Nancy N. Nichols,1 Bruce S. Dien,1 Rodney J. Bothast,2 and Michael A. Cotta1

1 Fermentation Biotechnology Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, U. S. Department of Agriculture,[4] Peoria, Illinois

2 National Corn-to-Ethanol Research Center, Southern Illinois University-Edwardsville, Edwardsville, Illinois

CONTENTS

Introduction…………………………………………………………………………………………………….. 60

Corn Ethanol and the Energy Balance……………………………………………….. 61

Economics of Corn Ethanol……………………………………………………………….. 61

Theory behind Conversion of Corn to Ethanol……………………………………………….. 62

Structure and Composition of the Corn Kernel………………………………….. 62

Conversion of Starch to Glucose………………………………………………………… 63

Fermentation of Glucose to Ethanol…………………………………………………… 63

Processes for Converting Corn to Ethanol………………………………………………………. 66

Wet-Milling and Dry-Grind Corn Processes for Ethanol

Fermentation………………………………………………………………………………………. 66

Dry-Grind Ethanol Production……………………………………………………………. 67

Starch Conversion…………………………………………………………………….. 67

Fermentation…………………………………………………………………………….. 68

Distillation and Dehydration……………………………………………………… 68

Stillage Processing and Feed Products……………………………………….. 68

Ethanol Production by Wet Milling…………………………………………………….. 69

Steeping…………………………………………………………………………………….. 69

Oil, Fiber, and Gluten Separation………………………………………………. 69

Starch Conversion…………………………………………………………………….. 70

Coproducts from Wet Milling……………………………………………………. 70

Future Directions in the Corn Ethanol Industry………………………………………………. 70

Alternative Feedstocks……………………………………………………………………….. 71

Corn Fiber………………………………………………………………………………….. 71

Corn Stover and Alternative Starches……………………………………….. 71

Other Types of Biomass……………………………………………………………. 71

New Products from Wet-Milled Corn Starch………………………………………. 72

Modifying the Corn Dry-Grind Process………………………………………………. 73

Quick Fiber and Quick Germ Processes…………………………………….. 73

Very-High-Gravity Fermentations…………………………………………….. 73

New Processing Technology to Decrease Energy Use……………….. 73

Alternate Uses for DDGS…………………………………………………………………….. 74

Zein Protein from Corn Dry Grinding or Wet Milling………………………….. 74

Hybrid and Strain Development…………………………………………………………. 75

Conclusions……………………………………………………………………………………………………… 75

References……………………………………………………………………………………………………….. 75

INTRODUCTION

Ethanol production is a growing industry in the United States, where corn is the feedstock used to produce approximately 90% of fuel ethanol. Approximately 1.26 billion bushels of corn, equal to 11% of the total U. S. corn crop, were processed to ethanol in 2004 according to the Renewable Fuels Association. Globally, the only crop used to produce more ethanol is sugar cane. Approxi­mately 61% of world ethanol production is from sugar crops, with the remainder being made primarily from corn [1]. The success of corn as a feedstock for ethanol production can be directly tied to the huge and sustained improvements in yields realized in the United States; corn yields per acre quadrupled in the fifty years from 1954 to 2004. According to the U. S. Department of Agriculture-National Agricultural Statistics Service, the average U. S. corn grain yield in 2004 was 160.4 bushels per acre and the average price was $1.95 per bushel.

