Combustion process

Abstract

To provide a combined combustion and post-combustion process whereby both the combustion in a main combustion zone and the post-combustion in a post-combustion zone are optimized.SOLUTION: Provided is a combined combustion and post-combustion method whereby flue gas is generated by combustion in a main combustion zone 10, the flue gas 17 being evacuated from the main combustion zone 10 and introduced into a post-combustion zone 19 where the flue gas 17 is subjected to post-combustion and post-combusted gas 23 is obtained which is evacuated from the post-combustion zone 19, whereby a first level of one or more combustible substances in the flue gas 17 evacuated from the main combustion zone 10 and / or a second level of one or more combustible substances in the post-combusted gas 23 evacuated from the post-combustion zone 19 are monitored, whereby a control signal is generated on the basis of monitored levels and whereby a post-combustion oxidant injection rate or a stoichiometric excess of post-combustion-oxidant with respect to a post-combustion fuel is adjusted according to the control signal.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to the field of combustion, more particularly to a combustion process in which fuel is combusted with an oxidizer in a combustion zone of a furnace, hereinafter referred to as the main combustion zone, and the flue gases leaving said combustion zone are subjected to post-combustion in a post-combustion zone downstream of the main combustion zone in the direction of flow of the flue gases. [Background technology]

[0002] For maximum heat production in the main combustion zone, it is generally preferable to achieve complete combustion within the main combustion zone, in which case the flue gases from the combustion zone are free of residual combustibles.

[0003] To achieve complete combustion in the primary combustion zone, the combustion oxidant must be supplied to the primary combustion zone in at least a stoichiometric amount relative to the fuel. In actual practice, a slight excess of oxidant relative to the stoichiometric amount is required to achieve complete combustion. A large excess of oxidant also allows for complete combustion, but dilutes the combustion and therefore reduces the flame temperature. Furthermore, when oxygen-enriched air or oxygen is used as the combustion oxidant, a large excess of oxidant also increases the costs associated with supplying the oxidant to the process.

[0004] In practice, complete combustion in the main combustion zone is not always possible or even desirable.

[0005] For example, in some processes, a reducing atmosphere in the furnace is required to avoid undesirable oxidation of the charge being heated in the main combustion zone, as is often the case in furnaces for melting non-ferrous metals. When a reducing atmosphere is required, operating the main combustion zone with excess oxygen is clearly excluded.

[0006] Furthermore, in some processes, such as waste incineration facilities or secondary aluminum melting furnaces where contaminated aluminum, such as painted beverage cans, is melted, combustibles are released, at least in part, by the charge in uncontrolled states and amounts, and as a result, it is not possible to accurately predict the amount of oxidizer that will correspond to the stoichiometric amount at different times throughout the process.

[0007] To optimize the combustion process in such furnaces, monitoring methods have been proposed that monitor changes in the amount of combustibles released by the charge in the main combustion zone and allow the furnace operator to respond quickly by adjusting the controlled supply of combustion oxidizer and / or fuel to the main combustion zone. Examples of such monitoring processes are described in ES-A-2201885, ES-A-2207389, EP-A-0949477, WO-A-2010022964, WO-A-2011131880, and JP 2021-025687.

[0008] These methods respond to actual changes in the amount of combustibles emitted or the amount of residual combustibles in the flue gas and therefore may reduce the level of such residual combustibles, but may not completely prevent their presence, particularly, but not exclusively, when the main combustion zone is operated in a reducing atmosphere.

[0009] Blowing residual combustibles into the atmosphere is often not permitted for environmental reasons. Furthermore, venting residual combustibles from the process represents a loss of heat energy that would have been generated if all the combustibles had been completely burned.

[0010] For these reasons, it is becoming increasingly common to use post combustors in which residual combustibles present in the flue gas are completely or substantially completely combusted with a post-combustion oxidizer before the flue gas is released into the atmosphere.

[0011] Post-combustion of flue gas can be achieved by injecting only post-combustion oxidizer into the post-combustion chamber, or by injecting both post-combustion oxidizer and post-combustion fuel into the post-combustion chamber with a stoichiometric excess of post-combustion oxidizer relative to the post-combustion fuel. Injecting both post-combustion oxidizer and post-combustion fuel into the post-combustion chamber is particularly useful when residual combustibles are present in the flue gas at relatively low concentrations and when injection of post-combustion fuel into the post-combustion chamber is required to raise the temperature so that the residual combustibles will ignite and burn.

[0012] Advantageously, the post-combustor comprises energy recovery means for recovering and utilizing the thermal energy produced by the post-combustion. Summary of the Invention

[0013] It is an object of the present invention to provide such a combined combustion and post-combustion process with optimized post-combustion.

[0014] It is a further object of the present invention to provide a combined combustion and post-combustion process in which combustion in the main combustion zone and post-combustion in the post-combustion zone are both optimized.

[0015] Furthermore, the present invention proposes a multi-step combined combustion and post-combustion method.

[0016] General Description In the first step of the method of the present invention, hereinafter referred to as "step a)," a nominal post-combustion operating mode is defined for the post-combustion zone. When the post-combustion zone is an oxidizer-only post-combustion zone, i.e., a post-combustion zone that is not equipped or adjusted to inject post-combustion fuel, but is equipped or adjusted only to inject post-combustion oxidizer, the defined nominal post-combustion operating mode indicates the nominal post-combustion oxidizer injection rate into the post-combustion zone.

[0017] On the other hand, when the post-combustion zone is an oxidizer-fuel post-combustion zone, i.e., a post-combustion zone equipped or adjusted to inject both a post-combustion oxidizer and a post-combustion fuel, the nominal post-combustion operating mode refers to both a nominal post-combustion oxidizer injection rate and a nominal post-combustion fuel injection rate into the post-combustion zone, where the nominal post-combustion oxidizer injection rate and the nominal post-combustion fuel injection rate are such that a nominal stoichiometric excess of post-combustion oxidizer over post-combustion fuel is defined.

[0018] In a further step of the method of the present invention, hereinafter referred to as step b), fuel and combustion oxidant are supplied to the main combustion zone. The rate at which fuel is supplied to the main combustion zone in step b) is referred to as the "actual fuel supply rate" and the rate at which combustion oxidant is supplied to the main combustion zone is referred to as the "actual oxidant supply rate."

[0019] In step c), the fuel thus supplied (i.e., fuel supplied to the main combustion zone at the actual fuel supply rate) is combusted in the main combustion zone with the oxidizer supplied (i.e., oxidizer supplied to the main combustion zone at the actual oxidizer supply rate), thereby producing heat and flue gases. As mentioned above, the flue gases may contain residual combustibles.

[0020] In step d) of the method, flue gases are discharged from the main combustion zone and introduced into a post-combustion zone.

[0021] In step e), an oxidizer required for the post-combustion of the discharged flue gases (called "post-combustion oxidizer") or a combination of post-combustion oxidizer and fuel (the latter called "post-combustion fuel") with a stoichiometric excess of post-combustion oxidizer over the post-combustion fuel is introduced into said post-combustion zone.

[0022] Thus, in step e), when the post-combustion zone is an oxidizer-only post-combustion zone, post-combustion oxidizer is injected into the post-combustion zone at the actual post-combustion oxidizer injection rate, and no post-combustion fuel is injected.

[0023] When the post-combustion zone is an oxidizer-fuel post-combustion zone, both a post-combustion oxidizer and a post-combustion fuel are injected into the post-combustion zone in step e). The post-combustion oxidizer is injected at an actual post-combustion oxidizer injection rate, and the post-combustion fuel is injected at an actual post-combustion fuel injection rate. The actual post-combustion oxidizer injection rate and the actual post-combustion fuel injection rate together define an actual stoichiometric excess of post-combustion oxidizer relative to the post-combustion fuel, which is not used for combustion of the post-combustion fuel and is therefore available for combustion of residual combustibles of the flue gas within the post-combustion zone.

[0024] In step f), the discharged flue gas is subjected to post-combustion in said post-combustion zone with a post-combustion oxidant, or an excess amount of a post-combustion oxidant, during which residual combustibles present in the flue gas discharged from the main combustion zone are combusted, resulting in post-combusted gas.

