Applicability of Dimethylether to Solid Oxide Fuel Cells

Transcription

1 17 Nov. 2011, 7th Asian DME Conference Applicability of Dimethylether to Solid Oxide Fuel Cells ~ Reforming and Cell Performance in Anode Off-gas Recycle ~ Yohei Tanaka, Katsutoshi Sato, Akihiko Momma, Ken Nozaki, Tohru Kato, and Atsushi Yamamoto National Institute of Advanced Industrial Science and Technology (AIST), Japan 1

2 Introduction 2

3 Solid oxide fuel cell (SOFC) ~ single cell and stack ~ Ceramics-based technol. Operated at ºC Various structures SOFC.pdf ew/pdf/451/ pdf /news/2007/0103.html 700W μchp system (ENEFARM) on sale since Oct in Japan ess/2004/09/17.htm 8/pdf/ pdf 3

4 Electrical efficiency plot for small power generators SOFC PEFC SOFC+GT GT: Gas Turbine GE: Gas Engine DE: Diesel Engine PEFC: Polymer Electrolyte Fuel Cell PAFC: Phosphoric Acid Fuel Cell MCFC: Molten Carbonate Fuel Cell SOFC: Solid Oxide Fuel Cell SOFC+GT: SOFC-GT combined system SOFC systems can achieve highest electrical efficiency at 1-10,000 kw Theoretically, 60-70% are expected at a system level. 4

5 SOFC system with reforming process Chemical energy of fuel ( ΔH ) Reforming e.g. 650ºC Chemical energy of H 2 and CO Electrochemical reactions Electrical energy ( ΔG ) + Thermal energy Heat + Oxidant (H 2 O, CO 2 ) Part of HT waste heat Fuels are reformed to H 2 and CO for electrochemical reactions on anode (fuel electrode) to proceed easily. SOFC can deal with not only H 2 but also CO, leading to a simple fuel processing. Use of high temp. (HT) waste heat from SOFC for the endothermic reforming etc. leads to higher electrical efficiency. 5

6 DME (CH 3 OCH 3 ) as a fuel of SOFC 1. Easily liquefied and vaporized No fuel pump or blower is needed for fuel supply Portable and stationary applications Unlike LPG, gas composition will be stable against time, leading to easier control of system 2. No C-C bond Carbon deposition in reforming may be less probable than one for LPG 3. No Sulfur No need for desulfurization, leading to simpler fuel processing Reports on DME reforming mainly target LT fuel cells and lowering reforming temperature. DME-fueled SOFC performance including reforming is not clear at a practical-sized cell level. 6

7 Objectives of this work SOFC performance with reformate of DME and other fuels will be discussed at practical-sized single-cell level Possibility of improving electrical efficiency by anode off-gas recycle (AGR) is investigated to discuss attainable electrical efficiency with DME 7

8 Experimental 8

9 SOFC testing method at an atmospheric pressure Generation of synthesized reformate gas (anode gas) H 2 O 2 CO 2 Air MFC MFC MFC MFC Catalytic H 2 combustor > 200ºC H 2, H 2 O Catalytic equilibrium reactor ºC Cell H 2 -Hassembly 2 O-CO-CO 2 -CH 4 Anode Electrolyte Cathode ºC Disc-type Ni-YSZ/YSZ/YDC/LSCF single cell (electrode area 100 cm 2 ) For the case assuming steam reforming, S/C or O/C was set to 3.0. (e.g. for 6H 2 O + C 2 H 6 O, S/C= 3.0 and O/C= 3.5) 9

10 DME-reforming-catalyst performance test for AGR DME Synthesized anode off-gas (H 2 O, CO 2, H 2, CO) Reforming catalyst ºC Electric furnace Gas analysis (μgc, FTIR) Catalyst: Süd-chemie FCR4 (ca. 1.2 ml or 1.4 g) Base conditions: DME 20 NmL/min (GHSV= 1000 h 1 ), 650ºC On-line analysis of reformate gases Post-analysis: estimation of carbon amount deposited on a used catalyst surface by TPO (Temperature Programmed Oxidation ) 10

11 Results and discussion 11

12 Cell voltage / V Cell voltage / V Energy Technol. Res. Inst. Fuel Cell System Group Effect of fuel specie on SOFC performance@750ºc ~ steam reforming case ~ S/C = 3.0 O/C = 3.0 DME CH 4 C 3 H DME (S/C = 2.5) CH 4 (S/C = 3.0) C 3 H 8 (S/C = 3.0) Practical operating point Practical operating point Current density / ma cm Current density / ma cm -2 Cell voltage for DME, CH 4, C 3 H 8 is comparable at practical A cm -2. Perfect DME reforming makes little difference in SOFC performance. 12

