There's a lot of information on the Internet about, well, anything you want to know about. It can feel like information overload at times. So we wanted to go back to basics and give a little primer about the issue of biodiesel storage life. When we consider the question of the shelf life for biodiesel, there's no stock answer that applies to every situation all of the time. As much as we'd like there to be, instead the best answer to the question is a variable one that could range from days to years, depending on the storage conditions.
All fuels degrade over time. The old school diesel fuels from the fifties could be kept relatively fresh for 1-2 years. The advent of ultra low sulfur diesel cut this storage to 3-6 months. When you blend biodiesel into the mix, the figure changes again. So there's no one stock answer to the question. A fuel's storage life is going to depend on a host of factors related to storage conditions.
The influencers of storage life for both conventional diesel and biodiesel would be contamination with microbes and/or chemicals, light, storage temperature, oxygen exposure, and the type of biodiesel feedstock that the fuel was made from.
Microbial contamination is a big key for biodiesel storage life, because microbes love to feed on biodiesel fuel. Any time you get water in a fuel storage tank, the possibility of microbial contamination inevitably follows. And this can reduce the usable storage life of a fuel from months to (literally) hours. When this happens, you can kill the microbes within the fuel, but the fuel itself may already be degraded to the extent that its inability is compromised. And no biocide or treatment would reverse that damage.
Oxidative stability and damage is the other big influencer to the storage life of biodiesel fuel. Biodiesel, because it's made from a vegetable or animal fat, is more susceptible to attack and breakdown from oxygen exposure. Even though they are chemically processed to burn like diesel fuel in an engine, biodiesel molecules aren't exactly the same in structure as petroleum molecules. They have more areas on their molecular structure that are weak points for oxygen and free radicals to attack, reacting and breaking the molecules apart. Acids then form, the fuel becomes thicker and you get corrosion to storage tanks and components, not to mention the formation of sediments.
We mentioned feedstock earlier as an influencing factor. Some feedstocks (like palm and coconut oil and tallow) produce fuel feedstocks that are more stable than others (like soybean or canola). What you can find, interestingly, is that the feedstocks which are most stable tend to have the worst cold weather handling properties, and vice-versa. So biodiesel users in Florida or Hawaii would be best off using fuel made from tallow or palm oil, since those geographic area stay warmer. But biodiesel users in Minnesota might elect to go the opposite way and use soy or canola fuels that perform better in cold weather. They just have to deal with more instability issues as a tradeoff.
Does anything else contribute to stability or lack thereof? Besides exposure to oxygen, light can accelerate decomposition reactions by contributing energy to drive the reactions along. It's the same reason fuel goes bad quicker in hot environments - the air temperature provides the necessary energy for the chemical reactions in the fuel. And we mentioned chemical contamination earlier, by which we really mean exposure to certain types of metals like copper and brass. These metals act as catalysts to increase chemical breakdown reactions in the fuel.
If you're storing biodiesel fuel, whether B100 or a lower blend, it's recommended to treat the biofuel with both a biocide and a stability agent. The biocide will prevent the establishment of microbial contamination while the stability agent blocks the initial chemical reactions that lead to the chain reactions that result in stored fuel losing its quality.
This post was published on June 3, 2014 and was updated on October 19, 2020.