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Improving Biodiesel Stability with Fuel Additives

By Glenn Kenreck | January 24, 2007
Blends of biodiesel are now well-adopted fuels that are gaining market share and popularity. Biodiesel is a fuel comprised of mono-alkyl esters of long-chain fatty acids derived from vegetable oils or animal fats that meets the requirements of ASTM D 6751. However, several unique characteristics of biodiesel, also known as fatty acid methyl esters (FAME), or B100, may cause some operability problems. Low-temperature fluidity, water haze, and thermal and oxidation stability are the most significant issues facing the marketers and end users of biodiesel blends.

To date, there's been a significant amount of research done in these areas, and many articles have been written about these topics. In fact, it's now possible to examine biodiesel properties and recommended specifications through mainstream literature reviews.

"Cold flow improver" technology is evolving but, because of the composition of B100 and especially soy oil esters, the improvement using additives is limited to a few degrees Fahrenheit. Understanding the chemical makeup of biodiesel and the measurement of the metrics that could impact operability are key to understanding the root causes of instability and are helpful in formulating solutions to overcome stability problems. The work by J. Andrews Waynick of Southwest Research Institute13 appears to be the most comprehensive and inclusive document on this subject. Now that fuels containing B100 are being used, experiential information and data can be introduced into the analyses of these critical areas. The variables that have been identified include glycerol content, free fatty acid content, metals and carbon-carbon double bond types, and configurations (degree and level of unsaturation).

The manufacturing process is a significant potential source of stability problems, and process controls are the first line of defense to achieve and maintain stability of biodiesel. Fuel blending strategies and fuel additives are now gaining interest to meet market demands for viable fuels containing blends of biodiesel. Fuel additives are successfully being used to improve operability by preventing oxidation and thermal degradation of B100 and biodiesel blends.

B100 is sometimes produced in a batch process by transesterification of methanol with triglycerides, also known as glycerol fatty acids, in the presence of caustic. The "degree of completion" of the reaction to replace the glycerol with an alcohol such as methanol was recognized as important to fuel quality early in the development of biodiesel. The reaction is conducted at elevated temperatures of 120 to 150 degrees Fahrenheit to complete the reaction. The mixture is then neutralized with an acid and washed to remove excess methanol, salts and free glycerol. Effective neutralization is required to remove alkali metals from the oil phase. If washing isn't effective, the salts and glycerol may remain in the B100. Minimizing glycerol levels is required to ensure quality B100 production. Maximizing the degree of completion of the esterification reaction minimizes free fatty acids that can contribute to instability. Removal of alkali metal salts from the process and drying is also important to meet the quality standard of finished B100 product. Process controls and quality testing are required to achieve the quality standards established in ASTM D 6751.

Why is Stability Important?
Oxidation and polymerization reactions have been recognized throughout the refining and automotive industries as causes of filter plugging, fuel injector deposits, injector coking and corrosion.4 Oxidation by-products such as acids and carbonyls, cause corrosion and accelerate the degradation process. Degradation by-products may be insoluble in the diesel blends and cause filter plugging in fuel systems.

Fuel filter plugging and engine deposit formation may be related to the formation of "total insolubles," but B100 doesn't produce significant insolubles when tested by storage stability tests.6 This may be due to the solubility of the polar degradation by-products in the somewhat polar B100, but these by-products can then become insoluble when the biodiesel is blended into the nonpolar diesel fuels. Insolubles are expected to become a more significant problem as ultra-low sulfur diesel (ULSD), which is even more nonpolar, is introduced in the marketplace.

Some truckers are complaining that fuel filter clogging is increasing as animal fat and soybean-based fuels are being introduced nationwide.14 Fuel system problems, including filter plugging, have been documented in the United States, along with fuel filter plugging in vehicles that use B100 or biodiesel blends. According to a survey conducted by the Minnesota Trucking Association, 62 percent of 90 fleets said they had experienced fuel filter plugging. 10

Operability problems increase maintenance costs and decrease equipment reliability. Subsequently, problems related to biodiesel use could prevent the growth and adoption of biodiesel in the fuel marketplace. So preventing operability problems improves the viability of biodiesel as a fuel.