[0025] In step g), the gases thus post-combusted are discharged from the post-combustion zone.

[0026] In step h) of the method according to the invention, a first level of one or more combustible substances in the flue gases discharged from the main combustion zone and / or a second level of one or more combustible substances in the post-combustion gases discharged from the post-combustion zone is monitored.

[0027] Immediately thereafter, in step i), a first control signal is generated based on the (first or second) level monitored in step h), or based on one or both of the first and second levels monitored in step h).

[0028] In step j) the post-combustion in the post-combustion zone is adjusted.

[0029] When the post-combustion zone is an oxidizer-only post-combustion zone, the actual post-combustion oxidizer injection rate into the post-combustion zone is adjusted in step j) in response to the first control signal.

[0030] When the post-combustion zone is an oxidizer-fuel post-combustion zone, the actual stoichiometric excess amount of post-combustion oxidizer is adjusted in step j) in response to the first control signal via the actual post-combustion oxidizer injection rate and / or the actual post-combustion fuel injection rate.

[0031] The first control signal is thus generated for the purpose of regulating post-combustion, and so long as this is accomplished, the nature (digital (electronic) or analog (pneumatic, etc.)) or value of the first control signal is not important.

[0032] The monitoring in step h) may be monitoring of a single combustible substance in the corresponding gas stream or monitoring of multiple combustible substances in the gas stream. Examples of such combustible substances are H2, CO, and VOCs (volatile organic compounds). Various sensors for monitoring these combustible substances in gas streams are commercially available. The substance or substances to be monitored will be selected depending on the type of process taking place in the main combustion zone and the type and level of combustible substances that may be present in the flue gas emitted from the flue gas. The essential point in this regard is that the level of the monitored combustible substance or substances must be easily and quickly measurable and provide a clear indication of the completeness or level of incompleteness of the main combustion (when the emitted flue gas is monitored) or post-combustion (when the post-combusted gas is monitored). The substance or substances to be monitored may also be selected depending on environmental regulations, in particular to ensure that the level of the substance or substances in the post-combusted gas remains below the limit values ​​imposed by the environmental regulations.

[0033] The monitoring may be direct monitoring, in which the level of one or more substances in the gas stream is measured, for example, in situ, i.e., in the gas stream itself or through sampling. The monitoring may also be indirect monitoring of one or more substances, in which a property correlated to the level of one or more combustible substances is measured, such as the intensity of a flame produced when one or more combustible substances in the gas stream are brought into controlled contact with an oxidizer, for example, via an air gap or through controlled injection of an oxidizer such as air or oxygen into the monitored gas, as described, for example, in EP-A-2561295. Indirect monitoring via a property correlated to the level of one or more combustible substances may be based on the temperature or temperature change or evolution in the monitored gas when one or more combustible substances in the monitored gas stream are brought into controlled contact with such an oxidizer, as described, for example, in ES-A-2201885, ES-A-2207389, and WO-A-2006117336.

[0034] Combustion flue gases are frequently discharged from the main combustion zone at high temperatures. In that case, the flue gases contain a significant amount of residual heat. Furthermore, post-combustion of combustible materials in the discharged flue gases also generates heat. Therefore, a heat recovery facility can be advantageously provided within and / or downstream of the post-combustion zone. The thermal energy recovered by the heat recovery facility can be used, for example, as a heat source for pre-heating fuel and / or oxidant upstream of the main combustion zone and / or post-combustion zone, for heating or drying the charge fed to the main combustion zone, for generating steam, or for generating mechanical or electrical power.

[0035] Post-burn adjustment based on the level of combustibles in the post-burned gases According to one embodiment of the method, step h) includes monitoring a level, referred to as a second level, of one or more combustible substances in the post-combustion gases discharged from the post-combustion zone, and a first control signal is generated in step i) based on said second monitored level (also referred to as "signal B" or "level B").

[0036] In that case, step a) may comprise defining an upper threshold value B1up for said second monitoring level, and when the second monitoring level of residual combustibles in the post-combusted gas exceeds the upper threshold value B1up, the first control signal generated in step i) is further configured to be: 1) In the case of an oxidizer-only post-combustion zone, adjusting the actual post-combustion oxidizer flow to be higher than the nominal post-combustion oxidizer flow; or 2) In the case of an oxidizer-fuel post-combustion zone, the actual stoichiometric excess of post-combustion oxidizer is adjusted to be greater than the nominal stoichiometric excess of post-combustion oxidizer.

[0037] In the following, this post-combustion operating mode (described in 1) and 2) above) will be referred to as the "boosted post-combustion" operating mode.

[0038] Alternatively, or in combination with the above, step a) may include defining an upper positive threshold RB1up for the rate of change of the second monitored level (signal B). The rate of change corresponds to an increase or decrease per unit time of the monitored level of the monitored substance. The change is positive in the case of an increase in the monitored level of one or more combustible substances and negative in the case of a decrease in said monitored level. When the monitored level is constant, the change is zero (0). Thus, the upper positive threshold RB1up corresponds to an upper threshold for an increase (per unit time) of the second monitored level.

[0039] When the second monitoring level increases at a rate greater than the upper threshold RB1up, the first control signal generated in step i) causes the post-combustion zone to operate in a boosted post-combustion operating mode in step j).

[0040] Responding to the rate of increase of the first monitoring level, as described in WO-A-2010 / 022964, allows a more efficient response to significant peaks in the level of combustibles in the post-combusted gases.

[0041] Also, as indicated above, monitoring the level of one or more combustible substances may be direct monitoring, where the level of one or more substances in the gas stream is measured, or indirect monitoring, where a property correlated to the level of one or more combustible substances is measured. In the latter case, the rate of change of the monitored level may be compared to a threshold value RB1up by comparing the measured rate of change of the correlated property to a threshold value for change of the correlated property that corresponds to the threshold value RB1up for the rate of change of the monitored level.

[0042] It may be useful to combine several criteria for adjusting after-combustion, in particular the thresholds B1up and RB1up.

[0043] In the boosted post-combustion operating mode, when the post-combusted gases contain a higher level of combustible matter, more post-combustion oxidant is available for post-combustion of the discharged flue gases, thus ensuring more complete post-combustion of the discharged flue gases in the post-combustion zone than in the nominal post-combustion mode.

[0044] Injecting above-nominal flow of post-combustion oxidizer, or above-nominal excess of post-combustion oxidizer, into the post-combustion fuel is generally only economically justified when this is necessary to obtain the desired / required level of post-combustion and to limit the concentration of residual combustibles in the post-combusted gases.

[0045] Therefore, during the course of the boosted post-combustion operating mode, when the monitoring levels return to "normal" levels of combustibles in the post-combusted gases, post-combustion in the post-combustion zone should return to the nominal post-combustion operating mode.

[0046] According to one embodiment, step a) may include defining a predetermined duration Δt of boosted post-combustion operation, i.e., the time between the start of boosted post-combustion operation and the return to nominal post-combustion operation. In other words, according to such an embodiment, when the post-combustion zone has been in boosted post-combustion operation for a period Δt, a first control signal is generated in step i) such that in step j)

[0047] 1) In the case of an oxidizer-only post-combustion zone, the actual post-combustion oxidizer flow is adjusted to equal the nominal post-combustion oxidizer flow, or 2) The actual post-combustion oxidizer flow and the actual post-combustion fuel flow are adjusted to equal the nominal post-combustion oxidizer flow and the nominal post-combustion fuel flow, respectively.

[0048] In other words, the after-combustion is then operated in the nominal after-combustion operating mode.

[0049] In order to avoid instabilities in the operation of the post-combustion zone (i.e., frequent switching), it may also be useful in such embodiments to define in step a) a predetermined duration Δtpclag between the return of the post-combustion zone to nominal post-combustion operation and the next adjustment of the post-combustion according to step j), where the duration Δtpclag is typically only an integer fraction of the duration of Δtpcboost. In practice, a duration Δtpclag of 8 to 20 seconds, preferably 8 to 12 seconds, has proven useful.

