System and method for laser ignition of fuel in a coal-fired combustor

Abstract

A system and method for igniting a coal-air-fuel mixture includes a burner (14) having a burner tube (100) and a first flow guide (118), the burner tube being operable to deliver a flowing fuel-air mixture to a furnace for combustion therein, the first flow guide disposed within the tube and operable to guide a first portion of the flowing fuel-air mixture to a specific location within the burner tube. The system also includes a laser igniter (17) within the burner tube (130), the laser igniter comprising a laser tube (110) and a laser source (150), the laser tube having a first end with a laser input and a second end with a light output, the laser source being operatively coupled to the laser input. The laser source includes a laser. The laser igniter guides photons from the light output to the location within the burner tube to ignite at least a portion of the first portion of the fuel.

Description

Technical Field

[0001] The embodiments described herein generally relate to evaluation and control methods and systems for laser ignition of coal. More specifically, a method and system for controlling the ignition and combustion of fuel in a coal-fired boiler system. Background Technology

[0002] A boiler typically comprises a furnace in which fuel is burned to generate heat to produce steam. The combustion of fuel produces heat energy, or thermal energy, which is used to heat and evaporate liquids such as water, producing steam. The resulting steam can be used to drive turbines to generate electricity or to provide heat for other purposes. Fossil fuels such as pulverized coal and natural gas are conventional fuels used in many combustion systems of boilers. When fuel is burned, heat is generated and soot and flue gas are formed.

[0003] Today's electricity market is shifting from baseload to cyclic and peak loads due to the increasing participation of renewable energy. A new challenge facing many grid systems is grid stability associated with the sudden and cyclical nature of electricity production from such renewable sources. As more renewable energy enters the grid, there is a growing need to operate fossil fuel incineration power plants at low power and / or improve rapid start-up to help stabilize the grid. Enabling plants to operate at partial load for longer periods and having multiple cold starts will allow more renewable electricity to penetrate the grid and reduce stress and fatigue in components due to load cycling. However, operating plants at low power and making large plants more responsive to load changes presents challenges. For example, large power plants exhibit large thermal masses, requiring time to heat up and having limited thermal gradient capabilities. Furthermore, burner flame stability decreases for coal combustion at low plant loads.

[0004] To ensure operability under low-power conditions, in many cases, additional support or ignition flames are generated by burning oil. However, oil igniters present several problems, which are amplified by the operating characteristics of modern coal plants. These include increased corrosion due to the presence of sulfur in the oil, increased logistical complexity due to the need for plants to maintain adequate reserves of flammable oil or natural gas, and increased operating costs.

[0005] In some systems, plasma igniters have been commercially deployed to overcome the need for oil-based igniters that generate flames. These igniters directly ignite coal with the aid of multiple ignition stages and maintain the flame across the entire air / fuel ratio and fuel flow level. In a plasma igniter, a high-voltage source (present in the ignition zone) generates plasma (high-energy-band charged ions). The plasma bombards coal particles, heating them and ultimately igniting them. However, plasma igniters also present challenges that lead to higher costs. For example, plasma igniters require high auxiliary power requirements (120kW to 150kW), medium-voltage cables with significant electrical insulation, bulky equipment, frequent replacement of corroded electrodes, and ignition nozzles that impede the coal flow more than desired.

[0006] Therefore, there is a need for a new method and system for generating additional support or starting flames without the need for oil or complex plasma igniters. Summary of the Invention

[0007] In one embodiment, a burner assembly is disclosed. The burner assembly includes: a fuel source; a furnace; a fuel delivery pipe for delivering fuel from the fuel source to the furnace; and a laser igniter configured to ignite the fuel delivered to the furnace via the fuel delivery pipe, the laser igniter including: a laser source; a laser tube for delivering a laser beam from the laser source to the fuel in the fuel delivery pipe; and an ignition tube disposed within the fuel delivery pipe to receive a portion of the fuel flowing through the fuel delivery pipe, wherein the laser beam heats and ignites the fuel in the ignition tube, wherein the ignition of the fuel in the ignition tube occurs in multiple ignition stages.

[0008] In another embodiment, a system is provided. The system includes: a coal fuel source; an air source; a heating furnace; a fuel delivery pipe for delivering a mixture of coal from the coal fuel source and air from the air source to the heating furnace to cause combustion of the mixture; and a laser igniter configured to ignite the coal-air mixture delivered to the heating furnace via the fuel delivery pipe, the laser igniter including: a laser source; a laser tube for delivering a laser beam from the laser source to the coal-air mixture within the fuel delivery pipe, wherein a portion of the laser tube is disposed within the fuel delivery pipe; and an ignition tube. The system includes a plurality of flow guiding devices, each configured to guide a portion of the coal-air mixture flowing through the fuel delivery pipe to flow around the ignition tube for irradiation by the laser beam. The laser beam is disposed within the fuel delivery pipe at a predetermined interval from the portion of the laser tube disposed within the fuel delivery pipe. The system also includes a plurality of flow guiding devices, each of which is configured to guide a portion of the flow of the coal-air mixture around the ignition tube for irradiation by the laser beam.

[0009] In another embodiment, a method for igniting fuel for combustion in a burner assembly is disclosed. The method includes: conveying a flow of a mixture of fuel from a fuel source and air from an air source to a heating furnace via a fuel delivery pipe; guiding a portion of the flow of the fuel-air mixture through an inlet of an ignition tube disposed within the fuel delivery pipe; directing a laser beam toward the fuel-air mixture being conveyed through the ignition tube; heating and igniting the fuel-air mixture being conveyed through the ignition tube in multiple stages; and supplying the heated and ignited fuel-air mixture from the ignition tube to the heating furnace.

[0010] Additional features and advantages are achieved through the technology disclosed herein. Other embodiments and aspects of this disclosure are described in detail herein. For a better understanding of this disclosure and its advantages and features, refer to the specification and accompanying drawings provided herein. Attached Figure Description

[0011] The embodiments will be better understood by referring to the following description of non-limiting embodiments, in which:

[0012] Figure 1 This is a simplified schematic diagram of a power generation system with a boiler and ignition system according to an implementation plan;

[0013] Figure 2 This is a cross-sectional illustration of an exemplary ignition tube and igniter according to one embodiment;

[0014] Figure 3 It is based on an implementation plan. Figure 2 More detailed illustrations of the ignition system and ignition tube;

[0015] Figure 4 This is a flowchart illustrating an ignition method according to an implementation plan. Detailed Implementation

[0016] The exemplary embodiments described herein will now be referenced in detail, examples of which are shown in the accompanying drawings. Wherever possible, the same reference numerals used throughout the drawings refer to the same or similar parts. While various embodiments are described herein with reference to pulverized coil boilers in heat recovery steam generation systems, this reference is merely illustrative. Generally, the embodiments described can be applied to any application of fuel combustion systems, including but not limited to pulverized coal burners that can be used in pulverized coal power plants. Other systems may include other types of plants employing coal combustion systems, including but not limited to chemical plants, power plants, and boilers, furnaces, and incinerators utilizing various fuels (including but not limited to coal). For example, envisioned boilers include, but are not limited to, pulverized coal boilers with both T-type and wall-fired combustion, circulating fluidized bed (CFB) and bubbling fluidized bed (BFB) boilers, stoker boilers, suspension burners for biomass boilers (including controlled circulation, natural circulation, and supercritical boilers), and other heat recovery steam generation systems.

