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Resonantly stabilized radicals, such as propargyl, cyclopentadienyl, benzyl, and indenyl, play a vital role in the formation and growth of polycyclic aromatic hydrocarbons (PAHs) that are soot precursors in engines and flames. Pyrene is considered to be an important PAH, as it is thought to nucleate soot particles, but its formation pathways are not well known. This paper presents a reaction mechanism for the formation of four-ring aromatics, pyrene and fluoranthene, through the combination of benzyl and indenyl radicals. The intermediate species and transition structures involved in the elementary reactions of the mechanism were studied using density functional theory, and the reaction kinetics were evaluated using transition state theory. The barrierless addition of benzyl and indenyl to form the adduct, 1-benzyl-1H-indene, was found to be exothermic with a reaction energy of 204.2 kJ mol. The decomposition of this adduct through H-abstraction and H-loss was studied to determine the possible products. The rate-of-production analysis was conducted to determine the most favourable reactions for pyrene and fluoranthene formation. The premixed laminar flames of toluene, ethylbenzene, and benzene were simulated using a well-validated hydrocarbon fuel mechanism with detailed PAH chemistry after adding the proposed reactions to it. The computed and experimentally observed species profiles were compared to determine the effect of the new reactions for pyrene and fluoranthene formation on their concentration profiles. The role of benzyl and indenyl combination in PAH formation and growth is highlighted.

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Acid gas (H 2 S and CO 2 ) is produced in large volumes worldwide from the desulfurization of hydrocarbon fuels and is utilized in sulfur recovery units (SRUs) to produce sulfur. However, the hydrogen content of acid gas is wasted as lowgrade steam, which highlights the need for the efficient utilization of this resource. The production of H 2 from acid gas is desired, as it is an inexpensive feedstock. In this work, a kinetic study is conducted on H 2 production from acid gas in an industrial SRU to utilize its built-in inertia, while saving on the capital cost and enhancing the processing capacity of the SRU. The thermal energy generated during the combustion of acid gas in the reaction furnace (RF) is used for acid gas pyrolysis in the waste heat boiler (WHB) of the SRU. While this technique has been investigated previously, its realization at the industrial scale is hindered by low H 2 yield. This paper presents suitable means of enhancing H 2 production via operational modifications in RF and WHB. A detailed reaction mechanism, developed for acid gas combustion and pyrolysis and validated using experimental data from industrial furnaces and reactors, is used for the kinetic simulations of the SRU thermal unit. The results show that RF operational changes such as the extent of H 2 S oxidation and feed preheating can increase H 2 yield from 3% to 38% in the WHB without changing the composition of the acid gas stream. This significant improvement in H 2 yield can help in realizing its production from acid gas in SRUs.

An increase in the exploitation of sour reservoirs with a high content of acid gas (mainly H 2 S and CO 2 ) and the enforcement of stringent emission standards have triggered demand for high sulfur recovery efficiency in sulfur recovery units (SRUs). The high amount of H 2 O that is present in feed gases to the SRU can act as a diluent in the thermal section of the SRU and can also pose operational difficulties and have substantial chemical effect on the equilibrium-limited reactions of sulfur production and the destruction of impurities. This paper explores the role of H 2 O (0−12% in the furnace feed) in sulfur recovery and in the fate of undesirable aromatics, COS, and CS 2 in the thermal section using a detailed reaction mechanism and industrial SRU operating parameters. An increasing H 2 O concentration in the feed decreased the sulfur recovery efficiency in the thermal section significantly, as the equilibrium conversion of H 2 S to sulfur was affected. The higher H 2 O concentrations increased the concentrations of H 2 S, H 2 , and SO 2 at the furnace exit because of the production of OH radicals from enhanced chemical decomposition of H 2 O by active radicals such as SH, O, and H. It also increased the quantity of unwanted aromatics exiting the Claus furnace, but COS and CS 2 production declined because of the fall in the decomposition of CO 2 and feed hydrocarbons as the furnace temperature decreased. This study provides guidelines to increase sulfur production in the thermal section and reduce the number of catalytic stages to decrease SRU operational cost.