WSRC-STI-2007-00545 - 1APP

Max Gorensek, PhD, PESenior Fellow Engineer

Computational and Statistical Science

OLI Simulation ConferenceHyatt Hotel, Morristown, NJ

October 23-24, 2007

Performance Estimates for Sulfur-based Thermochemical Hydrogen Cycles using OLI

WSRC-STI-2007-00545 - 2APP

Outline

Background The Sulfur Cycles Thermal Efficiency Considerations HyS Flowsheet Analysis Using OLI-MSE

– Properties Modeling Challenges– Application Example– Results of Analysis

WSRC-STI-2007-00545 - 3APP

Our Energy Future

World energy needs are growing rapidly There is a finite supply of oil and gas Alternative energy supplies need to be

developed soon Environmental concerns are increasing America needs energy security & diversity

– Petroleum imports will exceed 75% by 2025WE NEED A SUSTAINABLE ENERGY SYSTEM

WSRC-STI-2007-00545 - 4APP

The “Hydrogen Economy” Could Be One Solution

Broad-based use of hydrogen as a fuel– Energy carrier analogous to electricity– Produced from variety of primary energy sources– Can serve all sectors of the economy: transportation, power,

industry and buildings– Replaces oil and natural gas as an end-use fuel– Makes renewable and nuclear energy “portable”

Advantages:– Inexhaustible– Clean– Universally available to all countries

WSRC-STI-2007-00545 - 5APP

Hydrogen Can Be Made From Domestic Resources

WSRC-STI-2007-00545 - 6APP

Hydrogen Economy Will Need a Lot of Hydrogen

National Academy of Engineering Report (2004) estimates:– Use of H2 for all light-duty vehicles in 2050 will require 110

MM tons per year– 12-fold increase over current use– Energy content = 13.5 Quad– Power content = 450 GWth

Will require multiple primary sources– Fossil fuels with CO2 sequestration– Renewable energy with electrolysis– Nuclear water-splitting

WSRC-STI-2007-00545 - 7APP

Time of Day/MonthH2 Storage

High Capacity Pipeline

Industrial H2 Users

Hydrogen Fueled Future

Distributed Power

Transport Fuel

Centralized Nuclear Hydrogen Production Plant

Nuclear Hydrogen Future

Thermochemical Process H2O → H2 + ½ O2

WSRC-STI-2007-00545 - 8APP

DOE Nuclear Hydrogen Initiative

Parallel development of technology to make hydrogen from nuclear energy via– Electrolysis (electricity input)– High Temperature Electrolysis (heat and electricity input)– Thermochemical cycles (heat input)– Hybrid thermochemical cycles (heat and electricity input)

What is a thermochemical cycle?– Chemical process– Series of chemical reactions that combine to split water– All intermediate species regenerated– True thermochemical cycles use only heat to drive process– Hybrid cycles use both heat and electricity

WSRC-STI-2007-00545 - 9APP

Status of Thermochemical Cycles

Major design challenges due to large material flows, corrosive chemicals, impurities, reactant separation, high temperature heat exchange, and costs

Currently in lab-scale development stage Two leading cycles:

– Sulfur-Iodine (SI) process– Westinghouse or Hybrid Sulfur (HyS) process

WSRC-STI-2007-00545 - 10APP

Sulfur-Iodine (SI) Thermochemical Cycle for Production of H2

H2SO4 ½O2 + SO2 + H2O> 800°C

Heat

H2SO4+ 2HI I2 + SO2 + 2H2O< 120°C

Heat SO2 + H2OH2SO4 (H2O)

H2O

H2

½O2

2HI I2 + H2> 300°C

HeatI2 (H2O)2HI (I2, H2O)

Inputs:

• Water

• Heat (>800°C)

Outputs:

• Hydrogen

• Oxygen

• Waste heat

H2SO4 ½O2 + SO2 + H2O> 800°C

Heat

H2SO4+ 2HI I2 + SO2 + 2H2O< 120°C

Heat SO2 + H2OH2SO4 (H2O)

