Århus Maskinmesterskole 1 Hydrogen for the future?

Århus Maskinmesterskole 2 Hydrogen was produced first by Cavendish in 1766 by reacting metals with acids. Is hydrogen the ideal fuel for the future? FuelkWh/kg H2H2 33 Natural Gas13.9 Gasoline12.7 Diesel11.6 Hydrogen is one of the most common substances in the Universe. However, we need to produce it from either fossil fuels (reforming), water (by electrolysis) or biomass (for instance gasification).

Århus Maskinmesterskole 3 Transition to a Hydrogen Economy?

Århus Maskinmesterskole 4 The Hindenburg accident (6th of maj 1937) Hydrogen safety… Source: ”The Hindenburg tragedy revisited: The fatal flaw found”, Addison Bain & W. D. Van Vorst, 1999 Safety facts about hydrogen (H 2 ) Flame appearance Very light blue to invisible SmellNone Explosion limits (in air) 4-74% vol. (if ignited) (CH 4 limit is very narrow) DiffusivityExtremely fast (CH4 is quite slow) Ignition energy0.02mJ Flame velocity m/s (risk of back-flash) (CH 4 ~ 37-45m/s) Minimum flame diameter0.6mm (CH 4 ~ 2mm) Flame temperature2050°C

Århus Maskinmesterskole 5 Hydrogen: Pro-et-Contra Pros: ∙Hydrogen can be transported cheaply by pipelines ∙Used in fuel cells, only pure water is product ∙Heat of combustion per unit weight is larger than for any other fuel ∙Leaks dissipate rapidly under conditions of good ventilation ∙It is less toxic than other fuels

Århus Maskinmesterskole 6 Hydrogen: Pro-et-Contra Contra: ∙A secondary fuel source is needed to produce hydrogen! ∙Storage requires high pressure, liquefaction, or extensive chemical systems ∙Liquefaction temperature is extremely low (20.3 K) ∙The heat of combustion per unit volume is very small ∙The ignition energy in air mixtures is low ∙The range of flammability in air mixtures is very broad

Århus Maskinmesterskole 7 Hydrogen for the future?

Århus Maskinmesterskole 8 Hydrogen Sources:

Århus Maskinmesterskole 9 Hydrocarbon Reforming Principles: Steam Reforming Hydrocarbons react with steam (endothermic) ~ 75% H 2 in gas Partial Oxidation Hydrocarbons react with oxidant (exothermic) ~ 40% H 2 Autothermal Reforming Hydrocarbons react with steam and oxidant (H~0) ~ 60% H 2 Synthesis Gas H 2, CO, CO 2, CH 4 & Water (+N 2 ) Reformer Fuel Air Products (Reformate or Syngas) H 2, CO 2, CO, CH 4, N 2 CnHmOpCnHmOp (Chemical Reactor) Water

Århus Maskinmesterskole 10 Steam Reforming (SR - Typically Natural Gas) Methane: Shift Conversion: (500°C-800°C) (Step-Wise) 95% of all hydrogen is produced like this today! Carbon Formation IF there’s absence of air or steam! And risk for Reactions (endothermic)

Århus Maskinmesterskole 11 Chemical Equilibrium Reactor Modeling ”Steam To Carbon Ratio” Steam Reforming Of DNG, p=1.013 bar, S/C= Temperature, [K] Molar Fractions, [-] CO H2 CO2 CH4 H2O C(gr) N2 NH3 Steam Reforming Of CH4, p=1.013 bar, S/C= Temperature, [K] Molar Fractions. [-] CO H2 CO2 CH4 H2O C(gr)

Århus Maskinmesterskole 12 Equilibrium Constant for Water-Gas-Shift Reaction

Århus Maskinmesterskole 13 Equilibrium for Carbon Formation C(gr): Carbon formation takes place above 650°C mainly via the pyrolysis reaction: IF there’s absence of air or steam!

Århus Maskinmesterskole 14 An Industrial Steam Reformer:

Århus Maskinmesterskole 15 Shape of various catalysts and supports:

Århus Maskinmesterskole 16 Partial Oxidation (Exothermic) Methane: Shift Conversion: (1200°C-1500°C) Autothermal Reforming Methane: Shift Conversion: Methane:

Århus Maskinmesterskole 17 Autothermal Reforming of Danish Natural Gas:

Århus Maskinmesterskole 18 Hydrogen for the future?

