COMBUSTION PROCESS

Abstract

Combined combustion and post-combustion method whereby flue gas is generated by combustion in a main combustion zone, the flue gas being evacuated from the main combustion zone and introduced into a post-combustion zone where the flue gas is subjected to post-combustion and post-combusted gas is obtained which is evacuated from the post-combustion zone, whereby a first level of one or more combustible substances in the flue gas evacuated from the main combustion zone and/or a second level of one or more combustible substances in the post-combusted gas evacuated from the post-combustion zone is/are monitored, whereby a control signal is generated on the basis of the monitored level(s) and whereby the post-combustion oxidant injection rate or the stoichiometric excess of post-combustion-oxidant with respect to post-combustion fuel is regulated in function of said control signal.

Claims

1. A combined combustion and post-combustion method comprising: a) defining a nominal post-combustion operation mode for a post-combustion zone with either: (1) a nominal post-combustion-oxidant injection rate into the post-combustion zone, when the post-combustion zone is an oxidant-only post-combustion zone which is not equipped for the injection of post-combustion fuel therein, or (2) a nominal post-combustion-oxidant injection rate and a nominal post-combustion-fuel injection rate into the post-combustion zone, when the post-combustion zone is an oxidant-fuel post-combustion zone which is equipped for the injection of both post-combustion oxidant and post-combustion fuel therein, the nominal post-combustion-oxidant injection rate and the nominal post-combustion-fuel injection rate defining a nominal stoichiometric excess of the post-combustion oxidant with respect to the post-combustion fuel, b) supplying fuel and combustion oxidant to a main combustion zone at respectively an actual fuel supply rate and an actual oxidant supply rate, c) combusting the supplied fuel with the supplied oxidant in the main combustion zone, thus producing heat and flue gas, which may contain residual combustible matter, d) evacuating the flue gas from the main combustion zone and introducing the evacuated flue gas into the post-combustion zone, e) injecting, into the post-combustion zone: (1) post-combustion oxidant at an actual post-combustion-oxidant injection rate and no post-combustion fuel, when the post-combustion zone is an oxidant-only post-combustion zone, or (2) post-combustion oxidant at an actual post-combustion-oxidant injection rate and post-combustion fuel at an actual post-combustion-fuel injection rate, when the post-combustion zone is an oxidant-fuel post-combustion zone, the actual post-combustion-oxidant injection rate and the actual post-combustion-fuel injection rate defining an actual stoichiometric excess of post-combustion oxidant with respect to the post-combustion fuel, f) post-combusting the evacuated flue gas in the post-combustion zone with: (1) the post-combustion oxidant in the case of an oxidant-only post-combustion zone, or (2) the actual stoichiometric excess of post-combustion oxidant in the case of an oxidant-fuel post-combustion zone, thereby generating a post-combusted gas, g) evacuating the post-corn busted gas from the post-combustion zone, h) monitoring a first level of one or more combustible substances in the flue gas evacuated from the main combustion zone and/or a second level of one or more combustible substances in the post-combusted gas evacuated from the post-combustion zone, i) generating a first control signal on the basis of the level or on the basis of one or both of the levels monitored in step h), and j) regulating, in function of the first control signal: (1) the actual post-combustion-oxidant injection rate in the case of an oxidant-only post-combustion zone, or (2) the actual stoichiometric excess of post-combustion-oxidant through the actual post-combustion-oxidant injection rate and/or the actual post-combustion-fuel injection rate, in the case of an oxidant-fuel post-combustion zone.

2. The method according to claim 1, whereby the first control signal is generated on the basis of the second monitored level.

3. The method according to claim 2, whereby step a) comprises: i. defining an upper threshold B1up for the second monitored level, and whereby, when the second monitored level exceeds the upper threshold B1up, the generated first control signal causes, in step j) (1) the actual post-combustion-oxidant flow to be higher than the nominal post-combustion-oxidant flow, in the case of an oxidant-only post-combustion zone, or (2) the actual stoichiometric excess of post-combustion-oxidant to be greater than the nominal stoichiometric excess of post-combustion-oxidant, in the case of an oxidant-fuel post-combustion zone.

