The effect of operating temperature on the performance of Molten Carbonate Fuel Cell systems

Transcription

1 Te effect of operating temperature on te performance of Molten Carbonate Fuel Cell systems A. Musa, H.J. Steeman, M. De aepe (Department of Flow, Heat and Combustion Mecanics, Gent University- UGent, Sint-ietersnieuwstraat 41, 9000 Gent, Belgium, ABSTRACT: Molten carbonate fuel cells (MCFC) are a promising alternative power source for distributed or building power plants. In order to better understand te termodynamics of fuel cells and cycles termodynamic models are built in Aspen Customer Modeller for te externally reformed (ER) MCFC and internally reformed (IR) MCFC. Tese models are integrated in Aspen lus TM. In tis article te performance of internal and external reforming molten carbonate fuel cell systems is investigated. Steam-carbon ratio (S/C) and load variation are studied. Te simulations sow tat te MCFC cycles are more efficient in case of S/C ratio=.5 compared to S/C ratio=4. For part load te gross efficiency and net efficiency of te MCFC cycles are iger in case of lowering te fuel utilization to 0.5 compared to te lowering te fuel flow rate to te anode. KEYWORDS: fuel cells, Molten carbonate fuel cell, simulations, performance. 1. Introduction Fuel cells are considered to become major players on te energy and automotive markets, because of teir interesting caracteristics. Major advantages of fuel cells, among oters, are ig efficiency, low on-site emission, clean and quiet operation and fast load response. Many types of fuel cells ave been developed so far. Te fuel cells used for stationary energy production are typically ig temperature cells suc as molten carbonate fuel cells (MCFC). Because of teir ig operating temperatures tese cells can be used in combined eat and power (CH) plants. Te molten carbonate fuel cell (MCFC) is one of te fuel cells of wic te concept is proven and several of tem are in operation [1]. Little information is found in literature about te operational aspects of tese MCFC s. In [], te influence of operating temperature on te efficiency of a combined eat and power fuel cell plant is analyzed. Flowseet calculations sow tat te influence of te operating temperature of te fuel cell on te overall system efficiency is very small in te operating temperature range between 650 C and 700 C and tat te fuel cell system performs best at 675 C wit a net electrical efficiency of 51.89%. In [3], te effects of system configuration and operating conditions on te system efficiency of a 100 kw MCFC system efficiency as been analyzed. Te results sow tat te configuration of te unit operators in a system as a great effect on system efficiency, wile system size and efficiency of unit operators ave less effect. In tis paper a termodynamic model for an MCFC is developed in Aspen Customer Modeller and ten integrated in Aspen lus TM to be able to study te operational conditions of te cell. Empasis is laid on te difference in operational modes between internally and externally reformed MCFCs.. MCFC systems description Molten Carbonate Fuel Cells are considered to be ig temperature cells, aving an operation temperature lying between 600 C and 700 C [1]. Te cells currently in operation are fuelled wit natural gas. In order to provide te cell wit te necessary ydrogen gas, te metane is converted to ydrogen in a 1/1

