In the northern parts of the United States and other cold regions of the world, one of the major concerns among biodiesel users is its unfavorable cold flow properties. In cold climates, it can be a challenge to fuel vehicles with high blends of biodiesel because biodiesel tends to gel (freeze) at higher temperatures than does conventional diesel. The actual temperature at which biodiesel freezes depends on the type of oil or fat from which it is made.
A diesel fuel’s cold-weather characteristics are measured by the cloud point (CP), the cold filter plugging point (CFPP), and the pour point (PP). The cloud point is the temperature of the fuel at which small, solid crystals can be observed as the fuel cools. The cold filter plugging point is the temperature at which a fuel filter plugs due to fuel components that have crystallized or gelled. The pour point refers to the lowest temperature at which there is movement of the fuel when the container is tipped.
Compared to petro-diesel, biodiesel tends to have a much narrower range of temperatures between the cloud point and the pour point. Those who are used to dealing with petro-diesel may be surprised at this narrow range. While there may be a 20-degree difference between the cloud point and the pour point of petroleum diesel, biodiesel may have a difference of only a few degrees.
Testing for Cloud Point and Pour Point
The tests for cloud point and pour point are relatively quick and easy, so they are used to estimate the cold filter plugging point, which is the actual temperature above which a fuel can be used. It is possible to test a fuel for its cold filter plugging point, but the test is expensive and tedious. Therefore, most people use the cloud point and pour point to bracket the temperature at which the fuel will start to fail. A fuel may still work in an engine even if the temperature is below the cloud point. However, the fuel will definitely not work below the pour point (after it has gelled).
It would be expected that the cloud point would always be higher than the pour point. However, because the cloud point and pour point of biodiesel can be very close, and because of the way the cloud point and pour point are reported, it is possible that the pour point could be higher than the cloud point. The cloud point is reported at intervals of 1°C, and the pour point is reported at intervals of 3°C. As the fuel is cooled, it is checked every three degrees to see if the surface of the fuel moves when the container is tipped. When no movement is detected, then three degrees are added to the temperature to get the lowest temperature at which the fuel will pour. Therefore, for a fuel that starts to cloud at -1°C and that gels at -2°C, the cloud point would be reported as -1°C, and the pour point at 0°C.
The cloud point of soybean biodiesel is about 34°F (1°C), whereas the cloud point for No. 1 diesel is about – 40°F (-40°C) and for No. 2 diesel between -18°F (-28°C) and +20°F (-7°C). Usually, when biodiesel nears the cloud point temperature, changes must be made to the fuel, such as the addition of anti-gel additives or No. 1 diesel fuel, to prevent filters from clogging. However, it should be kept in mind that fuel additives recommended for diesel may not be effective for biodiesel.
Low blends of biodiesel tend to perform the same as diesel fuel in cold weather. Studies funded by the National Biodiesel Board indicate that blends of B2 or B5 have minimal or no effect on cold-flow properties of diesel blends.
For higher blends, the cloud point of the blend can be estimated by multiplying the percentage of each fuel with that fuel’s cloud point and then adding the answers. For example, soy biodiesel has a cloud point of 1°C, and No. 1 diesel has a cloud point of -40°C. A B20 blend of soy biodiesel and No. 1 diesel would therefore have a cloud point of -31.8°C (20% x 1°C = .20°C; 80% x -40°C = -32°C; -32°C + .20°C = -31.8°C).
Cloud Point and Pour Point of Different Types of Biodiesel
Although biodiesel can be made from any oil feedstock that allows ASTM D6751 standards in the United States or EN14214 standards in Europe to be met, all properties of biodiesel are not the same. Biodiesel made from various crop oils have unique cold-weather characteristics that can vary up or down by as much as 15 degrees Celsius (see table below).
ASTM D6751 does not specify the required cloud point for biodiesel, but requires that the cloud point be reported to the customer. If a customer is not careful to select an appropriate biodiesel feedstock, the fuel may gel unexpectedly in cold weather. While about 80% of biodiesel in the United States is soy biodiesel, which has a cloud point of about 1°C, some U.S. biodiesel is made from animal fat (tallow) or waste oils and could have a significantly higher cloud point.
Biodiesel made from different feedstocks may have different cloud and pour points:
Sources: Moser (2008), Vyas et al. (2009), Mittelbach and Remschmidt (2005).
Methods to Lower Cloud and Pour Points
Biodiesel performance in cold weather can be enhanced by using fuel-line heaters or in-tank fuel heaters, by insulating fuel filters and fuel lines, and by storing the diesel-powered equipment in heated buildings.
In addition, some additives and processing methods can lower the cloud point and pour point of the fuel itself.
“Winterization” is the process of removing saturated methyl esters by cooling the fuel to cause crystallization and then separating the high melting components by filtration. Lee et al. (1996) found that the CP of a common soybean biodiesel could be reduced to -7.1°C through winterization. However, because a portion of the biodiesel is separated, there is a yield loss of 26%. Davis et al. (2007) used soybean methyl ester fractionation by urea and methanol for producing modified biodiesel with a CP as low as -45°C. The process takes advantage of clathrates which form between urea and long-chain saturated methyl esters. In either case, a significant amount of high CP biodiesel is removed.
Winterization is generally not an efficient way of improving cold flow properties because of the high yield loss. It is also not practical in many cases to store the high CP fraction which was separated for summer use or to transport it to a warmer climate region, so it must be used as a lower-value fuel.
The CP of biodiesel can also be reduced by using a branched-chain alcohol instead of methanol during processing. Isopropyl and 2-butyl esters of normal soybean oil crystallized 7° to 11° and 12° to 14°C lower, respectively, than the corresponding methyl esters (Lee et al., 1995). However, use of isopropyl alcohol is more expensive, and the reaction is harder to complete than with methanol.
A variety of fuel additives for diesel and biodiesel are commercially available to improve the cold flow properties. Dunn et al. (1996) studied the effects of 12 cold flow additives for petroleum diesel on the cold flow behavior of biodiesel. They concluded that the additives significantly improved the PP of diesel/biodiesel blends but did not affect the CP greatly. Many additives contain some proprietary components such as copolymers of ethylene, vinyl acetate, or other olefin-ester copolymers. Because of these proprietary compounds, the impact of cold flow additives on biodiesel from different types of feedstock such as canola, mustard, and used vegetable oil needs to be determined experimentally.
University of Idaho scientists studied the effects of four commercially available cold flow additives on biodiesel made from soy oil, mustard oil, and used vegetable oil. Of these three types of biodiesel, mustard biodiesel responded best to the additives. Still, the researchers found that the additives did not work as well for biodiesel as for petro-diesel. The average reduction in CP and PP for 100% mustard biodiesel was 0.3°C and 7.2°C, respectively. However, the additives reduced the PP of petroleum diesel by at least 16°C, to below -36°C in all cases studied .
Another way to improve the cold temperature performance of biodiesel is to blend it with another biodiesel with a lower cloud point. This has been shown to be an effective technique for reducing the cloud point of palm oil. Moser (2008) was able to obtain CFPP values for palm oil at or less than 0°C through blending with other methyl esters.