WO2005087903A1

WO2005087903A1 - Blends of synthetic distillate and biodiesel for low nitrogen oxide emissions from diesel engines

Info

Publication number: WO2005087903A1
Application number: PCT/US2005/007663
Authority: WO
Inventors: Etop Esen; Andre L. Boehman; David P. Morris
Original assignee: Conocophillips Company; The Penn State Research Foundation
Priority date: 2004-03-09
Filing date: 2005-03-09
Publication date: 2005-09-22
Also published as: US20050210739A1; US20090293345A1

Abstract

This invention shows how to make and use a biodiesel-based fuel in diesel engines without incurring the NOx penalty. Embodiments primarily relate to an optimum range of bulk modulus of compressibility for biodiesel blends, which results in generating 'NOx neutral' biodiesel blends or to formulate biodiesel blends with lower NOx emissions than conventional petroleum diesel fuel. These biodiesel blends preferably comprise synthetic paraffinic middle distillate derived from a hydrocarbon synthesis to generate synthetic environmentally-friendly diesel fuels.

Description

BLENDS OF SYNTHETIC DISTILLATE AND BIODIESEL FOR LOW NITROGEN OXIDES EMISSIONS FROM DIESEL ENGINES BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to the blending of synthetic products derived from synthesis gas and biomass for the production of environmentally-friendly diesel fuels, which generate very low levels of NOx emissions when used in compression ignition engines.

Background of the Invention A number of performance specifications have been established for diesel fuels of different grades depending upon service application. A number of different properties are set out in these specifications including, for example, flash point, cloud point, pour point, viscosity, sulfur content, distillation range, gravity and ignition quality. Of these, the ignition quality is an important parameter and is usually expressed as cetane number (CN) determined by the standard ASTM test method D-613. Diesel fuel properties given the most attention are cetane number, aromatics content, and sulfur content. Internal combustion engines produce emissions containing water vapor, carbon dioxide, carbon monoxide, unburned hydrocarbons, oxides of nitrogen (NOx), carbonaceous soot and other particulate matter. Federal and state regulations dictate the amount of these and other pollutants, which may be emitted. Oxides of nitrogen, products of incomplete combustion, and particulates are considered atmospheric pollutants. Sulfur is, of course, associated with the production of acidic oxides of sulfur, an atmospheric pollutant. These compounds have been reported to contribute to "acid rain." During combustion of fuels that contain sulfur compounds, oxides of sulfur (SO_x), such as sulfur dioxide (S0₂), and sulfur trioxide (S0₃) are produced as a result of oxidation of the sulfur. Some fuels may contain nitrogen compounds that may contribute to the formation of oxides of nitrogen (NO_x). NO_x are primarily formed at high temperatures by the reaction of nitrogen and oxygen from the air used for the reaction with the fuel. Aromatics in diesel fuels are also considered undesirable, not only for their adverse effect on ignition quality, but also because they have been implicated in the production of significant amounts of particulates in the engine exhaust. Current environmental regulations are setting stricter specifications on diesel fuels, especially to address sulfur, nitrogen, and particulate emissions. Federal and state legislative bodies and agencies have issued a number of rules applicable to the production of clean diesel fuel in attempts to reduce emissions. Many technologies have been developed for reduction of SO_x and NO_x, but few can remove both types of pollutants simultaneously in a dry process or reliably achieve cost effective levels of reduction. A rapid series of diesel fuel improvements has been introduced in most parts of the developed world to provide reductions in particulates and NO_x from the vehicle fleets in current operation as well as to facilitate the introduction of after-treatment devices. Reducing the sulfur content and the "heavy end" of the fuel have been the key changes. Reducing the sulfur content typically involves the reduction of fuel sulfur via hydrotreating to levels as low as 10 parts-per- million (or ppm) as for example, in Swedish Mk 1 fuel. Other fuel parameters such as aromatics and cetane have also been the subject of investigation. Because of increasingly stringent federal and state regulations, demand for clean diesel fuels for compression ignition engines that contain virtually no sulfur and nitrogen, and with lower aromatic content, will likely increase. Clean diesel fuels having relatively high cetane number are particularly valuable. Clean distillates can be produced from petroleum-based distillates through severe hydrotreating at great expense. Such severe hydrotreating imparts relatively little improvement in cetane number and also adversely impacts the fuel's lubricity as discussed in an article by Booth & Wolveridge in Oil & Gas Journal, Aug.16, 2003, p 71-16. Fuel lubricity, required for the efficient operation of fuel delivery system, can be improved by the use of costly additive packages. Specially manufactured fuels and the incorporation of special fuel components such as biodiesels and Fisher Tropsch diesels, have been gaining attention. The production of clean, high cetane number distillates from Fischer-Tropsch synthesis has been discussed in the open literature. Generally, Fischer-Tropsch synthesis converts a mixture of hydrogen and carbon monoxide (called syngas) to a multitude of hydrocarbon molecules from 1 to 100 or more carbon atoms. Sources of synthesis gas can be obtained from reaction of methane or natural gas with an oxidant (water and/or molecular oxygen) and/or from gasification of coal, petroleum coke or biomass. The mixture of hydrocarbons from Fischer-Tropsch synthesis can be distilled, and its fractions submitted to various hydroprocessing schemes to generate valuable products such as gasoline, diesel, wax, and/or lubricants. Since the Fischer-Tropsch synthesis tends to produce primarily linear hydrocarbons (i.e., normal paraffins), the Fischer-Tropsch derived diesel product (FT diesel) is of particularly good quality. Fischer-Tropsch derived diesels typically have high cetane number (greater than 70), and have very low sulfur, nitrogen and aromatic contents. Research from Clark and coworkers disclosed in a 1999 article in Society of Automotive Engineers (SAE, Warrendale, PA) Technical Paper No. 1999-01-2251, and other studies have shown that FT diesel fuels yield lower NO_x emissions. Even though the low aromatic content of FT diesel yields good thermal stability and reduced tendency to form deposits in engine; the absence of aromatics can result however in swelling of elastomers in vehicle fuel systems, and hence seal leakage problems may arise. Moreover, the disclosed hydroprocessing schemes for preparing Fischer- Tropsch derived distillates also leave the diesel lacking in one specific property, e.g., lubricity. The Fischer-Tropsch derived distillates would require blending with other less desirable stocks or the use of costly additives, such as a lubricity-enhancing additive. Renewable diesel fuels are fuels that are used in diesel engines in place of or blended with petroleum diesel, but are made from renewable resources such as vegetable oils, animal fats, or other types of biomass, such as grasses and trees. Today Fischer-Tropsch diesel is made from fossil fuels (coal and natural gas), but a "biosyngas", a synthesis gas generated from biomass, could be used to make clean liquid fuels in the future. Biodiesel is an example of a renewable diesel fuel that is used across the world today. Biodiesel can be manufactured from vegetable oils, animal fats, waste vegetable oils (such as recycled restaurant greases, called yellow grease), microalgae oils, or any combination thereof, which are all renewable. These feedstocks can be transformed into biodiesel using a variety of esterification or transesterification technologies. Biodiesel use is growing rapidly, increasing from about 7 million gallons in 2000 to more than 20 million gallons in 2001, with additional production capacity available to quickly accommodate further growth. Current U.S. biodiesel production is based largely on soybean oil and used cooking grease, both of which are abundant feedstocks. The most frequently used biodiesel feedstock in Europe is rapeseed (canola) oil. No matter what the process or the feedstock used, the produced biodiesel must meet rigorous specifications to be used as a fuel. Fuel-grade biodiesel must be produced to strict industry specifications, as is described in the American Society for Testing and Materials method, ASTM D-6751, in order to insure proper performance in diesel engines. Technically, biodiesel is defined as a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, designated B100, and meeting the requirements of ASTM method D-6751. Fatty-acid alkyl esters are actually long chains of carbon molecules (8 to 22 carbons long) with an alcohol molecule attached to one end of the chain. Biodiesel refers to the pure fuel without blending with a diesel fuel derived from fossil fuels. The biomass-derived ethyl or methyl esters can be blended with conventional diesel fuel or used as a neat fuel (100% biodiesel). Biodiesel blends are denoted as "BXX" with "XX" representing the percentage of biodiesel contained in the blend (i.e. : B20 is 20% biodiesel, 80% petroleum diesel; B100 is pure biodiesel). Pure biodiesel typically requires special treatment in cold weather, due to a high pour point. Biodiesel, as defined in ASTM D-6751, is registered with the U.S. Environmental Protection Agency (EPA) as a fuel and a fuel additive under Section 211(b) of the Clean Air Act. Biodiesel is used, mostly as a 20% blend (B20) with petroleum diesel, in federal, state, and transit fleets, private truck companies, ferries, tourist boats, and launches, locomotives, power generators, home heating furnaces, and other equipment. Biodiesel is non-toxic and biodegradable. It is safe to handle, transport, and store, and has a higher flash point than petroleum diesel. Biodiesel can be stored in diesel tanks and pumped with regular equipment except in colder weather, where tank heaters or agitators may be required.

