Quality Control Aspects of Biodiesel: Ensuring Engine Safety

Linda Benedict, Iqbal, Javed, LaCaze, Shelly  |  11/17/2009 3:35:22 AM

Javed Iqbal and Shelly L. Martin
Making biodiesel fuel from vegetable oil or animal fat is a simple process. Rudolph Diesel used raw vegetable oil when he invented the first diesel engine in Augsburg, Germany, in 1893. Our modern engines, however, cannot digest the viscous oil used then. To avoid viscosity-related problems, used vegetable oils must undergo a chemical reaction called transesterification to be converted into biodiesel. In this process, oil molecules called triglycerides react with alcohols in the presence of a base catalyst such as lye. The product of the reaction is biodiesel, chemically named as mono-alkyl esters. Glycerin is a byproduct of the reaction. Theoretically, 100 gallons of oil or fat would react with 20 gallons of alcohol (usually methanol) in the presence of a catalyst (sodium or potassium hydroxide) to yield 100 gallons of biodiesel and approximately 22 gallons of glycerin.

Although no modification is needed to a diesel engine to use biodiesel, there is concern about the effects. Since 1993, manufacturers make diesel engines using materials compatible with biodiesel. Diesel engines made before 1993, however, may have natural rubber seals and fuel lines susceptible to slow degradation with biodiesel. Replacing such parts with current synthetic fuel lines is recommended to solve this problem. Rubber lines in new vehicles are mostly resistant. The switch to low-sulphur diesel fuel in the early 1990s caused wear and failure of injection pumps, and the manufacturers switched to resistant components, which are also resistant to biodiesel.

Poor quality biodiesel can cause short- and long-term engine and equipment problems. The American Society for Testing and Materials (ASTM) has, therefore, developed specifications for biodiesel fuel to ensure a certain quality of biodiesel for your vehicles. Poor quality biodiesel cannot leave the engine without problems. Though the effect will probably not be immediately noticeable, corrosion and damage can accumulate until the engine fails. 

To avoid problems using biodiesel, it is advisable to understand the basic chemistry of biodiesel production, fuel quality testing, diesel equipment mechanics and fuel system problems. It is advisable to try using homemade biodiesel in older, inexpensive equipment before fueling newer, more expensive equipment. By all means, keep non-ASTM certified biodiesel fuel out of equipment in which the warranty may be voided by its use. Those who own diesel equipment under warranty are advised to check with their manufacturers regarding recommended percentage limits for biodiesel blends. Good quality biodiesel can be assured if the following conditions are met:

The reaction is complete.

You have to make sure the vegetable/fat oil molecules are completely broken down and the viscous glycerin is free. An incomplete reaction results in unreacted oil left dissolved in the biodiesel. The presence of unreacted oil is determined by dissolving a small amount of the biodiesel in methanol (See Figure 1). If it does not completely dissolve in the methanol, you know the reaction was incomplete. The undissolved fraction determines the rate of conversion, e.g., one out of four parts of biodiesel undissolved in methanol shows 25 percent of unreacted oil. The unreacted products in 100 percent biodiesel should be less than 0.22 percent or 2,200 parts per million. Poorly converted oils/fats have been shown to form gum-like deposits around injector tips and valve heads. The presence of an unreacted product can also affect the cold-flow properties of biodiesel, resulting in gelling at higher temperatures and causing filter plugging and injector damage after long-term use.

The biodiesel is washed well.

Free glycerin is the byproduct of a complete reaction and is laden with the catalyst (lye) used in the process. Both of these along with excess alcohol settle down to the bottom of the reactor before being drained out. Biodiesel is then washed with water to remove any leftover glycerin and catalyst. Glycerin is limited by the ASTM specifications as 0.02 percent. Free glycerin has the potential to form gum-like deposits around injector tips and valve heads while the catalyst/lye can damage the injector pump. It also affects the viscosity of the final product.

The biodiesel is dry enough.

Moisture contents in biodiesel, as leftover from the wash process, can cause corrosion of the engine’s fuel system components through rusting. Moisture can also cause the biodiesel to become acidic over time causing corrosion of the fuel storage tanks. Water can facilitate microbial and algal growth leading to storage problems in the tanks. For these reasons, biodiesel is dried enough to reduce the moisture to 0.05 percent.

Other issues to consider when producing and using biodiesel are:

Methanol (or ethanol), the unifying force

As a solvent, methanol keeps biodiesel, glycerin, lye and water together. Removing methanol helps separate glycerin, lye and water from biodiesel, which helps meet the ASTM specifications. Methanol drastically affects the biodiesel flashpoint (the temperature at which it ignites), which is typically more than that of petro-diesel. Biodiesel’s flashpoint is about 266 degrees F, while the flashpoint of petro-diesel is about 143 degrees F. For comparison, methanol has a flashpoint of 52 degrees F. As little as 1 percent methanol in biodiesel can lower the flashpoint to less than 100 degrees F, a safety hazard for storage. Residual methanol can directly affect an engine’s fuel pumps and seals and can result in poor combustion properties. The ASTM standard hence limits the amount of residual alcohol in biodiesel to less than 0.2 percent, which is too small to affect the fuel’s performance.

Biodiesel, a good solvent

Biodiesel has a better solvent potential than petro-diesel. It therefore can clean out fuel tanks, storage tanks and the fuel lines when used for the first time. Biodiesel and its blends act as a detergent and will eventually clean out most of the sediment and sludge that has built up in the fuel system and tanks. Because fuel filters catch most of it, it is recommended that they be replaced more frequently when biodiesel is started initially. Once all the sediment and sludge is out, there is no need to change the filter more than routine maintenance. Starting with lower biodiesel blends can also help alleviate sudden loads on the fuel filters and the engine.

Winter starting

The biggest issue with running biodiesel or even petro-diesel in cold weather is their tendency to gel. Petro-diesel tends to gel between 5 and 40 degrees F, while biodiesel prepared from vegetable oil tends to gel between 26 and 54 degrees F. Petro-diesel fuel is adjusted for the season (winterized) at its distribution point to maintain the cold weather flow characteristics of the fuel. To avoid gelling of biodiesel in cold weather, it must be mixed with winterized diesel in a percentage suitable for that weather and climate. Another way to fix this problem is to add an anti-gel additive. The term "cloud point" is used to describe the temperature at which the fuel turns into gel-like crystals. Cloud point is dependent upon the oil used. Animal fat gels at higher temperatures compared to vegetable oil. The cloud point can be estimated by subjecting a biodiesel sample to a cold temperature (See Figure 3). One way is to put the sample into a clear vial, preferably glass, and churn it around in a bowl containing ice cubes dusted with table salt (the old way of make ice cream in the kitchen). Use a low temperature stem thermometer to note the temperature of the biodiesel when it turns to gel.

Good quality biodiesel ensures excellent engine performance comparable to petro-diesel while providing important environmental and economic benefits to society. There are proven solutions to the problems encountered with using biodiesel, e.g. cold-weather performance, solvent properties and long-term storage. Poor-quality biodiesel, on the other hand, has many negative effects on an engine. To protect against this, it is recommended to use biodiesel that complies with the ASTM biodiesel specifications. Home biodiesel brewers should have their product pass these standards.

Additional information is available at the LSU AgCenter Web site, www.lsuagcenter.com/callegari , and the National Biodiesel Board’s site at www.biodiesel.org.  

Javed Iqbal, Laboratory Manager, and Shelly L. Martin, Extension Associate, W.A. Callegari Environmental Center, LSU AgCenter, Baton Rouge, La

(This article was published in the fall 2009 issue of Louisiana Agriculture.)
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