Category Archives: Biofuels Refining and Performance
Methanol-fueled vehicles emit less CO2 and other polluting gases compared to gasoline-fueled vehicles. Therefore, methanol use maintains good air quality. For a higher compression ratio compared to gasoline, a higher level of NOx can be achieved. But low flame temperature and latent heat of vaporization tend to decrease NOx emissions. The overall effect is a lower level of NOx emissions.
The power-conditioning system is an integral part of a fuel cell system. It converts the dc electric power generated by the fuel cell into regulated dc or ac for consumer use. The electrical characteristics of a fuel cell are very far from that of an ideal electric power source. The dc output voltage of a fuel cell stack varies considerably with the load current (see Fig. 9.15), and it has very little overload capacity. It needs considerable auxiliary power for pumps, blowers, and so forth, and requires considerable start-up time due to heating requirements. It is slow to respond to load changes, and its performance degrades considerably with the age of the fuel cell. The various blocks of a fuel cell power-conditioning system are shown in Fig. 9.16.
The dc voltage generated by a fuel cell stack is usually low in magnitude (<50 V for a 5- to 10-kW system, <350 V for a 300-kW system) and varies widely with the load. A dc—dc converter stage is required to regulate and step up the dc voltage to 400-600 V (typical for 120/240-V ac output). Since the dc—dc converter draws power directly from the fuel cell, it should not introduce any negative current into the fuel cell and must be designed to match the fuel cell ripple current specifications. A dc—ac conversion (inverter) stage is needed for converting the dc to ac power at 50 or 60 Hz (see Fig. 9.17). Switching frequency harmonics are filtered out using a filter connected to the output of the inverter to generate a high-quality sinusoidal ac waveform suitable for the load.
Whether they are green algae (chlorella) or the higher plants, autotrophs in general are gifted in nature to fix carbon dioxide and produce biomass. In ecologic terms, these are producers. The dominant autotroph is phototrophic. Photosynthesis has two distinct aspects: the light dependent step, where photolysis of water takes place:
ADP + Pi ——^ ATP
During this reaction, oxygen is set free, Co II is reduced and phosphorylation of ATP takes place. The photoenergy is chemically utilized twofold.
In the next step, through a very complex enzymic sequence, CO2 is incorporated into the existing metabolite pool and higher carbohydrates are biosynthesized. This step of the reaction finds variation in different species; carbohydrates, proteins, and lipids are biosynthesized. Then, the first part of the reaction makes the autotrophs unique. Light falling on chloroplasts develops an electrical field across the membrane.
In the presence of the pigments in chloroplasts, the light energy is trapped and activates water and lyses it. Ideally, water, if converted into its elemental components, requires (at 25°C) 68.3 kcal/mol (from liquid) or 57.8 kcal/mol (from vapor). Thermal energy is not sufficient to bring this change. During photolysis, the plant pigment augments electron flow, and the electron flow system culminates in the two energy-rich chemical products (reduced Co II, ATP, and O2), as already mentioned (see Fig. 1.7).
Lester Packer’s group at the University of California at Berkeley has shown the steps of the pathway with chloroplasts from spinach leaves,
ferredoxin from Spirulina, and hydrogenase of Clostridium pasteuri — anum .
2H, O ^ 4H + 4e + O2
4 Ferredoxin+ + 4e~ ^ 4 Ferredoxin
4H+ + 4 Ferredoxin ^ 2H2 + 4 Ferredoxin+
O2 + Glucose ^ Gluconate + H2O2 H2O2 + Ethanol ^ 2H2O + Acetaldehyde The overall reaction is
Glucose + Ethanol ^ Gluconate + Acetaldehyde + H2
Two H2 are produced for each O2 produced (if not consumed by an oxidase-type reaction as shown previously). Dr John Benemann of the same university has also suggested that hydrogen and methane production is possible by designing a two-stage system separated from each other (see Fig. 1.8).
Dibromothymoquinone blocks the natural electron flow system at plastocyanin level (see Fig. 1.9). Thus, in the presence of an artificial donor or acceptor, the photo systems I and II can be separated at pre — and post-blocking points.
