on-board fuel processing for a fuel cell−heat engine hybrid system †

On-Board Fuel Processing for a Fuel Cell-Heat Engine HybridSystem†

Osman Sinan Suslu*,‡ and Ipek Becerik‡,§

Energy Institute, and Chemistry Department, Istanbul Technical UniVersity,Maslak, Istanbul 34469, Turkey

ReceiVed May 15, 2008. ReVised Manuscript ReceiVed January 16, 2009

Fuel cells operated with hydrogen are more efficient than internal combustion engines, because the combustionin the internal combustion engine is less reversible than the electro-oxidation of hydrogen in the fuel cell.Hydrogen can be produced out of hydrocarbons, such as natural gas, or renewable resources at stationaryfacilities, but fuel cells operated with pressurized hydrogen stored on board require advanced hydrogeninfrastructure for commercialization. An alternative to on-board storage of hydrogen is on-board processing ofa liquid hydrocarbon to hydrogen via steam reforming. However, on-board reforming of hydrogen is lessenergy-efficient than centralized production of hydrogen. Moreover, on-board reforming, purification, andsubsequent oxidation of the reformate in a fuel cell is not more efficient than a hybrid electric vehicle technologyassisted internal combustion engine. We propose a fuel cell-heat engine hybrid system, which consists of amembrane reformer, a fuel cell, and a reciprocating internal combustion engine, and estimate the efficiency ofthe proposed hybrid system. Steam reforming of a hydrocarbon requires additional heat input, which can berecovered from the waste heat of an internal combustion engine. On the other hand, the retentate of the membranereformer can be used in the internal combustion engine to further increase the system efficiency. Methanol isproposed as the fuel for the membrane reformer because the temperature level required is low enough torecover waste heat of reciprocating internal combustion engines for steam reforming of methanol. The hybridsystem proposed is more flexible than a fuel cell with an on-board reformer, because additional fuel can bedirectly combusted in the internal combustion engine at cold start or rapid load increase. Because fuel cellefficiency decreases with load and internal combustion engine efficiency increases with load, the overall systemefficiency is less load-dependent compared to the efficiencies of each of these technologies. The power of anautomobile engine is considered as a benchmark for the system proposed.

1. Introduction

The importance of efficient power generation systems withlow emissions increases with the global demand for energy.Besides the economical aspects of fossil energy carriers, suchas coal, gas, and oil, their accelerated consumption increasesthe amount of greenhouse gases in the atmosphere, whichincreases average temperatures, endangering life on earth.Although fuel cells (FCs) have characteristic sources of internalirreversibility, such as ohmic losses and activation and polariza-tion losses, FCs do not involve the far more irreversiblecombustion process in internal combustion engines (ICEs).Consequently, their implementation could increase the thermalefficiency of power systems, which is defined as the ratio ofthe power produced to the chemical energy rated by the lowerheating value of the fuel depleted. The increase of thermalefficiency decreases CO2 emissions per kilojoule of mechanicalenergy produced by a power system. Moreover, FCs allow fora clean, silent, and direct conversion from chemical to electricalenergy.

The above-defined efficiency of FCs can be further increasedif their waste heat can be used in a heat engine (HE) or for fuel

processing. Sung et al.1 report in their thermodynamic analysisof FC-HE hybrid systems that the losses caused by theirreversibility in FCs can be recovered by HEs converting thewaste heat of the FC into additional work. In the case of a hybridsystem with a reversible FC, the produced work is exactly thesame as the work of a FC operating at ambient temperature, ifthe waste heat of the FC can be recovered by operating a CarnotHE.1 However, high-temperature FCs, such as solid oxide FCs(SOFCs) or molten carbonate FCs, are more suitable for sucha process, integrating the FC, fuel processing, and a gas turbinein a combined process.

Because the requirements for a new technology, such as FCsin mobile systems to be competitive with the highly developedICE, are much more demanding in terms of power density(power/volume),2 operational flexibility, and low investmentcost,3 the above-mentioned type of cycles do not fulfill theprimary requirements for mobile applications of FCs, which arethe power density, specific power (power/mass),2 and flexibilityof on-board energy conversion. Penner et al. have comparedthe specific power and working temperature of SOFC (0.1 kW/kg, 950 °C), molten carbonate (0.04 kW/kg, 650 °C), phosphoricacid (0.09 kW/kg, 190 °C), and proton exchange membrane FCs(PEMFCs) (0.8 kW/kg, 80 °C), respectively. Only PEMFCs aresuitable for the requirements of mobile applications: low-

† From the Conference on Fuels and Combustion in Engines.* To whom correspondence should be addressed. Telephone: 90-212-

2877129. Fax: 90-212-2335921. E-mail: [email protected].‡ Energy Institute.§ Chemistry Department.

(1) Sung, T. R.; Jeong, L. S. J. Power Sources 2007, 167, 295–301.(2) Larminie, J.; Dicks, A. Fuel Cell Systems Explained; John Wiley

and Sons Ltd.: New York, 2000; p 15.(3) Donitz, W. Int. J. Hydrogen Energy 1998, 23, 611–615.

Energy & Fuels 2009, 23, 1858–18731858

10.1021/ef8003575 CCC: $40.75 2009 American Chemical SocietyPublished on Web 03/24/2009

temperature operation allows for arbitrary interruption ofoperation without major energy losses, and the specific poweris significantly higher than that of the other FCs.4 Murphy etal. report that the specific power density of PEMFC can furtherreach values of 1 kW/kg, which could compete with the specificpower of ICE.5

Although the above values do not account for the weight ofthe storage system (fuel + storage tank), the portion of theweight of the storage system could be smaller in the total weight,if the FC technology is used in bigger transportation vehicles,such as buses and trucks, compared to automobiles. Todemonstrate the ability of the FC technology in meeting publictransport requirements, 27 Citaro buses powered by Ballard fifth-generation FC technology were put into regular service in nineEuropean cities within the scope of the Clean Urban Transitfor Europe (CUTE) demonstration program.6 According to theopinions of the bus drivers, the operation of FC-powered buseswith on-board stored hydrogen was improved in exhaustemissions, smell, comfort, and safety in comparison to diesel-engine-powered buses, while speed and acceleration of the FCbuses were worse.7 This perceptional opinion about performancecould result from the fact that the settling time of a FC whengenerating 1 kW of maximum output is about 3 s.8

In the case of automobiles, the operation with FCs constrainsthe establishment of a hydrogen infrastructure, while buses forpublic transportation require only one central hydrogen produc-tion facility, where the refueling of a whole fleet may bescheduled to operate the facility efficiently. Moreover, the weightof the storage tank represents a higher portion in the total weightof a smaller vehicle, such as an automobile. Therefore, forautomobiles, fuel processing to hydrogen is also considered, ifthose are intended to be operated with FCs. Demirdoven et al.have determined and compared the tank to wheel efficiency(TTW) of a conventional ICE, a hybrid ICE (HICE), and a FChybrid electric vehicle (FCHEV) according to the federal urbandrive schedule (FUDS). The TTW efficiency of the ICE, HICE,and FCHEV vehicles were 12.6, 27.2, and 26.6%, respectively.9

Because they have used the same fuel, gasoline having anestablished infrastructure, to constrain the same well to tank(WTT) efficiency for the compared systems, the TTW efficiencyof the hybrid FCHEV is unexpectedly low, because the gasolineprocessing to hydrogen with subsequent use of the latter in theFC had an efficiency of only 35% in their calculation.

If hydrogen stored on board is used instead of processedhydrogen from gasoline as fuel for the FC in contradiction tothe above comparison, the TTW efficiency would be higher thanthe above given value, because fuel processing would not benecessary. On the combined FUDS and federal highway drivingschedule (FHDS) used in environmental protection agency(EPA) tests, the simulated fuel economy of the direct hydrogenFCHEV (1920 kg) is 2.9 times the published fuel economy of

the HICE (1695 kg) on the same platform, provided that FCHEVhas equal performance in acceleration with HICE.10

However, on a commercial level, hydrogen stored on boardposes a number of problems, such as hydrogen infrastructure,mechanical problems involved with refueling, limited drivingrange, high weight involved with the size of the storage cylinder,and safety concerns of carrying a high-pressure cylinder ofhydrogen.11 Despite the high lower heating value (LHV) ofhydrogen (120 kJ/g) compared to methanol (21 kJ/g), ethanol(28 kJ/g), and gasoline (45 kJ/g), its global gravimetric energydensity12 is lower. The global gravimetric energy density is theratio of the energy of the fuel to the sum of the weights of thefuel and its storage tank. If hydrogen, methanol, ethanol, andgasoline storage systems are assumed to weigh 50 kg, with theweight of the fuel inclusive, hydrogen compressed at 5000 psia(340 atm) weighs only 7.5% of the fully refueled tank. Theliquid fuel tanks with 12 kg tare weight and 13 gallons13 (50L) fuel volume weigh 51.35, 51.45, and 49 kg, if those are fullyloaded with methanol (12 + 50 × 0.787 kg/L), ethanol (12 +50 × 0.789 kg/L), and gasoline (12 + 50 × 0.74 kg/L),respectively. Consequently, the global gravimetric energydensity of compressed hydrogen, methanol, ethanol, and gasoline(modeled with octane) can be calculated to 9 (120 × 7.5%),16.2 (21 × 50 × 0.787/51.35), 21.3 (28 × 50 × 0.789/51.45),and 33.9 (45 × 50 × 0.74/49) kJ/g tank, respectively. As aconsequence, larger vehicles (vans, buses, trains, ships, etc.)provide enough storage volume to be operated as “zero-emissionvehicles” with hydrogen stored on board.3

The term “zero-emission vehicle” is restricted on TTWemissions, but the WTT emissions depend upon the primaryenergy resource and its stationary processing to hydrogen,including the distribution to the fuel station. Consequently, well-to-wheel (WTW) emissions (total emission) of such vehiclesare only zero if the sum of both WTT and TTW emissionsequals zero. WTT CO2 emissions have to be determined beforeconsidering FCs with hydrogen fuel cleaner than the state ofthe art ICEs with current fuels. Although hydrogen is not theonly fuel for direct FCs, running on formic acid,14 medium-sized hydrocarbons,15 and alcohols,16 those are currently as-sociated with many technological problems, including coking,catalyst poisoning, low catalytic activity, fuel crossover, andpolarization losses. The same concerns about WTT emissionsexist for all alternative fuels, even if produced out of renewableresources. Some well-known fuels, secondary energy resources,and their primary energy resource have been evaluated in termsof climate relevant emissions in g of CO2 kW-1 h-1 with respectto the production costs in euros kW-1 h-1 in Figure 1.

Another concern is the variability of the load demand inmobile engines, which has a negative effect on the averageefficiency, because an engine is operated only occasionally atits most efficient load. Whereas HEs commonly yield maximumefficiencies near the maximum power, FCs yield maximumefficiencies near zero power. Projections based on realistic

(4) Penner, S. S.; Appleby, A. J.; Baker, B. S.; Bates, J. L.; Buss, L. B.;Dollard, W. J.; Fartis, P. J.; Gillis, E. A.; Gunsher, J. A.; Khandkar, A.;Krumpelt, M.; O’Sullivan, J. B.; Runte, G.; Savinell, R. F.; Selman, J. R.;Shores, D. A.; Tarman, P. Energy 1995, 20, 331–470.

(5) Murphy, O. J.; Cisar, A.; Clarke, E. Electrochim. Acta 1998, 43,3829–3840.

(6) Ballard. Case Study. Fuel Cell Transit Application. Fuel CellSolutions To Power Our Future, http://www.ballard.com/files/pdf/ Spec_Sheets/bus_case_study_v9.pdf.

(7) Daimler Chrysler. HyFLEET-CUTE European Bus DemonstrationProgram, 2007.

(8) Obara, S.; Kudo, K. Trans. Jpn. Soc. Mech. Eng., Ser. B 2005, 71,1678–1685.

(9) Demirdoven, N.; Deutch, J. Science 2004, 305, 974–976.

(10) Ahluwalia, R. K.; Wang, X.; Rousseau, A. J. Power Sources 2005,152, 233–244.

(11) Lindstrom, B.; Pettersson, L. J. Int. J. Hydrogen Energy 2001, 26,923–933.

