Compendium of Burners for Refining Heating Furnaces

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A burner is a device that introduces fuel and air into a heating furnace at optimal flow rates, with the desired level of turbulence and air/fuel ratio, in order to ignite and maintain a stable combustion. A burner generally consists of three main components: a fuel nozzle, an air regulator, and a combustion chamber. Modern burners also include sound-insulating elements as well as safety systems for ignition and operation, all in consideration of environmental protection and safety.

1. Classification of Burners
Burners can be classified based on the type of fuel used, the method of air supply, emission requirements, and flame shape. Based on the fuel used, they can be divided into gas burners, oil burners, and combined oil-gas burners (refining heating furnaces do not use solid fuels). According to the method of air supply, they can be categorized as naturally ventilated burners or forced-ventilation burners (including high-intensity burners). Based on emission requirements, they can be classified as burners with conventional NOx emissions or low-NOx burners. In terms of flame shape, there are cone-shaped flame burners, fish-tail (fan-shaped) flame burners, flat-flame burners, and radiant wall flame burners. Gas burners can be further divided into external mixing type, premixed type, and semi-premixed type, depending on when the fuel and air are mixed. Oil burners can be classified into those using medium (steam or air) for atomization, pressure atomization, or mechanical atomization. The burners commonly used in refining heating furnaces are often combinations of these different types; for example, naturally ventilated low-NOx gas burners (which typically have a cone-shaped flame unless otherwise specified), forced-ventilation pressure-atomized oil burners, naturally ventilated combined oil-gas burners, naturally ventilated flat-flame gas burners, and naturally ventilated semi-premixed radiant wall flame gas burners.

2. Gas Burners
Gas burners have three types of fuel nozzles: external mixing type, premixed type, and semi-premixed type. In the external mixing type, the fuel gas and air mix together outside the nozzle during combustion. The combustion rate and completeness depend mainly on physical processes, namely the diffusion and mixing of the fuel gas and air. In the premixed type, the fuel gas and air are mixed evenly inside the nozzle, so the combustion process is determined by the chemical kinetics of the combustion reaction rather than the physical mixing process. As a result, its combustion rate is much faster, and the combustion is more complete (requiring less excess air). In the semi-premixed type, about 30% to 70% of the air is mixed inside the nozzle, while the remaining air mixes outside the nozzle. Its combustion characteristics lie between those of the external mixing type and the fully premixed type. The flame of an external mixing type burner is yellow, while that of a premixed type burner is blue. A semi-premixed type burner has two conical flame surfaces: the outer cone is yellow, and the inner cone is blue.

2.1 External Mixing Type Gas Burners
External mixing type gas burners can operate under a wide range of conditions regarding the composition, density, and calorific value of the fuel gas. Fuel gases with high hydrogen content or a large proportion of high-molecular hydrocarbons can be used. However, they are not suitable for fuel gases containing liquid droplets or large amounts of unsaturated hydrocarbons, as these can cause carbon deposits or polymers to form in the nozzle, blocking the orifices. External mixing type gas burners can operate at a wide range of fuel gas pressures. At the designed heat release rate, the pressure of the gas before the fuel nozzle is usually set at 0.1–0.15 MPa. This value accommodates the pressure levels in the fuel gas networks of refining plants and also ensures appropriate nozzle sizes, reducing the risk of clogging due to dirt. The operating range of external mixing type gas burners depends on the composition, density, calorific value of the gas, and the available pressure. When the gas composition remains constant, the adjustment ratio (the ratio of maximum to minimum heat release) can reach 5:1. For naturally ventilated external mixing type gas burners, air supply relies primarily on the draft force generated within the furnace near the burner. To ensure stable combustion, a draft force of at least 40–50 Pa is required at the height of the burner bricks. In naturally ventilated external mixing type gas burners, when the fuel nozzle is located in the center of the burner and the flame is cylindrical or conical, the length of the flame depends on the opening angle of the fuel nozzle: a smaller opening angle results in a longer flame； The opening angle is large and the flame is short and thick. The typical flame length is 1~2m/MW. The heat release of natural ventilation external mixing gas burner is usually 0.3~4.5MW. The heat release of forced draft burner is usually 1~6MW. The heat release of