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Dynamic Modeling and Control of a Steam Reformer-Fuel Cell Power System Operating on LPG for Vehicular Applications

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A publication of CHEMICAL ENGINEERING TRANSACTIONS VOL. 29, 2012 Guest Editors: Petar Sabev Varbanov, Hon Loong Lam, Jiří Jaromír Klemeš Copyright 2012, AIDIC Servizi S.r.l., ISBN ; ISSN
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A publication of CHEMICAL ENGINEERING TRANSACTIONS VOL. 29, 2012 Guest Editors: Petar Sabev Varbanov, Hon Loong Lam, Jiří Jaromír Klemeš Copyright 2012, AIDIC Servizi S.r.l., ISBN ; ISSN The Italian Association of Chemical Engeerg Onle at: DOI: /CET Dynamic Modelg and Control of a Steam Reformer-Fuel Cell Power System Operatg on LPG for Vehicular Applications Dimitris Ipsakis a, Spyros Vetakis* a, Simira Papadopoulou a,b, Panos Seferlis a,c, Costas Elmasides d, Krystalia Papadaki e, Spyros Mastrogeorgopoulos e, Alexios Kyriakides a a Chemical Process Engeerg Research Institute (C.P.E.R.I., Centre for Research and Technology Hellas (CE.R.T.H., P.O. Box 60361, 57001, Thermi-Thessaloniki, Greece b Automation Department, Alexander Technological Educational Institute of Thessaloniki, P.O. Box 141, Thessaloniki, Greece c Department of Mechanical Engeerg, Aristotle University of Thessaloniki, P.O. Box 484, Thessaloniki, Greece d Systems Sunlight SA, 67200, Neo Olvio, Xanthi, Greece e Department of Chemical Engeerg, Aristotle University of Thessaloniki, P.O. Box 1517, Thessaloniki, Greece Spyros Vetakis, The core aim of this study is to develop a control scheme based on a rigorous mathematical model that will be able to capture the dynamic features of a 1 kw p fuel cell power system based on LPG reformg that satisfies acceptably power variations vehicular applications. The tegrated system consists of an LPG steam reformer followed by a water-gas-shift reactor. A high temperature PEM fuel cell accompanies the system and receives the produced hydrogen havg high tolerance CO levels (up to 1000 ppm. A burner that exploits the anode off-gas and an additional supply of fresh LPG meets system s heat requirements, while further stream heat tegration and dividual coolers complement system autonomy and efficiency. Material and energy balances fully apply system reactors and fuel cell (no axial/radial distributions are troduced, while energy balances for the cold and hot streams are developed for the tensive heat exchangg network. Model validation with available experimental data and thermodynamic results confirm the accuracy of the proposed mathematical modelg scheme. A set of simulations of the tegrated system cludg closed loops of predefed conventional PI controllers is applied order to evaluate the effectiveness of the respective control scheme. 1. Introduction LPG (liquefied petroleum gas is a widely used propane-butane mixture that is readily available from petroleum referies, is convenient storage and transportation and recently provided low prices (Zeman et al., A medium scale pilot plant unit based on LPG reformg was presented by Recupero et al. (2005, and highlighted the effect of let composition and operatg temperature on LPG conversion rates. Further thermodynamic analysis identified desired operatg ranges for steam/carbon ratios and operatg temperatures for autonomous LPG reformg units (Wang et al., 2010; Kale et al., Despite providg valuable sights on the overall process of LPG reformg, Please cite this article as: Ipsakis D., Vetakis S., Papadopoulou S., Seferlis P., Elmasides C., Papadaki K., Mastrogeorgopoulos S. and Kyriakides A., (2012, Dynamic modelg and control of a steam reformer-fuel cell power system operatg on lpg for vehicular applications, Chemical Engeerg Transactions, 29, the aforementioned studies fall short describg complex dynamic teractions that could be exploited developg effective control schemes that could mimize system operation & matenance costs and simultaneously ensure safety durg