Dynamic Modeling and Control Issues on a Methanol Reforming Unit for Hydrogen Production and Use in a PEM Fuel Cell

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1 Dynamc Modelng and Control Issues on a Methanol Reformng Unt for Hydrogen Producton and Use n a PEM Fuel Cell Dmtrs Ipsaks 1,, Spyros Voutetaks 1, Panos Seferls 3, Smra Papadopoulou 1,4 1 Chemcal Process Engneerng Research Insttute (C.P.E.R.I.), CEntre for Research and Technology Hellas (CE.R.T.H.), P.O. Box 6361, 571 Therm-Thessalonk, Greece (Tel: ; e-mal:pars@cper.certh.gr). Department of Chemcal Engneerng, Arstotle Unversty of Thessalonk, P.O. Box 1517, 5414 Thessalonk, Greece (Tel: ; e-mal:psaks@cper.certh.gr). 3 Department of Mechancal Engneerng, Arstotle Unversty of Thessalonk, P.O. Box 484, 5414 Thessalonk, Greece (Tel: ; e-mal:seferls@cper.certh.gr). 4 Department of Automaton, Alexander Technologcal Educatonal Insttute of Thessalonk, P.O. Box 14561, 5411 Thessalonk, Greece (Tel: ; e-mal:shmra@tethe.gr). Abstract: The presented research work focuses on the mathematcal descrpton and control analyss of an ntegrated power unt that uses hydrogen produced by methanol autothermal reformng. The unt conssts of a reformer reactor where methanol, ar and water are co-fed to produce a hydrogen rch stream through a seres of reactons. The hydrogen man stream s fed to a preferental oxdaton reactor (PROX) for the reducton of CO at levels below 5ppm wth the use of ar. In the end, the PROX outlet stream enters the anode of a PEM fuel cell where power producton takes places to serve a load demand. The operaton of the two reactors s descrbed by a combnaton of partal dfferental equatons (mass and energy balances) and non-lnear equatons (knetc expressons of the reactons), whle the power producton n the fuel cell s based on the nlet hydrogen flow and on operatonal characterstcs. A smple case sceanro s employed when a step change on methanol flowrate s mposed. Man target s to dentfy and analyze the changes occurng n the man varables of concern (H, CO and temperature levels) that affect the overall system operaton. Based on the results, an nsght on the challengng control scheme wll be appled n order to dentfy possble ways of settng up a relable and robust control structure accordng to the developed mathematcal model. Keywords: methanol reformng, preferental oxdaton, hydrogen, PEM fuel cell, dynamc modelng 1. INTRODUCTION Hydrogen can be consdered an energy carrer for the future and when derved from renewable energy sources can be totally non-pollutng when used n fuel cells (Ipsaks D. et al., 8). Fuel cells advantages nclude (Larmne J. and Dcks A., 3) the low operaton temperatures (~ 8 o C), the CO tolerance by the electrolyte, the fast cold start and ther few movng parts, whch enhances ther role as back-up unts n vehcles. Nevertheless, the man dsadvantage of fuel cells refers to the hydrogen supply. Natural gas, gasolne and hgher hydrocarbons have been proposed for hydrogen producton va steam reformng, but methanol carres the most advantages from all (Lndström B. and Petterson L.J., 1). Methanol s a lqud that does not requre specal condtons of storage, whle t s also free from hgh reformng temperatures and sulphur oxdes that are met n methane and gasolne reformng. Moreover, methanol has a hgh H:C rato and no C:C rato and thus, prevents the soot formaton (Lndström B. and Petterson L.J., 1), whle bomass resources can be used to produce methanol (bomethanol). Producton of hydrogen from methanol can be acheved n three ways: () steam reformng of methanol, () partal oxdaton of methanol and () autothermal reformng of methanol (Lndström B. and Petterson L.J., 1). Autothermal reformng has the asset of elmnatng the dsadvantages of steam reformng (endothermc process whch requres a heatng source) and partal oxdaton (hghly exothermc process whch leads to the formaton of hot spots n the catalyst) by properly selectng the reactants ratos n such a way, so that adabatc condtons can be acheved. One of the drawbacks of hydrocarbons reformng however, s the producton of CO at hgh levels that degrade the electrochemcal performance of low temperature PEM fuel cells. Several processes used for the mnmzaton of CO content at acceptable levels (less than 5ppm) have been dscussed n the past, where among them preferental oxdaton s consdered to be the smplest and the least expensve method (Cptì F. et al., 7).