H2O

H2

½O2

2HI I2 + H2> 300°C

HeatI2 (H2O)2HI (I2, H2O)

H2SO4 ½O2 + SO2 + H2O> 800°C

Heat

H2SO4+ 2HI I2 + SO2 + 2H2O< 120°C

Heat SO2 + H2OH2SO4 (H2O)

H2OH2O

H2H2

½O2½O2

2HI I2 + H2> 300°C

HeatI2 (H2O)2HI (I2, H2O)

Inputs:

• Water

• Heat (>800°C)

Outputs:

• Hydrogen

• Oxygen

• Waste heatAt least 115 different thermochemical cycles have been proposed

WSRC-STI-2007-00545 - 11APP

Hybrid Sulfur (HyS) Cycle

H2SO4 ½O2 + SO2 + H2O> 800°C

Heat

H2 + H2SO4 SO2 + 2H2O100°C

Electric Energy SO2 + H2OH2SO4 (H2O)

H2OH2O

H2H2

½O2½O2

Inputs:

• Water

• Heat (>800°C)

• Electricity

Outputs:

• Hydrogen

• Oxygen

• Waste heatThe only 2-step, all-fluids thermochemical cycle – based on sulfur oxidation and reduction

WSRC-STI-2007-00545 - 12APP

Efficiency Benchmark for Thermochemical Cycles

Alkaline electrolysis (AE) provides an efficiency benchmark– Mature technology– 65-70% LHV efficiency

Coupled with PWR/LWR – Thermal efficiency of Rankine

cycle power generation is ~33%

– Net thermal efficiency is 21-23% Coupled with HTGR

– Thermal efficiency of Brayton cycle power generation is ~46%

– Net thermal efficiency is 30-32% More complex TC cycles become

attractive compared to PWR/LWR- (or HTGR-) coupled AE when LHV efficiency is ≥ 26-29% (or 36-40%)

Alkaline Electrolysis

2 H2O(l) + 2 e– → 2 OH– + H2(g) 2 OH– → ½ O2(g) + H2O(l) + 2 e–

H2O(l) → H2(g) + ½ O2(g)

WSRC-STI-2007-00545 - 13APP

HyS Electrolyzer Requires Much Less Power than Alkaline Electrolyzer

Water electrolysis reactions:H2O(l) → ½ O2(g) + 2 H+ + 2 e– anode reaction

2 H+ + 2 e– → H2(g) cathode reaction

H2O(l) → H2(g) + ½ O2(g) net reaction

Standard cell potential, E° = -1.229 V at 25°C SO2-depolarized electrolysis (SDE) reactions:

2 H2O(l) + SO2(aq) → H2SO4(aq) + 2 H+ + 2 e– anode reaction

2 H+ + 2 e– → H2(g) cathode reaction

2 H2O(l) + SO2(aq) → H2SO4(aq) + H2(g) net reaction

Standard cell potential, E° = -0.158 V at 25°C

= -0.173 V in 30% H2SO4

= -0.262 V in 50% H2SO4

WSRC-STI-2007-00545 - 14APP

Actual SDE Potentials Look Promising

• Lu and Ammon (Westinghouse) data for 50% H2SO4 anolyte sat’d with SO2 at 1 atm, 50°C

• Sivasubramanian et al. (University of South Carolina) data for dry, gaseous SO2 feed at 1 atm, 80°C

• Steimke and Steeper (SRNL) data for 32% H2SO4 anolyte sat’d with SO2 at 4 atm, 70°C

• Current development goal is 0.6V at 0.5 A/cm2, 20 bar, and 90°C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.1 0.2 0.3 0.4 0.5 0.6Current Density, A/cm2

Cell P

oten

tial,

V

Lu and Ammon data (1980)Sivasubramanian et al. (2007)Steimke and Steeper (2006)Development Goal

WSRC-STI-2007-00545 - 15APP

Energy Allowance for H2SO4 Decomposition

SDE cell voltage thermal equivalent at 46% conversion efficiency (Qth to We)

