The deployment of low-emission alternative fuels is crucial to decarbonise the transport sector. A number of alternatives like hydrogen or dimethyl ether/methanol synthesised using CO₂ as feedstock for fuel production (hereafter refer to “CO₂-based fuels”) have been proposed to combat climate change. However, the decarbonisation potential of CO₂-based fuels is under debate because CO₂ is re-emitted to the atmosphere when the fuel is combusted; and the majority of hydrogen still relies on fossil resources, which makes its prospects of being a low-carbon fuel dependent on its manufacturing process. First, this paper investigates the relative economic and environmental performance of hydrogen (produced from conventional steam methane reforming and produced via electrolysis using renewable energy), and CO₂-based fuels (dimethyl ether and methanol), considering the full carbon cycle. The results reveal that hydrogen produced from steam methane reforming is the most economical option and that hydrogen produced via electrolysis using renewables has the best environmental profile. Whereas the idea of CO₂-based fuels has recently gained much interest, it has for the foreseeable future rather limited practical relevance since there is no favourable combination of cost and environmental performance. This will only change in the long run and requires that CO₂ is of non-fossil origin, i.e. from biomass combustion or captured from air. Second, this paper address unresolved methodological issues in the assessment of CO₂-based fuels, such as the possible allocation of emissions to the different sectors involved. The outcomes indicate that implementing different allocation approaches substantially influences the carbon footprint of CO₂-based fuels. To avoid allocation issues, expanding the boundaries including the entire system and is therefore recommended.

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CO 2 utilization is increasingly considered a greenhouse gas abatement strategy alternatively to CO 2 storage. Existing indicators that assess the performance of CO 2 utilization options often provide an incomplete perspective and are unsuitable to compare different utilization options with different functionality (e.g. plastics and fuels). This study introduces a new performance indicator for CO 2 utilization options: Specific Primary Energy Consumption per unit of Fossil feedstock Replaced (SPECFER). This indicator, expressed in MJ/MJ, provides a proxy for the energy efficiency of which CO 2 conversion options can replace fossil feedstock required in conventional processes. Three CO 2 utilization case studies (CO 2 based methanol, polyols and dimethyl ether) are used to show the application and effectiveness of the SPECFER indicator. Among the case studies, only CO 2 conversion into polyol appears particularly efficient (SPECFER of 0.05 MJ/MJ), while the other options are not (SPECFER of > 1 MJ/MJ). The paper shows that the SPECFER indicator adds key insights compared to conventi onal indicators to the effectiveness of CO 2 utilization options and is a promising indicator complementary to CO 2 emissions reduction or life cycle greenhouse gas reduction potential. The SPECFER thus improves the understanding of the performance of CO 2 utilization and enables the possibility to distinctly compare different CO 2 converting utilization technologies.@en

This paper assesses whether a cleaner and more sustainable production of the chemical building blocks methyl crotonate (MC) and methyl acrylate (MA) can be obtained in an innovative process in which resource consumption, waste generation and environmental impacts are minimized by using polyhydroxybutyrate (PHB) produced from wastewater as feedstock. For this purpose, conceptual process design, process modelling, economic and carbon footprint assessments of five conversion alternatives for wastewater-based PHB to MC or MA are performed. The PHB conversion step is modelled based on experimental data for both intracellular and extracellular feedstock. Results show that, despite the lower reaction selectivity of the direct conversion of intracellular PHB to MC or MA, this route is economically and environmentally preferred because no intensive downstream process is required for cell release after wastewater fermentation. The lowest total production costs are achieved when dry intracellular PHB is used as feedstock: 1.31 /kg MC and 2.89 /kg MA. However, the use of aqueous PHB as starting material leads to minimal carbon footprint due to lower energy demand: 3.25 kg CO₂-eq/kg MC and 8.78 kg CO₂-eq/kg MA, respectively. A sensitivity analysis is conducted to evaluate the eco-efficiency of the PHB conversion routes when the co-products methyl 3-hydroxubutyrate, crotonic acid and propylene are sold.

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