Sludge formation in stored fuels has always been a problem to reckon with. But fuel changes in the last 5-10 years have made it even more of an issue. Besides the transition to ultra low sulfur diesel, guess what else has happened in that time? The widespread introduction of biodiesel fuels. Most diesel fuels can have up to 5% biodiesel content and still be called regular diesel fuel.
First issue to be reckoned with – why be concerned at all? Answer – sludge formation means the biodiesel has become unstable and is breaking apart. It forms heavy polymers that drop out of suspension and settles everywhere the biodiesel fuel sits for periods of time. This typically means in storage tanks. When the engine tries to burn that sludge, it doesn’t combust as well as fresh, healthy fuel. You get injector deposits – the number one cause of drops in performance and mileage.
Sludge prevention in the refinery
Fuel blenders can do certain things to influence fuel properties through the blending of the fuel before it leaves. That helps do things like reduce the chances of sludge formation. Research in Japan in 2008 and 2009 that was presented to the SAE showed there was a relationship in biodiesel blends (i.e. fuel that is a blend of x percent biodiesel with y percent #2 diesel fuel) with respect to the formation of sludge and the “aromatic composition” of the base diesel fuel. If the percent of aromatics in the diesel rises above 30%, it reduces actual sludge formation from the biodiesel itself.
But that fact bumps up against a couple of problems. Most commercially-produced diesel fuel settles around 25% aromatics. The maximum legal limit you can have is 35%. And the higher the aromatic content, the greater chance of harmful carbon emissions, which is something the EPA doesn’t want. So they don't have much margin to play with there.
Consider also that the end user – the one who is going to be storing biodiesel and dealing with the sludge problems – they have no control over the fuel composition. So “just increase the aromatic composition” is irrelevant to them. They probably don’t even know what that means.
So what can the biodiesel user do?
For facilities that blend biodiesel on-site (usually by splash blending the biodiesel with the diesel in the main fuel storage tank), they can choose biodiesel made from oils that are more stable. The funny thing about types of biodiesel is that there’s an inverse relationship between cold flow and stability (the two biggest problems biodiesel users wrestle with). Biodiesel made from soy and chicken fat, have reasonably good cold flow properties, but not so good stability. Inversely, biodiesels made from tropical oils like coconut and jatropha are more stable (form less sludge) but fare less well in the cold. Sunflower and canola biodiesels seem to fall in the middle.
The caveat to these facts is that many times you are hostage to what the market gives you. And that’s understandable. Biodiesel is extremely price sensitive, and a large part of the price comes from the cost of getting the feedstock oil to make the biodiesel. If the producer can get chicken fat for less than they can get sunflower, they’ll do that.
Since sludge formation comes from chemical chain reactions in the biodiesel, the end user can use a biodiesel stabilizer that stops these reactions in both the biodiesel portion and the diesel base portion. If you can stop the initial reactions from happening, it stops the chain reactions from getting started. And it keeps the biodiesel healthy and ready to use in storage.
The most important consideration in applying this is to treat the fuel in the beginning. Just as you have to put an anti-gel in the fuel before it gets cold (before it gels), you have to stop the chemical reactions before they start by treating the fuel when it is fresh.
Check out these related posts on Biodiesel:
This post was published on May 20, 2015 and was updated on May 20, 2015.