Publisher’s version / Version de l'éditeur:
Environmental Process and Sustainable Energy, 28, 3, pp. 380-385, 2009-10
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.
https://nrc-publications.canada.ca/eng/copyright
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.
https://doi.org/10.1002/ep.10394
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Thermal modeling and simulation of an integrated solid oxide fuel cell
and charcoal gasification system
Colpan, C. Ozgur; Yoo, Yeong; Dincer, Ibrahim; Hamdullahpur, Feridun
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=3055728d-d04a-45e7-a5a5-4ec1bb370ce0 https://publications-cnrc.canada.ca/fra/voir/objet/?id=3055728d-d04a-45e7-a5a5-4ec1bb370ce0Thermal Modeling and
Simulation of an Integrated
Solid Oxide Fuel Cell and
Charcoal Gasification System
C. Ozgur Colpan,
aYeong Yoo,
bIbrahim Dincer,
cand Feridun Hamdullahpur
aa
Mechanical and Aerospace Engineering Department, Carleton University, Ottawa, Ontario, Canada;
ozgurcolpan@yahoo.com (for correspondence)
b
National Research Council of Canada, Institute for Chemical Process and Environmental Technology,
Ottawa, Ontario, Canada
c
Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, Ontario, Canada
Published online 21 August 2009 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10394
In this study we propose a novel integrated char-coal gasification and solid oxide fuel cell (SOFC) sys-tem, which is intended to produce electricity and heat simultaneously. This system mainly consists of an updraft gasifier using air and steam as the gasifi-cation agents, a planar and direct internal reforming SOFC and a low temperature gas cleanup system. The performance of this system is assessed through numerical modeling using a pre-developed and vali-dated heat transfer model of the SOFC and thermody-namic models for the rest of the components. These models are used to simulate the performance of the cell and system for a case study. In addition, a para-metric study is conducted to assess the effect of Reyn-olds number at the fuel channel inlet of the SOFC on the cell performance, e.g., fuel utilization and power density, and the system performance, e.g., electrical efficiency, exergetic efficiency, and power to heat ratio. The number of stacks is also calculated for dif-ferent Reynolds numbers to discuss the economical feasibility of the integrated system. The results show that the electrical efficiency, exergetic efficiency and power to heat ratio of this system are 33.31%, 45.72%, and 1.004, respectively, for the base case.
The parametric study points out that taking the Reyn-olds number low yields higher electrical and exergetic efficiencies for the system, but it also increases the cost of the system.Ó2009 American Institute of Chemical Engineers Environ Prog, 28: 380–385, 2009
Keywords: SOFC, fuel cell, charcoal, gasification, modeling, simulation, efficiency, Reynolds number
INTRODUCTION
There has been a greater interest on alternative fuel and energy systems recently because of the depletion of fossil fuel and increasing concerns on global warming. Among these alternative systems, fuel cells are one of the most promising energy tech-nologies of the future because of their merits such as high efficiency and low environmental impacts. There are various fuel cell types that basically differ from each other according to the type of electrolyte and fuel used, e.g., proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC), alkaline fuel cell (AFC), molten carbonate fuel cell (MCFC) and SOFC. Among these fuel cell types, SOFC and MCFC are known as the high temperature fuel cells.
The SOFC can be designed to operate between 5008C and 10008C. The high operation temperature provides several advantages to SOFC over the low Ó2009 American Institute of Chemical Engineers
temperature fuel cells such as usage of a wide range of fuel, e.g., methane, carbon monoxide, methanol, syngas, and ammonia, internal reforming of the gases, and integration to bottoming cycles, e.g., gas turbine and gasification systems. However, there are challenges with construction and durability. In addi-tion, carbon deposition and sulphur poisoning can be problems depending on the fuel type. Carbon deposition is generally avoided by adjusting the steam to carbon ratio at the inlet of the SOFC, which can be done in two ways: sending external water vapor to the SOFC or recirculating the depleted fuel gas stream to the fuel channel inlet. A well-designed gas cleanup system should be used to eliminate the sulphur content of the fuel according to the SOFC’s impurity tolerance level.