Hemicelluloses are heterogeneous polymers of pentoses (e. g., xylose and arabinose), hexoses (e. g., mannose, glucose, and galactose), and sugar acids. Unlike cellulose, hemicelluloses are not chemically homogeneous. Hemicelluloses are relatively easily hydrolyzed by acids to their monomer components consisting of glucose, mannose, galactose, xylose, arabinose, and small amounts of rhamnose, glucuronic acid, methylglucuronic acid, and galacturonic acid. Hardwood hemicelluloses contain mostly xylans, whereas softwood hemicelluloses contain mostly glucomannans. Xylans are the most abundant hemicelluloses. Xylans of many plant materials are heteropolysaccharides with homopolymeric backbone chains of 1,4- linked ^-D-xylopyranose units. Xylans from different sources, such as grasses, cereals, softwood, and hardwood, differ in composition. Besides xylose, xylans may contain arabinose, glucuronic acid or its 4-O-methyl ether, and acetic, ferulic, and p-coumaric acids. The degree of polymerization of hardwood xylans (150-200) is higher than that of softwoods (70-130) [14, 15].
Lignin is a very complex molecule. It is an aromatic polymer constructed of phenylpropane units linked in a three-dimensional structure. Generally, softwoods contain more lignin than hardwoods. Lignins are divided into two classes, namely, “guaiacyl lignins” and “guaiacyl — syringyl lignins.” Although the principal structural elements in lignin have been largely clarified, many aspects of their chemistry remain unclear. Chemical bonds have been reported between lignin and hemi — cellulose, and even cellulose. Lignins are extremely resistant to chemical and enzymatic degradation. Biological degradation can be achieved mainly by fungi, but also by certain actinomycetes [15, 17].
Crop description. Gossypium spp., commonly known as cotton, belongs to the family Malvaceae and is native to the tropical and subtropical regions (see Fig. 4.4). Four separate domesticated species of cotton grown in various parts of the world are G. arboreum L., G. herbaceum L., G. hirsutum L., and G. barbadense L. Cotton shrubs are annual and found in the United States, Australia, Asia, and Egypt. Some have been grown for many years in southern Europe, mainly the Balkans and Spain. It can grow up to 3 m high [61-64].
Main uses. Cotton is a major world fiber crop. Its fiber grows around the seeds of the cotton plant and is used to make textile, which is the most widely used natural-fiber cloth. The seeds yield a valuable oil used for the production of cooking oil or margarine. The fatty acid composition includes mainly palmitic acid (21%), stearic acid (2.4%), oleic acid (19.5%), linoleic acid (54.3%), and myristic acid (0.9%). Cottonseed oil, cake, meal, and hulls for feeding are other uses of the by-products. Whole cottonseed may be used as a feed for mature cattle. Cottonseed meal is an excellent protein supplement for cattle. The limitations on effective utilization of this product in rations for swine and poultry are of minor significance to ruminant animals. Cottonseed meal has a relatively low rumen degradability and is therefore a good source of by-pass protein and is especially useful in rations for milking cows [61-64].
Figure 4.4 Gossypium spp. (Photo courtesy of Prof. Jack Bacheler [http:/ /ipm. ncsu. edu/ cotton/InsectCorner/photos/ images/Open_cotton_plant. jpg].)
Kose et al. investigated the transesterification of refined cottonseed oil, using primary and secondary alcohols (oil-alcohol molar ratio 1:4) in the presence of an immobilized enzyme from Candida antarctica (30% enzyme, based on oil weight). The reaction was carried out at 50°C for 7 h, showing that conversion using secondary alcohols was more effective
 . Some authors have also proposed the use of lipase with methanol
 . Royon et al. used the same catalyst in a t-butanol solvent. Maximum yield was observed after 24 h at 50°C with a reaction mixture containing 32.5% t-butanol, 13.5% methanol, 54% oil, and 0.017 g of enzyme per g of oil . Recent tendencies propose the use of ultrasonically assisted extraction transesterification to increase ester yield .
