WO2015036914A1

WO2015036914A1 - Combustion method and industrial burner

Info

Publication number: WO2015036914A1
Application number: PCT/IB2014/064339
Authority: WO
Inventors: Christian ATZENI
Original assignee: Atzeni Christian
Priority date: 2013-09-11
Filing date: 2014-09-09
Publication date: 2015-03-19
Also published as: EP3044509A1; ES2730887T3; ITMI20131507A1; RS58795B1; EP3044509B1

Abstract

A method for burning fuel comprises conveying a fuel flow (7) in a combustion chamber (2), conveying an oxidizing agent flow (9) into the combustion chamber (2), removing combustion gases/fumes (11) from the combustion chamber (2), withdrawing a partial flow (15) of combustion gases/fumes (11) from the combustion chamber (2) and supplying the first partial flow (15) in the oxidizing agent flow (4) upstream of the combustion chamber (2) by means of using the oxidizing agent flow as the driving fluid for suctioning the partial flow (15) of combustion gases/fumes.

Description

"Combustion method and industrial burner"

DESCRIPTION

[0001] It is the object of the present invention a combustion method and an industrial fuel burner for carrying out the method, particularly a fuel burner with recirculation of the combustion fumes.

[0002] The industrial fuel burners are used in the metallurgical field, for example in the combustion chambers, in fume scrubbers, in forging furnaces, heat treatment furnaces, furnaces for heating and melting, in a continuous or discontinuous cycle, for the processing of ferrous and non-ferrous metallic materials.

[0003] In the early 1980s, compact combustion burners have been developed, which perform a very high preheating of the combustion air and which, consequently, operate at very high temperatures, typically at 1350 °C, for example, for the heat treatment lines of continuous steel extrudates and for aluminum melting furnaces.

[0004] Despite the excellent energy efficiency of the combustion with energy recovery by means of pre-heating of the combustion agent or fuel, the formation of pollutant nitrogen oxides (NOx) during a high temperature combustion is incompatible with the environmental and health protection regulations and prevents the use thereof on a large scale.

[0005] In order to reduce the formation of nitrogen oxides, it has been generally attempted to lower the temperature of the so-called flame core, which at least partially erases the energy efficiency increase achieved by pre-heating the fuel or oxidizing agents.

[0006] US-A-5092761 and EP 1 20 188 B1 disclose methods for the reduction of nitrogen oxides by a partial recirculation of the combustion fumes, withdrawn directly from the combustion chamber of a burner and re-supplied into a fuel flow.

[0007] The premixing of combustion fumes with the fuel fluid involves a dilution and dispersion of the fuel itself and of the flame and, consequently, a lowering of the combustion temperature, known as the "flameless" combustion effect.

[0008] These known methods have proven effective for lowering the NOx levels, but they do not lower the fuel consumption, and lead to problems of practical implementation. The combustion fumes that are withdrawn directly from the combustion chamber turn out to be too hot to be suctioned and supplied to the fuel flow by means of a fan or a pump. The subtraction of hot fumes from the combustion chamber before they have completed the heat exchange with the environment per se reduces the burner energy efficiency. A suctioning of the fumes by the Venturi effect, by using the fuel flow as the motor fluid, as described in EP1203188B1 , can recirculate only a very reduced volume of fumes, since the ratio between the fuel gas and the oxidizing agent air is usually in the order of 1 :10 or even lower.

[0009] Therefore, the object of the present invention is to provide a combustion method and an industrial burner for the above-mentioned applications, having such characteristics as to obviate the drawbacks mentioned with reference to the prior art.

[0010] A particular object of the invention is to provide a combustion method and an industrial burner, particularly for the above-mentioned applications, having an improved energy efficiency and such characteristics as to reduce the NOx formation.

[0011] These and other objects are achieved by an industrial combustion method according to claim 1 and by an industrial combustion burner according to claim 10.

[0012] The dependent claims relate to advantageous embodiments of the invention.

[0013] In order to better understand the invention and appreciate the advantages thereof, some embodiments thereof will be described below, referring to the appended figures, in which:

- Fig. 1 is a schematized partial sectional view of an industrial burner according to an embodiment of the invention;

- Fig. 2 illustrates the flows of oxidizing agent, fuel, and combustion fumes, as well as the formation of a flame in the burner of Fig. 1 , in accordance with an embodiment.

