Biodiesel Fuels

Natural glycerides have been investigated as alternative fuels for the compression-ignition engine since the late 1800s. Rudolph Diesel, the inventor of the engine that bears his name, demonstrated a diesel-cycle engine fueled with peanut oil in 1900 at the Paris Exposition. In 1912, Diesel wrote: “The use of vegetable oils may become in the course of time as important as petro­leum and the coal tar products of the present time.” This obviously has not happened, since the majority of heavy-duty farm and construction vehicles and large trucks are powered by diesel engines that operate on petroleum diesel fuels. Of the two basic types of diesel engines, direct injection, in which the diesel fuel is injected directly into the combustion chamber, and indirect injection, in which the fuel is injected into a precombustion chamber, the direct injection design is more commonly used for the larger vehicles. The direct-injected engines are easier to start and less expensive than the more elaborate but quiet indirect-injected engine (Quick, 1989).

Many natural glycerides can be used with little or no difficulty as diesel fuels for vehicles equipped with indirect-injection engines. Several problems arise when they are used to fuel direct-injection engines. One of the problems is caused by the higher viscosity and lower volatility of natural glycerides as compared with the corresponding properties of petroleum diesel fuels (cf. Krawczyk, 1996). A comparison of selected properties of No. 2 diesel fuel, soybean and rapeseed oils, their corresponding transesterification products with methanol and ethanol, and the fatty acid makeup of the glyceride oils and esters is shown in Table 10.1.

Performance of the glycerides as a diesel fuel is improved by conversion of the fatty acid moieties of the glycerides to the corresponding methyl or ethyl esters. Fouling problems are significantly reduced, and the viscosities, pour points, and combustion characteristics of the esters in blends with diesel fuel or as neat liquids are superior to those of the natural glycerides. The cetane numbers of the methyl and ethyl esters are about 50 to 65, and they have lower ash, sulfur, and volatilities and higher flash points than conventional diesel fuels. The cetane numbers of the methyl and ethyl esters of the pure fatty acids in the oils have been correlated with the chain length and degree of saturation (Freedman et ah, 1990; Clements, 1996; Knothe, Bagby, and Ryan, 1996). For the ethyl esters of the C18 fatty acids, stearic, oleic, linoleic, and linolenic acids, the reported cetane numbers are 77, 54, 37, and 27, respectively. The corresponding cetane numbers determined for the free acids

TABLE 10.1 Comparison of Some Typical Properties of Diesel, Soybean Oil, Rapeseed Oil, and Ester Fuels11 Property No. 2 diesel Soybean oil Rapeseed oil Oil Methyl ester Ethyl ester Oil Methyl ester Ethyl ester Specific gravity 0.8495 0.92 0.886 0.881 0.91 0.880 0.876 Viscosity at 40°C, mm2/s 2.98 33 3.891 4.493 51 5.65 6.17 Cloud point, °С -12 -4 3 0 0 -2 Pour point, °С -23 -12 -3 -3 -21 -15 -10 Flash point, °С 74 188 171 179 124 Boiling point, °С 191 339 357 347 273 Water & sediment, vol % <0.005 <0.005 <0.005 <0.005 <0.005 Carbon residue, wt % 0.16 0.068 0.071 0.08 0.06 Ash, wt % 0.002 0 0 0.002 0.002 Sulfur, wt % 0.036 0.01 0.012 0.008 0.01 0.012 0.014 Cetane number 49 38 55 53 32 62 65 Copper corrosion 1A 1A 1A 1A 1A Higher heating value, MJ/kg 45.42 39.3 39.77 39.96 40.17 40.54 40.51 MJ/L 38.58 36.2 35.24 35.20 36.60 35.68 38.00 Fatty acid composition, wt % Palmitic (16:0) 9.8 9.9 10.0 1 2.2 2.6 Stearic (18:0) 2.4 3.8 3.8 0.9 0.9 Oleic (18:1) 28.9 19.1 18.9 32 12.6 12.8 Linoleic (18:2) 50.7 55.6 55.7 12.1 11.9 Linolenic (18:3) 6.5 10.2 10.2 15 8 7.7 Eicosenoic (20:1) 0.2 0.2 7.4 7.3 Behenic (22:0) 0.3 0.3 0.7 0.7 Erucic (22:1) 0.0 0.0 50 49.8 49.5 Others 1.7 0.8 0.9 2 6.3 6.6 “Adapted from Cruz, Stanfill, and Powaukee (1996); Peterson et al. (1995); Reed (1993); Shay (1993); Stumborg et al. (1993); Auld, Peterson, and Korns (1989). The figures in parentheses after each fatty acid denote the number of carbon atoms and double bonds in the fatty acid. The figures for “Oil” are

are 62,46,31, and 20. These trends would be expected because of the increasing degree of unsaturation from stearic to linolenic acid and the fact that more paraffinic hydrocarbons usually have higher cetane numbers. Similar correla­tions were found for other esters.

