ABSTRACT

ABST R AC T Traditionally, energy processing facilities have used a number of metrics and indices to assess process sustainability. Among these indicators is the ratio of output energy of the facility to the input energy. With the growth of the biofuels industry and the increasing emphasis upon integrating mass and energy resources associated with the life cycle analyses of these facilities, it is desirable to “close the loop” for energy usage (and other resources such as water, nutrients and air emissions) of a biofuel product. As the infrastructure and process economics associated with biofuels evolve, it is possible to reduce (or eliminate) substantially the use of fossil fuels in the life cycle of a biofuel product. Sufcient amounts of a biofuel from a given facility can provide the energy needed for the process and for the other life-cycle activities (e.g., transportation) and sell the excess quantity as the net product. This poses a dilemma for the denition of energy efciency (output energy of the process/input energy to the process) as it becomes innite when the material balance loop is closed. The paper addresses this issue and introduces two new concepts. The rst one is to involve the mass ows of the feedstocks as energy carriers and identify energy equivalents.