– 0.6 V ≈ 252 kJ/mol H2

– 0.7 V ≈ 294 kJ/mol H2

Energy consumption for different net thermal efficiencies

– 36% LHV ≈ 672 kJ/mol H2

– 40% LHV ≈ 605 kJ/mol H2

H2SO4 decomposition heat requirement

– at 0.6 V, 672 – 252 = 420 kJ/mol H2 allowance for 36% LHV efficiency

– 605– 252 = 353 kJ/mol H2 allowance for 40% LHV efficiency

– H2SO4 decomposition heat input needs to be ≤ ~420 kJ/mol to achieve net thermal efficiencies of 36% or higher (LHV basis)

– Can consume up to ~420 kJ/mol and still be attractive

WSRC-STI-2007-00545 - 16APP

OLI-MSE Model an Obvious Choice for Modeling Sulfuric Acid Properties

Sulfur cycles involve sulfuric acid over wide T-p-x range– 0-900°C– 0.01-90 bar

– 0-98 wt% H2SO4

OLI-MSE model fits sulfuric acid VLE data accurately over entire range

OLI-MSE can be used in conjunction with Aspen Plus™ for Sulfur-based cycle flowsheet simulations

P. Wang et al., J. Molecular Liquids 125 (2006) 37-44.

WSRC-STI-2007-00545 - 17APP

0

20

40

60

80

100

120

140

0.0 0.2 0.4 0.6 0.8 1.0

mole fraction water

tem

pera

ture

, °C

Maass and Maass (1928)Spall (1963)van Diepen and Berkum (1979)OLI MSE model

SO2-H2O LLE Inevitable for HyS at High Pressures

SO2-H2O binary exhibits complex phase behavior– Hydrates– Immiscibility

LLE possible at conditions typical for H2SO4 decomposition product– 15-120°C temperatures– 2-35 bar pressures

HyS temperatures too high for hydrate formation

OLI-MSE model predicts SO2-H2O LLE with reasonable accuracy

WSRC-STI-2007-00545 - 18APP

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0 20 40 60 80 100

mole percent H2SO4 (SO2-free basis)

mol

e S

O2

diss

olve

d pe

r m

ole

acid

Miles and Carson (1946)

Miles and Fenton (1920)

Kuznetsov (1941)

OLI-MSE model

Solubility of SO2 in Sulfuric Acid at 20°C, 1 atm Partial Pressure

SO2-H2O-H2SO4 ternary also has complex phase behavior

Solubility of SO2 in sulfuric acid varies with concentration– Broad minimum at 30-50 mol%– Increases rapidly with

concentration above 50 mol%

OLI-MSE model reproduces this behavior reasonably well – Predicts extra peak at low

temperatures for SO2 solubility in sulfuric acid

– Speciation may need revision

WSRC-STI-2007-00545 - 19APP

SO2 Immiscibility Possible Over Entire H2SO4 Concentration Range

SO2

SO2

LLE occurs at sufficiently high pressures and SO2 levels

Ternary diagrams published by Francis (1965)*.– SO2 solubility is 26% in pure H2O, H2O

solubility is 1.5% in pure SO2

– SO2 minimum solubility is 17% in 94-96% H2SO4

– Dashed line in diagram 16 indicates isopycnic (both liquid phases have equal density)

– SO2 completely miscible with 30% oleum– HyS process operates in 0-95% H2SO4 range

* A.W. Francis, "Ternary Systems of liquid Sulfur Dioxide," J. Chem. Eng. Data, 10(1), 45 (1965).

