BR102022026199A2 - COMBUSTION PROCESS - Google Patents

Abstract

Combined combustion and post-combustion method whereby waste gas is generated by combustion in a main combustion zone 10, the waste gas 17 being evacuated from the main combustion zone 10 and introduced into a post-combustion zone 19, where the residual gas 17 is subjected to afterburning and afterburning gas 23 is obtained, which is evacuated from the afterburning zone 19, whereby a first level of one or more combustible substances in the residual gas 17 is evacuated from the main combustion zone 10 and/or a second level of one or more combustible substances in the post-combustion gas 23 evacuated from the post-combustion zone 19 is/are monitored, whereby a control signal is generated based on the level(s). is) monitored and by which the post-combustion oxidizer injection rate or the stoichiometric excess of the post-combustion oxidizer in relation to the post-combustion fuel is regulated as a function of said control signal.

Description

COMBUSTION PROCESS

[001] The present invention relates to the field of combustion. The present invention relates more specifically to combustion processes, in which the fuel is subjected to combustion with oxidizer in the combustion zone of a furnace, hereinafter referred to as the main combustion zone, and in which the residual gases evacuated from said combustion zone are subjected to post-combustion in a post-combustion zone downstream of the main combustion zone in the direction of waste gas flow.

[002] For maximum heat generation in the main combustion zone, it is generally preferable to achieve complete combustion within the main combustion zone. In this case, the residual gas from the combustion zone is free from residual combustible matter.

[003] To achieve complete combustion in the main combustion zone, the combustion oxidizer must be supplied to the main combustion zone at least in stoichiometric quantity in relation to the fuel. In current practice, a small excess of oxidant over the stoichiometric amount is required to achieve complete combustion. A large excess of oxidizer also allows complete combustion, but dilutes the combustion and thus reduces the flame temperature. Furthermore, when oxygen or oxygen-enriched air is used as a combustion oxidant, a large excess of oxidant would also increase costs related to supplying oxidant to the process.

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

[005] For example, for certain processes, a reducing furnace atmosphere is required, for example to prevent unwanted oxidation of a charge that is heated within the main combustion zone. This is often the case with furnaces for melting non-ferrous metals. If a reducing atmosphere is required, operation of the main combustion zone with excess oxygen is obviously excluded.

[006] Furthermore, in some processes, such as waste incineration plants or secondary aluminum melting furnaces in which contaminated aluminum, such as painted beverage cans, is melted, the combustible matter is, at least in part, released in a manner and quantity uncontrolled by the load. As a consequence, it is not possible to accurately predict the amount of oxidant corresponding to the stoichiometric amount at different times throughout the process.

[007] In order to optimize the combustion process in these furnaces, monitoring methods have been proposed that monitor changes in the amount of combustible matter released by the charge into the main combustion zone, so as to allow the furnace operator to respond quickly by adjusting the control supply of combustion oxidizer and/or fuel to the main combustion zone. Examples of such monitoring processes are described in ES-A2201885, ES-A-2207389, EP-A-0949477, WO-A-2010022964, WO-A2011131880 and JP-A-2021025687.

[008] As these methods respond to a real change in the amount of combustible matter released or in the amount of residual combustible matter in the waste gas, they allow to reduce the level of this residual combustible matter, but cannot totally prevent its presence, in particular, but not only, when the main combustion zone is operated with a reducing atmosphere.

[009] Ventilation of residual combustible matter into the atmosphere is often not permitted for environmental reasons. Furthermore, evacuating the residual combustible matter from the process also corresponds to a loss of thermal energy, which would have been produced if all the combustible matter had been completely burned.

[0010] For these reasons, it is increasingly common to use an afterburner in which the residual combustible matter present in the waste gas is completely or substantially completely subjected to combustion with an afterburner oxidizer before the waste gas is released into the atmosphere.

[0011] Post-combustion of waste gas can be achieved by injecting post-combustion oxidizer only into the afterburner or by injecting post-combustion oxidizer and post-combustion fuel into the afterburner with a stoichiometric excess of post-combustion oxidizer relative to the post-burner fuel. -combustion. Injecting post-combustion oxidizer and post-combustion fuel into the afterburner is particularly useful when residual combustible matter is present in the waste gas in relatively low concentration and when injection of post-combustion fuel into the afterburner is necessary to increase the temperature to ignite and subject residual combustible matter to combustion.

[0012] The afterburners are advantageously equipped with energy recovery means to recover and exploit the thermal energy generated by the afterburner.

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

[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 after-combustion zone are optimized.

[0015] To this end, the present invention proposes a combined method of combustion and post-combustion in several stages.

General description

[0016] In a first step of the method of the invention, hereinafter referred to as “step a)”, a nominal post-combustion mode of operation is defined for a post-combustion zone. When the post-combustion zone is an oxidizer-only post-combustion zone, that is, an after-combustion zone that is not equipped or regulated for post-combustion fuel injection, but only for oxidant injection post-combustion mode in the same, the defined nominal post-combustion mode of operation features a nominal rate of post-combustion oxidizer injection into the post-combustion zone.

[0017] When, on the other hand, the post-combustion zone is an oxidizing fuel post-combustion zone, that is, an after-combustion zone equipped or regulated for the injection of post-combustion oxidizer and fuel. post-combustion, the nominal post-combustion mode of operation features both a nominal post-combustion oxidizer injection rate and a nominal post-combustion fuel injection rate into the post-combustion zone. The nominal post-combustion oxidizer injection rate and the nominal post-combustion fuel injection rate are such that a nominal stoichiometric excess of the post-combustion oxidizer over the post-combustion fuel is defined.

[0018] In another step of the method of the invention, referred to below as step b), the fuel and combustion oxidizer are supplied to the main combustion zone. The rate at which fuel is supplied to the main combustion zone in step b) is called the “actual fuel supply rate” and the rate at which the combustion oxidizer is supplied to the main combustion zone is called the “real rate oxidizer supply”.

[0019] In step c), the fuel so supplied (i.e., the fuel that is supplied to the main combustion zone at the actual fuel supply rate) is burned with the supplied oxidizer (i.e., the oxidant that is supplied to the main combustion zone at actual oxidant supply rate) within the main combustion zone where heat and waste gases are produced. As explained previously, said waste gas may contain residual combustible matter.

[0020] In step d) of the method, the residual gas is evacuated from the main combustion zone and introduced into the post-combustion zone.

[0021] In step e) an oxidizer (referred to as “post-combustion oxidant”) or a combination of post-combustion oxidizer and fuel (the latter being referred to as “post-combustion fuel”), with an excess of stoichiometric oxidant of post-combustion oxidizer in relation to post-combustion fuel, necessary for the post-combustion of the evacuated residual gas is introduced into said post-combustion zone.

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

[0023] When the post-combustion zone is an oxidizing fuel post-combustion zone, both the post-combustion oxidizer and the 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 over post-combustion fuel, said actual stoichiometric excess of post-combustion oxidizer is not used for combustion of the post-combustion fuel and is therefore available for the combustion of residual combustible matter from the waste gas within the post-combustion zone.

[0024] In step f), the evacuated residual gas is subjected to post-combustion in said post-combustion zone, with the post-combustion oxidizer, respectively, the excess post-combustion oxidizer. During said post-combustion, the residual combustible matter present in the waste gas evacuated from the main combustion zone is burned, resulting in an after-combustion gas.

[0025] In step g) the post-combustion gas is therefore evacuated 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 residual gas evacuated from the main combustion zone and/or a second level of one or more combustible substances in the gas afterburner evacuated from the afterburner zone are respectively monitored.

[0027] Then, 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 afterburner in the afterburner zone is regulated.

[0029] When the post-combustion zone is an oxidizer-only post-combustion zone, the actual post-combustion oxidant injection rate into the post-combustion zone is regulated in step j) as a function of the first control signal.

[0030] When the post-combustion zone is an oxidizing fuel post-combustion zone, the actual stoichiometric excess of post-combustion oxidant is regulated in step j) as a function of the first control signal through the actual post-oxidant injection rate. -combustion and/or the actual post-combustion fuel injection rate.

[0031] The first control signal is thus generated for the purpose of regulating the post-combustion. To the extent that this is achieved, the nature (digital (electronic) or analog (such as pneumatics)) or value of the first control signal is not important.

