Methanol-natural gas dual-fuel collaborative combustion system and method

Abstract

The invention provides a methanol-natural gas dual-fuel collaborative combustion system and method.The methanol-natural gas dual-fuel collaborative combustion system comprises a dual-fuel supply subsystem, a turbulent burner set, a heating surface collaborative subsystem, a flue gas purification and air induction subsystem and a DCS closed-loop control subsystem; the method has the beneficial effects that collaborative optimization of main steam temperature increasing and carbon emission reducing is achieved, through methanol-natural gas dual-fuel collaborative combustion, the characteristics of high oxygen content, high latent heat of vaporization and radiation of methanol fuel are fully utilized, and the main steam temperature is effectively increased on the premise that the structure of a boiler heating surface is basically not changed; meanwhile, emission is reduced due to the low-carbon property of methanol, and unification of efficient operation and low-carbon emission is achieved.

Description

Technical Field

[0001] This invention relates to the field of low-carbon coupled combustion technology for power plant boilers, and particularly to a methanol-natural gas dual-fuel synergistic combustion system and method. Background Technology

[0002] In practical applications, some studies and practices have explored the combustion characteristics of methanol in boilers. For example, some studies have conducted in-depth analyses of the combustion characteristics of methanol boilers through numerical simulation and experimental verification, exploring the effects of factors such as different excess air coefficients and different cracked gas blending ratios on boiler combustion and emission characteristics (Zeng Mingtao. Research on the Characteristics of Methanol and its Cracked Gas in Boilers [D]. Wuhan: Huazhong University of Science and Technology, 2021). In addition, some regions are also exploring the application of methanol fuel in heating boilers, especially in northern regions, where the demand for heating is high, and methanol boilers are of great significance in replacing scattered coal and reducing environmental pollution.

[0003] However, current research on systems and methods for co-firing methanol in gas-fired boilers to increase main steam temperature and achieve synergistic carbon reduction is relatively limited. While some existing research involves methanol combustion in boilers, most focuses on the combustion characteristics of single fuels or applications in small boilers. Further exploration and innovation are needed to effectively co-fire methanol in large gas-fired boilers to simultaneously increase main steam temperature and reduce carbon emissions. Therefore, developing a combustion system and method for co-firing methanol in gas-fired boilers that meets these requirements is of significant practical importance for promoting the development of low-carbon coupled combustion technology in power plant boilers.

[0004] Existing technologies have explored the application of methanol in combustion systems to some extent, but they are mainly focused on engines or coal-fired boilers, which differs from the present invention's goal and technical approach of co-firing methanol in gas-fired boilers to increase the main steam temperature and synergistically reduce carbon emissions. Summary of the Invention

[0005] To overcome the aforementioned problems in the prior art, the present invention provides a methanol-natural gas dual-fuel co-combustion system and method.

[0006] This invention discloses a methanol-natural gas dual-fuel co-combustion system, comprising a dual-fuel supply subsystem, a swirl burner group, a heating surface coordination subsystem, a flue gas purification and induced draft subsystem, and a DCS closed-loop control subsystem. The dual-fuel supply subsystem includes independent natural gas supply branches and methanol supply branches. The swirl burner group includes at least two natural gas burners and at least one methanol burner, arranged in stages within the furnace space. The heating surface coordination subsystem includes a screen-type superheater, a high-temperature superheater, a low-temperature superheater, an economizer, and an air preheater arranged sequentially along the flue gas flow path. The flue gas purification and induced draft subsystem includes an induced draft fan, a dust collector, and a chimney. The DCS closed-loop control subsystem is configured to execute a multivariate decoupled control strategy, using the main steam temperature as the core controlled variable, the methanol to natural gas mass flow ratio as the key controlled variable, and the furnace outlet oxygen content, NOx concentration, and... Concentration is used as a constraint.

[0007] Based on this, the natural gas supply branch includes a main gas header and an ignition gas header connected to the natural gas pipeline in the plant area. The main gas header is connected to each natural gas burner through multiple independent branches. Both the main gas header and the ignition gas header are equipped with dual redundant pneumatic quick-cut valves that are hardwired to the main fuel trip (MFT) system. The methanol supply branch includes a methanol storage tank, a transfer pump, and an atomizing device connected in sequence. The methanol storage tank is equipped with a nitrogen sealing device and a heating device. The outlet of the transfer pump is equipped with a self-regulating reflux regulating valve. The atomizing device is a pressure atomizing nozzle or a dual-fluid atomizing nozzle.

