In liquid-phase processes (Cybulski 1994; USDOE 1999), the heat transfer between the solid catalyst and the liquid phase is highly efficient, and therefore the process temperature is very uniform and steady. A gas phase delivers reactants to the finely divided catalyst and removes the products swiftly. This allows high conversions to be obtained without loss of catalyst activity. The higher conversion per pass (compared to fixed-bed technology) eliminates the need for a recycle loop, which implies less auxiliary equipment, fewer energy requirements, smaller volumetric flow through the reactor (Katofsky 1993). An additional advantage is the ability to withdraw a spent catalyst and add a fresh catalyst without interrupt­ing the process.

Different reactor types are possible for liquid-phase methanol production, such as fluidized beds and monolithic reactors. Air Products and Chemicals, Inc. invented a slurry bubble column reactor in the late 1970s, which was further developed and demonstrated in the 1980s and 1990s. From 1997 to 2003, a 300- tonne/day demonstration facility at Eastman Chemical Company in Kingsport, TN produced about 400 million liters methanol from coal via gasification (Hey — dorn et al. 2003).

In the slurry bubble column reactor (Figure 2.6), reactants from the gas bubbles dissolve in the liquid and diffuse to the catalyst surface, where they react. Products then diffuse through the liquid back to the gas phase. Heat is removed by generating steam in an internal tubular heat exchanger.

Commercial Cu/Zn/Al catalysts developed for the two-phase process are used for the three-phase process. The powdered catalyst particles typically measure 1 to 10 pm and are densely suspended in a thermostable oil, chemically resistant to components of the reaction mixture at process conditions, usually paraffin. Catalyst deactivation due to exposure to trace contaminants is a point of concern (Cybulski 1994).

Conversion per pass depends on reaction conditions, catalyst, solvent, and space velocity. Experimental results show 15-40% conversion for CO rich gases and 40-70% CO for balanced and H2 rich gases. Computation models predict future CO conversions of over 90%, up to 97% respectively (Cybulski 1994; Hagihara et al. 1995). Researchers at the Brookhaven National Laboratory have developed a low temperature (active as low as 100°C) liquid phase catalyst that can convert 90% of the CO in one pass (Katofsky 1993). With steam addition the reaction mixture becomes balanced through the water gas shift reaction. USDOE claims that the initial hydrogen to carbon monoxide ratio is allowed to vary from 0.4 to 5.6 without a negative effect on performance (USDOE 1999).

The investment costs for the liquid-phase methanol process are expected to be 5-23% less than for a gas-phase process of the same methanol capacity. Operating costs are 2-3% lower; this is mainly due to a four times lower electricity consumption (USDOE 1999).

A laboratory process to ferment highly concentrated mash, with greater than 30% solids, has been developed [38]. Very-high-gravity (VHG) fermentations produce 21 to 23% ethanol under optimal fermentation conditions. The VHG process requires a mash with high sugar concentration and low viscosity, which can be achieved by adding enzymes (e. g., proteases, glucanases, and amylases) or by double mashing, in which the solids are removed from an initial mash, and the liquids are used to prepare the second (VHG) extract. VHG fermentations use less water than conventional fermentations. Energy costs also decrease because there is less volume to cool for fermentation and then distill.

In 2000, 29.9 billion liters of ethanol were produced worldwide (13). The majority of the production comes from Brazil and the United States. In 2003, 2.8 billion gallons of ethanol were produced in the United States alone (2). Production in 2005 is expected to be approximately 4.0 billion gallons of ethanol (8). The top four producers of ethanol are Iowa (575 million gallons per year), Illinois (523 million gallons per year), Minnesota (486 million gallons per year), and Nebraska (454 million gallons per year). These four states produce approximately 72% of the total ethanol for the United States. The demand for ethanol is approximately divided into 68% fuel, 21% industry, and 11% food and beverages (3). Over 95% of the fuel ethanol produced in the United States was used to make E10, however, a small portion is used for the ever-increasing E85 market.

In comparison, 120 billion gallons of gasoline are sold in the United States each year (2) while only 2.8 billion gallons of ethanol are produced, so the United States does not produce enough ethanol for all gasoline sold to be E10 (maximum ethanol concentrations allowed by the U. S. Environmental Protection Agency) (4). Currently, E10 represents 8% to 10% of the total gasoline sales in the United States (4). This ethanol production shortage is likely to be a major problem as MTBE is phased out and there is more demand for ethanol as an oxygenate. This increase in demand will likely result in a dramatic increase in production of ethanol in the United States.