[0050] Embodiments using a predetermined duration of boosted post-combustion operation Δtp may be used in particular for well-understood processes, in particular processes where flue gas from a main combustion zone is known to exhibit one or more combustible peaks, the durations of said peaks are approximately known, and the duration Δtp defined in step a) is selected in response to said known approximate durations, for example, equal to or greater than said known approximate durations. The approximate durations of the peaks may be known from earlier monitoring results of the same main combustion zone and process, or combined main and post-combustion zones and processes, obtained from one or more main combustion zones and processes and / or combined main and post-combustion zones or processes through simulation results of such main combustion or combined main and post-combustion zones and processes, or a combination thereof.

[0051] According to an alternative embodiment, step a) comprises defining a lower threshold value B1low for the second monitoring level, i.e. for the monitoring level of combustible substances in the post-combusted gas, such that B1low≦B1up. When the second monitoring level is below said lower threshold value B1low, the first control signal generated in step i) is such that in step j)

[0052] 1) In the case of an oxidizer-only post-combustion zone, the actual post-combustion oxidizer flow is adjusted to equal the nominal post-combustion oxidizer flow, or 2) The actual post-combustion oxidizer flow and the actual post-combustion fuel flow are adjusted to equal the nominal post-combustion oxidizer flow and the nominal post-combustion fuel flow, respectively.

[0053] That is, as a result, the after-combustion is operated in the nominal after-combustion operating mode.

[0054] Alternatively, or in combination with the above embodiment, step a) may include defining a lower threshold value B2low and a corresponding time period ΔtB for the second monitoring level, where B2low≦B1up.

[0055] When the second monitoring level remains below the lower threshold B2low for at least the time period ΔtB, again, the first control signal generated in step i) is such that in step j) the afterburning is adjusted to operate in the nominal afterburning operating mode.

[0056] Thus, defining the lower threshold B1low provides a mechanism for the afterburning to return to the nominal afterburning mode when the monitoring level of the combustible substances in the afterburned gas drops to a sufficiently low level (below B1low). Defining the lower threshold B2low and the corresponding time period ΔtB provides a mechanism for the afterburning to return to the nominal afterburning mode when the monitoring level of the combustible substances in the afterburned gas remains consistently (i.e., for at least ΔtB) sufficiently low (below B2low). The latter feature is particularly interesting for processes where the monitoring level of the combustible substances in the afterburned gas tends to vary frequently, in that applying this criterion over the period ΔtB increases the stability of the afterburning adjustment. When the two options are combined, B1low is usually selected to be lower than B2low.

[0057] B1low or B2low may be equal to B1up. In that case, the afterburning is adjusted in the boosted afterburning operating mode when the second monitoring level is higher than the threshold B1up, and is adjusted in the nominal afterburning operating mode when the second monitoring level is below the threshold B1up or remains below the threshold B1up for at least the duration of ΔtB, according to the principle.

[0058] To reduce the frequency of switching from the boosted afterburning operating mode to the nominal afterburning operating mode while still providing effective adjustment, B1low or B2low can be selected to be less than B1up, i.e., B1low, B2low < B1up.

[0059] The method of the present invention, in which post-combustion is adjusted based on monitored levels of combustibles in the post-combusted gases, is useful for a wide range of primary combustion processes, including those in which the efficiency or productivity of a transformation process (such as a melting or sintering process) occurring in the primary combustion zone takes priority over control of the composition of the flue gases leaving the primary combustion zone. Such adjusted post-combustion substantially reduces or even eliminates emissions of combustibles to the environment, while allowing for additional energy recovery.

[0060] Adjusting post-combustion based on monitored levels of combustibles in the post-combusted gases is particularly useful when it is impractical to monitor the levels of one or more combustibles in the flue gases emitted from the main combustion zone upstream of the post-combustion zone. This may be the case, for example, when the main combustion zone and the post-combustion zone are located within the same reactor, e.g., the main combustion zone is located at the bottom of the reactor and the post-combustion zone is located above the main combustion zone near the top of the reactor.

[0061] Post-combustion adjustment based on the level of combustibles in the flue gas The first control signal used to adjust post-combustion in step j) may be generated in step i) based on a first monitored level (Level C) of one or more combustible substances in the flue gases discharged from the main combustion zone and upstream of the post-combustion zone.

[0062] Such monitoring of the first level of one or more combustible substances in the discharged flue gas can be easily achieved when the discharged flue gas flows from the main combustion zone to the post-combustion zone through a gas duct connecting these two combustion zones.

[0063] The different embodiments described above relating to the adjustment of post-combustion based on the second monitored level of combustibles in the post-combusted gases can be applied in the same way to the post-combustion adjustment based on the first monitored level.

[0064] Thus, step a) may further comprise defining an upper threshold C1up for the first monitoring level. When the first monitoring level exceeds the upper threshold C1up, in step i), a first control signal is generated in step j) for adjusting after-combustion in a boosted after-combustion operating mode.

[0065] Alternatively, or in combination with the above, step a) may comprise defining an upper positive threshold RC1up for the rate of change of the first monitoring level (level C), which corresponds to an upper threshold for the increase per time unit of the first monitoring level.

[0066] When the first monitoring level increases at a rate greater than the upper threshold RC1up, the first control signal generated in step i) causes the post-combustion zone to operate in a boosted post-combustion operating mode in step j).

[0067] For a return from a boosted post-combustion mode of operation to a nominal post-combustion mode of operation, step a) may include defining a predetermined duration of boosted post-combustion operation, Δtpcboost, as already described above. When the post-combustion zone has been in boosted post-combustion operation for the duration Δtpcboost, step i) generates a first control signal that operates the post-combustion zone in the nominal post-combustion mode of operation in step j).

[0068] Alternatively, post-combustion in the post-combustion zone may be returned to the nominal post-combustion operating mode when the monitoring levels return to "normal" levels of one or more combustible materials during the boosted post-combustion operating mode.

[0069] Furthermore, step a) i) defining a lower threshold C1low for the first monitoring level, and / or ii) It may comprise defining a lower threshold C2low for the first monitoring level and a corresponding time period ΔtC.

[0070] When the first monitoring level is below the threshold C1low or remains below the lower threshold C2low for at least the time period ΔtC, the first control signal generated in step i) causes the after-combustion to be adjusted in step j) to operate in the nominal after-combustion operating mode.

[0071] The above comments regarding thresholds B1low, B2low, ΔtB, B1up, and RB1up apply mutatis mutandis to C1low, C2low, ΔtC, C1up, and RC1up.

[0072] Adjusting post-combustion based on monitored levels of combustibles in the discharged flue gases is particularly useful when, during at least some stages of the combined main combustion and post-combustion process, the levels of monitored combustibles in the post-combusted gases are very low and the sensitivity of the monitoring method and / or apparatus is insufficient to allow accurate detection of the levels of monitored combustibles, and as a result, accurate adjustment of post-combustion is not possible during said stages.

[0073] Adjusting post-combustion based on monitored levels of combustibles in the exhausted flue gases, compared to adjustments based on monitored levels of combustibles in the post-combustion gases, also has the advantage of allowing for a faster response in that the adjustments are based on gas flow entering the post-combustion zone rather than leaving the post-combustion zone.

[0074] Post-burn adjustment based on the level of combustibles in the flue gas and post-burned gas It is also possible to combine the two above-mentioned possibilities and their advantages by generating in step i) a first control signal that is used to regulate post-combustion in step j) based on both a first monitoring level (level C) of one or more combustible substances in the discharged flue gases upstream of the post-combustion zone and a second monitoring level (level B) of one or more combustible substances in the post-combusted gases.

[0075] In that case, step a) can be, for example: i) defining an upper threshold C1'up for a first monitoring level of one or more combustible substances in the emitted flue gas; and ii) It may be useful to provide for defining an upper threshold B1'up for the second monitoring level of one or more substances in the post-combusted gases.

[0076] When the first monitoring level exceeds the upper threshold C1'up or when the second monitoring level exceeds the upper threshold B1'up, the first control signal generated in step i) operates the after-combustion in a boosted after-combustion operating mode in a after-combustion adjustment step j).

