The rapid growth in biodiesel use has created an unexpected shortage in certain feedstocks used to manufacture the fuel. In the United States the shortage of soybean oil for biodiesel production has resulted in steep price increases for this critical raw material. As a result, U.S. biodiesel producers have sought alternative feedstock sources for production.

As a feedstock, soybean oil is attractive due to its abundance and reasonably attractive cold flow properties. Some producers are now considering blending their principal soybean feedstock with available alternatives. Other vegetable oils, such as corn, cottonseed, canola (rapeseed), flax, sunflower and peanut can be used but these seed oils are generally more expensive than soybean oil. Attractive lower-cost alternatives include poultry fat, yellow grease (used cooking oil), choice white grease (lard) and tallow.

In part one of this report ("Improving the Quality of Biodiesel Through the Use of Antioxidants and Metal Chelators" in the February 2008 issue of Biodiesel Magazine), stability issues associated with the use of different feedstocks, biodiesel production variability, metal contamination, storage life and transport were addressed by utilizing specific combinations of additives. In this article the use of these same additive systems to improve the stability of biodiesel blends is examined.


Figure 1. Oxidative stability index data for antioxidants at a 300 parts-per-million treatment rate.
SOURCE: ALBERMARLE CORP.

Biodiesel Selection
Before looking at how blends respond to stabilizers to improve stability, it is important to understand the response seen with single biodiesel systems. Figure 1 shows the oxidative stability index response to various additives in biodiesel produced from feedstocks of undistilled soybean, distilled canola, yellow grease and poultry fat. The additives used in this study are butylated hydroxyl toluene (BHT), TBHQ/citric acid blend in solvent and Ethanol 4760E, which is a blend of synergistic antioxidants plus metal chelator.

The results imply that different feedstocks show drastically different responses to additives and certain additives respond much more favorably. Ethanox 4760E shows a very powerful response in yellow grease and poultry fat. The specially formulated combination of antioxidants plus metal chelator in Ethanox 4760E leads to the robust performance seen in all the biodiesel tested versus the other stabilizers.

Based on the above data and feedstock trends in North America, a stability study was performed utilizing blends of biodiesel produced from soybean oil, poultry fat and yellow grease feedstocks. Soybean oil-based biodiesel was selected as the base because soybean oil is the principal biodiesel feedstock in the United States. Poultry fat and yellow grease biodiesel were chosen due to the abundance and low cost of these underutilized feedstocks. Some estimates state that 2.3 billion pounds per year of poultry fat may account for 300 MMgy of biodiesel, while 2.75 billion pounds per year of yellow grease may account for similar quantities of biodiesel. These estimates suggest that future biodiesel production may see much greater use of these alternative feedstocks.


Table 1. Feedstock blends used in Rancimat testing (figures listed as percentages)
SOURCE: ALBEMARLE CORP.


Figure 2. Oxidative stability index results from blend study
SOURCE: ALBEMARLE CORP.


Figure 3. Oxidative stability index improvement as a cost/performance estimate
SOURCE: ALBEMARLE CORP.


Figure 4. Contour plot showing Ethanox 4760E performance
SOURCE: ALBEMARLE CORP.

Blend Study
Biodiesel blends were prepared as shown in Table 1. Each of the blends was treated with 300 parts per million of various antioxidants (BHT, TBHQ/citric acid blend in solvent, and Ethanox 4760E). The blends were tested in the Rancimat to determine the oxidative stability index value. The blends without antioxidants were also tested as a reference. The oxidative stability index results are shown in Figure 2. The data shows that Ethanox 4760E performs superior to all the other antioxidants tested, whether evaluated in a pure biodiesel or biodiesel comprised of varying feedstocks. A significant performance gain from Ethanox 4760E relative to pure soybean biodiesel is seen whenever poultry fat or yellow grease biodiesel is used in the blend (compare B4, B5 and B7 to B2).

Cost-performance economics are critical when trying to select a stabilization system for biodiesel. This type of analysis generally requires additive performance information as well as additive pricing. It's well known that additive pricing can be affected by many factors including production costs, purchased volumes and shipping costs. Figure 3 describes the improvement in oxidative stability index achieved when using 300 ppm of each of the listed stabilizers. As an estimate, a greater improvement in oxidative stability index corresponds to a more cost effective stabilization system (of course this assumes equal additive cost, which is typically not the case). Note in this type of analysis Ethanox 4760E is typically the most effective option whether a single biodiesel is used or whether blends of biodiesel are used.

Estimating Oxidative Stability Index Results
The data for each antioxidant in Figure 2 can be illustrated in the form of a contour plot that visualizes the predicted oxidative stability index results for biodiesel blends not actually included in the study. Figure 3 shows such a contour plot for Ethanox 4760E oxidative stability index data. The labels B1 through B7 indicate the locations on the contour plot where the actual tested blends are located. By moving on the surface of the contour plot, predictions of oxidative stability index can be made on blends not actually tested. The contour plot clearly shows the trend for the various biodiesel and biodiesel blends where stability follows: poultry is greater than yellow grease is greater than soybean oil.

The formula embedded in this plot is a useful tool for determining oxidative stability index when blending biodiesel from different sources. From these three particular biodiesel sources we have the following quadratic equation to estimate oxidative stability index data:

Oxidative stability index (hours) = 0.29A + 0.07B + 0.13C - 0.002AB - 0.00053AC - 0.00065C

"A" is the percentage of poultry fat-based biodiesel. "B" is the percentage of soybean oil-based biodiesel, and "C" is the percentage of yellow grease-based biodiesel.

For example, if a biodiesel producer typically uses 80 percent soybean oil and 20 percent poultry fat, the equation would predict an oxidative stability index of 8.2 hours when using 300 ppm of Ethanox 4760E. If this particular producer decided to reduce costs by using 60 percent soybean oil, 20 percent yellow grease and 20 percent poultry fat feedstock, the equation would predict an oxidative stability index of 9.2 hours when using 300 ppm of Ethanox 4760E. Thus for the specific biodiesel samples used in this study, the change in feedstock composition would predict an improvement in oxidative stability index.

Summary
As the demand for biodiesel increases, alternative feedstocks other than soybean will need to be utilized. Poultry fat and yellow grease represent attractive options. The above study shows that a properly formulated stabilization system such as Ethanox 4760E can meet the challenges of stabilizing biodiesel produced from multiple feedstock sources as well as feedstock blends.

Emily Schneller and Vincent Gatto are with Baton Rouge, La.-based Albemarle Corp. Reach Schneller at emily_schneller@albemarle.com and Gatto at vince_gatto@albemarle.com.