Biomass-powered Solid Oxide Fuel Cells Ming Liu
Experimental and Modeling Studies
for System Integrations
Biomass-powered Solid Oxide Fuel Cells
Ming Liu
INVITATION
To the public PhD
defense of
Ming Liu
Titled:
Biomass-powered
solid oxide fuel cells
The ceremony will take
place on Monday 25
March 2013 at 10:00
in the Senaatszaal,
Aula TU Delft
Mekelweg 5,
Delft
At 9:30 I will shortly
introduce my
dissertation
Ming Liu
195577-os-Liu.indd 1 1-3-13 9:34Biomass-powered Solid Oxide Fuel Cells
Experimental and M odeling Studies for System Integrations
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op maandag 25 maart 2013 om 10:00 uur
door
Ming LIU
Master of Science in Mechanical Engineering, Tianjin University Geboren te Feidong, China
Prof. dr. ir. A. H. M. Verkooijen
En de copromotor: Dr. ir. P.V. Aravind
Samenstelling promotiecommissie:
Rector Magnificus voorzitter
Prof. dr. ir. A. H. M. Verkooijen, Technische Universiteit Delft, promotor
Dr. ir. P. V. Aravind, Technische Universiteit Delft, copromotor
Prof. dr. ir. E. S. Lora, Federal de Itajuba
Prof. dr. ir. J. Kiel, ECN
Prof. dr. ir. D. M. J. Smeulders, Technische Universiteit Eindhoven
Prof. dr. ir. B.J. Boersma, Technische Universiteit Delft
Dr. ir. K. Hemmes, Technische Universiteit Delft
Financial support from the China Scholarship Council ([2008] 3019), CEMIG, Brazil (Project No.237), and the NUFFIC/CAPES program is deeply appreciated.
ISBN/EAN: 978-94-6186-137-5
Copyright @2013 by M. Liu (lming928@hotmail.com)
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without the prior permission of the author
I
Summary
Biomass-powered solid oxide fuel cells
Experimental and modeling studies for system integrationBiomass is a sustainable energy source which, through thermo-chemical processes of biomass gasification, is able to be converted from a solid biomass fuel into a gas mixture, known as syngas or biosyngas. A solid oxide fuel cell (SOFC) is a power generation device that directly converts the chemical energy of a fuel to electricity. Therefore, biomass-powered SOFCs could be highly efficient.
Typically, in addition to carbon dioxide and water vapor, the major components of syngas produced from biomass gasification include hydrogen, carbon monoxide and methane which are potential fuels for SOFCs, which make integration possible between SOFCs and biomass gasifiers. However, the syngas is also comprised of trace species such as tars, H2S, HCl, and
alkali compounds, among others, which could be detrimental to SOFCs if they are contained within the feeding syngas stream. Therefore, the syngas must be pretreated in order to reduce these trace species to a level that SOFCs are able to tolerate. With various gas treatments, the overall system performance would fluctuate, and therefore, the influence of the gas treatment methods on the system performance must be understood.
The most prominent among the trace species is tar. The effect of tars on the performance of SOFCs has yet to be studied, however, it is known that, even though tar can possibly poison the fuel cell through carbon deposition, it may also become a fuel for SOFCs. Furthermore, SOFC systems are currently designed in general for employing natural gas. Due to the fact that SOFC systems are very sensitive to the fuel types, it is necessary to completely understand the system response when switching from natural gas to biosyngas to enable a better controllability for future experiments.
The research scope of this thesis is limited to the aforementioned issues. The objective of this thesis is to provide a fundamental study to ensure a safe and efficient system integration. The study is limited to an existing downdraft fixed-bed gasifier and a 5 kWe SOFC CHP system due to these two units
entering the commercial market. The approach utilized, however, could be further adopted for the large scale power plants based on biomass gasifiers and SOFCs.
The research begins with the evaluation of technologies involved biomass-powered SOFCs in chapter 2. Technologies regarding biomass gasification, gas cleanup and fuel cells are discussed based on literature surveys. The review begins by briefly summarizing conventional gasifiers including
fixed-II
bed and fluidized bed gasifiers, which are implented for biomass gasification. Following that, details are indicated for SOFC performance affected by the trace species such as particulates, H2S and available cleaning technologies.
The combination of biomass gasifiers with fuel cells including proton exchange membrane fuel cells (PEMFC), molten carbonate fuel cells (MCFC), and SOFCs is then reviewed with an emphasis on the development of SOFC technology and the study of integration between biomass gaisifers and SOFCs.
Chapter 3 presents a thermodynamic study of the influence of cleaning technology on the energetic and exergetic performance of the integrated gasifier–SOFC system with distinctive system configurations. Two gas cleaning systems, specifically, a combined high and low temperature gas cleaning system and a high temperature gas cleaning system are considered to connect the gasifier with the SOFC system. The influence of the steam addition for the suppression of carbon deposition and various heat sources for steam generation on the system performance is evaluated. The performance of the SOFC system operating with natural gas and biosyngas is also compared.
The installed SOFC system, particularly the embedded pre-reformer and anode off-gas recirculation was initially designed for natural gas. This design is desirable as it effectively uses the steam in the anode off-gas and the heat generated in the stack. As SOFC performance is very sensitive to gas composition and operating conditions, both of which are affected by the anode recirculation, an evaluation of the recirculation behavior on safety issues regarding carbon deposition and nickel oxidation and system performance are presented in chapter 4. An important finding is that, by not implementing the recirculation, the biosyngas-fueled SOFC system effectuates a much higher net electrical efficiency, less initial investment and simpler system configuration in comparison to that when recirculation is implemented.
Tolerance of SOFCs to the trace species from biomass gasification is not yet fully understood. The influence of biomass gasification tars on SOFC performance and mitigation of carbon deposition are experimentally evaluated in chapter 5&6.
Well-controlled operational conditions assist in the suppression of carbon deposition. Chapter 5 presents the influence of operating conditions including steam levels, current density and time on stream on the performance of SOFCs with Ni–YSZ anodes fueled by tar-containing biosyngas at 800 °C. Changes in impedance spectra and polarization curves of SOFCs following tar exposure were analyzed to assess the cell performance. The biosyngas composition and the tar concentration employed in these measurements were identical to those measured from the commercial air-blown biomass gasifier that is to be connected to the studied SOFC system. Operating this type of
III SOFC with the tar concentrations could result in severe damage to the cell due to carbon formation on the anodes. Scanning Electron Microscopy (SEM) indicated carbon deposition which affected the performance of the SOFC, as is exhibited by the impedance spectra and anode polarization curves of the cells after exposure to tars. However, the risk of carbon deposition could be alleviated by increasing steam levels and current loads.
Chapter 6 presents a similar study of the effects of tar on SOFC performance, but possesses a focus on Ni–GDC anodes and various operating temperatures levels (700, 800 and 900 °C) under both dry and wet conditions. Polarization behavior, electrochemical impedance spectroscopy, and cell voltage degradation were analyzed to evaluate the cell performance. It is most likely that the cells with Ni–GDC anodes did not suffer from carbon deposition under the wet conditions studied. Dry tar-containing syngas for SOFCs is unlikely to cause carbon formation under a mild current load; however, it may induce carbon formation at open circuit. The effect of carbon dioxide that is capable of suppressing carbon deposition was experimentally investigated, and an enhanced performance was observed under the conditions studied. Under carbon risk-free operating conditions, the cell voltage increases when raising the feeding tar concentration, indicating that tar performs as fuel for SOFCs.
