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Recently, interest in biodiesel use in the U.S. has been growing due to: 1) its potential to reduce dependence on imported petroleum [1], 2) its potential to help mitigate possible negative impacts of global climate change by lowering net CO₂ emissions from the transportation sector (though there is some debate on this issue [2,3]), and 3) tax incentives and publicity that have resulted from the efforts of biodiesel advocates [4,5]. Researchers at Sandia National Laboratories’ Combustion Research Facility (CRF) have been studying biodiesel for a number of years to better understand how and why biodiesel use could impact in-cylinder processes and, as a result, the efficiency and emissions of advanced compression-ignition engines.

What is biodiesel? – Biodiesel is defined as “a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats” [6]. Biodiesel is typically created by reacting fatty acids with an alcohol in the presence of a catalyst to produce the desired mono-alkyl esters and glycerin. After reaction, the glycerin, catalyst, and any remaining alcohol or fatty acids are removed from the mixture. The alcohol used in the reaction is typically methanol, although ethanol and higher alcohols also have been used. The majority of the biodiesel currently produced in the U.S. is made from soybean oil [7], and soy biodiesel typically consists of the five methyl esters shown in Fig. 1 [8]. While neat (i.e., 100%) biodiesel can be used, a blend of between 2 and 20% (by volume) of biodiesel with diesel fuel is recommended to avoid engine-compatibility problems [6].

Biodiesel advantages and disadvantages – Biodiesel has a number of attractive attributes in addition to the aforementioned potential to reduce dependence on imported petroleum and to lower net greenhouse-gas emissions. Engine testing programs have shown that biodiesel fueling typically leads to lower emissions of particulate matter, unburned hydrocarbons, and carbon monoxide [9]. Biodiesel typically has improved lubricity and ignition quality relative to diesel fuel in the U.S. [10]. Biodiesel is sulfur-free, so it won’t poison catalytic aftertreatment systems [6]. Furthermore, biodiesel is renewable; that is, it’s not a fixed resource like fossil fuels that could be completely consumed.

Biodiesel is not a panacea, however. One notable disadvantage is that diesel-engine emissions of nitrogen oxides (NO_x) tend to increase by approximately 1% for every 10 vol% of biodiesel that is blended into diesel fuel [9]. Biodiesel also can create problems in cold-weather conditions, because certain of its constituent compounds can form crystals in the fuel. These crystals can cause undesired effects like plugging of fuel filters so that fuel cannot travel to the engine. In addition, biodiesel is often more susceptible to oxidative and biological instabilities than conventional diesel fuel, though these issues generally can be avoided by using the fuel promptly or by adding small amounts of stabilizer and biocide [6]. Finally, impurities such as unreacted fatty acids or alcohol, as well as glycerin or catalyst left over from the production process, can lead to accelerated wear or corrosion of engine components. Proper quality control is critical for avoiding unnecessary problems during the introduction of biodiesel into widespread use [11].

Past and current biodiesel research at the CRF – Early biodiesel research at the CRF showed that the elevated latent and specific heats of biodiesel compounds can lead to longer liquid penetration lengths within the combustion chamber than would be measured for diesel fuel injected into the same conditions; see Fig. 2 and Ref. [12]. This could cause impingement and adhesion of liquid fuel on in-cylinder surfaces, which could lead to increased fuel consumption and emissions, as well as oil degradation. A later study investigating molecular-structure effects on the soot-reduction characteristics of different oxygenated fuel compounds found that an ether structure was more effective than an ester structure (like that found in biodiesel) at attenuating in-cylinder soot concentrations in experiments. Subsequent reaction-path analysis provided an explanation by showing that more than 30% of the oxygen in the ester structure was unavailable for the prevention of soot-precursor formation [13].

