In dry/hot gas cleaning, residual contaminations are removed by chemical absor­bents at elevated temperature. In the methanol process, hot gas cleaning has few energy advantages as the methanol reactor operates at 200-300°C, especially when preceding additional compression is required (efficient compression requires a cold inlet gas). However, dry/hot gas cleaning may have lower oper­ational costs than wet gas cleaning (Mitchell 1998). Within ten years hot gas cleaning may become commercially available for BIG/CC applications (Mitchell 1998). However, requirements for methanol production, especially for catalyst operation, are expected to be more severe (Tijmensen 2000). It is not entirely clear to what extent hot gas cleaning will be suitable in the production of meth­anol.

Tars and oils are not expected to be removed during the hot gas cleaning since they do not condense at high temperatures. Therefore, they must be removed prior to the rest of the gas cleaning, as discussed above.

For particle removal at temperatures above 400°C, sliding granular bed filters are used instead of cyclones. Final dust cleaning is done using ceramic candle filters (Klein Teeselink et al. 1990; Williams 1998) or sintered-metal barriers operating at temperatures up to 720°C; collection efficiencies greater that 99.8% for 2-7 pm particles have been reported (Katofsky 1993). Still better ceramic filters for simultaneous SOx, NOx, and particulate removal are under development (White et al. 1992).

Processes for alkali removal in the 750-900°C range are under development and expected to be commercialized within a few years. Lead and zinc are not removed at this temperature (Alderliesten 1990). High-temperature alkali removal by passing the gas stream through a fixed bed of sorbent or other material that preferentially adsorbs alkali via physical adsorption or chemisorption was dis­cussed by Turn et al. (1998). Below 600°C alkali metals condense onto particu­lates and can more easily be removed with filters (Katofsky 1993).

Nickel-based catalysts have proved to be very efficient in decomposing tar, ammonia, and methane in biomass gasification gas mixtures at about 900°C. However, sulfur can poison these catalysts (Hepola et al. 1997; Tijmensen 2000). It is unclear if the nitrogenous component HCN is removed. It will probably form NOx in a gas turbine (Verschoor et al. 1991).

Halogens are removed by sodium and calcium-based powdered absorbents. These are injected in the gas stream and removed in the dedusting stage (Ver — schoor et al. 1991).

Hot gas desulfurization is done by chemical absorption to zinc titanate or iron oxide-on-silica. The process works optimally at about 600°C or 350°C, respectively. During regeneration of the sorbents, SO2 is liberated and has to be processed to H2SO4 or elemental sulfur (Jansen 1990; Jothimurugesan et al. 1996). ZnO beds operate best close to 400°C (van Dijk et al. 1995).

Early compression would reduce the size of gas cleaning equipment. How­ever, sulfur and chloride compounds condense when compressed and they may corrode the compressor. Therefore, intermediate compression to 6 bar takes place only after bulk removal and 60 bar compression just before the guardbed.

As stated previously, wet-milling operations have the capacity and flexibility to make more products than a dry-grind ethanol facility, because the individual parts of the kernel are fractionated. Products resulting directly from wet milling are corn oil for cooking, CO2, which may be captured and sold for carbonation of beverages, and corn gluten meal and corn gluten feed, which are sold as animal feeds. Additional products may be obtained from starch, by siphoning part of the sugar stream into alternate products. The product mix from a wet mill can be changed (within limits) in response to market conditions, and has grown to include products that were formerly synthesized by chemical processes. Alternative fer­mentation products include organic acids, amino acids, sugar alcohols, polysaccharides, pharmaceuticals, nutraceuticals, fibers, biodegradable films, sol­vents, pigments, enzymes, polyols, and vitamins [24]. The simple sugars derived from starch can also be converted enzymatically to sweeteners including high fructose corn syrup, which is the primary food use of corn in the United States.