The fuel properties of biodiesel are strongly influenced by the properties of the individual fatty esters as well as those of some minor components. Both moieties, the fatty acid and alcohol, have considerable influence on fuel properties such as CN, with relation to combustion and exhaust emissions, cold flow, oxidative stability, viscosity, and lubricity. It therefore appears reasonable to enrich (a) certain fatty ester(s) with desirable properties in the fuel, in order to improve the properties of the whole fuel. For example, from the presently available data, it appears that isopropyl esters have better fuel properties than methyl esters. The major disadvantage is the higher price of isopropanol in comparison to methanol, besides modifications needed for the transesterification reaction. Similar observations likely hold for the fatty acid moiety. It may be possible in the future to improve the properties of biodiesel by means of genetic engineering of the parent oils, which could eventually lead to a fuel enriched with (a) certain fatty acid(s), possibly oleic acid, that exhibits a combination of improved fuel properties.
At first glance vegetable oil offers a favorable CO2 balance. However, when the extra N2O emission from biofuel production is calculated in “CO2-equivalent” global warming terms, and compared with the quasicooling effect of “saving” emissions of fossil fuel derived CO2, the outcome is that production of commonly used biofuels can contribute as much or more to global warming by N2O emissions than cooling by fossil fuel savings . In addition, widespread use of vegetable oil fuels is limited by high viscosity, low volatility, poor cold flow behavior, and lack of oxidation stability during storage [6, 7]. Partial conversion of vegetable oil to hydrocarbons offers the possibility to preserve the favorable environmental characteristics of vegetable oil-based fuels while improving viscosity and cold flow behavior [34, 35]. Figure 8.2 depicts thermogravimetry of vegetable oil without pure oil (dashed line) and in the presence of a Y-zeolite (Koestrolith). The dotted line represents the first derivative from the catalyzed conversion reaction.
The efficiency of the decarboxylation effect of Y-zeolite activity on pure vegetable oil at T = 450°C may be seen by comparing the IR spectrum of pure vegetable oil fuel in Fig. 8.3 with the corresponding spectrum of the conversion product in Fig. 8.4. The carbonyl band at around 1700 cm 1 is an indicator for conversion efficiency.
Table 8.3 summarizes physical and chemical parameters of vegetable oil fuel and conversion products at different temperatures. The change
TABLE 8.3 Characteristics of Commercial Vegetable Oil Fuel and Its Y-Zeolite Conversion Product
in viscosity is quite remarkable. In accordance with Fig. 8.2, a reaction temperature of T = 450°C is preferred.
A typical natural phenomenon, probably a unique mating signal by the “firefly,” also exists in other living species, namely, bacteria, protozoa, fungi, and worms, in the forms that emit visible light. In most cases, the
nature of the luminescent light varies in color and intensity; but chemical pathways are, to a great extent, common. The chemical products responsible for giving out different colors are different and are not yet fully known.
A heat-labile simple protein enzyme luciferase (MW 105) makes a complex (luciferyl adenylate E) with reduced luciferin, in the presence of ATP (Mg2+), which subsequently breaks down into different products in the presence of molecular oxygen. This results in the excitation of luciferin to a high-energy state. On return of the same to the ground state, emission of visible light produces bioluminescence (see Fig. 1.13).
LH2 + ATP (Mg2+) + E ^ LH2 — AMP — E + PPi
LH2 — AMP — E + O2 ^ Products + Light
The phenomenon appears to be insignificant but a substantial supply of luciferin, ATP (Mg2+), and a little enzyme can deliver an appreciable luminescence of practical use. Whether luciferin, luciferase, and ATP may also be harvested from animal resources, or the chemical components may be synthesized economically and the enzyme can be procured from flies, remains a matter of investigation and development. Like bee-keeping, culture of “fireflies” is very likely to become a profitable art. The dream of producing high voltage by animal tissues, imitating the electric eel, may come true in the near future; the fundamentals are known, but economic viability is not assured, hence not discussed here.
C—O — P—O—Ribose-Adenine
Figure 1.13 Firefly bioluminescence.