Measuring Stability
The stability of biodiesel can be measured using several different analytical procedures. A commonly cited problem in literature is that biodiesel stability may be controlled by several mechanisms and no single analytical test is capable of predicting actual performance in engine systems.

The oxidative stability index (OSI) Rancimat method has been identified as a procedure to measure stability by quantifying the acids formed during the degradation process when biodiesel is exposed to elevated temperatures and oxygen. Namely, it has been used to predict the oxidative stability of biodiesel.7 Research has shown that little correlation exists between stability and insolubles. This conclusion is true for linear correlations. However, the correlation coefficient is higher when using an exponential decay relationship, although the correlation is still weak. Tests such as the ASTM-525 and ASTM 6418 measure the thermal stability of biodiesel by quantifying the color degradation and gum formation. However, stability measurements can't always predict the formation of solids that cause filter plugging, one of the more common problems with biodiesel.

Long-term storage tests such as ASTM D 4625 have been used to determine oxidative stability. Using this test, various samples of biodiesel have been found to be susceptible to oxidation. Likewise, research has shown that sunlight can accelerate the degradation process, although light shouldn't be a problem in the fuel applications.3 The effects of aging in long-term storage tests can be measured by changes in viscosity. This correlation is relatively high, indicating that predictions can be based on available data.

Iodine value (IV) can be used to quantify the number of double bonds, a major cause of instability. However, the position and number of double bonds per molecule are important factors in the stability of B100, and the IV can't distinguish these parameters. The esterification efficiency along with free acids, glycerols, and partially transesterified oils are also important variables that can't be quantified using the IV, so the IV wouldn't be expected to be a good predictor of stability. Today, the composition and molecular structures of various oil feedstocks that B100 is commonly derived from have been clearly identified. Therefore, IV testing would be useful primarily for determining changes in the base oils from degradation.

The results of aging can be measured by IV, total acid number, anisidine value (aldehydes) and viscosity.6 Acid number increases as B100 ages; this is the evidence of degradation and not a measure of stability.4 These metrics can only quantify the degree of degradation, but they don't predict the stability of B100 or biodiesel blends.

Glycerols
Glycerols have been identified as a potential cause of biodiesel instability. Glycerol is produced via transesterification of triglycerides with methanol. The measurement of free and total glycerols indicates the removal effectiveness and conversion of the triglycerides to methyl esters. Reaction conditions could result in the formation of oxidized by-products that cause discoloration, especially when glycerol is exposed to either high temperature or alkaline pH. Glycerol can oxidize or thermally degrade to form diols or acids that can contribute to the instability of biodiesel. These degradation by-products may catalyze polymerization of unsaturated fatty acids. The unreacted triglycerides may be oxidized or dissociated over time, resulting in the formation of free glycerol that could polymerize or oxidize. Long-term storage can result in exposure to oxygen or metals that facilitate the oxidation process and dissociation of the glycerol to form free fatty acids. These molecules tend to polymerize, forming gums that can foul injection systems and prevent proper fuel atomization or distribution. These gums or insolubles may also plug fuel filters. Free glycerol and oxidized glycerol by-products may form sediments or cause corrosion. 4

According to John Hausladen, when the excess glycerol in biodiesel gets cold, it suspends and never goes back into the liquid, contributing to fuel filter clogging problems. 10

Glycerol resulting from partially reacted or unreacted oils is measured as total and free glycerol. The glycerol specification in ASTM D 6751 was established to maintain a minimum standard for B100 and prevent the detrimental effects of glycerol in biodiesel. B100 should meet the required specifications of either ASTM D 6751 or the appropriate European specification.8 Not meeting these specifications can cause elevated levels of glycerol or soap that could result in insolubles or deposit formation in the fuels and, subsequently, engine operability problems. The most valuable control measure for glycerol is the manufacturing process. Certification of quality that includes the glycerol content and esterification can be required to ensure that the purchased lot of B100 meets the minimum quality standards.