WSRC-STI-2007-00545 - 20APP

HyS Cycle Simplified Flowsheet

Power Generation

HTGR Nuclear Heat Source

Sulfuric Acid Decomposition

Electrolyzers and Auxiliaries

Sulfur Dioxide / Oxygen Separation

Thermal Energy

H2O, SO2, O2

H2SO4, H2O

H2O FeedO2 By-product

H2O, SO2

Electric Power

H2 Product

WSRC-STI-2007-00545 - 21APP

SRNL SDE Configuration

• Nafion® or other proton exchange membrane

• Gas diffusion carbon electrodes

• Membrane electrode assembly (MEA) construction

• Porous carbon flow fields• Recirculating acid anolyte• No catholyte needed

Anode

(+) (−)

Cathode

PEMSeparator

2 H+

2 e−

2 H+ + 2 e− → H2(g)

H2(g)

SO2(aq) + H2O(l) → SO3(aq) + 2 H+ + 2 e−

SO2(aq), H2O(l)

SO3(aq) + H2O(l) → H2SO4(aq)

H2SO4(aq)

WSRC-STI-2007-00545 - 22APP

Bayonet Decomposition Reactor Design

Tube Heated

With

Electrical Heat Source

Inlet

Outlet

Catalyst

Heat InputHeat Input

Boiler/Vaporizer/SuperheaterBoiler/Vaporizer/Superheater

Section With RecuperationSection With Recuperation

SiC Tube

DecompositionDecomposition

SectionSection

ManifoldManifold

Tube Heated

With

Electrical Heat Source

Inlet

Outlet

Catalyst

Heat InputHeat Input

Boiler/Vaporizer/SuperheaterBoiler/Vaporizer/Superheater

Section With RecuperationSection With Recuperation

SiC Tube

DecompositionDecomposition

SectionSection

ManifoldManifold

Tube Heated

With

Electrical Heat Source

Inlet

Outlet

Catalyst

Heat InputHeat Input

Boiler/Vaporizer/SuperheaterBoiler/Vaporizer/Superheater

Section With RecuperationSection With Recuperation

SiC Tube

DecompositionDecomposition

SectionSection

ManifoldManifold

Bayonet reactor consists of one closed ended tube co-axially aligned with an open ended tube to form two concentric flow paths

Heat applied externally Liquid fed to annulus, vaporized,

passed through catalyst bed Product returns through center,

heats feed through recuperation Advantages include internal heat

recuperation, only one connection at cool end, corrosion resistance, and low fabrication cost

Silicon carbide bayonets are an off-the-shelf item (thermocouple tubes)

Developed at Sandia Nat’l Lab’s

* Evans, R.J., “Nuclear Hydrogen Initiative Thermochemical Cycles”, AIChE Spring National Meeting, Orlando, FL, April 24, 2006.

