ASTM D6751: Standards for Biodiesel Contamination and Quality

Gear case (3)ASTM International, known as the American Society for Testing and Materials (ASTM) sets standards for the production of Biodiesel. ASTM D6751 includes allowed amounts of certain biodiesel contamination. The contaminates we are concerned with are: methanol, water, glycerine, acids and ash. Cloud point and viscosity are important, but we really do not see related break downs At Karmakanix, as we are located in the Bay Area, and do not have the same temperature drops as other areas.

A rather bulky pdf of ASTM D6751 – 12 can be purchased online directly from ASTM itemizing the current standards. A copy of the older, but useful, standards is available online at US Dept of Energy Website.

Methanol in Biodiesel

Biodiesel manufacturing entails using some alcohol as the strata to accept esters broken off the oils using lye. Typically methanol is employed due to its price and volatility. Left over methanol is removed through a water process, but a tiny bit is always left behind even in ASTM Certified Manufacturing. The ASTM standard is .2% maximum.

Straight up, methanol eats everything except glass and stainless steel. Everything. How badly it damages a material mostly depends on the concentration, or percent, of methanol present. Thankfully, modern biodiesel has very little methanol in it. Home made biodiesel can be another story. Other issues with biodiesel contamination can also be present.

There is some question about long term biodiesel usage versus the slow degradation of the plastic parts and senders inside the different fuel pumps. Without going through the chemistry, biochemistry and physics involved, we believe the long term biodiesel usage from Certified sources is fine in terms of methanol issues. Further information on our Karmakanix diesel motor webpages.

Water in Biodiesel

Biodiesel may contain small quantities of water. The water content comes in two varieties. Although biofuel is only slightly miscible with water it is hygroscopic, meaning it absorbs water from the air slowly. The reason biodiesel can absorb water is the presence of mono and diglycerides left over from an incomplete reaction. There may also be water that is residual to processing or resulting from condensation while inside a storage tank. Biofuel made from used cooking oils has a higher propensity for water content than biofuel made from new oil stock such as GMO soybean oil. Water reduces the heat of combustion, causing smoke, harder starting, and reduced power. Water can cause corrosion of fuel pumps, fuel lines, and injectors. Microbes in water cause the paper element filters to rot and fail, causing failure of the fuel pump. At cold winter temperatures, water freezes to form ice crystals that provide sites for nucleation, accelerating gelling of the fuel. Water also participates in the long term chemical reactions that oxidize old fuel and cause the formation of contaminates that can clog fuel filters and check valves.

Glycerine in Biodiesel

Glycerine, or Glycerol, is a byproduct of normal biodiesel production. It’s what’s left of the oils after the esters are removed and joined with the alcohol. This form of glycerine is dark and syrupy. Occasionally we see slight amounts in a fuel tank or car boy. The odds of having a glycerol problem are nearly as small as a methanol attack. We do see one glycerin related diesel fuel system contamination every few years, usually in combination with high water and acid content, possibly an issue brought about from old fuel or combining fuels.

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