Method and device for heatina a furnace

The present invention relates to a method for heating a furnace. Furthermore, the invention relates to a method for retrofitting an existing furnace for more flexible heating operation. The invention also relates to such a furnace.

Industrial heating furnaces for heating metal materials, such as steel slabs, are conventionally heated by a fuel, which is combusted together with an oxidant. Normally, a main part of such combustion takes place in a main heating zone. The combustion products may then flow, counter-currently to the heated metal material, downstream in the furnace towards a flue gas exit at a metal material charging opening. The downstream zone through which the combustion products flow is conventionally denoted a “dark zone”.

Hence, the metal material being transported through the furnace, in its upstream direction, is preheated by the hot combustion products in the dark zone on its way from the charging opening to the main heating zone.

For thick materials, it is many times desired to heat the material as early as possible in the furnace, in order for the central parts of the material to be properly heated as soon as possible. The centre of a slab, for instance, needs to be heated to near equilibrium before it can be removed from the furnace for rolling or forging.

This problem has conventionally been solved by adding extra burners in the dark zone, so as to increase heating power there. This increases the flue gas temperature, which may lead to problems in any upstream heat recuperation or regeneration equipment used to harvest thermal energy from the flue gases.

With the aim of not heating the flue gases too much, it has been proposed to use so-called oxyfuel burners (that is, burners operated using a high-oxygen oxidant) in the dark zone. However, this is typically a complex and expensive solution.

Therefore, it is desired to more efficiently be able to heat a metal material in the dark zone, without such a solution being overly complex or expensive.

Furthermore, it is desirable for such a solution to offer increased flexibility in terms of heating powers in a furnace. The latter is particularly true for existing heating furnaces, that otherwise are very expensive to upgrade for providing a more flexible heating power.

The present invention solves the above described problems.

Hence, the invention relates to a method for heating a furnace with a longitudinal direction and a cross plane which is perpendicular to the longitudinal direction, which furnace is arranged with at least one heating zone which is heated using combustion of a fuel with an oxidant, and which furnace is further arranged with a dark zone downstream of said heated zone, to which dark zone no fuel is supplied directly, which method is characterised in that the fuel and oxidant supplied to the heating zone is substoichiometric, in that between 10% and 40% of the total oxidant for achieving stoichiometric or near stoichiometric combustion is supplied directly to the dark zone, in that a flue gas temperature is measured in and/or downstream of the dark zone, and in that the share of the total oxidant supplied to the dark zone is controlled so as not to exceed a predetermined maximum measured such temperature.

Furthermore, the invention relates to a method for retrofitting an existing furnace for operation according to any one of the preceding claims, which existing furnace has a longitudinal direction and a cross plane which is perpendicular to the longitudinal direction, which existing furnace is arranged with at least one heating zone which is heated using combustion of a fuel with an oxidant, and which existing furnace is further arranged with a dark zone downstream of said heated zone, to which dark zone no fuel is supplied directly, which method is characterised in that the method comprises providing a separate oxidant lance arranged to provide oxidant directly to the dark zone; connecting the separate oxidant lance to a source of oxidant; modifying the furnace to supply the fuel and oxidant supplied to the heating zone substoichiometrically to provide between 10% and 40% of the total oxidant for achieving stoichiometric or near stoichiometric combustion by supplying oxidant directly to the dark zone; providing a flue gas temperature sensor arranged to measure the flue gas temperature in and/or downstream of the dark zone; and modifying the furnace to control the share of the total oxidant supplied to the dark zone so as not to exceed a predetermined maximum measured such temperature.

