Modeling and Analysis of an Integrated Power System Based on Methanol Autothermal Reforming

Transcription

1 17th Medterranean Conference on Control & Automaton Makedona Palace, Thessalonk, Greece June 4-6, 009 Modelng and Analyss of an Integrated Power System Based on Methanol Autothermal eformng Dmtrs Ipsaks 1,, Spyros Voutetaks 1, Panos Seferls 3, Smra Papadopoulou 1,4, Mchael Stoukdes 1 Chemcal Process Engneerng esearch Insttute (C.P.E..I.), CEntre for esearch and Technology Hellas (CE..T.H.), P.O. Box 60361, Therm-Thessalonk, Greece (Tel: ;emal:pars@cper.certh.gr). Department of Chemcal Engneerng, Arstotle Unversty of Thessalonk, P.O. Box 1517, 5414 Thessalonk, Greece (Tel: ; emal:psaks@cper.certh.gr). Abstract: The ntegrated power system under consderaton, conssts of the fuel processor (reformer and preferental oxdaton reactors), the fuel cell and the heat management system. In the reformer reactor, methanol, ar and water are co-fed to produce hydrogen under autothermal condtons. The produced hydrogen due to the hgh content of CO (>5000ppm), s treated n the preferental oxdaton reactor (POX) for the CO mnmzaton at acceptable levels (<50ppm). After the oxdaton clean-up step, the anode of the polymer electrolyte membrane (PEM) fuel cell s fed wth the reformate gas (~60-65% H, ~15-5% CO, ~15-0% N, ~1-3%CH 3 OH and traces of CO). The present paper s focused on the mathematcal analyss of the man subsystems of the ntegrated power unt. The two reactors are modeled va a system of partal dfferental equatons (PDE s) and the speces flowrates and reactor temperature are analyzed along the length of each reactor. Moreover, the PEM fuel cell voltage-current characterstc s modeled va a non-lnear equaton that depends on the mass & energy balances (ordnary dfferental equatons) of the concerned speces. Fnally, the heat management system s analyzed n order to provde nsghts for future control studes that wll depend on the developed mathematcal model (model-based control). Keywords: ntegrated systems; dynamc modelng; methanol reformng; preferental oxdaton; PEM fuel cell; hydrogen; I. INTODUCTION Alternatve methods for cleaner energy producton requre the explotaton of renewable and alternatve energy sources. Solar and wnd energy explotaton s of usual nterest n stand-alone applcatons [1], but for portable and transportaton usages, bomass resources are used. A characterstc of alternatve ntegrated power systems s the use of hydrogen as an ntermedatte energy source. Hydrogen s used for power generaton n fuel cells and ts producton s usually fulflled by hydrocarbons reformng, such as methanol []. Methanol advantages nclude the flexble storage abltes, the presence of hgh H:C rato and no C:C rato and also the absence of hgh reformng temperatures and sulphur oxdes that are usually met n natural gas and 3 Department of Mechancal Engneerng, Arstotle Unversty of Thessalonk, P.O. Box 484, 5414 Thessalonk, Greece (Tel: ; emal:seferls@cper.certh.gr). 4 Department of Automaton, Alexander Technologcal Educatonal Insttute of Thessalonk, P.O. Box 1456, Thessalonk, Greece (Tel: ; emal:shmra@tethe.gr). gasolne reformng. Steam reformng, partal oxdaton and autothermal reformng, are the three man processes for producng hydrogen from methanol [3]. Steam reformng leads to a hgh level of hydrogen at the outlet stream (~75%), but s a reacton that s endothermc and requres heat to be provded by an external source (e.g. burner). Partal oxdaton elmnates the use of a burner, but the hgh temperatures lead to the formaton of hot spots n the catalyst, whle the produced hydrogen remans at low levels (~45%). In the autothermal steam reformng, oxygen, water and methanol are co-fed to the reformer and the reactants ratos are chosen n such a way, so that a slghtly exothermc or thermally neutral reacton scheme can be acheved [4]. Hence, autothermal reformng elmnates the dsadvantages of the above procedures and s consdered the most effectve method for hydrogen producton at an autonomous power unt that uses methanol. On the other hand, hydrocarbons reformng cause the producton of CO at levels that poson the anode of PEM fuel cells. Among all processes reported for the mnmzaton of CO content, preferental oxdaton s consdered to be the smplest and the least expensve method [5]. evew n lterature reveals a scarce number of studes dealng wth ntegrated systems. Most studes focus on each subsystem and POX reactors have been modelled usng D steady-state models [6], whle steam reformers have also been modelled usng axal dstrbuton models [7]. In one of the very few papers on ntegrated systems, Stamps A. T. and Gatzke E.P. [8], developed and mplemented a system level model of a vehcular reformer-pem fuel cell stack power system wthout focusng on the system dynamc nteractons. Hence, the man target of ths research work s to present a complete study of an ntegrated system under operaton. Am s the effectve ntegraton of the developed mathematcal models of the subsystems. Based on them the system dynamc results wll be presented and dscussed for smple case scenaros appled. Moreover, an analyss on the heat management system wll be gven n order to provde nsghts for future control studes /09/$ IEEE 141