Numerical simulation is an efficient tool for the evaluation of SOFCs’ response when switching fuels. Chapter 7 presents such a numerical study with the focus on the evaluation of kinetic models for methane steam reforming for SOFCs operation with multiple fuels. Three frequently employed kinetic models were selected in order to examine their impacts on the performance of a tubular SOFC. The resulting thermo-electrochemical behaviors derived from these models were compared. It was discovered that all three kinetic models are reasonably accurate in terms of the polarization behavior, but they significantly affected the local thermo-electrochemical performance. A more rapid kinetic model was adopted based on the evaluation of these three kinetic models in order to evaluate the performance of the tubular SOFC in terms of local electrochemical performance, anode oxygen partial pressure and overall SOFC performance when performing with multiple fuels.
Chapter 8 draws the conclusions regarding the work presented in this dissertation, and recommendations are suggested for future research activities.
V
Samenvatting
Biomassa als brandstof voor een vast oxide brandstofcel
Experimentele and model studies voor systeemintegratieBiomassa is een duurzame energiebron. Biomassavergassing kan een vaste brandstof omzetten in een gasmengsel, dat syngas of biosyngas wordt gemoemd, door thermo-chemische processen. Een vast oxide brandstofcel (Solid Oxide Fuel Cell, SOFC) is een apparaat dat direct de chemische energie van een brandstof omzet in elektriciteit en daarmee de mogelijkheid van een hoog rendement biedt.
Naast koolstofdioxide en waterdamp zijn de belangrijkste componenten van syngas, geproduceerd door biomassavergassing, waterstof, koolmonoxide en methaan, die potentiële brandstoffen zijn voor SOFC’s. Dit maakt de integratie mogelijk van SOFC’s met biomassavergassers. Het syngas bevat echter ook sporen van teer, H2S, HCl, alkalicomponenten
enzovoort, die schadelijk zijn voor SOFC’s als ze in de syngasvoedingstroom aanwezig zijn. Daarom zal het syngas moeten worden voorbehandeld, om deze sporencomponenten terug te brengen tot een niveau dat SOFC’s kunnen verdragen. Met verschillende gasbehandelingen veranderen de prestaties van het totale systeem en daarom is het belangrijk de invloed van de gasbehandelingsmethoden op de prestaties van het systeem te kennen. Onder deze sporencomponenten is teer een belangrijke component. Het effect van teer op de prestaties van SOFC’s is momenteel onderwerp van studie, aangezien het een brandstof voor de SOFC kan zijn, maar het is ook mogelijk dat het de brandstofcel vergiftigt door koolstofafzetting. Verder worden SOFC-systemen tegenwoordig gewoonlijk ontworpen voor het gebruik van aardgas; aangezien de systemen erg gevoelig zijn voor de typen brandstof, is het noodzakelijk goed het systeemgedrag te leren kennen, bij het overschakelen van de brandstof van aardgas naar syngas, om toekomstige experimenten beter beheersbaar te maken.
Het onderzoeksdoel van deze dissertatie is beperkt tot de bovengenoemde onderwerpen. Het doel van deze dissertatie is een fundamentele studie voor een veilige en efficiënte systeemintegratie. De studie is beperkt tot een bestaande vastbedvergasser, met neerwaartse stroming van het gas, en een op een SOFC gebaseerde warmtekrachtkoppelinginstallatie van 5 kWe, aangezien deze twee
componenten de commerciële markt hebben bereikt. De gebruikte benaderingkan evenwel verder worden toegepast voor grootschalige centrales, gebaseerd op biomassavergassers en SOFC’s.
Het onderzoek begint in hoofdstuk 2 met de evaluatie van technieken die betrokken zijn bij de conversie van biomassa in elektriciteit met de
VI
brandstofcel als generator. Technologieën over biomassavergassing, gasreiniging en brandstofcellen worden besproken op basis van literatuurstudie. De evaluatie begint een korte samenvatting van traditionele vergassers, inclusief vastbed- en wervelbedvergassers voor biomassavergassing. Vervolgens worden de door verontreinigingen (zoals stofdeeltjes, H2S enzovoort) beïnvloede prestaties van SOFC’s en de
beschikbare reinigingstechnieken daarvoor besproken. Daarna wordt de combinatie van biomassavergassers met brandstofcellen, met inbegrip van proton exchange membrane fuel cells (PEMFC), gesmolten carbonaat brandstofcellen (MCFC) en SOFC’s besproken. De nadruk ligt op de ontwikkeling van de SOFC-technologie en de bestudering van de integratie van biomassavergasser en SOFC’s.
Hoofdstuk 3 behandelt een thermodynamisch onderzoek naar de invloed van de reinigingstechnologie op de energetische en exergetische prestaties van het geïntegreerde vergasser–SOFC–systeem in verschillende systeemconfiguraties. Twee gasreinigingssystemen, het ene een combinatie van processen bij hoge en lage temperatuur en het andere geheel bij hoge temperatuur worden beschouwd als verbindingsschakel tussen de vergasser en het SOFC-systeem. De invloed van stoomtoevoeging, om koolstofdepositie tegen te gaan, op de systeemprestaties is vergeleken voor verschillende warmtebronnen voor de stoomproductie. De prestaties van het SOFC-systeem werkend met aardgas en met syngas is eveneens vergeleken. Het geïnstalleerde SOFC-systeem, in het bijzonder de geïntegreerde pre-reformer en de recirculatie van gas aan de anodezijde, is oorspronkelijk ontworpen voor aardgas. Dit ontwerpaspect is wenselijk omdat het efficiënt gebruik maakt van de waterdamp in het uitlaatgas van de anode en van de warmte die in de stack wordt ontwikkeld. De stack reageert erg gevoelig op de gassamenstelling en bedrijfscondities, die beide worden beïnvloed door de recirculatie aan de anode. Daarom wordt in hoofdstuk 4 een evaluatie gepresenteerd over de invloed van de recirculatie op veiligheidsaspecten, met betrekking tot koolstofdepositie en nikkeloxidatie, en op de systeemprestaties. Een belangrijke bevinding is dat zonder recirculatie het SOFC-systeem met syngas een veel hoger elektrisch rendement haalt, lagere investeringen vergt en een eenvoudiger systeemconfiguratie heeft, in vergelijking met een systeem met de recirculatie.
De tolerantie van SOFC’s voor sporencomponenten uit biomassavergassing moet nog goed worden uitgezocht. De invloed van teer, die bij de vergassing ontstaat, op de prestaties van SOFC’s en vermindering van de koolstofafzetting zijn experimenteel onderzocht in hoofdstuk 5 en 6.
