Utilizing landfill gas to manufacture methanol has many significant advantages over the current suppliers of methanol. •

• “Green methanol” will allow the manufacturer to take advantage of various tax credits and incentives passed by the United States Congress to promote renewable products and fuels. One example is the 60-cents — per-gallon “alcohol fuel credit” that applies to methanol used in trans­portation fuels.

• The utilization of landfill gas in the manufacture of methanol results in the sequestering of methane and carbon dioxide, this process is credited with reducing greenhouse gas emissions.

The emerging markets for renewable methanol are becoming more defined and mature as demand is being driven by environmental statutory requirements and U. S. government mandates regarding the reduction of global warming gases and ending U. S. reliance upon foreign oil and energy. To that end, landfill gas to methanol offers a compelling opportunity to develop a significantly large domestic industry to support the growing demand for domestic renewable fuels and energy.

For the purposes of this chapter, the high temperature, dilute acid pretreatment and subsequent enzymatic saccharification method will be examined in more detail as a conversion technology for ethanol production from alfalfa stem fractions. The high temperature, dilute acid pretreatment is designed to remove noncellulosic cell wall polysaccharides and lignin, because these constituents will interfere with the cellulase enzyme cocktails used for hydrolysis of the cellulose. One design goal of this pretreatment is to reduce the pH of the feedstock reaction mixture to 1.3-1.5 prior to heating (National Renewal Energy Laboratory, Golden, CO; Laboratory Analytical Procedure-007, May 17, 1995). The amount of sulfuric acid required to reach this pH target for alfalfa stems was 8.1 mmol g-1 biomass DM in a 1% solids slurry, compared to 6.4 mmol for switchgrass and corn stover (Jung, unpublished). Maturity of alfalfa stems and switchgrass did not influence the acid requirement. Dien et al. (2005) observed that the sulfuric acid loading required to maximize release of nonglucose sugars from alfalfa stems when heated at 121°C for 1 h was 2.5% (wt/vol), whereas 1.5% was sufficient for switchgrass. The higher acid require­ment for alfalfa stems is most likely due to the greater pectin content of alfalfa cell

Immature Mature

walls compared to grasses; however, the hemicellulose content is lower and lignin content is similar in alfalfa stems compared to the grasses (Dien et al., 2005). Torget et al. (1990, 1992) also observed that legume feedstocks are more recalcitrant to acid pretreatment than grasses.

The efficiency of glucose release by acid pretreatment, followed by enzymatic saccharification from cell wall polysaccharides (cellulose and xyloglucans), declined as alfalfa stems became more mature (Figure 5.2). While efficiency of glucose conversion declined with maturity, the total yield of glucose was not altered (Figure 5.2), because cellulose content increased in more mature alfalfa stems. Similar declines in efficiency with maturity were observed for switchgrass and reed canary grass (Dien et al., 2005), but the efficiency of glucose release from the grasses was greater than from alfalfa stems. This may reflect the higher concentration of lignin in the alfalfa stems because across all three species, efficiency of glucose release was negatively correlated with lignin content of the feedstock. Increasing the temperature of the acid pretreatment resulted in improved efficiency of glucose release from alfalfa stems (Dien, personal com­munication). While efficiency of glucose release was lower for alfalfa than for grasses, total yield of glucose was very similar between the feedstocks. This again reflects the interaction of efficiency with glucose content of the feedstocks.

Pure ethanol becomes difficult to vaporize when cold, leading to poor cold startability. In fact ethanol will not form an air/fuel vapor mixture high enough to support combustion below 11 °C.12 Therefore, gasoline is added to the ethanol in order to support cold startability and increased cold start enrichment is used to achieve combustible vapor air mixtures in the engine.

Additional difficulties when cold starting with E85 can be attributed to its high conductivity. During cold starts, the spark plug electrodes can become wetted with fuel. Since E85 is much more conductive, this can leading to plug shorting and misfire.8

These problems have been addressed by the major automakers through better cold-start fuel calibrations. Most manufacturers now report good cold starts at temperatures below 0°F (-18°C) when using E85 in a winter blend (E70).

Safety

E85 has wider flammability limits on the rich side, and it has a higher flame speed compared to gasoline. This increases the probability of encountering flash­back or fuel vapor ignition during fuel filling.17 Because of this, vehicles using

E85 require a flame arrestor, which is installed into the fuel filling tube. This device will extinguish any flame that might occur.

Since enzymes are highly selective, there are limited problems associated with fuel crossover from the anode to the cathode in a biofuel cell. If an anode and cathode are both selective, then a polymer electrolyte membrane is no longer required to separate the anode and cathode solutions. Topcagic et al. have devel­oped the first ethanol membraneless biofuel cell. At the cathode, bilirubin oxidase has been chosen to replace platinum as the reducing catalyst to increase specificity of the cathode. A schematic showing the simplicity of a membraneless biofuel cell can be seen in Figure 12.7.

