Design of a Methanol Reformer for On-Board Production of Hydrogen As Fuel for a 3Kw High-Temperature Proton Exchange Membrane Fuel Cell Power System

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Design of a Methanol Reformer for on-board Production of Hydrogen as Fuel for a 3kW High-temperature Proton Exchange Membrane Fuel Cell Power System

Vladimir Gurau Georgia Southern University, [email protected]

Adedayo Ogunleke Georgia Southern University, [email protected]

F. Strickland

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Recommended Citation Gurau, Vladimir, Adedayo Ogunleke, F. Strickland. 2020. "Design of a Methanol Reformer for on-board Production of Hydrogen as Fuel for a 3kW High-temperature Proton Exchange Membrane Fuel Cell Power System." International Journal of Hydrogen Energy, 45 (56): 31745-31759: Elsevier. doi: https://doi.org/ 10.1016/j.ijhydene.2020.08.179 source: https://www.sciencedirect.com/science/article/pii/ S0360319920332067?via%3Dihub#! https://digitalcommons.georgiasouthern.edu/manufact-eng-facpubs/104

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Design of a methanol reformer for on-board production of hydrogen as fuel for a 3 kW High- Temperature Proton Exchange Membrane Fuel Cell power system

* V. Gurau a, , A. Ogunleke b, F. Strickland a a Georgia Southern University, Department of Manufacturing Engineering, Statesboro, GA, 30458, USA b Georgia Southern University, Mechanical Engineering Department, Statesboro, GA, 30458, USA highlights graphical abstract

CFD is an essential tool for deter- mining process parameters of methanol reformers. Operating conditions of MEAs are critical constraints in methanol reformer design. Reformer design must result in high conversion efﬁciency and small reformer volume.

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Article history: The method of Computational Fluid Dynamics is used to predict the process parameters Received 31 January 2020 and select the optimum operating regime of a methanol reformer for on-board production Received in revised form of hydrogen as fuel for a 3 kW High-Temperature Proton Exchange Membrane Fuel Cell 28 May 2020 power system. The analysis uses a three reactions kinetics model for methanol steam

Accepted 21 August 2020 reforming, water gas shift and methanol decomposition reactions on Cu/ZnO/Al2O3 cata- Available online 15 September 2020 lyst. Numerical simulations are performed at single channel level for a range of reformer operating temperatures and values of the molar ﬂow rate of methanol per weight of Keywords: catalyst at the reformer inlet. Two operating regimes of the fuel processor are selected Methanol reformer which offer high methanol conversion rate and high hydrogen production while simulta- High-temperature proton exchange neously result in a small reformer size and a reformate gas composition that can be membrane fuel cell tolerated by phosphoric acid-doped high temperature membrane electrode assemblies for Methanol steam reforming proton exchange membrane fuel cells. Based on the results of the numerical simulations, Fuel cell system optimization the reactor is sized, and its design is optimized. Computational ﬂuid dynamics © 2020 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

* Corresponding author. E-mail address: [email protected] (V. Gurau). https://doi.org/10.1016/j.ijhydene.2020.08.179 0360-3199/© 2020 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY-NC- ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 31746 international journal of hydrogen energy 45 (2020) 31745e31759

3 Nomenclature Vcat Volume of the active catalyst bed ðm Þ V/cell Voltage delivered by a single cell in the fuel cell A Active area of the fuel cell MEA (cm2) : Â stack (V) aref Specific area density of the reformer catalyst (3 5 W=F ; Inverse molar flow rate of methanol at inlet per 108m2=m3) CH3 OH in weight of catalyst (kg s=mol) cp Specific heat capacity at constant pressure (J=kg K) ÀX xi Molar fraction of component i C1 À C7 Adsorption coefficients of surface spices (bar ) y Mass fraction of component i T; T i CS1 CS1A Total surface concentration of active sites for MSR and WGS reactions (mol=m2) Greek symbols T; T ε CS2 CS2A Total surface concentration of active sites for MD V Volume porosity of the catalyst bed (0.36) reaction (mol=m2) ε Area porosity of the catalyst bed (0.36) 2= Di Diffusion coefficient of species i (m s) z Methanol conversion rate F Faraday constant (96,485 C/eq) h Reformer thermal efficiency (%) h Specific enthalpy (J=kg) l Thermal conductivity (W=mK) 2 l i Fuel cell current density (A/cm ) i Stoichiometric ratio of component i in À9 2 KV Permeability of catalyst bed (1:0 Â 10 m ) electrochemical reaction (1.2 for hydrogen, 2.0 for