Goed geregelde bedrijfscondities helpen om de koolstofdepositie te onderdrukken. Hoofdstuk 5 toont de invloed van bedrijfscondities, inclusief stoomhoeveelheden, stroomdichtheid en bedrijfstijd, op de prestaties van
VII SOFC’s met anodes van Ni–YSZ met teerhoudend biosyngas als brandstof, bij 800 °C. Veranderingen in impedantiespectra en polarisatiecurves van SOFC’s, na blootstelling aan teer, zijn geanalyseerd om de celprestaties vast te stellen. De samenstelling van het biosyngas en de gebruikte teerconcentratie in deze metingen waren identiek aan die, gemeten aan de commerciële biomassavergasser, met lucht als vergassingsmiddel, die gebruikt moet gaan worden met het bestudeerde SOFC-systeem. Het werken van dit type brandstofcel onder de gebruikte teerconcentraties zou kunnen resulteren in ernstige schade van de cel ten gevolge van koolstofafzetting op de anodes. Koolstofafzetting werd waargenomen door Scanning Electron Microscopy (SEM) en beïnvloedde de prestaties van de SOFC, zoals gepresenteerd met de impedantiespectra en anode-polarisatiecurves van de cel na blootstelling aan teren. Niettemin kan het risico van koolstofafzetting worden verminderd door toename van de stoomhoeveelheid en de stroombelasting.
Hoofdstuk 6 beschrijft een vergelijkbare studie over het gebruik van teerhoudend syngas in SOFC’s, maar met de nadruk op Ni–GDC anodes en verschillende temperatuurniveaus (700, 800 en 900 °C), waarbij zowel droge als natte condities worden beschouwd. Polarisatiegedrag, elektrochemische impedantiespectroscopie en degradatie van de celspanning werden gebruikt voor de analyse van de prestaties. De cellen met NI–GDC anodes vertoonden geen koolstofafzetting bij de drie beschouwde temperatuurniveaus bij natte condities. Droog, teerhoudend syngas veroorzaakte geen koolstofafzetting in SOFC’s bij een milde stroomsterkte; er was echter wel koolstofvorming bij open klemmen. Het effect van koolstofdioxide, dat in staat is de koolstofafzetting tegen te gaan, werd experimenteel onderzocht en onder deze condities werden verbeterde prestaties geconstateerd. Onder risicovrije condities, wat koolstof betreft, heeft de celspanning de neiging te stijgen voor een hogere teerconcentratie, waaruit blijkt dat de teer bijdraagt aan het vermogen van SOFC’s.
Numerieke simulatie is een efficiënt instrument voor de beoordeling van de responsie van SOFC’s bij het wisselen van brandstof. Hoofdstuk 7 beschrijft een dergelijk numerieke studie met de nadruk op beoordeling van kinetische modellen voor het reformeren van methaan met stoom, om SOFC’s met verschillende brandstoffen te laten werken. Drie vaak gebruikte kinetische modellen zijn geselecteerd om hun invloed op de prestaties van een buisvormige SOFC te bepalen. Het resulterende thermo-electrochemische gedrag, verkregen uit deze modellen, is vergeleken. Hieruit bleek dat alle drie kinetische modellen met redelijke nauwkeurigheid kunnen worden gebruikt in termen van polarisatiegedrag, maar ze beïnvloedden de locale thermo-elektrochemische prestaties in belangrijke mate. Een sneller kinetisch model werd gebruikt, naar aanleiding van de evaluatie van deze drie kinetische modellen, om de prestaties van de buisvormige SOFC te beoordelen, in
VIII
termen van locale elektrochemische prestaties, partiële druk van de zuurstof en overall prestaties van de SOFC, wanneer verschillende brandstoffen worden gebruikt.
In hoofdstuk 8 worden conclusies getrokken betreffende het werk da tin deze dissertatie is gepresenteerd, waarnaast aanbevelingen worden gedaan voor toekomstige onderzoeksactiviteiten.
IX
Nomenclature
A Cell active area/ejector section area m2
A Specific active area m2 m−3
Cp Specific heat of gas at constant pressure J kg−1 K−1
D Diameter m
Di,eff Effective gas diffusion coefficient of species i m2 s−1
Di,j Binary diffusivity m2 s−1
Di,k Knudsen diffusivity m2 s−1
Di,m Molecular diffusivity of component i in the mixture m2 s−1
E Energy kJ mol−1 k−1
Ex Exergy kW
F Faraday constant C mol−1
i Current density A m−2
i0 Exchange current density A m−2
I Current A
Current contribution from hydrogen in domain i A
ICO,i Current contribution from carbon monoxide in domain i A
k
Current contribution ratio between hydrogen and carbon monoxide/Pre-exponential factor for volumetric reaction −/mol bar−0.5 m−2 s−1(mol bar−1 m−2 s−1) f sr
k Forward reaction rate of steam reforming mol m−3 s −1
b sr
k Backward reaction rate of steam reforming mol m−3 s −1
f wgs
k Forward reaction rate of water gas shift reaction mol m−3 s −1
b wgs
k Forward reaction rate of water gas shift reaction mol m−3 s −1
K Viscous resistance/equilibrium constant m−2/−
l Thickness m
m Mass flow rate kg s−1
M Mach number −
Ma Molecular weight kg mol−1
n Number of electrons transferred/species molar flow rate −/mol s−1
n0
i Molar flow rate of species i at the reformer outlet mol s−1
Pe Electrical Power W
P Pressure Pa
pi Partial pressure of species i Pa
Pcr Threshold critical pressure in the critical mode Pa
Psub Threshold subcritical pressure in the subcritical mode Pa
Qreac Heat generated by the oxidation of H2 and CO inside the
stack
W
Qfuel Heat duty for preheating fuel W
Qloss Heat generated by irreversibilities of the stack
polarization
W
Qref Heat transmitted to steam reformer W
Qair Heat duty for preheating air W
Qwgs Heat released by the water gas shift reaction W
QH Transmitted heat of the heat exchanger W
r Volumetric reaction rate/Radius mol m−3 s
−1/m
R Electrical resistance/Universal gas constant /Radius Ω m2/J mol−1 K−1/m
X
Req Equivalent resistance Ω·cm2
Sm Mass source term kg m3 s−1
ST Heat source term kJ m3 s−1
Su Momentum source term N m−3
Si Mass source term for species i kg m3 s−1
T Temperature K
T Temperature difference
UA Heat transfer capacity rate at the design condition W K-1
UA* Heat transfer capacity rate at the off-design condition W K-1
UF Fuel utilization − T Temperature K V Velocity/Cell voltage m s−1/V V Velocity vector m s−1 0 rev
V Standard reversible voltage for hydrogen V
ΔVx Voltage loss V
w Width of interconnection and Ni-felt m
yi Mole fraction of species i −
Yi Mass fraction of species i −
Greek letters
α Coefficient/Exponent for the off-design of the heat
exchanger −
γ Specific heat ratio of gas/Pre-exponential factor for
exchange current density −
por Mean pore radius m
ε Electrode porosity −
η Isentropic coefficient of primary
flow/Overpotential/Energy efficiency −/V/−
ηx Exergy efficiency −
λ Thermal conductivity w m−1 k−1
μ Dynamic viscosity kg m−1 s−1
ρ Gas density /electrical resistivity kg−3/Ω m
σ Electrical conductivity Ω−1m−1
τ Tortuosity −
Φ The electrical potential/Mass flow rate of anode inlet gas V/kg s−1
ω Entrainment ratio kg kg−1
