Sulfur compounds in fuels

Sulfur in cmde oil is mainly present in organic compounds such as mercaptans (R-SH), sulfides (R-S-R ) and disulfides (R-S-S-R ), which are all relatively easy to desulfurize, and thiophene and its derivatives (Fig. 9.2). The latter require more severe conditions for desulfurization, particularly the substituted dibenzothiophenes, such as that shown in Fig. 9.2. Sulfur cannot be tolerated because it produces sulfuric add upon combustion, and it also poisons reforming catalysts in the refinery and automotive exhaust converters (particularly those for diesel-fueled cars). Moreover, sulfur compounds in fuels cause corrosion and have an unpleasant smell. 

For all fuel cells, except those running on high-purity hydrogen, some form of fuel treatment is required. The main problem with fuel supplies intended for conventional combustion systems is the presence of minor contaminants containing ash-making chemicals and sulfur compounds. In fuel cell applications, the sulfur compounds form corrosive substances that poison the catalysts in the reformer stages and the fuel cell itself. 

Qian, E.W., Dumeignil, F., Amano, H., and Ishihara, A. Selective removal of sulfur compounds in fuel oil by combination of oxidation and adsorption. Preprints of Papers—American Chemical Society, Division of Petroleum Chemistry, 2005, 50, 430. 

The presence of sulfur compounds in fuels is also undesirable from the perspective of catalyst poisoning during refining, and therefore much research is being conducted into desulfurization processes. Particular attention needs to be focused on aromatics which many experiments have indicated are the hardest compounds to desulfurize. 

Ranoval of Sulfur Compounds in Fuels by Adsorption Processes... 117... 

Sasaki K (2(X)8) Thermochemical stabUity of sulfur compounds in fuel cell gases related to fuel impurity poisoning. J Fuel Cell Sci Technol 5,031212-1/031212-8... 

Control of SO is intrinsic to the MHD process because of the strong chemical affinity of the potassium seed in the flow for the sulfur in the gas. Although the system is operated fuel-rich from the primary combustor to the secondary combustor, the predominant sulfur compound in the gas is sulfur... 

Sulfur It is now well established that sulfur compounds in low ppm (parts per million) concentrations in fuel gas are detrimental to MCFCs (74,75,76,77,78). The tolerance of MCFCs to sulfur compounds (74) is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, gas cleanup). The principal sulfur compound that has an adverse effect on cell performance is H2S. At atmospheric pressure and high gas utilization (-75%), <10 ppm H2S in the fuel can be tolerated at the anode (tolerance level depends on anode gas composition and partial pressure of H2), and <1 ppm SO2 is acceptable in the oxidant (74). These concentration limits increase when the temperature increases, but they decrease at increasing pressures. 

In the context of the desirability of removing sulfur compounds from fuels, a bacterial strain has been identified that will metabolize thianthrene to water-soluble products under aerobic conditions (83MI5). A thermophilic organism, Sulfolobus acidocaldarius, removed 38% of the sulfur, as measured by sulfate release, in 4 weeks at 70°C (87MI2). 

The replacement of conventional catalytic processes for purification of mixtures by processes involving extraction with ionic liquids has also been envisioned. For example, the extractive removal of sulfur compounds from fuels has shown promising leads for high selectivity for aromatic sulfur compounds, and removal of such compounds has been a major challenge in conventional heterogeneous hydrodesulfurization catalysis (27,296). 

Copper can be present in fuel systems in the form of heating coils, cooling coils, brass fittings, or bronze parts. Copper is quite resistant to corrosion by water but can be attacked by ammonia and sulfur compounds. Finished fuels usually do not contain ammonia unless the ammonia somehow carries over from refining process operations. Sulfur compounds such as hydrogen sulfide and possibly elemental sulfur are more frequently the cause of copper corrosion problems in fuel systems. 

Hydrogen sulfide, mercaptans, active elemental sulfur, inorganic acids, and ammonia can all attack and corrode copper. The presence of these compounds in fuel can lead to destruction of copper heating lines, cooling coils, and nonferrous metal fittings. Also, hydrogen sulfide and mercaptans can contribute to fuel odor problems. 

High concentration of olefins or oxygen-, sulfur-, and nitrogen-containing compounds in fuel... 

Plants to process alternate sources of energy will make low-sulfur fuels from high-sulfur feeds such as coal, oil shale, tar sands, or heavy oil. Elemental sulfur will be a normal byproduct. Nearly all processing schemes first produce H2S from the sulfur compounds in the raw material, and then convert H2S to elemental sulfur. Usually this conversion step is labelled "Claus Process". 

Content of Sulfur Compounds in Diesel Fuel and Their Pseudo-First-Order Reaction Constants... 

These results show that, in equimolar concentrations, naphthalene would not be considered as a strong inhibitor toward direct sulfur extraction (A Do) for PASCs. However, as discussed earlier, the content of di- and trinuclear aromatics in diesel fuels and gas oils can be as high as 20-30%, whereas the level of sulfur compounds in today s diesel fuels is only 0.2% sulfur, or about 1 wt% PASCs. So the competition for the active site by aromatic hydrocarbons is very strong. Their effect on the direct desulfurization route will lower the rate to about one-third of the noninhibited rate in the case of dibenzothiophene and would lower that of 4,6-DMDBT even more. 

Elemental sulfur is obtained from underground deposits and recovered from natural gas and crude oil. Sulfur is removed from these fuels prior to burning in order to prevent pollution of the air with S02 and subsequent formation of acid rain (see the Interlude in Chapter 15). The sulfur compounds in gas and oil are first converted to H2S, one-third of which is then burned to give S02. Subsequent reaction of the S02 with the remaining H2S yields elemental sulfur ... 

The relative importance attached to sulfur compounds in petroleum may, at first, seem unwarranted, but the presence of sulfur compounds in any crude oil can only result in harmful effects. For example, the presence of sulfur compounds in finished petroleum products such as gasoline will cause corrosion of engine parts, especially under winter conditions when water containing sulfur dioxide (from the internal combustion) may collect in the crankcase. On the other hand, mercaptans cause the corrosion of copper and brass in the presence of air and also have an adverse effect on the color stability of gasoline and other liquid fuels. 

Removal of sulfur compounds from fuel oils is usually accomplished at the refinery. Although most of the sulfur in oil is in the high boiling fraction, some middle distillates have high sulfur contents. A better understanding of the chemistry of sulfur compounds in oil will also facilitate their efficient removal. 

Stringent regulations have been placed on the use of these fuels through the years these regulations have been tightened further and this process may well continue, in response to acid deposition issues, for example. Therefore, as a specific conclusion, it becomes increasingly important to characterize the sulfur compounds in fossil fuels in order to facilitate their efficient removal (38, 39). ... 

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