JP2007040698A - Combustion method of liquid fuel by stepwise atomization - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a combustion method of a liquid fuel and a burner overcoming a defect of a preceding technology. <P>SOLUTION: The method includes a process for generating liquid fuel spray 10 by injecting a main flow 3 of atomization gas in contact with the liquid fuel 4, and is characterized in that at least one secondary flow 1 of the atomization gas is injected near the spray 10 in order to generate secondary atomization of the spray before the spray is brought into contact with a jet of an oxidizer in order to burn the fuel. The burner is equipped with at least one liquid fuel injection means 7 combined with at least one atomization gas injection means in order to generate the fuel spray 10 and at least one oxidizer injection means, and is characterized by being equipped with at least one gas injection means 11, 12, 14 for secondary atomization arranged near the fuel injection means. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a liquid fuel combustion method and a burner for carrying out the method.

  For example, one important concern during assembly of combustion systems for industrial scale furnaces is to optimize combustion efficiency by optimally controlling flame characteristics, while addressing the need for heating methods in terms of heat transfer. Satisfied and to minimize the release of pollutants into the air.

  Two-phase combustion of liquid fuel with oxidizing gas involves two main steps before actual combustion occurs. In a first step, the liquid fuel stream is destabilized and the liquid fuel is atomized using an atomizing gas injected to form a fuel spray having a higher contact area with the oxidant. is necessary. The atomizing gas used can be, for example, air, oxygen, steam or even natural gas. In that case, it is necessary for the liquid fuel droplets thus formed to vaporize before the fuel is burned by the oxidant. Since the vaporization time of the fuel droplets is longer than the reaction time, the mixture of reactants, and thus the combustion characteristics, essentially depends on the characteristics of the fuel spray, in particular the size of the fuel droplets formed.

  The spray characteristics depend on the type of injector used. Most injectors use a high velocity difference between the atomizing gas and the liquid fuel to destabilize the fuel and form a spray. Two types of injectors are available depending on whether a high velocity flow is imposed on the fuel or the atomizing gas. That is, the fuel is supplied to high pressure, and the fuel jet flows at a relatively low speed through a small diameter orifice at high speed, and the fuel jet flows at a relatively low speed and is destabilized by the high-speed atomizing gas An injector called a "dual fluid" injector. However, mechanical injectors, on the one hand, have a limited flow range due to the higher cost of the compressor required to reach high liquid pressures on the one hand because they are more at risk of plugging the fuel injection orifice with deposits. Because of the little used.

  Dual fluid injectors may allow very high atomization gas flow rates. In this category, an air assisted atomizer operated at a very low exhaust gas flow rate and a spray atomizer characterized by a higher flow rate but at a lower atomization gas flow rate. Are distinguished. The two fluids can be mixed in a confined environment (internally mixed injector) or outside the injection system (externally mixed injector).

  Further, the liquid fuel stream can optionally be oriented to form a film on a suitable surface prior to contact with the atomizing gas stream. In this case, the injector is called a prefilming atomizer, which involves the pre-formation of a liquid film, in the other case a plain jet atomizer. With full liquid flow.

The most commonly used injectors in oxycombustion are dual flow type injectors with external mixing and spraying at atomizing gas velocities below 200 m · s ⁻¹ , and plain jet injectors .

  The combustion characteristics of the spray produced by such injectors are controlled primarily by changing the relative atomizing gas and liquid fuel velocities. In so doing, the average droplet size of the fuel spray is changed, so that the vaporization time compared to the combustion reaction time also changes. Droplet size and vaporization time are very important combustion parameters that control the mixing of vaporized fuel and oxidant and therefore determine whether combustion is complete. However, controlling the injection rate helps to adjust the spray characteristics and the two-phase combustion characteristics to adapt them to the heated process, but such injectors still , Lacks flexibility.

  For example, it is difficult to effectively control the cooling of the injector. In fact, increasing the atomizing gas flow reduces the average drop size, resulting in a closer reaction zone and hence overheating of the injector.

