Di Methyl Ether (DME)

di-methyl-ether (CH₃-O-CH₃) has similar properties to ethanol for fuel. If we were to create a fuel for transport from the CO₂ in the atmosphere, then the total amount of CO₂ in the atmosphere would not increase.

One very good feature of DME is that is has no carbon-carbon bonds, so it will burn cleanly, with no particulates and no sulphur or nitrous oxides or soot.

DME is currently made using several methods:

• Indirect method that produces DME by dehydration reaction of methanol
• Direct method that synthesizes DME directly from “syngas” (gas mainly composed of H₂ and CO) which is produced from natural gas, coal, and biomass as raw materials.
• Catalytic method – there is also at least one method of making DME directly from H₂ and CO₂ using a catalyst – summary of the process is further down this page.

Most DME is currently produced by the Indirect method, from methanol. The problem is that the methanol is usually made from SynGas, which in turn is normally made from coal or methane – i.e. from fossil fuels.

Exon Mobil produce DME from methanol as the first step in their “Methanol to Gasoline” (MTG) process, which historically they have used to produce “gasoline” from methane (in New Zealand) or from coal (a process licensed to China), both of which are classed as “synthetic fuels”.

If the syngas is made from biomass then that is better, though typically syngas from biomass has oxides of nitrogen and sulphur in it as well, which are not wanted when synthesising a liquid fuel.

A group in Japan has worked on the technical development of the Direct method, creating DME from syngas, expecting its higher efficiency as it would not require the production of the interim methanol – but if this is ultimately just a way of burning carbon, it is not good.

We really want to extract CO₂ from the atmosphere, not to “burn” carbon to produce DME.

If we have biomass (eg waste food) which would otherwise produce methane, it might be beneficial to produce DME from that methane instead. Methane is about 84 times worse than CO₂ for global warming. However its half-life, and therefore how long it lasts in the atmosphere is about 10 years, which is much lower than that of CO₂, which is about 100 years. This means that over a 100 year time frame, the Global Warming Potential of Methane is 28 times worse than CO₂. Therefore, crazy as it might seem, it is better to burn methane, or convert it to H₂ and CO₂, than to let it escape into the atmosphere. 

For CO₂ capture we either need to pursue a way to produce Methanol from atmospheric CO₂ – which is possible, or we need to pursue the Catalytic method of producing DME from CO₂ and H₂.

In the UK, if BP could use all their chemical engineering ability to concentrate on making this process an industrial reality, it would be really beneficial.

Given its similarity in terms of energy density and physical properties to ethanol and LPG, designing internal combustion engines to use DME, should not be a particularly difficult task.

Obvious question:

What would the effect of DME be if it escaped into the atmosphere – would it have more of a warming effect than the CO₂ we are extracting?

Thankfully the answer is no – if DME were to escape or spill, it would break down in the atmosphere to CO₂ and water within a few days, so it would be no worse than the CO₂ which had been used to create it in the first place.

The energy density in kWh per kg for various fuels can be found in Wikipedia. Some numbers for comparison are: