A New Solar ThermochemicalWater-Splitting Cycle for

Hydrogen Production

Cunping Huang & Ali T-RaissiFlorida Solar Energy Center

University of Central Florida1679 Clearlake Road, Cocoa, FL 32922, USA

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IntroductionIntroduction

• Thermochemical water splitting cycles (TCWSCs) employing solar energy as a heat source present a viable option for future production of hydrogen

• TCWSCs can be highly efficient processes compared to other hydrogen production methods based on water splitting

• Using sulfuric acid decomposition process as the oxygen generating step is attractive, because H2SO4decomposition is a highly endothermic process

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Sulfur–Halogen TCWSCSulfur–Halogen TCWSC

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Sulfur Family CyclesSulfur Family Cycles• Ispra Mark 11 (Westinghouse)

2H2SO4(g) → 2SO2(g) + 2H2O(g) + O2(g) 850 OC (Thermochemical step)SO2(g) + 2H2O(g) → H2SO4(aq) + H2(g) 77 OC (Electrochemical step)

• Ispra Mark 132H2SO4(g) → 2SO2(g) + 2H2O(g) + O2(g) 850 OC (Thermochemical step)2HBr(aq) → Br2(l) + H2(g) 77 OC (Electrochemical step)Br2(aq) + SO2(g) + H2O(l)→ 2HBr(g) + H2SO4(aq) 77 OC (Thermochemical step)

• Ispra Mark 162H2SO4(g) → 2SO2(g) + 2H2O(g) + O2(g) 850 OC (Thermochemical step)I2(l) + SO2(g) + 2H2O(l) → 2Hl(l) + H2SO4(aq) 120 OC (Electrochemical step)2HI(l) → I2(l) + H2(g) 450 OC (Thermochemical step)

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Westinghouse TCWSC (1)Westinghouse TCWSC (1)

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Westinghouse TCWSC (2)Westinghouse TCWSC (2)

• Advantages1. The cycle is an all fluid, two step process2. Thermodynamic properties of chemical species are

well known3. The flowsheet is fully developed

• Disadvantages1. Difficult in oxygen separation from SO2/O2 mixture 2. Possible environmental impact due to SO2

emission3. Low efficiency of electrolytic process due to the

low H2SO3 concentration

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S-NH3 TCWSC (Reactions)S-NH3 TCWSC (Reactions)

• (NH4)2SO3 + H2O → (NH4)2SO4 + H2 80 oC (photo-electrocatalytic step)

• (NH4)2SO4 → 2NH3 + H2SO4 350 oC (thermochemical step)

• H2SO4 → SO3 + H2O 400 oC (thermochemical step)

• SO3 → SO2 + 1/2O2 850 oC (thermochemical step)

• SO2 + 2NH3 + H2O → (NH4)2SO3 25 oC (chemical adsorption)

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S-NH3 TCWSC (Process)S-NH3 TCWSC (Process)

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A New S-N3 TCWSC (Flowsheet)A New S-N3 TCWSC (Flowsheet)

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Solubility of (NH4)2SO3 & (NH4)2SO4Solubility of (NH4)2SO3 & (NH4)2SO4

153

60.5

1.297

144

59

1.220

104

51

0.881

78.4

43.9

0.664

68.8

40.8

0.583

60.8

37.8

0.515

54.0

35.1

0458

47.9

32.4

0.406

Solubility(g/100 g H2O)

Weight %

Molar conc.(mol/100 g H2O)

(NH4)2SO3

103

50.7

0.769

95

48.7

0.709

88

46.8

0.657

81

44.8

0.604

78.0

43.8

0.582

75.4

43.0

0.563

73

42.4

0.545

70.6

41.4

0.527

Solubility(g/100 g H2O)

Weight %

Molar conc.(mol/100 g H2O)

(NH4)2SO4

1008060403020100Temperature (oC)

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Advantages of S-NH3 CycleAdvantages of S-NH3 Cycle

• Separation of O2 from (NH4)2SO3 aqueous solution is much easier than separation of O2 from SO2

• No acid gas emission

• (NH4)2SO3 aqueous solution has a higher concentration of SO3

2- than that of H2SO3 thus insuring higher H2 production step efficiency

• No side reactions are expected during the photoelectrochemical oxidation of (NH4)2SO3based on possible cathode NH4

+ reactions

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Open Issues for theS-NH3 TCWSC

Open Issues for theS-NH3 TCWSC

• NH3 may be partially decomposed into H2 + N2 during (NH4)2SO3 decomposition process

• Full flowsheet development is still underway

• Efficiency for the process is not yet available

• High temperature photoelectrochemical process can have high efficiency for H2 production step

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AcknowledgmentsAcknowledgments

Funding for this workwas provided by a grant from

NASA Glenn Research Center