System-Wide Perspective for Life Cycle Assessment of CO -based C1-Chemicals

In recent years, the conversion of CO2 to basic chemicals with one carbon atom (C1-chemicals) such as methane and methanol has gained increasing interest. The major motivation for the utilization of CO2 is the reduction of global warming and fossil depletion impacts. However, these reductions are not guaranteed because all C1-chemicals require hydrogen besides the abundantly available CO2. Thus, the goal of this thesis is the life cycle assessment of CO2-based C1-chemicals (methane, methanol, carbon monoxide and formic acid). The assessment is based on a system-wide perspective, which means that for limited resources such as renewable electricity also the utilization of the limited resources is in other processes is considered. First of all, the CO2-based processes are compared to fossil-based processes for C1-chemicals. Formic acid has the highest potential to reduce global warming and fossil depletion impacts followed by carbon monoxide, methanol and methane. Even if hydrogen is supplied by fossil-based steam reforming, formic acid reduces global warming and fossil depletion impacts. All other CO2-based C1-chemicals require hydrogen from electrolysis using renewable electricity. In the following, the supply of hydrogen by electrolysis is analyzed in more detail. The CO2-based processes for carbon monoxide and methane required about 60 % and 88 % renewable electricity (in 2020 in the EU-27) to reduce global warming impacts compared to the fossil-based processes. If 100 % renewable electricity is used, all CO2-based C1-chemicals reduce global warming and fossil depletion impacts compared to the fossil-based processes. For the assessment of these reductions, also alternative utilization options for renewable electricity (Power-to-X) are analyzed such as electricity storage systems, battery electric vehicles and heat pumps. The highest reductions per electricity used are achieved for heat pumps followed by battery electric vehicles and electricity storage systems. Then, the CO2-based C1-chemicals follow. Since renewable electricity is used more efficiently outside the chemical industry, also biomass-based methane and methanol are analyzed. The utilization of biomass achieves the highest reductions if coal-fired power plants are substituted followed by the production of methanol. For methanol and methane, the yield per biomass can be increased if additional hydrogen is used.