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28 Nickel Based Catalysts 3.5 Pretreatment to reduce tendency towards coke formation The reactor is heated up under constant flow of N2 (750Nl/h). The activation of the catalyst is achieved by flushing hydrogen for 1-2h (400 Nl/h N2 + 350 Nl/h H2). After the reduction steam treatment is being applied (750 Nl/h N2 +0.6 l/h H2O) so the active sites of the catalyst are occupied by water. This procedure reduces the initial activity of the catalyst but the tendency towards coke formation is reduced. With the increase of time on steam (approximately 2h) the catalyst regains its activity and its selectivity towards CO (Pfeifer,2008). 3.6 Calcination of Ni catalysts It has been reported that calcinated Nickel catalysts are highly effective (95% methane conversion with dry reforming) in tar decomposition and have long lifetime feasibility (no sintering and little carbon deposition
Iron Based Catalysts 29 4 Iron Based Catalysts Introduction: Although nickel based catalysts are the ones that have been most widely studied the past 20 years, due to their disadvantages researchers have turned towards alternative catalyst investigation in order to achieve sufficient tar cracking and reforming. Development of iron based catalysts seems to have gained a lot of attention as it is a promising technology for the decrease of the tar content in the product gas, while catalyzing the water gas shift reaction also. One of their basic advantages is that they can be applied in situ as they are not so prominent to be deactivated by carbon deposition, compared to nickel catalysts. This is very important as the use of a second reactor to place the catalyst increases a lot the cost of a facility (Virginie,2010b). In addition, they are cheaper and the disposal of used iron catalysts is not toxic, as it is in the case of nickel based catalysts (Azhar Uddin,2008). Of course they can be regenerated and iron is naturally abundant (Virginie,2010b), which makes their use appealing. However, Ni catalysts have much higher reforming abilities for hydrocarbons decomposition than metallic Fe catalysts due to the high ability of these catalysts to decompose the C-H and C-C bonds on their metallic surface. So, iron catalysts need to be much more investigated (Guan,2012). So far, many different types of Fe-based catalysts have been studied. Most commonly, researches focus on the impregnation of iron onto olivine or onto a support that seems to enhance the activity of the catalyst. These types of catalysts are quite effective (65% conversion at 850 ⁰C), inexpensive, non-toxic and have a good ageing behavior. They can also act as an oxygen carrier that transfers oxygen from the combustor to the gasifier to burn volatile compounds (Virginie,2012). 4.1 Mechanism of biomass tar decomposition over Fe-based catalysts There are three basic catalytic methods for the use of Fe-based catalysts. The most common one is the impregnation of iron on a bed material (usually olivine) for in situ use. Alternatively the catalytic material can be used in the freeboard of the gasifier or it can be placed in a secondary reactor (Nordgreen,2012). Ferrous materials are able to catalyze the reactions of the main components of the fuel gas (H2, CO, CO2, H2O) pyrolysis and tar decomposition. They can also decompose NH3 and HCN efficiently (Simell,1992). The mechanism of tar reforming includes the decomposition of tar over the iron based catalyst followed by Water Gas Shift Reaction (Azhar Uddin,2008), (Noichi,2010). Steam reforming over a Fe-Based catalyst (Sarvaramini,2012): H2O + Fe ↔ FeO + H2 ΔH⁰=14.4 Kj/mol (800⁰C) (31) C6H6 + H2O → 6CO + H2 (32) Phenol was used as a tar model compound for decomposition over Fe catalysts to describe the mechanism of tar cracking. The phenol is adsorbed separately on the iron oxide surface to form surface phenoxy species. The derived hydrocarbon fragments from the phenoxy adsorbed species are oxidized by labile, -O and/or – OH species of support via a back spillover process and by –OH groups residing on iron oxide surfaces to form H2, CO, CO2 and WGSR (Virginie,2010b). Steam reforming of phenol: C6H5OH + 5H2O→6CO + 8H2 (33) CO + H2O↔CO2 + H2 (34)
Page 1: Betreuer der Arbeit: Dipl. Ing. Mat
Page 5: Acknowledgements This work was acco
Page 8 and 9: II Content Content Acknowledgements
Page 10 and 11: IV List of Figures List of Figures
Page 12 and 13: VI List of Abbreviations List of Ab
Page 14 and 15: VIII Notation Notation ΔH [Kj/mol]
Page 17 and 18: Introduction 1 1 Introduction The d
Page 19 and 20: Introduction 3 1.2 Tasks The presen
Page 21 and 22: Biomass Gasification 5 2 Biomass Ga
Page 23 and 24: Biomass Gasification 7 Spouted Bed
Page 25 and 26: Biomass Gasification 9 2.2 Gas Clea
Page 27 and 28: Biomass Gasification 11 types of ca
Page 29 and 30: Biomass Gasification 13 Mechanical
Page 31 and 32: Biomass Gasification 15 Partial oxi
Page 33 and 34: Biomass Gasification 17 The followi
Page 35 and 36: Biomass Gasification 19 Groups of C
Page 37 and 38: Nickel Based Catalysts 21 even show
Page 39 and 40: Nickel Based Catalysts 23 The conte
Page 41 and 42: Nickel Based Catalysts 25 ing press
Page 43: Nickel Based Catalysts 27 3.4 Proce
Page 47 and 48: Iron Based Catalysts 31 Measurement
Page 49 and 50: Iron Based Catalysts 33 The percent
Page 51 and 52: Iron Based Catalysts 35 4.6 Catalys
Page 53 and 54: Iron Based Catalysts 37 Reduction i
Page 55 and 56: Iron Based Catalysts 39 and MgO (Di
Page 57 and 58: Precious Metal Catalysts 41 5 Preci
Page 59 and 60: Precious Metal Catalysts 43 Bimetal
Page 61 and 62: Experiments 45 Table 3: Biomass fee
Page 63 and 64: Experiments 47 6.3.1Experimental Pr
Page 65 and 66: Experiments 49 6.3.2 Tar Sampling M
Page 67 and 68: Results and Discussion 51 7 Results
Page 69 and 70: Results and Discussion 53 7.2 Sampl
Page 71 and 72: Results and Discussion 55 Wet Gas C
Page 73 and 74: Results and Discussion 57 er, the v
Page 75 and 76: Results and Discussion 59 7.3.1. Ef
Page 77 and 78: Results and Discussion 61 The conve
Page 79 and 80: Results and Discussion 63 7.3.3 App
Page 81 and 82: Results and Discussion 65 7.4 Blank
Page 83 and 84: Summary/Outlook 67 The aim is to pr
Page 85 and 86: References 69 (Aznar,1993) Aznar, M
Page 87 and 88: References 71 (Hepola,1997b) Hepola
Page 89 and 90: References 73 (Sarvaramini,2012) Sa
Page 91 and 92: 75 Appendix Appendix A: Iron based

Dipl. Ing. Matthias Mayerhofer Technische Universität München ...

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