lines are closed by said one or more valves, where 1≦x≦n−1, the first distributor divides the flow of first fuel into the n−x first flexible lines which are open for its injection into the combustion zone by the first-fuel rod or rods connected to said first open flexible lines.

The burner assembly according to the invention therefore makes it possible, during the maintenance of one or, if necessary, several of its first-fuel rods, to keep the burner assembly in operation and to do so without having to reduce the level of the power supplied by said burner assembly to the combustion zone. In this manner, the effect of the maintenance of the first-fuel rods on the thermal profile in the furnace, and therefore of the productivity of the furnace and/or the quality of the product originating from the furnace, is greatly reduced.

Clearly, when the number x of first flexible lines closed by said one or more valves is equal to n−1, the totality of the flow of first fuel is sent into the only first flexible line left open by the first distributor. When all the first flexible lines are closed, no flow of first fuel is injected into the combustion zone.

Each first flexible line may, for example, be furnished with a valve making it possible to open and close this flexible line. The first fuel-supply device may also comprise a valve which makes it possible to close and open selectively one or more of the first flexible lines.

The burner assembly may comprise a central control unit for the opening and the closure of the first flexible lines by the valve(s) of the first fuel-supply device and/or means for the manual operation of this or these valves.

In the present context, it is considered that a flexible line is closed when it does not allow a flow of fuel to pass and that a flexible line is open when it allows a fuel flow to pass through the flexible line.

The first fuel may in particular be a gaseous fuel (such as natural gas) or a liquid fuel, such as fuel oil.

According to one embodiment, the number n of first-fuel rods is equal to the number m of fuel-rod passageways in the burner unit.

According to an alternative embodiment, the number n of first-fuel rods is equal to the number m of fuel-rod passageways+1.

Therefore, for a burner assembly having, in normal operation, an injection of first fuel into the combustion zone by means of m first-fuel rods, this embodiment makes it possible, when one of the first-fuel rods is taken out of service for its maintenance by the closure of the corresponding first flexible line, to replace in the burner unit the rod under maintenance with the m+1^st first-fuel rod and to open the first flexible line that corresponds to this m+1^st rod so as to be able to operate the whole burner with m first-fuel rods during the maintenance of one of the first-fuel rods. In this manner, the effect of the maintenance of the first-fuel rods on the thermal profile in the furnace, and therefore on the productivity of the furnace and/or the quality of the product coming from the furnace, is again more clearly reduced, or even eliminated.

It is also possible to provide a source of fuel called a reserve source, linked by means of a reserve-fuel supply device to a reserve-fuel rod so as to be able, for a burner assembly having, in normal operation, an injection of first fuel into the combustion zone by means of m first-fuel rods, when one of the m first-fuel rods is taken out of service for its maintenance by the closure of the corresponding first flexible line, to replace in the burner unit the first fuel-rod under maintenance with this reserve-fuel rod and thus be able to operate the burner assembly with m⁻¹ first-fuel rods and the reserve-fuel rod during the maintenance of one of the first-fuel rods. Such an operating mode is particularly recommended in the case of a furnace or other industrial installation comprising a considerable number of burner assemblies.

The reserve fuel may also be a gaseous or liquid fuel. The reserve fuel is advantageously a liquid fuel, notably because of the ease of storage and transport of such a fuel.

The reserve fuel may be of the same type (gaseous, liquid, etc.) as the first fuel or of a different type. The composition of the reserve fuel may match or be different from the composition of the first fuel.

The present invention also relates to a burner assembly as described above and which also comprises:

• • p second-fuel rods, where p>1 and
  • a second fuel-supply device.

Each of said second-fuel rods is able to be mounted in the or in one of the fuel-rod passageways and to be removed from said fuel-rod passageway via the inlet face, as already indicated above for the first-fuel rods.

The second fuel device is capable of transporting a flow of a second-fuel from a second one or fuel source to the burner unit for its injection into the combustion zone through one or the fuel passageway(s). The second fuel-supply device comprises a second supply line, a second flow meter, a second distributor and p second flexible lines connecting the second distributor to the p second-fuel rods.

The second supply line is capable of transporting the flow of the second fuel from the second-fuel source to the second distributor. The second flow meter is capable of regulating the rate of flow of the second-fuel from the second-fuel source to the second distributor. The second distributor is capable of dividing the flow of second fuel into p subsidiary flows on the p second flexible lines.

