EP3365603B1 - Firing system and method for operating same - Google Patents

Description

The invention relates to a firing system for the combustion of solid fuel supplied to a fuel bed with a primary combustion stage with a first supply device for supplying a first oxygen-containing reaction gas and carrying out an incomplete combustion process with generation of a first volume flow and a secondary combustion stage downstream of the first combustion stage with one of a second volume flow of a second oxygen-containing reaction gas supplied to a second supply device into a second supply device supplying an exhaust gas space above the fuel bed.

Firing systems, that is to say plants which convert chemically bound energy into thermal energy, such as waste incineration plants, are sufficiently known from the prior art. In this case, solid fuel is transported to a fuel bed and, if necessary, burned with the aid of an additional fuel source with liquid or gaseous fuel with the supply of reaction gas, for example air or, for example, oxygen-enriched air, by means of a first supply device, for example a fan, in a first combustion stage. In such combustion systems, the aim is to minimize the emission of pollutants in their exhaust gases, for example in order to comply with or fall below the legally applicable limit values. For example, the first combustion stage can be followed by a second combustion stage, which in the exhaust gas chamber downstream of the first combustion stage supplies a second reaction gas by means of a second feed device in order to oxidize afterburning incompletely oxidized pollutants, for example carbon monoxide in carbon dioxide or incompletely burned hydrocarbons. It has been shown here that nitrogen oxides are reduced with a constant supply of reaction gas, for example air in the second combustion stage in a staged operating mode, this reduction is often only possible with great effort in order to comply with current limit values.

It is therefore in the DE 103 47 340 A1 proposed a device and a method for optimizing the exhaust gas burnout in combustion plants, in which a second secondary supply of the second reaction gas takes place via nozzles which are controlled as a function of sensors for the detection of incompletely burned compounds for the introduction of reaction gas.

From the DE 20 2006 005 464 B3 A method for reducing nitrogen oxide on the primary side in a two-stage combustion process is known, in which the temperature of the exhaust gas when the fuel leaves the exhaust gas burnout zone is adjusted by supplying a water-gas mixture in such a way that fewer nitrogen oxides are formed.

From the WO 94/24484 A1 A method for reducing all emissions from the incineration of waste is known, in which part of the flue gas generated in the incineration plant is recycled and pure oxygen is added to it. Furthermore, the state of the art for preventing nitrogen oxide formation is combustion with excess air, that is to say with a stoichiometric supply of the reaction gas, in order to achieve better mixing of the (gaseous) fuel and the first reaction gas in the form of the combustion air. The excess air leads to better mixing, but the excess air (oxygen) also leads to a stronger oxidation of the fuel and thus to an increased formation of nitrogen oxides.

The WO 03/083370 A1 describes a method and a device for regulating the primary and secondary air injection of a waste incineration plant.

The DE 43 01 082 A1 describes a method for supplying an O2-containing combustion gas for the combustion of lumpy combustible material, in particular garbage, into a combustion chamber with an associated grate of an incinerator, in which part of the combustion gas is supplied as a primary gas through the grate and the other part of the combustion gas at least as Secondary gas is supplied in a quantity-controlled manner in at least one jet, the carbon monoxide content being recorded by means of a measuring sensor and being fed to an evaluation and control unit. The object of the invention is the development of a firing system and a method for controlling it, in which the nitrogen oxide contents are reduced in a simple manner. In particular, a method for operating a firing system is to be proposed which can be applied to existing firing systems without major modifications.

The object is achieved by the device of claim 1 and the method of claim 6. The claims dependent on these claims represent advantageous embodiments of the device of claim 1 or the method of claim 6.

