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20 Nickel Based Catalysts 3 Nickel Based catalysts Introduction: The most commonly preferred for the reforming of methane and hydrocarbons. General Composition of Ni-based Catalysts, which is shown schematically below, Figure 10: a) Ni element which is considered as the active site of the catalyst b) Support to give mechanical strength and protection c) Promoter to ensure the economical operations (Mg, alkali metals such as potassium). CnHm+H2O CeO2 Promoter Ni γ-Al2O3 CO+CH4+H2 Figure 10: Composition of Ni-based catalysts Support Although in some studies it was reported that the Ni content of the catalyst affects the decomposition of the hydrocarbons. The catalyst A1 containing ~13 wt % Ni was more effective (90%) than the catalyst B which contained ~2 wt% Ni (Hepola,1997a). In other the contradictory result was concluded, all olivine impregnated catalysts resulted in similar tar decomposition (~76%) although the Ni content was different (3 or 6 wt% Ni) (Zhang,2007). It is important to mention that in both cases synthetic gas was used with toluene as a tar model compound. Supports protect the catalyst from sintering and carbon deposition if a strong nickel-support interaction exists initially, as they provide a high surface area. The most commonly tested supports for steam reforming are: i. Zirconia based which is active in tar decomposition and has a minor tendency towards coking and deactivation by sulfur. However, it has moderate effect on ammonia decomposition and a low activity regarding lower hydrocarbons (Rönkkönen,2011b). ii. Olivine contains iron so the stabilization of Ni is enhanced. The hardness, density and basicity of the catalyst are compatible with the gasification environment. It also accelerates the reaction of steam with absorbed gaseous species. Olivine is has natural characteristics, such as hardness, density and basicity, that could be combined with high effectiveness of nickel to obtain a very promising catalyst (Zhang,2007). In some tests it
Nickel Based Catalysts 21 even showed good ageing behavior, at least 260h at 800 °C and no sintering of nickel particles or carbon deposition was evident (Zhang,2007), (Courson,2000). iii. a-alumina supports are the most commonly used supports because they are cheap and sufficiently active (Abu El-Rub,2004). But they are not stable and deactivate easily (Pfeifer,2008). A-alumina supports have a favorable surface area, provide mechanical strength and its surface chemistry allows dispersion of metal phases. These supports accumulate coke and enhance the catalytic activity (Richardson,1997). iv. Dolomite when used as a support has very good efficiency; it is capable of 97% tar removal at 750°C (Świerczyński,2007), (Swierczynski,2008). Also it was reported that almost complete tar conversion can occur at 650 °C (Wang,2004). v. Calcium Oxide (CaO) is a well-known catalyst support used for both dry and steam reforming (Pfeifer,2008). Promoters are known for their ability to increase the activity and/or stability of the catalyst. They can also affect its reducibility, regenerability and coke resistance (Anis,2011). i. Potassium (K) has been reported to enhance significantly the Ni-based catalysts activity (Arauzo,1997). They also affect the neutralization of the support surface acidity and reduce the coke deposition on the catalyst (Abu El-Rub,2004). ii. Magnesium (Mg) reinforces the stabilization of the Ni crystallite size (Abu El-Rub,2004) and improves the resistance to attrition but increases the coke production (Arauzo,1997). iii. Cerium Oxide CeO2 is very effective in preventing carbon deposition. As the surface of the available O2 content is higher due to the increased crystal oxygen on the catalyst surface the redox reaction is being favored during steam reforming. It adsorbs water and dissociates it resulting in –O and –OH transferring to the nickel and reacting with carbon on the catalysts surface to form CO or CO2 (Zhang,2007). iv. Calcium Oxide (CaO) is also used as a promoter to reduce carbon deposition (Anis,2011). v. Chromium (Cr) it is mostly known for its ability to inhibit the encapsulation of nickel crystallites by inactive carbon filaments. It also increases the number of active nickel sites and has a significant effect on the surface metal dispersion and pore dimensions (Bangala,1998). Generally, the containment of oxygen in the support or the promoter can have a very positive effect in tar reforming due to the occurrence of the reduction-oxidation (Redox) Eq. (27); all chemical reactions in which atoms have their oxidation state changed. CnHm + O2 → n CO2 + Characteristics of Ni-based catalysts: H2O (27) Advantages: They are 8-10 times more active than other commonly used catalysts, such as calcinated dolomites. It has been reported that complete tar elimination is achieved for catalytic reaction around 900°C and they are also capable of increasing the yields of either of CO and H2O by the water-shift reaction (Abu El-Rub,2004). According to Le Chatelier’s principle, when the temperature increases the equilibrium shifts towards CO/H2O, whereas when it decreases, the equilibrium shifts towards H2/CO2 production. Methane formation reaction is favored in lower temperatures (Anis,2011) and reverse ammonia conversion reaction occurs too (Dayton,2002). As a result they can be extensively used for high-temperature steam reforming reactions of hydrocarbons and ammonia (Hepola,1997b).
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: Biomass Gasification 19 Groups of C
Page 39 and 40: Nickel Based Catalysts 23 The conte
Page 41 and 42: Nickel Based Catalysts 25 ing press
Page 43 and 44: Nickel Based Catalysts 27 3.4 Proce
Page 45 and 46: Iron Based Catalysts 29 4 Iron Base
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
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Dipl. Ing. Matthias Mayerhofer Technische Universität München ...

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