Thermal Processing OME Synthesis: A New Route to Diesel Fuels?

From Dominik Stephan* |

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Power-to-X is on everyone’s lips: as energy storage, alternatives to battery-electric mobility or basic raw materials for industrial processes, the conversion of green electricity into material form is a cornerstone of all defossilization strategies. But how realistic is the vision of fuel from the socket? A test plant in Lower Bavaria, Germany, shows that the necessary technologies have long been in the starting blocks.

OME would provide an alternative to diesel that not only burns cleaner, but can also be produced sustainably. Suitable processes already exist, as shown by a project at the Straubing campus in Germany.
OME would provide an alternative to diesel that not only burns cleaner, but can also be produced sustainably. Suitable processes already exist, as shown by a project at the Straubing campus in Germany.
(Source: Jan Troeschel/TUM)

It doesn’t take much to turn sunlight into visions: Tomorrow’s value chain is supposed to be climate-neutral, at any rate sustainable, and preferably without fossil raw materials. But the technology that promises prosperity for all without environmental pollution will still be a long time coming. Today, no one disputes that it is possible in principle to produce fuels and basic chemicals from renewable energy and waste gases. However, these processes also have to be economic. And so institutes, companies and politicians seek the ‘philosopher’s stone’ in chemical synthesis.

“The processes we need are actually already prepared,” says someone who should know: Jakob Burger, Professor of Chemical and Thermal Process Engineering at the Technical University of Munich (TUM), is researching the direct synthesis of artificial fuels with his team at the TUM campus in Straubing in Lower Bavaria, Germany. The means for this is a reactor about the size of a table, plus distillation columns several metres high and a downstream membrane unit. A typical pilot plant set-up — but one that packs a punch.


In Straubing, oxymethylene ether (OME) is produced from methanol and formaldehyde in a multi-stage process — a synthetic mixture that, as an alternative to diesel fuels, could help to make the much-criticized compression ignition engine emission-neutral. In the reactor, Prof. Burger’s team reacts methanol molecules on an acid catalyst to form dimethyl ether. Formaldehyde is then added to create OME. In practice, however, some effort is required to obtain the desired molecules with the right chain length (n = 3 or 5): “You can imagine it as similar to petroleum: The mixture that comes out of the reactor consists of different components that are present in different boiling fractions. We are interested in the medium-boiling one,” Burger explains.

Enter the two distillation columns, which are up to 10 metres high: This equipment, designed, manufactured and supplied by the Iludest company from Waldbüttelbrunn near Würzburg, Germany, is used to separate the lighter and heavier boiling fractions. The company, which jokingly describes itself as an “engineering office with attached manufacturing”, is a specialist in custom plants, as Udo Interwies, Managing Director Engineering, explains: “Distillation is 1,000 years old. The challenge is to implement it on a small scale and at low cost.” The process requires high-pressure and low-pressure distillation equipment, made of metal and glass respectively, that is integrated into the overall process. This is not always easy, says Interwies: “Pilot-scale projects require instrumentation and control technology just like a large-scale plant. The challenge is to make the technology safe, but at the same time simple and practicable.”

What’s behind this synthetic diesel fuel?
OME in focus

OME, actually (poly)oxymethylene dimethyl ether, is not a pure substance, but a series of organic compounds with the formula CH3-O-(CH2-O)n-CH3. The colorless and flammable liquids have so far only been obtained industrially from coal gasification in China. But this could change: OME with a chain length of n = 3 or n = 5 can be used as an alternative to conventional diesel fuel made from petroleum. Not only could the carbon-hydrogen-oxygen compound be obtained from carbon dioxide or biogenic carbon and water using electrical energy, but the substance also burns much cleaner than conventional fuel. Conventional diesel fuel has a trade-off between low NOx and low soot emissions. OME, on the other hand, has no C-C bond in the molecule, which means that these effects can be decoupled. However, the high binding enthalpy of the carbon-carbon bond cannot be utilized in OME, which is why its calorific value is significantly lower than that of conventional diesel fuel.