[0008] Based on this, the methanol supply branch also includes an adaptive module; when the water content of the methanol fuel exceeds a preset threshold, the adaptive module is an alkaline washing tower located at the outlet of the methanol storage tank; when the ambient temperature is lower than a preset threshold, the heating device is replaced by a steam coil heat tracing device, and the atomizing device is replaced by a steam-methanol dual-fluid atomizing nozzle.

[0009] Based on this, the methanol burner is arranged in the upper middle part of the furnace walls on both sides of the boiler, and the natural gas burner is concentrated in the lower middle part of the front wall of the boiler; the methanol burner adopts a double swirl coupling structure, including adjustable axial swirl blades and radial swirl blades, which are used to premix methanol and air downstream of the nozzle; the air distribution system implements a three-stage air supply, including primary air supplied to the root of the methanol flame, secondary air for organizing natural gas combustion, and tertiary air introduced from the bottom of the furnace to reduce the peak flame temperature.

[0010] Based on this, the economizer adopts a two-stage arrangement, including an upper economizer located below the low-temperature superheater and a lower economizer located above the air preheater; the screen-type superheater adopts an H-type finned tube structure, and its radiation absorption characteristics match the radiation spectrum of the methanol flame.

[0011] Based on this, the induced draft fan is driven by a permanent magnet synchronous variable frequency motor; the dust collector is a bag filter or an electrostatic precipitator / bag filter hybrid dust collector; and the chimney outlet is equipped with a monitoring device. Online monitoring devices for NOx and particulate matter concentrations.

[0012] Based on this, the flue gas purification and induced draft subsystem also includes an SCR denitrification device, which is located after the air preheater.

[0013] Based on this, the control algorithm executed by the DCS closed-loop control subsystem includes: a feedforward control unit, which calculates the theoretical demand for methanol and natural gas based on the boiler load command; a closed-loop feedback unit, which uses the main steam temperature deviation as input to adjust the methanol flow rate through a PID controller and adjusts the natural gas flow rate accordingly; and a constraint control unit, which prioritizes adjusting the combustion air distribution when the NOx concentration exceeds the standard, and adjusts the combustion air distribution when the NOx concentration exceeds the standard. When emissions exceed standards, priority should be given to increasing the proportion of methanol blending.

[0014] Based on this, the DCS closed-loop control subsystem also includes a safety interlock unit, which includes receiving the MFT signal and triggering the interlocking closure of the dual fuel main pipe quick-cut valve and the methanol supply pump, and triggering the shut-off of the individual burner fuel valve based on the flame detection signal of the "two out of three" logic.

[0015] Another aspect of the present invention discloses a method for operating the methanol-natural gas dual-fuel co-combustion system, comprising the following steps:

[0016] Step 1: Cold start, use natural gas for ignition, and after the furnace temperature rises to the set value, start the methanol supply. The methanol blending ratio will slowly increase from the initial value according to the set slope.

[0017] Step 2: Variable load operation. During the load increase process, the methanol blending ratio increases accordingly for each certain percentage increase in load; during the load decrease process, the methanol blending ratio decreases accordingly for each certain percentage decrease in load, and the upper limit of the methanol blending ratio is limited at low load.

[0018] Step 3: Normal shutdown. First, reduce the methanol blending ratio to zero, purge the methanol pipeline, then cut off the natural gas supply and perform furnace purging.

[0019] Step 4: Emergency handling. In the event of a methanol leak, immediately stop the methanol supply and purge with inert gas. In the event of a fire, trigger the MFT and cut off all fuel supply.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] (1) This invention achieves synergistic optimization of main steam temperature increase and carbon emission reduction: This invention utilizes the methanol-natural gas dual-fuel synergistic combustion to fully leverage the high oxygen content, high latent heat of vaporization, and radiation characteristics of methanol fuel, effectively increasing the main steam temperature without significantly altering the boiler's heating surface structure; simultaneously, the low-carbon properties of methanol reduce carbon emissions. This approach achieves a balance between efficient operation and low carbon emissions.

[0022] (2) This invention makes full use of the existing main structure of the gas boiler and upgrades the function by adding a methanol supply system, modifying the burner and optimizing the control system. It avoids large-scale modifications such as expanding the heating surface, significantly reducing the modification cost and engineering difficulty. Methanol is usually cheaper than natural gas, and blending it can effectively reduce fuel costs and improve the economic efficiency of the power plant.

