Steam reforming General

In industry, large steam reformers generally produce between 20 000 and 100 000 Nm3/h of hydrogen. These reformers can be scaled down to 1000 Nm3/h. Their disadvantages are their large size and a high cost for materials, imposed by the conditions of pressure and temperature. Compact steam reformers have been developed for use with fuel cells. These reformers operate at a lower pressure and temperature (3 bar, 700 °C) the requirements for materials are thus less. For these units, energy conversion efficiency can reach 70%-80%. 

Partial oxidation and steam reforming generally generate 3 and 4 moles of hydrogen per mole of methane, respectively (Tables 2 and 4). The difference is released in the form of heat in the case of partial oxidation. 

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. 

The carbon monoxide concentration of gas streams is a function of many parameters. In general, increased carbon monoxide concentration is found with an increase in the carbon-to-hydrogen ratio in the feed hydrocarbon a decrease in the steam-to-feed-carbon ratio increase in the synthesis gas exit temperature and avoidance of reequiUbration of the gas stream at a temperature lower than the synthesis temperature. Specific improvement in carbon monoxide production by steam reformers is made by recycling by-product carbon dioxide to the process feed inlet of the reformer (83,84). This increases the relative carbon-to-hydrogen ratio of the feed and raises the equiUbrium carbon monoxide concentration of the effluent. 

The steam reforming catalyst is very robust but is threatened by carbon deposition. As indicated in Fig. 8.1, several reactions may lead to carbon (graphite), which accumulates on the catalyst. In general the probability of carbon formation increases with decreasing oxidation potential, i.e. lower steam content (which may be desirable for economic reasons). The electron micrograph in Fig. 8.4 dramatically illustrates how carbon formation may disintegrate a catalyst and cause plugging of a reactor bed. 

However, some contradictory results were obtained in several studies. For instance, in the CH4-NO reaction, some authors have reported that N20 was the primary product [95] while others found that ammonia was first produced [96], The presence of water can play a decisive role since H20 allows generating H2 by WGS or steam reforming [59], Olefins generally show a higher activity than alkanes. Propene for instance has been found more reactive than propane. Some exceptions should be quoted, ethylene has been found less reactive than CH4 in NO reduction at stoichiometry [97],... 

Shu, ]., B.P.A. Grandjean, and S. Kaliaguine, Methane steam reforming in asymmetric Pd- and Pd-Ag/porous SS membrane reactors, Appl. Catal. A General, 119, 305-325,1994. 

Fuel-reforming process should be understood in a broader sense as including all options such as partial oxidation (POX), steam reforming (SR), and their combination, i.e., autothermal reforming (ATR). In general, the fuel-reforming process can be represented by the following equation ... 

In the case of synthesis gasoline or diesel fuel from natural gas (GTL), synthesis gas is produced by a combination of steam reforming and partial oxidation processes (combined reforming) to achieve a H2 CO ratio of generally 2.1 1. This means that the overall process energy demand can be reduced to its minimum. The individual reactions are ... 

D. Srinivas, C. V. V. Satyanarayana, H. S. Poddar, and P. Ratnasamy, Structural studies on NiO-Ce02—Zr02 catalysts for steam reforming of ethanol, Appl. Catal. A General 246, 323—334 (2003). 

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. 

Comparison of pyrolysis and air-steam gasification shows that the yield of hydrogen from biomass is generally higher by air-steam gasification than that by pyrolysis, because with interaction of water and char from decomposition of biomass intermediate products are formed, which leads to more hydrogen-rich gas yield by the steam reforming. 

Steam reforming is generally the preferred process for hydrogen production.Particularly for portable hydrogen production, the requirement of an external heat source can be addressed through the advanced heat and mass transfer provided by microreactors. 

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