US20100183989A1 - Air-Gas Pilot Burner that can Operate with Oxygen - Google Patents

Abstract

A system includes at least one pilot burner and one more powerful main burner. The pilot burner includes at least a first tube open at an end and including at least a first feedline capable of injecting a combustible gas into the first tube. The pilot burner further includes at least a second tube open at an end and including at least a third feedline capable of injecting an oxygen-rich gas into the second tube. The pilot burner further includes at least a second feedline capable of feeding the first tube with air.

Description

• This application claims the benefit of priority under 35 U.S.C. §119 (a) and (b) to French Application No. 0950256, filed Jan. 16, 2009, the entire contents of which are incorporated herein by reference.
BACKGROUND
• 1. Field of the Invention
• The present invention relates to a pilot burner for igniting or maintaining a flame of a main burner, to an installation comprising a boiler or furnace capable of implementing the technology referred to as “oxy-fuel combustion” and such a pilot burner, and to a process for employing said installation, especially for making it easier to capture the carbon dioxide, or CO₂, in the flue gases exiting the boiler or furnace.
• 2. Related Art
• A pilot burner is a device capable of generating what is called a “pilot” flame intended to ignite or maintain the flame of at least one more powerful main burner. Typically, a main burner releases 5 to 1000 times more energy than a pilot burner. In one particular configuration, the pilot burner releases a power of about 4 to 10% of that of the main burner. According to another particular configuration, the pilot burner releases a power of less than 4% of that of the main burner. A pilot burner may be permanently lit, for example to stabilize the combustion of a difficult fuel in the main burner, or to deliver an ignition complement under abnormal fuel delivery conditions to a main burner. The pilot burner may also be lit only to start the burner, or to stop it, or at certain moments, for example during tricky operating conditions. It may also be used for completely or partly raising the temperature of a boiler or furnace during its start-up, with or without the main burner or burners being simultaneously ignited.
• In the field of pilot burners, models suitable for boilers or furnaces operating in air-combustion or aero-combustion mode have been known for a long time. Many techniques have been developed for the purpose of producing a pilot flame which to a greater or lesser extent is effective, reliable and inexpensive.
• In general, a pilot burner flame must be a reliable source of ignition. In particular, it must be extremely stable.
• Document GB-A-2 328 735 describes a pilot burner operating with air that has only one duct for the combustible gas. The oxidizer used for the pilot flame is the air present in the combustion chamber. This pilot burner does not provide a very stable flame under all operating conditions.
• EP-A-972 163 describes a more sophisticated pilot burner with air and combustible gas mixed together prior to their exit from the pilot burner. This configuration increases the stability of the flame when operating with air. However, in this configuration it is not possible to operate in oxy-fuel combustion mode, i.e. with an oxygen-rich gas as oxidizer.
• More recently, oxy-fuel combustion technology has been developed in various industrial fields, in particular that of energy production. It makes it possible to introduce the smallest possible amounts of elements deleterious to possible CO₂capture, such as nitrogen. Pilot burners more suitable for oxy-fuel combustion have been developed.
• For example, document U.S. Pat. No. 6,196,834 discloses a pilot burner for starting oxy-fuel burners. In fact, such a pilot burner cannot operate correctly with any type of oxidizer. In particular, although it operates correctly with an oxygen-rich oxidizer, which is its purpose, it will not operate well with air as oxidizer. This is because, all other things being equal, the stoichiometric air flow rate necessary for combustion of the combustible gas is approximately five times greater than the stoichiometric oxygen-rich gas flow rate necessary. It is difficult for such a change in oxidizer to be compatible with the reliability required of a pilot flame, which reliability determines the safety of the boiler or furnace.
• Therefore, this pilot burner must always operate with the same oxygen-rich gas as oxidizer. However, it may prove to be very troublesome to have to feed the pilot burner or burners for these main burners with oxygen-rich gas as it may require an air separation unit to be started up or maintained in operation.
SUMMARY OF THE INVENTION
• The object of the present invention is to alleviate some or all of the drawbacks of the prior art mentioned above, i.e. in particular to obtain a high-quality pilot flame having the required stability and safety, while operating both with air and with an oxygen-rich gas as oxidizer for the pilot burner.