Since the methanol and ethanol transesterification products of a variety of commercial biomass-derived oils, such as soybean, rapeseed, peanut, palm, and sunflower oils, and even waste animal fats have generally been found to be suitable as additives or neat fuels for diesel-powered engines without engine modifications, programs were started, first in several European countries and then the United States, to develop and market what has been termed “biodiesel.” Biodiesel is defined as monoalkyl esters of long-chain fatty acids derived from renewable feedstocks, such as vegetable oils and animal fats, for use in compres­sion ignition engines. The emphasis in European biodiesel programs has been on the methyl ester of rapeseed oil, and in the United States, the methyl esters of soybean and rapeseed oils have received the most attention (с/. Krawczyk, 1996). In Europe, the most advanced program is in Austria. Pilot plants, small — scale farm cooperatives, and industrial-scale plants (10,000 to 30,000 t/year capacities) have been built, and fuel standards for rapeseed oil methyl ester have been adopted. In Malaysia, the methyl ester of palm oil is being developed as biodiesel, and in Nicaragua, biodiesel applications of the oil from Jatropha curcas, a large shrub or tree native to the American tropics, are under investiga­tion (с/. Jones and Miller, 1991).

Many projects are underway in the United States to complete the U. S. database on the properties and performance of biodiesel. Included in this work are studies on the emissions and performance of engines and vehicles fueled with biodiesel as additives, neat fuels, and fuel blends; the effects of oxygenated additives in biodiesel on cetane number; the effects of interesterification of different vegetable oils followed by transesterification with methanol on bio­diesel properties; and the low-temperature flow properties, long-term storage effects, and toxicides of a variety of methyl and ethyl esters. The information being developed in this work continues to support the marketing of biodiesel. Some engine test results and fields trials with diesel vehicles indicate that a few issues must be addressed before biodiesel can be successfully commercial­ized. In order of importance and frequency of occurrence, these issues concern fuel quality, fuel filter plugging, injector failure, material compatibility, and fuel economy (Schumacher and Van Gerpen, 1996; Schumacher, Howell, and Weber, 1996). It appears that the fledgling biodiesel industry is approaching a stage similar to that of the petroleum industry many years ago, when detailed motor fuel specifications and test procedures were needed to facilitate fuel compatibility and interchangability from different suppliers across the country. The tentative specifications being developed for biodiesel by the American

“Schumacher, Howell, and Weber (1996); Weber and Johannes (1996). These are tenative specifi­cations for 100% biodiesel and are being evaluated by the American Society for Testing and Ma­terials as of March 4, 1996. A considerable amount of experience exists in the United States with blends of 20% biodiesel and 80% diesel fuels. Although biodiesel can be used in this form, the use of blends over 20% biodiesel should be evaluated on a case-by-case basis until further expe­rience is available. The GC method for glycerol is the Austrian update (Christiana Plane) of the U. S. Department of Agriculture method.

Society for Testing and Materials (ASTM) are shown in Table 10.2. After fine — tuning and approval, the ASTM standards are expected to eliminate many of the potential problems that can result from the large number of vegetable oils and animal fats that can be used to produce biodiesel. In other words, when a particular manufacturing source supplies biodiesel that meets the ASTM standards, it is assumed it will be fully compatible with all other sources of biodiesel and exhibit the same end-use characteristics even though the feed­stock may not be the same at each manufacturing site.

Market projection studies indicate that biodiesel which meets the ASTM standards will be manufactured and distributed initially for niche market applications such as government bus and truck fleets where the mandates of the Clean Air Act of 1990 require the use of cleaner burning alternative fuels. A major difficulty regarding large-scale marketing of biodiesel is consumer cost. This subject will be examined later in Section IV, but is not expected to be an insurmountable barrier in the long term.