Superscripts and subscripts
0 Nozzle inlet −
1 Primary flow at the nozzle section −
2 Nozzle outlet −
3 Mixing chamber inlet −
4 Diffusion chamber inlet −
XI
5 Diffusion chamber outlet −
act Activation −
an Anode −
aux Auxiliary −
bulk Gas channel (outside the electrode diffusion layer) −
ca Cathode −
cr Critical −
diff Diffusion −
eff Effective −
ele Electricity /Electrolyte −
eq Equivalent −
ic Interconnector −
ohm Ohmic −
p Primary flow (i.e., inlet fresh fuel) −
reac Electrochemical reactions −
ref Reference −
rev Reversible −
rs Reaction site −
s Secondary flow (i.e., anode recycled gas) −
spe Species −
sr Steam reforming −
stack Fuel cell stack −
sub Subcritical −
t Throat −
th Thermal −
tot Total −
Abbreviations
AFC Alkaline fuel cell −
BoP Balance of plant −
CE Counter electrode −
BFB Bubbling fluidized bed
CFB Circulated fluidized bed −
CHP Combined heat and power −
ECS Electronic control system
EIS Electrical impedance spectroscopy −
ER Equivalent ratio −
ESP Electrostatic precipitators −
ESB Erbia-stabilized bismuth oxide −
FU Fuel utilization −
GC Gas Chromatograph
GDC Gadolinia doped ceria −
XII
GT Gas turbine
HHV Higher heating value −
HDS Hydrodesulfurization −
LSM Lanthanum strontium manganite −
LHV Lower heating value kJ kg−1
LSM Lanthanum Strontium Manganese Oxide −
MCFC Molten carbonate fuel cell −
Ni–GDC Nickel-gadolinium doped ceria −
OCV Open circuit voltage −
OTCR Oxygen to carbon ratio −
ppm Parts per million −
PAFC Phosphoric acid fuel cells −
PAH Polyaromatic hydrocarbons −
PEMFC Polymer membrane −
PSA Pressure swing adsorption −
RE Response electrode −
RE0 Reference electrode −
RME Rapemethylester −
SEM Scanning electron microscope −
SCO Selective catalytic oxidation −
SR Steam reforming −
SOFC Solid oxide fuel cell −
SWPC Siemens Westinghouse Power Corporation −
TP Thermocouple point −
VPS Versa Power Systems −
WES Water electrostatic scrubber −
WGS Water gas shift −
XIII
Table of Contents
Summary ... I Samenvatting ... V Nomenclature ... IX Table of Contents ... XIII
1. Introduction ... 1
1.1 Introduction ... 2
1.2 Motivation ... 3
1.3 Research scope and outline ... 4
1.4 Reference ... 5
Background and technology ... 7
2. 2.1 Biomass gasification ... 8
2.1.1 Fixed bed gasification ... 9
2.1.2 Fluidized gasification ... 11
2.1.3 Other gasification technologies ... 12
2.2 Gas cleaning technology ... 12
2.2.1 Particulates ... 13
2.2.2 Alkali Compounds ... 16
2.2.3 Tars ... 17
2.2.4 Chlorides ... 19
2.2.5 Sulfur ... 20
2.2.6 Nitrogen-containing compounds and others ... 22
2.3 SOFC overview ... 23
2.3.1 Fuel cell generals ... 23
2.3.2 SOFC types ... 26
2.3.3 SOFC Materials ... 27
2.3.4 SOFC Systems and development ... 29
2.4 Biomass-powered fuel cells ... 33
2.4.1 Gasifier–PEMFC systems... 33
2.4.2 Gasifier–MCFC systems ... 34
2.4.3 Gasifier–SOFC systems ... 35
2.5 Summary ... 36
2.6 Reference ... 36
System thermodynamic evaluations ... 45
3. 3.1 Introduction ... 46
3.2 System configuration ... 46
3.2.1 Gasifier ... 46
3.2.2 Gas cleaning system ... 47
XIV
3.3 Modeling ... 49
3.3.1 Gasifier ... 49
3.3.2 Anode recirculation ... 50
3.3.3 Gas processing ... 50
3.3.4 Fuel cell model ... 50
3.3.5 Energy and exergy efficiency ... 51
3.3.6 System modeling process ... 52
3.3.7 Natural gas fueled SOFC CHP system ... 52
3.3.8 Biosyngas fueled SOFC CHP system ... 55
3.4 Results and discussion ... 58
3.4.1 Gasification ... 58
3.4.2 Fuel cell model validation ... 58
3.4.3 Carbon deposition ... 59
3.4.4 System performance ... 60
3.4.5 Influence of fuel utilization ... 65
3.4.6 Influence of current density ... 67
3.4.7 Influence of anode recirculation ... 68
3.5 Conclusions ... 69
3.6 Reference ... 70
Anode recirculation studies... 73
4. 4.1 Introduction ... 74
4.2 Ejector and SOFC system description ... 75
4.3 Model specifications ... 77
4.3.1 Recirculation ... 77
4.3.2 Steam reforming ... 79
4.3.3 Electrochemical model ... 80
4.3.4 Combustion and heat exchange ... 84
4.3.5 Energy balance ... 86
4.4 Calculation procedure and model validation ... 87
4.4.1 Calculation procedure ... 87
4.4.2 Model validation ... 89
4.5 Results and discussion ... 90
4.5.1 Recirculation behavior ... 90
4.5.2 Effects of recirculation ... 92
4.5.3 Carbon deposition and nickel oxidation analysis ... 96
4.5.4 System performance optimization ... 101
4.6 Conclusions ... 103
4.7 Reference ... 103
The effects of tar on Ni–YSZ anodes ... 107
5. 5.1 Introduction ... 108
XV
5.2 Experimental ... 108
5.2.1 Sample preparation ... 108
5.2.2 Experimental procedure ... 109
5.2.3 Operating conditions ... 111
5.3 Results and Discussion ... 111
5.3.1 Characterization of the Starting Anode ... 111
5.3.2 SOFC operation with tar-free biosyngas ... 113
5.3.3 Effect of biosyngas steam content on cell performance ... 114
5.3.4 Effect of current density on cell performance ... 115
5.3.5 Longer term operation with tar-containing biosyngas ... 117
5.4 Conclusions ... 119
5.5 Reference ... 119
The effects of tar on Ni–GDC anodes ... 121
6. 6.1 Introduction ... 122
6.2 Role of temperature in carbon deposition ... 124
6.3 Experimental ... 126
6.3.1 SOFC with Ni–GDC anode ... 126
6.3.2 Model tar and tar evaporator ... 127
6.3.3 SOFC test station ... 128
6.3.4 Operational procedure ... 129
6.4 Results and discussion ... 130
6.4.1 Open circuit voltage ... 130
6.4.2 Syngas with tar under wet conditions ... 131
6.4.3 Syngas with tar under dry conditions ... 134
6.4.4 Long-term operation ... 136 6.5 Conclusion ... 137 6.6 Reference ... 138 A numerical analysis ... 141 7. 7.1 Introduction ... 142
7.2 The model descriptions... 143
7.3 The kinetic models ... 147
7.3.1 Methane steam reforming ... 147
7.3.2 The water gas shift reaction ... 148
7.3.3 The specific active area ... 149
7.4 The model assumptions and methods ... 150
7.5 Results and discussion ... 153
7.5.1 A comparison of the kinetic models ... 153
7.5.2 Validation of the model and the polarization behavior ... 156
XVI
7.6 Conclusions ... 163
7.7 Reference ... 163
Conclusions and recommendations ... 167
8. 8.1 Conclusions ... 168
8.2 Recommendations ... 170
Appendices ... 173
Biomass gasification experiments ... 173
A.1 A.1.1 Preparation ... 173
A.1.2 Operational procedure ... 174
A.1.3 General results ... 174
Biomass gasification tar and measurements ... 175
A.2 A.2.1 Tar definition, composition, and classification ... 175
A.2.2 Tar content quantification ... 176
A.2.3 Reference ... 178
Gas cleaning measurements ... 179
A.3 A.5.1 System description ... 179
A.5.2 Sample analysis ... 180
A.5.3 Results ... 180 A.5.4 Discussion ... 183 A.5.5 Reference ... 183 Publications ... 185 Acknowledgement ... 187 Curriculum Vitae ... 189
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1.