The proposed method is intended for combustion technology in multi-stage combustion systems. Multi-stage combustion systems as proposed are advantageously used in combustion systems for solid fuels, in which the solid fuel in the first combustion stage is converted into an exhaust gas and thus a first volume flow and the first volume flow in one while supplying a first reaction gas such as air or oxygen-enriched air a further combustion stage, for example with a second volume flow of a second reaction gas, for example air and / or at least partially recirculated exhaust gas, optionally with further gas additives, for example ammonia, and / or water vapor. These multi-stage, for example two-stage processes can be provided, for example, in the area of grate furnaces, for example in waste incineration plants, biomass furnaces, special waste incineration plants or the like with rotary kiln, fluidized bed, fixed bed, deck oven technology or the like. The second firing stage essentially serves the and over-stoichiometric treatment of the exhaust gases of the first volume flow of the first combustion stage with the second volume flow of the second reaction gas and thus the most complete possible burnout of gas species such as carbon monoxide and organic hydrocarbons with air or recirculated flue gas. In this case, the combustion of the fuel in the first combustion stage takes place sub-stoichiometrically, so that a residual content of incompletely or not fully oxidized components, for example carbon such as soot, carbon monoxide and ammonia, can remain in the first volume flow. In the afterburning of the second combustion stage, these incompletely oxidized components serve as reducing agents or catalysts for the reduction of nitrogen oxides. For example, improved comproportionation to nitrogen can be promoted between the remaining ammonia and nitrogen oxides during the pulsating supply of the second reaction gas in the second volume flow and thus under and over-stoichiometric conditions during the mixing of the volume flows.

Components of the two volume flows include gases or compounds that are supplied and that arise during a reaction, for example the combustion of the fuel, and solids carried in the volume flows of simple or complex composition. For example, the components carbon monoxide, carbon dioxide, water vapor, ammonia, nitrogen oxides, hydrocarbons, residual oxygen and soot can be contained as exhaust gases in the first volume flow. For example, air, oxygen with higher proportions than in the air, water vapor, ammonia and proportions of the exhaust gas can be present in the second volume flow. A third volume flow supplied as the first reaction gas can contain as components air, oxygen-enriched air, oxygen and optionally further components.

In the proposed firing system, in order to reduce the nitrogen oxide content, the supply of a volume flow of the second reaction gas is controlled in a pulsating manner by means of the second supply device during a combustion process. Such a time-pulsating metering of the volume flow can be provided in new systems of firing systems and can be easily retrofitted in existing firing systems by adapting the second supply device. For example, the second feed device can be provided with a pinch valve, cellular wheel sluices or the like, which interrupt or continuously change the volume flow of the second reaction gas at a predetermined or predeterminable frequency and thus lead to a temporal volume flow gradation.

The pulsation is applied from the outside by means of a control. Pulsation impressed from the outside is to be understood here to mean, for example, an oscillating or intermittent change in the volume flow, which subsequently causes the exhaust gas to be burned in the same way. The pulsation of the volume flow can be represented, for example, by a sawtooth or rectangular profile. It goes without saying that in addition to controlling the volume flow of the second reaction gas, other regulations, for example pressure control of the second reaction gas, are also included in the proposed solution to the problem. In particular, all possibilities are provided for pulsatingly changing the stoichiometry of the components of the exhaust gas and the components of the second reaction gas in the post-combustion process of the second combustion stage. This can also be understood to mean an additional pulsating operation of the first reaction gas. The proposed supply options deviating from the pulsating operation of the volume flow are therefore to be subsumed under the pulsating operation of the volume flow.

For example, an oscillating supply of the second reaction gas in the form of air or air enriched with oxygen, water vapor, ammonia and / or the like, a mixture of these with recirculated exhaust gas, pure exhaust gas or the like, which is capable of pulsating properties, is proposed. To change the oxidative and reducing properties of the mixture of exhaust gas and reaction gas in a pulsating manner. In this way, the proportion of nitrogen oxides can be reduced, for example, by dis- and / or comproportionation reactions or oxidation and reaction. In addition, this stoichiometric behavior of the components of exhaust gas and second reaction gas, which changes over time, can be achieved by means of an oscillating supply of the first reaction gas, for example air or air enriched with oxygen, water vapor or a mixture of oxygen-containing gases when solid fuels are converted in the first combustion stage to reduce nitrogen oxide be supplemented and improved.