2 reformer prior to introduction of te fuel to te cell. Steam is used to reform CH 4 to H according to eq. (13). Heat for te reaction is coming from te combustor. Te ig temperatures inside te cell stack also make it possible to reform te metane directly inside te cell if steam is provided at te inlet. Te eat necessary for tis reforming reaction is delivered by te electrocemical reaction in te cell. Te MCFC systems considered in tis work are externally reformed ER-MCFC and internally reformed IR- MCFC systems. Te different cell types ave different internal cemical reaction scemes wic are discussed furter in te paper. Te different cells also need different configurations of surrounding eat excangers, pre-eaters, pumps and compressors. In tis paper two typical configurations are compared, looking from te same point of view. Te fuel cell is integrated into a system, containing te necessary compressors, pump and eat excangers for providing fuel and steam, reforming, CO and eat recovery. A combustor is added delivering eat for reforming. A turbine recovers energy in te exaust. Fuel is provided at atmosperic conditions. Te fuel is pure metane (CH 4 ). Te fuel mass flow rate being burned in te combustor is varied. Steam for reforming is produced at 4 bar and temperature depending on te operational condition of te fuel cell. Te fuel cell systems are simulated in two ways. Firstly te performance wit varying steam to carbon ratio of bot MCFC systems is analysed for a current density of 150 ma/cm and corresponding fuel utilisation of 0.7. Secondly te load is varied by lowering te current density to 107mA/cm. Tis means tat te fuel utilisation or te fuel flow rate decreases. Te caracteristics of te fuel cell system are given in Table 1. Te steam-carbon ratio is defined as te ratio between te mass flow rate of steam and te CH4 mass flow rate to te anode. Operation temperature is defined as te average of outlet and inlet temperature and pressure is kept constant at 4 bar. Te catode inlet temperature is controlled to be 600 C. CO is recovered from te anode exaust..1 IR-MCFC system In te IR-MCFC system, water and metane are admitted into te eat excangers H/E and H/E3 to generate steam and to preeat te metane (Fig. 1.). Te pre-eated metane is mixed wit steam. Te mixture passes to te pre-eater, were it is eated to a given temperature, and ten enters into te anode side in te stack, see Fig.1. Te remaining anode gas is recycled to te combustor to produce CO gas for te catode. art of te eat released in te combustor (stream ) is used in te pre-eater; te remaining eat is used to eat up te burned gas. Tis burned gas from te combustor passes to five eat excangers H/E1, H/E, H/E3, H/E4, and H/E5 respectively. In tree of tese eat excangers te ot effluent of burned gas releases te eat necessary to preeat te catode inlet gases, generate steam, and preeat te metane. Te combustor exit gas wic contains a large portion of H O, enters into te flas tank at 4 bar, 30 C. In te flas tank te combustor exit gas is separated into water and remaining gas wic contains a large portion of CO. art of tis water is supplied to te eat excanger (H/E6). Te remaining combustor exit gas is supplied to te eat excanger (H/E4), were it is eated to 400 C. art of te compressed air from te compressor (COM) is mixed wit part of te remaining catode gas and wit part of te remaining combustor exit gas to form te catode inlet gas. Tis catode inlet gas is supplied to te eat excanger (H/E1), were it is eated to 600 C, and ten enters into te catode side of te stack. art of remaining catode gas and combustor exit gas are mixed, te mixture is sent to a turbine and eat /1

3 excanger (H/E6) for recovering energy. Te boundary conditions for all te eat excangers are summarised in Table.. ER-MCFC system Te ER-MCFC system is similar to te IR-MCFC system (Fig.1), except tat instead of te pre-eater in te IR-MCFC system tere is a metane steam reformer. In tis case te pre-eated metane and steam are fed to te reformer, were ydrogen, carbon dioxide and carbon monoxide are produced. Tis ydrogen-ric gas is used as fuel at te anode of stack. Table 1 Input parameters of te MCFC systems Fuel cell A cell 50 m S/C ratio 0.5,.5 and 4 p 4 bar u f 50 and 70% m CH4 m ot water T catode 600 C oter devices λ (air factor)combustor 1.1 η for compressor 0.8 η for turbine 0.85 T react combustor 900 C 8.7 and 40 kg/r 6000 kg/r CH4 5 C COM1 H/E4 m fuel VAR H O Exaust gasses CH4 40 kg/r H/E3 H/E6 H/E re-heater CO 400 C ANODE Cold water H/E5 30 C Hot water 6000 kg/r Flas F 5 C Air Fuel Combustor H/E1 600 C CATHODE H O Turbine m Air VAR COM Air 100 kg/r Fig.1. Scematic diagram of internal reformed MCFC system (COM: compressor; H/E: eat excanger). 3/1