[0017] The embodiments described herein relate to a power generation system having a combustion system and a laser-based ignition and control scheme for the combustion system. In particular, a method for generating a start-up flame and additional flame support without the complexity of oil or plasma ignition. In one embodiment, the flame is generated inside a custom-designed coal burner where a high-power laser beam impinges on flowing pulverized coal particles. The temperature of the particles increases by absorbing photons from the laser and ignites upon reaching a desired critical temperature. Furthermore, the energy released from the ignition event of a single particle is absorbed by adjacent coal particles. These particles are also heated and ignited. This chain process continues until a stable flame is produced.

[0018] Figure 1A power generation system 10 is shown, which includes a combustion system 11 with a boiler 12, as may be employed in power generation applications according to several embodiments. The boiler 12 may be a tangential combustion boiler (also known as a T-type combustion boiler) or a wall-mounted combustion boiler. Fuel and air are introduced into the boiler 12 through a burner assembly 14 and / or nozzles associated therewith. The combustion system 11 includes a fuel source, such as, for example, a pulverizer 16, which is configured to grind fuel such as coal to a desired fineness. Pulverized coal is delivered from the pulverizer 16 to the boiler 12 using primary air. An air source 18 supplies primary, secondary, or combustion air to the boiler 12, in which the primary, secondary, or combustion air is mixed with fuel and combusted. In the case of oxygen combustion in the boiler 12, the air source 18 may be an air separation unit that extracts oxygen from an incoming air stream or directly from the atmosphere. In one embodiment, the burner assembly 14 includes a fuel source from the pulverizer 16 and a laser igniter 17, as will be described in further detail herein. The laser igniter 17 includes, but is not limited to, a power supply and cooling system for a high-power laser, a high-power laser, and a mechanism such as an optical fiber for delivering light to the burner location. The laser igniter 17 and the laser tube deliver the laser to the flowing pulverized coal for ignition and flame stabilization, as will be further described herein. The laser will be protected with precautions to prevent any accidental leakage of high-power radiation.

[0019] Boiler 12 includes a hopper zone 20 located below the main burner zone 22, from which ash is collected for subsequent removal. A grating may be provided at the bottom of boiler 12 for two purposes. First, the grating is used to introduce combustion, suspension, or fluidizing gas (for bed-type boilers), referred to as primary air or combustion air, which is pumped into boiler 12 by fan 34 via air preheater 35. Second, the grating facilitates the removal of bottom ash and other debris from boiler 12. Boiler 12 also includes: the main burner zone 22 (also called a bellows), in which air and an air-fuel mixture are introduced into boiler 12; and a burnout zone 24, in which any unburned air or fuel in the main burner zone 22 is burned.

[0020] Furthermore, boiler 12 includes a superheater zone 26 with a superheater 27 and an economizer zone 28 with an economizer 31, in which combustion flue gas can superheat steam, and in which water can be preheated before entering the mixing ball or boiler drum 25. In the main burner zone 22, a controlled flow of primary air, pulverized coal, and secondary air is introduced into the combustion system 11 to achieve the formation of a rotating fireball. Boiler feedwater entering economizer 31 originates from the use of steam turbine 50 and condenser 57 downstream of steam turbine 50. The condensate is first heated by steam using one or more low-pressure preheaters (not shown) before entering economizer 31. Pump 40 is used to assist in circulating water to the water wall 23 and through boiler 12.

[0021] Fuel is burned within boiler 12 with primary and secondary air to produce a flue gas flow, which is ultimately processed and discharged through a chimney downstream of economizer zone 28. The final, typically final, step of heat harvesting from the flue gas occurs in combustion air preheater 35, where the flue gas heat is used to heat the air used as combustion air in combustion system 11. Following air preheater 35 in the flue gas path is an electrostatic precipitator / dust collector or bag filter (not shown), which separates any solid particles remaining in the flue gas before it is discharged into the atmosphere via the chimney. As used herein, directions such as “downstream” refer to the general direction of the flue gas flow. Similarly, the term “upstream” is used in the opposite direction to “downstream,” i.e., opposite to the direction of the flue gas flow.

[0022] In the typical operation of the power generation system 10 and (more specifically) the combustion system 11, the combustion of fuel in the boiler 12 heats the water in the water wall 23 of the boiler 12, which then passes through a steam drum (or equivalent) referred to below as the boiler drum 25. The heated steam is then directed to a superheater 27 in the superheater zone 26, where additional heat is imparted to the steam by flue gas. The superheated steam from the superheater 27 is then directed via a piping system generally shown as 60 to the high-pressure section 52 of the turbine 50, where the steam expands and cools to drive the turbine 50, thereby turning the generator 58 to generate electricity. The expanded steam from the high-pressure section 52 of the turbine 50 can then be returned to the reheater 29 to reheat the steam, which is then directed to the intermediate-pressure section 54 of the turbine 50 and finally to the low-pressure section 56 of the turbine 50, where the steam continuously expands and cools to drive the turbine 50.

[0023] like Figure 1As shown, the combustion system 11 includes a series of sensors, actuators, and monitoring devices to monitor and control the ignition and combustion processes and the resulting consequences for boiler operation. For example, temperature and pressure monitors, generally and commonly shown as 36 and 37, are employed throughout the system and interface with the control unit 200 to ensure proper control, operation, and that the operating limits of the combustion system 11 and boiler 12 are not exceeded. In another example, the combustion system 11 may include a plurality of fluid flow control devices 30, also interfaced with the control unit 200, which supply secondary air for combustion to each fuel inlet nozzle associated with the burner assembly 14. In one embodiment, the fluid flow control devices 30 may be electrically actuated air dampers that can be adjusted to vary the amount of air supplied to each fuel inlet nozzle associated with each burner assembly 14.