[0032] Monitoring in step h) can be monitoring a single combustible substance in the corresponding gas flow or monitoring multiple combustible substances in said gas flow. Examples of such combustible substances are H2, CO and VOCs (Volatile Organic Compounds). Various sensors for monitoring these combustible substances in a gas stream are commercially available. The substance or substances to be monitored will be selected depending on the type of process occurring in the main combustion zone and the type and levels of combustible substances that may be present in the gases waste evacuated from them. Essential in this regard is that the level of the monitored combustible substance(s) must be easily and quickly measurable and must provide a clear indication of the completeness or incomplete level of the main combustion (when the evacuated residual gas is monitored). , respectively from post-combustion (when post-combustion 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 said substance or substances in the gas post-combustion remains below the threshold value imposed by the aforementioned environmental regulations.

[0033] Monitoring can be direct monitoring, whereby the level of the substance or substances in the gas stream is measured, for example in situ, that is, in the gas stream itself, or by means of sampling. Monitoring may also be indirect monitoring of the substance or substances, whereby a property correlated with the level of one or more combustible substances is measured, such as the intensity of a flame generated when one or more combustible substances in the stream of gas is(are) placed in controlled contact with an oxidant, for example through an interstice or through controlled injection of an oxidant, such as air or oxygen, into the monitored gas, as for example described in EP-A -2561295. Indirect monitoring through a property correlated to the level of one or more combustible substances can also be based on a temperature or temperature change or evolution in the monitored gas when one or more combustible substances in the monitored gas stream are brought into control. contact with such an oxidant, as for example described in ES-A-2201885, ES-A-2207389 and WO-A-2006117336.

[0034] Waste gas is often evacuated from the main combustion zone at high temperatures. In this case, said waste gas contains significant amounts of waste heat. Furthermore, the post-combustion of combustible substances in the evacuated waste gas also generates heat. Consequently, a heat recovery installation can advantageously be provided in and/or downstream of the post-combustion zone. The thermal energy recovered by said heat recovery installation can be used as a heat source, for example, for the preheating of fuel and/or oxidizer upstream of the main combustion zone and/or the post-combustion zone, to heat or dry a charge supplied to the main combustion zone, for generating steam, or for generating mechanical or electrical power.

Post-combustion regulation based on a level of combustible substance(s) in the post-combustion gas

[0035] According to one embodiment of the method, step h) includes monitoring the level, referred to as the second level, of one or more combustible substances in the post-combustion gas evacuated from the post-combustion zone and the first control signal is generated in step i) based on said second monitored level (also referred to as 'signal B' or 'level B').

• 1) o fluxo real de oxidante pós-combustão seja regulado de modo a ser maior do que o fluxo nominal de oxidante póscombustão, no caso de uma zona de pós-combustão somente de oxidante ou
• 2) o excesso estequiométrico real do oxidante póscombustão seja regulado de forma a ser maior que o excesso estequiométrico nominal do oxidante pós-combustão, no caso de uma zona de oxidante combustível pós-combustão.

[0036] In this case, step a) may include defining an upper threshold B1upper for said second monitored level. When the second monitored level of residual combustible matter in the post-combustion gas exceeds the upper threshold B1upper, the first control signal generated in step i) causes, in step j)

• 1) the actual post-combustion oxidizer flow is regulated to be greater than the nominal post-combustion oxidizer flow in the case of an oxidizer-only post-combustion zone or
• 2) the actual stoichiometric excess of the post-combustion oxidizer is regulated to be greater than the nominal stoichiometric excess of the post-combustion oxidizer, in the case of a post-combustion combustible oxidizer zone.

[0037] In the following, this post-combustion mode of operation (as described in 1) and 2) above) is referred to as the “enhanced post-combustion” mode of operation.

[0038] Alternatively or in combination with the above, step a) may include defining a positive upper threshold RB1upper for a rate of change of the second monitored level (signal B). The rate of change corresponds to an increase or decrease per unit time in the monitored level of the monitored substance(s). The change is positive in case of an increase in the monitored level of one or more combustible substances and negative in case of a decrease in said monitored level. The change is zero (0) when the monitored level is constant. The positive upper threshold RB1superior thus corresponds to an upper threshold for the increase (per unit of time) of the second monitored level.

[0039] When the second monitored level increases at a rate greater than the upper threshold RB1upper, the first control signal generated in step i) causes, in step j) the afterburner zone to operate in the afterburner operating mode reinforced.

[0040] As described in WO-A-2010/022964, responding to a rate of increase of the first monitored level allows for a more efficient response to pronounced spikes in the level of combustible substances in the post-combustion gas.

[0041] As also indicated previously, monitoring the level of one or more combustible substances can be direct monitoring, whereby the level of the substance or substances in the gas stream is measured, or indirect monitoring, whereby a property correlated with the level of one or more combustible substances is measured. In the latter case, it is possible to compare the rate of change of the monitored level with the RB1superior threshold by comparing the rate of change of the measured correlated property with a threshold value for the change of the correlated property corresponding to the RB1superiorm threshold for the rate of change of the monitored level .

[0042] It may be useful to combine several criteria for post-combustion regulation, in particular the B1upper and RB1upper thresholds.

[0043] In the intensified post-combustion mode of operation, more post-combustion oxidizer is made available for the post-combustion of the evacuated waste gas when the post-combustion gas contains a higher level of combustible substance(s), thus ensuring a more complete afterburning of the waste gases evacuated into the afterburner zone than would be the case with the nominal afterburner mode.

[0044] Injection above nominal post-combustion oxidizer flows, respectively, an above-nominal excess of post-combustion oxidizer into post-combustion fuel is generally only economically justifiable when this is necessary in order to obtain the desired/required levels post-combustion gas and to limit the concentration of residual combustible matter in the post-combustion gas.

[0045] Therefore, when, in the course of said enhanced afterburner mode of operation, the monitored level returns to a “normal” level of combustible substances in the afterburner gas, the afterburner in the afterburner zone shall return to nominal post-combustion operating mode.

• 1) o fluxo real de oxidante pós-combustão é regulado para ser igual ao fluxo nominal de oxidante pós-combustão, no caso de uma zona de pós-combustão somente de oxidante ou
• 2) o fluxo real de oxidante pós-combustão e o fluxo real de combustível pós-combustão são regulados para serem iguais, respectivamente, ao fluxo nominal de oxidante póscombustão e ao fluxo nominal de combustível pós-combustão.

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

• 1) the actual post-combustion oxidizer flow is regulated to be equal to the nominal post-combustion oxidizer flow in the case of an oxidizer-only post-combustion zone or
• 2) the actual post-combustion oxidizer flow and the actual post-combustion fuel flow are regulated to be equal, respectively, to the nominal post-combustion oxidizer flow and the nominal post-combustion fuel flow.

[0047] In other words, the afterburner is then operated in the nominal afterburner operation mode.

[0048] In order to avoid instability (i.e. frequent switching) in the operation of the post-combustion zone, it may, in the case of such a modality, also be useful to define in step a) a predetermined duration Δtpca delay between the return of the post-combustion zone afterburner to nominal afterburner operation and the next afterburner regulation according to step j), wherein the duration Δtptraso is typically only a fraction of the duration Δtpcreforce. In practice, a duration Δtpca delay between 8 and 20 seconds, preferably between 8 and 12 seconds, has been found to be useful.

[0049] Embodiments with a predetermined duration Δtpcreforce of enhanced post-combustion operation may, in particular, be used for well-understood processes, in particular processes whereby it is known that the residual gas from the main combustion zone presents peaks of one or more of combustible substances and for which the duration of said peaks is approximately known, the duration of Δtpcreforce defined in step a) being selected depending on said approximate known duration, for example equal to or greater than said approximate known duration. The approximate duration of the peaks can be known from previous monitoring results from the same main combustion zone and process or combined main and post-combustion zone and processes, previous monitoring results obtained from one or more zones and processes main combustion zones and/or main combustion zones and post-combustion zones or processes, through simulation results of main combustion or combined main combustion and post-combustion zones and processes, or combinations thereof.

• 1) o fluxo real de oxidante pós-combustão é regulado para ser igual ao fluxo nominal de oxidante pós-combustão, no caso de uma zona pós-combustão somente de oxidante ou
• 2) o fluxo real de oxidante pós-combustão e o fluxo real de combustível pós-combustão são regulados para serem iguais, respectivamente, ao fluxo nominal de oxidante póscombustão e ao fluxo nominal de combustível pós-combustão, isto é, de modo que a pós-combustão seja operada no modo nominal de operação de pós-combustão.

