H 2 Production for Fuel Cells and Catalyst Deactivation

Transcription

1 KE Environmental catalysis H 2 Production for Fuel Cells and Catalyst Deactivation D.Sc. Reetta Kaila Helsinki University of Technology (TKK) Department of Biotechnology and Chemical Technology December 2 nd 2008

2 Energy consumption & Reduction of Emissions

3 Energy consumption Energy demand continues to climb as the world population grows CO 2 emissions increase with the energy consumption Source: International Energy Agency (IEA), CO2 Emissions (M tons) Energy consumption (quadrillion btu) CO2 emissions 100 Energy consumption

4 CO 2 emissions Source:

5 World Primary Energy Supply by Fuel (Mtoe) from 1971 to 2005 International Energy Agency: KEY WORLD ENERGY STATISTICS 2007.

6 Renewable Energy Sources World Marketed Energy Use by Energy Type, World Oil Consumption by Sector, Fossil fuels should be replaced with renewable fuels to reduce emissions!

7 Reduction of emissions Emissions and exhaust gases (e.g., CO 2 ) X Greenhouse effect (global warming) Developing of more efficient technologies is important especially for mobile applications Fuel cells High energy efficiency Lower emission levels H 2 + O 2- H 2 O + 2 e - Fuel Exhaust gas Source: Hydrogen Energy and Fuel Cells A Vision for Our Future (EU, 2003)

8 Fuel cells

9 What is a fuel cell (FC)? It will produce energy in the form of electricity and heat as long as fuel is supplied. In principle, a fuel cell operates like a battery. Unlike a battery, a fuel cell does not run down or require recharging. On the catalyst the hydrogen atom splits into a proton and an electron. A fuel cell consists of two electrodes and an electrolyte.

10 FC characteristics Fuel cell type PEMFC PAFC MCFC SOFC (solid oxide) ELECTROLYTE Polymer Phosphoric Molten Carbonate Ceramic Acid Salt OP.TEMPERATURE 80 C 190 C 650 C 1000 C FUELS H2 Reformate H2 Reformate H2/CO/ Reformate H2/CO2/CH4 Reformate REFORMING External External External/Internal External/Internal OXIDANT O2/Air O2/Air CO2/O2/Air O2/Air EFFICIENCY (HHV) 40-50% 40-50% 50-60% 45-55%

11 SOFC reactions Working principle of the SOFC (Wärtsilä). Topsoe Fuel Cell (SOFC) MEA stands for Membrane Electrode Assembly. (Image: Xcellsis)

12 On-board reformer + FC e - Liquid hydrocarbon fuels (e.g., gasoline, diesel) Air (O 2 + N 2 ) H 2 O On-board reformer H 2 -rich fuel gas (H 2,CO,CO 2, H 2 O,CH 4 ) Anode Electrolyte Cathode Fuel Cell (SOFC) Air (O 2 + N 2 ) Exhaust gas (H 2 O + CO 2 )

13 FC system driven on MeOH DaimlerChrysler, 2001, Methanol (25 l ) CH 3 OH+ H 2 O = 3 H 2 + CO 2

14 Car makers will use fuel cells in the next decade to replace the internal combustion engine "We started with the premise, what if we were inventing the automobile today rather than a century ago? What might we do differently," said GM president and CEO Rick Wagoner.

15 Fuel Cells Hydrogen Energy and Fuel Cells A Vision for Our Future (EU, 2003)

16 Hydrogen H2 + ½ O2 = H2O + energy

17 Hydrogen Hydrogen is considered as a clean fuel no GHG emissions Hydrogen is not available in the nature Energy is needed to produce H 2 from primary fuels (H 2 O, HCs) H 2 is consumed e.g. in fuel cells for energy production Figure 1: Emission less hydrogen fuel cycle. The only emissions are oxygen at the electrolysis plant and water vapour at the site of consumption. (Image: US DoE/EREN)