Charcoal is a substance that contains mainly car-bon, as well as low quantities of volatile materials and ash. It is generally produced by the slow pyroly-sis of biomass, e.g., wood, sugarcane waste, and municipal solid waste. The charcoal, a renewable replacement for coal, has much higher gravimetric energy density than biomass, resulting in its economic distribution. Gasification of this substance produces a gas with high heating value, so called syngas containing negligible impurities such as tars and sulphur. Therefore, the integration of SOFC with charcoal gasification can simplify the gas cleanup processes to reduce impurities that may cause fast performance degradation of SOFC. Different gasifiers can be used to produce syngas such as fixed bed, e.g., downdraft and updraft, and fluidized bed, e.g., bubbling, circulating and twin. Among these types, downdraft or updraft gasifiers are generally used for small-scale applications. In these applications, updraft gasifier can be preferred because of its simple design and very high carbon conversion efficiency, whereas downdraft gasifier can be advantageous because of its high exit temperature and low particulates at the exit. However, both of these gasifiers have scale-up and feed size–related limitations. Circulating fluidized beds can be preferable for large applications because of their advantages such as good temperature control and scale-up potentials, and greater range of toler-ance to particle size. However, they have high levels of particulates at the exit and their cost is high.
The performance of integrated SOFC systems can be assessed through numerical modeling. The integra-tion of SOFC with gas turbine cycles has been widely studied, e.g., [1–3]. There are also few studies on the modeling of integrated gasification and SOFC systems. For example, Kivisaari et al. [4] studied the feasibility of a 50-MW integrated system consisting of an entrained flow gasifier, a low temperature gas cleanup, a steam turbine and a high-temperature SOFC. Their results showed that the plant has 47% electrical efficiency and 85% fuel utilization efficiency. Kuchonthara et al. [5] analyzed an integrated system mainly consisting of a coal gasification system, a gas turbine system and a SOFC. They found that the cycle efficiency as high as 46% can be achieved. Panopoulos et al. [6] analyzed an integrated SOFC and allothermal biomass steam gasifi-cation system. They found the electrical efficiency of
the system to be 36%. Cordiner et al. [7] studied the integration of a downdraft gasifier fueled with wood and a SOFC. The electrical efficiency of the system was calculated as 45.8%. Athanasiou et al. [8] studied the integration of a SOFC, steam turbine and gasifier sys-tem. The electrical efficiency of the system was found to be 43.3%.
In this study, a novel integrated charcoal gasification and SOFC system is proposed and then the perform-ance of the cell, stack and system is assessed through a case study. The system consists of an updraft gasifier using air and steam as the gasification agents, a low-temperature gas cleanup system and a planar and direct internal reforming SOFC. The performance assessment parameters are taken as electrical effi-ciency, exergetic efficiency and power to heat ratio. A parametric study is also conducted to study the effect of Reynolds number at the fuel channel inlet on the performance and economical feasibility of the system. Description of the System
The proposed integrated SOFC and charcoal gasifi-cation system is shown in Figure 1. In this system, it is assumed that charcoal is produced from slow pyrolysis of biomass in a separate system, and then it is fed to the integrated system. It should be noted that the py-rolysis of biomass is not considered in this study. The gasifier type is chosen as updraft and the gasification agents are taken as preheated air and steam. The heat necessary to increase the temperature of these agents is supplied from the depleted air and fuel streams of the SOFC. The syngas produced from the gasification is cleaned according to the impurity tolerance levels of the SOFC in the cold gas cleanup system. The temper-ature of the exit of the cleanup system, which is taken same as the temperature of the gasifier exit, is increased to the SOFC operating temperature using a heat exchanger. The SOFC is assumed to include many stacks according to its power requirement. The gas composition at the fuel channel inlet of the SOFC is controlled by anode recirculation to prevent the car-bon deposition. If any carcar-bon deposition problem is detected, the anode recirculation ratio is increased up to a point that this problem is avoided. Because there is some amount of unused fuel at the exit of the SOFC, an afterburner uses this unused fuel and increases the
temperature of the exit gases from the SOFC. The exit of the afterburner has high enthalpy flow rate, which is sufficient to supply heat to several streams such as the gas streams exiting the gas cleanup system, the blowers and the pump through the heat exchangers and the heat recovery steam generator (HRSG) consist-ing of an evaporator and a superheater. Then, this stream is emitted to the atmosphere.
Modeling
The performance of the proposed integrated SOFC and charcoal gasification system is assessed using a heat transfer model for SOFC and thermodynamic models for the rest of the components. For the SOFC, a pre-developed and validated model developed by the author [9] is used. In this model, a transient quasi-2D approach is used to find the distributions of molar gas compositions, carbon activity and current density through the flow direction and the distribu-tion of temperature at the flow and thickness direc-tions. The continuity and heat transfer equations are applied to a control volume enclosing a repeat ele-ment found in the middle of a stack. It is assumed that the solid parts of this repeat element have adia-batic boundary conditions. All the heat transfer mech-anisms, e.g., conduction, convection and radiation, and all the polarizations, e.g., ohmic, activation, and concentration, are considered in this model.