[0023] (3) In this invention, a closed-loop-feedforward composite control strategy based on DCS is adopted, with the main steam temperature as the core controlled variable to achieve real-time and precise adjustment of the methanol-natural gas ratio. A multi-variable constraint mechanism is introduced to ensure the stability of the combustion process and compliance with environmental protection standards. The safety interlock design ("two out of three" flame detection and MFT hard-wired protection) greatly improves the safety of the system.

[0024] (4) This invention designs a backup plan for methanol with high water content and low temperature environment, which broadens the range of fuel sources and application areas. The standardized operation process clarifies the operation requirements under different working conditions, ensuring that the blending process is stable and controllable, and is suitable for the variable load peak shaving operation of large power plant boilers.

[0025] (5) This invention is the first to achieve the multi-objective synergy of methanol co-firing, main steam temperature increase and carbon reduction in a large gas-fired boiler, providing a complete and reliable technical path for the low-carbon transformation of traditional gas-fired power plants, which is of great significance for promoting the clean and low-carbon transformation of energy. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the system structure of the present invention;

[0027] In the diagram: 1. Natural gas pipeline, 2. Main gas header, 3. Ignition gas header, 4. Methanol storage tank, 5. Methanol burner, 6-9. Natural gas burner, 10. Steam pipeline, 11. Screen-type superheater, 12. Methanol nozzle, 13-16. Gas nozzle, 17. Valve, 18 and 25. Exhaust fan, 19. High-temperature superheater, 20. Low-temperature superheater, 21. Upper economizer, 22. Lower economizer, 23. Air preheater, 24. Variable frequency motor, 26. Bag filter, 27. Chimney. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0029] Example 1

[0030] Based on four levels—system-level fuel reconfiguration, furnace-level combustion reconfiguration, heating surface-level heat flux reconfiguration, and control-level coupling reconfiguration—this invention discloses a methanol-natural gas dual-fuel synergistic combustion system. Its core objective is to achieve, without modifying the pressure-bearing components and heating surfaces of existing large-scale gas-fired boilers, "increase main steam temperature, reduce natural gas consumption, and reduce [other factors]" by "adding a new methanol fuel supply branch, modifying a set of dual-swirl burners, and embedding a closed-loop-feedforward composite control algorithm." The triple technological, economic, and environmental benefits of "emissions".

[0031] refer to Figure 1 The methanol-natural gas dual-fuel co-combustion system comprises five hardware components: a dual-fuel supply subsystem, a swirl burner assembly, a heating surface co-combustion subsystem, a flue gas purification and induced draft subsystem, and a DCS closed-loop control subsystem. On the software side, a system is constructed with main steam temperature as the core controlled variable, the methanol / natural gas mass flow ratio as the key controlled variable, and furnace outlet oxygen, NOx, and... A multivariate decoupling control strategy with concentration as a constraint.

[0032] Natural gas supply relies on the existing natural gas pipeline 1 within the plant area. After entering the boiler room, the natural gas is divided into two routes: one is the main gas header 2, which supplies the majority of the gas during normal boiler operation; the other is the ignition gas header 3, with a lower flow rate, specifically used for the ignition phase during cold start-up of the boiler. In this embodiment, after being distributed via header 2, the main gas is connected to four natural gas burners 6, 7, 8, and 9 through four independent branches, which are connected to gas nozzles 13, 14, 15, and 16 respectively, entering the furnace. Both the main gas header 2 and the ignition gas header 3 are equipped with valves 17. In this embodiment, dual-redundant pneumatic quick-cut valves are used. Valve 17 is hardwired to the main fuel trip (MFT) system to ensure instantaneous and reliable gas supply cutoff in any emergency, ensuring system safety and forming a highly reliable gas control and safety cutoff system. Ignition gas header 3 is only activated during the cold start-up phase. After successful ignition and stable furnace conditions, it automatically exits operation through interlocking with the DCS closed-loop control subsystem to prevent accidental injection and fuel waste.

