Comparative LCA of multi-product processes with non-common products: a systematic approach applied to chlorine electrolysis technologies

Multi-product processes are one source of multi-functionality causing widely discussed methodological problems within life cycle assessment. A multi-functionality problem exists for comparative life cycle assessment (LCA) of multi-product processes with non-common products. This work develops a systematic workflow for fixing the multi-functionality problem caused by the non-common products. A novel technology for chlor-alkali electrolysis is analyzed and compared to the industrial standard technology to illustrate the approach and to benchmark the new technology's environmental impact.

A matrix-based workflow for comparative LCA of multi-product systems is presented. Products are distinguished in main products and by-products based on the reason of process operation. We argue that only main products form the reference flows of the compared multi-product systems. Fixing the multi-functionality problem follows directly from the chosen reference flows. The framework suggests system expansion to fix the multi-functionality problem if non-common main products exist. Non-common by-products still cause a multi-functionality problem. These by-products are systematically identified and the multi-functionality problem is fixed with avoided burden and allocation. A case study applies the workflow for comparing environmental impacts of the standard chlorine electrolysis to a novel process using oxygen-depolarized cathodes. Three scenarios are derived and evaluated. The assessed impact categories are cumulative energy demand, global warming potential, acidification potential, photochemical ozone creation potential, eutrophication potential, and human toxicity potential.

The proposed workflow minimizes the methodological choices. The multi-functionality problem is systematically fixed based on the distinction between the main products and by-products. Inconsistent solutions are prevented by rigorous identification of unequal by-products within the compared systems. Selecting avoided burden processes or allocation factors is the remaining ambiguous choice common to the standard methods. The case study demonstrates the applicability of the workflow to comparative LCA of multi-product systems. The case study results show lower environmental impacts for the novel electrolysis technology in all practically relevant scenarios and impact categories.

The framework for comparative LCA of multi-product systems with non-common products adds systematic clarity to the general ISO standards. The approach reduces the subjective choices of LCA practitioners to the identification of reason of process operation. This reason is defined if the site-specific economic conditions are known. The matrix-based formulation allows identification of inconsistencies caused by multi-functionality. For the novel electrolysis technology, the results indicate significant potential for environmental impact reduction.