4. The method according to claim 2, whereby step a) further comprises: 1′. defining a lower threshold B1low for the second monitored level, and/or ii′. defining a lower threshold B2low for the second monitored level and a corresponding time period ΔtB, whereby, when the second monitored level is below threshold B1low or remains below lower threshold B2low during at least time period ΔtB, the generated first control signal causes in step j): (1) the actual post-combustion oxidant flow to equal to the nominal post-combustion-oxidant flow, in the case of an oxidant-only post-combustion zone or (2) the actual post-combustion oxidant flow and the actual post-combustion fuel flow to be equal to respectively the nominal post-combustion oxidant flow and the nominal post-combustion fuel flow in the case of an oxidant-fuel post-combustion zone.

5. The method according to claim 1, whereby the first control signal is generated on the basis of the first monitored level.

6. The method according to claim 5, whereby step a) further comprises: i. defining an upper threshold C1up for the first monitored level, and whereby, when the first monitored level exceeds the upper threshold C1up the generated first control signal causes, in step j) (1) the actual post-combustion-oxidant flow to be higher than the nominal post-combustion-oxidant flow, in the case of an oxidant-only post-combustion zone, or (2) the actual stoichiometric excess of post-combustion-oxidant to be greater than the nominal stoichiometric excess of post-combustion-oxidant, in the case of an oxidant-fuel post-combustion zone.

7. The method according to claim 5, whereby step a) comprises: i. defining a lower threshold C1low for the first monitored level, and/or ii. defining a lower threshold C2low for the first monitored level and a corresponding time period ΔtC, whereby, when the first monitored level is below threshold C1low or remains below lower threshold C2low during at least time period ΔtC, the generated first control signal causes in step j): (1) the actual post-combustion oxidant flow to equal to the nominal post-combustion-oxidant flow, in the case of an oxidant-only post-combustion zone, or (2) the actual post-combustion oxidant flow and the actual post-combustion fuel flow to be equal to respectively the nominal post-combustion oxidant flow and the nominal post-combustion fuel flow, in the case of an oxidant-fuel post-combustion zone.

8. The method according to claim 1, whereby the first control signal is generated on the basis of both the first and the second monitored level.

9. The method according to claim 8, whereby step a) comprises: i. defining an upper threshold C1′up for the first monitored level, and ii. defining an upper threshold B1′up for the second monitored level, and whereby when the first monitored level exceeds upper threshold C1′up or when the second monitored level exceeds upper threshold B1′up, the generated first control signal causes, in step j) (1) the actual post-combustion-oxidant flow to be higher than the nominal post-combustion-oxidant flow in the case of an oxidant-only post-combustion zone or (2) the actual stoichiometric excess of post-combustion-oxidant to be greater than the nominal stoichiometric excess of post-combustion-oxidant, in the case of an oxidant-fuel post-combustion zone.

10. The method according to claim 9, whereby step a) comprises: i. defining a lower threshold C1′low for the first monitored level and a lower threshold B1′ low for the second monitored level, and/or ii. defining a lower threshold C2′low for the first monitored level and a corresponding time period ΔtC′ and a lower threshold B2′low for the second monitored level and a corresponding time period ΔtB′, whereby, when the first monitored level is below lower threshold C1′low and the second monitored level is below threshold B1′low or when the first monitored level remains below lower threshold C2′low during at least time period ΔtC′ and the second monitored level remains below lower threshold B2′low during at least time period ΔtB′, the generated first control signal causes in step j): (1) the actual post-combustion oxidant injection rate to be equal to the nominal post-combustion-oxidant injection rate, in the case of an oxidant-only post-combustion zone (19), or (2) the actual post-combustion oxidant injection rate and the actual post-combustion fuel injection rate to be equal to respectively the nominal post-combustion oxidant injection rate and the nominal post-combustion fuel injection rate, in the case of an oxidant-fuel post-combustion zone.