Introduction
This chap ter p resents the m otivation, research scop e, and stru ctu re of this PhD thesis. All the w ork and resu lts p resented in the follow ing chap ters are su bjected to this gu id eline.
2
1.1 Introduction
The modern economy possesses an extreme dependency on the consumption of fossil fuels; therefore, the worldwide primary energy consumption is increasing rapidly. Approximately 80 % of the total energy demand will continue to be achieved with fossil fuels [1]. The electricity generation from fossil fuels accounts for 70 % of the world market, whereas less than a 3 % share is derived from renewable sources [1]. In addition to the diminution of fossil fuels, global warming which is the result mainly from carbon dioxide, has become a concern of mankind. It is estimated that the continued use of fossil fuels will increase the global temperature by 1.0– 3.5 °C in the coming 50–100 years [2]. The fewer reserves of fossil fuels and a greater environmental awareness are accelerating the energy research into alternative and sustainable ways for energy production.
Figure 1-1 Biomass conversion pathways
All organic material stemming from plants such as wood, as well as sewage sludge, could be labeled as biomass [3]. The use of biomass results in a limited long-term effect on the environment as CO2 production is
balanced by its consumption. Therefore, the application of biomass for energy production reduces the emission of carbon dioxide and prevents the climate change, and it is considered to be a renewable energy source. Biomass conversion could be categorized into thermo-chemical conversion, bio-chemical conversion, and mechanical extraction. The thermo-bio-chemical conversion includes combustion, liquefaction, pyrolysis, and gasification. Within the bio-chemical conversion process, digestion for biogas production
3 and fermentation for ethanol production are the two options. Mechanical extraction, sometimes followed by chemical extraction, is a technology utilized for the production of bio-diesel. Typical biomass conversion pathways are illustrated in Figure 1-1 [4]. The choice for an actual conversion route is dependent on the type of biomass and the end-use product: heat, electricity, fuel, or a combination of them.
Among the biomass conversion pathways, biomass gasification could achieve efficiency up to 85 % [5] when converting biomass into a secondary energy carrier, i.e. biosyngas. Biosyngas is mainly comprised of hydrogen, carbon monoxide, methane, carbon dioxide, water vapor, and nitrogen (when air is used as the gasifying agent). Their content varies when different gasification technologies are applied. In addition to the major components, biosyngas also contains certain trace species including particulates, tars, alkalis, sulphur compounds, and hydrogen chloride, among others. These minor species can deteriorate all types of processes and downstream equipment such as internal combustion engines [6] and SOFCs [7]. Therefore, the gas must be pretreated before use in downstream facilities.
Fuel cells are electrochemical devices that directly convert gaseous fuels into electricity without combustion processes, and thus they feature a higher conversion efficiency than conventional paths, most of which involve combustion processes. Different fuel cell types exist, and each type has its own operating window and fuel requirements. Among the fuel cells, SOFCs can potentially operate with a wide range of fuels including natural gas, hydrogen, syngas, biogas, ammonia, and hydrocarbons under certain conditions; futhermore, they are more tolerable to contaminants [8]. Therefore, the energy systems based on biomass gasifiers and SOFCs is able to produce power in a sustainable mannar with high efficiency.
1.2 Motivation
Because the combination of SOFCs with biomass gasification could result in highly efficient energy conversion systems [9], the objective of this research is to study the fundamentals for the development of biomass-powered SOFC systems.
In order to employ biosyngas in powering SOFC systems safely and efficiently, certain key questions require being answered:
1. What are the key factors that affect the system performance? 2. Are modifications required for the existing SOFC systems? 3. What are the effects of tars on SOFCs?
4. What are the responses of SOFCs when switching fuel to biosyngas? This thesis attempts to answer such questions.
4
1.3 Research scope and outline
Development of gasifier–SOFC systems for future residential combined heat and power (CHP) generation is promising. Two lab scale (semi- commercialized) units, i.e. a downdraft fixed-bed gasifier and a 5 kWe SOFC
CHP system, have been employed to study the fundamentals for system integrations.
As previously mentioned, syngas must be pretreated to reduce the trace species to a level that is tolerable by SOFCs. With different gas treatments, the overall system performance would vary, and thus the influence of the gas treatment approaches on the system performance must be understood. Among these trace species, tar is a major component. The effect of tars on the performance of SOFCs is not yet fully understood because, even though it may become a fuel for SOFCs, it can also poison the fuel cell via carbon deposition. Furthermore, SOFC systems are, generally, currently designed for using natural gas. Because SOFC systems are very sensitive to fuel types, it is necessary to fully understand the system response when switching the fuel from natural gas to biosyngas in order to have a better controllability for future experiments.
The scope of this thesis is limited to the aforementioned issues. The objective of this thesis is conducting fundamental studies toward creating a safe and efficient system integration. The aforementioned two lab scale units were employed for this study.The utilized approach, however, could be further adopted for large scale power plants based on biomass gasifiers and SOFCs.
Figure 1-2 depicts the main structure of this thesis, which has 8 chapters as follows:
Chapter 2 presents the evaluation of technologies involved in biomass-powered SOFCs. This overview contains four sections. The first section describes biomass gasification. The second analyzes gas cleaning technologies that are available for the reduction of trace species’ content. The third section presents an overview of SOFCs from electrodes to systems. Biomass-powered fuel cell energy conversion systems are described in the fourth section.
Chapter 3 presents a thermodynamic study regarding the influence of cleaning technology on the energetic and exergetic performance of the integrated gasifier−SOFC system with various system configurations.
Chapter 4 presents an analysis of the anode recirculation behavior and its effects on the performance of the SOFC system when operating with natural gas and biosyngas.
Chapter 5&6 describes the tar-containing biosyngas used in SOFCs, focusing on the analysis of SOFC response to fuels containing tar and carbon
5 deposition mitigation by controlling operational conditions. SOFCs with Ni– YSZ anode running at different current and steam levels are experimentally presented in chapter 5, whereas SOFCs with Ni–GDC anodes running at
different temperature levels and under both dry and wet conditions are presented in chapter 6.
Chapter 7 presents a numerical study focused on the evaluation of kinetic models for methane steam reforming for an SOFC that operates with multiple fuels. Three frequently used kinetic models were selected in order to examine their influence on the SOFC performance, and the thermo-electro-chemical behavior of the SOFC when operating with multiple fuels was analyzed.
Finally, chapter 8 draws the conclusions based on the studies presented in
chapter 1–7. Furthermore, some recommendations for future research are suggested.
Figure 1-2 The main structure of this thesis
1.4 Reference
[1] Renewables 2010 global status report. 2010.
[2] Kessel DG. Global warming —facts, assessment, countermeasures. Journal of Petroleum Science and Engineering. 2000;26:157-68.
[3] McKendry P. Energy production from biomass (part 1): overview of biomass. Bioresource Technology. 2002;83:37-46.
