Cerium dioxide redox cycle for fuel production

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1 Cerium dioxide redox cycle for fuel production Thermodynamic modelling B. Bulfin, F. Call, M. Lange, C. Sattler, R. Pitz-Paal, and I. V. Shvets July 1, 2015

2 Contents Introduction Motivation Solar Fuels and Thermochemical redox cycles Model of the ceria redox system Development of model from equilibrium data Other ceria based oxides Thermodynamics In-depth thermodynamic evaluation Efficiency maximisation and practicality of the cycle Introduction Contents July 1, / 27

3 Contents Introduction Motivation Solar Fuels and Thermochemical redox cycles Model of the ceria redox system Development of model from equilibrium data Other ceria based oxides Thermodynamics In-depth thermodynamic evaluation Efficiency maximisation and practicality of the cycle Introduction Contents July 1, / 27

4 Contents Introduction Motivation Solar Fuels and Thermochemical redox cycles Model of the ceria redox system Development of model from equilibrium data Other ceria based oxides Thermodynamics In-depth thermodynamic evaluation Efficiency maximisation and practicality of the cycle Introduction Contents July 1, / 27

5 Fuels of modern society Water H 2 O O 2 Air CO 2 N 2 Can these chemicals be sustainably converted into the fuels of modern society. Introduction Motivation July 1, / 27

6 Fuels of modern society Water H 2 O O 2 Air CO 2 N 2 Can these chemicals be sustainably converted into the fuels of modern society. Hydrocarbons Alcohol Fertiliser NH 3 Introduction Motivation July 1, / 27

7 Solar Fuels We are interested in the thermochemical method of fuel production. Introduction Motivation July 1, / 27

8 Two step thermochemical redox cycle MO x MO x δ + δh 2 O Endothermic MO x δ + δ 2 O 2 Exothermic MO x + δh 2 Reduction - T rd Oxidation - T ox MO x is a metal oxide. M = (Zn, Sn, Fe, Ce... etc.) Introduction Thermochemical redox cycles July 1, / 27

9 Heat engine Q in T H Heat engine W Q out T C Efficiency of heat engine η = W Q in, with a maximum of η Carnot = 1 T C T H. Introduction Thermochemical redox cycles July 1, / 27

10 Heat engine Q in T H Q in T rd Reduction O 2 Heat engine W H 2 O MO x MO x δ H 2 Oxidation Q out T C Q out T ox Efficiency of heat engine η = W Q in, with a maximum of η Carnot = 1 T C T H. Efficiency of the thermochemical cycle is η = HHV H 2, with a maximum Q in η max = HHV H 2. H rd HHV H2 is the higher heating value of hydrogen - Total energy available including latent heat of vaporisation of steam. Introduction Thermochemical redox cycles July 1, / 27

11 Cycle properties G = H T S. Reaction spontaneous for G < 0. Reduction + Oxidation = Water splitting: G rd (T ) + G ox(t ) = G ws(t ) Introduction Thermochemical redox cycles July 1, / 27

12 Cycle properties G = H T S. Reaction spontaneous for G < 0. Reduction + Oxidation = Water splitting: G rd (T ) + G ox(t ) = G ws(t ) Grd G T kjmol Gws Trd Gox Tox H rd > H ws and S rd > S ws Introduction Thermochemical redox cycles July 1, / 27

13 CeO 2 redox cycle SUN H 2 O + CO 2 Solar reactor CeO 2 CeO 2 δ + δ 2 O 2 CeO 2 δ + δh 2 O CeO 2 + δ H 2 CeO 2 δ + δco 2 CeO 2 + δco Solar collector Q solar CO + H 2 O 2 Fischer-Tropsch (2n+1)H 2 + nco C nh 2n+2 + nh 2 O Liquid hydrocarbons Introduction Thermochemical redox cycles July 1, / 27

14 CeO2 redox cycle Introduction Thermochemical redox cycles July 1, / 27

15 Why Ceria? Ceria reduction: H 440 kjmol 1 Water splitting: H 250 kjmol 1 CO 2 splitting: H 280 kjmol 1 Ceria releases oxygen without any phase change ( 17 %). [Kümmerele 1999] Fuel production has already been demonstrated. [Chueh 2010] Ceria is relatively abundant (similar to copper). Introduction Thermochemical redox cycles July 1, / 27