2 Most of the smulaton and modelng studes on methanol reformng, focus on each subsystem as an ndvdual part. Steam reformers have been modelled usng axal dstrbuton models (Suh J. S. et al., 7), whle PROX reactors have also been modelled usng D models (Cptì F. et al., 7). In one of the very few papers that deal wth an ntegrated system, Stamps A. T. and Gatzke E.P. (6), developed and mplemented a system level model of a vehcular reformer PEM fuel cell stack power system wthout the use however, of a hydrogen purfcaton system, whle the dynamc operaton results were not provded explctly. As can be seen, lterature references lack n studes that deal wth the detaled mathematcal descrpton of all the nvolved subsystems and on ther overall dynamc nteractons. The objectve of ths study s to analyze the dynamc behavour of the two reactors based on a step change mposed n methanol flowrate. The reactors temperature and CO content wll be contnuously montored, snce they mostly affect the power producton of the fuel cell. In the end, an analyss on the demandng task of the control of such an ntegrated power system wll be appled, n order to dentfy all the necessary actons need to be taken for the development of a robust control scheme.. DESCRIPTION OF THE INTEGRATED POWER SYSTEM Fgure 1, shows the overall process flow dagram of the methanol autothermal reformng unt. The autothermal reformng of methanol s based on the effectve selecton of the reactants ratos at the nlet of the reformer. For the current study, t was found from steadystate smulatons usng Aspen Plus (Ouzoundou M. et al., 8a) that the optmum values for H O/CH 3 OH and O /CH 3 OH are between and respectvely. Steam reformng s consdered as the sum of the water gas shft (R3) and methanol decomposston (R4) reactons. The reducton of CO after steam reformng, takes place at the PROX reactor (.1m length and.1m dameter). There, besdes CO oxdaton, H oxdaton also takes place, but t s kept at low rate through the effectve selecton of the catalyst. The reactons of the PROX can be seen n table : Table. Reactons takng place at the PROX CO +.5O CO R, 98 = -83kJ/mol, (R5) H +.5O H O R, 98 = -4kJ/mol (R6) The man objectve of the PROX reactor s to keep the CO concentraton at a maxmum lmt of 5 ppm by usng effcently the O /CO rato. From ASPEN Plus smulatons (Ouzoundou M. et al., 8a), ts optmum value s 1, but could vary severely wth CO concentraton. The PROX reactor due to the presence of the two hghly exothermc reactons s surrounded by a jacket to cool the system from undesrable extreme heat generaton. The knetc expressons for all the nvolved reactons can be found n (Ouzoundou M. et al., 8b) where an analyss on ther selecton s provded. Fnally, the hydrogen rch stream s fed to the anode of the fuel cell where t onses releasng protons (H + ) and electrons (e - ). Protons pass through the proton exchange membrane to the cathode, whle electrons flow through an external electrc crcut and produce current. In the cathode, oxygen s fed va ar supply and reacts wth the protons and electrons to form water (Table 3). Table 3. Reactons takng place at the PEM fuel cell H H + +e - Anode, (R6).5O + H + +e - H O Cathode, (R7) Fg. 1: Overall operaton scheme of the methanol autothermal reformng unt. Methanol, ar and water are co-fed to the reformer (.15m length and.1m dameter) after they are preheated by the effluent gases of a burner. The man reacton that takes place at the reformer s the steam reformng of methanol (R1), but because t s an endothermc reacton, oxygen also reacts wth methanol (R) to provde the necessary heat to the system. Table 1. Reactons takng place at the reformer CH 3 OH + H O CO + 3H CH 3 OH +.5O CO + H R, 98 = 49kJ/mol, (R1) R, 98 = -193kJ/mol, (R) CO + H O CO + H CH 3 OH CO + H R, 98 = -41.kJ/mol, (R3) R, 98 = 9.1kJ/mol, (R4) 3. MATHEMATICAL DESCRIPTION OF THE SUBSYSTEMS In ths secton, the mathematcal models of the nvolved subsystems wll be presented and dscussed. All parameters, varables and symbols are explaned n the nomenclature secton. gproms was used to smulate the operaton of the ntegrated power system (PSE, ). 3.1 Reformer and PROX Reactors The model equatons for the catalytc reactors consst of the standard materal and energy balances for a pseudohomogeneous system (packed-bed type). These equatons are n partal dfferental form wth spatal and radal dstrbutons. In order to smplfy the model and reduce the computatonal effort needed to smulate all the subsystems, a number of assumptons have been made. It s hghlghted, that the descrpton of the system does not lose ts accuracy