Also, the invention relates to a heating furnace with a longitudinal direction and a cross plane which is perpendicular to the longitudinal direction, which furnace is arranged with at least one heating zone which is heated using combustion of a fuel with an oxidant, and which furnace is further arranged with a dark zone downstream of said heated zone, to which dark zone no fuel is arranged to be supplied directly, which furnace is characterised in that the furnace is arranged to supply fuel and oxidant to the heating zone substoichiometrically, in that the furnace is arranged to supply between 10% and 40% of the total oxidant for achieving stoichiometric or near stoichiometric combustion directly to the dark zone, in that the furnace comprises a flue gas temperature sensor arranged to measure a flue gas temperature in and/or downstream of the dark zone, and in that the furnace is arranged to control the share of the total oxidant supplied to the dark zone so as not to exceed a predetermined maximum measured such temperature.

In the following, the invention will be described in detail, with reference to exemplifying embodiments of the invention and to the enclosed drawings, wherein:

Figure 1 is a simplified side view of a furnace according to the invention;

Figure 2 is a simplified top view of the furnace illustrated in Figure 1 ;

Figure 3 is a simplified detail view of an oxidant lance in accordance with the present invention;

Figure 4 is a flow chart illustrating a method according to the invention for heating a furnace of the type generally illustrated in Figure 1 ; and

Figure 5 is a flow chart illustrating a method according to the invention for modifying or retrofitting an existing furnace for operation according to the flow chart illustrated in Figure 4.

Figure 6 is a simplified view of a side-fired furnace according to the invention.

Flence, Figure 1 shows an industrial furnace 100 having a longitudinal direction D and a cross plane C which is perpendicular to the longitudinal direction D. The furnace 100 comprises at least one, but possibly several heating zones 120, 130, 140, through

which a metal material 104 is transported, preferably in the longitudinal direction D, whereby the material 104 is heated on its way from an entry door 101 to an exit door 102. The furnace 100 is further arranged with a dark zone 110, arranged near the entry door 101 , to which dark zone 110 no fuel is supplied directly.

The furnace 100 may be a continuous reheating furnace, and the material 104 may be a metal material, such as steel. Preferably, the metal material 104 is at least 10 cm, such as at least 20 cm, thick. In general, the material 104 is preferably heated to temperatures above about 1 000 °C.

Each zone 110, 120, 130, 140 may in general comprise both an upper and a lower zone, including the dark zone 110. 121 denotes a baffle arranged to delimit the dark zone 110 from the heating zone 120.

The furnace 100, and in particular the one or several heating zones 120, 130, 140 which are not the said dark zone 110, is heated using combustion of a fuel with an oxidant, both of which are provided directly to the heating zone 120, 130, 140 in question.

The fuel may be a gaseous, liquid or solid fuel. The oxidant supplied to the heating zone 120, 130, 140 in question is preferably an oxidant comprising at least 85% oxygen, and is more preferably industrially pure oxygen, but may in certain embodiments also be air or any other oxidant. For instance, one or several of the burners 122, 123, 132, 133 arranged in the heating zone 120, 130 in question may be adapted for high-oxygen oxidant supplementation, by a respective separate primary oxidant lance 124 (see Figure 2) being installed at a distance from the respective burner 122 in question, fed with such high-oxygen oxidant from a control device 160 of the furnace 100 via a line 1. The lanced high-oxygen oxidant, forming a jet 124a substantially in a downstream direction in the longitudinal direction D, may be the only oxidant used in the non-dark zone heating zones 120, 130, however it may also be used in addition to oxidant which is supplied via the burner 122 itself. In general, the heating zones 120, 130, 140 may be used with any combination of oxyfuel and air burners, even if the present invention is particularly advantageous for furnaces 100 being heated by at least one oxyfuel burner.

Each of the burners 122 so supplemented using a respective primary oxidant lance 124 may in itself be an existing burner 122, such as an air burner, which has been retrofitted by said lance 124, during which retrofitting some or all a previously used oxidant (such as air) was replaced by the said high-oxygen oxidant.

In the arrangement according to figures 1 and 2 the burners 122, 132 essentially fire in the longitudinal direction D, i.e. the oxidant and the fuel are supplied to the heating zone 120, 130 essentially in the longitudinal direction D. The fuel, the oxidant and the combustion products are essentially parallel to the metal material to be heated. Such furnaces are also referred to as front or back fired furnaces.