16 Why Ceria? Ceria reduction: H 440 kjmol 1 Water splitting: H 250 kjmol 1 CO 2 splitting: H 280 kjmol 1 Ceria releases oxygen without any phase change ( 17 %). [Kümmerele 1999] Fuel production has already been demonstrated. [Chueh 2010] Ceria is relatively abundant (similar to copper). Introduction Thermochemical redox cycles July 1, / 27

17 Analytical model Ceria can be partially reduced at high temperatures. CeO 2 heat CeO 2 δ + δ 2 O 2 δ depends on P O2 and T, δ(p O2, T ). [Panlenar 1975] No phase change implies a simple model. Analytical model Model July 1, / 27

18 Analytical model k reduction CeO 2 + CeO δ 2 δ k oxidation 2 O 2 (1) k a = A a exp ( ) E a RT δ = [V Ö ] [Ce] Set a maximum value for δ, say δ max. Rate = Reduction Oxidation ( ) dδ dt = (δmax δ)a E rd rd exp δpo n RT 2 A ox exp ( ) E ox RT Analytical model Model July 1, / 27

19 Analytical model k reduction CeO 2 + CeO δ 2 δ k oxidation 2 O 2 (1) k a = A a exp ( ) E a RT δ = [V Ö ] [Ce] Set a maximum value for δ, say δ max. Rate = Reduction Oxidation ( ) dδ dt = (δmax δ)a E rd rd exp δpo n RT 2 A ox exp ( ) E ox RT Analytical model Model July 1, / 27

20 Model schematic Ceria δ P O 2 O 2 atmosphere oxidation n A ox exp -E RT ox reduction - E red A exp max red RT 3 /4 Ce O 2 - O V O 2 Analytical model Model July 1, / 27

21 Equilibrium At equilibrium dδ dt = 0 ( ) ( ) δ = A rd P n E O δ max δ A 2 exp ox RT Rearrange to get ( ) δ log = n log(p O2 ) + log δ max δ E = E rd E ox ( ( A rd exp A ox )) E RT Plots of log(δ) vs. log(p O2 ) are common in the literature. Analytical model Equilibrium data July 1, / 27

22 Equilibrium At equilibrium dδ dt = 0 ( ) ( ) δ = A rd P n E O δ max δ A 2 exp ox RT Rearrange to get ( ) δ log = n log(p O2 ) + log δ max δ E = E rd E ox ( ( A rd exp A ox )) E RT Plots of log(δ) vs. log(p O2 ) are common in the literature. Analytical model Equilibrium data July 1, / 27

23 Equilibrium data ( ) δ log = n log(p O2 ) + log δ max δ ( ( A rd exp A ox )) E RT 1 Isothermal plots of equilibrium data. 0 log(δ/(δmax δ)) log(p O2 ) [bar] From the literature we have δ(p O2, T ) [Panlenar 1975, Zinkevich 2006] The slope of each line is the parameter n. Analytical model Equilibrium data July 1, / 27

24 Equilibrium data ( ) δ log = n log(p O2 ) + log δ max δ ( ( A rd exp A ox )) E RT 1 Isothermal plots of equilibrium data. 0 log(δ/(δmax δ)) log(p O2 ) [bar] From the literature we have δ(p O2, T ) [Panlenar 1975, Zinkevich 2006] The slope of each line is the parameter n. Analytical model Equilibrium data July 1, / 27

25 Model vs. literature equilibrium ( ) δ 0.35 δ = P 0.22 O 2 exp ( ) [kj mol 1 ] RT (2) δ vs. Temperature Vacancy concentration δ P O2 P O2 P O2 P O2 P O2 = 100 Pa = 10 Pa = 1 Pa = 0.1 Pa = 0.01 Pa 1,200 1,400 1,600 1,800 2,000 Temperature [ o C] Isobaric plots of model Points are literature equilibrium data E = ± 1.2 kjmol 1 A rd = ± Pa n A ox δ max = 0.35 n = 0.22 ± Analytical model Equilibrium data July 1, / 27