[0077] To regulate the return to the nominal post-combustion operating mode, step a) comprises: i) defining a lower threshold value C1'low for a first monitoring level of one or more combustible substances in the discharged flue gases and a lower threshold value B1'low for a second monitoring level of one or more combustible substances in the post-combusted gases; and / or ii) It may also include defining a lower threshold C2'low for the first monitoring level and the corresponding time period ΔtC', and a lower threshold B2'low for the second monitoring level and the corresponding time period ΔtB'.

[0078] When the first monitoring level is below the lower threshold C1'low and the second monitoring level is below the threshold B1'low, or when the first monitoring level remains below the lower threshold C2'low for at least the time period ΔtC' and the second monitoring level remains below the lower threshold B2'low for at least the time period ΔtB', the first control signal generated in step j) adjusts the after-combustion so as to operate in the nominal after-combustion operating mode.

[0079] Again, the above comments regarding thresholds B1low, B2low, ΔtB, and B1up apply mutatis mutandis to C1'low, C2'low, ΔtC', and C1'up, as well as B1'low, B2'low, ΔtB', and B1'up.

[0080] In order to regulate the return to nominal post-combustion operation, it is also technically possible to combine the lower threshold value C1'low for the first monitoring level with the lower threshold value B2'low for the second monitoring level and the corresponding time period ΔtB', or to combine the lower threshold value B1'low for the second monitoring level of one or more combustible substances in the post-combusted gas with the lower threshold value C2'low for the first monitoring level and the corresponding time period ΔtC'.

[0081] When post-combustion is adjusted based on the level of combustibles in both the flue gas and the post-combusted gas, it is of course also possible to define a predetermined duration of boosted post-combustion operation Δtpcboost in step a), so that when the post-combustion zone has been in boosted post-combustion operation for the period Δtpcboost, in step i), a first control signal is generated that operates the post-combustion zone in the nominal post-combustion operation mode in step j).

[0082] Step a) may then comprise defining a predetermined duration Δtpclag between the return of the post-combustion zone to nominal post-combustion operation and the next adjustment of the post-combustion according to step j).

[0083] The adjustment of the post-combustion may be made independently of any adjustment of the main combustion.

[0084] The modulation of post-combustion according to any of the above embodiments may be combined with modulation of the main combustion in the main combustion zone where the flue gas is produced. In particular, the modulation of post-combustion may advantageously be combined with modulation of the main combustion to limit the level of combustibles in the flue gas produced by the main combustion.

[0085] In that case, step a) of the method according to the present invention may include defining a nominal main combustion operating mode for the main combustion zone with a nominal fuel supply rate and a nominal oxidizer supply rate to the main combustion zone, while step i) of the method further includes generating a second control signal based on the first level (Level C) and / or the second level (Level B) monitored in step h).The method then also includes step k) of adjusting the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone in response to the generated second control signal.

[0086] Primary combustion adjustment based on the level of combustibles in the flue gas According to one such embodiment, a second control signal for adjusting the main combustion is generated in step k) based on a first monitoring level (level C) of one or more residual combustible substances in the flue gases discharged from the main combustion zone.

[0087] In such a case, step a) may further comprise defining an upper threshold A1up for the first monitoring level (level C). When the first monitoring level exceeds the upper threshold A1up, the second control signal generated in step i) causes step k) to adjust the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone so that the ratio of the actual oxidizer supply rate to the actual fuel supply rate becomes higher than the ratio of the nominal oxidizer supply rate to the nominal fuel supply rate, thereby making additional oxygen available in the main combustion zone for combustion of combustibles present in the charge being processed in the furnace, for example. As a result, the amount of residual combustibles discharged from the main combustion zone together with the flue gases is reduced.

[0088] In the following, such operation of the main combustion in the main combustion zone is referred to as a "boosted main combustion" mode of operation.

[0089] Alternatively, or in combination with the above, step a) may include defining an upper positive threshold RA1up for the rate of change of the first monitored level (level C). The rate of change corresponds to an increase or decrease per unit time of the monitored level of the monitored substance, for example expressed in ppm / second. The rate of change is positive in the case of an increase and negative in the case of a decrease. When the monitored level is constant, the rate of change is zero (0). Thus, the upper positive threshold RA1up corresponds to an upper threshold for an increase (per unit time) of the first monitored level.

[0090] When the first monitoring level increases at a rate greater than the upper threshold RA1up, the second control signal generated in step i) causes the main combustion zone to operate in a boosted main combustion mode of operation in step k).

[0091] Responding to the rate of increase of the first monitoring level, as described in WO-A-2010 / 022964, allows for a faster and more efficient response to changes in the composition of the emitted flue gases and adjustment of the main combustion operating mode.

[0092] As indicated above, indirect monitoring is also possible, where a characteristic that is correlated to a monitoring level is measured and monitored.

[0093] It is preferable to combine both said criteria, ie to combine the thresholds A1up and RA1up.

[0094] As previously described with respect to the post-combustion adjustment performed in the post-combustion zone, step a) may include defining a predetermined duration Δtmcboost of boosted main combustion operation, i.e., the time between the start of boosted main combustion operation and the return to nominal main combustion operation. According to such an embodiment, when the main combustion zone has been in boosted main combustion operation for a period of Δtmcboost, step i) generates a second control signal to operate the main combustion zone in nominal main combustion operation mode in step k), regardless of the level monitored at that time in step h).

[0095] To avoid instabilities in the operation of the main combustion zone (i.e., excessively frequent switching), in such embodiments it may also be useful to define in step a) a predetermined duration Δtmclag between the return of the main combustion zone to nominal main combustion operation and the next adjustment of the main combustion according to step k). The duration Δtmclag is typically only an integer fraction of the duration of Δtmcboost. Δtmclag may be, for example, 8 to 20 seconds, preferably 8 to 12 seconds.

[0096] Embodiments using a predetermined duration of boosted main combustion operation Δtmcboost are particularly useful for well-understood processes, in particular processes where the flue gas from the main combustion zone is known to exhibit one or more combustible peaks, the duration of said peaks is approximately known, and the duration Δtmcboost defined in step a) is selected in response to said known approximate duration, for example at or above said known approximate peak duration.

[0097] The above comments regarding the predetermined duration Δtpcboost may be applied mutatis mutandis to the duration Δtmcboost.

[0098] As also previously described with respect to post combustion adjustments made in the post combustion zone, the main combustion may be returned to the nominal main combustion mode of operation when the monitored levels return to "normal" values.

[0099] Furthermore, when the main combustion is adjusted in response to the first monitoring level, step a) i. defining a lower threshold A1low for the first monitoring level; and / or ii. defining a lower threshold A2low for the first monitoring level and a corresponding time period ΔtA; Here, when the threshold A1low or the threshold A2low is used in combination with the threshold A1up, A1low or A2low≦A1up, preferably A1low or A2low <A1upである。

[0100] When the first monitoring level is below the lower threshold A1low and / or when the first monitoring level remains below the lower threshold A2low for at least the time period ΔtA, the second control signal generated in step i) causes the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone to be adjusted in step k) so that the actual oxidizer supply rate and the actual fuel supply rate correspond to the nominal oxidizer supply rate and the nominal fuel supply rate, respectively. In other words, the generated second control signal then operates the main combustion zone in the nominal main combustion mode of operation.

[0101] Main combustion adjustment based on the level of combustibles in post-burned gases As previously indicated, monitoring one or more combustible substances in the exhausted flue gases upstream of the post-combustion zone is not always readily feasible.

[0102] However, adjustment of the main combustion is also possible based on a second monitored level, namely, based on the monitored level of one or more combustible substances in the post-combusted gases exiting the post-combustion zone.

[0103] Furthermore, a second control signal may be generated in step i) based on a second monitoring level (Level B).

[0104] In particular, step a) may then comprise defining an upper threshold D1up for the second monitoring level, ie for the monitoring level of one or more combustible substances in the post-combusted gases.

