The combustion of fossil fuels such as coal, oil and natural gas has been identified as a major cause of the increase in the concentration of carbon dioxide in the earth’s atmosphere. Carbon dioxide and other so-called greenhouse gases (e.g., methane) allow solar energy to enter the earth’s atmosphere but reduce the amount of energy that can be radiated back into space, thus trapping energy and causing global warming.
One environmental benefit of replacing fossil fuels with biomass-based fuels is that the energy obtained from biomass does not add to the level of carbon dioxide in the atmosphere that causes global warming. Although the combustion of all types of fuel, including fuels produced from biomass, releases carbon dioxide into the atmosphere, the burning of biomass does not cause any net increase in carbon dioxide concentration. The reason for this is that plants use carbon dioxide from the atmosphere to grow (photosynthesis) and the carbon dioxide formed during combustion is balanced by that absorbed during the annual growth of the plants used as the biomass feedstock. In the case of fossil fuels, the carbon content of these fuels has been fixed and contained in the earth’s crust for millions of years. By burning these fossil fuels the formerly harmless carbon buried in the earth’s crust is released into the atmosphere as carbon dioxide resulting in a net increase in the carbon concentration that leads to global warming.
Although some fossil fuel-based energy may also be used to grow, produce and process the biomass feedstock, the general net result, based on a number of product life cycle studies, is still substantially reduced net greenhouse gas emissions. For example, although modern, high-yield corn production is relatively energy intensive, the net greenhouse gas emission reduction from making ethanol from corn grain can still be as much as 20%. If ethanol is produced from sugar cane where waste bagasse is used for steam and power production, the net reduction in greenhouse gas emission is even bigger. And making biodiesel from soybeans can reduce net emissions by nearly 80%. Finally, producing ethanol from cellulosic materials and using the non-fermentable lignin and other biomass for the production of steam, energy and electricity can reduce net carbon dioxide emission by close to 100%.
Since petroleum diesel and gasoline consist of blends of hundreds of different chemicals of varying hydrocarbon chains, many of these are hazardous and toxic. They include volatile compounds such as benzene, toluene and xylene that are identified as responsible for the health hazards and pollution associated with the combustion of petroleum-based fuels. Carbon monoxide (produced when combustion is inefficient or incomplete), nitrogen oxides (produced when combustion occurs at very high temperatures), sulfur oxides (produced when elemental sulfur is present in the fuel), and particulates that are generally produced during combustion are other specific emissions of concern. An important environmental and health benefit of using biofuels as an additive to or replacement of petroleum-based transportation fuels is a reduction in these harmful emissions, in particular sulfur oxides and particulates.
Since both ethanol and biodiesel contain oxygen in their chemical structure they can be used as fuel oxygenates to improve combustion characteristics. The presence of chemically-bound oxygen results in more complete combustion, which reduces carbon monoxide emissions. This is significant environmental and health benefit that can result from replacing petroleum fuels with biofuels. Ethanol is typically blended with gasoline to form an E10 blend (5%-10% ethanol and 90%-95% gasoline), but it can be used in higher concentrations such as E85 or in its pure form. Biodiesel is usually blended with petroleum diesel to form a B1 blend (1% biodiesel, 99% petroleum diesel) or B20 blend (20% biodiesel and 80% petroleum diesel), although other blend levels can be used up to B100 (pure biodiesel).
The global public debate about the use of ethanol as a world energy source has been going on for more than fifty years. On one hand, ethanol advocates have described ethanol as the “fuel of the future”. But critics question the feasibility and advisability of large-scale ethanol production in terms ranging from “net petroleum energy balance” to more general environmental and agricultural impacts. The opponents of widespread use of ethanol as fuel question the use of limited edible resources to meet the world’s energy needs. In the same manner, these opponents say that the need for cleaner-burning fuel sources for environmental and health reasons does not fully justify the use of edible plant oils such as soybean oil, palm oil and coconut oil. Many critics also argue that it is wrong to use grain for energy production instead of for food. In the final analysis, it is society that must decide whether the virtues and benefits of using ethanol or biodiesel as an energy source outweigh the potential negative consequences on available food supply of using grain or edible plant oils for biofuel. Society, in general, and the scientific community, in particular, must also resolve the debate about the energy balance of ethanol production, with different studies showing either a net gain or loss when the energy inputs to ethanol production are compared to the energy yield.
The future of agricultural and land use policies is complicated by this emerging potential for large-scale bio-energy production. The growing demand for cleaner-burning fuels, such as ethanol, is likely to generate changes in agricultural cropping patterns and land management practices. This will have serious impacts on the environment, possibly threatening the natural resource base. Both biofuels proponents and critics question how production decisions and policy formulation and development surrounding the rapid expansion of the biofuel industry will affect the environment, especially water quality and soil quality. In the same manner, they ask whether the long-touted climate benefits arising from the displacement of petroleum oil use will actually be realized. The answers to these questions are not yet certain. What is certain is that while energy policy will have a significant impact on how the markets for biofuels will develop, agricultural and land use policies will also significantly influence how those markets will impact the environment and the people’s health.