[4] Goldemberg J. World energy assessment: Energy and the challenge of sustainability. UNDP. 2000.
[5] Toonssen R. Sustainable power from biomass:comparison of technologies for centralized or decentralized fuel cell systems.PhD thesis, Delft: Delft university of technology; 2010.
6
[6] Hasler P, Nussbaumer T. Gas cleaning for IC engine applications from fixed bed biomass gasification. Biomass and Bioenergy. 1999;16:385-95.
[7] Aravind PV, Ouweltjes JP, Woudstra N, Rietveld G. Impact of biomass-derived contaminants on SOFCs with Ni/Gadolinia-doped ceria anodes. Electrochemical and Solid State Letters. 2008;11:B24-B28.
[8] Fuel cell handbook. EG&G Technical services; 2004. p. 1-49.
[9] Aravind PV, Woudstra T, Woudstra N, Spliethoff H. Thermodynamic evaluation of small-scale systems with biomass gasifiers, solid oxide fuel cells with Ni/GDC
7
Background and technology
2.
This chap ter p resents the backgrou nd and technologies involved in biom ass-p ow ered SOFCs. The review begins by briefly su m m arizing conventional biom ass gasification technologies. Follow ing that, d etails are ind icated for the p erform ances of SOFCs affected by the trace sp ecies and available cleaning technologies. The com bination of biom ass gasifiers w ith fu el cells is then review ed w ith an em phasis on the d evelop m ent of SOFC technology and the stu d y of integrations betw een biom ass gasifiers and SOFCs.
8
2.1 Biomass gasification
As a thermo-chemical process (Figure 1-1), biomass gasification is able to convert solid biomass to a gaseous energy carrier for downstream uses. In the gasification process, biomass is successively heated, dried, and pyrolyzed to produce gases and char [1]. These products react further when combined with a gasifiying agent, thereby producing syngas. This syngas is often referred to as biosyngas as it is obtained from biomass. A schematic view of the gasification process is presented in Figure 2-1 [2] and the major reactions involved in the gasification process are tabulated in Table 2-1 [3].
Figure 2-1 A schematic view of gasification process
The gasifying agent utilized for gasification includes air, pure oxygen, steam, or a mixture of them. Biosyngas produced from air blown gasifiers typically has a low heating value between 4000 and 6000 kJ m−3 [2] due to the dilution of the nitrogen, whereas syngas could have a heating value between 10000 and 20000 kJ m−3 [2] from oxygen- and steam-based gasifiers due to the elevated concentration of hydrogen and carbon monoxide. Generally, hydrogen, carbon monoxide, methane, carbon dioxide, water vapor, and nitrogen (when air is used as gasifying agent) are the major components of biosyngas. Additionally, it also contains trace species such as particulates, tars, alkali metals, ammonia, HCl, HCN, H2S, and COS, among others.
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Table 2-1 Major reactions in gasification processes
Name Stoichiometry ΔH0 (kJ mol−1)
Devolatilization
2
2 2 4 2 2
Biomass
...
char tar H O light gas
CO CO H CH C N >0 Char combustion Partial combustion C0.5O2CO −111 Complete combustion CO2 CO2 −394 Char gasification Boudouard reaction CCO2 2CO +173 Steam gasification CH O2 COH2 +131 Hydrogen gasification C2H2 CH4 −75
Homogeneous volatile oxidation Carbon monoxide oxidation 2 2 0.5 CO O CO −283 Hydrogen oxidation H2 0.5O2H O2 −242 Methane oxidation CH4 0.5O2 CO 2H2 −283
Water–gas shift reaction COH O2 CO2 H2 −41
Tar reactions (tar represented by CmHn)
Partial oxidation C Hm nn/ 2O2 n2H2mCO
+(200−300)
Dry reforming 2 2 2
2
m n n
C H mCO H mCO
Steam reforming C Hm nnH O2
mn2
H2mCOThermal cracking (mp C H) m n mCm p Hn q (mqpn) / 2H2
A variety of gasifiers are available for biomass gasification. Conventional types include fixed beds and fluidized beds.
2.1.1 Fixed bed gasification
The typical fixed bed gasifiers are partly comprised of a grate that supports the bed material and maintains a stationary reaction zone. They are relatively easy to design and operate and are useful for small to medium scale power and thermal energy uses (<a few MWs). Updraft, downdraft, and crossdraft
10
gasifiers are the three types of fixed bed gasifier. A schematic view of them is depicted in Figure 2-2 [4].
(a) (b) (c)
Figure 2-2[4] A schematic view of (a) updraft, (b) downdraft, (c) crossdraft fixed-bed gasifier
With updraft gasifiers, the fuel is fed into the top of the gasifier while the gasifying agent enters at the bottom and is drawn up through the fuel. The oxidation zone in updraft gasifiers contains a higher temperature of 1000 °C [5] due to the combustion process. The produced gas is cooled to 750 °C [5] when passing through the reduction zone. Finally, the gas passes through the pyrolysis zone, dries the incoming wet biomass and exits the reactor at a relatively low temperature of approximately 500 °C [1, 5]. Higher tar content (1–150 g Nm−3 ) is present in the produced gas as a consequence of the tars forming in the pyrolysis zone and being carried upward and exiting through the reactor along with the produced gas. Particulate levels in the raw gas are typically lower (0.1–3 g Nm−3 ) because of the non-turbulent conditions [6]. Ash is moved along with the solids in the opposite direction of the gas flow and is eventually withdrawn from the bottom of the gasifier. The updraft gasifier is able to handle biomass fuels with high contents of ash (up to 15 %) and moisture (up to 50 %) [7].
With downdraft gasifiers, the gasifying agent is introduced into the combustion zone. The pyrolysis products flow through the oxidizing zone which is able to crack the tars with a high operating temperature, resulting in a lower tar content (0.04–6 g Nm−3
). The raw product gas then exits the gasifier with a high temperature of 800 °C. Fewer particulate contents (0.01–10 g Nm−3 ) are present in the produced gas due to the absence of turbulence in the gasifier, but the gas may contain alkali-containing compounds as a result of passing through the hot reaction zone [6].
With crossdraft gasifiers, the fuel is fed from the top and the air is injected through a nozzle inthe side (Figure 2-2(c)). Unlike the downdraft and updraft types, the product exits from the side wall opposite from the air injection point. This process is primarily used for gasification of charcoal or pyrolyzed fuels in
11 small units and is less suitable for high-ash or high-tar fuels. This type of gasifier possesses a lower power capacity (<10 kWe) due to the small
reduction zone but features a quicker response time and a shorter startup time (5–10 minutes) compared to the updraft and downdraft gasifiers. However, crossdraft gasifiers are not used quite often as other gasifier designs offer more flexibility and better performance across fuel types.
2.1.2 Fluidized gasification
In a fluidized bed gasifier, inert materials and solid fuels are fluidized with the assistance of a gas that is distributed from below the bed. The gas, typically air, oxygen, and/or steam acts as the fluidizing medium and provides the oxidant for combustion and tar cracking. The fluidized bed gasifiers could be classified as bubbling fluidized beds (BFB) and circulating fluidized beds (CFB). The main difference between them is that, in the BFBG, the fluidizing velocity is approximately 2–3 m s-1, whereas it is 5–10 m s-1
in the CFB gasifiers [5]. The lower velocity in the BFB constrains the bed material in the lower section of the reactor in order to fluidize while maintaining the bed materials and chars in the reactor.