26 Model vs. literature equilibrium ( ) δ 0.35 δ = P 0.22 O 2 exp ( ) [kj mol 1 ] RT (2) δ vs. Temperature Vacancy concentration δ P O2 P O2 P O2 P O2 P O2 = 100 Pa = 10 Pa = 1 Pa = 0.1 Pa = 0.01 Pa 1,200 1,400 1,600 1,800 2,000 Temperature [ o C] Isobaric plots of model Points are literature equilibrium data E = ± 1.2 kjmol 1 A rd = ± Pa n A ox δ max = 0.35 n = 0.22 ± Analytical model Equilibrium data July 1, / 27

27 Mixed oxides based on ceria Analytical model Equilibrium data July 1, / 27

28 Thermodynamics Simple model which predicts δ(p O2, T ), and naturally P O2 (δ, T ) also. Reduction + Oxidation = Water splitting: G rd (T ) + G ox(t ) = G ws(t ) G rd (T ) = RT ln(p O2 (δ, T )) G ws(t ) is also known. The other thermodynamic properties for the reactions are known (C p, H... etc). Have all the information for a full thermodynamic analysis. Thermodynamics Introduction July 1, / 27

29 Thermodynamics Simple model which predicts δ(p O2, T ), and naturally P O2 (δ, T ) also. Reduction + Oxidation = Water splitting: G rd (T ) + G ox(t ) = G ws(t ) G rd (T ) = RT ln(p O2 (δ, T )) G ws(t ) is also known. The other thermodynamic properties for the reactions are known (C p, H... etc). Have all the information for a full thermodynamic analysis. Thermodynamics Introduction July 1, / 27

30 Heating efficiency heat = H CeO rd 2 δox CeO2 δrd + δ rd δ ox O 2 2 T rd P O2 CeO 2 δrd + (δ rd δ ox)h 2 O CeO 2 δox + (δ rd δ ox)h 2 T ox H 2 O T = T rd T ox δ eq = δ rd δ ox η max = HHV H 2. For ceria, η max 67%. H rd Additionally the temperature of ceria must be cycled. Q rd = δrd δ ox H rd (δ)dδ Q CeO2 = Trd T ox C pceo2 (T )dt [J mol 1 ] η heat = δeqhhv H 2 Q rd + Q CeO2, (3) Thermodynamics Heating July 1, / 27

31 Heating efficiency heat = H CeO rd 2 δox CeO2 δrd + δ rd δ ox O 2 2 T rd P O2 CeO 2 δrd + (δ rd δ ox)h 2 O CeO 2 δox + (δ rd δ ox)h 2 T ox H 2 O T = T rd T ox δ eq = δ rd δ ox η max = HHV H 2. For ceria, η max 67%. H rd Additionally the temperature of ceria must be cycled. Q rd = δrd δ ox H rd (δ)dδ Q CeO2 = Trd T ox C pceo2 (T )dt [J mol 1 ] η heat = δeqhhv H 2 Q rd + Q CeO2, (3) Thermodynamics Heating July 1, / 27

32 Heating efficiency heat = H CeO rd 2 δox CeO2 δrd + δ rd δ ox O 2 2 T rd P O2 CeO 2 δrd + (δ rd δ ox)h 2 O CeO 2 δox + (δ rd δ ox)h 2 T ox H 2 O T = T rd T ox δ eq = δ rd δ ox η max = HHV H 2. For ceria, η max 67%. H rd Additionally the temperature of ceria must be cycled. Q rd = δrd δ ox H rd (δ)dδ Q CeO2 = Trd T ox C pceo2 (T )dt [J mol 1 ] η heat = δeqhhv H 2 Q rd + Q CeO2, (3) Thermodynamics Heating July 1, / 27

33 Heating efficiency η heat = δeqhhv H 2 Q rd + Q CeO2, T rd set to 1500 C. Can increase δ eq by reducing P O2. Heating efficiency Η heat P O T C Decreasing P O2 comes with an energy cost. Thermodynamics Heating July 1, / 27

34 Heating efficiency η heat = δeqhhv H 2 Q rd + Q CeO2, T rd set to 1500 C. Can increase δ eq by reducing P O2. Heating efficiency Η heat P O T C Decreasing P O2 comes with an energy cost. Thermodynamics Heating July 1, / 27

