Syngas from What? Comparative Life-Cycle Assessment for Syngas Production from Biomass, CO₂, and Steel Mill Off-Gases

Syngas is an essential platform chemical for producing chemicals and fuels, whose market volume is expected to grow strongly. Today, syngas is mainly produced from fossil resources and thus emits large amounts of greenhouse gases (GHGs). The GHG emissions of syngas can be reduced by alternative feedstocks such as biomass, CO2, and steel mill off-gases (mill gas). However, since previous studies on alternative feedstocks differ in assumptions and methodological choices, their results are not comparable. Therefore, it remains unknown which feedstock is preferable from an environmental perspective. Herein, we present a consistent environmental assessment of alternative syngas pathways. We show that bio-based and mill gas-based syngas reduce GHG emissions the most, although the results strongly depend on the conventional feedstock use: if the feedstock is limited and already used elsewhere, its GHG mitigation potential decreases. Furthermore, producing syngas from mill gas or bio-waste leads to moderate environmental trade-offs, while other bio-feedstocks and CO2 can significantly increase other environmental impacts than GHG. Our results demonstrate that a consistent assessment of alternative pathways is required to make informed decisions on syngas decarbonization and highlight the importance of considering the conventional use of limited feedstocks in life-cycle assessments.