2 II. ANALYSIS OF THE INTEGATED POWE SYSTEM Fg. 1, shows the process flow dagram of the methanol autothermal reformng unt. In the fnal step, the hydrogen rch-stream s fed to the anode of the PEM fuel cell, where protons (H + ) and electrons (e - ) are released and react wth oxygen (fed va ar supply) at the cathode to form water [9]: Anode H H + +e - (7) Cathode 0.5O + H + +e - H O (8) II. MATHEMATICAL DESCIPTION OF THE SUBSYSTEMS Fgure 1. Smplfed process flow dagram of the methanol autothermal reformng unt. Methanol and water are evaporated and preheated wth the use of a burner before ther ntroducton to the reformer (0.15m length and 0.1m dameter). At the same tme, ar s preheated and also ntroduced to the reformer to provde the necessary heat to the system. The ratos of the reactants are chosen n such a way, so that the overall process n the reformer s consdered adabatc. From Aspen Plus smulatons t was found that the optmum values for H O/CH 3 OH and O /CH 3 OH are between and respectvely, unless stated otherwse. The reactons that take place at the reformer are: Steam eformng CH 3 OH + H O CO + 3H ΔΗ,98 =49kJ/mol (1) Partal Oxdaton CH 3 OH +0.5O CO + H ΔΗ,98 =-193kJ/mol () Water Gas Shft CO +H O CO + H ΔΗ,98 =-41.kJ/mol (3) Methanol Decomposton CH 3 OH CO + H ΔΗ,98 =90.1kJ/mol (4) Snce CO producton takes place at the reformer, ts mnmzaton s requred. At the POX reactor (0.1m length and 0.1m dameter), CO and H oxdaton take place: Carbon Monoxde Oxdaton CO +0.5O CO ΔΗ,98 =-83kJ/mol (5) Hydrogen Oxdaton H +0.5O H O ΔΗ,98 =-4kJ/mol (6) The man objectve of the POX reactor s to keep the CO concentraton at a maxmum lmt of 50 ppm by usng effcently the O /CO rato. From Aspen Plus smulatons, ts value for the current study s hgher than 1 for a CO converson >95%, but could vary severely wth the nlet CO concentraton. For all the aforementoned reactons, knetc expressons of the Arrhenus type are selected and ther man form s presented n []. A. eformer and POX reactors The model equatons for the two catalytc reactors consst of the standard materal and energy balances for a packedbed type reactor and are n partal dfferental form. In order to smplfy the model and reduce the computatonal effort needed to smulate all the subsystems, a number of assumptons s employed: 1) 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 3) constant reactor pressure (no system pressure drop) and flud velocty, 4) constant physcal propertes over the range of condtons n the system and 5) the temperature n the coolng jacket of the POX reactor s aprroxmately 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). Wth the above assumptons, the tubular reactors can be adequately descrbed by a pseudohomogeneous, two dmensonal mathematcal model. Materal Balance Equaton C t ε cat C + u C D r ( ε cat Energy Balance Equaton C D z 1 C + ) = r j= 1 N = 1 j = 1 ν, j N T N T ρ C + ρ C u p, p, = 1 t = 1 T T 1 T - k z k ( + ) = j ( ΔΗ r r Coolant Energy Balance Equaton, T, j j ) (9) (10) Tc ρ c Vc Cpc = Fc Cpc ( Tc, n Tc ) + Q (11) t Q = U A e actorlength o Ideal Gas Law ( T T c ) dz (1) P = C T (13) 14

