glycerides? who gives a drip!

Dear Ester~

How much is too much total glycerin in your biodiesel? Sure, the ASTM max is 0.24%, but where did that number come from? If you have 2.4%, is that really so much worse? Don't SVO'ers have 100%?

~Can Live On Glycerin

Dear CLOG~

Just because we can burn high levels of glycerin in diesel engines does not mean we should. Doing so over extended periods of time spells trouble.

My journey into grease began by putting a diesel engine on a gasoline tractor in college and converting it to run on cafeteria fryer oil. This was back in the early days when hardly a soul had heard of such a thing and an unknown hippie named Josh Tickell had just started selling a booklet called "From the Fryer to the Fuel Tank." The booklet had clearly been hand copied and bound at what appeared to be the same big chain copier that I used to make copies for my professors.

Over the years I went through a handful of old Volkswagen diesel cars and trucks, each one carefully converted to burn straight vegetable oil. It may be purely coincidental that I burned SVO and happened to spend a lot of time broken down on the side of the road, and that ultimately every one of those vehicles died of catastrophic engine failure. My ultimate solution was to stop using straight vegetable oil and start using on-spec biodiesel.

Fuel quality geeks have long been debating the merits and problems of high glycerin concentrations in fuel. An ASTM (American Society for Testing and Materials) committee determined that an acceptable limit of free glycerin in biodiesel is no more than 0.02% and no more than 0.24% for total glycerin. Free glycerin is a loner molecule that floats around by itself in the fuel. Total glycerin consists of both free and bound glycerin. Glycerin can also be bonded to between one and three fatty acid chains. These bound glycerin molecules are called mono-glycerides, di-glycerides or tri-glycerides and we call them mono's, di's and tri's for short.

Free glycerin is only a real problem if it exists in high enough concentrations to settle out of the biodiesel. Dr. Jon Van Gerpen of the University of Idaho found the solubility of free glycerin in biodiesel to be between 0.07% and 0.14%. Remember that ASTM has set a limit of 0.02% free glycerin because at this level it is soluble in the fuel and will not settle out over time. Van Gerpen's research suggests that, at least theoretically, biodiesel with slightly more than 0.02% free glycerin should not be a problem if it never settles out of the fuel phase. The range in solubility is due to tiny differences in residual methanol and dissolved water present in different biodiesel samples.

Burning free glycerin quickly leads to engine problems. However, achieving the 0.02% free glycerin spec is not difficult for the homebrewer so there's no reason to make and risk burning fuel with higher levels of free glycerin.

Recall that bound glycerin exists as a band of mono's, di's and tri's. If this band had a leader it would certainly be the mono's! On the good side, mono-glycerides have been identified as powerful contributors to lubricity. On the not-so-good side, monoglycerides of saturated fatty acids are not very soluble in biodiesel, have high melting points, tend not to combust and can polymerize lube oil. In the presence of even very small amounts of moisture, mono's tend to form crystals that can plug filters and lead to bunch of other nasty problems.

Because bound glycerin does not combust well, burning it can cause injector coking, engine deposits and increased emissions. Dr. Van Gerpen writes "increased viscosity adversely affects fuel injection duration, pressure, and atomization. The increased injection line pressure can lead to advanced injection timing, increasing combustion pressures and temperatures, and increased NOx formation in the exhaust."

Chances are that at first you won't notice any problems burning biodiesel with slightly more than 0.24% total glycerin. However, it won't take long for those mono's to hydrate and precipitate out of the fuel. Anyone who has noticed a slimy substance at the bottom of a storage tank is likely witnessing a problem-causing soup of hydrated mono-glycerides of saturated fatty acids. These mono's precipitate out of the fuel rapidly as temperature drops. ASTM has adapted a new test called the "Cold Soak" to predict the likelihood of filter clogging due, primarily, to these pesky mono's.

The mono slime does not combust and will leave deposits on injector nozzles. It also finds its way into crankcase oil, polymerizing it while it breaks down. Polymerized lube oil eventually clogs up channels in the engine that can lead to major damage like a piston poking through the engine side!

The National Renewable Energy Laboratory (NREL) sampled 56 biodiesel tanks out in the field and found the national average total glycerin to be 0.15%, well within the permissible level. Dr. Randall Von Wedel, a fuel quality expert who helped draft the original ASTM standards, recommends using biodiesel with a maximum 0.15% total glycerin. Dr. Van Gerpen recommends that commercial producers and homebrewers alike make 0.10% - 0.15% their target! These levels are completely within the reach of homebrewers if glycerin is decanted mid-reaction and enough catalyst is used.

If you like adventure, enjoy the challenge of tinkering with your car and can afford to be stranded on the side of the road every now and again, then burning fuel with higher levels of glycerin may be for you. Nowadays I take great care to ensure that my fuel contains less than 0.02% free glycerin and even less than 0.24% total glycerin. I love my adventure-free driving experience. S