Short, general public summary

How can agricultural and forestry residues help reduce industry’s climate impact? This thesis explored ways to turn these resources into chemicals and materials in France. It developed SCREEN-LCA, a method to identify promising bio-based products. The study identified ethylene and propylene, two key ingredients for plastics, as promising targets for replacement. Two ways of producing them from wheat straw were investigated: fermentation and gasification. Life cycle assessment showed that both routes can reduce climate impacts compared with fossil-based production. However, they also involve trade-offs, with potential increases in other environmental impacts. Finally, the study compared chemical production with a simpler use of straw: insulating buildings with straw bales. The results showed that using straw directly can offer even greater climate benefits than converting it into chemicals. More complex processing is not always the best environmental choice.

More extensive summary

The petrochemical and manufacturing industries rely heavily on fossil fuels and contribute substantially to climate change. The bioeconomy uses biomass as an alternative feedstock. Agricultural and forestry residues can be converted into bio-based products without dedicated cultivation, but are limited and face competing demand. This avoids additional cultivation of biomass, but residues are inherently limited and face competing demand. Life cycle assessment (LCA) evaluates the environmental impacts and trade-offs of bio-based production. Yet, consistent LCA models for comparing residual-biomass pathways remain scarce. Robust and transparent assessments are time- and data-intensive, hindering decision-making when the urgency of climate change necessitates rapid changes.

Therefore, the main research questions of this thesis are: Which pathways from residual biomass to bio-based products (chemicals and materials) can lead to high climate change mitigation in France? Among them, what are the process conditions and environmental trade-offs, also considering uncertainties?

To address the first question, the thesis proposes a novel approach to LCA case study selection named “SCREEN-LCA”. It brings a stepwise, goal-oriented and reproducible screening framework to identify high-carbon-footprint products (hotspots) and their practical bio-based alternatives. The screening identified ethylene and propylene, two major plastic monomers, in addition to PLA when it can PET. This work also resulted in a database of alternative bio-based pathways to fifty hotspots and a set of tools for estimating the footprint of products at a large scale.

These selected case studies were modelled and assessed via consequential LCA to address the second question. Two pathways for producing ethylene from wheat straw were investigated: fermentation and thermochemical conversion. Fermentation produces ethylene via lignocellulosic ethanol, while thermochemical conversion yields ethylene and propylene via gasification and methanol-to-olefins. Both models cover emissions and processes from harvest to final products, including co-product handling. Both pathways yield ethylene with lower climate impacts than fossil production, but differ in mitigation potential and other environmental trade-offs. Model sensitivity and uncertainties are documented and the Monte Carlo analysis shows robust conclusions over most of the environmental impacts. The study extends previous work through consequential LCA, updated data and transparent, mass-balanced, parameterized process models.

The last part of this thesis takes a step back by looking at contrasting uses of straw for bio-based products. It compares the conversion of straw to chemicals with a direct use of straw bales as building insulation materials. Straw-bale insulation requires little processing. Minimal processing of straw offers significantly greater climate mitigation potential than its conversion into chemicals. This comparison challenges conventional approaches to biomass valorization by highlighting the benefits of low-tech alternatives.

Future work could extend SCREEN-LCA to multiple objectives and incorporate economic and social dimensions into sustainability assessments. Further scenarios could cover PLA production and other materials, as well as alternative process options such as electrolysis and pretreatment.