Ethanol is a controversial fuel. The Renewable Fuels Association states that the ethanol fuel market adds $4.5 billion to farm revenue yearly, employs almost 200,000 people, and increases state tax revenue by $450 million (14). In the United States, there are four federal tax incentives for ethanol sold for fuel: (1) excise tax exemption, (2) blender’s tax credit, (3) income tax credit for businesses producing or selling ethanol, and (4) small-producers tax credit for farm co-ops. The first tax benefit is $0.52 per gallon, but the fourth tax benefit is only $0.10 per gallon. Over 30 states have also implemented tax incentives for ethanol as fuel. Most range from $0.20 to $0.40 per gallon. Although many argue assump­tions and data, researchers at Cornell University have calculated that a gallon of ethanol requires 29% more energy to produce than it contains as fuel (15). It has also been argued that ethanol production increases environmental degradation, because corn causes more soil erosion than any other farm crop (15). Although soil erosion is an issue, the environmental impacts of ethanol are considerably less than the toxic MTBE. The latest results from the U. S. Department of Agri­culture contradict researchers at Cornell University and show that corn ethanol is energy efficient and contains 34% more energy than is required to produce ethanol (16). Part of this dramatic increase in energy efficiency is due to lower energy use in the fertilizer industry and advances in fuel conversion technology over the last decade (16). Similar energy efficiency data has been shown by several other researchers (16-19).

Cost of production of ethanol is a function of plant location, feedstock, production scale, and end use. The choice of feedstock depends on the country. Brazil has used sugar cane as their primary feedstock. France has attempted to use Jerusalem artichokes, but later found that sugar beets and wheat were better for ethanol production. Sweden uses its surplus of wheat to produce the ethanol for their 6% ethanol-blended gasoline. However, in the United States, corn has been determined to be one of the best feedstocks. Approximately, 2.5 gallons of ethanol are produced from every bushel of corn (16), but the corn yield per acre varies as a function of state, along with the fertilizer and irrigation needs in that region. In a corn-based ethanol industry, the cost of the corn is approximately 50-60% of the cost of production of the ethanol (20). It is predicted that the cost of production of ethanol will decrease by $0.11 per liter over the next 10 years due to genetic engineering (21). Currently, the cost of production of ethanol from corn is $0.88 per gallon versus $1.50 gallon from cellulose-based biomass (22). As gas prices rise, the cost of ethanol and ethanol blends becomes more com­petitive with gasoline.

A single-chamber solid-oxide fuel cell (SC-SOFC), which operates using a mix­ture of fuel and oxidant gases, provides several advantages over the conventional double-chamber SOFC, such as simplified cell structure (no sealing required) and direct use of hydrocarbon fuel [15,16]. Figure 11.5 shows a schematic diagram of SC-SOFC operation. The oxygen activity at the electrodes of the SC-SOFC is not fixed and one electrode (anode) has a higher electrocatalytic activity for the oxidation of the fuel than the other (cathode). Oxidation reactions of a hydrocarbon fuel can be represented with a simplified multistep, quasi-general mechanism as follows:

On the other hand, the cathode has a higher electrocatalytic activity for the reduction of oxygen according to the reaction:

1/2O2 + 2e- ^ O2- (11.4)

These reactions lead to a low oxygen partial pressure at the anode locally, while the oxygen partial pressure at the cathode remains relatively high. As a

Furnace thermocouple

———— •

FIGURE 11.5 Schematic diagram of a single-chamber solid-oxide fuel cell operating with a mixture of fuel and air.

result, an electromotive force (emf) between two electrodes is generated with a mixed fuel and air mixture. Due to the presence of oxygen at the anode, SC-SOFC is not affected by the problems associated with carbon deposition, which is a significant drawback for double-chamber SOFCs when Ni-cermet is used as anode material.

The fuel/air mixtures for SC-SOFC were generally chosen to be richer than the upper explosion limits, yet they were fuel-lean enough to prevent the carbon deposition, which has been a significant problem in double-chamber SOFCs [17]. However, variations in the ratios of the local fuel-air mixture were also dependent on catalytic activity and test conditions that affect the performance of the fuel cell [15]. An ideal SC-SOFC has the same open circuit voltage (OCV) and I-V output as a double-chamber cell, given a uniform oxygen partial pressure. The difference in catalytic properties of the electrodes must be sufficient to cause a significant difference in oxygen partial pressure between the anode and the cathode. For the ideal SC-SOFC, one electrode would be reversible toward oxygen adsorption and inert to fuel, while the other electrode would be reversible toward fuel adsorption and completely inert to oxygen [18]. If the electrode materials are not sufficiently selective, a parasitic reaction creates mixed poten­tials at the electrodes, which reduces the efficiency of the cell. Compared to traditional double-chamber fuel cells, parasitic reactions in a single-chamber fuel cell have historically reduced the OCV by about half. This is analogous to a leak that allows the fuel to seep into the oxidizer side of a double-chamber fuel cell [19]. Advances in electrode catalyst materials for SC-SOFC may lead to a similar performance as a conventional double-chamber SOFC with a substantial reduction in complexity and cost of the fuel cell.

