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Carbon Deposition Avoidance: The processing of hydrocarbons always has the potential to form coke. Coke formation is influenced by the composition of the fuel, the catalyst, and the process conditions (e.g., partial pressure of steam). Coke causes the greatest problems in gas flow paths and on catalyst. Carbon deposition not only represents a loss of carbon for the reaction, but more importantly also results in deactivation of the catalyst due to deposition at the active sites. Thermal cracking46 in over-heated preheaters and manifolds can easily form carbon. If the fuel conversion reactor is not properly designed or operated, coking is likely to occur. Thermo dynamic equilibrium provides a first approximation of the potential for coke formation. Free carbon in hydrocarbon fuels forms according to the three equations, (8-1), (8-2), and (8-3). Figures 8-4 and 8-5 show the effect of increasing steam on carbon deposition for methane and octane, respectively. Increasing steam, hydrogen, and carbon dioxide concentrations alleviates carbon deposition. Low contents of aromatics and alkenes help to maintain the activity of the catalyst (10). No carbon deposits at low temperatures (~600 °C) in mixtures containing at least two atoms of oxygen and four atoms of hydrogen per atom of carbon. At these conditions, all carbon is present as CO2 or CH4 (7). Higher hydrocarbon fuels show a greater tendency for carbon formation than does methane. One method to alleviate carbon deposition problems in the fuel processor is to use special catalysts either containing alkali or based on an active magnesia support. With a highly active catalyst, the limit permitted on the final boiling point of the hydrocarbon feedstock is related mainly to the possibility of desulfurizing the feed to below 0.1 ppm, rather than to the reactivity of the hydrocarbons. With proper desulfurization, it has been possible to convert light oil into syngas with no trace of higher hydrocarbons in the reformate gas (17). Coke formation resulting from higher hydrocarbon fuels can also be eliminated with an adiabatic pre-reformer. The adiabatic reformer is a simple fixed bed reactor. By adiabatic pre-reforming, all higher hydrocarbons are converted at low temperature (below ~500 °C) with steam into methane, hydrogen, and carbon oxides at conditions where carbon formation does not occur. It is possible to use a high pre-heating temperature (650 °C or above) for internal reforming in MCFC and SOFC without the risk of carbon formation. For natural gas containing only minor amounts of higher hydrocarbons, adiabatic pre-reforming at a steam to carbon ratio as low as 0.25 mole/atom has been demonstrated. For heavier feedstocks such as naphtha, operation at a steam to carbon ratio of 1.5 has been proven in industry. Pilot tests have been carried out at a steam to carbon ratio of 1.0 with reformate recycle. 46. Thermal cracking is the breaking of a hydrocarbon carbon-carbon bond through the free-radical mechanism. Cracking may result in the formation of lower chained hydrocarbons, the original "cracked" hydrocarbon, or further cracking of the hydrocarbon to soot. 8-23PDF Image | Fuel Cell Handbook (Seventh Edition)
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