The global biodiesel industry is among the world's fastest -growing markets, with production volumes in some cases tripling over the past year. Spurred by environmental and national economic benefits, biodiesel continues to gain global popularity. World capacity, production and consumption grew 32 percent on average annually from 2000 to 2005. Production volumes were just under 2 million metric tons in 2004. Global demand reached 6.9 million metric tons in 2006 and is projected to reach 44.8 million metric tons by 2010.

With industry growth-fuel quality-already the focus of numerous studies, has also taken on increased importance. To ensure the success and sustainability of the industry, the National Biodiesel Board (NBB) and other industry stakeholders-including original equipment manufacturers, and chemical and oil companies-are trying to manage and ensure that consistent quality fuel makes it onto the market and to the consumer. Fuel quality has been under added scrutiny since Minnesota's 2005 biodiesel quality issues, which resulted in filter clogging incidences. Additional scrutiny resulted from recent findings published by the National Renewable Energy Laboratory (NREL), which found that half of U.S. biodiesel samples pulled from November 2005 to July 2006 didn't ASTM standards.

Although there are a number of factors that impact biodiesel quality, including the inherent stability of the feedstock, two key factors are the influence of metal contamination on oxidative stability and the storage stability of biodiesel, which will be reviewed in more detail.

Feedstock Developments
The primary feedstock for biodiesel in the United States is soybean oil. In Europe, it is rapeseed oil. In Asia, a variety of feedstocks are used depending on the region. As the biodiesel industry continues to grow, manufacturers in each region have started to deal with issues of supply and sustainability. As a result, producers have started to evaluate and qualify a broader range of feedstocks. This is critical as the fatty acid profiles have a significant impact on the fuel's property and stability.


Figure 1

Figure 1 highlights some of the common feedstocks currently used for the production of biodiesel. Fatty acid alkyl esters age more rapidly than traditional petroleum diesel. Sunflower and soybean oil, which have a high level of polyunsaturation, are much more susceptible to oxidation than more saturated oils, such as palm. As the differing proportions of fatty acids affect the biodiesel fuel properties, including cold flow performance and resistance to oxidation, feedstock selection and the use of blends will continue to play an important role in understanding biodiesel stability and in optimizing the fuel's performance.

Furthermore, as the demand for biofuels continues to increase and more farmland is dedicated to biofuel-related crop production, the biofuel industry will start to compete with crop production for animal and human nutrition. Vegetable oil prices have been influenced by the growing demand from biodiesel production. Rapeseed oil has been tracking crude oil prices since 2004. With the rapid growth of biodiesel in the United States, soybean oil prices are also expected to increase. This is another reason producers are looking to multi-feedstock systems and the ability to use an increasing array of oils to offset some of these challenges.

Balancing the need for achieving optimum quality and performance, in combination with availability and affordability, will be a key driver for future feedstock development.

Storage Stability and the Influence of Stabilizers
As indicated earlier, oils and esters containing higher levels of polyunsaturation will have lower overall stability and be at a greater risk of oxidation. The products that arise during oxidation and subsequent degradation can lead to the formation of lower molecular weight acids, peroxides and polymers. Reports from equipment manufacturers, such as Bosch, Cummins and others, have shown that deposit formations and filter clogging are some of the potential results of aged or degraded biodiesel. For engine and fuel injection equipment manufacturers, one of the main concerns is the formation of insoluble gums and sediment deposits in the fuel systems that can influence vehicle operability.

If left unprotected, stored fuel will degrade over time. Along with proper handling, storage and distribution, the use of stabilizers will significantly reduce oxidation and help minimize degradation. Oxidation is further accelerated by the exposure to heat, sunlight, contaminants and a variety of metals that can act as pro-oxidants.


Figure 2

Figure 2 demonstrates that storing biodiesel, even at ambient outdoor temperatures and conditions, can have a significant impact in reducing the oxidative stability of biodiesel. What is even more notable is that, without protection, even high-quality biodiesel that initially meets ASTM D 6751 specifications drops off in stability after only nine weeks in storage. With increasing requirements to store, transport and export biodiesel, storage stability-with respect to oxidation-will be even more significant for fuel suppliers, terminal operators and distributors.