Significant improvement in the performance of single-chamber solid-oxide fuel cells has been achieved in recent years [15, 20-22]. Since SC-SOFC does not require high-temperature sealing materials to prevent the mixing of fuel gas and oxygen at operation temperatures, it offers a robust and more reliable alter­native to double-chamber SOFC for special applications. As further advances are made toward controlling the catalytic activity of electrode materials, electrolyte resistance particularly at lower operating temperatures, optimizing of the gas flow rate and the cell configuration, SC-SOFCs may find widespread implementation as compact power sources in the future.

Several recent studies on the development of SC-SOFCs have been conducted in our laboratory to improve their performance and understand complex electrode reactions [20,23-26]. Initial experiments were carried out using fuel cells pre­pared by deposition of YSZ thin-film electrolytes (1-2 pm thickness) on the NiO-YSZ anode as a substrate with (La, Sr)(Co, Fe)O3 (LSCF) as the cathode (Figure 11.6). A power density of 0.12 W cm-2 was obtained at an OCV of >0.8 V using a methane-air gas mixture as a fuel [23].

In another study, the effect of mixed gas flow rates on the performance of SC-SOFCs has been investigated using a cell that consists of a 18-pm thickYSZ porous electrolyte on a NiO-YSZ anode substrate with a (La, Sr)(Co, Fe)O3 cathode. Higher gas flow rates led to an increase of cell temperature due to

increasing methane reaction rate, which resulted in improved cell performance. Figure 11.7 shows that optimization of gas flow rate (linear velocity) lead to a decrease of the operating temperature effectively and increased cell performance as well as fuel efficiency. At a cell temperature of 744°C (furnace temperature: 606°C), an OCV of ~0.78 V and a maximum power density of ~660 mW cm-2 (0.44 V) were obtained. The results indicated that a porous ion-conducting mem­brane provides sufficient separation of oxygen activity at the electrodes by selec­tion of an optimum operation temperature and a gas flow rate. Thus, it appears that SC-SOFCs with porous electrolyte provide opportunities to design thermally and mechanically more robust stacks by utilizing hydrocarbon fuels. It also allows fabrication of the cells at lower temperatures using conventional processing tech­niques such as screen printing, since densification of the electrolyte at high sintering temperatures is not required.

Through normal daily operations, the project will generate a large quantity of high-nutrient-level organic matter, i. e., vegetable trimmings, ash, leaves, grass clippings, and fish offal not suitable for feed. This material will be composted in specially designed containers that conserve nutrients. It will then be tested and adjusted for nutrient balance and ph, and used in separate greenhouses for the production of root crops. Fish tank effluent will be used to irrigate these crops as well. Since the effluent will not return to the tanks in this type of system, it will be replaced with fresh water. However, the plant growth stimulating effect of the fish effluent will still be in force, resulting in vigorous growth of the plants.

Methanol (chemical formula CH3OH and also known as methyl alcohol or wood alcohol) is a clear, colorless liquid that is water soluble and readily biodegradable. Methanol occurs naturally in the atmosphere as a by-product of biomass and landfill decomposition. As an industrial product, methanol is a liquid petrochem­ical that can be made from renewable and nonrenewable fossil fuels containing carbon and hydrogen. Since natural gas costs account for the major portion of the operating costs of domestic methanol producers and are followed in impor­tance by distribution costs and operating costs, virtually all new methanol pro­duction has been moved offshore near low cost or “stranded” natural gas locations.

Large world-scale or megamethanol plants are being built in these stranded gas areas, such as Chile, Trinidad, Qatar, Equatorial Guinea, and Saudi Arabia.

Each plant can produce 300 million gallons annually of methanol and costs more than $350 million to construct. The construction of these megaplants has rein­forced the characterization of methanol as a “typical” commodity as cycles of oversupply resulting in lower prices and idled capacity are followed by periods of shortage and rapidly rising prices as demand catches up and exceeds supply until increased prices justify new plant investment.

