Advocates of the corn ethanol industry note that agricultural business and rural economies benefit from ethanol production. Rural economies gain new jobs and an expanded tax base. The local impact of a dry grind facility producing 40 million gallons of ethanol per year is an estimated $56 million spent annually for feedstock (corn), labor, and utilities, and also $1.2 million in state and local tax revenues [8]. Farmers benefit from the establishment of a guaranteed market for a set number of bushels, with an increased value of $0.25-$0.50 per bushel in the national corn price plus an additional $0.05-$0.10 per bushel in the locality of ethanol plants. A strengthened domestic market for corn also prevents the United States from flooding the global corn market.

Federal and state tax support for ethanol production has driven growth of the ethanol industry. Federal ethanol tax incentives for ethanol-blended fuels are directed at gasoline marketers in one of two ways. The excise tax exemption reduces the federal excise tax, paid at the terminal by refiners and marketers, by 5.1 cents per gallon of 10% ethanol-blended fuel. Or, gasoline refiners can claim an income tax credit of 5.1 cents per gallon of gasoline blended with 10% ethanol. At the state level, tax incentives generally are directed to benefit small ethanol producers, which are typically farmer-owned cooperatives [9]. In some cases, states directly support new ethanol facilities with cash payments to help defray construction costs. State incentives for users range from cents-per-gallon tax exemptions for ethanol blends to rebates for purchase of alternative fuel vehicles and grants to fuel stations selling alternative fuels. Several states mandate use of ethanol blends or flexible fuel vehicles for state-owned fleets. Minnesota and Hawaii have renewable fuel standards requiring use of E10 blends in cars, and Minnesota’s E20 law offers two options for increasing ethanol use to 20 percent of the gasoline sold in the state by 2013. Montana’s E10 law states that most of the gasoline sold in the state must include 10% ethanol when annual production of ethanol in the state reaches 40 million gallons. A few other state legislatures are considering similar measures.

Amylolytic Enzymes and Solventogenic Clostridia

The solventogenic clostridia, like all clostridia, are Gram positive, spore forming, obligate anaerobes. These bacteria can change to a variety of morphologies during fermentation, with motile rod-shaped cells present during the exponential growth phase and dormant oval-shaped endospores formed when the culture encounters adverse conditions. The maintenance of cellular growth (like other heterotrophic bacteria) and butanol production by solventogenic clostridia depends on the utilization of nutrients obtained from the surroundings. Corn is principally com­posed of starch, and starch is made up of amylose and amylopectin. Amylose is composed of a linear polymer of glucose with links exclusively in the a-1, 4 orientation. On the other hand, amylopectin is a highly branched polysaccharide consisting of linear chains of a-1, 4-linked D-glucose residues, joined by a-1, 6-glucosidic bonds. The branch points occur on the average of every 20-25 D — glucose units, so that amylopectin contains 4-5% of a-1, 6-glucosidic linkages (Jensen and Norman, 1984). High-molecular-weight macromolecules like starch from corn are too large to be assimilated by the bacterial cells and therefore need to be hydrolyzed into low-molecular-weight products by specific extracellular depolymerases, which can then be taken into the cells via specific transport systems. Solventogenic clostridia have the ability to utilize a wide spectrum of carbohydrates through the secretion of several extracellular amylolytic enzymes.

Several amylolytic enzymes with different modes of action necessary for efficient and complete breakdown of starch to glucose have been identified in the solventogenic clostridia. They include a-amylase, p-amylase, glucoamylase, a — glucosidase, and pullulanase, and their mode of action and linkages hydrolyzed in the starch molecule and products formed are summarized in Table 6.1.