[0050] According to an alternative embodiment, step a) comprises defining a lower threshold B1lower, where B1lower ≤ B1upper, for the second monitored level, that is, for the monitored level of combustible substance(s). ) in the post-combustion gas. When the second monitored level is below said lower threshold B1lower, the first control signal generated in step i) is such that in step j),

• 1) the actual post-combustion oxidizer flow is regulated to be equal to the nominal post-combustion oxidizer flow in the case of an oxidizer-only post-combustion zone or
• 2) the actual post-combustion oxidizer flow and the actual post-combustion fuel flow are regulated to be equal, respectively, to the nominal post-combustion oxidizer flow and the nominal post-combustion fuel flow, i.e. so that the afterburner is operated in the nominal afterburner operating mode.

[0051] Alternatively, or in combination with the above embodiment, step a) may include defining a lower threshold B2lower for the second monitored level and a corresponding time period ∆tB, where B2lower ≤ B1upper.

[0052] When the second monitored level remains below the lower threshold B2lower for at least the time period ∆tB, again, the first control signal generated in step i) is so that in step j) the afterburner is regulated to operate in the nominal post-combustion operation mode.

[0053] Setting the B1lower threshold thus provides a mechanism for the afterburner to return to the nominal afterburner mode when the monitored level of combustible substance(s) in the afterburner gas has fallen to a level sufficiently low (below B1inferior). Setting the lower threshold B2lower and the corresponding time period ∆tB provides a mechanism for the afterburner to return to the nominal afterburner mode when the monitored level of combustible substance(s) in the afterburner gas has consistently (i.e. , for at least ∆tB) remained sufficiently low (below B2inferior). This last characteristic is of particular interest for processes in which the monitored level of combustible substance(s) in the post-combustion gas tends to vary frequently, as applying the criterion during a period ∆tB increases the stability of post-combustion regulation. When the two options are combined, B1lower is typically selected below B2lower.

[0054] B1inferior or B2inferior can be equal to B1superior. In this case, the afterburner is regulated according to the principle that when the second monitored level is above the upper threshold value B1, the afterburner is regulated in the enhanced afterburner operating mode, and when the second level monitored is below the B1upper threshold value, or remains below the B1upper threshold value for a duration of at least ∆tB, the afterburner is regulated in the nominal afterburner operating mode.

[0055] In order to reduce the switching frequency from the boosted afterburner mode of operation to the nominal afterburner mode of operation, while still providing effective regulation, B1lower or B2lower may be selected below B1upper, i.e. B1inferior, B2inferior < B1superior.

[0056] A method in accordance with the present invention by which afterburning is regulated based on a monitored level of combustible substance(s) in the afterburning gas is useful for a wide range of major combustion processes , including those main combustion processes whereby the efficiency or productivity of a transformation process, (such as a melting or sintering process) conducted in the main combustion zone takes precedence over the control of the composition of the waste gas exiting said zone main combustion engine. Post-combustion regulated in this way substantially reduces or even eliminates emissions of combustible substances into the environment, while at the same time allowing greater energy recovery.

[0057] Afterburner regulation based on the monitored level of combustible substance(s) in the afterburner gas is particularly useful when monitoring the level of one or more combustible substances in the waste gas evacuated from the main combustion zone to upstream of the post-combustion zone is impractical. This may be the case, for example, when the main combustion zone and the post-combustion zone are located within the same reactor, with, for example, the main combustion zone located at the bottom of the reactor and the post-combustion zone. combustion being located near the top of the reactor above the main combustion zone.

Post-combustion regulation based on a level of combustible substance(s) in the waste gas

[0058] The first control signal, used to regulate the post-combustion in step j), can also be generated in step i) based on the first monitored level of one or more combustible substances in the waste gases evacuated from the combustion zone main and upstream of the post-combustion zone (level C).

[0059] Such monitoring of the first level of one or more combustible substances in the evacuated waste gas can be readily carried out when the evacuated waste gas flows from the main combustion zone to the post-combustion zone through a gas duct connecting the two zones.

[0060] The different modalities described above with respect to post-combustion regulation based on the second monitored level of combustible substance(s) in the post-combustion gas can be applied in a manner analogous to post-combustion regulation on the basis of first monitored level.

[0061] Thus, step a) may further comprise defining an upper threshold C1upper for said first monitored level. When the first monitored level exceeds the upper threshold C1superior, in step i) the first control signal is generated which causes, in step j), the afterburner to be regulated in the reinforced afterburner operating mode.

[0062] Alternatively or in combination with the above, step a) may include defining a positive upper threshold RC1upper for a rate of change of the first monitored level (level C). The positive upper threshold RC1upper corresponds to an upper threshold for the increase per unit time of the first monitored level.

[0063] When the first monitored level increases at a rate greater than the upper threshold RC1upper, the first control signal generated in step i) causes, in step j) the afterburner zone to operate in the afterburner operating mode reinforced.

[0064] For the return from the boosted afterburner operation mode to the nominal afterburner operation mode, step a) may, as already described above, include defining a predetermined duration Δtpcreforce of the afterburner operation. enhanced combustion. When the afterburner zone is in boosted afterburner operation for a period Δtpcreforce, a first control signal is generated in step i) which causes in step j) the afterburner zone to operate in the nominal boost mode. post-combustion operation.

[0065] Alternatively, the afterburner in the afterburner zone may be made to return to the nominal afterburner mode of operation when, in the course of said enhanced afterburner mode of operation, the monitored level returns to a level “ normal” to one or more combustible substances.

[0066] To this end, step a) may comprise:
i) define a lower threshold C1lower for the first monitored level,
and/or
ii) define a lower threshold C2lower for the first monitored level and a corresponding time period ∆tC

[0067] When the first monitored level is below the C1lower threshold or remains below the lower C2lower threshold for at least the time period ∆tC, the first control signal generated in step i) causes the afterburner to be regulated at the step j) to operate in the nominal post-combustion operation mode.

[0068] The comments made above regarding the thresholds B1inferior, B2inferior, ∆tB, B1superior and RB1superior apply mutatis mutandis to C1inferior, C2inferior, ∆tC, C1superior and RC1superior.

[0069] After-combustion regulation based on a monitored level of combustible substance(s) in the evacuated waste gas is particularly useful when, at least during certain phases of the combined main combustion and after-combustion process, the level of the monitored combustible substance(s) in the post-combustion gas is so low that the sensitivity of the monitoring method and/or device is insufficient to allow accurate detection of the level of the substance(s) fuel(s) monitored, so that no precise regulation of after-burning is possible during said phases.

[0070] Compared to regulation based on the monitored level of combustible substance(s) in the post-combustion gas, regulation of post-combustion based on the monitored level of combustible substance(s) in the evacuated waste gas It also has the advantage of allowing a faster response, where regulation is based on the flow of gas entering, rather than leaving, the post-combustion zone.

Post-combustion regulation based on a level of combustible substance(s) in the waste gas and post-combustion gas

[0071] It is also possible to combine the two possibilities above and their advantages, generating, in step i), the first control signal, which is used to regulate the post-combustion in step j), based on both the first monitored level of one or more combustible substances in the waste gas evacuated upstream from the post-combustion zone (level C) and the second monitored level of one or more combustible substances in the post-combustion gas (level B).

• i) definir um limiar superior C1’superior para o primeiro nível monitorado de uma ou mais de substâncias combustíveis no gás residual evacuado e
• ii) definir um limiar superior B1’superior para o segundo nível monitorado de uma ou mais de substâncias no gás póscombustão,

[0072] In this case, step a) may, for example, usefully comprise:

• i) define an upper threshold C1'upper for the first monitored level of one or more combustible substances in the evacuated waste gas and
• ii) define an upper threshold B1'upper for the second monitored level of one or more substances in the post-combustion gas,

[0073] When the first monitored level exceeds the upper threshold C1'upper or when the second monitored level exceeds the upper threshold B1'upper, the first control signal generated in step i) causes, in the post-combustion regulation step j), the afterburner is operated in enhanced afterburner operation mode.