Prior to the 1980s, methanol was produced and consumed locally in North America primarily as an intermediate feedstock for derivatives such as formal­dehyde, acetic acid, and plastics recycling in packaging. Limited international trade was seen mostly through U. S. exports. U. S. natural gas feedstock, at that time, was reasonably competitive. Today methanol is a global commodity and the United States has gone from the position of largest producer in the world, to the largest net importer in less than a decade.

The American Methanol Institute, the trade organization for the methanol industry, has changed its name to the Methanol Institute and predicts that there will be no U. S.-produced methanol within five years due primarily to high and volatile natural gas prices. With the controversy surrounding methyl-tertiary — butyl-ether (MTBE), significant demand for methanol has disappeared from the domestic market. While it was widely predicted that methanol prices over the past several years would be extremely weak, the recovery of the international economy and demand from China and India have virtually made up for the rapid decline for the demand of methanol for MTBE.

MTBE was developed in the early 1990s as an oxygenate to improve more complete combustion of gasoline. This was done at the behest of the EPA. However, leaking underground storage tanks at automobile service stations caused ground water contamination and MTBE was found to be a possible carcinogen. As a result, California, New York, and several other states have banned MTBE. It has largely been replaced by ethanol as an oxygenate for gasoline.

Ethanol production depends on fermentation of simple sugars by microorganisms. The yield of potentially fermentable sugars from the conversion process is the critical response variable in assessing the value of alfalfa as an ethanol production feedstock. Potentially fermentable sugar yield is a function of both carbohydrate composition and concentration (discussed earlier), and the efficiency with which the cell wall polysaccharides are converted to simple sugars through processing. The results of two pretreatment methods have been reported previously. Ferrer et al. (2002) described parameters of ammonia processing of whole dried alfalfa hay that influenced the susceptibility of the fiber to subsequent enzymatic hydrol­ysis. The ammonia loading, moisture, time and temperature of treatment were varied and then the treated material digested with a mixture of cellulase, cello — biase, and xylanase. Conditions of 2 g ammonia g-1 DM, with 30% moisture and processing at 85°C for five minutes was shown to convert 76% of the theoretical yield of reducing sugars in the fiber. Approximately 200 mg sugars g-1 DM was obtained (Ferrer et al., 2002); however, the yield of ethanol produced from this material remains to be determined.

Liquid hot water (LHW) pretreatments of the fiber fraction obtained after wet fractionation of alfalfa have been optimized for maximum sugar conversion (Sreenath et al., 1999) and ethanol production (Sreenath et al., 2001). The LHW pretreatment was found to solubilize hemicellulose, and the resulting extract contained significant amounts of acetic acid and formic acid (Sreenath et al., 1999). The remaining fiber fraction (raffinate) when treated with cellulase released 59 g of reducing sugars from 100 g of substrate. Addition of dilute acid (0.07% sulfuric acid) to the LHW decreased the amount of reducing sugars released by cellulase treatment to 24 g 100 g-1 substrate (Sreenath et al., 1999). Fermentation of the raffinate fraction after LHW pretreatment was tested with two strains of Candida shehatae in a simultaneous saccharification and fermentation (SSF) process as well as a separate hydrolysis and fermentation (SHF) process (Sreenath et al., 2001). The yield of ethanol was 0.45 g ethanol g-1 sugar with SSF and 0.47g ethanol g-1 sugar with SHF. The extract from the LHW pretreatment was also used in fermentation experiments and was poorly fermented, most likely due to the presence of organic acids. Addition of dilute acid to the LHW treatment resulted in fractions that were poorly fermented. Although untreated fiber sub­strate was shown to yield 51 g reducing sugars from 100 g of substrate (Sreenath et al., 1999), the yield of ethanol by SHF and SSF was 0.25 and 0.16 g ethanol g-1 sugar, respectively (Sreenath et al., 2001). These experiments demonstrate the impact of pretreatment on saccharification and ethanol production as well as the requirement to optimize processes for each lignocellulosic feedstock.

Ethanol has long been used in racing because of its desirable properties for increasing engine power output. E85, too, increases the power and torque capa­bility of engines compared with gasoline. Most spark-ignited engines used in on­road vehicles operate with air to fuel mixtures at or near the stoichiometric condition. Since the stoichiometric air to fuel ratio of E85 is less than gasoline, engines operating on E85 can use about 1.48 times more E85 for the same amount of air. Remembering that about 1.4 times more E85 is required to equal the energy of gasoline on a volume basis, this leads to about a 6-7% increase in power.