4 Table Te boundary conditions for all te eat excangers of te MCFC systems Heat excanger Specification Value H/E1 Cold stream outlet temperature 600 C H/E Hot inlet cold outlet temperature difference 50 C H/E3 Hot inlet cold outlet temperature difference 50 C H/E4 Cold stream outlet temperature 400 C H/E5 Hot stream outlet temperature 30 C H/E6 Hot inlet cold outlet temperature difference 50 C 3. Energy balance of te MCFC and system Figure gives a scematic view of te MCFC. Incoming flows into te control volume are te anode and catode inlet gasses, outgoing is te anode and catode outlet flows. Electrical power is transported over te boundaries and eat can also be extracted. Te First Law of Termodynamics applied to te MCFC stack in steady state conditions, ten gives Q + el a out = a in a out a in a out a in + c out c in c out c in c out c in (1) el is te electrical power, te indexes a and c stand respectively for te anode and catode gases. Q is te eat transfer rate between te MCFC stack and te surroundings. If te cell is adiabatic tis eat transfer rate becomes zero and, assuming tat anode and catode gasses leave at te same temperature, (T energy balance simplifies to: el = a out a in out a in a out a in + c out c in out c in c out c in out), te Tis equation yields te outlet temperature T out if te electrical power el is given. Te electrical power can be determined by te model described in te next paragrap. Te gross system efficiency ( η gross eating value ( LHV ) of te total amount of fuel ( ) is defined as te ratio of power produced by te fuel cell to te lower Q = m LHV ) supplied to te system [3]. tot tot * el η gross = (3) Qtot Were mtot is te sum of te mass flow rate of CH4 supplied to te fuel cell and to te combustor. Te net system efficiency ( η net ) is defined as te ratio of te power produced by te fuel cell and te turbine, minus compressors power, to te ( LHV ) of te total amount of fuel ( Q ) supplied to te system. el comp + turb η net = (4) Q tot tot () 4/1

5 el Q Anode T a_in Catode T c_in MCFC Anode T out Catode (a) Fig.. Scematic of control volume (a) and te equivalent electrical circuit of te MCFC stack (b). (b) 4. Fuel Cell Stack Model Te model of te stack used in tis paper is based on an existing isotermal MCFC Co-flow model [6]. An equivalent electrical circuit depicted in Fig. a is used, were te cell is considered to be a sequence of voltage sources and resistors (quasi omic resitance) in a continuous way along te flow pat. Te model expresses all variables as a function of X, te distance along te flow pat of te anode and catode gas cannels. Te model is extended in a way tat bot externally and internally reformed cells can be simulated. Tis requires te introduction of te correct cemical equations as discussed in te next paragrap. Following assumptions are made: Te fuel cell is stationary and isotermal Te operating temperature is te average of inlet and outlet temperatures Canges in te composition of te anode and catode gases are only significant in te flow direction ressurized system operating at 4 bar Quasi-omic resistance r is independent of position Nernst potential is independent of ydrostatic pressure graduations Te fuel utilization factor u f = n Hconsumed / ( n Hin + n COin + 4*n CH4in ) In te original model on wic te model used in tis paper is based all losses were modelled using one value for te local quasi-omic resistance in te entire cell [6]. In tis article local values for te resistance are used and a distinction is made between irreversible losses due to internal resistance and irreversible losses due to polarization. Te omic cell resistance is calculated using an Arrenius equation as function of te operating temperature (eq 6) and te correlations for te polarization losses (eq.7-8) are obtained from [7]. r = r + r + r (5) om pol, anode pol, catode 1 1 r = 0.5exp 3016 (6) om T 93 cell r,.7*10 exp pol anode = H CO HO Tcell (7) r, 7.505*10 exp pol catode = O CO Tcell (8) 5/1

