The term biodiesel has no unambiguous definition. It stands for neat vegetable oils used as DF as well as neat methyl esters prepared from vegetable oils or animal fats and blends of conventional diesel fuel with vegetable oils or methyl esters. With increasing emphasis on the use of esters as DF, however, the term “biodiesel” increasingly refers to alkyl esters of vegetable oils and animal fats and not the oils or fats themselves. In an article on proposed ASTM standards, biodiesel was defined (9) as “the mono alkyl esters of long chain fatty acids derived from renewable lipid feedstock, such as vegetable oils or animal fats, for use in compression ignition (diesel) engines.” Nevertheless, clear distinction between these different vegetable oil-based or — derived alternative diesel fuels is necessary.

For use in the United States, the U. S. Department of Energy has stated (10), “that biodiesel is already covered in the statutory and proposed regulatory definitions of “alternative fuel” which refer to any “fuel, other than alcohol, that is derived from biological materials.” The Department, therefore, is considering amending the proposed definition of “alternative fuel” specifically to include neat biodiesel.” The definition of biodiesel was not extended to include biodiesel blends, with the Department of Energy stating that “the issue of including biodiesel mixtures or blends comprised of more than 20 percent biodiesel is currently under study. However, this subject is complex and will require significantly more data and information, and a separate, future rulemaking, before DOE can make a determination as to whether to include them in the definition of “alternative fuel.”

Ayaaki Ishizaki, Naohiko Taga, Toshihiro Takeshita, Toshikazu Sugimoto,
Takeharu Tsuge, and Kenji Tanaka

Department of Food Science and Technology, Faculty of Agriculture,
Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812, Japan

Fermentative production of biodegradable plastic material, poly-D-3- hydroxybutyrate, P(3HB) from CO2 or agricultural wastes is expected to contribute to the solution of global environmental pollution problems. The practical cultivation systems to produce P(3HB) from CO2, H2 and O2 by hydrogen-oxidizing bacterium were developed by maintaining O2 concentration in gas phase below the lower limit for explosion. P(3HB) productivity was increased by improving gas mass transfer with the use of air-lift fermentor and the addition of 0.05 %CMC to the culture medium. P(3HB) was also produced from xylose via L-lactate by two-stage culture method using Alcaligenes eutrophus and Lactococcus lactis 10-1. P(3HB) productivity was increased by a pH-stat fed-batch culture method with feeding the substrate solution so as to control L-lactate concentration at very low level.

Polyhydroxyalkanoates, PHAs are potential raw materials for manufacturing biodegradable plastics(l). Alcaligenes eutrophus is a hydrogen-oxidizing bacterium that is able to grow autotrophically using H2, O2 and CO2 and heterotrophically using organic acids as substrate with the accumulation of poly-D-3-hydroxybutyric acid, P(3HB) in the cell under nutrient-limited conditions(Fig. l). The growth rate of this hydrogen-oxidizing bacterium is much higher than that of other autotrophs such as photosynthetic organisms and hence the bacterium has the potential to be used in industrial processes. Production of P(3HB) from CO2 or organic acids derived from agricultural wastes by A. eutrophus, could contribute to the solution of two environmental pollution problems of increased CO2 levels in the atmosphere and that of the disposals of non-biodegradable plastic waste. Here, we describe a strategy and system set-up for fermentative production of P(3HB) from CO2 and agricultural waste materials.

© 1997 American Chemical Society

M. Gfllbe, M. Larsson, K. Stenberg, C. Tengborg, and G. Zacchi1

Department of Chemical Engineering I, Lund University,
S-221 00 Lund, Sweden

Ethanol is an attractive alternative for replacing gasoline as a motor fuel and it can be produced from a number of cellulosic materials. However, processes in which biomass is involved are often very complex. By a combination of computer simulations and experiments in a bench-scale unit different process strategies can be evaluated, and the results from experiments can serve as feed-back to the process simulations. The results in this study show the impact of recycling process streams and the effect of different energy-integration options.

Ethanol, which is believed to be an interesting alternative for replacing gasoline as a motor fuel can be produced from a number of cellulosic materials including agricultural products (1-4). The main reason for turning towards processes utilising cellulosic materials is the abundance of various cellulose sources, such as forest waste (4-6). Ethanol production from sugar — or starch-containing crops is an industrially well-established technique which has been used for many years, mainly for the production of alcoholic beverages. However, the process technology for the conversion of cellulosic materials into ethanol has not yet been fully optimised. A number of different technologies have been proposed, the major difference being the way in which the material is hydrolysed and the fermentable sugars extracted.