Since E85 burns cleaner and the engine spark timing can be advanced due to the increased octane number, an engine operating on E85 can actually achieve higher increases in power.

Additionally, E85 has a higher heat of vaporization than gasoline. This is important in spark-ignited engines as the fuel is inducted into the intake manifold. As the fuel vaporizes due to the heat of the engine, it displaces air, reducing the ability of the engine to draw in fresh air. This reduces the volumetric efficiency of the engine, reducing the power. By increasing the heat of vaporization, E85 increases the engine volumetric efficiency, allowing more air to be drawn into the engine. This additional air allows the engine to use additional fuel, leading to increased power.

Further, an engine can be designed with a higher compression ratio to take full advantage of the increased octane of E85. All of these factors can be combined to increase power by more than 25% compared with the same-sized gasoline engine.

More recent research in developing long-term stability in biofuel cell systems has focused on studying a new enzymatic system. Initial studies have been successful in utilizing PQQ-dependent ADH as a catalyst at the anode of a biofuel cell. Pyrroloquinoline quinone (PQQ)-dependent alcohol dehydrogenase (ADH) has been chosen to replace NAD+-dependent ADH in order to extend the lifetime and simplicity of the fuel cell. PQQ is the coenzyme of PQQ-dependent ADH, and it remains electrostatically attached to PQQ-dependent ADH; therefore, the enzyme and coenzyme will remain in the membrane leading to an increased lifetime and activity for the biofuel cell. Also, PQQ-dependent ADH possesses desirable electrochemistry (it has the ability to transition between its oxidized and reduced state). The coenzyme PQQ has quasireversible electrochemistry and a low overpotential at an unmodified carbon electrode. This eliminates the need for an electrocatalyst layer, thereby simplifying the process of forming high — surface-area bioanodes.

PQQ-dependent ADH bioanodes fabricated and tested by Treu et al. using the same procedures and methods as for the NAD+-dependent ADH system had desirable results. The PQQ-dependent ADH enzymatic system has shown life­times of greater than 1 year of continuous use, open circuit potentials of 1.0 V and power densities of up to 3.61 mW/cm2 for a 1.0 mM ethanol solution at room temperature in a static system [25].

The PQQ-dependent ADH-based biofuel cell outperformed the NAD+-depen — dent ADH-based fuel cell in all aspects of traditional systems. For the traditional system of PQQ-dependent ADH anode coupled with a traditional platinum cath­ode, the PQQ-based enzymatic system increased the overall lifetime by greater than 711%, increased open circuit potential by 67%, and increased power density by 251% [25]. There was no correlation between power and different pHs for the PQQ-dependent ADH biofuel cell [26].

PQQ-dependent ADH has optimum selectivity for ethanol, but will oxidize other alcohols. Similar fuel cells have been developed for methanol, butanol, and propanol using PQQ-dependent ADH bioanodes. The performance of those fuel cells is shown in Table 12.3. PQQ-dependent ADH is a desirable substitute for a biocatalyst at the anode of a biofuel cell. Eliminating the poly(methylene green) layer simplifies the fabrication of a bioanode and lowers the IR drop leading to an increase in performance. Research has shown that PQQ-dependent ADH enables simple and timely electrode fabrication, along with impressive open circuit potentials, current densities, power densities, and lifetime for a complete ethanol/oxygen biofuel cell.

The U. S. Department of Energy’s Annual Energy Review, published September 7, 2004, provides a wealth of statistics and research into the use, production, and availability of energy throughout the world. The “holy grail” of alternative energy

FIGURE 15.1 Energy consumption by source. Source: Department of Energy, Energy Information Administration: Annual Energy Review, 2003. With permission.

sources is to address the use of petroleum since it accounts for 39.4% of energy consumption followed by coal and natural gas, each accounting for 23.2% (Figure 15.1) [1].

The presidential prime-time address was not purely political; the United States is by far the largest user of petroleum, followed by Japan whose consumption is quickly being surpassed by China’s budding development (Figures 15.2 and 15.3) [1].

The United States’ geographic nature of dispersed cities and urban centers has created the reliance on personal transportation and thus the leading consump­tion of petroleum. Despite efforts to institute mass transit, there has been little or no effect on the increasing use of this form of energy. The current adminis­tration believes the answer to be hydrogen fuel cells, stating the desire for a child born in 2003 to have a fuel-cell vehicle as their first car (January 28, 2003). However, there are nearer-term opportunities, as described in this book, which should be the focus of the country’s efforts.