3 and the crucal phenomena can be easly descrbed by the developed mathematcal model. The deal gas law s appled for all gas components. No dffuson phenomena are assumed to take place from the gas phase to the surface of the catalyst. Constant reactor pressure and flud velocty. Constant physcal propertes (component densty and heat capacty) over the range of condtons. The temperature n the coolng jacket of the PROX reactor s approxmately unform and the resstance to heat transfer occurs prmarly between the reactor contents and the wall of the tube (beng at the coolng medum temperature). Materal Balance Equaton C C C u D z t z cat z C 1 C D r ( ) cat r r r N R 1 j 1, j R Energy Balance Equaton N T N T C C u p, p, 1 t 1 z R T T 1 T - k z k ( ) R j ( r z r r r Coolant Energy Balance Equaton c j1 Tc Vc Cpc Fc Cpc ( Tc, n Tc ) Q t j R, T, j (3) Q U A Re actorlength o Ideal Gas Law ) (1) () ( T T c ) dz (4) P V n R T P C R T (5) Speces Flowrate o F C Q (6) F, n R Qo u S Preactor T Boundary Condtons (Eqs. 1-4) n z= C =C,n and T=T n, r [,R] (8) z=l C z T z and (7), r [,R] (9) r= r C r=r r C T r and, z (,L) (1) T r and k h ( T T ),z(,l) (11) r In the soluton of the reformer, h w s and for the PROX. For the dscretzaton of the dstrbutons the method of centered fnte dfference ( nd order) was used. The dscretzaton of the axal and radal dstrbuton was performed for 5 and 5 ntervals, respectvely. More ntervals showed that the results are not affected, but the ncrease n the requred computatonal effort leads to neffcent soluton procedure. 3. PEM Fuel Cell Larmne J. and Dcks A., (3), presented an equaton that relates the power producton wth the hydrogen flow and the operatonal characterstcs of the fuel cell: P n F V (1) fc F H e F cell The hydrogen flow predcted by the reactors mathematcal model s used n the above equaton to predct the power producton as a functon of tme. V cell s usually around.7v/cell (Larmne J. and Dcks A., 3). The same authors also concluded that f all the enthalpy of reacton of a hydrogen fuel cell was converted nto electrcal energy then the output voltage would be 1.48V (water n lqud form) or 1.5V (water n vapour form). Therefore, the dfference between the actual cell voltage and ths voltage represents the energy that s converted nto heat nstead. For the vapour case n our fuel cell, heat s calculated as: Q fc P fc 1.5 ( 1) V cell 3.3 Model valdaton w c (13) Based on the expermental results presented n (Ouzoundou M. et al., 8b), valdaton wth the above mathematcal model (knetc parameters and the axal and radal dstrbuted paramaters were estmated) has been performed, where t has been found that the model accurately smulates the performance of both reactors. Fgures and 3 show the comparson between the expermental data and mathematcal model for the two reactors, respectvely and the devaton between smulated and expermental values s wthn the expected error (less than 5%). Only n low PROX temperatures a hgher devaton s detected but consdered neglgble, snce PROX operates n temperatures >15 o C.

4 Methanol Converson, % 95 9 EXP SIM Flowrate, mol/s.1 H CH3OH Tme, s Fg. 4. Methanol nlet and hydrogen outlet flowrates Fg.. Comparson between smulated and expermental results for the reformer CO Converson, % Fg. 3. Comparson between smulated and expermental results for the PROX 4. SIMULATION RESULTS FROM THE DYNAMIC OPERATION OF THE INTEGRATED POWER SYSTEM The dynamc operaton of the ntegrated power unt wll be presented based on an mposed step change. The condtons of the smulated case study are: nlet methanol at.mol/s, reformer temperature at 3 o C, PROX temperature at o C, H O/CH 3 OH at 1.5, O /CH 3 OH at.14 and O /CO at. It s noted that the flowrates of the reactants and reactor temperature at the nlet of the reformer do not have a sharp constant value at the start of the smulaton tme, but a value that ncreases smoothly wth tme and reaches ts steady state value after a few seconds. Fg.4 shows the 5% step change on the nlet methanol flowrate at t=3s (nlet water and oxygen flowrates are also ncreased based on the selected ratos). As can be seen, at the same tme an ncrease at the ext hydrogen flowrate starts to appear and after 8s the ncrease n hydrogen flowrate s calculated at 49.6%, whch shows that methanol converson s practcally unaffected by the ncrease n the methanol flowrate for the selected condtons. EXP SIM Smlarly Fg.5 shows the CO content (ppm) where a 13% decrease s observed at the tme that the step change occurs. Ths decrease s excpected due to the fact that water flowrate s ncreased (accordng to the methanol step change) and the water gas shft reacton s favored. Ths can also be concluded by the fact that CO flowrate ncrease s calculated at.5% whch dffers sgnfcantly from the 5% ncrease detected n other products, such as hydrogen. CO Content, ppm Tme, s Fg. 5. Carbon monoxde content at the reformer ext Fg.6 shows the reformer nlet and outlet temperature levels. The nlet gas temperature follows a smooth ncrease n order to prevent hot spots due to the rgorous exothermc partal oxdaton of methanol Tme, s z=m z=.15m Fg. 6. Temperature levels at the reformer nlet and outlet Ths smooth ncrease s based on the operaton of the assumed preheater that heats the reactants mxture. On the

