Hydrogen Sulfide Combustion:  Relevant Issues under Claus Furnace Conditions

The major chemical paths for the combustion of hydrogen sulfide under conditions typical of the Claus furnace (i.e., fuel-rich conditions) are presented. The manuscript begins with a brief survey of recently published research that involves sulfur chemistry in high-temperature environments, including the results of sensitivity analysis for some of the systems involved. Recommended values for the heats of formation of sulfur species are included. The reaction mechanism that is presented consists of more than 150 reactions. Issues such as the formation and destruction of COS and CS2 are presented: new chemical paths for the formation of COS and CS2 (not involving elementary carbon) are illustrated, on the basis of sound thermochemical and kinetic considerations. The formation of COS and CS2 is of great importance in the design of sulfur plants in industry. Possible reactions of COS and CS2 with SO2, and CO2 with H2S and sulfur species, also are discussed, prompted by experimental observations in flow reactors. The mechanism can explain the formation of hydrogen, which also is an important issue in sulfur plant design and associated tail gas units. Species such as H2S2 seem to have an important role during the combustion of hydrogen sulfide. Higher-molecular-weight linear H2Sx species are also considered, and it is concluded that their role is possibly minor. The chemical steps leading to the formation of Sx species by molecular growth are presented. The ring structure of some of the Sx species is discussed, as well as intramolecular ring conversions for S8, S7, S6, and S5. The possibility of H,OH radical recombination catalyzed by oxygenated sulfur species may explain the delayed oxidation of hydrocarbon species in the Claus furnace that has been observed in previous experiments by other authors. This could be an important design consideration for Claus plants to minimize the coking of catalyst beds in the process. The most likely chemical paths for the radical quench are presented and based on past observations. Controversy persists in regard to the actual mechanism and the rate constants of the reactions involved in the radical recombination, as well as the thermochemistry of some of the oxygenated sulfur species involved. More studies are needed to resolve the issues. The study also reveals the lack of high-temperature data for the kinetic coefficients of some of the reactions. Much rate data are based on atmospheric studies, rather than high-temperature oxidation. Similarly, better thermodynamic data are lacking for some important oxygenated sulfur species in the mechanism. This is most important for temperature- and pressure-dependent reactions, such as unimolecular reactions and chemically activated reactions. Studies that involve hydrogen sulfide flames under fuel-rich conditions are lacking. Most of the studies have been limited to the impact of sulfur species on the formation of other species, such as CO and NOx, in flames or reactors.