<p>In order to secure the energy supply to an increasing population and at the same time limit the damage to Earth, i.e. avoiding a fatal climate change as a result of anthropogenic emissions of greenhouse gases (primarily CO<sub>2)</sub>, immediate action is necessary. This includes reducing the energy consumption, increasing the energy conversion efficiency, and using renewable energies. The transport sector is the one most dependent on fossil energy and it stands for a significant part of the energy consumption in the world. For instance, in EU-25 transportation stands for 30 % of the total final energy consumption and relies to 98 % on oil. Being the only renewable energy possible to convert into liquid fuels biomass, as a means for reducing the CO<sub>2</sub> emissions from the transport sector, will play an important role in the near future.</p><p>The conversion of biomass into transportation fuels is preferentially done via gasification followed by the fuel synthesis. The whole production chain from biomass to final fuel is very dependent on R&D, in order to become competitive with the fossil fuels. Fischer-Tropsch (FT) diesel made from biomass is a viable option for reducing the CO<sub>2</sub> emissions from transportation since it may be blended with conventional diesel in any concentration. Furthermore, since its composition is almost the same as of petroleum-based diesel (although cleaner) the same distribution system and engines may be used, which facilitates its introduction on the market. This thesis presents the results of the laboratory work performed in 2003 – 2007 at the Department of Chemical Technology, KTH, and at the Department of Chemical Engineering, NTNU (the Norwegian University of Science and Technology) in Trondheim. Part of the work has been performed in close cooperation with the Department of Chemical and Biological Engineering at Chalmers University of Technology.</p><p>All FT experiments were performed in a fixed-bed reactor at 210 ºC and 20 bar. Pure mixtures of H<sub>2</sub>, CO and N<sub>2</sub> were used as feed to the reactor. Steam was also occasionally introduced. Selectivity to C<sub>5+</sub> was used as a measure of the catalysts’ ability to grow long-chain hydrocarbons, which is desirable when diesel is the product aimed for.</p><p>The first part of the thesis deals with the direct conversion of a H<sub>2-</sub>poor syngas, which is obtained upon gasification of biomass, into FT hydrocarbons. “H<sub>2-</sub>poor” means that the H<sub>2</sub>/CO ratio is lower than what is required by the stoichiometry (~ 2.1) of the FT synthesis (reaction 1). In order to increase the H<sub>2</sub>/CO ratio to the required one, internal water-gas-shift (WGS) is needed (reaction 2).</p><p>FT: CO + 2H2 “-CH2-“ + H2O (1)</p><p>WGS: CO + H2O CO2 + H2 (2)</p><p>The H<sub>2</sub>/CO usage ratio has been used as a measure for the internal WGS activity, it is defined as follows:</p><p>where S is the selectivity (of total C-containing products), and the factor <i>F</i> indicates the number of H<sub>2</sub> moles required for one CO mole to form the product (e.g. for producing high molecular weight <i>n</i>-paraffins, 2 moles of H<sub>2</sub> per mole of CO are required). For the fraction C<sub>2</sub> – C<sub>4</sub>, F will have different values depending on the selectivity to C<sub>2</sub>, C<sub>3</sub> and C<sub>4</sub>, and it also depends on the olefin/paraffin ratios for those hydrocarbons. In order to reach the highest once-through conversion of the syngas, the H<sub>2</sub>/CO usage ratio should be equal to the inlet H<sub>2</sub>/CO ratio. The lower the usage ratio, the higher the relative WGS activity.</p><p>The combined FT and WGS reactions with H<sub>2</sub>-poor syngas have been tested for 12 wt% Co and 12 wt% Co – 0.5 wt% Re catalysts supported on γ-Al<sub>2</sub>O<sub>3</sub>. It was found that with lower H<sub>2</sub>/CO ratios in the feed, the syngas conversion and the CH<sub>4</sub> selectivity decreased, while the C<sub>5+</sub> selectivity and olefin/paraffin ratio for C<sub>2</sub>-C<sub>4</sub> increased slightly. The WGS activity was low for all catalysts, implying a H<sub>2</sub>/CO usage ratio close to the stoichiometric one (2.1), even for inlet H2/CO ratios of 1.5 and 1.0.