Biofuels

The global energy market provides humans with about 370 exajoules of energy per year, which is equivalent to about 170 million barrels of oil per day (Box 1) or about 11.73 terrawatts (TW) per hour [1]. Approximately 95% of this energy comes from fossil fuels. Additionally, the International Energy Agency suggests that direct combustion of plant biomass provides about one-third of the energy needs in Africa, Asia and Latin America, and as much as 80 to 90% in the poorest countries of these regions [1]. Combining estimates of the magnitude of this form of biofuels consumption with the relatively small amount for which market numbers are available suggests that biofuels currently provide about 10% of all human energy use. There has recently been an upsurge of interest in the use of liquid biofuels for transportation in the developed world. This has been stimulated by a very rapid increase in the price of petroleum, strategic concerns about dependence on politically unstable regions of the world, and concerns about global climate change. The probable trajectory of this interest is beyond the scope of this article. But it is worth noting that there is no compelling evidence that even half of the recoverable petroleum has been used. Also, the regions that consume most of the energy are endowed with abundant coal reserves that are projected to be adequate to meet human energy needs for several hundred years. Coal can be converted into a wide variety of liquid fuels that can substitute for petroleum. Thus, it would be prudent to view the recent discontinuity in historical fossil fuel price trends as a transient imbalance rather than an indicator that fossil fuels are nearing depletion [1]. If concerns about climate change are ignored, there is not a pressing motivation to develop biofuels. Primer The linkage between climate change and biofuels arises from the fact that biofuels can be carbon neutral sources of energy. Energy from sunlight is collected by the photosynthetic system of plants and used to reduce and condense atmospheric CO2 into the chemicals that comprise the body of plants. When plants are burned, the energy resulting from oxidation is released as heat and the CO2 is recycled. If the biomass is simply burned, about 85% of inherent energy is available as heat and if that heat is used to produce steam for generators, approximately 35% of the energy can be recovered as electricity. With highly productive plants, such as Miscanthus giganteus, growing on good soils with adequate rainfall and favorable mean temperature, such as are found in central Illinois (Figure 1), more than 2% of annual incident solar insolation can be harvested as biomass [2]. If we use a value for average solar insolation of 120,000 TW, 2% solar conversion efficiency, and an energy recovery value of 50%, we could meet all human energy needs — 11.73 TW at the present level of consumption — by growing a plant such as Miscanthus on about 3.2% of the terrestrial surface area. Similar numbers can be obtained from actual yield measurements (Box 2). The calculation provides a tangible way of envisioning global bioenergy capacity. Current goals are much more modest, however; the US Secretary of Energy has established a goal for the US of obtaining 30% of transportation fuels from biomass by 2030. The reason for the current focus on using biomass for liquid fuels rather than for direct combustion is that coal is abundant and inexpensive, it is less expensive Box 1