Abstract
Current liquid-fuel supplies in the United States are derived primarily from relatively inexpensive fossil fuels. The low cost and widespread availability of petroleum has, over the last 150 years, facilitated enormous growth in the U.S. and global economies, in their respective human populations and resource consumption, and in their attendant impacts on ecosystems worldwide. Concerns are growing, however, as to whether alternative fuel sources can fill the void left by future declines in the supply of this inexpensive and energy-rich resource, concepts expressed in phrases such as “Peak Oil,” “end of cheap oil,” and “second half of the age of oil.”
A key concept in the assessment of current and future fuel supplies is the Energy Return on Investment (EROI), the amount of energy used in the extraction and processing of a fuel source divided by the energy gained from these activities. The EROI of petroleum was as high as 80:1 — 100:1 in the first third of the 20th century. However, both the quantity of oil remaining to be exploited and its EROI are declining to from 10:1 to 20:1. Biomass is being promoted as a renewable feedstock that can be used to produce liquid fuels as a domestic alternative to current petroleum-based liquid fuels, which are derived mostly and increasingly from imported sources. However, our summary shows that the EROI of liquid fuels from different alternative sources varies from less than 1:1 to about 10:1, far less than even present day petroleum. In addition, the potential supply from these sources is limited and is projected to replace no more than 30% of current petroleum use in the U.S. by 2030 under very optimistic assumptions. These lower grade resources are expected to have greater impacts on wildlife per unit delivered to society than present fuels, primarily due to the extensive nature of the resource and the potential conversion of forested lands to agriculture. While the production of large amounts of biomass feedstocks has the potential to add a modest amount of badly needed liquid fuel to our national supplies, there are sustainability, biodiversity, climate change, and water resource issues that need to be addressed in order to ensure that these resources are used as effectively as possible and with the least negative impact. Here we review the literature on several major potential biomass-based liquid fuels for use in the U.S.: corn ethanol, sugar-cane ethanol, lignocellulosic ethanol, and vegetable oil biodiesel. For each, we summarize the fuel’s EROI, potential magnitude, and potential impacts on the environment and wildlife.