Use of corn to make fuel ethanol dates back to the earliest days of automobiles [2]. The Model T was originally designed to run on ethanol, although use of ethanol for fuel ceased with the development of the petroleum industry. The ethanol industry was briefly revived during World War II and during the oil crisis in the 1970s. The current renaissance of the ethanol industry dates to the 1980s, when the U. S. Environmental Protection Agency mandated adding fuel oxygen­ates to gasoline to reduce automotive emissions. Ethanol has been used as an oxygenate primarily in the Midwestern United States whereas MTBE, which is derived from petroleum, has been used on both coasts. However, MTBE is now recognized as a hazard to water supplies, and its use is being phased out by all states. The opening of coastal markets to ethanol has led to tremendous growth in the domestic ethanol industry, which is expected to produce five billion gallons of ethanol per year by 2012. Whether this target of five or more billion gallons will be achieved because of oxygenate requirements, or be directly mandated as part of a National Energy Bill—which will allow gasoline refiners to trade ethanol credits—is unclear. What appears certain is that the corn ethanol industry can look forward to continued and steady growth. Growth is ensured by the increas­ingly appreciated advantages of corn ethanol: reduced oil imports, reduced auto­motive-associated net CO2 emissions, and a stabilized corn market.

Although commercial biorefining of alfalfa remains undeveloped, alfalfa has tremendous potential as a feedstock for production of ethanol and other products. Alfalfa is widely adapted and produces large amounts of biomass over the course of four or more years. The production costs of alfalfa are low and cultivation of the crop has numerous environmental benefits. Importantly, alfalfa leaves contain the majority of the protein in the plant and are easily separated from stems through processing. Leaf meal is a valuable coproduct in its own right as animal feed, as well as a potential source for human nutritional supplements and products derived from transgene expression. The stem fraction of alfalfa is rich in cell wall polysaccharides that can be used as a source of fermentable sugars to produce ethanol and other bioproducts. A biomass-type of alfalfa is being developed that is more upright in growth habit and performs well in a reduced frequency harvest management system, maximizing the yield of both leaf and stem fractions while lowering production costs. Incorporation of enhanced compositional traits such as more cellulose, less lignin and valuable transgenic protein products into this alfalfa biomass type through traditional breeding and using the tools of biotech­nology will add to the value alfalfa brings to biofuels and bioproduct systems.

ACKNOWLEDGMENTS

The authors thank members of the USDA-ARS-Plant Science Research Unit (Drs. Carroll P. Vance, Michael P. Russelle, and John Gronwald) and Dr. Sandra Austin — Phillips (University of Wisconsin-Madison) for stimulating conversations, insights, and for providing literature citations.

E85 is safer than gasoline to store, transport, and refuel. Ethanol is water soluble and biodegradable. Land and water spills of pure ethanol are usually harmless, dispersing and decomposing quickly. In E85 spills, the gasoline portion of a spill is still a problem; however, the total volume of gasoline in the spill is reduced. This is significant as it is estimated that the amount of oil leaked from vehicles into rivers, lakes, and groundwater is estimated to be six times the annual volume of oil spills.23

Finally, since E85 has fewer highly volatile components than gasoline, it produces lower levels of emissions resulting from evaporation. This leads to lower vehicle evaporative emissions during fueling and when the vehicle is not being used. This, of course, is what causes the problems in cold starting a vehicle using pure ethanol, which is why E85 is used in vehicles instead of pure ethanol.

Sustainability

Ethanol can be made from biomass resources containing sugar. The main source of ethanol in the United States is corn. Over the years, considerable debate surrounded the energy balance for the production of fuel ethanol from corn. An early study,24 found that it takes more energy to produce and distribute fuel ethanol than is recovered when combusting the fuel. This led critics to proclaim that corn ethanol is not a fossil energy substitute and its use would actually increase petroleum use. More recent studies have shown that ethanol production is net energy positive, that is burning ethanol liberates more energy than is consumed in its production.22 One of the most comprehensive studies from an independent source found that 1.34 BTU of energy are delivered for each 1 BTU input.25 By contrast, gasoline delivers about 0.8 BTU for each 1 BTU input. Thus, E85 use provides about 1.25 BTU for every 1 BTU of energy input.

