Fuel Processing for High-Temperature High-Efficiency Fuel Cells

Ind. Eng. Chem. Res. 2010, 49, 7239–7256

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Fuel Processing for High-Temperature High-Efficiency Fuel Cells Khaliq Ahmed* and Karl Fo¨ger Ceramic Fuel Cells Ltd., Noble Park, Victoria 3174, Australia

Fuel processing for fuel cells has received considerable attention in the literature. However, most of the reported work focused on the production of hydrogen. With internal reforming fuel cells, the fuel processor can operate at relatively low temperatures to generate a mixture of gases containing hydrogen, methane, and the carbon oxides. The primary challenge for any fuel cell system is to select the most effective reforming scheme for a particular application considering factors such as electric efficiency, operating parameters, system complexity, and costs. In this paper we briefly review fuel processing technologies for fuel cells with particular emphasis on fuel pretreatment for internal reforming fuel cells, and discuss concepts and investigations we have pursued at Ceramic Fuel Cells, Ltd. (CFCL) on processing of gaseous and liquid hydrocarbons for application in CFCL’s solid oxide fuel cells. 1. Introduction Fuel cells are a promising technology for the generation of electricity at high electrical efficiencies and low emissions of both the greenhouse gas CO2 and environmental pollutants such as NOx and SOx. Unlike thermal generators, electrochemical devices such as fuel cells are not constrained by Carnot’s rule and thus distinguish themselves by their potential for very high electrical efficiency and a relatively flat efficiency versus load curve. The fuel cell principle has been known for over 160 years, and notwithstanding several periods of intense research and development, civil applications did not emerge until the 1990s (use in the space program has occurred since the late 1960s). The main reasons are materials and production technology limitations, high costs, and the need to further improve their reliability and life. Fuel cells are commonly classified into two main groups, and are named after their electrolyte material: 1. Low-temperature fuel cells operating at temperatures 600 °C such as molten carbonate (MCFC) and solid oxide (SOFC) fuel cells. The general fuel cell reactions involve ion species migrating through the electrolyte membrane: in the low-temperature fuel cells, protons (H+) in PEMFC, DMFC, and PAFC and hydroxyl ions (OH-) in AFC; in the high-temperature fuel cells, carbonate ions (CO32-) in MCFC and oxide ions (O2-) in SOFC. With the exception of the direct methanol fuel cell, hydrogen is the preferred fuel for all fuel cells, but in high-temperature fuel cells (TOP > 600 °C) this hydrogen can be produced inside the fuel cell from internal reforming of hydrocarbons such as methane. The oxidation medium is oxygen or air. The different fuel cell types are shown in Table 1 along with operating temperatures and fuel cell efficiency. Fuel cells can be applied wherever an electricity generator is needed, but each application differs in specificationsvolume, weight, fuel choice, operating parameters (start-stop time, number of thermal cycles, temperature), life expectations, and costs. Thus the characteristics of each fuel cell type determine * To whom correspondence should be addressed. Tel.: +613 9554 2353. Fax: +613 9554 2953. E-mail: [email protected].

the suitability for particular applications. Commonly, lowtemperature fuel cells are characterized by fast start-up and are therefore well matched to transport and mobile applications and in the form of microfuel cells for battery replacement in consumer electronics, whereas high-temperature fuel cells need longer times to arrive at operating temperatures and are therefore better suited as stationary power generators. Fuel cells are not unique in their function, and must compete against wellestablished technologies, such as internal combustion engines in mobile applications, thermal power generation technologies in stationary generation, and various types of batteries in the secure power, portable, and consumer electronics market segments. For successful market introduction, fuel cell developers need to focus on the primary competitive advantages of fuel cells over their competitors. Almost every automotive company has developed test fuel cell vehicles (using PEMFC and liquid or compressed hydrogen as fuel), and companies such as Daimler and Honda have launched or are launching small market testing series; for example, Honda leases its FCX Clarity (100 kW PEMFC) for 600 USD/month to selected customers in California. Over 100 hydrogen fuel cell buses are or have been in operation, and a fleet of hydrogen buses was used during the 2010 Winter Olympics in Vancouver. Functionally, the technology is largely proven, but costs of 50 USD per kW (similar to IC-Engine power trains) are still a significant challenge. Stationary power systems demonstrated to date range in size from 500 W to a few megawattssunder 5 kW (residential market) primarily with PEMFC and SOFC, in the 100-500 kW range with predominantly PAFC and MCFC and a few SOFC demonstrator systems, and in the megawatt range predominantly MCFC. With commonly available fuels such as natural gas, only the hightemperature fuel cells MCFC and SOFC have reached electrical efficiencies of g50% lower heating value (LHV). Table 1. Types of Fuel Cells fuel cell

electrolyte

operating temperature, °C

electrical efficiency, %

DMFC PEMFC PAFC AFC SOFC MCFC

direct methanol proton exchange membrane phosphoric acid alkaline solid oxide molten carbonate

50-120 80-120 100-220 150-220 600-1000 600-700

∼30 ∼30-35 ∼35-40 ∼40 ∼45-50%. In this paper we briefly review the current state of fuel processing technologies for fuel cells, including desulfurization and the conversion of hydrocarbon-based fuels to mixtures suitable for high-temperature fuel cell applications. We focus on concepts and investigations in fuel processing catalysis that we have pursued at Ceramic Fuel Cells, Ltd. on processing of gaseous and liquid hydrocarbons to a methane-rich mixture for application to high efficiency internal reforming type solid oxide fuel cells. 2. Fuel Processing for High-Temperature Fuel Cells Gaseous fuel can be directly delivered to the fuel processor, but liquid fuels need to be gasified prior to the pre-reformer or reformer and the fuel cell (fuel vaporization step). For all types of fuels, a first fuel processing step, fuel cleanup, involves removal of impurities that are poisons for the fuel cell anode and for the pre-reforming catalyst, particularly transition metal-based pre-reforming catalysts. For conventional hydrocarbon fuels such as natural gas, LPG, and higher hydrocarbons this involves removal of all sulfur compounds from the fuel through a process usually referred to as desulfurization. In the next step of the process, hydrocarbons higher than C1 (methane) in the hydrocarbon-based fuels are converted to a mixture of methane and hydrogen (with some CO and CO2 formed as byproducts) to avoid coke formation from heavier hydrocarbons (C2+) in downstream components and on the Nibased anodes. For high stack and system efficiencies prereforming should only convert the higher hydrocarbons in the fuel, leaving all or most of the methane unconverted.13 2.1. Desulfurization. Hydrocarbon feedstocks such as natural gas and LPG contain a number of different kinds of sulfur compounds, which include inorganic sulfur species such as COS, H2S, and CS2 and light organic sulfur compounds such as methyl mercaptan, ethyl mercaptan, dimethyl sulfide, and diethyl sulfide. In addition, gas odorants such as mercaptans and tetrahydrothiophene (THT) are added by gas suppliers at concentrations that vary according to local regulations. These sulfur compounds lead to deactivation of the fuel processing catalysts used in prereforming and of the fuel cell anode, and in most cases the poisoning is irreversible. Even with reversible poisoning, as with H2S, the catastrophic decline in activity in a matter of hours or days, at best, makes it impractical to operate real commercial systems by switching to S-free gas during operation to reverse the poisoning. All sulfur compounds must therefore be removed from the gas supply to the fuel cell system to parts per billion levels in order to achieve a commercially viable operating life of the pre-reforming catalyst and the fuel cell stack. In principle, two desulfurization technologies are used: (i) hydrodesulfurization (HDS) (ii) adsorptive desulfurization In large-scale syngas technologies, the traditional desulfurization is a two-step process: (i) Hydrodesulfurization converts organic sulfur compounds to H2S and a hydrocarbon compound by a hydrogenation reaction at 200-400 °C, over a Co-Mo or Ni-Mo based catalyst: R-SH + H2 ) R-H + H2S

(1)

Ind. Eng. Chem. Res., Vol. 49, No. 16, 2010

(ii) This is followed by adsorption of H2S on a high-surfacearea zinc oxide at 200-400 °C, through the following reaction: ZnO + H2S ) ZnS + H2O

(2)