5 other hand, the outlet temperature of the reformer (at t=s the reformer s assumed to be at 3 o C) s ntally decreased due to the fact that low temperature gas mxture exts the reactor at t<1s, but as the reformer operaton proceeds, the ext temperature s gradually ncreased to 6-7 o C. As can be seen for the present condtons, the endothermc reactons preval and the ext temperature s 3-4 o C lower than the nlet (at steady-state condtons) and also the step change seems to affect the temperature levels by lowerng them by 1 o C. Unlke the reformer, where neglgble changes are observed n the radal doman, n the PROX the speces concentraton and temperature vares severely along the reactor radus. Fg.7 shows the CO content at the wall of the reactor, at ts center and the average value that ndcates the ext flow. As can be seen, the CO content s hgher away from the center due to the lower temperature at that regon (see Fg.8). As we move to the reactor center, the CO levels are qute low (practcally zero at the reactor center) due to the ncreased temperature whle the average value s always lower than 5ppm for the current condtons. It s hghlghted that the CO at the ext of the reformer s measured and accordng to the selected O /CO rato, the ar feed rate s manpulated and ntroduced to the PROX reactor, so as to always provde a constant O /CO rato. If the O flow was constant based on the ntal condtons, then the CO levels would have been hgher ndcatng a possble fuel cell deteroraton. CO Content, ppm r=r Average r= Tme, s Fg. 7. Carbon monoxde content at the PROX wall, center and average value n the radal doman Fg.8 shows the temperature levels at the wall of the reactor, at the center and the average (ext) value. Intally (t=s), the reactor s assumed to be at o C and as the oxdatons take place, the reactor center s found to have ncreased temperature n contrast wth the reactor wall temperature that s mantaned at low levels due to the presence of the coolng medum. Eventually, the ext (average) temperature s ntally decreased, but as the gas mxture approaches the reactor ext, the temperature levels are ncreased. It can also been sad, that at t=3s, a small ncrease s detected due to the ncreased flowrate at the nlet of the PROX whch s not consdered severe (less than 15 o C). The presence of a controller to mantan the reacton temperature at specfc levels s of prmary mportance and the manpulated value wll be the coolant flowrate. It s noted that the hydogen man stream s assumed to be cooled down before enterng the PROX and the fuel cell, but the dynamc smulaton of the heat exchangers s omtted for ths study, snce we focus on the man subsystems Tme, s r= r=r Average Fg. 8. Temperature levels at the PROX wall, center and average value n the radal doman Fnally, Fg. 9 shows the power and heat producton levels based on the nlet hydrogen flow. As can be seen, at t=3s a 49.8% ncrease n power and heat producton s detected, whch should be taken nto consderaton at the developed control scheme that wll always try to meet the load demand (see next secton). Heat & Power Producton, W Tme, s Fg. 9. Power and heat producton n the fuel cell Heat Power 5. CONTROL ISSUES ON THE INTEGRATED POWER SYSTEM As t s obvous from the above analyss, the operaton of the ntegrated power system, requres the development of a robust control scheme. The varables that constantly need to be montored are: the reactor temperatures, the CO levels and fnally, the power to be suppled to the load. In the reformer, the temperature wll be controlled by the O /CH 3 OH rato and n the PROX through the coolant flowrate. The CO composton wll be controlled by the effectve selecton of the H O/CH 3 OH and O /CO ratos. Nevertheless, the man varable of concern s the power that needs to be provded to the load. Changes n the demanded load power level wll be handled by manpulatng the methanol flowrate n order to produce the hydrogen needed n the fuel cell to operate. As was presented, step changes n methanol flowrate affect the overall operaton and model predctve control (MPC) s the more sutable control scheme of such an ntegrated power system. The control algorthm must also satsfy the bounds for CO and temperature levels n order to protect the varous subsystems from deteroraton (manly PROX and fuel cell).