The syngas is assumed to be at chemical equilib-rium at the exit of the reaction zone of the gasifier. Considering this assumption, six equations are solved simultaneously to calculate the syngas composition and the molar ratio of air to charcoal. These equa-tions consist of atom balances for C, H and O, equi-librium relations for steam reforming of methane and water-gas shift reaction, and the energy balance around the control volume enclosing the gasifier. It should be noted that molar ratio of steam to charcoal is taken as an input parameter in the gasifier model.
Thermodynamic principles and laws are applied to the rest of the components of the system to calculate the enthalpy flow rates of the streams, steam generated in the HRSG, work input to the auxiliary components, such as blower and pump, and the net power output of the system. Finally, the performance assessment pa-rameters, which are taken as electrical efficiency, exer-getic efficiency, and power to heat ratio, are calculated. These parameters are given in Eqs. 1–3.
hel¼ð _WnetÞsystem _nfuel LHV (1) e¼W_netþ D _Exprocess _ Exfuel (2) PHR¼ð _WnetÞsystem D _Hprocess (3) CASE STUDY
As a case study, the performance of an integrated SOFC and charcoal gasification system is investigated using the input data given in Table 1. The input for the
gasifier is based on some preliminary design data of an updraft-bed gasifier produced by Alterna Energy. For the SOFC, the manufacturing type is selected as elec-trolyte-supported and the thicknesses of the cell com-ponents are taken accordingly. As a result of this selec-tion, operating temperature of the SOFC is taken high enough, e.g., 8508C, to overcome the ohmic losses of the electrolyte. Thermomechanical considerations are taken into account in determining the excess air coeffi-cient for the SOFC not to cause excessive internal stresses in the cell [9]. Reynolds number plays an im-portant role on the performance of the system. For the base case, the value for this number is chosen low enough to get high fuel utilization. Discussions on the
Table 1. Input data.
Environmental temperature 258C
Type of fuel Charcoal
Gasifier
Mass ratio of steam entering to gasifier to charcoal
1 Temperature of gasification
agents, i.e., air and steam
3008C Reaction zone temperature 8508C Temperature of syngas
exiting gasifier
708C SOFC
Power requirement of SOFC 10 kW Number of cells per stack 50 Temperature of syngas entering SOFC 8508C Temperature of air entering SOFC 8508C Pressure of the cell 1 atm
Cell voltage 0.65 V
Reynolds number at the fuel channel inlet
3 Excess air coefficient 7
Active cell area 10 3 10 cm2 Number of repeat
elements per single cell
18 Flow configuration Co-flow Manufacturing type
Electrolyte-supported Thickness of air channel 0.1 cm Thickness of fuel channel 0.1 cm Thickness of interconnect 0.3 cm Thickness of anode 0.005 cm Thickness of electrolyte 0.01 cm Thickness of cathode 0.005 cm Balance of plant Temperature of exhaust gas leaving the system
1278C Pressure ratio of blowers 1.18 Isentropic efficiency
of blowers
0.53 Pressure ratio of pump 1.2 Isentropic efficiency
of pump
0.8 Inverter efficiency 0.95
effect of this number on the performance and cost of the system can be found in Results and Discussion. Some typical values are taken for the remaining data for the components of the system.
RESULTS AND DISCUSSION
In this section, the results and discussion of the case study are presented for the performance of the cell, and the performance and economical feasibility of the system.
Performance of the SOFC
The input parameters given in Table 1 are used for simulating the performance of the SOFC. The molar composition of the syngas, which is an input parame-ter in the SOFC model, is first calculated using the gasifier model. It is found that the syngas consists of: 0.53% CH4, 21.61% H2, 34.28% CO, 3.91% CO2, 3.14%
H2O and 36.53% N2. Then the distributions of the
current density and carbon activity of the SOFC are found using the heat transfer model of the SOFC for this syngas composition and the recirculation ratio of zero. These distributions are shown in Figure 2. As can be seen in this figure, the carbon activities for all the locations are less than one, which means that there is no carbon deposition possibility; even we do not recirculate any depleted fuel. It should be noted that only thermodynamic equilibrium is considered in evaluating the carbon deposition problem. The cur-rent density distribution is also shown in this figure. This distribution shows that the current density has the highest value at the inlet and it decreases through the channel length. The average current density for the given conditions is 0.383 A/cm2.