35 Fuel production H 2 H 2 O ox Q ox T ox P amb T amb P amb N 2 +O 2 rd Q CeO2 Reactor N 2 +O 2 N 2 Heat Exch. Q gas Q rd N 2 Q N2 T rd P N 2 Prod. η fuel = η plant δ eqhhv H2 Q rd + Q CeO2 + Q N2 + Q gas + Q ox. Energy stored in fuel δ eqhhv H2. Yield by the higher heating value of the fuel. Thermodynamics Fuel production July 1, / 27

36 Fuel production H 2 H 2 O ox Q ox T ox P amb T amb P amb N 2 +O 2 rd Q CeO2 Reactor N 2 +O 2 N 2 Heat Exch. Q gas Q rd N 2 Q N2 T rd P N 2 Prod. η fuel = η plant δ eqhhv H2 Q rd + Q CeO2 + Q N2 + Q gas + Q ox. η plant depends on solar concentration and reduction temperature T rd Thermodynamics Fuel production July 1, / 27

37 Fuel production H 2 H 2 O ox Q ox T ox P amb T amb P amb N 2 +O 2 rd Q CeO2 Reactor N 2 +O 2 N 2 Heat Exch. Q gas Q rd N 2 Q N2 T rd P N 2 Prod. η fuel = Reduce Q rd and heat ceria Q CeO2. η plant δ eqhhv H2 Q rd + Q CeO2 + Q N2 + Q gas + Q ox. Thermodynamics Fuel production July 1, / 27

38 Fuel production H 2 H 2 O ox Q ox T ox P amb T amb P amb N 2 +O 2 rd Q CeO2 Reactor N 2 +O 2 N 2 Heat Exch. Q gas Q rd N 2 Q N2 T rd P N 2 Prod. η fuel = η plant δ eqhhv H2 Q rd + Q CeO2 + Q N2 + Q gas + Q ox. Produce Q N2 and heat Q gas the sweep gas. Thermodynamics Fuel production July 1, / 27

39 Fuel production H 2 H 2 O ox Q ox T ox P amb T amb P amb N 2 +O 2 rd Q CeO2 Reactor N 2 +O 2 N 2 Heat Exch. Q gas Q rd N 2 Q N2 T rd P N 2 Prod. η fuel = η plant δ eqhhv H2 Q rd + Q CeO2 + Q N2 + Q gas + Q ox. Heat the oxidiser Q ox Thermodynamics Fuel production July 1, / 27

40 Ambient fuel production η fuel = η plant δ eqhhv H2 Q rd + Q CeO2 + Q N2 + Q gas + Q ox. (4) Fuel production efficiency with T = 500 C. Fuel efficiency Ηfuel T rd Log P O2 bar η fuel can be maximised w.r.t. T and P O2. Maximum is 4.5%. At maximum δ eq is very small. Thermodynamics Fuel production July 1, / 27

41 Ambient fuel production η fuel = η plant δ eqhhv H2 Q rd + Q CeO2 + Q N2 + Q gas + Q ox. (4) Fuel production efficiency with T = 500 C. Fuel efficiency Ηfuel T rd Log P O2 bar η fuel can be maximised w.r.t. T and P O2. Maximum is 4.5%. At maximum δ eq is very small. Thermodynamics Fuel production July 1, / 27

42 Fuel production pumping H 2 H 2 O T amb P amb ox Q ox T ox P amb O 2 Reactor Pump Q pmp rd Q CeO2 Q rd O 2 Heat Exch. Q gas T rd P η fuel = η plant δ eqhhv H2 Q rd + Q CeO2 + Q ox + Q gas + Q pmp. Q gas is now a small quantity of released heat. Thermodynamics Pumping July 1, / 27

43 Fuel production: pumping H 2 H 2 O T amb P amb ox Q ox T ox P amb O 2 Reactor Pump Q pmp rd Q CeO2 Q rd O 2 Heat Exch. Q gas T rd P η fuel = η plant δ eqhhv H2 Q rd + Q CeO2 + Q ox + Q gas + Q pmp. Q pmp is the heat energy required to pump the oxygen. Thermodynamics Pumping July 1, / 27