The invention may also be applied to so-called side fired furnaces where the burners for heating the heating zone are arranged in the side walls of the furnace. The burners fire in an essentially horizontal direction perpendicular to the longitudinal direction D. According to the invention oxidant lances are preferably arranged in the side walls of the dark zone of the furnace.

Figure 6 shows a side fired continuous reheating furnace 600 having a longitudinal direction D and a cross plane C which is perpendicular to the longitudinal direction D. The furnace 600 is similar to furnace 100 shown in figures 1 and 2. The furnace 600 comprises several heating zones 620, 630, 640, through which a metal material 104 is transported and heated. The furnace 600 is further arranged with a dark zone 610, arranged near the entry door 601 , to which dark zone 610 no fuel is supplied directly.

The furnace 600, and in particular the one or several heating zones 620, 630, 640 which are not the said dark zone 610, is heated using combustion of a fuel with an oxidant, both of which are provided directly to the heating zone 620, 630, 640 in question.

The fuel may be a gaseous, liquid or solid fuel. The oxidant supplied to the heating zone 620, 630, 640 is preferably an oxidant comprising at least 85% oxygen, and is more preferably industrially pure oxygen, but may in certain embodiments also be air or any other oxidant.

In one embodiment one or several of the burners 622, 623, 632, 633 arranged in the heating zone 620, 630 are adapted for high-oxygen oxidant supplementation, by a respective separate primary oxidant lance 624 being installed at a distance from the respective burner 622 in question, fed with such high-oxygen oxidant from a control device 660. The lanced high-oxygen oxidant may be the only oxidant used in the non dark zone heating zones 620, 630, however it may also be used in addition to oxidant which is supplied via the burner 622 itself. In general, the heating zones 620, 630, 640 may be used with any combination of oxyfuel and air burners.

Each of the burners 622 so supplemented using a respective primary oxidant lance 624 may in itself be an existing burner 622, such as an air burner, which has been retrofitted by said lance 624, during which retrofitting some or all a previously used oxidant (such as air) was replaced by the said high-oxygen oxidant.

According to a preferred embodiment the heating zone 620, 630, 640 of the furnace 600 is heated by burners 622, 623, 632, 633 wherein all burners 622, 623, 632, 633 heating the heating zone 620, 630, 640 are located in the side walls of the furnace 600.

In another preferred embodiment the burners 622, 623, 632, 633 located in the side walls of the heating zone 620, 630, 640 are supplemented with an oxidant lance arranged close to the burner 622, 623, 632, 633 for supplying a share of the oxidant to the heating zone 620, 630, 640.

Preferably, for front- or back-fired furnaces 100 as well as for side-fired furnaces 600 all oxidant is supplied to the dark zone 110, 610 via lances arranged in the side walls of the furnace 100, 600.

The following advantageous embodiments are valid for front-fired, back-fired and side-fired furnaces.

The temperature in the heating zones 120, 130, 140, 620, 630, 640 of furnaces 100, 600 may preferably be at least 1000°C. The flue gases run counter-currently to the transport direction through the furnace 100, 600 of the material 104.

According to the invention, the fuel and oxidant supplied to the non-dark zone heating zone 120, 130, 140, 620, 630, 640 is controlled to be substoichiometric, meaning that there is a fuel surplus in relation to the available oxidant in the said heating zone 120, 130, 140, 620, 630, 640 as a whole. In particular, the flue gases reaching the downstream part of the heating zone 120, 620 arranged just upstream of the dark zone 110, 610 comprises a combustible fuel surplus, a consequence of which is that the flue gases flowing into the dark zone 110, 610 carry such a fuel surplus.

