The use of biodiesel in a conventional diesel engine also results in substantial reduction of unburned hydrocarbon derivatives, carbon monoxide, and particulate matter. Emissions of nitrogen oxides are either slightly reduced or slightly increased depending on the duty cycle and testing methods. The higher oxygen content of biodiesel enables a more complete combustion of the solid carbon fraction to carbon dioxide, while soluble hydrocarbon derivatives are unaffected or even increased.
Producers and users of biodiesel use the “B” factor system to state the amount of biodiesel in a fuel mix, similar to the “E” system used for fuel containing ethanol. For example, fuel containing 20% biodiesel is labeled B20, whereas pure biodiesel is referred to as B100.
See also: Bioalcohols, Biofuels – First Generation, Biofuels – Second Generation, Biofuels – Third Generation, Biogas, Vegetable Oil.
Biodiesel Feedstocks
A major focus has been and continues to be on biofuels produced from crops, such as corn, sugar cane, and soybeans, for use as renewable energy sources. Though it may seem beneficial to use renewable plant materials for biofuel, the use of crop residues and other biomass for biofuels raises many concerns related to major environmental problems, including food shortages and serious destruction of vital soil resources. Examples of feedstock for the production of biodiesel are (i) waste vegetable oil, (ii) animal fat including tallow, lard, yellow grease, chicken fat, and the by-products of the production of fatty acids from fish oil, and soybeans which are also used as a source of biodiesel (Table B-4).
Table B-4 Composition of various oils and fats used for biodiesel production.
| Oil or Fat | 14:0* | 16:0 | 18:0 | 18:1 | 18:2 | 18:3 |
|---|---|---|---|---|---|---|
| Corn oil | 1-2 | 8-12 | 2-5 | 19-49 | 34-52 | Trace |
| Cottonseed oil | 0-2 | 20-25 | 1-2 | 23-35 | 40-50 | Trace |
| Linseed oil | 4-7 | 2-4 | 25-40 | 35-40 | 25-60 | |
| Olive oil | 9-10 | 2-3 | 73-84 | 10-12 | Trace | |
| Peanut oil | 8-9 | 2-3 | 50-60 | 20-30 | ||
| Safflower oil – Hi linoleic acid | 5.9 | 1.5 | 8.8 | 83.8 | ||
| Safflower oil – Hi oleic acid | 4.8 | 1.4 | 74.1 | 19.7 | ||
| Rapeseed oil – Hi oleic acid | 4.3 | 1.3 | 59.9 | 21.1 | 13.2 | |
| Rapeseed oil – Hi erucic acid | 3.0 | 0.8 | 13.1 | 14.1 | 9.7 | |
| Soybean oil | 6-10 | 2-5 | 20-30 | 50-60 | 5-11 | |
| Tallow | 3-6 | 24-32 | 20-25 | 37-43 | 2-3 | |
| Yellow grease | 2 | 23 | 13 | 44.3 | 7 | 0.7 |
| *Indicates the number of carbon atoms in the alkyl chain and the position of the double bond. | ||||||
However, for a given production line, the comparison of the feedstocks includes several issues which are (i) chemical composition of the biomass, (ii) cultivation practices, (iii) availability of land and land use practices, (iv) use of resources, (v) energy balance, (vi) emission of greenhouse gases, acidifying gases, and ozone depletion gases, (vii) absorption of minerals to water and soil, (viii) injection of pesticides, (ix) soil erosion, (x) contribution to biodiversity and landscape value losses, (xi) farm-gate price of the biomass, (xii) logistic costs such as transport and storage of the feedstock), (xiii) direct economic value of the feedstocks taking into account the co-products, (xiv) creation or maintenance of employment, and (xv) water requirements and water availability. In addition, different types of feedstocks have different types of fatty acids. The fatty acids are different in relation to the chain length, degree of unsaturation, or presence of other chemical functions, such as fatty acids.
Tropical countries have the highest potential to produce bio-fuel crops: higher energy yields, better greenhouse gas balance if properly produced, lower costs, and in some countries, large reserves of uncultivated cropland. Sugar cane and palm are the highest yielding tropical biofuel crops and consequently provide the greatest carbon offsets. Industrialized countries with biofuels targets (such as the United States and the