METHOD FOR HEATING A FURNACE

Abstract

The invention relates to a method for heating a furnace, wherein the furnace comprises an inner chamber and a outer chamber. A process gas is introduced into the inner chamber. A fuel is combusted with an oxidant to produce combustion gases in a combustion chamber. The combustion gases are passed through the outer chamber, and recirculated to the combustion chamber. The process gas is pre-heated by indirect heat exchange with the combustion gases.

Claims

1. A method for heating a furnace, wherein the furnace comprises an inner chamber and an outer chamber, wherein the outer chamber at least partly surrounds the inner chamber, wherein a fuel is combusted with an oxidant to produce combustion gases, wherein the combustion gases are passed through the outer chamber, wherein the fuel is combusted with the oxidant in a combustion chamber which is external to the furnace, in that the combustion gases are recirculated from the outer chamber to the combustion chamber.

2. The method according to claim 1, wherein a process gas is introduced into the inner chamber and that the process gas is pre-heated by indirect heat exchange with the combustion gases.

3. The method according to claim 1, wherein the oxidant comprises at least 30% by volume oxygen, at least 50% by volume oxygen, or at least 80% by volume oxygen, preferably more than 90% by volume oxygen, preferably more than 98% by volume oxygen.

4. The method according to claim 1, wherein the combustion of the fuel and the oxidant is flameless.

5. The method according to claim 1, wherein the combustion gases are recirculated by means of a fan.

6. The method according to claim 4, wherein the combustion gases are cooled upstream of the fan.

7. The method according to claim 1, wherein the fuel and/or the oxidant are pre-heated in indirect heat exchange with the combustion gases.

8. The method according to claim 1, wherein the pressure of the recirculated combustion gases is controlled by withdrawing a part of the combustion gases from the recirculated combustion gas stream.

9. The method according to claim 1, wherein the temperature of the recirculated combustion gases is controlled by adding combustion gases in the combustion chamber.

10. The method according to claim 1, wherein the outer chamber has two or more inlet openings and that the combustion gases are distributed to at least two of the inlet openings.

11. The method according to claim 1, wherein the furnace is a bell furnace with a base, an inner hood and an outer hood and wherein the base and the inner hood define the inner chamber and wherein the base and the outer hood define the outer chamber.

12. The method according to claim 1, wherein the fuel is continuously combusted with the oxidant or in that fuel and the oxidant are intermittently combusted with each other.

Description

[0033] The invention as well as further details of the invention shall be explained with reference to

[0034] FIG. 1 which shows a bell furnace heated according to the invention.

[0035] FIG. 1 shows a bell furnace 1 which comprises an inner hood 2, an outer hood 3 and a base 4. The base 4 and the inner hood 2 form an inner chamber 5. A part 7 to be heat-treated, for example a coil of metal strip, is placed in the inner chamber 5. The goods or parts 7 may be annealed or subjected to another heat treatment process, preferably in a defined process atmosphere. Therefore, the inner chamber 5 is provided with a process gas inlet 8 and a process gas outlet 9. A process gas, for example an inert gas such as nitrogen, is introduced into the inner chamber 5 via process gas inlet 8 in order to generate inside the inner chamber 5 a defined process gas atmosphere with a defined pressure and composition.

[0036] The space between the inner hood 2 and the outer hood 3 is used as outer chamber 6. By passing a hot gas through the outer chamber 6 the inner chamber 5 can be indirectly heated.

[0037] A combustion chamber 10 is provided with a burner 11 which is supplied with a fuel 12 and an oxidant 13. The oxidant 13 has preferably an oxygen content of at least 80% by volume or at least 90% by volume. In a preferred embodiment the burner 11 is an oxy-fuel burner and the oxidant is technically pure oxygen.

[0038] The combustion chamber 10 is separate from and external to the furnace 1. The combustion chamber 10 is connected with the outer chamber 6 by means of a pipeline 14 and a pipeline 15. The hot combustion gases produced by the burner 11 can be circulated from the combustion chamber 10 through pipeline 15 into the outer chamber 6 and then recirculated via pipeline 14 to the combustion chamber 10. A pump, a fan or a compressor 16 is provided in the pipeline 14 to circulate the combustion gases through the recirculation circuit 15, 6, 14, 10. The pressure of the recirculated combustion gases is preferably controlled by withdrawing a part of the combustion gases from the outer chamber 6. The outer chamber 6 is provided with an outlet 17 and a damper 18 which opens when the pressure exceeds a certain limit.

[0039] The pipeline 14 is further provided with a heat exchanger 19. A process gas supply line is passed through the heat exchanger 19 and connected to the process gas inlet 8.

[0040] In operation a fuel 12 and an oxidant 13, in particular pure oxygen, are reacted in the burner 11 to generate combustion gases 21. The hot combustion gases 21 are then passed through the outer chamber 6, through the heat exchanger 19 and recirculated back to the combustion chamber 10.

[0041] In the outer chamber 6 the combustion gases 21 are in heat exchange with the inner hood 2 and indirectly with the process gas atmosphere in the inner chamber 5. The process gas atmosphere in the inner chamber 5 is heated by the combustion gases 21 to a pre-defined temperature. The pump, fan or compressor 16 circulates the combustion gases 21 through the recirculation cycle 10, 15, 6, 14.

[0042] The hot combustion gases 21 are passed from the outer chamber 6 through the heat exchanger 19. Within the heat exchanger 19 the combustion gases 21 are in indirect heat exchange with the process gas which is passed through the process gas supply line 20 and introduced into the inner chamber 5. Thereby, during start-up of the heat treatment process it is possible to speed up the temperature increase in the inner chamber 5 and during the heat treatment process any process gas entering the inner chamber 5 is already pre-heated so that the temperature uniformity in the inner chamber 5 is increased.

[0043] The burner 11 operates continuously at a low rate in order to add a small amount of hot combustion gases to the recirculated combustion gases 21 so that the temperature of the combustion gases 21 remains within the desired range. It is also possible to operate the burner 11 intermittently and to combust the fuel 12 and the oxidant 13 only when the temperature of the recirculated combustion gases has fallen below a certain level and when additional heat shall be added to the recirculated combustion gases 21.