- Fig. 3 is a schematized partial sectional view of an industrial burner according to a further embodiment of the invention;

- Figs. 4 and 5 are schematized views of an industrial burner according to embodiments of the invention.

[0014] With reference to the figures, an industrial burner 1 comprises a combustion chamber 2, a fuel duct 3 in fluidic connection with the combustion chamber 2, an oxidizing agent duct 4 in fluidic connection with the combustion chamber 2, and a discharge duct 5 in fluidic connection with the combustion chamber 2. The industrial burner 1 further comprises means (e.g., a fan or a pump 6) for conveying a fuel flow 7 through the fuel duct 3 into the combustion chamber 2, means (e.g., a fan 8 or a pump) for conveying an oxidizing agent flow 9 (e.g., air, air mixed with oxygen or only oxygen) through the oxidizing agent duct 4 into the combustion chamber 2, as well as ignition means 10 for ignition, where in the combustion chamber 2 there are not the conditions for a self-ignition, a combustion reaction of the fuel 7 with the oxidizing agent 9 within the combustion chamber 2, so that the combustion generates hot combustion gases/fumes 11 that, possibly after aver giving at least part of the heat, are removed from the combustion chamber 2 through the above- mentioned discharge duct 5.

[0015] According to an aspect of the invention, the burner 1 further comprises one or more first recirculation ducts 12 that put in fluidic communication a withdrawing point 13 of the combustion chamber 2 (Fig. 4) or of the discharge duct 5 downstream of the combustion chamber 2 (Fig. 5) with a re-input point 14 of the oxidizing agent duct 4 upstream of the combustion chamber 2, in which, at the re- input point 14, the oxidizing agent duct 4 is configured as a Venturi tube, so that the oxidizing agent flow 9 acts as the driving fluid, generating a recirculation of a first partial flow 15 of combustion gas withdrawn from the combustion chamber 2 or from the discharge duct 5 and suctioned through the first recirculation duct(s) 12 into the oxidizing agent duct 4, where the first partial flow 15 of combustion gases/fumes mixes with the oxidizing agent flow 9.

[0016] In accordance with an embodiment (Fig. 1), the burner 1 may comprise a mixing and injection body 16 defining:

[0017] - an inner chamber 18; [0018] - at least one fumes inlet aperture 17 in communication with the inner chamber 18 and to which a tube of the first recirculation duct 12 is connected,

[0019] - at least one oxidizing agent inlet opening 19 in communication with the inner chamber 18 and to which a tube of the oxidizing agent duct 4 is connected, [0020] - at least one oxidizing agent outlet opening 21 in communication with the inner chamber 18 for outputting a oxidizing agent-combustion fumes mixture, as well as

[0021] - a fuel passage duct 22, to which a tube of the fuel duct 3 is connected, and forming a fuel outlet opening 23 for outputting the fuel 7.

[0022] The at least one oxidizing agent inlet opening 19 can be formed in a tubular collector 20 projecting in the inner chamber 18 and having a narrow flow cross- section to accelerate the oxidizing agent flow 9 at the outlet from the collector 20 and generate the Venturi effect that is necessary for suctioning the first partial flow 15 of combustion gases/fumes in the oxidizing agent flow 9. The part of the mixing and injection body 16 forming the oxidizing agent 21 and fuel 23 output openings directly opens into the combustion chamber 2 and it can be made of a refractory and/or thermal insulation material or, alternatively, of stainless steel, to withstand high thermal stresses.

[0023] The combustion chamber 2 itself may form an annular, e.g., converging or diverging frusto-conical diffusion surface 26, about the oxidizing agent 21 and fuel 23 output openings of the mixing and injection body 16.

[0024] Fig. 2 illustrates the operation of the burner 1 in accordance with an embodiment. The flow of oxidizing agent, passing through the collectors 20, is accelerated and suctions the first partial flow 15 of combustion gases/fumes through the fumes inlet aperture 17 in the inner chamber 18, where it mixes with the flow of oxidizing agent in a mixing zone 24 downstream of the collectors 20. The oxidizing agent-fumes/gases mixture then passes through a diffuser zone 25, which may have a frusto-conical profile, of the oxidizing agent outlet opening 21 from which it reaches the combustion chamber 2.