[0105] When the second monitoring level exceeds the upper threshold D1up, the second control signal generated in step i) causes the main combustion zone to operate in a boosted main combustion mode of operation in step k).

[0106] For a return to the nominal, primary combustion mode of operation, step a) may include defining a predetermined duration Δtmcboost of boosted primary combustion operation, i.e., the time between the start of boosted primary combustion operation and the return to nominal primary combustion operation. According to such an embodiment, when the primary combustion zone has been in boosted primary combustion operation for a period of Δtmcboost, step i) generates a second control signal to operate the primary combustion zone in the nominal, primary combustion mode of operation in step k), regardless of the level monitored at that time in step h).

[0107] As already explained, step a) may then also include defining a predetermined duration Δtmclag between the return of the main combustion zone to nominal main combustion operation and the next adjustment of the main combustion according to step k).

[0108] Alternatively, step a) comprises: i. defining a lower threshold D1low for the second monitoring level, and / or ii. defining a lower threshold D2low for a second monitoring level and a corresponding time period ΔtD; D1low or D2low≦D1up, preferably D1low or D2low <D1upである。

[0109] When the second monitoring level is below the threshold D1low or remains below the lower threshold D2low for at least the time period ΔtD, the second control signal generated in step i) operates the main combustion zone in the nominal main combustion operating mode in step k).

[0110] When the method includes adjusting both post-combustion and main combustion, and the first and second control signals are based on the same monitoring level, it may be possible to use the same criteria to switch to the respective boosted operating modes and to switch to the respective nominal operating modes for both main combustion and post-combustion, in which case a single control signal may be used as both the first and second control signals.

[0111] The nominal operating mode defined in step a) corresponds to the operating mode of combustion in the corresponding combustion zone (post-combustion zone or main combustion zone) without the feedback / feedforward adjustment proposed by the present invention.

[0112] Thus, the term "nominal" is synonymous with the terms "target" or "set" or "preset" as used by those skilled in the art, for example, in the expressions "target flow rate" or "set flow rate."

[0113] Therefore, the nominal operating mode and the corresponding nominal parameters are established by an experienced furnace operator depending on the nature / type / parameters of the process (e.g. incineration, melting, steam production, etc.) and the equipment (both main and post-combustion equipment) to which the method according to the invention is applied.

[0114] Therefore, the nominal main combustion operating mode depends on the furnace in which the main combustion zone is located, the nature of the process occurring therein, and the production rate of the furnace. Because the post-combustion zone receives flue gases produced in the main combustion zone, the nominal post-combustion operating mode depends on the nature or characteristics and volume of the flue gases exiting the main combustion zone, which in turn depends on the furnace in which the main combustion zone is located and the nature of the process occurring therein. Specific environmental goals or regulations regarding atmospheric emissions may also have to be considered when defining the nominal post-combustion mode.

[0115] The nominal operating mode and corresponding nominal parameters may be constant over time, for example, in a furnace that is operated continuously without intentional changes to the charge or product to be produced, combustion heat production rate, overall production rate, etc.

[0116] Additionally, the nominal operating mode and corresponding nominal parameters may change over time due to, for example, changes in production rate or changes in the charge introduced into the furnace, the process carried out therein, or the nature of the product to be produced thereby.

[0117] In particular, in the case of a batch or semi-batch process, the nominal operating mode and corresponding nominal parameters may also be varied periodically over time, one cycle for each batch. An example of such a batch process is a batch melting process, in which each batch introduced into the furnace undergoes a heating stage, a melting stage, and a refining stage, or at least two of the foregoing stages including the melting stage.

[0118] Therefore, the nominal post-combustion operating mode defined by the skilled artisan in step a) can vary widely, and it is a particular advantage of the method according to the invention that it is suitable and useful for such a wide range of operating modes.

[0119] Considering that the gases subjected to post-combustion in the post-combustion zone are flue gases discharged from the main combustion zone, whether the post-combustion must be adjusted to operate in a boosted post-combustion operating mode or in a nominal post-combustion operating mode ultimately depends on the main combustion taking place in the main combustion zone, and this is regardless of the monitoring level used to generate the first control signal.

[0120] Therefore, when the method according to the invention comprises both a step j) for adjusting the post-combustion and a step k) for adjusting the main combustion, and both the post-combustion and the main combustion are operating in corresponding boosted operating modes, it is advisable to generate the first and second control signals in step i) such that the main combustion mode returns to its nominal operating mode first when a condition is met, and only afterwards is the post-combustion allowed to return to its nominal operating mode, also when a condition is met.

[0121] If the return of main combustion from a boosted main combustion mode of operation to a nominal main combustion mode of operation, i.e., a decrease in the availability of oxygen for main combustion, causes the level of residual materials in the flue gas discharged from the main combustion zone to rise again and therefore requires additional oxygen for effective post-combustion of the discharged flue gas, this approach allows post-combustion to remain in a boosted post-combustion mode of operation, thereby maintaining the level of combustible materials in the post-combusted gas at an acceptable level, which is particularly important for complying with applicable emission standards. Only if the monitored level of residual combustible materials in the discharged flue gas remains low after the main combustion zone has returned to nominal operation is the post-combustion zone then allowed to return to its nominal mode of operation.

[0122] Such an embodiment would of course require a corresponding selection of criteria for switching the main and post-combustion zones to their respective nominal operating modes.

[0123] Thus, in embodiments of the present invention that employ both modulation of post-combustion (step j) and modulation of main combustion (step k), when both main combustion and post-combustion are in boosted modes of operation and both meet the criteria for switching to their respective nominal modes of operation: First, switch the main combustion zone to the nominal main combustion operating mode according to step k), Then wait for the lag time period Δtlag, At the end of the lag time period Δtlag, if the criteria for switching the post-combustion zone to the nominal post-combustion operating mode are still fulfilled, it may be advantageous to switch the post-combustion zone to the nominal post-combustion operating mode according to step j).

[0124] The lag time period Δtlag is then likewise defined in step a) of the method according to the invention.

[0125] In step a), the lag time period Δtlag is defined so that the effect of the main combustion zone returning to its nominal operating mode on the composition of the flue gases leaving the main combustion zone is reflected by the monitoring level used to generate the first control signal used to regulate post-combustion in step j).The length of the lag time period Δtlag is therefore defined taking into account the nature of the furnace in which the main combustion zone is located, the nature of the process carried out therein, and the length of time required for the flue gases leaving the combustion zone to reach the point at which monitoring takes place.

[0126] The invention and its advantages are illustrated in the following examples with reference to the figures. [Brief explanation of the drawings]

[0127] [Figure 1] Schematic diagram of an industrial furnace with an afterburner. DETAILED DESCRIPTION OF THE INVENTION

[0128] The figure shows a furnace 10, such as a furnace for melting scrap aluminum, where the scrap aluminum may be contaminated with flammable contaminants such as paint and lacquer on aluminum cans and oil.

[0129] The furnace 10 defines a primary combustion zone therein, which is heated by the combustion of a fuel with an oxidizer, referred to as the "primary combustion."

[0130] Furnace 10 further includes one or more burners 12 (although only one burner is shown, there may be multiple burners) 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.

[0131] Fuel and oxidant are supplied in a controlled manner, i.e., at regulated flow rates, to one or more burners 12. Primary combustion of fuel with oxidant within furnace 10 produces heat and combustion gases within a primary combustion zone of furnace 10. (The primary combustion is represented diagrammatically by flame 11, although the combustion may be in the form of multiple flames or flameless combustion.)

[0132] Instead of, or in combination with, burners 12 injecting both fuel and oxidizer, fuel and oxidizer may be supplied separately to the main combustion zone, for example, for staged or flameless combustion.

[0133] For a charge 15 of scrap aluminum to be melted in the main combustion zone, a reducing atmosphere 16 is desired above the charge 15 to limit aluminum metal loss due to oxidation. Therefore, a less than stoichiometric amount of oxidizer (relative to the amount of fuel) is supplied to the burner 12. As a result, the flue gases 17 exiting the furnace 10 contain combustible materials.