Biodiesel mixes readily with petroleum diesel at any blend level, making it a very flexible fuel additive. One of the unique benefits of biodiesel is that it significantly reduces air pollutants that are associated with petroleum diesel exhaust. It can help reduce greenhouse gas emissions, as well as sulfur emissions since biodiesel contains only trace amounts of sulfur, typically less than the new U.S. EPA rule finalized in 2001 that will require that sulfur levels in diesel fuel be reduced from 500 ppm to 15 ppm, a 97% reduction, by 2006. However, NO_x emissions are an exception, since in the case of biodiesel fueling there is a well documented increase of 2-4% in NOx emissions with a blend of 20 vol.% methyl soyate in petroleum diesel fuel such as is described by Graboski and McCormick in an article in Progress in Energy and Combustion Science (1998) vol. 24(2), pp. 125-164, hereafter referred to as the 'Graboski paper'. Researchers have been looking for the underlying causes of the biodiesel "NOx effect". Heywood has shown in "Internal Combustion Engine Fundamentals". 1988, McGraw-Hill, New York, p. 864, that advancing injection timing can lead to an increase in NOx emissions from diesel engines. Several researchers have reported an advance in the fuel injection timing when biodiesel is being used, such as is described by Choi and coworkers in a 1997 SAE Technical Paper No. 970218 hereafter referred to as the 'Choi paper', and by Monyem and coworkers in an article in Transactions of the ASAE (2001) vol. 44(1), pp. 35-42 hereafter referred to as the 'Monyem paper'. It was further demonstrated by the 'Monyem paper' that there was a linear dependence on NOx and the actual start of injection, regardless of the fuel used. Szybist and Boehman described in their 2003 SAE Technical Paper No. 2003-01-1039, which is incorporated herein by reference in its entirity, the use of a combination of spray visualization, laser attenuation, fuel line pressure sensing and heat release analysis to study the impact of biodiesel blending on the start of injection, end of injection and ignition delay in an air-cooled single cylinder direct injection diesel engine. They made comparisons of injection timing and duration for diesel fuel and a range of biodiesel blends (B20 to B100). Shifts in injection timing were observed between the fuel blends, amounting to a 1 crank angle (CA) degree difference between diesel fuel and pure biodiesel (B100). Combustion studies were also performed to see how the shift in injection timing affected the timing of the combustion process. There was an advance in ignition of up to 4 CA degrees with B 100, which can be attributed, at least in part, to the advanced injection timing. Some researchers suggested that differences in the physical properties of the biodiesel, such as its increased kinematic viscosity as described in the 'Choi paper' or a higher bulk modulus of compressibility as described in the 'Monyem paper', could lead to variations in fuel injection timing. Biodiesel has indeed a higher viscosity (ca. 4.1 mm²/s at 40°C) compared to diesel (ca. 2.6 mm²/s at 40°C). Viscosity directly influences the amount of fuel that leaks past the plunger in the fuel pump and the needle in the fuel injection nozzle. High viscosity fuels, such as biodiesel, lead to reduced fuel losses during the injection process compared to lower viscosity fuels, leading to a faster evolution of pressure, and thus an advance in fuel injection timing as shown by Tat and Van Gerpen (2003) in their NREL report NREL/SR-510-31462, p. 114, hereafter referred to as the 'Tat 2003 paper'. Tat and coworkers have previously suggested in an article in Journal of the American Oil Chemists Society (2000) vol. 77(3), pp. 285-289, hereafter referred to as the 'Tat 2000 paper', that a difference in the bulk modulus of compressibility could be responsible for the difference in fuel injection timing; and that the NOx increase with biodiesel fueling is attributable to an inadvertent advance of fuel injection timing. The effect of viscosity on fuel injection timing relative to the effect of bulk modulus is currently unknown. A diesel fuel injection system was modeled by Rakopoulos and Hountalas and described in their article in Energy Conversion and Management, 1996, vol. 37(2), pp. 135-150. The system was separated into five control volumes. A constant pressure throughout four of the control volumes could be assumed at any given time: the pumping chamber, delivery valve chamber, the injector main volume, and the sac volume. For these volumes the only fuel property that was important in modeling the pressure as a function of time was the bulk modulus of compressibility. A less compressible fuel will result in a faster rise in pressure in the chamber. Constant pressure throughout the fifth control volume, the fuel line from the pump to the injector, could not be assumed. The only fuel properties that were important in modeling the pressure as a function of time in the fuel line were the density and speed of sound, which is dependent on the bulk modulus of compressibility. It was not necessary to take fuel viscosity into account to in order to validate the model. In a similar model of the fuel injection system, a sensitivity analysis on the effect that the bulk modulus had on the fuel injection timing was performed by Arcoumanis and coworkers and described in their 1997 SAE Technical Paper No. 97034. A 10% increase in the compressibility of the fuel advanced the fuel injection timing 0.5 CA degrees. By comparison, they found that a fuel density difference had a negligible effect on the start of fuel injection. Fuel viscosity was not found to affect the fuel injection timing. Some researchers have reported that certain compositions in biodiesel were more prone to the "NO_x effect". Biodiesel fuel composition depends on the feedstock that is subjected to the transesterifcation process. McCormick and co-workers observed in a paper in Environmental Science and Technology (2001) vol. 35, pp. 1742-1747, hereafter referred to as the 'McCormick 2001 paper', that the unsaturated methyl and ethyl esters of fatty acids, produced from soybean and linseed oils, yield the highest NO_x increase in a diesel engine. In contrast, the most highly saturated methyl and ethyl esters of fatty acids, produced from tallow or by hydrogenating the ethyl and methyl esters, yield much lower NOx emissions, in some cases even lower than for diesel fuel. McCormick and co-workers (2001) stated that the fuel chemistry was at the root of the fuel properties and the increased NO_x emissions. Efforts to combat the "biodiesel NOx effect" have included blending of biodiesel with various fuel stocks, selection of different biodiesel feedstocks, and use of cetane improvers. McCormick's group showed in a 2002 SAE Technical Paper No. 2002-01-1658 hereafter referred to as the 'McCormick 2002 paper', that a roughly "NO_x neutral" B20 (soybean oil-derived) biodiesel fuel could be obtained by blending with the biodiesel with: a diesel blend consisting of 46% Fischer-Tropsch diesel fuel, a diesel basestock containing 10% aromatics, 1 vol.% di-tert-butyl peroxide (DTBP), 0.5 vol.% ethyl-hexyl nitrate, or 500 ppm by volume of ferrocene. In each blend, it appeared that the reduction in NOx from the base of B20 blended with conventional diesel fuel arose from enhancing the cetane number of the fuel, which each of the above cases should provide. Consequently, the problem remains that biodiesel (B100 and B20) increases NO_x emissions from diesel engines. There is still a need for clean alternate fuels for compression ignition engines in order to reduce NOx emissions. This invention provides a more efficient and effective method for blending synthetic distillates and biofuels to meet current NO_x specification for diesel specification or to formulate biodiesel blends with reduced NO_x emissions as well as very low sulfur emissions, without altering other important fuel specifications related to injection quality (cetane number). SUMMARY OF THE INVENTION These and other needs in the art are addressed in one embodiment by a method for forming a synthetic environmentally friendly diesel fuel or diesel fuel blendstock with reduced sulfur and NO_x emissions when used in compression ignition engines. This invention relates to the blending of a synthetic middle distillate derived from synthesis gas and a biodiesel derived from biomass for the production of environment-friendly diesel fuels, which generate very low levels of NO_x and sulfur emissions when used in diesel engines. The invention relates to a synthetic environmentally-friendly fuel for use in compression ignition engines, wherein the synthetic environmentally-friendly fuel comprises a mixture of a synthetic liquid distillate and a liquid biofuel, and wherein the synthetic environmentally-friendly fuel is characterized by a sulfur level less than 20 ppmS; a specific gravity equal to or less than 0.84; a bulk modulus of compressibility between about 1300 megapascals (MPa) and about 1600 MPa measured at 15 MPa and 37.8 degrees C (°C), and a cetane number greater than 55. The environmentally-friendly diesel fuel preferably has a specific gravity between about 0.78 and about 0.84. In some embodiments, the synthetic environmentally-friendly fuel comprises primarily the synthetic liquid distillate and the liquid biofuel, and may further contains minor components, such as fuel additives. In alternate embodiments, the synthetic environmentally- friendly fuel comprises essentially a mixture of the synthetic liquid distillate and the liquid biofuel. As used herein, "biofuel" is defined as a liquid energy source that is derived from agricultural crops or residues or from forest products or byproducts and can be substituted for liquid or gaseous fuels derived from petroleum or other fossil carbon sources. Biodiesel is one type of biofuel. In preferred embodiments, the environmentally-friendly fuel comprises a volume fraction of the liquid biofuel between about 1 percent and about 45 percent. In alternate embodiments, the environmentally-friendly fuel comprises a liquid biofuel volume fraction between 1 percent and 20 percent; or between about 20 percent and 45 percent; or at about 45 percent. Other embodiments for the environmentally-friendly fuels for compression ignition engines include blends of a synthetic distillate with two or more biofuels from different feedstocks.; blends of two or more synthetic distillates with a biofuel from one feedstock; or any combination thereof. The invention further relates to a method for forming a synthetic environmentally- friendly diesel fuel or diesel fuel blendstock, said method comprising blending a liquid biofuel with a synthetic liquid distillate so as to form a fuel for compression ignition engines, said fuel being characterized by a bulk modulus of compressibility measured at 15 MPa and 37.8 °C between about 1300 MPa and about 1600 MPa, a cetane number greater than about 55, and wherein the environmentally-friendly diesel fuel comprises less than 20 ppm sulfur. The specific gravity of the resulting blend is preferably equal to or lower than 0.84. The method may further include adjusting the volumetric ratio of synthetic distillate to biofuel in the fuel so as to meet a specific gravity of the fuel of less than 0.84. The synthetic middle distillate preferably is preferably derived from a Fischer-Tropsch synthesis. The method for forming a synthetic environmentally-friendly diesel fuel may further include the following steps: feeding a synthesis gas to a hydrocarbon synthesis reactor, wherein the synthesis gas is reacted under conversion promoting conditions to produce a hydrocarbon synthesis product; optionally, hydroprocessing at least a portion of said the hydrocarbon synthesis product to a hydroprocessing unit, wherein the hydrocarbon synthesis product is hydroprocessed; fractionating the hydrocarbon synthesis product to at least generate a synthetic middle distillate, wherein the synthetic middle distillate has a boiling range comprising a 5% boiling point between about 170°C and about 210°C and a 95% boiling point between about 320°C and about 350°C. The synthetic distillate utilized in the environmentally-friendly fuel is further characterized by a cetane number greater than 75, by a sulfur content less than 10 ppm sulfur; by a paraffin content greater than about 90 percent; and by a specific gravity less than about 0.8. The liquid biofuel in the environmentally-friendly fuel is preferably characterized by a sulfur content less than 30 ppm sulfur, and by a specific gravity greater than about 0.86. The liquid biofuel is derived from biomass such as vegetable oils, animal fats, waste vegetable oils, and/or microalgae oils. Preferably, the biofuel comprises an organic compound selected from the group consisting of esters of fatty acids, hydrogenated esters of fatty acids, pure vegetable oils, and combinations thereof. The liquid biofuel more preferably comprises primarily alkyl esters of fatty acids, wherein said fatty acids are characterized by having between 8 to 22 carbons atoms. The liquid biofuel is preferably derived by the esterifcation and/or transesteriflcation of one or more vegetable oils. The invention further relates to a method for forming a "NO_x neutral" fuel formulation comprising a synthetic liquid distillate and a liquid biofuel by adjusting the ratio of synthetic liquid distillate to liquid biofuel to achieve a bulk modulus of compressibility similar to that of a petroleum diesel formulation. Additionally, the invention relates to a method for forming a synthetic diesel fuel formulation comprising primarily of a synthetic distillate and a biofuel within a biofuel volumetric fraction within an optimum range which comprises a bulk modulus of compressibility in said synthetic diesel fuel formulation lower than that of a conventional (crude derived) diesel fuel such that the synthetic diesel fuel formulation generates reduced NO_x emissions compared to petroleum diesel fuel formulations. The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions^" do not depart from the spirit and scope of the invention as set forth in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings. Figure 1 illustrates the measured bulk modulus as a function of pressure for biodiesel (B100), FT diesel, blends thereof (B20-FT and B40-FT), 20% biodiesel blend in petroleum diesel fuel (B20), a ultra-low sulfur diesel (BP-15), a paraffinic solvent (Norpar^®), and soy oil; Figure 2 illustrates the relation between the bulk modulus of compressibility at 15 MPa and 37.8°C (100°F) of a diesel blend with biodiesel and FT diesel with