[0024] The boiler 12 may also include other individually controllable air dampers or fluid flow control devices 30 at various spatial locations around the heating furnace and boiler 12. Each of the flow control devices 30 can be independently controlled by the control unit 200 to ensure the desired air-fuel ratio and flame temperature are achieved for each nozzle position. Furthermore, the power generation system 10 may also include a plurality of fluid flow control devices 66 to control, for example, the flow rate of water or steam in the system 10. In one embodiment, the fluid flow control device 66 may be a plurality of electrically actuated valves that can be adjusted to change the amount of fluid passing through them. Each of the fluid flow control devices (e.g., 66) can be individually controlled by the control unit 200.

[0025] Figure 1 Also shown is the downstream flue (or downstream ventilation section) 33 of the boiler 12 downstream of the superheater 27, reheater 29, and economizer 31 in economizer zone 28. The downstream flue 33 may also be equipped with monitoring devices 37. Monitoring devices 37, such as gas sensors 37, are optionally configured to measure and assess the types of gases within the downstream flue 33, such as carbon monoxide (CO), carbon dioxide (CO2), mercury (Hg), sulfur dioxide (SO2), sulfur trioxide (SO3), nitrogen dioxide (NO2), nitric oxide (NO), oxygen (O2), etc. SO2 and SO3 are collectively referred to as SOx. Similarly, NO2 and NO are collectively referred to as NOx.

[0026] The operation of boiler 12 continues, optionally with a predetermined ratio of fuel to air supplied to each burner assembly in burner assembly 14 for combustion. As the fuel / air mixture burns within the furnace and generates flue gas, the combustion process and the generated flue gas are monitored. Specifically, various parameters of the fireball and flame, furnace wall conditions, and flue gas parameters are sensed and monitored. These parameters can be transmitted to combustion control unit 200, where they are analyzed and processed according to control algorithms stored in memory and executed by a processor. Control unit 200 is configured to control the fuel supplied to boiler 12 and / or the air supplied to boiler 12 based on one or more monitored combustion and flue gas parameters and furnace wall conditions.

[0027] Figure 2 A simplified block diagram of a main burner assembly 14 and a laser ignition system 17, as part of a combustion system 11 according to one embodiment, is depicted. In one embodiment, the burner assembly 14 includes, but is not limited to, a tube 100 that carries pulverized coal particles (generally shown as 102) from a pulverizer 16 and primary air from an air source 18, similar to the primary air in a conventional burner assembly. In the figures, the tube 100 is depicted with a circular cross-section; however, this depiction is for illustrative purposes only. The tube 100 may have any kind of construction and / or cross-section, including but not limited to circular, square, rectangular, triangular, or polygonal shapes, without limiting the scope of the embodiments described herein.

[0028] The tube 100 of the main burner assembly 14 is also equipped with one or more flow guiding devices, generally shown as 118, and specifically as 118a, 118b, 118c, etc. The flow guiding device 118 is operable to help guide the flow of pulverized coal particles 102 and air in the tube 100. The flow guiding devices may be distributed circumferentially around the tube 100 or the ignition tube 130, or both. In one embodiment, the flow guiding device 118 operates to guide and concentrate the coal particles 102 in the burner assembly 14. The function of the flow guiding device 118 is to mechanically transfer a controlled portion of this fuel to a selected location in the tube 100 for ignition by an igniter (e.g., the laser igniter 17 of the described embodiment). The magnitude of the fuel flow rate injected into the burner is determined by the desired operating point of the burner. The flow guiding device 118 is designed to ensure that the coal particles 102 spend the maximum amount of time within the spatial envelope of the focused or collimated laser beam, as described in more detail herein. In one embodiment, the flow guiding device 118 may be a controllable Venturi port. In another embodiment, the flow guide 118 may be a static or controllable baffle or blade. In one embodiment, the flow guide 118 may be implemented as a static or controllable structure with a variable shape that causes redirection of the flow in the tube. For example, the flow guide may have a straight or curved leading edge to apply variable regulation or correction to the flow of air and coal particles in tube 100 or ignition tube 130. In one embodiment, the flow guide 118 operates to primarily direct the flow of coal particles 102 to the center of tube 100 for direct laser impact from laser igniter 17, as further described herein. In another embodiment, the function of the flow guide 118a is to transfer a controllable portion of the fuel and air flow in tube 100 into optional ignition tube 130 via mechanical means, as will be described in further detail herein.

[0029] In one embodiment, tube 100 is configured with a laser igniter 17 enclosed therein. In another embodiment, tube 100 is configured with a laser igniter 17 that is substantially, but not necessarily, concentric therein. The laser igniter 17 includes a second tube 110, referred to as laser tube 110, which surrounds a laser beam guided through laser tube 110 and optional ignition tube 130. Laser tube 110 has a laser input end 112 and an air input end 113 at a first end 114, and a focusing lens 119 at a second end or outlet end 116. Air input end 113 guides air along the length of laser tube 110 to provide cooling and to keep laser tube 110 under positive pressure and flow to ensure that laser tube 110 remains clean and to prevent any coal particles 102 from entering from tube 100.

[0030] Laser input 112 may include, but is not limited to, input from a laser source 150 operatively connected to a controllable power supply. Laser source 150 may include a laser diode (not shown), a fiber laser, or any high-power CW or pulsed laser, from which light is guided through selected lenses, gratings, couplers, etc., to couple to fiber optic 154. The laser is optically coupled to laser input 112 via fiber optic 154. Advantageously, by employing one or more fibers 154 to couple energy from the laser source to the laser igniter 17, the laser diode source 150 and associated optics are allowed to be located at a distance from the challenging environment of the laser igniter 17 and the combustion system 11. In one embodiment, multiple fibers 154 can be used to deliver laser light from lower-power lasers, resulting in a higher overall power of the laser system. For example, multiple lower-power lasers can be used together to generate a high-intensity laser or a laser beam with a large beam volume. In this way, the accumulated energy heats the flowing coal particles 102, and not necessarily in the form of a “single focal spot” of the laser as in the case of laser processing. Advantageously, this modular construction reduces the cost of the laser igniter and makes the system flexible and scalable. The total power of the system can be easily adjusted or increased by adding more fibers and / or lasers. Furthermore, this approach improves system robustness by eliminating any single point of failure within the laser source 150 in the entire ignition system of the combustion system 11. Although the use of a continuous wave (CW) fiber-coupled laser as described herein is convenient, allowing the laser source 150 to be placed at a distance away from the combustion system 11, this description is for illustrative purposes only. Other implementations and configurations are possible. For example, a high-power free-space coupled light beam can be employed through a series of mirrors and / or lenses.