11. The method according to claim 3, whereby step a) further comprises defining a predetermined duration Δtpcboost and whereby, when, in step j) (1) the actual post-combustion-oxidant flow has been higher than the nominal post-combustion-oxidant flow, in the case of an oxidant-only post-combustion zone or (2) the actual stoichiometric excess of post-combustion-oxidant has been greater than the nominal stoichiometric excess of post-combustion-oxidant, in the case of an oxidant-fuel post-combustion zone, for a period Δtpcboost, a first control signal is generated in step i) which causes in step j) (1) the actual post-combustion oxidant injection rate to be equal to the nominal post-combustion-oxidant injection rate, in the case of an oxidant-only post-combustion zone, or (2) the actual post-combustion oxidant injection rate and the actual post-combustion fuel injection rate to be equal to respectively the nominal post-combustion oxidant injection rate and the nominal post-combustion fuel injection rate, in the case of an oxidant-fuel post-combustion zone.

12. The method according to claim 1, whereby: step a) includes defining a nominal main combustion operation mode for the main combustion zone with a nominal fuel supply rate and a nominal oxidant supply rate to the main combustion zone, step i) includes generating a second control signal on the basis of the first level and/or second level monitored in step h), and whereby the method further comprises: a step k) of regulating the actual oxidant supply rate and the actual fuel supply rate to the main combustion zone in function of the second control signal.

13. The method according to claim 12, whereby in step i) the second control signal is generated on the basis of the first monitored level.

14. The method according to claim 13, whereby step a) further comprises: i. defining an upper threshold A1up for the first monitored level, and/a ii. defining a positive upper threshold RA1up for a rate of change of the first monitored level, and whereby when the first monitored level exceeds the upper threshold A1up, and/or when the first monitored level increases at a rate greater than upper threshold RA1up, the generated second control signal causes, in step k), the actual oxidant supply rate and the actual fuel supply rate to the main combustion zone to be regulated so that the ratio between the actual oxidant supply rate and the actual fuel supply rate is higher than the ratio between the nominal oxidant supply rate and the nominal fuel supply rate.

15. The method according to claim 13, whereby step a) comprises: i. defining a lower threshold A1low for the first monitored level, and/or defining a lower threshold A2low for the first monitored level and a corresponding time period ΔtA, and whereby: when the first monitored level is below lower threshold A1low and/or when the first monitored level remains below lower threshold A2low for at least the time period ΔtA, the generated second control signal causes, in step k), the actual oxidant supply rate and the actual fuel supply rate to the main combustion zone to be regulated so that the actual oxidant supply rate and the actual fuel supply rate correspond respectively to the nominal oxidant supply rate and the nominal fuel supply rate.

16. The method according to claim 12, whereby in step i) the second control signal is generated on the basis of the second monitored level.

17. The method according to claim 16, whereby step a) comprises: i. defining an upper threshold D1up for the second monitored level, and whereby, when the second monitored level exceeds the upper threshold D1up, the generated second control signal causes, in step k), the actual oxidant supply rate and the actual fuel supply rate to the main combustion zone to be regulated so that the ratio between the actual oxidant supply rate and the actual fuel supply rate is higher than the ratio between the nominal oxidant supply rate and the nominal fuel supply rate.

18. The method according to claim 17, whereby step a) further comprises: ii. defining a lower threshold D1low for the second monitored level, and/or iii. defining a lower threshold D2low for the second monitored level and a corresponding time period ΔtD, whereby, when the second monitored level is below threshold B1low or remains below lower threshold B2low during at least time period ΔtB, the generated second control signal causes in step k), the actual oxidant supply rate and the actual fuel supply rate to the main combustion zone to be regulated so that the actual oxidant supply rate and the actual fuel supply rate correspond respectively to the nominal oxidant supply rate and the nominal fuel supply rate.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0144] The present invention and its advantages are illustrated in the following example with reference to the figure, which is a schematic presentation of an industrial furnace equipped with a post combustor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0145] The figure shows a furnace 10, such as a furnace for melting scrap aluminium, whereby the scrap aluminium may be contaminated with combustible contaminants, such as paint and lacquer on aluminium cans and oil.