[0074] In order to regulate the return to the nominal post-combustion mode of operation, step a) may also include:
i) define a lower threshold C1'lower for the first monitored level of one or more combustible substances in the evacuated residual gas and a lower threshold B1'lower for the second monitored level of one or more combustible substances in the post-combustion gas,
and/or
ii) define a lower threshold C2'lower for the first monitored level and a corresponding time period ∆tC' and a lower threshold B2'lower for the second monitored level and a corresponding time period ∆tB'

[0075] When the first monitored level is below the lower threshold C1'lower and the second monitored level is below the lower limit B1'or when the first monitored level remains below the lower threshold C2'lower for at least the period of time ∆ tC' and the second monitored level remains below the lower threshold B2'lower for at least the time period ∆tB', the first control signal generated causes the afterburner in step j) to be regulated to operate in the nominal mode of operation of post-combustion.

[0076] Again, the comments made above regarding the limits B1inferior, B2inferior, ∆tB and B1superior apply mutatis mutandis to C1'inferior, C2'inferior, ∆tC' and C1'superior, as well as to B1'inferior, B2'inferior, ∆tB' and B1'superior.

[0077] It is also technically possible to combine the lower threshold C1'lower for the first monitored level with the lower threshold B2'lower and the corresponding time period ∆tB' for the second monitored level or to combine the lower threshold B1'lower for the second monitored level of one or more combustible substances in the post-combustion gas with the lower threshold C2'lower and the corresponding time period ∆tC' for the first monitored level to regulate the return to nominal post-combustion operation.

[0078] When after-combustion is regulated based on a level of combustible substance(s) in both the residual gas and the after-combustion gas, it is naturally also possible to define in step a) a predetermined duration Δtpcreforce of the operation enhanced post-combustion operation, wherein, when the post-combustion zone is in enhanced post-combustion operation for a period of Δtpcreforce, a first control signal is generated in step i) which causes in step j) the post-combustion zone -combustion operates in the nominal post-combustion mode of operation.

[0079] Step a) may also include defining a predetermined duration Δtpca delay 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).

[0080] Post-combustion regulation can be carried out independently of any main combustion regulation.

[0081] Post-combustion regulation, according to any of the embodiments described above, can also be combined with main combustion regulation in the main combustion zone, where the residual gas is generated. In particular, post-combustion regulation can be advantageously combined with a main combustion regulation, so as to limit the level of combustible substances in the residual gas generated by the main combustion.

[0082] In this case, step a) of the method according to the present invention may include defining a nominal mode of main combustion operation for the main combustion zone with a nominal fuel supply rate and a nominal fuel supply rate. nominal oxidizer supply 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 second level (level B) monitored in step h) . The method also comprises a step k) of regulating the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone as a function of the second control signal generated.

Main Combustion Regulation Based on the Level of Combustible Substance(s) in the Waste Gas

[0083] According to one of these embodiments, the second control signal for regulating the main combustion is generated in step k) based on the first monitored level (level C) of one or more residual combustible substances in the waste gas evacuated from the zone main combustion engine.

[0084] In this case, step a) may additionally comprise defining an upper threshold A1upper for the first monitored level (level C). When the first monitored level exceeds said upper threshold A1upper, the second control signal generated in step i) causes, in step k), the actual oxidizer supply rate and the actual fuel supply rate for the zone of main combustion are regulated such that the ratio of the actual oxidizer supply rate to the actual fuel supply rate is greater than the ratio of the nominal oxidizer supply rate to the nominal fuel supply rate, making available thus additional oxygen in the main combustion zone, for example, for the combustion of combustible matter present in the charge being treated in the furnace. As a consequence, the amount of residual combustible substances evacuated from the main combustion zone together with the residual gas is reduced.

[0085] 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.

[0086] Alternatively or in combination with the above, step a) may include defining a positive upper threshold RA1upper for a rate of change of the first monitored level (level C). The rate of change corresponds to an increase or decrease per unit of time in the monitored level, expressed for example in ppm/s of the monitored substance(s). The rate of change is positive in case of increase and negative in case of decrease. The rate of change is zero (0) when the monitored level is constant. The positive upper threshold RA1superior thus corresponds to an upper threshold for the increase (per unit of time) of the first monitored level.

[0087] When the first monitored level increases at a rate greater than the upper threshold RA1upper, the second control signal generated in step i) causes, in step k), the main combustion zone to operate in the reinforced mode of operation of main combustion.

[0088] As described in WO-A-2010/022964, responding to a rate of increase of the first monitored level allows for a faster and more efficient response to changes in the composition of the evacuated residual gas and adjustment of the main mode of combustion operation.

[0089] As previously indicated, indirect monitoring is also possible, whereby a property correlated to the monitored level is measured and monitored.

[0090] It is preferable to combine both said criteria, that is, to combine the A1upper and RA1upper thresholds.

[0091] As already described in relation to the regulation of the afterburning occurring in the afterburning zone, step a) may include defining a predetermined duration Δtmcreboost of the enhanced main combustion operation, i.e., the time between the start of the operation of enhanced main combustion and return to nominal main combustion operation. According to such an embodiment, when the main combustion zone is in boosted main combustion operation for a period of Δtmcreforce, a second control signal is generated in step i) which causes in step k) the main combustion zone to operate in the nominal mode of main combustion operation, regardless of the level or levels monitored in step h) at that time.

[0092] In order to avoid instability (i.e. excessively frequent switching) in the operation of the main combustion zone, it may, in the case of such an embodiment, also be useful to define in step a) a predetermined duration Δtmca delay between the return of the combustion zone main combustion to nominal main combustion operation and the next main combustion regulation according to step k). The duration Δtmcatraso is typically only a fraction of the duration of Δtmcreforce. Δtmcatraso can, for example, be between 8 and 20 seconds, preferably between 8 and 12 seconds.

[0093] Embodiments with a predetermined duration Δtmcreboost of the enhanced main combustion operation are particularly useful for well-understood processes, in particular processes whereby the residual gases from the main combustion zone are known to present peaks of one or more combustible substances and wherein the duration of said peaks is approximately known, the duration Δtmcreforce defined in step a) being selected as a function of said approximate known duration, for example equal to or greater than said approximate known peak duration.

[0094] The comments made previously regarding the predetermined duration Δtpcreforce, can be applied mutatis mutandis to the duration Δtmcreforce.

[0095] As already described regarding the regulation of afterburning occurring in the afterburning zone, the main combustion can be made to return to the nominal mode of operation of the main combustion when the monitored level returns to “normal” values.

[0096] To this end, when the main combustion is regulated depending on the first monitored level, step a) may also include:
i. set a lower threshold A1lower for the first monitored level,
and/or
ii. set a lower threshold A2lower for the first monitored level and a corresponding time period ∆tA,
whereby, when the A1lower limit or the A2lower limit is used in combination with the A1upper limit, then A1lower, respectively A2lower ≤ A1upper, preferably A1lower, respectively A2lower < A1upper.

[0097] When the first monitored level is below the lower threshold A1lower and/or when the first monitored level remains below the lower threshold A2lower for at least the time period ∆tA, the second control signal generated in step i) causes, in step k), the actual oxidizer supply rate and the actual fuel supply rate for the main combustion zone to be regulated, so that the actual oxidizer supply rate and the actual fuel supply rate correspond respectively at the nominal oxidizer supply rate and the nominal fuel supply rate. In other words, the second control signal generated then causes the main combustion zone to operate in the nominal main combustion operating mode.

Main Combustion Regulation Based on the Level of Combustible Substance(s) in the Afterburner Gas

[0098] As previously indicated, it is not always easily feasible to monitor one or more combustible substances in the waste gas evacuated upstream of the post-combustion zone.

[0099] Regulation of the main combustion is also possible based on the second monitored level, that is, based on the monitored level of one or more combustible substances in the after-combustion gas evacuated from the after-combustion zone.

[00100] For this, the second control signal can be generated in step i) based on the second monitored level (level B).

[00101] In particular, step a) may then comprise defining an upper threshold D1upper for the second monitored level, that is, for the monitored level of one or more combustible substances in the post-combustion gas.

[00102] When the second monitored level exceeds the upper threshold D1upper, the second control signal generated in step i) causes, in step k), the main combustion zone to operate in the reinforced mode of main combustion operation.

[00103] For returning to the nominal main combustion operation mode, step a) may include defining a predetermined duration Δtmcreboost of the boosted main combustion operation, i.e., the time between the start of the boosted main combustion operation and return to nominal main combustion operation. According to such an embodiment, when the main combustion zone is in boosted main combustion operation for a period of Δtmcreforce, a second control signal is generated in step i) which causes in step k) the main combustion zone to operate in the nominal mode of main combustion operation, regardless of the level or levels monitored in step h) at that time.