The reaction is favored at lower temperatures by thermodynamic equilibrium, and its extent is reduced by the presence of moisture. HDS is a proven technology with long-standing experience, having demonstrated reliable operation with high sulfur pickup and delivering high quality sulfur-free fuel. Its main limitations are high-temperature operation requiring energy input and the requirement of a recycle hydrogen stream from the plant. For small-scale hydrogen generation and for fuel cell technologies, particularly the internally reforming types, where H2 is not freely available, HDS is not a viable option and adsorptive desulfurization is most attractive. Activated carbon based desulfurization is one of the earliest adsorptive technologies with its ease of operation at ambient temperatures. It can be purged in air and does not require H2 or a reducing atmosphere, but its main drawbacks are low sulfur pickup, adsorption of higher hydrocarbons, and leakage of some low boiling point sulfur species such as H2S and COS. H2S can be adsorbed on a downstream ZnO bed, but COS cannot be completely removed as its adsorption on ZnO is slow with a shallow adsorption profile.14 Low sulfur pickup leads to large bed sizes and/or frequent changeover. Adsorption is not very selective, so other compounds compete with sulfur. In particular, high benzene or moisture concentrations will reduce sulfur pickup significantly. Activity and adsorption capacity can be improved somewhat by doping with base metal oxides or with transition metals such as copper. For example, with a metalimpregnated activated zinc oxide, it is possible to establish chemisorption equilibrium corresponding to H2O/H2S 1 or 2 orders of magnitude lower than the bulk phase, making it possible to reduce the sulfur content of light hydrocarbon feedstocks to less than 10 ppb. The next-generation adsorptive desulfurization is based on special adsorbents targeting different sulfur species. These can operate at ambient temperatures to 50-100 °C. Multi metal oxide based adsorbents remove inorganic sulfur and some light organic sulfur compounds. Zeolite-based adsorbents remove THT and mercaptans. These special adsorbents are tailored for increased chemisorption of sulfur compounds on active centers of the catalyst with decreased physisorption of higher hydrocarbons on the surface of the catalyst. They also result in reduced benzene adsorption compared to activated carbon, which renders the spent adsorbents less hazardous or toxic than spent activated carbon. Due to high sulfur pickup, bed sizes are an order of magnitude lower than with activated carbon. Alternatively, longer life can be achieved with the same desulfurizer size. 2.2. Routes for Conversion of Hydrocarbon Fuels. There are three main processing routes for converting higher hydrocarbons to syngas, a H2-rich gas, or a CH4-rich gas mixture: 1. catalytic steam reforming (SR) 2. catalytic partial oxidation (CPOX) 3. catalytic autothermal reforming (ATR) 2.2.1. Catalytic Steam Reforming. Steam reforming of higher hydrocarbons is conventionally known as pre-reforming and involves the following overall reactions. CnHm + nH2O f nCO + (n + m/2)H2

(-∆H°298 < 0) (3)

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CH4 + H2O / CO + 3H2 (-∆H°298 ) -206.158 kJ/mol) (4) CO + H2O / CO2 + H2

(-∆H°298 ) 41.137 kJ/mol) (5)

Reaction 3 is usually considered irreversible for all higher hydrocarbons provided sufficient catalyst activity exists, followed by the establishment of the equilibriums of the exothermic methanation reaction (4) and water gas shift reaction (5). The thermodynamic equilibrium of reaction 3 and the forward reaction of reaction 4 are favored by high temperature and low pressure, whereas the thermodynamics of the forward reaction of reaction 5 is favored by low temperature and unaffected by pressure. In practice, however, the kinetics of the reactions are favored by high pressure and high temperature. Further methane-forming reactions are CO2 + 4H2 / CH4 + 2H2O (-∆H°298 ) 247.295 kJ/mol) (6) 2CO + 2H2 / CH4 + CO2

(-∆H°298 ) 165.021 kJ/mol) (7)

Undesirable reactions during steam reforming are CO + H2 / C + H2O 2CO / C + CO2 CH4 / C + 2H2

(-∆H°298 ) 131.286 kJ/mol) (8) (-∆H°298 ) 172.423 kJ/mol) (9) (-∆H°298 ) -74.873 kJ/mol) (10)

CnHm f nC + (m/2)H2

(11)

The thermodynamic minimum steam to carbon (S/C) ratio for all hydrocarbons is 1.0. However, practical minimum S/C ratios required to avoid the undesirable carbon-forming reactions generally tend to be upward of 1.7 for natural gas and higher for the higher hydrocarbons, e.g., 2.2 for naphtha.14 In industrial practice, excess steam is used with S/C ratios ranging from 3 for the lighter hydrocarbons to 7 for the higher hydrocarbons. Hydrocarbon feedstocks ranging from natural gas to heavy naphtha can be converted to useful chemicals by steam reforming.14-19 The process is carried out in a fixed-bed adiabatic pre-reformer by making use of a highly active prereforming catalyst. As a result, higher hydrocarbons are completely converted in the pre-reformer to a mixture of carbon oxides, hydrogen, and methane. Common catalysts for steam reforming are group VIII metals, in which Ni is the most widely used. Rh, Ru, and other noblemetal catalysts are more active but due to their cost are not economical. Nickel steam reforming catalysts have proven industrial performance over many decades with regard to their ease of manufacture, activation, stabilization, and chemical reactivity. The hydrocarbon fuel reacts with steam over a supported nickel catalyst. The overall process consumes heat and can therefore be used for thermal management of the fuel cell stack. The product spectrum varies depending on process conditions. When carried out at relatively low temperatures, the product gas contains hydrogen, carbon dioxide, carbon monoxide, and methane, a suitable fuel mixture for internal reforming fuel cells. At high temperatures, the product gas is hydrogen-rich and contains little or no methane and is suitable for all fuel cells after appropriate gas cleaning.

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Even though steam reforming is a very mature technology on an industrial scale, scaling down the technology to the levels and conditions required for solid oxide fuel cell systems poses serious risks that are not encountered with the large-scale systems used in the petrochemical industry. For example, steam reforming is usually carried out at steam/carbon ratios of 3-5 to avoid carbon deposition. Such high S/C ratios are unattractive for fuel cell systems as the high steam content lowers the system electrical efficiency by lowering the Nernst voltage through steam dilution. Suppression of carbon formation can be achieved by the addition of small amounts of additives to the catalyst formulation. For example, the addition of alkali to the catalyst enhances the adsorption of steam and hence lowers the tendency for carbon formation.19 The alkali, however, lowers the reforming activity and has a tendency to leave the catalyst surface, which can promote stress corrosion cracking in downstream components.19 Since acidic supports promote cracking of hydrocarbons, suppression of carbon formation can also be achieved by basic supports e.g., MgO,14,19 CeO2,20 and Ce-ZrO2.21 MgO-supported nickel catalysts are highly active for dissociation of water, and as a result the amount of OHy species is increased, enhancing the removal of CHx and retarding the full dehydrogenation of CHx to Cx.22 Heavier rare earth metal oxides such as ceria enhance coke gasification dramatically, while maintaining or only slightly increasing steam reforming activity supposedly caused by strong metal-support interaction (SMSI) between nickel and ceria.20 Yet another option is the use of noble metal catalysts such as rhodium and ruthenium, which have high reforming activity and lower carbon formation as they do not dissolve carbon to the same extent as common group VIII transition metals,23 but their high costs and dwindling reserves make them commercially unattractive. Alloy formation with multivalent metals such as bismuth, which has an electronic structure similar to carbon, would be expected to concentrate at the surface of the nickel and thereby prevent the dissolution of carbon by blocking the formation of nickel carbide.23 Use of trace amounts of sulfur in the feed to minimize coking by ensemble size control is a well-established process.24,25 Copper also appears to inhibit carbon formation by ensemble size control, but it is not possible to prevent carbon formation as copper-nickel alloys do not provide the required high coverage.26,27 The rate of carbon formation is significantly decreased with more than 10 atom % Cu in Ni, but is increased with smaller additions of 1 atom % Cu. An electronic effect of the influence of various alloying elements on the chemisorption of methane on Ni may explain the result.27 Additions of small amounts of molybdenum and tungsten have also shown to be effective in reducing carbon formation.28 The mechanisms for such a promotional effect is not well understood.29 Promotional effects of a number of alloys have been reported.30 Better understanding of the reforming reactions31-33 and of the mechanisms of carbon formation and minimization34-36 have led to the development of catalysts with better control of carbon limits37,38 for a variety of hydrocarbon feedstocks at relatively low steam to carbon ratios. Ni-based steam pre-reforming catalysts are also intolerant to sulfur impurities. A Ni catalyst exposed to a sulfur-containing gas not only loses its activity for the reforming reaction through the blocking of active Ni sites by adsorption of sulfur and through formation of bulk nickel sulfide, a sulfur-poisoned catalyst is also more susceptible to coking, which results in further often irreversible deactivation. For example, typical Nibased pre-reforming catalysts will lose their activity dramatically if the sulfur concentration exceeds 0.5 ppm. With sulfur

