CN105247285B - Combustion method for low NOx premixed gas burner - Google Patents

Abstract

The method of the invention comprises a premix burner formed by a set of premix nozzles (2) arranged in a circle of diameter DB around a central nozzle (4) on a central axis a of the burner (1), wherein the central nozzle is intended to generate a radial flame (5), wherein the oxidant to fuel ratio (R) of the set of nozzles (2, 4) is between 1.3 and 1.75. The radial flame (5) is used for cross-ignition of different premixing nozzles (2, 4). The annular flame is also provided by premixed combustion, so as to maintain the ratio (R) in favour of a low yield of hot nitrogen oxides produced at the intersection of the radial flame and the flame from the nozzle.

Description

Combustion method for low NOx premixed gas burner

technical field

The present invention relates to industrial gas burners. These burners emit nitrogen oxides (NOx), which are a source of pollution.

Therefore, reducing NOx from combustion is an important consideration during the development of industrial burners. The combustion of fuel gas (natural gas or other gases) produces nitrogen oxides through high-temperature oxidation of nitrogen in the air. These nitrogen oxides are commonly referred to as thermal NOx.

The rate of nitrogen oxide formation obeys the Arrhenius law and is highly dependent on: the local oxygen content,

local nitrogen content,

· Local temperature (significantly greater than 1500°C).

Background technique

At present, industrial gas burners are basically diffusion flame burners, which means that in the burner, the mixing of oxidant and fuel occurs all within the combustion chamber. With these diffusion flame burners, the most widely used techniques for reducing thermal NOx are generally based on local variations of oxygen and fuel content either by using an air stage to provide the content or by using a fuel stage.

In the case of flame combustion with premixed oxidizer and fuel, the temperature in the center of the flame is more uniform and depends very strongly on the oxidizer/fuel ratio. Therefore, as a direct consequence, nitrogen oxide emissions are also highly sensitive to this ratio.

The method and the burner that form the subject of the present invention relate to low NOx gas burners based on premix technology. Experience has shown that nitrogen oxides can vary by more than a factor of 10 depending on the oxidizer/fuel ratio. For example, when the oxidant is ambient air and the fuel gas is a rich gas (with a low calorific value, 6kWh/Nm ³ ), the amount of oxidant required for complete combustion of the gas and the low yield of nitrogen oxides in the chemical Between 140% and 170% of the metered. That being the case, the emissions of nitrogen oxides from premixed combustion become very low compared to those obtained from diffusion flames.

However, excess oxidant is an impediment to the use of this premixed technology in processes such as boilers, industrial furnaces, etc., where combustion requires more than 100% stoichiometric A significant excess of oxidant produces additional heat loss.

Contents of the invention

The burner that forms the subject of the present invention uses premixing technology for its low NOx performance and additional means for reducing the overall excess air at the burner without increasing thermal NOx emissions.

In this burner, tests have confirmed that the oxidant/fuel ratio or more specifically the ratio (R) in the premix is very important in order to simultaneously ensure that the temperature of the combustion is sufficiently high and that the thermal NOx produced is very low.

LCV is the lower calorific value, that is, the calorific value of energy condensation is not considered.

CP is the heat capacity in kJ/kg.

The flow rate is in Kg/h.

The combustion method according to the invention comprises a premix burner consisting of a set of premix nozzles arranged on a circle of diameter DB around a central nozzle on the central axis of the burner, wherein The central nozzle is intended to generate a radial flame, wherein the set of nozzles has an oxidizer/fuel ratio (R) between 1.3 and 1.75.

The burner consists of two types of nozzles. A set of premix nozzles is arranged on a circle of diameter DB and is primarily intended to generate axial flames. These peripheral nozzles are placed around a central nozzle arranged on the central axis of the burner and intended to generate radial flames.

The radial flame through the central nozzle is intended to ensure inter-ignition between the individual premixing nozzles called "periphery". The radial flame is also generated by premixed combustion with a ratio (R) between 1.3 and 1.75 at the nozzle.

