Biodiesel Fuel Management in Cold Weather

Javed Iqbal  |  2/10/2010 11:51:53 PM

Figure 1

Cold-weather performance is one of the biggest challenges for the success of biodiesel as fuels in the automobile industry. At low operating temperature the fuels may thicken and might not flow properly, affecting the performance of fuel lines, fuel pumps and injectors. The low-temperature performance is commonly defined by cloud point, pour point and cold filter plug point. The cloud point is especially important because it limits the cold-flow properties of the resulting biodiesel blend. The cloud point is the temperature at which a cloud of wax crystals first appears in biodiesel. The intent of the cloud point measurement is to obtain the temperature at which the liquid fuel begins to change from a single-phase liquid to a two-phase system. The chemical composition of some biodiesel feedstock leads to biodiesel (B100) that may have higher cloud point. The cloud point can be predicted with knowledge of the esters (biodiesel) composition; however, it is hard to determine the esters composition of used vegetable oil used as the feedstock unless it is analyzed. The saturated methyl esters components of biodiesel (e.g. methyl palmitate, C16-0, and methyl stearate, C18-0) are the first to precipitate. The amounts of these esters are hence the determining factors for cloud point. B100 generally has a higher cloud point than petroleum based diesel fuel. This test can be in an automated instrument. The sample is cooled in a 1.5 +/- 0.1 oC/min device over the range from +70 to –40 oC while being continuously illuminated by a light source.

Parameter

Cloud point

ASTM D2500

Pour point

ASTM D97

Cold Filter plug point

Fuel’s Feedstock

oF

oC

oF

oC

oF

oC

Soy methyl esters

38

3

25

-4

28

-2

Canola methyl esters

26

-3

25

-4

24

-4

Lard methyl esters

56

13

55

13

52

11

Edible tallow methyl esters

66

19

60

16

58

14

Inedible tallow methyl esters

61

16

59

15

50

10

Yellow grease methyl esters

46

8

43

6

34

1


    Table 1
: Cold Flow Data for various B100 fuels
The cloud point can be modified in two ways -- through the use of additives retarding the formation of solid crystals in B100 by various mechanisms and by blending feedstock that are relatively high in saturated fatty acids with feedstock that have lower saturated fatty acid content. The result is a net lower cloud point for the mixture. The cloud point of biodiesel produced from waste cooking oil at W.A. Callegari Environmental Center is found approximately –2 oC (28.4 oF), where the principal feedstock was canola oil. Fatty acid chains, particularly the saturated fatty acid chains, therefore, play an important role in determining the cold-flow properties of biodiesel fuel. A relationship exist between the cold-flow properties of biodiesel and saturated fatty acid methyl esters. As the content of the saturated fatty acid methyl esters increases, the cloud point of biodiesel occurs at higher temperature. The cloud point of animal fats occurs approximately between 14oC to –1oC. If the biodiesel is derived from rapeseed oil, the cloud point is approximately between – 2 oC (28.4 oF). Waste cooking oil, which has less saturated fatty acids, performs better than animal fats or other oils with higher amounts of saturated fatty acid chains.

Commonly used biodiesel blends are B2, B5, B10 and B20 (numbers indicate the percentage of biodiesel in the blend), where B2 or B5 have minimal or no effect on cold-flow properties of the finished blend. B20, however, freezes about 3 to 5 degrees Fahrenheit faster than No. 2 petroleum diesel, depending on the cold-flow properties of the biodiesel and the cold-flow properties of the petroleum diesel.

As indicated in Figure 1, the cloud point of B100 starts at -2.8 oC (27 °F) for waste cooking oil, mostly canola oil that is made up primarily of mono- or poly-unsaturated fatty acid chains. Most of the animal fats or frying oils that are highly saturated have cloud points as high as 66 °F or higher.

The graph below demonstrates that biodiesel, B100, produced at W.A. Callegari Center can safely be used during cold weather at around -2 oC (28.4 oF) or higher without using additives. However, because of strong solvent potential and to maintain the integrity of the fuel lines, B50 or lower is recommended which reduces could point further down to -7oC (19.4 oF), a temperature rarely observed in the South.