[0025] The fuel flow 7, whether it is solid, liquid, or gaseous, reaches the combustion chamber 2 through the fuel passage duct 22 and the fuel outlet opening 23 and, in the area defined by the conical diffusion surface 26, the fuel mixes with the oxidizing agent/fumes mixture, giving rise to the combustion.

[0026] The fuel outlet opening may comprise a centrifugal end dispenser 27 so shaped as to induce a centrifugal acceleration and to further promote the mixing of the fuel with the oxidizing agent/fumes mixture in the zone (14).

[0027] In accordance with an embodiment (Fig. 3), the burner may comprise a second recirculation duct 28, extending for example externally along the fuel passage duct 22, from a fuel withdrawing point 30 adjacent to the fuel outlet opening 23 up to the re-input point 14 (at the exit of the oxidizing agent from the collectors 20) so as to withdraw a second partial flow of fuel 29 from the combustion chamber 2 in the fuel withdrawing point 30 and re-input the second partial flow 29 at the re-input point 14 into the oxidizing agent duct, by using the oxidizing agent flow 9 as the driving fluid to suction also the second partial flow 29 of fuel. This allows to stabilizing and controlling the flame shape by virtue of a "pilot flame" effect, and carrying out the two-stage combustion. [0028] In practice, the second recirculation duct 28 allows exploiting the depression created by the motion of the oxidizing agent to suction small amounts of fuel, for example, to a max. 30% of the total flow rate of the fuel. The fuel suctioned mixes with the oxidizing agent and the fumes, giving rise to a low-temperature partial combustion, which is completed upon combining with the remaining fuel when the oxidizing agent exits the oxidizing agent outlet opening 21.

[0029] The burner 1 hereto described by way of exemplary embodiment example can be used and further adapted and configured to carry out a method for burning a fuel, which generally speaking comprises:

[0030] - conveying a fuel flow 7 through the fuel duct 3 into the combustion chamber 2;

[0031] - conveying an oxidizing agent flow 9 through the oxidizing agent duct 4 into the combustion chamber 2;

[0032] - starting a combustion reaction of the fuel 7 with the oxidizing agent 8 within the combustion chamber 2, said combustion generating combustion gases/fumes 11 ;

[0033] - removing the combustion gases/fumes 11 from the combustion chamber 2 through the discharge duct 5,

[0034] - withdrawing a first partial flow 15 of combustion gases/fumes 11 from the combustion chamber 2 and supplying the first partial flow 15 of combustion gases/fumes into the oxidizing agent duct 4 in a re-input point 14 upstream of the combustion chamber 2, to mix the first partial flow 15 of combustion gases/fumes with the oxidizing agent flow 9, [0035] - configuring the oxidizing agent duct 4 at the re-input point 14 as a Venturi tube, and using the oxidizing agent flow as the driving fluid for suctioning the first partial flow 15 of combustion gases/fumes.

[0036] In accordance with an embodiment, the oxidizing agent flow 9 is conveyed by a conveyor of oxidizing agent fluid, for example, air, 8, particularly a fan or pump, which is arranged in the oxidizing agent duct 4, and the first partial flow 15 of combustion gases/fumes is supplied into the oxidizing agent duct 4 downstream of the conveyor 8.

[0037] In an embodiment, the method provides for conveying the oxidizing agent flow 9 without fuel at , the re-input point 14 of the oxidizing agent duct 4, so as to pre- mix the first partial flow 15 of combustion gases/fumes only with the oxidizing agent.

[0038] In a further embodiment, the method provides for using, as the driving fluid for suctioning and recirculating the first partial flow 15 of the combustion gases/fumes, more than 80% of the total volume, preferably substantially the whole total volume, of the oxidizing agent supplied into the combustion chamber 2.

[0039] As already mentioned above, it is advantageous to withdraw from the combustion chamber 2, at a fuel withdrawing point 30 adjacent to an outlet opening 23 of the fuel duct 3, a second partial flow 29 of fuel, and to re-input the second partial flow 29 at the re-input point 14 into the oxidizing agent duct 4, by using the oxidizing agent flow 9 as the driving fluid to suction also the second partial flow 29 of fuel.

