Laurea in Scienza dei Materiali Materiali Inorganici Funzionali. Hydrogen production and storage: an overview

Transcription

1 Laurea in Scienza dei Materiali Materiali Inorganici Funzionali Hydrogen production and storage: an overview Prof. Dr. Antonella Glisenti -- Dip. Scienze Chimiche -- Università degli Studi di di Padova

2 Bibliography 1. J.D. Holladay, J. Hu, D.L. King, Y. Wang; Catalysis Today 139 (2009) Review: An overview of hydrogen production technologies 2. S. Abanades, P. Charvin, F. Lemont, G. Flamant; International Journal of Hydrogen Energy, 33 (2008) Novel two-step SnO 2 /SnO water-splitting cycle for solar thermochemical production of hydrogen

3 H 2 PRODUCTION 1. Reforming Reforming of hydrocarbons Reforming of alcohols Reforming of CO, CO 2 2. Pyrolisis 3. Hydrogen from water Electrolysis Thermolysis Photoelectrolysis 4. Hydrogen from biomass Gasification Bio-hydrogen Direct photolysis Fermentation Bio-catalysed electrolysis

4 Fuel processing of gaseous, liquid, and solid fuels for hydrogen production.

5 REFORMING Reforming Hydrocarbons Alcohols Steam reforming Partial oxidation Autothermal reforming

6 OTHER REACTIONS Water Gas Shift (WGS) Oxidation Coke Formation Minimum reaction temperatures required for avoiding coke formation during isooctane reforming at thermodynamic equilibrium

7 Steam Reforming Modest temperature required for SR (> 180 C for methanol, DME, oxygenated hydrocarbons, > 500 C for hydrocarbons) Precious metals and non precious metals as catalysts Mass and heat transfer limitations: kinetics and catalysts are rarely the limiting factors in conventional reactors microchannel-based reactors and Rh based (or Ni based) catalysts K, Mg to limitate C formation

8 Partial oxidation Non catalytic POX C Catalysts can be added to lower temperature Hard control of temperature (exthermal) thermal efficiency = 60-75% Autothermal reforming, OX, ATR, WGS Higher H 2 production with respect to POX Temperature control with the O 2 /H 2 O flow Favorable gas composition for Fischer Tropsch synthesis

9 Preferential oxidation and water-gas-shift Reforming: gas mixture with significant amounts of CO POX reactors followed by WGS reactor (to decrease CO) High temperature = fast kinetics = CO selectivity + Low Temperature Reactor to reduce CO amount Cu based catalysts, Molybdenum carbide, Ptbased, Fe-Pd + Reactor for CO methanation Higher complexity because carefully measured concentrations of air must be added

10 PYROLYSIS Hydrocarbon is decomposed into H 2 and C (without water or oxygen present). C n H m nc + 1/2m H 2 H = hydrocarbon dependent any organic material (fuel flexibility) production of hydrocarbons and carbon nanotubes and spheres. no carbon oxides are formed: no secondary reactors (WGS, PrOx, etc.); simplicity and compactness, emissions reduction.

11 PLASMA REFORMING Reactions = same as conventional reforming (POX, ATR, SR); Energy and free radicals are provided by a plasma (H, OH, and O radicals + electrons create conditions for reductive and oxidative reactions to occur) Overcomes many limitations of conventional techniques (cost and deterioration of the catalysts, limitations on H 2 production from heavy hydrocarbons) Can operate at lower temperatures If no catalysts are used the process is highly sulfur tolerant. Electrical requirements High electrode erosion Methane conversion as a function of power input Empty reactor: plasmatron air = 0.4 g/s, fuel = 0.27 g/s, additional air = 0.7 g/s. Water addition, g/s H 2 O added. Catalytic case: plasmatron air = 0.35 g/s, fuel = g/s, additional air = g/s. Water addition, g/s water.

12 AMMONIA REFORMING Ammonia cracking is endothermic and is the reverse of the synthesis reaction (500 C and 250 atm): cracking operates at T = C, and low pressures are preferred N 2(g) + 3H 2(g) 2 NH 3(g) H = -92.4kJ/mol Typical catalysts: iron oxide, molybdenum, ruthenium, and nickel. Inexpensive fuel (fertilizer production) with an extensive distribution system High energy density (8.9 kwh/kg), higher than methanol (6.2 kwh/kg), but less than diesel (13.2 kwh/kg) Simple leak detection In SOFCs, ammonia can be fed directly to the fuel cell without any reforming; purification processes for PEM

13 NON-REFORMING H 2 PRODUCTION Hydrogen from biomass Gasification Bio-hydrogen Direct photolysis Dark fermentation Photo fermentation Microbial electrolysis cell (MEC)

14 NON-REFORMING H 2 PRODUCTION Hydrogen from biomass Biomass: the most likely renewable organic substitute to petroleum. Second only to hydropower as a primary energy source among renewables. animal wastes, municipal solid wastes, crop residues, short rotation woody crops, agricultural wastes, sawdust, aquatic plants, short rotation herbaceous species, waste paper, corn,

