Effects of Raw Materials and Production Practices on Biodiesel Quality and Performance

The demand for transportation fuels is increasing around the world, especially the demand for petroleum-based fuels. To cope with rising demand and dwindling petroleum reserves, alternative motor fuels such as biodiesel are at the forefront of commercialization. Biodiesel is an environmental renewable clean burning fuel. Biodiesel is a replacement for diesel in compression-ignition engines. Biodiesel is composed of mono-alkyl esters of long chain fatty acids. These esters are produced when virgin vegetable oils, i.e., soy, canola, palm and rapeseed oil, animal fats from tallow, poultry offal and fish oils or used cooking oils and trap grease from restaurants are reacted with an alcohol. The major chemical components of vegetable oils, fats and greases are triacylglycerols. The chemical reaction of converting triacylglycerols into methyl esters is termed transesterification. A stochiometric excess of alcohol and a catalyst is required for the effective transesterification of triacylglycerols into alkyl esters. The transesterification reaction is depicted in Figure 1. The alcohol used for producing biodesel is usually methanol. Methanol is the least expensive alcohol and therefore the alcohol of choice. The catalyst can be an acid or a base depending on the amount of free fatty acids present. The catalyst bases most commonly used are NaOH or KOH. The acid catalyst is usually H2SO4. In order to be commercially available in the United States and Canada, biodiesel must meet the specifications in ASTM D6751, Standard Specification for Biodiesel Fuel (B100) Blend Stock for Distillate Fuels. In Europe they follow the requirements and test methods for fatty acid methyl esters (FAME). The requirements are specified in EN 14214. The requirements for these two standards are given in Table 1. These specifications are designed to meet the requirements necessary for the proper performance of compression-ignited engines. Feedstock, feedstock quality and production practices can influence the quality of the biodiesel and therefore, the performance and commercial approval of the final product.


Introduction
The demand for transportation fuels is increasing around the world, especially the demand for petroleum-based fuels.To cope with rising demand and dwindling petroleum reserves, alternative motor fuels such as biodiesel are at the forefront of commercialization.Biodiesel is an environmental renewable clean burning fuel.Biodiesel is a replacement for diesel in compression-ignition engines.Biodiesel is composed of mono-alkyl esters of long chain fatty acids.These esters are produced when virgin vegetable oils, i.e., soy, canola, palm and rapeseed oil, animal fats from tallow, poultry offal and fish oils or used cooking oils and trap grease from restaurants are reacted with an alcohol.The major chemical components of vegetable oils, fats and greases are triacylglycerols.The chemical reaction of converting triacylglycerols into methyl esters is termed transesterification.A stochiometric excess of alcohol and a catalyst is required for the effective transesterification of triacylglycerols into alkyl esters.The transesterification reaction is depicted in Figure 1.The alcohol used for producing biodesel is usually methanol.Methanol is the least expensive alcohol and therefore the alcohol of choice.The catalyst can be an acid or a base depending on the amount of free fatty acids present.The catalyst bases most commonly used are NaOH or KOH.The acid catalyst is usually H 2 SO 4 .In order to be commercially available in the United States and Canada, biodiesel must meet the specifications in ASTM D6751, Standard Specification for Biodiesel Fuel (B100) Blend Stock for Distillate Fuels.In Europe they follow the requirements and test methods for fatty acid methyl esters (FAME).The requirements are specified in EN 14214.The requirements for these two standards are given in Table 1.These specifications are designed to meet the requirements necessary for the proper performance of compression-ignited engines.Feedstock, feedstock quality and production practices can influence the quality of the biodiesel and therefore, the performance and commercial approval of the final product.

Feedstock
As previously stated, the feedstock sources can be virgin vegetable oils, animal fats and greases.The virgin vegetable oils that are commonly used are soybean, canola, rapeseed, sunflower and palm.Soybean vegetable oil, fats and yellow grease are mainly used in the United States [1].Canola is used in Canada.Rapeseed and sunflower oil are the primary feedstock in Europe [2].Palm oil, which is mainly produced in the tropics, is the main feedstock used there [3,4].The feed stock source can influence the cetane number, oxidation stability, cold soak filterability (deposition), and cold flow properties.

Cetane Number
The performance of diesel engines depends on the compression ratio, injection timing, fuel/air mixture and ignition delay.The cetane number is a measurement based on the ignition delay of compression-ignition engines (the lower the ignition delay, the higher cetane number).ASTM D613 and EN ISO 5165 are the standard procedures for determining cetane number.The lower the ignition delay, the better the compression-ignition engines performs.The low ignition delay increases power, engine efficiency and the engine's ability to start at lower temperatures.The composition of the biodiesel influences the cetane number.The minimum acceptable cetane number necessary for acceptable performance in modern compression-ignition engines is 40 [5].
The chemical composition of the triacylglycerols from different feedstocks varies in chemical composition.Therefore, the methyl esters produced from different feedstocks varies according to the source.The cetane numbers of the methyl esters from different feedstocks are given in Table 2.

