Solar syngas production from CO 2 and H 2 O in two-step thermochemical cycles based on

Transcription

1 Solar Fuels & Materials Page 215 Solar syngas production from CO 2 and H 2 O in two-step thermochemical cycles based on Zn/ZnO redox reactions Dr. Peter G. Loutzenhiser Department of Mechanical and Process Engineering, ETH Zurich, Switzerland Outline Motivation Thermodynamic analysis Solar ZnO dissociation Non-solar Zn+CO 2 /H 2 O reaction Conclusions 2 1

2 Solar Fuels & Materials Page 216 Solar Fuels CONCENTRATED SOLAR POWER Absorption Heat Reactants Chemical (CO 2 +HO) 2 Reactor Liquid Fuels Products Synthesis Gas (CO + H 2 ) Electricity & Transportation 3 11 MW-electric/ 55 MW-thermal(Sevilla, ES) 4 2

3 Solar Fuels & Materials Page 217 CO 2 capture and sequestration scenarios CO 2 capture from flue gases CO 2 capture from air Sequestration Ocean Depths Deep Surface Geological Mineral Media Carbonation M. Bachau, D. Bonijoly, J. Bradshaw, R. Burruss, S. Holloway, N.P. Christenson, O.D. Mathiassen Int. J. Greenhouse Gas Control 1 (2007) Alternative to sequestration with concentrated solar energy Chemically reduce mixtures of H 2O and CO 2 to synthesis gas using concentrated solar irradiation for conversion to liquid fuels Solar fuels decouple the intermittency and location and allow for storage and transportation Replaces fossil fuels without major changes in technology Creates carbon-neutral fuels for CO 2 capture from the air Alternative to CO 2 sequestration for direct capture from flue gases; CO 2 is never introduced into the atmosphere 6 3

4 Solar Fuels & Materials Page 218 Two-step CO 2 splitting cycle CO 2 captured from flue gases CO 2 captured from air CONCENTRATED SOLAR POWER Direct thermolysis CO 2 CO+½O 2 T=2700 K 30% theoretical dissociation CO 2 CO, ½O 2 7 Two-step CO 2 splitting cycle CO 2 captured from flue gases CONCENTRATED SOLAR POWER CO 2 captured from air SOLAR REACTOR y x y x 2 2 MO M O xm y 2 O2 M x O y OXIDIZER yco +xm M O yco 2 x y yco 2 yco 2 Power production y 2 O2 yco 8 4

5 Solar Fuels & Materials Page 219 CO 2 /H 2 O splitting with Zn/ZnO redox reactions CO 2 H 2 O SOLAR REACTOR ZnO Zn + ½O 2 ZnO CONCENTRATED SOLAR POWER Zn Zn OXIDIZER Zn+CO 2 /H 2 O ZnO+CO/H O2 CO H 2 Solar reaction ZnO Zn+½O 2 ΔH K = 351 kj mol -1 Non-solar reaction Zn+CO 2 /H 2 O ZnO+CO/H 2-68 kj mol -1 < ΔH K < -65 kj mol -1 Net reaction CO 2 /H 2 O CO/H 2 +½O kj mol -1 < ΔH K < -286 kj mol -1 9 Motivation Thermodynamic analysis Solar ZnO dissociation Non-solar Zn+CO 2 /H 2 O reaction Conclusions Outline Equilibrium compositions Second law analysis 10 5

6 Solar Fuels & Materials Page 220 Equilibrium compositions for the ZnO (+Ar) reaction at 1 bar 11 Equilibrium compositions for the 2Zn+CO 2 +H 2 O reaction at 1 bar 12 6

7 Solar Fuels & Materials Page 221 Second law analysis of a theoretical cycle Solar reactor n CO2 where = n n Assuming C=5000 suns I 1 kw m DN n ZnO CO 2 H 2O 2 1 mol s 1 P.G. Loutzenhiser, A. Steinfeld, 2011 International Congress on Hydrogen, submitted 13 Absorption efficiency Solar reactor 4 reactor absorption 1 T IC Q solar Q re-radiation Q Q Q solar net solar P.G. Loutzenhiser, A. Steinfeld, 2011 International Congress on Hydrogen, submitted 14 7