According to a particular feature, the central nozzle is a premix nozzle, which guarantees a better distribution of the ratio (R).

According to another feature, the radial flame is a premixed flame of 3% to 20% of the total flow rate of the fuel discharged by the nozzle.

According to one particular arrangement, both the central nozzle and the peripheral nozzles are premix nozzles and each have an oxidizer/fuel ratio (R) between 1.3 and 1.75. Each nozzle has an optimal ratio (R), the amount of NOx emitted by each nozzle is low, and the intersection area between the flames of the individual nozzles also emits very little NOx.

According to another arrangement, the central nozzle is removable. The central nozzle is removable and can be easily replaced by a liquid fuel injection gun, giving the burner the option of being a mixed gas and liquid burner.

According to a particular feature, the premix nozzle has a deflection element that imparts an exit angle to the premix relative to the central axis A of between -45° and +45°. Premix nozzles have axial or inclined outlets with respect to the axis of the burner and the outlet angle of the nozzles varies between -45° and +45° to adapt the shape of the flame to various combustion chamber geometries , and change the flame length/diameter ratio by a factor greater than 4.

According to a particular arrangement, the gas injectors are placed on the outer periphery of the burner around the peripheral nozzle with a diameter DL. In order to reduce excess air/oxidant at the burner without increasing NOx emissions, a certain ratio of gas needs to be injected outside the premix nozzle. The additional injection of gas takes place using injectors arranged between each pair of nozzles or every two or three nozzles.

According to a particular feature, gas injectors are placed on the outer periphery of the burner around the peripheral nozzle with diameter DL, and the injectors inject 20% to 50% of the total injected fuel.

According to another feature, the injector has an exit angle of between 0° and 40° relative to the exit angle of the peripheral nozzle. The additional gas injected at a diameter DL close to the outer diameter DB of the nozzle is given or accounts for 10%, and is injected at an angle between 0° and 40°, and preferably between 10° and 30°, relative to the axis of the premix nozzle . This makes it possible to locally increase the ratio (R) without excessive rapid mixing between these additional injection gases and the premix coming out of the nozzle. The gas is injected in such a way that it first mixes with the flue gas circulating at the outer periphery of the flame. The advantage of this configuration is that it makes it easier to adjust to the opening of the combustion chamber and/or boiler, especially existing combustion chambers or boilers.

According to a particular feature, the diameter DL has a dimension less than or equal to the following value in mm: [P(MW)×20]450. P Power in megawatts. This dimension makes it possible to optimize the size of the burner and thus the size of the combustion chamber and/or the opening of the boiler.

According to another arrangement, the injector is equipped with a Venturi system. The gas is injected using a venturi system that accelerates the premixing of the flue gas with the additional injected gas at the outer periphery of the gas flame.

According to another feature, the diameter DL is between 1.2 and 1.6 times the diameter DB of the peripheral nozzle. The injector injects gas with a diameter larger than the diameter DB of the nozzle. The additional injection of gas is introduced via a straight injector through the wall of the combustion chamber and/or boiler, or in the case of a combustion chamber without a straight lance through the wall, through an elbow injector mechanically rotated from outside the combustion chamber introduce.

According to another feature, the injector has an exit angle of between -20° and +20° relative to the axis of the burner.

According to an alternative, the nozzle has a flame stabilizer representing 0.1 to 0.4 times the cross-section of said nozzle. Because the stability of the premixed flame is sensitive to local velocity gradients, reliable means must be used that will "hold" and stabilize the flame. This also required the development of high-performance flame holding and stabilization devices using stabilizers inserted at the exit of each nozzle, allowing the stability of the premixed flame to extend to a greater ratio (R), and to extend the stability of the premixed flame when there is no flame retreat into the premixed nozzle The risk of internal burner flow rate changes without increasing the burner. For a change in feed to the burner to be greater than 6, and for an overall burner pressure drop of 300 DaPa, the surface area of the flame holder needs to be between 0.1 and 0.4 relative to the surface area of the nozzle. Stabilizers can be used in the center of the nozzle or on the outside of the nozzle.