Figure 3

As shown in Figure 3, by adding additives at varying concentrations, the same high-quality biodiesel can be maintained over the same nine-week period. The optimal concentration will ultimately be dependent on the biodiesel, as well as the length and condition of storage. Not all antioxidants and stabilizers on the market will exhibit the same level of effectiveness.

To maximize protection and to provide assurance that the fuel quality is maintained through storage and delivery to the customer, it is critical that additives are usedas early in the process as possible. Once biodiesel starts to degrade, the resulting acids, peroxides or polymers that are formed can't be reversed. Other factors to consider when selecting the optimum stabilizer include product form and multifunctional performance. Liquid stabilizers are usually easier to handle and dose than solids, requiring less effort and energy to process. Some stabilizers are also multifunctional and can protect against other influences impacting stability, such as metals, providing another level of protection.

Impact of Metals on Fuel Stability
In addition to storage stability, the presence of metals in biodiesel can have a significant impact on the quality of biodiesel. Certain metals have been found to cause operability problems due to the formation of deposits and the poisoning of emission control devices. Certain metals can also catalyze reactions that cause undesired products. Recent updates to ASTM D 6751 - and specifications included in both the U.S. and European standards - have limited the concentration of metals, including magnesium and calcium, which are used as adsorbents for purifying biodiesel, and sodium and potassium, which are used as catalysts in the manufacturing process. There are other metals, such as copper, iron and zinc, that are particularly active in catalyzing the oxidation of polyunsaturated methyl esters. These metals, if present even in small quantities, may contribute to biodiesel instability. Figure 4 illustrates that less than one part per million (ppm) consisting of metals such as copper and iron can drastically decrease the stability of biodiesel when evaluated by the Rancimat method. (EN 14112)


Figure 4

Although it is recommended that copper, brass and zinc metals are avoided in tanks and fittings, it isn't always possible to eliminate all potential exposure from production to distribution and use. For instance, copper may be introduced through the use of brass valves or fittings. In addition, steel-which is commonly used for storage tanks or in reactors-contains iron. Figures 5 and 6 illustrate the efficacy of several stabilizers in addressing the impact of copper- and iron-induced degradation. These metals can lead to decreased fuel stability and potential sediment formation. The levels of sediment from metal-catalyzed degradation could be even higher than otherwise observed with regular biodiesel. As not all stabilizers are able to protect biodiesel from the influences of trace-level metals, it is essential to look for stabilizers that are also able to offer protection against these metals during additive selection.


Figure 5


Figure 6

Conclusions
Driven by new regulations and incentives for manufacturers to employ fuels from renewable sources, the biodiesel industry has experienced remarkable growth. One of the key factors influencing long-term market sustainability will be the industry's ability to supply consistent and acceptable biodiesel. While product standards have improved, they will continue to be reviewed and updated to meet the changing requirements and needs for fuel quality.

With the adoption of an oxidative stability specification in ASTM D 6751 in 2007, the EN 14214 and ASTM D 6751 standards now mandate a minimum level of stability. Even so, NREL has reported that even if a fuel meets current standards, engine manufacturers and fuel consumers are concerned about the durability of engine and fuel system components. Although enhanced measures for assessing and understanding the impact of fuel quality may eventually lead to more stringent requirements for the future, proactive measures must be taken by manufacturers and distributors to fully ensure fuel quality and stability during storage and distribution. The use of stabilizers provides an easy and essential means to ensure the long-term stability and quality of biodiesel.

With the use of the appropriate additives, producers of biodiesel are able to meet regulated specifications and address performance-related concerns. Such additives provide solutions to poor oxidative stability and create a uniform fuel source regardless of the original feedstock, even if presented with negative influences, such as metals. Improved quality can be achieved and assured through the adoption of and adherence to appropriate specifications and, more importantly, in undertaking activities that will allow the industry to achieve long-term, sustainable growth.

Natalie Li is the global marketing manager for Ciba Specialty Chemicals. For more information, contact biodiesel@cibasc.com or (914) 785-2304.