Cheong Ying Chan Professor of Engineering & Environment Chair - - PowerPoint PPT Presentation

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

Picture sources: Wikipedia & Gov HK

Biogas Heat engine Annual biogas yield from sewage treatment works in HK: 9.67 million m3

Issues: low energy efficiency; harmful gas emissions

Biogas Heat engine Electricity (CH4 )

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: (LHVbiogas≈5, 000 kcal/m3)



Emission



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.

7000 people

twice Every year

Emissions Heat engines SOFCs NOx Yes negligible SOx Yes negligible Heat engines μe=30%-40% Electricity yield: 1.45-1.93×1010 kcal SOFCs μe=55-75% Electricity yield: 3.38-3.62×1010 kcal

Extra electricity

Research Objectives

 To design and construct a lab-scale biogas fuel cell;  To test the fuel cell that uses biogas from sewage treatment works

Waste water

Clean water

Biogas

Zero emission

Electricity Heat

SOFC Plant Sewage Treatment Works

An integrated STW and SOFC system

Lab-scale fuel cell system

CO2 CH4

MFC MFC Evaporator

SOFC

Electrochemical Interface & Impedance Analyzer

Power output

H2O Ventilation Peristaltic pump T Temperature controller Gas flow control system Temperature control system

Anode side: CH4 and CO2 were supplied to the anode, 30 ml min-1, p=1 atm, the composition of CH4 65% and CO2 35 %; Cathode side: exposed to ambient air; H2 will be input for 2 hours to reduce the anode before CH4+CO2 input. H2S: injected into the fuel inlet as pulse input. The average concentration

• f the H2S is about 6 ppm, higher at the beginning and lower with time.

H2

MFC

Cell components

Cathode:

1. Thin layer where oxygen reduction takes place 2. Electron conductive 3. Oxygen-ions conductive 4. Chemical compatibility with electrolytes at high temperature

Anode:

1. Porous structure to allow fuel transport 2. Electron conductive 3. Smaller grains to reduce ionic resistance 4. Thickest part to provide mechanical strength

Electrolyte:

1. Dense thin layer of ceramic that conducts oxygen ions 2. Electron isolative 3. Crucial influence on cell performance

Cell materials

LSM/YSZ YSZ Ni-YSZ

Electrolyte Anode Cathode Components Materials Electrolyte YSZ Cathode LSM Anode Ni-YSZ

Cell configuration

Preliminary results – with CH4+CO2

Voltage dropped ~40% after 10 hours operation

20 40 60 80 100 0.0 0.2 0.4 0.6 0.8 1.0

Ni/YSZ

Cell voltage, V/V Time/h

Voltage drop

Preliminary Results – with H2S injected

379 mW cm-2

500 1000 1500 0.0 0.2 0.4 0.6 0.8 1.0

Cell voltage (V) Current density CH4 + CO2 CH4 + CO2 + H2S

100 200 300 400 500

Power density (mW cm

• 2)

103 mW cm-2

Power density decreased to less than one third with H2S injected

Issue #1: Carbon deposition

Mechanism:

• Nickel catalyzes the cracking of C-C bonds→ free radicals

CH4 → C· + 2H2

• Polymerization of free radicals
• Deposition on the anode → block the pores

Ni-YSZ anode Deposited C filament SEM Images of Ni-YSZ Catalyst after 30h

• f Methane Steam Reforming Reaction

Issue #2: Sulfur poisoning

Possible degradation mechanism of cell performance caused by H2S contamination of biogas.

Int J of Hydrogen Energy 33 2008 6316 Energy & Environ. Sci 4 2011 4380

electrolyte

Mechanism:

• At the anode, H2S can be oxidized as:

H2S + O2

• → S(s) + H2O
• The produced sulfur will block triple-phase boundaries

Our approach to tackle the issues

Cu-modified Ni/YSZ: Inactive to breaking of C-C bonds; high electronic conductivity. MnO-modified Ni/YSZ: High catalytic activity for hydrocarbon oxidation

Components Materials Electrolyte YSZ Cathode LSM Anode Cu/Ni-YSZ MnO/Ni-YSZ

Advanced Anode Materials

Results – with CH4 + CO2

The both modified anodes show higher power density than the commercial anode, because of a catalytic effect and a reduced ohmic resistance

200 400 600 800 1000 1200 1400 1600 1800 2000 0.0 0.2 0.4 0.6 0.8 1.0

Cu-Ni/YSZ MnO-Ni-YSZ in CH4+CO2+H2S Power density, P/mWcm

• 2

Cell Voltage, V/V Current density, i/mAcm

• 2

645 mWcm

• 2

521 mWcm

• 2

100 200 300 400 500 600 700

After injecting H2S, the power density almost remains the same.

Better sulfur tolerance

Results – with CH4 + CO2 + H2S

Results – Long Term Test

500 1000 1500 2000 2500 0.0 0.2 0.4 0.6 0.8 1.0

Cu-Ni/YSZ MnO-Ni/YSZ

Cell voltage, V/V Time/h

Run out of gas change gas cylinder Run out of gas change gas cylinder H2S injection H2S injection H2S injection H2S injection H2S injection

Our modified anodes enable the fuel cell to achieve a long-term stable

• peration of up to 2500 h with no degradation even with H2S 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

• 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.

scaling up

Biogas pre-treatment for large-scale fuel cell

SOFC Bulk Desulfurization

Raw Biogas Bulk H2S H2S Adsorption methods: Activated carbon, molecular sieves, or other catalysts such as Mo or Co supported on alumina Air H2S < 10 ppm Recuperator Hot air (500 ⁰C)

Start-up & thermal management

 Extremely careful attention should be paid to start-up and shut-down

• perations.

 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.