Stability
Long-term storage stability of B5 and B20 blends has recently been examined using the D4625 test that simulates storage at normal ambient temperature, roughly 21°C (70°F). The test is accelerated by a factor of 4, such that one week of test time simulates one month of actual storage. This test was carried out for 39 weeks (simulating 3 years of biodiesel blend storage) with four biodiesels of varying stability blended with two diesel fuels. Results for B5 blends are shown in Figure 11. For B100 with a three-hour or higher induction time, the D4625 test shows no indication that oxidation is occurring in B5 blends under these test conditions. Based on these results, B5 blends made from in-specification B100 can be stored for one year or longer. However, as storage conditions can influence stability, we recommend that you consider adding a synthetic antioxidant and monitoring the condition of the fuel periodically if you store fuel for longer than 6 months.
Figure 12 shows the results for B20 blends. All of the B20 blends, with one exception, remained within specification by the end of 39 weeks and showed no signs of oxidation beyond a decrease in induction period. The results for the lowest stability blend were notably different between the two base diesels. In the case of Diesel A, this blend went out of specification for induction period by 23 weeks and remained at a constant value for the remaining storage time. This blend showed no signs of oxidation by the end of test. The blend prepared with Diesel B went out of specification by 17 weeks and showed a steady drop in induction period from 31 to 39 weeks, eventually reaching zero. At the end of the test, the peroxide value of this blend had increased significantly and the acid number was out of specification. Based on these results, B20 blends made from in-specification B100 can be stored for one year and possibly longer. The base diesel used to prepare a blend also influences the storage stability of the blended fuel. Again, we recommend the use of a synthetic antioxidant and monitoring of the fuel periodically if you store fuel for longer than 6 months.
As biodiesel ages in storage, the induction period decreases, the peroxide value increases, the acid number eventually increases and goes out of specification, gums and varnish can form, and the viscosity can increase. Induction period, acid number, viscosity, and water and sediment tests can be used to ensure your biodiesel blend meets ASTM specifications for either B6 to B20 (D7467) or diesel fuel (D975). The results of the study presented here did not detect any statistically significant insoluble material formation in the B20s during storage. Previous studies have shown that when the acid number of a biodiesel blend increases to above the specification limit, a considerable amount of insoluble material can form. Some data suggest that when oxidized or aged biodiesel is blended with diesel to make B20, some of the sediments and gums soluble in the B100 become insoluble and come out of solution, forming sediments. This information is presented as a warning only. You should never blend out-of-specification B100 into diesel to make B20. Make sure the induction period, water and sediment, acid number, and viscosity values are all in specification before blending. Fuel should be monitored for changes in induction period and acid number during storage. Blends that have gone above the maximum acid number of 0.3 mg KOH/g should not be used.
Thermal stability is generally meant to indicate that the fuel is degrading when it is subjected to high temperatures for a short period, similar to what would be experienced in the fuel injector or fuel system of a modern diesel engine. If the fuel degrades in a hot engine, the primary concern is the potential for fuel pump and injector fouling or corrosion. The data suggest thermal stability should not be a concern with biodiesel.