44 Pumping efficiency The efficiency of vacuum pumps decreases with pressure Efficiency of some Bosch pumps Log(η elec to pump ) data Analytical Log(P ) Current vacuum pumps not designed for efficiency. Pumping efficiency, η pmp(p ) = 0.4 P Thermodynamics Pumping July 1, / 27

45 Pumping efficiency The efficiency of vacuum pumps decreases with pressure Efficiency of some Bosch pumps Log(η elec to pump ) data Analytical Log(P ) Current vacuum pumps not designed for efficiency. Pumping efficiency, η pmp(p ) = 0.4 P Thermodynamics Pumping July 1, / 27

46 Reduced reduction pressure η fuel = η plant δ eqhhv H2 Q rd + Q CeO2 + Q gas + Q ox + Q pmp. (5) Efficiency with T = 500 C. Fuel efficiency Η fuel T rd Dashed line P = P O2, which means Q N2 = Q gas = 0 η fuel can be maximised w.r.t. T and P. Maximum is 7.5%. δ eq improved compared to ambient operation. Thermodynamics Fuel production: pumping July 1, / 27

47 Reduced reduction pressure η fuel = η plant δ eqhhv H2 Q rd + Q CeO2 + Q gas + Q ox + Q pmp. (5) Efficiency with T = 500 C. Fuel efficiency Η fuel T rd Dashed line P = P O2, which means Q N2 = Q gas = 0 η fuel can be maximised w.r.t. T and P. Maximum is 7.5%. δ eq improved compared to ambient operation. Thermodynamics Fuel production: pumping July 1, / 27

48 Ceria heat recuperation Using the hot ceria from the reduction step to heat the relatively cool ceria from the oxidation step is a popular idea. [Miller 2008, Lapp 2013] N 2 +O 2 CSP Reduction N 2 Ceria Recuperation Insulation Oxidation H 2 O H 2 Why bother? Instead use the heat as high temperature process heat for Q pmp and Q ox. Thermodynamics Heat recuperation July 1, / 27

49 Ceria heat recuperation Using the hot ceria from the reduction step to heat the relatively cool ceria from the oxidation step is a popular idea. [Miller 2008, Lapp 2013] N 2 +O 2 CSP Reduction N 2 Ceria Recuperation Insulation Oxidation H 2 O H 2 Why bother? Instead use the heat as high temperature process heat for Q pmp and Q ox. Thermodynamics Heat recuperation July 1, / 27

50 Ceria heat recuperation Assume the hot ceria can be used to decrease any of the other heat requirements with effectivness ε sld = Fuel efficiency Η fuel T rd Sharp peak occurs when Q pmp + Q ox becomes larger than the available recycled process heat. Maximising efficiency w.r.t. T and P gives η fuel 11 % Thermodynamics Heat recuperation July 1, / 27

51 Practicality of the ceria cycle. Amount of ceria needed to produce a mole of fuel is 1 δ eq. ṅ CeO2 = Q out δ eqhhv H2 [mol s 1 ] m CeO2 Q out = t cycm(ceo 2 ) ṅ CeO 2 Q out [kg kw 1 ] Assuming t cyc = 10 min, T rd = 1500 C and ε sld = 0.6 Optimised ambient pressure, η fuel 7.5 %, m CeO 2 Q out Toyota Prius need 3750 kg kg kw 1. To power a Optimised pumped system, η fuel 11 %, m CeO 2 Q out 13 kg kw 1. Very efficient pump η pmp = 0.5P 0.27 (Transonic axial flow), η fuel 18 %, m CeO2 Q out 7 kg kw 1. Solar electrolysis is % efficient. Thermodynamics Practicality July 1, / 27