3 Speces Flowrate F, n Qo = u S = Preactor o T n (14) F = C Q (15) Boundary Condtons (Eqs. 1-4) C =C,n and T=T n, rε[0,] (16) z=l C = 0 r=0 C = 0 r= C = 0 T and = 0, rε[0,] (17) T and = 0, zε(0,l) (18) T and kr = hw ( T Tc ), zε(0,l) (19) In the soluton of the reformer, h w s 0 and for the POX 0. 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 50 and 5 ntervals, respectvely. B. PEM Fuel Cell The deal fuel cell voltage s equal to the thermodynamc potental, dvded by the flow of the charge for the two electrons that are released [9]: - ΔG f E = (0) n F e However, the actual voltage s less due to actvaton losses (V act ), ohmc losses (V ohmc ) and concentraton losses (V mtl ) [9, 10]: V fc = E V V (1) V act ohmc V act, V ohmc and V mtl losses, depend on the partal pressures of the reactants and on the fuel cell temperature [9, 10]. The mass balance at the anode and cathode s gven by Eq.(): d(van/cat C ) = Q dt an / cat ( C mtl, n n c I C ) n () F n F The last part of the equaton s postve when water producton s consdered and negatve for hydrogen and e oxygen consumpton. The thermal model descrbng the fuel cell temperature s [10]: C dt ( T T fc fc amb t = nc I ( a + nt ) (3) dt t a and nt are rerstances that are related to actvaton and ohmc losses respectvely, and are gven by non-lnear equatons [10]. Based on the expermental results presented n [], valdaton wth the above mathematcal model has been performed both for the reformer and POX. Fgures and 3 show the comparson between the expermental data and mathematcal model for the two reactors, respectvely. The devaton between smulated and expermental values s wthn the expermental error (less than 5%) and only n low POX temperatures a hgher devaton s detected but consdered neglgble, snce POX operates n temperatures >150 o C []. Methanol Converson, % Fgure. Comparson between smulated and expermental results for the reformer CO Converson, % EXP Fgure 3. Comparson between smulated and expermental results for the POX III. DYNAMIC ESULTS FOM THE OPEATION OF THE INTEGATED POWE SYSTEM For the presented case study, the reformer operates at 300 o C and the selected ratos are H O/CH 3 OH:1.5 and O /CH 3 OH:0.14. Moreover the POX temperature s 00 o C and the O /CO s. The flowrates of the reactants and the EXP SIM SIM ) 143

4 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 methanol flowrate consumpton along the reactor length where at the end of the reformer, methanol converson reaches approxmately 99%. An nterestng result s that methanol s consumed very fast at the short lengths (<0.05m) of the reformer and more slowly through the rest of reactor. Carbon Monoxde Concentraton, ppm Methanol Flowrare, mol/s Tme, s Fgure 4. Methanol flowrate as a functon of tme and reformer length. Smlarly, hydrogen and carbon doxde flowrates (Fg. 5), ncrease wth tme and along the length reactor due to the consumpton of methanol and reach ther steady value after ~15s. 0 Tme, s Fgure 6. Carbon monoxde concentraton as a functon of tme and reformer length Fnally, Fg. 7 presents the temperature profle n the reformer, where t can be seen that ntally the temperature n the varous postons n the reactor s decreased because low temperature gas mxture passes, but after ~3s, the temperature s moved to hgher levels. In general, t can be sad that the endothermc reactons preval and the temperature s moved to lower levels, whle the smooth ncrease of temperature at the nlet of the reactor nhbts the hot spots that are usually detected at the short lengths of the reformer due to the exothermc reacton of partal oxdaton H & CO Flowrates, mol/s Tme, s H m.05m.1m.15m Fgure 5. Hydrogen and carbon doxde flowrates as a functon of tme and reformer length Fg. 6 shows that CO content ncreases rapdly at the short lengths of the reformer and through the rest of the reactor ts ncrease s consdered to take place more slowly. The reason behnd ths behavor s that most of the methanol has been consumed and CO has nearly reached equlbrum condtons. CO Tme, s Fgure 7. Temperature as a functon of tme and reformer length. The nlet flowrates of the POX are equal to the outlet flowrates of the reformer, but the reformer ext s assumed to be cooled to the POX operaton temperature. However, the dynamcs of heat exchangers are omtted for ths study. Fg. 8 shows the carbon monoxde concentraton along the POX length. As can be seen, carbon monoxde decreases along the length of POX wth tme and eventually at the end of POX, carbon monoxde levels are at acceptable levels for use n PEM fuel cells. Specal cauton however should be gven to the O /CO rato that manly affects the CO converson. Based on the CO content at the reformer ext, the ar feed rate at the POX s manpulated accordngly n order to always have a constant value at the rato of O /CO (an equvalent of a feedforward control scheme). 144

