Generation of Hydrogen Production Cycles Through Water-Splitting

1. Generation of Hydrogen Production Cycles Through Water-Splitting Jeff Jenneman James Phan Quang Nguyen Miguel Bagajewicz

2. Background • Decreasing supply of fossil fuels. Increasing demand from large developing countries like China and India. • Need for reduced CO2 output and other greenhouse gases. • Hydrogen Economy – Argues hydrogen fuel is the answer for the replacement of fossil fuels. Clean, environmentally friendly, form of energy

3. Background • Water-Splitting Cycles • Main overall reaction: H2O  H2 + 1/2O2 • Thermal decomposition of water occurs at about 2500 K • Electrolysis of water found to be only 30 – 45 % efficient • Instead, to lower temperatures, a set of reactions with the net products of H2 and O2 • Other materials are continually used and regenerated

4. Outline • Introduction and Purpose • Code for cycle generation • Restraints • Configurations • Thermodynamic data • Results • Cycle generation • Cycle analysis • Separation Work • Functional Group Method • Summary

5. Introduction and Purpose • Previous methods of hydrogen production employ • High temperatures • Nuclear reactors assumed as high temperature heat sources • Metallic/inorganic reactants and products

6. Previous Work at OU • “Nuclear Production of Hydrogen Using Thermochemical Water Splitting Cycles” by Brown et. al • Over 100 potential water splitting cycles analyzed and reduced to 25 cycles for detailed study • Cycle criteria included: • Number of reactions and/or separation steps in the cycle • Number of elements in the cycle • Are non-stationary solid reactants involved? • Corrosive properties of chemicals used, etc.

7. Previous Work at OU • Most of the work done in previous water splitting projects has dealt with thermodynamic analysis and efficiency screening for currently developed cycles • Analysis of 10 cycles compared cycle configurations, excess reactants, ideal separation work, pinch analysis, etc. • First project aimed at developing new, unique, lower temperature water splitting cycles

8. Previous Work at OU • Conclusions • Hydrogen production cycles can quickly be ranked based on efficiency • The best configuration depends on the considered cycle • Phase separation and good cascade properties

9. Purpose of this Work • Find unique cycles that operate at lower temperatures employing non-metallic/inorganic reactants • Methods • Use a pool of molecules and discover cycles by enumeration • Use functional groups instead of a pool of initial molecules

10. Outline • Introduction and Purpose • Code for cycle generation • Restraints • Configurations • Separation work • Thermodynamic data • Results • Cycle generation • Cycle analysis • Functional Group Method • Summary

11. Code for Cycle Generation • Pool of 100 molecules • Cyclic and non-cyclic compounds • Choose 4 unique molecules • Each must be unique H2O + aA + bB  cC + dD + H2/ ½O2 cC + dD  aA + bB + H2/ ½O2

12. Constraints • Atomic balance • Stoichiometric coefficients • Coefficients are < 5 • Gibbs Energy (< 50) • Varying temperature • ΔG < 0 – Reaction favorable • 0 < ΔG < 50 – Reaction possibly favorable

13. Atomic Balance H2O + aA + bB  cC + dD +H2/ ½O2 cC + dD  aA + bB + H2/ ½O2 Oxygen Balance: OA + OB – OC – OD = 0 Hydrogen Balance: HA + HB – HC – HD + 2 = 0 Atom X balance: XA + XB – XC – XD = 0 Atom Y balance: YA + YB – YC – YD = 0 Coefficient Matrix Variable Matrix Solution Matrix

14. Kinetics • The reaction kinetics were not directly studied in this project. Some constraints were built into the code to ensure the cycles generated had the greatest kinetic viability • The coefficients in each reaction were limited to 5 • Successfully generated cycles that required more than 5 total molecules in any one reaction were further eliminated due low kinetic feasibility.

15. Populate Atomic Balance Atom check of products and reactants If not satisfied  Next species Calculate equilibrium constant and heat of RXN If determ = 0  Next species Build matrix Pinch analysis to obtain utilities If Coeff >5 or <0.5  Next species Calculate coefficients Calculate efficiency If Gibbs > 50 and Temp > 1000  Next species Calculate Gibbs Energy Print results If Gibbs > 0 and Temp < 1000  Change Temp Flow sheet for Code

16. Pinch Analysis – Minimum Heat Duty • No heat can transfer from cold streams to hot streams • Minimum approach temperature of 10 K • Divides streams in reactor network into sections in which heat can transfer.

17. Pinch Analysis2 Reactant/2 Product Interval Reaction 2 ∆Hr2 = E2 kJ 1 (Rxn 2) T2 + ∆Tmin T2 2 T2 T2 - ∆Tmin C D 3 A B H2 4 (Rxn 1) Reaction 1 ∆Hr1 = E1 kJ T1 + ∆Tmin T1 5 T1 T1- ∆Tmin 6 ½O2 H2O 298 298 + ∆Tmin 7 298

18. Thermodynamic Data Sources • Gibbs and Enthalpy of formation as well as heat capacity correlations • Chemical Properties Handbook by C.L. Yaws • JANAF Thermochemical Tables

19. Comparison of Yaws Handbook and JANAF Thermochemical Tables Some disagreement with Yaws correlations and JANAF data. In these cases curve fitting to JANAF data was done to develop accurate correlations in pinch calculations.