[0146] Furnace 10 defines a main-combustion zone therein, which is heated by the combustion of fuel with oxidant, referred to as “main combustion”.

[0147] Thereto, furnace 10 is equipped with one or more burners 12 (only one burner is shown, even though multiple burners may be present) fluidly connected to a fuel source 13 and an oxidant source 14. The oxidant supplied by oxidant source 14 is preferably an oxygen-rich oxidant (i.e. an oxidant having an oxygen content higher than that of ambient air), such as oxygen-enriched air or oxygen.

[0148] The fuel and the oxidant are supplied to the one or more burners 12 in a controlled manner, i.e. at a regulated flow rate. The main combustion of the fuel with the oxidant in furnace 10 generates heat and combustion gases inside the main-combustion zone of furnace 10 (said main combustion being schematically represented by flame 11, even though said combustion may be in the form of multiple flames or flameless combustion).

[0149] Instead of or in combination with burners 12, which inject both fuel and oxidant, fuel and oxidant may also be supplied separately to the main-combustion zone, for example for the purpose of staged or flameless combustion.

[0150] In the case of a charge 15 of scrap aluminium to be melted in the main-combustion zone, a reducing atmosphere 16 is desired above charge 15 so as to limit any loss of aluminium metal due to oxidation. A less-than-stoichiometric amount of oxidant (compared to the amount of fuel) is therefore supplied to the burner(s) 12. As a consequence, the flue gas 17 which is evacuated from furnace 10 contains combustible substances.

[0151] As the charge 15 of contaminated aluminium is heated, combustible contaminants are typically also released by charge 15 into the atmosphere 16 of the main combustion zone in an uncontrolled manner, i.e. with peaks and dips in the amount of combustibles released. Said released combustible contaminants contribute to the level of combustible substances in flue gas 17.

[0152] Other furnaces operate with a neutral (i.e. an atmosphere which is neither oxidizing, nor reducing) or with an oxidizing atmosphere in the main-combustion zone. In such a case, the baseline of the level of combustible substances in the flue gas from the main combustion zone is typically zero or near zero, while occasional peaks of combustible substances may be observed in the evacuated flue gas.

[0153] In step a) of the method according to the invention, a nominal main combustion operation mode is defined for the main combustion zone with a corresponding nominal fuel supply rate and a corresponding nominal oxidant supply rate to the main combustion zone via its burner(s) 12.

[0154] For example, in the case of a batch scrap aluminium melting process, the furnace operation may comprise an initial heating phase, in which the solid charge 15 is heated to the aluminium melting temperature, a melting phase, during which the solid charge 15 is progressively melted, and a refining phase, during which the molten charge 15 is refined and then maintained at its tapping temperature. For each phase a constant or evolving nominal fuel supply rate and nominal oxidant supply rate to furnace 10 are defined. As explained above, in order to avoid oxidation of the aluminium charge 15, the nominal oxidant supply rate may be kept substoichiometric with respect to the nominal fuel supply rate, in particular during the melting and refining phase of the process (as the molten charge is more susceptible to oxidation).

[0155] The flue gas 17 which has been evacuated from the main combustion zone of furnace 10 is transported via conduct 18 to post combustor/post-combustion zone 19. Post-combustion oxidant and post-combustion fuel are injected into post-combustion zone 19 in a controlled manner (i.e. at regulated flow rates) in order to combust combustible substances present in the evacuated flue gas 17 with a controlled stoichiometric excess of post-combustion oxidant with respect to the post-combustion fuel. The thus obtained post-combusted gas 23 is evacuated from post-combustion zone 19.

