Life cycle optimization of energy-intensive processes using eco-costs

Purpose

This study provides a general methodology to integrate LCA into a single- or multi-objective process design optimization context. It uses specific weightings for foreground emissions, for preventable background emissions and for unpreventable background emissions, for each impact category. It is illustrated for a natural gas combined cycle power plant with three scenarios to reduce its carbon dioxide emissions: CO₂ capture and sequestration, fuel substitution with biogas or fuel substitution with synthetic gas from wood.

Methods

Assuming that the opportunity to prevent emissions elsewhere is an implicit part of the process design decision space, the optimal solution cannot waste such opportunities and is shown to minimize total life cycle costs, including emission avoidance costs based on the optimal combination of prevention and compensation measures in the background system. In the case study, background emissions are inventoried from the ecoinvent database, their compensation costs are derived from the Ecocosts 2007 impact assessment method and their prevention costs are estimated from the literature. The calculated avoidance costs (weightings) then show how the background system affects the final choice of CO₂ reduction scenario.

Results and discussion

In the case study, all three options partially shift environmental burdens to the background system, which can be prevented or compensated. The corresponding minimum avoidance cost is highest overall for the biogas option, thus putting it at a disadvantage. For a vast majority of ecoinvent processes, energy efficiency is important to minimize total avoidance costs because they are dominated by background CO₂. Furthermore, prevention cost data gathering can be simplified in some cases, without distorting design decisions, using a CO₂-only background inventory. The non-CO₂ background inventory is more useful after process design, for procurement decisions.

Conclusions

Over-investing in design modifications cannot achieve the same background impact reductions as a sensible green procurement policy. Thus, the proposed weighting methodology ensures that all types of design decisions integrate LCA without incorrectly assuming that emissions are necessarily unavoidable when in the background. Within a context of future emission taxes or tradable permits, the weightings can also anticipate the after-tax cost passed on by suppliers—a marketable benefit of LCA.

Recommendations

Since many LCA studies are equivalent to design optimization problems, the proposed weighting methodology provides a single-score impact method relevant to decision-making as well as a straightforward approach to LCA interpretation in terms of detailing the optimal combination of applicable design modifications, prevention measures and compensation measures.