Alternatives for platinum found in the literature typically use laccase enzyme as studied by Heller’s Group [27]. Laccase lowers the power of a biofuel cell due to the maximum turnover rate of laccase occuring at pH 5.0 and deactivation in the presence of chlorine ions. Bilirubin oxidase has been chosen as a catalyst for future studies, because it has optimum performance in a physiological envi­ronment (near-neutral pH and presence of various ions). The second problem associated with many biocathodes in the literature is that electrodes are osmium — based creating a toxicity hazard to the surrounding environment. Topcagic et al. have replaced the osmium-based mediator with a ruthenium-based complex of

similar structure that is less toxic and has a higher self-exchange rate. A third problem associated with anodes and cathodes in the literature is a technique of immobilizing the enzyme at the electrode surface. Literature enzyme immobili­zation employs covalent bonding of the enzyme to the surface of the electrode or to the mediator. This method does not protect the enzyme from its surroundings and its optimal activity is lowered due to the conformational change that resulted from physically attaching the enzyme to the surface to the electrode [23]. How­ever, instead of physically attaching the enzyme, Topcagic et al. have immobilized it in modified Nafion®; therefore optimum enzyme activity is retained and the enzyme is protected from the surrounding environment.

The membraneless biofuel cell operates at room temperature, which varies from 20-25°C in a phosphate buffer pH 7.15 containing 1.0 mM ethanol. Since the polymer electrolyte membrane has been eliminated, the electrodes have to be specific enough to work in same compartment. Both electrodes, bioanode and biocathode, consisted of immobilized enzyme casting solution at the surface of the 1-cm2 carbon fiber paper. The performance of NAD+-dependent ADH and PQQ-dependent ADH bioanodes coupled to biocathodes was also studied and is

Electrical Load

summarized in Table 12.3. The bioanode used for most studies had PQQ-depen — dent alcohol dehydrogenase enzymes, while the biocathode had bilirubin oxidase, bilirubin with Ru(bpy)3+2 immobilized at the surface of the electrode. Maximum open circuit potential is 1.04 V with maximum current density of 8.47 mA/cm2 [28]. For the membraneless system comprised of a PQQ-dependent ADH anode and bilirubin oxidase biocathode, the fuel cell has an increased lifetime of greater than 353%, increased open circuit potential of 15%, and increased power density of 97% compared to the NAD+-dependent ADH bioanode [28].

CONCLUSIONS

Research has succeeded in increasing the stability of enzymes at the electrode surface, which in turn increases the open circuit potential, current and power of a biofuel cell. Also, by eliminating the need for an electrocatalyst layer and a polyelectrolyte membrane, it reduces the cost of production of the current biofuel cell and simplifies fabrication. Replacing NAD+-dependent ADH with PQQ — dependent ADH is a step toward reaching the goal of increasing the overall lifetime of the biofuel cell for future use in multiple power applications. The most important phenomenon to examine is the increase in lifetime of the mem­braneless system with PQQ-dependent bioanodes. The PQQ-dependent bioanodes are more stable than NAD+-dependent bioanodes. In addition to better data results, PQQ-dependent ADH serves as an invaluable replacement for NAD+-dependent ADH due to the simplicity it offers in bioanode fabrication.

There are a number of opportunities for new power sources aside from replacing petroleum for transportation. Beginning at the small end of the scale, alternatives to today’s battery chemistry are in critical demand. Proliferation of portable electronics and increasing functionality has exceeded the capabilities of the lith­ium-ion battery. Indeed, the features able to be offered to consumers by the major electronics development companies are limited by the battery, an example being the delay of mass introduction of the 4G cell phone. Many outside of Japan may not be familiar with these phones, known as “power eaters,” which last for all of 15 minutes when being used to watch television or movies. The Japanese suffer through this shortfall because their long daily work commutes are brightened by the entertainment. Batteries’ limitations have at least created a new market oppor­tunity, known as “juice bars,” where the Japanese cannot only receive liquid refreshment, but for a fee can recharge their cell phones as well.

This, of course, is a temporary solution to an obvious problem: the need for new portable power sources. Fuel cells have been studied for over 40 years as a potential battery replacement technology. With a potential $4-billion-plus lithium battery market takeover opportunity, there are a plethora of entities working to deliver portable fuel cells [2]. The Direct Methanol Fuel Cell (DMFC) has probably received the majority of attention in this area. Despite hundreds of millions in research and development dollars, in early 2005 there is no consumer product on the market. DMFC has applications and viability in other markets, but the size, cost, and performance requirements for rechargeable battery replace­ment have thus far proved insurmountable. New catalyst developments as well as new membrane technology may push DMFCs over the hump to widespread commercialization; however, toxicity will remain a serious obstacle.

Enter the opportunity for fuels other than methanol in the portable battery market. Ethanol is the obvious choice because it is already widely available in day-to-day life and on airlines as well, a necessity. Additionally, ethanol has certain chemical properties making it desirable as a fuel, such as higher energy density than methanol while remaining a small enough molecule for good diffu­sion properties. As discussed in this book, innovations in catalysts are required to employ ethanol. Efficacy has been demonstrated; however, catalyst stability and operating temperature must still be addressed for metal-based systems.