• For this purpose, the invention relates to a system comprising at least one pilot burner and at least one more powerful main burner, said pilot burner comprising:
• at least a first tube open at an end and including at least a first feedline capable of injecting a combustible gas into said first tube; and
• at least a second tube open at an end and including at least a third feedline capable of injecting an oxygen-rich gas into said second tube, characterized in that said pilot burner further comprises at least a second feedline capable of feeding said first tube with air.
• The pilot burner according to the invention comprises at least two tubes open at one of their ends. The first tube includes at least one feedline intended to be connected to a combustible gas source and at least one other feedline intended to be connected to an air source.
• According to one particular embodiment, the combustible gas is natural gas, propane or butane. According to another embodiment, this combustible gas is liquefied petroleum gas (LPG) fed in the gas phase into the pilot burner. The second tube includes at least one feedline intended to be connected to a source of oxygen-rich gas, i.e. one comprising at least 80% oxygen by volume. According to one particular embodiment, the oxygen-rich gas comprises at least 90% oxygen by volume. According to another particular embodiment, this gas comprises at least 95% oxygen by volume.
• The sources may be liquefied or pressurized gas containers, or installations intended to generate these gases on demand, or else a combination of these two methods of production. The oxygen-rich gas is optionally produced on site by a cryogenic distillation unit. The air may have undergone treatments, for example filtration and/or degreasing and/or compression treatments. In one particular configuration, this air comes from an instrument air network. The combustible gas may come from a container or from a distribution network.
• Upon stopping the pilot burner, a flow of purging gas (typically air that may have undergone treatments) may be injected into both tubes.
• Ignoring the connections made for connecting them to the gas sources, the two tubes of the pilot burner have a geometric shape isomorphic with a cylinder open at one end and closed at the other. They may have one or more common walls. According to one particular embodiment, they are pipes with no common lateral walls. Each of these tubes may be manufactured as one or more parts, welded or joined together using methods known to those skilled in the art.
• The tubes may especially be made of carbon steel, stainless steel or a refractory material. They may also be formed from alloys having a high nickel content and/or alloys containing chromium and/or molybdenum. Their ends may have parts made of ceramic materials.
• The flow of gases in these tubes, from the aforementioned feedlines to the open ends of these tubes, defines a flow direction or axial direction. The cross section of the tubes is therefore defined as the intersection of these tubes with a plane orthogonal to the main flow direction. These tubes may have a variable cross section. Corresponding to each cross section is a flow bore, defined as the area within the cross section, through which the gases can flow. The flow rate of these gases may be defined as the area of this bore multiplied by the average velocity of the gases.
• The pilot burner may further include members for regulating the flow rates of the gases injected into the tubes. These members may for example be controlled opening valves located on the feedlines connecting said tubes to the gas sources. They may also include on/off isolating valves, purge devices, vents, non-return valves or other valves or rupture discs. These members are such that, when the combustible gas and the air are injected into the first tube, the flow rate of oxygen-rich gas injected into the second tube is zero. The burner is then in air mode and operates with premixing. Furthermore, during this operation in air mode, a flow of purging gas (typically air that may have undergone treatments) may flow through the second tube.
• According to another configuration, corresponding to oxy-fuel combustion mode, or operating mode with oxygen-rich gas, the members for regulating the flow rates of the gases injected into the tubes are such that the flow rate of air injected into the first tube is zero and the flow rate of oxygen-rich gas injected into the second tube is non-zero. Thus, the combustible gas and the oxygen-rich gas are injected into different tubes. In order for a pilot flame to be able to develop, the combustible gas and the oxygen-rich gas must come into contact with each other at the outlets of the two tubes.
• Moreover, embodiments of the invention may include one or more of the following features:
• said pilot burner comprises electrodes capable of producing at least one electric arc or a spark in order to ignite said pilot burner;
• said pilot burner comprises a device designed for and capable of controlling the presence of a flame emanating from said pilot burner;