high-intensity burner is through Usually 4.5~20MW. 2.2 Premixed gas burner Natural ventilation The air supply of the premixed gas burner is sucked in by fuel gas pressure through a venturi tube, so it is also called a pilot gas nozzle. Under the design heat release, the fuel gas is in front of the nozzle The pressure is 0.1~0.25MPa. When the heat release is given, changes in the fuel gas composition may change the fuel operating pressure. This directly affects the amount of combustion air inhaled. Therefore, the premixed burner is not suitable for fuels with frequently changing gas compositions. Under all operating conditions, the speed of the gas-air mixture at the nozzle hole should be greater than the flame propagation speed. Otherwise, the flame will return to the venturi tube to burn or flash, causing backfire or backfire explosion. When using high flame propagation speed fuel gas (such as hydrogen) , low-load operation should be strictly limited. When the hydrogen content (mole fraction) of the fuel gas exceeds 70%, the use of premixed burners is not recommended. In addition, preheating the air will accelerate the flame propagation speed. Therefore, the premixed air is prohibited from being heated, otherwise it will frequently backfire and fail. Normal operation. Premix burners can operate at low excess air. The excess air in a single burner can reach 5%~10%. For a certain fuel gas composition, the turndown ratio of premix burners is generally limited to 3:1. Premix burners It can achieve stable combustion at very low pumping force. When fully premixed, the pumping force at the altitude where the burner is located can be as low as 13~25Pa. Considering that the size of the Venturi ejector should not be too large, the heat release of the fully premixed gas burner should not exceed 1MW. ｡The flame volume of the premixed type is smaller than that of the external mixed type. The flame shape depends on the design of the gas/air mixture nozzle and the shape of the refractory bricks. It can produce conical flames, fishtail flat flames and radial flames spread on the burner bricks and furnace wall refractory materials. 3. The combustion process of fuel oil in fuel burners includes the physical process of evaporation of fuel oil and the diffusion and mixing of oil vapor and air, as well as the chemical process of combustion reaction between fuel molecules and oxygen molecules in the air. Compared with the combustion process of fuel gas, there is an additional physical process of "evaporation". Therefore, its combustion rate is relatively slow and the flame volume is relatively large. The evaporation of fuel oil The combustion process is directly related to its specific surface area. Increasing the specific surface area can intensify the combustion process. In order to increase the combustion rate and strengthen the combustion process, many methods of expanding the specific surface area have been used. For example, initially, fuel oil was poured on randomly stacked refractory bricks for combustion, and later oil dripper combustion was used. It was not until the end of the 18th century that the atomization combustion method appeared.* * Strengthen the combustion process of fuel oil. If 1kg of fuel oil is spherical, its surface area is 0.0483m3, and it is atomized into oil droplets with a diameter of 30μm, the total surface area is 200m2, and the specific surface area increases by 4140 times. Atomization is the process of breaking liquid into extremely fine droplets to improve fuel-air mixing and enhance combustion. There are many ways to atomize fuel oil, the common ones are medium atomization, pressure atomization and mechanical atomization. Medium atomization is divided into steam atomization, pressure atomization Compressed air atomization and blower air atomization. Steam atomization is divided into internal mixing type, external mixing type and intermediate mixing type according to the mixing position of steam and oil. Refining heating furnaces basically use internal mixing steam atomization. Compressed air atomization is rarely used, generally only in light burning It is recommended to use oil (such as naphtha) to prevent steam lock. Air atomization by blower is generally used in the machinery manufacturing industry to burn diesel. Pressure atomization and mechanical atomization are used in oil field heating furnaces and long-distance pipeline heating furnaces that lack atomization media to burn crude oil. The steam pressure of internal mixed steam atomizing oil nozzles used in refining heating furnaces is generally 0.7~1MPa, and the fuel oil pressure is usually 0.1~0.2MPa lower than the steam pressure. The steam must be dry or slightly superheated, usually 200~2 10℃. Wet steam will form water droplets in the pipes and fuel nozzles. The evaporation of the water droplets will absorb heat and form sparks, resulting in difficulty in ignition and incomplete combustion. Superheated steam can vaporize part of the fuel oil in the fuel nozzle, causing vapor lock and affecting the flame. Stability. The amount of steam required to atomize each kilogram of fuel oil is called steam