13 Calculated DC electrical O/C= 3, 355 ma cm 2 DC electrical efficiency = V cell / (Q i / n F) U f V cell : cell voltage, Q i : lower heating value of fuel, n : number of electron transferred for electrochem. reaction of a molecular of fuel, F: Faraday s constant, U f : fuel utilization Fuel V cell (V) Q i / n F (V) U f (%) Efficiency (%) DME CH C 3 H Thermodynamic and electrochemical constants Due to the difference in the constant Q i / n F, efficiency for DME is lowered at a single-cell level. 13

14 Anode off-gas recycle (AGR) f R = r f a,out = r f ex / (1 r) Recycle ratio r H 2, H 2 O, CO, CO 2 DME f f Reformer Anode f a,out f ex Air Cathode A method to improve fuel utilization (U f ) Fuel is reformed by the recycled gas. Recycle ratio and temperature will influence reforming. Possibly leading to improvement of electrical efficiency Recycled and anode-inlet gases at steady state were synthesized by our testing system to evaluate reforming and SOFC performances in AGR 14

15 CO 2 + CO formation rate / sccm Energy Technol. Res. Inst. Fuel Cell System Group Effect of recycle ratio on DME reforming in AGR Effect of recycle ratio on DME conversion and carbon deposition in reforming at 650ºC for 5 h Recycle ratio, r (O/C) (1.3) (1.6) (2.0) (2.3) (2.5) DME conversion (%) Carbon deposition (mg/g-cat.) 84.0 ± ± ± ± ± 0.04 Ratio of carbon deposition to carbon in DME feed (%) Assumed SOFC operation conditions: 750ºC and 90% fuel utilization TPO profile r = 0.30 Conversion of DME to H 2 -H 2 O-CO-CO 2 - CH 4 were as high as 96% or better r = 0.40 r = 0.55 r = 0.65 r = Higher r led to decrease in C deposition r = 0.65 or higher will be required in AGR Temperature / ºC 15

16 DC electrical efficiency / %-LHV Energy Technol. Res. Inst. Fuel Cell System Group Improvement of electrical efficiency by AGR With AGR at r = 0.65 Without AGR 60 U f = 90% 55 AGR at r = Current density / ma cm -2 U f = 76% DC electrical efficiency will be enhanced by 9-8 percentage points, or 16%(rel) with AGR at ma cm 2 to 64-59%(LHV) for the used cell 16

17 Cell voltage / V DC electrical efficiency / %-LHV Energy Technol. Res. Inst. Fuel Cell System Group Optimization of SOFC operating conditions in AGR Important operative parameters: cell temp., fuel utilization (U f ), air utilization (U air ), recycle ratio (r) cell temp. dependency U f dependency U f = 95% U f = 90%, r = 0.65, U air = 30% U f = 92.5% U f = 90% U f = 85% U f = 80% ºC 750ºC 700ºC 650ºC ºC, r = 0.65, U air = 30% Current density / ma cm Current density / ma cm -2 For the used cell, ºC and U f = 92.5% are optimal at single-cell level. 17

18 Cell voltage / V Cell voltage / V Energy Technol. Res. Inst. Fuel Cell System Group Optimization of SOFC operating conditions in AGR (2) Important operative parameters: cell temp., fuel utilization (U f ), air utilization (U air ), recycle ratio (r) U air dependency r dependency ºC, U f = 90%, r = 0.65, 750ºC, U f = 90%, U air = 30% U air = 15% U air = 30% U air = 45% 0.75 r = 0.55 r = 0.65 r = % Current density / ma cm Current density / ma cm -2 For the used cell, U air dependency was small. Effect of r on cell voltage was 2%. 18

19 Conclusions 1. As long as DME is reformed to equilibrium, SOFC performance (V-I curve) is comparable to one for methane and propane. However, DC electrical efficiency will be lowered by 10%(rel.) than one for propane even at identical cell voltages. This is due to thermodynamic and electrochemical constants (heating value and n). 2. Anode off-gas recycle for DME can enhance DC electrical efficiency by 16%(rel) at practical SOFC operating conditions to attain 59-64% efficiency for the used cell. More detailed: Recycle ratio, r should be 0.65 or higher to suppress carbon deposition in a reformer. 92.5% fuel utilization and ºC are optimal conditions at a single-cell level for the used cell. 19

20 Acknowledgement Part of this work was supported by Ministry of Industry, Economy and Trade (METI), Japan. The support is appreciated. 20

21 Supplementary slides 21

22 Fuel Cell ~ Converter of chemical energy to electrical energy ~ Chemical energy of fuel : (ΔH at combustion) Electrochemical reactions Electrical energy (ΔG) + Thermal energy (+TΔS) Fuel combustion ΔH ΔG Thermodynamic electrical efficiency, th = ΔG ΔH TΔS Thermodynamically, electrical energy can be obtained as much as ΔG out of ΔH at fuel combustion. 22

