One of the main benefits from future use of biofuels would be the reduction of greenhouse gases compared to the use of fossil fuels. Carbon dioxide, a greenhouse gas that contributes to global warming, is released into the air from combustion. Twenty-four percent of worldwide energy-related carbon emissions in 1997 were from the United States. Carbon and due to rising energy consumption, are expected to increase 1.3 percent per year through 2015.
When plants grow, they adsorb carbon dioxide from the atmosphere. If these plants are used for biofuels, the carbon dioxide released into the atmosphere during combustion is that which was adsorbed from the atmosphere while they were growing. Therefore the net balance of carbon dioxide from the use of biofuels is near zero. Since some fossil fuel use is required in both the planting and the production of bioenergy, there are some net carbon dioxide and other greenhouse gases released into the atmosphere. In determining the net carbon dioxide balance, important variables include growth rates, type of biomass, efficiency of biomass conversion, and the type of fossil fuel used for production. The amount of carbon accumulated in the soil and the amount of fertilizers used also have a large effect on the carbon balance. In particular, nitrous oxide (N2O), a powerful greenhouse gas, can be released as a result of fertilizer application. Estimates for the amount of greenhouse emissions recycled using biomass for energy production range from a low of 20 to a high of 95 percent. Wood and perennial crops have higher greenhouse gas reduction potential than annual crops. Using biomass to replace energy intensive materials also can increase the carbon balance in favor of energy crops. It is estimated that the nation’s annual carbon dioxide emissions could be reduced by 6 percent if 34.6 million acres were used to grow energy crops.
There is some greenhouse gas benefit from planting forests or other biomass and leaving the carbon stored in the plants by not harvesting. However, over the long term, increased carbon dioxide benefits are realized by using land that is not currently forested for growing some energy crops such as fast-growing poplar. The added benefits come from the displacing fossil fuels biofuels usage, since energy crops can be repeatedly harvested over the same land.
In the calculation of greenhouse gas benefits of planting energy crops, many assumptions are made. Among them is that the land will be well managed, appropriate crops for the region will be used, there will be careful use of fertilizers and other resources, and efficient production methods will be employed to get the maximum amount of energy from the biomass. Most importantly, it is assumed that biomass is grown in a sustainable manner. Harvested biomass that is not replanted increases greenhouse gas emissions in two ways: Carbon dioxide emissions that had been previously stored in trees is released in the atmosphere, and future carbon fixation is stopped.
To comply with carbon reduction goals, some countries impose taxes on carbon dioxide emissions. Since biofuels have lower full-cycle carbon dioxide emissions than fossil fuels, biofuels are more cost-competitive with fossil fuels in regions where these taxes are imposed.
Another advantage to using biomass as an energy source is a possible increase in energy security for countries that import fossil fuels. More than two-thirds of the oil reserves are in the Middle East. More than half of the oil consumed in the United States is imported and oil accounts for approximately 40 percent of the trade deficit of the United Sates. A substantial biofuels program could help to the increase energy independence of importing nations and lessen the impact of an energy crisis.
There are some disadvantages with the use of biofuels as well. Some of the high-yield energy crops also have significant removal rates of nutrients from the soil. Each year the cultivation of row crops causes a loss of 2.7 million metric tons of soil organic matter in the United States. However, there are exceptions: Through the use of good farming practices, Brazilian sugarcane fields have had minimal deterioration from the repeated planting of sugar-cane. Moreover, using switchgrass and other grasses increases soil organic matter and thus can help in reducing the soil erosion caused by the cultivation of row crops. Research is being conducted into improving sustainable crop yield with a minimal of fertilizer application. Possible solutions include coplanting energy crops with nitrogen-fixing crops to maintain nitrogen levels in the soil.
It is estimated that biomass is cultivated at a rate of 220 billion dry tons per year worldwide. This is about ten times worldwide energy consumption. Advocates suggest that by 2050, better use of cultivated biomass could lead to biomass providing 38 percent of the world’s direct fuel and 17 percent of electricity generation. However, a large increase in bioenergy seems unlikely. When the U.S. Energy Information Administration (EIA) does not include any new greenhouse gas legislation into its energy utilization projections, only limited growth for renewable energy is predicted.