</p><p>By incorporating significant amounts of Fe (20 % of total metal) in the Co catalyst referred to above (by co-impregnation) in order to achieve a 12 wt% bimetal loading on γ-Al<sub>2</sub>O<sub>3</sub>, a slightly lower usage ratio was obtainable (1.92) for an inlet ratio of 1.0 for dry conditions. Different Fe:Co ratios ranging from 100 % Fe to 100 % Co (12 wt% bimetal) were tested for an inlet H<sub>2</sub>/CO ratio of 1.0. The characterisation results indicated that Fe was enriched at the surface, hence covering the more FT-active Co sites, even at low percentage Fe. Not even upon significant replacement of Co by Fe (≥ 20 %) were the usage ratios for dry conditions lowered to any significant extent.</p><p>The WGS reaction at the low temperature used could be boosted by addition of external water. However, since the catalysts with the highest WGS activity had surface enrichment of Fe, high water partial pressures negatively affect the FT rate and also lead to rapid deactivation by re-oxidation of the FT-active iron phases (iron carbides) or by sintering. Surprisingly, also the WGS activity rapidly deactivated at high partial pressures of water, although Fe<sub>3</sub>O<sub>4</sub> is believed to be the WGS-active phase. Possibly, surface oxidation of Fe<sub>3</sub>O<sub>4</sub> into γ-Fe<sub>2</sub>O<sub>3</sub> may take place during these water-rich conditions.</p><p>The WGS reaction (reaction 2) is thermodynamically favoured at low temperatures. However, the WGS kinetics over the tested catalysts is far too slow at the FT reaction temperature used in typical low-temperature FT (LTFT) applications, with diesel as the desired product.</p><p>The second part of this thesis deals with the hydrocarbon selectivity over Co-based catalysts in the FT synthesis with stoichiometric H<sub>2</sub>/CO feed. An alternative catalyst preparation technique, the microemulsion (ME) technique, was used as a complement to the conventional incipient wetness (IW) impregnation. A great deal of effort was put into the development of the synthesis of Co particles of monodisperse size in a ME and the subsequent deposition of these onto a porous support material. By using the ME technique it was possible to prepare relatively small Co particles in a large-pore support such as TiO<sub>2</sub>, which is not an easy task using the conventional impregnation technique.</p><p>With the prepared ME catalyst on TiO<sub>2</sub> it was possible to study the effect of Co particle size on FT selectivity irrespective of the pore size of the support. It was found that, at least for Co particles above 10 nm, it is not the Co particle size that is the principal parameter determining the selectivity of a catalyst, but rather the physical and/or chemical properties of the support.</p><p>Due to the smaller Co particle size (~ 12 nm) of the ME-TiO<sub>2</sub> catalyst as compared to a corresponding IW catalyst (~ 26 nm) supported on TiO<sub>2</sub>, the FT activity of the former was 100 % higher as fresh catalyst. After 120 h on stream the ME catalyst still gave 60 % higher rate to C<sub>5+</sub> (expressed as gC<sub>5+</sub>/gcat,h) compared to both an IW-TiO<sub>2 </sub>and an IW-γ-Al<sub>2</sub>O<sub>3 </sub>catalyst, all catalysts having the same composition (12 wt% Co, 0.5 wt% Re).</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:kth-4440 |
| Date | January 2007 |
| Creators | Lögdberg, Sara |
| Publisher | KTH, Chemical Engineering and Technology, Stockholm : Kemiteknik |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Licentiate thesis, comprehensive summary, text |
| Relation | Trita-CHE-Report, 1654-1081 ; 2007:36 |
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