15 GASIFICATION partial oxidation of the materials (biomass and coal) into a mixture of hydrogen, methane, carbon monoxide, carbon dioxide, and nitrogen is very mature and commercially used with or without a catalyst + steam and/or oxygen = steam reforming and production of a syngas stream (H 2 to CO ratio of 2:1), Fischer-Tropsch higher hydrocarbons, or WGS for hydrogen production

16 GASIFICATION: disadvantages Low thermal efficiency (moisture contained in the biomass must also be vaporized) Significant amount of tar: Rh/CeO 2 /M (M = SiO 2, Al 2 O 3, and ZrO 2 ) catalyst to reduce the tar formation Air: significant dilution of the products, production of NOx: Low cost, efficient oxygen separators Resources to gather biomasses to the central processing plant, high logistics costs = smaller efficient distributed gasification plants Production yield for thermal decomposition and superheated steam reforming

17 BIOGAS PRODUCTION

18 Fermentation The biomass used needs to be biodegradable available in high quantities, inexpensive, high carbohydrate content The pathways, yields, rate of production are dependent on the type of bacteria Current efficiency 2-3%; theoretical limit = 68% Standard fermentation Dark fermentation Photo fermentation (IR)

19 Solar energy hydrogen production methods

20 DIRECT PHOTOLYSIS Photosynthesis uses solar energy to convert carbon dioxide and water to carbohydrates and oxygen. For some organisms, excess solar energy is used to produce hydrogen via direct photolysis of water. Researchers are trying to engineer algae and bacteria so the majority of the solar energy is devoted to hydrogen production Direct photolysis process.

21 DIRECT PHOTOLYSIS: Advantages and Disadvantages Primary feed is low cost Significant surface area to collect sufficient light The microorganisms in addition to producing hydrogen, produce oxygen Stop of hydrogen production Significant safety and separation issues occur To identify or engineer less oxygen sensitive organisms To separate the hydrogen and oxygen cycles To change the ratio of photosynthesis to respiration

22 MICROBIAL ELECTROLYSIS CELL - MEC bioelectrochemically assisted microbial reactor (BEAMR), Conversion of waste into Energy

23 MICROBIAL ELECTROLYSIS CELL - MEC Use electrohydrogenesis to directly convert biodegradable material into hydrogen. Exoelectrogens microorganisms, decompose (oxidize) organic material and transfer electrons to the anode. The electrons combine at the cathode, after traveling through an external load, with protons and oxygen forming water. A MEC operates in anaerobic state and an external voltage is applied The theoretical potential for hydrogen production at ph 7 is V, (vs. Ag/AgCl). Exoelectrogens generate an anode potential of approximately V an = -0.5 V. Therefore the minimum applied potential is 0.11 V (applied voltage is > 0.3 V due to electrode overpotentials and ohmic resistance) The design of MEC systems initially used similar components as used in PEM fuel cells.

24 Hydrogen production rate of different types of bio-hydrogen processes Bio-hydrogen system H 2 synthesis rate Bio-reactor volume (m 3 ) for 5kW PEMFC Direct photolysis 0.07 mmol H 2 /l h 1707 Photo-fermentation 0.16 mmol H 2 /l h 747 Dark fermentation mmol H 2 /l h MEC 5.8 mmol H 2 /l h 21

25 NON-REFORMING H 2 PRODUCTION Hydrogen from water Thermochemical water splitting Photoelectrolysis Electrolysis Alkaline electrolyzers Proton exhange membrane electrolyzers Solid oxide electrolysis cells.

26 Solar energy hydrogen production methods

27 THERMOCHEMICAL WATER SPLITTING

28 THERMOCHEMICAL WATER SPLITTING Heat alone is used to decompose water to hydrogen and oxygen; efficiencies close to 50% are achievable Water decomposition T = 2500 C, Chemical reagents to lower T; more than 300 water splitting cycles referenced in the literature

29 THERMOCHEMICAL WATER- SPLITTING CYCLES A thermochemical cycle: multi-step decomposition of water into hydrogen and oxygen using only heat. Thermochemical cycles are expected to be more efficient than electrolysis for H 2 production Avoids greenhouse gas emission; allows complete recycling of chemicals. Solves the problem of long-term efficient thermal energy storage, by converting solar radiation in storable and transportable chemical fuels. The produced solar H 2 fuel can be processed for heat and power generation, supplied as a clean energy carrier.

30 TWS: TWO STEPS CYCLES Involves reactions of metal-oxide redox pairs. First step (water-splitting): oxidation of the active redox material (reduced state of a metal oxide) by water steam produces hydrogen. Second step (driven by solar thermal energy), reduction of the metal oxide = active material regeneration with release of oxygen. Schematic representation of the two-step water-splitting cycle

31 ADVANTAGES (i) The maximum temperature of the cycle (1200 to 1800 C) is compatible with renewable solar thermal energy; (ii) Water and heat are the only inputs, hydrogen and oxygen the only outputs; (iii) H 2 ando 2 are produced separately; (iv) The other chemicals and reagents are recycled in a closed cycle; (v) The produced H 2 (PEMFC). is pure and can be directly processed, e.g., Switzerland (ETHZ, PSI), Germany (DLR, EC funded project Hydrosol ), France (CNRS-PROMES), USA (DOE funded projects) and Japan.

