LOW NOx EMISSION BURNER FOR FIRING LIQUID FUELS AND METHOD OF OPERATION

Petroleum refining and petrochemical processes frequently involve heating process streams in a furnace. The interior chamber of the furnace contains tubes which contain the process streams. The interior chamber is heated by a plurality of burners which receive a fuel which combusts to produce heat.

One area of concern for burners is the production of NOx gases. It is believed that there are at least three principal NOx formation mechanisms in combustion processes: thermal NOx, fuel NOx, and prompt NOx. See, “Nitrogen Oxides (NOx), What and How They are Controlled,” EPA Technical Bulletin November 1999 (available at: https://www3.epa.gov/ttncatc1/dir1/fnoxdoc.pdf).

It is known that NOx formation in gas burners can be mitigated by staging fuel and air and creating primary and secondary combustion (flame) zones. Staged air and staged fuel burners work primarily on the thermal NOx and prompt NOx formation processes. The highest flame temperatures, and thereby the greatest potential for thermal NOx formation, is achieved when gaseous fuels and combustion air are thoroughly mixed and rapidly combusted in or near stoichiometric proportions.

Accordingly, the staged air and staged fuel burners seek to lower the temperature of the flame and thereby lower the NOx production. Classic staged fuel or staged air burners produce two combustions zones for off-stoichiometric combustion. In the case of the staged fuel burner, all the combustion air passes through the primary combustion zone and into the secondary combustion zone with the partial combustion products of the primary combustion zone. The staged air burner reverses the staging process and introduces all the fuel gas for combustion in the primary combustion zone and only a portion of the combustion air.

More recently, internal flue gas recirculation burners have utilized flue gas within the heater or furnace combustion chamber. The flue gas, which is relatively cool, passes into and through the combustion zones thereby further cooling the combustion zone and reducing thermal NOx formation. The water vapor in the flue gas also serves to mitigate NOx created via the prompt NOx mechanism by solvating and catalyzing hydrocarbon combustion.

When firing liquid fuels, alone or in combination with fuel gases, the nitric oxide (NOx) emissions can be quite high. NOx refers to oxides of nitrogen, principally comprised of nitric oxide, NO, and nitric dioxide, NO, and is formed from thermal mechanisms. The high flame temperatures of liquid fuels generate higher NOx emissions. When the liquid fuel contains nitrogen compounds (i.e., “bound” nitrogen), which liquid fuels often do, the NOx emissions can be well over 200 ppm in many service conditions. Liquid fuels with no nitrogen in the liquid fuel (i.e., no “bound” nitrogen) can have NOx emissions of greater than 150 ppm in normal operating conditions, while a comparable low NOx gas burner may produce less than 30 ppm in similar service.

Therefore, there is a need for burners for liquid fuels or for liquid fuel and fuel gas having reduced NOx emissions, and for processes for operating the low NOx burners.

The burner of the present invention delivers low NOx emissions for processes involving the combustion of liquid fuel which it achieves through the targeted injection of a NOx reducing medium. The low NOx emissions may reduce permitting difficulties and may reduce or eliminate the need to purchase offsetting NOx credits (if available).

A conventional liquid fuel burner may comprise a centrally located liquid fuel injector (also known as an “oil gun”). Liquid fuels include, but are not limited to, hydrocarbons which are normally liquid at ambient conditions including naphtha, alcohols, diesel fuel, bunker fuel oil, distillate oil, crude oil, and the like.

Liquid fuel must be atomized and changed through a phase shift to gaseous form by the atomizing medium in order to burn. The liquid fuel injector is a device for mixing the liquid fuel with the atomizing medium. The atomizing medium may be steam, air, nitrogen, fuel gas, and the like. Steam is the most commonly used atomizing medium, followed by air.

The liquid fuel injector is often centrally located in a concentric assemblage of cylinders which make up the burner assembly. The central liquid fuel injector is often surrounded by or directly adjacent to a flame stabilization device such as an oil tile, flame diffusor, flame holder, bluff body or other devices used to stabilize the base of the flame front. The flame is established downstream of the liquid fuel injector on or near the flame stabilization device. This is the injection point for the liquid fuel and the atomizing medium. A portion of the combustion air is directed into and around the liquid fuel injector and the flame stabilization device. The remaining portion of the combustion air is injected around and outside of the centrally located liquid fuel injector and flame stabilization device. In some cases, a portion of the combustion air is injected at the periphery of the burner assembly for delayed mixing with the fuel. This periphery air injection is termed “staged air” and is used to lower NOx emissions by delaying combustion and by reducing the partial pressure of oxygen immediately surrounding the liquid fuel injector. "Combustion air” means any oxygen-containing gas including, but not limited to air, oxygen, flue gas, and the like.

