Evaluation and Evaluation and Design of Water- - Design of Water Splitting Cycles Splitting Cycles Scott Mullin, Jacob Tarver, Uchenna Uchenna Odi Odi Scott Mullin, Jacob Tarver, University of Oklahoma May 2006
Overview Overview � Need for hydrogen Need for hydrogen � � Water Water- -splitting cycles as solution splitting cycles as solution � � Current evaluation methods Current evaluation methods � � Efficiency defined Efficiency defined � � Our methodology as improvement Our methodology as improvement � � Results of our analysis Results of our analysis � � Economics Economics � � Conclusions Conclusions �
Accomplishments Accomplishments � Novel methodology Novel methodology � � Rapidly screen cycles without detailed Rapidly screen cycles without detailed � process flowsheets flowsheets process � Optimize T, P and excess reactants for non Optimize T, P and excess reactants for non- - � spontaneous reactions spontaneous reactions � Scoping algorithm Scoping algorithm � � Calculations refined for best cycles Calculations refined for best cycles � � Found better cycles than currently favored Found better cycles than currently favored � Sulfur- -Iodine and UT Iodine and UT- -3 3 Sulfur
Hydrogen Economy Hydrogen Economy � Currently 11 million tons/year Currently 11 million tons/year � � In H In H 2 economy † † : : 2 economy � � 200 million tons/year for transportation 200 million tons/year for transportation � � 450 million tons/year for all non 450 million tons/year for all non- -electric electric � � H H 2 is not a natural resource 2 is not a natural resource � � Must be produced Must be produced � � Steam reformation of methane Steam reformation of methane � � CO CO 2 output 2 output � � Rising fuel prices Rising fuel prices � † K. R. Schultz 2003, General Atomics, DOE grant
Alternative H 2 Production Alternative H 2 Production Petroleum Petroleum � CO CO 2 , expensive 2 , expensive � Electrolysis, high T electrolysis Electrolysis, high T electrolysis � Premature, inefficient Premature, inefficient � Photocatalytic reactors reactors Photocatalytic � Premature Premature � Thermochemical cycles cycles Thermochemical � Efficient, established processing techniques Efficient, established processing techniques �
Abundant heat, electricity 2 2 2
Water- -Splitting Cycles Splitting Cycles Water � “ “New New” ” technology, chosen by DOE technology, chosen by DOE � through Nuclear Hydrogen Initiative through Nuclear Hydrogen Initiative � Efficient hydrogen production Efficient hydrogen production � � 50 50- -60% currently, 80 60% currently, 80- -90%+ possible 90%+ possible � � Use 950 Use 950º ºC or cooler process heat C or cooler process heat � � 202 cycles known, but few researched 202 cycles known, but few researched � � Others can be found, as described by Others can be found, as described by � Holiastos and and Manousiouthakis Manousiouthakis 1998 1998 Holiastos
Economics Economics � $1 billion for water $1 billion for water- -splitting facility splitting facility � � $100 million range annual energy costs $100 million range annual energy costs � � Which cycle is best? Which cycle is best? � � Few cycles researched in detail Few cycles researched in detail � � Process design too complex Process design too complex � � Efficient cycles desirable Efficient cycles desirable � � Justify increased equipment costs Justify increased equipment costs � Bottom line: saving few % efficiency has huge Bottom line: saving few % efficiency has huge savings over plant lifetime savings over plant lifetime
Cycles Cycles � Most are Most are thermochemical thermochemical, some hybrid electric , some hybrid electric � � Any number of reactions, species Any number of reactions, species � � Named after institutions or chemicals Named after institutions or chemicals � � Steady Steady- -state operation state operation � O 2 T 1 A Sample 2-step cycle ⎯⎯ → T 1 T 1 A B + C + O 2 T 2 T 2 B, C ⎯⎯ → B + C + H O A + H 2 2 H 2 T 2 H 2 O
Efficiency Efficiency � Theoretical, 1 mol basis for cycle comparison Theoretical, 1 mol basis for cycle comparison � � Minimum reversible energy (heating and work) Minimum reversible energy (heating and work) � requirement requirement � Performance limit Performance limit � � Thermodynamics: JANAF tables for state Thermodynamics: JANAF tables for state � functions, pure component averages functions, pure component averages � Δ H (H O) Q is total heat requirement η = f 2 + W is separation, electric and shaft work † Q W † Shaft work (pumping, compression) small compared to other terms