Figure 1: Cloud point of various biodiesel and petrodiesel blends using ASTM2500 method
(SEE FIGURE 1 IMAGE ABOVE)

In order to ensure engine safety, B100 tanks and fuel lines should be designed for the cold-flow properties of the biodiesel being used and the climate they will see. Fuel pumps, lines and dispensers need to be protected from cold and wind chill with properly approved heating and/or insulating equipment. Fuel in above-ground tanks should be heated in a range that fluctuates between 5°F to 10°F above the fuel cloud point. Once crystals begin to form, they should go back into solution as the fuel warms up. However, that process could be slow if the fuel warms only marginally or very slowly. Crystals formed in biodiesel or diesel fuel can drift to the bottom of the tank and begin to build up a gel layer. Slow agitation can prevent crystals from building up on the tank bottom, or once they are present in the fuel, agitation can help to dissolve crystals back into solution. If B100 has gelled completely, it may be wise to bring the B100 temperature up to 100°F to 110°F to melt the most highly saturated biodiesel components if the fuel needs to be used right away. Lower temperatures can be used if enough time is allowed for the mixture to come to its equilibrium cloud point. Further work is being done in this arena. Some additive manufacturers have data that show their cold-flow additives can reduce the pour point of a B100 by as much as 12°C (30°F), but the treatment rate is in excess of 10,000 ppm. At more typical treatment rates (1,000 ppm), benefits were about 3°C. B100 found in the United States cannot be managed effectively with current cold-flow additives like some petrodiesel or European rapeseed oil-based biodiesel, especially in the northern regions. The U.S. oils and fats contain too high a level of saturated compounds for most additives to be effective. Cold-flow additive effectiveness also can change dramatically depending on the exact type of biodiesel or the chemical process it has undergone (Table 2); much like the situation found with diesel fuel. Cold-flow additives have been used much more successfully with biodiesel blends. Contact the major additive manufacturers for more information.

Mixing No.1 diesel fuel with biodiesel can help reduce most fuel-gelling problems. Other measures may include the addition of fuel-line heaters or in-tank fuel heaters, along with the use of anti-gel additives. Insulating the fuel filters and fuel lines from the cold also will help. These measures should eliminate most cold-weather operational problems associated with biodiesel, especially in the South, assuming the biodiesel meet the American Society of Testing and Materials (ASTM) specifications for biodiesel designated as ASTM D 6751. This specification covers pure biodiesel (B100) for blending with petroleum diesel at levels up to 20 percent by volume. The ASTM specification for petroleum diesel is ASTM D 975. Biodiesel that meets the American Society of Testing and Materials specifications is a safe and reliable fuel that can be used in most diesel engines. However, it is important to check with engine manufacturers about any impact of biodiesel use on engine warranties.

Best management practices in cold weather include:
  • Keep your tank close to full; a large amount of fuel will gel more slowly than a small amount.
  • Blend biodiesel with at least 50% diesel when using it in cold climates.
  • Use a cold-weather additive with biodiesel in the winter, e.g. pour-point depressants, flow improvers.
  • If your fuel lines do plug, try pouring hot water on them. Do not continue to crank your engine if your fuel system is plugged. This can damage the fuel pump.
  • Proper fuel management and a clear understanding of fuel’s cold flow properties.
  • Use quality fuel meeting ASTM D 6751 specifications.
  • Blending biodiesel with kerosene (#1-D), which has excellent cold-flow properties.

Testing your biodiesel quality:

The LSU AgCenter's W.A. Callegari Environmental Center offers affordable ASTM analysis for biodiesel fuels. For further information, call the lab at 225-765-5155 or send us an e-mail.

Oil

Alkyl group

CP (°F)

PP (°F)

Canola

Methyl

33.8

15.8

Canola

Ethyl

30.2

21.2

Soybean

Methyl

32

28.4

Soybean

Ethyl

33.8

24.8

Safflower

Methyl

--

21.2

Safflower

Ethyl

21.2

21.2

Sunflower

Methyl

35.6

26.6

Sunflower

Ethyl

30.2

23

Rapeseed

Methyl

28.4

15.8

Rapeseed

Ethyl

28.4

5

Mustard Seed

Ethyl

33.8

5

No.1 Diesel

-35

-45

No.2 Diesel*

Variable

Variable

CP - Cloud Point; PP - Pour Point
*The cloud and pour point of the fuel varies based on the ambient (outside) temperature of where the fuel is used. This is determined and specified by the fuel supplier.


    Table 2.
Cold-flow properties of (B100) Biodiesel (Methyl and Ethyl Esters) (The Biodiesel Handbook)
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