[0033] The methanol fuel supply is a newly added subsystem. Methanol enters the methanol burner 5 via a transfer pump driven by a methanol storage tank 4. The transfer pump is a variable frequency gear pump, but a centrifugal methanol-specific pump or a plunger pump can also be used. The methanol storage tank 4 is equipped with a nitrogen sealing device, a flame arrestor, and an electric heating coil to ensure fuel storage safety and low-temperature adaptability. Before entering the furnace, the methanol fuel is treated by an atomization device. In this embodiment, the atomization device is a methanol nozzle 12, which uses a Y-type mechanical pressure atomizing nozzle. The atomization pressure is set to a range that can form stable, fine droplets, meeting the requirements for rapid vaporization and stable ignition of methanol. The average diameter is controlled within the micron range, which is conducive to rapid vaporization and stable ignition, to ensure that the methanol vaporization in the furnace is short and achieves synchronous and stable ignition with the natural gas flame. A dual-fluid atomizing nozzle (using compressed air or steam as the atomizing medium) can also be used, especially under conditions with high fuel viscosity or a wider range of load variations, to provide better atomization fineness and adjustment characteristics.

[0034] In this embodiment, if the water content of methanol fuel exceeds 5%, an alkaline washing tower can be added at the outlet of the storage tank to neutralize and remove any formic acid that may be generated, thus preventing corrosion of the pipeline. If the ambient temperature is below -10°C, the electric heat tracing will be replaced with steam coil heat tracing, and a steam-methanol dual-fluid atomizing nozzle will be used instead of a mechanical pressure atomizing nozzle to improve atomization performance under low-temperature conditions.

[0035] The burners adopt a spatially hierarchical arrangement strategy. The newly added methanol burner 5 is arranged at specific elevations on both sides of the boiler, while the original natural gas burners 6, 7, 8, and 9 are concentrated at different elevations on the front wall. In this embodiment, the two methanol burners 5 are respectively arranged in the upper middle part of the furnace walls on both sides of the boiler, at an elevation of 19 meters (the figure only shows one side). The natural gas burners 6, 7, 8, and 9 are concentrated on the front wall, with two located at an elevation of 9 meters and the other two located at an elevation of 12 meters. Alternatively, all methanol burners 5 can be arranged on the front or rear wall, staggered with the natural gas burners. Although this may slightly affect the mixing uniformity, it can still achieve the basic function.

[0036] The methanol burner 5 maintains an appropriate distance from the center of the adjacent natural gas burner to avoid mutual interference and entrainment between the flames of different fuels, ensuring combustion stability. Both the methanol burner 5 and the natural gas burners 6, 7, 8, and 9 employ a dual-swirl coupling structure. The methanol fuel channel outlet is equipped with 6-8 adjustable axial swirl blades, while the natural gas channel outlet has 12-16 radial swirl blades. The two airflows swirl perpendicularly to each other, and the orthogonal swirl fields generate strong shear and turbulence within 100mm downstream of the nozzle. This achieves rapid and uniform premixing of methanol and natural gas at the molecular scale, significantly improving combustion efficiency and suppressing unstable combustion. This is the foundation for achieving efficient and stable combustion and low pollutant generation.

[0037] The air distribution system implements a three-stage air supply. The primary air is directly injected into the root of the methanol flame to create a local oxygen-rich environment, ensuring that the methanol with high latent heat of vaporization can ignite quickly and stably. The secondary air is heated by the air preheater 23 and then enters the annular area of ​​the natural gas flame tangentially to organize combustion and control the flame shape. The tertiary air (i.e., burnout air) is introduced from the bottom of the furnace to reduce the peak flame temperature, thereby suppressing the formation of thermal NOx.

[0038] High-temperature flue gas flows sequentially through the furnace, screen-type superheater 11, high-temperature superheater 19, low-temperature superheater 20, upper economizer 21, lower economizer 22, air preheater 23, and bag filter 26, finally exiting through chimney 27. The heat transfer surface synergistic subsystem achieves heat flow reconstruction through the synergistic optimization of radiation and convection heat transfer. The methanol flame has a high temperature and radiation intensity, with its peak radiation wavelength in the micrometer range, which matches the radiation absorption characteristics of the H-type finned tubes used in the screen-type superheater 11, significantly improving the proportion of radiation heat absorption. At the same time, since the total amount of methanol combustion flue gas is less than that of natural gas, the flue gas velocity flowing through the convective heating surface increases, and the convective heat transfer coefficient is improved, thus jointly promoting the increase of the main steam temperature. After the high-temperature main steam flows out from the high-temperature superheater 19, it is transported to the user through the steam pipeline 10. The economizer system adopts a two-stage arrangement. The upper economizer 21 is located below the low-temperature superheater 20 and is used to recover the waste heat of the medium-temperature flue gas. The lower economizer 22 is arranged above the air preheater 23 to further reduce the exhaust gas temperature and improve the boiler thermal efficiency. The low exhaust gas loss characteristics of methanol fuel itself, combined with the synergistic effect of the two-stage economizer, significantly improve the overall thermal efficiency of the boiler.

