Studies on High Efficiency Energy Systems
based on Biomass Gasifiers and
Solid Oxide Fuel Cells with Ni/GDC Anodes
Studies on High Efficiency Energy Systems based on Biomass Gasifiers
and Solid Oxide Fuel Cells with Ni/GDC Anodes
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus,prof.dr.ir.J.T.Fokkema, voorzitter van het College voor Promoties,
in het openbaar te verdedigenop maandag 26 november 2007 om 10.00 uur
door
Aravind Purushothaman VELLAYANI
Master of Science from Faculty of Physics
Department of Energy and Semiconductor Research (Renewable Energy) University of Oldenburg (Germany)
Dit proefschrift is goedgekeurd door de promotoren: Prof. Dr-Ing. H. Spliethoff
Prof. Dr.Dr.h.c. J. Schoonman
Samenstelling promotiecommissie: Vice- Chancellor, Voorzitter
Prof. Dr-Ing. H. Spliethoff, Technische Universiteit Delft, promoter Prof.Dr.Dr.h.c. J. Schoonman, Technische Universiteit Delft, promoter Prof. dr. ir. Ad H.M. Verkooijen, Technische Universiteit Delft
Prof. Dr. H.J. Veringa, Universiteit Twente
Prof. dr. ir. J.J.H. Brouwers, Eindhoven Technische Universiteit Dr. R Rosenberg, Technische Onderzoek Centrum Finland (VTT) Dr. GJM Janssen, Energieonderzoek Centrum Nederland (ECN)
Reservelid:
Prof. Dr-Ing J Gross, Technische Universiteit Delft
Een speciaal woord van dank aan ir.N.Woudstra voor de steun en het advies tijdens dit onderzoek.
ISBN-13: 978-90-9022534-0
Copyright © 2007 by Aravind Purushothaman Vellayani
Summary
Importance of biomass as a sustainable primary energy source is widely acknowledged. Solid oxide fuel cell-gas turbine systems (SOFC-GT systems) operating on fuels such as hydrogen and methane are expected to have electrical efficiencies of the order of 60-80%. Gasification offers a technology for converting solid biomass into a gaseous fuel known as biosyngas. The main components in the biosyngas such as hydrogen, carbon monoxide, and methane are fuels for both SOFCs and gas turbines. However, the contaminants in biosyngas such as particulates, tar, H2S, HCl and others are widely
suggested to be detrimental to the smooth operation of SOFCs and gas turbines. If the gas can be cleaned to have the contaminant levels low enough and compatible for both SOFCs and gas turbines, this introduces an excellent opportunity for generating electricity using SOFC-GT systems at high efficiencies.
Extensive studies are required for developing such systems operating at high efficiencies. As the topics required to be studied spread over a wide range of subjects, this becomes a multi-disciplinary research activity. The results from such studies are expected to lead to the development of clearer concepts for achieving high efficiencies with gasifier-SOFC-GT systems, employing system components, which are expected to be technically and economically viable in the future. This thesis reports the results from an attempt to evaluate the technical feasibility of biomass gasifier-SOFC-GT systems and to generate technical knowledge regarding design and operation of such systems. The work done in this thesis can be broadly divided into three categories, i.e., 1) Introduction to the topic and preliminary technical analysis, 2) Electrochemical studies with synthetic gas mixtures, and 3) Studies on issues related to system integration such as gas cleaning, thermodynamic system modeling and integrated experiments in which SOFCs were connected to a real gasifier.
Preliminary technical analysis
A brief analysis on the relevance of SOFC-GT systems based on the information from the literature is provided in the Chapter 1. It appeared that the gasifier-SOFC-GT systems have significant advantages over competing systems such as the gasifier-gas engine systems especially at low power levels of few hundred kW.
Electrochemical experiments with synthetic gas mixtures.
Electrochemical Impedance Spectroscopy (EIS) was used for studying fuel oxidation on SOFCs with Nickel/Gadolinium doped Ceria anodes (Ni/GDC) and the results are presented in Chapter 3. Symmetrical test cells under single gas atmosphere were employed for the experiments. Theoretical understanding of the impedance spectra recorded on Ni/GDC anodes was developed. A model for DC diffusion resistance (gas phase) was developed and the results from model calculations were validated with experimental results. Impedance due to gas phase processes were separated from impedance due to processes at the surface or in the bulk of the anode. The knowledge thus generated was used for planning the experiments for studying the influence of biomass-derived contaminants on Ni/GDC anodes reported in Chapter 4. Experimentally observed anodic impedance with various biosyngas compositions were comparable with the impedance obtained with humidified hydrogen.
The impedance measurements were also carried out with three contaminants namely H2S,
HCl, and naphthalene at 1123 K and 1023 K and the results are presented in Chapter 4. (Appendix 4a presents results from similar experiments with KCl). Chemical equilibrium computations were carried out to analyze the possible interactions between these contaminants and anode materials. The results obtained from the experiments and chemical equilibrium calculations indicated that, at the levels at which the contaminants were added (H2S and HCl up to 9 ppm and naphthalene at 110 ppm), these contaminants
had no significant impact on the anodic performance.
The electrochemical performance of planar, state-of-the-art SOFC cell with Ni/GDC anode was assessed with synthetic biosyngas compositions and results are presented in Chapter 5. I-V and electrochemical impedance measurements were carried out. At 80% fuel utilization, stable electrochemical performance was obtained, with a power density of 2600 W/m2 at 1123 K, and 3000 W/m2 at 1193 K Sulfur deactivated the Ni/GDC anode for methane reforming significantly, but not for the oxidation of hydrogen and carbon monoxide up to 9 ppm H2S. Similar was the case with tar (Appendix 5a)
components such as naphthalene. They also have (at few tens of ppm levels) a significant influence on the methane reforming capability of the anode but only a minor influence on hydrogen or carbon monoxide oxidation. AC impedance measurements reported in Appendix 5b revealed that the cell resistance (excluding gas phase resistance) amounts 0.6 Ohm-cm2 at 1123 K and 0.3 Ohm-cm2 at 1193 K. At low AC frequencies, an additional polarization arc was seen in the impedance spectra, which has been attributed to gas phase processes. It is also observed that the slope of the I-V curve is partially due to gas phase processes.
System integration issues
based on Ni/GDC anodes at temperatures above 873 K (based on equilibrium calculations). Though information from literature and results from chemical equilibrium studies were sufficient to put forward a conceptual design for a high-temperature gas cleaning system, detailed experiments are required to be carried out to finalize the design for the cleaning series and to evaluate its long-term performance. Preliminary experiments were carried out to study alkali metal removal using alkali getters and results are presented in Appendix 6a. An experimental gas-cleaning unit comprising different reactors for different contaminants was designed and fabricated and the details are presented in Appendix 6 b. (Detailed gas cleaning experiments are currently being carried out)
System calculations were carried out to study the thermodynamic performance of small-scale gasifier-SOFC-GT systems (SOFCs with Ni/GDC anodes) of the order of 100 kW and the results are presented in Chapter 7. The results indicate that high system electrical efficiencies above 50% are achievable with these systems. If the gas cleaning systems employed in such power plants are at lower temperatures when compared to gasification temperature, additional steam will have to be added to have conditions thermodynamically unfavorable for carbon deposition. To analyze the influence of gas cleaning at lower temperatures and steam addition on system efficiency, additional system calculations were carried out. It was observed that steam addition did not have significant impact on system electrical efficiency. However, generation of additional steam using heat from gas turbine outlet had decreased the thermal energy and exergy available at the system outlet thereby decreasing the total system efficiency. With the gas cleaning at atmospheric temperature, there was a decrease in the system efficiency of the order of 3-4% when compared the efficiency of the systems working with medium to high gas cleaning temperatures.