While the energy balance is important, the type of energy used and produced is perhaps even more important. For example, ethanol is a high-energy density liquid fuel. E85 can be used to displace the use of fossil energy. This approach focuses the energy balance issue, looking at the energy value of the liquid fossil fuels used in the production of ethanol. Using this approach, 1 BTU of fossil energy (typically petroleum) generates between 4.07 and 6.34 BTU of ethanol fuel energy.2625 Converting to E85, this means each 1 BTU of fossil fuel generates between 3.46 and 5.39 BTU of E85 fuel. According to a 1998 report from Argonne National Laboratory, each gallon of E85 used reduces petroleum use by 73-75%.21

This section will show several options of integration of the steam reforming of ethanol in overall processes that are energetically favored. Some of these pro­cesses have been proposed for the production of hydrogen from biomass-derived ethanol.

Figure 13.1 shows a schematic based on that proposed by Verikyos and colleagues [7], in which bioethanol could come from different sources: plants, agroindustrial residues, and the organic fraction of municipal solid waste. Besides bioethanol, biogas is produced. Aqueous solutions of ethanol initially generated could be concentrated prior to the steam reforming by a conventional distillation procedure. The efficiency of the overall process has been estimated to be twice that of the process producing electricity by a conventional method from biomass combustion [7].

This is a heat-integrated process, in which the heat necessary in the steps of distillation and steam reforming reactions is supplied by the heat evolved from several exothermic chemical reactions. Among these, the electrochemical reaction in the fuel cell may contribute to the energy balance of the total process. For instance, in the case of an MCFC, one of the limiting factors of the technology for a high-yield operation is the recovery of heat generated at the anode [8]:

CO32- + H2 ^ CO2 + H2O + 2e — AH < 0.

The use of this heat to produce H2 by steam reforming may optimize the operation of the cell, and the direct contact of catalyst with vapor carbonates may be avoided by placing a chamber with the catalyst adjacent to the anode.

In MCFC and SOFC fuel cells, the catalyst may be placed in the anode compartment and then the internal steam reforming of ethanol occurs. In this case, formation of carbon residues must be avoided since the electrode structure may break down [9]. In this respect, it must be highlighted that the presence of some of the by-products, such as ethylene, has been related to the deactivation of catalysts by carbon deposition.

Moreover, for stationary applications, the use of heat generated by the com­bustion of methane (biogas) is proposed:

CH4 + 2O2 ^ CO2 + 2H2O AH0 = -803 kJ mol-1

Recently, catalytic methane combustion coupled to ethanol steam reforming across an autothermal wall, which eliminated heat transfer boundary layers, was reported [10]. On the other hand, if oxygen is introduced in the ethanol:water mixture, the coupling of two oxidation reactions may take place:

CH3CH2OH + 3O2 ^ 2CO2 + 3H2O AH°= -1280 kJ mol-1

H2 + 1/2O2 ^ H2O AH° = -242 kJ mol-1

By tuning the amount of oxygen, using air as a carrier and an appropriate catalyst, the catalytic partial oxidation of ethanol can be effective:

CH3CH2OH + 3/2O2 ^ 2CO2 + 3H2 AH0 = -545 kJ mol-1

Although the combination of these reactions with steam reforming will pro­duce a decrease in the hydrogen yield, all of them are fast reactions and highly exothermic [11,12]. For mobile applications, there are certain advantages to the combination of endothermic and exothermic reactions, which overcomes the difficulty of quenching the exothermic process by cooling. The endothermic reaction may control the temperature of the exothermic process, alleviating the need for additional cooling equipment.

Thus, the operation of the global process under autothermal conditions can be proposed [13,14]. The O2/H2O/C2H5OH molar ratio can be adjusted so that the combined steam reforming and oxidation reactions could come close to a thermally neutral process:

CH3CH2OH + xO2 + (3 — 2x)H2O ^ 2CO2 + (6 — 2x)H2

for which

AH° = ((3 — 2x)/3)173 — (x/1.5)545) kJ mol-1

Thermodynamic calculations under autothermal conditions, considering CH3CH2OH, O2, and H2O as reactants and H2, CO, and CO2 as products, have recently been carried out [4]. Other thermodynamically possible reaction prod­ucts, such as methane, ethylene, etc. were not taken into account. Results for different fuels (methane, methanol, dimethylether, and gasoline) have been compared. The calculations lead to a maximum hydrogen content in the product of 41-43% when ethanol is used as fuel. This maximum is achieved between 530-600 K with a water/ethanol molar ratio of 1.6-2.9; under these conditions, the CO content in the effluent is approximately 5-10% [4].

Several studies are centered on the optimization of fuel cells that are fueled by H2 produced in the ethanol steam reforming [3,15-17]. Ethanol has been claimed to be a promising alternative to CH4 as a source of H2 for these systems, and an efficiency for the SOFC system of 94% has been calculated for ethanol, which compares well with the efficiency values calculated for CH4, 96%; meth­anol, 91%; and gasoline, 83% [16].

Perhaps somewhat counterintuitive to the layperson, the first commercial fuel cells have been introduced for large-scale applications. Stationary power plants are being installed all across the globe. As the cost of such systems decreases and reliability increases, large-scale fuel cells will begin to be used for residential power. One fairly obvious operating concern is how to provide the fuel to resi­dential areas. It is doubtful that such systems would operate on direct hydrogen, just as the large-scale industrial fuel cells being used today do not. Possibilities include using natural gas or piping in other liquid fuels such as methanol or ethanol using the existing infrastructure.

CONCLUSION

It would be hard for anyone to deny that energy is one of the most important issues at the start of 21st century. Energy is at the root of the major conflicts of our time as well as the catalyst for previously disadvantaged society’s emergence into modern culture. As energy demand increases at staggering rates, the murmur for alternative energy technologies is quickly turning into a scream.

REFERENCES

1. U. S. Department of Energy, Energy Information Administration, Annual Energy Review, 2003.

2. Darnell Group Report to the U. S. Fuel Cell Council, Jan. 2003.

3. Valdes, J., The World Congress on Industrial Biotechnology and Bioprocessing, Orlando, FL, Apr. 21-23, 2004.

[1] This chapter is broadly based on Hamelinck, C. N. and Faaij, A. P.C., Future prospects for production of methanol and hydrogen from biomass, Journal of Power Sources, 111, 1, 1-22, 2002.

[2] In Europe methanol may be blended in regular gasoline up to 5% by volume without notice to the consumer. Higher blends are possible like M85 (85% methanol with 15% gasoline) but would require adaptations in cars or specially developed cars. Moreover, blends higher than 5% require adaptations in the distribution of fuels to gas stations and at the gas stations themselves. Pure methanol is sometimes used as racing fuel, such as in the Indianapolis 500.

[3] Methanol can be the source for hydrogen via on board reforming. Direct methanol fuel cells are under development that can directly process methanol (van den Hoed 2004).

[4] Mention of trade names or commercial products in this chapter is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U. S. Department of Agriculture.

[5] Author to whom correspondence should be addressed.

When the tars and BTX are removed, the other impurities can be removed by standard wet gas cleaning technologies or advanced dry gas cleaning technologies.

Wet low-temperature synthesis gas cleaning is the preferred method for the short term (van Ree et al. 1995). This method will have some energy penalty and requires additional waste water treatment, but in the short term it is more certain to be effective than hot dry gas cleaning.

A cyclone separator removes most of the solid impurities, down to sizes of approximately 5 pm (Katofsky 1993). New generation bag filters made from glass and synthetic fibers have an upper temperature limit of 260°C (Perry et al. 1987). At this temperature particulates and alkali, which condense on particulates, can successfully be removed (Alderliesten 1990; Consonni et al. 1994; Tijmensen 2000; van Ree et al. 1995). Before entering the bag filter, the synthesis gas is cooled to just above the water dew point.

After the filter unit, the synthesis gas is scrubbed down to 40°C below the water dew point, by means of water. Residual particulates, vapor phase chemical species (unreacted tars, organic gas condensates, trace elements), reduced halogen

Removes NH3, HCI, metal, part HCN, HS

/ / Removes Removes Removes Removes traces NH3 HCI, HCN H2S, and COS HCN, H2S, NH3, COS FIGURE 2.4 Three possible gas cleaning trains. Top: tar cracking and conventional wet gas cleaning; middle: tar scrubbing and conventional wet gas cleaning; and bottom: tar cracking and dry gas cleaning.

gases and reduced nitrogen compounds are removed to a large extent. The scrub­ber can consist of a caustic part where the bulk of H2S is removed using a NaOH solution (van Ree et al. 1995) and an acid part for ammonia/cyanide removal. Alkali removal in a scrubber is essentially complete (Consonni et al. 1994).

With less than 30 ppm H2S in the biomass derived synthesis gas, a ZnO bed may be sufficient to lower the sulfur concentration below 0.1 ppm. ZnO beds can be operated between 50 and 400°C, the high-end temperature favors efficient utilization. At low temperatures and pressures, less sulfur is absorbed; therefore, multiple beds will be used in series. The ZnO bed serves one year and is not regenerated (Katofsky 1993; van Dijk et al. 1995). Bulk removal of sulfur is thus not required, but if CO2 removal is demanded as well (see page 23), a solvent absorption process like Rectisol or Sulfinol could be placed downstream, which also removes sulfur. H2S and COS are reduced to less than 0.1 ppm and all or part of the CO2 is separated (Hydrocarbon Processing 1998).

Starch is washed to remove residual protein and is converted to a glucose syrup. First, the starch is jet-cooked and held for liquefaction at 90°C with alpha-amylase. In contrast to the dry-grind process, all of the alpha-amylase is added prior to jet cooking. Then glucoamylase and pullulanase, a a-1^6 debranching enzyme, are added to convert dextrin polymers to sugars. Addition of pullulanase ensures good conversion of dextrins to glucose by decreasing formation of isomaltose, a glucose dimer. Isomaltose is present at starch polymer branching points and can also be formed by “reversion” of glucose to isomaltose in a reaction catalyzed by glucoamylase. After saccharification, glucose is fermented to ethanol by an industrial strain of S. cerevisiae. In wet mills, the fermentation is often run using a series of fermenters in a semi­continuous process. Approximately 2.5 gallons of ethanol are produced from a bushel of corn in the wet-milling process.

Pervaporation is a technique that allows selective removal of volatiles from model solution/fermentation broths using a membrane. The volatile or organic compo­nent diffuses through the membrane as a vapor followed by recovery by conden­sation. In this process, a phase change occurs from liquid to vapor. Since it is a selective removal process, the desired component requires a heat of vaporization at the feed temperature. The mechanism by which a volatile/organic component is removed by pervaporation is called solution-diffusion. In pervaporation, the effectiveness of separation of a volatile is measured by two parameters called selectivity (a measure of selective removal of volatile) and flux (the rate at which an organic/volatile passes through the membrane per m2 membrane area). A schematic diagram of the pervaporation process is shown in Figure 6.4. The details of pervaporation have been described in the literature (Maddox, 1989; Groot et al., 1992; Qureshi and Blaschek, 1999b).

Application of pervaporation to batch butanol fermentation has been described by Groot et al. (1984), Larrayoz and Puigjaner (1987), Qureshi and Blaschek (1999a), and Fadeev et al. (2001). Pervaporation has also been used for the removal of butanol from the fermentation broth in fed-batch reactors (Qureshi and Blaschek, 2000b; Qureshi and Blaschek, 2001a). In the fed-batch reactors concentrated sugar solutions have been used to reduce the process stream volume. It is interesting to note that acids did not diffuse through the membranes used by the above authors. Qureshi et al. (1992) used a polypropylene membrane through which diffusion of acids occurred, however, at high acid concentration in the fermentation broth.

In an attempt to improve membrane selectivity, Matsumura et al. (1988) applied a combination of liquid-liquid extraction and pervaporation to recover butanol. The extraction solvent used for this process was oleyl alcohol, which formed a liquid layer (also known as a thin membrane) on a microporous 25 pm thick polypropylene flat sheet. The oleyl alcohol also got impregnated into the sheet pores. In this combination of liquid-solid membrane, oleyl alcohol dissolved butanol relatively quickly followed by diffusion through the polypropylene mem­brane. The advantage of combining the liquid and solid membrane was that a high butanol selectivity (180) was achieved in comparison to a low selectivity (10-15) when using polypropylene film alone. It was estimated that if this solid — liquid pervaporation membrane were used for butanol separation, the energy requirement would be only 10% of that required in a conventional distillation. Unfortunately, the membrane was not stable as the oleyl alcohol that formed a thin film and was impregnated into the polypropylene film pores diffused out of the membrane.

In order to develop a stable and highly selective membrane, Qureshi et al. (1999) employed two techniques known as adsorption and pervaporation. It has been reported that adsorption of butanol onto silicalite and molecular sieves is a quick and selective process (Ennis et al., 1987). Qureshi et al. (1999) synthesized a membrane in which silicalite, an adsorbent, was included into a silicone mem­brane. By combining these, butanol selectivity was improved from 40 (silicone membrane) to 209. The membrane developed was called a silicalite-silicone membrane. The membrane was found to be stable with a working life of three years. This membrane was used with both butanol model solutions and fermen­tation broths (Qureshi et al., 1999; 2001). A comparison of various membranes suggested that this membrane may be superior to other pervaporation membranes used for butanol separation (Qureshi and Blaschek, 1999b). Some of the details
of the use of this membrane have been given in the section fed-batch fermentation (Qureshi et al., 2001) where 155 gL-1 ABE was produced in the integrated fermentation and product recovery process as compared to <20 gL-1 in a batch process. This membrane was so efficient that a butanol concentration up to 700 gL-1 was achieved in the permeate.

New materials are being developed for use as separators/ionic conductors in DMFCs. These materials generally fall into one of two categories, fluorinated and nonfluorinated. Here some current developments in both areas are presented.

In an effort to develop an inexpensive and effective separator for DMFCs and fuel cells operated on other fuels, Melman et al. developed a nanoporous proton­conducting membrane (NP-PCM) [63]. The NP-PCM is made of polyvinylidene fluoride (PVDF) and SiO2. Fuels are mixed into 3 M sulfuric acid electrolyte and circulated past the anode. Characteristics of the NP-PCM separator that bests Nafion include: membrane cost lowered by 2 orders of magnitude; pore sizes roughly 50% that of Nafion; methanol crossover cut in half; as much as 4x greater ionic conductivity; and a membrane insensitive to heavy metal corrosion products that allow for less expensive hardware and catalysts. The maximum power achieved with the cell was 85 mW cm-2 at ~243 mV on oxygen at atmospheric pressure, and cell temperature of 80°C. One disadvantage of this type of mem­brane is the peripheral systems must be corrosion resistant.

Nafion is made by copolymerizing tetrafluoroethylene (TFE) and perfluorovi — nyl ether (PSEPVE) containing a sulfonyl fluoride. Whereas Nafion provides good performance in PEFCs at temperatures below 100°C, the material is expen­sive due to the relatively difficult polymerization and expense of the monomer PSEPVE. Also, chemical modifications to tailor Nafion properties has proven difficult. Yang and Rajendran describe an effective copolymerization strategy of TFE and ethylene to produce less expensive melt-processable terpolymers that are easily hydrolyzed and acidified to give polymers of high ionic conductivity [42]. The conductivity is similar to or slightly higher than that of Nafion. The membrane also uptakes more water (by weight) than Nafion under the same conditions. An unoptimized 5-mil thick membrane tested in a DMFC yielded performance comparable to a DMFC made with Nafion 115. The power output of the DMFCs were quite similar, though the methanol crossover through the experimental membrane is 9% higher than for Nafion 115 (i. e., 10.9 x 10-4 vs. 9.9 x 10-4 g min-1 cm-2). The authors speculate that the PEM may be optimized by making a more homogeneous thickness and cross-linking.

A review of nonfluorinated PEMs for use in DMFCs was prepared by Roziere and Jones [38]. The number of nonfluorinated polymer materials for application in higher-temperature fuel cells (i. e., > 80°C) is limited by thermal instability. Thermally stable polymers tend to have either polyaromatic or polyheterocyclic repeat units. Examples of these include polybenzimidazole (PBI), poly(ether ketone)s (PEK), poly(phenyl quinoxaline (PPQ), polysulfone (PSU), and poly(ether sulfone) (PES). The chemical structures of some common nonfluori — nated polymers are shown in Figure 9.5. These polymers are thermally stable but are poor ionic conductors until modified. Modification methods include acid and base doping of the polymer, sulfonation of the polymer backbone, grafting phos- phonated or sulfonated functional groups onto the polymer (where sulfonated polymers generally contain “s” in the acronym), graft polymerization onto the polymer followed by functionalization of the graft material, and total synthesis. Many of these membranes exhibit good proton conductivity (0.01 to 0.1 S cm-1) when well-hydrated but a balance between the sulfonation level imparting ionic conductivity, adequate membrane strength, and membrane swelling must be met before long-term use in fuel cells is possible. Roziere and Jones suggest that sPEKs and related polymer blends hold the most promise for use in DMFCs. They cite unpublished performance data of a DMFC made using polyaromatic membrane with comparable power density to Nafion cells and little degradation after start/stop regimes at temperatures >100°C over a period of weeks.

There is a limited amount of DMFC performance data for fuel cells made with these PEMs, and the conditions under which the PEMs are tested in DMFCs are quite varied. Relevant DMFC performance data included here are listed in the “Performance Targets and Efficiencies” section of this chapter. A more rig­orous set of evaluations for candidate PEMs in operational DMFCs should include durability tests including startup and shutdowns over extended periods.

An engaging two-paper study by Silva et al. evaluates inorganic-organic hybrid membranes made from sulfonated poly(ether ether ketone) (sPEEK)

PEK

[64,65]. One goal of the study is to demonstrate a systematic and complete approach to DMFC membrane development, characterization, and, ultimately, the real-world testing in DMFCs that is thus far lacking for other promising materials. The sPEEK membranes have a sulfonation degree of 87% and zirconium oxide content that varies between 2.5 and 12.5 wt%. The group evaluates the membranes using standard analytical techniques (e. g., impedance spectroscopy (proton con­ductivity and proton transport resistance), pervaporation (permeability to meth­anol), and water swelling) and as PEMs in DMFCs. The organic/inorganic hybrid sPEEK/ZrO2 membranes exhibit good proton conductivity and the addition of ZrO2 particles can tailor the electrochemical performance of the membranes. This makes the organic/inorganic hybrid sPEEK/ZrO2 membranes a possible alterna­tive to perfluorinated membranes. It was found the proton transport resistance increased as the wt% ZrO2 in the membrane increased and that proton conduc­tivity followed the opposite trend. Water uptake decreases as the inorganic com­ponent increases, following the same trend as the proton conductivity. This supports the observation of the importance of sorbed water in proton conduction in sulfonated matrices. Methanol permeability decreases as the inorganic content of the membrane increases. The permeability of O2, CO2, and N2, reduction products and oxidant stream constituents were evaluated as a function inorganic content of the membrane. It was found that as ZrO2 content increases, the per­meability of O2 and CO2 decreases and N2 permeability is unaffected. In general, the desirable characteristic of decreasing permeability of reactants, products, and oxidant stream constituents as the ZrO2 content increases were observed but with a corresponding decrease in proton conductivity.

Jorissen et al. evaluated DMFCs made with nonfluorinated PEMs (sPEEK and sPEK using PBI and basically substituted PSU (bPSU)) prepared by a thin- film method [24], in which the catalytic layer is sprayed directly onto the PEM. The authors used unsupported electrocatalysts at relatively high loadings (1.5 to 6.5 mg cm-2) to overcome the effects of flooding at both electrodes. The fuel cells were operated at 110°C with little or no cathode humidification and perfor­mance was compared to DMFCs made with Nafion 117 and 105. None of the DMFCs prepared from the test PEMs had optimized catalyst layer/PEM inter­faces, yet a number of the test cells performed similarly to the control cell made with Nafion 105. The best performing of these cells was a membrane composed of PEK, PBI, and bPSU that generated 85% of the power density (230 mW cm-2 @ 500 mV) of the Nafion 105 cell (270 mW cm-2 @ 500 mV). The control cell made from Nafion 117 exhibited poor performance generating less than half the power density of Nafion 105 cell under the same conditions.

Another nonfluorinated material of promise is poly(vinyl alcohol) (PVA). The material is chemically and thermally robust and quite inexpensive relative to Nafion. In a recent article by Khan et al., the synthesis and characterization of PVA-based membrane is described [66]. Membranes were based on PVA and its ionic blends with sodium alginate (SA) and chitosan (CS). The membrane ion exchange capacity (IEC, in milliequivalents of ion per gram dry polymer) were determined to be ~0.5 for PVA, 0.6 for PVA-CS and 0.8 for PVA-SA. All of the PVA-based membranes were found to have lower methanol permeability than Nafion. The PVA-CS membrane had the lowest permeability at 6.9 x 10-8 cm2 s-1 as compared to Nafion 117 with a permeability of 2.76 x 10-7 cm2 s-1. The other two membranes had permeability intermediate to PVA-SA and Nafion. Contrasting the desirable characteristic of lower methanol permeability than Nafion 117, the proton conductivities of the PVA-based membranes are signifi­cantly lower (~0.01 S cm-1) than that for Nafion 117 (0.1 S cm-1). The investi­gators hold out the possibility of improving proton conduction in the PVA mem­branes through doping.

Lee et al. also investigated the use of PVA-based materials. The group pre­pared membranes made from PVA, SiO2, and sulfosuccinic acid (SSA) [67]. The SSA acted both as the cross-linking agent and as the source of hydrophilic sulfonate groups. The SSA content of the membranes was varied from 5 to 25 wt %. Both the proton conductivity and methanol permeability decreased as the wt % of SSA increased to 20 wt %. At over 20 wt % SSA both trends reversed. Similar to what Khan et al. found, the membranes had proton conductivities on the order of 1 x 10-3 to 1 x 10-2 S cm-1 and methanol permeability of 1 x 10-8 to 1 x 10-7 cm2 s-1. In a related study, Lee and his group expanded the previous work by adding poly (acrylic acid) (PAA) to the hybrid membranes [68]. The thought was the addition of unreacted carboxylic acid groups may improve proton conductivity, but the PAA did not markedly improve upon the original hybrid membranes.

The roof of the aquaponics greenhouse is designed to capture rainwater and to melt and capture snow. Water is then transported through pipes to a central water — storage tank. Since this geographic area is subject to periodic droughts, the need to keep a supply of water is essential during the summer months.

Floor of the Structure

The floor of the aquaponics greenhouse is 61 1 thick concrete fitted with special plumbing that allows for the passage of hot water during the winter and cold water during the summer. It is connected to both the outdoor furnace and the recouperators on the microturbines. It has 31 1 of special insulation below it. Sump tanks, which gather flood water from the growbeds, are beneath the floor and accessible through a manhole in the top. It is then pumped back into the fish tank.

FUEL FLOW CHART

FIGURE 14.1 Flow chart of system.

Balanced Diet

Many types of plants can be grown in this system, if all of their cultural require­ments are met (light, heat, nutrients, and water). There are no weeds, so each plant can reach its full potential. The beds are comfortable to work with since they are hip height and require no stooping. As such, the project can potentially be used to provide a complete diet. Initially, the project will produce lettuce, broccoli, tomatoes, parsley, oregano, basil, cucumbers, peppers, onions, swiss chard, and spinach.