(12) Sampaio, M. R.; Rosa, L. P.; D’Agosto, M. d. A. RenewableSustainable Energy ReV. 2007, 11, 1514–1529.

(13) Thomas, C. E.; Sims, R. Society of Automotive Engineers (SAE)Proceedings, Fuel Cell for Transportation TOPTEC, Arlington VA, 1996.

(14) Hebbling, C.; Heinzel, A.; Golombowski, D.; Meyer, T.; Muller,M.; Zedda, M. Int. Conf. Microreact. Technol. 1999, 3, 338–401.

(15) Sammes, N. M.; Boersma, R. J.; Tompsett, G. A. Solid State Ionics2000, 135, 487–491.

(16) Lamy, C.; Lima, A.; LeRhun, V.; Delime, F.; Coutanceau, C.; Leger,J. M. J. Power Sources 2002, 105, 283–296.

Fuel Cell-Heat Engine Hybrid System Energy & Fuels, Vol. 23, 2009 1859

developments suggest that FCs will operate overall with higherefficiencies than HEs when load factors are below 45%.17

Designing ICEs with lower performance also increases ef-ficiency. However, the consumers rate their engines accordingto performance besides fuel consumption, because full load isalso required during acceleration occasionally. Because bothsystems have different efficiency characteristics with respect tothe load, a superposition of their efficiency functions will dependless upon the load. Consequently, the efficiency of the hybridsystem will be more independent from the load than the case ifthe FC or ICE were operated separately.

Therefore, a hybrid system comprises the efficiency lossesof the ICE at a lower cost than a FC, because the portion of thefull load provided by the cheaper ICE decreases the investmentcosts compared to the case if the full load is provided by themore expensive FC only. This is also an opinion that has beenexpressed by Obara et al., who proposed a hybrid co-generationsystem (HCGS) consisting of a PEMFC and a hydrogen mixinggas engine (NEG) to optimize the capacity of a hybrid systemfor combined heat and power production, run with city gas inJapan. It operates a gas engine in parallel with a FC stack fedby a reformer, which can feed the gas engine too, if the powerdemand exceeds the power capacity of FC. Because PEMFCand NEG are operated according to a base load and a fluctuatingload, respectively, the hybrid system has the maximum powergeneration efficiency of PEMFC and the partial load charac-teristic of NEG.18 Consequently, an immediate power demandincrease of a mobile vehicle could be responded by the ICE ofthe hybrid power system, while the FC works efficiently atpartial load or idling of the system. Although this concept isrealized for a stationary system, it could also be a moreeconomic solution for mobile systems, because the moreexpensive FC system will not be dimensioned for the full powerdemanded only occasionally.

The cold start of the fuel processor-FC (FP-FC) systemsis another challenge besides load fluctuations. With no externalheat input, the time to achieve a bench-scale temperature andreformate composition is approximately 175 s for a methanolreformer.19 Thanks to the absence of fuel processing kinetics,cold start and load fluctuations affect the direct hydrogen FCless than the FP-FC system. Zur Megede proposes an ICE-FCdriven system with a FP in his U.S. patent.20 Besides a processor

for the FC, an ICE is supplied with a liquid fuel to start thevehicle. Until the exhaust gases of ICE heat the FP to its workingtemperature to generate hydrogen for the FC, the traction powerof the vehicle is supplied by ICE. The ICE and FC are jointlyconnected to the same coolant circulating system, which is moreeconomic and saves weight. After the cold start of the ICE, theFC is already heated by the ICE to the working temperature bymeans of the cooling system of the ICE. If the ICE is shut downafter the FC has started, a restart of the operation of the ICEfor assisting the electric motor takes place at a higher operatingtemperature, because the waste heat of the FC keeps the coolantwarm, avoiding the generation of pollutants at a cold restart.20

The use of the waste heat of an ICE for fuel processing hasalso been proposed by Obara et al. for co-generation. They haveproposed a hybrid system for co-generation using exhaust heatof a woody-biomass stirling engine (WB-SEG) for the steamreforming of city gas, which supplies the produced reformedgas to a PEMFC. The waste heat and power of both WB-SEGand PEMFC are used for residential heating and electric powerproduction, respectively.21

Besides using waste heat of an ICE for fuel processing in aFC-HE hybrid system, the system efficiency can further beincreased, if the flue gases from the anode of a FC arecombusted in the ICE. Such a low-temperature FC-ICE hybridsystem has been proposed by Morgenstern et al. to use low-temperature ethanol reformate in both the FC and ICE of aFC-HE hybrid system.22 They report a new type of copper-nickel catalyst, which catalyzes low-temperature reforming ofethanol, reversibly dehydrogenated to acetaldehyde followed bydecarbonylation of the latter to form CO and methane. LHV ofmethane along with other residual gases could be captured bya downstream ICE, exhaust gases of which would provide therequired heat for the proposed low-temperature endothermicreaction. Their proposed power train has a bypass line, in whichmethane and residual gas fed to ICE is supplemented withethanol, providing peak power. Unfortunately, their catalyst hadlow activity for the water-gas shift reaction (WGSR), so thata direct oxidation of the reformate at the anode of PEMFCwould poison its catalyst.22

The PEMFC anode catalyst is comprised of Pt or more CO-tolerant Pt alloys (PtRu), at which the acceptable CO reformateconcentration is below 10 and 100 ppm, respectively.23 Abovethose concentrations, CO is strongly adsorbed irreversibly,preventing H2 adsorption (catalyst poisoning). Although themildly exothermic WGSR is equilibrium-limited and does notreach such low CO concentrations with commercial catalysts,24

it is used before further purification, because it increases H2

selectivity and removes CO simultaneously. Methanation of COwith H2 competes with methanation of CO2, decreasing H2

selectivity. Although methane could be used in an ICE, thetemperature window of methanation is too narrow to decreasethe CO amount in energy-efficient, single-stage compact puri-fiers. Preferential oxidation (PROX) makes use of the higheroxidation potential of CO (LHVCO ) -283 kJ/mol) and otherhydrocarbons in comparison to hydrogen (LHVh ) -242 kJ/mol) by bleeding air (∼2%) into the fuel stream entering the

(17) Glazebrook, R. W. J. Power Sources 1982, 7, 215–256.(18) Shin’ya Obara, S.; Itaru Tanno, I. Int. J. Hydrogen Energy 2007,

32, 4329–4339.(19) Kumar, R.; Ahmed, S.; Krumpelt, M. Fuel Cell Seminar Program

and Abstracts, 1996; pp 750-753.(20) Zur Megede, D. U.S. Patent 6,276,473, 2000.

(21) Obara, S.; Itaru Tanno, I.; Kito, S.; Hoshi, A.; Sasaki, S. Int. J.Hydrogen Energy 2008, 33, 2289–2299.

(22) Morgenstern, D. A.; James, P.; Fornango, J. P. Energy Fuels 2005,19, 1708–1716.

(23) Vielstich, W.; Lamm, A.; Gasteiger, H. A. Handbook of Fuel Cells:Fundamentals Technology and Applications; John Wiley and Sons Ltd.:New York, 2003; Vol. 3, p 211.

(24) Trimm, D. L. Appl. Catal., A 2005, 296, 1–11.

Figure 1. Fuel production costs versus respective climate-relevantemissions (Zuberbuehler et al.37).

1860 Energy & Fuels, Vol. 23, 2009 Suslu and Becerik

reactor in the presence of noble metals, such as Pt, Ru, and Rh,supported on Al2O3 to selectively oxidize CO in mobile FPs.25

Besides the operating conditions, the efficiency of a PEMFCdepends upon the purity of the hydrogen produced by thereformer. Bench-scale tests of a low-temperature, catalyticmethanol reformer, which combines partial oxidation (POX),steam reforming, decomposition, and WGSRs to producehydrogen, used ambient temperature methanol, water, and airat the inlet and produced reformate at 200 °C. The reformatecontained 50% hydrogen, 29% nitrogen, 20% carbon dioxide,and 1% carbon monoxide. Hydrogen production was ap-proximately 90.9% of the theoretical hydrogen production, aresult of the incomplete reaction of methanol.19 After PROX,the reformate is cooled to the FC-operating temperature byevaporative cooling, which also humidifies the reformate priorto entering the FC anode. Dehydration of the membrane resultsin increasing resistive losses, while excessive water leads toflooding of the cathode.26

Ogden et al. have compared direct hydrogen FCs with on-board reformed hydrogen FC systems.27 In their comparison,the CO removal step after reforming or POX and subsequentWGSR is PROX. For compressed gas hydrogen storage, thefeed gas to the FC anode is pure hydrogen. For the case ofmethanol steam reforming and gasoline POX, the hydrogencontent is about 75% and 35% by volume, respectively. Thehigher the hydrogen content, the better the FC performance andthe greater its power density. Moreover, the use of purehydrogen for the FC enables recycling of the anode off gas.Because reformate contains also other gases (N2 and CO2) thanhydrogen and water, the anode off gas has to be purged intothe catalytic burner of the reactor, after hydrogen contained inthe reformate is oxidized at the anode of the FC.

Consequently, vehicles with on-board FPs are heavier,because the FP adds weight and the FP-FC system is lessenergy-efficient than a pure hydrogen system; therefore, a largerFC is needed to provide the same power output. The methanolFC vehicle weighs about 10% more than the hydrogen vehicle,and the gasoline POX vehicle weighs about 19% more. For 100units of primary energy input, about 66 units of methanol, 76units of hydrogen, and 94 units of gasoline energy are deliveredto vehicles. Because of the absence of any impurities, includingCO, the PEMFC stack can produce 30-60% more power thanthe case when the CO content is 20 ppm.27 Despite such claims,Zur Megede considers FPs essential for a successful com-mercialization of FC vehicles in a mass consumer market,28

because a liquid fuel, such as methanol, allows for an easierintroduction and market penetration for FC technology. How-ever, a precondition for using FPs is the availability of a compacton-board reforming and gas-cleaning system. In comparison tothe well-known reformer techniques in the chemical industry,this gas production system must be very compact and, inaddition, highly dynamic. It is estimated that a power densityof 0.4 kW/L can be reached.3

To meet the challenges in developing such FPs, catalyticmembrane reactors (CMRs) are considered to increase the powerdensity and efficiency by simultaneously decreasing the numberof purification steps and increasing hydrogen purity, respec-

tively. Membrane separation by a nonporous CMR can be usedas a hydrogen purification technology, which separates, besidesCO, all reaction products other than hydrogen, like inert gases,such as CO2, N2, and hydrocarbons, and comprises highhydrogen feed concentrations for the FC. A CMR is a combina-tion of a heterogeneous catalyst and a perm-selective membrane,which is a thin film or layer that allows one component of amixture to selectively permeate through it.29 The side to whichhydrogen is permeated is flown by a sweep gas, to which thedesorbed hydrogen from the membrane wall diffuses. Usingsteam as a sweep gas for hydrogen removal from the membranewall, wet H2 stream could be directly suitable for the FC.30 Onedistinct advantage of a CMR is that combining the functions ofcatalysis and purification lowers the costs of separation andelimination of unwanted reaction products, while appropriatereactor design can improve product yield or reaction selectiv-ity.31 The generation of the sweep gas requires the evaporationof distilled water provided by the condensation of steam fromthe cathode off gas of the PEMFC, which is a reaction productof hydrogen electro-oxidation. The heat for the evaporation ofwater to generate the sweep gas can be recovered from the wasteheat (exhaust gas or engine coolant) of the ICE of the hybridsystem.