Ki Equilibrium constant for reaction i air) 2= ki Rate constant of reaction i (m mol s) m Dynamic viscosity (kg=ms) ð = Þ u_ = 3 LHV Lower heating value MJ kg i Source term for species i (g m s) = 3 Mi Molecular weight of species i (kg kmols) r Density (kg=m ) _ = mi Mass ﬂow rate of species i (kg s) F Relative humidity of the reformate gas (%)

n Number of cells in the fuel cell stack FW Parameter relating the gas composition to the _ = ni Molar ﬂow rate of species i (kmol s) WGS equilibrium p Pressure (Pa) Subscripts pi Partial pressure of species i (bar) MD Methanol decomposition reaction Pe Electrical power provided by the fuel cell stack MSR Methanol steam reforming reaction (3000 W) _ WGS Water gas shift reaction Q Source term for energy equation (W=m3) in Property at reformer inlet q_ Wall heat ﬂux (W=m2) out Property at reformer outlet R Universal gas constant (8314 J=kmol K) _ = 2 eq Equivalent property rj Rate of reaction j (mol m s) mix Property of the gas mixture S/M Steam to methanol molar ratio at reformer inlet cat Catalyst T Absolute temperature (K) U Velocity vector (m/s)

monoxide concentrations up to 3% [4], compared to less than Introduction 100 ppm in low-temperature PEMFCs. These benefits make HT- PEMFCs particularly suitable for reformate gas-operating auto- There is a need to demonstrate power systems with a high fuel- motive systems with a simplified design, in which the prefer- to-electricity conversion efficiency used to extend the endur- ential oxidation stage in the fuel processing line can be ance of autonomous terrestrial vehicles and unmanned aerial eliminated. Also, they result in fuel cell power systems with a e vehicles (UAV) [1 3]. Fuel cells are energy conversion devices significant reduction in cost and complexity resulting from a that convert the chemical energy of hydrogen directly to elec- smaller radiator in the cooling loop and the elimination of the tricity at higher conversion efficiencies than other systems. humidifiers in the gas feed loops. In addition to these advan- Proton Exchange Membrane Fuel Cells (PEMFCs) deliver high- tages, HT-PEMFCs offer the ability to use the fuel cell stack waste power density and offer the advantages of low weight and vol- heat to boil water or heat space when used as a combined heat ume, rapid start-up, and better durability compared with other and power (CHP) system, increasing thus the system efficiency fuel cells, features that make them particularly suitable for substantially when compared to conventional low temperature automotive, underwater and aerial applications. High- fuel cells power systems. Temperature PEMFCs (HT-PEMFCs) are capable of operating Even though hydrogen gas has the highest heat of com- between 120 C and 180 C without external humidification, bustion (MJ/kg), its energy density (MJ/m3) is lower compared which renders them significant benefits over the low- to other fuels. Despite having a higher energy conversion ef- temperature PEMFCs. These benefits include simplified water ficiency than other systems, a long-endurance automotive and thermal management, faster electrode kinetics for both fuel cell power system using compressed hydrogen gas would electrode reactions, and an improved anode tolerance to carbon international journal of hydrogen energy 45 (2020) 31745e31759 31747