18 H 2 as an energy carrier Hydrogen (H 2 ) is not a primary source for energy, but an energy carrier: Natural gas, crude oil, biomass, water Energy supply (e.g., solar, wind, hydro, nuclear) H 2 Stationary energy production (FC s) Transportation (FC s) Energy released Industry (Reactant) (MeOH, NH 3, oil refinery) Energy released

19 Hydrogen (H 2 ) Hydrogen Energy and Fuel Cells A Vision for Our Future (EU, 2003)

20 Production methods for H 2 Steam reforming of natural gas (NG) (industry!), hydrocarbons and alcohols Partial oxidation of hydrocarbons Gasification and pyrolysis of coke or biomass Electrolysis of water Photo electrolysis Biological production

21 Reactions of CH 4 to H 2 Steam reforming (SR) CH H O H Partial oxidation (POX) CH 0.5O H CO CO Δ r H 298 H 2 /CO kj/mol 3-36 kj/mol 2 Dry reforming (DR) CH + CO H 2CO kj/mol 1

22 SR of natural gas (NG) Industrial process for H2 production Tubular fixed bed reactor Ni-based catalysts SR is an endothermic reaction consumes energy requires high temperatures ( C) + Stationary systems (industry) + In the vicinity of natural gas pipelines -Transportation of natural gas or hydrogen (liquidized or compressed) is challenging Liquid fuels (e.g., C x H y ) become attractive for mobile applications

23 Liquid fuels as source for H2

24 Liquid hydrocarbons (C x H y ) Advantages of commercial fuels (e.g., diesel): Availability (infrastructure) Easy to deliver and store High volumetric H 2 density n-hexadecane (C 16 H 34 ) H atom C atom C16 H H 2O 25H CO ΔH 700C = 2806 kj/mol Fuel Disadvantages of commercial fuels (e.g., diesel): The SR reaction enthalpy increases with the C-chain length Demand for energy increases A complex mixture of aliphatic and aromatic hydrocarbons (PAHs) Undesired side reactions are possible Sulfur compounds poison the conventional reforming catalyst

25 SR of higher hydrocarbons C x H y + x H 2 O x CO + (x + y/2) H 2 ΔH > 0 C 7 H H 2 O 7 CO + 15 H 2 ΔH 298 =1107 kj/mol C 12 H H 2 O 12 CO + 25 H 2 + Water gas shift reaction (WGS): CO + H 2 O CO 2 + H 2 + Methanation: CO + 3 H 2 CH 4 + H 2 O ΔH 298 =1866 kj/mol ΔH 298 = -41 kj/ mol ΔH 298 = -206 kj/mol

26 H 2 density in liquid fuels Fuel Volumetric H 2 density Enthalpy of SR ΔH 700 C kmol/50 dm 3 Fuel kj/mol Fuel kj/mol H2 Hydroge n gas, 700 bar (CGH 2 ) liquid, -253 C (LH 2 ) Fossil fuels Natural gas (NG) gas, 700 bar liquid, -162 C 3.5 Gasoline (C 7 H 14 ) Diesel (C 16 H 34 ) Renewable Methanol fuels Ethanol nd generation biodiesel (BTL, C 16 H 34 ) Water * * Electrolysis: H 2 O = H 2 + ½ O 2, ΔG = 237 kj/mol H2. CGH 2 = compressed gaseous hydrogen LH 2 = liquidised hydrogen

27 Renewable fuels 399 gco 2 /kwh 43 gco 2 /kwh Biomass Fossil fuels Bio oils Renewable fuels Fermentation Gasification Transesterification FTsynthesis Ethanol (aq) Biogas BTL-diesel Biodiesel Biodiesel Glycerol (aq) ATR Biochemicals Synthesis gas FTsynthesis SOFC Bioenergy Are e.g.water or methanol, biofuels?

28 Terminology for Energy Commodities International Energy Agency: ENERGY STATISTICS MANUAL 2005.

29 Autothermal Reforming

30 Autothermal reforming (ATR) Combines endothermic SR with exothermic partial oxidation (POX) FUEL e.g. diesel (C 16 H 34 ) Heat (Q) H 2 O O 2 O 2 Endothermic Steam reforming (SR) dh700 = 2806 kj/mol Exothermic Partial Oxidation (POX) dh700 = kj/mol Complete Oxidation (OX) Thermal cracking H 2 + CO = Thermoneutral ATR CO 2 +H 2 O CH 4 +C 2 H 4 +C x H 2x Optimise: 1) ΔHPOX X POX + ΔHSR X SR = 0 Coke deposition 2) Heat prod. H 2 production Reaction conditions important!

31 Autothermal reforming (ATR) Hydrocarbon or alcohol fuels H 2 O Autothermal reforming (ATR) O 2 O 2 Endothermic Steam reforming (SR) heat Exothermic Partial Oxidation (POX) Oxidation (OX) Thermal cracking H 2 + CO CH 4 + C 2 H 4 + C x H 2x H 2 O + CO 2 H 2 O Water gas shift (WGS) H 2 O + CO H 2 + CO 2 Methanation 3 + CO CH + H O H Boudouard CO CO + C 2 2 ( s ) Carbon deposition C H y 2 H + C x y / 2 ( s ) C (s) H 2 O SR H O C s H ( ) CO Dry reforming (DR) CO + C s 2 CO 2 ( ) H 2 + CO 2 CH 4 + H 2 O CO 2 H 2 + CO CO H 2 In addition to reaction conditions,also the catalyst plays a crucial role in ATR of liquid hydrocarbons!

32 Thermodynamic equilibrium The limits of chemical reactions are set by the thermodynamic equilibrium Product flow composition (mol-%) H 2 O CH 4 C C 3 H 6 CO 2 CO C 4 H Temperature ( C) The thermodynamic equilibrium gives the direction of a reaction, but a catalyst is needed to speed up the reaction and to reach the goal. H 2

33 Reforming catalysts

34 Definition of a catalyst Energy Reactants Catalyst Products Balance ~ Thermodynamic equilibrium Reaction

35 Demands for a ATR catalysts Active catalyst for ATR (high conversions) Selective catalyst for ATR (produces synthesis gas with no undesired side products) Thermally stable catalyst ( C) Tolerate sulfur present in fossil fuels Prevent coke deposition (blockage of catalyst bed must be avoided) Economically viable ATR catalyst low metal loading e - long life time Commercial, liquid fuels (e.g., gasoline, diesel) Air (O 2 + N 2 ) H 2 O On-board Reformer (ATR) H 2 -rich fuel gas Anode Electrolyte Cathode Exhaust gas (H 2 O + CO 2 ) Air (O 2 + N 2 ) Fuel Cell (SOFC)

36 Heterogeneous catalysis Reactants The reaction takes place on the active sites of the catalyst surface: 10 mm Catalyst bed Support Products Active sites Surface area (e.g., m 2 /g)

37 Active sites of a catalyst 2 µm Metallic sites Support 2 mm = 2000 µm

38 Preparation of a heterogenous ATR catalysts Grinding of the support Support calcination Catalyst impregnation Catalyst drying Catalyst calcination Impregnated catalyst Calcined support Noble metal precursors Fresh catalyst

39 Catalyst testing in lab-scale I Reactor Catalyst bed

40 Catalyst testing in lab-scale II Flow control Reactants Pressure gauge Temperature control ( C) Products Analysis (GC, FTIR)

41 Catalyst testing in lab-scale III Nitrogen (g) FC N 2 purge and dilution Argon (g) CO 2 (g) H 2 (g) Air (g) FC FC FC FC Mixing vessel Filters Flow controllers PI Furnace, reactor and the catalyst bed TIC FT - IR Vaporiser TIC TIC PI Filters FIC Cooling water TI Hydrocarbons (l) H 2 O (l) FIC HPLC-pumps by-pass TIC Liquids FI Dry gas meter

42 Reforming catalysts The conventional reforming catalysts (Ni-based) Highly active for SR of natural gas Do not tolerate sulfur Strong carbon formation Fresh Ni catalyst Inert SiC Used Ni catalyst The risk for carbon formation increases if the fuel contains C-C bonds The risk for carbon formation is high, especially with ethanol! Methanol (C1-compound) becomes interesting. However, it is very poisoneous! For ATR of liquid fuels, more stable catalysts are needed.