Current biodiesel research in the CRF’s Advanced Fuels Optical Engine Laboratory is focused on determining the underlying reason(s) behind the observed increased NO_x emissions with biodiesel fueling. Hypotheses in the literature to explain the elevated biodiesel NO_x emissions include: 1) increased bulk modulus of biodiesel causing an advanced start of combustion, 2) larger premixed-burn fraction, and 3) increased stoichiometric adiabatic flame temperature. Recently, a study was conducted wherein a hydrocarbon (i.e., non-oxygenated) reference fuel was formulated to have the same ignition delay as neat biodiesel at a given operating condition. Figure 3 shows that, even when start of combustion and premixed-burn fraction were matched between the two fuels, NO_x emissions were still found to be ~10% higher for the neat biodiesel fuel. Furthermore, computed stoichiometric adiabatic flame temperatures were found to be identical for the reference fuel and a number of other oxygenated and non-oxygenated fuel components, including a biodiesel surrogate (methyl oleate). These results show that the three hypotheses above cannot fully explain the increased NO_x emissions with biodiesel fueling [14]. Experiments to identify the primary source(s) of increased biodiesel NO_x emissions are currently underway.

Figure 1. Molecular structures of the five methyl esters that typically comprise soy biodiesel [8].

Figure 2. Superimposed images of Mie scattering from the fuel-jet core showing the liquid-penetration length, and schlieren images showing the jet spreading angle. Image pairs are shown for neat biodiesel and conventional #2 diesel fuel. The liquid-penetration length is more than 30% longer for neat biodiesel than for #2 diesel fuel at this condition, which could lead to impingement of liquid fuel on in-cylinder surfaces. The ambient temperature and density into which the fuel is injected are 1000 K and 14.8 kg/m³, respectively [12].

Figure 3. Measured NO_x emissions are ~10% higher on average when fueling with neat biodiesel relative to fueling with a hydrocarbon reference fuel, even when the start of combustion and premixed-burn fraction are matched between fuels (inset) [14]. The engine speed (800 rpm) and intake conditions also were the same for both fuels at each load point.

References

1. Energy Policy Act of 2005 (Public Law 109-58), Library of Congress web site http://thomas.loc.gov/cgi-bin/bdquery/z?d109:h.r.00006:

2. National Renewable Energy Laboratory, “Life Cycle Inventory of Biodiesel and Petroleum Diesel for Use in an Urban Bus,” NREL/SR-580-24089, web site http://www.nrel.gov/docs/legosti/fy98/24089.pdf

3. Pimentel, D. and Patzek, T.W., “Ethanol Production Using Corn, Switchgrass, and Wood; Biodiesel Production Using Soybean and Sunflower,” Natural Resources Research 14: 65-76 (2006).

4. National Biodiesel Board, “Tax Incentive,” web site http://www.biodiesel.org/news/taxincentive/

5. “Willie Nelson’s Biodiesel – Source of Farm Fresh BioDiesel,” web site http://www.wnbiodiesel.com/

6. “Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels,” ASTM D 6751 (2003).

7. National Biodiesel Board, web site http://www.biodiesel.org

8. NIST Chemistry WebBook, “NIST Standard Reference Database Number 69, June 2005 Release,” web site http://webbook.nist.gov/

9. Environmental Protection Agency, “A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions,” EPA420-P-02-001 (2002).

10. Grabowski, M.S. and McCormick, R.L., “Combustion of Fat and Vegetable Oil Derived Fuels in Diesel Engines,” Prog. Energy Combust. Sci. 24:125-164 (1998).

11. National Biodiesel Accreditation Commission, “BQ-9000 Quality Management Program,” web site http://www.bq-9000.org/

12. Higgins, B.S., Mueller, C.J., and Siebers, D.L., “Measurements of Fuel Effects on Liquid-Phase Penetration in DI Sprays,” SAE Paper 1999-01-0519, SAE Trans. 108 (1999).

13. Mueller, C.J., Pitz, W.J., Pickett, L.M., Martin, G.C., Siebers, D.L., and Westbrook, C.K., “Effects of Oxygenates on Soot Processes in DI Diesel Engines: Experiments and Numerical Simulations,” SAE Paper 2003-01-1791, SAE Trans. 112 (2003).

14. Cheng, A.S., Upatnieks, A., and Mueller, C.J., “Investigation of the Impact of Biodiesel Fuelling on NO_x Emissions Using an Optical Direct Injection Diesel Engine,” Int. J. Engine Res. 7:297-318 (2006).