Biodiesel is produced through a series of physical and chemical processing of the fruits, seeds, or parts of oil-containing plants. The first step usually involves the extraction of the crude oil. This step may involve several different types of operations or processes depending on the kind of feedstock. For example, the process of extracting crude oil from jatropha may involve only one or two simple mechanical operations whereas the process of extracting oil from coconut may involve many. The detailed description of the various unit operations and processes involved in the extraction of crude oil from different types of plant oils such as palm oil, soybean oil, coconut oil and others is outside the scope of this book. However, the two main processes for extracting oil from seed feedstock are mechanical press extraction and solvent extraction. In mechanical press extraction, the oil seed feedstock (e.g., jatropha seed, sunflower seed, etc.) is first heated to about 110°F. The oil seed is then crushed in a screw press. After most of the oil is removed, the remaining seed meal can be used as an animal feed. The solvent process extracts more of the oil contained in the oil seed feedstock but requires more costly equipment. The process uses a solvent to dissolve the oil. After extraction, a distillation process separates the oil from the solvent. The solvent condenses and can be recycled and reused in the process. Solvent extraction produces vegetable oil with a higher degree of purity than the mechanical press process.
The central problem in using plant oil as diesel fuel is that plant oil is much more viscous (thicker) than conventional diesel fuel. It is 11 to 17 times thicker. Plant oil also has very different chemical properties and combustion characteristics to those of conventional diesel fuel. If the fuel is too thick it will not atomize properly when the fuel injectors spray it into the combustion chamber and it will not combust properly–the injectors may get coked up, leading to poor performance, higher exhaust emissions and reduced engine life. The process of transesterification reduces the high viscosity of plant oil, resulting in a higherquality fuel. In the transesterification process, vegetable oil reacts with alcohol (methanol or ethanol) in the presence of a catalyst (usually sodium hydroxide). The oil molecules (triglycerides) are broken apart and reformed into methyl esters and glycerin, which are then separated from each other and purified.
Biodiesel, which is a mixture of fatty acid alkyl esters, can be produced not only from plant oils, such as coconut oil, palm oil and soybean oil, but also from animal fats or recycled cooking oil and greases. Esters are compounds of alcohol and organic acids. Fatty acid methyl ester is made by bonding methanol to animal fat or plant oil. The process is relatively straightforward, but must consistently achieve prescribed standards to minimize the risk of damaging expensive diesel engines. Biodiesel can be used as a fuel for vehicles in its pure form, but it is usually used as a petroleum diesel blend to reduce levels of particulates, carbon monoxide, hydrocarbons and air toxins from diesel-powered vehicles.
Because it is oxygenated, biodiesel dramatically reduces air toxins, carbon monoxide, soot, small particles, and hydrocarbon emissions by 50% or more, reducing the cancer-risk contribution of diesel up to 90% with pure biodiesel. Air quality benefits are roughly proportional for diesel/biodiesel mixtures. Biodiesel’s superior lubricity helps reduce engine wear, even as a small percentage additive. The most common use of biodiesel is as B20 (20% biodiesel, 80% diesel) and B2 (2% biodiesel, 98% diesel) or B1 (1% biodiesel, 99% diesel). The use of these blends requires no engine modifications. But because it gels at higher temperatures than petroleum diesel, however, pure biodiesel requires special management in cold climates. Biodiesel contains slightly less energy than petroleum diesel, but it is denser, so fuel economy tends to fall 7% for every 10% biodiesel in a fuel blend.
When applied in its pure form, biodiesel has a higher flash point than fossilbased diesels, and its viscosity results in a more intense pulverization, which is another disadvantage. Nevertheless, if properly treated, these shortfalls can be abated–and the advantages of biodiesel greatly make up for its disadvantages. It is nonhazardous, nontoxic and biodegradable, and it reduces air pollutants such as particulates, carbon monoxide, hydrocarbons and air toxins. It burns more efficiently than petroleum diesel, and its higher lubricity can reduce engine wear, prolonging its lifetime.