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Adiabatic Diesel Pre-reforming
Gepubliceerd door: Publicatie datum:
ECN Waterstof en Schoon Fossiel 3-7-2008
ECN publicatienummer: Publicatie type:
ECN-E--08-046 ECN rapport
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The pre-reforming process converts higher hydrocarbons into a mixture of hydrogen and C1 components: CO, CO2, and methane. It reduces the risk of coke formation in downstream re-forming processes. Pre-reforming is practiced at industrial scales with hydrocarbon fuels rang-ing from natural gas up to naphtha. More recently, attention has been drawn to pre-reforming at small scales or with heavier feedstock such as diesel. Direct steam reforming of heavy hydro-carbon fuels may cause severe carbon deposition problems at elevated temperature. Because it has a lower operating temperature, pre-reforming may be required in order to achieve a stable process. The adiabatic pre-reforming process is based on a set of reactions: steam reforming of hydro-carbons, followed by water-gas shift and methanation reactions. Steam reforming is irreversible for all higher hydrocarbons. No intermediate products are generally formed and complete conversion is normally be attained. Water-gas shift and methanation are limited by thermodynamic equilibrium. While steam reforming is strongly endothermic, water-gas shift and methanation are exothermic. The overall heat of reaction may therefore be negative, zero, or positive and the process, in contrast to other reforming processes, can be operated adiabatically. For heavy feed-stock such as diesel, this results in a characteristic temperature profile. It displays a decrease in temperature followed by an increase in temperature along the reactor axis. The shape of the temperature profile depends on the nature of the feedstock and the operating pressure (which affects the equilibrium of methanation). The shape of the axial temperature profile changes during the process, because of catalyst deactivation, which cannot normally be prevented. As a result, the temperature profile moves along the reactor axis in the direction of flow, while the width of the profile broadens. Group VIII metals are catalytically active for the steam reforming reaction. Nickel is the active metal in almost all industrial catalysts. Precious metals are very active for the steam reforming as well. Mechanistic and kinetic studies can be found in literature for both classes of catalysts. Precious metals may offer advantages in terms of a better sulphur tolerance and reduced rates of coking. Nevertheless, both nickel and precious metal pre-reforming catalysts degrade over time. Critical issues are the poisoning by sulphur and the formation of carbonaceous deposits, in particular gum at low temperatures and whiskers at higher temperatures. The limits of gum and whisker formation result in a temperature window, in which a catalyst can be operated at acceptable deactivation rates. As the deactivation of nickel catalysts at favourable operating conditions approaches a constant rate over time, predictive modelling is possible. For precious metal catalysts, deactivation data is largely lacking.

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