[0039] The flue gas purification and induced draft subsystem is responsible for processing the flue gas generated by the boiler and ensuring a precise supply of air required for combustion. Induced draft fans 18 and 25 are driven by permanent magnet synchronous variable frequency motors 24, featuring short dynamic response time. The DCS closed-loop control subsystem rapidly adjusts the induced draft fan speed based on real-time monitoring of furnace negative pressure and flue gas oxygen content signals, thereby achieving high-precision control of the air volume required for the combustion process. The bag filter 26 employs pulse-jet cleaning, exhibiting high collection efficiency for submicron-sized particles generated by methanol combustion. The chimney 27 outlet is equipped with... Online monitoring devices for NOx and particulate matter concentrations upload monitoring data directly to the carbon trading platform, providing a basis for enterprises' carbon asset management.

[0040] The induced draft fan system can be configured with one in use and one on standby; the dust collector can be upgraded to an electrostatic precipitator-bag filter hybrid dust collector according to actual emission requirements; if further reduction of NOx emissions is required, an SCR denitrification device can be added after the air preheater.

[0041] The DCS closed-loop control subsystem collects real-time data on boiler load, main steam temperature, dual fuel flow, flue gas oxygen content, NOx, and... A multivariate control model was constructed based on six types of signals, including concentration, with the optimization objectives of "raising temperature, reducing carbon, and stabilizing combustion." The control algorithm includes:

[0042] Feedforward control: The theoretical demand for methanol and natural gas is directly calculated based on the boiler load command to achieve rapid response to load changes;

[0043] Closed-loop feedback: The methanol flow rate is adjusted by a PID controller based on the main steam temperature deviation as input, while the natural gas flow rate follows according to a preset ratio to ensure stable steam parameters;

[0044] Constraint and control: When the NOx concentration exceeds the set limit, adjustment is preferentially made by reducing the swirl intensity or increasing the burnout air volume; when If emissions exceed standards, priority should be given to increasing the proportion of methanol blending.

[0045] The safety interlock logic fully covers hazardous operating conditions: MFT signal triggering will directly interlock and shut down valve 17 and methanol supply pump; the burner flame detection adopts the "two out of three" logic, and once a fire is detected, the fuel valve of the burner will be immediately shut off.

[0046] The algorithm can be extended to fuzzy PID or model predictive control (MPC) to improve control quality under complex operating conditions; the hardware platform is also compatible with PLC and RTU combinations, which facilitates the transformation of old plant control systems.

[0047] Example 2

[0048] A method for operating a methanol-natural gas dual-fuel co-combustion system includes the following steps:

[0049] Step 1: Cold start. First, use natural gas to ignite the gas main pipe 3. Once the furnace temperature rises to the set temperature value that is sufficient to ensure stable ignition of methanol, start the methanol pump. The initial methanol blending ratio starts from a low value and gradually increases at a relatively slow slope. At the same time, the natural gas flow rate is reduced accordingly to ensure that the combustion conditions in the furnace smoothly transition to the predetermined state. In this embodiment, the methanol blending ratio starts from 5% and increases slowly at a slope of 1% per minute.

[0050] Step 2: During the load increase process, as the boiler load increases, the methanol blending ratio is increased by a certain margin for each certain load increase, and the swirl blade angle of the burner is dynamically adjusted simultaneously to optimize the combustion structure under different loads. In this embodiment, for every 10% increase in load, the methanol blending ratio is increased by 2% to 3%, and the swirl blade angle is dynamically adjusted to optimize the combustion structure.

[0051] Step 3: During the load reduction process, as the boiler load decreases, the methanol blending ratio is reduced accordingly for each certain load reduction. When the boiler load drops to a lower threshold, the methanol blending ratio is limited to a lower level, with maintaining combustion stability as the primary objective. In this embodiment, the methanol ratio is reduced by 2% to 3% for every 10% decrease in load; when the load is below 30%, the methanol ratio is limited to less than 10%.