result in a system having a substantial volume due to the size It can be noted that only two of these reactions are linearly of the gas cylinders. However, methanol is an energy carrier independent and any one of them can be expressed as the with energy density seven times higher than that of com- algebraic sum of the other two. Because of this, there has been pressed hydrogen gas available using today’s technology. disagreement in the past regarding the reactions that must be Methanol steam reformers convert methanol solution into a included in a kinetic model of the process of methanol steam hydrogen rich gas that contains as well carbon monoxide, reforming on Cu/ZnO/Al2O3 catalyst. Previous numerical carbon dioxide and traces of unused water and methanol models have used one or two of the three possible reactions vapors in a catalytic reaction at temperatures as low as [7,8,13e17] and have considered that the other reactions were 200 Ce350 C and using inexpensive copper-based catalysts either at equilibrium, or that their reaction rates were negli- [5]. The reforming reaction is endothermic and requires the gible. Peppley [18] and Peppley et al. [19,20] have shown that input of heat to sustain the process. This heat can be produced the rate expressions for all three reactions (1e3) must be using the catalytic combustion of methanol vapor in excess of included in the kinetic model to accurately predict the air in a Pt-based catalyst. The entire combustion/reforming of composition of the product gas and that reaction models methanol can be obtained onboard and therefore can be in- which involve only one or two of the possible three reactions tegrated in the same power system with the fuel cell. Liquid are unable to explain the experimentally observed variation in methanol can be easily stored and carried onboard without the product composition. Furthermore, they showed that the requiring special technologies. The exhaust hot combustion MD reaction (3) occurs on a different type of catalyst sites than gases and the reformate gas can be used to evaporate the the other two reactions. The three - equation kinetic model of methanol solution before entering the reformer reactor. Since Peppley [18] and Peppley et al. [19,20] has been used in the past the reforming process takes place at temperatures only in Computational Fluid Dynamics (CFD) studies of methanol slightly higher than the fuel cell operating temperature, the steam reformers [6,11,12,21e26]. Other three - equation ki- exhaust gases from the reformer may be used to preheat the netic models have been used in Ref. [27,28]. The interested fuel cell for a faster startup. HT-PEMFC fed with hydrogen reader may find other methanol reforming technologies for from a methanol reformer would constitute a DC power sys- production of hydrogen in Ref. [29e34]. tem with high energy conversion efficiency and simplified The objective of this study is to develop and use a CFD thermal management that has not received sufficient atten- model of a methanol steam reformer to calculate the opti- tion in the past. mum operating regime and to size and optimize the design of While HT-PEMFCs are tolerant to carbon monoxide in a fuel processor for on-board production of hydrogen as fuel concentrations up to 3%, there is a penalty in their perfor- for a 3 kW HT-PEMFC power system. This endeavor represents mance resulting from the dilution of hydrogen in the refor- an initial step in the fabrication of a power system with a high mate gas. Hydrogen can be separated from the reformate gas fuel-to-electricity conversion efficiency used to extend the before entering the fuel cell using an electrochemical hydrogen endurance of autonomous terrestrial vehicles and unmanned pump, which is essentially a PEMFC operating in reverse. aerial vehicles (UAVs). Since the reactions in a methanol steam reformer are The reminder of this paper is organized as follows: Section overall endothermic and the catalyst particles have a rela- Mathematical model presents the CFD model used in this tively low thermal conductivity, the process in a fixed-bed study. Section Model Validation compares numerical simula- steam reformer is characterized by a non-uniform tempera- tions obtained in this work to experimental results presented ture field which contains a cold region in its core where the by Peppley [18] in order to calibrate and validate the numerical methanol conversion efficiency is lower. To improve the model. In Section Results and Discussion we perform nu- methanol conversion efficiency through better thermal man- merical simulations and based on these calculations and on agement, one uses a system of micro-channels machined in a the operating constraints of high-temperature membrane highly thermally conductive material. Two methanol electrode assemblies (MEAs), we select optimum operating reformer designs with micro-channels have been studied: regimes. In Section Reformer Design and Sizing we size the with parallel channels fabricated in a flat plate [6e12], or with fuel processor based on the numerical results and optimize its a bundle of tubular channels fabricated in a cylindrical body design. [13,14]. The second design which was adopted in this work offers better technical solutions for maintaining the catalyst particles within the reformer channels. The three overall reactions that occur in a methanol steam Mathematical model reformer are the methanol steam reforming (MSR) reaction (1), the water gas shift (WGS) reaction (2) and the methanol The mathematical model used in the present analysis uses a decomposition (MD) reaction (3): three reaction model that accounts for methanol steam À Á reforming (1), water gas shift (2) and methanol decomposi- 0 CH3OH þ H2O 4 CO2 þ 3H2 DrH ¼ 49; 500 J = mol 298 MSR tion (3) reactions over a Cu/ZnO/Al2O3 catalyst. The 3D (1) computational domain consists of a single cylindrical À Á channel filled with active catalyst and having entry and exit þ 4 þ D 0 ¼À ; = CO H2O H2 CO2 rH298 WGS 41 100 J mol (2) regions filled with inert particles. A mixture of water and methanol vapor enters the flow domain through inlet. The À Á 4 þ D 0 ¼ ; = domain is heated uniformly through the surrounding walls. CH3OH 2H2 CO rH298 MD 90 600 J mol (3) 31748 international journal of hydrogen energy 45 (2020) 31745e31759