respect to the biodiesel volume fraction in the diesel blend; Figure 3 illustrates the relation between injection timing (Crank Angle Variation) and bulk modulus of compressibility measured at 6.89 MPa and 100°F for biodiesel (B100), biodiesel blend with baseline diesel (B20), baseline petroleum diesel (BP-15), biodiesel blend with FT diesel (B20-FT) and a paraffin solvent (Norpar^®); Figure 4 illustrates the relation between specific gravity and bulk modulus of compressibility of various fuels measured at 6.89 MPa and 37.8°C (100°F); Figure 5 illustrates a high pressure housing for bulk modulus measurements; and Figure 6 illustrates the schematic diagram of a spray visualization chamber connected to a direct injection diesel engine with access for digital imaging and laser attenuation measurements of fuel injection timing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS To more clearly illustrate the present invention, several drawings are presented. However, no limitations to the current invention should be ascertained from the drawings presented herein below. This invention shows how to use a biodiesel-based fuel in diesel engines without incurring the NO_x penalty, that should ultimately assist in deploying the use of biodiesels in areas where ozone non-containment is a problem and a hindrance to biodiesel utilization. Even though there exist physical as well as chemical reasons for the changes in NO_x emissions with fuels, the Applicants believe that trends in NO_x emissions can be related primarily to the bulk modulus of compressibility of the fuel. There exists therefore an optimum range of bulk modulus of compressibility for biodiesel blends, which results in generating "NO_x neutral" biodiesel blends or to formulate biodiesel blends with lower NO_x emissions than conventional diesel fuel. A related factor to the compressibility of the fuel is the speed of sound. The speed of sound of a fuel can affect the performance of some types of fuel injection systems, reducing the time for the pressure pulses to travel from the injection pump to the injector. All injection systems compress more fuel that is injected. The bulk modulus and speed of sound of fuels are related to each other and to fuel density through mathematical relationships. By measuring a fuel's speed of sound and density, the adiabatic bulk modulus can be calculated. A higher bulk modulus of compressibility, or speed of sound, in the fuel blend, leads to a more rapid transferal of the pressure wave from the fuel pump to the injector needle and an earlier needle lift. This property, the bulk modulus of compressibility, can then be associated with fuel injection performance on biodiesel and blends. Tat and coworkers measured and disclosed the bulk modulus of compressibility for biodiesel and petroleum diesel in the 'Tat 2000 paper' from atmospheric pressure to 35 MPa, and showed that the bulk modulus of compressibility increased linearly with pressure. Additionally, for the pressure range they studied, the bulk modulus of biodiesel was always 5-10% higher than diesel fuel. One can therefore conclude that biodiesel is less compressible than petroleum diesel. The accompanying Figure 1 shows the measured bulk modulus of compressibility for some fuel samples: a biodiesel fuel (B100; methyl soyate), a biodiesel blend B20 with a petroleum diesel, an unrefined soybean oil, a commercially-available normal paraffin mixture from Cπ - ₅ (Norpar^®- 13), a Fischer-Tropsch diesel, two biodiesel blends with FT diesel (B20-FT; B40- FT), and an ultra-low sulfur baseline diesel fuel (BP-15). Norpar^® is the trademark for a line of hydrocarbon fluids with very high normal paraffin content (>95%) and relatively narrow boiling ranges, commercially available from ExxonMobil corporation. It will be understood that other such hydrocarbon fluids can be substituted therefor. The bulk modulus of compressibility for B20, B100 and the soybean oil are higher than that of the baseline diesel fuel, consistent with the results reported in the 'Tat 2000 paper'. Figure 1 also shows that the bulk modulus of compressibility for B20 was slightly above that of the baseline diesel. One should note that the "B20" fuel from the 'Graboski paper' had between 2 and 4% higher NOx emissions than the diesel fuel. We can conclude that the increase in NOx emissions observed for B20 by Graboski and McCormick in the 'Graboski paper' was due to an increase in the bulk modulus of compressibility of the B20 formulation. Figure 1 also shows that the bulk modulus of the B40-FT blend is slightly below that of the baseline diesel, and that the B20-FT blend and FT diesel have significantly lower bulk moduli of compressibility than the baseline diesel. Figure 1 therefore shows that FT diesel and blends of FT diesel comprising up to 40% biodiesel volume fractions are therefore more compressible than baseline petroleum diesel. Figure 2 represents the bulk modulus of compressibility for biodiesel blends with FT diesel versus the biodiesel volumetric fraction of the blends, 0% being pure FT diesel and 100% being pure B100. Figure 2 can be utilized to determine what blend ratio would be equivalent in bulk modulus to the baseline diesel and what blend ratio would yield a bulk modulus lower than that of the baseline diesel. A B45-FT blend corresponds to a bulk modulus of compressibility at 15 MPa of 1580 MPa, equivalent to that of the baseline diesel. Hence the B45-FT blend would comprise a "NOx neutral" biodiesel blend. Accordingly, the biodiesel-FT blends with less than 45% of biodiesel should give a reduced level of NO_x emissions when used in a diesel engine compared to a petroleum diesel or a B20 blend with petroleum diesel, or B 100 biodiesel. Thus, one strategy for combating the biodiesel "NO_x effect" is to use highly paraffinic diesel fuels, such as FT diesel as the diesel basestock, in such as way that the bulk modulus of compressibility is within a desirable range to result in NO_x emissions equating current petroleum diesel fuel or to result in even lower NO_x emissions in compression ignition engines than current petroleum diesel fuel. The plot in Figure 2 also indicates that the bulk modulus of methyl soyate biodiesel blends with FT diesel increases in a linear fashion with respect to the biodiesel volumetric fraction in the blend. This linear fit of bulk modulus of biodiesel blends and biodiesel fractions should be quite helpful in determining the optimal range of biodiesel/FT diesel ratios, once the bulk modulus of compressibility is determined for the pure biodiesel and for pure FT diesel. A correlation can be established to calculate the biofuel volume fraction needed in a biofuel / synthetic distillate blend to make the environmentally-friendly diesel fuel with a desired bulk modulus of compressibility selected within an optimum range of bulk moduli of compressibility once the measured bulk moduli of compressibility of the two main components of the blends: biofuel and synthetic distillate are measured. This correlation is shown by Equation (1): x_b = 100 (B -B_sd) / (B_b - B_sd) (1) wherein B represents the desired bulk modulus of compressibility of the fuel blend at a specific pressure and a specific temperature; B_sd represents the measured bulk modulus of compressibility of the synthetic distillate at the same pressure and temperature settings; B represents the measured bulk modulus of compressibility of the liquid biofuel at the same pressure and temperature settings; and x_b represents the calculated biofuel volume fraction in the resulting fuel blend to achieve the desired bulk modulus B of compressibility of the fuel blend. Figure 3 indicates that there is an advance of fuel injection timing (positive CA change) for the diesel-biodiesel (B20) and pure biodiesel (B100). B100 has the most advance in fuel injection timing and also has the highest bulk modulus of compressibility as shown in Figure 3. The advanced in injection timing confirms the work described in the 'Choi paper' and the 'Monyem paper'. The 'Choi paper' reported an advance in fuel injection timing, 0.6 CA degrees with a 40% volume blend of biodiesel with a petroleum diesel. The 'Monyem paper' reported an advance in fuel injection timing, based on the fuel line pressure, of 2.3 CA degrees with neat biodiesel, and 0.25 to 0.75 CA degrees with a 20% volume blend of biodiesel using a John Deere 4276 Dl engine. Figure 3 also shows that the purely paraffinic solvent, Norpar^®, and the 20% blend of B 100 and FT diesel (B20-FT) retard the fuel injection timing by 0.5 CA and 0.1 CA respectively, compared to the baseline diesel (BP-15). Norpar^® has the most retardation in fuel injection timing and also has the lowest bulk modulus of compressibility as shown in Figure 3. This retardation in injection timing associated with the paraffinic mixture gives support to the proposition that variation in injection timing due to the lower bulk modulus of compressibility is a contributing factor in the reductions in NO_x emissions observed with FT diesel fuels. And, the retarded injection timing provides an explanation for the reduced NO_x emissions measured by McCormick' s group and reported in the 'McCormick 2002 paper' when they blended FT diesel with conventional diesel to produce a partly paraffinic base for a B20 blend. The data in Figures 1 and 3 for bulk modulus and injection timing show that there is a trend on which one can base a judgment about the potential impact of a fuel on injection timing and emissions. There is an increase in bulk modulus with increasing density. Figure 4 shows the specific gravity of various fuels versus their respective bulk modulus, and the specific gravity of the fuels is correlated directly with the bulk modulus. The fuels represented in Figure 4 include data from several published sources: the 'McCormick 2001 paper'; the 'Tat 2003 paper', and an article by Ofner et al. in Inst. Mech. Engr. J. (1996) "A fuel injection system concept for dimethyl ether," vol. 22, pp. 275-287 hereafter referred to as the 'Ofner paper'. The 'Tat 2003 paper' presented a survey of the bulk moduli of the various methyl and ethyl esters at 6.89 MPa (1000 psi), that are common in biodiesel fuels albeit measured at a slightly higher temperature of 40°C (104°F). The lowest density in Figure 4 represents dimethyl ether with the lowest bulk modulus (R-450 MPa at 3.4 MPa pressure), which originates from the 'Ofner paper'. Hence, Figure 4 shows a definitive trend between bulk modulus and fuel density, since the bulk modulus generally seems to increase with an increase in fuel specific gravity. That trend can be represented by a linear relationship as shown in Equation (2) with a correlation coefficient R² = 0.925 B = -2,666 + 4,966 *SG (2) where B is the bulk modulus of compressibility in MPa and SG is the specific gravity. Although there seems to be a correlation between specific gravity and bulk modulus of compressibility for these fuels, it is to be noted however that fuels of similar specific gravity can differ by 20 MPa or more, or 50 MPa or more, or sometimes 100 MPa or more in bulk modulus of compressibility, and that the bulk modulus of compressibility of a fuel could be lower than that of a fuel with a lower specific gravity, as illustrated by the scatter of data points for the various fuels with specific gravity between about 0.84 and 0.89. Similarly, the actual bulk modulus for dimethyl ether (with the lowest specific gravity) is much lower than would have been predicted with Equation (2). The NO_x emissions trends observed in the 'McCormick 2001 paper' for the speciated biodiesel constituents and various biodiesel feedstocks can be explained, in light of the present work, on the basis of the variation of the bulk modulus of the fuels, consistent with the observations by Van Gerpen and co-workers disclosed in the 'Monyem paper' and the 'Tat 2002 paper.' These same observations have relevance to the formulation of reformulated diesel fuels, FT diesel fuels, biodiesel fuels (B20, B100, etc.) and blends thereof. These observations on the correlation of bulk modulus with density are consistent with the historical literature on the bulk modulus of hydrocarbons. Bridgman in his 1958 book "The Physics of High Pressure", published by G. Bell and Sons (London), pp. 116-149 described the methodologies for testing the compressibility of fluids and presented a summary of results that were available at that time for various fluids, including normal alkanes, iso-alkanes, alcohols, halogenated compounds and water. Bridgman asserted that the compressibility of fluids at lower pressures was due to consumption of free space between the loosely packed molecules. At higher pressures, the compressibility is less and is due to compression of the molecules themselves that would be opposed by intermolecular repulsion. Thus, the bulk modulus should increase with increasing pressure as the resistance to further compression increases. Similar trends in compressibility of hydrocarbons were observed by Cutler and coworkers in their paper in the Journal of Chemical Physics (1958) vol. 29, pp. 727-740 hereafter referred to as the 'Cutler paper', who considered a variety of pure hydrocarbons including normal alkanes from Cj₂ to Cis, branched alkanes, cycloalkanes and aromatic compounds. The 'Cutler paper' disclosed that compressibility was greatest for normal alkanes, which have a less rigid structure, and decreased with increasing rigidity of molecular shape. Thus, compressibility increased as molecular structure varied from multi-ring aromatic, to aromatic, to cycloalkane, to branched alkane, and to straight-chain alkane. Since the bulk modulus is inversely related to the compressibility, the trend for bulk modulus would be to increase with increasing rigidity. Following Bridgman's logic, the bulk modulus would also increase with increasing density, because a more dense fluid would possess less of the free space to be consumed during compression. For the various biodiesel fuel stocks considered in the 'McCormick 2001 paper', there appears to be a strong correlation between NO_x emissions and density. An underlying reason for the trend of increasing NO_x with increasing density (and Iodine Number, which is an indication of the degree of unsaturation) is that as the density of the biodiesel feedstock increases, its bulk modulus increases and leads to advanced injection timing. The Applicants believe that the higher bulk modulus of compressibility of vegetable oils and their methyl esters leads to advanced injection timing. Advanced injection timing has been shown in the literature to contribute to the NO_x emissions increase with the use of biodiesel fuel.