Basically, the production of ethanol from, for example, wood can be performed by hydrolysing the material, thus releasing fermentable sugars. The sugars are fermented to ethanol using various micro-organisms, and the diluted ethanol is recovered in a distillation unit. Hydrolysis can be performed either by the use of dilute or concentrated acids (7-9), or by using cellulose-degrading enzymes (3,10-12). The advantages of acid hydrolysis are the well-established technology and short reactor residence times. The main drawbacks are the corrosive action of the acid on equipment, the production of large amounts of salts, such as gypsum, from the neutralisation of the acid and the non-selectivity of the acid. Enzymatic hydrolysis has

Corresponding author

© 1997 American Chemical Society

the potential to give higher yields, and it is performed at temperatures only slightly above room temperature, 40-50°C. However, enzymatic hydrolysis requires pre­treatment in order to open up the compact structure of the cellulosic material (13-16).

When developing a new industrial process for ethanol production based on enzymatic hydrolysis, it is essential to have access to reliable experimental data. Most data found in the literature are based on laboratory-scale equipment, and determined under carefully controlled conditions, although some pilot facilities have been used to verify some of the laboratory data (17-19). Most of the data are also for individual steps, e. g. pretreatment, hydrolysis or fermentation, and are not always valid for a fully integrated process. This leads to many assumptions when estimating the ethanol production cost. This is reflected in the large discrepancy between estimated production costs. Several cost estimates for the production of ethanol can be found in the literature with prices ranging from 0.18 to 1.51 US$/L (20-22). Accurate design requires mid-scale process facilities with integrated process steps, e. g. bench-scale or pilot plants. A bench-scale unit is attractive, due to its flexibility and low equipment cost, for investigations of a variety of process parameters and process configurations. The more expensive pilot-plant facilities are, however, necessary, for final scale-up of the process.

Process development also requires technical and economic calculations in order to estimate equipment and energy requirements, to assess the overall production cost and to discriminate between various process configurations. Process simulations are also valuable for the identification of cost-critical steps and for the planning and configuration of experimental investigations in small-scale equipment. The data thus gathered can be used to refine the technical and economic models for more accurate calculations.

This chapter presents the combined use of a bench-scale unit and process simulation for the development and optimisation of an integrated bio-ethanol plant based on enzymatic hydrolysis of lignocellulosics.

James Weifu Lee and Elias Greenbaum[3]

Chemical Technology Division, Oak Ridge National Laboratory,
Oak Ridge, TN 37831-6194

Present energy systems are heavily dependent on fossil fuels. This will eventually lead to the foreseeable depletion of fossil energy resources and, according to some reports, global climate changes due to the emission of carbon dioxide. In principle, hydrogen production by biophotolysis of water can be an ideal solar energy conversion system for sustainable development of human activities in harmony with the global environment. In photosynthetic hydrogen production research, there are currently three main efforts: (1) direct photoevolution of hydrogen and oxygen by photosynthetic water splitting using the ferredoxin/hydrogenase pathway, (2) dark hydrogen production by fermentation of organic reserves such as starch that are generated by photosynthesis during the light period, and (3) Two-stage hydrogen production in a combined fermentative and light-driven algae/bacteria system. In this chapter, the advantages and challenges of these approaches for hydrogen production are discussed in relation to a new opportunity brought by our recent discovery of a new photosynthetic water-splitting reaction [Nature, 373, 438-441 (1995); Science, 273, 364-367 (1996)], which, theoretically, has twice the energy efficiency of conventional water splitting via the two-light-reaction Z-scheme of photosynthesis.

Hydrogen is a versatile, clean, and environmentally acceptable energy carrier. It can be produced by photolysis of water, an inexpensive and inexhaustible raw material. Photolysis can be performed using either inorganic systems such as semiconductors or living organisms such as cyanobacteria or green microalgae. It is now clear that green algae are probably better for H2 production than cyanobacteria, since the latter use the more energy-intensive enzyme, adenosine triphosphate (ATP)-requiring nitrogenase, for production of H2. Based on a recent feasibility analysis (7), H2 production by green algae can be more cost-effective than semiconductor photovoltaic electronics. The discussion in this article is focused on H2 production by photosynthetic water splitting.

