Steam reforming, carbon oxide formation

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

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. 

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. 

A structured ruthenium catalyst (metal monolith supported) was investigated by Rabe et al. [70] in the ATR of methane using pure oxygen as oxidant. The catalytic activity tests were carried out at low temperature (<800 ° C) and high steam-to-carbon ratios (between 1.3 and 4). It was found that the lower operating temperature reduced the overall methane conversion and thus the reforming efficiency. However, the catalyst was stable during time on-stream tests without apparent carbon formation. 

In the steam-reforming process (Fig. 1), the hydrocarbon feedstock is first desulfurized by heating to 370°C in the presence of a metallic oxide catalyst that converts the organosulfur compounds to hydrogen sulfide. Elemental sulfur can also be removed with activated carbon absorption. A caustic soda scrubber removes the hydrogen sulfide by salt formation in the basic aqueous solution. 

Since ATRs combine some of the best features of steam reforming and partial or full oxidation, some groups have developed compact catalyst systems to eliminate the need for a robust burner and mixer design. The catalyst system also reduces the formation of carbon and soot. Farrauto et al.6 and Giroux et al.7 discussed ATR-based systems for fuel cell applications in detail. 

The technologies for production of syngas from hydrocarbons are based on either steam-reforming or partial oxidation. In the former case, the hydrocarbons react with steam with considerable addition of heat to produce a syngas with a H2/CO ratio of 3 or more. Partial oxidation may be carried out either thermally or catalytically (or by a combination) to produce a syngas with an H2/CO ratio less than 2. Regardless of technology, CO2 may be added to the feed to adjust the gas composition to a low H2/CO ratio. In all cases, limits for the formation of carbon on catalysts or soot in the condensate must be considered to avoid rapid deactivation and low on-stream factors. 

---