[00104] As already explained, step a) may also include defining a predetermined duration Δtmca delay between the return of the main combustion zone to the nominal main combustion operation and the next regulation of the main combustion according to step k).

[00105] Alternatively, step a) may include:
i. set a lower threshold D1lower for the second monitored level,
and/or
ii. define a lower threshold D2lower for the second monitored level and a corresponding time period ∆tD, with D1lower, respectively D2lower ≤ D1higher, preferably D1lower, respectively D2lower < D1higher.

[00106] When the second monitored level is below the D1lower threshold or remains below the lower D2lower threshold for at least the time period ∆tD, the second control signal generated in step i) causes, in step k), the main combustion zone is operated in the nominal main combustion operating mode.

[00107] When the method comprises regulation of post-combustion and main combustion and the first and second control signals are based on the same level(s) monitored, in certain cases it may be possible to use the same criteria for switching to the respective boosted operating modes and for switching to the respective nominal operating modes for both main combustion and afterburning. In this case, a single control signal can be used as the first and second control signals.

[00108] The nominal operating mode or modes defined in step a) correspond to the combustion operating mode in the corresponding combustion zone (post-combustion zone, respectively main combustion zone) without regulation of feedback/feedforward proposed in accordance with the present invention.

[00109] The term “nominal” is therefore synonymous with the terms “target” or “defined” or “predefined” as used by the skilled person in, for example, the expressions “target flow” or “defined flow”.

[00110] The nominal operating modes and corresponding nominal parameters are thus established by the qualified combustion furnace operator depending on the nature/type/parameters of the process (e.g. incineration, melting, steam production etc.) and the equipment (both main combustion equipment and post-combustion equipment) to which the method is applied, according to the invention.

[00111] The nominal mode of operation of the main combustion therefore depends on the furnace in which the main combustion zone is located, the nature of the process conducted therein and the production rate of the furnace. As the post-combustion zone receives the waste gases generated in the main combustion zone, the nominal mode of post-combustion operation depends on the nature or properties and volume of the waste gas evacuated from the main combustion zone, which in turn depends on the furnace in which where the main combustion zone is located and the nature of the process conducted there. Specific environmental targets or regulations regarding air emissions may also have to be taken into account when setting the nominal afterburner mode.

[00112] The nominal mode(s) of operation, and the corresponding nominal parameters, may be constant over time, for example, in a furnace operated continuously without intentional changes in the load or product to be produced, combustion heat production rates, total production rates, etc.

[00113] Nominal operating modes and corresponding nominal parameters may also vary over time, for example, due to changes in the production rate or the nature of the load introduced into the furnace, the process conducted therein or the product to be produced by the same.

[00114] In particular, in the case of batch or semi-continuous processes, the nominal modes of operation and the corresponding nominal parameters may also vary cyclically over time, one cycle for each batch. An example of such a batch process is a melting batch process, wherein each batch introduced into the furnace passes through a heating phase, a melting phase and a refining phase, or at least two of said phases, including a fusion.

[00115] The nominal modes of post-combustion operation, as defined by the person qualified in step a), can therefore vary widely. It is a particular advantage of the method according to the present invention to be suitable and beneficial for such a wide range of modes of operation.

[00116] Given that the gas that is subjected to afterburning in the afterburner zone is the waste gas evacuated from the main combustion zone, whether or not the afterburner should be regulated to operate in the nominal or boosted mode of operation afterburner depends largely on the main combustion occurring in the main combustion zone, and this is independent of the monitored level used to generate the first control signal.

[00117] For this reason, when the method according to the present invention comprises both step j) for regulating the post-combustion and step k) for regulating the main combustion, and both the post-combustion and the main combustion are operating in the corresponding intensified operating mode, it may be advisable to generate the first and second control signals in step i) so that the main combustion mode first returns to its nominal mode of operation, when the conditions for this are met, and only then, and again when the conditions are met, can the afterburner return to its nominal mode of operation.

[00118] If the return of main combustion from the enhanced mode of main combustion operation to the nominal mode of main combustion operation, i.e. with reduced oxygen availability for main combustion, causes the level of residual substances in the gases to evacuated residual gas from the main combustion zone rises again, requiring additional oxygen for the effective afterburning of said evacuated residual gas, this approach allows the afterburner to remain in the enhanced afterburner mode of operation, thus maintaining any levels of substance(s) ) fuel(s) in the afterburner gas at an acceptable level. This is particularly important to comply with applicable emission standards. Only when the monitored level of residual combustible substance(s) in the evacuated waste gas remains low after the return to nominal operation of the main combustion zone, the post-combustion zone in turn can return to normal. its nominal mode of operation.

[00119] Such a modality naturally requires a corresponding selection of the criteria for switching the main combustion zone and the post-combustion zone to their respective nominal operating modes.

• ● primeiro para mudar a zona de combustão principal para o modo nominal de operação de combustão principal, de acordo com a etapa k),
• ● depois disso, esperar por um período de atraso ∆tatraso,
• ● quando, no final do período de atraso ∆tatraso, os critérios para mudar a zona de pós-combustão para o modo nominal de operação de pós-combustão ainda forem atendidos, para mudar a zona de pós-combustão para o modo nominal de operação de pós-combustão de acordo com a etapa j).

[00120] Thus, in embodiments of the present invention with post-combustion regulation (step j) and main combustion regulation (step k), it may be advantageous when the main combustion and post-combustion are in a reinforced operation mode and both meet the criteria for a switch to their respective nominal operating modes:

• ● first to switch the main combustion zone to the nominal main combustion operation mode according to step k),
• ● after that, wait for a delay period ∆tadelay,
• ● when, at the end of the delay period ∆tadelay, the criteria for switching the afterburner zone to the nominal afterburner mode of operation are still met, to switch the afterburner zone to the nominal mode of operation post-combustion according to step j).

[00121] Delay period ∆tadelay is then defined in the same way in step a) of the method according to the invention.

[00122] In step a), the delay period ∆tadelay is defined, so that the effect of the return of the main combustion zone to its nominal mode of operation on the composition of the residual gas leaving the main combustion zone is reflected by the monitored level or levels used to generate the first control signal used to regulate the afterburner in step j). The duration of the delay period ∆tadelay is thus defined taking into account the nature of the furnace in which the main combustion zone is located, the nature of the process conducted there and the length of time necessary for the residual gases leaving the combustion zone to combustion are to reach the point or points where monitoring occurs.

Example

[00123] The present invention and its advantages are illustrated in the following example with reference to the figure, which is a schematic representation of an industrial furnace equipped with an afterburner.

[00124] The figure shows a furnace 10, such as a furnace for melting aluminum scrap, in which the aluminum scrap can be contaminated with combustible contaminants, such as paint and lacquer in aluminum cans and oil.

[00125] The furnace 10 defines a main combustion zone inside it, which is heated by the combustion of fuel with oxidizer, referred to as “main combustion”.

[00126] To achieve this, the furnace 10 is equipped with one or more burner(s) 12 (only one burner is shown, although several burners may be present) fluidly connected to a fuel source 13 and a source of oxidant 14. The oxidant supplied by the oxidant source 14 is preferably an oxygen-rich oxidant (that is, an oxidant with an oxygen content greater than that of ambient air), such as oxygen-enriched air or oxygen.

[00127] The fuel and oxidizer are supplied to one or more burner(s) 12 in a controlled manner, that is, at a regulated flow rate. The main combustion of the fuel with the oxidizer in the furnace 10 generates heat and combustion gases within the main combustion zone of the furnace 10 (said main combustion is schematically represented by the flame 11, although said combustion may be in the form of multiple flames or flameless combustion).

[00128] Instead of or in combination with the burners 12, which inject fuel and oxidizer, the fuel and oxidizer can also be supplied separately to the main combustion zone, for example, for staged or flameless combustion purposes.

[00129] In the case of a charge 15 of aluminum scrap to be melted in the main combustion zone, a reducing atmosphere 16 above the charge 15 is desired in order to limit any loss of metallic aluminum due to oxidation. A lower than stoichiometric amount of oxidizer (compared to the amount of fuel) is therefore supplied to the burner(s) 12. As a consequence, the waste gas 17 which is evacuated from the furnace 10 contains combustible substances.

[00130] As the contaminated aluminum charge 15 is heated, combustible contaminants are also typically released by the charge 15 into the atmosphere 16 of the main combustion zone in an uncontrolled manner, that is, with peaks and drops in the amount of fuels released. Said released combustible contaminants contribute to the level of combustible substances in the waste gas 17.

