Stability

Stability can refer to two issues for fuels: long-term storage stability or aging and stability at elevated temperatures and/or pressures as the fuel is recirculated through an engine’s fuel system. For petroleum diesel, long-term storage stability is commonly referred to as oxidative stability. Thermal stability is the common term for the stability of fuels at elevated fuel system temperatures. For B100, storage stability is the paramount concern; thus, D6751 includes an oxidation stability requirement.

The oxidation stability test, EN15751 (also referred to as the Oil Stability Index or the Rancimat test), involves heating a specified quantity of B100 to 110°C (230°F) while air is bubbled through the sample at a controlled flow rate. After bubbling through the B100, the air bubbles through a water bath that collects volatile acids formed by oxidation of the biodiesel. A conductivity meter is used to monitor the water. A stable B100 can go for many hours under these conditions without forming volatile oxidation products. This period of time, before oxidation products form, is called the induction time or induction period. The stability requirement in D6751 is that the B100 have a minimum three-hour induction time. Because this requirement applies at the time of blending, many biodiesel producers make B100 with higher induction times.

In biodiesel, fuel aging and oxidation can lead to high acid numbers, high viscosity, and the formation of gums and sediments that clog filters. If the oxidation stability, acid number, or viscosity measurements exceed the limits in ASTM D6751, the B100 is degraded to the point where it is out of specification and should not be used. Biodiesel with high oxidation stability (longer induction time) will generally take longer than biodiesel with low oxidation stability to reach an out-of-specification condition. Monitoring the induction time and acid number of B100 over time can provide an indication of oxidation. B100 should be tested at receipt to ensure it is within specification. If the biodiesel will be stored prior to blending, the induction time and acid number should be monitored at regular intervals to ensure the biodiesel is not oxidizing.

In some cases, deposits from the cleaning or solvent effect of B100 have been confused with gums and sediments that could form in storage as the B100 ages. Although sediment can clog a filter in either case, care should be taken to make sure the reason for the clogging is properly identified. For example, if oxidation stability and acid number are within specification, sediments are most likely due to the cleaning effect and not to aging or oxidation.

Guidelines to help identify biodiesel and storage conditions that will provide the highest levels of stability follow:

• The higher the level of unsaturation, the more likely that the B100 will oxidize. Saturated fatty acidesters are fairly stable, and each time the level of unsaturation increases (for example, from monounsaturated to polyunsaturated), the stability of the fuel decreases exponentially. The points of unsaturation on the biodiesel molecules can react with oxygen, forming peroxides that break down into acids, sediments, and gums.
• Heat and sunlight will accelerate this process.
• Certain metals such as iron, rust, copper, brass,bronze, lead, tin, and zinc will accelerate the degradation process and form even higher levels of sediment. B100 should not be stored in systems that contain these metals.
• Some types of feed stock processing and biodiesel processing can remove natural antioxidants,potentially lessening inherent stability. Plant oils and fats are produced with natural antioxidants—nature’s way of protecting the oil from degradation.Bleaching, deodorizing, or distilling oils and fats,either before or as part of the biodiesel process, can remove these natural antioxidants.
• Antioxidants, whether natural or incorporated as additives, can significantly increase the storage life or stability of B100.
• Keeping oxygen from the biodiesel reduces or eliminates fuel oxidation and increases storage life. Commercially, this is done by using a nitrogen blanket on storage tanks or storing biodiesel insealed drums or totes with minimal head space.

The ASTM D4625 test is used to simulate storage at ambient temperature, roughly 21°C (70°F). The test is accelerated by a factor of 4 for petroleum fuels, that is, one week of storage at D4625 conditions (43°C or 110°F, open to air) simulates one month of storage at 21°C (70°F). This acceleration factor has not been validated for B100, but it is still a useful guide. ASTM D4625 data (see Figure 4)16 indicate that B100 will lose oxidation stability over time under these storage conditions. Higher initial induction period values can provide longer storage time before biodiesel goes out of specification. Figure 4 also shows that, as the oxidation stability is reduced to near zero, the material will oxidize due to a loss of “oxidation reserve.” This is evident in the increase in peroxide values. Acid numbers remain relatively constant until peroxide values become very high. Once sufficient peroxides have formed, the acid number increases rapidly due to peroxide degradation. Measurement of insoluble material in these B100s was not statistically significant during D4625 storage for 13 weeks (simulating 1 year of storage); however, highly oxidized biodiesels—having acid numbers well above the D6751 limit of 0.5 mg KOH/g—have historically been shown to form insoluble materials.

B100 should not be stored longer than four months unless it has been treated with synthetic antioxidants and has an oxidation stability of 6 hours or longer. Non-oxidizing storage conditions in containers with little head space or under a nitrogen blanket will also be helpful. In fact, when B100 is being stored longer than about two months, it should be tested for oxidation stability every two weeks. One of the best ways to stabilize biodiesel is to blend it with petroleum diesel.