Conservation equations u_ ¼ðÀ _ À _ Þ , , CH3OH rMSR rMD MCH3OH aref (11) The CFD model consists of the following governing equations: The mass conservation equation: u_ ¼ð , _ þ , _ þ _ Þ , , H2 3 rMSR 2 rMD rWGS MH2 aref (12) v ðε rÞþV , ðεr Þ¼ V U 0 (4) u_ ¼ð_ þ _ Þ , , vt CO2 rMSR rWGS MCO2 aref (13) The momentum conservation equations: u_ ¼ðÀ _ À _ Þ , , H2O rMSR rWGS MH2 O aref (14) v m ðεVrUÞ¼ ÀεVVp þ εV U (5) vt KV _ _ _ uCO ¼ðrMD À rWGSÞ , MCO,aref (15) The species conservation equations:

v À Á À Á À Á ε ry þ V , εrUy À V , εrD Vy ¼ ε u_ i ¼ CH OH; H O; H ; CO ; CO (6) vt V i i i i V i 3 2 2 2

The energy conservation equation: The heat source in the energy conservation Eq. (7) represents the heat of reactions: v À Á À Á À Á ε r þ V , εr À V , εl V ¼ ε _ ÂÀ Á v V cpT UcpT eq T VQ (7) _ t Q ¼À D H0 , r_ À r Á298 MSR MSRÀ Á Ã þ D 0 , _ þ D 0 , _ , Reaction kinetics model rH298 WGS rWGS rH298 MD rMD aref (16) m The thermodynamic properties , l and cp of the gas In this study we used the reaction kinetics model of Peppley mixture are calculated as mass fraction weighted averages of [18] and Peppley et al. [19,20] which consists of reversible the thermodynamic properties of gas species: Langmuir-Hinshelwood reaction rate expressions for each of X e the reactions (1 3) involved in the process of methanol steam Fmix ¼ yi,Fi F ¼ m; l; Cp (17) i reforming over Cu/ZnO/Al2O3 catalyst:

" !# . p3 ,p 1=2 1 H CO2 T T k ,C , p p , 1 À , 2 ,CS ,CS MSR 1 CH3OH H2 K p ,p 1 1A MSR CH3 OH H2O _ ¼ rMSR . . = (8) = = = 1 2 1 þ C , p p1 2 þ C ,p ,p1 2 þ C p p1 2 1 þ C ,p 1 CH3OH H2 2 CO2 H2 3 H2O H2 4 H2

" !# . , À Á 0 = pH pCO 2 , , , 1 2 , À 1 , 2 2 , T kWGS C3 pCO pH2O pH 1 K p ,p CS1 The expressions used in this study for the thermodynamic 2 WGS CO H2O _ ¼ . . rWGS 2 properties of the gas species as function of temperature are 1=2 1=2 1=2 1 þ C , p p þ C ,p ,p þ C , p p 1 CH3OH H2 2 CO2 H2 3 H2 O H2 shown in Table 2. The density of the gas mixture is calculated (9) as:

" !# r ¼ Pp . y (18) p2 ,p i 1=2 1 H CO T T RT i k ,C , p p , 1 À , 2 ,CS ,CS Mi MD 5 CH3OH H2 K p 2 2A MD CH3 OH _ ¼ rMD . . = The equivalent thermal conductivity of the porous catalyst = = 1 2 1 þ C , p p1 2 þ C , p p1 2 1 þ C ,p 5 CH3 OH H2 6 H2 O H2 7 H2 region is the weighted average of the thermal conductivities of (10) the gas mixture and of the solid matrix:

The expressions for the reaction kinetics parameters in Eq. leq ¼ εVlmix þð1 À εVÞlcat (19) 8e10 are shown in Table 1. The values of the physical properties of the catalyst are shown in Table 3. Constitutive relations The partial pressures in the expressions for the reaction rates (8e10) are calculated as a function of mass fractions as: The sources for chemical species are calculated from the reaction rates (8e10): international journal of hydrogen energy 45 (2020) 31745e31759 31749