[00131] Other furnaces operate with a neutral atmosphere (that is, an atmosphere that is neither oxidizing nor reducing) or with an oxidizing atmosphere in the main combustion zone. In this case, the reference level of combustible substances in the waste gas of the main combustion zone is typically zero or close to zero, while occasional spikes of combustible substances may be observed in the evacuated waste gas.

[00132] In step a) of the method according to the invention, a nominal main combustion mode of operation is defined for the main combustion zone with a corresponding nominal fuel supply rate and a nominal oxidizer supply rate corresponding to the main combustion zone through its burner(s) 12.

[00133] For example, in the case of a batch aluminum scrap melting process, the furnace operation may comprise an initial heating phase, in which the solid charge 15 is heated to the aluminum melting temperature, a melting, 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 pouring temperature. For each phase, a constant or evolving nominal fuel supply rate and nominal oxidizer supply rate for the furnace 10 are defined. As explained above, to prevent oxidation of the aluminum charge 15, the nominal oxidizer supply rate may be maintained substoichiometric with respect to the nominal fuel supply rate, in particular during the smelting and refining phases of the process (as the charge melt is more susceptible to oxidation).

[00134] The residual gas 17 that has been evacuated from the main combustion zone of the furnace 10 is transported through the conduit 18 to the afterburner/afterburner zone 19. The post-combustion oxidizer and the post-combustion fuel combustion are injected into the post-combustion zone 19 in a controlled manner (i.e., at regulated flow rates) to subject combustible substances present in the evacuated waste gas 17 to combustion with a controlled stoichiometric excess of post-combustion oxidant relative to the post-combustion fuel. The post-combustion gas thus obtained 23 is evacuated from the post-combustion zone 19.

[00135] In the illustrated embodiment, the afterburner 19 is equipped with a burner 20 and a separate afterburner oxidizer injector 25. The afterburner fuel is supplied to the burner 20 together with a stoichiometric amount of afterburner oxidizer. Additional afterburner oxidizer is supplied to injector 25 so as to provide a stoichiometric excess of afterburner oxidizer (compared to afterburner fuel) in afterburner 19. Again, while only one afterburner 20 and one injector 25 are shown, the afterburner 19 may be equipped with multiple burners 20 and/or multiple injectors 25. The use of multiple injectors 25 may, in particular, be useful for ensuring intimate mixing of the stoichiometric excess of afterburner oxidant with residual gas 17 that enters afterburner 19.

[00136] In the illustrated embodiment, the post-combustion zone 19 is separated from the main combustion zone by the conduit 18 through which the residual gas 17 is transported. In other embodiments, the main combustion zone and the post-combustion zone may be located in different parts of the same casing, the residual gas generated in the main combustion zone of the casing moving to the post-combustion zone of said casing. In this case, the post-combustion zone is advantageously located above the main combustion zone, so as to benefit from the natural upward draft of the waste gas generated.

[00137] In the illustrated example, the level of combustible substances, such as H2, CO and/or VOCs, in the waste gas evacuated 17 in conduit 18 is determined using sensor 21.

[00138] In the illustrated embodiment, sensor 21 is a sensor, as described in co-pending patent application EP21152977. The disclosure in said co-pending patent application of said sensor and its operation are incorporated herein by reference. Valve 103 controls the flow of oxidant from oxidant source 14 which is supplied to sensor 21.

[00139] Other sensors and monitoring devices and methods for monitoring the levels of combustible substances in waste gas 17 are commercially available and can be used in the context of the present invention.

[00140] The level of combustible substances in the evacuated residual gas 17 detected by the sensor 21 is transmitted to the central control unit 22 and compared with a reference value stored therein. Said reference value corresponds to the level of combustible substances that the residual gas 17 would normally be expected to present at a given stage of the batch process and operating parameters, including actual oxidant supply rate and fuel supply rate to the ) burner(s) 12 of oven 10.

[00141] When said comparison by control unit 22 reveals that the detected level of combustible substances in waste gas 17 is significantly higher than the reference level, this is indicative of a peak in the release of fuels by load 15 in the main combustion zone .

[00142] The level of combustible substances in the waste gas 17 detected by the sensor 21 is, for example, considered by the control unit 22 as significantly higher than the reference value when the detected level of combustible substances in the waste gas 17 is higher than a value upper threshold A1superior also defined in step a), where the threshold value A1superior is greater than the reference value.

[00143] In this case, the control unit 22 generates a control signal, referred to as a 'second control signal', which regulates the main fuel controller 101 and the main oxidizer controller 102 so that the furnace is operated in boost mode of main combustion operation, wherein the ratio of the actual oxidizer supply rate to the actual fuel supply rate to the burner(s) 12 exceeds the ratio of the nominal oxidizer supply rate to the rate nominal fuel supply. In this way, more oxygen is made available within the main combustion zone for the combustion of the combustible matter released within said main combustion zone, without generating an oxidizing atmosphere 16 above the load 15.

[00144] According to one embodiment, after a predetermined duration Δtmcreforce, a second different control signal is output such that the main fuel controller 101 causes the actual rate of fuel supply to the burner(s) es) 12 corresponds to the nominal fuel supply rate and such that the main oxidizer controller 102 causes the actual oxidant supply rate to the burner(s) 12 to correspond to the nominal oxidizer supply rate.

[00145] Alternatively, the return to nominal main combustion may be based on the level of combustible substances in the evacuated residual gas 17 detected by sensor 21. For example, when comparison by the central control unit 22 between the detected level of combustible substances in the gas evacuated residual 17 sensor 21 is substantially equal to or even lower than the reference value stored in the central control unit 22, the control unit 22 generates a second control signal which, in the manner described above, causes the main combustion in the furnace 10 return to nominal main combustion operating mode.

[00146] Considering that the above-described regulation of the main combustion can maintain the levels of combustible substances in the waste gases 17 leaving the furnace 10 within certain limits, it does not solve the problem of the release into the atmosphere of the detected levels of combustible substances in gas residual evacuated 17.

[00147] This problem is solved by subjecting the evacuated residual gas 17 to post-combustion in the post-combustion zone 19.

[00148] When the temperature of the waste gas 17 entering the post-combustion zone 19 and the nature and concentration of combustible substances in the waste gas 17 are such that ignition of the combustible substances is ensured, the post-combustion of the gas residual 17 can be achieved by injecting only post-combustion oxidant into the post-combustion zone 19, for example through injector 25.

[00149] In many cases, both the afterburner oxidizer and the afterburner fuel will be injected into the afterburner zone 19 to create a permanent afterburner flame in zone 19. The afterburner of combustible substances in the gas residual 17 is then achieved in the post-combustion zone 19 by a stoichiometric excess of post-combustion oxidizer relative to the post-combustion fuel. In the illustrated embodiment, the post-combustion fuel and corresponding stoichiometric amount of post-combustion oxidizer are supplied to the burner 20, while the excess post-combustion oxidizer for the post-combustion of the waste gas is injected into the post-combustion zone 19 through the injector 25.

[00150] In step a) of the method, according to the invention, a nominal mode of post-combustion operation is defined for the post-combustion zone 19, with a corresponding nominal post-combustion oxidant injection rate and nominal rate post-combustion fuel injection. The nominal total post-combustion oxidizer injection rate and the nominal post-combustion fuel injection rate, and in particular the ratio between the two, is defined based on the temperature and composition that the evacuated residual gas 17 would normally have at the enter the afterburner/afterburner zone 19, given the phase and parameters of the batch process in the furnace 10, and taking into account any limitations on the level of combustible substances that a gas to be released to the atmosphere imposed by, for example, environmental regulations. In other words, the nominal post-combustion oxidizer and fuel injection rates are such that they would result in the effective reduction by post-combustion of the combustible substances in the waste gas 17 to the level normally expected at a given stage of furnace operation 10.

[00151] In the illustrated embodiment, another control signal, referred to as 'first control signal', is generated by the control unit 22 based on the above-described comparison between the level of combustible substances in the evacuated residual gas 17 detected by the sensor 21 and the reference value stored in the central control unit 22. Said first control signal is sent to the first flow controller 104, which regulates the excess post-combustion oxidizer to the injector 25.

[00152] The post-combustion fuel flow and the corresponding stoichiometric flow of post-combustion oxidizer to post-combustion burner 20 are regulated respectively by controller 106 and controller 105.

