In a chemical plant, we often come across a “stack” standing beside the factory building, constantly emitting flames. Some might wonder if this continuous burning doesn’t consume a lot of fuel and is truly a pity. Indeed, this raises a question: Why do they do this?
In fact, that was not the ordinary chimney we usually see. Its official name is flare stack system.
Why is the flare stack system necessary?
During the production process in a chemical plant, numerous flammable and explosive gases, most of which are harmful to human health, are inevitably produced. Most of these gases are heavier than air. If they are directly discharged into the atmosphere, they will gradually accumulate on the ground and reach high concentrations, causing serious environmental pollution and threatening people’s health. What is even more worrying is that once exposed to a fire source, they may cause fires or even explosions.
To prevent these potential risks, people choose to concentrate these harmful gases and burn them for treatment. The flare system is precisely designed for this purpose. It can efficiently convert these flammable, explosive, toxic and corrosive gases into harmless carbon dioxide, water or other non-toxic and low-toxic substances.
Although direct combustion of fuel does involve a waste of resources, factories are still actively seeking technical means to recover the flare gas in an attempt to convert it into reusable energy. However, even though the recovery technology is quite mature, the safety flare remains indispensable in chemical production. In the event of an accident, it is still the key facility that ensures safety, capable of quickly and effectively burning large amounts of flammable gas to guarantee production safety.
Therefore, the flare stack system is regarded as the last line of defense in the safety system of the chemical plant.
Why are the shapes of the flare stack system different?
This is mainly because there are two main types of flare system: elevated flare system and ground flare system, each with its own unique features and applicable scenarios.
The combustion process mainly takes place near the ground. The type of the flare system may adopt the traditional unsealed burner or a steel sealed structure with a refractory lining, and it is equipped with multiple burners inside. This design has the advantages of low operating costs and convenient maintenance. However, the processing capacity of ground-based flare systems is relatively small, and they cannot burn toxic gases. At the same time, the construction cost is also relatively high.
On the other hand, the flames of the elevated system burn at the top of the flare stack, far above the ground. At the top of the flare stack, there is a special flare tip equipped with a perpetually burning light. Once the perpetually burning light is ignited by the igniter, it will continue to burn. The exhaust gas reaches the flare tip and is immediately ignited. The elevated flare system performs well in handling a large amount of exhaust gas and has a lower construction cost. However, it should be noted that due to the fast exhaust gas outlet speed, the noise generated during combustion cannot be avoided. At the same time, the high-intensity flames and heat radiation at the flare tip pose a significant threat to nearby workers and equipment. In addition, the elevated flare system can be further divided into various types such as self-supported, derrick supported, and guyed-supported flares types.
Comparative analysis of self-supported, derrick supported, and guyed-supported flares
Self-supported flare systems have attracted considerable attention for their unique structural design. Requiring no external support, these flares rely on their inherent mechanical properties for stability, saving material costs while ensuring ease of use. However, their design and manufacturing are relatively complex, placing stringent demands on material properties.
In contrast, a guyed-supported flare system utilizes a cable system for stable support. This design offers greater flexibility, adapting to complex terrain and harsh environments. It also offers high stability and safety, ensuring long-term, reliable operation. However, it should be noted that the cable system can be relatively complex to maintain and service.
Another common option is a derrick-supported flare system. These towers offer stable support, resulting in a robust structure and high load-bearing capacity. Furthermore, the tower design effectively reduces noise and heat radiation during combustion, ensuring the safety of personnel and equipment. However, tower construction and maintenance costs are relatively high, requiring careful consideration of the overall project budget and requirements.
What types of gases are treated by flare stack system?
The range of gases treated by flare system is wide, depending on the specific plant location and process requirements. In refineries, flares are commonly used to treat hydrogen sulfide gas in sulfur plants. Flare stack system also treat a variety of other gases, including refinery purge and waste gases, non-recoverable gases discharged with crude oil from oil wells, blast furnace exhaust, unusable gases from coke ovens, and waste gases from chemical processes. These gases have complex compositions and may contain low-molecular-weight, high-calorific value hydrocarbons, as well as low-calorific value inert components and CO. Flares are also widely used to combust waste gases from sites such as sewers, coal gasification, rocket engine testing, nuclear power plants equipped with sodium/water heat exchangers, heavy water plants, and ammonia fertilizer plants.
What does a typical flare system look like?
A typical flare system consists of several key components. First, it includes a gas header and a network of pipes that collect gases from various process units. In addition, the system is equipped with a buffer tank (or de-entrainment tank) for removing and storing condensable or entrained liquids. To prevent exhaust gas backflow, dedicated sealing tanks, water seal tanks, or corresponding facilities are also essential. The flare system also includes single or multiple burners and flare tubes, as well as gas pilots and igniters when necessary to ignite the exhaust gas and air mixture. For smokeless flares, external power delivery, such as steam purge or forced ventilation, is also required. Natural gas, fuel gas, inert gas, or nitrogen can all be used as purge gas.
What is the structural composition of a flare system?
Next, we’ll explore the specific structural components of an elevated, non-detachable flare. First, the flare tip, one of the core components of the flare system.