Biosyngas from Delft CFBG was used for feeding electrolyte supported SOFCs with Ni/GDC anodes. Results from the integrated experiments are presented in Chapter 8. Biosyngas was cleaned using a medium temperature gas cleaning system developed by TU Graz before feeding to SOFCs. Experiments with several hours of duration were carried out with clean tar free biosyngas (after tar reforming) as well as with tar rich biosyngas (tar reformer bypassed). The tar load in the gas with no tar reforming was few thousand mg/Nm3. When the cell was operated with tar rich biosyngas, the difference between Nernst voltage and the measured cell voltage increased by few percentages. This was probably due to the lack of the methane conversion on the anode surface under tar load as discussed in Chapter 5 and Appendix 5a of this thesis. No significant degradation in the SOFC performance was observed during this operation period.
Samenvatting
Het belang van biomassa als een duurzame primaire energiebron wordt breed erkend. De verwachting is dat combinatiesystemen van een vastoxidebrandstofcel met een gasturbine (SOFC-GT-systemen) met brandstoffen als waterstof en methaan een elektrisch rendement van 60 tot 80% bereiken. Vergassing biedt een technologie om vaste biomassa om te zetten in een gasvormige brandstof, bekend als biosyngas. De belangrijkste componenten van biosyngas, zoals waterstof, koolmonoxide en methaan zijn brandstoffen die zowel voor SOFC’s als voor gasturbines geschikt zijn. In het algemeen echter worden de verontreinigingen in biosyngas, zoals vaste deeltjes, teer, H2S, HCl en
andere, beschouwd als schadelijk voor de goede werking van SOFC’s en gasturbines. Als het gas zodanig kan worden gereinigd dat het verontreinigingsniveau laag genoeg is voor toepassing in zowel SOFC’s als gasturbines, biedt dit een goede mogelijkheid voor het produceren van elektriciteit met SOFC-GT-systemen met hoge rendementen.
Er zijn uitgebreide studies nodig om zulke systemen met hoge rendementen te ontwerpen. Aangezien de te bestuderen onderwerpen verspreid zijn over een breed veld aan thema’s, wordt dit een multidisciplinaire onderzoeksactiviteit. De resultaten van zulke studies zullen naar verwachting leiden tot de ontwikkeling van duidelijker concepten om hoge rendementen te bereiken met SOFC-GT-systemen, met gebruikmaking van systeemcomponenten die, naar verwachting, technisch en economisch levensvatbaar zullen zijn in de toekomst. Dit proefschrift beschrijft de resultaten van een poging om de technische haalbaarheid van biomassavergassing SOFC-GT-systemen te evalueren en technische kennis te genereren met betrekking tot het ontwerpen en bedrijven van zulke geïntegreerde systemen. Het verrichte werk in dit proefschrift kan globaal worden verdeeld in drie categorieën, te weten 1) Inleiding tot het onderwerp en voorbereidende technische analyse, 2) Elektrochemische studies met kunstmatige gasmengsels, en 3) Studies over onderwerpen met betrekking tot systeemintegratie zoals gasreiniging, thermodynamische systeemmodellering en geïntegreerde experimenten waarin SOFC’s waren gekoppeld aan een concrete vergasser.
Voorbereidende technische analyse
Een korte analyse met betrekking tot het belang van SOFC-GT-systemen, gebaseerd op informatie uit de literatuur is gegeven in hoofdstuk 1. Het bleek dat de vergasser-SOFC-GT-systemen een significant voordeel hebben boven concurrerende systemen, zoals de vergasser-gasmotorsystemen, vooral bij lage vermogensgroottes van enkele honderden kW.
Elektrochemische experimenten met synthetische gasmengsels
"Electrochemical Impedance Spectroscopy" (EIS) is gebruikt voor het bestuderen van brandstofoxidatie in SOFC's met Nickel / Gadolinium doped Ceria (Ni/GDC) en de resultaten zijn gepresenteerd in hoofdstuk 3. Voor de experimenten zijn symmetrische testcellen gebruikt onder een enkele gasatmosfeer . Theoretisch begrip van impedantiespectra, vastgelegd voor Ni/GDC-anodes, is hieruit ontwikkeld. Een model voor DC diffusieweerstand (DC diffusion resistence) (gasfase) is ontwikkeld en de resultaten van modelberekeningen zijn gevalideerd met experimentele resultaten. Impedantie als gevolg van processen in de gasfase is afzonderlijk beschouwd van de impedantie als gevolg van processen aan het oppervlak of in de bulk van de anode. De aldus verkregen kennis is gebruikt voor het plannen van de experimenten voor het bestuderen van de invloed van verontreinigingen vanuit de biomassavergassing, op de Ni/GDC-anodes, zoals weergegeven in hoofdstuk 4. De impedantie die experimenteel is waargenomen aan de anodezijde voor diverse samenstellingen van het biosyngas is vergelijkbaar met de impedantie, verkregen met bevochtigde waterstof.
De impedantiemetingen zijn ook uitgevoerd met drie soorten verontreiniging, namelijk H2S, HCl, en naftaleen bij 1123 K en 1023 K en de resultaten zijn gepresenteerd in
hoofdstuk 4 (appendix 4a vermeldt de resultaten van vergelijkbare experimenten met KCl). Berekeningen van het chemische evenwicht zijn uitgevoerd om de mogelijke interacties tussen deze verontreinigingen en het anodemateriaal te analyseren. De uit de experimenten verkregen resultaten en die uit de chemisch evenwichtberekeningen laten zien dat bij de gebruikte niveaus van de verontreinigingen (H2S en HCl tot 9 ppm en
naftaleen bij 110 ppm), deze geen significante invloed hadden op de prestaties van de anode.
De elektrochemische prestaties van vlakke SOFC’s met Ni/GDC-anode zijn vastgesteld met een synthetische biosyngassamenstelling en de resultaten zijn gepresenteerd in hoofdstuk 5. Er zijn I-V-metingen en elektrochemische impedantiemetingen uitgevoerd. Bij een brandstofutilisatie van 80% is een stabiele elektrochemische prestatie gevonden, met een vermogensdichtheid van 2600 W/m2 bij 1123 K en van 3000 W/m2 bij 1193 K. Zwavel deactiveerde de Ni/GDC-anode aanzienlijk voor methaanreforming, maar, tot 9 ppm H2S, niet voor de oxidatie van waterstof en koolmonoxide. Dit was ook het geval
Aspecten van systeemintegratie
In hoofdstuk 6 is een analyse gepresenteerd van systemen om biosyngas te reinigen bij hoge temperatuur, om een brandstof te bereiden voor SOFC's met Ni/GDC-anodes. Resultaten uit de literatuur en uit de analyse van het chemische evenwicht zijn gebruikt om de mogelijkheden van gasreiniging te evalueren. Op grond van de resultaten van deze evaluatie mag worden verwacht dat het mogelijk is om met hoge temperatuur gasreiniging te voldoen aan de eisen die worden gesteld door vastoxidebrandstofcellen, gebaseerd op anodes van Ni/GDC en bij temperaturen boven 873 K. Hoewel er genoeg informatie is uit de literatuur en resultaten van studies met betrekking tot het chemische evenwicht om een conceptueel ontwerp van een gasreinigingssysteem voor hoge temperatuur voort te zetten, is het nodig gedetailleerde experimenten uit te voeren, om het ontwerp voor het reinigingssysteem te voltooien en om het gedrag op de lange termijn te kunnen evalueren. Om het verwijderen van alkalimetalen met alkaligetters te bepalen zijn voorbereidende experimenten uitgevoerd en de resultaten zijn gepresenteerd in appendix 6a. Een experimentele gasreinigingseenheid met verschillende reactoren om verschillende verontreinigingen te verwijderen was ontworpen en gefabriceerd en de details zijn gepresenteerd in appendix 6b (gedetailleerde gasreinigingsexperimenten worden momenteel uitgevoerd).