R. M. Worden1, M. D. Bredwell1, and A. J. Grethlein2

department of Chemical Engineering, Michigan State University,
East Lansing, MI 48824
2Athena Neurosciences, 800 Gateway Boulevard,

South San Francisco, CA 94080

Biomass-derived synthesis gas can be readily converted into fuels and chemicals by anaerobic microorganisms. However, synthesis — gas fermentations typically exhibit low volumetric productivities due, in part, to low cell densities, production of unwanted by­products, and slow transfer of the synthesis gas into the liquid phase. Engineering approaches to improve bioreactor productivities are discussed, and recent advances in this area are summarized. Particular emphasis is placed on the use of bioreactor design to increase biocatalyst concentrations, development of metabolic models to study pathway regulation and the use of microbubble dispersions to enhance synthesis-gas mass transfer.

Synthesis gas, which consists primarily of carbon monoxide (CO), and hydrogen (H2), is produced by the partial oxidation of an organic feedstock at high temperature in the presence of steam. Although coal and petroleum have historically been the most commonly used feedstocks for synthesis-gas production, several new gasification plants have recently been based on biomass (7). Biomass offers several advantages over the traditional feedstocks. First, it has a much lower sulfur content than many coals. Synthesis gas produced from wood chips at the GE gasification plant in Schenectady, NY contained 28 ppm H2S (2), compared to 1-2% for coal-derived synthesis gases (3). Purification steps to remove sulfur from coal-derived synthesis gas are energy-intensive and add significantly to the product costs (4). Second, biomass materials are more reactive and thus require lower gasification temperatures and/or residence times. Fluidized-bed coal gasifiers are typically run at 1000°C using residence times of 0.5-3.0 h. By comparison, a temperature of 850°C was sufficient to gasify biomass using a residence time of 30 s to 5.0 min (5). Third, gasification of wood waste has the potential to solve a disposal problem while producing a valuable product. When wood waste is in

© 1997 American Chemical Society

short supply, short-rotation forestry can serve as a steady source of feedstock.

Synthesis gas can be catalytically converted into chemical products (e. g., methanol) in reactors operated at high temperatures and pressures. The status of such catalytic processes has recently been reviewed (6). Production of higher molecular weight alcohols (e. g., butanol) from synthesis gas is problematic, because existing catalysts yield a broad mix of alcohols. Anaerobic bioconversion of synthesis gas into fuels and chemicals represents an alternative approach that offers the advantages of lower temperatures and pressures, higher reaction specificity of the biological catalysts, and higher tolerance to sulfur compounds. A variety of anaerobes are able to convert synthesis-gas components into fuels and chemicals, including ethanol, butanol, acetic acid, butyric acid, and methane. The pathways of most of these microbes involve the conversion of CO or CO2 and H2 to the intermediate acetyl-CoA, which serves as a branch point for production of cell mass and two- and four-carbon alcohols and acids. Several reviews of microbial CO metabolism have been published (7,8,9,10). Whole-cell biocatalysts capable of converting synthesis gas to fuels and chemicals can tolerate orders of magnitude higher H2S concentrations (greater than 1%) than iron and nickel catalysts (1-10 ppm) used for Fischer Tropsch conversion of synthesis gas (11,12,13).

Synthesis-gas fermentations have potential for commercial development, and some of the engineering issues have been addressed (14,15,16). Low bioreactor productivity is a major obstacle to commercialization. Several factors contribute to the low productivity, including low cell density, an inability to regulate branched pathways to obtain only the most desirable product, inhibition of the biocatalysts by the reactants and products, and low rates of transfer of CO and H2 from the gas to the liquid phase. Several engineering approaches have been used in the past few years to overcome these limitations. The purpose of this paper is to summarize these approaches and discuss their impact on the feasibility of producing fuels and chemicals via synthesis gas fermentations.

We have reviewed critical features of the key unit operations necessary to economically operate bioethanol production plants; however, these technologies cannot be used to plan or model actual production plants until the process is integrated at reasonable scale. This view is commonly overlooked by researchers and planners involved in the important details of unit operations. Pilot plant verifications of biomass to ethanol processes have "discovered" critical problems that result from fully integrated operation that were not realized at bench or simulated integrated operation, especially contamination of cellulase production and SSF unit operations (164, 73) and mass/02 transfer problems that result from lignocellulose utilization at large scale (73). Other unforeseeable scale-up issues may have equal impact on process economics; for example, the problems encountered in scaling pretreatment equipment are reflected (to some extent) only by experience in the pulp and paper industry.