concentrations up to 0.1 ppm the predicted catalyst life is 6 months to 2 years, depending on operating conditions. Longer lifetimes of up to 5 years required for stationary fuel cell applications can be achieved if the sulfur concentration is reduced to 20 ppb or less. Commercial Ni-based pre-reforming catalysts are usually supplied in the oxidized state, as nickel is pyrophoric in the “active” reduced state. This leaves the user with two options: (1) load the pre-reformer with the oxidized catalyst and carry out in situ reduction using hydrogen or a hydrogen-rich stream and/or (2) reduce and passivate or stabilize the catalyst ex situ and then reactivate the catalyst in situ under mildly reducing atmospheres. Option 1 is generally not feasible for small-scale solid oxide fuel cell systems with no readily available source of hydrogen. Furthermore, if a hydrogen-rich mixture is used for reduction, the reduction may not be ideal depending on the composition of this reducing mixture. In operation, there is the added risk of thermal cracking of the heavier hydrocarbons until the catalyst is fully reduced. Option 2 is the preferred option for the high-temperature fuel cell systems as a mildly reducing atmosphere can be generated as the system is heated with the supply fuel. Some catalyst manufacturers supply their catalysts in the reduced and stabilized state at a premium price. The process of stabilizing the catalyst after reduction involves thermally controlled exposure of the reduced catalyst to small concentrations of oxygen or air and is an art usually closely guarded by the catalyst manufacturers. Carbon formation is an important consideration in the conversion of hydrocarbon feedstocks. The two main routes of carbon formation during steam reforming, partial oxidation, and autothermal reforming are (i) hydrocarbon cracking or thermal decomposition and (ii) the Boudouard reaction. Both reactions are catalyzed by transition metals and can lead to the formation of large deposits of carbon on catalysts and metal surfaces. The Boudouard reaction, dissociating CO to carbon and CO2, is thermodynamically favored at temperatures below 700 °C and is not kinetically favored at temperatures below 300 °C. While methane decomposition is favored at high temperatures (>650 °C), decomposition of higher hydrocarbons occurs at much lower temperatures: the higher the carbon number, the lower their activation temperature and hence higher the propensity for carbon formation. Olefins and aromatics have even higher thermodynamic potential for carbon formation. Carbon formation can also occur from gas phase reactions, particularly with heavier hydrocarbon fuels. As it is not adsorbed on catalyst surfaces, gas phase coke does not lead to catalyst deactivation. However, accumulation of gas phase coke leads to an increase in pressure drop and may eventually lead to partial or complete blockage of the reactor. Calculations can be carried out to determine the thermodynamic propensity of carbon formation from reactions 8-11 for any particular hydrocarbon. Notable among the various commercial software available for carrying out such thermodynamic analyses are FACT39 and HSC.40 However, there are notable differences in the form and activity of carbon in the databases of the different software, and these lead to differences in predictions of carbon formation. The accuracy and reliability of these calculations depend on reliable thermodynamic data. Figure 1 shows a comparison of free energies of thermal cracking of various hydrocarbons calculated using the commercially available software HSC.40 Acetylene cracking has the highest negative free energy at low temperatures up to 300 °C, followed by benzene, ethylene, and propylene. Thermal cracking of methane is thermodynamically feasible above 550 °C, and


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Figure 1. Free energies of thermal cracking of hydrocarbons.

Figure 2. Rate of carbon formation from different hydrocarbons. [Reproduced from ref 41.]

for ethane and propane the corresponding temperatures are 200 and 100 °C. These thermodynamic trends do not necessarily correlate well with the rates of carbon formation measured during steam reforming of various hydrocarbons.41 Figure 2 shows higher rates and earlier breakthroughs of carbon formation from ethylene than benzene even though thermal cracking of benzene has a higher thermodynamic potential for carbon formation. There are a number of reasons for this. First, thermodynamic calculations cannot be used to predict carbon formation when the system is not at equilibrium. In real applications, whether carbon is formed and the extent of carbon formation is not merely a question of thermodynamic limitations of temperature and pressure but is rather dictated by kinetic conditions (catalyst, reactor wall) and transport limitations (space velocities, catalyst and reactor bed porosities, axial and radial dispersion effects, temperature gradients and hot spots, flow uniformity, and channeling). For example, the impact of pore distribution on carbon formation was illustrated by a study of steam reforming of naphtha.42 Thermal cracking of hydrocarbons can occur during the catalytic reaction of hydrocarbons with steam or oxygen, if the reactant mixture is heated to

excessively high temperatures before achieving a significant conversion of the hydrocarbons via the catalytic route. In fuel cell systems, thermal integration of the reformer with other balances of plant components such as the heat exchanger and afterburner is beneficial for supplying the endothermic heat of the steam reforming reaction. Thus the ideal cylindrical reactor is usually not attractive as it is not easily amenable to thermal integration. Practical and economic considerations dictate reactor configuration and catalyst size. Plate-type reactors including especially microchannel reactors with their advantages of small external diffusion resistance, enhanced mixing, and heat transfer,43-45 are particularly attractive for highly endothermic reactions such as steam reforming and lead to very compact reactors with substantially improved heat exchange, but careful design is required in particular for plate-type geometries to eliminate nonuniformities46 and ensure even flow distribution in order to avoid local overheating or higher hydrocarbon slip. A high aspect ratio (reactor length/diameter or length/width) is desirable to minimize axial dispersion. However, this also leads to high linear gas velocity and increased pressure drop. Therefore, a compromise is usually made between these two configurations. In packed-bed reactors, the ratio of width of the reactor to catalyst diameter is usually kept to >4 to minimize channeling. While channeling and maldistribution of flow can lead to carbon formation, a too-high ratio increases the risk of radial gradients. Nickel and other supported metal catalysts experience a continual slow decline in activity due to “aging”. The principal underlying process of this deactivation is the sintering of metal particles that occur during normal operation and especially under extreme conditions. Sintering is a temperature-dependent process, usually with a slow time constant. Sintering occurs via two mechanisms.47 At low temperatures, metal particles grow as they move over the support, collide, and coalesce, whereas at high temperatures, particles grow through metal atom transport. Sintering rates depend on process conditions, particularly steam content (steam/hydrogen ratio), catalyst composition, metal dispersion, initial particle size distribution, and pore structure. In practice, the internal pores of the catalyst are

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Figure 3. Pre-reforming of natural gas in a flat-bed pre-reformer at S/C ) 1.5 under fuel cell (0.5-2.0 kW) operating conditions (GHSV ) gas hourly space velocity).

exposed to a gas composition that is different from that at the external surfaces and bulk-gas phase due to the progress of reaction. Furthermore, the composition changes along the length and width or radius of the reactor. This leads to variation in sintering and coking conditions. Metal particle growth via sintering not only deactivates the catalyst through the loss of active area, it also influences resistance to carbon formation and sulfur poisoning.48 The useful life of the catalyst is therefore closely related to the extent, nature, and rate of sintering. Both increasing temperature and increasing steam content accelerate the sintering process significantly.49 A key challenge in designing a fuel pre-reformer for fuel cell systems is accounting for the different operating regimes. Degradation of fuel cell stacks and the requirements of power modulation have the consequence of changing the dynamics of the pre-reformer operating conditions. As stacks degrade, the thermal integration envelope changes between the pre-reformer and the heat exchanger utilizing waste heat from the fuel cell stack. Increased resistive losses of the fuel cell with changes in cooling demands can have a significant impact on the temperature profiles in a thermally integrated pre-reformer. During power modulation, the space velocity through the pre-reformer changes, leading to different temperature profiles. The temperature of the fuel processing reformer cannot be controlled independently of the stack or balance of plant (BoP), easily and economically. These temperature changes can significantly impact higher hydrocarbon slip, carbon formation, and sulfur poisoning dynamics even with parts per billion levels of sulfur in the feed. Any form of catalyst deactivation not only changes the intrinsic activity of the catalyst, it also affects product selectivity by altering the kinetics of competing reactions. Depending on the type of carbon formed (whisker, gum, pyrolytic), deactivation of the catalyst may or may not occur. In the event carbon formation only leads to fouling, its consequent effect of increased pressure drop and uneven flow distribution disrupts normal operation of the pre-reformer and may eventually lead to reactor blockage. Carbon formation can only be avoided by selecting “safe” operating conditions. Therefore, operating conditions may need to change as stacks degrade with time or during power modulation by changing

parameters such as S/C ratio and space velocity as these parameters determine catalyst performance at any given temperature. Analysis of trends in gas composition data with operating time can reveal information about the routes of carbon formation and gasification. If carbon is formed by the methane decomposition reaction (10), the amounts of carbon oxides in the product gas will decrease over time. The ratios of H2/CO and H2/(CO + CO2) will increase and the ratio of CO/CO2 should be unaffected. If carbon is formed by the Boudouard reaction (9), both H2/CO and CO2/CO will increase. If carbon is gasified by H2O, H2/CO will decrease if H2/CO > 1, which is the case for CH4 reforming, and CO2/CO will decrease. If carbon is gasified by H2, methane content will increase and both H2/CO and H2/ (CO + CO2) will decrease, with CO2/CO remaining constant. A comparison of the results of pre-reforming of natural gas in a flat-bed pre-reformer at 0.5-2.0 kW fuel cell system operating conditions, with equilibrium calculations based on Gibbs free energy minimization of reactions 3-5 are shown below (see Figures 3 and 4).50 1. At lower S/C, CH4 content is lower than predicted and H2 content is higher than predicted, with a better approach to equilibrium at higher flow rates. Higher flow rates result in lower temperatures, which favors methanation. At lower flow rates, with higher temperatures, more CO2 and H2 are formed but less recombine to form methane compared to the higher flow rate situation. The deviation from equilibrium in this case is dictated by the approach of the methanation reaction to equilibrium. 2. At higher S/C, CH4 content is higher than predicted and H2 content is lower than predicted. In this case the methane reforming kinetics is lowered due to its negative order dependency on steam concentration. As a consequence, there is less methane reforming, which results in higher methane content than predicted from complete equilibrium. In this case approach to equilibrium is better at the lower flow rate, which is consistent with further lowering of the methane reforming rate at higher space velocities. 2.2.2. Catalytic Partial Oxidation. In catalytic partial oxidation the hydrocarbon reacts with a substoichiometric amount of oxygen over a catalyst to produce a mixture of hydrogen


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Figure 4. Pre-reforming of natural gas in a flat-bed pre-reformer at S/C ) 2.5 under fuel cell (0.5-2.0 kW) operating conditions (GHSV ) gas hourly space velocity).

and carbon monoxide. In practice, air is used as the source of oxygen, as a consequence of which the product gas is diluted with residual nitrogen, resulting in reduced cell voltage and electrical efficiency. Some carbon dioxide is formed as some of the hydrocarbon goes to complete combustion. The reaction is exothermic and attractive for fast heat-up applications. The overall partial oxidation reaction for hydrocarbon is shown in eq 12 and takes place at temperatures of 400-900 °C, depending on the hydrocarbons contained in the fuel. CnHm + (n/2)O2 / nCO + (m/2)H2

(12)

Typically, the catalyst is based on one of the noble metals Pt, Pd, Ru, or Rh supported on a monolithic refractory metal oxide support. Alternatively, the partial oxidation catalyst may be oxide-based. The catalyst is more tolerant to sulfur. With heavy hydrocarbon fuels such as diesel, the partial oxidation reactor may be fitted with a heated element to generate enough heat to raise the temperature of the reactor to above the lightoff temperature of diesel. Product gas composition will depend on the establishment of equilibrium between the accompanying reactions of methane steam reforming (4) and the water gas shift reaction (5). In addition, the carbon limits are established by the potential of the carbon-forming reactions (8)-(11). The knowledge of partial oxidation of hydrocarbons is primarily attributed to the decades of work devoted to catalytic partial oxidation (CPO) of methane or natural gas,51-54 and not much attention has been given to CPO of higher hydrocarbons.55-57 Liquid hydrocarbons are activated at much lower temperatures than methane but are more challenging due to increased risk of carbon formation. At Ceramic Fuel Cells Ltd. (CFCL) we have focused on the development of products for stationary applications and in particular the residential microCHP (combined heat and power) market. CFCL believes that high electrical efficiency (g50% LHV) is critical for success in these market segments. Therefore, we have not pursued catalytic oxidation primarily due to the fact that this leads to lowering of the stack electrical efficiency owing to the lower heating value of the product gas from the partial oxidation

reactor and the dilution of the fuel by nitrogen from the air used for partial oxidation, which results in lowering of the operating voltage. In contrast, Hexis AG uses partial oxidation as reforming option in their 1 kW micro-CHP system and has demonstrated net ac electrical efficiencies of e30%. However, for applications such as auxiliary power units and other portable power generators where fast heat-up is critical, the competing technologies struggle to reach efficiencies of 20%, and system simplicity (no water treatment and steam generation) is key, CPO may be the choice reforming option. Figure 5 shows that, for an SOFC operating on diesel at 85% fuel utilization and 70% cell voltage efficiency, the overall electrical efficiency ranges from 30 to 40% with the partial oxidation route at practical partial oxidation temperatures. In contrast, electrical efficiency of 57% is achieved with steam reforming at S/C ) 2.58 2.2.3. Catalytic Autothermal Reforming. Autothermal reforming (ATR) combines endothermic steam reforming and exothermic partial oxidation reactions. The heat produced by the partial oxidation reaction is used in the steam reforming reaction to generate hydrogen and carbon monoxide. It is desirable to balance the reactions in such a way that it is essentially thermoneutral, requiring no net input of heat. The overall reaction of autothermal reforming can be described by the following equation. CnHm + aO2 + 2(n - a)H2O f nCO + (2n - 2a + m/2)H2 (13) The thermoneutrality of the autothermal reactor depends on the values of n, m, and a. The reaction proceeds through a combination of partial oxidation and steam reforming reactions. In an ideal configuration, the first portion of the reactor is filled with a partial oxidation catalyst, followed by steam reforming catalyst in the second part of the reactor.57 In this concept, the heat generated from the exothermic partial oxidation reaction is strategically coupled with the heat requirement of the endothermic steam reforming reaction. With novel autothermal reforming catalysts developed in the past decade,59-61 only small amounts of the catalyst are required to initiate oxidation at low

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Figure 5. SOFC efficiencies with steam reforming and partial oxidation of diesel.

temperatures and to produce the heat required for the steam reforming reaction. Two different classes of materials have been examined: Group VIII transition metals (e.g., platinum, rhodium, or nickel) supported on an ion-conducting oxide (e.g., ceria, gadolinium-doped ceria), and mixed nonnoble metal oxides with the ABO3 stoichiometry and the perovskite structure. Development of a low-cost and efficient fuel cell system hinges, to a large extent, on the development of small, compact, and cost-effective reformers with reduced startup times. Autothermal reforming is particularly attractive from these considerations. However, the design of autothermal reactors is not trivial. The coupling of exothermic and endothermic reaction changes the dynamic behavior of the reactor, which can be quite complex with respect to parametric sensitivities. Heat loss is minimized by integration of reaction and heat exchange, but making the reactor less sensitive to perturbations remains a challenge as these disturbances influence the catalytic activity in different zones along the axis of the reactor which in turn strongly affects reactor performance. Design must therefore involve optimization of the operating conditions and reactor/ catalyst structure.62 In actual practice, the reactor temperatures will depend on process conditions. During autothermal reforming of natural gas, S/C ratio, O2/C ratio, and space velocity affect catalyst bed temperatures and product compositions.11 Reducing the O2/C ratio resulted in less hydrogen production and decreased methane conversion due to less oxygen available for combustion. Reducing the S/C ratio had a similar effect with a reduction in both hydrogen production and methane conversion due to less available steam for steam reforming. However, a reduction in the S/C ratio also showed an increase in CO due to lack of steam for the water gas shift reaction. At low S/C ratio there is a higher risk of carbon formation primarily due to the decomposition of methane and catalyst deactivation. No reaction was observed for an ATR feed inlet temperature of less than 300 °C. Once the light-off temperature was reached, spontaneous combustion occurred, as observed by the rapid increase in catalyst bed temperature. At the light-off temperature, the immediate addition of air results in a spontaneous combustion reaction, observed by rapid increase in temperature. It was also shown that the majority of combustion occurs at the bottom of the catalyst bed followed by the endothermic steam reforming, resulting in a reduction in temperature along the length of the catalyst bed.