The pulsation is controlled by means of time intervals of the same length or of different lengths, in which no or little second reaction gas is metered in at first time intervals and more reaction gas is metered into the exhaust gas space in second time intervals. Alternatively or additionally, the metering can be frequency-dependent, that is to say dependent on the repetition rate of maxima and minima of the reaction gas over time and / or dependent on the amplitude of these maxima or minima. According to the invention, the frequency is controlled depending on the carbon monoxide content after the second firing stage. For example, a medium half-hourly value from 100 mg / Nm ³ carbon monoxide preferably to an average half-hour value of less than 50 mg / Nm ³ carbon monoxide. Here, the oscillation frequency of the second reaction gas can be controlled, for example, in such a way that the half-hourly value of the carbon monoxide is less than 50 mg / Nm ³ and the nitrogen oxide contents are reduced, preferably minimized. An oscillation frequency can be dependent on further parameters, for example the amplitude of the oscillation frequency, the oxygen content, additional components such as ammonia, water vapor and possibly an added gas quantity from the exhaust gas recirculation, the thermal output of the combustion system and / or local conditions. A range of the oscillation frequencies can be provided, for example, between 0.1 Hz and 10 Hz, preferably 0.5 Hz and 5 Hz.

The oscillation or pulsation of the second reaction phase can only be provided during a combustion phase and can be suspended, for example, during a start-up phase of the combustion system. In the start-up phase, the second reaction gas can be supplied continuously or switched off. For example, the pulsation of the second reaction gas can be activated when a predetermined nitrogen oxide content in the exhaust gas is reached or exceeded, for example when the NO _x concentrations are above 400 mg / Nm ³ . For this purpose, the NO _x concentration can be continuously detected, for example by a sensor or detector.

In summary, the object is achieved by a combustion system for the combustion of solid fuel supplied to a fuel bed with a primary combustion stage with a first supply device for supplying a first oxygen-containing reaction gas and a secondary combustion stage downstream of the first combustion stage with a second oxygen-containing reaction gas into an exhaust gas space above the Solved fuel bed supplying second feed device, wherein a volume flow of the second reaction gas is pulsed in time controlled by means of the second feed device during a combustion process. The volume flow can be set to oscillate or intermittently. The volume flow can be clocked in the form of a sawtooth profile or rectangular profile. The second feed device can be provided with a time-controlled pinch valve or a cellular wheel sluice. The second reaction gas can contain oxygen and / or water vapor. Furthermore, the object is achieved by a method for operating a combustion system for burning a solid fuel supplied to a fuel bed with a first combustion stage with a first supply device for supplying a first oxygen-containing reaction gas and a second combustion stage with a second supply device for supplying a second oxygen-containing reaction gas in one of the first Combustion stage following exhaust gas space solved, the fuel being oxidized in the first combustion stage under substoichiometric conditions and by periodically varying the supply of the second reaction gas, afterburning of exhaust gases of the first combustion stage is carried out alternately under substoichiometric and superstoichiometric reaction conditions. A volume flow of the second reaction gas can be increased and weakened at alternating time intervals. The time intervals for an increase in the volume flow can be equal to or different from the time intervals for a weakening of the volume flow. The volume flow can be varied in a rectangular or sawtooth shape depending on the time. A frequency of the volume flow such as the oscillation frequency can be controlled depending on a carbon monoxide content of the exhaust gas. The frequency can be set to a carbon monoxide content of less than 100 mg / Nm3, preferably less than 50 mg / Nm3. Ammonia can be added to the second reaction gas. Steam can be added to the second reaction gas. Portions of the exhaust gas from the combustion system can be admixed with the second reaction gas or the second reaction gas can be formed from the exhaust gas from the combustion system. The first reaction gas can also be operated in a modulated manner, such as pulsed, oscillating or intermittent.

Figur 1

eine schematische Darstellung eines Feuerungssystems,

Figur 2

eine schematische Darstellung eines Feuerungssystems mit gegenüber dem Feuerungssystem der Figur 1 verringerten Ausmaßen,

Figur 3

eine systematische Darstellung eines pulsierenden Betriebs der Zufuhreinrichtung zur Zufuhr des zweiten Reaktionsgases,

Figur 4

ein Diagramm zur Darstellung eines Ablaufs eines Brennvorgangs des Feuerungssystems der Figur 2
und

Figur 5

ein Diagramm der Kohlenmonoxid- und Stickoxidgehalte im Abgas des Feuerungssystems abhängig von der Frequenz des zweiten Reaktionsgases.

The invention is based on the in the Figures 1 to 5 illustrated embodiments explained in more detail. Show:

Figure 1

a schematic representation of a firing system,

Figure 2

is a schematic representation of a firing system with compared to the firing system of Figure 1 reduced dimensions,

Figure 3

a systematic representation of a pulsating operation of the supply device for supplying the second reaction gas,

Figure 4

a diagram illustrating a sequence of a burning process of the combustion system of the Figure 2
and

Figure 5

a diagram of the carbon monoxide and nitrogen oxide contents in the exhaust gas of the combustion system depending on the frequency of the second reaction gas.

