Hierarchical thermodynamic metrics for evaluating the environmental sustainability of industrial processes

Industrial progress toward sustainability requires meaningful, practical, and scientifically sound metrics. Most existing metrics rely on information about material and energy inputs and emissions from the main process and selected processes in its life cycle. Such metrics often result in multiple conflicting variables, making it difficult to use them for decision making. Furthermore, sustainability metrics need to be scientifically rigorous and capable of evaluating the broader economy and ecosystem scale impacts of selected processes and products. This paper proposes a framework for evaluating the environmental sustainability of industrial processes that satisfies these needs. This framework uses exergy analysis to combine different types of material and energy streams in a thermodynamically sound manner. Exergy analysis is also combined with end‐point life‐cycle impact assessment methods for evaluating the impact of emissions. This results in metrics for a selected system with different levels of aggregation ranging from multiple to single dimensions. The challenge of analyzing a process at life cycle and coarser spatial scales is met by combining exergy analysis, life cycle assessment, input–output analysis, and both economic and ecological aspects. The result is a doubly nested hierarchy, which analyzes processes at multiple spatial scales of process, life cycle, economy, and ecosystem. Each scale contains another hierarchy based on the degree of aggregation of the metrics. A case study of the ammonia process illustrates the characteristics of the proposed approach. © 2004 American Institute of Chemical Engineers Environ Prog, 2004