Previous Surveys Previous Surveys � Brown et al 2000 scored cycles based on Brown et al 2000 scored cycles based on � known characteristics known characteristics � Good starting point, but not reproducible Good starting point, but not reproducible � � Arbitrary criteria, no emphasis on efficiency Arbitrary criteria, no emphasis on efficiency � � Elemental abundance, Elemental abundance, “ “corrosivity corrosivity” ”, # elements , # elements � � Rejects cycles with Rejects cycles with “ “too positive too positive” ” free energies free energies � � Favors well Favors well- -researched cycles researched cycles �
Score † † Score 0 1 2 3 # reactions 6 - - 5 # separations 10 9 8 7 # elements 7 - 6 - Least Ir Rh, Tc, Os, Pt, Bi, Ag, In, Pt, Bi, Ag, In, abundant Ru, Re, Au Pd, Hg, Pd, Hg, Cd, Cd , Sb Sb, , element Se Tm, Tl Tl, Lu , Lu Se Tm, Brown’s method is good at identifying cycles based on estimated process complexities, but is not quantitative or reproducible. What happens if you change the weights, or add further scoring criteria? Adapted from Brown et al 2000 † † Adapted from Brown et al 2000
Previous Surveys cont’ ’d d Previous Surveys cont � Cycles are complex, so Lewis et al 2005 Cycles are complex, so Lewis et al 2005 � developed systematic approach developed systematic approach � Scoping method based on efficiency Scoping method based on efficiency � � Quantitative, standard basis Quantitative, standard basis � � Oversimplifications Oversimplifications � � Requires detailed Requires detailed flowsheets flowsheets � � Not truly scoping Not truly scoping � � Assumes 50% loss of all work energy Assumes 50% loss of all work energy � � Does not estimate real separation energy Does not estimate real separation energy � Our method is truly scoping, based on Our method is truly scoping, based on theoretical requirements theoretical requirements
General Methodology General Methodology � Cyclic nature couples all calculations Cyclic nature couples all calculations � � Decouple the problem Decouple the problem � � Find realistic estimates for Q, W Find realistic estimates for Q, W � � Refine calculations for best cycles Refine calculations for best cycles � � Account for additional energy requirements Account for additional energy requirements � � Economic analysis of best cycles Economic analysis of best cycles � � Apply methodology to all cycles Apply methodology to all cycles � � Evaluate the 202 from literature Evaluate the 202 from literature � � Find unknown cycles Find unknown cycles �
Equilibrium Equilibrium � Excess reactants added to shift reactions to Excess reactants added to shift reactions to � the right the right � How do we handle excess after the How do we handle excess after the � reaction? reaction? � Requires optimization, coupled equations Requires optimization, coupled equations � v ⎛ ⎞ i n ∏ ∑ ⎜ ⎟ i ⎜ ⎟ n ⎝ ⎠ ∑ ∑ ν ν products = = i K K P P eq x v ⎛ ⎞ i n ∏ ⎜ ⎟ i ∑ ⎜ ⎟ n ⎝ ⎠ reactants i
Excess Reactant Handling Excess Reactant Handling Immediate recycle : full separation energy costs ⎯⎯ → T 1 T 1 A B + C + O 2 T 2 T 2 ⎯⎯ → B + C + H O A + H 2 2 No recycle : saves separation energy, but negatively shifts equilibrium in most cases and increases heat cascade requirement We optimize T, P, # excess mols and their handling
Cycles cont’ ’d d Cycles cont O 2 B T 1 � Methodology Methodology � A accounts for arbitrarily accounts for arbitrarily complex cycles complex cycles H 2 T 2 D, H 2 O ⎯⎯ → T 1 T 1 A + B C + O 2 ⎯⎯ → T 2 T 2 D + H O E + F + H H 2 O T 3 2 2 C ⎯⎯ → T 3 T 3 C + H O B + F 2 ⎯⎯ → T 4 T 4 E + F A + D + H O 2 T 4 E, F Conditions optimized for each reactor
Heat Requirements Heat Requirements Maximize heat recovery from exothermic reactions and cooling Maximize heat recovery from exothermic reactions and cooling � � streams streams Pinch occurs when there is not enough heat to power reactions or Pinch occurs when there is not enough heat to power reactions or � � heat streams, requiring input from the hot utility heat streams, requiring input from the hot utility
Generic Heat Integration Generic Heat Integration H hot is total enthalpy of cooling streams H cold is total enthalpy of heating streams
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