According to a particular feature, the flame holder determines the exit angle of the premixture. The exit angle of the premixture is imparted by a deflection element, which may or may not be associated with a flame holder.

According to another arrangement, the peripheral nozzles have a flame holder comprising a radial portion arranged at the center of each peripheral nozzle. In order to better maintain the flame at the tip of each premixing nozzle, the stabilizer includes a radial stabilizing section which ensures a better spread of the radially central flame towards the center of each peripheral nozzle. Rapid ignition of the premix exiting the peripheral nozzles at the stabilizer is ensured for a wide range of premix nozzle exit velocities and thus for a wide range of burner power variations.

The invention also relates to the burner used by the method.

Description of drawings

Still other advantages may occur to those skilled in the art after reading the following examples given by way of example and described with reference to the accompanying drawings.

- Figure 1 is a cross-section through a burner of the prior art, shown with peripheral nozzles installed,

- figure 2 shows an upwardly facing view of the burner of figure 1,

- Figure 3 shows a cross-section of a burner according to the invention, showing the arrangement of the central nozzle and the spread of the radial flame,

- Figure 4 is an upwardly facing view of the burner of Figure 3,

- Figure 5 is a detailed view of a nozzle with a stabilizer according to a first embodiment,

- Figure 6 is a detail view of a second embodiment of the nozzle according to the invention,

- Figure 7 is a detail view of a third embodiment of the nozzle according to the invention with stabilizer and deflection elements,

- Figure 8 is a detail view of a nozzle according to a fourth embodiment,

- Figure 9 is a detail view of a peripheral nozzle with a central nozzle,

- figure 10 is a cross-sectional view of a burner according to a second embodiment with additional gas injection,

- Figure 11 is an upwardly facing view of the burner of Figure 10,

- Figure 12 shows a detail of the injector seen from the side,

- Figure 13 is an upwardly facing view of the injector of Figure 12,

- Figure 14 is a cross-sectional view of a burner according to a third embodiment,

- Figure 15 is a cross-sectional view of a fourth embodiment of a burner according to the invention.

detailed description

A prior art burner 1 , shown in FIG. 1 , comprises a premixing nozzle 2 that is circular about the axis A of the burner 1 . Each nozzle 2 is preferably cylindrical into which substantially all of the oxidant (usually air) is introduced. The fuel itself is injected into these nozzles 2 via a series of main injectors, the number of which can vary from 1 to 16 depending on the burner power and the available gas pressure.

Each main injector 3 diffuses the gas using a series of holes (not depicted) distributed radially over the main injector 3 and at 90° to 45° relative to the axis of the nozzle 2 . The position of the main injector 3 in the nozzle 2 and the stepped arrangement of the holes are defined in such a way that the gas is diffused as evenly as possible through the entire surface area of the nozzle 2, avoiding areas where the jets intersect and thus avoid areas where the gas collects, And avoid areas where there is no gas in the premix.

A burner 1 according to the invention is depicted in FIGS. 3 and 4 , comprising a peripheral nozzle 2 and a central nozzle 4 . The premixing nozzle 2 is preferably circular around a central nozzle 4 generating a radial flame 5 . This radial flame 5 has the task of ensuring mutual ignition between the individual premixing nozzles 2 . The radial flame is also generated by combustion with premixing so that the ratio (R) is kept in favor of low thermal NOx generation at the intersection between the radial flame 5 and the axial flame emanating from the nozzle 2 .

The central nozzle 4 is removable and can be replaced by another type of nozzle that gives the burner 1 some mixing function.

Various alternative forms of the nozzle 2 shown in Figures 5 to 8 include a stabilizer 6 arranged to:

- at the center of the nozzle 2 in Fig. 5,

- on both sides of the nozzle 2 in Fig. 6,

- at the center in Fig. 7 and with fins 60,

- In FIG. 8 , the stabilizer 6 is placed at the top end of the circular duct 61 .

