The continuous culture technique is often used to improve reactor productivity and to study the physiology of the culture in steady state. A number of studies exist for continuous fermentation of butanol, and they all give some insight into butanol fermentation and the behavior of the culture under these conditions. Because of the production of fluctuating levels of solvents and the complexity of butanol fermentation, the use of a single-stage continuous reactor does not seem to be practical at the industrial scale. In continuous culture, a serious problem exists in that solvent production may not be stable for long time periods and ultimately declines over time, with a concomitant increase in acid production. In a single-stage continuous system, high reactor productivity may be obtained, however, at the expense of low product concentration compared to that achieved in a batch process. In a single-stage continuous reactor using C. acetobutylicum, Leung and Wang (1981) produced 15.9 gL-1 total solvents (ABE) at a dilution rate of 0.1 h-1 resulting in a productivity of 1.6 gL-1h-1. The productivity was improved further to 2.55 gL-1h-1 by increasing the dilution rate to 0.22 h-1. It should be noted that the product concentration decreased to 12.0 gL-1. In a related continuous fermentation process using a hyperbutanol producing strain of C. beijerinckii BA101, Formanek et al. (1997) was able to produce 15.6 gL-1 ABE at a dilution rate of 0.05 h-1 resulting in a productivity of 0.78 gL-1h-1. However, solvent concentration decreased to 8.7 gL-1 as dilution rate was increased to 0.2 h-1. This resulted in an increase in productivity to 1.74 gL-1h-1.

As a means of increasing product concentration in the effluent and reducing fluctuations in butanol concentration, two or more multistage continuous fermen­tation systems have been investigated (Bahl et al., 1982; Yarovenko, 1964). Often, this is done by allowing cell growth, acid production, and ABE production to occur in separate bioreactors. In a two-stage system, Bahl et al. (1982) reported a solvent concentration of 18.2 gL-1 using C. acetobutylicum DSM 1731, which is comparable to the solvent concentration in a batch reactor. This type of mul­tistage bioreactor system (7-11 fermenters in series) was successfully tested at the pilot scale and full plant scale level in the Soviet Union (now Russia) (Yarovenko, 1964). However, 7-11 fermenters in series add to the complexity of the system for a relatively low-value product such as butanol. It is viewed that such a multistage system would not be economical.

Immobilized and Cell Recycle Reactors

Increased reactor productivity results in the reduction of process vessel size and capital cost thus improving process economics. In a butanol batch process, reactor productivity is limited to less than 0.50 gL-1h-1 due to a number of reasons including low cell concentration, down time and product inhibition (Maddox, 1989). Increasing cell concentration in the reactor is one of the methods to improve reactor productivity. Cell concentration can be increased by one of two techniques namely, “immobilization” and “cell recycle.” In a batch reactor a cell concentration of <4 gL-1 is normally achieved. In an attempt to improve the reactor productivity, Ennis et al. (1986a) were among the early investigators to use the cell immobilization technique for the butanol fermentation. These authors used cell entrapment technique and continuous fermentation with limited success in productivity improvement. The same group investigated another technique involving cell immobilization by adsorption onto bonechar and improved reactor productivity to approximately 4.5 gL-1h-1 (Qureshi and Maddox, 1987) followed by further improvement to 6.5 gL-1h-1 (Qureshi and Maddox, 1988). The culture that was used in these studies was C. acetobutylicum P262. In an attempt to explore clay bricks as an adsorption support for cells of C. beijerinckii, Qureshi et al. (2000) were able to improve reactor productivity to 15.8 gL-1h-1. In another approach, Huang et al. (2004) immobilized cells of C. acetobutylicum in a fibrous support, which was used in a continuous reactor to produce ABE. In this reactor a productivity of 4.6 gL-1h-1 was obtained.

Cell recycle technique is another approach to increase cell concentration in the reactor and improve reactor productivity (Cheryan, 1986). Using this approach, reactor productivities up to 6.5 gL-1h-1 (as compared to <0.5 gL-1h-1 in batch fermentation) have been achieved in the butanol fermentation (Afschar et al., 1985; Pierrot et al., 1986). In a similar approach, Mulchandani and Volesky (1994) used a single-stage spin filter perfusion bioreactor in which a maximum productivity of 1.14 g L-1 h-1 was obtained; however, the ABE concentration fluctuated over time.