Free Fatty Acids
Acids are a known cause of instability in vegetable oils and hydrocarbon fuels. These may be present in B100 if the transesterification reaction was incomplete and alkali metal salt of fatty acid was present in the neutralization and wash process. The alkali metals are removed by acid, and the free fatty acid is formed. Free fatty acids may degrade or cause corrosion and thermal instability. They can adversely impact oxidative stability. Alkali can undergo oxidation and break down into aldehydes, ketones, epoxides and alcohols. 2 Oxidation stability decreases with decreasing molecular weight and increasing amounts of double bonds present, as well as the number of double bonds per molecule.1 These reaction by-products may be of lower molecular weight than the fatty acid. They likely will retain the double bonds, and because they are in lower molecular weight, they are more susceptible to polymerization processes. These acids and degradation by-products can act as a chain initiator for polymerization of organic compounds, especially unsaturated compounds that contain multiple carbon bonds and are more susceptible to polymerization.

Finally, alkali metal soaps of fatty acids can catalyze instability, as well as increase insolubles or water haze.

Driving the transesterification reaction to completion can minimize free acid or partial esterification that leads to free fatty acid catalyzed instability. However, when the possibility of free fatty acids exist, neutralizers can be added to B100 or biodiesel blends to minimize free acid levels and, subsequently, the impacts of free acids on degradation. Antioxidants can be used to prevent, inhibit or terminate the polymerization mechanisms, such as hyperperoxide reaction or diene condensations. The synthetic antioxidants appear to be more effective than naturally occurring antioxidants, which may be removed during the B100 manufacturing process. 8

Metals
Metals are known to catalyze oxidation and polymerization reactions of hydrocarbons. Heavy metals promote autooxidation. 2 Transition metals, such as copper or iron, may be present if corrosion occurs in the manufacturing process, and they could act as a Lewis acid to catalyze polymerization of polyunsaturated hydrocarbon molecules found in the B100 or biodiesel blends. Fungible diesels include cracked stocks light-cycle oil, which may contain metals and high levels of olefins. These cracked stocks, when combined with metals from B100, may decrease stability of the final biodiesel blends. Alkali metals can form sediments and cause injector failures.4 Furthermore, alkali metals could form soaps and contribute to insolubles or water haze in the diesel blends. Metals have been shown to have a significant effect in the OSI test, and copper-known to cause instability in petroleum-based fuels-has a significant impact, as well.1

Production controls including effective neutralization, washing and water separation can prevent alkali metal in B100. Proper materials of construction for B100 production equipment minimizes corrosion and the subsequent introduction of metals into the biodiesel. Copper alloys should be avoided in B100 process equipment. If metal contamination occurs, metal deactivators can be used to chelate transition metals, and inhibit the catalytic oxidation and polymerization effects of the metals in the biodiesel.

Unsaturates
Much information has been published about the effects of double bonds in various oil feedstocks. Unsaturates are well-known as root causes of fuel instability. These molecules are susceptible to oxidation, polymerization, and gum formation. Diesel fuels are blended from cracked feedstocks that contain reactive olefins. The composition of unsaturates in vegetable oils and tallow commonly used in manufacturing B100 are well-identified. Vegetable oils are known to contain polyunsaturated fatty acids that are susceptible to oxidation and polymerization reactions. The quantity of double bonds can be measured by IV, but the location and number of double bonds per molecule can't be determined. Tallow, often used in the United States, has fewer double bonds and is free of the multiple double bonds. U.S. biodiesel typically has less than two-hour stability, as measured by the OSI test.7 This is well below the European specification of six hours. The primary reasons for this difference are the number and position of double bonds.

