Comparative well-to-tank life cycle assessment of methanol, dimethyl ether, and oxymethylene dimethyl ethers via the power-to-liquid pathway

Abstract

Energy systems are currently transforming into configurations with a significant share of renewable energies to reduce greenhouse gas (GHG) emissions. One frequently discussed option to reduce CO2 and simultaneously provide temporary storage for fluctuating renewable energy is coupling the energy and the transport sector in a Power-to-Liquid (PtL) approach by producing synthetic fuels. The syntheses for methanol, 1-step and 2-step dimethyl ether and oxymethylene dimethyl ethers (OME3–5) are modeled using captured CO2 and hydrogen. Hydrogen is produced by electrolysis using German electricity mixes (2021, 2030) and wind power. To evaluate the environmental impacts of the different PtL pathways, a comparative well-to-tank Life Cycle Assessment (LCA) is conducted. Aspen process simulations are performed to provide material and energy balances for the LCA. The results show that the production of all PtL fuels is highly emission intensive, when using the current electricity mix and is – from an environmental point of view – not a viable option today. The GHG emissions are estimated to be between 9.3 – 16 kg CO2 equivalents/l diesel equivalent which is five to eight times that of a conventional diesel system. But already the electricity mix 2030 can drastically reduce the GHG emissions to 2.5 – 6 CO2 equivalents/l diesel equivalent. Wind will decrease these values further to 0.6 – 3.6 CO2 equivalents/l diesel equivalent. Dimethyl ether and methanol generate considerably lower impacts than OME3–5 as their production is less energy intensive. The conventional 2-step dimethyl ether synthesis and the more innovative 1-step process hardly differ.