Many of the nonporous CMRs studied to date have used Pdalloy. The retentate gas from CMR, which is also called bleedgas, consists of unconverted reactants and residual reactionproducts, such as CO, CO2, H2O, hydrocarbons, and unrecoveredH2. Except CO2 and H2O, these are used as fuel for thecombustion catalyst placed at an adjacent channel and supplyheat for the steam reforming reaction. Because the catalyticcombustion takes place with excess air at a moderately lowtemperature just high enough to provide the required heatexchange, exhaust from the catalytic combustion contains neitherpollutants, such as CO and NO, nor any other combustiblehydrocarbons. By adjusting the system pressure, hydrogenrecovery by the metal membrane can be maintained at anoptimum level, in which bleed gas is produced just enough tobalance the heat requirement by the reformer. Hydrogenproduced from the system has a purity of 99.9999% or better.Because only pure hydrogen is supplied to the anode of thePEMFC stack, anode off-gas recycling is possible by increasingfuel (hydrogen) use to 100%. This is not possible in PROXreactors, the output of which contains diluents, such as CO2

and N2, besides H2. The total volume and weight of this designare 40 L (0.75 kW/L) and 50 kg (0.6 kW/kg), respectively,32 aconsiderable increase compared to the value of 0.4 kW/L.3

Moreover, the absence of diluents increases the Nernst potential.Wieland et al. have introduced a circuit diagram of a CMR

feeding hydrogen to a FC stack.33 The oxidation product ofhydrogen in the FC is steam further used for reforming ofmethanol and sweeping of hydrogen from the membrane wallat the permeate side of CMR, after it has been condensed fromcathode off gas. Therefore, only a small flash tank is used insteadof an extra tank for water storage, because water is a byproductof the FC. Moreover, the anode off gas is directly fed to the

(25) Haji, S.; Malinger, K. A.; Suib, S. L.; Erkey, C. Fuel CellTechnology, Reaching Towards Commercialization; Springer: Berlin,Germany, 2006; pp 165-202.

(26) Bao, C.; Ouyang, M.; Baolian, Y. Tsinhua Sci. Technol. 2006, 11,54–64.

(27) Ogden, J. M.; Steinbugler, M. M.; Kreutz, T. G. J. Power Sources1999, 79, 143–168.

(28) Zur Megede, D. J. Power Sources 2002, 106, 35–41.

(29) Armor, J. N. Appl. Catal. 1989, 49, 1–25.(30) Basile, A.; Gallucci, F.; Paturzo, L. Catal. Today 2005, 104, 244–

250.(31) Falconer, J. L.; Noble, R. D.; Sperry, D. P. Catalytic Membrane

Reactors, Membrane Separations Technology, Principles and Applications;Elsevier: Amsterdam, The Netherlands, 1995; pp 670-712.

(32) Han, J.; Kim, I.; Choi, K. J. Power Sources 2000, 86, 223–227.(33) Wieland, S.; Melin, T.; Lamm, A. Chem. Eng. Sci. 2002, 57, 1571–

1576.


FC anode, increasing the fuel use of the FC to 100%, as theCMR produces pure hydrogen.33

2. Elements of the Proposed Hybrid System

A double-sided heat and gas recovery according to Morgensternet al.22 is also apparent in our proposed hybrid system. The flowsheet in the work of Wieland et al.33 is adapted to our hybrid systemto purify hydrogen for the FC. Hydrogen not permeated to the FCcycle is used in an ICE. In such a system, the cold-start difficultiesare also solved, such as in the work of Zur Megede.20 The flowsheet of the FC-HE hybrid model proposed is depicted in Figure2. It contains a reciprocating ICE, a membrane reformer, and aFC. The membrane reformer has in its outer shell a heat exchangerto provide heat for endothermic steam reforming recovered fromexhaust gas. The catalyst is situated between the shell and themembrane, which selectively removes hydrogen from the reactorinto the anode cycle of the FC. Instead of burning the bleed gas onthe shell side of the reactor to supply heat for evaporation andreforming of the fuel-steam mixture,32 an ICE is introduced,combusting the residual gases of CMR to increase the systemefficiency by means of further power production.

According to our knowledge, the combination of feeding an ICEwith the bleed gas of a CMR has not been proposed in the literatureof FC-HE hybrid systems yet. Our theoretical model has not beenbuilt yet, but a simple thermodynamic calculation was performedon the basis of the efficiencies of the system elements assumed ordetermined according to references. The selection of the appropriatefuel out of many alternatives has a vital affect on the system designaccording to the properties of the fuel. Therefore, some quantitativeparameters for a fuel to be reformed to hydrogen have beenintroduced to determine the most appropriate one, before the unitsof the proposed design, the CMR, the FC, and the ICE, arepresented.

2.1. Fuel. A suitable primary fuel for a mobile FP-FC system(for automotive or low-power applications) has to fulfill severalselection criteria. The fuel should have a high energy content,be easy to handle (be liquefiable at moderate pressures), and becheap. Furthermore, the fuel of choice should provide easyrefueling and create little health, safety, and environmentalhazards, whereas the conversion process must be robust and easyto downsize.11 Gasoline and diesel fuel fall into the category offuels with a well-developed infrastructure, whereas methanol andhigher alcohols require the establishment of a modified distribu-tion network.34

Liquid fuels have an important advantage in handling andrefueling compared to gaseous fuels. Although the heat ofvaporization required to evaporate the fuel for the reaction inthe CMR may seem to be a disadvantage for a liquid fuel,evaporation of most liquid fuels takes place at a temperaturelevel appropriate to recover the exhaust gas waste heat or coolingload of the ICE; thus, the heat of evaporation can be fullyrecovered. Unfortunately, the temperature level of a PEMFCcooling is too low to use it either for evaporation or reformingof the fuel; however, its waste heat could be used to preheat the

fuel before evaporation. However, the amount of heat to preheatthe fuel and water is negligible compared to the cooling load ofthe FC; therefore, the PEMFC is mostly cooled by a radiator.FCs with higher working temperatures could be cooled byevaporation of liquid fuels and water.

We have chosen steam reforming for fuel processing as aresult of the higher hydrogen concentration in reformate, becausethe permeation of hydrogen increases with the hydrogen partialpressure gradient between retentate and permeate sides of themembrane. Other options would be POX enabling lighterconstructions or autothermal reforming. However, these pro-cesses have lower efficiencies as a result of lower partial pressureof hydrogen in reformate, because this will be diluted by nitrogenoriginating from air used as an oxidizer. Moreover, they do notallow heat recovery for reforming. We examined steam reformingof compressed natural gas (CNG), liquefied petroleum gas (LPG),gasoline, methanol, and ethanol in terms of the energy requiredfor reforming. Because the first three fuels are rather mixturesthan pure substances, those have been idealized by methane,propane, and octane, respectively.

This examination is performed in Table 1, where severalparameters are defined and calculated for the reforming reactionsof the fuels to be discussed. The enthalpy of the reforming reactionis determined by the application of the Hess rule to the reactionsof fuel combustion and water dissociation (reverse reaction ofhydrogen combustion). This methodology is used to determine thereaction enthalpy; it is obvious that the actual reaction mechanism,involving catalytic intermediate reactions, proceeds in a differentmanner. Because oxygen does not take part in the reformingreaction, the stoichiometric values of the second reaction (waterdissociation) are multiplied by the ratio of the stoichiometriccoefficients of oxygen of both reactions, becoming the stoichio-metric coefficient of hydrogen nH2 (mol H2/mol fuel) in thereforming reaction. Both reactions are summed to cancel oxygenout of the reaction equation. The sum determines the reformingreaction with its stoichiometric coefficients and the overall reactionenthalpy (∆Hr°), depending upon nH2 and LHVs of fuel (LHVf)35

and hydrogen (LHVh) on a molar basis referred to standardtemperature (T0 ) 25 °C) and pressure (p0 ) 1 atm) (STP), in eq1. LHV in these calculations is the absolute value of the combustionenthalpy on a molar basis. If heating values are listed on a massbasis, those have to be multiplied with the molar mass of the fueland hydrogen to determine LHVs on a molar basis.

Steam reforming of ethanol requires more heat per mole offuel than methane does according to Table 1. On the other hand,the heat input required for 1 mol of hydrogen is less for ethanolreforming than for methane reforming. Because hydrogenproduction is our objective, specific reforming enthalpy (∆Href

0 )is defined in eq 2, which is the energy amount required for 1mol of hydrogen to be reformed from a fuel. Consequently, fora given amount of (waste) heat to be used as reaction enthalpyof steam reforming, the hydrogen yield increases with thedecrease of ∆Href

0 .

Another parameter is the total vaporization enthalpy r0, requiredfor evaporation of both liquid reactants. This is equal to the sum

(34) Pettersson, L. J.; Westerholm, R. Int. J. Hydrogen Energy 2001,26, 243–264.

(35) Levenspiel, O. Chemical Reaction Engineering; Wiley: New York,1972.

Figure 2. FC-HE hybrid model.

CcHhOo +nH2

2O2 S cCO2 + h

2H2O ∆Hr,c

0 ) -LHVf

+ nH2× (H2O S

12

O2 + H2 ∆Hr,d0 ) LHVH2)

CcHhOo + (2c - o) H2O S cCO2 + nH2H2

nH2) h

2+ 2c - o ∆Hr

0 ) nH2LHVH2

- LHVf ( kJmol fuel)

(1)

∆Href0 )

∆Hr0

nH2

) LHVh -LHVf

nH2

(kJ/mol of H2) (2)


of enthalpy of vaporization of the fuel rf0 and water rH2O

0 multipliedby its stoichiometric coefficient.

This amount of heat can be supplied by the ICE more easilythan the heat amount for reforming. If the total vaporization enthalpyis divided by the reaction enthalpy of liquid reactants, which is thesum of the reaction enthalpy and total vaporization enthalpy, thequotient shows the percentage of the energy consumed only forthe evaporation of liquid reactants. Consequently, this quotientshould be as high as possible for the fuel to be chosen, because thetemperature level of evaporation depends upon the pressure of thereformer and not the required temperature level of the reformingcatalyst to maintain its activity for the reaction, with the latter beinghigher than the former.

Alcohols seem to be more suitable for on-board fuel processingby steam reforming, because their specific reforming enthalpy isless than that of alkanes according to Table 1, because oxygen inthe fuel decreases the amount of steam required to oxidize the fuel.Steam reforming of methanol requires the least amount of energyfor generating 1 mol of hydrogen (16.43 kJ/mol H2) under the fuelslisted, because its oxygen/carbon ratio is 1, the highest under thefuels listed. Although the gravimetric energy density of methanol(16.2 kJ/g of tank) considering the weight of the fuel and tank isabout half of gasoline (33.9 kJ/g of tank), gasoline requires 2.3times as much heat (37.2 kJ/mol H2) to produce the same amountof hydrogen compared to methanol. Consequently, for steamreforming of methanol, gasoline, and ethanol to 1 mol of H2, 1.01 g(16.43/16.2), 1.10 g (37.2/33.9), and 1.36 g (28.9/21.3) fuel tankweight is required, respectively, if a 50 L tank with 12 kg tareweight is fully loaded with liquid fuel. Thus, the gravimetric energydensity of methanol and gasoline tanks with respect to the energyof hydrogen (LHV ) 241.84 kJ/mol H2) generated for the FC willbe 238 kJ/g of tank [(241.84 kJ/mol of H2)/(1.01 g of tank/molH2)] and 220 kJ/g of tank (241.84/1.1), respectively. Therefore,for the same weight of the two liquid fuel tanks, methanol willgenerate at least 8% (238/220 - 1) more H2 energy compared togasoline. The actual difference in energy density referred to H2

production from methanol and gasoline will be higher, becausegasoline reforming takes place at a higher temperature thanmethanol reforming, requiring more heat to preheat the reactants.

Methanol has also the highest ratio of evaporation (62.4%) heatto the total reaction enthalpy, considering also the phase change ofreactants from liquid to gaseous. Also, the absence of C-C bondsthat are more difficult to break by catalysis causes a lowertemperature level for steam reforming of methanol, further decreas-ing the availability (exergy) of the required heat to be supplied permole of hydrogen. Besides lower quantity, lower quality of energyis required for the on-board generation of hydrogen out of methanol.Therefore, methanol has been chosen as the fuel to be processedto hydrogen in the FC-HE hybrid system.

