FUEL PROCESSING FOR PEM FUEL CELLS

Transcription

1 FUEL PROCESSING FOR PEM FUEL CELLS ENGINEERING HURDLES & SCIENCE OPPORTUNITIES National Science Foundation Workshop Engineering Fundamentals of Low Temperature Fuel Cells Arlington, VA November 14-15, 2001 Richard Bellows HydrogenSource 60 Bidwell Road South Windsor, CT USA

2 Presentation Outline Need for Fuel Processing Conventional Hydrocarbon Fuel Processing Performance Targets Catalyst Issues Less Conventional Alternatives Sulfur Removal WGS Membrane Reactors Balance of Plant (BOP)

3 AVAILABILITY: ESSENTIAL FOR COMMERCIALIZING FUEL CELLS Fuel Processing System (FPS): Convert hydrocarbons into -rich feed for fuel cells, O O Fuel Rich Stream FPS Components: - Reactors/catalysts - Heat exchangers - Feed & control elements - System integration & control algorithms DS ATR WGS PROX

4 FPS Development Targets Specification Residential Commercial Automotive Scale (kwe) Fuel natural gas natural gas gasoline FPS Cost Target ($/kwe) {1500} {1500} {60} FPS Volume (L/kWe) ~20 ~20 ~1 FPS Weight (kg/kwe) ~20 ~20 ~1 Life (hours) K 40-80K 3-6K Start-up/Transient (min.) 120/- 5-10/- 1/.01

5 Conventional Fuel Processing Strategy CO Rectification CO Mitigation O O 2 Fuel O Autothermal C, CO Water Gas Shift C 20% 16% CO 35% 1% CO Preferential Oxidation C C 8 H O O = CO CO + O = CO 2 + CO + 1/2 O 2 = CO 2 Bleed 34% <10 ppm CO Fuel Cell C Electricity O

6 Reactor Volume Distribution 120 WGS PROX 100 Catalyst Volume (%) Automotive Systems Residential

7 Overall System Efficiency for Autothermal Train (Excludes Auxiliary Losses) Ideal: C 8 H Ideal ATR Train O/C = 0.8, H2O/C = 1.2 (100% Theoretical H2 ) PEFC (V cell = 1.2V, Util = 100%) η train 85.9% X η fc 95.9% = η ideal Practical: 82.4% ATR WGS PROX O2 Bleed PEFC (0.3% CH 4 slip) (1% CO slip) (2X O 2 ) (200X O 2 ) η CH4 X η CO X η prox X η bleed 96.8% 97.3% 97.3% 97.7% X (V cell = 0.7V) ( Util = 85%) η voltaic X η H2 util. 58.3% 85% X η ideal 82.4% = η pract. 36.6%

8 Catalyst Development Issues CO Rectification CO Mitigation O O 2 Fuel O Autothermal C, CO 20% 16% CO Water Gas Shift C 35% 1% CO Preferential Oxidation C C 8 H O O = CO CO + O = CO 2 + CO + 1/2 O 2 = CO 2 - Mixing Zone Effects & Thermal Stability Bleed - Avoid carbon by effective reactant mixing (CFD) - Avoid pre-ignition by rapid mixing (free radical) Fuel Cell - Rapid start requires higher air (catalysis) C - High exotherms - thermal stress (simulation) - High temperatures and sudden thermal gradients (support materials) Water Gas Shift (WGS) - Volume at High CO Conversion - Equilibrium limited conversion requires staging - Higher activity at low temperatures: C (catalysis) Preferential Oxidation (PROX) Transient Excursions & Volume - High CO (100s ppm) during up transients (reactor design) - Better understanding needed (CFD, catalysis, simulation) 34% <10 ppm CO Electricity O

9 Less Conventional Fuel Processing Strategies Plasma Reforming WGS Membrane Reactors O Purification Methanation Fuel HO 2 Autothermal, CO Water Gas Shift Preferential Oxidation Adsorption Sulfur Traps Bleed O 2 Enrichment Electricity Fuel Cell HO 2

10 Sulfur Target - Reduced Below 10 ppbv O Fuel HO 2 Autothermal, CO Water Gas Shift Preferential Oxidation Sulfur Traps Pre-Reforming: Bleed Electricity Ni + RS => NiS + R = Fuel Cell HO 2 Post Reforming: S + ZnO => ZnS + O

11 Analysis - Sulfur Removal Options Options Pros Cons At Refinery Known Refinery Technology Low S Refinery Streams Refractory S Compounds Known Economic Tradeoffs Low S may be 1 ppmw Pre-Reforming Known Refinery Technology Elongated Adsorption Fronts Low Space Velocity Limited Capacity Post-Reforming Refractory S Converted to S O Limits S Equilibrium Elongated Adsorption Fronts Low Space Velocity Limited Capacity

12 Selective Membrane Reactors H2O Fuel O Primary, CO, N2 CO + O = CO 2 + ( ), N2 Membrane Options - Pd Alloys - Knudsen Diffusion - Free-Volume Bleed Fuel Cell Electricity O

13 Analysis - Selective Membrane Reactors Options Pros Cons Pd Alloys Simplified CO Cleanup! Loss % Fuel! Eliminated Reactor Volumes Compressor Costs Improved Utilization - 96%! Pd Cost - Thickness 5-20µ! Nernstian Voltage ~30 mv Delamination - T & P Cycling! Purity (CO, NH 3, S, HC)! Slow Transport in Non-Pd Alloys

14 Balance of Plant (BOP) Issues Heat Exchangers Volume and Thermal Mass - Sizing controlled by gas phase film coefficients - Micro-channel approach limited by pressure drop - Manufacturing: Cost vs thermal cycling stresses - Optimal thermal integration Compressors/Expanders - Low Efficiency - Efficiency at turn-down (20-30%) Pumps/Blowers - Manufacturing costs in volume Sensors - Eliminate to reduce costs Controllers - Eliminate sensors with smarter algorithms - Intrinsically simple designs - Reduce CO transients with reactor modeling

15 Summary Scientific Opportunities in Fuel Processing New Materials Higher activity catalysts => Lower cost, lower thermal mass Non-NM catalysts on Ceria with promoters Resistance to thermal stress during start-up Better Components mixing zone effects Sulfur removal processes (<ppb) WGS membrane reactors (non-pd alloys) Heat exchangers thermal stresses vs cost Compressor/expanders - efficiency at turn down vs. cost Better Systems Understanding Optimum thermal integration Optimum control strategies