Liquid-liquid extraction has been considered an important technique for the recovery of ABE from fermentation broths. Usually, a water-insoluble organic extractant is mixed with the fermentation broth. Butanol is more soluble in the organic (extractant) phase than in the aqueous (fermentation broth) phase. There­fore, butanol selectively concentrates in the organic phase. Since the extractant and fermentation broth are immiscible, the extractant can easily be separated from the fermentation broth after butanol extraction. Liquid-liquid extraction is able to remove fermentation products without removing substrates, water, or nutrients.

In order to improve substrate utilization and productivity, liquid-liquid extrac­tion must be integrated with butanol fermentation such that simultaneous fermen­tation and butanol removal from the fermentation broth is achieved. The choice of extractant is critical because an extractant with low partition coefficient will not be efficient in the recovery of butanol and a toxic extractant will inhibit or kill the bacterial cells. Unfortunately, most extractants with high partition coef­ficient are toxic to the clostridia. The extractant of choice among researchers has been oleyl alcohol because it is nontoxic and a good extractant as well (Evans and Wang, 1988, Groot et. al., 1990).

A general observation made while surveying the DMFC literature is the unsettling lack of replicates in many studies. Frequently, one test cell is compared to one control cell and no statistics are provided to gauge cell performance. The incom­plete reporting of test condtions further confounds comparison. While impractical for some studies, an analysis of cell performance over extended periods of time and with intermittent startup and shutdowns should be the standard. As a result of the limited data, broad conclusions drawn from these studies must be assessed with caution.

As can be seen from the breadth of topics covered in this chapter, many avenues are being pursued to make DMFC technology a practical reality. Effort is expended in the engineering of hardware and fuel delivery systems and in developing PEMs impermeable to methanol. That being said, the outlook for DMFC technology appears mixed. At this point, the development of alternative (e. g., less expensive, more effective, more robust) electrocatalysts appears to be the foremost obstacle in making DMFCs a practicable power generation alternative.

An important study by Zelenay et al. shows that use of the highly active PtRu electrocatalyst present in the great majority of DMFC anodes (see Tables 9.2 and 9.3) will contaminate the cathode with Ru [43]. The migration of Ru occurs under nearly all operating conditions, including conditions where the cell is only humid­ified with inert gases and no current is drawn from the cell. Cathode contamination with Ru inhibits oxygen reduction kinetics and reduces cathode electrocatalyst tolerance to methanol crossover. The degree to which Ru contamination occurs depends on such factors as anode potential and operating lifetime of the cell. Depending upon the degree of Ru contamination at the cathode, the associated loss of cell performance is estimated to be from as little as ~40 mV to as much as 200 mV Figure 9.6 shows data for a Pt-only cathode subject to Ru contamination.

The top plots of Figure 9.6, marked “a” and “b,” include CO stripping scans (a) and cyclic voltammograms recorded in the absence of CO (b) for a DMFC cathode made with Pt only as the electrocatalyst. The DMFC cathode was part

of a 22-cell stack run intermittently for 6 months that experienced voltage reversal. The plots include data for control electrodes, Pt only (short dashed lines) and PtRu (long dashed lines). As can be seen in (a), the Pt-only DMFC cathode behaves in a near identical fashion to the PtRu control, consistent with PtRu in the electrocatalyst layer of the cathode. The CVs of plot (b) shows a similar trend with the traces for the DMFC cathode morphologically similar to the PtRu control.

The bottom plot of Figure 9.6 shows how cell performance can diminish with Ru migration. The plot is the iR corrected voltage-current plot for the DMFC cathode described above. The cell was fed a 0.3-M methanol solution, cell temperature is 70°C, dry air is the oxidant, and no backpressure is applied at the cathode. The solid line plots the initial performance of the cell and the dashed line plots the performance of the cell after inclusion in the DMFC stack for 6 months.

It should be noted the fate of the cathode shown in Figure 9.6 is a rather extreme example of Ru migration and the conditions the cell experienced are not ideal, however, as the researchers found, Ru migration occurs even under the most benign conditions. Similar electrocatalyst migration in the form of agglom­eration was found by Yi et al. following a 75-hour DMFC life test [29]. The PtRu electrocatalyst used by Yi differed from the Zelenay study in that the anode catalyst is carbon supported PtRu rather than PtRu black and suggests that all forms of PtRu electrocatalysts are likely susceptible to migration in DMFC.