• said first tube is at least partially located inside said second tube;
• said second tube is at least partially located inside said first tube;
• at least one portion of said first tube and at least one portion of said second tube have gas flow bores of constant area;
• said portions of said tubes having gas flow bores of constant area have concentric circular cross sections;
• the ratio of the minimum area of the bore for flow of said combustible gas close to the outlet of said first tube to the minimum area of the bore of said oxygen-rich gas close to the outlet of said second tube is not less than 0.15 but does not exceed 5.5;
• said first tube comprises a convergent portion as outlet, which converges in the flow direction of said combustible gas;
• said second tube comprises a convergent portion as outlet, which converges in the flow direction of said oxygen-rich gas;
• said second tube comprises elements intended to reduce the bore of said second tube.
• The invention may also relate to any alternative device or method comprising any combination of the above features or those mentioned later.
• The electrodes mentioned above serve to ignite the pilot burner. Generally, use is made of an electrode and a counter electrode that are suitably isolated and connected to a source of electricity, so as to be able to generate an electric arc or a spark between the two electrodes, according to methods known to those skilled in the art. This arc must be located at the end of the tubes, in the zone where the combustible gas leaving the first tube encounters the oxygen-rich gas leaving the second.
• In one particular configuration, the electrode is placed at the outlet of the first tube, at the point where the combustible gas exits the tube, so as to ensure ignition as rapidly as possible upon opening the combustible-gas control member. According to one embodiment, the energy released by the electric arc or the spark is equal to or greater than 8 J (joules), preferably between 9 and 15 J and ideally between 10 and 12 J.
• The means for controlling the presence of a pilot flame may differ in nature. According to one advantageous embodiment, said electrodes are used as a flame detector. Specifically, the presence of a flame modifies the resistivity of the medium lying between the two electrodes. By measuring the current at the terminals of the electrodes, it is possible to detect the presence or absence of a flame. According to another embodiment, a separate electrode is used for flame detection, so as to separate the operating function from the safety function.
• Other principles for detecting the presence of a flame may be used, for example by measuring the ultraviolet (UV) radiation. This is because combustion of the hydrogen present in the fuel for the pilot burner emits substantial radiation in the UV spectrum. Another principle is based on detecting the infrared (IR) radiation emitted by the combustion. These methods may be employed without distinction. The principle of this pilot burner makes it possible to use the same flame detector for the oxy-fuel combustion operating mode and for the operating mode using air. Different flame detection means specific to each of these modes may also be chosen. The detection means are positioned according to the prior art, in general relative to the position of the flame root in order to allow reliable detection.
• According to one particular geometry of the burner, one of the two tubes lies at least partly inside the other. For example, the terminal portion of the first tube, i.e. that having the open end, lies inside the second tube. The outlet of the first tube is at the same level as that of the second tube, or else slightly upstream or downstream thereof.
• According to another embodiment of the invention, it is the second tube that lies at least partly inside the first. For example, the terminal portion of the second tube, i.e. that having the open end, lies inside the first tube. The outlet of the second tube may be at the same level as that of the first tube, or slightly upstream or downstream thereof.
• The tubes may be held in their relative positions by centring elements, these elements may also serve to hold the electrode and/or the counter electrode.
• According to an even more particular geometry, at least one portion of each tube has a circular cross section. According to a preferred embodiment, the tubes have a constant cross section and are concentric in this portion. They therefore take the form, in this portion, of coaxial cylinders.
• Independently of their general shape, one or both of the tubes may have a convergent terminal portion, i.e. the flow bores narrow down in the flow direction before the gas outlet. The tubes may also have a constriction near their outlet. The expression “near the outlet” means the terminal portion of the tube that goes from its outlet back to 5 times the hydraulic diameter of the outlet cross section of the tube. This constriction allows the gas ejection velocity to be increased and prevents flash-back.