Currently, more than 75% of methanol is produced from naturalgas.36 The production is based on three fundamental steps: naturalgas reforming to produce synthesis gas (syngas), conversion ofsyngas into crude methanol, and distillation of the crude methanolto achieve the desired purity. However, the synthesis gas, as thefirst step for the synthesis of methanol, can also be generated fromprimary energy sources, such as coal and renewable biomass.Because of a wide range of feedstock availability, fuels generatedfrom synthesis gas have the highest potential to provide theincreasing transportation fuel demand from renewable resources,such as biomass. Besides fuel costs, the use of renewable fuelsdepends upon resource availability and environmental concerns.An overview of different fuels concerning their CO2 emissionequivalents and estimated specific costs is shown in Figure 1.37

For the evaluation of renewable fuels, not only commercialaspects should be taken into account. Furthermore, their CO2

reduction potential, biodegradability, the security of supply, andeven ethical aspects should be considered. The conversion technolo-gies that lead to high yields per hectare area are fermentativeprocesses for the production of methane-containing product gases[substitute natural gas (SNG)] and thermochemical gasificationprocesses for the production of hydrogen and synthetic fuels, suchas methanol, from synthesis gas.38 The ethical aspects are reflectedin the specific fuel yield per hectare cultivated area because of thelimited availability of agricultural area for food production in aworld with an increasing population. Whereas bioethanol andbiodiesel production requires the cultivation of plants dedicated forfuel production, biomethanol may also be produced from municipal,agricultural, and forest wastes via thermochemical gasification.

According to a recent study about the energy choices in thewestern U.S. by Vogt et al., the conversion of available biomassfrom municipal, agricultural, and forest wastes to biomethanol canresult in significant environmental and economic benefits. In thewestern U.S., forest materials are the dominant waste sources inIdoha, Montana, Oregon, and Washington, while in California, thegreatest amount of available biomass is from municipal wastes. Inthe state of Washington, thinning “high-fire-risk” small stems,namely, 5.1-22.9 cm diameter trees, from wildfire-prone forestsand using them to produce biomethanol as a gasoline substitute,3.3-6.6 tons of C/ha of carbon emissions can be avoided. If thesesame “high-fire-risk” woody stems were burned during a wildfire,7.9 tons of C/ha would be emitted in the state of Washington alone.If all of the harvestable high-fire-risk wood was converted intomethanol, 36.005-71.499 ML of biomethanol could be producedin these five states every year, supplementing gasoline by biometha-nol with a lowest range of 36-102% and a highest range of72-204%.39 An economic benefit in collecting high-fire-risk woodfor biomethanol production would help to reduce the risk of

(36) Roan, V.; Betts, D.; Twining, A.; Dinh, K.; Wassink, P.; Simmons,T. An InVestigation of the Feasibility of Coal-Based Methanol forApplication in Transportation Fuel Cell Systems; University of Florida:Gainesville, FL, 2004.

(37) Zuberbuehler, U.; Specht, M.; Bandi, A. Proceedings of the 2ndWorld Conference and Technology Exhibition on Biomass for Energy,Industry and Climate Protection, 2004, http://www.zsw-bw.de/info/ papers/renevable/Renevable.pdf.

(38) Specht, M.; Zuberbuehler, U.; Bandi, A. NoVa Acta Leopold. 2004.(39) Vogt, K. A.; Vogt, D. J.; Patel-Weynand, T.; Upadhye, R.; Edlund,

D.; Edmonds, R. L.; Gordon, J. C.; Suntana, A. S.; Sigurdardottir, R.; Miller,M.; Roads, P. A.; Andreu, M. G. Renewable Energy 2009, 34, 233–241.

Table 1. Endothermic Reforming Reactions of Some Commercial Hydrocarbons

LHVf nH2 ∆Hr0 a ∆Href

0 rf0 nH2O r0 b

kJ mol of H2 kJ kJ kJ mol of H2O kJ F

fuel reforming + shift mol of fuel mol of fuel mol of fuel mol of H2 mol of fuel mol of fuel mol of fuel (%)

CNG CH4 + 2H2O f CO2 + 4H2 802.27 4 165.07 41.27 2 88.02 34.78LPG C3H8 + 6H2O f 3CO2 + 10H2 2043.16 10 375.19 37.52 15.06 6 279.12 42.66gasoline C8H18 + 16H2O f 8CO2 + 25H2 5115.79 25 930.09 37.20 41.46 16 745.62 44.50methanol CH3OH + H2O f CO2 + 3H2 676.22 3 49.29 16.43 37.90 1 81.91 62.43ethanol C2H5OH + 3H2O f 2CO2 + 6H2 1277.55 6 173.46 28.91 42.34 3 174.37 50.13

a LHVh ) 241.84 kJ/mol. b rH2O0 ) 44.01 kJ/mol.

r0 ) rf0 + (2c - o)rH2O

0 (kJ/mol of fuel) (3)

F ) r0

r0 + ∆Hr0

(4)


catastrophic forest fires and to increase the cultivation of newforests, offering the added benefit of contributing significantlytoward mitigating C emissions.

Methanol from biomass is at least 2-3 times more expensivethan fossil-fuel-based methanol. But the cost difference betweentaxed gasoline and untaxed renewable methanol may be rathersmall.40 According to Figure 1, biomethanol production costs about0.1 euro kW-1 h-1, causing climate-relevant emissions of about70 g of CO2 kW-1 h-1 energy produced, whereas production ofcrude oil based fuels, such as diesel and gasoline, cost about 30%of the biomethanol production at a crude oil price of $30/barreland cause about 300 g of CO2 kW-1 h-1 energy produced. Methanolsynthesized from natural gas is slightly more expensive and causesslightly more climate-relevant emissions compared to the productionof gasoline and diesel from crude oil.

With regard to the economic reasons of the methanol synthesis,the temporary integration of fossil raw material to biomass shouldalso be considered. For a maximum methanol yield from biomass,hydrogen addition to or CO2 removal from the synthesis gas hasto be performed, which increases the costs and decreases theefficiency of the process, because the H2 amount in biomass is nothigh enough to reach an optimum stoichiometry factor of S ) 2 [S) pH2 - pCO2/(pCO + pCO2)] in the synthesis gas generated out ofbiomass.38

An alternative is the mixing of natural gas to increase the H2

amount, because the synthesis gas generated by steam reformingof natural gas contains more hydrogen than is necessary for themethanol synthesis. If synfuels are produced from natural gas,including additional regenerative primary energy, this process isnot CO2-neutral any more. However, such synthetic fuels includea decreased fossil CO2 portion corresponding to the regenerativeenergy portion. The Hynol process is proposed to meet the demandfor an economic process of methanol production with reduced CO2

emission. This new process consists of three reaction steps: (a)hydrogasification of biomass, (b) steam reforming of the producedgas with additional natural gas feedstock, and (c) methanol synthesisof the H2 and CO produced during the previous two steps. TheH2-rich gas remaining after methanol synthesis is recycled to gasifythe biomass in an energy-neutral reactor, so that there is no needfor an expensive oxygen plant, as required by commercial steamgasifiers. The methanol production cost is $0.43/gallon for a 1085million gallons/year Hynol plant, which is competitive with currentU.S. methanol and equivalent gasoline prices.41 An overall 41%reduction in CO2 emission can be achieved in comparison to theuse of conventional gasoline fuel.

An increase of crude oil prices favors the production of coal-and biomass-based fuels. Although production of coal-basedmethanol is less expensive than biomethanol, climate-relevantemissions of coal-based methanol are higher than crude-oil-basedgasoline and diesel emissions. On the other hand, gasification ofcoal prior to the combustion of the synthesis gas in a gas turbineis more viable for a clean process, because the smaller gas volumecompared to combustion reduces the required capacity (and hen-ce cost) of any gas cleanup equipment needed downstream of thereactor.42 For a more efficient and clean process, the LPMEOHprocess uses fine catalyst particles slurried in an inert mineral oil.The catalyst is kept in suspension by reactant gas, which bubblesup through the catalyst slurry. This type of reactor is typicallyreferred to as a slurry bubble column reactor (SBCR). An importantfeature of the LPMEOH technology is its potential integration witha combined cycle power plant, i.e., integrated gasification combinedcycle (IGCC) power generation. With this integration, methanolcould be produced from the excess syngas not committed for power

generation.43 At peak power demand, electricity can be generatedfrom synthesis gas, whereas methanol can be produced during off-peak times to increase capacity use. The cost of methanol from aLPMEOH unit integrated into a coal-based IGCC system isestimated to be in the range of 50-60 cents per gallon on a currentdollar basis or 40-50 cents per gallon on a constant dollar basis.43

2.2. Reformer. Methanol and steam in the presence of an Al2O3-supported CuO/ZnO catalyst at temperatures greater than 160 °Creact to form a hydrogen-rich gas, containing CO2 and minorquantities of CO.44 Higher hydrogen selectivities are reached attemperatures between 200 and 300 °C and 1-3 bar, where copperis the active component.45 During steam reforming of methanol,two further side reactions occur; methanol decomposition, D in eq6, and WGSR, W in eq 7, take place besides methanol reforming,R in eq 5, being algebraically the sum of the two side reactions.46

Peppley et al. developed a comprehensive surface mechanismfor methanol-steam reforming over a 40% CuO-40% ZnO-20%Al2O3 catalyst to be able to account for the variation of the COcontent in the product gas as well as the rate of production ofhydrogen. They determined rates of reforming, WGSR, anddecomposition reactions, which let them predict methanol conver-sion and other reaction products, including CO, an undesirablebyproduct.47 If the product gas is to be cleaned by PROX beforeentering the FC anode, the CO concentration determined accordingto their work can be used to dimension the PROX reactor.

Because, in this work, a perm-selective membrane is chosen forhydrogen purification, a less complex power law expression thanthe one by Peppley et al. has been chosen. Lee et al. carried out akinetic study of methanol reforming over a similar catalyst (64%CuO-10% ZnO-24% Al2O3 with 2% MgO as the promoter) atatmospheric pressure and at temperatures in the range from 160 to260 °C. This temperature range is appropriate for the temperatureof the exhaust gas waste heat of ICE. Therefore, besides steamgeneration, reaction enthalpy of steam reforming can be used fromthe exhaust gas. They did not only do a kinetic study; moreover,they have also considered the influence of the porous structure ofthe catalyst pellets on the mass transport by determining the internaleffectiveness factor (0 < ηi < 1) of the design equation48 of thereformer in eq 8, which decreases with the temperature and thepartial pressure of the reactants and increases with the partialpressure of the products.

Fj (mol of j/s) and νj (mol of j/mol of fuel) are the molar flowand stoichiometric ratio of reactants and products. The variable jstands for m, w, c, and h, which are abbreviations of methanol,steam or water, carbon dioxide, and hydrogen, respectively. W (kg)and rR [mol of fuel s-1 (kg of catalyst)-1] are the catalyst load andreaction rate of reforming, respectively.

(40) Specht, M.; Bandi, A. The methanol cyclesSustainable supply ofliquid fuels. Centre for Solar Energy and Hydrogen Research (ZSW),Stuttgart, Germany, 1999 (available at www.zsw-bw.de, 2005).

(41) Dong, Y.; Steinberg, M. Int. J. Hydrogen Energy 1997, 22, 971–977.

(42) Larson, E. D.; Worrell, E.; Chen, J. S. Resour. ConserV. Recycl.1996, 17, 273–298.

(43) U.S. Department of Energy (DOE). Commercial-scale demonstra-tion of the liquid phase methanol (LPMEOH) process. NETL-2004/1199,DOE, Washington, D.C. 2004.

(44) Christiansen, J. A. J. Am. Chem. Soc. 1921, 43, 1670–1672.(45) Brown, L. F. Int. J. Hydrogen Energy 2001, 26, 381–397.(46) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Appl.

Catal., A 1999, 179, 21–29.(47) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Appl.

Catal., A 1999, 179, 31–49.(48) Fogler, H. S. Elements of Chemical Reaction Engineering, 4th ed.;

Prentice Hall PTR International Series in the Physical and ChemicalEngineering Sciences: Upper Saddle River, NJ, 2006.