(a) (b)
Figure 2-3 Typical fluidized bed gasifier (a) bubbling (b) circulating
In contrast, the entire CFB is involved in the fluidization of the bed materials, and a fraction of sand and char could be circulated with the assistance of the gas stream and a cyclone. A schematic view of them is indicated in Figure 2-3 [5]. The exiting gas from fluidized bed gasifiers typically contains a high concentration of particulates consisting of both ashes originating from biomass and fine particulates of the bed material (such as coase sand, limestone) due to the turbulence from the cyclones in the gasifier. As the gas is hot, it may also contain vaporized alkali salts. The cyclones, therefore, are usually an integral part of the fluidized bed gasifiers that separate these coarse particulate matters from the gas stream. Typical
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fluidized bed gasifiers produce lower tar concentration than the fixed-bed updraft type but higher than the fixed-bed downdraft type [2]. The content of particulates and tars generated from different types of gasifiers is shown in
Table 2-2 [6].
2.1.3 Other gasification technologies
In addition to the general gasifiers described above, other types of gasifiers that have also been proposed, designed, and tested for gasification. For instance, the entrained flow gasifier is frequently employed for coal gasification; however, for a high carbon conversion, it requires a relatively significant amount of oxidant with an elevated operating temperature (>1200 °C) and pressure (> 20 bar) [5]. Unique gasifiers, e.g. the gasifier formed by the combination of a fluidized bed and an entrained flow gasifier [8] are also available. Many other designs such as molten salt gasifiers [9] and supercritical fluid gasifiers [10] have also been investigated. Additionally, other emerging gasification technologies such as plasma gasifiers and heat pipe gasifiers have also attracted much attention, but these designs are more reactor-specific and not widely applied; thus, they are not further detailed in this work.
Table 2-2 Particulate and tar contents from different biomass gasifiers Gasifier
type
Particulate loading (g Nm−3 ) Tar loading(g Nm−3 )
Low High Representative
range Min. Max.
Representative range Downdraft 0.01 10 0.1–0.2 0.04 6 0.1–0.2 Updraft 0.1 3 0.1–1 1 150 20–100 FB 1 100 2–20 <0.1 23 1–15 CFB 8 100 10–35 <1 30 1–15
In summary, each gasifier type demonstrates individual features toward power capacity, gas composition, component type, and performance. In general, fixed beds are favored for electricity generation on a small scale (a few hundrend kWe [11]), whereas FB gasifiers, especially CFB gasifiers are
favorable for a large scale biomass gasification.
2.2 Gas cleaning technology
Gas cleaning is important for the commercialization of advanced energy conversion systems based on biomass gasification as pointed out by Maniatis [12]. Downstream applications dictate different requirments on fuel quality. Generally, Fischer-Tropsch and advanced power generation processes require a more stringent fuel quality. As this work is limited to the development of gasifier−SOFC systems, the review of cleaning technologies focuses mainly on the treatment of biosyngas for the use in SOFCs. However, it should be noted that these technologies, in fact, are initially applied to other applications. Hence, the review cannot be completely excluded from the use
13 of biosyngas in other processes. For the use of raw biosyngas in SOFCs, the gas cleaning systems may require dealing with five primary compounds which are particulates, alkali compounds, tars, chlorides, sulfur, and nitrogen-containing components. The impacts of the impurities on SOFCs and available technologies for the biosyngas cleanup are detailed below.
2.2.1 Particulates
Particulates within the biosyngas generally include inorganic material derived from mineral matters in the biomass feedstock, unconverted biomass in the form of char, and possible bed materials [6]. Particulates, although unavoidable in biomass gas streams, are undesirable as they can abrade and damage downstream equipment; additionally, they are harmful for the environment [6]. Information in open literature regarding the impact of particulates on the operation of SOFCs with biosyngas is very limited. Hofmann et al. [13] reported the effect of particulates from biomass gasifiers on SOFCs with Ni–GDC anodes. Even with a very brief test, as reported, their post-mortem with SEM (Scanning Electron Microscope) revealed that particulates up to ~10 μm in diameter deposited on the surface of the anode were observed to decrease the performance of the SOFC during the measurement. At present, it is not clear what the minimum concentration of particulates is that SOFCs are able to tolerate. However, it is certain that particulates must be reduced to the lowest possible levels, even down to a few ppmw levels, for smooth and long-term SOFC operation. This estimation requires experimental confirmations in the future.
Cyclones, wet scrubbers, filters, and electrostatic precipitators have been commonly employed either in a single stage or in multi-stages for particulate removal.
Cyclone
Cyclones are considered to be very powerful and inexpensive for initial gas cleanup, and they are especially effective in removing larger particulates. They are able to remove more than 90 % of particulates with diameter larger than 5 µm at a minimal pressure drop of 1000 Pa [6]. Partial removal of the particulates in the 1−5 µm range is also possible, but the efficiency of removal rapidly deteriorates for the particulates with a diameter smaller than 10 μm [14]. Conventional cyclones process at a lower collection efficiency for the smaller particulates (submicrons), so approaches to increase efficiency include improving the design and/or using multi-cyclones. Alternative cyclones which promote partial recirculation can achieve high removal efficiences comparable with the bag filter [15, 16]. Lee et al. [17] have tested a novel cyclone to remove the smaller particulates. This novel design cyclone is the combination of a cyclone with a swirl scrubber and assists in the removal of particulates with the diameter smaller than 2.5 µm. The removal efficiency
14
was more than 85 %, which is 15 % higher than the conventional high efficiency cyclone [18].
Scrubber
In a wet scrubber, particulates are removed by capturing the solid in droplets of a spray liquid [19]. The scrubbers’ removal efficiency is 70–99 %, which is higher than that of cyclones, and they are able to remove the particulates with a diameter larger than 0.5 μm [20]. The collection efficiencies of lab scale scrubbers [21] are dependent on many variables including particulate size distribution, gas velocity, spray liquid rate, and the type of injected liquid.
Venturi and packed bed scrubbers are widely studied for particulates removal as their removal efficiencies are greater than 98 % [22-24]. The venturi scrubber for particulates removal has been reported on many studies [25-27]. In general, they are effective when the particulate diameter is larger than 2.5 µm. Spray scrubbers are more commonly used to remove acid compounds, but can also be used to remove particulates [23]. Spray scrubbers with a novel design are very effective for particulates removal. Mohan and Meikap [28] reported on a novel spray-bubble scrubber that is able to remove particulates in the range of 1–500 µm with the efficiency of 75–99 %. Mohan et al. [29] employed a counter current spray column utilizing water as the scrubbing liquid to remove the particulates of 2–200 µm and discovered that efficiency is able to increase up to 94 %. The water electrostatic scrubber (WES) is an alternative for the removal of the particulates in submicron diameters. Carotenut et al. [30] developed a model and reported that removal efficiency was approximately 99.5 % for particulates with the diameter from 100 nm to 5 µm.
Filter
The barrier filters contain a range of porous materials that allow gases to pass through while preventing the passage of particulates. These filters effectively remove particulates with a smaller diameter (0.5–100 μm) from gas streams [31]. Examples of barrier filters include bag filters (fabric filters- baghouses), rigid barrier filters (ceramic and metal material), and packed bed filters [31-33]. Major fiter types and their fabrication resources are demonstrated in Table 2-3 [14]. Depending on several factors such as gas filtration velocity, particulates characteristics, and cleaning mechanisms [18], their efficiency varies from 95 to 99.9 %.