5 Carbon Monoxde Concentraton, ppm m.05m.1m 0 Tme, s Fgure 8. Carbon monoxde concentraton as a functon of tme and POX length. The temperature profle of POX s presented n Fg.9. The hghly exothermc reactons cause the sharp ncrease nsde the reactor and thus, makng the control system essental n controllng the temperature at acceptable levels. Eventually, temperature s moved to lower temperatures due to the coolant effect. It s hghlghted that the temperature levels presented refer to an average value n the radal doman calculated by Eq.10. At the wall of the tube the temperature s lower than 00 o C whch manly results n the ntal temperature decrease Tme, s Fgure 9. Temperature as a functon of tme and POX length. Fg. 10, shows the voltage-tme descrpton of the PEM fuel cell. 1.4 The operaton current s set at 100 whch s assumed to reach ts steady-state value rapdly (<10sec). At the start of the smulaton tme, the voltage decreases due to the sharp ncrease of the operaton current (<10sec), but as the temperature ncreases (Fg. 11), the voltage ncrease s favored and reaches ts steady-state value after about 300sec. Temprature, o C Tme, s Fgure 11. Fuel cell stack temperature as a functon of tme. A. Case Scenaros Table I presents the effect of the H O/CH 3 OH rato on CO and hydrogen producton at the reformer. The ncrease n the water content, ncreases the hydrogen producton because methanol converson s ncreased, but the most mportant mpact s the decrease of CO content due to the water-gasshft reacton whch favors the POX operaton. TABLE I. SENSITIVITY OF THE SYSTEM IN THE H O/CH 3 OH ATIO H O/CH 3 OH CO, ppm H, mol/sec Smlarly, table II presents the effect of the O /CH 3 OH rato on CO and hydrogen producton and also to the peak temperature n the reformer. As can be seen, the ncrease of the O /CH 3 OH rato, causes an ncrease n hydrogen producton snce methanol converson ncreases (reaches ~100%). Nevertheless, ncreased oxygen content causes an ncrease n the temperature levels that favor the CO producton. Cell Voltage, Volt Tme, s Fgure 10. Cell voltage as a functon of tme. TABLE II. SENSITIVITY OF THE SYSTEM IN THE O /CH OH ATIO O /CH 3 OH CO, ppm H, mol/sec T peak, o C As t s obvous the selecton of H O/CH 3 OH:1.5 and O /CH 3 OH:0.14 s consdered satsfactory as the hydrogen producton remans at hgh levels and CO at low ones, whch makes easer the mnmzaton of CO at the POX reactor. 145

6 Moreover, hgh temperatures are excluded wth the use of a lower O /CH 3 OH rato. IV. ANALYSIS ON THE HEAT MANAGEMENT SYSTEM OF THE INTEGATED POWE SYSTEM As descrbed n Fg.1, heat management s requred n varous postons of the ntegrated system. The outlet of the two reactors needs to be cooled down before proceedng to the next subsystems, but the most challengng part n the modelng of the heat management system s the burner. In the burner, the fuel cell anode-off gas (contans unreacted hydrogen and a mxture of CO, N, CH 3 OH and traces of CO) enters the burner and provdes heat to be used n the nlet reactants. However, as was found from Aspen Plus smulatons, the produced heat s lower than system heat requrements and addtonal methanol s needed to meet the demands. The addtonal methanol wll be hgher at the startup of the system (no anode-off gas wll be present), but gradually the addtonal methanol wll be decreased. Hence, the mathematcal model needs to be accurate enough to be used n future control studes. Model predctve control s a sutable control scheme to be appled n such a power system, snce the ntegrated unt wll be studed under the developed mathematcal model. The requred power profle wll be gven as nput and methanol flowrate wll be manpulated to produce the hydrogen needed to serve the load demand. In the meantme, CO and temperature levels wll be restrcted n bounds n order to protect the varous subsystems from deteroraton (manly POX and fuel cell). All these consequent changes wll be decded based on the mnmzaton of an objectve functon that wll take nto account all the system necessary constrants. V. 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 and the PEM fuel cell was well found to smulate the dynamc operaton of the unt where all the subsystems are connected to each other. The next step wll be the ntegraton of the heat management system (burner and heat exchangers) where the developed control scheme wll try to mantan the 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 : specfc heat capacty of the component, J/K kg C t : heat capacty of fuel cell, J/ o C D r : radal effectve dffusvty, m /s D z : axal effectve dffusvty, m /s E: thermodynamc cell voltage, Volt F: Faraday 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 n: nlet condtons : component that takes part at the system I: operaton current, A j: reacton ether n reformer or POX k r : radal thermal conductvty, J/m s K k z : axal thermal conductvty, J/m s K n c : number of cells of the PEM fuel cell n e : number of electrons n F : Faraday 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, J/s Q an/cat : anode/cathode flowrate, m 3 /s Q o : volumetrc flow, m 3 /s r: radus of the reactor, m j : knetc expresson of the reacton j, mol/kg cat s t : thermal resstance at the fuel cell, o C/W S: cross secton of the reactor, m t: tme, s T: temperature, K T c : coolant temperature, K T fc : PEM fuel cell temperature, K T amb : ambent temperature, K u: superfcal gas velocty, m/s U: overall heat transfer coeffcent, W/m K V an/cat : anode/cathode volume, m 3 V c : coolant jacket volume, m 3 V fc : operaton cell voltage, Volt z: length of the reactor, m ΔG f : Gbbs free energy of formaton, J/mol ΔΗ,T, j : enthalpy of reacton j at temperature T, J/mol ε cat : vod fracton of the catalyst ν,j : coeffcent of the component n the reacton j ρ : densty of the component, kg/m 3 ρ c : coolant densty, kg/m 3 EFEENCES [1] D. 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