R : CH3OH + H2OS-kR

kR

CO2 + 3H2 ∆Hr ) 49.3 kJ/mol

(5)

D : CH3OHS-kD

kD

CO + 2H2 ∆Hr ) 90.5 kJ/mol (6)

W : CO + H2OS-kW

kW

CO2 + H2 ∆Hr ) -41.2 kJ/mol (7)


According to their experimental results, the reaction rateincreased with an increase in the methanol partial pressure butdecreased with an increase in the hydrogen partial pressure,indicating hydrogen inhibition, while CO2 partial pressure had noeffect on methanol conversion. They further conclude from theirexperimental results that the reforming rate is not affected by thewater partial pressure, as long as its partial pressure exceeds themethanol partial pressure (over stoichiometric). The products werealmost exclusively H2, CO2, and a small amount of CO (<1%),significantly less than the equilibrium CO concentration. Thisindicates a reaction sequence of steam reforming followed by thereverse WGSR for the CO output, suppressed by the increase ofwater partial pressure.49

They neglected in their kinetic analysis the reaction rates ofdecomposition and WGSR and predicted with their power lawexpression only methanol conversion regarding the methanolreforming reaction. This is sufficient to determine H2 output ofCMR, because the CO production (<1%) rate has only a marginaleffect on membrane permeability at this concentration, with noeffect on the hydrogen selectivity, FC efficiency, and stability aswell, thanks to the dense membrane. The adsorbed amount of H2

on the Pd surface may be decreased in the presence of CO, but theH2 diffusivity through the bulk of Pd, as the rate-determining stepof H2 permeation, may hardly be affected. If an appropriate amountof H2 can be adsorbed even in the presence of CO, the H2 permeancewill not be seriously affected.50

If water partial pressure is substoichiometric, the rate of reverseWGSR will increase, which will increase CO selectivity, resultingin competitive adsorption of CO on the Pd membrane. Thisdecreases hydrogen permeance over the membrane. Han et al. reportthat the hydrogen flux through the membrane is reduced by 25%,if the membrane reactor operates with reformate compared to apure hydrogen feed within the reactor.32 Moreover, catalyst cokingwill occur as a result of CO reduction by hydrogen, which decreasesthe amount of catalytic sites for methanol reforming. An appropriatesteam/carbon ratio higher than the stoichiometric ratio will hinderthe catalyst to coke by decreasing CO production.25

In general, the rate r of any gas-solid-catalyzed reaction in eq10 can be expressed as the product of the apparent rate coefficientk and a pressure-dependent term f(pj),51 where pj is the partialpressure of reacting gases.

The pressure-dependent term mostly consists of a product ofexperimentally determined exponential functions of the partialpressures of reacting gases; reactants and products have positiveand negative exponents, respectively. If they have no influence onthe rate at the apparent reaction conditions, this exponent equalszero. The rate coefficient k will change as the prevailing reactionconditions vary. Consequently, it is convenient to use the Arrheniusequation in eq 11, where k0 [mol (s kgcat kPar)-1] is a temperature-independent pre-exponential factor and Ea (J/mol) is the apparentactivation energy of the catalytic reaction. The exponent r in theunit of the pre-exponential factor is a real number, the sum of theexponents of the products and reactants.

To determine methanol conversion rR [mol s-1 (kg catalyst)-1]by solving the design equation, we used the power law model ineq 12 according to Lee et al.,49 where pm and ph are the partialpressures of methanol and hydrogen, respectively.

2.3. Membrane. Palladium exhibits high catalytic activity forthe adsorption and dissociation of hydrogen into atoms enteringthe membrane and recombination of the atoms into molecularhydrogen exiting the membrane.52 According to the Richardsonequation in eq 13 for dense membranes, the hydrogen flux (Jh)through the membrane is inversely proportional to the membranethickness (δM) and directly proportional to the product of thehydrogen permeability (kperm) and the hydrogen partial pressuregradient across the membrane, which is determined by the hydrogenpartial pressures on the reactor (ph) and permeate (ph,perm) sides.

The hydrogen permeability k of palladium increases with thetemperature according to an Arrhenius-type relation because theendothermic activation energy for diffusion dominates the exother-mic adsorption of hydrogen on palladium.53 The activation energyEa,perm and the constant kperm

0 can be determined by least-squaresminimization fitting of experimental results.

For thick membranes (δM ∼ 1 mm) and low hydrogen partialpressures, the exponent of 0.5 reflects the dissociation of the gaseoushydrogen molecule into two hydrogen atoms that diffuse into themetal, after they form an ideal solution in it.54 The optimum valuesof the exponent, which fit experimental results best, vary between0.5 and 1; the value increases with the feed pressure55 and decreaseswith the membrane thickness, indicating that surface effectsincreasing the exponent are also involved besides the solutionprocess of hydrogen in palladium,56 each having a stronger rate-determining step character according to the reactor conditions duringhydrogen diffusion.

Pd alloys, such as PdCu and PdAg, have advantages comparedto bulk Pd. One reason for using PdAg instead of Pd is that thehydrogen permeation rate can be enhanced with a certain PdAgcomposition. The hydrogen solubility is larger in silver than inpalladium, while the diffusion coefficient is lower. For PdAg, thehydrogen permeability increases with silver content to reach amaximum at 23 wt % Ag.57 The mechanical strength also increaseswhen alloying palladium with silver. Pure Pd becomes brittle afterrepeated R- cycles, which does not occur in the PdAg alloy.58

Besides high-purity hydrogen production, the membrane reactorhas also the advantage that the conversion increases with thepressure gradient; the MR exceeds the equilibrium conversion of a

(49) Lee, J. K.; Ko, J. B.; Kim, D. H. Appl. Catal., A 2004, 278, 25–35.

(50) Uemiya, S.; Kato, W.; Uyama, A.; Kajiwara, M.; Kojima, T.;Kikuchi, E. Sep. Purif. Technol. 2001, 22-23, 309–317.

(51) Thomas, J. M.; Thomas, W. J. Principles and Practice ofHeterogeneous Catalysis; VCH Verlagsgesellschaft: Weinheim, Germany,2005; Vol. 3, p 26.

(52) Nowick, A. S.; Burton, J. J. Diffusion in Solids, Recent DeVelop-ments; Academic Press: New York, 1975; pp 232-295.

(53) Buxbaum, R. E.; Kinney, A. B. Ind. Eng. Chem. Res. 1996, 35,530–537.

(54) Buxbaum, R. E.; Marker, T. L. J. Membr. Sci. 1993, 85, 29–38.(55) Hurlbert, R. C.; Konecny, J. O. J. Chem. Phys. 1961, 34, 655–

658.(56) Morreale, B. D.; Ciocco, M. V.; Enick, R. M.; Morsi, B. I.; Howard,

B. H.; Cugini, A. V.; Rothenberger, K. S. J. Membr. Sci. 2003, 212, 87–97.

(57) Uemiya, S.; Matsuda, T.; Kikuchi, E. J. Membr. Sci. 1991, 56, 315–325.

(58) Bohmholdt, G.; Wicke, E. Z. Phys. Chem. Neue Folge 1967, 56,133.

dFj

dW) ηiνjrR [mol of j s-1 (kg of catalyst)-1]

j ) m, w, c, and h (8)

coking : 2CO S C + CO2 or CO + H2 S C + H2O (9)

r ) k f (pj) [mol s-1 (kg of catalyst)-1] (10)

k ) k0e(-Ea/RT) [mol (s kgcat kPar)-1] (11)

KM(T) ) 2.19 × 109e-103000/RT

rR ) KMpm0.564(11.6 kPa + ph)

-0.647 (12)

Jh ) kperm

(ph0.5 - ph,perm

0.5 )

δM(mol s-1 m-2) (13)

kperm ) kperm0 e-(Ea,perm/RT) (mol m s-1 m-2 kPa-1) (14)


conventional reformer if the pressure is increased to 20 bar at thefeed side and if the pressure level of the permeate side is heldconstant at 1 bar. Because of the high permeation rate at highpressure, a CMR with a 75:25 Pd/Ag membrane almost attains thetheoretical maximum of the hydrogen recovery.33

To determine the hydrogen flux through a dense PdAg mem-brane, we used the parameters proposed by Basile et al.59 Theactivation energy and the permeation constant in eq 14, fitting theirexperimental results best, were 33 310 (J/mol) and 1.66 × 10-5

(mol m s-1 m-2 kPa-0.5), respectively. Hydrogen flow permeated(Fh,perm) through the membrane wall can be determined by integrat-ing the hydrogen molar flux over the shell surface A [m2] of themembrane in eq 15.

The above equation has to be solved simultaneously with thedifferential equation system of the reformer in eq 8, because thepermeated hydrogen flux in eq 13 is a function of the hydrogenpartial pressure of the reformer. This flux (JhdA) is subtracted fromthe hydrogen flow (dFh) in eq 16.

Once hydrogen output of the membrane reformer, permeated tothe FC cycle, has been determined according to eq 15, themembrane reformer efficiency can be determined. This efficiencyis defined in eq 17, which is the ratio of the thermal energy of thehydrogen flow (Fh,perm) at the exit of the permeate side to the thermalenergy of the initial flow of methanol (Fm0). Both energies arespecified with the product of the molar flows (Fh,perm and Fm0) andmolar LHVs (LHVh and LHVm) of the species.60

In a conventional reformer, all reacted hydrogen is included inthe numerator of the reformer efficiency, because all hydrogen issent to the FC after subsequent purification steps. In a membranereformer, only the purified hydrogen that permeates through themembrane wall is used in the FC. The unrecovered portion ofhydrogen in the retentate is not regarded in the numerator. However,the reforming process is endothermic; therefore, in a conventionalreactor, additional fuel has to be fired at an adjacent combustionchannel to maintain the reaction, which should be added to thechemical energy of the fuel in the denominator. In the case of themembrane process, this is not required, because the retentate embedschemical energy, which can be liberated in the adjacent combustionchannel to recover heat for endothermic steam reforming.

We propose in our hybrid system in Figure 2 to further usehydrogen left in the retentate stream with the unreacted methanolin an ICE to increase the overall system efficiency. In this case,the reformer efficiency is not a real thermal efficiency. It is just aratio showing the fraction of the chemical energy sent to the FC,because the rest of the chemical energy in the retentate stream isalso converted to mechanical energy with the efficiency of the ICE.Finally, subsequent use of the waste heat of the ICE exhaust gasesfor endothermic steam reforming further increases the systemefficiency.

2.4. Fuel Cell. The thermal efficiency (η) definition of a HEcan be adapted to the definition of the energetic efficiency (ηFC) ofa FC. The energy input is the chemical energy Qh,perm of thepermeated hydrogen through the membrane to the FC cycle, andthe output is the electrical power (PFC) of the FC stack, which is

the product of the voltage V (V) and electrical current I (A) of theFC stack.61

All of the hydrogen entering the FC cannot always react at itsanode; fuel use (0 < µf < 1) of a FC is defined as the ratio of thereacted hydrogen flow (Fh,reac) to the total flow at the anode entrance(Fh ) Fh,perm).

For every mole of hydrogen reacted (Fh,reac) at the anode, 2 molof electrons flow through the external circuit, of which the electriccharge (2NAe- ) 2F, where F ) Faraday constant) causes theelectric current I. The electric current is therefore the product ofFh,reac and the electric charge flown through the external circuit ofthe FC associated with the reaction of hydrogen at the anode. Theamount of hydrogen reacted at the anode is called fuel usage ofthe FC.

Fh,perm determined as a function of the current in eq 20 can beset into eq 18 to obtain the FC efficiency as a function of its voltageand fuel use in eq 21. The denominator is a constant related to afictive potential, which would theoretically be gained, if all of thechemical energy, LHVh, could have been converted into electricenergy. Consequently the determination of the FC efficiency isrelated to the calculation of the operational FC voltage Vc in volts.

If a FC is fed with pure hydrogen, the FC anode can be flownin a dead-end mode.2 Consequently, all of the hydrogen sent to theFC anode is reacted, increasing the fuel use to unity. In the case offeeding the FC anode with the permeated hydrogen from amembrane reactor, which has a dense membrane with an infiniteselectivity toward hydrogen, the anode exit flow can be recycledto the anode entrance or membrane entrance. The feed containsbesides hydrogen only steam used as sweep gas, maintaining alsothe required humidity in the membrane electrolyte of the FC.References given about membrane reformers feeding the FC anodepropose either feeding the anode in a dead-end mode62 or recyclingthe anode exit flow to the anode entrance.33

Steam in the anode feed is driven to the cathode by the electro-osmotic drag of the protons, reacting with oxygen to steam at thecathode. Both sweep gas and oxidation product steam can berecycled either as sweep gas or for steam reforming of methanolafter separation from unreacted air by condensation. The sweepgas amount sent to the membrane and the FC has to be controlledboth to maintain the required humidity of the membrane electrolyteof the FC and to prevent flooding of the anode. The open circuitvoltage (E0) of the FC at STP is the ratio of the Gibbs enthalpy ofsteam formation to the electric charge of the generated electrons.

