Fuel cell sulfur compounds

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

A method for simultaneous desulfurization and desalting of fossil fuels by mixing the aqueous biocatalytic solution with the feed under conditions for both processes to occur. Both inorganic salts, the originally present in the fuel and the produced from the conversion of the organic sulfur compounds are solubilized by the added water, resulting in an aqueous phase with a salt concentration greater than 0.5 wt%. The biocatalyst consists of Rhodococcus sp. ATCC 53968, its mutants or cell-free fractions. 

Syngas cleanup system - low or high temperature and processes used to remove sulfur, nitrogen, particulates, and other compounds that may impact the suitability of the syngas for specific applications (i.e., turbine and fuel cell for electric power generation, hydrogen production, liquid fuel production, or chemical production). 

Figure 6.3 provides a simplified block diagram of a fuel cell power plant system. Gasification is used to convert the solid fuel to gas, which is processed to remove sulfur compounds, tars, particulates, and trace contaminants. The clean gas is then converted to electricity in the FC. Waste heat from the FC is used to generate steam, which can be used to run the gasification process and to generate additional power in the bottoming cycle. 

Global has also designed and built a dual-stage, low-temperature adsorbent desulfurizer. Sulfur in propane can exceed as much as 300-ppm compared to natural gas, which ranges from 2 to 15-ppm sulfur and it must be removed to block any poisoning of the fuel cell. The test results indicated that no sulfur compounds were present in the outlet gas of the desulfurizer. The system design uses a modular assembly and layout, including a circular hot box where the fuel cell stacks and the fuel processor are located and easily accessed. 

The concentrations of impurities entering the PAFC are very low relative to diluents and reactant gases, but their impact on performance is significant. Some impurities (e.g., sulfur compounds) originate from fuel gas entering the fuel processor and are carried into the fuel cell with the reformed fuel, whereas others (e.g., CO) are produced in the fuel processor. 

Like MCFCs, SOFCs can integrate fuel reforming within the fuel cell stack. A prereformer converts a substantial amount of the natural gas using waste heat from the fuel cell. Compounds containing sulfur (e.g., thiophene, which is commonly added to natural gas as an odorant)... 

Fuel cells generate electricity, heat, and distilled water by reacting H2 with oxygen (air). This process emits no C02, no carbon monoxide, no sulfur dioxide, no volatile organic compounds, and no fine particles. The only byproduct of the oxidation of H2 is distilled water. There are some 1,035 firms that are active in the field of fuel cell development and manufacturing (http //www. fuelcells.org/directory). This field of technology is fast advancing, the delivery of new units increased by 75% to approximately 100,000 units in 2007. 

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. 

Other metals, such as copper, nickel, or silver, have been used as electrode materials in connection with specific applications, such as the detection of amino acids or carbohydrates in alkaline media (copper and nickel) and cyanide or sulfur compounds (silver). Unlike platinum or gold electrodes, these electrodes offer a stable response for carbohydrates at constant potentials, through the formation of high-valence oxyhydroxide species formed in situ on the surface and believed to act as redox mediators (40,41). Bismuth film electrodes (preplated or in situ plated ones) have been shown to be an attractive alternative to mercury films used for stripping voltammetry of trace metals (42,43). Alloy electrodes (e.g., platinum-ruthenium, nickel-titanium) are also being used for addressing adsorption or corrosion effects of one of their components. The bifunctional catalytic mechanism of alloy electrodes (such as Pt-Ru or Pt-Sn ones) has been particularly useful for fuel cell applications (44). 

The selective ODS has shown many potential advantages for deep desulfurization of the fuels for fuel cell applications, because the process usually has higher desulfurization capacity than the adsorption desulfurizaton, and also can run at mild operating conditions without the use of H2. For ODS of liquid hydrocarbons fuels, direct use of oil-soluble peroxides or 02 as oxidants in an ODS process is greatly attractive, as the process does not involve a complicated biphasic oil-aqueous solution system. The key in ODS is how to increase the oxidation selectivity for the sulfur compounds. 

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