dynamic transitions. Several modelg studies on hydrocarbon reformg systems are presented literature (Wu and Pai, 2009; L et al., 2006; Ipsakis et al., 2012, with ma itiative the development of accurate dynamic models that are able to provide a rigorous framework advanced process control and optimization studies. Followg such specifications, the proposed study is organized two levels. First, an accurate dynamic mathematical model of an tegrated LPG reformg system is presented and evaluated. Secondly, accordg to engeerg knowledge and process availability a number of PI control loops of specific system variables are cluded the system dynamics. The control actions are imposed by selected manipulated variables order to operate with the required operatg limits. 2. Process Flowsheet Description The ma objective is to design, simulate and control a power system based on LPG reformg that could provide efficiently and unterruptedly power to a forklift or other vehicles through a fuel cell. The power requirements are considered to vary significantly durg a simple operatg day and therefore, dynamic transients and control flexibility policy is of primary importance such a complex problem. The chemical system is necessary to be accompanied by a Li-Ion battery for absorbg power excess from the system (chargg and for providg power deficit (dischargg durg extreme operatg load demands (Ipsakis et al., As seen from Figure 1, water is evaporated heat exchanger E1 with the use of the burner effluent. The gas mixture water-lpg (mixer is further heated E2 by the reformer let before enterg the plug flow reformer for hydrogen production. The reformer let (after E2 is air-cooled E3 and enters the high temperature shift reactor (HTS for CO mimization (less than 1000 ppm. Due to significant amount of water contaed at HTS let, a condenser is utilized for water removal and simultaneous heatg of the hydrogen rich stream (~75 % E4 before enterg the anode of the high temperature fuel cell. There, power generation takes place and the anode effluent along with fresh LPG is used as ma fuels the burner. The overall reaction scheme is shown Table 1. LPG storage TI_02 TI_03 POWER Li-Ion Accumulator TI_01 E-2 HTS REFORMER BURNER E-3 E-4 CONDENSER H2O CATHODE INLET POWER Forklift MIXER ANODE INLET FC COOLANT JACKET POWER E1 H2O storage Flue gas to vent TI_04 CATHODE OUTLET ANODE OUTLET Figure 1: LPG reformg and fuel cell power system Table 1: Reaction scheme of the LPG reformg and fuel cell power system Subsystem Reaction Subsystem Reaction Reformer C 3H H 2O 3 CO + 7 H 2 C 4H H 2O 4 CO + 9 H 2 CΟ +H 2O Η 2 + CO 2, CΟ +3 H 2 CΗ 4 + H 2O Burner C 3H 8 + 5O 2 3CO 2 + 4H 2O C 4H O 2 4CO 2 + 5H 2O CH 4 + 2O 2 CO 2 + 2H 2O CO + 0.5O 2 CO 2 H O 2 H 2O Water Gas Shift CΟ +H 2O Η 2 + CO 2 Fuel Cell H O 2 H 2O 50 3. Mathematical Modelg The nonlear dynamic model consists of: a component molar balances, b energy balances that identify temperature dynamics of streams and subsystems and c constitutive equations that fully complement the mathematical modelg. The assumptions that follow the overall mathematical model refer to: a ideal gas behavior, b no spatial variation is considered, c negligible system pressure drop and e pseudo-homogeneous ketics. Equations 1, 2 provide the molar and energy balances respectively: dni d( Ci, V C i, C i, v r i, j i, j (1 d ( cpvt c p ( T T th (2 where n i the ith component moles mol, C i the ith component concentration mol/m 3, V the mixture volume m 3, the volumetric flowrate m 3 /s, r i,j the j reaction rate of component i mol/m 3 s, ν i,j the stoichiometric coefficient of i reaction j, T is the fluid let stream temperature K, c p the specific heat capacity J/K kg, ρ the mixture total density kg/m 3 and Σ th the sum of the total heat exchange (e.g. environmental losses, heat radiation, heat of reaction, heat due to electrochemical phenomena, heat exchange between streams W. In the case of reformer-burner couplg an additional set of equations is needed order to derive the