For bioethanol production processes, in which yield is crucial to economic operation (78), all process-related diversions of fermentable sugars from ethanol are catastrophic. In the pretreatment section, for example, effects of wear and normal aging on equipment contacted by hot, acidic biomass are difficult to model Char buildup on piping and reactor surfaces may pose a threat to biomass yield from unpredicted adsorption or condensation reactions, as well as the better understood effects of heat transfer reduction.

Another aspect of fully integrated plant operation not always considered is the consequence of allowing sugars, especially mono — and disaccharides, extended residence times following initial prehydrolysis or enzyme saccharification (765). Reducing sugars are reactive under virtually all conditions encountered in a biomass conversion facility! Mildly acidic conditions and moderate temperatures (i. e., conditions consistent with those following dilute acid pretreatment and storage) permit reversion and transglycosylation of glucose to generate populations of all possible configurations of alpha — and beta-linked disaccharides (i. e., 1-1, 1-2, 1-3, 1-4, and 1-6) and even higher-order oligomers in yields that approach 10% (766). Many of these oligosaccharides are not fermentable, and the nonreducing sugars, such as 1-1 beta — and alpha-linked glucose (a — and p-trehalose), are quite stable even after dilution and neutralization. When enzymes are added to the process, as in SHF or hybrid SSF schemes, reversion and transglycosylation reactions produce all possible configurations of disaccharides consistent with the anomeric requirement of the enzyme; i. e., a-glucosidase produces only alpha products and P-glucanases produce only beta products, but many linkage combinations are possible. Clear precedent exists for efficient production (20% to 37% yield) of transglycosylation products from 10% solutions of glycosyl donor (cellobiose) using a Fusarium oxysporum P-glucosidase (767) and from 10% solutions of maltose using an A. niger a-glucosidase (168). Enzymatic transglycosylation is also possible at glycosyl donor concentrations as low at 2% (769). Because most amylases and cellulases have active sites that can tolerate some diversity in the glycosyl acceptor group, it may be possible to find disaccharides or higher forms of glucose-xylose, glucose-galactose, and even glucose-mannose in biomass hydrolysis process streams. Of further concern to bioethanol plant efficiency are the observations that transglycosylation reactions that involve glycosyl transferases have been reported that utilize non-carbohydrate species as glycosyl acceptors, including alcohols (methanol ethanol and propanol) (767) and lignin model compounds (veratryl and vanillyl alcohols) (170). An unfortunate conclusion easily drawn from the literature is that the precise nature (chemical composition) of reversion and transglycosylation products formed during proposed bioethanol process operations is not currently known, nor is the resultant impact of this loss on fermentable sugars taken into account in process models. One critical reason for this dilemma lies in the advanced level of analytical capability required to properly detect and identify these products in biomass processing streams. A related and contributing problem is the inability of most laboratories to adequately assess fermentation process mass balances; thus, failure to achieve theoretical yields is often attributed to microbial or enzymatic performance problems, not to the presence of nonfermentable and unassayable sugars.

Until World War I, the major chemical route to acetone was the thermal conversion of "grey acetate of lime," i. e., calcium acetate derived from neutralizing pyroligneous acid (wood distillate) with lime (14). An excellent early kinetic study of this reaction was performed by Ardagh et al. (15) although the reaction is difficult (16).

The thermal conversion of calcium acetate, propionate, and butyrate to 2-propanone (acetone), 3-pentanone, and 4-heptanone, respectively, is a first-order reaction (17). The rate constants follow an Arrhenius dependence on temperature with the constants shown in Table IV. The Arrhenius constants for each species are similar except for calcium propionate below 398°C. The reactions are very rapid; at 440°C, it takes less than one minute to achieve 90% conversion.

The thermal decomposition is quite selective to liquid products with theoretical yields over 93% for calcium acetate and butyrate, and over 87% for calcium propionate. Gaseous products account for the remaining material. In the liquid product, over 90% is the primary product (2-propanone, 3-pentanone, 4-heptanone) with other ketones being the dominant by-products. These by-products will also contribute to the value of the final fuel. The composition of the liquid product was remarkably independent of the temperature, even at very high temperatures, indicating that the products are stable at the reaction conditions.