In summary, the principle challenges to commercializing DMFC technology are effective and stable electrocatalysts tolerant to methanol contamination of the cathode and membranes significantly less permeable to methanol. The most commonly used separator, Nafion, is not sufficient for the task. Nor are Pt and PtRu electrocatalysts. The outlook for membrane development for the near future seems to be somewhat static. However, promising electrocatalyst developments such as relatively inexpensive Pd-based catalysts that are stable and reaction specific may be the breakthrough that allows for the use of a less than ideal separator such as Nafion.

ABBREVIATIONS

Whereas the aquaponics system is water-based, the system as a whole is very efficient in the use of water to distribute nutrients. There is a small amount of evaporation from the fish tanks and growbeds, but the majority of water used goes into plant and fish growth, i. e., the only water actually removed from the system is in the form of plant material and fish flesh.

Power Generation

The aquaponics greenhouse will have microturbines placed inside during the winter to generate electricity. A microturbine is an aircraft engine that has been miniaturized and turns an alternator. The turbines are inside because the exhaust from burning the alcohol fuel is composed of carbon dioxide and water vapor — at a temperature of around 1300°F. An 80-kw turbine will generate nearly 900,000 BTUs per hour in its exhaust. The carbon dioxide “fertilizes” the air for the plants, increasing growth potential. The plants in turn release oxygen into the air. Each 301 bay of the greenhouse will require approximately 40 kw of power for lighting during the winter. The lighting is essential even on sunny days due to the low angle of incidence of insolation (incoming solar radiation). The sunlight bounces off the roof of the structure and does not penetrate in enough candlepower to benefit the plants. Without lights, there is a very real danger of nitrogen poisoning in green leafy vegetables grown inside. Heat and carbon dioxide levels will be monitored continuously. Should either exceed acceptable limits, the exhaust from the microturbines will be switched to a recouperator — gathering excess heat — and then outside. The recouperator stores excess heat in the thermal mass of the floor.

Dimethyl ether (CH3OCH3) is generally produced by dehydration of methanol. At large scale, the methanol production and dehydration processes are combined in one reactor, such that the dimethyl ether is produced directly from synthesis gas slightly more efficiently than methanol. The previously mentioned slurry bubble column reactor of Eastman Chemical Company in Kingsport, TN, has been demonstrated to be able to produce DME as well. The LPDME™ Process uses a physical mixture of a commercial methanol catalyst and a commercial dehydration catalyst in a single slurry reactor (Heydorn et al. 2003).

Like methanol, DME has promising features as a fuel candidate for both auto and diesel engines. With small adaptations to engine and fuel system, DME can be used in blends with diesel (10-20%), leading to higher fuel efficiency and lower emissions. In auto engines, DME can be used with LPG (any %) and neat. Since DME is as easily reformed as methanol, it has a big potential as fuel of fuel cell vehicles (van Walwijk et al. 1996). DME can be easily pressurized and handled as a liquid (Ekbom et al. 2003).

A number of attributes make alfalfa an attractive crop for production of biofuels and for biorefining. Alfalfa has a long history of cultivation around the world. It was introduced several times into North America during the 1700s and 1800s and is currently grown across the continent (Russelle, 2001). In the United States, alfalfa is the fourth most widely grown crop with over 9.3 million hectares of alfalfa harvested in 2003 (USDA-NASS, 2004). It is a perennial plant that is typically harvested for four years (an establishment year plus three subsequent years). Depending on location, alfalfa is harvested three or more times each year by cutting the stems near ground level. On average across the United States, alfalfa yields 7.8 Mg of dry matter (DM) per hectare each year, although yields can vary by location from 3.4 (North Dakota) to 18.4 (Arizona) Mg ha-1 (USDA- NASS, 2004). In 2003 the national harvest of alfalfa was over 69 million metric tons (USDA-NASS, 2004). The technology for cultivation, harvesting, and storing alfalfa is well established, machinery for harvesting alfalfa is widely available, and farmers are familiar with alfalfa production. There is a well-developed indus­try for alfalfa cultivar development, seed production, processing, and distribution. Alfalfa breeders have utilized the extensive germplasm resources of alfalfa to introduce disease and insect resistance, expand environmental adaptation, and improve forage quality. Nonetheless, alfalfa cultivation requires fertile, deep, well-drained soils of near neutral pH and is limited to humid areas with adequate rainfall. In arid or semi-arid areas, irrigation is essential for crop production. Despite breeding efforts that have increased disease and pest resistance, alfalfa yields have not increased substantially over the past 25 years (Brummer, 1999).