An opposite trend, a retarding of injection timing, is observed with paraffinic mixtures because they have a lower bulk modulus of compressibility than conventional diesel fuels. This supports the observation that paraffinic fuels such as Fischer-Tropsch diesel fuels yield lower NO_x emissions.

Thus, the biodiesel "NOx effect" can be attributed to variations in the bulk modulus of the fuel or fuel blend, and these effects correlate for biofuels and paraffinic fuels quite well with fuel density. The present work also shows that a 45 vol.% blend of a methyl soyate biodiesel and a FT diesel displays the same bulk modulus of compressibility as a ultra-low sulfur petroleum diesel fuel.

Thus, one strategy for combating the biodiesel "NOx effect" is to use highly paraffinic diesel fuels, such as FT diesel as the diesel basestock.

Blends of biofuel and synthetic distillate The invention relates to a synthetic environmentally-friendly fuel or diesel fuel blendstock (generally referred to collectively herein as a "fuel") for use in compression ignition engines, the synthetic fuel comprising a mixture of a synthetic liquid distillate and a liquid biofuel, said mixture being characterized by a sulfur level less than 20 ppm S; a specific gravity equal to or less than 0.84; a bulk modulus of compressibility between about 1300 MPa and about 1600 MPa measured at 15 MPa and 37.8°C, and a cetane number greater than 55. The environmentally- friendly diesel fuel preferably has a specific gravity between about 0.78 and about 0.84. Preferably, the environmentally-friendly diesel fuel has a cetane number greater than about 60; more preferably, a cetane number greater than about 65. In some embodiments, the synthetic environmentally-friendly fuel comprises primarily at least one synthetic liquid distillate and at least one liquid biofuel (i.e., greater than 90% volume comprises both components), and may further contain minor components, such as fuel additives. In alternate embodiments, the synthetic environmentally-friendly fuel comprises essentially of a mixture of at least one synthetic liquid distillate and at least one liquid biofuel (i.e., greater than 95% volume comprises both components). Since biofuels tend to increase the bulk modulus of compressibility, and the paraffinic fuels tend to decrease the bulk modulus of compressibility, there exists an optimum biofuel volume fraction in the synthetic fuel blends that yields a bulk modulus of compressibility about equal to that of a petroleum diesel fuel so as to achieve "NO_x neutral" synthetic fuels. The preferred biodiesel blends of the present invention comprise a synthetic middle distillate, such that the NO_x effect is mitigated by the selection of an optimum range of bulk modulus of compressibility for each biodiesel blend. This optimum range of bulk modulus of compressibility is primarily dependent on a specific range of volumetric ratios of synthetic middle distillate to biodiesel in the biodiesel blend, and the respective bulk modulus of compressibility of pure synthetic middle distillate and pure biodiesel. The environmentally-friendly fuel preferably comprises a volume fraction of liquid biofuel between about 1 percent and about 45 percent. In these preferred embodiments, the environmentally-friendly fuel comprises a bulk modulus of compressibility measured at 15 MPa and 37.8°C between about 1300 MPa and about 1600 MPa, and a cetane number greater than 55. In alternate embodiments, the environmentally-friendly fuel comprises a liquid biofuel volume fraction between 1 percent and 20 percent; or between about 20 percent and 45 percent; or at about 45 percent. The environmentally-friendly fuel comprising a volume percent of liquid biofuel between 1 percent and 20 percent has a bulk modulus of compressibility measured at 15 MPa and 37.8°C between about 1300 MPa and about 1500 MPa, and a cetane number greater than 70. In alternate embodiments, the environmentally-friendly fuel comprises a volume percent of liquid biofuel between about 20 percent and 45 percent has a bulk modulus of compressibility measured at 15 MPa and 37.8°C between about 1500 MPa and about 1600 MPa; a cetane number greater than about 60; and a specific gravity between about 0.8 and about 0.84. In alternate embodiments, the environmentally-friendly fuel comprises a volume percent of liquid biofuel of about 45 percent so that the fuel has a bulk modulus of compressibility measured at 15 MPa and 37.8°C between about 1560 MPa and about 1600 MPa. In additional embodiments, the environmentally- friendly fuel comprises a biofuel to synthetic distillate volumetric ratio between 0.01 and about 0.45; or between 0.01 and about 0.2; between 0.2 and about 0.45; or at about 0.45. In preferred embodiments, the synthetic distillate is a FT diesel distillate. FT Diesel distillate can be combined with the biofuel in any ratio suitable to reduce the bulk modulus of compressibility of the blend to a desired value so as to produce a diesel fuel with "NOx neutral" effect which produced reduced NOx emissions when used in a diesel engine compared to a conventional petroleum diesel. In alternative embodiments, FT diesel distillate can be combined with biofuel to reduce the density of the resulting blend for any desired reason. For instance, a biofuel can be combined with FT diesel distillate to satisfy density specifications for a diesel fuel. Typically, these specifications determine allowable uses of diesel fuel, classifications of diesel fuel, and the like, which are all well known. Examples of allowable uses of a diesel fuel include on-road use, off-road use, and the like. For instance, regulations may require that the diesel fuel have a density within a specified range to qualify as an on-road use diesel fuel. The regulations may also require the diesel fuel to comprise other properties, such as cetane number, sulfur content, aromatics content, and the like, within a specified range to qualify as the on-road use diesel fuel. An example of classifications for diesel fuels includes specifications for a No. 2 diesel fuel. These classifications are well known and include World-Wide Fuel Charter classifications, ASTM classifications, European classifications, and the like. For instance, the December 2002 World-Wide Fuel Charter recommends a density range measured at 15°C of about 820 kg/m³to about 850 kg/m³ (the minimum limit can be relaxed to 800 kg/m³ when ambient temperatures are below -30°C) for a No. 2 diesel fuel. To bring an off- spec synthetic mixture such as a FT diesel and a biofuel to within the density specifications of the 2002 World-Wide Fuel Charter specifications for a No. 2 diesel fuel, FT diesel distillate can be combined with a biofuel comprising mono-alkyl esters of fatty acids to form a diesel product in a desired ratio to bring the density of the diesel product within the specification limits of 815 kg/m³ to about 850 kg/m³, as long as the bulk modulus of compressibility is within or below the "NO_x neutral" zone for diesel. The FT diesel distillate can be further blended with the biofuel to achieve a specific gravity of the blend below 0.84 and to adjust at least one other property of the blend, wherein the other properties include the cetane number, lubricity, iodine number, viscosity, and the like. With the FT diesel distillate preferably having a density at 15°C from about 0.76 g/cm³ to about 0.80 g/cm³, more preferably between about 0.77 g/cm³ to about 0.79 g/cm³, FT diesel distillate can be combined with a biofuel having a higher bulk modulus of compressibility than FT diesel distillate to produce a synthetic diesel fuel having a bulk modulus of compressibility lower than that of the biofuel. The FT diesel distillate can be combined with biofuel by any known method. In the present embodiment, the FT diesel distillate is combined with the biofuel in a vessel. Preferably, the environmentally-friendly fuel has a sulfur level less than 15 ppm S; more preferably less than 10 ppm S; still more preferably less than 5 ppm S. Preferably, the environmentally-friendly fuel for compression ignition engines has a boiling range with a 5% boiling point between about 320°F and 350°F (about 160-177°C) and a 95% boiling point between about 600°F and 650°F (about 315-343 °C). In another embodiment, the environmentally-friendly synthetic liquid fuel has a boiling range having a 5% boiling point between about 340°F and about 410°F (or between about 170°C and about 210°C) and a 95% boiling point between about 570°F and about 645°F (or between about 300°C and about 340°C). Alternatively, the environmentally-friendly synthetic liquid fuel has a boiling range having a 5% boiling point between about 355°F and about 420°F (or between about 180°C and about 215°C) and a 95% boiling point between about 600°F and 650°F (or between about 315°C and 343°C).

