T.S. Zhao Cheong Ying Chan Professor of Engineering & Environment Chair Professor of Mechanical & Aerospace Engineering Director of the HKUST Energy Institute
Outline Background Lab-scale biogas fuel cell system Preliminary Results Issues with the use of biogas in the fuel cell Our approach to tackle the issues Challenges in scale up of the fuel cell system Conclusions
Current utilization of biogas in HK (CH4 ) Heat engine Biogas Annual biogas yield from sewage treatment works in HK: 9.67 million m 3 Heat engine Biogas Electricity Issues: low energy efficiency; harmful gas emissions Picture sources: Wikipedia & Gov HK
Solid Oxide Fuel Cell (SOFC): from chemicals to electricity directly A SOFC is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. High efficiency Fuel flexibility Long-term stability Zero emissions Relatively low cost
Fuel cells vs. Heat engines Energy efficiency: (LHV biogas ≈5, 000 kcal/m 3 ) Every year Heat engines SOFCs μ e =30%-40% μ e =55-75% Extra electricity Electricity yield: Electricity yield: 1.45-1.93 × 10 10 kcal 3.38-3.62 × 10 10 kcal 7000 people twice Emission Emissions Heat engines SOFCs NOx Yes negligible SOx Yes negligible Fuel requirement: conventional heat engines cannot run with methane content less than 45%, but the fuel cell can Noise level: Fuel cell operation is much more quiet Data are based on Chapter 13, High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications 2007; Hong Kong Energy Statistics, 2014 Annual report and J Fuel Cell Sci and Tech 11 2014 031003.
Research Objectives To design and construct a lab-scale biogas fuel cell; To test the fuel cell that uses biogas from sewage treatment works An integrated STW and SOFC system Sewage Electricity SOFC Treatment Waste water Biogas Plant Works Heat Clean water Zero emission
Lab-scale fuel cell system Power output MFC Electrochemical Interface & MFC Impedance Analyzer Temperature control system MFC SOFC Temperature H 2 controller CO 2 CH 4 T H 2 O Evaporator Peristaltic pump Ventilation Gas flow control system Anode side: CH 4 and CO 2 were supplied to the anode, 30 ml min -1 , p =1 atm, the composition of CH 4 65% and CO 2 35 %; Cathode side: exposed to ambient air; H 2 will be input for 2 hours to reduce the anode before CH 4 +CO 2 input. H 2 S: injected into the fuel inlet as pulse input. The average concentration of the H 2 S is about 6 ppm, higher at the beginning and lower with time.
Cell components Anode: Electrolyte : Cathode: 1. Porous structure to 1. Dense thin layer of 1. Thin layer where oxygen allow fuel transport ceramic that reduction takes place 2. Electron conductive conducts oxygen 2. Electron conductive 3. Smaller grains to reduce ions 3. Oxygen-ions conductive ionic resistance 2. Electron isolative 4. Chemical compatibility 4. Thickest part to provide 3. Crucial influence on with electrolytes at high mechanical strength cell performance temperature
Cell materials Components Materials Electrolyte YSZ Cathode LSM Anode Ni-YSZ Cathode LSM/YSZ Electrolyte YSZ Anode Ni-YSZ
Cell configuration
Preliminary results – with CH 4 +CO 2 1.0 Ni/YSZ 0.8 Cell voltage, V /V 0.6 Voltage drop 0.4 0.2 0.0 0 20 40 60 80 100 Time/h Voltage dropped ~40% after 10 hours operation
Preliminary Results – with H 2 S injected 500 CH 4 + CO 2 1.0 CH 4 + CO 2 + H 2 S 379 mW cm -2 400 -2 ) 0.8 Power density (mW cm Cell voltage (V) 300 0.6 200 0.4 103 mW cm -2 100 0.2 0.0 0 0 500 1000 1500 Current density Power density decreased to less than one third with H 2 S injected