Table 2-3 Major types of filters
Type Fabrication resource
Bag filters fabric fiber materials, textile, plastics, ceramic
Rigid barrier filters metal or sintered ceramic, powder or fibers
15 The bag filters have been used widely on an industrial scale since the 1970s and have an almost 100 % efficiency in removing those particulates larger than 1 µm [32, 34, 35]. For the removal of the particulates with the diameter less than 1 µm, the pressure drop within such filters is prohibitively high [36]. The bag filters are constructed of porous fabrics that remove fine solid particulates from gas streams [19]. A variety of fibers (cotton, paper, nylon, dacron, glass, graphite, etc.) are available, but which one is better suited depends on the nature of the gas, particulate matter, temperature, and pressure of the gas, among others [37]. Generally, the bag filters operate at a temperature between 120 to 180 °C but can also effectuate at a high temperature [19].
The rigid filters are used commonly at high temperatures [38] to obtain quality performance, but it is also possible to use them at a low temperature (<450 oC) [35]. Rigid filters including ceramic (monolithic, composite) and metallic (iron aluminide, hastelloy-x sintered, haynes sintered fiber alloy) are available. The removal efficiency of rigid filters is approximately 100 % with the particulates loadings less than 0.1 ppm at the gas outlet [35, 39]. The ceramic filters are commercially available for the application of gasification. Candle filters ordinarily remove particulates from the raw syngas at temperatures between 300 and 500 °C.
The granular bed filters (or packed bed filters) are usually cylindrical vessels filled with granular material such as sand or granular adsorbent in a relatively small diameter. Particulates deposit on the bulk bed when the gas flows through [40]. The bed could be one of three different types including fixed bed, moving bed, and fluidized bed. A major disadvantage of the conventional fixed bed technology is the gas flow must be stopped for cleaning. Fluidized beds are able to operate continuously, but they are less effective in removing small particulates than the fixed and moving beds [41]. In general, the moving bed filters are often used at a high temperature [42].
Table 2-4 An overview of different systems for particulates removal
Technique Efficiency (%) Pressure drop (Pa) Operating temperature (°C) Power consumption Cyclone 90–99 500–2000 <1200 Low
Rigid filter >99.8 20000 <1000 Low
Bag filter >99 20000 <250 Low
Packed bed filter 99 500–1500 <1000 Low
Scrubber 95 200–20000 <100 0.2–6 kWh/1000 m3
ESP >99 498–2491 <1200 Low
Electrostatic precipitators (ESP)
ESPs perform by creating a high voltage electrical field around the discharge electrodes to ionize the flowing gases and the particulates. The
16
charged particulates deposit on the neutral collecting plates from which they are then dislodged [43]. ESPs are highly efficient filtration devices (>99 %) that easily remove the fine particulates from the gas stream. In contrast to wet scrubbers which apply energy directly to the flowing stream, an ESP applies energy only to the particulate matters being collected and, therefore, is very energy-efficient. The cost and operational costs for these devices is a comparatively expensive investment, making them less attractive for small-scale CHP applications.
An overview of particulate removal processes is presented in Table 2-4
[44-46].
2.2.2 Alkali Compounds
Biomass feedstock may contain significant amounts of alkali salts, particularly potassium and sodium. The normal biomass gasification temperature can be higher than the temperature required for vaporizing alkali salts (700 °C or higher [6, 14]); thus, vaporized alkali salts may be present in the syngas exiting from the gasifier. The alkali release is often correlated with the chlorine in the solid fuel [40], and KCl content appears as the highest among the alkali release [47].The total vapor-phase alkalis measured after a cyclone but before gas coolers was in the range of 1–10 ppm [48]. Nurk et al. [49] reported that 6 ppm KCl in the feeding stream could decrease the performance of SOFCs, indicating that the alkali content that SOFCs is able to tolerate may be less than a few ppm.
Alkali could be separated by physical adsorption at low temperatures (<600 °C) or chemical reactions at high temperatures (>600 °C). For the removal at a low temperature, the aforementioned technologies are generally applied for the removal of particulates. The high temperature process uses fixed bed systems with selective sorbents such as quartz, aluminosilicates, clay, bauxite, kaolinite, emathlite, and diatomaceous earth which are essentially composed of Al2O3 and SiO2 [40, 50, 51], but it is under
development as it has many instabilities [52] such as undesired HCl being released during the process. An economic approach to remove the alkali is first by condensation/reaction with the particulates present in the gas stream, and then they are removed together by the facilities for particulates removal. However, the coldest component in this hot gas path is the heat exchanger that cools the gases prior to entering into the filter. The information available for alkali and trace metal species all indicate that lowering the filter temperature reduces the amount of vapor phase species occupying the remaining fuel gas stream [53]. Unfortunately, filter operating temperatures cannot just be lowered to the levels required to remove most of these vapor species as the lower limits for filter operation are dictated by other gasification processes factors, e.g. tars must remain in the vapor phase to avoid blocking the filters.
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2.2.3 Tars
Tar is a complex amalgamation of condensable hydrocarbons including one-ring to 5-ring aromatic compounds, other oxygen-containing hydrocarbons, and complex polyaromatic hydrocarbons (PAH). It could be defined as an organic compound with a molecular weight larger than benzene [54]. Tar is generally undesirable as it may lead to condensation in pipes, heat exchangers, and particulate filters, resulting in choking and attrition. These problems can diminish system efficiency and increase overall cost [55]. The allowable limit for tar is highly dependent on process types and the end user application. For instance, tars could be directly fed as a fuel for burners, whereas the preferable tar and dust loads for engines is less than 10 mg Nm−3 [56]. Under SOFC conditions, the effects of tars on SOFC performance are not yet understood.
Tar may become a fuel for SOFCs without causing solid damages [57], however, such an application is highly associated with certain key factors including anode types, operating conditions, and tar compounds. Previously reported scientific results indicate that the level of tar is most likely required to be decreased to a few ppm for SOFC stacks for long-term operation. There are indications that even larger quantities of tar may not affect SOFC performance and even that they may be reformed at the SOFC anodes. Further detailed research is required to solidly confirm this and for selecting suitable SOFC operating conditions for long-term operation. Chapter 4 and 5
study the impacts of tar on the performance of SOFCs with Ni/YSZ and Ni/GDC anodes, respectively.
Table 2-5 Influence of operating conditions on tar content reduction
Operating conditions Tar concentration (g Nm−3 ) Source
Temperature 15 (T=700 °C) 0.54 (T=800 °C) Li et al. [58]
19 (T=700 °C) 5 (T=800 °C) Narváez et al.[59]
ER 7.2 (ER=0.37) 2.0 (ER=0.26) Narváez et al. [59]
Pressure 4.3 (P=8 bar) 3.3 (P=21.4 bar) Knight [60]
● Removal inside a gasifier
Table 2-6 Tar removal efficiency of wet scrubbers in biomass gasification systems
Technology Tar removal efficiency
Spray tower 11–25 % heavy Tars 40– 60 % PAH 0–60 % phenolics 29 % heavy Tars Venturi scrubber 50–90 %
Venturi and spray scrubber 83–99 % condensable material
Venturi plus cyclonic demister 93–99 % condensable organics