Er zijn systeemberekeningen uitgevoerd om de thermodynamische prestaties van kleinschalige vergasser-SOFC-GT-systemen (SOFC's met Ni/GDC anodes) op een schaal van 100 kW te bestuderen en de resultaten zijn vermeld in hoofdstuk 7. De resultaten wijzen op hoge elektrische systeemrendementen van meer dan 50% die haalbaar zijn met deze systemen. Als de in deze elektrische productie-eenheden toegepaste gasreinigingssystemen bij lagere temperatuur werken in vergelijking met de vergassingstemperatuur, is additionele stoombijmenging noodzakelijk, om condities te creëren die thermodynamisch ongunstig zijn voor koolstofdepositie. Om de invloed van gasreiniging bij lagere temperaturen en stoombijmenging op het systeemrendement te bepalen, zijn aanvullende systeemberekeningen uitgevoerd. Hieruit bleek dat stoombijmenging geen invloed van belang had op het elektrische systeemrendement. Wel heeft de productie van de bijgemengde stoom, gebruikmakend van de warmte in de uitlaatgassen van de gasturbine, de beschikbare energie en exergie aan de uitlaat van het systeem verminderd en daarmee het totale rendement (als WKK). Met een gasreiniging op omgevingstemperatuur was de vermindering van het systeemrendement circa 3 tot 4%, vergeleken met het rendement van de systemen met middelgrote tot hogere gasreinigingstemperaturen.
gas zonder teerreforming bedroeg enkele duizenden mg/mn3. In bedrijf met teerhoudend
biosyngas nam het verschil tussen de Nernstspanning en de gemeten celspanning met een paar procent toe. Dit was mogelijk het gevolg van het gebrek aan methaanconversie aan het anodeoppervlak onder invloed van de teer, zoals besproken in hoofdstuk 5 en appendix 5a van dit proefschrift. Gedurende de gehanteerde bedrijfsperiode werd geen opvallende degradatie in de prestaties van de SOFC geconstateerd.
Table of Contents
Chapter 1: Introduction1.1 Changing world energy scenario and requirement of new technologies 1 1.1.1 Renewable energy sources and relevance of biomass 1 1.1.2 Dutch future renewable energy situation 2
1.2 Biomass as an energy source 3
1.2.1 Important benefits of biomass utilization 3
1.2.2 Biomass availability 3
1.2.3 Energy from biomass 4
1.3 Fuel Cells 5
1.3.1 Different types of fuel cells 6
1.3.2 High-temperature fuel cell - gas turbine systems 8
1.3.3 Biomass gasifier-SOFC-GT systems 8
1.3.4 Decentralized energy generation and biomass gasifier - SOFC-GT systems
8
1.4 Motivation and the scope of this work 9
1.5 Conclusion Appendix 1a
10 11
References 12
Chapter 2: High-Efficiency Energy Systems with Biomass Gasifiers and Solid Oxide Fuel Cells- A preliminary technical analysis
2.1 Introduction 15
2.2 Biomass Gasifiers 16
2.2.1 Basic chemistry of biomass gasification 16
2.3 Solid Oxide Fuel Cells 18
2.3.1 Anode, electrolyte and cathode 20
2.3.2 Stack design 22
2.3.3 Fuels for Solid Oxide Fuel Cells (SOFCs) 22
2.4 SOFC- Biomass Gasifier Combination 23
2.4.1 SOFC operation with biosyngas and impact of gas composition 23 2.4.2 Impact of biomass derived contaminants and gas cleaning options
on SOFC Operation
25 2.4.3 Operational experience with SOFCs connected to biomass
gasifiers
32
2.5 SOFC-Gas Turbine systems 33
2.6 Biomass gasifier - Solid Oxide Fuel Cell- Gas Turbine Systems 35 2.7 Summary of the discussions and research plan 36
Chapter 3: Impact of biosyngas and its components on Ni/GDC anode for SOFCs
3.1 Introduction 45
3.2 SOFC anode Impedance 45
3.3 Diffusion limitation at Ni/GDC anodes 46
3.4 Experimental aspects 47
3.5 Analysis of the results 49
3.6 Dilution with CO and CO2 62
3.7 Experiments with Simulated Biosyngas 67
3.8 Carbon Deposition 70
3.9 Discussions and Conclusions 71
Appendix 3a
Appendix 3b
72 73
References 75
Chapter 4: Impact of biomass derived contaminants on Ni/GDC Anodes
4.1 Introduction 77
4.2 Experiment 80
4.2.1 Test Setup 80
4.2.2 Experimental Procedure 81
4.3 Results and discussion 82
4.3.1 Experiments with H2S 85
4.3.2 HCl Tolerance Tests 89
4.3.3 Naphthalene Tolerance Tests 92
4.4 Conclusions 94
Appendix 4a 95
Appendix 4b 99
References 102
Chapter 5: Biosyngas Utilization in Solid Oxide Fuel Cells with Ni/GDC Anodes
5.1 Introduction 103
5.2 Experiments 103
5.3 Results 104
5.3.1 Endurance behavior with clean, synthetic wood gas 108
5.4 Conclusions 115
Appendix 5a 116
Appendix 5b 122
Chapter 6: Evaluation of high temperature gas cleaning options for power plants based on biomass gasifiers and solid oxide fuel cells with Ni/GDC anodes
6.1 Introduction 127
6.2 Biomass derived contaminants and their cleaning. 128
6.2.1 Tars 128
6.2.2 Particulates 130
6.2.3 Hydrogen Chloride 132
6.2.4 Alkali Compounds 135
6.2.5 Sulfur 140
6.2.6 Nitrogen Containing Contaminants 144
6.3 Summary of the results and a configuration for the cleaning system 144
6.4 Conclusions 146
Appendix 6a 147
Appendix 6b 151
Reference 155
Chapter 7: Thermodynamic evaluation of small scale systems with biomass gasifiers, solid oxide fuel cells with Ni/GDC anodes and gas turbines
7.1 Introduction 161
7.2 Considerations regarding selected technologies 162
7.3 Component models and system efficiencies 169
7.4 Process calculations with the base case system 172
7.5 Results and discussion 175
7.6 Gas cleaning at low temperatures and impact of steam addition 178
7.7 Gas cleaning at atmospheric temperature 182
7.8 Conclusions 186 Appendix 7a 188 Appendix 7b 189 Appendix 7c 191 Appendix 7d 193 Reference 194
Chapter 8: Performance of high temperature Ni-GDC/YSZ/LSM SOFC connected to Delft circulating fluidized bed gasifier
8.1 Introduction 197
8.2 Experimental set up 197
8.2.1 Gasifier and gas cleaning 197
Chapter 1: Introduction
1.1 Changing world energy scenario and requirement of new
technologies
World energy demand is rising and is projected to increase by almost two thirds in 2030 from today, reaching around 16 billion tonnes of oil equivalent [1]. Most of the energy requirement to date is met with fossil fuels like oil, natural gas, and coal. Natural gas is relatively cleaner than coal and oil, and is therefore starting to replace oil and coal consumption in a significant way. With the so called “oil peak” anticipated to be surpassed in the near future, the era of cheap oil will most probably come to an end in the decades to come. With global warming and its consequences starting to affect human life, along with the prevailing thought that consumption of fossil fuels is most probably playing a significant role in causing global warming, it is expected that attempts to meet the energy requirements with decreasing fossil fuel consumption will attract substantial attention in the years to come. This will need significant developments in new technologies or changes into less energy intensive lifestyles, which is a matter of social acceptance and requires broad attention.