Biofuel The biofuel or blendstock should comprise one or more organic compounds selected from the group consisting of esters of fatty acids, hydrogenated esters of fatty acids, pure vegetable oils, and combinations thereof. The liquid biofuel more preferably comprises primarily alkyl esters of fatty acids, each having between 8 to 22 carbon atoms. Prefened esters of fatty acids are methyl esters, ethyl esters, or combinations thereof, of fatty acids, said fatty acids comprising between 8 and 22 carbon atoms. Examples of suitable fatty acid esters are methyl laurate, methyl palmitate, methyl stearate, ethyl stearate, methyl oleate, methyl linoleate, ethyl linoleate, methyl linolenate, and the like. The biofuel preferably comprises a density higher than about the density of synthetic distillate. The biofuel preferably comprises mono-alkyl esters of fatty acids which include products of esterification and/or transesterifϊcation of vegetable oils, animal fats, and yellow grease, said products comprising very small amounts of glycerol and/or alcohol. More preferably, the liquid biofuel comprises an alkyl ester of one or more vegetable oils selected from the group consisting of canola oil, cotton oil, sunflower oil, coconut oil, palm oil, soya oil, and combinations thereof. Still more preferably, the liquid biofuel comprises alkyl esters of fatty acids produced from soybean oil and/or canola oil, wherein the alkyl esters are methyl esters, ethyl esters, or combinations thereof. In some embodiments, the liquid biofuel comprises methyl soyate. The liquid biofuel can be characterized by a bulk modulus of compressibility measured at 15 MPa and 37.8°C greater than about 1600 MPa, preferably between about 1650 MPa and about 2100 MPa. Furthermore, the biofuel has a very low sulfur content, i.e., less than 50 ppm sulfur, preferably less than 30 ppm sulfur, preferably less than 10 ppm sulfur. Biodiesel comprising canola oil and/or canola alkyl esters, may have a sulfur content slightly higher than from other feedstocks. The biofuel should also have very low aromatic and nitrogen contents. In addition, the liquid biofuel is preferably substantially free of glycerin, i.e., the total glycerin content of the liquid biofuel should be less than 0.24 percent by weight. The 'free' glycerol content is preferably less than 0.02 percent by weight. The free and total glycerin can be measured employing the ASTM method D-6584 "Test Method for Determination of Free and Total Glycerine in B-100 Biodiesel Methyl Esters by Gas Chromatography" . In some embodiments, the liquid biofuel comprises a substantially glycerin-free product of the esterification of soya oil, canola oil or mixtures thereof. Preferably, the liquid biofuel has a specific gravity greater than about 0.86; more preferably between about 0.86 and about 0.91; still more preferably between about 0.86 and about 0.89. The liquid biofuel is not meant to comprise essentially alcohols, such as methanol and ethanol, which also can be derived from biomass (renewable resources). The liquid biofuel preferably should contain a very low content of 'free' (i.e., unbound) alcohol molecules, i.e., less than about 10 percent by weight. In the case when the method of making the liquid biofuel includes the use of an alcohol during the esterification process described later, the flash point for biodiesel is used as the mechanism to limit the level of un-reacted alcohol remaining in the finished fuel. The flash point specification for biodiesel is intended to be 100°C minimum. Typical values are over 160°C. Due to high variability with the ASTM Method D-93 "Standard Test Methods for Flash-Point by Pensky-Martens Closed Cup Tester" as the flash point approaches 100°C, the flash point specification has been set at 130°C minimum to ensure an actual value of 100°C minimum. The flash point of the liquid biofuel is preferably greater than 100°C; more preferably greater than 130°C. The liquid biofuel preferably has a boiling range with an initial boiling point between about 300°C and about 330°C and a final boiling point between about 350°C and about 370°C according to the ASTM distillation method D-86 "Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure"; or a boiling range with an initial boiling point between about 310°C and about 350°C and a final boiling point between about 400°C and about 480°C according to the ASTM vacuum distillation method D-1160 "Standard Test Method for Distillation of Petroleum Products at Reduced Pressure" . The kinematic viscosity at 40°C of the liquid biofuel can be between about 1.9 mm²/s (cSt) and about 6 cSt, but preferably between 3 cSt and 6 cSt. The kinematic viscosity at 40°C is preferably measured by the ASTM method D-445 "Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) ". The cetane number of the liquid biofuel is preferably greater than 43; more preferably between about 45 and about 65; still more preferably between about 50 and about 60. The cetane number is preferably measured by the ASTM method D-613 "Standard Test Method for Cetane Number of Diesel Fuel Oil". The density at 15°C of the liquid biofuel is preferably between about 0.86 g/cm³ and about 0.91 g/cm³; more preferably between about 0.87 g/cm³ and about 0.89 g cm³. The density at 15°C can be measured by the ASTM method D-4052 "Standard Test Method for Density and Relative Density of Liquids by Digital Density Meter". The specific gravity of the liquid biofuel is preferably between about 0.86 and about 0.91' more preferably between about 0.87 and about 0.89 as measured by the ASTM method D-1298 "Standard Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method'. In preferred embodiments, the liquid biofuel meets the specifications of the ASTM method

D-6751 specifications "Standard Specification for Biodiesel Fuel (B100) Blend Stock for Distillate Fuels". Methods of synthesizing biofuel The liquid biofuel is preferably manufactured from vegetable oils, animal fats, waste vegetable oils (such as recycled restaurant greases, called yellow grease) and microalgae oils, or any combination thereof. Methods of preparation of biodiesel are well known. The feedstocks can be transformed into biodiesel using a variety of esterification or transesterification technologies. Oils and fats are composed principally of triglycerides, composed of three long-chain fatty acids of 8 to 22 carbons attached to a glycerol backbone, and free fatty acids, which fatty acid chains break off the triglycerides. The biofuel is preferably synthesized by esterification and/or transesterification of one or more feesdstocks selected from the groups consisting of vegetable oils (e.g., canola oil, soybean oil, linseed oil, and the like), animal fats (e.g., beef tallow, pork lard), waste vegetable oils (e.g., yellow grease), and microalgae oils. Biodiesel feedstocks are classified based on the free fatty acid (FFA) content: refined oils