6 5. Termocemical aspects: water gas sift reaction and metane reforming In bot models te cemical reactions are assumed to be in equilibrium, wic according to [6] is an acceptable approximation. Tis means tat te reactions occur instantaneously and reac te equilibrium condition spontaneously at eac position X. For bot models te electrocemical reaction and water gas sift reaction are implemented: CO catode (9) 1 + O + e CO3 H + CO3 H O + CO + e anode (10) H 1 + O H O overall reaction (11) CO + + H O CO H gas sift reaction (1) For te IR-MCFC an additional reaction equation is taken into account for te fuel reforming CH H O CO 3H Te electrocemical and water gas sift reactions are exotermic and, on te oter and, fuel reforming is a strongly endotermic reaction..by consequence te eat produced in te combustor sould be used in te reformer of te ER-MCFC system. In case of IR-MCFC system te eat generated by te electrocemical reaction and water gas sift reaction is used in te reforming reaction. Te equilibrium potential V eq used in Figure b is given by te Nernst equation resulting from te reactions (9) to (11): V eq ph ( u) ( ) ( ) u pco u RT cell 1 / RTcell ( u ) = E + ln( p ( u) p ( u) ) + ln 0 O CO (14) F F ph O 6. Effect of operating temperature and steam-carbon ratio on te performance of MCFC systems. For all te fuel configurations studied te operating temperature of MCFC systems is varied. Aspen lus TM Design-spec function [8] is used to vary te fuel burnt in te combustor to control te temperature of anode gas inlet. In te IR-MCFC system simulations te gas temperature at te anode inlet will be varied in a range between 540 C and 980 C. In ER-MCFC system simulations te gas temperature at te anode inlet will be varied in a range between 430 C and 880 C. In tese two ranges of te gas temperature at te anode inlet te operating temperature in bot MCFC systems will range between 600 C and 700 C. In a first paragrap te performance wit varying steam to carbon ratio of bot MCFC systems is analysed for a current density of 150 ma/cm and corresponding fuel utilisation of 0.7. Secondly te load is varied by lowering te current density to 107mA/cm. Tis means tat eiter te corresponding fuel utilisation is lowered to 0.5 or te fuel flow rate is lowered from 40 kg/r to 8.7 kg/r. (13) 6/1

7 6.1. Effect of steam-carbon ratio on te performance of te MCFC systems in case of current density (150 ma/cm ). Figure 3 sows te effect of decreasing te steam-carbon ratio on te cell voltage of te MCFC. Te ER- MCFC as a iger cell potential tan te IR-MCFC. Tis is caused by te difference in H concentration in te two cell types if for bot cells te steam flow rate is kept constant. In te IR-MCFC tere is always CH 4 present in te cell, resulting in a lower partial pressure of H in te cell. From equation (14) follows tat a reduction in partial pressure of H results in a reduction of te V eq. As operating temperature goes up in bot cases te cell voltage goes up. Tis is mainly caused by te reduction of te cell omic and polarization resistance wit rising temperature (equations 5-8). Te IR-MCFC cell voltage curve as a steeper slope. If temperature goes up, te reforming reaction moves to te H side, resulting in a iger H concentration inside te cell. From equation (14) again follows tat voltage will go up. By decreasing te steam-carbon ratio, te cell voltage goes up. For te ER-MCFC reducing te steamcarbon ratio results in a lower H O flow rate going into te cell. Looking again at eq. (14), tis causes a raise in cell voltage. For te IR-MCFC cell it can be noticed tat for different steam-carbon ratios te cell voltage as te same value around 65 C, wile te difference in te cell voltage becomes more pronounced at 700 C wen a cell wit very low steam carbon ratio (S/C=0.5) is compared wit cells wit steam-carbon ratios of.5 and 4. As long as more tan te stoicometric amount of H O is added to te IR-MCFC lowering te steam-carbon ratio as little effect because te amount of CH 4 wic is reformed and te partial pressures of H and H O vary little. If te H O flow rate is lower tan te stoicometric amount te reforming reaction uses te H O produced by te electrocemical reaction causing a beneficial sift of te reforming reaction equilibrium. As te operating temperature as a strong influence on te equilibrium of te reforming reaction, sifting it to more H and less H O wit iger temperatures, te cell voltage goes up wit temperature. Figure 4 sows te CH 4 concentration in te outlet of te anode. By decreasing te steamcarbon ratio te CH 4 concentration curves moves upwards. Tis means an increase in te mass flow rate of steam results in moving te reforming and gas-sift reaction equilibrium to te H side S/C=4 IR S/C=.5 ER S/C=.5 IR S/C=4 ER S/C=0.5 IR Cell voltage (V) Fig.3. Effect of steam-carbon ratio on te cell voltage at I=150 ma/cm. 7/1