A very few fungi and yeast produce succinate. The fermentation of filter paper cellulose by several anaerobic fungi has been studied in the absence and presence of methanogenic bacteria. In the absence of methanogens, Neocallimastix sp. strain L2,

N. frontalis RE1, N. patriciarum CX, Piromonas communis P and Sphaeromonas communis FG10 have been found, respectively, to produce 0.48, 0.59, 0.39, 0.81 and

O. 26 mol succinate/10 mol hexose. In the presence of methanogenic bacteria, the production of succinate is reduced significantly for all the fungi, a result attributed to the process of hydrogen transfer between the fungi and bacteria (137).

Muratsubaki elucidated the pathway by which succinate is produced by the anaerobic growth of Saccharomyces cerevisiae on glucose (138). The activity of fumarate reductase, which catalyzes the conversion of fumarate to succinate, is three times greater under anaerobic conditions than under aerobic conditions. However, succinate dehydrogenase activity is completely lost after ten hours of fermentation.

These observations indicate that for this organism the citric acid cycle has been modified to become a reductive pathway leading to succinate production during the anaerobic growth of S. cerevisiae on glucose (138).

Genetic Engineering of Klebsiella axytoca and other Bacteria for the SSF Process.

Many different bacteria have the native ability for cellobiose uptake and metabolism. However, none of these organisms produce ethanol efficiently without genetic modification. Research at the University of Florida has focussed on the genetic engineering of improved organisms for die SSF process using the portable ethanol pathway derived from Z. mobilis. Three organisms have been initially targeted: 1) Klebsiella oxytoca (35), an abundant organism in pulp and paper waste; 2) Erwinia which cause soft-rot of plant tissue (39); and 3) Bacillus (40). All three organisms utilize cellobiose and thus do not require supplemental B-glucosidase. The latter two organisms also secrete endoglucanases which can assist cellulose hydrolysis. Most of our published studies have focussed on K. oxytoca P2 in which the ethanol pathway genes have been chromosomally integrated (41). In this organism and in plasmid­bearing derivatives of Erwinia, ethanol is produced from solubilized sugars at greater than 90% of theoretical yield. Other studies indicate that no major barriers exist to prevent the transfer of the ethanol pathway to Bacillus, although optimal strains have not been reported (40).

Optimization of SSF using K. axytoca P2. Commercial fungal cellulases produced by Trichoderma are most active under conditions too extreme (45-50°C, pH 4.0-5.0) for the growth of strain P2. A series of experiments was conducted with Sigmacell 50 (crystalline cellulose; Sigma Scientific Company, St. Louis, MO) to identify the optimal conditions for an SSF process with this biocatalyst (37). Spezyme CE and Spezyme CP cellulases were generously provided by Genencor International (South San Francisco, CA). Temperature was varied from 30°C-40°C at pH 5.0-pH 6.0. Surprisingly, ethanol yields exceeded 70% of the theoretical maximum (0.568 g ethanol/g cellulose) in most cases. The highest yield and rates of ethanol production were obtained at pH 5.2 and 35°C. Under these conditions, cellulase enzymes from Trichoderma are very stable and continue to be active for many days. Further studies were conducted to examine the dose dependence of cellulase enzymes. With 100 g cellulose/L, a loading of 1000 filter paper units of cellulase (FPU)/L (ie., 10 FPU/g cellulose) approached saturation. Under these conditions, the overall efficiency for fermentation plus saccharification was 72% of the theoretical maximum (Figure 4).

Comparison of SSF with Different Substrates

Time (h) Figure 4. Production of ethanol from different substrates by SSF using K. oxytoca strain P2. Fermentations contained 100 g substrate/L and 1,000 FPU cellulase/L.

Further studies were conducted using sugar cane bagasse which had been treated with dilute acid to remove hemicellulose (Figure 4) (42). Pretreatment was essential for saccharification and fermentation. However, this material was much less digestible than Sigmacell 50 and required approximately twice as much cellulase enzyme to achieve high yields. Partial saccharification of acid-treated bagasse (pH 4.8 and 48°C) for 12 h with enzymes alone (without biocatalysts) improved mixing and fermentability with a modest benefit to yield. In many cases, SSF with acid treated bagasse stopped after less than 50% of the cellulose had been digested. A brief heat treatment (followed by re-inoculation) was found to rejuvenate the saccharification process. Although the basis for this effect is not understood, it is possible that brief heating to around 60°C allows cellulases which are bound at nonproductive sites to be released. Subsequent binding to at new sites may allow saccharification to resume.