Nafion has been the workhorse PEM of choice for PEFCs and DMFCs for the past 20 years. Its structure is shown in Figure 9.4. While well-suited for use in PEFCs run on hydrogen and oxygen, Nafion is not well-suited for use in DMFCs in large part due to methanol permeability. Efforts to develop a more appropriate PEM for use in DMFCs continues. The ideal membrane is impermeable to methanol, allows facile proton conduction, has good ionic conductivity, can operate over a wide variety of temperatures (e. g., >100°C), and is mechanically and chemically robust. Efforts to develop PEMs appropriate for DMFCs fall roughly into two categories, one focused with developing entirely new PEM materials, the other focused on tailoring the properties of Nafion [24,55,56].

[(CF2CF2)n-CF2CF-]

(OCF2CF-)m ocf2cf2so3h

CF3

FIGURE 9.4 Chemical structure of Nafion where m is usually 1 and n varies from 6 to 14.

Aquaponics is the joining together of two food-producing systems, aquaculture (food fish farming) and hydroponics (soilless vegetable farming). When these two systems are joined, they form a symbiotic relationship with each other (each benefits from the other). Fish breathe in the same water in which they eliminate, creating an overabundance of ammonia waste and a deficiency of oxygen. If the oxygen is not replaced and the ammonia waste not removed, the fish will die. Using the effluent from the fish tanks to grow plants does two things: first, the plants remove the nitrogenous wastes from the water through their roots and use it for growth, second, the clean water is then oxygenated and returned to the fish tank. The only nutrient input into the system is fish feed. The dimensions for the concrete grow-out tank are 41 high, 201 wide and 601 long. This tank has the capability to produce 1 metric ton of fish weekly (2200 pounds), and will carry an average of 18,000 pounds of fish at all stages of growth. The tank is a modified raceway, which is folded back on itself, i. e., it is a 101 wide raceway folded back, which now makes it 201 wide with a divider in the middle. The water flow is straight through. The return from the grow beds enters on the right side of the tank and flows all the way around to exit on the left side, carrying solid wastes with it. A 21 1 recess in the tank floor on the left side allows the solids to accumulate and be pumped to the grow beds. Insulated plumbing will connect the tanks to the aquaponic grow beds in other parts of greenhouse. These beds use pea gravel as a growing medium and measure 41 x 8′ x 1, and are elevated to hip height, eliminating stooping. These beds are required to provide adequate biofiltration for the fish tank and will provide approximately 9888 square feet of plant growing area. The surface of the gravel will provide growing space for nitrifying bacteria, which convert the fish wastewater to a useable form for the growing plants to absorb. The growing beds, therefore, act as a biofilter to cleanse the water for the fish and the fish provide nutrients for the plants, which are so stimulating to the plants that days-to-maturity are often reduced by 1/3 to 1/2. The grow beds are flooded to 11 ‘ beneath the top surface of the gravel every hour for 3-5 minutes. The water is then drained by gravity into the sump tanks and pumped back into the fish tank. Project research has not discovered any explanation for this astound­ing growth rate, so it remains a mystery. However, empirical evidence is very real as observed by the effect on field-grown red raspberries (the reader is invited to see picture documentation on pantry homepage at: www. oneaccordfoodpan- try. org and specific weeds, i. e., Queen Anne’s Lace — or wild carrot, nettles, and goldenrod — which attained heights of approximately 9. The effect was also evident on strawberries which reached hipheight and had stems as thick as one’s little finger.

If the caloric value of the unconverted synthesis gas is too low for (direct) combustion in a gas turbine, this could be compensated for by cofiring natural gas. Besides raising the heating value of the gas, the application of natural gas can also increase the scale, thermal efficiency, and economics of the gas turbines.

Natural gas can also be applied as cofeeding in the entire process. Or, vice versa, the large scale of existing methanol production units could be utilized by plugging in a biomass gasifier and gas make-up section. The product can be considered partially of biomass origin.

Black Liquor Gasification

Pulp and paper mills produce huge amounts of black liquor as a residue. They are the most important source of biomass energy in countries such as Sweden and Finland, representing a potential energy source of 250-500 MW per mill. As modern kraft pulp mills have a surplus of energy, they could become key suppliers of renewable fuels in the future energy system, if the primary energy in the black liquor could be converted to an energy carrier of high value.