[0134] As the contaminated aluminum charge 15 heats, combustible contaminants are also typically released by the charge 15 into the main combustion zone atmosphere 16 in an uncontrolled manner, i.e., with peaks and declines in the amount of combustibles released, which contribute to the level of combustibles in the flue gas 17.

[0135] Other furnaces operate in a neutral (i.e., neither oxidizing nor reducing) or oxidizing atmosphere in the main combustion zone. In such cases, the baseline level of combustibles in the flue gas from the main combustion zone is typically zero or near zero, while occasional peaks of combustibles may be observed in the discharged flue gas.

[0136] In step a) of the method according to the invention, a nominal main combustion operating 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 burners 12.

[0137] For example, in the case of a batch scrap aluminum melting process, furnace operation may include an initial heating phase in which the solid charge 15 is heated to an aluminum melting temperature, a melting phase in which the solid charge 15 is gradually melted, and a refining phase in which the molten charge 15 is refined and then maintained at its tapping temperature. For each phase, a constant or evolving nominal fuel feed rate and a nominal oxidant feed rate to the furnace 10 are defined. As mentioned above, to avoid oxidation of the aluminum charge 15, particularly during the melting and refining phases of the process (because the molten charge is more susceptible to oxidation), the nominal oxidant feed rate may be kept sub-stoichiometric with respect to the nominal fuel feed rate.

[0138] Flue gases 17 discharged from the main combustion zone of furnace 10 are transported via conduit 18 to a post-combustion / post-combustion zone 19. A post-combustion oxidizer and post-combustion fuel are injected in a controlled manner (i.e., at a regulated flow rate) into post-combustion zone 19, along with a controlled stoichiometric excess of post-combustion oxidizer relative to the post-combustion fuel, to combust combustible materials present in the discharged flue gases 17. The resulting post-combustion gases 23 are discharged from post-combustion zone 19.

[0139] In the embodiment shown, the post-combustion oxidizer 19 comprises a burner 20 and a separate post-combustion oxidizer injector 25. Post-combustion fuel is supplied to the burner 20 along with a stoichiometric amount of post-combustion oxidizer. Additional post-combustion oxidizer is supplied to the injector 25 to provide a stoichiometric excess of post-combustion oxidizer (relative to the post-combustion fuel) within the post-combustion oxidizer. Again, while only one post-combustion burner 20 and one injector 25 are shown, the post-combustion oxidizer 19 may comprise multiple burners 20 and / or multiple injectors 25. The use of multiple injectors 25 can be particularly useful to ensure an intimate mix of the flue gas 17 and the stoichiometric excess of post-combustion oxidizer entering the post-combustion oxidizer.

[0140] In the embodiment shown, the post-combustion zone 19 is separated from the main combustion zone by a conduit 18 through which the flue gases 17 are transported. In other embodiments, the main combustion zone and the post-combustion zone may be located in different parts of the same enclosure, with the flue gases generated in the main combustion zone of the enclosure moving 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 updraft of the generated flue gases.

[0141] In the example shown, the level of combustibles, such as H 2 , CO, and / or VOCs, in the discharged flue gas 17 in conduit 18 is determined using sensor 21 .

[0142] In the embodiment shown, sensor 21 is the sensor described in co-pending patent application EP 21152977. The disclosure of said sensor and its operation in said co-pending patent application is incorporated herein by reference. Valve 103 controls the flow of oxidant from oxidant source 14 to sensor 21.

[0143] Other sensors and monitoring devices and methods for monitoring the level of combustibles in flue gas 17 are commercially available and may be used in the context of the present invention.

[0144] The level of combustibles in the discharged flue gases 17 detected by the sensors 21 is transmitted to a central control unit 22 and compared to reference values ​​stored therein, which correspond to the level of combustibles in the flue gases 17 that would normally be expected to be present at a given stage of the batch process, as well as operating parameters including the actual oxidant and fuel feed rates to the burners 12 of the furnace 10.

[0145] When said comparison by the control unit 22 reveals that the detected level of combustibles in the flue gas 17 is significantly higher than the reference level, this indicates a peak in the emission of combustibles by the charge 15 in the main combustion zone.

[0146] The level of combustible substances in the flue gas 17 detected by the sensor 21 is considered by the control unit 22 to be significantly higher than the reference value, for example, when the detected level of combustible substances in the flue gas 17 is higher than an upper threshold A1up also defined in step a), where the threshold A1up is greater than the reference value.

[0147] In that case, the control unit 22 generates control signals, referred to as "second control signals," that adjust the main fuel controller 101 and the main oxidizer controller 102 so that the furnace operates in a boosted main combustion operating mode in which the ratio of the actual oxidizer supply rate to the burners 12 exceeds the ratio of the nominal oxidizer supply rate to the nominal fuel supply rate. In this way, more oxygen is made available in the main combustion zone to combustibles released therein without creating an oxidizing atmosphere 16 above the charge 15.

[0148] According to one embodiment, after a predetermined duration Δtmcboost, different second control signals are issued such that the main fuel controller 101 causes the actual fuel supply rate to the burner 12 to correspond to the nominal fuel supply rate, and such that the main oxidizer controller 102 causes the actual oxidizer supply rate to the burner 12 to correspond to the nominal oxidizer supply rate.

[0149] Alternatively, the return to nominal primary combustion may be based on the level of combustibles in the discharged flue gas 17 as detected by the sensor 21. For example, when a comparison by the central control unit 22 indicates that the detected level of combustibles in the discharged flue gas 17 by the sensor 21 is substantially equal to or even lower than a reference value stored in the central control unit 22, the control unit 22 generates a second control signal to return primary combustion in the furnace 10 to the nominal primary combustion operating mode in the manner described above.

[0150] The above-described adjustment of the main combustion can keep the level of combustible materials in the flue gas 17 leaving the furnace 10 within certain limits, but does not in itself solve the problem of detectable levels of combustible materials in the exhausted flue gas 17 being released into the atmosphere.

[0151] This problem is addressed by subjecting the exhausted flue gases 17 to post-combustion in a post-combustion zone 19 .

[0152] When the temperature of the flue gas 17 entering the post-combustion zone 19 and the nature and concentration of the combustible material in the flue gas 17 are such that ignition of the combustible material is guaranteed, post-combustion of the flue gas 17 can be achieved, for example, by injecting only a post-combustion oxidizer into the post-combustion zone 19 via an injector 25.

[0153] In many cases, both a post-combustion oxidizer and a post-combustion fuel will be injected into post-combustion zone 19 to create a persistent post-combustion flame in post-combustion zone 19. Post-combustion of the combustible material in flue gas 17 is then achieved by a stoichiometric excess of post-combustion oxidizer relative to the post-combustion fuel in post-combustion zone 19. In the embodiment shown, corresponding stoichiometric amounts of post-combustion fuel and post-combustion oxidizer are supplied to burner 20, while an excess amount of post-combustion oxidizer for the post-combustion of the flue gas is injected into post-combustion zone 19 by injector 25.

[0154] In step a) of the method according to the invention, a nominal post-combustion operating mode is defined for the post-combustion zone 19 with corresponding nominal post-combustion oxidizer injection rates and nominal post-combustion fuel injection rates. The total nominal post-combustion oxidizer injection rate and nominal post-combustion fuel injection rate, and in particular the ratio of the two, are defined based on the temperature and composition that the discharged flue gases 17 would normally have when they enter the post-combustion / post-combustion zone 19, given the stages and parameters of the batch process in the furnace 10, and bearing in mind limitations on the level of combustibles in the gases to be released into the atmosphere, imposed, for example, by environmental regulations. In other words, the nominal post-combustion oxidizer and fuel injection rates are such that post-combustion will result in effective suppression of combustibles in the flue gases 17 at levels normally expected at a given stage of operation of the furnace 10.

[0155] In the embodiment shown, a further control signal, referred to as a "first control signal", is generated by the control unit 22 based on the comparison of the level of combustibles in the exhausted flue gas 17 detected by the sensor 21 with a reference value stored in the central control unit 22. The first control signal is sent to a first flow controller 104, which adjusts the excess amount of post-combustion oxidant to the injector 25.