The Figure 1 shows a schematic representation of the combustion system 1 with the fuel bunker 2 and the loading table 4 with plunger 3, which transports the solid fuel 5 onto the fuel bed 6 designed as a grate. On the fuel bed 6, the fuel 5 is burned in the first combustion stage 7 while supplying the first reaction gas 8 via the first supply device 9 under substoichiometric conditions, that is to say oxidized. Air or oxygen-enriched air is preferably used as the first reaction gas 8. The solid fuel 5 can be formed from waste, biomass, coal, coke or mixtures thereof. The first feed device 9 is designed here, for example, as a blower. The ash from the first firing stage 7 is discharged into the ash box 10.

Above the first combustion stage 7, the second combustion stage 12 for afterburning incompletely oxidized components of the first volume flow in the form of the exhaust gas of the first combustion stage 7 is arranged in the exhaust pipe 11. The second supply device 13 for supplying the second reaction gas 14 is provided on the second firing stage 12. The second feed device 13 doses the second volume flow at least temporarily in a pulsating manner with a preferably adjustable repetition rate such as oscillation frequency, for example 0.1 Hz to 10 Hz, preferably 0.5 Hz to 5 Hz. The second reaction gas 14 is formed from air, air enriched with oxygen, air enriched with ammonia, water vapor or the like, partly from air mixed with exhaust gas from the combustion system 1, or completely from exhaust gas. The second feed device 13 has a device for forming the pulsation of a volume flow of the second reaction gas 14, for example a pinch valve, a cellular wheel sluice or the like. The resulting pulsating changing stoichiometry between the incompletely burned components of the first combustion stage 7 and the components of the second reaction gas 14, in particular oxygen, has a positive influence on the special reaction chemistry of the nitrogen oxides carried in the exhaust gas, so that their content drops, for example by Oxygen deficiency can be reduced to nitrogen. At the same time, with excess oxygen, the oxidation of the other, not completely burned components of the exhaust gas of the first combustion stage 7, such as carbon monoxide and hydrocarbons, can advantageously be influenced by the pulsation, so that their content decreases.

The Figure 2 shows that compared to the combustion system 1 of FIG Figure 1 Firing system 1a produced with reduced dimensions in a schematic illustration with the combustion chamber 3a operated in a batch process, which is filled with fuel 5a. Via the fuel bed 6a in the form of a grate, the first reaction gas is introduced from below in the direction of arrow 15a and the first combustion stage 7a is thus formed. Via the exhaust pipe 11a, the exhaust gas resulting from a substoichiometric combustion taking place in the first combustion stage 7a reaches the afterburning chamber 16a, into which the second reaction gas is pulsed in the direction of the arrow 17a to form the second combustion stage 12a. The introduction of the second reaction gas can in principle be provided on all combustion systems in an adjustable vertical or at any other angle with respect to the direction of movement of the exhaust gas with or against the direction of movement. Here, a targeted mixing of the exhaust gas and the second reaction gas can be controlled become.