Further according to the invention, between 10% and 40%, preferably between 25%-40%, of the total oxidant for achieving stoichiometric or near stoichiometric combustion is then supplied directly to the dark zone 110, 610, such as via lances 151 , 152, 153, 154. It is noted that these relative amounts relate to the amount of oxygen in the respective oxidant. In the exemplifying embodiment illustrated in Figures 1 and 2 and in figure 6, there are two pairs 151 , 152; 153, 154 of such dark zone 110, 610 oxidant lances, one pair on each lateral side of the furnace 100, 600 and pointing into the dark zone 110, 610 to deliver oxidant directly to the dark zone 110, 610, such as horizontally from locations arranged in its side walls.

In one preferred embodiment, the above described reallocation of the oxidant from the heating zones 120, 130, 140, 620, 630, 640 to the dark zone 110, 610 results in that a gas volume flow (number of molecules) passing into the dark zone 110, 610 remains the same or substantially the same, while the gas mass flow decreases due to the reallocated oxidant.

In an alternative embodiment, the respective temperature in the heating zones 120, 130, 140, 620, 630, 640 in question is maintained, as compared to before said reallocation of oxidant supply, by increasing firing power in the heating zone 120, 130, 140, 620, 630, 640. For instance, this may encompass increasing the amount of provided fuel per time unit. It has turned out that the net effect, at constant power, in general is positive by performing the reallocation of oxidant according to the present invention, even if the temperature in the heating zone 120, 130, 140, 620, 630, 640 is thus maintained.

Claims

1. Method for heating a furnace (100, 600) with a longitudinal direction (D) and a cross plane (C) which is perpendicular to the longitudinal direction (D), which furnace (100, 600) is arranged with at least one heating zone (120, 130, 140, 620, 630, 640) which is heated using combustion of a fuel with an oxidant, and which furnace (100, 600) is further arranged with a dark zone (110, 610) downstream of said heated zone (120, 130, 140, 620, 630, 640), to which dark zone (110, 610) no fuel is supplied directly, c h a r a c t e r i s e d i n that the fuel and oxidant supplied to the heating zone (120, 130, 140, 620, 630, 640) is substoichiometric, in that between 10% and 40% of the total oxidant for achieving stoichiometric or near stoichiometric combustion is supplied directly to the dark zone (110, 610), in that a flue gas temperature is measured in and/or downstream of the dark zone (110, 610), and in that the share of the total oxidant supplied to the dark zone (110, 610) is controlled so as not to exceed a predetermined maximum measured such temperature.

2. Method according to claim 1, characterised in that all oxidant supplied to the dark zone (110, 610) is supplied via one or more lances (151,152,153,154) located in the side walls of the furnace (100, 600).

3. Method according to claim 1 or 2, c h a r a c t e r i s e d i n that all fuel and all oxidant supplied to the heating zone are supplied via one or more burners and/or via one or more lances located in one or more side walls of the furnace.

4. Method according to claim 3, c h a r a c t e r i s e d in that a portion of the oxidant supplied to the heating zone is supplied via at least one lance.

5. Method according to any of the preceding claims, c h a r a c t e r i s e d i n that the share of the total oxidant supplied to the dark zone (110, 610) is regulated so as to achieve a predetermined measured such temperature.

6. Method according to any of the preceding claims, c h a r a c t e r i s e d i n that the total amount of oxidant supplied to the heating zone (120, 130, 140, 620, 630, 640) and the dark zone (110, 610) is regulated so as to achieve a predetermined heating power.

7. Method according to any of the preceding claims, c h a r a c t e r i s e d i n that the oxidant supplied to the heating zone (120, 130, 140, 620, 630, 640) and to the dark zone (110,610) are supplied from the same source (166).

8. Method according to any one of the preceding claims, c h a r a c t e r i s e d i n that the oxidant supplied to the heated zone (120, 130, 140, 620, 630, 640) and/or to the dark zone (110, 610) comprises at least 85% oxygen, and preferably is industrially pure oxygen.