[0156] The flow of post-combustion fuel to post-combustion burner 20 and the corresponding stoichiometric flow of post-combustion oxidizer are regulated by controller 106 and controller 105, respectively.

[0157] When the comparison by the control unit 22 reveals that the detected level of combustibles in the flue gas 17 is significantly higher than the reference value, for example by comparing the detected level with the upper threshold level C1up defined in step a), the control unit 22 generates a first control signal to the first flow controller 104 to increase the amount of post-combustion oxidizer excess to the injector 25 to a boosted excess amount of post-combustion oxidizer that is higher than the post-combustion oxidizer excess amount during the nominal post-combustion operating mode, so that the ratio of the actual total post-combustion oxidizer injection rate to the actual post-combustion fuel injection rate is higher than the ratio of the total nominal post-combustion oxidizer injection rate to the nominal post-combustion fuel injection rate. In this way, extra available post-combustion oxidizer is made available to post-combust the peak of combustibles present in the discharged flue gas 17 (boosted post-combustion operating mode).

[0158] The criteria for a "significantly high" level for adjusting post-combustion (such as C1up) may be the same as or different from the criteria for a "significantly high" level for adjusting main combustion (such as A1up).

[0159] When the comparison by the central control unit 22 finds that the level of combustibles in the discharged flue gas 17 detected by the sensor 21 is substantially equal to or even lower than the corresponding reference value stored in the central control unit 22, where again the same or different criteria for “substantially equal” for the adjustment of the main combustion and the adjustment of the post-combustion may apply, the first control signal generated by the control unit 22 causes the second flow controller 104 to adjust the actual post-combustion oxidizer injection rate to the injector 25 so that the actual stoichiometric excess amount of post-combustion oxidizer injected into the post-combustion zone 19 corresponds to the nominal stoichiometric excess amount of post-combustion oxidizer (the actual post-combustion fuel flow to the burner 20 and the actual post-combustion oxidizer flow to the burner 20 are adjusted by the controllers 106 and 105, respectively, also based on the first control signal, so that the actual post-combustion fuel flow to the burner 20 corresponds to the nominal post-combustion fuel flow rate, and so that the post-combustion oxidizer flow to the burner 20 is stoichiometric with the actual / nominal post-combustion fuel flow).

[0160] In the embodiment shown, a second sensor 24 of the same type as the first sensor 21 is present in the exhaust of the post-combustion chamber 19 to monitor the level of one or more combustible substances in the post-combustion gases 23 leaving the post-combustion zone 19, providing extra safety for the control of post-combustion in the post-combustion zone 19.

[0161] An upper threshold value B1up for the level of combustibles in the post-combusted gases 23 has been determined in step a), said upper threshold value B1up being higher than a reference value for the monitored level of combustibles in the post-combusted gases 23. said reference value corresponds to the level of combustibles in the post-combusted gases 23 that would normally be expected to be present at a given stage of the batch process, as well as to operating parameters of the main combustion zone and of the post-combustion zone.

[0162] Also, the upper threshold B1up is at most equal to, and preferably less than, the maximum level of said combustible material allowed by local environmental regulations.

[0163] The control unit 22 compares the level detected by the second sensor 24 with an upper threshold B1up, and when the level detected by the second sensor 24 is higher than the threshold B1up, the control unit 22 generates a first control signal to operate the post-combustion zone 19 in boosted post-combustion operation as described above, regardless of the level detected by the first sensor 21.

[0164] Similarly, when the level detected by the first sensor 21 is higher than the threshold level C1up, the central control unit 22 generates a first control signal to operate the post-combustion zone 19 in boosted post-combustion operation, regardless of the level detected by the first sensor 24.

[0165] Only when both the level detected by the first sensor 21 and the level detected by the second sensor 24 are substantially equal to or even lower than the corresponding reference values ​​stored in the central control unit 22 does the control unit 22 generate a first control signal, based on which the controllers 104, 105, and 106 adjust the post-combustion fuel flow and the total post-combustion oxidizer flow to the post-combustion zone 19 to correspond to their respective nominal flow rates, thereby supplying stoichiometric flows of post-combustion fuel and oxidizer to the burner 20, while supplying a stoichiometric excess of post-combustion oxidizer to the injector 25, as described above.

[0166] Although the use of a central control unit 22 is shown, it should be understood that the method according to the present invention may also be implemented in a distributed or modular system using a PLC or the like to compare detected values ​​with reference values ​​and / or to regulate fuel and / or oxidant supply.

[0167] The above embodiment provides maximum control of the level of combustibles in the post-combusted gases and therefore the level of combustibles that tend to be released into the atmosphere.

[0168] When suppression of combustibles in the post-combusted gases is less important, a less sophisticated control system may be used for post-combustion, based solely on the level of combustibles detected by, for example, one of sensors 21 and 24. [Explanation of symbols]

[0169] 10 furnace 11 Flame 12 Burner of furnace 10 13 Fuel source 14 Oxidant Source 15 Charge 16 Atmosphere inside furnace 10 17. Exhausted flue gases 18 Flue gas duct 19 Post-combustion chamber / post-combustion zone 20 Burner of the post-combustion chamber 19 21 First Sensor 22 Central Control Unit 23 Post-burned gas 24 Second Sensor 25 Oxidizer 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 oxidizer 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

Claims

1. 1. A combined combustion and post-combustion method comprising: a) the nominal post-combustion operating mode for the post-combustion zone (19), (1) defining a nominal post-combustion oxidizer injection rate into the post-combustion zone (19) when the post-combustion zone (19) is an oxidizer-only post-combustion zone that is not equipped to inject post-combustion fuel; (2) or, when the post-combustion zone (19) is an oxidizer-fuel post-combustion zone that is equipped to inject both post-combustion oxidizer and post-combustion fuel, using a nominal post-combustion oxidizer injection rate and a nominal post-combustion fuel injection rate into the post-combustion zone (19), wherein the nominal post-combustion oxidizer injection rate and the nominal post-combustion fuel injection rate define a nominal stoichiometric excess of the post-combustion oxidizer relative to the post-combustion fuel; b) supplying fuel and combustion oxidant to the primary combustion zone (10) at an actual fuel supply rate and an actual oxidant supply rate, respectively; c) burning the supplied fuel in said main combustion zone (10) with the supplied combustion oxidant, thus producing heat and flue gases which may contain residual combustibles; d) discharging the flue gases from the main combustion zone (10) and introducing the discharged flue gases (17) into the post-combustion zone (19); e) in said post-combustion zone (19), (1) when said post-combustion zone (19) is an oxidizer-only post-combustion zone, injecting post-combustion oxidizer at the actual post-combustion oxidizer injection rate and not injecting post-combustion fuel; or (2) when the post-combustion zone (19) is an oxidizer-fuel post-combustion zone, injecting a post-combustion oxidizer at an actual post-combustion oxidizer injection rate and injecting a post-combustion fuel at an actual post-combustion fuel injection rate, wherein the actual post-combustion oxidizer injection rate and the actual post-combustion fuel injection rate define an actual stoichiometric excess of post-combustion oxidizer relative to the post-combustion fuel; f) treating the flue gas (17) discharged in the post-combustion zone (19) with (1) In the case of an oxidizer-only post-combustion zone, post-combustion is performed using the post-combustion oxidizer; or (2) in the case of an oxidizer-fuel post-combustion zone, post-combustion with said actual stoichiometric excess of post-combustion oxidizer; thereby producing post-combusted gas (23); g) discharging the post-combustion gases (23) from the post-combustion zone (19); h) monitoring a first level of one or more combustible substances in the flue gases (17) discharged from the main combustion zone (10) and / or a second level of one or more combustible substances in the post-combustion gases (23) discharged from the post-combustion zone (19); i) generating a first control signal based on the levels monitored in step h), or based on one or both of the levels; j) in response to the first control signal, (1) in the case of an oxidizer-only post-combustion zone (19), by adjusting the actual post-combustion oxidizer injection rate; or (2) in the case of an oxidizer-fuel post-combustion zone (19), adjusting the actual stoichiometric excess of post-combustion oxidizer via the actual post-combustion oxidizer injection rate and / or the actual post-combustion fuel injection rate; A method for providing

2. The method of claim 1 , wherein the first control signal is generated based on the second monitoring level.

3. Step a) i. defining an upper threshold value B1up for the second monitoring level; When the second monitoring level exceeds the upper threshold B1up, the generated first control signal is (1) in the case of an oxidizer-only post-combustion zone (19), the actual post-combustion oxidizer flow is higher than the nominal post-combustion oxidizer flow; or (2) The method of claim 2, wherein, for an oxidizer-fuel post-combustion zone (19), the actual stoichiometric excess of post-combustion oxidizer is greater than the nominal stoichiometric excess of post-combustion oxidizer.

