The proposed approach enhances the metrics described in Schwarz et al. (2002). Exergy is used for reducing dimensionality on the input side of Figure 1 by combining material and energy streams in a theoretically rigorous manner.

Figure 1. Sustainability metrics of AIChE-CWRT (Schwarz et al., 2002).

This combination is possible because the utility of any material or energy stream is in its ability to do work, and exergy represents this useful part of any material or energy stream. On the output side, the dimensionality may be reduced in two ways. The exergy of all the output streams can be calculated, as for the inputs, and may roughly correspond to the impact of the outputs on the environment (Ayres et al., 1998; Seager and Theis, 2002). Unfortunately, the relationship between the exergy of emissions and their impact is often tenuous. Consequently, if end-point impact assessment methods are available (Bare et al., 2000), then the impact of emissions may be represented in terms of exergy loss of the impacted system, or its ability to do useful work. Finally, the input and output side results may be combined to yield a single aggregate metric. This approach results in a hierarchy of metrics at different levels of aggregation as shown in Figure 2.

Figure 2. Hierarchical structure of sustainability metrics for a selected system.

Information for developing the aggregation hierarchy at the process scale may be readily obtained from mass and energy balances and cost information about the process from simulation or literature sources. Developing the aggregation hierarchy for the process scale would permit a conventional thermodynamic analysis, and inclusion of the impact of emissions allows consideration of some broader life cycle aspects.

Expanding the analysis to the life cycle scale involves selection of the most important processes in the life cycle. This approach is analogous to that used for process LCA, and may utilize the extensive life cycle inventory databases included in various software packages.

The coarser economy scale considers activities in the entire economy to satisfy the requirements of the processes selected in the life cycle scale. This analysis relies on combining economic input-output LCA (EIO-LCA) with process LCA, resulting in an approach analogous to a tiered hybrid LCA (Suh et al., 2004).

The ecosystem scale expands the analysis boundary to also account for the contribution of ecological goods and services. These inputs of natural capital form the basis of all economic activity, and ignoring them may result in transferring impacts to erosion of natural capital (Ekins et al., 2003). Determining the ecological cumulative exergy consumption throughout the life cycle for resources and impact of emissions is facilitated via the ratios of ECEC to money (Ukidwe and Bakshi, 2004).

The system boundary for sustainability metrics is expanded using process-based life cycle analysis. The expanded system includes a refinery process for the generation of natural gas from crude oil and a power plant for the generation of electric power from coal. The flow information for mass and exergy is based on data from Maple (2000) and Taftan Data (www.taftan.com). Such information could also be obtained from commercial life cycle inventory databases. Natural gas is produced in the refinery from crude oil, and is fed to the ammonia plant as raw material. Electric power for the ammonia plant is supplied from the power plant where coal is consumed to generate electricity. Figure 3 shows input and output flows of material and exergy for the process-based life cycle scale.

Figure 3. Mass and exergy flow for an ammonia plant and selected processes in its supply chain

From Figure 3, sustainability metrics for individual processes of the power plant, refinery and ammonia plant are shown at the process scale in Figure 4.

Figure 4. Figure 7. Multiscale hierarchy of sustainability metrics for ammonia process. The values of metrics are normalized by the production of ammonia.

The detailed calculation procedures can be found in Spreasheets file