[0040] In accordance with an embodiment (Fig. 4), the method may comprise the step of withdrawing the first partial flow 15 of combustion gases/fumes directly from the combustion chamber 2 (hence, still upstream of the fumes duct 11) and re- inputting the first partial flow 15 at the re-input point 14 into the oxidizing agent duct 4, by using the oxidizing agent flow 9 as the driving fluid for suctioning the first partial flow 15 of combustion gases/fumes. To this aim, the first recirculation duct 12 can extend from the combustion chamber 2 (upstream of the fumes duct 11) up to the re-input point 14 of the oxidizing agent duct 4.

[0041] In accordance with the embodiment illustrated in Fig. 5, the first partial flow 15 of combustion gases/fumes is withdrawn from the discharge duct 5 at a withdrawing point 13 downstream of the combustion chamber 2.

[0042] Advantageously, the oxidizing agent flow has, at the Venturi tube (when it exits the collectors 20) a speed ranging between 100 m/s and 270 m/s. By way of example, using an oxidizing agent pumped by a fan, the speed of the flow oxidizing agent at the Venturi tube (when it exits the collectors 20) ranges between 70 and 270m/s.

[0043] Under usual flow rate conditions, by way of example, the oxidizing agent- combustion gases/fumes mixture is output from the oxidizing agent outlet opening 21 (and then re-input into the combustion chamber 2) at a speed ranging between 30 m/s and 150 m/s. However, these values may sensibly vary based on the actual constructive embodiment.

[0044] In an advantageous embodiment, the oxidizing agent duct 4 is configured and supplied so as to create, at the re-input point 14, a depression ranging from 0 to 200 mmH₂0, preferably 0 to 90 mmH₂0.

[0045] The flow rate of the re-circulated combustion fumes may range, during the carrying out of the method, from 0 to beyond 100% of the volume of the fumes produced by the combustion.

[0046] For avoidance of doubt, the volume of the combustion gases/fumes produced by the combustion does not include also the volume of the fumes already recirculated before into the combustion chamber. Therefore, a part of the fumes that are present within the combustion chamber is always removed definitively from the process.

[0047] In accordance with an embodiment, based on the required thermal load, therefore as a function of the flow rate of oxidizing agent 9 in the oxidizing agent duct 4, the (Vfumi/Vcomb) ratio between the flow rate of the recirculated combustion fumes and the flow rate of the oxidizing agent 9 may vary or be varied or adjusted from 0:1 up to beyond 1 :1.

[0048] In accordance with a still further embodiment, the method comprises the step of recirculating the first partial flow 15 of combustion gases/fumes without interposing filters, so as to recirculate and re-burn also the dusts and the solid particulate which are contained in the combustion fumes. This allows reducing also the particulate emissions.

[0049] In accordance with a further embodiment, the method provides for adjusting the flame shape within the combustion chamber 2, adjusting the mutual inclination of the axes 36 of the fuel outlet openings 23 and the axis of the oxidizing agent outlet opening 21.

[0050] In accordance with a further embodiment, the first partial flow 15 of combustion gases/fumes has a temperature higher than 750° C, preferably higher than 1250° C, and a mixing of the first partial flow 15 of combustion gases/fumes with the oxidizing agent 9 increases the temperature of the oxidizing agent 9 to beyond 600°C, preferably beyond 1000°C.

[0051] In accordance with an embodiment, the fuel flow 7 may comprise a solid fuel, for example, in powder form, carried by a carrier fluid, for example, a gaseous or liquid fuel. In such a case, an end portion of the fuel duct, opening into the combustion chamber 2, may form a distributor 27 so shaped as to create a centrifugal acceleration of the fuel flow that promotes the distribution and mixing thereof with the oxidizing agent and with the fumes recirculated within the combustion chamber 2.

[0052] In accordance with an embodiment, the method may comprise the step of conveying into the oxidizing agent duct 4 all the oxidizing agent necessary to the process (hot or cold, such as, for example, atmospheric air, or mixtures thereof), withdrawn from the environment or from a passive mixing duct, without the aid of mechanical pumping means (e.g., fans, pumps, or compressors), by connecting an outlet end of the oxidizing agent duct 4 to the fuel duct 3 at an oxidizing agent suctioning point upstream of the combustion chamber 2, and configuring the fuel duct 3 at the oxidizing agent suctioning point as a Venturi tube, so as to use (preferably only) the fuel flow (for example, methane gas, GPL gas, coke oven gas, liquid fuels or mixtures thereof) as the driving fluid to suction and, hence, conveying the oxidizing agent flow 9.