The reversible cell voltage (E) at a temperature (T) and pressure(p) different from STP is called the Nernst voltage and is definedin eq 23 for activities (ai) other than unity.

(59) Basile, A.; Gallucci, F.; Paturzo, L. Catal. Today 2005, 104, 244–250.

(60) Boettner, D. D.; Paganelli, G.; Guezennec, Y. G.; Rizzoni, G.;Moran, M. J. J. Energy Resour. Technol. 2002, 124, 191–196.

(61) Boettner, D. D.; Paganelli, G.; Guezennec, Y. G.; Rizzoni, G.;Moran, M. J. J. Energy Resour. Technol. 2002, 124, 20–27.

(62) Manzolini, G.; Tosti, S. Int. J. Hydrogen 2008, 33, 5571–5582.

dFh,perm ) JhdA (mol of H2/s) (15)

dFj ) ηiνjrRdWdFh ) ηi3rRdW - JhdA (mol of H2/s) (16)

ηREF )Qh,perm

Qm0

)Fh,permLHVh

Fm0LHVm(17)

ηFC )PFC

Qh,perm

) VIFh,permLHVh

(18)

µF )Fh,reac

Fh,permf Fh,perm )

Fh,reac

µF(mol/s) (19)

I ) 2F Fh,reac ⇒ Fh,perm ) I2FµF

(mol of H2/s) (20)

ηFC ) µf

Vc

LHVh/2F) µf

Vc

1.25(21)

E0 )-∆gf,H2O

0

2F(V) (22)


For ideal gases, the activity of a gas is the ratio of its partialpressure (pi) to p0. The molar ratio (yi) of a gas in a mixture is theratio of its partial pressure to the absolute pressure of the mixture.

The case of the water produced in FCs is somewhat difficult,because this can either be steam or liquid. We assumed the relativehumidity to be unity in our calculation, which assures that thepolymer electrolyte is neither too wet nor too dry. The sweep gasflow rate has to be controlled to provide this condition bothconsidering steam formed at the cathode and oxygen stoichiometry,which is out of the scope of this work. Consequently, the molarratio of steam (yH2O) is the ratio of the saturation pressure at thetemperature concerned (psat) to the absolute pressure.

Hence, the activities are eliminated from eq 23 to express theNernst voltage as a function of T, p, and the molar ratio of thereactants in eq 26. Anodic and cathodic absolute pressures aremostly equal, minimizing internal stresses to simplify the design.

According to eq 26, the Nernst voltage can be increased byincreasing the total pressure of the reactants higher than the standardpressure (p0) or the molar ratios of the reactants (yH2 and yO2). Ifthe temperature is increased to decrease the activation losses, thesaturation pressure of the product steam increases. This increasesyH2O provided that the relative humidity remains unity, whichdecreases the Nernst voltage. An alternative to decrease yH2O is toincrease the total pressure higher than p0, but hydrogen recoveryfrom the membrane decreases in this case. The total pressure hasto be optimized considering both effects of the hydrogen recoveryand Nernst voltage.

The Nernst voltage according to eq 26 has a value of about 1.2V for a cell operating below 100 °C. However, the operational FCvoltage Vc decreases with the current when the external circuit isclosed (I > 0). This decrease results from four main irreversibilities:(a) fuel crossover and internal currents, (b) ohmic losses, (c)activation losses (activation polarization), and (d) concentrationlosses (concentration polarization). All of these irreversibilities arecombined in eq 27 to determine Vc from E and those irreversibilities.

Because fuel crossover and internal currents cause a voltage dropfrom the reversible voltage E, even when the circuit is open, thoseare represented with the internal current density in (mA/cm2) thatis summed up with the actual current density i (mA/cm2). Thecurrent density represents the current I flowing through the externalcircuit, referred to the surface of the electrodes. The ohmic voltagedrop is the product of the sum i + in with the area-specific resistancer (kΩ cm2). The term after the ohmic loss in eq 27 is the activationpolarization loss, caused by the slowness of the reactions, takingplace on the surface of the electrodes. For a slow reaction, theconstant A (V) is higher and the exchange current density i0 (mA/cm2) is lower, causing a higher voltage drop. A and i0 are determinedwith Tafel plots experimentally. The last term represents the voltagedrop caused by the concentration polarization, resulting from thechange in the concentration of the reactants at the surface of theelectrodes, as the fuel is used. For high concentration gradients ofthe reactants near the electrode surfaces, the constant B (V) andthe limiting current density il (mA/cm2) are higher, causing a higher

voltage drop. The internal current density (2 mA/cm2), exchangecurrent density (0.067 mA/cm2), limiting current density (900 mA/cm2), area-specific resistance (30 kΩ cm2), and the constants A(0.06 V) and B (0.05 V) are adopted from the reference to thefollowing calculation:2

Auxiliary components of the FC system include an air compres-sor to provide air flow at the desired air pressure entering thecathode, an expander to produce power from high-pressure airexiting the cathode, a hydrogen pump to recirculate unusedhydrogen exiting the anode, a pump to circulate coolant throughthe FC, and a fan to circulate air through the heat exchanger. Allof these components are necessary to support the operation of theFC stack.

The compression of the air from p1 to p2 increases its temperaturefrom T1 (K) to T2 (K), depending upon the isentropic efficiency(ηc) of the compressor and the isentropic exponent (γ) of the air.The power consumption PCp (W) of the compressor resulting fromthe temperature increase (∆Tcp ) T2 - T1) is determined with themolar flow of air Fa (mol/s) and its molar specific heat cp (J mol-1

K-1) in eq 28.In the hybrid system model proposed in Figure 2, the power

required by the compressor is provided by two turbo-chargedturbines mounted on the same shaft with the compressor, whichproduce the power PT by expanding the working fluid from p2 top1, resulting in a temperature decrease of ∆TT. The first one expandsthe reactor bleed gas from the reformer pressure to the superchargepressure of the ICE. The second turbine expands the cathode offgas from the FC pressure to the ambient pressure.

2.5. Internal Combustion Engine. The disadvantage of themembrane reactor not permeating all of the hydrogen produced inthe membrane reformer does not mean that the rest of the hydrogenexergy is lost, if an ICE uses the residual reformate of the reformer.Moreover, the sensible heat of the ICE exhaust gases is used inthe heater channel of the membrane reformer. If the residual gasof the membrane reformer was not used in ICE, it would have tobe combusted catalytically in the heater channel of the membranereformer, causing a higher exergy destruction but a higher exhaustgas temperature.

The use of the bleed gas of the reactor in an ICE affects theefficiency and exhaust gas quality of ICE as well. The bleed gasconsists of mainly H2, CO2, and some unconverted gaseousmethanol and steam if the conversion is not complete. Hydrogenis one of the best fuels that can be used in ICEs as pure or as apartial substitute of standard hydrocarbon fuels to improve perfor-mance and emissions. Tsolakis et al. have studied the applicationof exhaust gas fuel reforming in diesel engines experimentally asa way to assist the premixed charge compression ignition (PCCI)operation.63 When part of the main fuel was substituted withhydrogen-rich gas, they concluded that NOx and smoke emissionscan be reduced simultaneously. The use of exhaust gas recirculation(EGR) without reforming is an effective way of reducing NOx

emissions, but it is normally associated with higher smoke andparticulate emissions as well as with increased fuel consumption.The exhaust gas fuel reforming process involves hydrogen genera-tion by direct catalytic interaction of hydrocarbon fuels with engineexhaust gases. The technique involves the injection of hydrocarbonfuel into a catalytic reformer fitted into the EGR system, so that

(63) Tsolakis, A.; Megaritis, A. Int. J. Hydrogen Energy 2005, 30, 731–745.

E ) E0 + RT2F

ln[aH2aO2

1/2

aH2O ] (V) (23)

ai )pi

p0, yi )

pi

p⇒ ai ) yi

pp0

(24)

yH2O )psat

p(25)

E ) E0 + RT2F

ln[ yH2yO2

1/2

yH2O( ppO

)1/2] (26)

Vc ) E - (i + in)r - A ln[ i + in

i0] + B ln[1 -

i + in

il] (27)

∆TCp )T1

ηc((p2

p1)(γ-1)/γ

- 1), PCp ) Facp∆TCp

∆TT ) ηcT1((p2

p1)(γ-1)/γ

- 1), PT ) Facp∆TT

(28)


the produced gas mixture is fed back to the engine as reformedEGR (REGR).63

In contrast to their work, we only exchange the heat of theexhaust gas to the fuel-steam mixture to reform it, becausehydrogen partial pressure will be too low to purify hydrogen fromREGR. We further assume that the bleed gas from the reformerwill have a similar affect on the ICE, such as REGR, althoughsome hydrogen will have been permeated to the FC cycle, becausethe N2, CO2, and O2 (air/fuel ratio > 1) from the combustion willnot dilute the fresh charge prior to ICE entrance.

The chemical energy of the residual gas mixture is called thepremixed fuel energy rate (Qp) (kW). It is the rest of the unconvertedmethanol, hydrogen, and other chemical energy of the hydrocarbonsand is mixed with the charge air of the ICE. Although in our hybridsystem the fuel-steam mixture is only heated by the exhaust gas,we will use the same ratios of the PCCI engine, where the fuel ismixed with a recirculated portion of the exhaust gas. The premixedcombustion ratio (rp) is defined as the ratio of Qp (kW) to the totalfuel energy rate (Qt) combusted in ICE.64,65 Qp can be determinedafter the design equation of the membrane reformer has been solved,delivering the molar flows of combustible gases in the retentatestream of the reformer.

The total fuel energy rate (Qt) combusted in the diesel engine isthe sum of Qp and the energy (QICE) of the direct combusted fuelinjected into the ICE. QICE is determined depending upon Qp in eq30, after setting Qt from eq 29 into the sum of Qp and QICE.

The thermal efficiency of ICE (ηICE) is the ratio of its power(PICE) to Qt combusted in the engine.

3. Hybrid System Performance and Efficiency

The methodology used to estimate the efficiency of the hybridsystem is performed by describing and solving equations of theelements of the hybrid system. Thermo-physical data used inthese calculations are obtained from the software EngineeringEquation Solver (EES), containing thermo-physical functionsof several species, such as methanol and steam, determinedaccording to databases of the National Institute of Standardsand Technology (NIST) and Joint Army Navy Air Force(JANAF). The equations describing the hybrid system has beensolved by Mathematica, a symbolic mathematical software (seethe Supporting Information). The results evaluated in thatsoftware are exported into MS Excel, where charts are preparedto visualize how parameters, such as temperature and pressure,affect the performance of the proposed hybrid system, parts ofwhich are described in the references above. The models andparameters in those references are used in our hybrid systemmodel.

We used the equations given in the last section: the designequation for the reformer, the Richardson equation for themembrane, and the operational voltage equation for the FC. Thedifferential equations of the membrane reformer have been non-dimensionalized prior to solving them, to simplify the formula-

tion of boundary conditions, the solution, and the comparisonof the solutions with the experimental works in the references.Moreover, the non-dimensionalization of the design equationenables us to define some critical ratios, to scale up theperformance of the membrane reformer from laboratory condi-tions to a prototype.

In plug flow reactors (without catalyst), the space time τ (s)is the time necessary to process one reactor volume of fluidbased on entrance conditions. It is the ratio of the reactor volumeV (m3) to the volumetric flow V (m3/s) of the fluid. Conse-quently, the conversion in the reactor increases with the spacetime. Two reactors having different scales but the same spacetime defined in eq 32 reach similar conversions, if the reactantratios are the same.

In catalyzed packed bed reactors, the ratio of the catalyst massto the molar flow (W/Fm0) of the fuel can be related to the spacetime in plug flow reactors to scale up a catalyzed reactor. Thevolumetric flow can be determined with the total molar flow(FT0) at the reactor entrance, reaction pressure (pr), andtemperature (Tr), whereas the catalyst mass can be related tothe reactor volume (V) with the catalyst density (Fcat) andporosity (φ) in eq 33. Because temperature, pressure, and molarratios (Fm0/FT0) effect conversion, the reactor can be scaled upwith the ratio of (W/Fm0) without effecting the conversion byholding all other parameters constant. The scale factor of thecatalytic reactor (W/Fm0) increases the conversion in a way thatis similar to how τ impacts conversion in plug flow reactors.