The technologies required for reducing the dependency on fossil fuels, are categorized into two different groups in general, i.e. 1) Technologies for improving the efficiency of present day energy consumption and for reducing the emissions from fossil fuel utilization, and 2) Technologies for harnessing renewable energy sources.
Technologies of the first kind, i.e., clean fossil fuel applications, include advanced power plant technologies, technologies for improving efficiencies of various processes and for reducing the emission of pollutants such as CO2, particulates, NOx, sulfur
oxides and others. This can help in reducing local and regional environmental problems but the global problem will require sequestration of CO2, which is fairly complicated,
particularly in the transportation sector. The carbon capture processes available are costly and energy intensive. The sequestrated CO2 must be stored and leakage needs to
be excluded. It is also argued that sequestration is rather an action of mitigation, since the consumption of fossil fuels will continue and the areas available for storage are limited [2].
Technologies of the second kind will include all renewable energy technologies like solar energy, wind energy, hydropower, biomass etc. Nuclear technologies are widely debated among others but appear to be required, at least for a few decades and require safety concerns to be addressed sufficiently.
1.1.1 Renewable energy sources and relevance of biomass
important renewable energy technologies and comparative importance of biomass is presented here.
Solar energy: When sufficiently developed and economically viable, solar energy systems can provide an important share of the global energy requirement. This is anticipated to happen sometime in this century. However, utilization of solar energy requires energy storage as sunlight is not available during the night. The total installed capacity of solar photovoltaic power plants is increasing worldwide. Solar thermal systems also are steadily becoming popular both for thermal applications and for electricity generation. While solar thermal systems are already economically viable in many cases for thermal applications, like water heating, electricity generation from solar thermal power plants faces the same constraints of higher costs as solar photovoltaic systems as far as economic viability is concerned.
Wind energy: Wind energy has been used for more than 3500 years, for boats transporting goods in Mesopotamia. The Netherlands has a history of centuries for using windmills for, among others, water pumping. Advanced wind turbine installations for electricity production are increasing worldwide and a few GW installed capacity is being added every year. However, variations in wind energy availability again cause additional requirements of back-up and storage capacity.
It is widely accepted that hydropower is also a sustainable form of energy source and power plants of few GWs installed capacity are already installed worldwide. Hydropower capacity is not used optimally in many places and a significant improvement in hydropower capacity addition is expected in the future. The main drawbacks associated with hydropower are the environmental concerns associated with large construction projects in river basins rich in biodiversity and human settlements. Biomass energy sources include wood, agricultural residues, animal wastes, and others derived from biological sources. They currently account for about 14% of the world energy consumption [3]. Biomass is the main source of energy for many developing countries, where it is used with rather low efficiency mainly for domestic cooking applications. In many countries, this biomass is obtained from agricultural residues or from local plantations in a rather sustainable way. Improved cooking devices like high-efficiency stoves or gas-mode cooking, can help to bring down the biomass consumption for cooking and can help to partly meet biomass requirements for commercial energy production [4]. In well developed countries like The Netherlands, biomass is mainly available for commercial energy generation either as agricultural waste or as municipal solid waste [5]. In many countries, there exists a potential for biomass plantations too for sourcing biomass for energy applications.
Other various renewable energy technologies like geothermal energy, tidal energy, and wave energy are to date also attracting increased attention as fossil fuel prices are increasing and these technologies are getting increasingly matured. It is widely believed that the future energy systems will comprise of a mix of these technologies.
1.1.2 Dutch future renewable energy situation
renewable energy in 2020 and of this, biomass is expected to contribute for about 40%. The expected contribution of biomass thus amounts to 120 PJ. This would encourage the pursuit for developing advanced bio energy systems.
1.2 Biomass as an energy source
Biomass begins as energy from the sun, which is stored in plants through photosynthesis. And there exist different technologies for biomass conversion, which have their own advantages and disadvantages. The following section gives a brief overview of biomass utilization for energy generation.
1.2.1 Important benefits of biomass utilization
Environmental Benefits:
Energy from biomass is essentially CO2 neutral since the carbon in the biomass is
sourced from atmospheric CO2. It can help to mitigate climate change, soil erosion,
provide wildlife habitat, and help maintain forest health. However, sourcing biomass for energy generation will have to be done without disturbing the land requirement for other purposes, especially the land used for food generation.
Economic Benefits:
Since biomass is bulky and costly to transport, at least a part of biomass conversion facilities will have to be situated close to where it is generated. The number of jobs created (for production, harvesting, and use) and the industrial growth (from developing conversion facilities for fuel, industrial feed stocks, and power) would be significant. Rural economic development is one of the major benefits of biomass generation and utilization.
1.2.2. Biomass availability
In the Dutch scenario, it has been pointed out that approximately 90 PJ is the gross energetic potential from organic waste, which is already available [5]. This accounts for approximately 2.25 TJ/ square kilometer. Assuming that this is sourced from dry biomass, this could correspond to approximately 1.5-ton biomass availability per hector. If a high efficiency power plant of 50% energy efficiency is consuming this fuel, 1 kg of biomass can generate 2 kWh electricity, and a 100 kW power plant working 7500 hours an year will need approximately 375 tons of biomass per year and this can be sourced from an area of approximately 250 hectors. (It will be shown later in the thesis that such a potential exist, at least in theory, with gasifier-SOFC-GT systems having target efficiency around 50%). Probably in rural areas, a local farmer can operate such a plant for additional income generation. If one is selective for the biomass employed, the land required may be a few times higher. With the cost of transportation being a significant fraction of the biomass cost, (except for transport by ships which is comparatively cheaper) [9] such plants, possibly connected to grid, requiring minimum biomass transportation, may find certain advantages when compared to large scale power plants, which needs substantial biomass transportation.
If conditions in developing countries are considered, such as in India or in many of African countries, biomass is mainly used for cooking applications. Power plants based on biomass shall be considered either when additional land is available for bio-energy plantations or in parallel with introduction of technologies for cooking efficiency improvement. Improved wood stoves or biomass gasification systems with cooking gas supply and higher efficiency gas mode cooking are examples of technology options for cooking efficiency improvement [4]. In such circumstances, these plants may either be grid connected, or could be stand alone ones providing electric energy for local requirements. Intention of this brief discussion is not to argue for advantages of small-scale systems over large-small-scale systems. However, it is intended here to point out the fact that, since biomass is available in a decentralized manner, small-scale biomass based power plants may have techno-economical viability if other conditions are satisfactory, and development of such systems needs the attention of scientific community.
1.2.3 Energy from biomass
Commercial energy production from biomass is carried out using a variety of methods.