• In order for the burner to operate correctly both in oxy-fuel combustion mode and in air mode, the inventors have established that the ratio of the areas of the flow bores must have the following property. If the minimum flow bore of the first tube near the outlet of this first tube is called S1 and the minimum flow bore close to the outlet of this second tube is called S2, it is necessary for the ratio S1/S2 to lie within the closed interval 0.15-5.5. This ensures that the pilot flame will not be blown and will have the desired properties for guaranteeing safe and stable operation.
• To enhance the flame properties, one or both tubes may have a convergent portion that converges in the flow direction near its outlet.
• The flow bores of one or both tubes may be adjusted by introducing solid elements having openings or perforations. By suitably orienting these openings, it is possible to impress a rotational movement of the gases around the main flow direction. This effect may also be obtained by adding elements such as fins or grooves in the tubes, so as to deflect the flow of the fluids.
• At or near the outlet of one or both tubes, deflectors, swirlers (designed for and capable of impressing a rotational movement) or other devices may be fitted in order to increase the turbulence in the pilot flame formation zone.
• The invention also relates to a combustion installation using an oxidizer gas, comprising:
• at least one oxy-fuel combustion boiler or furnace designed for and capable of operating in oxy-fuel combustion mode with an oxygen-rich gas as oxidizer or in degraded mode with air used as oxidizer;
• at least one air separation unit capable of delivering an oxygen-rich gas; and
• at least one compressed-air feedline or at least one air compressor, characterized in that:
• said boiler or furnace comprises at least one system such as described above;
• said oxygen-rich gas feedline of said pilot burner is fluidically connected to said air separation unit so as to be able to receive oxygen-rich gas produced by said air separation unit; and
• said air feedline of said pilot burner is fluidically connected to said at least one compressed-air feedline or to said at least one air compressor.
• The installation mentioned above employs an oxy-fuel combustion boiler or furnace, i.e. one capable of burning at least one fuel in the presence of at least one oxygen-rich gas. Such a boiler may for example serve to generate steam and/or hot water or hot oil and/or electrical and/or mechanical power. Oxy-fuel combustion is the normal operating mode. However, this boiler or furnace may in general operate with air in what is called “degraded” mode, in the sense that the efficiency may be adversely affected or capture of the CO₂in the flue gases will be more difficult. This is because such oxy-fuel combustion installations are by definition optimized for operating with an oxygen-rich gas as oxidizer.
• Given the large amounts of oxygen-rich gas required, at least one air separation unit may prove to be necessary. It is also possible to use an oxygen-rich gas storage container fed by at least one air separation unit or by lorry. For operation with air, an existing compressed-air network is required, or else one or more compressors are used, with the possibility of incorporating filtration and/or degreasing operations.
• The inventors have determined that it is beneficial to equip this boiler or furnace with at least one pilot burner as described above and to connect it to the oxygen-rich gas source used for the boiler or furnace and to the air source.
• The pilot burner or burners may be placed, relative to the main burner with which it is associated, in various configurations. According to one particular configuration, a pilot burner is located inside a main burner, meaning that the outlets of the pilot burner or burners lie physically within the perimeter of the outlet of said main burner. According to another configuration, the pilot burner lies at a certain distance from the main burner. According to another configuration, the pilot burner lies at a certain distance from the main burner, in such a way that the pilot flame is located in or encounters the gas stream ejected by the main burner. According to one embodiment, the pilot flame converges towards the gases ejected by the main burner.
• The invention also relates to a combustion process, employing:
• at least one oxy-fuel combustion boiler or furnace comprising one or more main burners designed for and capable of operating in oxy-fuel combustion mode with an oxygen-rich gas used as oxidizer or in degraded mode with air used as oxidizer, said boiler or furnace comprising at least one pilot burner as described above;
• an oxygen-rich gas source comprising at least one air separation unit capable of delivering an oxygen-rich gas; and
• an air source comprising at least one compressed-air feedline or at least one air compressor,
• characterized in that it comprises at least the following step a):