The high biomass potential of alfalfa is based on underground, typically unobserved traits. Alfalfa develops an extensive, well-branched root system that is capable of penetrating deep into the soil. Root growth rates of 1.8 m a year are typical in loose soils (Johnson et al., 1996) and metabolically active alfalfa roots have been found 18 m or more below ground level (Kiesselbach et al., 1929). This deep root system allows alfalfa plants to access water and nutrients that are not available to more shallowly rooted annual plants, which enables established alfalfa plants to produce adequate yields under less than optimal rainfall conditions. Alfalfa roots engage in a symbiotic relationship with the soil bacterium Sinorhizobium meliloti. This partnership between the plant and bacterium results in the formation of a unique organ, the root nodule, in which the bacterium is localized. The bacteria in root nodules take up nitrogen gas (N2) and “fix” it into ammonia. The ammonia is assimilated through the action of plant enzymes to form glutamine and glutamate. The nitrogen-containing amide group is subsequently transferred to aspartate and asparagine for transport throughout the plant. On average, alfalfa fixes approximately 152 kg N2 ha-1 on an annual basis as a result of biological nitrogen fixation, which eliminates the need for applied nitrogen fertilizers (Russelle and Birr, 2004). Although a signif­icant proportion of the fixed nitrogen is removed by forage harvest, fixed nitrogen is also returned to the soil for use by subsequent crops. This attribute of increasing soil fertility has made alfalfa and other plants in the legume family crucial components of agricultural systems worldwide. Cultivation of alfalfa has also been shown to improve soil quality, increase organic matter, and promote water penetration into soil.

Responsible stewardship of agricultural lands has never been more important. Utilization of alfalfa as a biomass crop has numerous environmental advantages. There is an urgent need to increase the use of perennials in agricultural systems to decrease erosion and water contamination. Annual row crop production has been shown to be a major source of sediment, nutrient (nitrogen and phosphorus), and pesticide contamination of surface and ground water. Perennial crops such as alfalfa can reduce the nitrate concentrations in soil and drainage water, and prevent soil erosion (Huggins et al., 2001). In addition, energy costs associated with production of alfalfa are low. A recent study shows that energy inputs for production of alfalfa are far lower than for production of corn and soybean, and very similar to switchgrass (Kim and Dale, 2004), primarily because alfalfa does not require nitrogen fertilizer. Biorefining could increase the return on alfalfa production so that cultivation of the crop is more economically attractive, as well as environmentally beneficial.

An additional advantage of using alfalfa for biofuel production compared to other crops is the ability to easily separate leaves and stems to produce co­products. In fact, alfalfa herbage can almost be considered two separate crops because leaves and stems differ so dramatically in composition. On a dry weight basis, total alfalfa herbage contains 18-22% protein with leaves containing 26-30% protein and stems only 10-12% (Arinze et al., 2003). In some analyses, alfalfa protein has been valued highly, theoretically greatly reducing the cost of the lignocelluose fraction (Dale, 1983). Several different integrated processes for refining alfalfa have been proposed based primarily on the method of refining the protein fraction. From field-dried hay, leaves may be separated from stem material mechanically (see “Protein and Fiber Separation” below). The leaf meal could be used as a high-protein feed with the stems utilized for gasification and conversion to electricity (Downing et al., 2005) or fermentation to ethanol (Dale, 1983). Alternatively, protein could be extracted from total ground material and the residue used for fermentation. Fresh forage can be “juiced” to remove protein and the residue fermented to ethanol or other products (Koegel et al., 1999; Sreenath et al., 2001; Weimer et al., 2005). An economic analysis of these

alternatives is beyond the scope of this chapter. However, a comparison of the potential costs and revenues of different biobased feedstocks to produce ethanol and other products is clearly needed to advance biomass refining from the theo­retical to practical stages.