[0156] In the illustrated embodiment, the post combustor 19 is equipped with a burner 20 and a separate post-combustion oxidant injector 25. The post-combustion fuel is supplied to burner 20 together with a stoichiometric amount of post-combustion oxidant. Additional post-combustion oxidant is supplied to injector 25, so as to provide a stoichiometric excess of post-combustion oxidant (compared to the post-combustion fuel) in post combustor 19. Again, whereas only one post-combustion burner 20 and one injector 25 are shown, post combustor 19 may be equipped with multiple burners 20 and/or multiple injectors 25. The use of multiple injectors 25 may in particular be useful to ensure intimate mixing of the stoichiometric excess of post-combustion oxidant with flue gas 17 entering post combustor 19.

[0157] In the illustrated embodiment, post-combustion zone 19 is separated from the main-combustion zone by conduct 18 via which flue gas 17 is transported. In other embodiments, the main combustion zone and the post-combustion zone may be located in different parts of a same enclosure, the flue gas generated in the main combustion zone of the enclosure travelling to the post-combustion zone of said enclosure. In that case, the post-combustion zone is advantageously located above the main combustion zone, so as to benefit from the natural upward draft of the generated flue gas.

[0158] In the illustrated example, the level of combustible substances, such as H2, CO and/or VOCs, in evacuated flue gas 17 in conduct 18 is determined using sensor 21.

[0159] In the illustrated embodiment, sensor 21 is a sensor as described in co-pending patent application ER21152977. The disclosure in said co-pending patent application of said sensor and its operation is incorporated herein by reference. Valve 103 controls the flow of oxidant from oxidant source 14 is supplied to sensor 21.

[0160] Other sensors and monitoring devices and methods for monitoring levels of combustible substances in flue gas 17 are commercially available and may be used in the context of the present invention.

[0161] The level of combustible substances in evacuated flue gas 17 detected by sensor 21 is transmitted to central control unit 22 and compared with a reference value stored therein. Said reference value corresponds to the level of combustible substances which flue gas 17 would normally be expected to present at the given phase of the batch process and operation parameters, including actual oxidant supply rate and fuel supply rate to burner(s) 12 of furnace 10.

[0162] When said comparison by control unit 22 reveals that the detected level of combustible substances in flue gas 17 is significantly higher than the reference level, this is indicative of a peak in the release of combustibles by charge 15 in the main combustion zone.

[0163] The level of combustible substances in flue gas 17 detected by sensor 21 is, for example, considered by control unit 22 to be significantly higher than the reference value when the detected level of combustible substances in flue gas 17 is higher than an upper threshold value A1up also defined in step a), whereby threshold value A1up is greater than the reference value.

[0164] In that case, control unit 22 generates a control signal, referred to as ‘second control signal’, which regulates main fuel controller 101 and main oxidant controller 102 so that the furnace is operated in boosted main-combustion operation mode, whereby the ratio between the actual oxidant supply rate and the actual fuel supply rate to burner(s) 12 exceeds the ratio of the nominal oxidant supply rate to the nominal fuel supply rate. In this manner, more oxygen is made available within the main-combustion zone for the combustion of the released combustible matter within said main combustion zone, without generating an oxidizing atmosphere 16 above charge 15.

[0165] According to one embodiment, after a predetermined duration Δtmcboost, a different second control signal is emitted so that main fuel controller 101 causes the actual fuel supply rate to burner(s) 12 to correspond to the nominal fuel supply rate and so that the main oxidant controller 102 causes the actual oxidant supply rate to the burner(s) 12 to correspond to the nominal oxidant supply rate.

[0166] Alternatively, the return to nominal main combustion may be based on the level of combustible substances in evacuated flue gas 17 detected by sensor 21. For example, when the comparison by central control unit 22 between the detected level of combustible substances in evacuated flue gas 17 sensor 21 is substantially equal to or even lower than the reference value stored in central control unit 22, control unit 22 generates a second control signal which, in the manner described above, causes the main combustion in furnace 10 to return to the nominal main-combustion operation mode.