[0031] At the laser input end, photons emitted from fiber 154 are collimated and delivered to the ignition zone via laser tube 110, which is advantageously cooled and purified as described herein. Within laser tube 110, a lens 119 located at a certain distance within laser tube 110 focuses the photons to a compact spot size at a selected location near the entrance of ignition tube 130. In another embodiment, the photons are simply kept collimated and guided from laser tube 110 to ignition tube 130. Focused or collimated photons from the laser received via laser input end 112 are guided to ignition tube 130. Thus, compared to conventional plasma igniters, laser tube 100 maintains a simpler and smaller size and less intrusion into the coal flow. In particular, the purification within laser tube 110 and the simplified construction minimize fouling of laser tube 110 and increase the maintenance intervals of laser igniter 17 compared to conventional plasma igniters. In some embodiments, it is also advantageous to focus the laser deeper into ignition tube 130. For example, it might be desirable to focus the laser beam into the burner, rather than near the outlet end 116 of the laser tube 110. This could become desirable for achieving a certain volumetric heating of coal particles 102 using laser energy within a given burner geometry.

[0032] continue Figure 2In one embodiment, the laser igniter 17 further includes an optional ignition tube 130. The ignition tube 130 has an open end, one end 132 of which is closer to the laser tube 110. The ignition tube 130 is substantially concentric with and within the tube 100, and substantially on the same axis as the laser tube 110, although this is not necessarily the case. In one embodiment, the ignition tube 130 is axially located downstream of the laser tube 110 in the flow of air and coal particles 102. The coal particles 102 and the airflow in the tube 100 are guided into the ignition tube 130 by flow guides 118 disposed on the tube 100. In one embodiment, additional flow control devices (shown as 118b in this case) may also be disposed on the inner wall 136 within the ignition tube 130. These flow guides 118b can be used to further guide the flow of coal particles 102 and air as they flow through the ignition tube 130, thereby concentrating the coal particles 102 at approximately a selected location within the ignition tube 130. In one embodiment, coal particles 102 are substantially guided to the center of the ignition tube 130 to ensure that particles 102 are targeted and absorbed by as many photons from the laser as possible. In one embodiment, collimated or focused photons are substantially guided to the center of the ignition tube 130 to concentrate at the focal point of the photons. In one embodiment, it is desirable to achieve a distribution of photons from the laser beam and coal particles 102 such that a controlled or selected amount of coal particles 102 achieves critical ignition. Direction and ignition are controlled to ensure that the igniter 17 avoids a situation where too few coal particles 102 absorb far more laser energy than required, or conversely, too many coal particles 102 absorb laser energy, thus diverting that laser energy to too few coal particles 102 to be ignited to achieve the extent necessary for overall ignition and flame propagation. In one embodiment, this control is achieved by balancing the interaction between the laser beam geometry and the distribution of coal particles 102 flowing within the laser beam.

[0033] In one embodiment, it should be understood that tube 100 (and in embodiments where it is used) can be divided into multiple operating / ignition stages 160, whether or not it includes the optional ignition tube 130. It should also be understood that while several embodiments have been described using the optional ignition tube 130, such description is for illustrative purposes. The described functionality and operation of the combustion system 11 (more specifically, the laser igniter 17) can be achieved with or without the optional ignition tube 130. Figure 3A graphical depiction of the multi-stage ignition process as described in the embodiments herein is provided, and the various stages of ignition and combustion are depicted, generally shown as 160. In one embodiment, the first stage of ignition (depicted as 162) primarily requires laser photon absorption to ignite a small number of coal particles 102 via direct absorption of photon energy. Downstream of tube 100 or ignition tube 130, as depicted, in the second stage 164, the burning coal particles 102 generate a flame that heats and ignites more coal particles 102 that were not ignited in the first stage 162 (e.g., not yet ignited) and directly enter the second ignition stage 164. Thus, when all coal particles are ignited, subsequent ignition in the second stage 164 causes further expansion and propagation of the flame in and from the ignition tube 130 into the burner assembly 14, as depicted at 166.

[0034] Advantageously, the aforementioned embodiments overcome the need for an intermediate heat transfer medium (such as plasma or flame) to transfer energy to the coal particles. In these embodiments, energy is directed directly to the coal particles 102 by the laser photons themselves. When a photon strikes the coal particle 102, the coal particle absorbs the energy from the photon, heating and igniting it. In some embodiments, a preheating process can be employed to accelerate the ignition process. In one embodiment, the ignition tube 130 is preheated by allowing laser photons to strike the inner wall 136 of the ignition tube 130 (in the absence of coal particles 102 and airflow), thereby heating the ignition tube 130. In another embodiment, the ignition tube 130 is preheated by igniting the coal particles within it, which in turn heats the ignition tube 130. This preheating makes the ignition tube 130 hotter, preheats the coal particle 102 and air mixture, and thus reduces the laser power required to directly ignite the coal particles 102 by photon absorption. In another embodiment, to facilitate ignition, the primary air can be preheated to raise the temperature of the coal particles 102, thereby making laser ignition easier and requiring less laser power to raise the temperature of the coal particles 102 above the ignition point. Therefore, advantageously, the laser igniter 17 described herein is expected to require less laser power to ignite the coal burner 14 than existing plasma igniters.

[0035] In another embodiment, laser ignition of coal particles 102 can be further enhanced or promoted by further airflow and direction control. In one embodiment, the air velocity exiting the laser tube 110 can be slower than the air velocity flowing through the surrounding burner and tube 100. Under this condition, a recirculation zone (e.g., eddies in the flow) generally indicated as 163 in the ignition stage 162 will be generated in front of the laser tube 110 in the laser beam path. Coal particles 102 trapped in this recirculation zone 163 will pass through / cross the laser beam multiple times, thereby increasing the time the coal particles spend in the laser beam and absorbing photons for heating. Therefore, this recirculation will increase the probability of ignition of coal particles 102, and these particles 102 will subsequently further promote the ignition of remaining coal particles 102 in the burner. In one embodiment, the flow of the coal particle 102 mixture with the airflow through the tube 100 can be slowed or reduced to a predetermined minimum permissible speed in order to maximize the time the coal-air mixture spends within the laser beam emitted from the laser tube 110. In this way, the coal-air mixture can be irradiated for a longer period of time in the recirculation zone and in different stages of the ignition tube 130. In another embodiment, the flow control device 118 and the airflow rate in the laser tube 110 and / or tube 100 can be used to control the recirculation zone 163, and thus control the residence time of the coal particles 102 in the laser path for initial ignition and subsequent flame stabilization.