8 CH4 (kg/r) I=150 S/C=4 CH4 IR I=150 S/C=.5 I=107 CH4=8.7 kg/r I=107 CH4=40 kg/r, u=0.5 I=150 S/C=0.5 0 Fig.4. Te mass flow rate of remaining CH 4 in te anode exit. Figure 5 sows te effect of decreasing S/C ratio on te efficiencies of bot cycles. Te net and gross efficiencies of bot MCFC cycles are bigger in case of S/C ratio=.5. Te fuel consumption in te combustor (Fig. 6) is bigger in case of S/C ratio=4, causing te reduction in te efficiencies. Te combustor needs more fuel in case more steam as to be eated Gross S/C=4 IR Net S/C=4 IR Net S/C=.5 IR Gross S/C=.5 IR Gross S/C=4 ER Net S/C=4 ER Net S/C=.5 ER Gross S/C=.5 ER efficiencies Fig.5. Effect of steam-carbon ratio on te efficiencies at I=150 ma/cm. 8/1

9 60 50 fuel S/C=4 IR fuel S/C=4 ER fuel S/C=.5 IR fuel S/C=.5 ER fuel (kg/r) Fig.6. Effect of steam-carbon ratio on te fuel flow rate at I=150 ma/cm. Te efficiency of te IR cycle is in all cases bigger tan of te ER cycle. Toug te electrical output of te cell is bigger in te ER cycle, te fuel consumption is also bigger, causing te reduction in efficiency. Internal reforming uses eat directly produced in te fuel cell. Wit te ER-cycle te eat as to be recovered out of te exaust gasses. Tis is less efficient. As te power delivered by te turbine is bigger tan te power used by te compressors (Fig.7et efficiency (eq. (4)), is bigger tan gross efficiency (eq. (3)) turbine power S/C=.5 IR turbine power S/C=.5 ER compressors power IR compressors power ER power (kw) Fig.7. Turbine and compressor power at I=150 ma/cm. Tis analysis sows tat, altoug te cell voltage of te ER-cell is bigger tan of te IR-cell, te IR-cycle as te iger efficiency. Internal reforming is tus advantageous from te energy use point of view. Lowering steam-carbon ratio is also advantageous. 9/1

10 6. erformance of MCFC systems in case of current density (107 ma/cm ). Figure 8 sows te cell voltage for lower load of te Fuel cells, bot by reducing te fuel utilisation u to 0.5 and reducing te CH 4 flow rate from 40 kg/r to 8.7 kg/r. Cell voltage goes up in case u=0.5 compared to te case of ig current density because omic losses go down wit lower current density and H concentrations in te cell are iger. Te cell voltage of te MCFC is iger in case of lowering te fuel utilisation to 0.5 compared to lowering CH 4 flow rate. Cell voltage (V) Vcel u=0.5 IR Vcel CH4=8.7 kg/r IR Vcel u=0.5 ER Vcel CH4=8.7 kg/r ER 0.70 Fig.8. Cell voltage at I=107 ma/cm. In case of lowering te fuel flow rate, te fuel supplied to te combustor (Fig. 9) is increased compared to in case of I=150 ma/cm because te remaining fuel wic is not used in te fuel cell (Fig. 4) goes down wit te reduction of CH 4 mass flow rate to te anode and u=0.7. As less fuel is used in te fuel cell in case of lowering te fuel utilisation to 0.5 (Fig. 4) more can be burned in te combustor resulting in a reduction of extra fuel supplied to te combustor fuel u=0.5 IR fuel CH4=8.7 kg/r IR fuel u=0.5 ER fuel CH4=8.7 kg/r ER fuel (kg/r) Fig.9. Mass flow rate of fuel as a function of operating temperature at I=107 ma/cm. Bot gross and net cycle efficiency are reduced compared to I =150 ma/cm. As te power production goes down and te fuel flow rate only canges sligtly, efficiency goes down. Te net and gross efficiencies of 10/1