According to this embodiment of the invention, the second fuel-supply device comprises one or more valves making it possible to close and open the p second flexible lines one by one so that, when y of the p second flexible lines are closed by said one or more valves, where 1≦y≦p−1, the second distributor divides the flow of second fuel over the p−y second flexible line(s) which is/are open for its injection into the combustion zone by the second-fuel rod or rods connected to said second open flexible line(s).

Each second flexible line can, for example, be furnished with a valve making it possible to open and to close this flexible line. The second fuel-supply device may also comprise a valve which makes it possible to close and to open selectively one or more of the second flexible lines.

The burner assembly may comprise a central control unit for opening and closing the second flexible lines by one or the valves of the second fuel-supply device and/or means for the manual operation of this or these valves.

The second fuel may in particular be a gaseous fuel or a liquid fuel.

The second fuel may be of the same type (gaseous, liquid, etc.) as the first fuel, but in practice most frequently of a different type.

When use is made of a reserve fuel, the reserve fuel may also be a gaseous or liquid fuel. The reserve fuel is advantageously a liquid fuel, such as fuel oil, notably because of the ease of storage and of transporting such a fuel.

The number n first-fuel rods may be equal to the number p second-fuel rods.

The number p of second-fuel rods may be equal to the number m of fuel-rod passageways of the burner unit.

As already described above with reference to the first-fuel rods, the number p of second-fuel rods may be equal to the number m+1 of fuel-rod passageways.

Burner assemblies comprising 2 passageways of fuel (m=2) have been found to be particularly practical for a number of applications, and notably for melting furnaces.

The burner assembly according to the invention may be used in air combustion, but is particularly useful for applications in oxycombustion. The invention therefore relates in particular to a burner assembly in which the source of oxidant is a source of oxidant having an oxygen content of at least 80% vol, preferably of at least 90% vol.

The burner assembly according to the invention is notably advantageous for staged combustion.

The method of staged combustion of fuels consists in dividing the quantity of oxidant necessary for the total combustion of the fuel into at least two flows of oxidant inserted into the combustion zone at different distances from the injection of the flow or flows of fuel into this combustion zone. Therefore, at least one first flow of oxidant is injected very close to or together with the flow or flows of fuel. The oxidant injected by this or these flows of injected oxidant closest to or with the flow or flows of fuel is called primary oxidant. Notably it allows the partial combustion of the fuel at a controlled temperature which limits for example the formation of NOx. The or the other flow(s) of oxidant are injected into the combustion zone at a greater distance from the fuel than the primary oxidant. The oxidant thus injected makes it possible to achieve the combustion of the fuel that has not reacted with the primary oxidant. The oxidant injected by this or these latter flows is called secondary oxidant.

Document WO 02/081967 describes a method making it possible to apply this type of staged combustion method. The oxidant is separated into three distinct flows which are injected at different distances from the injection point of the fuel and at different speeds. Therefore, a first jet of oxidant is injected at a high speed at the center of the jet of fuel. Then, a second jet of oxidant is injected at a lower speed at a first distance from the jet of fuel. Finally, a third jet of oxidant is injected at a second distance from the jet of fuel, this second distance being greater than the first distance.

The invention therefore also relates to a burner assembly in which the oxidant-supply device is capable of transporting several oxidant flows from a source of oxidant to the burner unit for its injection through the at least one oxidant passageway into the combustion zone, at least one of said oxidant flows being a secondary oxidant flow injected into the combustion zone through an oxidant passageway situated at a distance ds>0 from the fuel-rod passageway(s) and at least one of said oxidant flows being a primary oxidant flow injected into the combustion zone through the or one of the fuel rod passageways or through an oxidant passageway situated at a distance dp from the fuel-rod passageway(s), where 0≦dp<ds.

According to one embodiment making it possible to stabilize the flame as much as possible in the combustion zone, at least one of the oxidant flows is a primary oxidant flow injected through the or one of the fuel-rod passageways around one or the fuel rod(s).

It is known practice to use an oxidant at a high temperature, namely a temperature of at least 100° C., or even of several hundreds of degrees in order to improve the energy efficiency of the combustion.

The inventors have in particular proposed, in application EP-A-1995543, heat exchangers making it possible to heat the oxygen satisfactorily.

The conveyance of a hot oxidant requires the use of specific means, usually fixed means, which do not lend themselves to the frequent maintenance operations mentioned above.