4. Step a) i'. defining a lower threshold value B1low for the second monitoring level, and / or ii'. defining a lower threshold value B2low for the second monitoring level and a corresponding time period ΔtB, When the second monitoring level is below a threshold B1low or remains below a lower threshold B2low for at least a time period ΔtB, the generated first control signal is in step j) (1) In the case of an oxidizer-only post-combustion zone (19), the actual post-combustion oxidizer flow is equal to the nominal post-combustion oxidizer flow; or (2) In the case of an oxidizer-fuel post-combustion zone (19), the actual post-combustion oxidizer flow and the actual post-combustion fuel flow are set equal to the nominal post-combustion oxidizer flow and the nominal post-combustion fuel flow, respectively.

5. The method of claim 1 , wherein the first control signal is generated based on the first monitoring level.

6. Step a) i. further comprising defining an upper threshold C1up for the first monitoring level; When the first monitoring level exceeds the upper threshold C1up, the generated first control signal is (1) in the case of an oxidizer-only post-combustion zone (19), the actual post-combustion oxidizer flow is higher than the nominal post-combustion oxidizer flow; or (2) In the case of an oxidizer-fuel post-combustion zone (19), the actual stoichiometric excess of post-combustion oxidizer is greater than the nominal stoichiometric excess of post-combustion oxidizer.

7. Step a) i. defining a lower threshold C1low for said first monitoring level; and / or ii. defining a lower threshold C2low for the first monitoring level and a corresponding time period ΔtC; When the first monitoring level is below a threshold C1low or remains below a lower threshold C2low for at least a time period ΔtC, the generated first control signal is in step j) (1) In the case of an oxidizer-only post-combustion zone (19), the actual post-combustion oxidizer flow is equal to the nominal post-combustion oxidizer flow; or (2) In the case of an oxidizer-fuel post-combustion zone (19), the actual post-combustion oxidizer flow and the actual post-combustion fuel flow are set equal to the nominal post-combustion oxidizer flow and the nominal post-combustion fuel flow, respectively.

8. The method of claim 1 , wherein the first control signal is generated based on both the first monitoring level and the second monitoring level.

9. Step a) i. defining an upper threshold C1'up for said first monitoring level; and ii. defining an upper threshold B1′up for the second monitoring level; When the first monitoring level exceeds an upper threshold C1′up or when the second monitoring level exceeds an upper threshold B1′up, the first control signal generated in step j) is: (1) in the case of an oxidizer-only post-combustion zone (19), the actual post-combustion oxidizer flow is higher than the nominal post-combustion oxidizer flow; or 9. The method of claim 8, wherein (2) in an oxidizer-fuel post-combustion zone (19), the actual stoichiometric excess of post-combustion oxidizer is greater than the nominal stoichiometric excess of post-combustion oxidizer.

10. Step a) iii. Defining a lower threshold C1′low for the first monitoring level and a lower threshold B1′low for the second monitoring level; and / or iv. defining a lower threshold C2′low for the first monitoring level and the corresponding time period ΔtC′ and a lower threshold B2′low for the second monitoring level and the corresponding time period ΔtB′; When the first monitoring level is below a lower threshold C1′low and the second monitoring level is below a threshold B1′low, or when the first monitoring level remains below a lower threshold C2′low for at least a time period ΔtC′ and the second monitoring level remains below a lower threshold B2′low for at least a time period ΔtB′, the generated first control signal in step j) is (1) in the case of an oxidizer-only post-combustion zone (19), making the actual post-combustion oxidizer injection rate equal to the nominal post-combustion oxidizer injection rate; or 10. The method of claim 9, wherein (2) for an oxidizer-fuel post-combustion zone (19), the actual post-combustion oxidizer injection rate and the actual post-combustion fuel injection rate are equal to the nominal post-combustion oxidizer injection rate and the nominal post-combustion fuel injection rate, respectively.

11. Step a) further comprises defining a predetermined duration Δtpcboost, and in step j), for a period Δtpcboost: (1) In the case of an oxidizer-only post-combustion zone (19), when the actual post-combustion oxidizer flow is higher than the nominal post-combustion oxidizer flow; or (2) In the case of an oxidizer-fuel post-combustion zone (19), when the actual stoichiometric excess of post-combustion oxidizer is greater than the nominal stoichiometric excess of post-combustion oxidizer; In step j), (1) in the case of an oxidizer-only post-combustion zone (19), making the actual post-combustion oxidizer injection rate equal to the nominal post-combustion oxidizer injection rate; or (2) For the oxidizer-fuel post-combustion zone (19), the actual post-combustion oxidizer injection rate and the actual post-combustion fuel injection rate are set equal to the nominal post-combustion oxidizer injection rate and the nominal post-combustion fuel injection rate, respectively.

10. The method of claim 3, 6 or 9, wherein a first control signal is generated in step i).

12. Step a) includes defining a nominal main combustion operating mode for the main combustion zone (10) at a nominal fuel supply rate and a nominal oxidizer supply rate to the main combustion zone (10); step i) includes generating a second control signal based on the first level and / or the second level monitored in step h); The method comprises: The method of any one of claims 1 to 3, further comprising the step of: k) adjusting the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone (10) in response to the second control signal.

13. 13. The method of claim 12, wherein in step i), the second control signal is generated based on the first monitoring level.

14. Step a) i. defining an upper threshold A1up for said first monitoring level, and / or ii. further comprising defining an upper positive threshold value RA1up for the rate of change of the first monitoring level. When the first monitoring level exceeds the upper threshold A1up, and / or When the first monitoring level increases at a rate greater than an upper threshold RA1up, 14. The method of claim 13, wherein the generated second control signal in step k) adjusts the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone (10) such that the ratio of the actual oxidizer supply rate to the actual fuel supply rate is higher than the ratio of the nominal oxidizer supply rate to the nominal fuel supply rate.

15. Step a) i. defining a lower threshold A1low for the first monitoring level; and / or ii. defining a lower threshold A2low for the first monitoring level and a corresponding time period ΔtA. when the first monitoring level is below a lower threshold A1low, and / or when the first monitoring level remains below a lower threshold A2low for at least the time period ΔtA; 14. The method of claim 13, wherein the generated second control signal in step k) adjusts the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone (10) such that the actual oxidizer supply rate and the actual fuel supply rate correspond to the nominal oxidizer supply rate and the nominal fuel supply rate, respectively.

16. The method of claim 12 , wherein in step i), the second control signal is generated based on the second monitoring level.

17. Step a) i. defining an upper threshold D1up for the second monitoring level; 17. The method of claim 16, wherein when the second monitoring level exceeds the upper threshold D1up, the generated second control signal causes, in step k), the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone (10) to be adjusted such that the ratio of the actual oxidizer supply rate to the actual fuel supply rate is higher than the ratio of the nominal oxidizer supply rate to the nominal fuel supply rate.

18. Step a) ii. defining a lower threshold D1low for the second monitoring level; and / or iii. defining a lower threshold D2low for the second monitoring level and a corresponding time period ΔtD; 18. The method of claim 17, wherein when the second monitoring level is below a threshold B1low or remains below a lower threshold B2low for at least a time period ΔtB, the generated second control signal causes in step k) the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone (10) to be adjusted such that the actual oxidizer supply rate and the actual fuel supply rate correspond to the nominal oxidizer supply rate and the nominal fuel supply rate, respectively.

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