Current limits of life cycle assessment framework in evaluating environmental sustainability – case of two evolving biofuel technologies

The growing need to use biofuel raw materials that do not compete with food and feed have resulted in a growing interest in lignocellulosic materials and microalgae. However, the life cycle environmental benefits of both biofuels have been questioned. The aim of this study was to evaluate how environmental sustainability of forest-based and microalgae biodiesel can be estimated by using the life cycle assessment framework. These biofuel chains were chosen because they are contrasting systems, as the first one is based on a “natural” feedstock production system, while the second one is an entirely anthropogenic system using an artificial infrastructure and external inputs to grow microalgae. This study focuses on life cycle impact categories still under methodological development, namely resource depletion, land use and land use change, water use, soil quality impacts and biodiversity. In addition, climate impacts were quantified in order to exemplify the uncertainty of the results and the complexity of estimating the parameters. This study demonstrates the difficulty to assess the absolute range of the total environmental impacts of the two systems. The results propose that the greenhouse gas emissions of microalgae biodiesel are higher than those of forest residue-based biodiesel, but the results of the microalgae chain are very uncertain due to the early development stage of the technology, and due to assumptions made concerning the electricity mix. On the other hand, the microalgae system has other advantages such as low competition on productive land and low biodiversity impacts. The findings help to recognise the main characteristics of the two production chains, and the main remaining research issues on bioenergy assessment along with the methodological development needs of life cycle approaches.