Perhaps a previously considered fringe effort, now gaining momentum, is the use of nontraditional catalysts, i. e., biological catalysts. The advantages are cost savings from elimination of precious metals, simplification of system design due to high selectivity for analyte, dramatically increased fuel options, and efficient operation at room temperature, among others discussed. Enzymes in particular, have the potential to compete (and in some cases already are competing) with DMFC and DEFCs and surpass their performance. It may be breakthroughs in this area of research that ultimately enable commercial applications. Elimination of PEMs, bipolar plates, and precious metal catalysts are significant advantages.

SOFC fuel cells have also had a resurgence of effort, most likely driven by military interest. Advances in catalysts and insulating materials are showing promise for portable applications, portable meaning 20-W systems.

Overall, it is the opinion of this author that the portable fuel cell effort emerged too early for its time. Hundreds of millions of dollars, if not billions, have been taken in by start-up companies through institutional, noninstitutional, and government sources over the past 10+ years with no product to show for it. The technology continues to suffer from essentially the same key hurdles that it did since the start of effort. Many investors are losing interest and have become calloused to the excitement surrounding portable fuel cells. That is unfortunate because we are nearing the opportune time for this market. Key breakthroughs are on the verge of occurring, which will hopefully burst the bubble and pave the way for commercial applications. Some venture firms recognize this oppor­tunity and are sticking with fuel cells in the belief that we are near the gold rush.

The raw synthesis gas produced by gasification contains impurities. The most typical impurities are organic impurities like condensable tars, BTX (benzene, toluene, and xylenes), inorganic impurities (NH3, HCN, H2S, COS, and HCl), volatile metals, dust, and soot (Tijmensen 2000; van Ree et al. 1995). These contaminants can lower catalyst activity in reformer, shift, and methanol reactor, and cause corrosion in compressors, heat exchangers and the (optional) gas turbine.

The estimated maximal acceptable contaminant concentrations are summa­rized in Table 2.2 together with the effectiveness of wet and dry gas cleaning, as described below.

The gas can be cleaned using available conventional technology, by applying gas cooling, low-temperature filtration, and water scrubbing at 100-250°C. Alter­natively, hot gas cleaning can be considered, using ceramic filters and reagents at 350-800°C. These technologies have been described thoroughly by several authors (Consonni et al. 1994; Kurkela 1996; Tijmensen 2000; van Dijk et al. 1995; van Ree et al. 1995). The considered pressure range is no problem for

TABLE 2.1 (CONTINUED) Characteristics of Gasifiers

1 640 ktonne dry wood annual, load is 8000 h.

2 Calculated from LHVwet = HHV^ X (1 — W) — Ew X (W + Hwet x mH2O); with Ew the energy needed for water evaporation (2.26 MJ/kg), Hwet the hydrogen content on wet basis (for wood Hdry = 0.062) and mH2O the amount of water created from hydrogen (8.94 kg/kg).

3 Wet biomass: 80/0.7 = 114 tonne/hr to dry biomass 80/0.85 = 94.1 tonne/hr for IGT П evaporate water 20.2 tonne/hr at 1.3 ts/twe in Niro (indirect) steam dryer. Calculation for BCL is alike. The steam has a pressure of 12 bar and a temperature of minimally 200°C (Pierik et al. 1995).

4 Pressure is 34.5, 25, or 1.2 bar, temperature is minimally 250, 240, or 120°C.

5 Calculated from the total mass stream, 188.5 tonne/hr.

6 Quoted from OPPA (1990) by Williams et al. (1995).

7 Knight (1998).

8 Compiled by Williams et al. (1995).

either of the technologies. Hot gas cleaning is advantageous for the overall energy balance when a reformer or a ceramic membrane is applied directly after the cleaning section, because these processes require a high inlet temperature. However, not all elements of hot gas cleaning are yet proven technology, while there is little uncertainty about the cleaning effectiveness of low temperature gas cleaning. Both cleaning concepts are depicted in Figure 2.4.

In contrast to the dry-grind process (where the whole corn kernel is ground and enters the fermenter) in wet milling, the kernel is first fractionated into separate components and only the starch enters the fermenter. Corn wet milling plants are often referred to as biorefineries, comparable to petroleum refineries, because wet milling fractionates corn into its components and then processes the components into more valuable products [20]. Wet milling separates the kernel into germ (oil), gluten (protein), fiber, and starch fractions, yielding corn oil, animal feed, and a variety of products derived from starch.

Steeping

The wet-milling process starts with steeping, which enables isolation of the different kernel fractions [20]. Corn is screened to remove foreign material, and soaked in dilute (0.12-0.20%) sulfurous acid at 52°C for (typically) 30-36 h. Lactobacillus and related bacteria growing in the steep water produce lactic acid and other metabolites that further acidify the medium. Steeping occurs in a series of large stainless steel tanks, with steep water recirculated countercurrently from tanks holding “older” corn that is nearing the end of the steeping process to “newer” corn that is beginning the steep. The effect of recycling the steep liquid is the progression of the corn up a SO2 concentration gradient. The combined action of sulfurous acid and lactic acid, and probably also direct effects of microbes growing in the steep, prepare the kernels for processing into fractions. Steeping softens and swells the kernels, disrupts disulfide bonds between the protein and starch in the endosperm, and releases the starch granules into solution.

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.