Compositions – Gaseous compositions
Reexamination Certificate
1998-10-19
2001-04-24
Griffin, Steven P. (Department: 1754)
Compositions
Gaseous compositions
C252S373000, C429S010000, C429S010000
Reexamination Certificate
active
06221280
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods of catalytic partial oxidation (CPOX) of hydrocarbon fuels and, more particularly, to improved methods of CPOX of heavy hydrocarbon fuels having a substantial sulfur content, such as commercial and logistic fuels.
Interest continues in methods of using hydrocarbon fuels to produce a gaseous product stream of hydrogen and carbon monoxide, as well as using the gaseous product stream to fuel a fuel cell system, such as a solid oxide fuel cell system (SOFC). The studies concerning hydrocarbon processing vis-a-vis fuel cells have been numerous and include S. Ahmed et al., “Partial Oxidation Reformer Development For Fuel Cell Vehicles,” Proceedings of the 32
nd
Intersociety Energy Conversion Engineering Conference, v.2, pp. 843-846 (1997); J. Bentley et al., “Reformer and Hydrogen Storage Development For Fuel Cell Vehicles,” Annual Automotive Technology Development Contractor's Coordination Meeting (1995); British Gas plc, “Evaluation of the Potential Use of Partial Oxidation in Solid Polymer Fuel Cell Systems” (1997); L. Brown, “Survey of Processes for Producing Hydrogen Fuel from Different Sources for Automotive-Propulsion Fuel Cells,” Los Alamos National Laboratory (1996); N. Edwards et al., “Fuel Cell System for Transport Applications including On-board HotSpot™ Reformer,” Johnson Matthey Technology Centre, Blount's Court, Sonning Common, Reading RG4 9NH, United Kingdom; Elangovan et al., “Planar solid oxide fuel cell integrated system technology development,” Journal of Power Sources, v. 71, pp.354-360 (1998); and T. Hirata et al., “Development of 60 kW Class Plate Reformer for Fuel Cell Plant,” IHI Engineering Review, v. 29, no. 2, pp. 53-58 (1996).
The processes of converting hydrocarbon fuels to hydrocarbon/carbon monoxide gas products that have been developed in the past generally fall into one of three classes—steam reforming, partial oxidation (catalytic and non-catalytic), and auto-thermal reforming (a combination of steam reforming and partial oxidation). All three hydrocarbon conversion methods have been considered for use in conjunction with fuel cells, although L. Brown, supra, suggests that partial oxidation alone has not been favored. Nevertheless, the contemplated uses of fuel cells have been many, but significant attention has recently been given to transport vehicles. In that regard, fuel cells have been considered as replacements for internal combustion engines due to the advantages of greater efficiency and reduced emissions.
Despite their advantages, each of the three hydrocarbon conversion processes has design barriers. In the steam reforming method, which is endothermic, there are space and weight issues. Because steam reforming involves an endothermic reaction, an external source of heat is needed and the required heat transfer processes are slow. Of course, with the need for steam comes a concomitant need for a water supply or recycling. Any such additional items only add to the size and weight of a vehicle that can, in turn, affect other design considerations.
On the other hand, partial oxidation is an exothermic process and, therefore, does not have the disadvantage of requiring heat input and related transfer inefficiencies. There has been progress in the partial oxidation of light hydrocarbons (i.e., molecules with up to 5 carbon atoms) in recent years. But the technology for the conversion of complex or heavy hydrocarbon fuels (molecules with greater than 5 carbon atoms) to hydrogen and carbon monoxide is still in its early development.
Of great interest for fuel cells is the conversion of refinery liquid hydrocarbon fuels, such as gasoline and naphtha, to hydrogen/carbon monoxide gas streams by partial oxidation processes. Gasoline typically has a minimum of 80%-90% hydrocarbons with greater than five or more carbon atoms per molecule. For military applications, the hydrocarbon fuels of greatest interest are the so-called logistic fuels, such as JP-8 jet fuel, JP-4 jet fuel, JP-5 jet fuel and No. 2 fuel oil. In logistic fuels, the number of carbon atoms in a molecule may typically range from at least six and up to about 20 or more. But higher numbers of carbon atoms tend to increase the potential problem of carbon formation in the conversion process.