[0036] In another embodiment, to facilitate laser ignition, the coal particle 102 and / or airflow velocity can be controlled, in this case, reduced. Reducing the velocity slows the coal particle 102, causing it to flow within the laser beam and absorb photons for a longer duration. This increased residence time allows the coal particle to absorb more energy from the laser beam. This method also allows for ignition of the coal particle 102 using lower power output, or even lower laser ratings, thereby reducing costs.

[0037] Another potential advantage of the laser igniter 17 of the described embodiment over the plasma igniter is that the energy input is directed only to the coal particles 102 that absorb laser radiation, and not to the air that does not absorb laser radiation. Therefore, the initial ignition of the coal particles 102 is achieved more efficiently compared to other igniters (such as oil, gas, or plasma igniters that ultimately also heat the surrounding air medium). Another advantage of the laser igniter 17 of the described embodiment is that, in oil, gas, or plasma igniters, the thermal energy transferred to the coal particles 102 is typically violent and turbulent. As a result, this turbulence disrupts the flow of the coal particles 102, making it difficult to simulate and design the initial ignition of the coal particles 102. Without interfering with the coal particles 102 and the airflow, laser photons are absorbed by the coal particles 102, thus making it easier to simulate, design, and control the ignition of the coal particles 102 in the coal-based combustion system 11. Therefore, improved ignition performance of the igniter 17 can be designed and implemented.

[0038] Now go to Figure 4 In the embodiment described, method 400 monitors ignition in combustion system 11 of boiler system 12. As depicted in process step 410, method 400 begins by supplying fuel and air to igniter 17 of burner 14. As described herein, the fuel may be pulverized coal in the form of coal particles 102 and air. In one embodiment, the coal particles 102 are sorted to have a selected size desired for ignition, as described herein. Method 400 continues by directing a first portion of the fuel-air mixture to a selected location within tube 100, as depicted in process step 420. Optionally, as depicted in process step 430, a first portion of the first portion of the fuel-air mixture is directed to ignition tube 130. Method 400 continues with process step 440 and directs photons from laser tube 110 to the selected location to ignite at least a portion of the first portion of the fuel. In an alternative embodiment, the selected location is within ignition tube 130. The selected location corresponds to the focal point of the photon as it is directed from laser tube 110. At process step 450, optionally, another portion of the first portion of the first part of the fuel-air mixture is directed into the ignition tube 130 to promote combustion of the fuel in the ignition tube. Method 400 continues at process step 460, in which the propagation of the flame in or through the burner 14 is controlled. Finally, as depicted in optional process step 470, method 400 may further include directing photons to the inner wall 136 of the ignition tube to promote heating of the ignition tube 130 and thereby ignite and burn the coal particles 102 in the ignition tube.

[0039] It should be understood that although the steps of method 400 are described in a specific order, they do not need to be in such an order, and the description in such an order is merely for illustrative purposes. Some steps can easily be performed in a different order. In addition to saving operation, the laser ignition system 17 of the described embodiment saves money, space, and energy compared to existing plasma igniters, and simplifies design and construction. In particular, closed-loop control of the laser igniter can be achieved using the control system disclosed herein to precisely control fuel ignition and combustion, thereby achieving optimal burner performance. For example, in one embodiment, the ignition of coal particles 102 can be controlled by a mechanically movable, electrically controlled flow guide 118 to control the flow, thereby achieving ignition with one selected configuration and maintaining ignition with another. Since the laser energy does not interfere with the flow of coal particles 102, this construction of the laser igniter 17 further improves operability and efficiency compared to existing laser igniters 17.

[0040] In one implementation, pressure and / or temperature sensor 120 ( Figure 2 The pressure sensor 120 can, but is not required, to monitor ignition in the laser igniter 17. While not essential, the use of this sensor 120 expands the capabilities of the laser igniter 17 and the entire combustion system 11, and can be used throughout the system 14, particularly in tube 100 and ignition tube 130. For example, the pressure sensor 120 in the burner tube 100 can provide an indication of the sudden expansion of gas due to ignition. This pressure change can serve as an indication of initial ignition, facilitating ignition control in the igniter. Similarly, thermocouples on the ignition stages (e.g., 162, 164) ensure sustained combustion due to the laser igniter 17. The measured temperature can be used to achieve controlled ignition and sustained combustion. For example, in one embodiment, the laser power can be maintained at a high level for a short period to achieve initial ignition of the coal particles 102, and then gradually reduced to achieve and maintain ignition. As a result, the average power required for the laser source 150 will be reduced, thereby reducing the cost of the laser source 150.

[0041] Control unit 200 may include necessary electronics, software, memory, storage devices, databases, firmware, logic / state machines, microprocessors, communication links, displays or other visual or audio user interfaces, printing devices, and any other input / output interfaces for performing the functions and achieving the results described herein. For example, as previously described, in one embodiment, control unit 200 may be implemented as a standalone or modular component of power generation system 10, including at least one processor or processing module (not shown) and a system memory / data storage structure, which may include random access memory (RAM) and read-only memory (ROM). The processor of this module may include one or more conventional microprocessors, microcontrollers, and one or more auxiliary coprocessors, such as math coprocessors. The data storage structures discussed herein may include suitable combinations of magnetic, optical, and / or semiconductor memories, and may include, for example, RAM, ROM, flash drives, optical disks such as compact disks, and / or hard disks or drives. Control unit 200 may be implemented as an integrated microcontroller, wherein each function may be integrated as needed into a single package, ASIC, or FPGA to interface with various sensors, control valves, modules, etc., to achieve the functionality, processing, and communication described herein.

[0042] Additionally, a software application that adapts the combustion system 11 and laser igniter 17 to perform the method 400 disclosed herein can be read from a computer-readable medium into the main memory of at least one processor. Therefore, the embodiments described herein can execute the methods disclosed herein in real time. Although in the embodiments, execution of a sequence of instructions in the software application causes at least one processor to perform the methods / processes described herein, hardwired circuitry can be used in place of or in combination with the software instructions for implementing the methods / processes. Therefore, the embodiments described herein are not limited to any particular combination of hardware and / or software.

