Determining the Optimum Composition of Biodiesel Fuel

Grant #: 94-09-01
Principal Investigator: Jon Van Gerpen
Co-Principal Investigator: Earl G. Hammond
Organization: Iowa State University
Participants: Kevin Schmidt, David Y.Z. Chang, Mustafa Canakci
Technical Area: Renewable Energy

Background and Significance:
Biodiesel is an alternative fuel for diesel engines that is produced by transesterification of oils and fats from plant and animal sources. The transesterification process combines the oil with an alcohol to produce chemical compounds known as esters along with a coproduct, glycerin. These esters are the biodiesel. Biodiesel is nontoxic, biodegradable and renewable. It also reduces the emissions of carbon monoxide, unburned hydrocarbons and particulates from most engines in which it is used.

Another important advantage of biodiesel is that, unlike virtually all other alternative engine fuels, it can be used in unmodified engines. Diesel engines can use pure biodiesel or any blend of biodiesel with conventional petroleum-based diesel fuel.

Biodiesel use is widespread in Europe where it is produced from rapeseed oil. It can be produced from almost any oil seed crop, as well as animal fats and restaurant waste grease. Much of the biodiesel interest in the United States is coming from the soybean industry and the producers of animal fats.

Project Objectives:
The objective of this project has been to develop solutions to the obstacles to commercialization of biodiesel in the United States. The primary obstacle to biodiesel commercialization is its high cost. The current price of bulk quantities of biodiesel is $3.50/gallon. About half of this cost is the soybean oil feedstock.

Summary of Work to Date:
In early phases of this project, we investigated alternative formulations of biodiesel that would maximize its value as a blending agent with diesel fuel. We identified a combination of 20% methyl palmitate and 80% methyl stearate as a biodiesel blend that provided the maximum emissions reductions and cetane number enhancement with the minimum impact on cold flow properties.

In later phases of this project, we have identified two additional obstacles to commercialization. First, the presence of processing contaminants from production of biodiesel can impact its performance in the engine and ultimately customer acceptance. Second, biodiesel is not as stable as conventional diesel fuel and can undergo reactions prior to combustion in the engine that can cause additional customer acceptance problems.

Biodiesel contaminants such as unreacted and partially reacted oil, water, glycerin and free fatty acids can cause problems such as fuel filter plugging, varnish formation and fuel system corrosion. We developed a set of standards to be used for the maximum allowable levels of these contaminants in biodiesel. These standards ensure that biodiesel can be used safely in engines without unnecessarily high production costs.

Our most recent work on biodiesel has been on biodiesel thermal and oxidative stability. Biodiesel from soybean oil contains a much higher level of unsaturated molecules than that made from rapeseed oil and most animal fats. This difference in chemical composition is responsible for soybean oil-based biodiesel being more prone to oxidation and the resulting varnish and sediment formation than biodiesel from other feedstocks. Our project has investigated the impact of this oxidation on engine fuel filters and on the development of techniques for characterizing the oxidation.

Earlier researchers have identified fuel filter plugging as one potential problem of biodiesel oxidation. We fabricated a fuel filter test stand to determine whether biodiesel would plug fuel filters and, if so, at what level of oxidation. In our experiment, we have determined that plugging is apparently due to other secondary factors such as catalytic reactions with the materials of the fuel storage system. We have also determined that the standard tests for fuel oxidative and thermal stability from the American Society for Testing and Materials (ASTM) are not suitable for biodiesel. These tests use a solvent washing technique on the filters that are used to capture sediments that does not remove biodiesel. Much of the published data for the stability of biodiesel is based on these tests and we believe it is erroneous. We are working to develop a better test procedure for characterizing sediment formation from biodiesel oxidation.

This project was originally scheduled to be completed June 30, 1997, but was carried over to December 31, 1997 to allow additional tests to be conducted.