INTEGRATION OF SOLID OXIDE FUEL CELLS WITH BIOMASS GASIFIERS

Transcription

1 ECN-RX INTEGRATION OF SOLID OXIDE FUEL CELLS WITH BIOMASS GASIFIERS P.V. Aravind N. Woudstra JP Ouweltjes J Andries W de Jong G Rietveld H Spliethoff Published in Proceedings 2nd World Conference on Biomass for Energy, Industry and Climate Protection, (2004), MARCH 2005

2 INTEGRATION OF SOLID OXIDE FUEL CELLS WITH BIOMASS GASIFIERS PV Aravind 1, N Woudstra 1, J P Ouweltjes 2, J Andries 1, W de Jong 1, G Rietveld 2, H Spliethoff 1 1. Section Energy Technology, TU Delft, 2628 CD Delft, The Netherlands 2. Energy research Center of the Netherlands ABSTRACT: Biosyngas from biomass gasifiers contains hydrogen, carbon monoxide, carbon dioxide, nitrogen, methane etc in addition to the various contaminants. This gas could be efficiently converted in solid oxide fuel cells. Information on the levels of contaminants in the fuel gas as well as the sensitivity of SOFC for all contaminants that will be found in the fuel gas is required for designing suitable cleaning systems. This paper presents a brief description of technologies related to such systems and the ongoing activities at University Delft in this topic in co-operation with ECN. Initial results are also presented. KEYWORDS. Biomass Gasifier, Solid oxide fuel cell, System Integration 1 INTRODUCTION World energy demand is growing and fossil fuels are providing the lion share of the energy required today. But they are not an infinite source of energy, which can meet the whole energy requirements of the future. The solar energy received by earth is sufficient to meet all the energy requirement of the mankind. But economically viable technologies for its conversion to electricity are not expected to emerge in the near future. A possible strategy to overcome the dual problems of energy shortage and environmental degradation is to utilize more efficient methods for conversion of the present primary energy sources. Fuel cells are such equipments with high conversion efficiency from chemical energy of the fuel to electricity. One variety of them, solid oxide fuel cells, besides giving better efficiency for conversion from chemical energy of the fuel to electrical energy, also provides thermal energy at high temperature. This heat energy could be converted into electrical energy using conventional technologies like gas turbines. Solid oxide fuel cells use hydrogen and carbon monoxide as fuels. Biomass gasification is a potential technology for generating low calorific value gas from biomass. The gas produced contains carbon monoxide and hydrogen, which may be used as a fuel for the solid oxide fuel cells if cleaned from the contaminants. Moreover biomass offers the advantage of being available for decentralized energy generation, which is expected to attain importance in the future. Several studies have been attempted on systems with SOFCs and biomass gasifiers, but most of them focus on system studies or feasibility studies with very few involving experimental studies [1,2,3,4,5,6,7]. 2 SOLID OXIDE FUEL CELLS. The solid oxide fuel cell (SOFC) has mainly three parts namely anode, electrolyte and cathode. The fuel enters the anode chamber and get oxidized. Oxygen enters the cathode chamber and gets transported through the electrolyte to the anode. The porous anode serves to provide electrochemical reaction sites for oxidation of the fuel, allow the fuel and byproducts to be delivered and removed from the surface sites, and to provide a path for electrons to be transported from the reaction sites to the interconnect in SOFC stacks. Anode should allow rapid transport of fuel and reactant gases. They must be fuel flexible, easy to fabricate, and of low cost for a wide range of commercial applications. The cathode of the fuel cell distributes the oxygen fed to it onto its surface and conducts the electrons. These electrons are needed for dissociation of oxygen molecules to oxygen ions, which pass across the electrolyte to the anode. The electrolyte prevents the two electrodes from coming into electronic contact by blocking the electrons and allows the flow of oxygen ions to maintain the overall electrical charge balance. The useful voltage output (V) under load conditions, that is, when a current passes through the cell, is given by V=E-IR- c- a Where E is the Nernst potential, I is the current passing through the cell, R the electrical resistance of the cell, and η c and η a the polarization losses associated with the cathode and anode, respectively. As it is the anode, which is affected by changing fuel properties, and to certain extend cathode (it depends: other fuels could result in other current distributions and temperature gradients) and electrolyte are immune to such changes, it is the anode of the cell which has to be mainly evaluated for operating with different fuels. 3 SOFC ANODE The electrical resistance of the anode is comprised 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 electrolyte or interconnect and is generally not affected by fuel variations. Concentration polarization is related to the transport of gaseous species through the porous electrodes. The 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 a critical parameter. 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 anode of SOFC is mainly limited to the physical triplephase boundaries (electrolyte/anode/fuel), if the anode exhibits solely electronic conduction. Nickel/Yttria Stabilized Zirconia (YSZ) is an example for such anodes. They are the most common

