A study by researchers at RWTH Aachen University in Germany has found that synthetic dimethoxymethane (DMM) is not only a promising fuel or blend component because of its outstanding combustion characteristics, but that it could be very attractive from a production point of view as well.

In an open-access paper in the RSC journal Energy & Environmental Science, the team reports its findings on the analysis of five DMM reaction pathways in terms of exergy efficiency, production cost, and climate impact.

Because the pathways have different technology readiness levels, the team developed a methodology to ensure consistent boundary conditions and model detail between pathways. The methodology enables a hierarchical optimization-based process design and evaluation.

The urgent need for introducing renewable energy into the mobility sector and the low energy density of state-of-the-art batteries call for alternative solutions to meet climate targets. Chemical energy carriers produced from renewable electricity—so called e-fuels—may contribute substantially to such a solution.

Oxymethylene ethers (OME_n) are particularly promising as they can not only be produced from carbon dioxide and renewable hydrogen. They can also drastically reduce hazardous emissions during combustion (such as nitrogen oxide and soot emissions) compared to fossil diesel. The commercial production of OME_n is however not sustainable and prevents its broad introduction into the transportation sector. Major inefficiencies are caused by the multitude of involved process steps already towards the first member of OME_n, dimethoxymethane (DMM).

To improve process performance, new catalysts have been developed enabling more direct and potentially sustainable pathways for DMM production. In order to evaluate how much these achievements in catalyst development improve sustainability—and finally estimate whether DMM can become a sustainable e-fuel—a combined techno-economic analysis (TEA) and life cycle assessment (LCA) of the these pathways is inevitable.

—Burre et al.

OME_n can be produced from renewable syngas via biomass gasification, or from renewable hydrogen and carbon dioxide, potentially achieving carbon neutrality over their entire life cycle. Their volumetric energy density (∼20 MJ L⁻¹) is about 40% lower than that of diesel, but it is similar to that of other e-fuels and about one order of magnitude higher than that of Li-ion batteries for BEV. This makes OME_n particularly suitable for long-distance and heavy-duty transportation, the researchers note.

Both OME₁ (methylal or dimethoxymethane, hereinafter referred to as DMM) and OME_3–5 offer outstanding combustion characteristics (e.g., high thermodynamic efficiency, low pollutant emissions ) but differ in production, infrastructure, and engine compatibility. Whereas OME_3–5 has more diesel-like properties and can be combusted in conventional diesel engines, DMM needs to be either mixed with additives to gain engine compatibility or blended with diesel. However, engine modifications seem to remain indispensable for both DMM and OME_3–5. In addition to the potential direct application in internal combustion engines, DMM is a key intermediate in OME_3–5 production via paraformaldehyde, trioxane, or in novel routes via gaseous formaldehyde.

—Burre et al.

The researchers examined five pathways: the established pathway of the condensation reaction of methanol and aqueous formaldehyde (FA); the direct oxidation of methanol to DMM; the direct reduction of CO₂ to DMM; the coupling of the dehydrogenation of methanol to FA with the acetalization of FA with methanol; and methanol transfer-hydrogenation.

The results showed that the non-oxidative (i.e., reductive, dehydrogenative, and transfer-hydrogenative) pathways consume stoichiometrically less hydrogen not only than the established and oxidative pathway, but also less than most other electricity-based fuels (e-fuels).

The higher resource efficiency of these pathways increases process exergy efficiency from 75% to 84%; production cost (2.1$ L_diesel-eq.⁻¹) becomes competitive to other e-fuels; and the impact on climate change reduces by up to 92% compared to fossil diesel, if renewable electricity is utilized.

With considerable catalyst improvements, a maximum exergy efficiency of 92% and minimum production cost of 2.0$ L_diesel-eq.⁻¹ are achievable.

—Burre et al.

Resources

• Jannik Burre, Dominik Bongartz, Sarah Deutz, Chalachew Mebrahtu, Ole Osterthun, Ruiyan Sun, Simon Völker, André Bardow, Jürgen Klankermayer, Regina Palkovits and Alexander Mitsos (2021) “Comparing pathways for electricity-based production of dimethoxymethane as a sustainable fuel” Energy & Environmental Science doi: 10.1039/D1EE00689D