Thermodynamic predictions based on average ATR temperature agreed closely with the observed results for the major component H2. However, less CH4 and CO2 are predicted and more CO is predicted. This indicates that the combustion/ conversion of methane is kinetically limited and the extent of partial oxidation is less than predicted (based on measured catalyst bed temperature), indicating, when reacted, more of the methane goes to complete combustion to CO2 than predicted by thermodynamics. However, the hydrogen concentration prediction based on the average ATR temperature is equal to the measured value. Interestingly, there is some slippage of O2 at temperature in the presence of substantial amounts of unreacted H2, indicating the need to lower the space velocity or the presence of channeling effects. Thermodynamic analysis63 applied to natural gas autothermal reforming showed that the optimal O2/C ratio and S/C ratio for maximum hydrogen yield under the constraints of carbon-free and minimized carbon monoxide and residual methane content in the reformed gases are 0.67 and 2.3-3.6, respectively. With this condition, the product’s temperature under the assumption of adiabatic reaction is 547-598 °C. In our tests11 carbon-free operation was achieved at a much lower S/C ratio of 1.0 with O2/C ratio and product temperatures similar to those reported by Chan and Wang.63 This result can be explained by a possible difference between the activity of the carbon in the thermodynamic calculations and that of the carbon formed in the reported experiments. Furthermore, the reactions involved are not necessarily under equilibrium, so the question of carbon formation is then a matter of kinetics. Additionally, the catalyst used in these experiments11 is a noble metal based catalyst which has a lower affinity for carbon. Each reforming route has its own advantages and disadvantages. Partial oxidation is a fast exothermic reaction with rapid start-up and dynamic response features. Moreover, the process is more tolerant of contaminants such as sulfur as a direct consequence of the presence of oxygen in the reaction mixture, which converts the catalyst-contaminating sulfur compounds to the innocuous sulfur dioxide. However, the risk of carbon formation is high for this reaction, and that could lead to fouling of the reactor and/or deactivation of the catalyst. Moreover, the nitrogen in the air dilutes the fuel supply to the fuel cell stack and generates substantial waste heat, thereby lowering the system electrical and overall efficiencies. Steam reforming is endothermic, requiring additional energy input, and suffers from


slow start-up and long response times. If the energy requirement can be met by waste heat from other parts of the system, e.g., by heat exchange with the anode off-gas stream, the steam reforming route can provide very high overall system efficiencies. However, no sulfur-tolerant steam reforming catalysts have been developed yet, resulting in the need for a very efficient sulfur removal unit upstream of the reformer. The autothermal reforming route combines the features of partial oxidation and steam reforming. It has rapid start-up and dynamic response features such as partial oxidation and depending on the reforming conditions can have high efficiencies. With respect to sulfur removal, the presence of steam in the reaction mixture reduces the effectiveness of converting sulfur compounds to sulfur dioxide, resulting in the formation of hydrogen sulfide instead. However, if the fuel does not contain very high levels of organic sulfur compounds, the levels of H2S formed may be tolerated by the catalyst with little or no poisoning. This will also depend on the type of material used in the catalyst. If a noble metal based catalyst is used for ATR, H2S can be removed by adsorption on a ZnO bed downstream of the ATR. However, this option is not feasible for catalysts that are poisoned by H2S.64 Regardless of the reforming route, there are certain design considerations for a reactor that make the design of compact pre-reformers for fuel cell systems a challenge. A suitable prereformer should be large enough to accommodate the asreceived catalysts in a manner that eliminates or minimizes mass and heat transfer limitations while remaining small enough to allow integration with other components of the balance of plant for efficient thermal management. Heat and mass transfer effects can never be completely eliminated, but useful criteria are available in the literature65 that help minimize these effects as far as practicable. These design criteria specify the ratio of the reactor diameter to the pellet diameter, the ratio of the reactor length to the reactor diameter, and the ratio of the reactor length to the pellet diameter to minimize channeling and radial and axial dispersion effects. 3. Fuel Processing Challenges for SOFC 3.1. Requirements of the Catalyst. An important consideration for fuel cell systems is the thermal cyclability of the fuel processing catalyst. A fuel cell system is expected to undergo a number of thermal cycles during its operating life. The number of thermal cycles over the lifetime of the system is strongly applications-dependent, with up to 2000 cycles for auxiliary power units (APU) and portable systems and 20-50 cycles for stationary systems (the pre-reformer is expected to last 5 years). The thermal cycling atmosphere is unlikely to be an “ideal” reducing environment for the catalyst. This is most relevant for metal supported catalysts, e.g., the Ni-based prereforming catalyst. For noble-metal-based ATR and CPOX catalysts, only the structural effects of thermal cycling on the support material apply. If the catalysts are prone to attrition or dusting, this will lead to an increase in pressure drop in the pre-reformer and may eventually lead to reactor blockage or catalyst pore blockage. For the Ni-based steam reforming catalyst the challenge is to avoid deep oxidation of the catalyst during the regime of shutdown where fuel is turned off. A mild surface oxidation is beneficial to protect the catalyst by forming a passive surface layer which can be easily converted to the metal during heat-up to reactivate the catalyst. This layer may be broken thermally or through the creation of a mildly reducing atmosphere.

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3.2. Catalyst Selection. Testing of commercial and new developmental catalysts in microreactors is usually carried out for three purposes: screening for selection of catalyst, kinetic studies, and catalyst deactivation studies. While the conditions required for generating kinetic data are well-defined from theory, the conditions for catalyst screening and for catalyst deactivation studies are not so easily defined. Commercial catalysts, designed for use in large-scale systems, are typically supplied in a size several millimeters in diameter. A common technique for screening of commercial catalysts is to crush the as-received pellets into particles and test them under standard kinetic experimental conditions. While this satisfies several testing criteria, it does introduce some uncertainty into the performance of the catalyst compared with the as-received pellet form. Some researchers66 have argued that catalyst screening tests using crushed pellets can be misleading. This is due in part to a change in the heat and mass transfer conditions. In addition to these heat and mass transfer effects, otherwise inaccessible sites can become available for undesirable side reactions, thus disturbing the delicate balance of reaction kinetics found at the surface and hence influencing the stability and selectivity of the catalyst over the long term. Conversely, if catalyst pellets are used, external heat and mass transfer limitations do not relate to plant-scale performance unless large reactors are used, thus negating the benefits of small scales and quick turnaround times inherent in microreactor systems. Additional benefits of small-scale experiments with crushed particles in a microreactor are the considerable savings in time and resources compared to performing this work with asreceived catalysts in larger scale reactors. Generating catalyst deactivation data is even more difficult both in laboratory- and plant-scale reactors and will not be addressed here. A recent review67 discusses catalyst deactivation in great detail. 3.3. Issues with Liquid Hydrocarbons. A variety of liquid hydrocarbons are promising candidates for SOFC. Fuel processing of these heavier hydrocarbons can be particularly challenging. There is a risk of carbon formation from thermal cracking during vaporization of the fuel. Fuel vaporization is an energyintensive process which places increased demands on the heating requirement for the endothermic reforming reaction. Owing to the presence of unsaturated hydrocarbons, aromatics, or cyclic compounds, there is a higher potential for catalyst deactivation from carbon formation. Moreover, there is an increased propensity for carbon formation from thermal cracking in the preheating zone. 3.3.1. LPG. LPG is a mixture of light hydrocarbons that are gases at normal temperatures and pressures, but liquefy at moderate pressures or reduced temperatures. LPG occurs naturally in crude oil and natural gas production fields and is also produced in the oil refining process. The main component gases of LPG are propane (C3H8) and butane (C4H10). Varieties of automotive LPG include 100% propane and mixes with varying proportions of propane and butane, but residential LPG (used for domestic cooking and heating) consists of propane only. Commercial LP gases invariably contain traces of lighter hydrocarbons such as ethane (C2H6) and ethylene (C2H4) and heavier hydrocarbons such as pentane (C5H12). Residential LPG is attractive for remote power applications, whereas automotive LPG is attractive for APU. The challenges of direct utilization of propane or LPG have been discussed in great detail,68 and alternative anodes for avoiding carbon formation have been reviewed, highlighting the findings that such anode materials which are resistant to carbon formation are often less electro-