Ethanol, biodiesel, and biogas
Ethanol is produced by converting the starch content of biomass feed stocks into alcohol by fermentation. Ethanol can be used as a fuel for transportation and is often blended with other fuels. Using ethanol even in low-level blends can have environmental benefits. Biodiesel is often sourced from bio-oils after transesterification and is mainly used as a transportation fuel. Biogas, which is a mixture of methane and CO2, can be
formed from biomass by bio-methanation. This gas is combustible and could be used in combustion devices like boilers, engines or fuel cells after cleaning. Bioethanol and biodiesel are often conceived as transportation fuels where as biogas is considered for stationary power generation.
Combustion and gasification
also produce heat, which is then captured for different applications. Co-firing with coal in existing boilers is another possibility. Gasification of biomass and utilization of the biosyngas generated in other energy conversion devices such as engines or turbines is another method to generate electricity from biomass. Gasification is basically partial combustion. Instead of simply burning the fuel, gasification captures about 65-80% of the energy in solid fuel by converting it into combustible gases. Biosyngas generated in a gasifier is a mixture of CO, H2, CH4, CO2, H2O, and N2. Various contaminants such as
tar, particulates, H2S, alkali compounds etc. are also present in the biosyngas. This gas
is first cleaned and is then burned to generate electricity or converted to synthetic fuels. The extent of gas cleaning required depends highly on the biomass used, type of gasifier employed and the downstream device to which the gas is fed. Gas cleaning is also done at different temperatures. Low temperature gas cleaning systems are considered to be more matured when compared with high temperature gas cleaning systems. High temperature gas cleaning systems are expected to increase the conversion efficiencies in different types of power plants and their development is a widely pursued research topic.
With biomass gasification and biogas generation offering two different ways for electricity generation and each of these technologies having their own advantages and disadvantages, in general biomass gasification is expected to have advantages of smaller reactor volumes due to fast reactions at high temperatures when dry biomass is considered as the fuel source.
Classification of gasifiers
Gasification methods can be classified into different groups considering the technology used for gasification. Choice of gasifying agent offers one way of classification. Air is the commonly used gasifying agent where as oxygen and steam are the other two.
Oxygen offers the advantage of better quality of gas and high calorific value but is expensive and the system becomes more complex. Steam also gives higher calorific value for the gas when compared to air but again makes the system more complicated. At atmospheric pressure, gasifiers offer more reliability and reasonably good quality for the gas produced. Gasification at higher pressures allows smaller reactor volume for the same power level. This also helps to reduce the size of the gas cleaning systems.
Another way of classification is into fixed bed gasifiers, fluidized bed gasifiers, and entrained flow gasifiers. In general, as of now, the fixed bed reactors offer the simplest designs and are suitable for small power level systems of few hundred kW [10]. They offer reasonably cleaner gas as the gas passes through a high temperature bed. Fluidized bed gasifiers offer better scale up potential and are suitable for higher power levels.
1.3 Fuel Cells
of ions from the anode/cathode through the electrolyte to the cathode/anode. The electrons that are created at the electrode/electrolyte interphase cause an external current. A schematic figure of a tubular fuel cell, and a tubular solid oxide fuel cell stack from Siemens-Westinghouse are given in Figure 1.1[11].
Air Flow Fuel Electrode Air Electrode Electrolyte Interconnection Fuel Flow
Figure 1.1. Left: single cell. Right: Siemens-Westinghouse tubular fuel cell system (Reproduced from reference 11 with permission; Copyright, American Society of Mechanical Engineers)
1.3.1 Different types of fuel cells
In general, fuel cells can be classified into two different types, i.e., high-temperature fuel cells and low-temperature fuel cells. Proton-exchange membrane fuel cells, phosphoric acid fuel cells, alkaline fuel cells, and direct methanol fuel cells are classified as low temperature fuel cells, while molten-carbonate fuel cells and solid oxide fuel cells are classified as high temperature fuel cells. Usually high-temperature fuel cells are often preferred for stationary applications and low-temperature fuel cells are preferred for mobile applications. The different types of fuel cells are briefly described here [12]. A brief discussion of the suitable type of fuel cell for gasifier-fuel cell power plants will be presented.
Proton Exchange Membrane Fuel Cell (PEMFC/SPFC)
PEMFCs operate at relatively low temperatures (about 353-373 K). They have high power density and can vary their output quickly to meet variations in power demand and are suited for applications such as in automobiles. The polymer membrane acts as the proton-conducting electrolyte. These membranes have on both sides porous electrodes with highly dispersed catalyst particles (mostly platinum). Hydrogen is fed to the anode side of the fuel cell where the hydrogen atoms release electrons and become protons. The electrons travel through the external circuit from anode to cathode generating electric current. At the same time, the protons diffuse through the membrane to the cathode, where they recombine and react with oxygen to produce water. They need pure hydrogen as fuel and contaminants such as CO in the fuel will easily affect the cell performance. PEMFCs are reported to have electrical efficiencies up to 50% when running with hydrogen as the fuel.
Direct Methanol Fuel Cells (DMFC)
itself accelerates the reaction to obtain hydrogen from the liquid fuel, i.e., methanol, eliminating the need for a fuel reformer. Electrical efficiencies of about 40% are expected with this type of fuel cell, which would typically operate at temperatures between 293-363 K. Higher efficiencies are usually achieved at higher temperatures. But using nafion does not allow temperatures above 353-363 K. In addition, nafion shows a large cross over of methanol, which is a disadvantage. So new polymer electrolytes are necessary.
Alkaline Fuel Cells (AFC)
Alkaline fuel cells have been developed for and widely used by NASA on space missions. They use aqueous potassium hydroxide as the electrolyte. They too are said to be costly for commercial applications, but several companies are examining ways to reduce costs and improve operating flexibility. Electrical efficiency of this type of fuel cells is around 50-60% percent with hydrogen and their operating temperature is up to 473 K.
Phosphoric Acid Fuel Cells (PAFC)
The phosphoric acid fuel cell (PAFC) uses phosphoric acid as the electrolyte. With hydrogen as fuel, the fuel cell electrical efficiency is around 40 percent and the operating temperature is about 473 K.
Molten-carbonate Fuel Cells (MCFC)
Molten-carbonate fuel cells have a molten mixture of alkali metal carbonates as electrolyte carrying oxygen from cathode to anode in the form of carbonate ions and fuel oxidation takes place at the anode. They have high fuel-to-electricity efficiencies and operate at about 873-973 K. Molten-carbonate fuel cells have been operated with hydrogen, carbon monoxide, natural gas, propane, landfill gas, marine diesel, and simulated coal gasification products. 10 kW to 2 MW Molten-carbonate fuel cells have been tested on a variety of fuels. Carbonate fuel cells for stationary applications have been successfully demonstrated in various places. MCFCs are reported to have electrical efficiencies up to 60% when running with hydrogen. A major problem with MCFC is the corrosion due to molten carbonate electrolyte.
Solid-Oxide Fuel Cells (SOFC)
Solid-oxide fuel cells (SOFCs) are also fuel flexible like MCFCs. They have an oxide ion conducting solid electrolyte and the fuel oxidation takes place at the anode side. They can be used in industrial and large-scale central electricity generating stations. Auxiliary power units (APUs) with SOFCs are another expected application. A solid-oxide system usually uses a gas-tight ceramic material, at high operating temperatures of 873-1273 K. A detailed description of SOFC operation is provided in a later part of this thesis and hence is not included here. SOFCs are reported to have electrical efficiencies up to 60% with hydrogen as fuel.
Choice of fuel cells for combining with biosyngas
temperature fuel cell systems are suitable but owing to the arguments before, SOFCs appear to have certain advantages.