Amylases are enzymes that act on starch, glycogen, and derived polysaccha­rides. They hydrolyze a-1, 4 or a-1, 6 glucosidic bonds between consecutive glucose units. a-Amylase (1,4-a-D-glucanohydrolase; EC 3.2.1.1) catalyzes the hydrolysis of a-1,4 glucosidic bonds in the interior of the substrate molecule (starch, glycogen and various oligosaccharides) and produces a mixture of glu­cose, maltose, maltotriose, maltotetraose, maltopentose, maltohexaose, and oli­gosaccharides in a ratio depending on the source of the enzyme (Ezeji, 2001). The p-amylase (1, 4-a-D-glucan maltohydrolase; EC 3.2.1.2) hydrolyzes a-1,4 glucosidic bonds in starch and oligosaccharides producing maltose units from

the nonreducing terminal end of the substrate. Glucoamylase (1, 4-a-D-glucan glucohydrolase; EC 3.2.1.3) hydrolyzes both a-1, 4 and a-1, 6 glucosidic linkages from the nonreducing terminal end of the glucose units in the starch molecule. a-Glucosidase (a-D-glucoside glucohydrolase; EC 3.2.1.20) catalyzes, like glu­coamylase, the hydrolysis of the terminal nonreducing a-1, 4-linked glucose units in the starch. The preferred substrates for a-glucosidases are maltose, maltotriose, maltotetraose, and short oligosaccharides. Furthermore, pullulanases (a-dextrin 6-glucanohydrolase; EC 3.2.1.41) are enzymes that cleave -1, 6 linkages in pullulan and release maltotriose, although pullulan itself may not be the natural substrate.

Synergistic action between pullulanase and a-amylase enzymes of C. ther — mosulfurogenes has been demonstrated (Spreinat and Antranikian, 1992) and an a-glucosidase of C. beijerinckii has been shown to hydrolyze both types of glucosyl linkages (a-1, 4 and a-1, 6) (Albasheri and Mitchell, 1995). In addition, Paquet et al. (1991) purified and characterized novel C. acetobutylicum 824 a — amylase, which possesses some glucoamylase activity (2.7%).

Drew C. Dunwoody, Hachull Chung,

Luke Haverhals, and Johna Leddy[5]

University of Iowa, Department of Chemistry,

Iowa City

CONTENTS

Introduction………………………………………………………………………………………………….. 155

Historical Perspective……………………………………………………………………………………. 157

Catalyst Development………………………………………………………………………. 158

Membrane Development………………………………………………………………….. 159

Thermodynamic Considerations…………………………………………………………………… 160

Thermodynamic Optimum……………………………………………………………….. 160

Realities of Surface Kinetics……………………………………………………………… 161

Performance Targets and Efficiencies………………………………………………………….. 164

General Optimization……………………………………………………………………………………. 166

Electrocatalysts and Supports………………………………………………………………………. 172

Membrane Technology…………………………………………………………………………………. 175

Nafion-Based Membranes……………………………………………………………….. 176

New Separators…………………………………………………………………………………. 178

Closing Thoughts…………………………………………………………………………………………… 182

Abbreviations………………………………………………………………………………………………… 184

References…………………………………………………………………………………………………….. 185

INTRODUCTION

This chapter presents aspects of current developments in direct methanol fuel cell (DMFC) technologies. In particular, the focus is on systems where the fuel stream is a solution of water and methanol fed directly to the fuel cell anode. These systems are the primary focus of the review as cells using reformed methanol as the fuel is a subset of the general class of polymer

electrolyte membrane (PEM) fuel cells that derive hydrogen from reformate sources such as alcohols and hydrocarbons. A diagram of a DMFC is shown in Figure 9.1. A short, historical, retrospective of relevant technologies and the thermodynamics that govern methanol-based fuel cell systems are pre­sented. A survey of the state of the art, largely in terms of performance, are presented for a variety of fuel cell categories. Efficiency and cost are often the driving force behind development efforts. Trends in DMFC design, elec­trocatalyst, and membrane development are discussed. Finally, some closing thoughts and discussion of one of the more challenging limitations of extant DMFC technology. The chapter focuses on the application side of the tech­nology, and, given space constraints, computer modeling studies are not cov­ered in depth. Save for historical references, pains are taken to use the most current references available. The majority range from 2003 to the present.

Many studies of ethanol steam reforming have been carried out over nickel-based catalysts, because these are used effectively on an industrial scale for the steam reforming of natural gas and higher hydrocarbons. Different carriers have been studied [7,31-34], a major concern being the catalyst deactivation by carbon deposition. In this context, basic supports give better results, concerning coke formation, than do acidic carriers.