[0167] Whereas the above-described regulation of the main combustion can keep the levels of combustible substances in flue gas 17 leaving furnace 10 within certain limits, it does not as such solve the problem of the release into the atmosphere of the detected levels of combustible substances in evacuated flue gas 17.

[0168] This problem is addressed by subjecting evacuated flue gas 17 to post combustion in post-combustion zone 19.

[0169] When the temperature of flue gas 17 entering post-combustion zone 19 and the nature and concentration of combustible substances in flue gas 17 is such that ignition of the combustible substances is assured, the post combustion of flue gas 17 can be achieved by the injection of only post-combustion oxidant into post-combustion zone 19, for example via injector 25.

[0170] In many cases, both post-combustion oxidant and post-combustion fuel will be injected into post-combustion zone 19 to create a permanent post-combustion flame in zone 19. The post combustion of combustible substances in flue gas 17 is then achieved in post-combustion zone 19 by a stoichiometric excess of post-combustion oxidant with respect to the post-combustion fuel. In the illustrated embodiment, the post-combustion fuel and the corresponding stoichiometric amount of post-combustion oxidant are supplied to burner 20, while the excess of post-combustion oxidant for the post-combustion of the flue gas is injected into the post-combustion zone 19 by means of injector 25.

[0171] In step a) of the method according to the invention, a nominal post-combustion operation mode is defined for post-combustion zone 19, with a corresponding nominal post-combustion oxidant injection rate and nominal post-combustion fuel injection rate. The total nominal post-combustion oxidant injection rate and the nominal post-combustion fuel injection rate and in particular the ratio between the two are defined on the basis of the temperature and composition which the evacuated flue gas 17 would normally have when entering post combustor/post-combustion zone 19, given the phase and parameters of the batch process in furnace 10, and keeping in mind any limitations to the level of combustible substances which a gas to be released into the atmosphere imposed by, for example, environmental regulations. In other words, the nominal post-combustion oxidant and fuel injection rates are such that they would result in the effective abatement by post-combustion of the combustible substances in flue gas 17 at the level normally to be expected at the given stage of operation of furnace 10.

[0172] In the illustrated embodiment, a further control signal, referred to as ‘first control signal’, is generated by control unit 22 on the basis of the above-described comparison between the level of combustible substances in evacuated flue gas 17 detected by sensor 21 and the reference value stored in central control unit 22. Said first control signal is sent to first flow controller 104, which regulates the excess of post-combustion oxidant to injector 25.

[0173] The flow of post-combustion fuel and the corresponding stoichiometric flow of post-combustion oxidant to post-combustion burner 20 are regulated respectively by controller 106 and controller 105.

[0174] When the comparison by control unit 22 reveals that the detected level of combustible substances in flue gas 17 is significantly higher than the reference value, for example by comparing the detected level with an upper threshold level C1up defined in step a), control unit 22 generates a first control signal, which causes first flow controller 104 to increase the excess of post-combustion oxidant to injector 25 to a boosted excess of post-combustion oxidant, which is higher than the excess of post-combustion oxidant during the nominal post-combustion operation mode, so that the ratio of the actual total post-combustion oxidant injection rate to the actual post-combustion fuel injection rate is higher than the ratio of the total nominal post-combustion oxidant injection rate to the nominal post-combustion fuel injection rate. In this manner, extra available post-combustion oxidant is made available for post-combusting the peak in combustible substances present in evacuated flue gas 17 (boosted post-combustion operation mode).

[0175] The criteria (such as C1up) for a “significantly higher” level for the regulation of the post-combustion may be the same as or different from the criteria (such as A1up) for a “significantly higher” level for the regulation of the main combustion.