9. Method according to any one of the preceding claims, c h a r a c t e r i s e d i n that the oxidant is supplied to the dark zone (110, 610) via at least one oxidant lance (151,152,153,154) operated at a lancing velocity of least at Mach 1.

10. Method according to any one of the preceding claims, c h a r a c t e r i s e d i n that the oxidant supplied in the dark zone (110, 610) is supplied to the 35% most upstream arranged part of the dark zone (110, 610).

11. Method according to any one of the preceding claims, c h a r a c t e r i s e d i n that the oxidant supplied to the dark zone (110, 610) is supplied through at least one lance (151,152,153,154) which is substantially parallel to the cross plane (C).

12. Method according to any of the preceding claims, c h a r a c t e r i s e d i n that the oxidant supplied to the dark zone (110, 610) is supplied via at least two lances (151,152,153,154) on either side of the furnace (100, 600), so that the two lanced streams (155) of oxidant either intersect or cooperate so as to give rise to a rotation of the dark zone (110,610) atmosphere.

13. Method according to any of the preceding claims, c h a r a c t e r i s e d i n that each of said lances (151 ,152,153,154;200) are arranged in a respective tube (210), through which tube (210) cooling air is supplied surrounding the respective lance envelope surface (211 ).

14. Method for retrofitting an existing furnace (100, 600) for operation according to any one of the preceding claims, which existing furnace (100, 600) has a longitudinal direction (D) and a cross plane (C) which is perpendicular to the longitudinal direction (D), which existing furnace (100, 600) is arranged with at least one heating zone (120, 130, 140, 620, 630, 640) which is heated using combustion of a fuel with an oxidant, and which existing furnace (100, 600) is further arranged with a dark zone (110, 610) downstream of said heated zone (120, 130, 140, 620, 630, 640), to which dark zone (110, 610) no fuel is supplied directly, c h a r a c t e r i s e d i n that the method comprises

providing a separate oxidant lance (151 ,152,153,154) arranged to provide oxidant directly to the dark zone (110, 610);

connecting the separate oxidant lance (151 ,152,153,154) to a source (166) of oxidant;

modifying the furnace (100, 600) to supply the fuel and oxidant supplied to the heating zone (120, 130, 140, 620, 630, 640) substoichiometrically to provide between 10% and 40% of the total oxidant for achieving stoichiometric or near stoichiometric combustion by supplying oxidant directly to the dark zone (110, 610);

providing a flue gas temperature sensor (168b) arranged to measure the flue gas temperature in and/or downstream of the dark zone (110, 610); and modifying the furnace (100, 600) to control the share of the total oxidant supplied to the dark zone (110, 610) so as not to exceed a predetermined maximum measured such temperature.

15. Heating furnace (100, 600) with a longitudinal direction (D) and a cross plane (C) which is perpendicular to the longitudinal direction (D), which furnace (100, 600) is arranged with at least one heating zone (120, 130, 140, 620, 630, 640) which is heated using combustion of a fuel with an oxidant, and which furnace (100, 600) is further arranged with a dark zone (110, 610) downstream of said heated zone (120, 130, 140, 620, 630, 640), to which dark zone (110, 610) no fuel is arranged to be supplied directly, c h a r a c t e r i s e d i n that the furnace (100, 600) is arranged to supply fuel and oxidant to the heating zone substoichiometrically, in that the furnace (100, 600) is arranged to supply between 10% and 40% of the total oxidant for achieving stoichiometric or near stoichiometric combustion directly to the dark zone (110, 610), in that the furnace (100, 600) comprises a flue gas temperature sensor (168b) arranged to measure a flue gas temperature in and/or downstream of the dark zone (110, 610), and in that the furnace (100, 600) is arranged to control the share of the total oxidant supplied to the dark zone (110, 610) so as not to exceed a predetermined maximum measured such temperature.

16. Heating furnace according to claim 15, c h a r a c t e r i z e d i n that one or more burners are arranged in the side walls of said at least one heating zone and that all fuel supplied to the heating zone is supplied via the burners.