Issue #1: Carbon deposition Mechanism: • Nickel catalyzes the cracking of C- C bonds→ free radicals CH 4 → C · + 2H 2 • Polymerization of free radicals • Deposition on the anode → block the pores Deposited C filament Ni-YSZ anode SEM Images of Ni-YSZ Catalyst after 30h of Methane Steam Reforming Reaction
Issue #2: Sulfur poisoning Mechanism: • At the anode, H 2 S can be oxidized as: - → S(s) + H 2 O H 2 S + O 2 • The produced sulfur will block triple-phase boundaries electrolyte Possible degradation mechanism of cell performance caused by H 2 S contamination of biogas. Int J of Hydrogen Energy 33 2008 6316 Energy & Environ. Sci 4 2011 4380
Our approach to tackle the issues Advanced Anode Materials Components Materials Electrolyte YSZ Cathode LSM Anode Cu/Ni-YSZ MnO/Ni-YSZ Cu-modified Ni/YSZ: Inactive to breaking of C-C bonds; high electronic conductivity. MnO-modified Ni/YSZ: High catalytic activity for hydrocarbon oxidation
Results – with CH 4 + CO 2 The both modified anodes show higher power density than the commercial anode, because of a catalytic effect and a reduced ohmic resistance
Results – with CH 4 + CO 2 + H 2 S Better sulfur tolerance 700 -2 645 mW cm 1.0 600 Power density, P /mW cm -2 521 mW cm Cell Voltage, V /V 0.8 500 400 0.6 300 0.4 200 Cu-Ni/YSZ 0.2 MnO-Ni-YSZ 100 in CH 4 +CO 2 +H 2 S -2 0.0 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 -2 Current density, i /mA cm After injecting H 2 S, the power density almost remains the same.
Results – Long Term Test 1.0 H 2 S H 2 S H 2 S H 2 S H 2 S injection injection injection injection injection 0.8 Cell voltage, V /V 0.6 0.4 Run out of gas Run out of gas change gas cylinder change gas cylinder 0.2 Cu-Ni/YSZ MnO-Ni/YSZ 0.0 0 500 1000 1500 2000 2500 Time/h Our modified anodes enable the fuel cell to achieve a long-term stable operation of up to 2500 h with no degradation even with H 2 S injection!
Findings Both the Cu-Ni/YSZ and MnO-Ni/YSZ anode showed good performance and long term stability because they can overcome the carbon deposition and sulfur poisoning problems; The MnO-Ni/YSZ anode has a better performance and is more cost-effective.
Challenges in scale up of the fuel cell system
Transport issues in large-scale fuel cells scaling up Geometric design (e.g., pore size, pore distribution) needs to be optimized. Transport of fuels and heat within the stack becomes a great challenge. Operating parameters (e.g., fuel pressure, fuel input rate) needs further investigations.
Biogas pre-treatment for large-scale fuel cell H 2 S Adsorption methods: Activated carbon, molecular sieves, or other catalysts such as Mo or Co supported on alumina Bulk H 2 S < 10 ppm Raw Biogas SOFC Desulfurization Hot air (500 ⁰C) Bulk H 2 S Recuperator Air
Start-up & thermal management Extremely careful attention should be paid to start-up and shut-down operations. A burner that combusts a small portion of fuel is integrated with fuel cell stack for start up. Self-sustain operation
Conclusions Our study shows that using fuel cells to produce electricity with biogas is much more efficient and much more environmentally friendly than using heat engines. A lab-scale fuel cell has been designed, fabricated and tested for the use of biogas derived from sewage treatment works in Hong Kong. A new and promising (MnO modified Ni/YSZ) anode has been developed to tackle the issues of carbon deposition and sulfur poisoning. The test results suggest that our developed cell can be run with the biogas derived sewage treatment works in Hong Kong. In the next-phase study, we wish to scale up the fuel cell and develop a big fuel cell stack for in-situ demonstration of the technology.
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