Table 1 e The parameters of the reaction kinetics model Table 3 e Physical properties of the catalyst. [18,20]. Property Value Parameter Expression Units r ,ð = 3Þ Density, cat kg m 1220 (from Ref. [39]) ; 2 KMSR 7 060 bar Thermal conductivity, l ,ðW =mKÞ 10 exp 24:39 À cat T Volumetric porosity of catalyst bed, εV 0.36 À ; e 2 9 KWGS 4 773 Permeability of catalyst bed, K ,ðm Þ 1.0 Â 10 exp À 4:67 þ V T 2 KMD 11; 833 bar exp 29:06 À T À ; 2 À kMSR 102 800 m =mol s ¼ Â 5 7:4 Â 1014exp pi pxi 10 (20) RT 2 kWGS À87; 600 m =mol s 5:9 Â 1013exp Boundary conditions RT 2 kMD À170; 000 m =mol s 3:8 Â 1020exp Inlet boundary conditions RT À0:5 The operation of catalytic reformers is usually analyzed as a C1 À41:8 20; 000 bar exp þ R RT function of two parameters at the reformer inlet: the inverse À1:5 C2 179:2 100; 000 bar of molar flow rate of methanol per weight of catalyst, exp À R RT W=F ; and the steam-to-methanol (or to carbon) molar CH3OH in À0:5 C3 À44:58 20; 000 bar exp þ ratio, S/M. The latter is defined as: R RT 0 À : _ _ C À44:58 20; 000 bar 1 5 nH o mH O MCH OH 3 exp þ S=M ¼ 2 ¼ 2 , 3 (21) R RT n_ m_ M CH3OH CH3OH H2O À1 C4 À100:8 50; 000 bar exp þ R RT The boundary conditions at the reformer inlet are there- À0:5 fore specified as functions of W=FCH OH;in and S/M. For the C5 30 20; 000 bar 3 exp þ R RT momentum equations, the mass flow rate of methanol/water À0:5 C6 30 20; 000 bar mixture at inlet is: exp þ R RT À Á À : ; À1 S=M,M þ M ,ð1 À ε Þ,V ,r C7 46 2 50 000 bar _ H2O CHÀ 3 OH ÁV cat cat exp þ min ¼ (22) R RT , ; 1000 W FCH3OH in T : Â À6 = 2 CS1 7 5 10 mol m T : Â À5 = 2 The boundary condition for the species conservation CS1A 1 5 10 mol m T : Â À6 = 2 equations are: CS2 7 5 10 mol m T : Â À5 = 2 CS2A 1 5 10 mol m 1 ; ¼ yCH3OH in M (23) = H2 O þ S M M 1 CH3 OH

Table 2 e The thermodynamic properties of the gas species as function of absolute temperature [35e38]. Component Property Expression

ð = Þ 2 CH3OH cp J kg K À 0:001T þ 3:419T þ 400:99 m ðkg =msÞ ½0:493 ,expð0:0025 ,TÞ Â 10À5 lðW =mKÞ 1:034 Â 10À5T À 1:653 Â 10À3 LHV,½MJ =kg 20.09 ð = Þ À9 5 À6 4 À3 3 2 H2O cp J kg K À 1:5071 Â 10 T þ 4:0048 Â 10 T À 4:2374 Â 10 T þ 2:2340T À 5:8718 Â 102T þ 6:3552 Â 104 m ðkg =msÞ ð0:0036T À 0:1016ÞÂ 10À5 l ðW =mKÞ 8:070 Â 10À5T À 6:269 Â 10À3 ð = Þ À6 3 À2 2 4 H2 cp J kg K 8:551 Â 10 T À 1:363 Â 10 T þ 7:491T þ 1:311 Â 10 m ðkg =msÞ ð0:0017T þ0:4094ÞÂ 10À5 l ðW =mKÞ 0:0004T þ 0:0688 LHV,½MJ =kg 119.96

CO2 cp ðJ =kg KÞ 331:33 Â lnðTÞÀ 1041:7 m ðkg =msÞ ð0:0039T þ0:3218ÞÂ 10À5 l ðW =mKÞ 8:001 Â 10À5T À 7:581 Â 10À3 2 CO cp ðJ =kg KÞ 0:0003 Â T À 0:1002 Â T þ 1041:7 m ðkg =msÞ ð0:0034T þ0:9092ÞÂ 10À5 l ðW =mKÞ 6:90 Â 10À5T þ 4:50 Â 10À3 31750 international journal of hydrogen energy 45 (2020) 31745e31759

Fig. 1 e The computational domain for model validation.

ﬂuxes are set to zero. The walls bounding the chemically 1 inactive entry and exit regions of the domain are adiabatic ; ¼ À yH2O in 1 M (24) = H2O þ (see Fig. 1). The heat ﬂux at the wall bounding the active S M M 1 CH3H catalyst region is assigned as: Z ¼ ¼ ¼ yH ;in yCO ;in yCO;in 0 (25) _ 2 2 QdV _ The temperature of the reactant gasses at inlet is known. q ¼ (26) areaheating wall

Wall boundary conditions In Eq. (26) the integral at the numerator is calculated over The walls bounding the channels are impermeable to gasses the volume occupied by the active catalyst region, Vcat. Since but are thermally conductive. The gas velocities and species the reaction is endothermic and the speciﬁc heat, cp of the

Fig. 2 e Present CFD model prediction vs experimental results of Peppley [18,20] at 1 atm and S/M ¼ 1. The results of the present model are plotted over the original graph in Ref. [20] (reprinted with permission from Elsevier). international journal of hydrogen energy 45 (2020) 31745e31759 31751

Fig. 3 e Present CFD model prediction vs experimental results of Peppley [18,20] The results of the present model are plotted over the original graph in Ref. [20] (reprinted with permission from Elsevier).