The molar compositions of the gas species found in the fuel channel of the SOFC are shown in Figure 3. As can been seen in this figure, the methane con-tent found in the syngas is fully consumed at a loca-tion that is very close to the inlet of the SOFC, and the molar compositions of the other gas species show a different trend until this location compared with the rest of the channel. This trend can be explained as follows. Steam reforming of methane reaction is con-sidered kinetically controlled and it occurs until this
location. After this location, the electrochemical and water-gas shift reactions are responsible for the change of the gas components.
The temperature distribution for a repeat element of the SOFC at the flow and thickness directions is shown in Figure 4. As can be seen in this figure, the change of temperature at the thickness direction is not very significant mainly because of the effect of conduction heat transfer between the PEN, i.e., solid structure consisting of anode, electrolyte, and cath-ode, and the interconnects. In the flow direction, there is a sharp temperature drop because of the endothermic steam reforming reaction at the very inlet, and then the temperature increases as a result of the exothermic water-gas shift reaction and heat generated because of the polarizations.
It is also found from the simulation of the case study that the fuel utilization and power density are 0.866 and 0.249 W/cm2, respectively. For the required
Figure 2. Carbon activity and current density distribution.
Figure 3. Distribution of molar gas composition of the gas species in the fuel channel. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
Figure 4. Temperature distribution in a repeat element of the SOFC. [Color figure can be viewed in the online issue, which is available at www. interscience.wiley.com.]
power from the SOFC, it is found that nine stacks are needed to generate this power.
Reynolds number is a non-dimensional number, which is a function of the mass flow rate, channel thickness and width and viscosity of the gas mixture, as shown in Eq. 4. ReDh ¼ _ m00fi ð2 tfc wgasÞ lmix ðtfcþ wgasÞ (4) As the Reynolds number increases, molar flow rates of the fuel entering SOFC and hydrogen that is used in the SOFC increase, which in turn increases the av-erage current density. In the modeling, because the cell voltage is assumed to be constant, power density becomes directly proportional to the current density; hence, the power density follows the same trend with the current density. On the other hand, the increase in molar flow rate of the fuel is higher than the molar flow rate of hydrogen used, hence fuel utilization decreases. These trends are shown in Figure 5.
Performance and Economic Feasibility of the Integrated System Studied
The performance assessment parameters, which are electrical efficiency, exergetic efficiency and power-to-heat ratio, are calculated for the base case. The electrical efficiency which compares the net elec-trical power output of the system with the lower heating value of the charcoal is found to be 33.31%. Exergetic efficiency evaluates the cogeneration sys-tem, taking into account both the quality and quantity of the energy forms. It is calculated that the proposed system has 45.72% exergetic efficiency. The final pa-rameter, power-to-heat ratio, compares the amount of power to useful heat generated. This value is found to be 1.003.
The effect of Reynolds number at the fuel channel inlet on the performance of the integrated system is investigated. It is shown in Figure 6 that the lower the Reynolds number is, the higher the electrical
effi-ciency is. This trend is mainly because the electrical efficiency of SOFC is directly proportional to the fuel utilization, hence the electrical efficiency of the inte-grated system shows the same behavior with that of the SOFC. In evaluating the efficiency of the cogener-ation system in terms of all the useful outputs, i.e., work and heat, exergetic efficiency is used instead of the classical efficiency term that does not distinguish the quality of the energy forms. If the classical effi-ciency was used, there would not be a significant change on this efficiency with the change of Reyn-olds number because the unused energy in the SOFC would affect the steam generation rate, and the ratio of the summation of the quantity of the work and useful heat to the lower heating value of the fuel would not change significantly. Because the quality of the net electrical work output is higher than that of the useful heat output, exergetic efficiency is higher for higher power-to-heat ratio, which is shown in this figure. The main purpose of operating this sys-tem should be producing electricity rather than heat because of the high purchase equipment cost of the
Figure 5. The effect of Reynolds number at the fuel channel inlet of the SOFC on the fuel utilization and power density of the SOFC. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
Figure 6. The effect of Reynolds number at the fuel channel inlet of the SOFC on the performance assessment parameters of the integrated system. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
Figure 7. The effect of Reynolds number at the fuel channel inlet of the SOFC on the number of stacks required for the given amount of power. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
SOFC. Because of this, higher power to heat ratio is required for the operation, which is also obtained at lower Reynolds number for this system. On the other hand, because the power density is lower for low Reynolds number conditions, more SOFC stacks must be used to generate the same required power for these conditions as shown in Figure 7. Using more SOFC stacks makes the integrated system less eco-nomically feasible because of the increase in the pur-chase equipment cost of the SOFC.