[0052] Step 4: Normal shutdown. First, gradually reduce the methanol blending ratio to zero and stop the methanol pump. Then, purge the methanol pipeline with an inert gas (such as nitrogen) to ensure that there is no residual methanol vapor in the pipeline. Next, cut off the natural gas supply and execute the furnace back-purge procedure to remove any combustibles that may have accumulated in the furnace. Finally, stop the operation of induced draft fans 18 and 25. In this embodiment, the methanol ratio is first reduced to zero, the methanol pipeline is purged for 30 seconds, then the natural gas is cut off, the furnace back-purge is executed for 5 minutes, and finally the induced draft fans 18 and 25 are stopped.

[0053] Step 5: Emergency Response. A clear emergency response procedure has been established to address potential emergencies such as methanol leaks or furnace fires. This includes immediately cutting off the fuel supply, activating the purging system, and other interlocking protection actions. Restarting is only permitted after manual confirmation following the elimination of the hazard. In the event of a methanol leak, the pump is immediately stopped, the valves are closed, and nitrogen purging is performed. In the event of a fire, the MFT (Mechanical Fuel Trip) is triggered, the quick-cut valve closes, and restarting is only permitted after manual confirmation.

[0054] The workflow of this invention is as follows: Natural gas is introduced through the existing natural gas pipeline 1 in the plant area, and methanol is supplied by a newly added storage and supply system. Both fuels are transported via their respective independent supply pipelines to dedicated burners located in specific positions within the furnace. Inside the burners, the fuel and preheated air are mixed in a controlled swirling field and ignited, generating high-temperature flue gas. The high-temperature flue gas flows sequentially through the existing heating surfaces of the boiler, transferring energy to the working fluid, ultimately producing high-temperature, high-pressure main steam. After leaving the heating surfaces, the flue gas is extracted by an induced draft fan, treated by dust removal and purification equipment, and finally discharged into the atmosphere through chimney 27.

[0055] Throughout the process, the DCS closed-loop control subsystem continuously collects key signals such as boiler load, main steam temperature, various fuel flows, and flue gas composition. By executing the embedded intelligent control algorithm, it dynamically adjusts key operational variables such as the blending ratio of methanol and natural gas and the air distribution volume, ultimately achieving the core objectives of increasing main steam temperature, reducing natural gas consumption, and reducing carbon dioxide emissions.

[0056] This invention is the first to integrate three major technologies—"methanol-natural gas dual-fuel combustion," "synergistic optimization of heating surface radiation and convection," and "closed-loop-feedforward composite control"—into a large-scale gas-fired boiler, filling a gap in low-carbon coupled combustion technology for power plant boilers. This system not only significantly improves main steam parameters and boiler thermal efficiency but also has the potential to reduce natural gas consumption and... Its emission reduction has significant engineering application value and economic and social benefits. Its modular design also facilitates retrofitting and application in existing power plants, providing an effective technical path for the low-carbon transformation of traditional gas-fired power plants.

[0057] In the description of this invention, it should be understood that the terms "coaxial," "bottom," "one end," "top," "middle," "other end," "upper," "side," "top," "inner," "front," "center," "both ends," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0058] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "setting," "connection," "fixing," "screw-in," "pad," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two components or the interaction between two components. Unless otherwise explicitly limited, those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0059] The foregoing description illustrates and describes preferred embodiments of the present invention. As previously stated, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the inventive concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A methanol-natural gas dual-fuel co-combustion system, characterized in that, It includes a dual-fuel supply subsystem, a swirl burner group, a heating surface coordination subsystem, a flue gas purification and induced draft subsystem, and a DCS closed-loop control subsystem; The dual-fuel supply subsystem includes independent natural gas supply branches and methanol supply branches; The swirl burner group includes at least two natural gas burners and at least one methanol burner, wherein the methanol burner and the natural gas burner are arranged in stages within the furnace space; The heating surface coordination subsystem includes a screen-type superheater, a high-temperature superheater, a low-temperature superheater, an economizer, and an air preheater arranged sequentially along the flue gas flow path. The flue gas purification and induced draft subsystem includes an induced draft fan, a dust collector, and a chimney; The DCS closed-loop control subsystem is configured to execute a multivariable decoupled control strategy, using the main steam temperature as the core controlled variable, the methanol to natural gas mass flow ratio as the key controlled variable, and the furnace outlet oxygen quantity, NOx concentration, and... Concentration is used as a constraint.