Potential Sources of Biodiesel

Various oilseed crops afford natural triglyceride esters several of which are under development as diesel fuels as already mentioned. Some biomass species produce natural esters of fatty acids and fatty alcohols, such as the desert shrub jojoba (Simmondsia chinensis [Link] Schneider) which yields esterification products of C20-C22 straight-chain, monounsaturated fatty alcohols and acids (National Academy of Sciences, 1977). These esters have not been considered as a source of biodiesel, but could be readily converted to normal paraffinic hydrocarbons. The largest source of feedstock for conversion to biodiesel is the seed oils. Seed oils are largely triglycerides and typically contain three long-chain fatty acids, each bound to one of the carbon atoms of glycerol via an ester linkage. The fatty acid distributions of soybean and rapeseed triglycer­ides in Table 10.1 are typical of most seed oils. An analysis of the average and potential annual yields of oilseeds and seed oils is shown in Table 10.3. With the exception of the Chinese tallow tree (Sapium sebiferum), the range in potential oil yields is about 400 to 1800 L/ha-year (2.5 to 11.3 bbl/ha-year), but average commercial yields are about 30 to 60% of these values. The estimates of potential yields are based on discussions with experts in breeding and agronomy of the individual oilseeds, analyses of potential yields under experimental conditions, and knowledge of crop improvement considerations (Lipinsky et ah, 1984). In addition, the confidence limits associated with potential yields of commercial oilseeds such as corn and soybeans are much narrower than are the estimates for the newer oilseed crops, such as the Chinese tallow tree.

Soybeans are the most widely planted oilseed in the United States and account for more than 50% of the world’s oilseed output. Annual soybean oil production in the United States and worldwide in the mid-1990s was about 7 million and 12 million tonnes, which in petroleum industry terms is about 48 million and 82 million bbl/year, a substantial amount of potential liquid fuel. Worldwide, annual commercial production in the mid-1990s of biomass — based glycerides, which includes nonseed oils such as palm, olive, and coconut oil production of about 14 million tonnes, was about 62 million tonnes. Essentially all production is used for foodstuffs and specialized industrial applications. So although the amounts of biomass glycerides that could poten­tially be converted to biodiesel are significant, it is obvious that large-scale biodiesel manufacture from existing commercial production under ordinary market conditions is problematic. Development of a large-scale biodiesel indus­try in almost any industrialized country would in all likelihood require dedi­cated, incremental production of biomass glycerides over that which is required for foodstuffs and industrial use.

“Adapted from Lipinsky et al. (1984). Growth is under dry-land conditions except for cotton, which is irrigated. The yield for the Chinese tallow tree is one reported yield equivalent to 6270 L/ha of oil plus tallow and is not an average yield from several sources. It is believed that the yield would be substantially less than this in managed dense stands, but still higher than that of conventional oilseed crops.

The Chinese tallow tree, which has been cultivated and naturalized in the South Atlantic and the Gulf States, deserves special consideration because the yields of potential biodiesel feedstocks are much higher than those of conventional oilseed crops, and because the seeds are not currently harvested for commercial processing or foodstuff usage in the United States. The tree is a member of the Euphorbiaceae family and is native to subtropical China, where it has been cultivated for 14 centuries as a specialty oilseed crop, medicinal plant, and source of vegetable dye, and for uses similar to those of linseed oils (Morgan and Schultz, 1981; Scheld, 1986). Plantations can be established from cuttings, seedlings, or seed. The most convenient and econom­ical method of stand establishment is direct planting of seeds by standard, mechanical, row-crop planters. The tree grows well in saline lands which are marginal for conventional agriculture. Dense plantations of 0.60 to 0.75 m spacing are practical, and coppicing is a feasible system of management. The seed pods yield both a hard vegetable tallow on the outside and a liquid oil on the inside, which together comprise 45 to 50 wt % of the seed. Expressed as weight percentages of the seed, the tallow is typically 25 to 30% of the seed, the oil is 15 to 20%, the hull is about 40%, and the remainder is seed protein and fiber. In some strains, the tallow mainly consists of a single triglyceride, the palmitic-oleic-palmitic triglyceride. The oil is composed largely of glycerides of oleic, linoleic, and linolenic acids and is chemically similar to linseed oil and dehydrated castor oil. Both the tallow and oil have good potential as a source of biodiesel. The annual yields of tallow and oil are also remarkably high, each probably near 14.8 bbl/ha-year from a planting with seeds of about 11,200 kg/ ha (10,000 lb/ac).

