66 Summary/Outlook 8 Summary/Outlook The global energy demand is growing, so the substitution of the fossil fuel is essential. The establishment of the biomass fuel gas for heat and electricity production seems to be more and more the key option nowadays due to environmental issues. However, to achieve global use of this alternative energy source it is crucial to purify the gas beforehand its use. Tar seems one of the greater problems as it can cause fouling and blocking erosion of the engine in which the product gas is used. Therefore, the development of catalysts that are able to fully decompose tar is an important research field. Commercial Nickel based catalysts have been widely tested and seem to have very high effectiveness. Due to their severe disadvantages though, it is necessary to turn towards the research of the effectiveness of other metallic oxides too. The use of iron based catalysts seems to be a promising solution. Although they are not as effective as nickel catalysts, because they are not easily deactivated they can be used in situ. Precious metal catalysts are not profoundly tested, due to high costs. The goal of this work was mainly to test these three catalyst types and compare their results. The most promising catalyst is the iron based, at it has a low cost and doesn’t deactivate by carbon deposition or sulfur poisoning. All the catalysts were similarly pretreated; they were reduced under a mixture of H2/N2 (10%/90%). The operating conditions tested were various, but the reference set point was chosen 800 ⁰C and space velocity 6500 1/h. This was chosen, because the iron based catalysts are meant to be used in situ at some point due their advantages that have been mentioned earlier, and these conditions are typical inside the reactor. The results indicated that iron based catalysts achieved ~40%, the nickel based ~98% and the precious metal catalysts ~97% tar conversion. These results are in total agreement with previous works. Long terms runs were not conducted to investigate how many hours on stream the nickel based catalysts could last on stream without being deactivated. As it is obvious from the results, the iron based catalysts didn’t reach very high conversion, even at 860 ⁰C, were the thermal decomposition is very high the conversion was ~70%; a lot lower compared to the other two types. Future work on the optimization of the quality of iron based catalysts is essential. Potentially, they could achieve a reasonable decomposition, so they can be used as an appropriate material for in situ tar conversion. However, regarding the catalysts particularly tested here, there was another drawback too; the particles of the catalyst were very fine. A reasonable amount was blown away from the catalytic bed during time on stream, despite the protective grid that was used to prevent it. Hence, the manufactures of these types of catalysts should also focus on the preparation of catalysts that have larger particles, at least the size of sand, so the material will not blow away during its use. As for the nickel based catalysts their great potentials have been known for the past 20 years. Their numerous disadvantages must still be resolved though. Regarding carbon deposition on their surface, it is inevitable. Hence, the only possible solution to this would be to regenerate the catalyst after some cycles of usage. It has been reported that this can be achieved if they are put under an oxygen stream to burn away the carbon, but the recovery is not full. Fortunately, the use of an appropriate sorbent for sulfur before the entrance of the product gas, has resolved the sulfur poisoning effect. In addition, the fact that they can’t be used in situ renders another drawback, as the use of a second catalytic bed increases a lot the cost of a facility. The precious metal catalysts have very good potentials too as they concentrate the advantages of the two previous mention ones; they achieve high conversion without being deactivated. The precious metal catalysts used here didn’t have fine particles enough, to consider them appropriate material for in situ use. Therefore, the manufacture of finer particle precious metal catalysts could be the solution to the need of a second catalytic bed. Plus, due to the fact that they have not been investigated thoroughly, it is not known how much they can actually last on stream. So, long term tests are important to be conducted. One last interesting point to mention for this type, is that although they have the same effectiveness in tar conversion with the nickel based catalysts, the amount of CH4 in the product gas is quite high, ~5% of the wet gas composition. Thus, they could be a useful material for methanation.
Summary/Outlook 67 The aim is to produce synthesis gas with very low impurities content for high efficiency power production in engine, turbine or fuel cell application. The catalytic tar decomposition is a crucial and challenging task that has to be investigated. The exact appropriate material to achieve the desirable results is still to be found. However, when the point of having tar free gas is reached, then biomass gasification will be considered as the most appropriate solution to overcome the problem fossil fuel depletion. .
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