High partial pressure of oxygen produces higher NOx emissions. In order to reduce the partial pressure, fuel gas may be injected outside of the liquid fuel injector (primary fuel gas) or at the outer periphery of the burner tile (staged fuel gas).

When gas firing capability is included in a liquid fuel burner, this is termed a “combination” liquid/gas burner, a “dual fuel” burner, or a hybrid fuel burner. The combination burner can fire 100% liquid fuel, 100% gaseous fuel, or any proportion of fuel gas and liquid fuel. The burner of the present invention can be incorporated in a combination liquid/gas fuel burner design.

In advanced low NOx burners, as more fully described in US 11,649,960 B2, a NOx reducing medium, such as steam, flue gas from a combustion zone or heater exhaust, nitrogen, carbon dioxide, or other “inert” media is introduced in specific targeted locations in the burner assembly.

In the present invention, combustion air and/or a NOx reducing medium are introduced at more than one location in the burner. A portion of the combustion air and the NOx reducing medium can be injected into the primary combustion zone through the NOx reducing medium conduit which surrounds the liquid fuel injector which injects atomized liquid fuel into the primary combustion zone. Another portion of the combustion air and the NOx reducing medium can be introduced into the primary combustion zone through a passage in the burner tile surrounding the NOx reducing medium conduit.

The burner comprises a plenum connected to a burner tile. The burner tile defines a primary combustion zone. The plenum has a NOx reducing medium inlet and a combustion air inlet. There is a passage through the burner tile from the plenum to the primary combustion zone. The burner includes a NOx reducing medium conduit surrounding a liquid fuel injector, and the outlet of the NOx reducing medium conduit and the outlet of the liquid fuel injector are in the primary combustion zone. The burner also includes a primary fuel gas conduit with one or more primary fuel gas outlet(s) which are utilized when the burner is used in a dual fuel arrangement. The primary fuel gas outlet(s) are in the primary combustion zone. In some instances, the burner may optionally also include one or more staged fuel gas conduit(s) with one or more staged fuel gas outlet(s). The staged fuel gas outlet(s) are located outside the burner tile. The staged fuel gas conduit(s) can be connected to the primary fuel gas conduit to provide fuel gas to the staged fuel gas outlet(s).

The NOx reducing medium is injected into the primary combustion zone from two places. A first portion of the NOx reducing medium enters the inlet of the NOx reducing medium conduit. A first portion of the combustion air is introduced into the NOx reducing medium conduit from the plenum through one or more combustion air inlet(s) in the NOx reducing medium conduit. The first portion of the NOx reducing medium and the first portion of the combustion air are mixed in the NOx reducing medium conduit and introduced into the primary combustion zone through the outlet of the NOx reducing medium conduit. The combustion air and the atomized liquid fuel react and produce a flame in the primary combustion zone.

A second portion of the NOx reducing medium and a second portion of the combustion air is introduced into the primary combustion zone through the passage in the burner tile. The second portion of the NOx reducing medium is introduced into the plenum through a NOx reducing medium inlet and the combustion air is introduced into the plenum through a combustion air inlet. The first portion of the combustion air from the plenum enters the NOx reducing medium conduit through one or more combustion air inlet(s) (e.g., openings or holes) in the conduit, as discussed above. The remainder of the combustion air and the second portion NOx reducing medium flows through the passage in the burner tile into the primary combustion zone.

The primary fuel gas from the primary fuel gas outlet reacts with the combustion air in the primary combustion zone and produces a flame.

There are one or more secondary fuel gas conduit(s) located outside the burner tile. The secondary fuel gas conduit(s) may be connected to the primary fuel gas conduit, or they can be separate. The secondary fuel gas from the secondary fuel gas outlet(s) reacts with combustion air downstream of the primary combustion zone.

The liquid fuel injector, the NOx reducing medium conduit, and the passage in the burner tile are typically in a concentric arrangement, although this is not required.