Two nickel-based systems, Ni/La2O3 and Ni/MgO can be highlighted [7,34]. Ni/La2O3 showed a good performance in terms of activity, selectivity, and stability [7]. No dehydration products were detected in the course of catalytic tests. Below 573 K, only dehydrogenation of ethanol occurred. The ethanol steam reforming takes place above 673 K. At this temperature, the presence of CO2 and CH4 is caused by the WGSR and methanation reaction, respectively:

CO + H2O о CO2 + H2 AH0 = -41.1 kJ mol-1 CO + 3H2 о CH4 + H2O AH° = -205.6 kJ mol-1

Then, at higher temperatures, the yield of H2 increases and the selectivity to CO2 and CH4 decreases. On the one hand, high temperature does not favor the WGSR. On the other hand, the reforming of CH4 with H2O and/or CO2 (dry reforming of methane) could take place, because these reactions become thermo­dynamically feasible above 823 K. Moreover, the Ni/La2O3 catalyst was seen to be very active for these reforming reactions:

CH4 + H2O о CO + 3H2 AH° = 205.6 kJ mol-1

CH4 + CO2 о 2CO + 2H2 AH° = 246.9 kJ mol-1

It is worth mentioning that there is good long-term stability of Ni/La2O3 when it is compared with other nickel-supported catalysts. This feature has been attrib­uted to the lack of formation of carbon deposits on its surface, and a model has been proposed for this [7]. A lanthanum oxycarbonate species is formed when CO2 reacts with a La2Ox species:

La2O3 + CO2 ^ La2O2CO3

La2O2CO3, which decorates the nickel particles, removes the surface carbon located at its periphery; the following reaction has been proposed:

La2O2CO3 + C ^ La2O3 + 2CO

The Ni/MgO system has been proposed as appropriate to carry out the steam reforming of bioethanol to supply H2 to MCFC [34]. The addition of alkaline ions produces metal particles of larger size and higher specific activity. The authors claimed that potassium addition stabilizes nickel catalysts by depressing the metal sintering under steam reforming conditions [34]. It has been suggested that the addition of potassium could change the electronic properties of Ni/MgO catalysts by electronic transfer from alkali-oxide moieties to nickel particles, which may depress the Boudouard reaction (2CO ^ CO2 + C) and hydrocarbon decomposition, which can lead to coke formation during steam reforming [31].

On the other hand, several studies have been carried out over Ni-Cu-based catalysts [35-39]. When alumina was used as support [35-37]. Potassium was added to avoid dehydration reactions. The study of catalyst generation as a function of the calcination step and the reducibility of different phases has been analyzed. Copper has been proposed to be responsible for the fast ethanol dehy­drogenation to acetaldehyde and nickel for the C-C rupture of acetaldehyde to produce methane and CO. A new aspect was recently introduced when Cu-Ni catalysts supported on SiO2 were considered: the formation of Cu-Ni, alloys which may prevent the deactivation of catalysts by carbon deposits [40].

As stated above, cobalt is also considered an appropriate active phase for the steam reforming of ethanol. An early study on supported cobalt-based catalysts was reported by Haga et al. [41]. This study provided evidence that the support strongly influences the catalyst performance. However, most of the cobalt-based catalysts that were tested (ZrO2-, MgO-, SiO2- and C-supported) showed high yields of methane, probably coming from ethanol or acetaldehyde decomposition or CO hydrogenation, and these reactions were highly suppressed over Co/Al2O3. Several later studies have been devoted to clarifying the role of support and cobalt phases in the behavior of catalysts [21,42-45]. Unsupported or ZnO-supported Co3O4 transforms under steam reforming conditions to small Co particles, which show a high catalytic performance in ethanol steam reforming [43,45]. Using bioethanol-like mixtures (H2O/CH3CH2OH = 13, molar ratio), these catalysts can yield up to 5.5 moles of hydrogen per mole of ethanol reacted at 623 K. The use of relatively low reaction temperature and an excess of H2O makes it possible to obtain almost exclusively CO2 and H2 as reaction products with a low presence of by-products [43,45]. The addition of alkaline metals on Co/ZnO catalysts has been found to have a promoter effect because they stabilize the catalyst by inhibiting coke formation [25].