[0043] As used herein, the term "computer-readable medium" means any medium that provides or participates in providing instructions for execution to at least one processor of control unit 200 (or any other processor of the device described herein). Such media can take many forms, including, but not limited to, non-volatile and volatile media. Non-volatile media include, for example, optical, magnetic, or optomagnetic disks, such as memory. Volatile media include dynamic random access memory (DRAM), which typically constitutes main memory. Common forms of computer-readable media include, for example, floppy disks, flexible disks, hard disks, solid-state drives (SSDs), magnetic tape, any other magnetic medium, CD-ROM, DVD, any other optical medium, RAM, PROM, EPROM, or EEPROM (electrically erasable programmable read-only memory), FLASH-EEPROM, any other memory chip or cartridge, or any other computer-readable medium.

[0044] In one embodiment, each sensor in the sensors (e.g., 36) can be hardwired to the control unit 200. In another embodiment, a low-power communication interface can be employed. The communication interface is coupled to an interconnect / network interface that interconnects components of system 10 with one or more controllers, such as control unit 200. The network can be a hybrid of wired and wireless components and can utilize communication networks including IP networks. It should be understood that the interconnect / network can include wired components or wireless components or combinations thereof. Such wired components can include conventional network cables, optical fibers, wires, or any other type of physical structure through which sensors 36, control valves 30, 66, control unit 200, and other devices of boiler system 10 can communicate. The network can include wireless components and can include radio links, optical links, magnetic links, acoustic links, or any other type of wireless link through which sensors 36, control valves 30, 66, and control unit 200 can communicate. In one embodiment, a wireless communication interface and a wireless network can be employed. For example, the communication interface can use various technologies, application technologies, and protocols to facilitate the implementation of the embodiments and is by no means limiting. For example, the communication interface and network can be implemented as Ethernet, NFC, etc. The network can be implemented using a hub and spoke type configuration or a mesh network configuration. In some implementations, a wireless mesh network can be used to allow multiple sensors and control valves 30, 66 deployed around boiler 12 to communicate with each other, coordinate measurements, and transmit data back to control unit 200.

[0045] It should be understood that although the boiler 12, and more specifically, the laser ignition system 17 or control unit 200, can be described as being implemented as comprising various separate modules for various components, such description is merely for illustration and example. In one or more embodiments, the functionality of all or some of the components can be readily integrated or combined as needed. For example, in one embodiment, the functionality of the sensor 36, control valves 30, 66, processing module 400, and communication interface / network, etc., can be integrated wholly or partially into a microcontroller, ASIC, FPGA, etc.

[0046] In one embodiment, a method for igniting fuel in a burner is described herein. The method includes: supplying a flowing fuel-air mixture to an igniter in the fuel burner; directing a first portion of the flowing fuel-air mixture to a location within the burner; and directing photons from a laser to that location to ignite at least a portion of the first portion of the fuel.

[0047] In addition to one or more of the features described above, or as an alternative, other embodiments of the method may include controlling the propagation of the flame in or through the burner based at least in part on guiding the flow of at least one of a first portion, a first portion of the first portion, and a second portion of the fuel-air mixture.

[0048] In addition to one or more of the features described above, or as an alternative, another embodiment of the method may include control based at least in part on the mixing of a second portion of the guided flow of the fuel-air mixture with at least a portion of the ignited first portion.

[0049] In addition to one or more of the features described above, or as an alternative, another embodiment of the method may include directing a first portion of the fuel-air mixture to a selected location within the ignition tube.

[0050] In addition to one or more of the features described above, or as an alternative, another embodiment of the method may include directing a portion of a first part of the fuel-air mixture to a selected location within the ignition tube.

[0051] In addition to one or more of the features described above, or as an alternative, another embodiment of the method may include preheating the ignition tube by directing photons from a laser to the inner wall of the ignition tube prior to guiding a first portion of the fuel-air mixture.

[0052] In addition to one or more of the features described above, or as an alternative, another embodiment of the method may include directing photons from a laser to a location in an ignition tube to ignite at least some of the fuel in a first portion of the fuel.

[0053] In addition to one or more of the features described above, or as an alternative, another embodiment of the method may include cooling the ignition tube by guiding another portion of the fuel-air mixture along the outer wall of the ignition tube.

[0054] In addition to one or more of the features described above, or as an alternative, another embodiment of the method may include guidance based on a flow control device configured to modify the direction of flow of fuel and air in the pipe.

[0055] In addition to one or more of the features described above, or as an alternative, another embodiment of the method may include enhancing the intensity of photons guided to the location by at least one of the following operations: collimating the photons with a collimator that uses a lens to focus the photons.

[0056] In another embodiment, this document also describes a system for igniting a coal-air-fuel mixture. The system includes: a burner having a burner tube operable to deliver a flowing mixture of fuel and air to a furnace for combustion therein; a first flow guide disposed within the tube, operable to guide a first portion of the flowing fuel-air mixture to a specific location within the burner tube; and a laser igniter located within the burner tube. The laser igniter includes: a laser tube having a first end with a laser input and a second end with a light output; and a laser source operably coupled to the laser input. The laser source includes a laser, and the laser igniter guides photons from the light output to the location within the burner tube to ignite at least a portion of the first portion of the fuel.

[0057] In addition to one or more of the features described above, or as an alternative, another embodiment of the system may include a second flow guide disposed within the tube, operable to guide a second portion of the flowing fuel-air mixture in the tube to control the propagation of the flame in or through the burner.

[0058] In addition to one or more of the features described above, or as an alternative, another implementation of the system may include: control of propagation based at least in part on the mixing of a second portion of the guided flow of fuel and air mixture with at least a portion of the ignited first portion.

[0059] In addition to one or more of the features described above, or as an alternative, another embodiment of the system may include an ignition tube within the tube, which is substantially concentric with the laser tube in the flowing fuel-air mixture and axially downstream of the laser tube.

[0060] In addition to one or more of the features described above, or as an alternative, another implementation of the system may include a first flow guide device that directs a first portion of the fuel-air mixture to a selected location within the ignition tube.

[0061] In addition to one or more of the features described above, or as an alternative, another embodiment of the system may include a second flow guide device disposed within the ignition tube, operable to guide at least a portion of the first part of the flowing fuel-air mixture to a certain position, and the laser igniter guides photons from the light output end to that position within the ignition tube to ignite at least a portion of the first part of the fuel.

[0062] In addition to one or more of the features described above, or as an alternative, another embodiment of the system may include a third flow guide disposed within the pipe, operable to guide at least another portion of the fuel-air mixture flowing in the pipe.

[0063] In addition to one or more of the features described above, or as an alternative, another embodiment of the system may include a third flow guide that is operable to control the propagation of the flame in or through the burner based at least in part on guiding the flow of at least one of a first portion, a first portion of the first portion, and a second portion of the fuel-air mixture.