Some of our most successful studies have been conducted with mixed office waste paper (38). This substrate is highly digestible with commercial cellulase (Figure 4). Enzyme loadings of around 8.3 FPU/g cellulose approach saturation with Spezyme CP. Over 40 g ethanol/L was produced after 96 h. As with bagasse, partial saccharification prior to inoculation improved mixing but was of little benefit to ethanol yield. Dilute acid pretreatment of mixed waste office paper solubilized approximately 10% of the dry weight and improved mixing in SSF experiments. However, this pretreatment does not appear essential. Yields of around 80% of the theoretical maximum were achieved in batch fermentations with 1000 FPU/L and 100 g MWOP/L (approximately 530 L ethanol/metric ton of mixed office waste paper) (Table III).

The re-utilization of cellulase enzymes in consecutive SSF processes with MWOP allowed a dramatic reduction in the requirement for fungal cellulase (Figure 5). Fungal endoglucanase and cellobiohydrolase have specific cellulose-binding domains which facilitate recycling (38). Cellulosic residue at the end of fermentation contains bound cellulase. By adding this residue to subsequent fermentations, both product yield and enzyme effectiveness were improved. With three consecutive recycles, 40 g ethanol was produced after each 80-h fermentation with 83% of theoretical yield using an average of only 570 FPU/L of fermentation broth. For this substrate, the estimated cost of cellulase enzyme produced on site is $0,085 per liter of ethanol, $0.32 per gallon of ethanol. Approximately 539 liters of ethanol per metric ton are projected using this approach (Figure 6).

Figure 6 shows a comparison of results from SSF fermentations with K. oxytoca P2 and yeasts. Further detail is provided in Table III. Two important parameters are highlighted in this comparison, ethanol production per 1000 FPU commercial cellulase and ethanol yield per metric ton of feedstock. Both represent major cost factors for a commercial process. This comparison illustrates the benefit of utilizing

K. oxytoca P2 containing a cellobiose uptake system for batch fermentations for recycling. SSF processes with P2 allow the maintenance of high ethanol yields with a fraction of the cellulase enzyme needed with yeast as a biocatalysts. However, the cost of fungal cellulase remains substantial and further efforts should be made to reduce the levels of these enzymes needed in bioconversion.

Most vegetable oils are triglycerides (TGs; triglyceride = TG). Chemically, TGs are the triacylglyceryl esters of various fatty acids with glycerol (Figure 1).

Some physical properties of the most common fatty acids occurring in vegetable oils and animal fats as well as their methyl esters are listed in Table I. Besides these fatty acids, numerous other fatty acids occur in vegetable oils and animal fats, but their abundance usually is considerably lower. Table II lists the fatty acid composition of some vegetable oils and animal fats that have been studied as sources of biodiesel.

The most common derivatives of TGs (or fatty acids) for fuels are methyl esters. These are formed by transesterification of the TG with methanol in presence of usually a basic catalyst to give the methyl ester and glycerol (see Figure 1). Other alcohols have been used to generate esters, for example, the ethyl, propyl, and butyl esters.

Selected physical properties of vegetable oils and fats as they relate to their use as DF are listed in Table III. For esters these properties are given in Table IV. Also listed in Table III are the ranges of iodine values (centigrams iodine absorbed per gram of sample) of these oils and fats. The higher the iodine value, the more unsaturation is present in the fat or oil.

That vegetable oils and their derivatives are suited as DF is shown by their CNs (Table III) which generally are in the range suitable for or close to that of DF. The heat

Continued on next page

a) Z denotes cis configuration.

b) The numbers denote the number of carbons and double bonds. For example, in oleic acid, 18:1 stands for eighteen carbons and one double bond.

c) Melting points and boiling points given in Ref. 28, pp. C-42 to C-553. Melting points and boiling points of 12:0 -18:0 and 18:3 esters given in Ref. 181.

d) Superscripts in boiling point column denote pressure (mm Hg) at which the boiling point was determined.

e) See Ref. 27.

f) Cetane number from Ref. 21. Number in parentheses indicates purity (%) of the material used for CN determinations as given by the author. Other CNs given in Ref. 21 not tabulated here (purities in parentheses): ethyl caprate (10:0) 51.2 (99.4); ethyl myristate (14:0) 66.9 (99.3); propyl caprate (10:0) 52.9 (98.0); isopropyl caprate (10:0) 46.6 (97.7); butyl caprylate (8:0) 39.6 (98.7); butyl caprate (10:0) 54.6 (98.6); butyl myristate (14:0) 69.4 (99.0).

g) CN from Ref. 17. CNs (lipid combustion quality numbers) deviating from Ref. 21 as given in Ref. 17: Methyl laurate 54, methyl myristate 72, methyl palmitate 91, methyl stearate 159.

contents of various vegetable oils (Table III) are also nearly 90% that of DF2 (11-13). The heats of combustion of fatty esters and triglycerides (14) as well as fatty alcohols (15) have been determined and shown to be within the same range.