consumption. The steam consumption is lower when burning light fuel oil, and higher when burning heavy fuel oil. When internal mixed steam atomization is used to burn vacuum residual oil, the general steam consumption (standard oil) is 0.15~0.3kg. /kg, up to 0.5kg/kg. The atomization effect is closely related to the viscosity of the fuel oil. In order to ensure a good atomization effect, the heavy fuel oil must be heated to obtain the appropriate viscosity. Steam atomization requires the design viscosity of the fuel oil to be 20mm2 /s (Engler viscosity 3°E), the maximum viscosity is limited to less than 45mm2/s (6°E). The turndown ratio of the oil burner depends on the available fuel pressure, the amount of heat release and the design of the atomizer. Typical internal mixed steam atomization The regulation ratio is 3:1. Since the combustion process of fuel oil has one more physical process of "evaporation" than that of fuel gas, its combustion rate is relatively slow and the flame volume is relatively large. Therefore, the distance from the center of the fuel burner to the center of the furnace tube specified in SH/T3036 The distance between the refractory material or the refractory material is larger than that of gas, and more excess air is required. The drilling of the oil nozzle determines the shape and length of the flame. The conventional included angle of the nozzle hole of the conical flame burner is 40~70°. For natural draft burners, when the included angle is At 50°, the flame length is about 2m/MW. The heat release of oil burners is usually 0.9~4.4MW in natural ventilation. It is usually 1.5~20.5MW in forced ventilation. High-intensity burners are usually 3~20.5MW. 4. Oil-gas combined burner The oil-gas combined burner is the most commonly used burner in oil refining heating furnaces. It is equipped with fuel nozzles and gas nozzles in the same combustion channel and air regulator. Burning oil alone or burning gas alone can achieve the designed heat release of the burner. Oil When oil-gas mixed combustion occurs, the total heat release of the two should not exceed the design heat release of the burner, otherwise incomplete combustion will occur due to insufficient air supply. In order to prevent incomplete combustion, the operating method of oil-gas mixed combustion is generally not used. Instead, all parts of the same furnace are burned. In some burners, one part burns oil alone, and the other part burns gas alone. It is worth noting that during operation, the burners burning oil and gas alone should be cross-distributed, and the fuel flow rate with more heat release should be used as the control of the temperature of the heated medium. Therefore, the temperature of the heated medium should be controlled. Therefore, the oil or gas burner should be cross-distributed. The heat release of the gas should not be equal or similar, otherwise the operation of the heating furnace may be difficult to control. 5. More than 90% of the NOx generated during the combustion process of the low NOx burner is NO. At lower temperatures, NO is oxidized into NO2. There are three mechanisms for NO generation.: Rapid conversion P-NOx (PromptNOx) is rapidly generated from N2 and O2 in the air through hydrocarbon groups at high temperatures in the flame surface when hydrocarbon-rich fuels are burned. ； Thermally converted T-NOx (thermal NOx) is generated by the reaction of N2 and O2 in the air downstream of the flame front at high temperatures ； Fuel-derived F–NOx (fuelNOx) is formed when nitrogen compounds in the fuel burn and are converted into NOx. The amount of P–NO generated is very small, usually one order of magnitude less than that of T–NO. The formation of T–NOx depends primarily on temperature, residence time, and excess oxygen: high temperatures facilitate the formation reaction of T–NOx ； The longer the residence time in the high-temperature zone, the more T–NOx is generated ； As the excess oxygen increases, the NOx concentration rises in external mixing gas burners, while it decreases in premixed burners. This is true under typical conditions in refinery heating furnaces, where the excess oxygen level is between 1% and 5% on a wet basis. If the excess air is increased further, the NOx concentration in external mixing gas burners reaches a maximum value; beyond this point, an increase in excess oxygen causes the NOx concentration to decline (due to dilution). This maximum value may occur at an excess air level of 60% to 70% (7% to 8% on a wet basis). Additionally, the furnace temperature, the temperature of the combustion air, and the hydrogen content in the fuel can all affect the temperature in the combustion zone, thereby influencing the amount of T–NOx generated. Fuel conversion-based F–NOx can be reduced by 30% to 50% using burners with air staging. Fuel denitration can effectively reduce the formation of F–NOx. The first generation of low-NOx gas burners were air-staged gas burners, which reduced the NOx concentration in the flue gases to 140 mg/m3. These burners are equipped with primary, secondary, and tertiary air inlets. All of the fuel is injected into the primary