23 Superiority of SOFC to lower-temp. fuel cells LT fuel cells SOFC Cell temp. << Reforming temp. Cell temp. > Reforming temp. ca. 100ºC ca. 650ºC ca. 800ºC ca. 650ºC DME Enthalpy 100 Reforming with partial fuel combustion & purification H 2 80 Electrical energy 40 DME Enthalpy 100 Reforming with HT waste heat H 2 + CO 115 DC = 40% DC = 58% Electrical energy 58 High-temperature waste heat from SOFC can be utilized for endothermic reforming to enhance chemical energy and electrical efficiency. 23

24 Chemical energy change by reforming of DME and CH 4 Steam reforming CO 2 reforming Steam reforming 2CO + 4H 2 3CO + 3H 2 DME CO + 3H 2 CH 4 ΔH (+15%) 1574 (+19%) ΔH (+26%) ΔG (+ 5%) 1457 (+ 7%) ΔG (+18%) ΔH : Enthalpy, or lower heating value at 25ºC, kpa (kj/mol-fuel) ΔG : Exergy, or maximum electrical energy extracted from chemicals (kj/mol-fuel) Endothermic reforming for SOFC enhances not only ΔH but also ΔG.

25 Change in enthalpy and exergy by steam reforming Fuel CH 4 C 3 H 8 Kerosene (C 12 H 24 (l)) CH 3 OH (l) DME (CH 3 OCH 3 ) Biodiesel (C 19 H 36 O 2 ) ΔH of fuel (kj/mol-fuel) ΔG of fuel (kj/mol-fuel) ΔH /n F (V) ΔH of reformate (kj/mol-fuel) 1008 (+26%) 2542 (+24%) 9199 (+24%) 767 (+20%) 1533 (+15%) (+24%) ΔG of reformate (kj/mol-fuel) 943 (+18%) 2372 (+14%) 8572 (+13%) 714 (+4%) 1429 (+5%) (+12%) ΔG reformate /ΔH fuel Lower-heating-value base, or H 2 O exists as gas. 25

26 DME to CH 4 reaction (SNG process) Overall: CH 3 OCH 3 1.5CH CO kj/mol (DME SR, shift, methanation over Ni catalyst at ºC, bar) DME SNG CH 4 SR 1.5CO + 4.5H 2 1.5CH 4 ΔH 1328 ΔG ( 9%) 1202 ( 12%) 1513 (+14%) 1414 (+4%) SNG process will be slightly inferior to DME steam reforming. But, DME decomposition with carbon formation may be avoided. 26

27 C-H-O diagram with equilibrium p O2 T. Takeguchi et al. / Journal of Power Sources 112 (2002) CH 4 + H 2 O = CO + 3H 2 H 2 + 1/2O 2 = H 2 O CO + 1/2O 2 = CO 2 2CO = C(s) + CO 2 C-H-O ratio affects equilibrium composition Oxygen partial pressure more influenced by O/C rather than by O/H 27

28 C-H-O diagram for steam reforming at S/C=3 or O/C=3 Carbon deposition C deposition boundary 28

29 C-H-O diagram for steam reforming of DME DME Steam reforming C ºC 650ºC 550ºC 450ºC 350ºC 250ºC O/C = H O O/C = 2.0 or higher is favored to suppress carbon deposition 29

30 Temperature / ºC Energy Technol. Res. Inst. Fuel Cell System Group Carbon deposition boundary for DME reforming in AGR Boundary Reforming experiment points No carbon deposition (thermodynamics) U f, DME = 90% Carbon deposition (thermodynamics) Recycle ratio, r From the reforming experiments, r = 0.65 (O/C= 2.3) or higher are required to prevent actual carbon deposition on the catalyst surface. 30

31 Temperature / ºC Energy Technol. Res. Inst. Fuel Cell System Group Fuel-specie dependency of C deposit. boundary No carbon deposition (thermodynamics) DME CH 4 U f = 90% Carbon deposition (thermodynamics) C 3 H 8 C 12 H Recycle ratio, r Temp. and r at C deposition boundary: DME~ CH 4 < C 3 H 8 < C 12 H 26 DME will be reformed at lower temperatures and r than the other fuels. 31

32 Temperature / ºC Energy Technol. Res. Inst. Fuel Cell System Group Effect of U f, DME on carbon deposition boundary Fuel: DME No carbon deposition U f = 80% U = 85% f U = 90% f U = 95% f 400 Carbon deposition Recycle ratio, r Higher utilization of DME leads carbon deposition boundary to lower temperatures and recycle ratios. 32

33 Cell voltage / V DC electrical efficinec / %-LHV Energy Technol. Res. Inst. Fuel Cell System Group Fuel dependency at realistic reforming conditions ~ cell performance and DC electrical efficiency ~ Fuel utilization / % Fuel utilization / % DME (O/C = 2) CH 4 (O/C = 3) C 3 H 8 (O/C = 4) Steam reforming at O/C= DME (O/C = 2) CH 4 (O/C = 3) C 3 H 8 (O/C = 4) Current density / A cm Current density / A cm -2 33