There are a number of physical and chemical characteristics of plant oils that affect their suitability as fuels. These include the heating value, pour point, cloud point, flash point, iodine value, viscosity, density, and cetane number. The characteristics that directly affect the efficiency of the fuel and the performance of the engine are summarized in Box 2.2. The typical values of these physical and chemical parameters for a number of plant oils are given in Table 2.6 while Table 2.7 compares the properties of canola oil with diesel and biodiesel.
The heating values of most plant oils vary within a narrow range, ranging from only 39,310 kJ/kg for linseed oil to 40,480 kJ/kg for crambe oil. These values compare well with diesel fuel with a heating value of 45,340 kJ/kg. On the other hand, the flash points of most plant oils, ranging from 2410C for linseed oil to 2770C for corn oil, are much higher than that of diesel fuel, which is only 520C. In addition to these physical and chemical characteristics, there are also other characteristics that do not have direct bearing on the actual performance of the engine, but are similarly important for environmental and other reasons. These include the ash, sulfur and potassium contents of the plant oil.
Plant oils with low iodine value are generally more combustible and more efficient fuels than oils with high iodine value. The disadvantage is that oils with low iodine value also have higher melting points and are usually solid at room temperature. For example, crude palm oil is usually solid at ambient temperature. As a consequence, biodiesel that is produced from oils with low iodine value also has a higher melting point and might only be suitable for use as fuel in warm tropical countries or during the summer months in temperate countries. High iodine value oils have lower melting points and make better cold weather biodiesel. The drawback, however, is that with high iodine value oils there is more risk of the biodiesel oxidizing and polymerizing (drying) into a tough, insoluble plastic-like solid. Therefore, biodiesel made from oils with high iodine value, such as linseed oil, soybean oil and sunflower oil, should be stored appropriately and used quickly.
Viscosity is one of the critical parameters in the use of plant oils as fuel since it affects injector lubrication and fuel atomization. Castor oil, for example, has a very high viscosity and therefore not suitable diesel fuel substitute or blending. Other plant oils have viscosities ranging from 31 mm2/sec for safflower oil to 54 mm2/sec for crambe oil. With proper processing, these viscosities can be reduced to a level close to that of diesel fuel, which is 3-5 mm2/sec. Viscosity is, of course, highly dependent on temperature – the higher the temperature, the lower the viscosity. Fuels with high viscosity may not provide sufficient lubrication for the precision fit of fuel injection pumps, resulting in leakage or increased wear. Preheating these types of fuels may render them suitable as diesel fuel substitute. Fuel atomization is also affected by fuel viscosity. Fuels with high viscosity tend to form larger droplets on injection which can cause poor combustion, increased exhaust smoke and emissions.
In the same manner, fuels with low cetane numbers, such as linseed oil (35) and rapeseed oil (38) will cause difficult starting and produce noise and thick exhaust smoke. In general, diesel engines will operate better on fuels with cetane numbers of around 50 or higher. Palm stearine (85), palm kernel oil (70), and palm olein (65) all have cetane numbers higher than 50. Ash content is also important. High concentrations of ash can cause injector tip plugging, combustion deposits and injection system wear. The ash content is important for the heating value, as heating value decreases with increasing ash content. The ash content of biofuels is typically lower than for most coals, and sulfur content is much lower than for many fossil fuels. Unlike coal ash, which may contain toxic metals and other trace contaminants, plant oil ash may be used as a soil conditioner to help replenish the nutrients removed by harvest.