52 Practicality of the ceria cycle. Amount of ceria needed to produce a mole of fuel is 1 δ eq. ṅ CeO2 = Q out δ eqhhv H2 [mol s 1 ] m CeO2 Q out = t cycm(ceo 2 ) ṅ CeO 2 Q out [kg kw 1 ] Assuming t cyc = 10 min, T rd = 1500 C and ε sld = 0.6 Optimised ambient pressure, η fuel 7.5 %, m CeO 2 Q out Toyota Prius need 3750 kg kg kw 1. To power a Optimised pumped system, η fuel 11 %, m CeO 2 Q out 13 kg kw 1. Very efficient pump η pmp = 0.5P 0.27 (Transonic axial flow), η fuel 18 %, m CeO2 Q out 7 kg kw 1. Solar electrolysis is % efficient. Thermodynamics Practicality July 1, / 27

53 Practicality of the ceria cycle. Amount of ceria needed to produce a mole of fuel is 1 δ eq. ṅ CeO2 = Q out δ eqhhv H2 [mol s 1 ] m CeO2 Q out = t cycm(ceo 2 ) ṅ CeO 2 Q out [kg kw 1 ] Assuming t cyc = 10 min, T rd = 1500 C and ε sld = 0.6 Optimised ambient pressure, η fuel 7.5 %, m CeO 2 Q out Toyota Prius need 3750 kg kg kw 1. To power a Optimised pumped system, η fuel 11 %, m CeO 2 Q out 13 kg kw 1. Very efficient pump η pmp = 0.5P 0.27 (Transonic axial flow), η fuel 18 %, m CeO2 Q out 7 kg kw 1. Solar electrolysis is % efficient. Thermodynamics Practicality July 1, / 27

54 Practicality of the ceria cycle. Amount of ceria needed to produce a mole of fuel is 1 δ eq. ṅ CeO2 = Q out δ eqhhv H2 [mol s 1 ] m CeO2 Q out = t cycm(ceo 2 ) ṅ CeO 2 Q out [kg kw 1 ] Assuming t cyc = 10 min, T rd = 1500 C and ε sld = 0.6 Optimised ambient pressure, η fuel 7.5 %, m CeO 2 Q out Toyota Prius need 3750 kg kg kw 1. To power a Optimised pumped system, η fuel 11 %, m CeO 2 Q out 13 kg kw 1. Very efficient pump η pmp = 0.5P 0.27 (Transonic axial flow), η fuel 18 %, m CeO2 Q out 7 kg kw 1. Solar electrolysis is % efficient. Thermodynamics Practicality July 1, / 27

55 Thermodynamic conclusions Thermodynamic efficiency for the ceria cycle was found to be quite low ( 11 %). Isothermal cycles will inevitably have very low efficiency - less than 2%. Improvement of vacuum pumps may be vital for success of the ceria cycle. Sensible to develop a method of converting heat stored in ceria to process heat. Doping Ceria or producing new materials could make a large difference to both efficiency and practicality Thermodynamics Thermodynamic conclusions July 1, / 27

56 Thermodynamic conclusions Thermodynamic efficiency for the ceria cycle was found to be quite low ( 11 %). Isothermal cycles will inevitably have very low efficiency - less than 2%. Improvement of vacuum pumps may be vital for success of the ceria cycle. Sensible to develop a method of converting heat stored in ceria to process heat. Doping Ceria or producing new materials could make a large difference to both efficiency and practicality Thermodynamics Thermodynamic conclusions July 1, / 27

57 Publications 1 An analytical model of CeO 2 oxidation and reduction. Brendan Bulfin, Arran J. Lowe, Kevin A. Keogh, Barry E. Murphy, Olaf Lübben, Sergey A. Krasnikov, and Igor V. Shvets, Journal of Physical Chemistry C 46 (2013) Thermodynamics of CeO 2 thermochemical fuel production. Brendan Bulfin, Friedemann Call, Mathias Lange, Christian Sattler, Robert Pitz-Paal, and I. V. Shvets, Energy and Fuels - 29 (2015) Publications July 1, / 27

58 Acknowledgements Thanks to my co-authors and to SFI and the Helmholtz institute for funding. I would also like to thank the DAAD program, who have funded my current fellowship.

Thermodynamics of CeO 2 thermochemical fuel production - Supporting Information

Thermodynamics of CeO 2 thermochemical fuel production - Supporting Information B. Bulfin, F. Call, M. Lange, O. Lübben, C. Sattler, R. Pitz-Paal, I. V. Shvets Abstract In this supporting document the