3 anode material for SOFC applications currently. 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 may increase ERZ due to its ionic conduction. In contrast, the use of anode materials which show mixed ionic and electronic conduction allows electrochemical reactions to take place at regions other than triple phase boundaries. This can lead to a significant drop in the activation polarization and yield improvement in electrical efficiency. Doped Ceria anodes come to this class. Doping Ceria with elements like Gadolinium and Samarium 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 as not enough to get required performance. A wide range of stable electronic conductors is being studied for this. 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 based anode. Catalytic activity of Nickel and mixed conductivity of doped ceria offers an excellent combination. Oxides with Perovskite structure are said to be attractive because of lower sulfur contamination and carbon deposition. High electrical conductivity of copper has tempted many studies on anodes containing copper. They are said to have of advantage when operated with hydrocarbon fuels. Low catalytic activity of copper reduces carbon deposition. They are used along with YSZ or ceria. Even though it seems that copper based anodes have advantages with low carbon deposition, their sintering at high temperature make them useful only at rather lower temperature operation around 600 o C. Anodes with these and other new materials are in developing stage and are yet be proven for reliable operation. State of the art cells are supported on electrolyte on which anode and cathode are applied. Anode supported cells are also being developed. Such anodes are preferred for low temperature operation as thin electrolyte helps to reduce the ohmic losses that can occur at low temperatures. Selection of proper anode and their operation parameters like working temperature will be a critical issue when SOFCs for operation with different fuels like biosyngas are considered. Nickel supported on Ceria is considered as the choice for present experiments. Anodes without Nickel will be studied in future. 4 BIOSYNGAS Gasification produces a combustible gas carrying chemical energy from the biomass by partial oxidation. Gasification technology employed determines quality and properties of the gas generated in the biomass gasifier. Air is the commonly used gasifying agent. 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. An example for the gas composition for the gas from a fixed bed down draft gasifier using air is 20% H 2, 20% CO, 12% CO 2, 2 % CH 4, 2.5 % H 2 O and the rest N 2 [9]. An example for gas from oxygen gasification is 32% H 2, 48% CO, 15% CO 2, 2% CH 4 and 3% N 2 [10] while that of the gas from steam gasification is approximately 38% CO, 35% H 2, 12% CO 2, 10% CH 4 and 5% other hydrocarbons [11]. Air gasification gives a gas with an HHV of 4 6 MJ/Nm 3. Gasification with oxygen gives a gas with an HHV of MJ/Nm 3 and with steam gives a range of MJ/Nm 3 [10]. A typical value of contaminant level in producer gas from a fluidized bed gasifier after two cyclones was 4000 ppm of NH 3, ppm of H 2 S, 5-20 g/nm 3 of tar, 5-30g /Nm 3 particles and 1 ppm wt alkali metals [12]. The gas also will contain few ppm halides [13]. There are different downstream cleaning methods either available or being developed to remove these contaminants further from the gas. As of now it is expected that tars and particulates can be cleaned to a few ppm level [10,14], acid gases and alkali compounds can be cleaned to sub ppm level [15,16] and H 2 S can be cleaned to around 1 ppm [5]. 5 SOFC OPERATION WITH BIOSYNGAS For realizing SOFC operation with biosyngas, there are many critical issues to be addressed. Most important will be defining the tolerance limits for various contaminants. Of the contaminants in biosyngas, it is known that NH 3 is a fuel [17] for SOFC and H 2 S affects performance when it is around 1 ppm or more [18]. No hard data is available for tolerance levels for other contaminants. Different anode materials will have different kinds of interactions with these contaminants and hence their tolerance levels for these contaminants may vary considerably. Their effectiveness in converting fuel into electricity also will be different. Hence proper selection of the suitable anodes and gas cleaning systems is very important in development of feasible and efficient designs of biomass gasifier-sofc systems. Since different gasification methods give different outlet gas compositions, selection of suitable fuel gas composition for SOFC operation will also be important. Hydrogen is the most promising fuel for SOFC. CO and methane are also proven as fuels. N 2 is expected to be inert. H 2 O and probably also CO 2 will help the shift reaction, and CH 4 will get reformed inside the cell. Fuels containing carbon can cause carbon deposition in certain circumstances. A careful control of anode gas composition in which the C-H-O equilibrium, which does not favor deposition, may be enough to avoid this problem. Weber et al [19] have reported that Ni/YSZ anodes can run with gas mixture of H 2, CO and CH 4. Complete CO feeding causes carbon deposition and may degrade the cell. Methane didn t cause problems when under load but had caused carbon deposition at open circuit condition. This may be because lack of oxygen ions available at anode under Open Circuit. Zhu et al has studied the performance of salt oxide composite anodes with biosyngas at intermediate temperatures and has measured the current voltage characteristics [6]. They have indicated reasonable performance of the cell except