[0176] When the comparison by central control unit 22 between the level of combustible substances in evacuated flue gas 17 detected by sensor 21 is substantially equal to or even lower than the corresponding reference value stored in central control unit 22, whereby again, the same or different criteria may be applied for “substantially equal” with respect to the regulation of the main combustion and the regulation of the post-combustion, the first control signal generated by control unit 22 causes second flow controller 104 to regulate the actual post-combustion oxidant injection rate to injector 25, so that the actual stoichiometric excess of post-combustion oxidant injected into post-combustion zone 19 corresponds to the nominal stoichiometric excess of post-combustion oxidant (the actual post-combustion fuel flow to burner 20 and the actual post-combustion oxidant flow to burner 20 being regulated, also on the basis of the first control signal, by respectively controllers 106 and 105 so that the actual post-combustion fuel flow to burner 20 corresponds to the nominal post-combustion fuel flow rate and so that post-combustion oxidant flow to burner 20 is stoichiometric with said actual/nominal post-combustion fuel flow).

[0177] In the illustrated embodiment, an extra security has been provided for the control of the post-combustion in post-combustion zone 19, in that a second sensor 24, of the same type as first sensor 21, is present in the exhaust of post combustor 19 and monitors a level of one or more combustible substances in the post-combusted gas 23 leaving post-combustion zone 19.

[0178] An upper threshold B1up for said level of combustible substance(s) in post-combusted gas 23 was determined in step a), said upper threshold B1up being higher than a reference value for the level of the monitored combustible substance(s) in post-combusted gas 23. Said reference value corresponds to the level of combustible substances which post-combusted gas 23 would normally be expected to present at the given phase of the batch process and the operation parameters of the main and of the post-combustion zone.

[0179] Upper threshold B1up is also at most equal to or preferably below the maximum level of said combustible substance(s) permitted by the local environmental regulations.

[0180] Control unit 22 compares the level detected by second sensor 24 with upper threshold B1up and, when the level detected by second sensor 24 is higher than the threshold B1up, control unit 22 generates a first control signal which causes the post-combustion zone 19 to operate in boosted post-combustion operation as described above, and this regardless of the level detected by first sensor 21.

[0181] Similarly, when the level detected by first sensor 21 is higher than the threshold level C1up, central control unit 22 generates a first control signal which causes the post-combustion zone 19 to operate in boosted post-combustion operation, regardless of the level detected by first sensor 24.

[0182] Only when both the level detected by first sensor 21 and the level detected by second sensor 24 are substantially equal to or even lower than the corresponding reference values stored in central control unit 22 does control unit 22 generate a first control signal on the basis of which controllers 104, 105 and 106 regulate the post-combustion fuel flow and the total post-combustion oxidant flow to post-combustion zone 19 to correspond to the respective nominal flow rates, whereby, as described above, the post-combustion fuel and the stoichiometric flow of oxidant are supplied to burner 20, while the stoichiometric excess of post-combustion oxidant is supplied to injector 25.

[0183] Whereas the use of a central control unit 22 is illustrated, it will be appreciated that the method according to the invention may also be implemented with a distributed or modular system using PLCs or the like for comparing detected values with reference values and/or for regulating fuel and/or oxidant supplies.

[0184] The above-described embodiment provides a maximum control of the level of combustible substances in the post-combusted gas and thus of the level of combustible substances liable to be released into the atmosphere.

[0185] When the abatement of combustible substances in the post-combusted gas is less critical, a less elaborate control system may be used for the post-combustion, for example, based only on the level of combustible substances detected by one of sensors 21 and 24.

REFERENCE NUMBERS IN DRAWING

[0186] 10: furnace - 11: flame - 12: burner of furnace 10 - 13: fuel source - 14: oxidant source - 15: charge - 16: atmosphere in furnace 10 - 17: evacuated flue gas - 18: flue gas conduct - 19: post combustor/post-combustion zone - 20: burner of post combustor 19 - 21: first sensor - 22: central control unit - 23: post-corn busted gas - 24: second sensor - 25: oxidant injector of post combustor 19 - 101: controller of fuel flow to burner 12 - 102: controller of oxidant flow to burner 12 - 103: controller of oxidant flow to first sensor 21 - 104: controller of oxidant flow to injector 25 - 105: controller of oxidant flow to burner 20 - 106: controller of fuel flow to burner 20 - 107: controller of oxidant flow to second sensor 24.

[0187] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.