Århus Maskinmesterskole 19 Fuel Processing: Source: J W Niemandsverdriet, 2005

Århus Maskinmesterskole 20 PEM-Steam Reforming System

Århus Maskinmesterskole 21 Automotive PEM-system with on-board reforming Power densities are set assuming automotive targets are met ~ 1kg/kW. Presently only very few experiences with control of such systems! PEM’s are poisoned by impurities in gas – i.e. CO:

Århus Maskinmesterskole 22 Desulphurization might be necessary: (Most sulphur compounds can be converted into H 2 S, which is captured in a ZnO bed):

Århus Maskinmesterskole 23 Selective Membranes... Feed Retentate Permeate Membrane

Århus Maskinmesterskole 24 Hydrogen for the future?

Århus Maskinmesterskole 25 Hydrogen Production By Electrolysis: ∙Electrolysis is an electrochemical process in which electrical energy is the driving force of chemical reactions. ∙Substances are decomposed, by passing a current through them. ∙High oil and natural gas prices could make this technology more viable in the future!

Århus Maskinmesterskole 26 Overall Principle: General Characteristics of voltaic and electrolytic cells. A voltaic cell generates energy from a spontaneous reaction (  G 0 ). In both types of cell two electrodes dip into electrolyte solution, and an external circuit provides the means for electrons to flow. Oxidation takes place at the anode, and reduction takes place at the cathode, but the relative electrode charges are opposite in the two cells. Oxidation Reaction A -  A + e- Oxidation Reaction X  X + + e- Reduction Reaction e- + Y +  Y Reduction Reaction e- + B +  B Overall (Cell) Reaction X + Y +  X + + Y,  G = 0 Overall (Cell) Reaction A - + B +  A + B,  G> 0 Voltaic Cell Energy is released from spontaneous redox reaction System does work on load (surroundings) Electrolytic Cell Energy is absorbed to drive nonspontaneous redox reaction Surrounding (power supply) do work on system (cell) Source: Dr. Fred Omega Garces, 2004

Århus Maskinmesterskole kW Alkaline Electrolyzer: Source: Professor Marcus Newborough, 2004

Århus Maskinmesterskole 28 Alkaline Electrolysers: ∙The operation of an alkaline electrolyser depends on the use of a circulating electrolyte solution (usually potassium hydroxide KOH) for transferring hydroxyl ions. ∙Alkaline electrolysers operate at relatively low current densities of <0.4 A/cm 2 and conversion efficiencies range from 60-90%. ∙Without auxiliary purification equipment, gas purities are typically 99.8% and 99.2% for H 2 and O 2 respectively. ∙Several large alkaline electrolysers of >100MW have been applied (e.g. in Egypt and Congo to utilise hydropower to generate ‘renewable hydrogen). ∙A modern alkaline electrolyser will achieve an efficiency of ~ 90% (consuming about 4kWh of electricity per m 3 of H 2 generated at NTP) and deliver gas at up to 30bar without auxiliary compression. ∙However, a significant post-electrolysis electricity consumption is incurred for gas compression to deliver H 2 and O 2 at the pressures required by industry and for storage on-board hydrogen vehicles ( bar). ∙The key factors favouring the alkaline electrolyser are that it obviates the need for expensive Platinum-based catalysts, it is well proven at large scale and it is usually of lower unit cost than a PEM electrolyser. Source: Professor Marcus Newborough, 2004

Århus Maskinmesterskole 29 PEM Electrolyzers: Source: Barbir et al., 2005

Århus Maskinmesterskole 30 Source: Barbir et al., 2005

Århus Maskinmesterskole 31 Polarization Curves for Electrolyzer:

Århus Maskinmesterskole 32 PEM Electrolyzers: ∙Expensive due to the use of precious metal catalysts (Platinum, Platinum/Ruthenium) and an expensive solid polymeric electrolyte for transferring protons. ∙PEM electrolysers have achieved >100,000 hours continuous operation without failure in critical environments (e.g. O 2 provision for nuclear submarines). ∙They can operate at much higher current densities than alkaline electrolysers (1-2 A/cm 2 ), with conversion efficiencies ranging from 50-90%, but cannot yet achieve high efficiencies at high current densities. ∙Without auxiliary purification equipment, gas purity is typically % both for H 2 and O 2. Operation at high pressure (including high differential pressure between the hydrogen and oxygen side at up to 200bar) is proven and the need for auxiliary gas compression is then considerably less than for the alkaline electrolyser. ∙High pressure gives risk of hydrogen permeation through membrane thus the efficiency is lowered. ∙The key factors favouring the PEM electrolyser are that it avoids the requirement to circulate a liquid electrolyte, it operates at a high current density, and it has the intrinsic ability to cope with transient variations in electrical power input (hence it has outstanding applications flexibility with respect to capturing intermittent renewable electricity supplies, such as wind and solar power). Source: Professor Marcus Newborough, 2004

Århus Maskinmesterskole 33 ∙Fuel Cell Systems Explained \\chapter 8\\chapter