[00153] When comparison by the control unit 22 reveals that the detected level of combustible substances in the waste gas 17 is significantly higher than the reference value, for example, comparing the detected level with an upper threshold level C1upper defined in step a) , the control unit 22 generates a first control signal, which causes the first flow controller 104 to increase the excess post-combustion oxidizer to the injector 25 to a boosted excess post-combustion oxidizer, which is greater than excess post-combustion oxidizer during the nominal mode of post-combustion operation such that the ratio of the total actual post-combustion oxidizer injection rate to the actual post-combustion fuel injection rate is greater than the ratio between the nominal total post-combustion oxidizer injection rate and the nominal post-combustion fuel injection rate. In this way, the extra available post-combustion oxidizer is made available for post-combustion of the peak in combustible substances present in the evacuated waste gas 17 (enhanced mode of post-combustion operation).

[00154] The criteria (such as C1upper) for a “significantly higher” level for afterburner regulation may be the same or different from the criteria (such as A1upper) for a “significantly higher” level for main combustion regulation. .

[00155] When the comparison by the central control unit 22 between the level of combustible substances in the evacuated residual 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, whereby, again , the same or different criterion(s) may be applied to “substantially the same” with respect to the main combustion regulation and the afterburner regulation, the first control signal generated by the control unit 22 does The second flow controller 104 regulates the actual post-combustion oxidant injection rate to the injector 25 so that the actual stoichiometric excess of the post-combustion oxidant injected into the post-combustion zone 19 corresponds to the nominal stoichiometric excess of the oxidant. post-combustion (the actual post-combustion fuel flow to burner 20 and the actual post-combustion oxidizer flow to burner 20 being regulated, also based on the first control signal, by controllers 106 and 105, respectively, from such that the actual post-combustion fuel flow to burner 20 corresponds to the nominal post-combustion fuel flow and such that the post-combustion oxidizer flow to burner 20 is stoichiometric with said actual/nominal post-combustion fuel flow ).

[00156] In the illustrated embodiment, extra security has been provided for the control of afterburner in the afterburner zone 19, in which a second sensor 24, of the same type as the first sensor 21, is present in the afterburner exhaust 19 and monitors the level of one or more combustible substances in the post-combustion gas 23 leaving the post-combustion zone 19.

[00157] An upper threshold B1upper for said level of combustible substance(s) in post-combustion gas 23 was determined in step a), said upper threshold B1upper being greater than a reference value for the level of the substance(s) fuel(s) monitored in post-combustion gas 23. Said reference value corresponds to the level of combustible substances that post-combustion gas 23 would normally be expected to present in a given phase of the batch process and to the operating parameters of the main zone and the post-combustion zone.

[00158] An upper threshold B1upper is also at most equal to or preferably below the maximum level of said combustible substance(s) permitted by local environmental regulations.

[00159] The control unit 22 compares the level detected by the second sensor 24 with the upper threshold B1upper and, when the level detected by the second sensor 24 is greater than the threshold B1upper, the control unit 22 generates a first control signal that causes the afterburner zone 19 to operate in the enhanced afterburner operation as described above, and this regardless of the level detected by the first sensor 21.

[00160] Similarly, when the level detected by the first sensor 21 is higher than the upper threshold level C1, the central control unit 22 generates a first control signal that causes the post-combustion zone 19 to operate in post-combustion operation. -reinforced combustion, regardless of the level detected by the first sensor 24.

[00161] Only when 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, the control unit 22 generates 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 oxidizer flow to the post-combustion zone 19 to correspond to the respective nominal flow rates, wherein, as described above, The post-combustion fuel and stoichiometric oxidizer flow are supplied to the burner 20, while the stoichiometric excess post-combustion oxidant is supplied to the injector 25.

[00162] Considering that the use of a central control unit 22 is illustrated, it will be appreciated that the method according to the invention can also be implemented with a distributed or modular system using PLCs or the like to compare detected values with reference values and/or to regulate fuel and/or oxidizer supplies.

[00163] The modality described above provides maximum control of the level of combustible substances in the post-combustion gas and thus the level of combustible substances capable of being released into the atmosphere.

[00164] When the reduction of combustible substances in the post-combustion gas is less critical, a less elaborate control system for post-combustion can be used, for example, based only on the level of combustible substances detected by one of the sensors 21 and 24.

[00165] Reference numbers in the drawing: 10:
furnace - 11: flame - 12: furnace burner 10 - 13: fuel source - 14: oxidizer source - 15: load - 16: atmosphere in the furnace 10 - 17: evacuated waste gases - 18: waste gas duct - 19 : afterburner/afterburner zone - 20: afterburner burner 19 - 21: first sensor - 22: central control unit - 23: afterburner gas - 24: second sensor - 25: gas oxidizer injector afterburner 19 - 101: fuel flow controller to the burner 12 - 102: oxidizer flow controller to the burner 12 - 103: oxidizer flow controller to the first sensor 21 - 104: oxidizer flow controller to the injector 25 - 105: oxidizer flow controller for the burner 20 - 106: fuel flow controller for the burner 20 - 107: oxidizer flow controller for the second sensor 24.

Claims (18)

Combined combustion and post-combustion method, characterized by the fact that it comprises the steps of:
a) define a nominal mode of post-combustion operation for an after-combustion zone (19) with

• (1) or a nominal rate of post-combustion oxidant injection into the post-combustion zone (19), when the post-combustion zone (19) is an oxidizer-only post-combustion zone that is not equipped for post-combustion injection. post-combustion fuel combustion in it,
• (2) or a nominal post-combustion oxidizer injection rate and a nominal post-combustion fuel injection rate into the post-combustion zone (19), when the post-combustion zone (19) is a fuel post-combustion zone oxidizer that is equipped for the injection of post-combustion oxidizer and post-combustion fuel into it, the nominal post-combustion oxidizer injection rate and the nominal post-combustion fuel injection rate defining a nominal stoichiometric excess of the post-combustion oxidizer in the which refers to post-combustion fuel,

b) supplying fuel and combustion oxidizer to a main combustion zone (10) at, respectively, an actual fuel supply rate and an actual oxidizer supply rate,
c) subject to combustion the fuel supplied with the oxidizer supplied in the main combustion zone (10), thus producing heat and residual gases, which may contain residual combustible matter,
d) evacuate the residual gas from the main combustion zone (10) and introduce the evacuated residual gas (17) into the post-combustion zone (19),
e) inject, into the post-combustion zone (19):

• (1) post-combustion oxidizer at an actual post-combustion oxidant injection rate and no post-combustion fuel, when the post-combustion zone (19) is an oxidizer-only post-combustion zone, or
• (2) post-combustion oxidizer at an actual post-combustion oxidizer injection rate and post-combustion fuel at an actual post-combustion fuel injection rate, when the post-combustion zone (19) is a post-combustion zone -oxidizing fuel combustion, the actual post-combustion oxidizer injection rate and the actual post-combustion fuel injection rate defining an actual stoichiometric excess of post-combustion oxidizer relative to the post-combustion fuel,

f) post-combustion of the evacuated residual gas (17) in the post-combustion zone (19) with:

• (1) the post-combustion oxidizer in the case of an oxidizer-only post-combustion zone or
• (2) the actual stoichiometric excess of post-combustion oxidizer in the case of an oxidizing fuel post-combustion zone, thereby generating an after-combustion gas (23),

g) evacuate the post-combustion gas (23) from the post-combustion zone (19),
h) monitoring a first level of one or more combustible substances in the residual gas (17) evacuated from the main combustion zone (10) and/or a second level of one or more combustible substances in the post-combustion gas (23) evacuated from the post-combustion zone (19),
i) generating a first control signal based on the level or based on one or both levels monitored in step h), and
j) regulate, depending on the first control signal:

• (1) the actual rate of post-combustion oxidant injection in the case of an oxidant-only post-combustion zone (19) or
• (2) the actual stoichiometric excess of post-combustion oxidizer through the actual post-combustion oxidizer injection rate and/or the actual post-combustion fuel injection rate in the case of an oxidizing fuel post-combustion zone (19).

