(48a) Dual Fluidized Bed Reforming Pilot Test Results: Tar Conversion and the Fate of Sulfur
Synthesis routes for future generation of clean liquid fuels from gaseous or solid fuel feedstock (Coal to liquid, Gas to Liquid) are currently investigated. Reforming of hydrocarbons is widely used to provide synthesis gas for these processes. Even though there have been ideas of using dual fluidized bed technology for hydrocarbon reforming in the early 1940-ies, methane steam reforming and autothermal reforming are carried out mainly in fixed bed catalytic reactors. The dual fluidized bed system is an alternative to the conventional reforming systems. The reactor system consists of two reactors, interconnected with loop seals. The reforming catalyst circulates between these two reaction zones. One reactor is operated as the reformer and the second reactor as the regenerator, which is operated with air and additional fuel to provide the necessary heat for the reformer.
Using syngas derived from biomass gasification as carbon source for 2nd generation biofuel production two components present in the gas stream can cause problems. These two components are tar and sulphur containing species like H2S. Since CO and H2 are the valuable components in the synthesis of biofuels, the presence of tar causes a loss of carbon and hydrogen and lowers the efficiency of the process. Tar also causes problems during the gas cleaning process since it condenses. Using standard reforming technology (fixed bed and nickel based catalysts) to convert the tar into CO and H2, the H2S content in the gas causes catalyst deactivation and lowers the system performance or even prohibits stable operation.
In this study the above mentioned setup of a dual circulating fluidized bed reformer is used. A gas mixture of carbon dioxide, methane, carbon monoxide and hydrogen is used as model syngas in the similar composition to the gas derived from biomass steam gasification. To investigate the influence of tar and H2S on a nickel-based catalyst, 1-methylnaphthalein is used as model tar. The sulphur tolerance is tested with H2S. The tar loading in the reformer feed is 27 g/Nm³ at a total gas feed of 15 Nm³/h. This gas feed corresponds to a thermal fuel power of about 45 kW. The H2S concentration in the feed gas is 260 ppmv.
In the reformer off gas the H2S concentration and in the regenerator the SO2 concentration is measured. Since the formation of SO2 in the reformer is very unlikely due to the highly reducing conditions and the presence of H2S in the oxidizing regenerator atmosphere should not be possible, the Sulphur balance can be closed around the whole system. XRF analysis of the used catalyst is done to identify a possible Sulphur accumulation on the particles.
The results of this study show stable operation of the reforming catalyst in the dual fluidized bed reformer under severe conditions over several hours. Full conversion of tar is always achieved. The sulphur fed to the reformer is transported to the regenerator. Only 2% of the Sulphur passed through the reformer, the rest is released as SO2 in the regenerator. Methane conversion is nearly complete under all conditions and shows only a slight decrease at the presence of Sulphur.
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