E85 is used to operate spark-ignited engines that have been modified to accept this fuel. Unlike E10, E85 cannot be used in spark-ignited engines that have not been modified. This has slowed the adoption of E85 because there is a supply and demand problem: consumers will not buy vehicles unless there is a readily available source of fuel, and fuel companies will not invest in the alternative fuels unless there is a large supply of vehicles that use the fuel. This has led to the development of a new type of vehicle, called a Flexible Fuel Vehicle, which can operate using various blends of alcohol.

Flexible Fuel Vehicles

Flexible Fuel Vehicles, or FFVs, can operate on ethanol-blends from 0% (gaso­line) up to 85% (E85) by volume. This eliminates the supply and demand problem as consumers can fuel with any combination of fuel, not worrying whether the correct fuel will be available. These vehicles are produced by most of the major automakers and represent the largest class of vehicles using E85. Most of these vehicles are sold without any cost penalty to the consumer. This is both an indication of the automakers’ desire to develop the market and the incremental costs required to produce these vehicles compared with their gasoline-fueled counterparts.

These vehicles are designed and manufactured using E85 compatible mate­rials. Further, due to the different fuel-air mixture requirements of gasoline and E85, the fuel delivery systems are sized to handle the increased volumes of fuel when using E85. Other changes are made to the control algorithms in order to optimize the vehicle for use with this fuel. Since E85 has a higher octane than gasoline, the spark timing can be advanced, improving engine performance. Further, the fuel injection pulsewidth or duration must be lengthened to increase the flowrate of E85 for given engine load and speed conditions.

Since consumers can fuel these vehicles with either E85 or gasoline (or any blend in between), these vehicles must determine the levels of ethanol present onboard the vehicle in order to ensure that the engine is operating at the best conditions for the given fuel blend. In order to accomplish this, current FFVs have a fuel sensor, which is located in the fuel delivery lines leading from the fuel tank to the engine. The fuel sensor measures the conductivity of the current fuel blend. Since ethanol and gasoline have vastly different levels of conductivity (ethanol is about 135,000 times more conductive than gasoline),8 this is a rela­tively easy task to accomplish with some precision. Older FFVs relied on the use of a feedback signal from an exhaust gas oxygen (EGO) sensor located in the exhaust stream. This sensor, already present on all spark-ignited vehicles, detects the presence of excess oxygen in the exhaust. Because of exhaust after-treatment requirements on-road vehicles generally operate using stoichiometric mixtures of fuel and air. Thus, the oxygen sensor is used to maintain stoichiometric combus­tion in an engine. This sensor can be used to determine fuel mixture as ethanol is an oxygenated fuel with a richer stoichiometric mixture; thus, the mixture used for gasoline will be too lean with E85 and lead to excess oxygen in the exhaust. Unfortunately, these sensors only function when warm, so they cannot be used to help during cold starts of the engine. Thus, going from gasoline to E85 results in a lean mixture until the EGO is functioning. This can lead to poorer quality cold starts and poor drivability under acceleration.9

Many environmental groups have been critical of using FFVs, since modify­ing gasoline-powered vehicles to operate using E85 puts E85 at an inherent disadvantage and it does not force the rapid buildup of an E85 fueling infrastruc­ture since consumers can continue to use gasoline.2 It is important to remember that the automakers need to produce vehicles that consumers will actually buy; most consumers will not buy a vehicle for which there are few fueling opportu­nities. This criticism is perhaps premature at this early stage of E85 development.