FIG. 9 shows that the form of the stabilizer has a radial portion 62 while allowing the stabilizer 6 to be arranged in the middle of the nozzle 2 . This radial portion 62 ensures a better spread of the radial central flame 5 towards the center of each peripheral nozzle 2 .

10 and 11 show a second embodiment of the burner 1 , in which the injector 7 is arranged at the outer circumference of the burner 1 with a diameter DL outside the diameter DB of the nozzle 2 . The injector 7 is placed between two nozzles 2 (see FIG. 11 ) or every two nozzles. This position allows easier mixing of the mixed gas from the nozzle 2 with the gas from the injector 7 .

In one embodiment, the injector 7 leads to a venturi 70 (see FIGS. 12 and 13 ) so that the gas emanating from the injector 7 can be accelerated to the low oxygen content combustion gas circulating at the outer periphery of the burner flame. mix. This arrangement allows a slow combustion of the additional gas injected by the injector 7 and limits the formation of NOx associated with this combustion. This venturi 71 has a semicircular shape such that it surrounds the top end 71 of the injector 7 .

The burner 1 shown in FIG. 14 is arranged on a wall 8 and has an injector 7 placed outside the diameter DB of the nozzle 2 . The injector 7 is straight and passes through the wall 8 . When it cannot pass through the wall 8, the injector 7 travels along the burner 1, past the wall 8 and then continues into an elbow section 73, allowing the gas to be injected at a selected distance. The injector 7 injects the gas at an angle between -20° and +20°, the injection angle being produced in a known manner, for example using an inclined nozzle (not depicted).

Claims (11)

1. A method of combustion comprising a premixed burner (1) consisting of a set of premixed peripheral nozzles (2) arranged around said burner The central nozzles (4) on the central axis (A) of (1) are arranged on a circle of diameter DB and said central nozzles are intended to generate radial flames (5), characterized in that the peripheral nozzles ( 2) and said central nozzle (4) have an oxidizer/fuel ratio R between 1.3 and 1.75, where Among them, LCV is the low calorific value, that is, the calorific value of energy condensation is not considered, CP is the heat capacity in kJ/kg, and The flow rate is in Kg/h. 2. The method according to claim 1, characterized in that said radial flame (5) is 3% of the total flow rate of fuel discharged by said peripheral nozzle (2) and said central nozzle (4) % to 20% premixed flame. 3. The method according to claim 1 or 2, characterized in that the central nozzle (4) and the peripheral nozzle (2) are both premix nozzles and each have an oxidant of between 1.3 and 1.75 / fuel ratio R. 4. The method according to claim 1, characterized in that the peripheral nozzle (2) of the premix has deflection elements (60, 61 ) which impart the premix with respect to the central axis (A ) into an exit angle between -45° and +45°. 5. The method according to claim 1, characterized in that a gas injector (7) is placed on the outer periphery of the burner (1) with a diameter DL around the peripheral nozzle (2), and the injection The injector (7) injects 20% to 50% of the total injected fuel. 6. A method according to claim 5, characterized in that the injector (7) has an exit angle of between 0° and 40° relative to the exit angle of the peripheral nozzle (2). 7. Method according to claim 5 or 6, characterized in that the diameter DL has a dimension in mm less than or equal to the following value: [P×20]+450, where P is the power in megawatts. 8. A method according to claim 5, characterized in that the diameter DL is between 1.2 and 1.6 times the diameter DB of the peripheral nozzle (2). 9. A method according to claim 5, characterized in that the injector (7) has an exit angle of between -20° and +20° relative to the central axis (A) of the burner . 10. The method according to claim 1, characterized in that the peripheral nozzle (2) has a flame stabilizer (6) occupying 0.1 to 0.4 times the cross-section of the peripheral nozzle (2). 11. A method according to claim 10, characterized in that the peripheral nozzles (2) have a flame holder (6) comprising a radial portion ( 62).