Carbon formation arises from the thermal cracking of hydrocarbons that can produce carbon-rich compounds (i.e., carbonaceous polymers) and, ultimately, coke. Thereby, system degradation can occur by, among other things, deposition of carbon on catalysts. In turn, the carbon deposition can lead to catalyst deactivation. Deposition on reactor walls can affect reactor performance and may lead to plugging. The problem of carbon formation has been extensively addressed in the past. Examples can be found in A. Dicks, “Hydrogen generation from natural gas for the fuel cell systems of tomorrow,” Journal of Power Sources, v. 61, pp. 113-124 (1996); and W. Houghtby et al., “Development of the Adiabatic Reformer to Process No. 2 Fuel Oil and Coal Derived Liquid Fuels” (1981).
In addition to carbon formation, the processes for liquid hydrocarbon fuel conversion to hydrogen/carbon monoxide gas streams may be affected by the sulfur that is usually present in these fuels. In both light and heavy hydrocarbon fuels, but particularly the latter, sulfur is present in varying amounts. The specifications for sulfur content in logistic fuels such as Jet A, JP-8, JP-4, JP-5, etc. is given by military specification MIL-T-5634M/N. These specifications require the maximum amount of total sulfur content in the fuel to be 0.3 wt. % (tested according to standardized methods D1266/D1522/D2622). Typically, however, commercially available jet fuels have a total sulfur content of about 0.05-0.07 wt. %. The compounds of sulfur which remain in the liquid refinery streams are usually the refractory benzothiophene, dibenzothiophene, and their derivatives [Lee et al. “Removal of Sulfur Contaminants in Methanol for Fuel Cell Applications,” Fuel Cell Seminar Poster Session, (1996)], which are essentially difficult to remove. As with carbon formation, sulfur can poison the catalyst and do so to a point where the catalyst becomes completely deactivated. Catalysts based upon nickel or platinum have appeared to be particularly susceptible to poisoning. It has been postulated that sulfur forms surface stable compounds with the catalyst. Thereby, catalyst active sites for oxidation are depleted and efficient production of hydrogen and carbon monoxide through catalytic partial oxidation is hindered.
One potential solution to the presence of sulfur has been to remove the sulfur prior to processing. Nickel or other transition metals, such as iron, have been known to remove sulfur from sulfur bearing organic compounds and are used in the laboratory. They have also been used as adsorbents to remove the thiophenic sulfur that remains in hydrocarbon fuels after hydro-desulfurization. These metals are very active for sulfur removal but suffer from the inability to adsorb a large quantity of sulfur because adsorption is limited only to the external surface of the metal due to the large size of the thiophenic molecule. Of these transition metals, Raney nickel seems to offer the best choice because of its high surface area. But experimental data shows that a ratio of about 100:1 by weight of Ni:S is needed for complete removal of the residual thiophenic sulfur from jet and diesel fuels. The required high Ni to S ratio limits this method of desulfurization to fuels with very small concentrations of sulfur, i.e., a few ppm of sulfur. For the removal of sulfur from logistic fuels that have hundreds of ppm of sulfur, desulfurization by nickel is costly and demanding in terms of metal weight and volume and, therefore, this method is impractical.
A solution around the sulfur problem in partial oxidation has been to omit the catalyst entirely (i.e., non-catalytic partial oxidation), particularly for converting heavy hydrocarbons on an industrial scale, and removing the sulfur,
Anumakonda Amarendra
Ferrall Joe
Yamanis Jean
Allied-Signal Inc.
Griffin Steven P.
Nave Eileen E.
Zak, Jr. Esq. William J.
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