[0064] In addition to one or more of the features described above, or as an alternative, other embodiments of the system may include at least one of the following: a collimator for collimating photons within a collimating laser tube and a lens for focusing photons at that location.

[0065] In addition to one or more of the features described above, or as an alternative, another implementation of the system may include a laser source located away from the laser input.

[0066] In addition to one or more of the features described above, or as an alternative, another implementation of the system may include a laser beam formed by a combination of multiple laser beams.

[0067] In addition to one or more of the features described above, or as an alternative, other implementations of the system may include varying the intensity of the laser under selected conditions, such as spike the laser intensity to a first level during initial ignition and then reducing the laser intensity thereafter to achieve stable ignition.

[0068] In addition to one or more of the features described above, or as an alternative, other implementations of the system may include different Venturi designs that can be configured to alter the flow of coal particles and air in at least one of the tubes and the ignition tube.

[0069] In addition to one or more of the features described above, or as an alternative, another implementation of the method may include slowing the coal flow for initial ignition.

[0070] In addition to one or more of the features mentioned above, or as an alternative, another implementation of the system may preheat at least one of the ignition tube, the tube, the coal particles, and the air directed to the ignition tube.

[0071] In addition to one or more of the features described above, or as an alternative, other implementations of the system may include controlling the flow direction and laser power for initial ignition, and then reverting to different configurations and laser power to obtain a stable flame.

[0072] As used herein, “electrical communication” or “electrical coupling” means that the components are configured to communicate with each other via direct or indirect signaling through direct or indirect electrical connections. As used herein, “mechanical connection” refers to any connection method capable of supporting the necessary forces for transmitting torque between components. As used herein, “operationally coupled” refers to a connection that can be direct or indirect. Connections are not necessarily mechanical attachments.

[0073] As used herein, elements or steps listed in the singular and beginning with the words “an” or “a” should be understood to not exclude multiple said elements or steps unless such exclusion is explicitly stated. Furthermore, references to “an embodiment” in the described embodiments are not intended to be construed as excluding the existence of additional embodiments that also include the listed features. Moreover, unless explicitly stated to the contrary, embodiments that “comprise,” “include,” or “have” one or more elements having a particular attribute may include additional elements that do not have that attribute.

[0074] Furthermore, while the dimensions and types of materials described herein are intended to define parameters associated with the described embodiments, they are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those skilled in the art upon review of the above description. Therefore, the scope of the invention should be determined by reference to the appended claims. Such descriptions may include other examples that would occur to those skilled in the art, and such other examples are contemplated within the scope of the claims if they have structural elements identical to the literal language of the claims, or if they include equivalent structural elements that are not substantially different from the literal language of the claims. In the appended claims, the terms “including” and “in which” are used as plain English equivalents of the corresponding terms “comprising” and “wherein”. Additionally, in the following claims, terms such as “first,” “second,” “third,” “upper,” “lower,” “bottom,” “top,” etc., are used only as designations and are not intended to impose numerical or positional requirements on their objects. Furthermore, any limitation in the following claims that is not written in the format of average plus function is not intended to be construed as such a limitation, unless and until such claims are limited to the description of the gap function of other structures, which is explicitly described by the phrase "in a manner for...".

Claims

1. A burner assembly (14), comprising: Fuel source (16); Air source; Heating furnace (12); A fuel delivery pipe (100) is used to deliver a mixture of fuel from the fuel source and air from the air source to the heating furnace; A laser igniter (17) is configured to ignite a fuel-air mixture transported to the heating furnace via the fuel delivery pipe, the laser igniter comprising: Laser source (150); Laser tube (110) for delivering a laser beam from the laser source to the fuel-air mixture in the fuel delivery pipe; and An ignition tube (130) is disposed within the fuel delivery tube to receive a portion of a fuel-air mixture flowing through the fuel delivery tube, wherein the laser beam heats and ignites the fuel-air mixture in the ignition tube, wherein the ignition of the fuel-air mixture in the ignition tube occurs in multiple ignition stages; and A flow control device is configured to reduce the velocity of the air exiting the laser tube to a velocity slower than the flow velocity of the fuel-air mixture in the fuel delivery pipe, creating a recirculation zone in the region between the outlet of the laser tube and the inlet of the ignition tube, wherein the fuel-air mixture carried in the fuel delivery pipe is recirculated to be more exposed to the laser beam in the recirculation zone before entering the ignition tube.

2. The burner assembly (14) according to claim 1, wherein the fuel delivery tube (100) includes a plurality of flow guides (118), wherein each of the flow guides is configured to guide a portion of the flow of the fuel-air mixture around the ignition tube (130) to be irradiated by the laser beam, or The inner wall of the ignition tube (130) includes a plurality of ignition guiding devices (118), each of which is configured to guide the flow of the fuel-air mixture to a concentrated position within the ignition tube for irradiation by the laser beam received from the laser tube, or It also includes an optical device (119) for guiding the laser beam toward the ignition tube (130).

3. The burner assembly (14) of claim 2, wherein the plurality of flow guiding devices (118) comprises at least one flow guiding device disposed around the inlet of the ignition tube (130) and at least one flow guiding device disposed around the outlet of the ignition tube, wherein the at least one flow guiding device disposed around the inlet of the ignition tube is configured to guide the flow of the fuel-air mixture in the delivery tube (100) toward the inlet, and wherein the at least one flow guiding device disposed around the outlet of the ignition tube is configured to guide any portion of the flow of the fuel-air mixture that avoids entering the inlet of the ignition tube toward the flow of the ignited fuel-air mixture exiting the outlet of the ignition tube to mix with the ignited flow and subsequently ignite, or wherein the plurality of flow guiding devices (118) comprises a Venturi device.

4. The burner assembly (14) according to claim 1, wherein a portion of the laser tube (110) is disposed within the fuel delivery tube (100), wherein the portion of the laser tube disposed in the fuel delivery tube is spaced apart from the ignition tube by a predetermined interval, wherein the predetermined interval between the portion of the laser tube disposed in the fuel delivery tube and the ignition tube (130) forms a recirculation zone (163), in which fuel particles transported in the fuel delivery tube are recirculated to be more exposed to the laser beam before entering the ignition tube, wherein the fuel particles in the recirculation zone absorb photons of the laser beam and are heated to cause initial ignition of some of the particles, the initial ignition causing subsequent absorption, heating and ignition of other fuel particles in the recirculation zone.