The suitability of fats and oils as DF results from their molecular structure and high energy content. Long-chain, saturated, unbranched hydrocarbons are especially suitable for conventional DF as shown by the CN scale. The long, unbranched hydrocarbon chains in fatty acids meet this requirement. Saturated fatty compounds have higher CNs. Other observations (16) are (i) that (a) double bond(s) decrease(s) quality (therefore, the number of double bonds should be small rather than large, (ii) that a double bond, if present, should be positioned near the end of the molecule, and (iii) no aromatic compounds should be present. A correlation to the statement on double bond position is the comparison of the CNs of methyl oleate (Table I), methyl petroselinate (methyl 6(Z)-octadecenoate and methyl cw-vaccenate (methyl ll(Z)-octadecenoate). The CN of methyl petroselinate (petroselinic acid occurs in less common oils such as parsley and celery seed oils) is 55.4 and that of methyl c/s-vaccenate (vaccenic acid occurs in fats such as butter and tallow) is 49.5 (17). In that study the CN of methyl

oleate was 47.2, the lowest of these 18:1 methyl esters. The double bond of methyl petroselinate is closer to one end of the molecule. It also has the longest uninterrupted alkyl chain of these compounds, which may play a role because alkanes have higher CNs as discussed above. This complements the observations in Ref. 16. Another possibility is benzene formation by a disproportionation reaction from cyclohexane, which in turn would arise from cleavage of methyl oleate (7 7). The low CN of benzene would account for the lower CN of methyl oleate. The other 18:1 compounds would not form cyclohexane due to the different positions of the double bond.

Table III. Fuel-related properties and iodine values of various fats and oilsa

Oil or Fat Iodine CN HG Viscosity CP PP FP Value (kJ/kg) (mm2/s) (°С) (°С) (°С)

a) Iodine values combined from Refs. 176 and 181. Fuel properties from Ref. 11. All tallow values from Ref. 177 (No CN given in Ref. 177, calcd. cetane index 40.15).

The combustion of the glyceryl moiety of the TGs could lead to formation of acrolein and this in turn to the formation of aromatics (76), although no acrolein was found in precombustion of TGs (18). This may be one reason why fatty esters of vegetable oils perform better in a diesel engine than the oils containing the TGs (16). On the other hand, as discussed above, benzene may arise from the oleic moiety also.

a) CN = cetane number; CP = cloud point, PP = pour point, FP = flash point, b) Some flash points are very low. These may be typographical errors in the references or the materials may have contained residual alcohols, c) Ref. 42. d) Ref. 55. e) Ref. 178. f) Ref. 17. g) Ref. 179. h) Ref. 177. i) Ref. 180. j) Ref. 95. k) Ref. 127. 1) Ref. 123.

However, the high viscosity of the TGs is a major contributing factor to the onset and severity of durability problems when using vegetable oils (19-20).

The above statements on CNs correlate with the values given in Tables I, III and IV. For example, corresponding to components of conventional DF, saturated fatty compounds show higher CNs than the unsaturated compounds. CNs generally increase with increasing chain length (21). The CNs of mixtures are influenced by the nature of their components. Correlation of data from Tables II, III and IV shows that major high — CN components lead to relatively high CNs of vegetable oils or their esters.

In some literature it is emphasized that biodiesel is an oxygenated fuel, thus implying that their oxygen content plays a role in making fatty compounds suitable as DF by “cleaner” burning. However, the responsibility for this suitability rests mainly with the hydrocarbon portion which is similar to conventional DF. Furthermore, the oxygen in fatty compounds may be removed from the combustion process by decarboxylation, which yields incombustible C02, as precombustion (18), pyrolysis and thermal decomposition studies discussed below imply. Also, pure unoxygenated hydrocarbons, like cetane, have CNs higher than biodiesel. Fatty alcohols, whose oxygen content is lower than that of the corresponding esters, also have CNs higher than the corresponding methyl esters as determined with ASTM D613. For example, the CN of 1-tetradecanol is 80.8 (22). The CNs of fatty alcohols also increase with chain length with 1-pentanol having a CN of 18.2 (22). The CNs of 1-hexadecanol and 1-octadecanol were not determined in this work due to their high melting points (22), but ignition delay with the constant volume combustion apparatus (CVCA) vessel discussed below was measured. The CNs of some fatty alcohols were lower when employing the CVCA. Fatty ethers (23) were also shown to have CNs higher than the corresponding fatty esters and were suggested as DF extenders. Their main disadvantage compared to esters is their less straightforward synthesis.

