Sugar-to-What? An Environmental Merit Order Curve for Biobased Chemicals and Plastics

Abstract

The chemical industry aims to reduce its greenhouse gas emissions (GHGs) by adopting biomass as a renewable carbon feedstock. However, biomass is a limited resource. Thus, biomass should preferentially be used in processes that most reduce GHG emissions. However, a lack of harmonization in current life cycle assessment (LCA) literature makes the identification of efficient processes difficult. In this study, 46 fermentation processes from literature are harmonized and analyzed on the basis of their GHG reduction compared with fossil benchmarks. The GHG reduction per amount of sugar used is defined as Sugar-to-X efficiency and used as a performance metric in the following. The analyzed processes span a wide range of Sugar-to-X efficiencies from -3.3 to 6.7 kg of CO₂ equiv per kg of sugar input. Diverting sugar from bioethanol production for fuels to the fermentation and bioconversion processes with the highest Sugar-to-X efficiency could reduce the chemical industry's GHG emissions by an additional 130 MT of CO₂ equiv without requiring any more biobased feedstocks.

Figure 1

System boundaries and functional unit used for the LCA. The functional unit is 1 kg of sugar used by the biobased process. The output of the fossil system is set to be equivalent to the output of the biobased system. If the biobased process produces more than one output, system expansion is used. The system boundaries are cradle to grave, assuming eventual release of all carbon as CO₂ at end-of-life and equal impacts for equivalent bio- and fossil-based products during the use phase.

Figure 2

Merit order curve for the S2X efficiency of biobased processes and value chains. The legend shows the fossil-based chemicals substituted by the biobased chemicals. The fossil-based chemical is not always substituted by a chemically identical biobased chemical. The dashed lines show the position of the zero lines if the feedstocks corn stover, sugar cane, or corn + iLUC would be considered instead of corn. Since all processes are normalized to 1 kg of sugar input, a change in feedstock would result in a constant offset for all products. Processes are labeled following the scheme: product, prefix, first author, year. The prefix is added if one author proposes multiple processes for the same product and is being kept close to the naming conventions of the original publication (Table 1). Tabulated results are available in the Supporting Information.

Figure 3

Relation between oxygen content of the substituted product and the S2X efficiency. The dashed line indicates the Pareto front of S2X efficiency and oxygen content from this study. However, the line does not reflect a fundamental law, and points above the line could exist even if they were not identified in this study. The oxygen content of petrol is assumed to be 2%.

Figure 4

Additional reduction in GHG emissions due to substituting bioethanol fuel production with biobased chemical production. Products with high S2X efficiency are assumed to substitute ethanol first until their current production volume is reached.