32 Cycle Criteria (i) Within the temperatures considered the G of the individual reactions must approach zero; (ii) The number of steps must be minimal (iii) Each individual step must have both fast reaction rates and rates which are similar to the other steps in the process (iv) The reaction products cannot results in chemical-by-products and any separation of the reaction products must be minimal in terms of cost and energy consumption; (v) Intermediate products must be easily handled

33 Volatile Metal Oxide Cycles Volatile ZnO/Zn: one of the best candidate for coupling with a solar energy source. ZnO/Zn ZnO Zn + ½ O 2 T = 1800 C Zn + H 2 O ZnO + H 2 T = 475 C Totale: H 2 O H 2 + ½ O 2 ZnO is decomposed near 1800 C in a solar reactor and Zn is recovered after quenching the product gases. Zn nanoparticles (size nm) = yielding up to 70% H 2 Energy efficiency of ZnO/Zn cycle = 45% The recombination of Zn and O 2 is a parasitic reverse reaction limiting the Zn yield after the solar step (separation, quenching, fast cool).

34

35

36

37 THERMOCHEMICAL WATER SPLITTING: The most promising cycles Ispra Mark-10 2H 2 O + SO 2 + I 2 + 4NH 3 2NH 4 I + (NH 4 ) 2 SO 4 T = 50 C 2NH 4 I 2NH 3 + H 2 + I 2 T = 630 C (NH 4 ) 2 SO 4 + Na 2 SO 4 Na 2 S 2 O 7 + H 2 O + 2NH 3 T = 400 C Na 2 S 2 O 7 SO 3 + Na 2 SO 4 T = 550 C SO 3 SO 2 + ½ O 2 T = 870 C Total: H 2 O H 2 + ½ O 2

38 THERMOCHEMICAL WATER SPLITTING: The most promising cycles Sulphuric acid decomposition H 2 SO 4 H 2 O + SO 2 + ½ O 2 T =? 2H 2 O + Br 2 + SO 2 H 2 SO 4 + 2HBr T =? 2HBr Br 2 + H 2 T =? SnO/Sn Total: H 2 O H 2 + ½ O 2 Solar Reduction Step SnO 2(s) SnO (g) + ½ O 2(g) T = 1600 C Hydrolysis Step SnO (s) + H 2 O (g) SnO 2(s) + H 2(g) T = 550 C Total: H 2 O H 2 + ½ O 2

39 Ceria and iron oxides easy redox couples

40 and perovskites? SOFC cathode mixed ionic-electronic conduction good surface exchange kinetics for Oxygen reduction reaction (ORR). Enhancement of ionic conductivity due to oxygen vacancy generation Large thermal reduction capabilities at relatively modest temperatures

41 and perovskites? Reducibility (δ and T) depend on composition and doping

42 O 2 production determined from TGA H 2 determined from TGA 1st and 2nd = after the first and second water-splitting step Legend of the histograms indicates the temperatures of reduction/reoxidation.

43 H 2 -Production Technologies: Summary Table

44 ALKALINE ELECTROLYZER Electrolyte = 30 wt% KOH or NaOH Cathode = Ni with a catalytic coating, Pt Anode = Ni or Co coated with metal oxides (Mn, W, Ru) Anode: 4OH - O 2 + 2H 2 O + 4 e - Cathode: 4H 2 O + 4 e - 2H 2 + 4OH - Total: 2H 2 O 2H 2 + O 2 The liquid electrolyte is not consumed in the reaction, but must be replenished because of losses during H 2 recovery. Current density = macm -2 Efficiencies = 50 60%

45 PROTON EXCHANGE MEMBRANE ELECTROLYZER Electrolyte = H-Conducting polymer (Nafion) Cathode, Anode = Pt black, Ir, Ru, and Rh Anode: 2H 2 O O 2 + 4H e - Cathode: 4H e - 2H 2 Total: 2H 2 O 2H 2 + O 2 Current density = 1600 macm -2 Efficiencies = 55 70%

46 SOLID OXIDE ELECTROLYSIS CELLS Solid oxide electrolysis cells (SOEC) are essentially solid oxide fuel cells operating in reverse. These systems replace part of the electrical energy required to split water with thermal energy Electrolyte = YSZ, Anode = Ni containing YSZ Cathode = metal doped La-based oxides Anode: 2O 2- O e - Cathode: 2H 2 O + 4 e - 2H 2 + 2O 2- Total: 2H 2 O 2H 2 + O 2 Efficiencies = 60, 85-90% Energy demand for water and steam electrolysis