The synthesis gas can contain a considerable amount of methane and other light hydrocarbons, representing a significant part of the heating value of the gas. Steam reforming (SMR) converts these compounds to CO and H2 driven by steam addition over a catalyst (usually nickel) at high temperatures (Katofsky 1993). Autothermal reforming (ATR) combines partial oxidation in the first part of the reactor with steam reforming in the second part, thereby optimally integrating the heat flows. It has been suggested that ATR, due to a simpler concept, could become cheaper than SMR (Katofsky 1993), although others suggest much higher prices (Oonk et al. 1997). There is dispute on whether the SMR can deal with the high CO and C+ content of the biomass synthesis gas. While Katofsky writes that no additional steam is needed to prevent coking or carbon deposition in SMR, Tijmensen (2000) poses that this problem does occur in SMR and that ATR is the only technology able to prevent coking.

Steam reforming is the most common method of producing a synthesis gas from natural gas or gasifier gas. The highly endothermic process takes place over a nickel-based catalyst:

Concurently, the water gas shift reaction (see below) takes place and brings the reformer product to chemical equilibrium (Katofsky 1993).

Reforming is favored at lower pressures, but elevated pressures benefit eco­nomically (smaller equipment). Reformers typically operate at 1-3.5 MPa. Typ­ical reformer temperature is between 830°C and 1000°C. High temperatures do not lead to a better product mix for methanol production (Katofsky 1993). The inlet stream is heated by the outlet stream up to near the reformer temperature to match reformer heat demand and supply. In this case less synthesis gas has to be burned compared to a colder gas input, this eventually favors a higher methanol production. Although less steam can be raised from the heat at the reformer outlet, the overall efficiency is higher.

SMR uses steam as the conversion reactant and to prevent carbon formation during operation. Tube damage or even rupture can occur when the steam-to — carbon ratio drops below acceptable limits. The specific type of reforming catalyst used, the operating temperature, and the operating pressure are factors that deter­mine the proper steam-to-carbon ratio for a safe, reliable operation. Typical steam to hydrocarbon-carbon ratios range from 2.1 for natural gas feeds with CO2 recycle, to 3:1 for natural gas feeds without CO2 recycle, propane, naphtha, and butane feeds (King et al. 2000). Usually full conversion of higher hydrocarbons in the feedstock takes place in an adiabatic prereformer. This makes it possible to operate the tubular reformer at a steam-to-carbon ratio of 2.5. When higher hydrocarbons are still present, the steam-to-carbon ratio should be higher: 3:5. In older plants, where there is only one steam reformer, the steam-to-carbon ratio was typically 5.5. A higher steam:carbon ratio favors a higher H2CO ratio and thus higher methanol production. However, more steam must be raised and heated to the reaction temperature, thus decreasing the process efficiency. Neither is additional steam necessary to prevent coking (Katofsky 1993).

Preheating the hydrocarbon feedstock with hot flue gas in the SMR convection section, before steam addition, should be avoided. Dry feed gas must not be heated above its cracking temperature. Otherwise, carbon may be formed, thereby decreasing catalyst activities, increasing pressure drop, and limiting plant throughput. In the absence of steam, cracking of natural gas occurs at temperatures above 450°C, while the flue gas exiting SMRs is typically above 1000°C (King et al. 2000).

Nickel catalysts are affected by sulfur at concentrations as low as 0.25 ppm. An alternative would be to use catalysts that are resistant to sulfur, such as sulphided cobalt/molybdate. However, since other catalysts downstream of the reformer are also sensitive to sulfur, it makes the most sense to remove any sulfur before conditioning the synthesis gas (Katofsky 1993). The lifetime of catalysts ranges from 3 years (van Dijk et al. 1995) to 7 years (King et al. 2000). The reasons for change out are typically catalyst activity loss and increasing pressure drop over the tubes.