According to one embodiment of the invention for staged combustion, the oxidant-supply device comprises means for the heating of the at least one secondary oxidant flow to a temperature of at least 100° C. upstream of the burner unit, said oxidant-supply device comprising no means for the heating of the at least one primary oxidant flow. Said primary oxidant, which is injected into the combustion zone through the or one of the oxidant-rod passageway(s) or through one or the oxidant passageway(s) situated at a distance dp from the fuel-rod passageway(s) is therefore not heated or preheated to a temperature of 100° C. or more at the inlet of the burner unit.

In the present context, the terms “heated” and “preheated” are used interchangeably.

Preferably, the secondary oxidant is preheated to a temperature at the inlet of the burner unit of between 100° C. and 650° C., preferably between 100° C. and 600° C., and again preferably between 350° C. and 550° C.

This embodiment makes it possible to improve energy efficiency through the use of a preheated oxidant for the secondary oxidant, while retaining the enhanced flexibility of the burner according to the invention, notably with respect to the maintenance of the fuel rods and/or the switch from a first fuel to a second fuel or to a combination of several fuels, through the use of a non-preheated oxidant for the primary oxidant.

For the system to have the advantage of the energy balance inherent in the use of a preheated oxidant, the non-preheated primary oxidant provides only a very small fraction of all the oxidant necessary for the total combustion of the fuel. This fraction is usefully less than 10% of all the oxidant. Advantageously, this proportion ranges between 1.5 and 7% of all the oxidant necessary to ensure the complete combustion of the fuel.

It is understood that the benefit of using a hot oxidant is not substantially affected by this very small fraction of unheated oxidant.

By unheated or non-(pre)heated, it should be understood that the (primary) oxidant is in the essential ambient temperature conditions that prevail on its path taking it to the furnace. On passing the refractory walls of the furnace, its temperature necessarily rises. The temperature of the non-(pre)heated oxidant is preferably the ambient temperature and must not exceed one hundred degrees Celsius, the temperature in the vicinity of the furnace nevertheless being substantially higher than that of the atmosphere at a distance from the furnace.

The primary oxidant and the secondary oxidant may have an identical or different composition. When the primary oxidant and the secondary oxidant have one and the same composition, they can be provided by one and the same source of oxidant, such as, for example, a unit for separating gases from the air, the secondary oxidant passing through, upstream of the burner unit, means for the preheating of said secondary oxidant, such as notably heat exchangers, while the primary oxidant does not pass through any means for preheating it upstream of the burner unit.

The burner unit may consist of a single brick. The burner unit may also advantageously consist of a set of several bricks, typically fire bricks. When the burner unit consists of several bricks, said bricks may be spaced out, notably when the burner assembly is mounted in a wall of a furnace. The use of a burner unit which consists of several bricks allows in particular a staging of the combustion that is more spaced out, that is to say the injection of at least one jet of secondary oxidant at a greater distance from the injection(s) of fuel. A burner unit which consists of several bricks may also allow a more spaced injection of different jets of fuel.

As already indicated above, the invention relates notably to a burner assembly in which the burner unit is mounted in a wall of a furnace, the combustion zone downstream of the outlet face being situated inside the furnace. The invention also relates to a furnace comprising such a burner assembly.

The invention also relates to the use of one or more of these burner assemblies for carrying out the combustion of oxidant and fuel in a combustion zone and in particular in such a combustion zone inside a furnace, and any combustion method by means of a burner assembly or of a furnace according to the invention.

The invention relates particularly to the use of one or more burner assemblies for carrying out staged combustion in the combustion zone and in particular staged combustion with one or more unheated primary oxidant flows and one or more heated secondary oxidant flows.

EXAMPLES

The advantages of the present invention appear more clearly from the detailed description given as an example below which refers to FIGS. 1 to 6 in which:

FIG. 1 is a schematic representation of the operation of the fuel- and oxidant-supply devices of a staged-combustion burner assembly according to the invention,

FIGS. 2 and 3 are schematic representations of the operation of the fuel- and oxidant-supply devices of one embodiment of a staged-combustion burner assembly according to the invention with a reserve rod of first fuel connected to the first distributor before and during a maintenance procedure on a first-fuel rod,

FIG. 4 is a schematic representation of the operation of the fuel- and oxidant-supply devices of one embodiment of a staged-combustion burner assembly according to the invention with a reserve rod connected to a source of reserve fuel,

FIGS. 5 and 6 are schematic representations of the operation of the fuel- and oxidant-supply devices of one embodiment of a staged-combustion burner assembly according to the invention during and after the change from a first fuel to a second fuel.

In FIG. 1, the first supply line 100 of the first fuel-supply device connects a source of first fuel (not illustrated) to the first distributor 101. The flow of first fuel from the source of first fuel to the first distributor is regulated by a first flow meter (not illustrated).