Method according to claim 1, characterized in that the first control signal is generated based on the second monitored level. Method according to claim 2, characterized by the fact that:
step a) comprises:
i. set an upper threshold B1upper for the second monitored level,
and whereby, when the second monitored level exceeds the upper threshold B1superior, the first control signal generated causes, in step j)

• (1) the actual post-combustion oxidizer flow is greater than the nominal post-combustion oxidizer flow in the case of an oxidizer-only post-combustion zone (19) or
• (2) the actual stoichiometric excess of post-combustion oxidizer is greater than the nominal stoichiometric excess of post-combustion oxidizer, in the case of an oxidizing fuel post-combustion zone (19).

Method according to claim 2 or 3, characterized by the fact that the step
a) additionally comprises:
i. set a lower threshold B1lower for the second monitored level, and/or
ii. set a lower threshold B2lower for the second monitored level and a corresponding time period ∆tB,
whereby, when the second monitored level is below the B1lower threshold or remains below the lower B2lower threshold for at least the time period ∆tB, the first control signal generated causes in step j):

• (1) the actual post-combustion oxidizer flow equals the nominal post-combustion oxidizer flow in the case of an oxidizer-only post-combustion zone (19) or
• (2) the actual post-combustion oxidizer flow and the actual post-combustion fuel flow are equal, respectively, to the nominal post-combustion oxidizer flow and the nominal post-combustion fuel flow in the case of an after-burn zone. combustion of oxidizing fuel (19).

Method according to claim 1, characterized in that the first control signal is generated based on the first monitored level. Method according to claim 5, characterized by the fact that the step
a) additionally comprises:
i. define an upper threshold C1upper for the first monitored level, and by which,
when the first monitored level exceeds the upper threshold C1upper, the first control signal generated causes, in step j)

• (1) that the actual post-combustion oxidizer flow is greater than the nominal post-combustion oxidizer flow in the case of an oxidizer-only post-combustion zone (19) or
• (2) that the actual stoichiometric excess of post-combustion oxidizer is greater than the nominal stoichiometric excess of post-combustion oxidizer, in the case of an oxidizing fuel post-combustion zone (19).

Method according to claim 5 or 6, characterized by the fact that the step
a) comprises:
i. set a lower threshold C1lower for the first monitored level, and/or
ii. set a lower threshold C2lower for the first monitored level and a corresponding time period ∆tC,
whereby, when the first monitored level is below the C1lower threshold or remains below the lower C2lower threshold for at least the time period ∆tC, the first control signal generated causes in step j):

• (1) the actual post-combustion oxidizer flow equals the nominal post-combustion oxidizer flow in the case of an oxidizer-only post-combustion zone (19) or
• (2) the actual post-combustion oxidizer flow and the actual post-combustion fuel flow are equal, respectively, to the nominal post-combustion oxidizer flow and the nominal post-combustion fuel flow, in the case of a post-combustion zone -combustion of oxidizing fuel (19).

Method according to claim 1, characterized in that the first control signal is generated based on the first and second monitored levels. Method according to claim 8, characterized by the fact that: the step
a) comprises:
i. set an upper threshold C1'upper for the first monitored level and
ii. set an upper threshold B1'upper for the second monitored level,
and whereby when the first monitored level exceeds the upper threshold C1'upper or when the second monitored level exceeds the upper threshold B1'upper, the first control signal generated causes, in step j)

• (1) the actual post-combustion oxidizer flow is greater than the nominal post-combustion oxidizer flow in the case of an oxidizer-only post-combustion zone (19) or
• (2) the actual stoichiometric excess of post-combustion oxidizer is greater than the nominal stoichiometric excess of post-combustion oxidizer, in the case of an oxidizing fuel post-combustion zone (19).

Method according to claim 9, characterized by the fact that:
step a) comprises:
iii. define a lower threshold C1'lower for the first monitored level and a lower threshold B1'lower for the second monitored level, and/or
iv. define a lower threshold C2'lower for the first monitored level and a corresponding time period ∆tC' and a lower threshold B2'lower for the second monitored level and a corresponding time period ∆tB',
whereby, when the first monitored level is below the lower threshold C1'lower and the second monitored level is below the lower limit B1'or when the first monitored level remains below the lower threshold C2'lower for at least the period of time ∆ tC' and the second monitored level remains below the lower threshold B2'lower for at least the time period ∆tB', the first control signal generated causes in step j):

• (1) the actual post-combustion oxidizer injection rate is equal to the nominal post-combustion oxidizer injection rate in the case of an oxidizer-only post-combustion zone (19), or
• (2) the actual post-combustion oxidizer injection rate and the actual post-combustion fuel injection rate are respectively equal to the nominal post-combustion oxidizer injection rate and the nominal post-combustion fuel injection rate , in the case of an oxidizing fuel post-combustion zone (19).

Method, according to any one of claims 3, 6 and 9, characterized by the fact that step a) further comprises defining a predetermined duration Δtpcreforce and, whereby, when, in step j)

• (1) the actual post-combustion oxidizer flow was greater than the nominal post-combustion oxidant flow in the case of an oxidizer-only post-combustion zone (19) or
• (2) the actual stoichiometric excess of the post-combustion oxidizer was greater than the nominal stoichiometric excess of the post-combustion oxidizer in the case of an oxidizing fuel post-combustion zone (19),

for a period Δtpcreforce, a first control signal is generated in step i) which causes in step j)

• (1) the actual post-combustion oxidizer injection rate is equal to the nominal post-combustion oxidizer injection rate in the case of an oxidizer-only post-combustion zone (19), or
• (2) the actual post-combustion oxidizer injection rate and the actual post-combustion fuel injection rate are respectively equal to the nominal post-combustion oxidizer injection rate and the nominal post-combustion fuel injection rate , in the case of an oxidizing fuel post-combustion zone (19).

Method according to any of the preceding claims, characterized by the fact that:
step a) includes defining a nominal main combustion mode of operation for the main combustion zone (10) with a nominal fuel supply rate and a nominal oxidizer supply rate for the main combustion zone (10) ,
step i) includes generating a second control signal based on the first level and/or second level(s) monitored in step h),
and whereby the method additionally comprises:
a step k) of regulating the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone (10) as a function of the second control signal. Method according to claim 12, characterized by the fact that in step i) the second control signal is generated based on the first monitored level. Method according to claim 13, characterized by the fact that step a) additionally comprises:
i. set an upper threshold A1upper for the first monitored level, and/or
ii. define a positive upper threshold RA1upper for a rate of change of the first monitored level, and whereby - when the first monitored level exceeds the upper threshold A1upper,
and/or
- when the first monitored level increases at a rate greater than the upper threshold RA1superior,
the second control signal generated causes, in step k), the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone (10) to be regulated, so that the ratio between the rate actual oxidizer supply rate and the actual fuel supply rate is greater than the ratio of the nominal oxidizer supply rate to the nominal fuel supply rate. Method according to claims 13 or 14, characterized by the fact that:
step a) comprises:
i. set a lower threshold A1lower for the first monitored level,
and/or
ii. set a lower threshold A2lower for the first monitored level and a corresponding time period ∆tA,
and whereby:
- when the first monitored level is below the lower threshold A1lower
and/or
- when the first monitored level remains below the lower threshold A2lower for at least the time period ∆tA,
the second control signal generated causes, in step k), the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone (10) to be regulated, so that the actual supply rate of oxidizer and the actual fuel supply rate correspond respectively to the nominal oxidizer supply rate and the nominal fuel supply rate. Method according to claim 12, characterized by the fact that in step i) the second control signal is generated based on the second monitored level. Method according to claim 16, characterized by the fact that:
step a) comprises:
i. set an upper threshold D1upper for the second monitored level,
and for which,
When the second monitored level exceeds the upper threshold D1upper, the second control signal generated causes, in step k), the actual oxidizer supply rate and the actual fuel supply rate to the main combustion zone (10) are regulated so that the ratio of the actual oxidizer supply rate to the actual fuel supply rate is greater than the ratio of the nominal oxidizer supply rate to the nominal fuel supply rate. Method according to claim 17, characterized by the fact that step a) additionally comprises:
ii. set a lower threshold D1lower for the second monitored level,
and/or
iii. set a lower threshold D2lower for the second monitored level and a corresponding time period ∆tD,
whereby, when the second monitored level is below the threshold B1lower or remains below the lower threshold B2lower for at least the time period ∆tB, the second control signal generated causes in step k), the actual supply rate of oxidizer and the actual fuel supply rate to the main combustion zone (10) are regulated so that the actual oxidizer supply rate and the actual fuel supply rate correspond respectively to the nominal oxidizer supply rate and the nominal fuel supply rate.

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