Biofuels for transportation include any products derived from living organisms, ranging from food crops, to plant and tree material, to microorganisms, that can be processed into substitutes for liquid transportation fuels. The use of biofuels for transportation dates back to the early years of the ICE, when Rudolf Diesel, inventor of the diesel engine, used peanut oil as a fuel in his early engine prototypes. One of Diesel’s motivations was indeed to create a source of mechanical power that could burn a wide variety of fuels, so that small businesses of the day would not be captives of the coal industry for their energy supplies. In recent years, interest in biofuels has surged as nations such as Brazil and the United States have sought to reduce petroleum imports. Also, many countries see biofuels as a way to prepare for anticipated pressures on the world’s petroleum resources by developing a renewable alternative.
In order to provide real benefit to society, a biofuel must achieve at least the following two objectives:
1. It must measurably reduce emissions of CO2 and other pollutants compared to petroleum, when taking into account life cycle energy inputs and emissions. These inputs include energy and emissions resulting from growing and harvesting crops, processing crops into finished fuels, and transportation of raw materials and finished fuels. Given the grave concern about climate change, a biofuel process that reduced CO2 without greatly increasing emissions of air pollutants might also be acceptable in some locations.
2. It must be available in sufficient quantity to displace a measurable fraction of the fuel currently derived from petroleum, without curtailing the ability of the world’s population to harvest sufficient grain for nutritional purposes. It is especially important to safeguard the survival of the poorest people in the world, who depend on food imports at times when locally produced food supplies fall short. Thus, even if it were economically attractive to shift a large part of the grain harvest of countries such as the United States from exports to domestic biofuel production, it would be morally untenable to pursue such a policy, not to mention contrary to the principles of sustainable development, if it led to starvation of many vulnerable people.
The presence of a positive net energy balance (NEB) is often used as a criterion for evaluating a biofuel, meaning that a biofuel that delivers more energy to the vehicle than it requires in the production life cycle. A biofuel with a positive NEB is considered worthwhile. While useful as a surrogate for a more complete analysis of environmental benefit and impact, NEB is imperfect, in that it does not consider the relative CO2 emissions of the energy source (e. g., coal versus natural gas for process heat). It also does not consider land use changes that might be necessary for producing the biofuel, for example, clearing forests and releasing a large quantity of CO2 in order to grow crops for biofuels. Even if energy input and NEB stay constant, environmental measures such as CO2 emissions are subject to change over time, either negatively, for example, if gas supplies diminish and biofuel producers switch to coal as an energy source, or positively, if renewable or nuclear energy with no GHG emissions is used as a major input into the production process. Thus, if possible, one should seek out measures of the target environmental concern (CO2, air pollutants, and the like), where available, in evaluating the net environmental benefits of a biofuel. NEB values are also subject to change over time due to improvements in technology and agricultural/industrial practices, so it is important to work with the most up-to-date values available when assessing biofuels.
The above discussion focuses on biofuels as an alternative to petroleum-derived liquid fuels, but they may also be seen as an alternative for short-term energy storage for renewable energy derived from the sun, as shown in Fig. 13-11. From this perspective, arriving solar energy on the left-hand side of the figure may be converted to electricity in a photovoltaic panel, and delivered via the grid to a recharging EV, where it is stored in the battery until used during the drive cycle. Also, since in a mature solar-to-EV system of the future, production of solar electricity might not exactly match demand for recharging in real time, an energy storage option is shown, for example, using pumped storage or conversion to hydrogen (for later reconversion back to electricity). Biofuels can achieve the same conversion-and-storage function, as shown on the right-hand side, through creation of oils and starches in plants using sunlight, water, and CO2. Thus the
Figure 13-11 Comparison of pathways to solar-derived energy for vehicles from PV panels and EVs (left) and from biofuels and ICEVs (right).
biofuel harvesting and processing system acts as a large battery bank, storing energy in biofuels and releasing it later through combustion in engines. When the biofuel is combusted, the CO2 is released back to the atmosphere, completing the cycle.
A limiting factor affecting all biofuels currently in use, and by extension affecting their ability to meet requirements 1 to 2 above, is the low retention rate of solar energy in plants and trees, for later conversion into biofuels. Of the incoming energy, which may average 100 to 250 W/m2 year round, taking into account diurnal cycles and variations in regional climate and latitude, less than 1% is available in the starches or oils as a raw material for conversion to fuel. This limit affects the ability of agriculture or forestry to provide sufficient raw materials to meet a major share of world transportation energy demand. Also, because the crops for biofuels accumulate energy at such low density, a large energy expenditure is required to gather the material together for processing, which hurts the balance of energy produced relative to energy input required. Much research work is therefore under way around the world to find ways to capture more of the solar energy in the biofuel crop and to use more of the biomass of the entire plant as a raw material. Breakthroughs in research along these lines are, in fact, critical if biofuels are to become a major source of energy for transportation; otherwise, using current crops and technology, they can play at best a moderate role in displacing petroleum consumption and reducing CO2 emissions.
In the remainder of this section, we focus most closely on the two most prevalent biofuels at present, namely, ethanol and biodiesel, and then conclude with a shorter discussion of some emerging alternatives.