The first distributor 101 is connected to the two first-fuel rods 110 and 111 by the two first flexible lines 120 and 121 (n=2).

Each of the two first-fuel rods 110 and 111 is inside a fuel-rod passageway 500, 501 between the inlet face and the outlet face 503 of the unit (m=2).

The first fuel-supply device also comprises two valves 130 and 131 making it possible to close and open the first flexible lines 120 and 121 individually.

The second supply line 200 of the second fuel-supply device connects a source of second fuel (not illustrated) to the second distributor 201. The second supply device comprises a second flow meter (not illustrated) for the regulation of a flow of second fuel from the source of second fuel to the second distributor. The second supply device also comprises two second flexible lines 220 and 221 in order to connect the second distributor to two second-fuel rods (not illustrated) (p=2), each second flexible line being able to be opened and closed by the valves 230 and 231.

The oxidant-supply device comprises secondary oxidant-supply lines 504 for transporting secondary oxidant to two secondary-oxidant passageways 506, 507 situated at a distance ds from the fuel passageways 110, respectively 111.

The oxidant-supply device also comprises a primary oxidant supply system. In the embodiment illustrated in the figures, the primary oxidant supply system comprises a primary oxidant supply line 508 and a primary oxidant distributor 509. The primary oxidant distributor 509 is connected to the fuel passageways 500 and 501 by pipes 510, 511. The primary oxidant is injected into the combustion zone 512 through an annular zone around the fuel rods 110 and 111 in the fuel passageways 500 and 501 (dp=0). Primary oxidant thus injected around a fuel rod is usually called “sheathing oxidant”. The two valves 530 and 531 make it possible to open and close the two primary oxidant pipes 510 and 511.

In order to change the type of fuel or to carry out maintenance of one of the m primary fuel rods (in this instance m=2), while keeping the burner alight and at a constant power, the following steps must be carried out:

• • (a) Close the first flexible line 120 (121) corresponding to the targeted first-fuel rod 110 (111). This closure is achieved by closing the valve 130 (131). The totality of the flow of first fuel, for example of natural gas, is then automatically diverted to the m−1 (in this instance m−1=1) first flexible lines 121 (120) that are still open and therefore to the corresponding m−1 first-fuel rods 131 (130). The power of the burner remains the same.
  • (b) Stop the flow of sheathing oxidant around the targeted first-fuel rod 120 (121) in order to be able to remove this rod from the fuel-rod passageway 500 (501) by closing the pipe of primary oxidant 510 (511) by means of the valve 530 (531). The totality of the flow of sheathing oxidant is then diverted to the fuel passageways 501 (500) containing the m−1 fuel rods 121 (120) that are still in operation (in this instance m−1=1).
  • (c) Remove the stopped first-fuel rod and optionally carry out the necessary maintenance.

In the case of a purely maintenance operation, after the maintenance or the replacement of the first-fuel rod in question, the reverse procedure is carried out: the stopped first-fuel rod 110 (111) is reinserted into its fuel-rod passageway 500 (501); reopen the flow of sheathing oxidant around this first-fuel rod 110 (111) in this fuel-rod passageway 500 (501) by opening the primary-oxidant pipe 510 (511) by means of the valve 530 (531) and open the first flexible line 120 (121) corresponding to this first-fuel rod 110 (111) by means of the valve 130 (131). The totality of the flow of first fuel is then automatically redistributed to the m (in this instance m=2) first flexible lines 121 and 120 and therefore to the m first-fuel rods 131 and 130.

Following this example, the burner assembly operates with m−1 first-fuel rods during the maintenance of one of the first-fuel rods. The power of the burner can remain constant during the maintenance procedure and the impact on the productivity of the furnace and/or on the quality of product coming out of the furnace is greatly limited.

The burner assembly illustrated in FIG. 2 makes it possible to further limit the impact of a maintenance procedure or change of fuel rods on the productivity of the furnace and/or on the quality of the product coming out of the furnace, notably if the operator wishes or is obliged to carry out long-term maintenance.

The first fuel-supply device of the burner assembly according to FIG. 2 comprises one m+1^st first-fuel rod 112, one m+1^st first flexible line 122 connected to the first distributor 101, and one m+1^st valve 132 making it possible to open and close the m+1^st first flexible line 122 (in this instance m+1=3).

Before the maintenance procedure, the first flexible line 122 is closed and the first-fuel rod 112 is out of service.

During the maintenance of, for example, the first-fuel rod 110, the operator may, after step (c) above:

(d) replace the first-fuel rod 110 with a rod 112 of the same fuel previously connected to the first distributor 101 of said fuel,

• • (e) reopen the flow of sheathing oxidant in the fuel-rod passageway 500, this time around the first-fuel rod 112 by opening the pipe of primary oxidant 510 by means of the valve 530, and
  • (f) open the first flexible line 122 corresponding to this first-fuel rod 112 by means of the valve 132.
    As shown in FIG. 3, the totality of the flow of first fuel is then automatically redistributed to the m (in this instance m=2) first flexible lines 122 and 121 and therefore to the m first-fuel rods 132 and 131. The maintenance of the rod 110 can then be carried out while the burner assembly operates at normal rate.

FIG. 4 shows an alternative embodiment of a burner assembly according to the invention.

If the operator desires or is obliged to carry out long-term maintenance of, for example, the first-fuel rod 110, the operator may, after step (c) above:

• • (g) replace the first-fuel rod 110 with a reserve-fuel rod 310 connected to a source of reserve fuel (not illustrated), such as, for example, a source of fuel oil. The reserve fuel rod 310 is a rod specifically designed for the injection of said reserve fuel,
  • (h) reopen the flow of sheathing oxidant in the fuel-rod passageway 500, this time around the reserve-fuel rod 310 by opening the primary-oxidant pipe 510 by means of the valve 530, and
  • (i) open the reserve flexible line 320 connected to the reserve-fuel rod by means of the valve 330 and regulate the flow of reserve fuel to the reserve-fuel rod 310 and the flow of first fuel to the m−1 first-fuel rods 111 connected to the m−1 first flexible lines 121 that are open (in this instance m−1=1) so as to achieve the desired power for the burner assembly.

In the precise case of fuel oil, or of other liquid fuels, as a reserve fuel, each fuel-oil rod 310 is also supplied by an atomization fluid. A flexible line 420 must also therefore be provided for this atomization fluid with its associated valve 430. The operator opens the valve 430 before opening the fuel-oil valve 330, which in this case is the reserve fuel. In order to achieve the nominal power, the operator reduces, manually or by programmable controllers, the power of the fuel 1 while increasing the power of the fuel 2 up to half of the burner power, all while complying with the flow-range setpoints of the atomization fluid.

As shown in FIG. 4, the burner assembly then operates at mixed rate with injection into the combustion zone on the one hand of a flow of first fuel and on the other hand of a flow of reserve fuel. The maintenance of the rod 110 can then be carried out while the burner assembly operates at mixed rate with a minimal impact on the productivity of the furnace and/or on the quality of the product coming out of the furnace.

In order to completely replace the first fuel with the second fuel in the embodiment illustrated in FIG. 1, it is necessary, as illustrated in FIGS. 5 and 6, to follow a similar scenario for the replacement of the m first-fuel rods (in this instance m=2) with the p second-fuel rods (in this instance m=2).

After step (c) above, the operator:

• • (j) replaces the first-fuel rod 110 with a second-fuel rod 210 connected to the second distributor 201 of the second fuel,
  • (k) reopens the flow of sheathing oxidant in the fuel-rod passageway 500, this time around the second-fuel rod 210 by opening the primary-oxidant pipe 510 by means of the valve 530, and
  • (l) opens the second flexible line 220 corresponding to this second-fuel rod 210 by means of the valve 230 while regulating the flows of first and second fuels to the fuel rods 111 and 210 connected to the first and second open flexible lines 121 and 220 so as to achieve the desired power for the burner assembly.

The burner assembly then operates (FIG. 5) at mixed rate with injection into the combustion zone on the one hand of a flow of first fuel and on the other hand of a flow of second fuel.

Steps (j) to (l) are then repeated for the replacement of the m−1 (in this instance m−1=1) other first-fuel rods 111 with second-fuel rods 121 in order to achieve an operation of the burner assembly with only one flow of second fuel (FIG. 6) and at the desired power.

Therefore, the burner assembly according to the invention makes it possible to switch from a first fuel to a second fuel, and even to a mixed rate of two fuels, without having to stop the burner assembly and without having to reduce the power of the burner assembly and with a minimal impact on the productivity of the furnace and/or on the quality of the product coming out of the furnace.

The above comments relating to the rods of liquid fuel clearly apply to all of the rods of liquid fuel which may be first-fuel rods, second-fuel rods or reserve rods.