5. The burner assembly (14) of claim 4, wherein the plurality of ignition stages of the ignition tube comprises a flame initiation stage and a flame growth and propagation stage, wherein the flame initiation stage receives fuel particles from the recirculation zone for further irradiation by the laser beam, the further irradiation causing combustion and flame generation, and wherein the flame growth and propagation stage receives the fuel particles that have undergone combustion and flame generation in the flame initiation stage for additional irradiation by the laser beam, the additional irradiation causing further flame growth and propagation of the flame away from the ignition tube toward the furnace.

6. A system comprising: Coal fuel source (16); Air source (18); Heating furnace; A fuel delivery pipe (100) is used to deliver a mixture of coal from the coal fuel source and air from the air source to the heating furnace for combustion of the mixture; A laser igniter (17) is configured to ignite the coal and air mixture transported to the heating furnace via the fuel delivery pipe, the laser igniter comprising: Laser source (150); Laser tube (110) for delivering a laser beam from the laser source to the coal and air mixture within the fuel delivery pipe, wherein a portion of the laser tube is disposed within the fuel delivery pipe; and An ignition tube (130) is disposed within the fuel delivery tube to receive a portion of the coal and air mixture flowing through the fuel delivery tube, wherein the inlet of the ignition tube is spaced apart by a predetermined interval from the portion of the laser tube disposed within the fuel delivery tube, wherein the laser beam heats and ignites the coal and air mixture at the predetermined intervals and within the ignition tube in a plurality of ignition stages; and Multiple flow guiding devices (118), wherein each of the flow guiding devices is configured to guide a portion of the flow of the coal-air mixture around the ignition tube for irradiation by the laser beam; and A flow control device is configured to reduce the velocity of the air exiting the laser tube to a velocity slower than the flow velocity of the coal-air mixture in the fuel delivery pipe, creating a recirculation zone in the area between the outlet of the laser tube and the inlet of the ignition tube, wherein the coal-air mixture transported in the fuel delivery pipe is recirculated to be more exposed to the laser beam in the recirculation zone before entering the ignition tube.

7. The system of claim 6, wherein the plurality of flow guiding devices comprises a pair of opposing flow guiding devices (118) disposed around the inlet of the ignition tube and a pair of opposing guiding devices disposed around the outlet of the ignition tube, wherein the pair of flow guiding devices disposed around the inlet of the ignition tube is configured to guide the flow of the coal-air mixture in the delivery pipe toward the inlet, and wherein the pair of guiding devices disposed around the outlet of the ignition tube is configured to guide any portion of the flow of the coal-air mixture that avoids entering the inlet of the ignition tube toward the ignited coal-air flow exiting the outlet of the ignition tube, to be mixed with the ignited coal-air flow and subsequently ignited, or The plurality of flow guiding devices (118) include Venturi-shaped devices.

8. The system of claim 6, wherein the predetermined interval between the portion of the laser tube (110) disposed in the fuel delivery tube (100) and the ignition tube forms a recirculation zone (163), in which coal particles in the coal-air mixture transported in the fuel delivery tube are recirculated to be more exposed to the laser beam before entering the ignition tube, wherein the coal particles absorb photons of the laser beam and are heated to cause initial ignition of some of the coal particles, and subsequent absorption, heating and ignition of other coal particles in the recirculation zone.

9. The system of claim 8, wherein the plurality of ignition stages of the ignition tube (130) comprises a flame initiation stage (162) and a flame growth and propagation stage (164), wherein the flame initiation stage receives the coal particles from the recirculation zone (163) for further irradiation by the laser beam, the further irradiation causing combustion and flame generation, and wherein the flame growth and propagation stage receives the coal particles that have undergone combustion and flame generation in the flame initiation stage for additional irradiation by the laser beam, the additional irradiation causing further flame growth and propagation of the flame away from the ignition tube toward the heating furnace.

10. The system of claim 6, wherein the inner wall of the ignition tube (130) includes a plurality of ignition guiding devices (118), wherein each of the ignition guiding devices is configured to guide the flow of the coal-air mixture to a concentrated position within the ignition tube for irradiation by the laser beam received from the laser tube.

11. The system according to claim 6, further comprising: Multiple sensors (36, 37) are located around the fuel delivery pipe and the laser igniter, each of which is configured to detect operating conditions associated with the heating and ignition of the coal-air mixture; and The control unit (200) is configured to receive detected operating conditions from the plurality of sensors and to control the heating and ignition of the coal and air mixture by the laser igniter based on the detected operating conditions.

12. A method (400) for igniting fuel for combustion in a burner assembly (14), comprising: A mixture of fuel from a fuel source and air from an air source is conveyed to the heating furnace via a fuel delivery pipe (100); A portion of the flow of the fuel-air mixture is guided through the inlet of the ignition tube (130) placed within the fuel delivery pipe; The laser beam from the laser tube is directed toward the fuel and air mixture being transported through the ignition tube into the fuel delivery tube; The mixture of fuel and air transported through the ignition tube is heated and ignited in multiple stages (162, 163, 164); The heated and ignited mixture of fuel and air is supplied from the ignition tube to the heating furnace; as well as The velocity of the air exiting the laser tube is reduced to a velocity slower than the flow velocity of the fuel-air mixture in the fuel delivery tube, creating a recirculation zone in the area between the laser tube outlet and the ignition tube inlet, wherein the fuel-air mixture transported in the fuel delivery tube is recirculated to be more exposed to the laser beam in the recirculation zone before entering the ignition tube.

13. The method (400) of claim 12, further comprising directing any portion of the flow of the fuel-air mixture that avoids entering the inlet of the ignition tube toward the ignited fuel flow exiting the outlet of the ignition tube, to mix with the ignited fuel flow and subsequently ignite, or It also includes reducing the flow velocity of the fuel-air mixture in the fuel delivery pipe (100) to a predetermined minimum permissible velocity, wherein the reduction in the flow velocity of the fuel-air mixture maximizes the amount of time the mixture is irradiated by the laser beam.

14. The method (400) of claim 12, wherein the fuel particles in the recirculation zone absorb photons from the laser beam during recirculation and are heated to cause initial ignition of some of the particles, the initial ignition causing subsequent absorption, heating and ignition of other fuel particles located around the inlet of the ignition tube.

15. The method (400) of claim 14, wherein heating and igniting the fuel delivered through the ignition tube (130) in multiple stages comprises: The recirculated fuel particles are received in the first stage of the ignition tube; The fuel particles are further irradiated by the laser beam in the first stage of the ignition tube, and the further irradiation causes combustion and flame generation; The fuel particles that undergo combustion and flame are received in the second stage of the ignition tube; and The laser beam further irradiates the fuel particles that have undergone combustion and flame generation in the second stage of the ignition tube, and this additional irradiation causes further flame growth and propagation.

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