On the combustion system 1a designed as a model system, the measuring points 18a, 19a, 20a designated here are provided at different points, the measuring point 19a allowing an optical access and the measuring points 18a, 20a an analysis of the components present at these points, for example after the first Allow firing stage 7a and after the second firing stage 12a. The post-combustion chamber 16a is followed by the heat exchanger 21a, the filter chamber 22a, the Venturi nozzle 23a, the carbon adsorber 24a and the blower 25a in the direction of movement of the exhaust gas. Figure 3 schematically shows the second volume flow with an oscillating supply of air for carrying out the second combustion stage, which is arranged downstream of the first combustion stage in the direction of movement of the exhaust gas. The second reaction gas is fed to the first volume flow such as exhaust gas from the first combustion stage by means of the second supply device in a pulsating manner over time t. The mixing of the components is of little or no importance here. Rather, the first combustion stage is operated sub-stoichiometrically with oxygen, i.e. with a combustion air ratio λ <1, so that in the second combustion stage, due to the pulsating operation of the second reaction gas, no or less oxygen in first time periods Δt ₁ and more oxygen in alternating second time periods Δt ₂ is introduced to the first volume flow. In the first time periods Δt _1, for example, components that are not completely oxidized, such as carbon monoxide (CO) and nitrogen compounds, such as ammonia (NH ₃ ) and nitrogen oxides (NO _x ), remain in the first volume flow from the first combustion stage, such as primary firing. If sufficient air or oxygen is added to the exhaust gas from the primary combustion in the second time periods Δt ₂ , so that the combustion air number λ> 1 results, carbon monoxide becomes carbon dioxide (CO ₂ ) and the nitrogen oxides with the ammonia in oxygen (O ₂ ) and water ( H ₂ O) implemented. If continuous operation with a combustion air ratio λ> 1 is known, the nitrogen oxides are further oxidized and cannot be reduced. The time segments .DELTA.t ₁ and .DELTA.t ₂ can be of different lengths, and the height of the amplitude .DELTA.A can vary. The residence time of the mixture of exhaust gas and second reaction gas can thus be set both over the time length of the time segments Δt ₁ , Δt ₂ , the amplitude ΔA and through the frequency, that is to say the repetition rate of the time segments Δt ₁ , Δt ₂ . According to the invention, the carbon monoxide content is used to control the oscillation frequency. For example, a currently valid half-hourly mean value of 100 mg / Nm ³ for waste incineration plants or preferably about 50% of the limit value, that is to say less than 50 mg / Nm ³ CO, can be regulated become. The oscillation frequency is preferably adjusted so that a carbon monoxide content of less than 50 mg / Nm ^{3 is} achieved and the nitrogen oxide content is reduced.

The Figure 4 FIG. 16 shows diagram 26 of one in the firing system 1a of FIG Figure 2 carried out model combustion process with different parameters over time t. Curve 27 shows the course of the volume flow of the first reaction gas - here air. Curve 28 shows the course of the second volume flow of the second reaction gas - here air. Curve 29 shows the curve of the oxygen content, curve 30 the curve of the carbon dioxide content, curve 31 the curve of the carbon monoxide content and curve 32 the curve of the nitrogen oxide content in each case at the measuring point 20a ( Figure 2 ). Curve 33 shows the course of the degree of nitrogen conversion from nitrogen oxide to nitrogen.

Between minute 14 and minute 45, the volume flow of the second reaction gas is operated in a pulsating manner. As a result, the oxygen content of the pulse minima decreases due to the system. The degree of nitrogen conversion increases. As a result, the nitrogen oxide content in the exhaust gas decreases significantly while the carbon monoxide content is low at the same time.

The Figure 5 shows the diagram 34 with the bars 35, 36, 37 for the carbon monoxide content and with the bars 38, 39, 40 for the nitrogen oxide content over different oscillation frequencies f. The bars 35, 38 show the contents with a continuous supply - that is, frequency f = 0 - of the second reaction gas into the firing system 1 a Figure 2 . With conventional afterburning, a sufficient reduction in the carbon monoxide contents, for example of approximately 10 mg / Nm ³ CO based on 11 volume percent oxygen, is possible. However, the nitrogen oxide levels remain at a high level of, for example, about 600 mg / Nm ³ NO _x based on 11 volume percent oxygen. If the second reaction gas with the oscillation frequency f = 1 Hz is pulsed into the second firing stage in the given test environment, the nitrogen oxide content in bars 39 is reduced to about half, but the carbon monoxide content in bars 36 increases many times over. If the oscillation frequency f = 2 Hz is set, the carbon dioxide content in bar 37 can be reduced to the original value during non-pulsating operation. At the same time, the content of nitrogen oxides in bars 40 remains in the region of half the content of nitrogen oxides during non-pulsating operation. These results assume that the contents during pulsating operation depend, among other things, on the system properties of the combustion system 1a ( Figure 2 ) and that the optimal oscillation frequencies must be determined separately for each firing system. To this extent, the proposed oscillation frequencies are not limiting for the invention.