The surface area/thickness ratio of the membrane increaseshydrogen recovery, similar to the way that the catalyst massincreases the conversion in catalytic reactors. The ratio (Amem/Fm0 δm) is the scale factor used to scale up the membrane toreach the same amount of hydrogen recovery. Because sweepgas flow rate on the permeate side effects hydrogen partialpressure, the ratio of Fm0 to Fs has to be maintained constantas well to upscale the membrane. The experimental results ofthe references concerning the reformer and membrane are scaledup by these factors to reach similar conversion and hydrogenrecoveries while generating a power output between 10 and 20kW with the FC.

4. Results and Discussion

The main purpose in using a FC-HE hybrid system is adouble-sided energy recovery. While the exhaust gases leavingthe feed side of the membrane reactor are mixed with the chargeair of ICE to decrease its fuel consumption, exhaust gases ofthe ICE are cooled in the heat exchanger of the reformer torecover waste heat for fuel processing. Moreover, the coolingload of ICE may be recovered for the evaporation of methanoland water used for methanol reforming. In this case, the coolingsystem of ICE has to be re-engineered to work under thepressure of the reformer. The radiator is used for cooling theFC. Consequently, the temperature of the engine cooling isincreased to the saturation temperature of steam at the opera-tional pressure of the reformer, so that the exhaust gas

(64) Suzuki, H.; Koike, N.; Adaka, M. SAE Tech. Pap. 970313, 1997.(65) Suzuki, H.; Koike, N.; Adaka, M. SAE Tech. Pap. 970509, 1997.

rp )Qp

Qt

(29)

Qt ) QICE + Qp ⇒ QICE ) Qp

1 - rp

rp(kW) (30)

ηICE )PICE

Qt

⇒ PICE ) ηICE

Qp

rp(kW) (31)

τ ) V

V0

(s) (32)

V ) W(1 - φ)Fcat

, V0 )FT0RTr

pr⇒

τ )pr/RTr

(1 - φ)Fcat

Fm0

FT0( WFm0

)(33)


temperature of ICE can also be increased to be sufficient forsteam reforming. Because steam has a higher saturation tem-perature than methanol at the same pressure (199 and 154 °Cat 15 atm, respectively), methanol can be partly evaporated bycooling the reactor bleed gas. The cooling of the reactor bleedgas also has a positive effect on the volumetric efficiency ofthe engine. The rest of methanol is evaporated in the engine,where water is also evaporated prior to entering the reformer.

The evaporation of reactants with the cooling load of ICEincreases the hybridization degree of the system, the ratio ofthe fuel cell’s power to the power of the hybrid system PFC/(PICE + PFC), because exhaust waste heat is used only for thereforming reaction. Moreover, the evaporation of the reactantswith the cooling load increases the temperature of the exhaustgas, resulting in decreased Carnot efficiency of the HE. On theother hand, the availability of waste heat is also increased tobe used for hydrogen production, resulting in a higher overallthermal efficiency.

The CMR considered for our work consists of three concentrictubes. The outer one is the heater, through which the exhaustgases of the ICE flows on the shell side of the catalyticreforming reactor. Inside the catalytic reactor, the methanol-steammixture flows counter-current to the exhaust gas of the ICE.Hydrogen reacted is permeated through the Pd-based membranefrom the reactor (feed side) into the membrane and thendesorbed into the inner tube (permeate side). The permeate sideis flown by steam as sweep gas, flowing co-current to the feedmixture.

Waste heat recovery in such a thermally coupled membranereformer to produce hydrogen brings some advantages besidesincreasing system efficiency during hydrogen purification. Thedriving force for the permeation through the membrane is thehydrogen partial pressure difference between the permeate sideand the feed side. As hydrogen permeates through the mem-brane, its partial pressure on the feed side decreases; therefore,methanol reforming progresses until a new equilibrium betweenpermeation and reforming is reached.

Water required for the steam reforming is condensed fromthe cathode off gas; therefore, only a small flash tank of waterwill be enough to maintain steam reforming of methanol.Because the engine cooling is partly realized by the evaporationof the condensed water prior to the reformer entrance, therewill be enough space in cooling channels of the engine to storerecycled water from cathode off-gas condensation besides asmall flash tank.

We propose steam as sweep gas for the membrane reactor,because steam also humidifies the FC. We do not propose atank filled with distilled water at the fuel station to drive steamreforming of methanol; water should be fully recycled from thecathode off gas if possible. The membrane should be dimen-sioned to recover enough hydrogen from the reformer, to feedthe FC with the required hydrogen amount, so that the FCproduces the water for the reformer besides electric power.Moreover, methanol, produced from synthesis gas, also containssome water; 100% pure methanol would be more expensive toproduce. The disadvantage of water in methanol from aninterrupted distillation process, which would be more energyefficient at the refinery, is the decrease of the power density ofthe fuel. On the other hand, water in methanol increases its flashpoint, hence decreasing the fire hazard.

Because steam diffuses with the protons through the mem-brane electrolyte of the FC, it has to be mixed to the anodecycle anyway, to maintain the humidity of the FC. Consequently,the sweep gas flow rate at the anode/membrane cycle should

be dimensioned according to the FC humidity to be maintainedat 100%. The circulation of the sweep gas with hydrogen isperformed with an ejector circulator, powered by the coolingpump of the engine. According to the temperature required atthe FC, a valve can mix some steam to the pressurized watercoming from the pump. The pressure difference betweenthe FC and reformer will be the driving force to maintain thecirculation of the ejector circulator. The evaporation of thepressurized water in the circulator cools the sweep gas comingfrom the hot membrane that was precooled in the internal heatexchanger (IHE). Such IHEs are also apparent in refrigeratorsand air conditioners.

If power demand exceeds power capacity of the FC, battery,and base capacity of ICE with bleed gas feed, ICE may combustadditional fuel directly. This fuel may be methanol or a well-established fuel with a higher energy density, such as gasolineor diesel, stored in a second tank to increase the specific energy(kJ/kg) of the total fuel energy stored on board. The dual fueloption enables operation of a hybrid system before the completeintroduction of a methanol infrastructure. The hybrid systemefficiency (ηHYB) is the ratio of the power produced by the FC(PFC) and the ICE (PICE) to the chemical energies of the fuelsconsumed by the reformer (Qm0) and ICE (QICE).

The design and the Richardson equations constitute a one-dimensional differential equation system, which does notconsider mass-transport effects in the radial direction. Thoseeffects have been estimated by an internal effectiveness factorthat has been taken from the reference for the apparent reactiontemperature. To solve the resulting equation, the differentialsdW and dA in eq 16 have to be related to a common integrationvariable (0 e e 1), the nondimensional form of the axiallength z (0 e z e L) of the reformer referred to the total lengthL of the reformer.

The total area Amem of the membrane through which hydrogenpermeates is the surface area of a cylindrical tube having anouter diameter of Dmem and length of L. Several membrane tubesmay also be operated in parallel to increase the total surface.The catalyst mass W is the product of the catalyst density Fand catalyst volume V ) Lπ(D2 - Dmem

2)/4. For the calculationof the catalyst mass, the porosity of the catalyst φ has beentaken into account, such as in eq 35.

Both the molar flows and partial pressures have to beexpressed by a common dimensionless function, for which thedifferential equation system will be solved. This dimensionlessfunction, called flow ratio fj of any gas j reacting in the reformer,is the ratio of the molar flow of the gas j to a characteristicmolar flow. The molar flow of methanol at the entrance of thereformer (Fm0) is chosen as the characteristic molar flow of thereformer.

The total molar flow of the gases through the reactor is FT,which changes during the reaction along the axial direction ofthe tube, if the sum of the stoichiometric ratios of the reactantsis not equal to the sum of the products. For an ideal gas, the

ηHYB )PFC + PICE

Qm0 + QICE

(34)

) z/L ⇒ dW ) Wd, dA ) Amemd

W ) (1 - φ)FV (kg) Amem ) πDmemL (m2)(35)

fj )Fj

Fm0⇒ Fj ) Fm0 fj (36)


ratio of its partial pressure pj (kPa) to the absolute pressure pr

in the reactor is equal to the ratio of its molar flow Fj to FT.

Setting eq 37 into eq 12 yields the non-dimensional form ofthe power law equation.

The differential equation of the permeate side must be solvedsimultaneously with the differential equation system of thereformer, because the molar flux of hydrogen through themembrane depends upon the partial pressure of hydrogen onthe permeate side (ph,perm). The ratio of ph,perm to the absolutepressure on the permeate side (pperm) is equal to the ratio of theflow of hydrogen (Fh,perm) to the total flow at the permeate side(Fh,perm + Fs0). The sweep gas flow rate Fs0 at the entrance ofthe sweep channel has been chosen to be the characteristic flowrate of the permeate side, remaining constant during thepermeation process. The choice of characteristic flow ratesshould simplify the formulation of boundary conditions.

The non-dimensional form of the design equation is obtainedby setting eq 36 into the design eq 16, yielding the followingdimensionless differential equation system:

Finally, the initial conditions for both the retentate andpermeate side have to be determined considering the definitionsof the flow ratios. The steam/methanol ratio sm is set equal tothe initial value of the steam flow ratio.

In Figure 3, the molar flows of the products, reactants, andpermeated hydrogen can be observed for co-current flow of thesweep gas and steam-methanol mixture. The pressures on theretentate (reactor) and permeate side are 15 and 1 atm,respectively. The reactor temperature Tr, ηi, sm, and sweep ratio(Fs0/Fm0) were 280 °C, 0.5, 1, and 0.05, respectively. The sweepratio has to be chosen according to FC conditions to realize anoptimum humidification. The scale factors of the reformer (W/Fm0) and membrane (Amem/δMFm0) were 39 (kgcat s mol-1) and4.6 × 106 (m s mol-1), respectively. Methanol feed is 0.13 mol/s, reaching a conversion (X) of almost 100% defined in eq 42.If hydrogen flux through the membrane wall becomes equal tohydrogen formation at any point along the axis, hydrogen flowhas a maximum at that value with respect to the axial length,which occurred at ) 0.7 in Figure 3.

The differential equation system of the permeate side withits boundary conditions is revised for the counter-current flowmode of retentate and permeate sides in eq 43. For counter-current streams, the dimensionless variable of the permeate side(1 - ) runs in the opposite direction of the variable of thereformer (); therefore, its differential (-d) is negative withrespect to the differential of the axial variable of the reformer(d). Moreover, the boundary value of the flow ratio ofpermeated hydrogen, fh,perm, must be guessed at ) 0 (exit ofthe permeate side), to reach fh,perm ) 0 at ) 1 (fh,perm[1] ) 0),where the sweep gas enters the sweep channel.

The membrane reformer efficiency according to its definitionin eq 17 is depicted as a function of the mean reactor temperatureTr in Figure 4. The permeate side pressure has been chosen asparameter. Other parameters affecting this efficiency have beenmaintained equal to the former evaluation in Figure 3. Anincrease of the reformer efficiency is obtained with the increaseof the reaction temperature and decrease of the permeate sidepressure. The effect of the temperature is significant, becauseit increases the reaction rate and consequently the formation ofhydrogen; however, it also increases hydrogen permeationaccording to an Arrhenius relationship in both eqs 11 and 14,respectively.

Hydrogen permeation through the membrane increases withthe hydrogen partial pressure gradient between the feed and

FT ) ∑j

Fj;pj

pr)

Fj

FT⇒ pj )

fj

fTpr (kPa) (37)

rR ) KM(pr

fT)-0.083 fm

0.564

(11.6fT/pr + fh)-0.647

(38)

fh,perm )Fh,perm

Fs0, ph,perm )

fh,perm

fh,perm + fspperm (39)

dfj

d) ηiνjrR

WFm0

dfh

d) ηi3rR

WFm0

-Amem

Fm0Jh

dfh,perm

d)

AmemJh

Fs0,dfs

d) 0

(40)

fm[0] ) fs[0] ) 1, fw[0] ) sm

fc[0] ) fh[0] ) fh,perm[0] ) 0 co-current (41)

X ) 1 - fm[1]/fm[0] (42)

Figure 3. Molar flows of reactants and products; retentate flows:Fh(H2), Fc(CO2), Fm(CH3OH), Fw(H2O); permeate flows: Fh,perm(H2),Fs(sweep gas).