6 All these consequent changes wll be decded based on the mnmzaton of an objectve functon (MPC) that wll take nto account all the system necessary constrants. Specal care, however, should be gven to the fact that that slow and fast dynamcs occur n the system (e.g. the fast PROX oxdaton versus the slow coolant effect) that mght need to be specally treated. Perturbaton Theory s proposed to allevate such problems, because t can be appled to mathematcal systems that combne non-lnear algebrac equatons and dfferental ones (Kumar A. and Daoutds P., 1999). 6. CONCLUSIONS An ntegrated power system for the producton of hydrogen va autothermal reformng of methanol has been studed n ths paper. The developed mathematcal model for the two reactors was valdated and used for the smulaton of the operaton of the power unt where a step change was mposed. The next step wll be the ntegraton of the heat management system (burner and heat exchangers), whle the developed control scheme wll try to mantan the operatonal varables of concern at ther desred values (set ponts). Nomenclature A: heat transfer area, m C : concentraton of the component, mol/m 3 Cp c : coolant specfc heat capacty, J/ K Kg C p : component specfc heat capacty, J/K kg D r : radal effectve dffusvty, m /s D z : axal effectve dffusvty, m /s F: Faraday s constant, Cb/mol F c : coolant flowrate, kg/s F : flowrate of the component, mol/m 3 h w : wall heat transfer coeffcent, W/m K : component that takes part at the system n: nlet condtons j: number of reacton at the reactors k r : radal thermal conductvty, W/m K k z : axal thermal conductvty, W/m K n c : number of cells of the PEM fuel cell n e : number of electrons n F : Faraday s effcency, % P : partal pressure of the component, bar P fc : fuel cell power, Watt P reactor : reactor pressure, bar Q: heat removed by the coolng jacket, Watt Q o : volumetrc flow, m 3 /s Q fc : heat, W r: radus of the reactor, m R: unversal gas constant bar m 3 / mol K R j : knetc expresson of the reacton j, mol/kg cat s S: cross secton of the reactor, m t: tme, s T: temperature, K T c : coolant temperature, K u: superfcal gas velocty, m/s U: overall heat transfer coeffcent, W/m K V c : coolant jacket volume, m 3 V cell : cell voltage, V/cell z: length of the reactor, m R,T, j : enthalpy of reacton j at temperature T, J/mol cat : vod fracton of the catalyst,j : coeffcent of the component n the reacton j : densty of the component, kg/m 3 c : coolant densty, kg/m 3 REFERENCES Cptì, F., Pno, L., Vta, A., Laganà, M., and Recupero V. (7). Model-based nvestgaton of a CO preferental oxdaton reactor for polymer electrolyte fuel cell systems. Internatonal Journal of Hydrogen Energy, Volume 3, Pages Ipsaks, D., Voutetaks, S., Seferls, P., Stergopoulos, F., Papadopoulou, S., and Elmasdes C. (8). The Effect of the Hysteress Band on Power Management Strateges n a Stand-Alone Power System. Energy, Volume 33, Pages Kumar A., and Daoutds P. (1999). Control of nonlnear dfferental algebrac equaton systems. Chapman & Hall/CRC, New York, Washngton D.C. Larmne J., and Dcks A. (3). Fuel Cell Systems Explaned. nd Edton, John Wley & Sons Ltd Lndström B., and Petterson L.J. (1). Hydrogen generaton by steam reformng of methanol over copperbased catalysts for fuel cell applcatons. Internatonal Journal of Hydrogen Energy, Volume 6, Pages Ouzoundou, M., Ipsaks, D., Voutetaks, S., Papadopoulou, S., and Seferls P. (8a). Expermental Studes and Optmal Desgn for a Small-Scale Autonomous Power System Based on Methanol Reformng and a PEM Fuel Cell. The AIChE 8 Annual Meetng, Phladelpha, PA, U.S.A., November 16-1, 8 Ouzoundou, M., Ipsaks, D., Voutetaks, S., Papadopoulou, S., and Seferls P. (8b). A combned methanol autothermal steam reformng and PEM fuel cell plot plant unt: Expermental and smulaton studes, (submtted n Energy, Specal Issue for PRES8) Process Systems Enterprse Ltd, gproms Introductory User Gude, London, Unted Kngdom, Release March. Stamps, T., and Gatzke, E.P., (6). Dynamc modelng of a methanol reformer PEMFC stack system for analyss and desgn. Journal of Power Sources, Volume 161, Pages Suh, J. S., Lee, M. T., Gref, R., and Grgoropoulos, C. P. (7). A study of steam methanol reformng n a mcroreactor. Journal of Power Sources, Volume 173, Pages