4 when there is a presence of more than 20% nitrogen. Baron et al has studied intermediate temperature SOFCs with Ni/GDC anodes for operation with biosyngas. They have reported higher anodic impedance when CO replaced hydrogen in the fuel stream [7]. Solid Oxide Fuel Cells have an efficiency of around 50 % from chemical energy of fuel to electricity. Since the fuel cell doesn t convert all the chemical energy of the inlet fuel, the unreacted hot flue gas coming out of the cell can be combusted. If the cell is operated at high pressures, this hot gas can be passed through a turbine for extracting mechanical energy, which could be converted to electrical energy with a generator. It has been theoretically shown that these systems can achieve electrical efficiencies as high as 70-80% [5]. When such systems are used in combination with biomass gasifiers, they offer a choice for sustainable and highly efficient energy systems. 6 PRESENT STUDIES The present study focus on understanding the effect of the biosyngas and contaminants present in it on SOFC anodes and developing concepts for design of high efficiency systems with solid oxide fuel cells and biomass gasifiers. Studies on anodes include Chemical equilibrium analysis of the interaction between contaminated fuel and SOFC anode. Electrochemical measurements on SOFCs are being carried out with different components of biosyngas including contaminants. The knowledge developed will be used for the development of a gas cleaning system for connecting SOFCs to gasifiers. Test cells will be tested connected to a circulating fluidisedfluidized bed gasifier which is being commissioned in the laboratory with the developed cleaning system attached. System studies will be carried out leading to selection of suitable system components for energy systems with SOFCs, gasifiers and gas turbines and development of concepts for reaching higher efficiencies with systems. Results from the studies on anodes and design of gas cleaning systems will be used in system calculations. In-house software CYCLE TEMPO will be used for thermodynamic evaluations. 6.1 Electrochemical characterization There are two main electrochemical methods for evaluation of the performance of SOFCs. Measurements of Current-Voltage characteristics and impedance measurements. I-V measurements give the variation of the voltage when current is varied. They can give the reasonable voltage or current range over which the cell can operate with optimum efficiency. Under realistic fuel utilization values they will show the operating current or voltage ranges where the each of three important losses in the cell, namely activation polarization, Ohmic losses and concentration polarization are dominant. Impedance of a cell indicates the resistance to current flow when an ac voltage is introduced. This resistance is in fact contributed by different factors. These include resistance for the electronic and ionic flows, charge transfer resistance and diffusion resistances etc. Many of these phenomena cause capacitance as well. It has been shown that many of these processes, which contribute to total impedance, become prominent at different frequencies of the induced voltage. Hence by applying a varying frequency signal and by analyzing the observed impedance it will be possible to understand the processes going on in the cell. When testing the anode performance with fuel gas carrying contaminants, the variation in spectra may indicate what specific changes do occur in the anode processes because of the contaminant influence. Hence results obtained could probably indicate the ways to overcome the problems due to such influences with more clarity when compared to I-V curve measurements. Hence it is decided to carry out impedance measurements for finding out the influence of biosyngas and the contaminants present in it on anode performance. 6.2 Preliminary results Impedance measurements were carried out on the symmetric samples with anode on both sides of electrolytes. The anode consists of the following three layers: an adherence layer of Gd 0.4 Ce 0.6 O mol% Co adjacent to electrolyte, a functional layer of NiO / Gd 0.1 Ce 0.9 O /35 weight percent, and finally a contact layer of NiO. The anodes were 22 mm in diameter and 35 micron in thickness and are printed to micron thick 3YSZ electrolytes with 25 mm diameter and sintered at 1200 o C. They are supported on ceramic supports in a single gas chamber through which the fuel gas is passed. A gas mixing station has been fabricated with provisions for mixing hydrogen, carbon monoxide, carbon dioxide, nitrogen, methane, and various contaminants. The gas is humidified in a temperature-controlled two chamber humidifier. Experiments were carried out at four different temperatures-1023 K, 1073 K, 1123 K, and 1173 K. The fuel was humidified at 30 o C. CO 2 was never passed through humidifier and when it was added to the gas mix, the humidifier temperature was adjusted to keep the humidity of total gas mixture same as with other mixtures. Impedance measurements were carried out in the frequency range of 100 khz to 0.1 Hz with humidified hydrogen and a gas mixture with 20% CO, 20% H2, 10 % CO and 50% N2 under zero bias. Impedance spectra obtained is given in Figure 1 after correction for series resistance. Z Imag (Ohm) Z Real (Ohm) Hydrogen 1173 K Hydrogen 1123 K Hydrogen 1073 K Hydrogen 1023 K Biosyngas 1173 K Biosyngas 1123 K Biosyngas 1073 K Biosyngas 1023 K Fig 1. Impedance plots with humidified hydrogen and biosyngas at different temperatures. Electrode area 3,8 cm 2. Y axis of the impedance plots gives the imaginary part of the impedance and the X axis gives the real part of the impedance. A rather simple way of interpreting the impedance spectra is to take the intercepts on the X-axis.

