Fuel steam reformer

In order to avoid carbon deposition, steam usually needs to be intentionally co-fed with the hydrocarbon vapor introduced into the reactor. Following initial fuel steam reforming, hydrogen, and to a lesser extent CO, are adsorbed on the high surface area fuel-side catalyst and react with oxygen anions to form H2O and CO2. The exothermic oxidation reactions produce heat which drives endothermic steam reforming reactions in a packed bed of catalyst adjacent to the membrane. Steam formed over the oxidation catalysts diffuses to the packed bed of steam reforming catalysts and reacts with hydrocarbon fuel by the reaction ... 

There are three primary techniques used to produce hydrogen or syngas from hydrocarbon fuels steam reforming (6.1), partial oxidation (6.2), autofliermal reforming (ATR) (6.3) [1-3]. 

Most hydrogen is derived from water by reaction with fossil fuels (steam reforming) rather than by electrolysis. 

Reformer. This unit chemically converts hydrocarbon or alcohol to synthesis gas (a mixture of hydrogen and carbon monoxide). The two most practical oxidants are steam and air. If air is used, the reformer is referred to as a partial oxidation (POX) reformer if steam is used, a steam reformer (SR), and if a mix of air and steam is used, an autothermal reformer (ATR). The choice of reformer type depends on a number of factors. Typically, POX reformers are smaller, cheaper, respond faster, and are suitable for a wide range of fuels. Steam reformers enable a higher system efficiency. ATRs and catalytic POX reformers (CPOX) share some of the advantages of each type ... 

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. 

A viable electrocatalyst operating with minimal polarization for the direct electrochemical oxidation of methanol at low temperature would strongly enhance the competitive position of fuel ceU systems for transportation appHcations. Fuel ceUs that directiy oxidize CH OH would eliminate the need for an external reformer in fuel ceU systems resulting in a less complex, more lightweight system occupying less volume and having lower cost. Improvement in the performance of PFFCs for transportation appHcations, which operate close to ambient temperatures and utilize steam-reformed CH OH, would be a more CO-tolerant anode electrocatalyst. Such an electrocatalyst would reduce the need to pretreat the steam-reformed CH OH to lower the CO content in the anode fuel gas. Platinum—mthenium alloys show encouraging performance for the direct oxidation of methanol. 

In general, the proven technology to upgrade methane is via steam reforming to produce synthesis gas, CO + Such a gas mixture is clean and when converted to Hquids produces fuels substantially free of heteroatoms such as sulfur and nitrogen. Two commercial units utilizing the synthesis gas from natural gas technology in combination with novel downstream conversion processes have been commercialized. 

High temperature steam reforming of natural gas accounts for 97% of the hydrogen used for ammonia synthesis in the United States. Hydrogen requirement for ammonia synthesis is about 336 m /t of ammonia produced for a typical 1000 t/d ammonia plant. The near-term demand for ammonia remains stagnant. Methanol production requires 560 m of hydrogen for each ton produced, based on a 2500-t/d methanol plant. Methanol demand is expected to increase in response to an increased use of the fuel—oxygenate methyl /-butyl ether (MTBE). 

Conventional Transportation Fuels. Synthesis gas produced from coal gasification or from natural gas by partial oxidation or steam reforming can be converted into a variety of transportation fuels, such as gasoline, aviation turbine fuel (see Aviation and other gas turbine fuels), and diesel fuel. A widely known process used for this appHcation is the Eischer-Tropsch process which converts synthesis gas into largely aHphatic hydrocarbons over an iron or cobalt catalyst. The process was operated successfully in Germany during World War II and is being used commercially at the Sasol plants in South Africa. 

In a typical PAFC system, methane passes through a reformer with steam from the coolant loop of the water-cooled fuel cell. Heat for the reforming reaction is generated by combusting the depleted fuel. The reformed natural gas contains typically 60 percent H9, 20 percent CO, and 20 percent H9O. Because the platinum catalyst in the PAFC can tolerate only about 0.5 percent CO, this fuel mixture is passed through a water gas shift reactor before being fed to the fuel cell. 

Carbon monoxide is a colourless, odourless gas and - without chemical analysis - its presence is undetectable. It is produced by steam reforming or incomplete combustion of carbonaceous fuels typical carbon monoxide concentrations in common gases are given in Table 5.30. 

In contrast to steam reforming, partial oxidation (POX) uses air instead of steam and, as its name implies, burns the fuel in restricted amounts of air so that it generates partially combusted products, including hydrogen. POX generates heat and can, therefore, potentially respond faster than a steam reformer. This is beneficial for load-following applications (c.g., transportation). 

In practice, this ratio is even lower than what is shown hy the stoichiometric equation because part of the methane is oxidized to carhon dioxide and water. When resids are partially oxidized hy oxygen and steam at 1400-1450°C and 55-60 atmospheres, the gas consists of equal parts of hydrogen and carhon monoxide. Table 4-2 compares products from steam reforming natural gas with products from partial oxidation of heavy fuel oil. 

Composition of synthesis gas from steam reforming natural gas and partial oxidation of fuel oil ... 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

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