Unsaturates and polyunsaturates are naturally occurring. The levels can be reduced by hydrotreating; however, this isn't economically feasible and would adversely affect the low-temperature fluidity. Antioxidants have been used extensively in the refining industry to prevent the reactions of unsaturates in fuels by inhibiting or interrupting the oxidation and polymerization reactions to prevent degradation, oxidation and gum formation. Additives can be used to prevent degradation of B100 and biodiesel blends, thus preventing operability problems.

In summary, literature documents the disconnect between stability and operability, including insolubles that cause system plugging. At this time, there isn't a single procedure that can evaluate or predict instability of biodiesel in general or the formation of insolubles in petroleum-based biodiesel blends that are causing operability problems in engines. The quality of B100 is essential to operability.
Reactions can occur in B100 or biodiesel blends to form polymeric gums and degradation by-products that may result in vehicle operability problems, including fuel filter pluging or injector fouling. B100 control during manufacturing is critical to preventing these problems. However, some variation in processes and feedstocks may result in B100 that has an increased tendency to form these degradation by-products. Fuel additives have been used for many decades to prevent and control oxidation and polymerization reactions that contribute to gum formation in petroleum fuels. They act to inhibit metal catalysis, prevent peroxide formation and neutralize acids that can catalyze polymerization reactions. They also act as chain terminators for olefinic polymerization processes. The mechanisms described in this article have been well-researched and documented.

Analytical techniques are available to determine the levels of components that may contribute to B100 instability. Biodiesel blends may become unstable because of factors that by themselves may not be significant but, when combined, are synergistic and increase the instability of the blend. Understanding the reaction mechanisms is necessary for correct application of stability improvers in B100 and biodiesel blends. Fuel additives can be used to counteract and prevent the mechanisms that cause oxidation, polymerization, and acid formation; mitigate the effect of insolubles; and improve the operability of biodiesel blends. n

Glenn Kenreck is a fuel additive products application manager for GE Water & Process Technologies. Reach him at glenn.kenreck@ge.com.

References
1. Knothe, G., Dunn R.O. Dependence of Oil Stability Index of Fatty Compounds on their Structure and Concentration and Presence of Metals. JOACS, 2003, 80 (10)

2. Wikipedia, the Free Encyclopedia, http//en.wikipedia.org/wiki/fatty_acid, Autoxidation and Rancidity

3. Prankl, H.; Schindlbauer, H. Oxidation Stability of Fatty Acid Methyl Esters. 10th European Conference on Biomass for Energy and Industry, June 8-11, 1998, Wurburg, Germany

4. Robert Bosch GmbH. Biodiesel Presentation, CARBbio_29Sept04

5. Brevard Biodiesel, Stability of Biodiesel & Iodine Value, www.brevardbiodiesel.com/10.html

6. Peckham, J. Biodiesel Blend Instability, Insolubles Formation-Tough to Predict, Fix; ULSD Makes it Worse. Diesel Fuel News, October 10, 2005, 7-11

7. ASTM Biodiesel Subcommittee Presentation. Biodiesel Quality; Test Method Update, Quality Update

8. Engine Manufacturers Association. Technical Statement on the Use of Biodiesel Fuel in Compression Ignition Engines, page 2

9. Presentation of BIOSTAB Project Results. Stability of Biodiesel Used as a Fuel for Diesel Engines and Heating Systems, July 3, 2003, Graz, Austria

10. Clogging Triggers Suspension, Minnesota Halts 2% Biodiesel Requirement Through Jan 13. The OilSpot News by DTN, December 29, 2005

11. System Lab Services. Determination of Biodiesel Oxidation and Thermal Stability-Final Report, February 12, 1997

12. Halldorsson, A. Doctorial Thesis, Science Institute, University of Iceland; Lipase Selectivity in Lipid Modification, May 2004

13. Waynick, J.A. Characterization of Biodiesel Oxidation and Oxidation Products, SwRI Project No. 08-10721, August 2005

14. PTSA Weekly Update. Truckers Complain that Biodiesel Fuel is Clogging Engine Filters, Friday December 9, 2005 TP1127EN 0604 Page 7
 

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