combustion zone. In the primary and secondary combustion zones, the fuel concentration is relatively high, while the oxygen concentration is correspondingly low ； In the third-stage combustion zone, the fuel concentration is relatively reduced, while the oxygen concentration is relatively increased. Such "rich and light" combustion,* * Slow down the combustion rate, reduce the temperature of the combustion zone, and significantly reduce the amount of NO generated. The second generation low NOx gas burner is a fuel staged burner, and the concentration of NOx in the flue gas is reduced to 80mg/m3. The fuel staged burner generally supplies the fuel gas in two stages (a few are also divided into three stages), and all combustion air is supplied in the first stage combustion zone. It also adopts "rich and light" combustion. The principle is to reduce the generation of NO. The third generation low NOx gas burner is a low NOx burner with internal flue gas reflow. The concentration of NOx in the flue gas is reduced to 50 mg/m3. Internal flue gas recirculation is the burner itself using the combustion air flow or fuel air flow to generate a low pressure area, which entrains the furnace gas (mainly flue gas) into the burner or combustion zone through the opening on the burner brick, diluting the oxygen concentration. , reduce the combustion rate and temperature of the combustion zone, thereby reducing the generation of NO. Most of the low NOx burners in use today adopt this technology. The enhanced internal flue gas recirculation low NOx gas burner put into industrial use at the beginning of this century uses graded fuel and inhales the furnace flue gas in stages, forming two combustion zones that both entrain flue gas, reducing the concentration of NOx in the exhaust flue gas to 35m g/m3 or less. External flue gas return is to use an induced draft fan (generally requiring a high-temperature induced draft fan) behind the furnace (usually a high-temperature induced draft fan) to suck out the flue gas and then send it to the burner. This method of reducing NOx generation has been tested domestically at Shanghai Gaoqiao Petrochemical Company and is expected to achieve certain results. This method should be the most effective for reducing NOx emissions from oil-fired heating furnaces. Low NOx heavy oil burners were developed and put into industrial use only at the beginning of this century. They mainly adopt the method of staged air supply to reduce the amount of NOx generated, so as to achieve a significant effect of reducing nitrogen oxides by 20% to 30%. Burning heavy oil containing 0.3% nitrogen (mass fraction), conventional combustion The NOx concentration in the flue gas of air-staged low-NOx heavy oil burners is generally above 600 mg/m3, and the NOx concentration in the flue gas of air-graded low-NOx heavy oil burners is between 400 and 500 mg/m3. After 30 years of research and development, the NOx emission reduction technology of gas burners has basically been perfected. And heavy oil burners have been basically perfected. NOx emission reduction technology for oil and residual oil burners is still in its infancy. The basic concept of low NOx combustion technology is to reduce the combustion rate and the temperature of the combustion zone, thereby reducing the generation of NOx. For a heating furnace with the same heat release amount, a low NOx burner requires a larger volume. Therefore, SH /T3036 The distance from the burner center to the center of the furnace tube and the exposed furnace wall specified for low NOx burners is larger than that of conventional NOx emission burners. When conventional burners are used, these distances can be reduced. 6. Combustion air supply Burners can be divided into two categories according to their air supply methods.: Natural ventilation and forced draft. The size of the burner depends on the pressure drop or suction loss of air passing through the burner. The former usually refers to forced ventilation, and the latter refers to natural ventilation. With the air regulator fully open, at least 75% of the pressure drop or suction loss of air passing through the burner must be used to convert into kinetic energy through the burner throat to intensify Mixing of fuel and air. In addition, the size of the burner, mainly the throat size, should consider the correction of ambient temperature and atmospheric pressure when designing. The burner throat refers to the part where burner bricks are used to form constraints on the air flow path to cause turbulent mixing of fuel and air. Natural draft burners are the simplest and cheapest, and are The most commonly used oil refining heating furnace. The combustion air of the natural draft burner is driven through the burner by the suction force of the furnace. The suction loss of the air passing through the burner should be at least 40~50Pa, but it should not be too high. For the bottom-fired vertical upward-drawing heating furnace, even if there is a considerable suction force due to the high furnace, the air ventilation should be designed The suction loss through the burner should not be greater than 