Autothermal reforming (ATR) combines steam reforming with partial oxida­tion. In ATR, only part of the feed is oxidized, enough to supply the necessary heat to steam reform the remaining feedstock. The reformer produces a synthesis gas with a lower H2.CO ratio than conventional steam methane reforming (Katof — sky 1993; Pieterman 2001).

An Autothermal Reformer consists of two sections. In the burner section, some of the preheated feed/steam mixture is burned stoichiometrically with oxygen to produce CO2 and H2O. The product and the remaining feed are then fed to the reforming section that contains the nickel-based catalyst (Katofsky 1993).

With ATR, considerably less synthesis gas is produced, but also considerably less steam is required due to the higher temperature. Increasing steam addition hardly influences the H2:CO ratio in the product, while it does dilute the product with H2O (Katofsky 1993). Typical ATR temperature is between 900°C and 1000°C.

Since autothermal reforming does not require expensive reformer tubes or a separate furnace, capital costs are typically 50-60% less than conventional steam reforming, especially at larger scales (Dybkjaer et al. 1997, quoted by Pieterman 2001). This excludes the cost of oxygen separation. ATR could therefore be attractive for facilities that already require oxygen for biomass gasification (Katof­sky 1993).

The major source of H2 in oil refineries, catalytic reforming, is decreasing. The largest quantities of H2 are currently produced from synthesis gas by steam­reforming of methane, but this approach is both energy and capital intensive. Partial oxidation of methane with air as the oxygen source is a potential alternative to the steam-reforming processes. In methanol synthesis starting from C1 to C3, it offers special advantages. The amount of methanol produced per kmol hydro­carbon may be 10% to 20% larger than in a conventional process using a steam reformer (de Lathouder 1982). However, the large dilution of product gases by N2 makes this path uneconomical, and, alternatively, use of pure oxygen requires expensive cryogenic separation (Maiya et al. 2000).

Reforming is still subject to innovation and optimization. Pure oxygen can be introduced in a partial oxidation reactor by means of a ceramic membrane, at 850-900°C, in order to produce a purer synthesis gas. Lower temperature and lower steam to CO ratio in the shift reactor leads to a higher thermodynamic efficiency while maximizing H2 production (Maiya et al. 2000).

There are a number of trends shaping the industry’s future. Wet millers will continue to develop value-added products for the starch stream and alternative uses for corn fiber. The dry-grind industry is looking at fractionating the corn prior to fermentation to realize better value from the corn, and at basic process modifications for lowering energy use [25, 26]. As is the case for corn fiber derived from wet milling, the dry-grind industry is also seeking alternate uses for its lowest value product, DDGS.

Alternative Feedstocks Corn Fiber

Wet mills have been investigating converting their least valuable residue from com processing (hulls and deoiled germ, known as corn fibers) into ethanol. Corn fiber is an attractive target for value-added research, because its value could be increased by converting it to ethanol instead of adding it to animal feed. The fiber is generated as a by-product at wet-milling facilities, and so there is no added cost for collection and transport of the material. Corn fiber contains 11-23% residual starch from wet milling and 12-18% cellulose (w/w, dry basis); the glucose in both polymers could be fermented to ethanol by traditional yeast. In addition, corn fiber contains 18-28% xylan and 11-19% arabinan. However, industrial yeast strains currently used for fermenting corn starch do not ferment arabinose and xylose, and the few naturally occurring yeast that do ferment pentoses produce low ethanol yields. Consequently, genetically engineered micro­organisms will be required for efficient conversion of pentose sugars to ethanol [27, 28]. A method to ferment the fiber to ethanol would increase ethanol yield from corn by 10%, while also generating feed coproducts with higher protein content [29]. Fermentation of corn fiber is being evaluated by Aventine Renewable Energy (Pekin, IL) to establish the process economics and robustness [30].