Oxidation Stability
All fuels, including biodiesel, have stability problems.Biodiesel is susceptible to oxidative degradation of the fuel quality.The oxidation degradation of the fuel is determined by the amount and position of the olefinic unsaturation in the fatty acid methyl ester molecular chains.All of the biodiesel feedstocks have polyunsaturated chains that are methyleneinterrupted in their triacylglycerols molecules.The oxidation proceeds at different rates depending on the number and position of the olefinic unsaturation [7].The fatty acids chemical composition of triacylglycerols used as feed stocks is given in Table 2. EN 15751 specifies a procedure to measure the propensity of biodiesel to oxidation.Oxidation stars by attacking the methylene carbons between the olefinic carbons.Hydrogen is removed and a hydroperoxide and conjugated dienes are formed.The hydroperoxide decomposes and interacts to form aldehydes, alcohols, carboxylic acids and high molecular weight polymers [9].Aldehydes detected in the oxidation process include hexenals [10], heptenals, propanal [11,12 ] and 2,4-heptadienal [12].Short chain aliphatic acids and alcohols have also been detected [13,14].Increase acidity due to formation of organic acids increases corrosion.Polymerization products from oxidation will increase viscosity of the fuel and therefore it will influence the performance.

Cold Soak Filterability
In cold weather, the most common problem associated with biodiesel or biodiesel blends is the plugging of the fuel filter.In 2008, a cold soak filtration test was added to the ASTM specifications, to address this problem.Cold soak filterability is a measurement of how well biodiesel flows when chilled and poured through a filter.Previous studies showed that the formation of precipitates during cold weather conditions depends on the feedstock, blend concentration and storage time [15,16].Most of the precipitate formed at lower temperatures will be re-dissolved when they are warmed to room temperature [17]; however, minor precipitate components remain as precipitates after warming to room temperature.Insoluble precipitates from soybean biodiesel can be attributed to sterols present in the soybean oil feedstock.Soybean oil contains approximately 0.36% sterols.Sterols are composed of a group of steroid alcohols present in plants.The culprit sterol was found to be sterol glucoside(SG) [15].Soybean oil may contain up to 0.23 % SG [16].
The insoluble precipitates from palm biodiesel are due to both sterol glucoside and monoacylglycerols; while, the precipitates from poultry fat biodiesel are due only to monoacylglycerols [15].

Cold flow properties
All diesel fuels, as well as biodiesel are subject to performance problems when they are subjected to cold temperatures.As a fuel is cooled, high molecular weight components present in the fuel begin to precipitate and this causes the fuel to start to solidify or gel.
The cold flow properties of the biodiesel are dependent on the fatty acids composition of the triacylglycerol feedstock.The transesterification does not change the chemical compositions of the fatty acids; it just makes methyl esters of these acids.Therefore, biodiesel made from triacylglycerol feedstock composed of high concentration of high molecular weight fatty acids will have poor cold flow properties.Tallow and palm biodiesel are the worst offenders.They start to have cold flow problems between 18 to 10°C.Canola, rapeseed, sunflower and soybean biodiesels start having problems around 0°C [18].

Feedstock Quality
Pure triacylglycerols feedstock is easy to convert to biodiesel.However, impurities that may be present in the feedstock can impact quality and cost of the final product.Common impurities present with the triacylglycerol feedstock are water, solids, free fatty acids and sulfur [19].

Water
In the production of biodiesel, it is important to keep water below 1%.The presence of water in the feedstock will produce soaps during the transesterication process and affect the completeness of the reaction.The soap and water can form a water in oil emulsion which will affect the final biodiesel fuel quality; since, it will create deposits, viscosity and engine performance problems.These water emulsions can be broken by heating.Therefore, the oil can be heated and the water allowed settling to the bottom of the container.Water removal is performed by pumping the water out from the bottom of the container from under the oil.

Solids
Insoluble particles can be present with the feedstock.This is a particular problem with yellow and trap grease.These particles can create fuel filter plugging and engine deposits.Therefore, it is recommended to filter the feedstock before transesterification.