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chemically active than the conventional carbon-formation-prone nickel-based anodes. With pre-reformed propane-fed SOFCs, the risk of carbon formation is eliminated or greatly reduced as was shown by Zha et al.69 In their study fuel cell performance was low and degraded due to carbon deposition when fed directly with propane; with pre-reformed propane, performance was greatly enhanced with no signs of deactivation and approached that of a hydrogen-fed SOFC. Technology for pre-reforming of LPG into a gas mixture containing methane, hydrogen, and carbon oxides, all of which can be fed directly to an internal reforming solid oxide fuel cell system, has been developed and proven through experiments at both the laboratory scale and a 5 kW size SOFC system.70 Six commercial catalysts from three catalyst manufacturers were tested in the laboratory reactor in the first instance over a range of reaction parameters, e.g., temperature, catalyst volume, catalyst particle size, and LPG/steam ratios. Two were found to be effective. Reaction parameters for effective operation were identified through a number of experiments that were aimed at determining the composition of the reformed LPG and the stability of the catalyst during operation. The data were used to design the LPG reformer. A major unknown during the reforming of LPG is the formation of carbonaceous deposits that gradually deactivate the catalyst or fill reactor spaces and catalyst pores with soot particles, leading, eventually, to reactor/plant shutdown. This issue was addressed first by thermodynamic modeling to identify a “thermodynamically safe” (carbon-free) operating regime for the catalysts, and second by demonstrating the technology over an extended period of time. A constant composition of a methane-rich mixture (55% methane, 25% hydrogen, and 20% carbon dioxide) was generated from the LPG reformer for the duration of a long-term test of over 500 h. There was no observed degradation in the activity of the catalyst. In an earlier work71 it was shown that conversion of propane increases with temperature but methane selectivity is reduced. Methane selectivity is increased by lowering the steam/carbon ratio in the range (1.0-0.5) where propane conversion is unaffected. Short-term tests indicated that it may be possible to operate at S/C ratios less than 1. For S/C g 1 propane conversion increases with decreasing S/C ratio, indicating negative dependence of the reforming rate with respect to steam concentration; methane selectivity is reduced slightly. For S/C < 1, propane conversion increases with increasing S/C ratio and methane selectivity is essentially unchanged. This suggests that two reactions are in operationscracking of adsorbed propane and steam reforming of propanesand there may be a critical S/C ratio (presumably not thermodynamic), beyond which steam reforming predominates and below which the cracking of propane prevails. Cracking is likely to predominate at low S/C ratio, so that with increasing S/C ratio in this region gasification of the carbon formed leads to increased concentration of products and an increase in conversion measured from a carbon balance of gaseous components. Furthermore, it was shown that the methane content of steam reformed liquid hydrocarbons can be increased by using a dualbed reformer where the second bed operating at a lower temperature than the first bed acts as a methanator, thereby generating a methane-rich mixture leading to higher fuel cell system efficiencies.72 3.3.2. Naphtha. A study of liquid hydrocarbon reforming has proven extremely challenging.73 Various hydrocarbons such as hexane, heptane, and hexadecane were investigated in a microreactor through a series of screening tests under different

Figure 6. Effect of S/C ratio on product distribution during steam reforming of heptane at 420 °C.

operating conditions to assess the activity of commercial prereforming catalysts for liquid hydrocarbon reforming. Because of the nature of the hydrocarbons under investigation, several reactor configurations were tested to determine the best method for dispensing the feeds to the reactor. Initial tests involved a single catalyst pellet, and the internal configuration of the microreactor was subjected to several modifications to minimize channeling effects. The process conditions were studied by carrying out steam reforming at various temperatures and steam to carbon ratios. However, these experiments proved to be extremely challenging with reproducibility in the microreactor being the major issue. A larger, bench reactor was designed and used in investigations of catalytic steam reforming of naphtha, ethanol, and diesel. This reactor configuration showed a tremendous improvement compared to the microreactor with high methane selectivity obtained for the various fuels. Moreover, the catalysts exhibited better stability (less coking) in the bench-scale-reactor experiments and the reforming could be performed for several days without loss of activity. In the microreactor experiments, regardless of whether whole catalyst pellets or crushed particles were used, two parameters played a significant role in the conversion of liquid hydrocarbons: (1) space velocity, which is primarily dictated by the dispensing pump, and (2) configuration of the reactor and particularly channeling effects. When channeling is minimized, the degree of conversion is greatly improved. It was interesting to observe that increased conversion was accompanied by an increase in methane selectivity. This indicates that as soon as the hydrocarbon was reformed abstracting heat for the endothermic steam reforming reaction, the temperature in the reaction zone was reduced, favoring recombination of the products via methanation. Steam reforming of heptane as a model compound for naphtha at various S/C ratios was studied in a microreactor using commercial Ni-based pre-reforming catalysts.73 In accordance with thermodynamic predictions, the production of hydrogen increases with increasing reaction temperature or S/C ratio, whereas methane selectivity decreases, as illustrated in Figures 6 and 7. Major issues with microreactor experiments were blockage of reactor due to coke formation and incomplete conversion of heptane at the range of space velocities reported. In experiments in a larger bench-scale reactor, complete conversion was achieved with the naphtha representative fuel composition shown in Table 2. Figure 8 shows that selectivity


Figure 7. Effect of temperature on product distribution during steam reforming of heptane at S/C ) 2.

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Figure 9. Product distribution of naphtha steam reforming at 440 °C, S/C ) 2.

Table 2. Naphtha Representative Composition

isopentane n-pentane 2-methylpentane 3-methylpentane n-hexane methylcyclopentane benzene cyclohexane n-heptane

volume, %

density, g/cm3

MW

boiling point, °C

22.43 21.98 10.67 6.10 15.37 8.68 5.57 7.27 1.93

0.62 0.626 0.653 0.664 0.659 0.749 0.874 0.78 0.684

72.15 72.15 86.18 86.18 86.18 84.16 78.11 84.16 100.2

27.85 36.07 60.27 63.28 68.95 71.80 80.10 80.74 98.42

of methane increased significantly with a decrease in S/C ratio from 1.5 to 1.2. The minimum temperature for which a total conversion was obtained was reported to be 400 °C.73 In a longer run at S/C ) 2 (Figure 9), stable performance was reported for over 500 h with methane and hydrogen contents of 54% and 24%, respectively. By adjusting the position of the reactor within the furnace, selectivity toward methane could be increased to up to 61%. The intent here was to make use of the capability of nickel-based catalysts to favor methanation. A gradient in temperature in the reactor (high temperature at the inlet and low temperature at the outlet) enables steam reforming in the high-temperature zone, while methanation is favored in the low-temperature zone.72 Methane and hydrogen contents of the pre-reformed gas can be tailored by appropriate design of the thermal integration aspects of the pre-reformer with other components in the fuel cell balance of plant.74

Figure 8. Effect of temperature on product distribution during steam reforming of naphtha at S/C ratios of 1.5 and 1.2.

Figure 10. Product distribution of naphtha steam reforming, at 420 °C at various S/C ratios.