Perstraction is a butanol recovery technique similar to liquid-liquid extraction that seeks to solve some of the problems inherent in liquid-liquid extraction. In a perstraction separation, the fermentation broth and the extractant are separated by a membrane (Qureshi et. al., 1992). The membrane contactor provides a surface area where the two immiscible phases can exchange the butanol. Since there is no direct contact between the two phases, extractant toxicity, phase dispersion, emulsion and rag layer formation are drastically reduced or eliminated. In such a system, butanol should diffuse preferentially across the membrane, while other components such as medium compositions and fermentation intermediates (acetic and butyric acids) should be retained in the aqueous phase. The total mass transport of butanol from the fermentation broth to the organic side depends on the rate of diffusion of butanol across the membrane. The net movement is measured as membrane flux or rate of movement, J = dQb/dt, where J = net flux and dQb/dt = diffusion rate (influx + efflux) of butanol. The membrane does, however, present a physical barrier that can limit the rate of solvent extraction.

Shelley D. Minteer

Department of Chemistry, Saint Louis University, Missouri

CONTENTS

Introduction………………………………………………………………………………………………….. 191

Direct Ethanol Fuel Cells……………………………………………………………………………….. 192

Ethanol Electrocatalysts……………………………………………………………………………….. 193

PtRu Catalysts………………………………………………………………………………….. 193

PtSn Catalysts………………………………………………………………………………….. 194

Conclusions…………………………………………………………………………………………………… 201

References…………………………………………………………………………………………………….. 201

Abstract This chapter details the background and performance of direct ethanol fuel cells (DEFCs). This chapter compares direct ethanol fuel cells to direct methanol fuel cells and other alcohol-based fuel cells. It discusses recent devel­opments in bimetallic electrocatalysts, membrane electrode assembly (MEA) fabrication techniques, temperature effects, and the effects of fuel concentration on the performance of the direct ethanol fuel cell.

INTRODUCTION

Portable power requires simplistic systems that operate at or near room temper­ature. Most research in fuel cells for use as portable power have employed polymer electrolyte membrane fuel cells. Polymer electrolyte membrane (PEM) fuel cells can be characterized into two categories: reformed and direct systems. Reformed systems require the use of an external reformer to reform a fuel (methane, methanol, ethanol, gasoline, etc.) into hydrogen for use in the fuel cell. In direct systems the fuel is oxidized at the surface of the electrode without treatment. Over the last 40 years, there has been extensive research on direct methanol fuel cells (DMFC) for portable power applications at low to moderate temperatures [1-4]. However, there are a number of problems associated with the use of methanol as a fuel for portable power supplies. Methanol is highly
toxic and could lead to long-term environmental problems because methanol is so miscible in water [5]. These limitations have led researchers to investigate other fuels. Ethanol is an attractive alternative to methanol as a fuel for a fuel cell. Ethanol is a renewable fuel and can be produced from farm products and biomass. Ethanol and its intermediate oxidation products have been shown to be less toxic than other alcohols [6]. The problem with ethanol as a fuel (in com­parison to methanol) is that complete oxidation of ethanol requires the breaking of a C-C bond, which is difficult at traditional Pt-based catalysts. This typically leads to incomplete oxidation of ethanol, which decreases the efficiency of the fuel cells and could provide toxic by-products or electrode passivation. This chapter focuses on the basics of direct ethanol fuel cells and the effects of catalyst, temperature, and fuel concentration on fuel cell performance.

Published results from researchers state that for every pound of fish produced, 50-75 pounds of vegetables are produced (3). Our goal is 35 pounds of vegetables, and each tank has the ability to produce 2200 pounds of fish a week at full capacity (we have room for 2-3 tanks). The project can be duplicated nearly anywhere in the world, and requires very little room. The yield from the aquaponics system is enormous. Square foot for square foot, the yield is 15 times that of traditional soil farming, given the same period of time.

Smaller Family-Sized Unit

The project will convert a smaller 171 x 481 greenhouse to a family-sized unit by scaling down the commercial unit. It will contain a smaller tank capable of producing 200 pounds of fish and enough growbed room to accommodate the waste from the fish. Additional growing area will be used for the growth of root crops in compost. The smaller unit will be able to meet the annual vegetable and fish needs for a family of 6. The technology will differ in this pilot project by using whole kernel corn to heat the radiant floor, and used vegetable oil (UVO or WVO) to directly fuel a diesel-powered electric generator.