5 In the spectra given in figure, the extreme right intercept gives the low frequency values and extreme left values give the high frequency intercept. It appeared that the anodic impedance increased with the lowering temperature and with diluted gas representing the biosyngas composition. At 1173 K the anodic polarization was approximately 0.9 ohm with humidified hydrogen whereas with a gas composition of 20% H2, 20 % CO, 10 % CO 2 and 50% N2 it has increased to approximately 1.3 ohm. But it can also be observed that a similar variation is observed when the temperature is lowered to 1123 K from 1173 K. Hence it can be assumed that the anodic polarization varies within reasonably acceptable limits when humidified hydrogen is replaced with a humidified gas mixture representing biosyngas. Impedance measurements are continuing at present to understand the effects of each of the components of biosyngas and the contaminants present in it. More details of these measurements and analysis of results are available elsewhere [20]. 6.3 Results from other ongoing activities Preliminary system studies have been carried out for SOFC-Gas Turbine integrated systems working using biosyngas from air gasification [5]. System scale has been selected as 100 kw approximately since these small scale systems are of particular interest for decentralized electricity generation. Preliminary results indicated an efficiency of more than 60% from HHV of biosyngas to electricity. Details of the calculations are available elsewhere [5]. When the gasification model is included, total system efficiency is slightly less than or around 50%. Detailed exergy analysis of the systems is being carried out to look for possible system configurations with higher exergy efficiencies. A circulating fluidized bed gasifier is in commissioning stage and various options for high temperature and low temperature gas cleaning are being evaluated. In near future a hot gas cleaning system will be installed in our laboratory. This cleaning system is expected to comprise of a ceramic filter, a catalytic tar cracker, an alkali getter and sorbents for acid gases. This system will be used for testing SOFC test cells connected to the gasifier. 8 REFERENCES [1] Makinenen et al, 8th European conference on Biomass for energy, Environment, Agriculture and Industry, 3-5 October 1994, Vienna. [2] Kartha. S et al, Proceedings of the 3rd Biomass conference of the Americans in Montreal, Quebec, Canada on August , Vol. 2, pp [3] Barchewitz, L. and Palsson, J., 4th European SOFC Forum. Lucerne, Switzerland, July [4] B J P Burhe, MSc Thesis Technical University Delft, [5] PV Aravind, MSc Thesis, University of Oldenburg, 2001 [6] B. Zhu et al, Int. J. Energy Res. 26 (2002) [7] Sylvia Baron et al, Journal of Power Sources 126 (2004) [8] W.Z. Zhu, S.C. Deevi, Materials Science and Engineering A362 (2003) [9] G. Sridhar et al., Biomass and Bioenergy 21 (2001) [10] A. V. Bridgwater, Fuel Vol 14 No. 5, , [11] C. Franco et al., Fuel 82 (2003) [12] Draelants D J et al, High temperature Gas Cleaning, pp , Vol. 2, 1999, University of Karlsruhe [13] A. van der Drift et al. Biomass and Bioenergy 20 (2001) [14] W. de Jong et al, Biomass and Bioenergy 25 (2003) [15] I.R. Fantom, Gas cleaning at high temperatures, Blackie Academic and Professionals, 1993, [16] Gopala Krishnan and Raghubir Gupta, Contract No DE-AC21-93MC , US Deapartment of Energy, 1999 [17] A. Wojcik et al., Journal of Power Sources 118 (2003) [18] Yoshio Matsuzaki and Isamu Yasuda, Solid State Ionics, Volume 132, Issues 3-4, 2 July 2000, Pages [19] Andre Weber et al, Solid State Ionics (2002) [20] PV Aravind et al (To be published), Proceedings of the sixth European solid oxide fuel cell forum, Lucerne, CONCLUSIONS High efficiency energy systems based on biomass gasifiers and solid oxide fuel cells are potentially attractive for future electricity generation. Basic knowledge on impact of biosyngas on SOFC performance is yet to be developed. Designing gas cleaning systems meeting the requirements of solid oxide fuel cells will be another challenging issue. Preliminary system studies have indicated possibility of achieving high electrical efficiencies with such systems. Anodic impedance variation seems to be within reasonable limits. Preliminary indications obtained with our studies so far are encouraging but it is apparent that significant technical issues will have to be solved before such systems are realized.