Design issues for acidic gas flares
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As the title suggests, I recently encountered a design for a flare for acidic gases; I haven’t worked with this type of flare before. I hope everyone can explain the structure of such flares and what considerations need to be taken into account in their design compared to those for ordinary fuels
The design of acid gas flares requires attention to the following points: 1. The water in the acid gas separation tank and the water seal tank must be able to flow by gravity to the low-level tank, where it can then be pumped away or pushed out using nitrogen; the water in these tanks must not be discharged directly into the sewer system； 2. Acid gas flares must be equipped with gas for blending combustion and gas for co-combustion. Burner gas is supplied to the top of the flare to ensure that the temperature of the acidic gas when it is flared exceeds 650°C. By mixing burner gas with the acidic gas, its calorific value is increased, thereby ensuring the appropriate temperature for the combustion of the acidic gas ； 3. The acid gas flare pipeline network must be equipped with heat tracing to maintain a temperature of over 80°C, thereby preventing the crystallization of sulfammonium salts in the acid gas flare lines ； 4. After the acidic gas is vented through the flare, it should be displaced with nitrogen to remove all acidic gas from the pipelines.
The venting system is an auxiliary system that ensures the safe operation of process equipment. Used to vent hydrocarbon gases and acidic gases. The torch includes a high-pressure vent system and a low-pressure vent system. It mainly handles the raw gas and acid gas emitted during plant startup, shutdown, and in case of emergency accidents. The emergency venting of sulfur-containing natural gas in the desulfurization unit belongs to the high-pressure venting system, with a venting pressure of 0.4 MPa (gauge). The emergency venting of acidic gases from desulfurization units and sulfur recovery units belongs to the low-pressure venting system, with a venting pressure of <0.025 MPa (gauge). The torch is 30 meters tall and has two burners at its top; it is constructed of stainless steel. At its base there is a combined gas-water separator. The torch is designed to operate at full gas capacity and is equipped with a flame arrestor to prevent backflow of flames. A stainless steel wind shield is installed at the top of the torch to protect the burner and the steady flame from being blown out by the wind, while also facilitating natural ventilation for the main burner to ensure proper combustion.
Since the acidic gas flare is a facility that does not operate under normal conditions, is its fire protection distance the same as that of the plant-wide flares? What is the specific basis?
As a supplementary note: the materials used for equipment in acidic flares are different from those used for ordinary vent gases. For acidic gas flares, 304 and 310 grades of material are typically required for the flare nozzles, and there are also specific requirements for the materials used in the liquid separation tanks and water seal tanks!
Refer to SH3029-1991 Code for Design of Exhaust Stack and Flare Towers in Petrochemical Enterprises
“Could you explain in detail the reason for ensuring that the temperature of acidic gases when they are released into the flare reaches over 650°C? If the calorific value of the acidic gas is greater than 2000 kcal/Nm3, is it still necessary to add combustion gas? Thank you!
Acid gas flares generally require a pilot flame for operation, and it is better if they can burn alongside another fuel. As for why a temperature of 650 degrees is required, it depends mainly on the combustion temperature of hydrogen sulfide.