The primary function of a flare tip is to ensure efficient and complete combustion of exhaust gases. Its design varies depending on the application. Common designs include straight-tube flare tips, commonly used in natural gas flares and coke oven gas flares, while inverted-cone flare tips are more commonly found in processes such as methanol and ammonia production using coal-water slurry as feedstock.
The photo shows the flare tip used in the 300,000 ton/year coal-to-ammonia unit. Its unique design features smoke suppression. Smoke suppression steam pipes are located both inside and outside the flare tip, a feature commonly found in petrochemical and refining industries. This ensures efficient and environmentally friendly combustion of waste gases from the production process.
The picture shows a flare tip with smoke suppression. Its sophisticated design features an igniter, ensuring rapid and safe flare ignition during production, thus ensuring efficient operation of the unit. This design plays a crucial role in chemical production, such as coal-to-ammonia production.
The flare igniter, an automatic ignition tool, plays a key role in chemical production thanks to its reliable ignition performance and long service life. Its specially designed ignition end is not only heat-resistant and anti-coking, but also self-cleaning, ensuring a stable and safe ignition process.
When the flare pressure reaches the preset value, the ignition control system activates and sends power to the high-voltage generator, raising the voltage to 10,000V before transmitting it to the igniter. This energizes the solenoid valve, opening gas to the ignition nozzle, and the igniter generates an arc, completing the ignition process.
When using the igniter, please note the following: First, the ignition device should be turned on before the oil (gas) injection ignition operation. The ignition time should generally not exceed 30 seconds. Second, before lighting the flare, it is crucial to verify the process and ensure that all gas valves except the starter igniter and pilot light are closed. Opening unrelated gas valves during the ignition process is strictly prohibited.
Molecular seals are also a crucial safety feature. During flare gas discharge, they utilize the force of another gas to effectively prevent the mixture of exhaust gas and air from returning to the flare, thereby preventing potential explosions. Labyrinth-type molecular seals are the most common type, suitable for gases lighter than air (such as nitrogen).
When using gases heavier than air, such as carbon dioxide, the labyrinth-type molecular seal requires an adjustment, requiring the structure to be inverted.
The flare body is a straight steel cylinder with flanges at both ends. Typically, its lower portion is connected to a vertical water seal tank, and its upper portion is connected to a molecular seal. This design effectively transfers the weight of the flare head and molecular seal.
Carbon steel is the most commonly used material for the flare body. However, thickened carbon steel or stainless steel may also be used depending on specific requirements. For flare systems handling acetic acid, the use of high-quality 316L stainless steel is particularly important.
Next, we’ll discuss another key component: the water seal tank.
Its core function is to prevent flashback and explosion risks in flare systems. Water seal tanks are typically constructed of carbon steel, though stainless steel is also sometimes used.
We’ll also further discuss the separator tank, a key component. The separator tank is designed to effectively separate droplets with diameters between 300μm and 600μm from the exhaust gas, thereby preventing the so-called “fire rain” phenomenon.
How are flare system flue gases removed?
To ensure complete combustion of these gases, sufficient combustion air must be provided and the flue gas and air must be mixed in the correct ratio. However, smoke may be generated during combustion, depending on the flue gas composition, flow rate, and distribution of the combustion air. Generally, flue gases containing methane, hydrogen, carbon monoxide, and ammonia do not produce smoke when burned; however, flue gases containing heavier hydrocarbons, such as straight-chain alkanes, hydrocarbons, and aromatics, do produce smoke when burned.
Elevated flare system typically use external auxiliary measures to abate flue gases, such as steam purges or air blowers, to enhance gas/flue gas mixing and agitation. Water sprays, high-speed vortexes, or natural gas injection can also be used to further suppress smoke. In contrast, ground flares rarely use these auxiliary measures for smoke suppression.
Why do plants need two flare systems?
Many plants have two flare systems, either arranged in parallel or in series. In a parallel flare system, if one flare is shut down for maintenance, the other can remain operational, ensuring continuous production. In a series flare system, a low-profile ground flare and an elevated flare are typically used. Ground flares are primarily used to handle low gas flows during routine operations, while elevated flares are designed for emergency situations or high-volume gas discharges.
Negative Impacts of Flare system
While noise and heat are unavoidable during flare system operation, measures can be taken to minimize these effects. Typically, flare system are located away from residential areas or in well-isolated areas.
Flare system emissions primarily consist of carbonaceous particulates (such as soot), unburned hydrocarbons, carbon monoxide (CO), and partially burned and converted hydrocarbons. If the flared gas contains sulfur compounds, such as hydrogen sulfide or mercaptans, sulfur dioxide (SO2) may also be present.
The hydrocarbon content in flare emissions is closely related to the degree of combustion, which in turn is influenced by the fuel-air ratio and mixing, specifically the flame temperature and stability. A properly operated flare system can achieve a burnout rate exceeding 98%, meaning that the hydrocarbon and CO content in the emissions is less than 2% of the total hydrocarbon gas flow.
After learning all the information about the flare system, are you ready to start building your own flare system? Ruichang is always ready to help you provide the most professional customized services for flare stack system.