The burner may include a flame stabilization device. Any suitable flame stabilization device can be used. Suitable flame stabilization devices include, but are not limited to, oil tiles, flame diffusers, flame holders, bluff bodies, or combinations thereof.

Another aspect of the invention is a method of reducing production of NOx gases at a burner. In one embodiment, the method comprises injecting a liquid fuel and an atomizing medium from a liquid fuel injector into a primary combustion zone defined by a burner tile forming an atomized liquid fuel. A first portion of NOx reducing medium is injected into the primary combustion zone from a NOx reducing medium conduit surrounding the liquid fuel injector. Combustion air and a second portion of NOx reducing medium is injected into the primary combustion zone through a passage in the burner tile. The combustion air and the atomized liquid fuel react and produce a flame in the primary combustion zone.

In some embodiments, the method further comprises passing a portion of the combustion air into the NOx reducing medium conduit. In this case, injecting the first portion of the NOx reducing medium into the primary combustion zone comprises injecting the first portion of the NOx reducing medium and the first portion of the combustion air into the primary combustion zone.

In some embodiments, the method further comprises injecting primary fuel gas through a primary fuel gas outlet into the primary combustion zone wherein the combustion air and the primary fuel gas react and produce a flame inside the primary combustion zone.

In some embodiments, the method further comprises injecting secondary fuel gas through a secondary fuel gas outlet into an area outside the burner tile wherein the combustion air from the passage and the secondary fuel gas react and produce a flame outside the primary combustion zone.

In some embodiments, the atomizing medium comprises steam, air, nitrogen, fuel gas, or combinations thereof.

In some embodiments, the method further comprises stabilizing the flame with a flame stabilization device in the primary combustion zone. In some embodiments, the flame stabilization device comprises an oil tile, a flame diffuser, a flame holder, a bluff body, or combinations thereof.

In some embodiments, injecting the combustion air and the second portion of NOx reducing medium into the primary combustion zone through the passage in the burner tile comprises introducing the combustion air and the second portion of NOx reducing medium into a plenum connected to the burner tile and wherein the combustion air and the second portion of NOx reducing medium pass from the plenum through the passage in the burner tile to the primary combustion zone.

In some embodiments, the passage in the burner tile surrounds the NOx reducing medium conduit.

In some embodiments, the first portion of NOx reducing medium, or the second portion of NOx reducing medium, or both comprise: flue gas from a combustion zone; flue gas from the exhaust of a heater, a boiler, or a furnace; steam; nitrogen; carbon dioxide; or combinations thereof.

In some embodiments, the method further comprises: monitoring at least one NOx value for the flame; and adjusting a flowrate of the first portion of NOx reducing medium, the second portion of NOx reducing medium, or both based on the at least one NOx value.

Based on operational needs and NOx reduction demand, a control system is designed to modulate the proportion of NOx reducing media directed to the any one of these locations, primary, secondary, or periphery. Passageways between the primary, secondary, or periphery injection locations may provide for the communication and passage of combustion air and/or NOx reducing media between the primary, secondary or periphery areas.

By designing for and actively controlling the injection rates, locations, localized stoichiometry, and NOx reducing medium introduced to the combustion process, the flame, the NOx production can be mitigated to extremely low values, e.g., less thanppmvd with relatively modest amounts of NOx reducing media, such as flue gas. By designing for and actively controlling the injection rates, locations, localized stoichiometry, and NOx reducing medium introduced to the combustion process, the flame and the NOx formation can be mitigated while maintaining good burner and flame stability and continuous operation.

While the lowest NOx emissions may be produced when the burner is receiving highest rates of NOx reducing medium, this may also be the incipient point of burner instability. This incipient instability can be detected by a high-speed pressure transmitter and associated instability detection software, similar to that described in U.S. Patent No. 7,950,919. However, unlike U.S. Patent No. 7,950,919 where principally combustion chamber oxygen is controlled and adjusted to react to instability, in this invention, the rate and location of NOx reducing medium can be controlled.

Generally, the NOx emissions from the heater may be monitored along with the stack oxygen and the combustion chamber pressure or draft. The rate, the amount of NOx reducing medium delivered, or both may be increased at the desired locations in the flame zone until the required NOx reduction is achieved. Once the desired NOx level is achieved, no additional NOx reducing medium may be introduced. If the burner becomes unstable, the rate and/or location of the NOx reducing medium can be controlled or the excess air, oxygen levels adjusted as suggested in U.S. Patent No. 7,950,919 until burner stability is achieved.