(such as refined soybean and canola oils with less than 1.5% FFA), low FFA yellow greases and animal fats (< 4% FFA), and high FFA yellow greases and animal fats (>20%). The production of biodiesel from low FFA fats and oils comprises a base catalyzed transesterification, wherein the triglycerides are transformed into biodiesel and glycerine under base conditions. The production of biodiesel from high FFA fats and oils comprises an acid esterification, wherein FFA are reacted with an alcohol (usually ethanol or methanol) in the presence of an acid (such as sulfuric acid) to form fatty esters such as ethyl or methyl esters. The esterification reaction is then followed by a transesterification. The liquid biofuel comprising alkyl esters is preferably produced by the base catalyzed reaction because it is the most economic for several reasons: low temperature (65°C or 150°F) and pressure (20 psi) processing; high conversion (98%) with minimal side reactions and reaction time; direct conversion to methyl ester with no intermediate steps; and no need for expensive materials of construction. The general process is depicted as follows: a fat or oil is reacted with an alcohol, like methanol or ethanol, in the presence of a catalyst to produce glycerin and alkyl esters (i.e., biofuel). The alcohol is charged in excess to assist in quick conversion and recovered for reuse. The catalyst is usually sodium or potassium hydroxide which has already been mixed with the methanol. An overview of present processes from various biodiesel feedstocks was described by Kinast in his Final Report to USDOE/National Renewable Energy laboratory, contact ACG-7- 15177-02, Sept. 1999. Additional processes for making biofuel are described by Canakci and Van Gerpen in their paper from Transactions of the American Society of Agricultural Engineers (2001) vol. 44, No. 6, pp. 1429-1436; as well as in their paper from Transactions of the American Society of Agricultural Engineers (1999) vol. 42, N^"o.5, pp. 1203-12; and by Freedman and coworkers in an article in the Journal of the American Oil Chemists' Society (1984) vol. 61, p. 1638. Synthetic distillate The synthetic liquid distillate preferably has a boiling range having a 5% boiling point between about 170°C and about 210°C and a 95% boiling point between about 320°C and about 350°C. These boiling points are based on the method ASTM D-86 from the American Society for Testing and Materials. The synthetic liquid distillate preferably has a density at 15°C of from about 0.76 g/cm³ to about 0.80 g/cm³; and more preferably between about 0.77 g/cm³ and about 0.79 g/cm³; most preferably at about 0.78 g/cm³. In addition, the synthetic liquid distillate is characterized by a paraffin content greater than about 90 percent, preferably greater than about 95 percent. The synthetic liquid distillate should have a sulfur content of less than about 10 ppm sulfur; preferably less than about 5 ppm sulfur; more preferably less than 1 ppm sulfur; still more preferably less than about 0.1 ppm sulfur. Moreover, the synthetic liquid distillate preferably has an aromatics content of less than about 1 percent by weight. Furthermore, the synthetic liquid distillate preferably has a cetane number greater than about 70, preferably greater than about 75. It is to be understood that the synthetic liquid distillate is not limited to the above-identified property values but can include higher or lower values depending on factors such as the synthesis conditions and the hydroprocessing scheme and conditions used to produce it. In some embodiments, the synthetic liquid distillate comprises very small amounts of olefins, i.e., less than 10 percent by weight, preferably less than 5 percent by weight. In alternate embodiments, the synthetic liquid distillate is characterized by a Bromine number of less than about 0.1 g/100 g as measured by the ASTM method D-1159 "Standard Test Method for Bromine Numbers of Petroleum Distillates and Commercial Aliphatic Olefins by Electrometric Titration" . Most of the synthetic liquid distillate used in the environmental-friendly fuel is preferably generated by a Fischer-Tropsch synthesis described herein. Methods of preparing synthetic distillate by Fischer-Tropsch process A syngas feed is fed to hydrocarbon synthesis reactor. Syngas comprises hydrogen, or a hydrogen source, and carbon monoxide. Hydrocarbon synthesis reactor comprises any reactor in which hydrocarbons are produced from syngas by Fischer-Tropsch synthesis, alcohol synthesis, and any other suitable synthesis. Hydrocarbon synthesis reactor is preferably a Fischer-Tropsch reactor. Preferably, the hydrogen is provided by free hydrogen, although some Fischer-Tropsch catalysts have sufficient water gas shift activity to convert some water (and CO) to hydrogen (and CO ) for use in the Fischer-Tropsch synthesis. It is preferred that the molar ratio of hydrogen to carbon monoxide in syngas feed 60 be greater than 0.5:1 (e.g., from about 0.67 to about 2.5). Preferably, when cobalt, nickel, and/or ruthenium catalysts are used, syngas feed contains hydrogen and carbon monoxide in a molar ratio of about 1.4:1 to about 2.3:1. Preferably, when iron catalysts are used, syngas feed 60 contains hydrogen and carbon monoxide in a molar ratio between about 1.4:1 and about 2.2:1. Syngas feed may also contain carbon dioxide. Moreover, syngas feed should contain only a low concentration of compounds or elements that have a deleterious effect on the catalyst, such as poisons. For example, syngas feed may need to be pretreated to ensure that it contains low concentrations of sulfur or nitrogen compounds such as hydrogen sulfide, hydrogen cyanide, ammonia and carbonyl sulfides. Syngas feed is contacted with the catalyst in a reaction zone. Mechanical arrangements of conventional design may be employed as the reaction zone including, for example, fixed bed, fluidized bed, slurry bubble column or ebullating bed reactors, among others. Accordingly, the preferred size and physical form of the catalyst particles may vary depending on the reactor in which they are to be used. A slurry bed reactor with catalyst particles with a weight size average between 30 and 150 microns is preferred. The catalyst in the reaction zone for hydrocarbon synthesis preferably comprises a catalytically active metal selected from the group consisting of cobalt, iron, ruthenium, and combinations thereof. More preferably, the catalyst in the reaction zone for hydrocarbon synthesis preferably comprises cobalt as one catalytically active metal. The catalyst may further comprise at least one promoter suitable for increasing the selectivity, stability, and/or activity of the reduced catalyst. Suitable promoters are preferably selected from the group consisting of ruthenium, rhenium, platinum, palladium, boron, manganese, magnesium, silver, lithium, sodium, copper, potassium, and combination thereof. The reduced catalyst may be supported or unsupported. The support for a supported catalyst preferably includes an inorganic oxide such as silica, alumina, titania, or any combination thereof. The Fischer-Tropsch reactor is typically run in a continuous mode. In this mode, the gas hourly space velocity through the reaction zone typically may range from about 50 to about 10,000 hr^"1, preferably from about 300 hr^"1 to about 2,000 hr^"1. The gas hourly space velocity is defined as the volume of reactants per time per reaction zone volume. The volume of reactant gases is preferably at but not limited to standard conditions of pressure (101 kPa) and temperature (0°C). The reaction zone volume is defined by the portion of the reaction vessel volume where the reaction takes place and which is occupied by a gaseous phase comprising reactants, products and/or inerts; a liquid phase comprising liquid/wax products and/or other liquids; and a solid phase comprising catalyst. The reaction zone temperature is typically in the range from about 160°C to about 300°C. Preferably, the reaction zone is operated at conversion promoting conditions at temperatures from about 190°C to about 260°C; more preferably, from about 205°C to about 230°C. The reaction zone pressure is typically in the range of about 80 psia (552 kPa) to about 1,000 psia (6,895 kPa), more preferably from 80 psia (552 kPa) to about 800 psia (5,515 kPa), and still more preferably, from about 140 psia (965 kPa) to about 750 psia (5,170 kPa). Most preferably, the reaction zone pressure is from about 250 psia (1,720 kPa) to about 650 psia (4,480 kPa). The product of hydrocarbon synthesis reactor primarily comprises hydrocarbons. Hydrocarbon synthesis product may also comprise olefins, alcohols, aldehydes, and the like. Hydrocarbon synthesis product primarily comprises paraffins (more than 80% paraffins). The hydrocarbon synthesis process should also comprise a fractionator in order for the product of hydrocarbon synthesis reactor to be separated into various fractions, including a gasoline fraction and middle distillate fractions (including diesel fraction). Methods of fractionation are well known in the art, and the feed to the fractionator can be separated by any suitable fractionation method. The fractionator preferably includes an atmospheric distillation column. The method for making the synthetic distillate may further comprise feeding the hydrocarbon synthesis product to a hydroprocessing unit, wherein the hydrocarbon synthesis product is hydroprocessed to produce a hydroprocessed product; fractionating the hydroprocessed product to produce a treated distillate; and combining the treated distillate with a liquid biofuel to produce a fuel, wherein the fuel has a bulk modulus within an optimum range and a cetane number greater than 55. Hydrocarbon synthesis product in part or in totality is preferably further hydroprocessed in order to generate an acceptable yield of liquid fuels such as gasoline and diesel. Hydroprocessing could be done on the totality or a portion of the hydrocarbon synthesis product. Hydroprocessing could comprise hydrotreatment, hydrocracking, hydroisomerization, dewaxing, or any combination thereof. In some embodiments, the hydroprocessing comprises a hydrotreatment to reduce the olefin content of the distillate so that the Bromine number (related to unsaturation of carbon-carbon bonds) is less than 0.1 g/lOOg as measured for example by ASTM method D-1159 "Standard Test Method for Bromine Numbers of Petroleum Distillates and Commercial Aliphatic Olefins by Electrometric Titration". The hydrotreatment should convert unsaturated hydrocarbons (such as olefins) to saturated hydrocarbons (such as alkanes). The hydrotreatment can take place over hydrotreating catalysts. The hydrotreating catalysts comprise at least one of a group VIB metal, such as molybdenum and tungsten, or a group VIII metal, such as nickel, palladium, platinum, ruthenium, iron, and cobalt. The nickel, palladium, platinum, tungsten, molybdenum, ruthenium, and combinations thereof are typically highly active catalysts, and the iron and cobalt are typically less active catalysts. The hydrotreatment is preferably conducted at temperatures from about 140°C to about 315°C. Other operating parameters of hydrotreatment may be varied by one of ordinary skill in the art to affect the desired hydrotreatment. For instance, the hydrogen partial pressure is preferably between about 690 kPa and about 6,900 kPa, and more preferably between about 2,060 kPa and about 3,450 kPa. Moreover, the liquid hourly space velocity is preferably between about 1 hr^"1 and about 10 hr^"1, more preferably between about 0.5 hr^"1 and about 6 hr^"1, and most preferably between about 1 hr^"1 and about 5 hr^"1. Alternatively or in addition, the hydroprocessing may comprise a hydrocracking step to convert heavy paraffins to lighter paraffins. Methods of hydrocracking are well known in the art, and hydrocracking of heavy distillate (such as wax) can include any suitable method. The hydrocracking preferably takes place over a platinum catalyst at temperatures from about 260°C to about 400°C and at pressures from about 3,550 kPa to about 10,440 kPa. The heavy distillate is preferably fed to a hydrocracker where its components are cracked into smaller hydrocarbon molecules, wherein a good portion of the cracked molecules are within the boiling range of diesel. The effluent of the hydrocracker is preferably recycled to a fractionator so the heavy hydrocarbons are recycled close to extinction. Alternatively or in addition, the hydroprocessing may comprise a hydroisomerization step to convert paraffins to more branched paraffins, so as to generate a synthetic distillate with at least one improved cold-flow property (such as lower pour point). It may be desirable to hydro isomerize during the hydrocracking step so as to form branched hydrocarbons, such as convert linear paraffins to isomers of paraffins (isoparaffins or branched paraffins); and/or convert monobranched paraffins to dibranched paraffins. Isoparaffins are known to improve cold flow properties in FT diesel, so increasing the relative amount of branched hydrocarbons in FT diesel should yield a diesel with decreased (improved) pour point. Paraffin isomerization catalysts and processes that can be used for producing low pour point diesel fuel can be found in U.S. patents 4,710,485; 4,589,138; 4,459,312; 5,149,421; 5,282,958, each of which is incorporated herein by reference in its entirety. Additional isomerization processes are described by Taylor and coworkers in Applied Catalysis A: General (1994) vol. 119, pp. 121-138. To further illustrate various illustrative embodiments of the present invention, the following example is provided. EXAMPLE The interaction between the bulk modulus of compressibility of various fuel samples and their effect on fuel injection timing was examined. The fuels considered ranged from biodiesel B100 (as methyl soyate, i.e., the methyl ester of soybean oil), unrefined soybean oil, paraffinic solvent, Fischer-Tropsch derived diesel, and ultra low sulfur diesel fuel. Both the impact on injection timing and the variation in the bulk modulus of compressibility were measured so that correlation between fuel composition, fuel properties and injection timing could be observed and quantified. Two different experimental systems were used: a high-pressure viscometer, capable of measuring the bulk modulus of compressibility with the use of a pycnometer; and a highly instrumented, single-cylinder direct injection (Dl) diesel engine, with an accompanying spray visualization chamber. The bulk modulus of various fuels was measured, and corresponding measurements were made of the impact of the fuel on the injection timing in the Dl diesel engine. High-Pressure Viscometer The high-pressure viscometer instrument was developed for studies of the viscosity and bulk modulus of hydraulic fluids that contained dissolved gases and described by J.A. O'Brien in his 1963 M.S. Thesis from Penn State University, PA, entitled "Precise Measurement of Liquid Bulk Modulus". The operation for this device was based on the following principle: when a fluid is exposed to higher pressures, the fluid has a reduction in volume. The isothermal bulk modulus of compressibility, Bj, and the isentropic bulk modulus of compressibility, Bs, are defined by Equations (3) and (4) respectively:

where v is the specific volume and p is the density. The isentropic bulk modulus of compressibility is related to the speed of sound, α, by the Equation [5]:

The experimental approach used in this experiment yielded a measurement for the isothermal bulk modulus of compressibility, B_τ, which will simply be referred to as B. The governing Equation (6) for the calculation of bulk modulus is:

where B is the isothermal bulk modulus, R is the measured pressure, Po is atmospheric pressure, Vo is the volume of the sample at atmospheric pressure and Fis the volume at the new pressure. Figure 5 shows a schematic diagram of the closed-bottom pycnometer and housing used in these studies. The measurement equipment 200 comprised a modified 21-R-30 Stainless Jerguson gauge 210 capable of handling pressures up to 4000 psi. Two panels with viewing windows 220 allowed for viewing of the sample. Each window glass had two gaskets, one on either side, to ensure a tight seal on the chamber. For pressures in the range of 0-1000 psi, a direct connection to a helium gas cylinder provided the necessary pressure via helium inlet 230. For pressures above 2000 psi, a 4.5-liter Aminco hydrogenation bomb (not shown) was filled with helium, and oil was pumped into the bomb to achieve pressures up to 16,000 psi. A constant temperature bath kept the pressure cell at a temperature of 100°F. Bulk modulus was measured via a change in height in the capillary 240 within the pycnometer tube 250, as the pressure in the cell was varied. The fuel sample was placed in reservoir 260 of the pycnometer tube 250. N-Octadecane was used as a calibration standard. Direct Injection Diesel Engine and Spray Chamber A Yanmar L40 AE D air cooled 4-stroke direct injection (Dl) diesel engine was coupled to an electric motor and motored at speed and fuel consumption conditions that simulated the G2 test modes from the ISO 8178-4.2 [ISO 8178: Reciprocating internal combustion engines - Exhaust emissions measurement - Part 4. Test Cycles for different engine applications. 1995, International Organization for Standardization]. Only results from a load setting of 25% at 3600 RPM are presented here. The experimental system is shown schematically in Figure 6. The fuel consumption was measured by a gravimetric method using an Ohaus Explorer balance, accurate to 0.1 g. The fuel injector 310 was removed from the cylinder head and placed into a spray chamber 320 with visual access to the fuel spray 330. The chamber 320 was positioned so that the original high pressure fuel line 340 could be used without modification of length, although it was necessary to bend the fuel line. The spray timing was monitored with a light attenuation method. A Uniphase 0.95 mW Helium-Neon laser 350 was positioned so that a laser beam 360 intersected the fuel spray at the injector orifice. During the spray event the laser was attenuated, changing the output voltage and enabling a clear transition at both the beginning and end of the spray. An AVL 364 shaft encoder mounted on the engine crank shaft enabled 0.1 crank angle (CA) degree resolution of the spray event. A data acquisition system 370 recorded the signals from the fuel line pressure sensor 380 and the phototransistor 390 located at the end of the laser beam 360. Fuel Samples The tested fuel samples were a biodiesel fuel (B100), a methyl soyate from World Energy, Chelsea, MA; an unrefined soybean oil (soy oil) from Agricultural Commodities, Inc., New Oxford, PA; Norpar^®-13 (Norpar^®), a normal paraffin mixture from Cπ - Cι₅ from ExxonMobil Chemicals, Houston, TX; a Fischer-Tropsch diesel (FT diesel) from ConocoPhillips Company, Houston, TX; a 15 ppm sulfur diesel fuel (BP-15) from British Petroleum - Fuels Technology, Naperville, IL; and a biodiesel/petroleum diesel blend (B20) consisting of methyl soyate from World Energy and ultra low sulfur diesel fuel from British Petroleum. Table 1 comprises properties of these fuel samples. Table 1: Properties of the fuel samples.

na: not available * all boiling points were determined by the ASTM D-86 method, except for B100 where the ASTM D2887 (SimDis) method was used. The FT diesel was obtained by converting a synthesis gas stream with a hydrogen-to-carbon monoxide molar ratio of about 2:1 over a Fischer-Tropsch cobalt-based catalyst in a slurry bubble reactor at a temperature of about 210-215°C and a pressure of about 450 psig (about 3200 kPa) so as to form a hydrocarbon synthesis product. The hydrocarbon synthesis product was then hydrotreated over a nickel-based catalyst so as to substantially transform all of the olefins and oxygenates to paraffins; then fractionated in an atmospheric distillation column to at least obtain a FT diesel fraction. The experiments were performed in order to examine two separate issues with regard to fuel formulation and engine emissions. The first was to study the difference in bulk modulus between biodiesel fuels and diesel fuels and the resulting effect on fuel injection timing. The second was the investigation of the potential impact of the use of paraffinic fuels, such as Fischer- Tropsch diesel fuels, on injection timing. Figures 1 and 3 show results from the bulk modulus of compressibility for diesel and biofuel blends, and the measurements of injection timing for biofuel B100, baseline diesel, B20-FT blend and the normal paraffin solvent (Norpar^®). In Figure 1, the bulk modulus of B20, B100 and the soy oil are higher than that of the baseline diesel fuel, consistent with the results reported in the 'Tat 2000 paper'. The bulk modulus of B20 was slightly above that of the baseline diesel fuel BP- 15. On the other end, the bulk modulus of FT diesel, both FT diesel/biodiesel blends, and the paraffinic solvent were lower than that of the baseline diesel fuel (BP-15). Blends of 20% and 40 vol.%) of (methyl soy) biodiesel and a FT diesel therefore displayed a lower bulk modulus of compressibility than the baseline diesel fuel, and should generate lower NOx emissions that the baseline diesel. A relative spray intensity of 0.2 was used as an indication of the beginning of light scattering by the fuel spray, providing a consistent means of quantifying the onset of the fuel spray. Accordingly, using 0.2 relative spray intensity as an indication of the start of fuel injection, Figure 3 indicates that there was a 0.2 CA advance of fuel injection timing for the petroleum diesel-biodiesel blend (B20), while there was an advance of 1.0 CA with pure biodiesel (B100). The purely paraffinic solvent, Norpar^®, retarded the fuel injection timing with the largest retardation of 0.5 CA, while the B20-FT blend shows a retardation of 0.1 CA in fuel injection timing. Norpar^® also showed the lowest bulk modulus of compressibility in Figure 1. Since the B20-FT blend with the synthetic FT diesel showed a retardation in fuel injection timing (-0.1 CA), the B20-FT blend is expected to generate less NOx emissions than the baseline fuel. On the other end, the B20 blend with the ultra-low sulfur petroleum diesel showed an advanced injection timing (+0.2 CA), the B20 blend is expected to generate more NOx emissions than the baseline fuel. In the present Example, the bulk moduli of blends of biodiesel and FT diesel were examined to determine what blend ratio would be equivalent in bulk modulus to the baseline diesel (ultra-low sulfur diesel fuel BP-15) and what blend ratio would be have a bulk modulus below that of the baseline diesel. Table 2 shows the respective bulk moduli extrapolated from Figure 1 at 5 MPa, 15 MPa and 25 MPa for paraffinic solvent Norpar^®, FT diesel, B20-FT, B40-FT, B20, B100 and soy oil. Table 2: Bulk modulus of compressibility at 100°F at different pressures extrapolated from Figure 1. B100 (methyl B20 with BP-15 B40- B20-

Pressure Soy Oil soyate) diesel diesel FT60 FT80 FT diesel Norpar^ 5 MPa 1980 1650 1500 1460 1430 1370 1350 1240 15 MPa 2070 1755 1605 1580 1555 1495 1455 1350 25 MPa 2150 1870 1730 1690 1670 1615 1555 1460