dynamics of the wall temperature teraction. dtburner, wall ( mcp burner UAburner, ( Tburner, Tburner, wall UAreformer, wall( Tburner, wall Treformer, wall (3 dtreformer, wall ( mcp reformer UAreformer, wall( Tburner, wall Treformer, wall UAreformer, ( Treformer, wall Treformer, (4 where m the subsystem mass kg, c p the subsystem specific heat capacity J/K kg, T burner,wall and T reformer,wall the subsystem wall temperature K, T burner, and T reformer, the fluid let temperature K (Eq.2, UA burner, and UA reformer, the overall heat transfer coefficient from bulk to wall W/K, and UA burner,wall and UA reformer,wall the overall heat transfer coefficient from wall to wall W/K The volumetric flowrate, concentration and molar flowrate are associated with the followg scheme: N Fi, / RT / i 1 (5 / P Fi, / Ci, / (6 / where / denote the let/let of a subsystem and F i the i-th component flowrate mol/s. In the case of the fuel cell, there is a lear dependence of current draw and hydrogen consumption via the Faraday s law: R n I c fc fc n (7 f ne F 51 where R fc the reaction rate mol/s, n c are the number of cells, I fc the operation current A, n e the number of electrons, F the Faraday s constant Cb/mol and n f is the fuel cell electrical efficiency. The fuel cell operatg voltage (V fc, Volt is based on a group of non-lear equations (Ipsakis et al., 2012 that is dependent on various system variables such as temperature (T fc, K, component concentrations (C i,fc, mol/m 3, operatg current (I fc, A, design characteristics (d and electrochemical parameters (p: V f ( T, C,, I, d, p fc fc i fc fc (8 4. Model Validation Model validation based on experimental data is a prerequisite stage of the mathematical model development. To this end, an experimental run regardg the followg operatg conditions was performed by (HELBIO S.A., 2012 LPG to reformer: 1.75ml/m, LPG to burner: 1.05 ml/m, water to reformer (liquid: 14.2 ml/m, air to burner: λ=1.4 (40 % excess. Furthermore, order to compare several results that are not measured or cannot be derived from experiments (heat exchange, various temperatures, etc and this way ensure the model validity, Aspen Plus simulations steady state mode were performed (Figure 1 and compared with the dynamic simulations. In Table 2, only the similar results are presented and comprise: experimental data/aspen Plus Simulation/Dynamic Simulation. As can be seen, the accuracy of results is acceptable and dicates the further use of the model control studies. Table 2: Comparison between simulated (dynamic model and Aspen Plus and experimental data Reformer HTS H / 72.7 / 72.5 % 73 / 75.8 / 75.8% CO / 12.1 / 12 % 23 / / 22.3 % CO 12.5 / 14.2 / 14.2 % 1.15 / 1.26 / 1 % CH / 1 / 1 % 1.5 / 0.9 / 0.9 % 5. Control Analysis The PI controllers (discrete velocity form that are used the mathematical model are troduced specific closed loops of the system accordg to current engeerg knowledge of the tegrated system. Table 3 presents the selected pairs of controlled and manipulated variables and the respective parameters of the cluded controllers the process flowsheet of Figure 1. Table 3: System Controlled and Manipulated variables for samplg time T s=5s Controlled Variables Manipulated Variables Controller Parameters Reformer operatg temperature LPG flow at burner K c=100, τ Ι,fast=60 s, τ Ι, slow =120 s HTS let temperature Coolant flow rate at E3 K c=10, τ Ι,fast =300 s, τ Ι, slow =3000 s Fuel cell operatg temperature Coolant flow rate at coolg jacket K c=50, τ Ι,fast =300 s, τ Ι, slow =3000 s Fuel cell let temperature Coolant flow rate at condenser K c=10, τ Ι,fast =200 s, τ Ι,slow =800 s As seen from Figures 2 and 3, arbitrary selected set-pot trajectories were imposed to the system and a set of closed loop simulation was performed. As was found, a very aggressive tegral action (fast action causes the system at the start up to promote a very high overshoot that could eventually deteriorate catalyst performance or material that operate at higher than desired operations. Also, manipulated variables are forced to crease their action, possibly near their maximum limits. After achievg steady state operation however, the aggressive PI action is considered quite satisfactory for this operatg scheme. Providg a slow action though at start-up, the overshoot is elimated but, steady state is achieved much later. To this end, a combed action of the two is proposed with slow action at first 2000 s and higher at the second stage as seen from controller parameters at Table 3. 