Direct thermal splitting of H2S into H2 and S within a separate ceramic heat exchange chamber, using only heat generated by combustion of the furnace feed gas, has been successfully demonstrated in the field using a mini-reaction furnace pilot facility. This process can be utilized in existing Claus front-end furnaces to thermally crack part of the acid gas feed to these units, so as to produce H, as well as sulfur. Modelling studies have shown that stoichiometric (i.e. 1/3) combustion of a rich (90% H2S) acid gas feed can support a net 25% cracking of all the feed H2S without affecting the primary sulfur recovery function of the unit. This result is conditional on recycling the uncracked H,S, after removal of the H2 back to the front-end furnace for normal Claus processing.
Recovery of elemental sulfur from acid gas during sour natural gas processing and refinery upgrading, is carried out on a worldwide scale using the modified-Claus reaction. This process, illustrated as eqns 1 to 3. converts H2S into S and H2O using a combination of thermal (eqn 1) and catalytic (eqn 2) steps.
Equation (1) (Available in full paper)
Equation (2) (Available in full paper)
Equation (3) (Available in full paper)
The initial combustion of up to a third of the H2O in the feed to produce SO2 within the front-end furnace of a Claus plant typically gives rise to temperatures of 1000 ° to 1200 ° C This non-essential process heat must be removed in a waste heat boiler prior to the subsequent catalytic stage. This step is necessary since the latter stage is operated at significantly lower temperatures in the 200 ° -315 ° C range in order to take advantage of equilibrium conversion. Thus the large amount of heat generated in the front-end furnace is currently only utilized to generate high pressure stearn.
While traditional Claus sulfur recovery is unarguably a highly efficient process for recovering elemental sulfur from hydrogen sulfide (>99% recovery in some cases), conversion of all the hydrogen present in the H2S feed into H2O, is wasteful. A modification to this process which would allow for recovery of part of this hydrogen in the form of molecular H2 would be attractive by virtue of providing two valuable products, in place of just sulfur. Such a scheme is in fact possible by taking advantage of the temperatures that normally exist in the front-end reaction furnace to directly crack part of the H2S feed into its elements in order to produce both H2 and sulfur (eqn 4).
Equation (4) (Available in full paper)
A key incentive for pursuing the production of H2, from H2S using such a scheme rests with the rising demand seen for H2 within refineries in the near future in order to carry out deeper desulfurization of higher sulfur feedstocks. This trend is one that has been established as part of the regulatory calls to lower sulfur fuels in order to meet more stringent emission standards.