Reference list

1

Firing system

1a

Firing system

2nd

Fuel bunker

3rd

Pestle

3a

Combustion chamber

4th

Loading table

5

fuel

5a

fuel

6

Fuel bed

6a

Fuel bed

7

first firing stage

7a

first firing stage

8th

first reaction gas

9

Feeder

10th

Ash pan

11

Exhaust pipe

11a

Exhaust pipe

12th

second burning stage

12a

second burning stage

13

second feeder

14

second reaction gas

15a

arrow

16a

Afterburner

17a

arrow

18a

Measuring point

19a

Measuring point

20a

Measuring point

21a

Heat exchanger

22a

Filter chamber

23a

Venturi nozzle

24a

Coal adsorber

25a

fan

26

diagram

27

Curve

28

Curve

29

Curve

30th

Curve

31

Curve

32

Curve

33

Curve

34

diagram

35

bar

36

bar

37

bar

38

bar

39

bar

40

bar

Claims (15)

1. Firing system (1, 1a) for the combustion of solid fuel (5, 5a) supplied to a fuel bed (6, 6a) with a primary combustion stage (7, 7a) having a first supply device (9) for supplying a first oxygen-containing reaction gas (8) and performing an incomplete combustion process by generating a first volume flow and a secondary combustion stage (12, 12a) downstream of the first combustion stage (7, 7a) with a second volume flow of a second oxygen-containing reaction gas (14) supplied by a second supply device (13) into an exhaust gas chamber above the fuel bed (6, 6a), wherein by means of the second supply device (13) a mixture of components of the volume flows is provided which has changed stoichiometrically over time, in that the second supply device (13) is controlled in a pulsating manner by means of a controller, characterised in that the controller is configured to increase and reduce the second volume flow of the second reaction gas (14) in alternating time intervals (Δt₁, Δt₂) such that an oscillation frequency (f) of the second volume flow is controlled as a function of a carbon monoxide content of the exhaust gas.
2. Firing system (1, 1a) according to claim 1, characterised in that the volume flow can be set to be oscillating
3. Firing system (1, 1a) according to claim 2, characterised in that the volume flow can be set to be intermittent.
4. Firing system (1, 1a) according to claim 2 or 3, characterised in that the second supply device (13) is provided with a clock-timed pinch valve or rotary feeder.
5. Firing system (1, 1a) according to any of claims 1 to 4, characterised in that at least one of the two volume flows contains ammonia and/or water vapour.
6. Method for operating a firing system (1, 1a) for combusting a solid fuel (5, 5a) supplied to a fuel bed (6, 6a) with a first combustion stage (7, 7a) having a first supply device (9) for supplying a first oxygen-containing reaction gas (8) and a second combustion stage (12, 12a) having a second supply device (13) for supplying a second oxygen-containing reaction gas (14) into an exhaust gas chamber following the first combustion stage (7, 7a), wherein the fuel (5, 5a) in the first combustion stage (7, 7a) is oxidised in substoichiometric conditions to a first volume flow and by supplying a second, periodically varied volume flow of the second reaction gas (14) a post-combustion takes place of exhaust gases of the first combustion stage (7, 7a) changing overtime in substoichiometric and superstoichiometric reaction conditions, characterised in that at changing time intervals (Δt₁, Δt₂) the second volume flow of the second reaction gas (14) is increased and decreased such that an oscillation frequency (f) of the second volume flow is controlled as a function of a carbon monoxide content of the exhaust gas.
7. Method according to claim 6, characterised in that the time intervals (Δt₂) of an increase of the second volume flow are equal to the time intervals (Δt₁) of a decrease of the second volume flow.
8. Method according to claim 6, characterised in that the time intervals (Δt₂) of an increase of the second volume flow are not equal to the time intervals (Δt₁) of a decrease of the second volume flow.
9. Method according to any of claims 6 to 8, characterised in that the second volume flow is varied as a function of time in rectangular form or saw tooth form.
10. Method according to any of claims 6 to 9, characterised in that the frequency is set to a carbon monoxide content of less than 100 mg/Nm³.
11. Method according to claim 10, characterised in that the frequency is set to a carbon monoxide content of less than 50 mg/Nm³.
12. Method according to any of claims 6 to 11, characterised in that an incomplete combustion of the fuel (5, 5a) is controlled to a residual content of ammonia in the first volume flow and/or ammonia is admixed to the second volume flow.
13. Method according to any of claims 6 to 12, characterised in that water vapour is admixed to the second reaction gas (14).
14. Method according to any of claims 6 to 13, characterised in that portions of the first volume flow are admixed to the second volume flow.
15. Method according to any of claims 6 to 14, characterised in that the first reaction gas (8) is modulated in operation.

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