CONCLUSIONS
In this paper, an integrated novel charcoal gasifica-tion and SOFC system is proposed and its perform-ance is assessed through numerical modeling. It is found that Reynolds number at the fuel channel inlet of the SOFC should be taken low enough to obtain high electrical and exergetic efficiencies for the sys-tem, but high enough not to increase the cost of the system significantly. This system is especially intended to be used in small-scale applications. For larger scale systems with a higher desired power to heat ratio, another power generating technology such as steam turbine should be added to this system, which is also expected to increase the performance of the integrated system. As a future study, the system proposed will be optimized to maximize the electrical and exergetic efficiencies, and minimize the cost of the products.
ACKNOWLEDGMENTS
The financial and technical support of an Ontario Premier’s Research Excellence Award, the Natural Sci-ences and Engineering Research Council of Canada, EcoEnergy Technology Initiative Program, AAFC-NRC Bioproducts Program, Carleton University and Univer-sity of Ontario and Institute of Technology is grate-fully acknowledged. In addition, the authors would like to acknowledge the contribution of Dr. Cyrille Dece`s-Petit at NRC-IFCI (Institute of Fuel Cell Innova-tion) and Mr. Phil Marsh, Chief Technology Officer of Alterna Energy Inc., in proposing the integration of charcoal gasification and SOFC and supplying the technical data for the gasifier.
NOMENCLATURE
E˙x exergy flow rate, W _
H enthalpy flow rate, W LHV lower heating value, J/mole
_
mfi mass flow rate at the fuel channel inlet, g/s
_n molar flow rate, mole/s PHR power to heat ratio
ReDh Reynolds number in an internal flow
tfc fuel channel thickness, cm wgas width of gas channel, cm
_
Wnet net electrical power output, W hel electrical efficiency
lmix viscosity of the mixture, g/s-cm
LITERATURE CITED
1. Winkler, W., & Lorenz, H. (2002). The design of stationary and mobile solid oxide fuel cell-gas turbine systems, Journal of Power Sources, 105, 222–227.
2. Koyama, M., Kraines, S., Tanaka, K., Wallace, D., Yamada, K., & Komiyama, H. (2004). Integrated model framework for the evaluation of an SOFC/ GT system as a centralized power source, Interna-tional Journal of Energy Research, 28, 13–30. 3. Palsson, J., Selimovic, A., & Sjunnesson, L. (2000).
Combined solid oxide fuel cell and gas turbine systems for efficient power and heat generation, Journal of Power Sources, 86, 442–448.
4. Kivisaari, T., Bjo¨rnbom, P., Sylwan, C., Jacquinot, B., Jansen, D., & de Groot, A. (2004). The feasibil-ity of a coal gasifier combined with a high-temper-ature fuel cell, Chemical Engineering Journal, 100, 167–180.
5. Kuchonthara, P., Bhattacharya, S., & Tsutsumi, A. (2005). Combination of thermochemical recupera-tive coal gasification cycle and fuel cell for power generation, Fuel, 84, 1019–1021.
6. Panopoulos, K.D., Fryda, L.E., Karl, J., Poulou, S., & Kakaras, E. (2006). High temperature solid oxide fuel cell integrated with novel allothermal biomass gasification Part I: Modelling and feasibility study, Journal of Power Sources, 159, 570–585.
7. Cordiner, S., Feola, M., Mulone, V., & Romanelli, F. (2007). Analysis of a SOFC energy generation sys-tem fuelled with biomass reformate, Applied Ther-mal Engineering, 27, 738–747.
8. Athanasiou, C., Coutelieris, F., Vakouftsi, E., Skou-lou, V., Antonakou, E., Marnellos, G., & Zabanio-tou, A. (2007). From biomass to electricity through integrated gasification/SOFC system-optimization and energy balance, International Journal of Hydrogen Energy, 32, 337–342.
9. Colpan, C.O. (2009). Thermal modeling of solid oxide fuel cell based biomass gasification sys-tems. PhD thesis, Department of Mechanical and Aerospace Engineering, Carleton University [In Press].