8 Solar Fuels & Materials Page 222 Solar reactor: Heat transfer rates Solar reactor 4 reactor T absorption 1 IC Q solar Qre-radiation Q Q Q Q solar H solar solar ZnO (+Ar) at T 1 amb Zn+ O 2 2 (+Ar) at Treactor absorption net P.G. Loutzenhiser, A. Steinfeld, 2011 International Congress on Hydrogen, submitted where = n I 1 kw m DN n ZnO n Assuming C=5000 suns CO2 n CO2 H2O 2 1 mol s 1 15 Zn oxidizer: Rates of work and heat transfer W W W W Maximum work cycle FC separator G FC 1 βco+(1-β)h2 O2βCO 2+(1-β)H2O 2 Q T S FC amb 1 βco+(1-β)h2 O2βCO 2+(1-β)H2O 2 W separator n ZnO RT amb 1 n Ar 1 0.5ln ln yo2 n ZnO yar where = n n n CO2 n CO2 H2O P.G. Loutzenhiser, A. Steinfeld, 2011 International Congress on Hydrogen, submitted 16 8

9 Solar Fuels & Materials Page 223 Theoretical cycle efficiencies Cycle efficiency cyle W Q solar cycle solar (without heat recuperation) W cycle cyle Q Q quench (with heat recuperation) P.G. Loutzenhiser, A. Steinfeld, 2011 International Congress on Hydrogen, submitted 17 Introduction Thermodynamic analysis Solar ZnO dissociation Non-solar Zn+CO 2 /H 2 O reaction Conclusions Outline Solar reactor Experimental results 18 9

10 Solar Fuels & Materials Page 224 Solar reactor ceramic insulation feeder rotary joint water/gas Inlets/outlets ZnO Zn + ½ O 2 cavity - receiver ZnO-sintered tiles quartz window 20%SiO 2 - Al 2 O 3 Concentrated Solar Radiation ZnO-sintered tiles 20%SiO 2 -Al 2 O 3 19 Exploded view of the solar reactor ZnO container Rotary joint ZnO feeder Data acquisition ZnO-tile cavity Porous Al 2 O 3 insulation Water-cooled reactor front Al 2 O 3 CMC front cone Ar nozzles Water-cooled Cu aperture Lateral front wall 20 10

11 Solar Fuels & Materials Page 225 Solar reactor: Schematic 21 Solar reactor ZnO feed cycle 22 11

12 Solar Fuels & Materials Page 226 Experimental results with five ZnO feedings feed: 120 g ZnO (each) K Tempe erature behind ZnO tiles, lar power input, W Sol solar power temperature O 2 from ZnO dissociation Molar flo ow of O 2, mol s Time, s 23 Motivation Thermodynamic analysis Solar ZnO dissociation Non-solar Zn+CO 2 /H 2 O reaction Conclusions Outlook Outline Kinetic analysis Aerosol reactor Novel packed-bed reactor 24 12

13 Solar Fuels & Materials Page 227 Analysis with thermogravimetry and gas chromatography Combination of isothermal and non-isothermal TG runs Zn sample ~100 mg Average diameter ~ 7 m Temperature range of K CO 2 partial pressure range of bars CO 2 H 2 O partial pressures bars Total flow of 100 ml N /min No carbon formation as corroborated by mass balance, XRD, and XPS 25 Isothermal TG experiments Temperature = 723 K Temperature = K 15-75% CO 2 -Ar/Zn 20% CO 2 -Ar/Zn Diffusion-controlled regime Interface-controlled regime 26 13

14 Solar Fuels & Materials Page 228 Rate laws and kinetic parameters Interface-controlled regime Diffusion-controlled regime 27 Power fit for determining order of reaction Resulting fits Arrhenius plots for rate constants in both regimes P. Loutzenhiser, M.E. Gálvez I. Hischier, A Stamatiou, A Frei, A. Steinfeld, Energy & Fuels 23 (2009)

15 Solar Fuels & Materials Page 229 H 2 O-CO 2 -Ar Isothermal experiments 5%H 2 O-Ar/Zn 5%CO 2 -Ar/Zn Two reaction regimes: 1) interface-controlled t and 2) diffusion-controlledi d Increase of reaction rate with reactive gas concentration Decrease of reaction extent with reactive gas concentration Strong effect of H 2 O(v) 29 Competitive kinetic model k1 CO + θ<*> CO + θ<o> 2 k2 HO+ θ<*> H + θ<o> O 2 2 k3 θ<o> + Zn ZnO 1 = θ<*> + θ<o> Steady-state d n k k y +k y r= = dt S k +k y +k y 3 1 CO2 2 H2O 3 1 CO2 2 H2O Ea where k = k0exp RT A Stamatiou, P.G. Loutzenhiser, A. Steinfeld, Energy & Fuels 24 (2010)

