Oxidation fuel-sulfur

Lamp Method the sample is burned in a closed system in an atmosphere of 70% CO2 and 30% oxygen in order to avoid formation of nitrogen oxides. This method was to have been abandoned as it takes three hours to carry out, but remains officially required for jet fuel sulfur analysis. 

Partial oxidation of heavy Hquid hydrocarbons requires somewhat simpler environmental controls. The principal source of particulates is carbon, or soot, formed by the high temperature of the oxidation step. The soot is scmbbed from the raw synthesis gas and either recycled back to the gasifier, or recovered as soHd peUetized fuel. Sulfur and condensate treatment is similar in principle to that required for coal gasification, although the amounts of potential poUutants generated are usually less. 

Exhaust emissions of CO, unbumed hydrocarbons, and nitrogen oxides reflect combustion conditions rather than fuel properties. The only fuel component that degrades exhaust is sulfur the SO2 concentrations ia emissions are directly proportional to the content of bound sulfur ia the fuel. Sulfur concentrations ia fuel are determined by cmde type and desulfurization processes. Specifications for aircraft fuels impose limits of 3000 —4000 ppm total sulfur but the average is half of these values. Sulfur content ia heavier fuels is determined by legal limits on stack emissions. 

Lead compounds were not found on the surrounding activated coating layer, rather only associated with the precious metal. The Pt sites are less poisoned by lead than are Pd or Rh sites because the Pt sites are protected by the sulfur in the fuel. Fuel sulfur is converted to SO2 in the combustion process, and Pt easily oxidizes SO2 to SO on the catalyst site. The SO reacts with the lead compounds to form PbSO, which then moves off the catalyst site so that lead sulfate is not a severe catalyst poison. Neither Pd nor Rh is as active for the SO2 to SO reaction, and therefore do not enjoy the same protection as Pt. 

Sulfur oxides resulting from fuel sulfur combustion often inhibit catalyst performance in Regions II, III, and a portion of Region IV (see Fig. 7) depending on the precious metals employed in the catalyst and on the air/fuel ratio. Monolithic catalysts generally recover performance when lower sulfur gasoline is used so the inhibition is temporary. Pd is more susceptible than Rh or Pt. The last is the most resistant. Pd-containing catalysts located in hotter exhaust stream locations, ie, close to the exhaust manifold, function with Httie sulfur inhibition (72—74). 

Fuel sulfur is also responsible for a phenomena known as storage and release of sulfur compounds. Sulfur oxides (S02,S02) easily react with ceria, an oxygen storage compound incorporated into most TWC catalysts, and also with alumina. When the air/fuel mixture temporarily goes rich and the catalyst temperature is in a certain range, the stored sulfur is released as H2S yielding a rotten egg odor to the exhaust. A small amount of nickel oxide incorporated into the TWC removes the H2S and releases it later as SO2 (75—79). 

Similarly, SO2 and SO3 (SOJ compounds are produced in combustion by the oxidation of sulfur compounds within the fuel source. SO , emitted into the atmosphere can be incorporated into aerosol particles and wet-deposited as corrosive sulfuric acid. Both NO , and SO , emissions contribute to acid rain content from wet deposition, due to their participation in the formation of nitric and sulfuric acid, respectively. 

SOx - Oxides of sulfur (SO2 and S03) are formed in the combustion of fuels containing sulfur and as a byproduct of chemicals production. 

Acid rain is caused primarily by sulfur dioxide emissions from burning fossil fuels such as coal, oil, and natural gas. Sulfur is an impurity in these fuels for example, coal typically contains 2-3% by weight sulfur.1M Other sources of sulfur include the industrial smelting of metal sulfide ores to produce the elemental metal and, in some parts of the world, volcanic eruptions. When fossils fuels are burned, sulfur is oxidized to sulfur dioxide (SO2) and trace amounts of sulfur trioxide (SC>3)J21 The release of sulfur dioxide and sulfur trioxide emissions to the atmosphere is the major source of acid rain. These gases combine with oxygen and water vapor to form a fine mist of sulfuric acid that settles on land, on vegetation, and in the ocean. 

FIGURE 2.28 Voltage drop due to sulfur poisoning versus CO concentration for an SOFC button cell operated on a HrCO fuel mixture containing 5 ppm H2S at 1000°C with current density of 200 mA/cm2. (From Sasaki, K. et al., Proceedings of the Ninth International Symposium on Solid Oxide Fuel Cells, 2005-07 1267-1275. Reproduced by permission of ECS-The Electrochemical Society.)... 

Summary of Previous Studies on Potential Sulfur-Tolerant Anode Materials for Solid Oxide Fuel Cells... 

Gong M, Liu X, Trembly J, and Johnson C. Sulfur-tolerant anode materials for solid oxide fuel cell application. J Power Sources 2007 168 289-298. 

Matsuzaki Y and Yasuda I. Effect of a sulfur-containing impurity on electrochemical properties of a Ni-YSZ cermet electrode. In Yokokawa H, Singhal SC, editors. Proceedings of the Seventh International Symposium on Solid Oxide Fuel Cells (SOFC-VII), Pennington, NJ The Electrochemical Society, 2001 2001(16) 769-779. 

Zha S, Cheng Z, and Liu M. Sulfur poisoning and regeneration in Ni-based anodes in solid oxide fuel cells. J Electrochem Soc 2007 154 B201-B206. 

Cheng Z, Zha S, and Liu M. Influence of cell voltage and current on sulfur poisoning behavior of solid oxide fuel cells. J Power Sources 2007 172 688-693. 

Kurokawa H, Sholkalapper TZ, Jacobson CP, De Johghe LC, and Visco SJ. Ceria nanocoating for sulfur tolerant Ni-based anodes of solid oxide fuel cells. Electrochem... 

Cheng Z and Liu M. Characterization of sulfur poisoning of Ni-YSZ anodes for solid oxide fuel cells using in situ Raman micro spectroscopy. Solid State Ionics 2007 178 925-935. 

Choi YM, Compson C, Lin MC, and Liu M. Ab initio analysis of sulfur tolerance of Ni, Cu, and Ni-Cu alloys for solid oxide fuel cells. J Alloys Compd 2007 427 25-29. 

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