Certain microalgae represent another source of natural triglycerides. Much research has been done on the culture and compositional characteristics of certain microalgae as a source of “algal oils” (cf. Klass, 1983, 1984, 1985; Brown, 1993). This work has been focused on the growth of the organisms under conditions that can promote glyceride formation. The oils are high in triglycerides and can be transesterified to form biodiesel in the same manner as other natural triglycerides (Hill and Feinberg, 1984). As mentioned in Chapter 4, the production of natural triglycerides from microalgae can some­times eliminate the high cost of cell harvest and extraction because it may be possible to separate the lipids by simple flotation or extraction from the culture media if they are leaked as extracellular products. As already mentioned, stressed growth conditions can often be used to increase the formation of natural lipids as shown for several microalgae in Table 10.4. The stress was caused by either the use of nutrient-deficient media or the addition of excess salt to nutrient-enriched media. The combination of both nutrient deficiency and salt enrichment appears to enhance lipid formation with Isochrysis sp., but to reduce it with Dunaliella salina. Interestingly, the free glycerol content can apparently be quite high for Dunaliella sp. This suggests either that all of the intracellular glycerol was not esterified, or that the esters were hydrolyzed after formation. Botryococcus braunii exhibited relatively high lipid contents under each set of growth conditions, but were the highest, 54.2 dry wt %, under nutrient-deficient growth conditions. Other microalgae have also been found to exhibit similar lipid contents. For example, the lipid content increased from 28 to over 50% of the cell dry weight with increasing nitrogen deficiency for Nannochloropsis sp. when grown in media under nitrogen-limited condi­tions (Tillett and Benemann, 1988). Lipid productivity was demonstrated to have a maximum of 150 mg/L-day at 5 to 6% cell nitrogen and was apparently independent of the light supply and cell density when the initial nitrogen concentration exceeded 25 mg/L. The key to the practical use of triglycerides from microalgae as feedstock for biodiesel production is the rate of triglyceride formation during the growth of microalgae. The yield on conversion of carbon

“Adapted from Tables 1-9 of National Renewable Energy Laboratory (1983). The culture media are denoted by FW, freshwater; NE, nutrient enriched; ND, nutrient deficient. The analyses were conducted on five different cultures of each species; statistical error analysis showed no standard error of more than 10%.

dioxide to microalgae is important, but high yields are not essential to obtain high triglyceride productivities. They are also a function of the growth rate of the particular microalgae and its triglyceride content. Commercial net produc­tivities of triglycerides from microalgae per unit growth area were projected to be 23 g/m2-day or 66 t/ha-year several years ago, and improvements through continued research were estimated to raise these figures to 43 g/m2-day or 124 t/ha-year (Hill and Feinberg, 1984). Although many species of microalgae have since been isolated and characterized, and small outdoor test facilities have been operated to collect data on optimal growth conditions, the sustained production of microalgae and the harvesting of the triglycerides in integrated, large-scale systems have not yet been demonstrated (Brown, 1993). Very high cell densities of the order of 46 million/ml have been achieved in a shallow, outdoor, 50-m2 raceway in Hawaii for the marine diatom Phaeodactylum tricor — nutum, the lipid content of which was found to be as high as 80% of its dry weight (Glenn, 1982; National Renewable Energy Laboratory, 1982; Shupe, 1982). This and similar projects on the growth of microalgae in fresh and saline water in California and the Southwest have since been terminated. Methods of maintaining optimal levels of nutrients, carbon dioxide, salinity, and temperature for continuous, outdoor microalgal growth, as well as eco­nomic methods of recovering triglycerides, must be developed and tested to allow design of large-scale systems.

Research on aquatic biomass by French researchers has resulted in several interesting results with the microalga B. braunii. In laboratory studies, this microalga, which under the growth conditions used is reported to have a hydrocarbon content as high as 75% of its dry weight, was reported to have been cultured at hydrocarbon productivities per unit growth area up to 15 g/ m2-day (Casadevall and Largeau, 1982). The dominant hydrocarbons produced are branched-chain olefins. In subsequent work, the French confirmed the high hydrocarbon content of B. braunii (Casadevall, 1984; Brenckmann, 1985; Brenckmann et al., 1985; Metzger et ah, 1985). Nitrogen limitation was not found to be necessary for high hydrocarbon production, the highest productivi­ties of which were observed during exponential growth. Light intensity did not affect the structure of the hydrocarbons, mainly C27, C29, and C3i alkadienes, which were produced in continuously illuminated batch cultures. But adjust­ment of the light intensity to the proper level gave maximum biomass and hydrocarbon yields. One strain was reported to yield straight-chain alkadienes and trienes as odd-numbered chains from C23 to C31, and another strain pro­duces triterpenes of the generic formula CnH2n_10 where n varies from 30 to 37.

The French results with B. braunii contrast sharply with the U. S. results and are unexpected because most of the literature on this organism describes lipids and not hydrocarbons as metabolites. The formation of lipids and hydro­carbons together for certain biomass species is not unique, as indicated in this chapter. It is surprising, however, that an organism is reported to produce higher lipid content under stressed growth conditions than that produced under nonstressed growth conditions with no mention of hydrocarbons, and high hydrocarbon production under nonstressed growth conditions with no mention of lipids. It may be that the unknown organic components in B, braunii (Table 10.4) are hydrocarbons, but it is unlikely. It is more probable that a major shift in biochemical metabolite formation can occur from one strain to another. In any case, it appears that certain microalgae are capable of high productivities of lipids or hydrocarbons. Such organisms could provide significant benefits in processes designed for the manufacture of liquid fuels from biomass.