  Furthermore, for excessively high atomization gas flow rates, the atomization mode is from fiber mode, ie with a fully deployed liquid cone, to superpulse mode, ie a short liquid cone. You can move to something with a table. In that case, the reaction zone tends to stabilize in the immediate vicinity of the burner and can cause its breakage. Furthermore, excessively high atomization gas velocities can also cause flame instability. In fact, the stability of the flame can be related to the ratio of the mixing time to the characteristic time of the combustion chemistry, and the flame has proven to be stable above this ratio. If the atomizing gas velocity increases excessively, the mixing time will be shorter than the characteristic time of combustion chemistry and the stability requirement will no longer be met.

  Another drawback is that the spray properties affect several parameters and thus make it impossible to control them independently of each other. That is, the spray droplet size and spray angle cannot be controlled independently, and the characteristics depend on both the ratio of atomizing gas momentum to fuel momentum. More specifically, when this momentum ratio is less than 5, an increase in atomizing gas flow simultaneously results in a decrease in drop size and an increase in spray angle, and the flames are those of adjacent burners. Interact with.

  Another drawback appears when using high viscosity fuels. In that case, a simple atomizing gas stream injected as a ring around the liquid fuel stream does not allow uniform atomization resulting in a spray having a uniform drop size. This creates irregular combustion and flame stabilization problems.

  The object of the present invention is to overcome the above drawbacks.

  That is, the present invention is a liquid fuel combustion method including the step of generating a spray of the liquid fuel by injecting a main flow of the atomizing gas in contact with the liquid fuel, Injecting at least one secondary stream in the vicinity of the spray to cause secondary atomization of the spray before it comes into contact with an oxidant jet for combustion of the fuel. A method for burning liquid fuel is provided.

  Thus, for the same total atomization gas flow rate, the flow rate of the first atomization stream is reduced, resulting in a play with larger sized drops near the injector. This first spray fuel drop then undergoes a second atomization, resulting in a modified spray having smaller drops. Since the first spray has already been produced, the second atomization is more effective, especially for high viscosity fuels, and entrains additional spray drops from the injector. With this staged atomization of liquid fuel, the combustion reaction takes place further away from the injector, thereby reducing the risk of burner overheating and failure. If the burner has a retractable head, it is important to allow the secondary atomization to operate the head with a higher retraction. Furthermore, the increased atomizing gas flow thereby enables more effective cooling of the burner. This increases the useful life of the injector and reduces its maintenance.

  This improvement in injector cooling is particularly advantageous when the atomizing gas used is oxygen, a gas with a high risk of overheating.

  In addition, the presence of secondary atomized flow reduces the dependence between drop size (flame length) and spray angle (flame width) by limiting the latter increase. Thus, the flame length can be more easily controlled while the risk of interaction with the adjacent burner flame is limited.

  A further advantage of the present invention is a reduction in the release of NOx compounds. In fact, the resulting flame temperature is more uniform, so NOx production is reduced. This process is accentuated by better dilution of the reactants and combustion products with the secondary atomizing gas stream.

  According to a first exemplary embodiment of the method, the secondary flow of atomizing gas is injected in a direction parallel to the liquid fuel injection direction.

  According to a second exemplary embodiment of the method, the secondary flow of atomizing gas is injected in an oblique direction converging towards the liquid fuel injection direction. This makes it possible to control the spray angle more precisely, and the changed spray angle is equal to twice the angle between the injection direction of the secondary atomizing gas flow and the fuel flow direction. Advantageously, the secondary atomizing gas stream is injected at an angle of 0-60 °, preferably at an angle of 0-30 °.

  Optionally, to orient the injection direction of the secondary atomization gas stream relative to the injection direction of the liquid fuel, if necessary, to increase the flame width and consequently increase the heat transfer to the heated process Adjustment means can be provided.

  Advantageously, the secondary flow of atomizing gas is injected to create turbulence. This turbulence improves the dilution of reactants and combustion products and further reduces NOx compound emissions.

  Preferably, the flow rate of the secondary atomizing gas constitutes 20% to 70% of the total flow rate of the atomizing gas.

  Advantageously, the secondary flow of atomizing gas is divided into a plurality of streams that are uniformly distributed and injected at equal intervals around the liquid fuel stream. This injection can be performed, inter alia, by concentric flow around the fuel injection, or by a plurality of apertures ending in the burner head and evenly distributed around the fuel injection. More precisely, the main atomizing gas stream is injected coaxially around the liquid fuel, while the secondary stream is injected through the openings.