Figure 4. Efficiency of the membrane reformer.

dfh,perm

d) -

AmemJh

Fs0,dfs

d) 0 counter-current

fs[0] ) 1, fh,perm[0] ) fh,guess ⇒ fh,perm[1] ) 0(43)


permeate sides. Hence, a decrease of the permeate side pressureincreases the hydrogen flux through the membrane wall,increasing the membrane reformer efficiency. On the other hand,the decrease of the permeate pressure decreases the Nernstvoltage of the FC. Therefore, a permeate side pressure of 0.1atm seems to be a bit curios, because this pressure level wouldalso be the working pressure of the FC. However, the workingpressure of the FC could be decoupled from the permeationpressure, if a compressor and expander are operated at the anodeentrance and exit, respectively. These are not depicted in Figure2. Power consumption of such a compressor must be less thanthe power gain, resulting in a differentiation of the FC andpermeation pressures. Such a cycle is the reverse of the Carnotcycle apparent in refrigerators, and an optimization of such acycle for this hybrid system is out of the scope of this work.

Once the outputs of the membrane reformer have beendetermined, the FC power and the ICE power can be estimated.The FC voltage depends upon the reversible Nernst voltage,which is a logarithmic function of the FC pressure accordingto eq 26. It is depicted in Figure 5 for a mean operational Tc of80 °C, at which the saturation pressure of steam is 0.46 atm.Once this voltage is determined, the operational FC voltage and,consequently, power density can be obtained. The Nernst voltageincreases with the working pressure of the FC.

The operational voltage according to eq 27 decreases withthe current density, while the FC has its maximum power densityat a current density of 0.8 A/cm2, corresponding to a voltageand power density of 0.5 V and 0.4 W/cm2, respectively. Toreach a FC maximum power of 14 kW, 100 cells each havingan area of 400 cm2 can be operated in series. For pperm ) 1 atmand Tr ) 280 °C, 0.144 mol/s hydrogen permeates through themembrane wall, resulting a current density of 0.63 A/cm2 at afuel use of 90%. This results in a FC voltage of 0.56 V. With100 cells in series each having 0.56 V and 250 A electric current,14 kW of FC power is obtained at an efficiency of 40%.

The same operating point of the reformer is used for theefficiency estimation of the hybrid system. The retentate streamof the reformer consisting of hydrogen Fh and unreactedmethanol Fm is the premixed fuel energy rate Qp used in ICE.There is also CO2 (Fc) and unreacted steam (Fw) in the retentate,which do not effect Qp. However, the sm ratio of the reformerhas to be maintained near unity to minimize the steam amountin retentate, which can otherwise negatively affect the engineperformance and water recycling.

Both a spark-ignition (SI) engine and compression-ignition(CI) engine can use the retentate gas. Because the exhaust gastemperature level of a SI engine does not so much depend uponthe load as the CI engine, the energy demand of the reformercan be more easily recovered by the SI engine, because its air/

fuel ratio (λ) is maintained at all loads near 1. At CI engines,this ratio increases with the decrease of the load, because theair amount charged into the engine remains constant for all loads,while the fuel amount is decreased to cope with the lower load,decreasing the exhaust gas temperature at lower loads.

The calculation of the exhaust gas temperature and the ICEefficiency depending upon the retentate composition is out ofthe scope of this work; we only calculate the energy level ofthe retentate Qp and estimate the hybrid system efficiency byassuming the engine efficiency (15-35%). We assume that theexhaust gas temperature of the SI engine (900/1000 K) is highenough to maintain a mean temperature level of the reformer(250/300 °C), because the reformer and exhaust gas channelare operated in a counter-current mode. We further assume thatthe cooling load of ICE is high enough to provide theevaporation of the reactants. Because 0.13 mol/s of methanoland water consumes 10 kW of latent heat at about 200 °C, 36kW of waste heat will be enough to be recovered from 60 kWof Qp, even with the best possible engine efficiency of 40%.

For rp ) 1 and ηICE ) 20%, PICE ) 12 kW is obtained out of60 kW Qp in a system power of 26 kW (12 + 14 ) 26 kW). Atotal of 0.13 mol of methanol has a chemical energy of 88 kJ,equivalent to a power input of 88 kW. The system efficiencyfor this operating point is 26/88 ) 29.5%. The FC power andthe power produced by ICE with Qp will be too low to maintaina maximum load of 50 kW, comparable to Necar3 of DaimlerBenz.28 Therefore, additional fuel (QICE) should be combustedin the ICE. The effect of rp on the hybrid system efficiency isexamined by means of an energy balance of the reformer todetermine the energy rate of the premixed fuel (Qp) as a functionof the fuel energy input into the reformer. The heat recoveredin the reformer (Qrec) is used from the exhaust gas of the ICEfor endothermic steam reforming of methanol.

The FC power can be obtained from Qm0 as well accordingto eqs 17 and 18.

Because QICE and PICE are determined as a function of Qp ineqs 30 and 31, the hybrid system efficiency becomes a functionof dimensionless variables rp, ηref, ηFC, and ηICE in eq 46. Theratio of ∆Hr/LHVm is 7.3% for methanol, showing the extentof exhaust heat recovery.

The efficiency of the hybrid system in eq 46 is depicted inFigure 7 for the FC and reformer efficiencies of 40 and 39.5%,respectively, which have been determined from the case studyand the solution of the design equation of the reformer. Theefficiency of ICE has been chosen as a parameter at 20, 30,and 40%. These values can be observed in the figure where thecurves intersect with the ordinate at rp ) 0. At this operatingpoint, ηHYB is equal to ηICE. The system efficiency increaseswith rp monotonically, because the increase of this valueincreases the energy sent to the FC, efficiency of which is higherthan ICE. Moreover, the increase of rp also increases heat

Figure 5. Reversible Nernst voltage. p, FC pressure; Erev, Nernstvoltage.

Qm0 + Qrec ) Qp + Qh,perm (kW)

Qrec ) Qm0∆Hr/LHVm, Qh,perm ) Qm0ηref

⇒ Qp ) Qm0(1 + ∆Hr/LHVm - ηref) (kW)

(44)

PFC ) Qm0ηrefηFC (kW) (45)

ηHYB )ηFCηREF +

(1 + ∆Hr/LHVm - ηref)ηICE

rp

1 + (1 + ∆Hr/LHVm - ηref)(1 - rp)/rp(46)


recovery from the exhaust gas, because more methanol isreformed at the reformer for the same amount of exhaust gasproduced by ICE. The efficiency increase with rp is higher, ifthe difference between ηFC and ηICE increases.

The efficiency losses of the ICE at partial load are evenedby efficiency gains of the FC at partial load, because at lowloads of the FC, its efficiency is even higher than theestimated 40%. The rest of the unpermeated hydrogen willstill be enough to drive the hybrid system at such efficienciesillustrated in the figure. At high loads, the efficiency of ICEwill be higher; additional fuel to maintain the higher powerdemand should be directly combusted in the ICE instead ofbeing reformed, to not decrease the FC power from itsmaximum value in Figure 6 at current densities higher than0.8 A/cm2. Consequently, the efficiency of the hybrid systemwill be more independent from the load than the case, if theFC or ICE were operated separately.

The efficiency of 29.5% in the case study is also confirmedin the figure at rp ) 1 for ηICE ) 20%. The FC and reformerefficiencies may be different than the values determined in thecase study at lower loads. However, both the FC efficiency andreformer efficiency will increase with the decrease of the load,because the former increases with the decrease of the powerdensity, while the latter will increase with methanol conversionto hydrogen as a consequence of decreasing methanol feed. X∼ (W/Fm0).

Burning residual energy contents in the ICE could complicatethe control of the process, if additional liquid fuel is injected ata higher power demand than 26 kW. Because the residual energycontent of the reformer is not measured, it is difficult to

determine the incremental fuel that should be injected into theengine. A way out of this problem is a variable permeatepressure by replacing the ejector circulator with a compressorin the anode cycle of the FC. The FC pressure is maintained ata constant pressure level, while the permeate side pressure iscontrolled according to the current density in the FC. If thepower demand is increased beyond the FC power capacity andmore fuel is sent into the reformer, the permeate side pressurewill be increased nearer the FC pressure, decreasing the reformerefficiency but maintaining hydrogen flux through the membraneat a constant level. The effect of variable permeate side pressureon the reformer efficiency is illustrated in Figure 4. In this case,no direct injection of additional fuel will be necessary.Consequently, the engine is always operated at rp ) 1,maximizing exhaust gas recovery for all load conditions.Moreover, the storage of a second fuel being more suitable fordirect combustion will also be unnecessary. On the other hand,direct combustion of a fuel is much more flexible compared toa preceding reforming prior to combustion to respond toimmediate load fluctuations.

5. Conclusion

FCs are planned to be introduced for traction of mobilevehicles within the next few decades. To avoid the establishmentof a hydrogen infrastructure and the associated investment costsfor the first commercial market entry of the FCs, processing ofliquid fuels to hydrogen could be an appropriate compromise.In general, the overall efficiency of a FP-FC system is worsethan a FC stack, powered by on-board stored hydrogen. TheFC-HE hybrid system introduced in this work tries to increasethe efficiency of fuel processing by heat recovery from an ICE.

The efficiencies of the FC, membrane reformer, and ICEare defined according to the references. The efficiency ofthe hybrid system has been approximated as a function ofthe efficiencies of the hybrid system elements and rp. Theefficiency of the hybrid system is expected to have a valuebetween the efficiencies of the FC and ICE. The hybridsystem efficiency is less load-dependent than the efficiencyof both elements, if appropriate load leverage is performedbetween the FC and ICE.

ηHYB is higher than the efficiency of a FP-FC system withouta membrane reformer, because the fuel availability of a FP-FCsystem decreases during the hydrogen purification. In addition,the hybrid system has a much better cold-start characteristic,because ICE will start the engine. WTT efficiencies of bothfuels, hydrogen and methanol, have to be evaluated andcompared to determine if the WTW efficiency of the hybridsystem is higher than the WTW efficiency of the direct hydrogenFC (on-board storage or on-board reforming).

Some parameters, such as specific reforming enthalpy andtotal vaporization enthalpy, are introduced to measure thecompatibility of the fuels for on-board fuel reforming. Thereactions and potential commercial fuels to be used in ahybrid system, such as in Figure 2, have been reported. Also,liquid fuels, such as methanol, ethanol, and gasoline, areanalyzed in terms of their global gravimetric energy densityreferred to the amount and energy of the reformed hydrogen.Intensive research takes place in the field of fuel reformingfor such hybrid systems, references of which are given inthe Introduction as well.

Alcohols seem to be more suitable for on-board fuel process-ing by steam reforming, because their specific reformingenthalpy is less than that of the other fuels. Especially, methanolstands out because of its low-temperature level of steam

Figure 6. FC voltage (Vc) and power density (PFC).

Figure 7. Hybrid system efficiency. ηICE, ICE efficiency; rp, premixedcombustion ratio.


reforming to be processed to hydrogen on board, easing wasteheat recovery for fuel reforming. Methanol is the fuel with thelowest specific reforming enthalpy according to Table 1, whichincreases its energy density referred to the processed hydrogenamount. Supposing a lower temperature level for reforming offuels with low specific reforming enthalpy does not necessarilyimply that reforming of such fuels is always easier, becausethis enthalpy does not give any statement about the propensityof the fuel to coke. However, the toxicity of methanol causestechnical challenges in safety precautions for refueling, storage,and distribution, to avoid leakages.

The advantage of the FC besides higher efficiency is itsbyproduct water used for steam reforming. Therefore, the wateramount used for reforming has to be minimized, provided thatno catalyst or membrane poisoning takes place. The control

strategy of such a system according to the load demand is surelya challenge, but the increase of free parameters also enablesopportunities to solve problems concerning efficiency andexhaust gas emissions simultaneously. Moreover, methanol canbe produced from renewable resources and less scarce fossilfuels compared to crude oil.

Acknowledgment. The authors thank Istanbul Technical Uni-versity for its support for this work.

Supporting Information Available: Calculations performed inthis work, evaluated in Mathematica codes. This material isavailable free of charge via the Internet at http://pubs.acs.org.

EF8003575


on-board fuel processing for a fuel cell−heat engine hybrid system †

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