The CNs of esters correlate well with boiling points (27). Quantitative correlations and comparison to numerous other physical properties of fatty esters confirmed that the boiling point gives the best approximation of CN (22).

ASTM D613 is used in determining CNs. For vegetable oil-derived materials, an alternative utilizes a CVCA (24). The amount of material needed for CN determination was reduced significantly with this bomb and it also allows studying materials with high melting points that cannot be measured by ASTM D613. Estimated cetane numbers (ECN) were determined on a revised scale permitting values greater than 100. In this case, the ECN of methyl stearate is 159 and that of methyl arachidate (20:0) is 196 (24). The ECNs of other esters were methyl laurate 54, methyl myristate 72, methyl palmitate 91, and methyl oleate 80. ECNs of fatty alcohols were 1 — tetradecanol 51,1 — hexadecanol 68, 1-octadecanol 81, oleyl alcohol 51, linoleyl alcohol 44, linolenyl alcohol 41, and palmitoleyl alcohol 46. The ECNs of the TGs trilaurin and trimyristin exceeded 100, while the ECN of tripalmitin was 89, tristearin 95, triolein 45, trilinolein 32, and trilinolenin 23. The term “Lipid Combustion Quality Number” with an accompanying scale was suggested instead of CN to provide for values in excess of CN 100.

Often the “cetane index” of a fuel is published and should not be confused with CN. This is an ASTM-approved alternative method for a “non-engine” predictive equation of CN for petroleum distillates (25 and references therein). Equations for predicting CNs are usually not applicable to non-conventional DFs such as biodiesel or other lipid materials (26) . Cetane indices are not given here. A method for estimating the cetane indices of vegetable oil methyl esters has been presented (27).

Besides CN, heat of combustion (HG) is another property of fatty compounds that is essential in proving the suitability of these materials as DF {14). Heats of combustion of fatty compounds, oils and fats as well as their methyl esters are listed in Tables I, III, and IV. For purposes of comparison, the literature values {28) for the heat of combustion of hexadecane (cetane), the high CN standard for conventional DF, is 2559.1 kg-cal (at 20°C). The data in Table I show that the heats of combustion of fatty compounds are similar to those of the compounds of similar CH content (long-chain, unbranched alkanes such as hexadecane) ideally comprising conventional DF. For example, the heat of combustion of methyl palmitate is 2550 kg-cal, that of methyl stearate is 2859 kg-cal, and that of unsaturated methyl oleate is 2828 kg-cal.

Even the combined CN and heat data do not suffice to determine the suitability of a material as DF. This is shown by the data in Tables III, which list the viscosities as well as cloud and pour points of numerous vegetable oils and fats. The viscosity of vegetable oils is approximately one order of magnitude greater than that of conventional DF. The high viscosity with resulting poor atomization in the combustion chamber was identified early as a major cause of engine problems such as nozzle coking, deposits, etc. {14, 29-31). Therefore, neat oils have been largely abandoned as alternative DFs.

Four possible solutions to the viscosity problem have been evaluated (52). The most common applied solution to this problem is the preparation of the methyl esters by transesterification. The three other solutions to the problem of high vegetable oil viscosity are dilution (blending) with conventional DF or other suitable hydrocarbons, microemulsification or (co-solvency), and pyrolysis. These processes are also discussed below. As shown in Table IV, the methyl esters of oils and fats have viscosities approaching that of DF2.

The methyl esters, however, have higher cloud and pour points than their parent oils and fats and conventional DF (Tables III and IV). This is important for engine operation in cold or cooler environments. The cloud point is defined as the temperature at which the fuel becomes cloudy due to formation of crystals which can clog fuel filters and supply lines. The pour point is the lowest temperature at which the fuel will flow. It is recommended by engine manufacturers that the cloud point be below the temperature of use and not more than 6 °С above the pour point.