Table 4 e Simulation results for model validation. Case # Operating Parameters Mass ﬂow rates of reformate components À at exit (g/s) x10 3 À1 _ _ _ _ _ z 4 Tin (K) p (atm) S/C W/FCH3OH,in (kg s mol ) mH2 mCO2 mCO mCH3 OH mH2 O W 1 513 1 1 2 3.46 26.32 0.140 172.30 96.91 0.100 0.23 2 513 1 1 4 2.59 20.22 0.084 81.06 45.60 0.153 0.47 3 513 1 1 6 2.2 17.59 0.057 51.10 28.75 0.199 0.75 4 513 1 1 8 2.02 16.59 0.062 20.20 35.91 0.250 1.05 5 513 1 1 10 1.85 15.56 0.058 27.11 15.24 0.292 1.24 6 533 1 1 4 4.52 37.87 0.355 68.36 38.46 0.286 0.411 7 533 1 1 10 3.39 25.67 0.624 19.28 10.85 0.496 0.92

_ reaction products are higher than that of the reactants, the follow the heat of reaction, Q, the reaction would eventually integrant at the numerator, expression (16) decreases as the seize. The integrant is therefore kept constant in time and is reaction advances in time until it reaches an equilibrium calculated from (16) for fresh reactant composition at the value. If the boundary condition (26) would be allowed to operating temperature.

e S/M ¼ W=F ¼ Fig. 4 Temperature distribution along the reformer, from inlet to outlet, at 533 K, 1 atm, 1 and. CH3 OH;in 4. 31752 international journal of hydrogen energy 45 (2020) 31745e31759

Fig. 5 e Mass fraction distributions of (a) methanol, (b) hydrogen, (c) carbon dioxide and (d) carbon monoxide, at 533 K, 1 atm, S/M ¼ W=F ¼ 1 and CH3 OH;in 10. international journal of hydrogen energy 45 (2020) 31745e31759 31753

e W=F Fig. 6 Methanol conversion rate as function of temperature and CH3 OH;in.

Parameters describing the reformer performance By convention, in this study the reformer operating temperature was considered to be the average temperature over The methanol conversion rate and the reformer thermal efﬁ- the active catalyst region: ciency are deﬁned as: Z ¼ 1 _ T _ Tdm (29) _ ; À _ ; m nCH3OH in nCH3 OH out z ¼ (27) active region _ ; nCH3 OH in and Model validation LHV ,m_ ; h ¼ À H2 H2 outÁ Â 100 LHV , m_ ; À m_ ; þ q_,area CH3OH CH3 OH in CH3 OH out heating wall The CFD calculations were performed using ANSYS-CFX (28) software with its High-Resolution Advection Scheme. The

e W=F Fig. 7 Mass ﬂow rate of hydrogen produced as function of temperature and CH3 OH;in. 31754 international journal of hydrogen energy 45 (2020) 31745e31759

e W=F Fig. 8 CO vol% in reformate gas as function of temperature and CH3OH;in.

domain grid (Fig. 1) was generated using ICEM CFD software entry and exit sections have adiabatic walls. The temperature and consists of 392,000 hexahedral elements with a maximum of the methanol solution at the reformer inlet was set equal to length ratio of 7.9. The CFD model was validated against the the operating reformer temperature. experimental results of Peppley [18,20]. The computational Simulations were run for operating temperatures of 513K domain (Fig. 1) corresponds to Peppley’s experimental setup and 533K, for an operating pressure of 1 atm, for a steam to [18] and consists of a tubular fixed-bed reactor having a methanol molar ratio, S/M of 1 and for different values of = 22.1 mm internal diameter, a 15 mm long non-reacting entry W FCH3 OH;in. Figs. 2 and 3 show an excellent agreement be- section filled with inert particles, a 40 mm long reactive sec- tween our simulations (numbered large red circles) and Pep- tion filled with Cu/ZnO/Al2O3 catalyst particles, followed by a pley’s experimental results [18,20] for a wide range of 6 mm long non-reacting exit section filled with inert particles. operating conditions. The curves in Figs. 2 and 3 represent the The active region of the reformer is heated, while the inert prediction of Peppley’s model. The numbers correspond to the

Fig. 9 e Relative humidity of the reformate gas after cooling to a fuel cell operating temperature of 180 C. international journal of hydrogen energy 45 (2020) 31745e31759 31755

Fig. 10 e Design of a single methanol reforming unit.

¼ = ¼ cases shown in Table 4. Fig. 3 represents the gas composition 1 atm, S/M 1 and W FCH3OH;in 4 (case 6 in Table 4). Since the relative to the water gas shift reaction as a function of meth- overall reaction in the reformer is endothermic and the ther- 4 anol conversion. Parameter W in Fig. 3 defined as: mal conductivity of the catalyst is relatively low, the temperature distribution is characterized by a colder region in the P P 1 4 ¼ CO2 H2 (30) reformer core, about 40 K below the temperature in the re- W P P K CO H2O WGS gions close to the heating walls. In this colder region the re- is a measure of the product composition relative to the action rates are lower and therefore the methanol conversion water gas shift reaction (when the WGS reaction is at equi- efficiency is lower. This expected result which is characteristic 4 ¼ librium W 1). In our calculations, the partial pressures of to fixed-bed catalytic reactors points out their disadvantage the gas species were calculated at reformer outlet and the and the need to use reformers with micro-channels fabricated equilibrium constant was calculated at the reformer operating in a material with high thermal conductivity. temperature. Fig. 5 presents the mass fraction distributions of the Fig. 4 presents the temperature field along the reformer reformate gas components along the reformer at an operating from inlet to outlet, at an operating temperature of 533 K,

Fig. 11 e Number of reformer units required for each simulated case. 31756 international journal of hydrogen energy 45 (2020) 31745e31759

¼ = ¼ temperature of 533 K, 1 atm, S/M 1 and W FCH3OH;in 10 (case

W 7inTable 4). 4 mes Reformer z Results and discussion Performance 0.368 73.9% 0.472 66.8%

The methanol reformer design adopted in this work consists of a bundle of 2 mm diameter, equally distanced tubular channels fabricated in a cylindrical body. This design offers a better thermal management and therefore higher methanol 2.5% 2.0% conversion efficiency when compared to fixed-bed reactors. The computational domain used in the analysis consists of a single 2 mm diameter, 60 mm long channel comprising a 5 mm long non-reacting entry region filled with inert particles,

Reformate quality a 50 mm long reactive region ﬁlled with catalyst and a 5 mm long non-reacting exit region ﬁlled with inert particles.

CO vol% Relative humidity Numerical simulations were performed for six different operating temperatures and ﬁve different molar ﬂow rates of O 2 methanol per weight of catalyst, W=F ; at the reformer H CH3OH in _ m inlet. In all cases, the operating pressure was 1 atm and the steam-to-methanol molar ratio, S/M ¼ 1. Higher ratios were not

OH considered in the analyses as it would have resulted in unac- 3

CH ceptable values of the reformate gas relative humidity. The _ m numerical results were used to select the optimum reformer operating regime and determine the process parameters. The CO

_ optimum operating regime is selected as a compromise be- m

at exit (g/s) tween high methanol conversion rate, high hydrogen produc-

2 tion and for a reformate gas composition that can be tolerated by

CO ® e

_ HT-PEM MEAs. Advent TPS are PA doped HT-PEM MEAs that m can operate between 120 Cto200C and can tolerate CO con-

2 centrations in the reformate gas up to 3%. However, the amount H _

m of water vapor in the anode and cathode reactant gasses must be 0.077 0.660 0.016 0.829 0.467 0.55% 0.077 0.688 0.030 0.570 0.321 1.20% Mass ﬂow rates of reformate components minimized to prevent the leach of PA out of the MEA and reduce their proton conductivity.

