→ S2 + 2 H2 (ΔH > 0) CH4 + 2 H2O → CO2 + 4 H2 The formation of carbonyl sulfide: H2S + CO2 → S=C=O + H2O The formation of carbon disulfide: CH4 + 2 S2 → S=C=S + 2 H2S Catalytic step The Claus reaction continues in the catalytic step with activated aluminum(III) or titanium(IV) oxide, and serves to boost the sulfur yield. More hydrogen sulfide (H2S) reacts with the SO2 formed during combustion in the reaction furnace in the Claus reaction, and results in gaseous, elemental sulfur. 2 H2S + SO2 → 3 S + 2 H2O (ΔH = -1165.6 kJ mol−1) One suggested mechanism is that S6 and S8 desorb from the catalyst's active sites with simultaneous formation of stable cyclic elemental sulfur. The catalytic recovery of sulfur consists of three substeps: heating, catalytic reaction and cooling plus condensation. These three steps are normally repeated a maximum of three times. Where an incineration or tail-gas treatment unit (TGTU) is added downstream of the Claus plant, only two catalytic stages are usually installed. The first process step in the catalytic stage is the gas heating process. It is necessary to prevent sulfur condensation in the catalyst bed, which can lead to catalyst fouling. The required bed operating temperature in the individual catalytic stages is achieved by heating the process gas in a reheater until the desired operating bed temperature is reached. Several methods of reheating are used in industry: Hot-gas bypass: which involves mixing the two process gas streams from the process gas cooler (cold gas) and the bypass (hot gas) from the first pass of the waste-heat boiler. Indirect steam reheaters: the gas can also be heated with high-pressure steam in a heat exchanger. Gas/gas exchangers: whereby the cooled gas from the process gas cooler is indirectly heated from the hot gas coming out of an upstream catalytic reactor in a gas-to-gas exchanger. Direct-fired heaters: fired reheaters utilizing acid gas or fuel gas, which is burned substoichiometrically to avoid oxygen breakthrough which can damage Claus catalyst. The typically recommended operating temperature of the first catalyst stage is 315 °C to 330 °C (bottom bed temperature). The high temperature in the first stage also helps to hydrolyze COS and CS2, which is formed in the furnace and would not otherwise be converted in the modified Claus process. The catalytic conversion is maximized at lower temperatures, but