1.3.2 High-temperature fuel cell - gas turbine systems
High temperature fuel cells, especially, SOFCs, are operated at high temperatures (873-1273 K) and the output gas stream has a temperature, which is higher than the inlet fuel gas temperature. As not all the fuel is utilized in this fuel cell, and the unutilized fuel can be burned in a combustion chamber. If the fuel cell is operated under high pressure, it is possible to supply the air required for the cathode by the air compressor of a gas turbine system. Then the output gas is combusted and expanded in the turbine so that mechanical energy will be generated. Subsequently, this is then converted into electrical energy using an electrical generator. It has been reported that system efficiencies about 65% are achievable with such systems even at a few hundred kW power level [13, 14]. Such systems have the advantages that they can operate with carbonaceous fuels too and also offer a possibility of combining SOFC-GT systems with biomass gasifiers, thereby promising the potential for a sustainable high-efficiency energy system.
1.3.3 Biomass gasifier-SOFC-GT systems
Composition of biosyngas from gasifiers will depend upon the gasification medium and the gasification technology involved as mentioned before. Air gasification gives a low calorific value gas with an HHV around 4-6 MJ/M3. This gas leaves the reactor at a high temperature, usually around 973-1073 K. At present, engines or turbines are used for electricity generation from biosyngas generated in gasifiers[15]. Solid-oxide fuel cells could utilize biosyngas as the fuel if the gas is sufficiently clean. Various gas-cleaning methods are being developed, which could probably provide sufficiently clean gas as required for fuel cells. Gasifier-fuel cell systems also shall be conceived employing gasifiers using other gasification mediums such steam and oxygen. Being convinced of the potential such systems offer, a few groups around the world have already started serious research on these kind of systems [16, 17].
1.3.4 Decentralized energy generation and biomass gasifier-SOFC-GT systems Biomass, being an energy source available in a decentralized manner, offers the possibility of conceiving decentralized power stations based on it. This will be especially true if modern electricity transmission and grid control systems make transport of electricity easier than transport of biomass. They also offer the possibility of applying combined heat and power generation, which means that the waste heat from electricity production is utilized for other applications. It is also argued that use of decentralized power generation may provide more reliability for electricity consumers. Developing countries have even more compelling reasons to apply decentralized power generation. In remote areas, the construction of an electric grid may require huge capital investment where as decentralised power generation could offer cheaper solutions. It is also argued that using decentralized power generation will stimulate the power generation market for more rapid introduction and commercialisation of new energy conversion technologies, which will be cleaner and more efficient.
simple systems, air gasification offers a fine choice. Target efficiencies for such systems with gasifiers, solid-oxide fuel cells, and gas turbines is above 50%, even in small scales, at which it is impossible to achieve with competing present day technologies. Though gasifier-SOFC-GT systems at high power levels are also attractive, the increase in system efficiencies that can be achieved at small scales with such systems (around one hundred kW) are significantly higher than the ones that can be achieved at high power levels of few megawatts or more. For comparison, it shall be pointed out that small-scale gasifier- gas engine systems have electrical efficiencies around 15-20% while large scale IGCC plants are expected to have electrical efficiencies around 30 -45%[10, 15, 18, 19].
1.4 Motivation and the scope of this work
The analysis presented here indicates the importance of systems with gasifiers and solid-oxide fuel cells. However, reported efforts for the development of such systems in literature are scarce. Extensive research is required to investigate means of developing such systems operating at very high efficiency. From the viewpoint of energy-system research, which is essentially a multi-disciplinary activity, this requires studying the topic from different viewpoints. The results from such studies will lead to the development of clearer concepts for achieving high efficiencies with gasifier-solid oxide fuel cell systems, employing system components, which are expected to be technically and economically viable in the future. One important step in this direction is to look for technical viability. Detailed investigation on the economic viability is outside the scope of the present work. However, there are indications available regarding the future costs of the system components such as solid-oxide fuel cells. For SOFCs, research goals have been often set to have them available at few hundred $/kW[20] in the coming decade. This is also expected to be true with other system components such as gasifiers and gas cleaning devices[13]. However, for employing such systems in future, if and when they are viable, both technically and economically, sufficient technical information regarding a range of issues such as selection of proper system components, their optimal operation points and the system integration etc. will have to be available at hand. The work presented in this thesis attempts to evaluate the technical feasibility of such systems and to generate technical knowledge regarding design and operation of such systems. To evaluate the technical feasibility of biomass gasifier-solid-oxide fuel cells systems, preliminary studies have to be carried out in such a way that the following questions can be answered.
What are the impacts of various biosyngas components including contaminants on SOFC performance?
What are the cleaning requirements for contaminants present in biosyngas? What types of cleaning devices shall be used for cleaning the gas?
What type of gasifier will be the most suitable one for using with SOFCs?
What type of SOFC, in terms of materials used, cell designs etc. are most suitable for operating with biomass gasifiers?
What are the additional system components (reformers, compressors etc.) required for achieving high efficiency in such energy systems?
What are the system integration issues to be addressed for assuring high efficiency system performance?
In the work presented in this thesis, it is attempted to provide an answer to some of these questions. While many of the areas mentioned are beyond the scope of a PhD work, care is taken that proposed research covers many of the above-mentioned areas. To select the specific areas of research, a literature survey is conducted as presented in Chapter 2. Specific research questions are then identified, based on their relevance and availability of resources to carry out the research.
1.5 Conclusion
Appendix 1a
Preliminary Economic analysis
To investigate the economic feasibility of gasifier-SOFC-GT systems, a preliminary indicative economic analysis is carried out. The fuel requirement is obtained based on a system model including a solid oxide fuel cell and a gas turbine running on biosyngas as fuel. Details of the system are published elsewhere[21]. The following assumptions are taken for the system. For the economic analysis conditions prevailing in India are taken. The total power level of the system is 103 kW. For the plant, a life span of 20 years is assumed. The following assumptions are taken for the costs. The fuel cell costs $1,500/kW. For the gas-cleaning $10,000 is taken based on the following argument. For a comparable IGCC system the cost of gas cleaning is given as 10- 15 % of the system. Here it is taken as about 5% of the system cost because of the following reasons, i.e., 1) The sulfur content in biomass is much smaller than in coal and 2) The total cost per kW for the proposed system is much higher than the average cost for a comparable IGCC system. System component replacement costs are included in the Operation and Maintenance costs. O and M costs are assumed to be 10% of the total capital cost. An interest rate of 8% per annum is assumed. Levelised cost per kWh of $0.084 is obtained for the electricity produced. This price is comparable to the price of electricity generated with other techniques and will come down if the cost of fuel cells comes down. Economic calculation details for the system are presented in Table 1a.1along with the assumed costs.
Table 1a.1. Economic Calculations (in US$)
Cost of fuel cell 1,14,000
Cost of micro turbine 30,000
Cost of gasifier 11,000
Gas cleaning equipments 10,000
Other expenses 10,000
Design and installation 20,000 Total Installation Cost 1,95,000 Fuel cost @0.04 $/ kg/year 10,320 O & M Cost @ 10% of IC/year 19,500 Total running cost/year 29,820
Net Present Value 5,11,199
Total Load in kWh/year 5,70,000 Net Present Load Value 60,44,052
References
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3. Bilgen, S., K. Kaygusuz, and A. Sari, Renewable Energy for a Clean and Sustainable Future. Energy Sources, 2004. 26: p. 1119–1129.
4. Aravind, P., J. Andries, and H. Spliethoff. Sustainable Energy Systems for Tropical Villages Using a Combination of Gasifier, Fuel Cells,Micro Turbines and Cooking Gas Supply. in Dubrovnik conference on sustainable development of energy, water and environment systems. 2002 Dubrovnik, Croatia.