43 Reactions on catalyst surface H 2 O C x H y H 2 O 2 CO 2 CO Active sites (Metallic clusters) C C C *CH 2 C CO Metal-support Interface (active) Support The reaction and the product distribution are affected by: The type of active site, the support and their interactions The physical and chemical properties of the catalyst Catalyst deactivation

44 Catalyst deactivation

45 Coke deposition Catalyst particles Catalyst monoliths Tested catalyst Fresh catalyst

46 Coke formation Dissociation of hydrocarbons: C x H y m C (s) + C x-m H y-2m +m H 2 (irreversible) CH 4 C (s) + 2 H 2 (reversible) 2 CO (g) CO 2 (g) + C (s) (Boudouard reaction) CO (g) + H 2 (g) H 2 O (g) + C (s) The type of coke depends among others from the reactant (aliphatic/aromatic HC), the reaction temperature and the catalyst. The formed coke can take part in the reactions.

47 Blockage of active sites Sulfur poisoning C x H y Coke deposition Active sites are blocked Reactions are hindered Side reactions become stronger, product distribution is degraded

48 Effect of H 2 S on products 4 b) Sequences with H 2 S H 2 Dry product flow (mmol/min) CO CO 2 CH 4 * Time on stream (min)

49 Effect of H 2 S on conversions Conversion (mol-%) a) X(DT) X(H 2 O) X(O 2 ) Sequences with H 2 S Time on stream (min) H2S decreases conversions of HC and water, but does not affect the conversion of O2! H2S*100 (ml/min)

50 Thermal stability Loss of active sites Sintering (migration) Breakage of the catalyst structure (encapsulation) Lower surface area and area of active sites Activity is decreased Side reactions become stronger, product distribution is degraded

51 Conclusions on deactivation Coke formation Poisoning Sintering of the active metal particles Sintering and encapsulation of metal particles

52 Implementation of Distributed Resources Multifuel plant CHP plant Hydro power Energy storage Multifuel plant Solar power Wind power Micro turbines Fuel cells Rolf Rosenberg, VTT

53 Literature Reviews 1. Cheekatamarla, P. K., Finnerty, C. M., Reforming catalysts for hydrogen generation in fuel cell applications, J. Power Sources 160 (2006) Naidja, A. et al., Cool flame partial oxidation and its role in combustion and reforming of fuels for fuel cell systems, Prog. Energy Combust. Sci. 29 (2003) Trimm, D. L., Önsan, Z. I., Onboard fuel conversion for hydrogen-fuel-cell-driven vehicles, Cat. Rev. - Sci. Eng. 43 (2001) Articles 1. Joensen, F., Rostrup-Nielsen, J. R., Conversion of hydrocarbons and alcohols for fuel cells, J. Power Sources 105 (2002) Palm, C. et al., Small-scale testing of precious metal catalyst in the autothermal reforming of various hydrocarbon feeds, J. Power Sources 106 (2002) Krumpelt, M. et al., Fuel processing for fuel cell systems in transportation and portable power applications, Catal. Today 77 (2002) Pettersson, L. J., Westerholm, R., State of the art of multi-fuel reformers for fuel cell vehicles: problem identification and research needs, Int. J. Hydrogen Energy 26 (2001) Brown, L. F., A comparative study of fuels for on-board hydrogen production for fuel-cell-powered automobiles, Int. J. Hydrogen Energy 26 (2001) Rostrup-Nielsen, J. R., Conversion of hydrocarbons and alcohols for fuel cells, Phys. Chem. Chem. Phys. 3 (2001) Books Rostrup-Nielsen, J. R., Catalytic Steam Reforming. in: Catalysis: Science and Technology, Vol 5, eds. J. R. Anderson, M. Boudart, Springler Verlag, Berlin, Heidelberg, New York, Tokyo 1984, pp

54 Web sites on H2 and FCs H FCs