11 bot MCFC cycles are iger in case of lowering te fuel utilisation to 0.5. Te fuel consumption in te combustor (Fig. 9) is bigger in case of lowering te fuel flow rate, causing te reduction in te efficiency Gross u=0.5 IR Net u=0.5 IR Gross CH4=8.7 kg/r IR Net CH4=8.7 kg/r IR Gross u=0.5 ER Net u=0.5 ER Gross CH4=8.7 kg/r ER Net CH4=8.7 kg/r ER efficiencies Fig.10. Efficiencies of te cycles at I=107 ma/cm, u=0.5 and CH 4 flow rate= 8.7 Nomenclature A cell active cell area (m ) E 0 F Cell potential (V) at 1 bar Faraday constant (C/mol) specific Entalpy (J mol -1 ) I current density (A/cm ) n mole flow rate (mol s -1 ) artial pressure (a) el comp tot Q electrical power compressor ower (W) total power of te cycle (W) eat (W) r total local resistance (Ωcm ) r om internal cell resistance (Ωcm ) r pol,anode resistance representing polarization losses at te anode (Ωcm ) R Universal gas constant (J/molK) r pol,catode resistance representing polarization losses at te catode (Ωcm ) T outlet temperature (K) T cell T catode u f V cell m CH4 X S/C ratio cell temperature (K) catode input mixture temperature ( C) total fuel utilization cell voltage (V) fuel flow rate to te anode coordinate in flow direction steam-carbon ratio 11/1

12 Greek symbols η Efficiency λ air factor combustor 7. Conclusion In tis article te performance of internal and external reforming molten carbonate fuel cell systems is investigated. Te simulations of MCFC cycles are performed in two ways. Firstly te performance is studied for varying steam to carbon ratio. Secondly te load is varied by lowering te current density to 107mA/cm. Te result indicate tat te cell voltage, te gross efficiency and te net efficiency of bot MCFC cycles are iger in case of S/C ratio=.5 compared to S/C ratio=4. Te cell voltage is iger in case of te ER-MCFC compared to IR-MCFC. Te cell voltage of IR-MCFC is more influenced by te operating temperature compared to ER-MCFC. Te efficiency of te IR-MCFC system is iger tan te efficiency of te ER-MCFC system. For part load te gross and net efficiencies of MCFC cycles are iger in case of lowering te fuel utilization to 0.5 compared to lowering te fuel flow rate to te anode. References: [1] Vielstic, W., Lamm, A., and Gasteiger, H., Fundamentals Tecnology and Applications, Wiley, Volume 4, New York, 004. [] Au, B., Mcail, S., Woudstra, N., K.Hemmes, 003, Te influence of operating temperature on te efficiency of a combined eat and power fuel cell plant, J. ower Sources 1: [3] Kang, B., Ko, J., Lim, H., 00, Effects of system configuration and operating condition on MCFC system efficiency, J. ower Sources 108: 3-38 [4] Au, S.F., Woudstra, N., Hemmes, K., 003, Study of multistage oxidation by flowseet calculations on a combined eat and power molten carbonate fuel cell plant, J. ower Sources 1: 8-36 [5] De Simon, G., arodi, F., Fermeglia, M., Taccani, R., 003, Simulation of process for electrical energy production based on molten carbonate fuel cells, J. ower Sources 115: [6] Standaert, F., Hemmes, K., Woudstra, N., 1996, Analytical fuel cell modelling, J. ower Sources 63: 1-34 [7] Yu, CY., Selman, JR., 1991, Te polarization of molten carbonate fuel cell electrodes 1 analysis of steady-state polarization data, J. Electrocem Soc 138: [8] Aspen lus TM 1.1 Users Guide, 003. Aspen Tec Ltd, Cambridge MA, USA 1/1