Low Temperature Operability of Biodiesel

Hannu Jääskeläinen

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• Introduction
• Factors Affecting Low Temperature Properties of B100
• Low Temperature Properties of Blends
• Precipitate Formation Above Cloud Point Temperature
• Cold Soak Filtration Test

Introduction

Low temperature operability is a major concern for all diesel-like fuels operating in low temperature environments. With petroleum diesel fuels, a range of options exists to adjust the low temperature operability limits of the fuel including varying fuel composition and use of low temperature operability additives. The wide possible range of hydrocarbons that can be used as diesel fuel allows considerable control of low temperature properties just by controlling the type and amount of some high boiling point components—by controlling the high end distillation temperature of the finished fuel for example. The range of compounds that compose biodiesel, however, are very limited and varying fuel composition provides a much narrower range of adjustment. To add to the challenge, most biodiesel contains a significant proportion of saturated long chain fatty acids that are very similar to the long chain paraffins whose concentration is undesirable in diesel fuels when low temperature operability range is to be extended. Thus, achieving acceptable low temperature operability limits with biodiesel blends can be a significant challenge.

There are several major sources of potential low temperature operability problems with biodiesel:

• Formation of waxes in the fuel—Long chain methyl esters, especially saturated ones, can solidify as the fuel temperatures drops. These can plug filters and cause engines to not start or stall shortly after start-up.
• Formation of precipitates from fuel contaminants—Some fuel contaminants such as saturated monoglycerides, sterol glucosides and soaps can lead to the formation of filter plugging precipitates at temperatures above the fuel’s cloud point.
• Fuel system icing—At sub-zero temperatures, water from fuel contamination can freeze, causing the precipitation of ice particles (water contamination is discussed in Biodiesel Standards & Properties).

A number of tests commonly used with petroleum fuels are available to measure low temperature operability including Cloud Point, Low Temperature Flow Test (LTFT), and Cold Filter Plugging Point (CFPP). Testing has shown that, provided the biodiesel fuel is of a high quality and precipitation problems above the cloud point are not encountered, these tests provide a reasonable estimate of low temperature operability in much the same way as they do for diesel fuel. Furthermore, a Cold Soak Filtration Test (CSFT) has been developed in an attempt to provide some measure of control over the formation of precipitates above the cloud point in biodiesel and to allow these conventional measures of low temperature operability to be used with more confidence.

The US biodiesel specification, ASTM D6751, requires that the cloud point be reported and that the fuel meet a maximum CSFT test limit of 360 s or 200 s if the expected fuel temperature is -12°C or less. It is then up to the blender to determine the appropriate actions that need to be taken to ensure trouble free operation when the biodiesel is blended with diesel fuel. Further guidance applicable to blends up to B5 and blends from B6-B20 can be found in ASTM D975 and ASTM D7467 respectively. Blending low levels of biodiesel into diesel fuel in cold weather was also studied by the Biodiesel Cold Flow Blending Consortium [1220] and others [1813][1827].

EN 14214 handles low temperature operability differently from ASTM 6751. While no specific requirements are applied to FAME, EN 14214 outlines CFPP values that automotive diesel fuel must meet for summer and winter grades.

Factors Affecting Low Temperature Properties of B100

Cold flow properties vary depending on the biodiesel feedstock, as illustrated in Figure 1 using a cloud point example. It is apparent that there are significant differences in the cloud point for biodiesel derived from different feedstocks.

The principal reason for the differences is the content of saturated fatty acid alkyl esters in the fuel [1499], which have a relatively high cloud point, Figure 2a. The cloud point of biodiesel is sensitive to the concentration of saturated alkyl esters when they constitute less than 20% of the mass fraction. The sensitivity increases dramatically as their concentration drops. While the cloud point of biodiesel can be reduced to extremely low levels through such processing as cold filtration or the formation of urea clathrates that reduce or even eliminate the saturated alkyl esters in the fuel (e.g., -57°C when saturates are 0%) [1855], fuel viscosity may become excessive at these low temperatures. Also, such processing can significantly reduce the yield of the final product because, in addition to saturates, some unsaturated fuel components are removed as well. In the example shown in Figure 2a where the cloud point was reduced to -57°C when all saturates were removed, only 43% of the original biodiesel was recovered after processing.

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