To illustrate the effect of biodiesel volume fractions on the bulk moduli of the biodiesel/FT diesel blends, a plot of the bulk moduli extrapolated at 15 MPa (and measured at 100°F) taken from Table 2 of neat FT diesel, FT diesel/biodiesel blends and neat biodiesel B100 versus the biodiesel volumetric fraction of the blends, is shown in Figure 2, wherein 0% represents neat FT diesel and 100% represents neat B100. From Figure 2, one can visualize a blend of FT diesel and biodiesel which would lead to a "NOx neutral" formulation with a biodiesel volume fraction that would correspond to a bulk modulus of compressibility at 15 MPa to be equal to about 1580 MPa, corresponding to the bulk modulus at 15 MPa for the baseline diesel. A B45-FT blend would correspond to a bulk modulus of compressibility at 15 MPa of about 1580 MPa, equivalent to that of the baseline diesel. Hence the B45-FT blend comprised a "NOx neutral" biodiesel blend. Accordingly, the biodiesel-FT diesel blends with less than 45% of biodiesel should generate a reduced level of NOx emissions when used in a diesel engine compared to conventional (crude derived) diesel formulations. The data in these tests also showed a trend of increasing bulk modulus of compressibility with increasing density. As Table 3 shows, the density of the fuels considered here correlated directly with the bulk modulus of compressibility. Table 3: Fuel injection timing, bulk modulus of compressibility at 1000 psi and 100°F, and specific gravity of various fuels.

negative (-) CA value represents a retardation in injection timing. ** B 100 is methyl soyate and B20 comprises 20 vol% of B 100.

The NOx emissions trends for the speciated biodiesel constituents and various biodiesel feedstocks shown in the 'McCormick 2001 paper' can be explained, in light of the present work, on the basis of the variation of the bulk modulus of the fuels, consistent with the observations of Van Gerpen and co-workers, such as are disclosed in the 'Monyem paper' and in the 'Tat 2000 paper'. These same observations have relevance to the formulation of reformulated diesel fuels, FT diesel fuels, biodiesel fuels (B20, B100, etc.) and blends thereof. The present Example demonstrated that the higher bulk modulus of compressibility of biodiesel (specifically a methyl ester of soybean oil) led to advanced injection timing. This advanced injection timing has been shown in the literature to contribute to the well-documented NOx emissions increase with the use of biodiesel fuel. An opposite trend, a retarding of injection timing, was observed with paraffinic fuels because they have a lower bulk modulus of compressibility than conventional diesel fuels. This supports the observation that paraffinic fuels such as Fischer-Tropsch diesel fuels yield lower NOx emissions. Thus, the observations of the biodiesel "NOx effect" reported in the literature can be attributed to variations in the bulk modulus of compressibility of the fuel or fuel blend, and these effects correlate for biofuels and paraffinic fuels quite well with fuel density. The embodiments set forth herein are merely illustrative. Many varying and different embodiments may be made within the scope of the present inventive concept, including equivalent structures hereafter thought of, permutations, substitutions, or combinations of features from the embodiments herein detailed in accordance with the descriptive requirements of the law. Many modifications may be made as well in these embodiments. Because of these reasons, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense. Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

What is claimed is:

1. A synthetic diesel fuel or diesel fuel blendstock characterized by producing low sulfur and NOx emissions when used in compression ignition engines, the synthetic diesel fuel comprising a mixture of a synthetic middle distillate and a liquid biofuel, said mixture being characterized by: a sulfur level less than 20 ppm sulfur; a specific gravity equal to or less than 0.84; a bulk modulus of compressibility measured at 15 MPa and 37°C between about 1300 MPa and about 1600 MPa; and a cetane number greater than 55.

2. The synthetic diesel according to Claim 1 wherein the synthetic middle distillate is characterized by a sulfur content less than 10 ppm sulfur and by a specific gravity less than about 0.8.

3. The synthetic diesel according to Claim 1 wherein the synthetic middle distillate is characterized by a cetane number greater than 70.

4. The synthetic diesel according to Claim 2 wherein the synthetic middle distillate is characterized by a cetane number greater than 75.

5. The synthetic diesel according to Claim 1 wherein the synthetic middle distillate is characterized by a paraffin content greater than 90 percent by weight.

6. The synthetic diesel according to Claim 1 wherein the synthetic middle distillate is characterized by a Bromine number of less than 0.1 gBr/lOOg.

7. The synthetic diesel according to Claim 1 wherein the synthetic middle distillate is characterized by an aromatics content of less than about 1 percent by weight.

8. The synthetic diesel according to Claim 1 wherein the synthetic middle distillate is characterized by a boiling range having a 5% boiling point between about 170°C and about 210°C and a 95% boiling point between about 320°C and about 350°C.

9. The synthetic diesel according to Claim 1 wherein the liquid biofuel has a sulfur content less than 30 ppm sulfur.

10. The synthetic diesel according to Claim 1 wherein the liquid biofuel comprises a compound selected from the group consisting of vegetable oils, alkyl esters of vegetable oils, mixtures of alkyl esters of fatty acids, and combinations thereof.

11. The synthetic diesel according to Claim 1 wherein the liquid biofuel comprises a mixture of alkyl esters of fatty acids.

12. The synthetic diesel according to Claim 1 wherein the liquid biofuel comprises an alkyl ester of a vegetable oil selected from the group consisting of canola oil, soybean oil, cotton oil, palm oil, sunflower oil, and combinations thereof.

13. The synthetic diesel according to Claim 1 wherein the liquid biofuel comprises an alkyl ester of a vegetable oil selected from the group consisting of canola oil, soybean oil, and combinations thereof.

14. The synthetic diesel according to Claim 1 wherein the amount of liquid biofuel comprises between about 1 percent by volume and 45 percent by volume of the diesel fuel.

15. The synthetic diesel according to Claim 1 wherein the amount of liquid biofuel comprises between about 1 percent by volume and 20 percent by volume of the diesel fuel.

16. The synthetic diesel according to Claim 1 wherein the amount of liquid biofuel comprises between about 20 percent by volume and 45 percent by volume of the diesel fuel.

17. The synthetic diesel according to Claim 1 wherein the synthetic middle distillate is derived from synthesis gas.

18. The synthetic diesel according to Claim 1 wherein the liquid biofuel comprises a total glycerin content less than 0.24 percent by weight of glycerin.

19. The synthetic diesel according to Claim 1 wherein the liquid biofuel has a flash point greater than 100°C.

20. The synthetic diesel according to Claim 1 wherein the liquid biofuel has an alcohol content less than 10 percent by weight.

21. The synthetic diesel according to Claim 1 wherein the liquid biofuel has a kinematic viscosity at 40°C between about 1.9 mm²/s and about 6 mm²/s.

22. The synthetic diesel according to Claim 1 wherein the synthetic diesel has a cetane number greater than about 60.

23. The synthetic diesel according to Claim 1 wherein the mixture of the synthetic middle distillate and the liquid biofuel comprises more than 90 vol.% of the synthetic diesel.

24. A synthetic environmentally-friendly diesel fuel that produces reduced sulfur and NO_x emissions when used in compression ignition engines, comprising: a synthetic liquid distillate characterized by a cetane number greater than 70, by a sulfur content less than 10 ppm sulfur; by a paraffin content greater than 90 percent by weight; and by a specific gravity less than about 0.8; and a liquid biofuel characterized by a sulfur content less than 30 ppm sulfur and a specific gravity greater than about 0.86, wherein the liquid biofuel is derived from a compound selected from the group consisting of vegetable oils, animal greases, vegetable oil wastes, microalgae oils, and combinations thereof; wherein the liquid biofuel is blended with said synthetic liquid fuel so as to form a synthetic diesel fuel, and wherein the synthetic diesel fuel has a bulk modulus of compressibility measured at 15 MPa and 37.8°C between about 1300 MPa and about 1600 MPa; a sulfur content less than 20 ppmS; and a specific gravity below 0.84.

25. The diesel fuel according to Claim 24 wherein the liquid biofuel comprises a compound selected from the group consisting of vegetable oils, alkyl esters of vegetable oils, mixtures of alkyl esters of fatty acids, and combinations thereof.

26. The diesel fuel according to Claim 24 wherein the liquid biofuel comprises a mixture of alkyl esters of fatty acids.

27. The diesel fuel according to Claim 24 wherein the liquid biofuel comprises an alkyl ester of a vegetable oil selected from the group consisting of canola oil, soybean oil, cotton oil, palm oil, sunflower oil, and combinations thereof.

28. The diesel fuel according to Claim 24 wherein the liquid biofuel comprises an alkyl ester of a vegetable oil selected from the group consisting of canola oil, soybean oil, and combinations thereof.

29. The diesel fuel according to Claim 24 wherein the synthetic diesel fuel comprises between about 1 percent by volume and 45 percent by volume of the liquid biofuel.

30. The diesel fuel fuel according to Claim 24 wherein the synthetic diesel fuel comprises between about 1 percent by volume and 20 percent by volume of the liquid biofuel.

31. The diesel fuel according to Claim 24 wherein the synthetic diesel fuel comprises between about 20 percent by volume and 45 percent by volume of the liquid biofuel.

32. The diesel fuel according to Claim 24 wherein the synthetic liquid distillate is derived from synthesis gas.

33. The diesel fuel according to Claim 24 wherein the synthetic liquid distillate is characterized by a boiling range having a 5% boiling point between about 170°C and about 210°C and a 95% boiling point between about 320°C and about 350°C.

34. The diesel fuel according to Claim 24 wherein the liquid biofuel comprises a total glycerin content less than 0.24 percent by weight of glycerin.

35. The diesel fuel according to Claim 24 wherein the liquid biofuel has a flash point greater than 100°C.

36. The diesel fuel according to Claim 24 wherein the liquid biofuel has an alcohol content less than 10 percent by weight.