[0053] In accordance with an embodiment (Figs. 6, 7), in order to increase the stability of the burner 1 in the initial steps, the at least one collector 20 can be displaced (for example, translatable) between a closure position (Fig. 6, forward position), in which the opening section of the re-input point 14 is closed or highly narrowed, and an opening position (Fig. 7, retracted position), in which the opening section of the re-input point 14 is at its maximum. In this manner, when the section 14 is closed (Fig. 6) in the mixing zone 14 downstream of the collectors 20 there is only the oxidizing agent air flow 9, allowing to start the combustion and to quickly reach a stable flame. When the flame is stabilized, the collectors 20 can be displaced from the closure position to the opening position, thereby opening the passage section 14 for suctioning the fumes 11 , which will be mixed with the oxidizing agent 9 (Figure 7).

[0054] The invention has a number of advantages. It allows suctioning, through a system of the Venturi type supplied by a pressurized fluid, high amounts of one or more passive fluids that are necessary to the technological process. By virtue of the suctioning through the Venturi effect, the need to have mechanical pumps, blowers, or fans at certain points along the fluid supply lines is obviated. This allows intaking hot or cold fluids, corrosives, oxidizing agents, fuels, inerts, or mixtures thereof.

[0055] By virtue of the use of the oxidizing agent flow and its high flow rate, it is possible to suction and recirculate large amounts of fumes and combustion gases, without the aid of further conveying means.

[0056] By virtue of mixing the combustion fumes with the oxidizing agent flow, the oxygen is highly diluted, and the flame cannot form an actual hot flame core anymore, but it diffuses in a more even manner into the combustion chamber ( "flameless combustion" effect); therefore, it is kept cooler. In this manner, also in the presence of very hot oxidizing air, the NOx emissions remain very low. By virtue of the withdrawal of combustion fumes that are already thermally exhausted downstream of the combustion chamber and, possibly, upstream of a heat exchanger associated with the discharge duct, it is possible to re-input in the burner also the residual thermal energy, which would be otherwise dissipated into the environment.

[0057] Therefore, it is possible to reconcile and meet the following needs:

[0058] - a high energy efficiency;

[0059] - a reduction of polluting substances, particularly nitrogen oxides, unburned gases, and particulate,

due to the lower combustion temperatures, and due to the recirculation and re- combustion of the combustion fumes containing possible residues of fuel and not completely burned substances.

[0060] It shall be apparent that, to the method and burner according to the present invention, those of ordinary skill in the art, with the aim of meeting contingent, specific needs, will be able to make further modifications and variations, all anyhow falling within the protection scope of the invention, as defined by the following claims.

Claims

1. A method for burning fuel in an industrial burner (1) having a combustion chamber (2), a fuel duct (3) in fluidic connection with the combustion chamber (2), an oxidizing agent duct (4) in fluidic connection with the combustion chamber (2) and a discharge duct (5) in fluidic connection with the combustion chamber (2), said method comprising:

- conveying a fuel flow (7) through the fuel duct (3) into the combustion chamber

(2);

- conveying an oxidizing agent flow (9) through the oxidizing agent duct (4) into the combustion chamber (2);

- igniting a combustion reaction of the fuel (7) with the oxidizing agent (9) within the combustion chamber (2), said combustion generating combustion gases/fumes (11);

- removing the combustion gases/fumes (11) from the combustion chamber (2) through the discharge duct (5),

- withdrawing a first partial flow (15) of combustion gases/fumes (11) from the combustion chamber (2) and supplying the first partial flow (15) of combustion gases/fumes into the oxidizing agent duct (4) in a re-input point (14) upstream of the combustion chamber (2), to mix the first partial flow (15) of combustion gases/fumes with the oxidizing agent flow (9),

- configuring the oxidizing agent duct (4) at the re-input point (14) as a Venturi tube, and using the oxidizing agent flow as the driving fluid for suctioning the first partial flow (15) of combustion gases/fumes.

2. The method according to claim 1 , comprising:

- conveying the oxidizing agent flow (9) by a mechanical conveyor (8), particularly a fan or pump, which is arranged in the oxidizing agent duct (4) and the first partial flow (15) of combustion gases/fumes is supplied into the oxidizing agent duct (4) downstream of the mechanical conveyor (8).