WSRC-STI-2007-00545 - 23APP

100

200

300

400

500

600

700

800

900

0

100,

000

200,

000

300,

000

400,

000

500,

000

600,

000

700,

000

800,

000

900,

000

1,00

0,00

0

Enthalpy, kW

Tem

pera

ture

, °C

Bayonet Reactor Feed Heating and Product Cooling Curves

Heat rejection target

High-temperature heating target

Recuperation

86 bar, 870°C, 80 mol% H2SO4 feed, 10°C min ΔT, 1-kmol H2/s production

328.7 MW

89.8 MW

WSRC-STI-2007-00545 - 24APP

High-temperature Heat Requirement for H2SO4 Bayonet Decomposition Reactor

300

350

400

450

500

550

600

650

700

750

800

30 40 50 60 70 80 90

wt% H2SO4 feed concentration

kJ/m

ol H

2 he

at r

equi

rem

ent

57 bar

70 bar

83 bar

870°C peak process temperature,900°C secondary helium temperature

WSRC-STI-2007-00545 - 25APP

Aspen-OLI Model of HyS Flowsheet Example

110.0

86.0031

254.4

86.0032

254.4

86.00

33

254.4

86.0043

40.0

86.00

34

40.0

86.0051

40.0

86.00

47

40.0

86.0035

-8.9

21.00

52

W2

W

-8.9

21.0053

-8.9

21.00

68

41.8

21.00

36

41.0

21.00

48

41.0

21.0059

41.0

21.00

49

41.8

21.0058

41.8

21.00

37

171.4

1.80

44

171.4

1.80

60

171.4

1.8045

40.0

1.80

61

40.0

1.8062

40.0

1.80

66

-8.6

21.00

54

-8.3

21.00

55

40.3

21.0067

40.1

21.00

106

80.0

21.0072

40.3

21.00100

40.3

21.00103

21.0094

41.1

21.00

77

40.3

21.00

87

40.5

21.00

56

41.3

21.0071

80.0

21.00

39

80.0

21.00

2R

80.3

21.00FEED

80.0

21.002

92.1

20.00

11

92.1

20.0012

92.1

20.0040

80.5

20.00

41

80.5

21.00

42

40.0

21.0069

40.0

21.0057

80.0

21.00

1

W1W

92.1

20.003

78.7

2.65

13

78.7

2.65

14

78.7

2.6573

40.0

2.6574

40.0

2.65

75

40.0

2.65

76

40.0

2.6578

219.9

21.00

79

40.0

21.00

80

40.0

21.0081

40.0

21.00

82

71.7

0.33

15

71.7

0.33

16

71.7

0.3383

96.2

0.33

17

53.9

0.04

18

53.9

0.0491

53.9

0.04

19

40.0

0.3384

40.0

0.33

85

40.0

0.33

86

40.0

0.33

88

40.0

0.0492

0.04

93

40.0

0.0495

222.4

2.65

89

40.0

2.6590

270.4

0.33

96

40.0

0.3397

53.9

0.27

20

109.2

0.27

21

109.2

0.27

22

109.2

0.2798

192.7

1.80

25

192.7

1.80101

192.7

1.80

26

109.2

1.80

23

192.7

1.80

24

192.7

1.8046

40.0

0.27

99

40.0

1.80

102

312.2

12.00104

40.0

12.00

105

312.2

12.00

29

319.1

86.0030

312.2

12.00

28

193.1

12.00

27

260.3

21.00

63

40.0

21.00

64

80.0

21.00AC ID

80.0

21.00SO2

39.9

21.0065

41.3

21.00

50

-3.7

21.0070

80.4

21.0038

40.0

20.00

4

40.0

20.00

5

40.0

20.00

7

40.0

20.008

40.0

20.00

6

40.0

20.001040.0

21.00

1R

40.0

20.00

9

RX-01

Q=325.987

KO-08

HX-07

Q=-39.024

KO-09

TE-01

W=-1.143KO-13

VV -04

VV -06

KO-12

KO-10

KO-11

VV -05

HX-10

Q=-39.725

KO-14

M X-07

PP-07

W=0.045

TO-01

TK-01

AN-M IX

SP-01

HX-08

Q=-62.498

PP-06

W=0.162

DR -02

AN

OD

E

CA

TH

OD

E

EL-01

VV -01

KO-02

HX-12

Q=-1.390

KO-15

CO-02

W=3.024

HX-13

Q=-12.638

KO-16

VV -02

KO-03

HX-02

Q=48.134

VV -03

KO-04

HX-14

Q=-1.538

KO-17 KO-18

HX-16

Q=-0.581

HX-15

Q=-1.504

CO-03

W=0.679

HX-17

Q=-63.247

CO-04

W=9.842

PP-02

W=0.003

HX-03Q=75.453

KO-05

KO-06

HX-04

Q=73.757PP-03

W=0.020

M X-02

HX-18

Q=-50.412

HX-09

Q=27.380

HX-19

Q=-63.808

PP-11

W=0.062

PP-12W=0.067

HX-20

Q=-82.824

PP-05

W=1.669

KO-07

HX-05

Q=128.497PP-04

W=0.268

CO-01

W=1.924

HX-11Q=-6.570

PP-13W=0.037

PP-10

PP-08

W=0.004

PP-09

W=0.070

M X-00

M X-10

M X-08

M X-11

M X-06

M X-05

M X-04

M X-03

M X-09

KO-01

DR -01

HX-01

Q=-38.396

PP-01

W=0.022

M X-01

HX-06

Q=-92.283

Temp eratu re (C)

Pressure (bar)

Q Duty (MW)

W Power(MW)

