Biofuels, like fossil fuels, come in a number of forms and meet a number of different energy needs. The class of biofuels is subdivided into two generations, each of which contains a number of different fuels that will be explored in this article.
First Generation Biofuels
First generation biofuels are made from sugar, starch, or vegetable oil. They differ from “second generation biofuels” in that their feedstock (the plant or algal material from which they are generated) is not sustainable/green or, if used in large quantity, would have a large impact on the food supply. First generation biofuels are the “original” biofuels and constitute the majority of biofuels currently in use.
Second Generation Biofuels
Second generation biofuels are “greener” in that they are made from sustainable feedstock. In this use, the term sustainable is defined by the availability of the feedstock, the impact of its use on greenhouse gas emissions, its impact on biodiversity, and its impact on land use (water, food supply, etc.). At this point, most second generation fuels are underdevelopment and not widely available for use.
Biofuel Table
This table breaks biofuels down by generation and then explores their uses, energy densities, and greenhouse gas impacts. Specific biofuels from the table are selected for further exploration on subsequent pages.| Fuel | Feedstock | Energy Density (megajoules/kilogram) | Greenhouse Gas CO2(kg/kg) | Notes |
| First Generation | ||||
|
Bioalcohol Ethanol Propanol Butanol |
Starches from wheat, corn, sugar cane, molasses, potatoes, other fruits | By Type 30 34 36.6 | By Type 1.91 N/A 2.37 | |
| Biodiesel | Oils and fats including animal fats, vegetable oils, nut oils, hemp, and algae | 37.8 | 2.85 | |
| Green Diesel | Made from hydrocracking oil and fat feedstock | 48.1 | 3.4 | Chemically identical to fossil fuel diesel |
| Vegetable Oil Castor Oil Olive Oil Fat Sunflower Oil | Unmodified or slightly modified | By Type 39.5 39 32 40 | By Type 2.7 2.8 N/A 2.8 | |
| Bioethers | Dehydration of alcohols | N/A | N/A | These are additives to other fuels that increase performance and decrease emissions, particularly ozone |
| Biogas | Methane made from waste crop material through anaerobic digestion or bacteria | 55 | 2.74 (does not take into account the direct effect of methane, which is 23X more effective as a GHG than CO2 | Same properties as methane from fossil fuels |
| Solid Biofuels Wood Dried plants Bagasse Manure Seeds | Everything from wood and sawdust to garbage, agricultural waste, manure | By Type 16-21 10-16 10 10-15 15 | By Type 1.9 1.8 1.3 N/A N/A | This category includes a very wide variety of materials. Manure has low CO2 emissions, but high nitrate emissions. |
| Second Generation | ||||
| Cellulosic ethanol | Usually made from wood, grass, or inedible parts of plants | |||
| Algae - based biofuels | Multiple different fuels made from algae | Can be used to produce any of the fuels above, as well as jet fuel | See specific fuels above | More expensive, but may yield 10-100X more fuel per unit area than other biofuels |
| Biohydrogen | Made from algae breaking down water. | Hydrogen compressed to 700 times atmospheric pressure has energy density of 123 | Does not have any greenhouse effect. | Used in place of the hydrogen produced from fossil fuels |
| Methanol | Inedible plant matter | 19.7 | 1.37 | More toxic and less energy dense than ethanol |
| Dimethylfuran | Made from fructose found in fruits and some vegetables | 33.7 | Energy density close to that of gasoline. Toxic to respiratory tract and nervous system | |
| Fischer-Tropsch Biodiesel | Waste from paper and pulp manufacturing | 37.8 | 2.85 | Process is just an elaborate chemical reaction that makes hydrocarbon from carbon monoxide and hydrogen |