The method according to the invention is a staged combustion method in which an oxidant is injected and divided into a plurality of streams introduced at various distances from the fuel spray, in particular as described in WO 2004/094942. Ideal. That is, the oxidant jet is divided into a primary oxidant jet and a secondary oxidant jet, and the primary oxidant jet is injected in the vicinity of the liquid fuel spray to generate the first incomplete combustion. The gas resulting from one incomplete combustion still includes at least a portion of the fuel, while the secondary oxidant jet is coupled to the portion of liquid fuel present in the gas resulting from the first incomplete fuel. Can be injected at a distance I ₂ from the liquid fuel spray that is longer than the distance between the liquid fuel jet and the primary oxidant jet closest to the liquid fuel spray. The present invention also includes the case where the third oxidant jet is injected at a distance I ₁ from the fuel spray that is longer than the distance I ₂ .

  The method according to the invention is that the atomizing gas is injected on the one hand at a rate that is at least 10 times faster than the fuel injection rate and on the other hand has an atomizing gas mass flow rate that is higher than 10% of the fuel mass flow rate. It is ideal for a two-phase burner.

  The invention further relates to a burner for carrying out the above two-phase combustion method, which burner is combined with at least one atomizing gas injection means to produce a fuel spray. And at least one oxidant injection means, and at least one secondary atomizing gas injection means disposed in the vicinity of the fuel injection means. In the case of a burner with a highly separated jet, such a burner is a primary oxidant injection means, a secondary oxidant arranged at a distance away from the fuel injection means longer than the distance of the primary injection means. Injecting means, and optionally, tertiary oxidant injecting means disposed on the edge of the burner.

  Advantageously, the secondary atomizing gas injection means is obtained by bypassing a common atomizing gas supply. The selection of the bypass diameter helps to define the ratio between the primary atomizing gas stream and the main stream. More advantageously, the secondary injection means are oriented parallel or oblique to the fuel injection means.

  The invention will be better understood from the following detailed description given with reference to the accompanying drawings.

The combustion method according to the invention is carried out by a burner comprising a spray-guided atomizing head 6 with external mixing, as shown in FIGS. This atomizing head 6
An ejection line 7 for a liquid fuel stream 4 fed by a fuel feed 2;
Concentric with the fuel discharge line 7 and connected to an atomizing gas feed 9 and in contact with the fuel stream 4 to produce a liquid fuel spray 10 under the action of the main atomizing gas stream 3. Gas atomization line 8
Is provided.

  Furthermore, the atomization head 6 has a fuel discharge line 7 and six secondary ejection channels 11 oriented parallel to the atomization gas discharge line 8. These six secondary discharge channels 11 are evenly distributed around the discharge lines 7, 8 and are designed to produce secondary atomizing gas branched from the atomizing gas supply 9 by the bypass channel 12. ing. Under the influence of these secondary atomizing gas streams 1, the liquid fuel spray 10 undergoes a second atomization and produces a modified spray 13. This modified spray 13 is then in contact with an oxidant stream (not shown), optionally divided into a primary stream and a secondary stream, to burn the fuel. Also, a tertiary oxidant supply can be provided at a relatively long distance from the atomizing head 6, which is injected with oxygen at a high rate to obtain sufficient dilution of the reactants before the main combustion zone. And thereby used to prevent excessive formation of hot NOx compounds. Experiments show that a burner injected with 50% of the total oxygen at the tertiary supply level leaves all other items equal and is approximately 4 times the flame stabilization distance of a conventional injector without a secondary atomizing gas stream. It proves to have worked to obtain a flame stabilization distance equal to twice. This longer stabilization distance serves to reduce overheating of the atomizing head 6. In addition, the resulting flame is also more uniform and longer due to delayed atomization, with the spray 10 having larger drops than the spray resulting from a conventional burner, while the modified spray 13 is smaller. It has been demonstrated that it has a drop size, has a better dilution and therefore more evenly distributed drops.