It was found73 that high selectivity in methane (>65%) and complete conversion could be obtained at temperatures ranging from 330 to 460 °C. However, for long-running experiments, deactivation of the catalyst would occur at the lower reaction temperature range from 330 to 390 °C. A reaction temperature of 420 °C afforded a stable run and yielded 65 mol % methane at S/C ) 2. A run was carried out73 at 420 °C while varying the S/C ratio from 1 to 3. The product distribution and temperature profile are illustrated in Figure 10. It can be seen that the lower the S/C ratio, the higher the methane content. The highest selectivity of methane is about 71 mol % at S/C ) 1. No long-term tests were carried out at these low S/C ratios. These experiments demonstrated that naphtha can be reformed with complete conversion of higher hydrocarbons to gas mixtures suitable for internal reforming SOFC and MCFC. 3.3.3. Diesel. Diesel composition and properties vary depending on end-use. Conventional HDS is not an option for diesel. Sulfur in diesel remains an issue for the foreseeable future, although mandated improvements in reduction of sulfur contents are in the pipeline. Thermodynamic analysis58 shows that low-temperature steam reforming without the thermodynamic potential for carbon formation can be achieved by using a steam/carbon ratio, S/C

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) 2. To use lower steam contents, the temperature must be raised to 580 and 680 °C for S/C ) 1.8 and 1.5, respectively. Higher methane content is desirable for internal reforming SOFCs, and this is achieved by low-temperature steam reforming. Both S/C ratio and temperature significantly affect carbon formation. At S/C ) 1.5, there is significant carbon formation propensity until the temperature is raised to 700 °C. It can be postulated that the predominant carbon-forming reaction is the Boudouard reaction, which is favored at lower temperatures. However, other carbon-forming reactions such as cracking of diesel components and the reversible carbon gasification reaction along with the change in relative contents of the carboncontaining species and carbon-gasifying species have an effect of the same order as that of the Boudouard reaction in the temperature range 300-500 °C. In particular, C6H6 and unsaturated hydrocarbons such as C2H2, C2H4, C3H4, C3H6, C4H6, C4H8, C5H10, and C6H12 may be formed favorably under certain conditions and contribute to coking. These compounds contribute to coking unless they are converted to desirable products. In real systems, the selectivity and kinetics of these reactions on the chosen catalyst will determine the extent of carbon formation. Thermodynamic potential for carbon formation is completely averted by autothermal reforming at S/C ) 1.8 and O2/C ) 0.25.58 The same can be achieved by autothermal reforming at S/C ) 1.5 and O2/C ) 0.5 By comparison, steam reforming at S/C ) 2 achieves the same without the effect of fuel dilution by nitrogen. To lower the S/C ratio to 1.5 for autothermal reforming with O2/C ) 0.25, the temperature must be raised to 580 °C. To lower the S/C ratio to 1.2 for autothermal reforming with O2/C ) 0.5, the temperature must be raised to 480 °C. Compared to the steam reforming route, selectivity to methane is lower for all autothermal conditions; this is an important consideration for internal reforming and thermal management of solid oxide fuel cell stacks. For the partial oxidation route, carbon formation can be avoided at O2/C ) 1 at T > 580 °C. To operate at O2/C ) 0.75, the temperature must be raised to 690 °C. To operate at O2/C ) 0.5, the temperature must be raised to >800 °C. However, very low methane selectivity, carbon formation risk, and fuel dilution by N2 make the partial oxidation route the least attractive of the three reforming routes. In microreactor experiments studying steam reforming of hexadecane73 using commercial pre-reforming catalysts, the variation of the amount of catalyst and the space velocity appeared to have no significant influence on the conversion of the hydrocarbon. Figure 11 illustrates the effect of the S/C ratio on product distribution at 500 °C. The S/C ratio does not significantly influence the selectivity of the different products. However, in accordance with the thermodynamic predictions, Figure 12 shows that increasing the temperature of the steam reforming reaction increases the production of hydrogen whereas that of methane decreases slightly. Table 3 shows the composition of a diesel representative fuel used in bench-scale reactor experiments.73 Complete conversion of diesel could not be achieved using commercial Ni-based pre-reforming catalysts, and the reaction was not stable for long-term runs. Figure 13 shows the product distribution and temperature profile during steam reforming of diesel of the composition shown in Table 3. Although a stable reaction temperature is obtained, the selectivity in methane was observed to decrease with time on stream. The lower conversion rate can be explained by the fact that the heavy hydrocarbons might not have enough time to be cracked and steam reformed on the surface of the catalyst pellets:

Figure 11. Effect of S/C ratio on product distribution during steam reforming of hexadecane at 500 °C.

Figure 12. Effect of temperature on product distribution during steam reforming of hexadecane at S/C ) 2. Table 3. Representative Components of Diesel Fuel Chosen for Experiments

n-tetradecane n-hexylbenzene 1-methylnaphthalene

volume, %

density, g/cm3

MW

boiling point, °C

73.8 1.4 24.9

0.762 0.857 1.001

198.39 162.27 142.2

253.70 226.00 244.64

the residence time may be too short for these hydrocarbons. On replacing the hydrocarbon mixture with hexadecane, and feeding the reactor from the bottom rather that from the top, the conversion increases up to 90%.73 The other possibility is due to the reforming ability of the catalyst for heavy hydrocarbons. Recently, Campbell et al.75 reported that catalytic cracking can also convert heavier hydrocarbons to light hydrocarbons (C1-C3) at high conversion with low coke yield, using two types of catalysts: commercially available zeolite and a novel manganese/alumina catalyst. Nibased pre-reforming catalysts may have a low activity for steam reforming of heavy hydrocarbons such as diesel. Indeed, analysis of the cold-trapped liquid showed the presence of the three initial components: no cracked intermediates were found in the analysis of the residual liquid.73


Figure 13. Product distribution of steam reforming of diesel substitute (Table 3), S/C ) 1.9, 460 °C. Table 4. Properties of Hydrocarbon Mixture (C7-C12)

boiling point (°C) MW density (g/mL) volume used (mL) normalized to 1 L

C7H16

C8H18

C9H20

C10H22

C11H24

C12H26

98 100.198 0.659 500 443.6

125.7 114.224 0.684 160 141.9

151 128.250 0.718 160 141.9

174 142.276 0.73 7.2 6.4

196 156.302 0.74 90 79.8

215.7 170.328 0.75 210 186.3

Steam reforming of a mixture of the intermediate components was also studied.73 Their properties and the mixture composition are listed in Table 4. Figure 14 shows the results of steam reforming of this mixture at S/C ) 2. The experiment was carried out in three consecutive days due to the limit of hydrocarbon stocks. Between the runs the reactor-catalyst system was kept overnight at 460 °C under an inert atmosphere. The product distribution profile reached steady levels by the third day. The percentage of methane was about 60 and 58 mol % when the reaction was carried out at 460 and 480 °C, respectively. The total conversion of the hydrocarbon mixture was about 84%. The liquid collected in the cold trap showed the presence of all six components. There were no signs of deactivation in this short-term test. A further test was carried out to confirm that there was no contribution to loss of activity or selectivity from the straight-

Figure 15. Product distribution of various hydrocarbons during steam reforming at 460 °C, S/C ) 2.2.

chain intermediate components and that such a loss was most likely due to the benzene and naphthalene compounds in Table 3. Figure 15 shows the experimental conditions and results with four types of fuels; full conversion was achieved for all four mixtures. On the fifth day, the diesel substitute was assessed one more time, and comparing to previous runs and Figure 13, similar results were obtained showing a decline in performance. In Figure 15, P1 and P5 refer to positions inside the furnace where the reactor is placed. P5 is at a higher temperature than P1. 3.3.4. Ethanol. Currently there are two commercial production routes for ethanol. The primary route is by hydrating ethylene, which is obtained from thermal cracking of petroleum naphtha. However, ethanol is also produced as a renewable fuel in countries with extensive plantations of sugarcane and corn, e.g., in some Latin American countries. Ethanol can also be produced as a clean renewable fuel from biomass. Ethanol produced in a renewable manner such as from lignocellulosic biomass is an attractive fuel for power generation, from energy density considerations. It has a 1.5 times higher energy density than natural gas or methane based on volume of fuel. Ethanol has other attractive features such as ease of storage, handling safety, and low toxicity. Very little research has been carried out on processing of ethanol for high-temperature fuel cells. Almost all of the reported work is focused on generation of hydrogen for PEM fuel cells, and only a few have investigated its applicability to hightemperature fuel cells, MCFC76,77 and SOFC.78 The technology of converting ethanol to hydrogen is the same as that for converting it to a fuel containing methane for application to internal reforming solid oxide fuel cells or molten carbonate fuel cells. As with conversion of natural gas, there are three routes for converting ethanol to the desired fuel, i.e., hydrogen or a methane-rich mixture. These routes are steam reforming, partial oxidation, and autothermal reforming. Ethanol is easily amenable to steam reforming and has a lower thermodynamic propensity for carbon formation than natural gas and other hydrocarbon fuels due to the presence of oxygen. Reforming ethanol by reacting it with steam requires one water molecule for each molecule of ethanol: C2H5OH + H2O / CH4 + 2H2 + CO2

Figure 14. Product distribution of diesel steam reforming at S/C ) 2.1.