52 Temperature, K Coolant flowrate, kg/s Temperature, K Coolant flowrate, kg/s Temperature, K Coolant flowrate, kg/s Temperature, K LPG flowrate, mol/s a b c d Figure 2: a Reformer temperature dynamics, b LPG feed flowrate manipulation, c HTS let temperature dynamics and d E3 coolant flowrate a b c d Figure 3: a Fuel cell let temperature dynamics, b condenser coolant flowrate manipulation, c Fuel cell operatg temperature dynamics and d coolant flowrate manipulation 53 6. Conclusions A control-oriented mathematical model for an tegrated LPG reformg and PEM fuel cell power generation system was presented this study. Experimental results were used to evaluate the accuracy of the proposed model scheme and specific PI control loops were troduced the process flowsheet. As was found, a clamped operatg control policy is required order to offset between high overshoots and low start-up times. Based on this come, the next step should be the development of a model-based advanced control framework on the premises of model predictive (nonlear control. Such an approach, aims to the matenance of process control targets specified trajectories by manipulatg a centralized scheme selected process variables. Fuel mimization and prolonged battery life is considered important hybrid applications (fuel cell and Li-Ion battery that volve complex teractions along with a combation of slow and fast dynamics. Until steady state is reached, battery as a fast subsystem could provide power to the system and afterwards the tegrated reformg-fuel cell system can support the overall operation. Acknowledgment The presented study is conducted on the framework of National Research Projects and Co-fanced by National Strategic Reference Framework (NSRF of Greece and the European Union, program Archimedes III (OPT-VIPS and program Cooperation 2009-ACT-I (09-ΣΥΝ The contribution from HELBIO, Hydrogen and Energy Production Systems is gratefully acknowledged. References HELBIO S.A., Hydrogen and Energy Production Systems www.helbio.com , Accessed Ipsakis D., Vetakis S., Papadopoulou S., Seferlis P., 2012, Optimal operability by design a methanol reformg-pem fuel cell autonomous power system, International Journal of Hydrogen Energy, doi:/ /j.ijhydene Ipsakis D., Giannakoudis G., Papadopoulos A.I., Vetakis S., Seferlis P., 2009, Design and optimization of a Stand-Alone Power System based on renewable energy sources, Chemical Engeerg Transactions, 18, Kale G.R., Kulkarni B.D., Joshi A.R., 2009, Thermodynamic study of combg chemical loopg combustion and combed reformg of propane, Fuel, 89(10, L S.T., Chen Y.H., Yu C.C., Liu Y.C., Lee C.H., 2006, Dynamic modelg and control structure design of an experimental fuel processor, International Journal of Hydrogen Energy,31(3, Recupero V., Po L., Vita A., Cipitı F., Cordaro M., Laganà M., 2005, Development of a LPG fuel processor for PEFC systems: Laboratory scale evaluation of autothermal reformg and preferential oxidation subunits, Ιnternational Journal of Hydrogen Energy, 30(9, Wang X., Wang N., Zhao J., Wang L., 2010, Thermodynamic analysis of propane dry and steam reformg for synthesis gas or hydrogen production, International Journal of Hydrogen Energy, 35(23, Wu W., Pai C.C., Control of a heat-tegrated proton exchange membrane fuel cell system with methanol reformg, Journal of Power Sources, 194(2, Zeman H., Url M., Hofbauer H., 2011, Autothermal reformg of hydrocarbon fuels, Chemical Engeerg Transactions, 24,
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