  The more uniform flame thus obtained thus reduces the temperature difference between the hot spot and the cold spot.

  A significant reduction in NOx compound emission by a burner with an atomizing head 6 according to the invention has also been demonstrated. Depending on the chosen oxidant distribution, a reduction of 1.2 to 1 to 1.5 can be obtained for the same total atomization gas flow. This reduction can be explained by the reaction zone lag obtained by the present invention.

  As a modification, the atomizing head 1 shown in FIG. 3 differs from the atomizing head 6 only in that it has a secondary discharge line 14 that is obliquely oriented with an angle α with respect to the fuel flow direction. The angle α is advantageously between 0 and 60 °, preferably between 0 and 30 °. These secondary discharge lines 14 deliver a secondary atomizing gas stream 5 which forms with the spray 10 a modified spray 13 having a spray angle of 2α. Such an atomizing head 1 is suitable for more accurate adjustment of the flame width.

  Although the invention has been described with reference to specific embodiments, the invention is not limited thereby, and the invention covers all technical equivalents of the means described, and combinations thereof. It is included if it is in the range.

1 is an enlarged schematic view of a longitudinal section of one atomizing head of one burner according to the present invention. FIG. The front view of the atomization head shown in FIG. FIG. 3 is an enlarged schematic view of a longitudinal section of another atomizing head of one burner according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 5 ... Gas flow for secondary atomization 2 ... Fuel supply 3 ... Gas flow for main atomization 4 ... Liquid fuel flow 6 ... Atomization head 7 ... Discharge line 8 ... Gas discharge line for atomization 9 ... For atomization Gas supply 10 ... Liquid fuel spray 11 ... Secondary discharge channel 12 ... Bypass channel 13 ... Modified spray 14 ... Secondary discharge line

Claims (10)

  A method for combusting liquid fuel comprising the step of generating a spray (10) of said liquid fuel by injecting a main stream (3) of atomizing gas in contact with the liquid fuel (4) At least one secondary stream (1, 5) of the working gas causes secondary atomization of the spray (10) before the spray comes into contact with the oxidant jet for combustion of the fuel. Therefore, the liquid fuel is burned in the vicinity of the spray (10).   The combustion method according to claim 1, characterized in that the secondary flow (1) of the atomizing gas is injected in a direction parallel to the injection direction of the liquid fuel.   The combustion method according to claim 1, characterized in that the secondary flow (5) of the atomizing gas is injected in an oblique direction converging toward the injection direction of the liquid fuel.   The combustion method according to claim 1, characterized in that the secondary flow (1, 5) of the atomizing gas is injected so as to produce a turbulent flow.   The flow rate of the atomizing gas in the secondary flow (1, 5) of the atomizing gas constitutes 20% to 70% of the total flow rate of the atomizing gas. The combustion method described.   2. The secondary flow (1, 5) of the atomizing gas is divided into a plurality of flows that are uniformly distributed and injected around the liquid fuel flow (4) at equal intervals. The combustion method of any one of -5. The oxidant jet is divided into a primary oxidant jet and a secondary oxidant jet, and the primary oxidant jet is injected in the vicinity of the liquid fuel spray to cause a first incomplete combustion, The gas resulting from incomplete combustion still contains at least a portion of the fuel, while the secondary oxidant jet is the portion of liquid fuel present in the gas resulting from the first incomplete fuel. In order to carry out combustion with the liquid fuel spray, and is injected at a distance I ₂ from the liquid fuel spray, which is longer than the distance between the liquid fuel jet and the primary oxidant jet closest to the liquid fuel spray. The combustion method according to any one of claims 1 to 6.   A burner comprising at least one liquid fuel injection means (7) combined with at least one atomizing gas injection means (8) to produce a fuel spray (10), and at least one oxidant injection means. A burner comprising at least one secondary atomizing gas injection means (11, 12, 14) disposed in the vicinity of the fuel injection means.   9. Burner according to claim 8, characterized in that the secondary atomizing gas injection means (11, 12, 14) are obtained by bypassing a common atomizing gas supply (9).   10. The secondary atomizing gas injection means (11, 12, 14) is parallel or obliquely oriented with respect to the fuel injection means (7). The burner described.

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