Rapeseed and soybean oil are the most-used feedstocks for biodiesel production in the European Union and the United States, respectively. However, the use of palm oil-based biodiesel is increasing due to strong production growth in tropical countries like Malaysia, Indonesia, Thailand, Nigeria and Colombia. Palm oil is a promising feedstock for biodiesel production because of its low cost and high productivity per unit of planted area.
Palm oil biodiesel, also known as palm oil methyl ester (PME), differs from other types of biodiesel in its grade of molecule unsaturation. PME is more saturated, which means it has a lower number of double carbon bonds in its molecules. For diesel engine applications, the degree of biodiesel molecule unsaturation represents a compromise. Saturated fuels such as PME have high-ignition quality. However, they also harden at higher temperatures, making them difficult to use in cold weather.
Since biodiesel is derived from renewable sources, its production and use are being promoted worldwide as a way to reduce oil dependency and decrease greenhouse gas emissions. Due to PME's rising importance as a biodiesel feedstock, it's important to consider its combustion and operational performance.
An Overview of Combustion
In diesel engines, air is highly compressed in order to reach conditions that cause the fuel to self-ignite. Fuel is injected at a point called the start of injection, which is toward the end of the piston's compression stroke. Before self-igniting, the fuel breaks up into droplets that are heated, vaporized and mixed with air. The period between the start of injection and the start of combustion is aptly named ignition delay. Initially, a phase of pre-mixed combustion takes place, where the fuel/air mixture prepared during the ignition delay ignites, causing a very rapid rise in pressure. Combustion then continues at the rate at which the mixture becomes available for burning.
Diesel combustion byproducts are a major source of urban air pollution. Diesel engines produce significant amounts of nitrogen oxide (NOx), gaseous hydrocarbon emissions (HC) and particulate matter (PM) emissions. However, carbon monoxide emissions are much lower than those from gasoline engines.
PME has a short ignition delay, which is represented by its high cetane number (68)[1]. Thus, when compared with fuels with a lower cetane number, less fuel ignites during pre-mixed combustion. This leads to a lower peak of in-cylinder pressure and temperature. NOx is reduced since it's strongly dependent on the flame temperature. HC emissions also tend to be reduced when high cetane fuels are combusted[2].
The most serious emission from diesel engines is PM, which consists primarily of soot (i.e., combustion-generated carbonaceous material). These particles are a health concern because they easily reach the deepest parts of the lungs, causing a series of significant health problems[3]. Studies using chemical kinetic modeling of diesel combustion with oxygenated fuels[4] show that oxygen atoms present on the biodiesel molecule prevent the carbon atom to which they are bonded to participate in the formation of soot precursor species, which leads to a reduction in soot emissions.
My Master's thesis work, conducted at Chalmers University of Technology in Sweden[5], showed a reduction in NOx, HC, and soot emissions from a research diesel engine fueled with PME compared to those emissions from Swedish low sulfur diesel fuel, also known as MK1. Reductions in NOx and HC emissions are explained by the better ignition quality of PME. The reduction in soot emissions is explained by the presence of oxygen atoms in the biodiesel molecule.
Operational Performance
While the benefits of using PME to reduce emissions have been proven in laboratory tests, its operational performance is still of concern to end users and the automotive industry. Oxidation stability and low-temperature fluidity are the main concerns.
PME oxidation stability has better performance than soy methyl esters and rapeseed methyl esters. PME also exceeds Europe's EN 14214 specification. Both are because PME has fewer unsaturated molecules susceptible to oxidation through its double carbon bonds. Oxidative stability is important to engine performance because oxidation byproducts can cause harmful effects such as filter plugging, deposits and corrosion.
On the other hand, saturated molecules harden at higher temperatures. As the biodiesel cools down, solid crystals form which eventually cause fuel filter plugging. The temperature at which this occurs is known as the cold filter plug point (CFPP). This property is the major difficulty facing the use of pure PME (B100) in cold weather conditions, since CFPP of PME is 12 degrees Celsius (54 degrees Fahrenheit). To reduce the impact of low CFPP, PME is mainly used in blends with diesel fuel.
In order to evaluate the operational performance of PME and to clear up the concerns about its use, a group of organizations from Colombia, led by the Colombian Oil Palm Research Center (CENIPALMA)[6], is conducting a long-term evaluation of transit buses from Transmilenio, which is the rapid transit system that serves the capital, Bogotá. The buses are scheduled to travel 100,000 kilometers (62,000 miles) on varying PME blends (from B5 to B50). During the study several operating parameters will be evaluated, such as specific fuel consumption, emissions and maintenance costs. The fact that tests take place in Bogotá is a challenge to PME performance: Bogotá is located at 2,600 meters (8,530 feet) above sea level, thus its climatic conditions are irregular with temperatures ranging from minus 8 to 20 degrees Celsius (18 to 68 degrees Fahrenheit). In addition, Bogotá has 8 million inhabitants and severe air contamination issues. B5 use will be mandatory in Colombia starting in 2008, and researchers expect results from testing to support the use of higher blends. This would greatly benefit a country which is one of the world largest producers of palm oil and whose regular diesel fuel has a sulfur content of 4,000 parts per million, which increases PM emissions.
In order to supply increasing biodiesel demand and reduce the cost of the production process, a wide variety of vegetable oils are being used as biodiesel feedstock, but their success will depend on low cost and good performance. PME emerges as a suitable alternative to diesel fuel due to its high ignition quality and high oxidation stability, though its low-temperature fluidity should be considered when used in cold weather conditions.
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