Poisoning steam reforming

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Steam reforming is the reaction of steam with hydrocarbons to make town gas or hydrogen. The first stage is at 700 to 830°C (1,292 to 1,532°F) and 15-40 atm (221 to 588 psih A representative catalyst composition contains 13 percent Ni supported on Ot-alumina with 0.3 percent potassium oxide to minimize carbon formation. The catalyst is poisoned by sulfur. A subsequent shift reaction converts CO to CO9 and more H2, at 190 to 260°C (374 to 500°F) with copper metal on a support of zinc oxide which protects the catalyst from poisoning by traces of sulfur. 

Figure 8.3.1 is a typical process diagram for tlie production of ammonia by steam reforming. Tlie first step in tlie preparation of tlie synthesis gas is desulfurization of the hydrocarbon feed. Tliis is necessary because sulfur poisons tlie nickel catalyst (albeit reversibly) in tlie reformers, even at very low concentrations. Steam reforming of hydrocarbon feedstock is carried out in tlie priiiiiiry and secondary reformers. 

The operation of a large synthetic ammonia plant based on natural gas involves a delicately balanced sequence of reactions. The gas is first desulfurized to remove compounds which will poison the metal catalysts, then compressed to 30 atm and reacted with steam over a nickel catalyst at 750°C in the primary steam reformer to produce H2 and oxides of carbon ... 

Remaining trace quantities of CO (which would poison the iron catalyst during ammonia synthesis) are converted back to CH4 by passing the damp gas from the scmbbers over a Ni methanation catalyst at 325° CO -t- 3H2, CRt -t- H2O. This reaction is the reverse of that occurring in the primary steam reformer. The synthesis gas now emerging has the approximate composition H2 74.3%, N2 24.7%, CH4 0.8%, Ar 0.3%, CO 1 -2ppm. It is compressed in three stages from 25 atm to 200 atm and then passed over a promoted iron catalyst at 380-450°C ... 

Natural gas consists mainly of methane together with some higher hydrocarbons (Tab. 8.1). Sulfur, if present, must be removed to a level of about 0.2 ppm prior to the steam reforming process as it poisons the catalyst. This is typically done by cata-lytically converting the sulfur present as thiols, thiophenes or COS into H2S, which is then adsorbed stochiometrically by ZnO, at 400 °C, upstream of the reactor. 

Fuel Hydrogen for PAFC power plants will typically be produced from conversion of a wide variety of primary fuels such as CH4 (e.g., natural gas), petroleum products (e.g., naphtha), coal liquids (e.g., CH3OH) or coal gases. Besides H2, CO and CO2 are also produced during conversion of these fuels (unreacted hydrocarbons are also present). These reformed fuels contain low levels of CO (after steam reforming and shift conversion reactions in the fuel processor) which cause anode poisoning in PAFCs. The CO2 and unreacted hydrocarbons (e.g., CH4) are electrochemically inert and act as diluents. Because the anode reaction is nearly reversible, the fuel... 

There are three major gas reformate requirements imposed by the various fuel cells that need addressing. These are sulfur tolerance, carbon monoxide tolerance, and carbon deposition. The activity of catalysts for steam reforming and autothermal reforming can also be affected by sulfur poisoning and coke formation. These requirements are applicable to most fuels used in fuel cell power units of present interest. There are other fuel constituents that can prove detrimental to various fuel cells. However, these appear in specific fuels and are considered beyond the scope of this general review. Examples of these are halides, hydrogen chloride, and ammonia. Finally, fuel cell power unit size is a characteristic that impacts fuel processor selection. 

As an application of Pt nanowires in heterogeneous catalysis, we performed preferential oxidation (PROX) of CO as a test reaction [32]. The PROX reaction is useful for PEM fuel cells for the selective removal of contaminating CO from hydrogen gas, because CO works as a strong catalyst poison for Pt electrode catalysts (Figure 15.24). H2 produced in steam-reforming and the water-gas shift reaction needs further to be purified in the PROX reaction to selectively oxidize a few% CO towards inert CO2 in a H 2-rich atmosphere, to reduce the CO content to <10ppm. Under the PROX conditions, the facile oxidation of H2 to H2O may also occur, thus the catalyst selectivity for CO oxidation over H2 oxidation is an... 

In addition to the direct use of ethanol as a fuel, its use as a source of H2 to be used with high efficiency in fuel cells has been thoroughly investigated. H2 production from ethanol has advantages compared vdth other H2 production techniques, including steam reforming of hydrocarbons and methanol. Unlike hydrocarbons, ethanol is easier to reform and is also free of sulfur, which is a well-known catalyst poison. Furthermore, unlike methanol, ethanol is completely renewable and has lower toxicity. 

Notably, since high-temperature steam reforming enhances the r-WGSR, which would produce CO, the undesirable poison of fuel cells, low-temperature reforming is preferable. Low temperatures can be achieved over strong acid catalysts, although the strong acid at the same time tends to cause deactivation by coke formation. 

Here we shall briefly summarize the effects of individual poisons on various catalytic reactions taking place on automotive catalysts. There are three main catalytic processes oxidation of carbon monoxide and hydrocarbons and reduction of nitric oxide. Among secondary reactions there are undesirable ones which may produce small amounts of unregulated emissions, such as NH3, S03 (6), HCN (76, 77), or H2S under certain operating conditions. Among other secondary processes which are important for overall performance, in particular of three-way catalysts, there are water-gas shift, hydrocarbon-steam reforming, and oxygen transfer reactions. Specific information on the effect of poisons on these secondary processes is scarce. 

One particular application for which supported Au catalysts may find a niche market is in fuel cells [4, 50] and in particular in polymer electrolyte fuel cells (PEFC), which are used in residential electric power and electric vehicles and operate at about 353-473 K. Polymer electrolyte fuel cells are usually operated by hydrogen produced from methane or methanol by steam reforming followed by water-gas shift reaction. Residual CO (about 1 vol.%) in the reformer output after the shift reaction poisons the Pt anode at a relatively low PEFC operating temperature. To solve this problem, the anode of the fuel cell should be improved to become more CO tolerant (Pt-Ru alloying) and secondly catalytic systems should be developed that can remove even trace amounts of CO from H2 in the presence of excess C02 and water. 

The main catalyst poison in steam reforming plants is sulfur that is present in most feedstocks. Sulfur concentrations as low as 0.1 ppm form a deactivating layer on the catalyst but the activity loss of a poisoned catalyst can be offset, to some extent, by raising the reaction temperature. This helps to reconvert the inactive nickel sulfide to active nickel sites. Nickel-free catalysts have been proposed for feedstocks heavier than naphtha. These catalysts consist mostly of strontium,... 

The catalysts used in the steam reforming process are poisoned by trace components in the hydrocarbon feed - particularly sulphur, chlorine and metal compounds. Sulphur is the most common problem. Chlorine compounds are... 

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