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(14)

However, this results in a steam/carbon ratio of 0.5 at the fuel reformer inlet which continually drops as ethanol is

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Figure 16. Thermodynamic simulation of steam reforming of ethanol at S/C ) 1 using HSC.40

converted and the water content diminishes to zero on full conversion. To minimize the risk of carbon formation and to generate a fuel stream entering the stack with methane and steam, an excess of steam must be used at the ethanol pre-reformer. The reaction is endothermic but less so compared to steam reforming of hydrocarbons such as methane. The heat of reaction for the steam reforming of ethanol is around +30 kJ/mol of hydrogen produced. Like steam reforming of hydrocarbons, low pressures favor the ethanol steam reforming equilibrium. An interesting feature of ethanol is that it is produced as an aqueous solution containing 8-10% (v/v) ethanol. As water is a coreactant in the reforming process, ethanol does not have to be fully purified for use in ethanol steam reformers. In practice, the product will also contain small amounts of CO from the reversible water gas shift equilibrium which occurs as a side reaction. The following compounds could also be formed depending on catalyst activity and kinetics: acetaldehyde, acetic acid, ethylene, and diethyl ether according to the following reactions C2H5OH / C2H4O + H2

(15)

2C2H5OH + H2O / C2H4O2 + 2H2

(16)

C2H5OH / C2H4 + H2O

(17)

2C2H5OH / (C2H5)2O + H2O

(18)

Thermodynamics predicts negligible concentrations of these undesirable products. However, in practice the actual amounts formed will depend on the selectivity of the catalyst for these products. Dehydration of ethanol leading to the formation of ethylene is to be avoided, particularly because ethylene provides a major route for carbon formation and a resultant decline in activity. If appreciable amounts of acetaldehyde are formed, selectivity and yield of the desired products H2 and CH4 will be reduced. Dehydrogenation catalysts would, therefore, seem to be most suitable. It has been shown79 that catalysts containing Ni favor the formation of methane by promoting the reactions

C2H4O / CO + CH4

(19)

C2H4O2 / CO2 + CH4

(20)

Therefore, selectivity to methane is higher with Cu/Ni/K/ Al2O3 and Ni/K/Al2O3 catalysts compared to Cu/K/Al2O3 catalysts, and the formation of undesirable products is reduced.80,81 Commercial Ni-based pre-reforming catalysts are potential candidates for exploring steam reforming of ethanol for SOFC applications. The processes of ethanol manufacture preclude the presence of sulfur compounds; therefore, desulfurization is not required for ethanol. Copper-based catalysts are strong candidates for the ethanol steam reforming reaction. CuO/ZnO, CuO/SiO2, CuO/Cr2O3, and CuO/NiO/SiO2 are reported to be the best catalysts.76,82-86 Preliminary tests on catalysts based on noble metals87,88 and carbides89 also showed acceptable performance as did studies on cobalt catalysts.90 From catalyst activity considerations, temperatures above 300 °C are required for ethanol steam reforming. Thermodynamic calculations were performed for steam/ ethanol ratios of 2-4, i.e., S/C ratios of 1-2 at temperatures of 200-600 °C. The results are plotted in Figures 16-18 and show that, with S/C ) 1, carbon formation occurs over the entire temperature range. At S/C ) 1.5-2, carbon formation is avoided by operating in this temperature range. At S/C ) 1.5, the methane content in the reformed wet gas is 25-27% at 400-300 °C while the steam content is 49-54%, such that a steam carbon ratio of ∼2.0 is maintained in the region of interest between 300 and 400 °C. At S/C ) 2, the methane content in the reformed wet gas is 20-23% at 400-300 °C while the steam content is 55-60%, such that a safe steam carbon ratio of >2.5 is maintained in the region of interest between 300 and 400 °C. Above this temperature, the methane content drops and the benefits of the endothermic internal steam reforming of methane for thermal management of the stack is reduced. From this analysis it is clear that operating the ethanol reformer in the temperature range 300-400 °C with an inlet steam/ethanol ratio of 3-4 would produce a fuel stream similar to pre-reformed natural gas that is ideally suited for internal reforming solid


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Figure 17. Thermodynamic simulation of steam reforming of ethanol at S/C ) 1.5 using HSC.40

Figure 18. Thermodynamic simulation of steam reforming of ethanol at S/C ) 2.0 using HSC.40

oxide fuel cells. Furthermore, the stack components and its peripherals for pre-reformed natural gas and pre-reformed ethanol can be the same. Figures 19 and 20 show the results of steam reforming of ethanol carried out in a bench-scale reactor with commercial Ni catalysts.73 Ethanol was totally converted and the highest production of methane was 58-60% with S/C ) 2 at 400 °C. In Figures 19 and 20 P9 and P11 refer to positions within the furnace where the reactor is placed. Position P9 results in a lower reactor exit temperature for the same nominal furnace control temperature, resulting in higher methane selectivity from methanation of the reformed products. These results confirm that ethanol can be totally converted, producing about 60 mol % methane in a pre-reformer, and is therefore suitable for in internal reforming SOFC.

4. Concluding Remarks The choice of reforming technology is strongly applications dependent. In cases where fast heat-up, system simplicity, and easy start-up are key product features, catalytic partial oxidation (CPOX) may be the optimum choice, but this choice significantly reduces the electrical efficiency of the fuel cell system. For optimum electrical efficiency, steam reforming (SR) is the choice reformer technology, and autothermal reforming lies in between CPOX and SR. In the high-temperature fuel cells for stationary applications, MCFC and SOFC, direct reforming of methane in the stack is desirable as it provides a mechanism for thermal management and the highest possible electrical efficiencies. This option requires a pre-reforming step with the objective of producing a

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Figure 19. Product distribution of ethanol steam reforming, S/C ) 2, catalyst A.

experiments with hydrocarbons and mixtures of hydrocarbons present in naphtha and diesel.70,71,73 Two commercial catalysts were used and the effects of reaction parameters, such as space velocity, S/C ratio, and temperature, on the reaction were studied. Experimental results show that lower reaction temperature (350-400 °C), low steam/carbon ratio (1-1.5), and specific space velocities are required to produce gas mixtures with high methane content. Conversion of diesel via steam reforming was incomplete with commercial Ni-based catalysts, possibly due to the fact that these catalysts are not designed for pre-reforming of heavy hydrocarbons.73 In the future alternative catalysts could be developed for diesel reforming. For example, mesoporous supports loaded with various metals or metal mixtures could be used for this process.73 The preliminary work performed on ethanol in this study shows that ethanol is a promising candidate for internal reforming SOFC. Literature Cited

Figure 20. Product distribution of ethanol steam reforming, S/C ) 2, catalyst B.

methane-rich fuel stream. Similar to full reforming, CPOX, ATR, and SR can be used for the pre-reforming process. Pre-reforming to methane-rich fuels was demonstrated by CFCL and others for several fuels. The key variables in prereforming are temperature, space velocity, S/C ratio (for SR and ATR), and O2/C ratio (for ATR and CPOX). These variables determine the extent of conversion and the propensity for carbon formation. In a fuel cell system the parameters S/C and O2/C ratios through their influence on fuel composition determine fuel cell efficiency. Also, we have shown that if high electrical efficiency is a key product requirement, steam reforming is the most preferred route for pre-reforming of diesel.58 Steam reforming is the most efficient fuel processing option with efficiencies above 90-95%. Although very high fuel processor efficiencies can be achieved with the steam reforming route, the reaction is highly endothermic and as such will require significant heat input to the reformer depending on the reforming temperature. With 10-15% of the heating value of the fuel still available for a typical target of 85-90% fuel utilization in the fuel cell stack, the steam pre-reformer can be heated in a system with a thermally integrated pre-reformer, fuel cell stack, burner, and heat exchanger, utilizing this waste heat. Trends in steam reforming of liquid hydrocarbon fuels into a methane-rich gas have been established from bench-scale

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ReceiVed for reView March 31, 2010 ReVised manuscript receiVed June 3, 2010 Accepted June 24, 2010 IE100778G

Fuel Processing for High-Temperature High-Efficiency Fuel Cells

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