2. The methanol-natural gas dual-fuel co-combustion system according to claim 1, characterized in that, The natural gas supply branch includes a main gas header and an ignition gas header connected to the natural gas pipeline in the plant area. The main gas header is connected to each natural gas burner through multiple independent branches. Both the main gas header and the ignition gas header are equipped with dual redundant pneumatic quick-cut valves that are hardwired to the main fuel trip (MFT) system. The methanol supply branch includes a methanol storage tank, a transfer pump, and an atomizing device connected in sequence. The methanol storage tank is equipped with a nitrogen sealing device and a heating device. The outlet of the transfer pump is equipped with a self-regulating reflux regulating valve. The atomizing device is a pressure atomizing nozzle or a dual-fluid atomizing nozzle.

3. The methanol-natural gas dual-fuel co-combustion system according to claim 2, characterized in that, The methanol supply branch also includes an adaptive module. When the water content of the methanol fuel exceeds a preset threshold, the adaptive module is an alkaline washing tower located at the outlet of the methanol storage tank. When the ambient temperature is lower than a preset threshold, the heating device is replaced by a steam coil heat tracing device, and the atomizing device is replaced by a steam-methanol dual-fluid atomizing nozzle.

4. The methanol-natural gas dual-fuel co-combustion system according to claim 1, characterized in that, The methanol burners are arranged in the upper middle part of the furnace walls on both sides of the boiler, and the natural gas burners are concentrated in the lower middle part of the front wall of the boiler. The methanol burners adopt a dual swirl coupling structure, including adjustable axial swirl blades and radial swirl blades, which are used to premix methanol and air downstream of the nozzle. The air distribution system implements a three-stage air supply, including primary air supplied to the root of the methanol flame, secondary air for organizing natural gas combustion, and tertiary air introduced from the bottom of the furnace to reduce the peak flame temperature.

5. The methanol-natural gas dual-fuel co-combustion system according to claim 1, characterized in that, The economizer is arranged in two stages, including an upper economizer located below the low-temperature superheater and a lower economizer located above the air preheater; the screen-type superheater adopts an H-shaped finned tube structure, and its radiation absorption characteristics match the radiation spectrum of the methanol flame.

6. The methanol-natural gas dual-fuel co-combustion system according to claim 1, characterized in that, The induced draft fan is driven by a permanent magnet synchronous variable frequency motor; the dust collector is a bag filter or an electrostatic precipitator / bag filter hybrid dust collector; the chimney outlet is equipped with a monitoring device. Online monitoring devices for NOx and particulate matter concentrations.

7. The methanol-natural gas dual-fuel co-combustion system according to claim 6, characterized in that, The flue gas purification and induced draft subsystem also includes an SCR denitrification device, which is located after the air preheater.

8. The methanol-natural gas dual-fuel co-combustion system according to claim 1, characterized in that, The control algorithm executed by the DCS closed-loop control subsystem includes: The feedforward control unit calculates the theoretical demand for methanol and natural gas based on the boiler load command. The closed-loop feedback unit uses the main steam temperature deviation as input to adjust the methanol flow rate through a PID controller and simultaneously adjusts the natural gas flow rate. The constraint control unit prioritizes adjusting the combustion air distribution when the NOx concentration exceeds the standard. When emissions exceed standards, priority should be given to increasing the proportion of methanol blending.

9. The methanol-natural gas dual-fuel co-combustion system according to claim 8, characterized in that, The DCS closed-loop control subsystem also includes a safety interlock unit, which includes receiving MFT signals and triggering the interlocking closure of valves and methanol supply pumps, and triggering the shut-off of individual burner fuel valves based on flame detection signals using "two out of three" logic.

10. A method for operating a methanol-natural gas dual-fuel co-combustion system as described in any one of claims 1-9, characterized in that, Includes the following steps: Step 1: Cold start, use natural gas for ignition, and after the furnace temperature rises to the set value, start the methanol supply. The methanol blending ratio will slowly increase from the initial value according to the set slope. Step 2: Variable load operation. During the load increase process, the methanol blending ratio increases accordingly for each certain percentage increase in load; during the load decrease process, the methanol blending ratio decreases accordingly for each certain percentage decrease in load, and the upper limit of the methanol blending ratio is limited at low load. Step 3: Normal shutdown. First, reduce the methanol blending ratio to zero, purge the methanol pipeline, then cut off the natural gas supply and perform furnace purging. Step 4: Emergency handling. In the event of a methanol leak, immediately stop the methanol supply and purge with inert gas. In the event of a fire, trigger the MFT and cut off all fuel supply.

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