The rise in use of portable electronic devices has been increasing steadily in the United States and abroad over the past few years and most likely will continue to increase over the years to come as the population becomes more dependent on multifunctional portable electronics. Harvesting energy from renewable resources has become an important focus in order to eliminate our dependency on oil and other nonrenewable resources necessary as primary power sources. It is well known that industrialized nations are the highest energy consumers and that there is a correlation between energy consumption and status of economic and technological development [9]. About 65% of the world’s primary energy was consumed in 1992 by industrialized countries and some of the more populated

Applications and Notes

Used in space vehicles, e. g., Apollo, shuttle

vehicles and mobile applications, and for lower-power systems

Suitable for portable electronics systems of low power, running or long times

Large numbers of 200-kW systems in use

Suitable for medium — to large-scale systems, up to MW capacity

Suitable for all sizes of CHP systems, 2-kW to multi-MW

countries, while developing areas consume more biomass energy such as wood and wood wastes.

The demand for energy is slowly increasing with developing technology, which in turn explains the extreme situation of the United States. The United States has only 5% of the world’s population and yet consumes about one quarter of the total global primary energy. The sources for energy production in the United States are usually obtained from coal, natural gas, and oil where oil is most common [9]. In order to minimize our dependency on oil, researchers are attempt­ing to harvest energy from renewable resources, such as alcohols, sugars, fats, and other biologically derived materials.

Biofuel cells are electrochemical devices in which energy derived from bio­chemical reactions is converted to electrical energy by means of the catalytic activity of microorganisms and/or their enzymes. Unlike metal catalysts, biocat­alysts are derived from biomatter, which is a renewable resource. Recent biofuel cell research has explored using enzymes as biocatalysts due to their availability and specificity. Enzymes are functional proteins whose purpose is to catalyze specific biochemical reactions by lowering the activation energy of the reaction, without undergoing a permanent chemical change itself. Enzymes can be manip­ulated and produced by genetic engineering or harvested and extracted from living organisms. Both means of acquiring enzymes are more cost effective than mining precious metals used as traditional catalysts. Biofuel cell catalysts are more environmentally friendly compared to heavy metal batteries due to the fact they naturally biodegrade. Another advantage of enzyme employment in biofuel cells is the enzyme specificity that pushes the fuel cell technology one step further. Specificity of the enzyme’s fuel utilization eliminates the need for employment of a salt bridge and therefore simplifies the fuel cell system [4].

The first biofuel cell was demonstrated by Potter in 1912 by employing glucose and yeast to obtain electrical energy [10]. This concept inspired scientists to investigate the metabolic pathways of power production [10]. Early biofuel cells employed microorganisms to oxidize the fuel for electricity generation; however, due to the slow mass transport of fuel across the cell wall, power densities are too low for practical applications. State-of-the-art microbial fuel cells developed by Lovley have shown greater than 40-day lifetimes, but power densities of 0.0074 mA/cm2 [11].

More recently, enzyme-based fuel cells were constructed employing enzymes in the solution. These fuel cells had higher power densities due to the elimination of cell walls that slowed the mass transport; however, their lifetime only extended from hours to a few days because of the enzyme’s stability. In contrast, higher power densities have been obtained with enzymatic fuel cells reaching up to 0.28 mW/cm2 for a glucose/oxygen membraneless biofuel cell at room temperature [12] and 0.69 mW/cm2 for a methanol/oxygen biofuel cell with a polymer elec­trolyte membrane [13]; however, enzymatic fuel cells are plagued with low lifetimes ranging from two hours [14] to seven days [15]. Table 12.2 depicts a brief history of biofuel cell technology.

Enzymes have been shown to be effective biocatalysts for biofuel cells com­pared to microbial biofuel cells. However, enzymes are very delicate catalysts. The optimal activity of enzymes depends on their three-dimensional configura­tion, which can be denatured with slight changes in pH or temperature. Therefore, it is necessary to develop an immobilization technique that will keep the enzyme active at the electrode surface in its optimal working conditions.

The project has specifically chosen an agricultural application in the hopes that some of this information may be used by operating farms as a means to create additional on-farm income or that it may encourage others to enter farming. Approximately 450 farms per year go out of business in New York State due to high energy costs and economic failure (6). With no remaining income, many farms fall prey to land developers, resulting in the loss of irreplaceable farmland forever.