20. Equilibrium constant and Efficiency • Equilibrium constant • Efficiency • = heat of formation of water at 298K • Hot Utility is found from the heat cascade • Equilibrium constant determines amount of conversion used for separation work calculations

21. Separation Work • Addition of ideal separation work to adjust efficiency calculation • Efficiency of 50% assumed • Excess reactants - Increased reactant to water ratio to 3:1 from stoichiometric feed.

22. Electrolysis/Hybrid Cycles • In cases where Gibbs energy greater than zero electrolysis possibility examined • Welectric= zF∆E (Nernst Equation) z is number of electrons transferred F is Faraday’s Constant ∆E is change in electric potential or electromotive force High Gibbs energy, electrolysis is used Cl2 + 2e-  2Cl- ∆E0 = 1.3601 V

23. Outline • Introduction and Purpose • Code for cycle generation • Restraints • Configurations • Thermodynamic data • Results • Cycle Configurations • New Cycles • Separation Work • Functional Group Method • Summary

24. Cycle Configurations • 1 Reactant/2 Products H2O + aA  bB + cC + H2/ ½O2 bB + cC  aA + H2/ ½O2 • 2 Reactants/1 Product H2O + aA + bB  cC + H2/ ½O2 cC  aA + bB + H2/ ½O2 • 2 Reactants/2 Products H2O + aA + bB  cC + dD + H2/ ½O2 cC + dD  aA + bB + H2/ ½O2 • 3 Reactants/2 Products • H2O +aA +bB +cC dD +eE + H2/ ½O2 dD +eE  aA + bB+cC + H2/ ½O2 • 3 Reactions • H2O + aA + bB cC + dD +½O2 • cC + dD  eE + fF • eE + fF  aA + bB + H2

25. Cycle Configurations

26. New Cycles2 reactants/2 Products

27. New Cycles3 reactants/2 Products This reaction still has 7 molecules reacting in the first reaction, so the kinetic feasibility is extremely low

28. Separation Work Separation work reduces efficiency about 10 %, excess reactant reduces 15 %; however, about 65 % increase in hydrogen conversion

29. Separation Work Separation work reduces efficiency about 7 %, excess reactant reduces 7 %; however, about 68 % increase in hydrogen conversion

30. 3 Reaction Cycle with Electrolysis/Excess Reactants Best efficiency and lowest temperatures of any cycle found. Kinetically, the low number of reactants in each reaction looks more realistic. However, large excess reactants, 10 to 1 ratio to water feed, in reaction 1 needed for significant conversion.

31. Cycles From Functional Groups • Generate molecules using a combination of functional groups • The same algorithm is then used • The generation of the molecules even for a small number of functional requires a large number of enumerations. • Large amount of enumerations required very long computing times, and the scope of this method was limited to only one reaction configuration. • Advantage is more elements can be integrated into search since molecules are not chosen by handpicked method

32. Populate If not satisfied then next species CheckConstraints If duplicate foundthen next species Check for Duplicates Print Results Functional Group Method • Flow sheet for the functional group method

33. Functional Group Method • Set of 15 functional groups • The first functional group is left blank to numerate molecules of less than 5 functional groups • For example: CH3 – F

34. Functional Group Method • Previous work by Joback and Stephanopoulos (1989) • Used a variety of computational techniques to find new compounds possessing improved properties for refrigerants • Evaluated a number of structural and chemical properties

35. Constraints • Our code used some of Joback and Stephanopoulos’s structural constraints. Examples are: • The number of groups having an odd number of free bonds must be even • There are 3 types of bonds: single, double, and triplethere must be an even number of each bond present in each molecule

36. Cycles produced using the functional group method • 308 unique molecular formulas were found using combinations of the 15 functional groups • Cycle were produced in the following configuration H2O + aA bB + H2-½O2 bB  aA + H2-½O2

37. Cycles produced using the functional group method Cycle 1: H2O + C2H6O  C2H6O2 + H2 C2H6O2  C2H6O + ½O2 • Both of the reactions in this cycle had highly positive Gibbs Energy of formation for the temperature range of 400 to 1000K Cycle 2: H2O + C2H4 C2H6 + ½O2 C2H6  C2H4 + H2 • The second reaction has a negative value for Gibbs energy at a temperature of 700 K • However, the first reaction has a highly positive Gibbs energy

38. Conclusions • Successfully generated and thermodynamically evaluated new water splitting cycles for a variety of cycle configurations • Efficiencies of non-electrolysis cycles reached a maximum of ~75% • Ideal separation work on average reduced total efficiency about 10% • Successful in identifying hybrid cycles and additional electric work. The most practical cycle found achieved 90% cycle efficiency

39. Questions?