Ekbom et al. (2003) have evaluated the production of methanol and DME (see below) from black liquor gasification (BLGMF process). This scheme could be realized against reasonable costs, if heat recovery boilers, which economic life has ended, are replaced by BLGMF. Using black liquor as a raw material for methanol/DME production would have the following advantages:

1. Biomass logistics are extremely simplified as the raw material for fuel making is handled within the ordinary operations of the pulp and paper plant.

2. The process is easily pressurized, which enhances fuel production efficiency.

3. The produced syngas has a low methane content, which optimizes fuel yield.

4. Pulp mill economics becomes less sensitive to pulp prices as the eco­nomics are diversified with another product.

5. Gasification capital cost is shared between recovery of inorganic chem­icals, steam production, and synthesis gas production.

Other Biofuels via Gasification

Gasification, gas cleaning, and make-up are important parts of the process to make methanol from biomass. These parts are also key to the production of hydrogen and Fischer-Tropsch liquids from biomass. Development of methanol from biomass thus offers synergy with development of hydrogen and Fischer — Tropsch liquids. Methanol can also be an intermediate in the production of other renewable fuels such as synthetic diesel, gasoline, and dimethyl ether.

Hydrogen

The production of hydrogen from synthesis gas is somewhat simpler and cheaper than the production of methanol. The gasification step should aim at maximizing the hydrogen yield, which can be further increased by reforming any methane left and a water-gas-shift reaction. Hydrogen separation takes place by pressure swing adsorption or (in future) membranes.

Hydrogen is already produced at large scale in the chemical and oil industry. It is often seen as the future fuel for the transportation sector and households.

With regard to feed coproducts, there is a need to diversify the markets for DDGS. Research aimed at modifying the amino acid composition, protein composition, and phosphorous content of DDGS should result in higher quality, consistent composition of feed ingredients and encourage expanded use of DDGS in poultry and swine rations. Nonfeed uses for DDGS have also been developed, prompted by the increased availability of DDGS resulting from increased ethanol produc­tion. Deicers, cat litter, and lightweight “ag-fiber” shipping containers can be produced from DDGS. DDGS could also be used to produce biogas, which could be recovered and used on-site to fuel the plant [26]. A plant fueled in this way would be less dependent on feed selling prices and power and natural gas purchase prices.

Since the 1980s, there has been increased interest in low concentration blending of ethanol with diesel fuel. Ethanol/diesel blends are commonly referred to as E — diesel. They generally contain from 10% to 15% ethanol and are used for many of the same reasons that ethanol/gasoline blends are used (decreased petroleum need and decreased emissions). Ethanol and diesel blending is more complicated than ethanol/gasoline blending, because of the low solubility of ethanol in diesel at low temperatures and the high flammability. At temperatures below 10°C, ethanol and diesel will separate [39]. The solution is either to add an emulsifier or a cosolvent. Boruff et al. has shown that approximately 2% surfactant (emul­sifier) is needed for every 5% of ethanol added to diesel fuel (40). The addition of the surfactant to the ethanol/diesel blend led to transparent solutions with no visible separation down to -15.5°C (40). Ethyl acetate has been studied as a co­solvent. Researchers have shown that adding 2.5% ethyl acetate for every 5% ethanol will ensure no separation down to 0°C (41). Cosolvents have been more popular than surfactants. The second issue with e-diesel is the increased risk of fire and explosions compared to plain diesel fuel. The National Renewable Energy Laboratory recommends solving this problem by equipping all fuel tanks with vents, better electrical grounding, and employing safer fuel tank level detectors (42). The physical properties of E-diesel compared to ethanol and diesel are shown in Table 7.5.

Blending ethanol with diesel fuel decreases emissions in a similar way to ethanol/gasoline blends. E-diesel has achieved reported 20% to 30% decreases

in carbon monoxide emissions and 20% to 40% decreases in particulate matter emissions (43). Miyamoto et al. showed that these improvements in emissions depend directly on the oxygen content (44). However, minimal decreases in NOx emissions have been reported (43) and an increase in hydrocarbon emissions have been reported (45-46). Table 7.6 shows the vehicle emissions from the use of 10% and 15% ethanol in diesel.

As far as engine use is concerned, the decrease in fuel viscosity and lubricity have been investigated for ethanol blends with diesel, but they do meet diesel specifications (42). Materials compatibility has also been investigated. E-diesel was found to have similar corrosive properties to typical diesel (42).

It is important to note that E-diesel fleet demonstrations have shown no fire or explosions incidents and no mechanical failures associated with the fuel system 43). Many studies of engine wear have been conducted and have shown no abnormal wear or deterioration due to the blending of ethanol with diesel at low concentrations (10-15%). E-diesel does shows a reduction in engine power, but this reduction is small and equivalent to the reduction in energy content of the ethanol versus diesel (39). The main engine performance issue with E-diesel is the leakage of fuel from the fuel injection pump due to slight decrease in viscosity of the blended fuel. Studies of engine power loss have shown decreases in power from 4% to 10% for ethanol/diesel blends ranging from 10% to 15% ethanol (46-48). Therefore, ethanol is a good choice as an oxygenate for diesel. It has minimal effect on engine power while dramatically decreasing particulate matter and carbon monoxide emissions.

TABLE 7.6

Vehicle Emissions from the Use of 10% to 15% Ethanol in Diesel

Emissions Range
(% ratio of blend/diesel)

73-80 59-70 96-100 95-100 80-160 73-140 171-200 175-210

Source: Data compiled from Hansen, A. C., Lye, P. W. and Zhang, Q., ASAE Paper No. 01-6048, Aug. 2001; Waterland, L. R., Venkatesh, S. and Unnasch, S., NREL/SR-540-34817, Sept. 2003; Spreen, K., Final Report for Pure Energy Corporation prepared at SRI, San Antonio, TX, 1999; Kass, M. D., Thomas, J. F., Sto­rey, J. M., et al., SAE Technical Paper 2001-01-2018, 2001.

CONCLUSIONS

Ethanol can be blended with gasoline to produce an oxygenated fuel with lower hydrocarbon emissions. Ethanol can also be blended with diesel to decrease carbon monoxide emissions and particulate matter emissions. Although green­house gas emissions are decreased with ethanol-blended fuels, emissions of certain aldehydes are increased, which could cause health issues. Automobiles can be operated on ethanol/gasoline blends from 5% to 25% and ethanol/diesel blends from 10% to 15% without need for any alterations in engine equipment or settings and with no effect on engine lifetime.

Sabina Topcagic, Becky L. Treu, and Shelley D. Minteer

Department of Chemistry, Saint Louis University, Missouri

CONTENTS

Introduction…………………………………………………………………………………………………… 216

Portable Electrical Energy Sources………………………………………………………………… 216

Batteries……………………………………………………………………………………………. 216

Fuel Cells…………………………………………………………………………………………… 217

Biofuel Cells……………………………………………………………………………………… 218

Enzyme Immobilization Techniques…………………………………………………………….. 220

Wired Technique………………………………………………………………………………. 221

Sandwich Technique………………………………………………………………………… 222

Entrapment Technique…………………………………………………………………….. 222

Nafion® Modification……………………………………………………………………… 223

NAD+-Dependent Alcohol Dehydrogenase Biofuel Cells…………………. 225

PQQ-Dependent Alcohol Dehydrogenase Biofuel Cells……………………. 226

Membraneless Biofuel Cells……………………………………………………………… 227

Conclusions…………………………………………………………………………………………………… 229

References…………………………………………………………………………………………………….. 230

Abstract There are three types of batteries: primary, secondary, and fuel cells. A fuel cell is an electrochemical device that converts chemical energy into electrical energy via catalysts. Fuel cells have many advantages over the two other types of batteries due to the fact they can be regenerated with the addition of fuel specific to the system. Traditional fuel cells employ heavy metal or precious metal catalysts, whereas biofuel cells employ biological catalysts (enzymes). Enzymes are highly specific catalysts, so they allow for the simplifi­cation of the fuel cell by eliminating the need for a polymer electrolyte membrane, which is one of the mostly costly parts of a fuel cell. Dehydrogenase enzymes have been employed at the anode of biofuel cells to oxidize alcohols. Methanol, ethanol, propanol, and butanol are examples of alcohols that can be used in biofuel cells. Long-term goals include investigating a variety of power applications for this technology ranging from portable electronics to sensors.