100Pa. This is because the design suction loss is too large, which means that the burner throat section is small and the flow rate is high. When the suction force in the furnace is insufficient, there will be insufficient air supply and incomplete combustion. When the suction force is too high, the high flow rate at the throat will cause defire, and even blow out the flame, making the combustion unstable. The air supply to the premixed gas burner used in the oil refining heating furnace is also naturally ventilated. The combustion air is sucked in through the venturi tube by fuel gas pressure, so it can operate normally even if the furnace draft is very low. It should be noted that semi-premixed gas burner The secondary air of the burner is sucked in by the furnace suction, so the furnace suction cannot be too low. The forced draft burner uses a blower to provide combustion air. The forced draft burner usually operates under the condition of air pressure exceeding 500Pa, so that the fuel and air The air is strongly mixed and combustion is enhanced. Therefore, the amount of excess air specified in SH/T3036 is 5% less in forced ventilation than in natural ventilation. Forced ventilation burners are often used in air preheating systems. When the oxygen source is supplied by a gas turbine, forced ventilation is also used. Air burner. The exhaust gas discharged from the gas turbine contains oxygen (volume fraction) 13%~17%, temperature 450~570℃, pressure 2500Pa. When pressure atomization or mechanical atomization with poor atomization effect is used to burn oil, forced atomization is often used. ventilation to improve its combustion effect. The operational disadvantage of the forced ventilation system is that its safety depends on the reliability of the fan and its driver. Failure of any one of them may cause the heating furnace and device to shut down. The solution is to add backup fans and drivers to ensure reliability, or allow the load to be reduced when the combustion air blower fails and is switched to natural ventilation operation. Most of the air preheating systems currently designed in China use natural ventilation burners. Although forced ventilation cannot be exerted in this way It has the effect of strongly mixing fuel and air, but when the fan fails, it can still switch to natural ventilation at full load operation without causing the heating furnace or device to shut down. The high-intensity burner is also a forced ventilation burner, but the volumetric heat intensity of its combustion is higher than that of ordinary The burner is much higher, generally above 10MW/m3. The pressure drop of the air passing through the burner is also much higher, generally 1000~2500Pa. Since this kind of burner is not used much in oil refining heating furnaces, it will not be introduced in detail. 7. Combustion stability Good mixing of fuel and air is one of the important conditions for stable combustion. It affects the fuel-air ratio, ignition temperature and combustion rate. The mixing energy is provided by the static pressure energy and kinetic energy of the fuel, atomizer and combustion air. The injection speed and flow direction of the fuel (including atomizer when burning oil) Can provide mixing energy. The energy of burning air in natural draft burners is very low, so they rely more on fuel energy for mixing than forced draft burners. When natural draft burners operate at low loads, due to low fuel energy and poor mixing, the stability of combustion is affected. This is why natural draft burners operate at low loads. Reasons for easy flameout. The mixing energy of the forced draft burner does not entirely rely on fuel energy. The high pressure difference of the combustion air at the throat is enough to provide the energy to cause turbulent mixing. Stable combustion requires a flame stabilization point, that is, the area in the burner where the flame is continuously ignited. The impact on combustion stability The biggest threat is "defire", that is, the flow rate of the fuel-air mixture exceeds the flame propagation speed and causes the flame to leave the stable point. Defire is usually the cause of flame extinguishing. Fuel combustion, premixed or external mixed gas combustion, all have the risk of defire. There are many measures to prevent defire, mainly the following points: 1) Place the flame root in the combustion channel, and the hot combustion channel refractory material will continuously force ignite the combustible mixture. 2) Use sudden changes in the combustion channel cross section, uneven combustion channels or flame attached walls to create vortices and make the hot flue gas return as a forcing point The heat source of combustion. The flame stabilizing flap of the radiant wall semi-premixed gas burner is a typical example. 