The production of high-quality sulfur starts with clean raw materials. The type, content and source of impurities in acid gas exert varying impacts on the operation of sulfur recovery units and finished sulfur products. Typical impurities include hydrocarbons, CO₂, H₂O and NH₃. Strict control of these impurity indicators is a must to manufacture high-grade sulfur.
1. Hydrocarbons and Amine Solvents
Entrainment of hydrocarbons and amine solvents in acid gas entering the furnace raises furnace temperature, increases heat load and thermal stress of waste heat boilers, and requires higher combustion air supply. The CO₂ and H₂O generated from combustion lower the partial pressure of H₂S and inhibit the Claus reaction. Higher hydrocarbon content leads to increased by-product COS and CS₂, reducing sulfur conversion efficiency.
Fluctuations in hydrocarbon concentration cause delayed air distribution and local oxygen deficiency. Heavy hydrocarbons, aromatic hydrocarbons and amine solvents crack under oxygen-lacking conditions to form carbon deposits, which contaminate sulfur products, clog catalyst beds, deactivate catalysts and raise system pressure drop. Severe blockages under extreme operating conditions may trigger unplanned shutdowns and extend carbon-burning maintenance cycles.
Control Specification: Hydrocarbon content in acid gas ≤ 3 vol%.
2. Carbon Dioxide (CO₂)
As an inert component, CO₂ reduces the partial pressure of H₂S and furnace flame temperature, boosting the generation of COS and CS₂ in process gas. The formation of organic sulfur is positively correlated with CO₂ and hydrocarbon concentrations. Incomplete hydrolysis of organic sulfur in the low-temperature catalytic section will simultaneously lower unit sulfur conversion and sulfur recovery rate, making high-purity sulfur unattainable.
3. Water Vapor
Though its inhibitory effect is milder than ammonia and CO₂, water vapor alters component partial pressure inside reactors and weakens Claus reaction efficiency, cutting sulfur output.
No water vapor is allowed to enter the furnace under normal operation. Front-end water carry-in causes sharp furnace temperature drop, furnace pressure surge and refractory lining damage; water leakage from waste heat boilers directly results in catalyst deactivation.
Control Specification: Water vapor content in acid gas: 2 vol% ~ 5 vol%.
4. Ammonia (NH₃)
Ammonia entrainment in feedstock brings severe adverse impacts:
①Ammonium salt crystals form and block front-end pipelines and equipment, hindering acid gas transportation;
②Combustion products N₂ and H₂O reduce H₂S partial pressure and sulfur recovery rate;
③Incomplete ammonia combustion generates NOₓ. Under aerobic conditions, NOₓ promotes the conversion of SO₂ to SO₃, forming sulfate crystals that block condensers at low-temperature zones, drastically lifting system pressure drop and even forcing unit shutdown;
④Ammonia reacts with alumina catalysts to deactivate them, while NOₓ accelerates equipment corrosion and catalyst poisoning;
⑤Ammonia accumulates in SCOT amine solution, weakening H₂S absorption in the absorber and desorption performance in the regenerator.
All above issues degrade total sulfur recovery efficiency.Control Specification: Ammonia content in feed gas ≤ 3 vol%.
5. Methanol
Methanol entrainment is a key control point for sulfur recovery units of coal chemical plants. Acid gas is prone to carry large volumes of methanol under fluctuating working conditions or abnormal incidents. Operators shall timely adjust combustion air supply to avoid carbon precipitation due to oxygen shortage, preventing carbon deposits from contaminating finished sulfur and clogging catalytic beds.