16 Solar Fuels & Materials Page 230 Reactor design criteria Flow through reactor Zn particle formation with high specific surface areas ZnO recycled back to the first step Aerosol flow reactor Homogeneous nucleation of Zn Subsequent heterogeneous reactions of Zn+CO 2 in the aerosol flow of the reaction zone Maximize ZnO/Zn depositions for recycling back to the first step 31 Configuration I Aerosol reactor FM Configuration II Filter CO 2 -Ar Flow GC Filter Reaction Zone Cooling Zone Evaporation Zone CO 2 Furnace 1 Furnace 1 FC CO 2 -Ar Flow Mixing Area FC Furnace 2 Furnace 2 Ar Zn Crucible Reaction Zone Quench Zone Evaporation Zone Zn Crucible Balance Ar Flow FC Ar Flow Balance 32 16

17 Solar Fuels & Materials Page 231 Quench flow inlets for Configuration II Annular inlet flow Radial inlet flow Mixing Area CO 2 -Ar Flow Zn(g)-Ar Flow 33 Configuration I Experimental setup Configuration II Reactant molar flow ratio of For annular inlet, the inlet CO 2 to Zn was varied from temperature was varied from K Reaction zone temperature was varied from K For the radial inlet, the inlet temperature was varied from K 34 17

18 Solar Fuels & Materials Page 232 Configuration I CO 2 :Zn molar flow ratio of unity Reaction zone temperature is 823 K Exemplarily run 35 Performance parameters for the aerosol reactor Conversion from Zn to ZnO nzno nco X Zn-to-ZnO n n Zn,i Zn,i Conversion from CO 2 to CO X CO2 -to-co n n CO CO 2,i Y Filter particle yield fzn 1 f Zn mfilter MZn MZnO n Zn,i Reaction outside the aerosol flow 1 f Zn mfilter M ZnO Z 1 n CO 36 18

19 Solar Fuels & Materials Page 233 Results from Configuration I Varied molar flow ratios of CO 2 to Zn Varied reaction zone temperatures 52% ~1 most effective use of reactants 823 K P.G. Loutzenhiser, M.E. Gálvez I. Hischier, A. Graf, A. Steinfeld, Chemical Engineering Science 65 (2010) Results from Configuration II Annular inlet flow Radial inlet flow 45% 51% 648 K 670 K P.G. Loutzenhiser, M.E. Gálvez I. Hischier, A. Graf, A. Steinfeld, Chemical Engineering Science 65 (2010)

20 Solar Fuels & Materials Page 234 Disadvantages The reaction occurred primarily on the reactor surfaces making ZnO/Zn recovery difficult A carrier gas is required to move reactants/products t t through h the reactor Zn must be evaporated Packed-bed reactor Prevent sintering by immobilizing particles Ensure efficient recovery of the ZnO for recycling 39 GC Tube Furnace TC Zn Sample with Support Packed-bed reactor Temperature below the Zn melting point ZnO Oparticles added to immobilize Zn particles to prevent sintering and maintain high specific surface areas Initial experiment with CO 2 as reactant gas FC CO 2 FC Ar P.G. Loutzenhiser, F. Barthel, A. Stamatiou, A. Steinfeld, AIChE Journal, in press 40 20

21 Solar Fuels & Materials Page 235 Temporal Zn-to-ZnO conversions 67 wt% Zn 100% CO 2 41 Temporal CO 2 -to-co conversions 42 21

22 Solar Fuels & Materials Page 236 Summary of overall conversions 43 Experiments for model validation Controlled the inlet temperature Controlled boundary conditions Goal was to look at the transient natural of the reaction Create a transient heat transfer model coupled with chemical kinetics Tested both CO 2 and mixtures of CO 2 /H 2 O Sample size between 3-9 g A. Stamatiou, P.G. Loutzenhiser, A. Steinfeld, AIChE Journal, in press 44 22

23 Solar Fuels & Materials Page 237 Exemplary runs with 50% H 2 O-CO 2 45 Conclusions The step-by-step methodology for investigated a two-step solar thermochemical cycle to produce syngas from H 2 O/CO 2 with Zn/ZnO redox reactions has been outlined This work represents determination of the fundamental parameters at a laboratory scale to initiate scale-up of both steps 46 23