3) Set up a burner stabilizer in the air flow at the base of the flame - a solid cone or an open hole flow limiter, which generates a vortex downstream of the burner stabilizer, causing the hot flue gas to return and force the ignition of the flame that may defire. The air flow rate of natural draft burners is very low, and a special burner stabilizer is generally not needed. Stable combustion can be obtained by using only the first two methods. Forced draft burners generally require a special burner stabilizer to achieve stable combustion. The swirl intensity generated by the burner stabilizer should be appropriate to achieve stable combustion. If the swirl intensity is too high, it will coke the fuel nozzle; if it is too small, it will not have the effect of stabilizing combustion. The swirl intensity is the ratio of the angular momentum of the airflow to the axial momentum, and is a quantitative description of the mixing and recirculation of the airflow. Another threat to the combustion stability is "backfire", that is, when the flame propagation speed exceeds that of the fuel-air mixture through the fuel injection When the nozzle speed is high, the phenomenon of flame propagating to the fuel-air mixture in the fuel nozzle occurs. It is commonly known as the flame "returns" to burn in the fuel nozzle, so it is called "backfire". Backfire generally produces backfire noise and flashback, and in severe cases, the combustion will be interrupted. Only premixed and semi-premixed gas burners have backfire. Fire hazard. The fuel of external mixed gas burners and oil burners is mixed with air outside the fuel nozzle, so there is no risk of flashback. The method to prevent flashback is based on the premise of reducing the flame propagation speed, increasing the fuel-air outflow speed, and ensuring a uniform velocity field. There are three main points:: 1) For high hydrogen fuel, especially when the hydrogen content (mole fraction) exceeds 70%, premixed and semi-premixed fuels are not recommended because the flame propagation speed of hydrogen is extremely fast. 2) The air premixed with the fuel must not be preheated, because the increase in temperature will accelerate the flame propagation speed. 3) The design of the fuel-air mixture outlet should ensure a uniform velocity field. If the velocity field is uneven, backfire will occur from the low velocity part. 8. Combustion safety protection system The simplest combustion safety protection system is to set up a permanent light. The permanent light is a small burner used to ignite the flame of the main burner. The fuel of the permanent light should be gas fuel. The fuel gas source of the permanent light should be an independent fuel source separate from the main fuel, preferably It is an independent natural gas supply system, and at least the fuel gas pipeline should be led independently from the fuel gas and water separation tank. In order to prevent rust from clogging the gas nozzle holes of the continuous light, the gas pipeline of the continuous light should be made of 18-8 stainless steel. SH/T3036 has detailed regulations on the continuous light.: “Each burner should have a gas continuous lamp. "The nominal heat release of the continuous lamp is 22kW." (The minimum heat release of the continuous lamp is 19kW according to the data.) "During the entire combustion process of the main burner, when the main burner fuel decreases, the pumping power decreases, the combustion air volume is unstable, and all other operations Under operating conditions, the continuous light should also remain stable. "The installation position of the continuous light should ensure that it can ignite any kind of fuel in the main burner." When the main fuel is temporarily interrupted, "the continuous light should be able to ignite the main burner again." From the perspective of instrument automatic control, the combustion safety protection system should be set as follows: When the heating furnace has only one burner, a redundant flame monitor (such as ultraviolet, or infrared, or ion rod) should be set up to continuously monitor the flame, and a fault alarm and emergency interlocking system should be set up. For a heating furnace with more than one burner and a continuous light, each light should be equipped with a flame ion rod. These ion rods should be connected to the interlock system and set up relevant alarm, logic and interlock protection functions. During the operation of the heating furnace, when the cut-off valve of the continuous light system is in the "open" position and it is detected that more than or equal to 50% of the continuous lights are not ignited, the flame ion rod should trigger the closing of all main fuel protection system valves. There should be a public alarm in the control room, which should alarm when one or more continuous lights are detected to be out. The alarm should have a "repeating flash" feature and will report when the first continuous light goes out. The alarm can then be alarmed again when other steady lights go out. A "voting off" switch is provided so that the operator can deactivate the steady light voting system when the furnace is operating stably. A "voting on" switch is provided to manually activate the voting system. When all main fuel gas valves are closed, the steady light voting system should be in an open state so that more than 50% of the continuous light voting systems are required to be lit when the main fuel gas valves are opened again.