(g/s) Fig. 6 through 9 are plots of the methanol conversion rate ðzÞ, _ ,ð = Þ of the hydrogen produced - mH2 mg s , of the CO vol% in the reformate gas and of the reformate gas relative humidity ð4Þ,all = as function of reformate operating temperature and W FCH3OH;in. Higher methanol conversion rates (Fig. 6) and hydrogen yields (Fig. 7) are obtained at higher operating temperatures. 2.049 1.686 The reformer operating regime will therefore be sought at the highest practical operating temperature. However, as shown in Fig. 8, the CO vol% in the reformate gas increases as well with temperature. Since commercial HT- PEM MEAs have a CO tolerance up to 3%, only the operating regimes in Fig. 8 situated below the threshold line are acceptable. Before delivering it to the fuel cell, the reformate gas must be cooled to the fuel cell operating temperature. Fig. 9 shows the relative humidity of the reformate gas calculated using the 19 15 Antoine equation for a fuel cell operating temperature of 180 C as:

. x ,p H2 O # of reformer units Consumption methanol solution 4 ¼ (31) : À 1810:94 , : 8 14019 244:485þ180 0 00131579 Fig. 11 10 Reformer size, consumption of methanol solution, reformate gas composition and quality and reformer performance parameters for the operating regi

e For all simulated operating regimes, the relative humidity of the reformate gas is below 5%, which is acceptable by PA- doped HT-PEM MEAs. We note that operating regimes 2 Table 5 selected in 1 Operating regime international journal of hydrogen energy 45 (2020) 31745e31759 31757

Fig. 12 e Methanol reformer consisting of 15 reformer units for on-board production of hydrogen as fuel for a 3 kW HT- PEMFC power system. corresponding to higher operating temperatures result in mass flow rate of hydrogen required to operate the fuel cell lower relative humidity of the reformate gas. stack (0.077 g/s) by the mass flow rate of hydrogen produced (Fig. 7) and is shown in Fig. 11. Two operating regimes that result in a practical reformer Reformer design and sizing volume (points 1 and 2 in Fig. 11) are selected and the reformate gas composition, the methanol consumption rate, the The reformer must produce sufficient hydrogen to operate a methanol conversion rate and the thermal efficiency of the 3 kW HT-PEMFC. The fuel cell stack sought to be used in the reformer are compared (Table 5). Note that all operating re- 2 power system is based on 166.25 cm active area Advent TPS® gimes corresponding to 613 K and most regimes correspond- 2 MEAs which can deliver a current density of 0.8 A/cm at 0.5 V/ ing to 593 K are not considered since their CO volume % is cell when operated with hydrogen and air at 1.2 and 2.0 stoi- higher than the 3% threshold (Fig. 8) acceptable by PA-doped chiometric ratios at 180oC[40]. The number of cells in the fuel HT-PEM MEAs. All other operating regimes corresponding to cell stack is calculate as: lower reformer operating temperature are disregarded as they result in an unpractical reformer size. Pe n ¼ (32) It is noted from Table 5 that operating regime 1 (T ¼ 593 K and i,V=cell,A = ¼ W FCH3 OH;in 2) results in a smaller reformer size (15 units), The number of cells in the fuel cell stack - 46 as provided by lower methanol consumption, better methanol conversion rate Eq. (32) is rounded up to the nearest integer. The mass flow ðzÞ, lower reformate relative humidity, but higher CO volume % rate of hydrogen (g/s) required to operate the fuel cell stack is and lower reformer thermal efficiency ðhÞ than operating regime calculated based on Faraday’s law: ¼ = ¼ 2(T 573 K and W FCH3OH;in 2). While in both cases the reformate gas quality (CO vol% and relative humidity) are i,A m_ ¼ l , ,n,M (33) acceptable for operation with PA-doped HT-PEM MEAs, the H2 H2 2F H2 methanol consumption rate and the reformer thermal effi- l ¼ : which for a hydrogen stoichiometric ratio - H2 1 2 results in ciency have different impacts on the overall fuel cell power 0.077 g/s of hydrogen. system efficiency. Both cases are retained for a further analysis The reformer design for the 3 kW power system comprises of a of the fuel cell power system efficiency. number of reactor units, each consisting of a bundle of 37 equally Fig. 12 shows the design of the methanol reformer con- spaced,50mmlongand2mmindiameterchannels(seeSection sisting of 15 reformer units for on-board production of Results and Discussion)fabricatedinahigh-temperature hydrogen as fuel for a 3 kW HT-PEMFC power system. conductive cylindrical body and filled with Cu/ZnO/Al2O3 catalyst particles for methanol reforming. They are surrounded by four peripheral channels filled with catalyst for the catalytic Conclusions combustion of methanol which provides heat to support the endothermic reforming reaction of methanol (Fig. 10). We present a Computational Fluid Dynamics model to deter- The number of reforming units for each case simulated in mine the optimum operating regime and the process param- Section Results and Discussion is determined by dividing the eters of a methanol reformer for on-board production of 31758 international journal of hydrogen energy 45 (2020) 31745e31759

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