3. The method according to claim 1 or 2, comprising the step of conveying the oxidizing agent flow (9) without fuel at the re-input point (14) of the oxidizing agent duct (4), so as to pre-mix the first partial flow (15) of combustion gases/fumes with only the oxidizing agent.

4. The method according to any of the preceding claims, wherein, as the driving fluid for suctioning and recirculating the first partial flow (15) of combustion gases/fumes, more than 80% of the total volume, preferably substantially the whole total volume, of the oxidizing agent supplied into the combustion chamber (2) is used.

5. The method according to any of the preceding claims, comprising:

- withdrawing from the combustion chamber (2), at a fuel withdrawing point (30) adjacent to an outlet opening (23) of the fuel duct (3), a second partial flow (29) of fuel, and re-inputting the second partial flow (29) at the re-input point (14) into the oxidizing agent duct (4) by using the oxidizing agent flow (9) as the driving fluid to suction also the second partial flow (29) of fuel.

6. The method according to any of the preceding claims, comprising:

- withdrawing said first partial flow (15) of combustion gases/fumes (11) directly from the combustion chamber (2) upstream of the fume discharge duct.

7. The method according to any of the claims 1 to 5, comprising:

- withdrawing the first partial flow (15) of combustion gases/fumes from the discharge duct (5) at a withdrawing point (13) downstream of the combustion chamber (2).

8. The method according to claim 1 , comprising the steps of:

- connecting an outlet end of the oxidizing agent duct (4) to the fuel duct (3) at an oxidizing agent suctioning point upstream of the combustion chamber (2), and

- configuring the fuel duct (3) at the oxidizing agent suctioning point as a Venturi tube, so as to use the fuel flow as the driving fluid for suctioning the oxidizing agent flow (9).

9. The method according to any of the preceding claims, comprising the steps of:

- in an initial step of combustion ignition and start, closing the passage section at the re-input point (14) in order to prevent the first partial flow (15) of combustion gases/fumes and the oxidizing agent flow (9) from mixing,

- in a combustion step after the initial step, opening the passage section at the re- input point (14) in order to mix the first partial flow (15) of combustion gases/fumes with the oxidizing agent flow (9).

10. An industrial burner (1) having a combustion chamber (2), a fuel duct (3) in fluidic connection with the combustion chamber (2), an oxidizing agent duct (4) in fluidic connection with the combustion chamber (2), as well as:

- means (6) for conveying a fuel flow (7) through the fuel duct (3) into the combustion chamber (2),

- means (8) for conveying an oxidizing agent flow (9) through the oxidizing agent duct (4) into the combustion chamber (2),

- a discharge duct (5) in fluidic connection with the combustion chamber (2) for removing the combustion gases/fumes (11) from the combustion chamber (2), at least one first recirculation duct (12) putting in fluidic communication a withdrawing point (13) of the discharge duct (5) downstream of the combustion chamber (2) with a re-input point (14) of the oxidizing agent duct (4) upstream of the combustion chamber (2), wherein, at the re-input point (14), the oxidizing agent duct (4) is configured as a Venturi tube, so that the oxidizing agent flow (9) acts as the driving fluid which generates a recirculation of a first partial flow (15) of combustion gases/fumes from the combustion chamber (2) into the oxidizing agent duct (4), where said first partial flow ( 5) mixes with the oxidizing agent flow (9).

11. An industrial burner (1) having a combustion chamber (2), a fuel duct (3) in fluidic connection with the combustion chamber (2), an oxidizing agent duct (4) in fluidic connection with the combustion chamber (2), as well as:

- a discharge duct (5) in fluidic connection with the combustion chamber (2) for removing the combustion gases/fumes (11) from the combustion chamber (2), at least one first recirculation duct (12) putting in fluidic communication a withdrawing point (13) of the combustion chamber (2) upstream of the discharge duct (5) with a re-input point (14) of the oxidizing agent duct (4) upstream of the combustion chamber (2), wherein, at the re-input point (14), the oxidizing agent duct (4) is configured as a Venturi tube, so that the oxidizing agent flow (9) acts as the driving fluid generating a recirculation of a first partial flow (15) of combustion gases/fumes from the combustion chamber (2) into the oxidizing agent duct (4), where said first partial flow (15) mixes with the oxidizing agent flow (9).