• a) air, as oxidizer, coming from said air source, is injected into the main burners of said boiler or furnace and into said air feedline of said pilot burner, said oxygen-rich gas feedline of said pilot burner not introducing oxygen-rich gas into said pilot burner.
• Moreover, methods of implementing the process according to the invention may have one or more of the following features:
• the process comprises the following step b):
• b) an oxygen-rich gas, as oxidizer, coming from said oxygen-rich gas source, is injected into the main burners of said boiler or furnace and into said oxygen-rich gas feedline of said pilot burner, said air feedline of said pilot burner not introducing air into said pilot burner;
• the process includes the following steps:
• c) a first fuel of gaseous type is injected into said combustible gas feedline of said pilot burner; and
• d) at least one main burner of said boiler or furnace is fed with a second fuel; and
• said first fuel of gaseous type has a composition identical to or different from that of said second fuel.
• According to alternative methods of implementation, steps a) and b) described above may be implemented in any order. Steps c) and d) themselves are simultaneous with steps a) and b).
• When the main burner or burners of the boiler or furnace are in oxy-fuel combustion mode, the pilot burner itself will in general be in oxy-fuel combustion mode (step b)).
• When the boiler or furnace is made to operate in air mode, for example during a start-up phase, it may be very advantageous and without any risk for the pilot burner also to be operated in air mode (step a)). This avoids consuming oxygen-rich gas and avoids having to start up the air separation unit just for the pilot burner.
• If the boiler or furnace is in oxy-fuel combustion mode and the air separation unit fails, it will be possible to continue using the pilot burner with air without any particular risk and thereby ensure that the pilot flame is present under all operating conditions of the boiler or furnace. Step a) may therefore follow step b) and will then for example correspond to a period of maintenance of the oxygen-rich gas source.
• Steps c) and d) may be in any order. According to one particular method of implementation, step c) starts before step d). In general, they are concomitant with steps a) or b).
• The boiler or furnace may employ, in the main burner or burners, one or more fuels such as natural gas, crude oil and its derivatives (such as bitumen, heavy or light fuel oil, petroleum residues, etc.), coal, lignite, peat, biomass, liquefied petroleum gas, slurries, sludge, etc.
BRIEF DESCRIPTION OF THE FIGURES
• Other particular features and advantages will become apparent on reading the following description, given with reference to the figures in which:
• FIG. 1 shows a pilot burner according to the invention in side or longitudinal view; and
• FIG. 2 shows the tubular portion of the same pilot burner in front or axial view, the ejected gases being directed towards the observer.
DETAILED DESCRIPTION OF THE INVENTION
• FIG. 1 shows thepilot burner1 in its entirety. It comprises afirst tube2 open at itsdownstream end2a, which may receive combustible gas via afirst feedline4 and air via asecond feedline5. Thepilot burner1 further includes asecond tube3 open at itsdownstream end3a, and may receive an oxygen-rich gas via afeedline6. The two tubes take the form of coaxial cylinders, thesecond tube3 surrounding thefirst tube2.
• Anelectrode7a, connected to a current source and suitably isolated electrically, except at its end, is placed at the centre of thefirst tube2. It is located on the axis of thetube2, its stripped end lying level with theopen end2aof thefirst tube2. Thesecond tube3 has acounter electrode7blocated at its outlet, in electrical contact with saidtube3. It also ensures electrical continuity with earth. The twoelectrodes7aand7bare placed so as to produce, between saidelectrodes7aand7b, an electric arc or aspark7ccapable of igniting the flame of thepilot burner1.
• Theseelectrodes7aand7balso serve as flame detectors. They form the terminals of a dipole, the current of which is measured by elements not shown in the figure.
• Thetube3 also has anelement8 near its outlet. This is designed to restrict the oxygen-rich gas flow bore in saidtube3. It takes the form of a gas flow obstacle, having perforations or channels from which the gas exits in the form of jets with a higher velocity than upstream of theelement8. Thebore2aof thetube2 at theelement8 has an area S2. If the area of the bore for flow of the oxygen-rich gas through theelement8 is S3, the ratio S2/S3 has a value of between 0.15 and 5.5. According to one particular embodiment, this ratio is between 0.15 and 2.0.
• FIG. 2 shows the flow bores2aand2bof thetubes2 and3 respectively. Thebore2ahas the form of a disc simply reduced by the presence of theelectrode7aat its centre. Thebore3awould be annular without the presence of theelement8, which restricts the flow. In the example shown, flow may take place only through four equal sectors of the ring. The flow and non-flow sectors alternate along thering3a.
• It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.