INTRODUCTION

Previous chapters of this book detail methods for producing ethanol from agri­cultural products and biomass. Although many of these methods are efficient, it is crucial to be able to efficiently convert energy to electrical power. As detailed in an earlier chapter, researchers have been attempting to develop direct ethanol fuel cells (DEFCs), but there have been problems because traditional precious metal catalysts (Pt-based catalysts) are unable to efficiently catalyze the oxidation ethanol and maintain an electrode with minimal fouling at low temperatures. However, living organisms are capable of efficiently catalyzing the oxidation of ethanol at 20-40°C. Living organisms, such as Pseudomonas aeruginosa [1], acetobacter [2], and gluconobacter [3], contain enzymes that can oxidize a variety of alcohols, including ethanol. Over the last 40 years, researchers have been working on employing living organisms and/or their enzymes in a fuel cell to convert chemical energy to electrical energy. This type of battery or fuel cell is referred to as a biofuel cell. The early research was plagued with enzyme stability problems and low power densities, but those issues are being overcome with current research. In this chapter, we will discuss the brief history, techniques, and applications of alcohol-based biofuel cells.

The Food Pantry has established a Web presence. During construction it will be possible to tape record and follow along step by step and document just how the greenhouses were sited, erected, equipped, and operated. As such, this will be a soup-to-nuts type of educational experience, giving students an in-depth picture of the entire project. The Website will be upgraded to an interactive site with periodic live teaching segments — broadcast directly from inside any of the outbuildings live over the Internet through streaming video — which will enable viewers to ask questions in real time. The segments will be archived for down­loading as a reference and offered for sale in CD format for a modest fee. In this way, the project can offer 1/2-hour teaching segments on every aspect of the system and from every location within the system through the use of a Web cam. Land lines will connect all the external buildings to project offices and computers. Everything then can be broadcast live over the Internet. A certain amount of upgrading will be required to make the offices suitable for this purpose.

The project will be promoting the use of this system worldwide, and modified to meet almost any climatic conditions. Phase 2 of this project will address the adaptation of this system to differing worldwide climatic needs, and other sources of renewable and sustainable fuels to power the project according to those needs. Through the Web page and e-mail, the project would be able to act as a resource to anyone anywhere in the world. Upon completion, it will be a powerful learning tool.

Ethanol (also known as ethyl alcohol) is the most common of alcohols. It is the form of alcohol that is in alcoholic beverages and is easily produced from com, sugar, or fruits through fermentation of carbohydrates. Its chemical structure is CH3CH2OH. It is less toxic than methanol. The LD50 for oral consumption by a rat is 7060 mg/kg [5]. The LD50 for inhalation by a rat is 20,000 ppm for 10 hours [6]. The NIOSH recommended exposure limit is 1000 ppm for 10 hours [7]. Ethanol is available in a pure form and a denatured form. Denatured ethanol contains a small concentration of poisonous substance (frequently methanol) to prevent people from drinking it. Ethanol is a colorless liquid with a melting point of -144°C and a boiling point of 78°C. It is less dense than water with a density of 0.789 g/ml and soluble at all concentrations in water. Ethanol is frequently used to form blended gasoline fuels in concentrations between 10-85%. More recently, it has been investigated as a fuel for direct ethanol fuel cells (DEFC) and biofuel cells. Ethanol was deemed the “fuel of the future” by Henry Ford and has continued to be the most popular alcoholic fuel for several reasons: (1) it is produced from renewable agricultural products (corn, sugar, molasses, etc.) rather than nonrenewable petroleum products, (2) it is less toxic than the other alcohol fuels, and (3) the incomplete oxidation by-products of ethanol oxidation (acetic acid (vinegar) and acetaldehyde) are less toxic than the incomplete oxi­dation by-products of other alcohol oxidation.

• Methanol fuel cells: transportation, stationary, and portable power.

• Production of biodiesel.

• Sewerage treatment denitrification.

• Fuel for standby turbine electric generators.

• Pulp and paper bleaching replacing chlorine.

• Stock-car racing fuel.

• Replacement for diesel fuel for stationary diesel engines.

• Converting methanol to hydrogen. Eliminating storage and trans­portation.

• Fuel additives for diesel fuel. Preventing gelling and fuel line freeze.

• Intermediate for production of dimethyl ether, acetic acid, and form­aldehyde.

• Environmental cleanup of perchlorate at military installations.

All referenced data in this chapter comes from the U. S. Environmental Pro­tection Agency (http://www. epa. gov).