Free fatty acids
Base catalyzed transesterefication of high free acid feedstock will react with the catalyst and produce soaps.Feedstock with more than 2% free fatty acid needs to be caustic striped before being used in base catalyzed transesterification.Feedstocks with characteristic high amounts of free fatty acids are tallow and yellow grease.These feedstocks usually contain over 15% free fatty acids.
On the other hand, acid catalyzed transesterification produces water as a byproduct of the reaction.Water needs to be removed in order to drive the reaction to completion.This reaction also requires higher temperatures and a higher ratio of alcohol to free fatty acids, usually around 20:1 to 40:1.
A combination of acid catalyzed esterification followed by a base catalyzed reaction offers a good alternative for biodiesel production from high free fatty acid feedstocks.In this case, the acid catalyst of choice is phosphoric acid, H 3 PO 4 .After esterication, the H 3 PO 4 is reacted with excess KOH.Finally, at the end of the process, the remaining KOH is reacted H 3 PO 4 .The K 3 PO 4 is dried and sold as fertilizer.

Sulfur
The EPA regulates the amount of sulfur in fuels.For on road fuels, the EPA mandates 15 ppm sulfur maximum.In Europe, the sulfur level in biodiesel has to be lower than 10 ppm.Biodiesel made from pure feedstocks has virtually no sulfur.However, sulfur levels in waste grease can reach to 200 -400 ppm.During production, the final sulfur concentration can be reduced by approximately 40 to 50%.Vacuum distillation can also reduce sulfur by 50%.Treatment with activated carbon can reduce sulfur in biodiesel to acceptable low levels.

Production Practices
Quality of the final product is also dependent on production practices.Good practices will insure completeness of the reaction, good separation of the glycerol from the reaction product, stripping of the alcohol, splitting of soaps and water and catalyst removal.

Reaction completeness
The trasesterication of triacylglycerols into biodiesel occurs by first producing a diacylglycerol, which in turn is converted to a monoacylglycerol and finally a glycerol molecule.Each of the reaction steps produces a molecule of fatty acid methyl ester.If left with the final product, they can produce cold flow problems and engine deposits and the biodiesel may not pass ASTM or EN specifications.However, there are absorbents in the marketplace that through filtration can selectively remove acylglycerols and glycerol.

Glycerol
Glycerol is an undesirable product in biodiesel production.It is insoluble in biodiesel and could be easily removed by settling to the bottom of the tank or by centrifugation.Excess methanol and high concentration of soaps will inhibit the separation.Glycerol in the biodiesel will create viscosity, engine combustion and filter plugging problems.Water washing or absorbents can reduce the concentration of glycerol in biodiesel to acceptable levels.

Alcohol
Biodiesel may contain up to 4% after glycerol separation.Excess methanol in the fuel will provide a dangerous explosive mixture in compression-ignited engines.The methanol present in the fuel influences the flash point.The change in flash point of fatty acid methyl ester biodiesel versus methanol and ethanol concentrations is given in Figure 2. Water washing or vacuum stripping will reduce alcohol to acceptable levels and meet ASTM and EN specifications.
Fig. 2. Flash point of methanol and ethanol versus concentration in biodiesel.

Soaps
Soaps have been previously discussed.They can form microemulsions and influence the performance of the fuel.Soaps can be removed by water washing of the final product.

Water and catalyst removal
Water can be present as microemulsion or dissolved in the fuel.Biodiesel can contain up to 0.15% dissolved water.Water can contribute to corrosion, microbiological grows, sedimentation, etc. Water can be removed by allowing it to settle to the bottom of the tank, boiling it off or by using solid absorbers.
Residual catalysts can form engine deposits and abrasion and wear of the fuel engine parts.
Catalyst is usually removed with the glycerol and with the final water wash of the fuel.

Conclusion
Biodiesel is a renewable fuel manufactured from feedstocks such as virgin and used vegetable oils, animal fats and recycled restaurant greases.It serves as a substitute for conventional diesel.Feedstocks, feedstock quality and production practices can influence the quality of the final product.However, by taking appropriate steps in the production of biodiesel, a high quality fuel can be produced.

Fig. 1 .
Fig. 1.The transesterification reaction for the production of Biodiesel from triacylglycerol.
could not leave this subject without mentioning BQ-9000.The National Biodiesel Accreditation Program is a cooperative and voluntary program for the accreditation of producers and marketers of biodiesel fuel called BQ-9000.The program is a unique combination of the ASTM standard for biodiesel, ASTM D 6751, and a quality systems program that includes storage, sampling, blending, shipping, distribution and fuel management practices.BQ-9000 is open to any biodiesel manufacturer, marketer or distributor of biodiesel blends in the U.S. and Canada.

Table 3
. Composition of tracylglycerols used as feedstock in biodiesel production.Percent by weight of total fatty acids [8].