Anolyte Prep Tank

Bayonet ReactorSO2-DepolarizedElectrolyzer

HydrogenProduct

WaterFeed

OxygenProduct

15.1 wt% SO2dissolved in

47.0 wt% H2SO4

50.2 wt% H2SO4

50.4 wt% H2SO4

50.6 wt% H2SO4

57.1 wt% H2SO464.5 wt% H2SO4

69.0 wt% H2SO4

72.3 wt% H2SO4

80.1 wt% H2SO4

Hybrid Sulfur Cycle FlowsheetPEM Electrolyzer with 47% H2SO4 feed

at 80°C and 21 bar containing 15.1% SO2;Bayonet decomposition reactor at 870 °C and 86 bar

with 80% H2SO4 feed

WSRC-STI-2007-00545 - 26APP

HyS Flowsheet Example Specifications

PEM electrolyzer with 47% H2SO4 feed containing 15.1% SO2 at 80°C and 21 bar makes 50% H2SO4 product at 92°C and 20 bar with 20% conversion at 600 mV

3 successive vacuum flash steps remove unreacted SO2

3 partial vaporization steps concentrate acid to 80 wt% at increasing pressure to allow recuperation

Bayonet decomposition reactor at 870°C and 86 bar with 80% H2SO4 feed makes H2O, SO2, and O2 at 47% conversion

Unconverted acid at 69% is recycled to concentration train Vapor product is cooled and let down to 21 bar

– SO2-H2O condensate collected and mixed with recycled anolyte– O2 vapor scrubbed with water removed in concentration train

WSRC-STI-2007-00545 - 27APP

0

50

100

150

200

250

300

350

0 100,000 200,000 300,000 400,000 500,000 600,000 700,000

Enthalpy, kW

Tem

pera

ture

, °C

Composite Curves for HyS Flowsheet Example (Excluding Bayonet Reactor) – 1-kmol H2/s Basis

Recuperation

Heat rejection target

High-temperature heating target

Pinch Point

79.2 MW

282.4 MW

WSRC-STI-2007-00545 - 28APP

Results of Efficiency Analysis

High-temperature heating target for flowsheet example– 328.7 MJ/kmol H2 needed for H2SO4 decomposition– Additional 79.2 MJ/kmol H2 needed for H2SO4 concentration– Total minimum heat requirement is 407.9 MJ/kmol H2

Power requirement for electrolysis and pumps/compressors– 115.8 MJ/kmol H2 needed for SDE – Additional 16.8 MJ/kmol H2 needed for shaft work– Total electric power requirement is 132.5 MJ/kmol H2

Efficiency estimates:– PWR/LWR-generated power at 33% efficiency gives 29.9% LHV efficiency– HTGR-generated power at 46% efficiency gives 34.7% LHV efficiency

Optimization (ongoing) should boost efficiency to desired range

WSRC-STI-2007-00545 - 29APP

Summary

Splitting water to make hydrogen using nuclear energy is one possible component of a sustainable energy future

Sulfur-based thermochemical cycles are being developed under the Nuclear Hydrogen Initiative as a means to split water using a high-temperature heat source like a nuclear reactor

Inherently more complex thermochemical cycles must outperform simpler water electrolysis to be attractive

Sulfur-based cycles present stream properties modeling challenges that can be successfully handled using the OLI-MSE model

Aspen-OLI is being used to simulate Hybrid Sulfur process flowsheets to evaluate and optimize the net thermal efficiency

Preliminary results based on electrolyzer and decomposition reactor performance extrapolated from development experiments suggest that sufficiently attractive net thermal efficiency should be attainable

WSRC-STI-2007-00545 - 30APP

Acknowledgements

This work sponsored by U.S. Department of Energy under Contract No. DE-AC09-96SR18500

Funding provided by the DOE Office of Nuclear Energy, Science & Technology under the Nuclear Hydrogen Initiative– Mr. Carl Sink

Consultation– Dr. William A. Summers (SRNL)– Dr. Edward J. Lahoda (Westinghouse Electric Co.)– Dr. Paul M. Mathias (Fluor Corp.)– Prof. John P. O’Connell (University of Virginia)– Prof. John W. Weidner (University of South Carolina)