Claims

1-15. (canceled)

16. A burner assembly comprising:

a burner unit having an inlet face and an outlet face, at least one oxidant passageway between the inlet face and the outlet face and at least one fuel-rod passageway between the inlet face and the outlet face, the burner unit being such that m fuel rods can be simultaneously mounted in the burner unit through the at least one fuel-rod passageway, where m>1,

n first-fuel rods, where n>1, each first-fuel rod being capable of being mounted in the or in one of the fuel-rod passageways, and of being removed from said fuel-rod passageway via the inlet face of the burner unit,

an oxidant-supply device capable of transporting a flow of oxidant from a source of oxidant to the burner unit for its injection through the at least one oxidant passageway into a combustion zone situated downstream of the outlet face,

a first fuel-supply device capable of transporting a flow of a first fuel from a first-fuel source to the burner unit for its injection into the combustion zone through one or the fuel passageway(s), the first fuel-supply device comprising a first supply line, a first flow meter, a first distributor and n first flexible lines connecting the first distributor to the n first-fuel rods,

i. the first supply line being capable of transporting the flow of the first fuel from the first-fuel source to the first distributor,

ii. the first flow meter being capable of regulating the rate of flow of the first fuel from the first-fuel source to the first distributor,

iii. the first distributor being capable of dividing the flow of first fuel into n subsidiary flows on the n first flexible lines,

wherein: the first fuel-supply device comprises one or more valves making it possible to close or open the n first flexible lines one by one so that, when x of the n first flexible lines are closed by said one or more valves, where 1≦x≦n−1, the first distributor divides the flow of first fuel into the n−x first flexible lines which are open for its injection into the combustion zone by the first-fuel rod or rods connected to said first open flexible lines.

17. The burner assembly of claim 16, wherein n=m.

18. The burner assembly of claim 16, wherein n=m+1.

19. The burner assembly of claim 16, further comprising:

p second-fuel rods where p>1, each second fuel rod being able to be mounted in the or in one of the fuel-rod passageways and to be removed from said fuel-rod passageway via the inlet face,

a second fuel-supply device capable of transporting a flow of a second fuel from a second-fuel source to the burner unit for its injection into the combustion zone through one or the fuel-rod passageway(s), the second fuel-supply device comprising a second supply line, a second flow meter, a second distributor and p second flexible lines connecting the second distributor to the p second-fuel rods,

i. the second supply line being capable of transporting the flow of the second fuel from the second-fuel source to the second distributor,

ii. the second flow meter being capable of regulating the rate of flow of the second fuel from the second-fuel source to the second distributor,

iii. the second distributor being capable of dividing the flow of the second fuel into p subsidiary flows on the p second flexible lines,

wherein: the second fuel-supply device comprises one or more valves making it possible to close and open the p second flexible lines one by one so that, when y of the p second flexible lines are closed by said one or more valves, where 1≦y≦p−1, the second distributor divides the flow of second fuel over the p−y second flexible line(s) which is/are open for its injection into the combustion zone by the second-fuel rod or second-fuel rods connected to said second open flexible line(s).

20. The burner assembly of claim 19, wherein n=p.

21. The burner assembly of claim 19, wherein p=m.

22. The burner assembly of claim 19, wherein p=m+1.

23. The burner assembly of claim 16, wherein m=2.

24. The burner assembly of claim 16, wherein the source of oxidant is a source of oxidant having an oxygen content of at least 80% vol, preferably of at least 90% vol.

25. The burner assembly of claim 16 for the staged combustion, wherein:

the oxidant-supply device is capable of transporting several oxidant flows from a source of oxidant to the burner unit for its injection into the combustion zone,

at least one of said oxidant flows being a secondary oxidant flow injected into the combustion zone through an oxidant passageway situated at a distance ds>0 from the fuel-rod passageway(s), and

at least one of said oxidant flows being a primary oxidant flow injected into the combustion zone through the or one of the fuel-rod passageways or though an oxidant passageway situated at a distance dp from the fuel-rod passageway(s), where dp<ds.

26. The burner assembly of claim 25, wherein at least one of the oxidant flows is a primary oxidant flow injected through the or one of the fuel-rod passageways and around one or the fuel rod(s).

27. The burner assembly of claim 16, wherein the burner unit is an assembly of several fire bricks.

28. The burner assembly of claim 16, wherein the burner unit is mounted in a furnace wall, the combustion zone downstream of the outlet face being situated inside the furnace.

29. A furnace comprising the burner assembly of claim 28.

30. A combustion method, comprising the step of combusting oxidant and fuel injected by the burner of the furnace of claim 29.