The variety of plants from which oil can be obtained provides considerable potential for nourishment and also for fuel; examples are rapeseed, beets, sunflowers, flax, olive trees, coconut palms, peanuts, soy, castor-oil plant, cocoa, and cotton. The extraction of oil using a mechanical press is well disseminated and not expensive. Generally, pressing is followed by refinement in stages to eliminate secondary substances. In the next stage, phosphates and slimes, and then free fatty acids are removed.
The long and branched oil molecules, which are the cause of this high viscosity, impair the combustion process by preventing oxygen atoms from the air from reaching the carbon atoms. This leads to coking of the injector holes, valves, and piston rings, causing impairment of operation or engine damage. A basic solution to this problem is to shorten the branched molecules, which can be accomplished by the procedure known as transesterification.
Sub stoichiometric oxidation of the coal tar gas over the dripping point, in a combustion chamber.
Injection of biocoke powder in the gasification medium: A synthetic raw gas is generated in an endothermic reaction. The gas is composed of CO2, CO, and H2. The subsequent Fischer–Tropsch synthesis produces so-called Sun Diesel, which has similar molecules to diesel fuel, from the synthesis gas (CO, H2) using a catalyst based on iron, magnesium oxide, thorium oxide, or cobalt .
The energy content per surface is three times higher than that of biomass fuel 1. Generation is based on rapeseed. A modern automobile with a diesel engine can achieve about 64,000 km, at a consumption of approximately 6 l/100 km, using the Sun Diesel that is obtained from 1 ha.
Hydrogenation (NexBtL): A vegetable oil is treated with phosphoric acid and caustic soda (H3PO4, NaOH) and subsequently hydrogenated at a temperature of 320–360 C and a pressure of 8 MPa. Such hydrogenation can be carried out in a typical refinery.
Biofuels of the second generation are broadly similar to classic diesel fuel. Oils obtained by transesterification (biofuels of the first generation) are less similar to diesel fuel: The oxygen content in the molecules of oils and oil esters lead to lower heat values and also to a lower stoichiometric air requirement.
In terms of the heat value of the mixture, the lower heat value is compensated for by a lower stoichiometric air requirement, which leads to a higher fuel participation in the mixture. However, an oil–air mixture cannot achieve the heat values of a diesel fuel–air mixture.
Oils and oil esters can be stored in a similar way to diesel fuels because of their similar density under ambient conditions. Non-esterified oils have a higher viscosity at low ambient temperature, which impedes normal engine operation. In some variants of storage and dosage systems, the oil is treated to diminishing the viscosity.
An additional problem when using non-esterified oils is the formation of fungi and slime in the complete fuel system, including filters. Just 15 % rapeseed oil content in diesel fuel is sufficient for such formation. The storage of oil esters is similar to that of diesel fuel.
Piston engines working in a Diesel cycle, with large or small swept volume of between 1.6 and 12 dm3 , with all configurations for mixture formation from pre-chamber to direct injection, with aspiration, or with super-/turbocharging, have been operated with oils and oil esters.
Applications and Results
The power and consumption of engines powered with non-esterified rapeseed oil and alternatively with diesel fuel are identical in all tested types. However, long operation with such pure oil is possible only in engines with a large swept volume equipped with a swirl chamber.
In automobile engines with relatively small swept volumes and with direct injection, the utilization of non-esterified oils is not recommended. The pollutant emission when using such oils is disadvantageous in comparison with the utilization of diesel fuel.
The utilization of oil esters in diesel engines for automobiles is possible, but the price of Tran’s esterification limits its large-scale application in the future. More interesting is the blending of crude oil with vegetable oils during refining. The resulting molecular structure is not different from the structure of diesel fuel.