Low-Temperature Properties

The low-temperature properties of biodiesel and conventional petroleum diesel are extremely important. Unlike gasoline, petroleum diesel and biodiesel can freeze or gel as the temperature drops. Different diesel fuel formulations are sold during the winter in many climates. If the fuel begins to gel, it can clog filters on dispensing equipment and may eventually become too thick to pump. B100 is commonly stored in heated aboveground tanks for blending in winter. Important low-temperature performance metrics for handling and blending of B100 are:

• Cloud point. The temperature at which smallsolid crystals are first visually observed as the fuelis systematically cooled. The crystals formed inB100 are not like the crystals formed in diesel fueland may behave differently in the fuel. Below thefuel’s cloud point, these crystals might plug filtersor could drop to the bottom of a storage tank.However, fuels can usually be pumped at temperatures below the cloud point.
• Pour point. The temperature at which the fuelcontains so many agglomerated crystals it is essentially a gel and will no longer flow. Distributors andblenders use pour point as an indicator of whetherthe fuel can be pumped, even if it would not be suitable for use without heating or taking other steps.

These guidelines should be followed for storing biodiesel (B100) in winter:

• B100 should be stored at temperatures at least2.5°C to 5°C (5°F to 10°F) higher than the cloudpoint. Because the cloud point of the B100 varies,the storage temperature will not be the same forall biodiesels.
• B100 from all but the highest cloud point feedstocks such as tallow or palm oil can be storedunderground in most cold climates without additional considerations because underground storagetemperatures are normally above 7°C (45°F).Aboveground storage and handling systems shouldbe protected with insulation, agitation, heatingsystems, or other measures if temperatures regularly fall below the cloud point. This precautionincludes piping, tanks, and pumping equipment.

The cloud point of B100 starts around -1°C to 0°C (30°F to 32°F) and can go as high as 20°C (68°F) or higher for biodiesel from highly saturated feedstocks (see Table 4 on next page). The pour point of B100 is usually only a few degrees lower than the cloud point, so once biodiesel begins to freeze, gelling can occur rapidly if the temperature drops only a few degrees further.

B100 tanks and fuel lines should be designed for the cold flow properties of the biodiesel being used and the local climate. Fuel pumps, lines, and dispensers must be protected from cold and wind chill with properly approved heating and insulating equipment. Fuel in aboveground tanks should be heated to 2.5°C to 5°C (5°F to 10°F) above the fuel cloud point.

In some rare cases, as B100 gets colder, impurities like SMGs may precipitate out of solution. SMGs exhibit an interesting behavior known as polymorphism,15 where the crystal form changes after precipitation. When SMGs first precipitate out of solution, they are in a less stable and more soluble form. Over time, the SMG crystals can transform into a more stable and less soluble form. These highly stable SMG crystals are difficult to bring back into solution, and the B100 must be heated well with adequate mixing.

Most of the time, once crystals form, they will generally go back into solution as the fuel warms. However, that process can be slow if the fuel is heated only slightly above the cloud point. Crystals formed in biodiesel or diesel fuel can drift to the bottom of the tank and begin to build up. Slow agitation can prevent crystals from building up on the tank bottom or, once present in the fuel, can help to bring the crystals back into liquid form. If B100 has gelled completely, the B100 should be heated to 38°C to 43°C (100°F to 110°F) to dissolve the most highly saturated biodiesel components if the fuel needs to be used immediately. Lower temperatures can be used if there is more time to allow the biodiesel to liquefy.

The low-temperature performance of B100 cannot be effectively managed with current cold flow additives, as can petroleum diesel and biodiesel blends. The level of saturated compounds in B100 is too high for most additives to be effective. Cold flow additives have been used much more successfully with biodiesel blends. You should work directly with the additive manufacturers on this issue.