5. Faaij, A., J. van Doorn, T. Curvers, L. Waldheim, E. Olsson, A. van Wijk, and C. Daey-Ouwens, Characteristics and availability of biomass waste and
residues in the netherlands for gasification Biomass and Bioenergy 1997. 12: p. 225-240.
6. Adoune, B., Low NOx emissions from fuel bond nitrogen in gas turbine combustors, in Section Energy Technology, Process and Energy Department. 2006, TU Delft.
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8. Berndes, G., M. Hoogwijk, and R. van den Broek, The contribution of biomass in the future global energy supply: a review of 17 studies. Biomass and
Bioenergy, 2003 25: p. 1 – 28.
9. Hamelinck, C., R. Suurs, and A. Faaij, International bioenergy transport costs and energy balance. Biomass and Bioenergy 2005. 29: p. 114–134.
10. Bridgwater, A.V., The Technical and Economic Feasibility of Biomass Gasification for Power Generation. Fuel-, 1995. 74: p. 631-653.
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12. http://fuelcellsworks.com.
13. Aravind, P.V., MSc Thesis. 2001, University of Oldenburg.
15. Sridhar, G., P.J. Paul, and H.S. Mukunda, Biomass derived producer gas as a reciprocating engine fuel—an experimental analysis Biomass and Bioenergy, 2001. 21(1): p. 61-72.
16. Aravind, P.V., J.P. Ouweltjes, E. de Heer, N. Woudstra, and G. Rietveld, Impact of biosyngas and its components on sofc anodes. Electrochemical Society Proceedings 2005. Vol. 7: p. 1459–1467.
17. Baron, S., N. Brandon, A. Atkinson, B. Steele, and R. Rudkin, The impact of wood-derived gasification gases on Ni-CGO anodes in intermediate temperature solid oxide fuel cells. Journal of Power Sources, 2004. 126: p. 58-66.
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21. Aravind, P.V., J. Andries, and N. Woudstra. Solid oxide fuel cell-gas turbine systems combined with thermo-chemical gasification of biomass in Transition Towards Sustainable Development in South Asia. 2003.
Chapter 2: High-Efficiency Energy Systems with Biomass
Gasifiers and Solid Oxide Fuel Cells – A
Preliminary Technical Analysis
2.1. Introduction
As described in Chapter 1 of this thesis, fuel cells are energy conversion devices with a high conversion efficiency. A high temperature fuel cell is the solid oxide fuel cell (SOFC), which besides having a high efficiency for the conversion of the chemical energy of the fuel to electrical energy, also provides thermal energy at high temperatures. This heat can be converted into electricity using conventional technologies, like gas turbines or steam turbines resulting in systems with even better efficiencies.
A combination of gasifiers, high-temperature fuel cells, and gas turbines is anticipated to lead to highly efficient and ultra-clean energy systems for the near future. Moreover, biomass offers the advantage of being available for decentralized energy generation, which is expected to attract world-wide attention in the near future. Realization of such systems helps to decrease the dependency on fossil fuels. As a consequence, a reduction of the emission of the green house gases is achieved, while politically the dependence on fossil fuel will also be smaller.
In the Solid Oxide Fuel Cells, the fuel is oxidized at the anode of the fuel cell. Chemical, electrochemical, and physical interactions between the anode components and the gas components determine the limitations of the SOFC to be operated with biosyngas from biomass gasifier as fuel. There are various choices for anode materials for SOFCs. Nickel/Yttria Stabilized Zirconia (Ni/YSZ), Nickel/Gadolinium Doped Ceria (Ni/GDC) are some of the examples for these materials. Interactions between the biosyngas components and these different anode materials are yet to be reported in literature in detail. Such studies will help to understand the comparative advantages and disadvantages of different anode materials for SOFCs for operation with biosyngas. These studies will also help to define most suitable operation parameters for SOFCs when used together with biomass gasifiers. Determination of these choices for SOFC materials and their operation conditions will help to determine the choices for other system components such as gasifiers, cleaning devices, gas turbines and their suitable operation parameters for arriving at high-efficiency system designs.
This chapter presents an overview of the technical issues related to the development of combined systems with biomass gasifiers and Solid Oxide Fuel Cells. Research requirements for further advancement with the development of such systems are pointed out .That forms the basis for the research plan set for the work presented in this thesis. Further on, the scope of this thesis is also presented.
This chapter presents brief reviews of 1. Gasification systems.
3. Available information on SOFC performance with biosyngas at the start of this work.
4. Available information on high-temperature gas cleaning systems for biosyngas at the start of this work.
5. Available information on the thermodynamic evaluation of SOFC-Gasifier systems.
6. Outline of the work presented in this PhD thesis.
Since a wide range of topics are covered in this chapter, to limit the scope of the discussion, only relevant and important results from the literature are briefly reviewed. Detailed discussions on each of these aspects are presented in the respective chapters of the present study.
2.2. Biomass Gasifiers
Gasification is the conversion of biomass into a gaseous fuel by heating biomass in a gasification medium such as air, oxygen, or steam. Unlike combustion, where oxidation is substantially complete, gasification produces biosyngas, a combustible gas carrying chemical energy from the biomass. Biosyngas from gasification consists of a mixture of carbon monoxide, carbon dioxide, methane, hydrogen, water vapor, higher hydrocarbons, and impurities. Exact gas compositions depend on various conditions. They include the type of biomass and gasification agent used, type of gasification method employed, and overall efficiency of conversion of biomass into chemical energy as carried in the gas is estimated to be around 80% (cold-gas efficiency). The use of biosyngas in energy conversion devices, like gas engines and gas turbines, or as a chemical feedstock to produce liquid fuels has been widely studied. Recently, broad interest is also being focused on the use of this gas as a fuel for high temperature fuel cells.
2.2.1 Basic chemistry of biomass gasification
The following are the important chemical reactions taking place in a gasifier [1]. Partial Oxidation C+½ O2↔CO ∆H= -268 MJ/kg mole
Complete Oxidation C+O2↔CO2 ∆H= -406 MJ/kg mole
Water-gas reaction C+H2O↔CO+H2 ∆H= +118 MJ/kg mole
Oxidation of carbon to carbon dioxide releases the maximum thermal energy, whereas the water-gas reaction is endothermic. Carbon monoxide, hydrogen, and steam can undergo further reactions during gasification as follows:
Water-gas shift reaction CO+H2O↔CO2+H2 ∆H = -42 MJ/kg mole
Methane formation CO+3H2↔CH4+H2O ∆H = -88 MJ/kg mole
Boudard reaction C+CO2↔2CO ∆H = +172 MJ/kg mole
with solid oxide fuel cells will have a significant impact on the overall system performance.
The choice of the gasifying agent offers one way of classifying biomass gasifiers. Air is the commonly used gasifying agent, while oxygen and steam are other options. Pure oxygen offers the advantage of a better quality of gas and high calorific value, but is expensive and the system becomes more complex (As a separate oxygen supply has to be established). Steam also gives a higher calorific value for the gas, if compared to air, but again makes the system more complicated. Gasifiers with air as gasifying agent generate biosyngas containing significant amounts of nitrogen, whereas biosyngas from oxygen or steam gasifiers contain minor amounts of nitrogen. Unlike the reaction with air/oxygen, the reaction of carbon with steam, i.e., the water gas reaction, is endothermic, requiring heat transferred at temperatures around 973 K, which is difficult to achieve. Table 2.1 gives a brief comparison between biosyngas generated with different gasifying agents [2-4].