Claims (11)

1. A system comprising at least one pilot burner and at least one more powerful main burner, said pilot burner comprising:

at least a first tube open at an end and including at least a first feedline capable of injecting a combustible gas into said first tube; and

at least a second tube open at an end and including at least a third feedline capable of injecting an oxygen-rich gas into said second tube,

characterized in that said pilot burner further comprises at least a second feedline capable of feeding said first tube with air.

2. The system ofclaim 1, wherein said pilot burner further comprises:

electrodes capable of producing at least one electric arc or a spark in order to ignite said pilot burner; and

a device designed for and capable of controlling the presence of a flame emanating from said pilot burner.

3. The system ofclaim 1, wherein:

said first tube is at least partially located inside said second tube; or

said second tube is at least partially located inside said first tube.

4. The system ofclaim 1, wherein at least one portion of said first tube and at least one portion of said second tube have gas flow bores of constant area.

5. The system ofclaim 4, wherein said portions of said tubes having gas flow bores of constant area have concentric circular cross sections.

6. The system ofclaim 1, wherein a ratio of the minimum area of the bore for flow of said combustible gas close to the outlet of said first tube to the minimum area of the bore of said oxygen-rich gas close to the outlet of said second tube is not less than 0.15 but does not exceed 5.5.

7. The system ofclaim 1, wherein:

said first tube comprises a convergent portion as outlet, which converges in the flow direction of said combustible gas; and/or

said second tube comprises a convergent portion as outlet, which converges in the flow direction of said oxygen-rich gas; and/or

said second tube comprises elements intended to reduce the bore of said second tube.

8. A combustion installation using an oxidizer gas, comprising:

at least one oxy-fuel combustion boiler or furnace designed for and capable of operating in oxy-fuel combustion mode with an oxygen-rich gas as oxidizer or in degraded mode with air used as oxidizer;

at least one air separation unit capable of delivering an oxygen-rich gas; and

at least one compressed-air feedline or at least one air compressor, characterized in that:

said boiler or furnace comprises at least one system according toclaim 1;

said oxygen-rich gas feedline of said pilot burner is fluidically connected to said air separation unit so as to be able to receive oxygen-rich gas produced by said air separation unit; and

said air feedline of said pilot burner is fluidically connected to said at least one compressed-air feedline or to said at least one air compressor.

9. A combustion process employing at least one oxy-fuel combustion boiler or furnace comprising one or more main burners designed for and capable of operating in oxy-fuel combustion mode with an oxygen-rich gas used as oxidizer or in degraded mode with air used as oxidizer, an oxygen-rich gas source comprising at least one air separation unit capable of delivering an oxygen-rich gas, and an air source comprising at least one compressed-air feedline or at least one air compressor, said boiler or furnace comprising at least one pilot burner as defined inclaim 1, said method comprising the step of:

a) air, as oxidizer, coming from said air source, is injected into the main burners of said boiler or furnace and into said air feedline of said pilot burner, wherein said oxygen-rich gas feedline of said pilot burner does not introduce oxygen-rich gas into said pilot burner.

10. The process ofclaim 9, further comprising the step of:

b) an oxygen-rich gas, as oxidizer, coming from said oxygen-rich gas source, is injected into the main burners of said boiler or furnace and into said oxygen-rich gas feedline of said pilot burner, wherein said air feedline of said pilot burner does not introduce air into said pilot burner and said steps a) and b) may be performed in any order.

11. The process ofclaim 9, further comprising the steps of:

c) a first fuel of gaseous type is injected into said combustible gas feedline of said pilot burner; and

d) at least one main burner of said boiler or furnace is fed with a second fuel.

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