H2S are burned in lances surrounding a central muffle by the following chemical reaction: 2 H2S + 3 O2 → 2 SO2 + 2 H2O (ΔH = -518 kJ mol−1) This is a strongly exothermic free-flame total oxidation of hydrogen sulfide generating sulfur dioxide that reacts away in subsequent reactions. The most important one is the Claus reaction: 2 H2S + SO2 → 3 S + 2 H2O The overall equation is: 2 H2S + O2 → 2 S + 2 H2O The temperature inside Claus furnace is often maintained above 1050°C. This ensures BTEX (Benzene, Toluene, Ethyl benzene and Xylene) destruction which otherwise would clog downstream Claus catalyst. Gases containing ammonia, such as the gas from the refinery's sour water stripper (SWS), or hydrocarbons are converted in the burner muffle. Sufficient air is injected into the muffle for the complete combustion of all hydrocarbons and ammonia. The air to the acid gas ratio is controlled such that in total 1/3 of all hydrogen sulfide (H2S) is converted to SO2. This ensures a stoichiometric reaction for the Claus reaction in the second catalytic step (see next section below). The separation of the combustion processes ensures an accurate dosage of the required air volume needed as a function of the feed gas composition. To reduce the process gas volume or obtain higher combustion temperatures, the air requirement can also be covered by injecting pure oxygen. Several technologies utilizing high-level and low-level oxygen enrichment are available in industry, which requires the use of a special burner in the reaction furnace for this process option. Usually, 60 to 70% of the total amount of elemental sulfur produced in the process is obtained in the thermal process step. The main portion of the hot gas from the combustion chamber flows through the tube of the process gas cooler and is cooled down such that the sulfur formed in the reaction step condenses. The heat given off by the process gas and the condensation heat evolved are utilized to produce medium or low-pressure steam. The condensed sulfur is removed at the liquid outlet section of the process gas cooler. The sulfur forms in the thermal phase as highly reactive S2 diradicals which combine exclusively to the S8 allotrope: 4 S2 → S8 Side reactions Other chemical processes taking place in the thermal step of the Claus reaction are: The formation of hydrogen gas: 2 H2S