A great number of molds are able to produce ethanol. The filamentous fungi Fusarium, Mucor, Monilia, Rhizopus, Ryzypose, and Paecilomyces are among the fungi that can ferment pentoses to ethanol . Zygomycetes are saprophytic filamentous fungi, which are able to produce several metabolites including ethanol. Among the three genera Mucor, Rhizopus, and Rhizomucor, Mucor indicus (formerly M. rouxii) and Rhizopus oryzae have shown good performances on ethanol productivity from glucose, xylose, and wood hydrolyzate . M. indicus has several industrial advantages compared to baker’s yeast for ethanol production, such as (a) capability of utilizing xylose, (b) having a valuable biomass, e. g., for production of chitosan, and (c) high optimum temperature of 37°C . Skory et al.  examined 19 Aspergilli and 10 Rhizopus strains for their ability to ferment simple sugars (glucose, xylose, and arabinose) as well as complex substrates. An appreciable level of ethanol has been produced by Aspergillus oryzae, R. oryzae, and R. javanicus.
The dimorphic organism M. circinelloides is also used for production of ethanol from pentose and hexose sugars. Large amounts of ethanol have been produced during aerobic growth on glucose under nonoxygen — limiting conditions by this mold. However, ethanol production on galactose or xylose has been less significant . Yields as high as 0.48 g/g ethanol from glucose by M. indicus, under anaerobic conditions, have been reported . However, the yield and productivity of ethanol from xylose is lower than that of P stipitis .
Although filamentous fungi have been industrially used for a long time for several purposes, a number of process-engineering problems are associated with these organisms due to their filamentous growth. Problems can appear in mixing, mass transfer, and heat transfer. Furthermore, attachment and growth on bioreactor walls, agitators, probes, and baffles cause heterogeneity within the bioreactor and problems in measurement of controlling parameters and cleaning of the bioreactor [66, 67]. Such potential problems might hinder industrial application of M. indicus for ethanol production. However, this fungus is dimorphic, and its morphology can be controlled to be yeast-like or pellet-like through fermentation .
Crop description. Brassica carinata, commonly known as Ethiopian mustard, is an adequate oil-bearing crop that is well adapted to marginal regions (see Fig. 4.15). This crop, which is originally from Ethiopia, is drought-resistant and grown in arid regions [127, 128]. Ethiopian mustard presents up to 6% saturated hydrocarbon chains. It is native to the Ethiopian highlands, is widely used as food by the Ethiopians, and pre-sents better agronomic performances in areas such as Spain, California, and Italy. This makes B. carinata a promising oil feedstock for cultivation in coastal Mediterranean areas, which could offer the possibility of exploiting the Mediterranean marginal areas for energetic purposes . Its fatty acid composition includes palmitic acid (3.6%), stearic acid (1.3%), oleic acid (14.8%), linoleic acid (12.2%), gadoleic acid (10.3%), and erucic acid (45.4%) .
Main uses. It is widely used as food in Ethiopia. Oil from wild species is high in erucic acid, which is toxic, although some cultivars contain little erucic acid and can be used as food. The seed can also be crushed and used as a condiment . There is a genetic relationship among
Figure 4.15 Brassica carinata.
B. carinata genotypes based on oil content and fatty acid composition. Genet et al. have generated information to plan crosses and maximize the use of genetic diversity and expression of heterosis . Dorado et al. found negative effects of singular fatty acids, such as erucic acid, over alkali-catalyzed transesterification reaction . These researchers described a low-cost transesterification process of B. carinata oil. An oil-methanol molar ratio of 1:4.6, addition of 1.4% of KOH, a reaction temperature in the range of 20-45oC, and 30 min of stirring are considered to be the best conditions to develop a low-cost method to produce biodiesel from B. carinata oil [39, 131]. Biodiesel from Ethiopian mustard oil could become of interest if a fuel tax exemption is granted . When compared with petroleum diesel fuel, Cardone et al. have found that engine test bench analysis did not show any appreciable variation of output engine torque values, while there was a significant difference in specific fuel consumption data at the lowest loads. Biodiesel produced higher levels of NOx concentrations and lower levels of particulate matter (PM), with respect to diesel fuel. Biodiesel emissions contain less soot .