It is further contemplated that visual field or infrared cameras may be used to monitor flame stability and quality aspects using artificial intelligence, AI, such as described in U.S. Patent Publ. No. 2020/0386404 (incorporated herein by reference). When instabilities or other anomalies in the flame image are detected with the AI, the amount and location of the NOx reducing medium and/or other control aspects of the heater controls system, such as excess oxygen, can be adjusted and controlled to simultaneously deliver the lowest level of NOx (or at least the required level) with good burner flame stability.

If there is a loss of NOx reducing medium at any time, the present burner will still work safely as a conventional low NOx burner. The NOx emissions may increase, but the burner will otherwise remain stable and continue to deliver heat reliably to the process in the heater, boiler or furnace. Further, the burner will operate, in the view of the burner operator, just as conventional burners operate with no special operational issues.

The introduction of NOx reducing medium may be by fixed static control devices or by automated computer control. Therefore, the burner operates, to the point of view of the operator, conventionally with draft and oxygen control as prescribed in API Recommended Practice, Third Edition, May 2014, Burners for Fired Heaters in General Refinery Service. Namely, draft and oxygen are controlled with the stack damper and burner air inlet register and/or the induced draft fan and forced draft fan control settings.

Process heaters are direct fired heaters, which are used mainly to heat process fluids to temperatures of more than°C in the radiation and convection sections. The typical flue gas temperature entering the convection section is more than°C; this temperature may vary with various heater designs and capacities.

Combustion air contains approximately 21% O₂ and 78% N₂ and is supplied to the burner to provide the oxidizing component (O₂) required in the combustion process. Nitrogen plays no part in the combustion of fuel gas and would, ideally, pass through the burner without reacting.

However, there are three primary processes that create NOx during fuel gas combustion: thermal NOx, which is temperature dependent, fuel NOx, which is fuel-bound nitrogen and local O, and prompt NOx, which is not of concern here.

Thermal NOx is formed at very high temperatures and is a result of the oxidation of nitrogen found in combustion air. Thermal NOx is the predominant type of NOx generated during combustion of natural gas and is a function of the temperature and residence time of the nitrogen at that temperature. The higher the temperature of the flame, the greater the formation of thermal NOx.

The primary focus of a control system for this invention is to reduce the amount of thermal NOx produced. The levels of excess air, the temperature of the air, and the way the air is mixed with the gas will affect the production of NOx.

The targeted De-NOx gas injection (TDGi) system includes the low NOx burner described above, a TDGi blower with an automated inlet isolation valve and manual outlet valves, instrumentation (e.g., flow meters, oxygen meters, temperature and pressure sensors and transmitters, and the like), and a variable frequency drive (VFD) control system. This unique design will utilize a TDGi technique to reduce the NOx emissions significantly.

The flame temperature may be reduced to lessen NOx production by adding a non-reactive gas into the burner. Suitable non-reactive gases include, but are not limited to, carbon dioxide, nitrogen, steam, flue gas, other inert gas, or combinations thereof.

One source for the non-reactive gas is a flue gas that, after the combustion chamber, will be inert and have a temperature substantially lower than the burner flame temperature.

The TDGi control system (TDGiCS) provides orderly and safe startup, operation, and shutdown of the TDGi system. The design of the TDGi incorporates blowers, isolation valves, critical analyzer, and sensors to execute the Start/Stop/Shutdown sequence of the TDGi. All necessary permissive, sequence status, and alarms related to TDGi can be displayed on an interface on a TDGi panel located in the field.

Prior to start up, the operator should ensure that all utilities (instrument air, power, etc.,) are available to the system. Low point drains should be drained to remove any condensate/liquid in the lower part of the TDGi ducting, and when drainage is completed, drain valves should be closed before starting operation of the TDGi system. Lastly, it should be verified that no TDGi related alarm conditions exist in the distributed control system (DCS).

The system should be purged to remove flammable vapors and gases that may have entered any portion of the system volume (e.g., the process Heater, the TGDi ducting section to stack exit, etc.) during the shutdown period. The process heater purging should be completed before starting TDGi purging.