~Biodiesel The "Least Stable" Among Fuels

The "least stable sample is the B20 biodiesel even [though] it contains a substantial amount of sulfur," the researchers found. "We conclude that the instability of B20 biodiesel blend with off-highway diesel fuel comes from an acid-catalyzed hydrolysis of the ester components from the biodiesel."

Nor are problems likely to arise solely from the deliberate mixing of used oil with fuel, as with the Cummins "Centinel" recycling system. While that system results in about a 3,000-ppm dose of lube oil in fuel, the newer high-pressure fuel injection systems also result in about 1,000-ppm of lube in fuel, Stehouwer explains.

A separate test of cetane improver in 20% biodiesel blend (B20) found even worse instability problems, the researchers found. This is of special concern because modern high-pressure injection systems circulate a lot of fuel through hot injectors (as a heat sink) for return to storage tank.

That's why engine makers are pressing for a tough thermal stability limit in American Society for Testing & Materials (ASTM) D975 diesel fuel standard, as well as similarly tough thermal stability limits for biodiesel blends.

With diesel fuel, careful fuel blending and proper additization could avoid stability problems, Stehouwer points out. Otherwise, thermally-unstable fuels could form gums that could plug filters and cause injectors to stick, resulting in costly repairs, he said.

COPYRIGHT 2003 PBI Media, LLC
COPYRIGHT 2004 Gale Group



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