Table 2.1. Comparison between biosyngas generated with different gasifying agents Gasifying
agent
Representative gas composition (Vol%)
HHV (Approx) MJ/Nm3 Air 20% HH 2, 20% CO, 12% CO2, 2% CH4, 2.5 %
2O, and the rest N2
4–6 Oxygen 32% HN 2, 48% CO, 15% CO2, 2% CH4, and 3%
2
10–15 Steam 38% CO, 35% H5% other hydrocarbons 2, 12% CO2, 10% CH4, and 12–18.
Allothermal gasifiers, unlike autothermal gasifiers, which are self sufficient in heat requirement, need an additional source for providing the energy required for the gasification process. For this reason, allothermal gasification offers a possibility of using the waste heat generated by a SOFC and could be of significance, if systems with gasifiers and SOFCs are considered and several studies already been reported on this integration [5].
also at the bottom. They provide rather low-quality gas (high tar content), as the gas is not passing through a very high temperature region (combustion zone) within the gasifier. On the other hand in down-draft co-current gasifiers, the biomass is fed from top along with the oxidizer. They offer reasonably cleaner gas as the gas passes through a high-temperature bed, thus causing cracking of a significant portion of tar components[3]. Recently, up-draft gasifiers also have started receiving attention especially for applications with solid oxide fuel cells. This stems from the argument that if the tar content in the gas is not a problem for SOFCs then up-draft gasifiers offer a better option, as they have simple designs and can work with relatively wet fuels[6]. Fluidized-bed reactors are suitable for higher power levels and offer relatively lower quality gas. They can be further classified as bubbling fluidized bed and circulating
fluidized-bed reactors. They are considered to be suitable for power levels up to 100 MW (2-100 MW) and are widely studied [3]. Another variety is entrained-flow
reactors. They do not use extra bed materials like other types and are operated at higher temperatures. The entrained-flow reactors are suitable for high power levels (5- 100 MW) and give very high quality gas, as they achieve very high temperatures in the reactor. They could be of significance when gasifier-SOFC system development reaches such a mature level that systems of power levels of a few MW can be conceived.
2.3. Solid Oxide Fuel Cells
Solid Oxide Fuel Cells are being developed for a few decades. As it is becoming clear that they will most probably be an important component of high efficiency energy systems in the future, at present considerable effort is being put into their development around the globe. Solid oxide fuel cells work at temperatures in the range of 873- 1273 K and can run with carbonaceous fuels, such a CH4, CO etc., in addition to
hydrogen[7].
In the SOFC, the fuel enters the anode chamber and is oxidised. Oxygen enters the cathode chamber and is ionized and transported through the electrolyte to the anode (Figure 2.1). The anode of the fuel cell disperses the fuel gas over at its interphase with the electrolyte, catalyzes the electrochemical reactions, and conducts the electrons that are freed from fuel molecules. These electrons flow through an external circuit producing electric power. The cathode of the fuel cell distributes the oxygen at its interphase with the solid electrolyte, and conducts the electrons from the external circuit where they reduce the oxygen molecules, producing oxide ions. Oxide ions diffuse through the solid electrolyte to the anode to form H2O or CO2 depending on the type of
fuel. The solid electrolyte contains many oxygen vacancies which make oxygen ions able to diffuse. The solid electrolyte mainly prevents the two electrodes to come into electronic contact by blocking the electrons and allows the flow of oxide ions from the cathode to the anode to maintain the overall electrical charge balance. They also play an important role in determining the operating temperature of the fuel cell.
H2, CO, CH4 → H2O, CO2 and unconverted fuel ←
← O2 → O2
anode electrolyte cathode O 2-e -Fuel inlet Fuel outlet Air inlet Air outlet Electrical Energy
Figure 2.1: Schematic diagram of the solid oxide fuel cell
Important reactions that take place in the solid oxide fuel cells are the following:
x . 2 o 2 o .. - x 2 o o H +O H O+V +2e 1 O V +2e2 O → + → .
-The Nernst equation giving the reversible voltage for the total cell reaction occurring in the fuel cell, when hydrogen is oxidized, is the following:
( )
2 2 2 0 *ln H *ln 1 O H O P RT RT E E P nF P nF ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ = +⎜ ⎟ ⎜⎜ ⎟⎟+⎜ ⎟ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ / 2 Where 0 ln 2 RT E K nF ⎛ ⎞ = ⎜⎝ ⎟⎠However, the useful voltage output (V) under load conditions, that is, when a current passes through the cell, is given by
c a
V=E-IR- - η η
Where I is the current passing through the cell, R is the internal electrical resistance of the SOFC, and c and a are polarization losses associated with the cathode and anode,
2.3.1 Anode, Electrolyte and Cathode
Anode
The anode polarization resistance comprises of internal resistance, contact resistance, concentration polarization resistance, and activation polarization resistance [8]. The internal resistance is the resistance to the transport of electrons within the anode. The contact resistance is caused by poor adherence between anode and solid electrolyte or interconnects and is generally not affected by fuel variations. Concentration polarization is related to the transport of gaseous species through the porous electrode. The porosity/ microstructure of the electrode is an important parameter affecting concentration polarization. Concentration polarization may become significant at higher current flows and fuel utilization. With dilute fuels like biosyngas, concentration polarization could be an important parameter affecting SOFC performance and this needs to be studied in detail. Activation polarization is related to the charge transfer processes at the anode and depends on the area of electrode/electrolyte/gas triple-phase boundaries (TPB) and the electrocatalytic activity of the electrode itself. The effective electrochemical reaction zone (ERZ) at the anode of a SOFC is mainly limited to physical triple-phase boundaries (electrolyte/anode/fuel), for Nickel/Yttria Stabilized Zirconia (Ni/YSZ) anodes which are the most common anode material for SOFC applications to date. Nickel serves as an excellent reforming catalyst and electrocatalyst for electrochemical oxidation of hydrogen. It also provides electronic conductivity for the anode. The YSZ constitutes a framework for the dispersion of Ni particles and is an oxide-ion conductor. In contrast, the use of anode materials that show mixed ionic and electronic conduction allows electrochemical reactions to take place at regions other than the triple phase boundaries. This can lead to a significant drop in the polarization losses at the anode and yields improvement in electrical efficiency. Doped ceria (this material becomes electronically conducting in reducing conditions) anodes come to this class. Doping ceria with elements like Gadolinia and Samaria for improving the performance and chemical stability is being widely studied. Ceria-based anodes are widely recognized to be effective in suppressing carbon deposition. Still the electronic conductivity of such anodes seems not sufficient to obtain the required performance. Research is focused to improve the electronic conductivity and hence they may probably offer a good choice as anode material for applications with fuels like biosyngas in the future. Doped ceria can also substitute YSZ in Ni-YSZ anodes. Catalytic activity and electronic conductivity of nickel and mixed ionic conductivity of doped ceria under reducing conditions offer an excellent combination. The fuel oxidation can then also take place on the ceria surface and catalytic activity of nickel may become less important. This combination of nickel and ceria may offer certain significant opportunities in using the anode with contaminated gases like biosyngas. In this case, it is probable that, certain kinds of anode contaminations, in which selective adsorption of the contaminant on nickel surface is believed to cause problems in SOFC operation, may no longer pose any significant threat. For example, the decrease in Ni/YSZ anode performance, while fuel gas contaminated with H2S is fed [9] is suggested as originating due to selective
adsorption of H2S on nickel surface .This may no longer pose a significant threat on
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