Fuels impurities

The emergence of commercial fuel cell cars will depend on developments in membrane technology, which are about one third of the fuel cell cost. Improvements are desired in fuel crossover from one side of a membrane to the other, the chemical and mechanical stability of the membrane, undesirable side reactions, contamination from fuel impurities and overall costs. 

Water Management Membrane Stability Fuel Impurities... 

Figure 5.36. Effects that long-term NH3 exposure has on H2-air fuel cell high-frequency resistance at 80°C. 30 ppm NH3 (g) was injected into the anode feed stream [39], (Reproduced by permission of ECS—The Electrochemical Society, from Uribe FA, Gottesfeld S, Zawodzinski Jr. TA. Effect of ammonia as potential fuel impurity on proton exchange membrane fuel cell performance.)...

Uribe FA, Gottesfeld S, Zawodzinski Jr TA (2002) Effect of ammonia as potential fuel impurity on proton exchange membrane fuel cell performance. J Electrochem Soc 149 A293-6... 

More efficient coal utilization can be realized with combined power plant cycles. For instance, the post combustion gases of a conventional combustor or an advanced MHD system can be further utilized to drive a gas or steam turbine. However, the sustained durability of downstream turbine or heat exchanger components requires minimal transport of corrosive fuel impurities. Control of mineral-derived impurities is also required for environmental protection. For the special case of open cycle-coal fired MHD systems, the thermodynamic activity of potassium is much higher in the seeded combustion gas (plasma) than in common coal minerals and slags. This results in the loss of plasma seed by slag absorption and is of critical concern to the economic feasibility of MHD. 

As a long-term objective, we aim to define the mechanisms by which inorganic fuel impurities (particularly K, Na, Cl, S, and heavy metals) and additives (e.g., K in MHD) are released to or removed from the environment, transported in a gas stream and deposited in cooler or chemically less reactive regions. To meet this objective, we are addressing the following basic tasks ... 

The solid oxide fue( cell (SOFC) have been under development during several decades since it was discovered by Baur and Preis in 1937, In order to commercialise this high temperature (600 - 1000°C) fuel cell it is necessary to reduce the costs of fabrication and operation. Here ceria-based materials are of potential interest because doped ceria may help to decrease the internal electrical resistance of the SOFC by reducing the polarisation resistance in both the fuel and the air electrode. Further, the possibility of using less pre-treatment and lower water (steam) partial pressure in the natural gas feed due to lower susceptibility to coke formation on ceria containing fuel electrodes (anodes) may simplify the balance of plant of the fuel cell system, and fmally it is anticipated that ceria based anodes will be less sensitive to poising from fuel impurities such as sulphur. 

SOFCs share with other fuel cell types a very low tolerance to fuel impurities of H2S (< 1 ppm), NH3 (< WYo), HCl and other halogens (< 1 ppm), but in contrast to the low-temperature cells accept fuel impiuities of CO in addition to the methane and hydrocarbons reformed internally (Dayton et ah, 2001). 

Finally, it should be realized that CO is not the only fuel (or fuel-derived) contaminant expected to affect anode performance in the PEFC. In a test of other possible contaminants that could result, in principle, from methanol reforming, Seymour et al. [27] reported strong and irreversible effects of formic acid at a PEFC platinum (high-loading) anode, whereas methanol, formaldehyde, and methyl formate were found to have much smaller and reversible effects. The fuel impurity aspects of coupling between natural gas (or gasoline) reformers of various types and a PEFC stack are even wider, and make it essential to probe and address, either by removal upstream or by use of modified catalysts, the possible detrimental effects of low levels of sulfur, H2S, COS, and NH3 [28]. 

Raask, E. Mechanism of Corrosion by Fuel Impurities Marchwood, UK Butterworth London, 1963 p. 145. 

Cross, N.L. Picknett, R.G. Mechanism of Corrosion by Fuel Impurities Marchwood, UK Butterworths London, 1963, p. 383. Dietzel, A. Emailwaren-Industrie. 1934, 11, 161. 

Preliminary data obtained with hydrogen sulfide, another antieipated reformed fuel impurity, show this eompound has an even more serious effeet on fuel eell performanee than CO. Hydrogen sulfide appears to poison the platinum eatalyst irreversibility. 

F. Uribe, S. Gottesfeld and T. Zawodzinski, "The Effect of Ammonia as Potential Fuel Impurity on Proton Exchange Membrane Fuel Cell Performance", J. Electrochem. Soc., 149, A293 (2002). 

F. Uribe and T. Zawodzinski, "The Effects of Fuel Impurities on PEM Fuel Cell Performance", Electrochemical Society Meeting, San Francisco, CA (2001). Abstract No. 339. 

Determine fuel and fuel impurity effects on fuel processor catalyst durability... 

R. Bonrup et al. Fuels and Fuels Impurity Effects and Fuel Processing Catalyst Proceed, of 2000 Fuel Cell Seminar, 2000, p. 288-291... 

Williams FA and Crawley CM, Impurities in Coal and Petroleum, The Mechanism of Corrosion by Fuel IMPURITIES. Butterworth, London, UK, 1963, p. 34. 

Anode contamination by fuel impurities such as traces of H2S or NH3 are more irreversible than CO poisoning. Catalyst deactivation by H2S can be partially relieved by bringing the anode to oxidizing potentials and subsequent operation under hot and humid conditions [43]. 

Impurities that adsorb mito the electrocatalysts surface inhibit the charge-transfer processes, resulting in performance loss. Common fuel impurities include carbon monoxide, ammonia, hydrogen sulfide, hydrogen cyanide, hydrocarbons, formaldehyde, and formic acid [93]. On the cathode side, ambient air may contain impurities such as sulfur dioxide, nitrogen oxides, and particulate matter (including salts) that can affect fuel cell performance [93]. 

Most of the aforementioned shortcomings are associated with the low operating temperature, limited mainly due to the electrolyte used. A PEMFC technology operating above 100°C is desirable. First of all, the kinetics of both electrode reactions are enhanced and this can in principle lead to the use of lower amount of the expensive noble metal on the electrodes. More importantly, tolerance of the catalyst to fuel impurities (CO, sulphide) is dramatically enhanced, broadening the options for the feed gas. The CO poisoning effect is temperature dependent due to the weaker CO adsorption on the Pt surface. The CO tolerance has been found to increase from 10-20 ppm of CO at 80°C to 1000 ppm at 130°C and up to 30000ppm at 200°C. This can simplify the overall power system, reduce its cost and volume and improve the transient response capacity, reliability or maintenance-free operation of the system. ... 

Finally, the possibility of an integrated Pd-based membrane system is also related to the possibility of separating hydrogen isotopes in nuclear reactors and in the ability to process fuel impurities such as tritiated water and methane. 

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