Low-Temperature Properties

Blending biodiesel with petroleum diesel moderates low-temperature operability problems of B100 by dilution, although the effects are not necessarily linear. Conventional low-temperature operability additives can be used with blends as these are believed to be effective in the petroleum portion of the blend. When biodiesel is blended with diesel fuel, the key variables are the cold flow properties of the diesel fuel, the properties of the biodiesel, the blend level, and the effectiveness of cold flow additives.

There are some critical metrics for low-temperature operability. Note that cold filter plugging point (CFPP) and low-temperature flow test (LTFT) are particularly useful for revealing the presence of additives. For blends, these include:

• Cloud point. The temperature at which small solid crystals are first visually observed as the fuel is cooled (ASTM D2500, D5771, D5772, or D5773).Below the cloud point of the blend, these crystals might plug filters and/or drop to the bottom of a vehicle or storage tank. Cloud point is the most widely used and most conservative estimate of the low-temperature operability limit. However, fuels can usually be pumped at temperatures below the cloud point.
• Pour point. The temperature at which the fuel contains so many agglomerated crystals it is essentially a gel and will no longer flow (ASTM D97, D5949, orD5950). Distributors and blenders use pour point as an indicator of whether the fuel can be pumped,even if it would not be suitable for use without heating or taking other steps.
• Cold filter plugging point. This is the temperature under a standard set of test conditions, as defined in ASTM D6371, where a fuel filter plugs. The CFPP test employs rapid cooling conditions. CFPP results more than 10°C (18°F) below the cloud point should be viewed cautiously as they may not reflect the true low-temperature operability limit. The test simulates the performance of an average or typical vehicle and is not protective of the most severe fuel system designs, which make up roughly one-third of heavy-duty vehicles or one-fifth of light-duty vehicles.
• Low-temperature flow test. This test also reports a temperature under a standard set of conditions,defined in ASTM D4539, where a fuel filter plugs. LTFT employs slow cooling at 1°C per hour and simulates the most severe (and common) fuel system designs in North American heavy-duty trucks from the standpoint of low-temperature operability.

It is strongly recommended that you consult Appendix X.5 to ASTM D975 or Appendix X.3 to ASTM D7467to understand the history and relative utility of tests forcloud point, CFPP, and LTFT.

B100 cold flow properties depend on FAME composition, which affects the cold flow properties of blends (Figure 6 and Figure 7). Measurements of cloud point and pour point are not exact, but have ±2°C (±3.5°F) repeatability. The same is true of diesel fuel. In addition, different No. 2 diesel fuels may have cloud points of -35°C to -5°C (-31°F to 23°F). Some fuels can have cloud points higher or lower than these figures. No. 1 diesel, jet A, or kerosene may have cloud points of -40°C to -51°C (-40°F to -60°F). A recent Coordinating Research Council study showed that biodiesel blends(B5 and B20), made from B100 meeting D6751-08a,would provide operability down to cloud point. Additives may allow operation at even lower temperatures.

From this same study, it was found that for biodiesel blends prepared from B100 meeting D6751-08a, the cloud point and LTFT will be nearly the same, and CFPP will be 2°C to 3°C (3.5°F to 5°F) lower, if no low-temperature flow improver additives are used. Additives do not usually alter cloud point, but can lower CFPP and LTFT. Thus, for additized fuels, CFPP or LTFT may be a better predictor of low-temperature operability.

Blends of No. 1 and No. 2 diesel fuel are frequently used to meet customer cold flow requirements (see Figure 8). Adjusting the blend of kerosene (or No. 1 diesel) in the diesel fuel alone or with additives can modify the cloud and pour point temperatures of B20. An accurate estimate of how B20 will perform in the winter months will require mixing the biodiesel with the winter diesel typically delivered in your area and testing the mixture.

Neither ASTM D975 nor ASTM D7467 has a specific requirement for the maximum or minimum cloud point, but the cloud point should be provided to the customer as part of the certificate of analysis. Appendix D shows a sample certificate of analysis. Cold flow properties needed for the fuel depend on where it is being used (for example, Michigan or Texas) and what time of year the fuel is being used (for example, January or July). A petroleum diesel or biodiesel fuel with a cloud point of -7°C (20°F) may be fine for a Texas summer, but not fora Michigan winter.

The appendices to ASTM Specifications D975 and D7467 contain maps of the 10th percentile minimum temperature for all states for October through March. At the 10th percentile temperature, only 10% of the days were colder during that month on average, based on data from several decades. Some users and distributors use the 10th percentile as the target for their low-temperature operability requirement. Many diesel fuel users will specify a cloud point in their purchase contract, for example, that the fuel cloud point be no higher than the 10th percentile minimum temperature. Some users do not monitor cold flow properties at all and rely on their distributors to make sure low-temperature operability is managed for their location.

The University of Minnesota Center for Diesel Research tested soy B20 made with various diesel fuels available in the region.20 Their database of biodiesel blends (0%, 2%, 5%, 10%, 20%, and 100%) shows how different diesel fuels and soy biodiesel blends alter cold flow properties (cloud point, pour point, and CFPP). CFPP is another measure of low-temperature operability that tends to predict an operability limit lower than cloud and may not protect in the most severe duty-operation vehicles. Some of the data are shown in Figure 9.

No. 1 diesel fuel typically costs more than No. 2, so blenders may prefer to use additives. Many cold flow additives are available for diesel fuel. Most reduce the size of crystals or inhibit crystal formation in some way. Most have limited effectiveness on B100, but work with varying degrees of effectiveness with B20.