What is Possible Today?

In addition, there is the membrane module supplied by DBI from Leipzig, Germany, in which the lighter-boiling fraction is separated from the water produced during ether synthesis. The result is clean OME, which can be used today as a diesel additive. “With a production capacity of about one kilo per hour, the plant is not yet on an industrial scale, but it proves that it is feasible in principle,” says Burger: “No one has built this process before: We are opening up a new route via a process that takes two upstream processes out of the value chain.” The researchers are not alone in this: the construction of the pilot plant is part of the Namosyn (“sustainable mobility through synthetic fuels”) project initiated by the German Federal Ministry of Education and Research with a budget of 24 million dollars.

Yet neither Namosyn nor the Technical University of Munich are the only ones working on OME synthesis. Especially in China, where oxymethylene ether has been known for a long time from coal chemistry, resourceful plant engineers have developed corresponding technologies and built plants with capacities of up to 50,000 tons per year — considerably more than the Lower Bavarian pilot plant production. In fact, explains Prof. Burger, Chinese OME would even have to be used in Europe for application trials with car manufacturers, as it would be available in corresponding quantities. Nevertheless, competition from the Middle Kingdom does not scare the German team: the Straubing process is not only simpler than most multi-stage processes used by its competitors, but also saves energy — a weighty argument for power-to-X processes.

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Critics counter that the unavoidable conversion losses stand in the way of efficient use of the scarce supply of green energy. However, although Germany’s share of renewably generated electricity is now gratifyingly high at almost 50 percent, measured in terms of total primary energy production it it is only about 15 percent. In short, despite declining primary energy demand, the amount of green electricity currently generated in Germany is hardly sufficient even for purely electric vehicles, if all other sectors are to become emissions-neutral at the same time.

With its globally unique process, the Namosyn project has an ace up its sleeve, Burger is sure: “We are proposing a more effective route to a new fuel that cannot yet be produced on a large scale,” the scientist explains. But he also warns: “We have to demonstrate feasibility now, otherwise we will lose our lead.”

Does OME Have a Chance?

All the technologies are already there. It would by no means be impossible to make a compression-ignition engine emission-neutral with climate-friendly synthetic fuel. So why do we still need naphtha for diesel, gasoline and the like? Prof. Burger knows that the stumbling block of all synthetic fuels has always been their economic viability: “Compared to conventional technologies there is a price gap that must be closed. To do so, we have to subsidize power-to-X products or make fossil fuels more expensive, which in the end amounts to the same thing.” Whether such technologies have a chance on a large scale will therefore depend on the framework conditions in which they operate. Cheap electricity from renewable or at least emission-neutral sources in hitherto unattained quantities is the essential basic prerequisite, since it is not just the transport sector and the chemical industry that have to end their dependence on fossil fuels.

Stefan Opis, Managing Director of Marketing and Sales at Iludest, says: “In this problem area, we are observing an increase in the activities and enquiries that are being addressed to us.” The plant manufacturer is not daunted by the challenges of synthetic fuels: “We already know the requirements from our plants that process ‘simple’ crude oil: With high-boiling fractions, substances start to solidify — so you have to guarantee continuous temperature control. Sometimes there are sublimations, and if you are not careful, solids form in the plant,” says Opis.

“When you work with electricity-based fuels, you always have conversion losses — and that inevitably affects the finances,” Burger explains. “Because of that, you would need a higher CO2 tax than would be necessary to replace coal with wind power, for example.” So climate tariffs could create the conditions for alternative fuels to break even, but would of course have social consequences that need to be considered. The question is whether there are real alternatives to this.

Those who already expect power-to-X to be economically viable without subsidies or new laws will inevitably be disappointed. But that should not prevent us from taking this path one step at a time. The signposts are clear to see — and not just in the Bavarian city of Straubing.

* * The author is an editor of PROCESS. Contact to TUM/Campus Straubing: Namosyn: Iludest: