It is further one of the most important oxidation products of natural gas, and represents (along with methanol and formic acid) one of the main reasons one cannot recover water from burning natural gas. Formaldehyde is also a common product in the decomposition of certain types of styrofoam, and many types of insulation.
I note that the decomposition of methane released into the atmosphere - although the process has a half-life of decades - involves formaldehyde intermediates.
All primary alcohols decompose into formaldehyde intermediates when they are burned. The main difference between ethanol and methanol in their pollution profiles is that, lacking a carbon-carbon bond, methanol has almost zero tendency to produce soot. Ethanol
does produce soot, although generally not as much as, say, diesel fuel.
Many other products give formaldehyde as a combustion product. The combustion of
wood, as you would expect from the structure of sugar, gives off formaldehyde. (In fact the historical method of producing methanol, which is also known as "wood alcohol" was the destructive distillation of wood under a vacuum. This same process takes place when wood is burning, and so it follows that there would naturally be some formaldehyde produced.) Thus DME has no advantage over natural gas
unless it is made from carbon dioxide or biomass.
In
combustion properties, it is difficult to imagine a liquid fuel with better burning properties than methanol. The big problem is its water toxicity, although I note that ethanol is very toxic to certain types of creatures, notably fish.
If methanol is transformed into DME, almost all of the toxicity issues go away completely. DME is
for practical purposes non-toxic.
An interesting review article covering most of what I say here can be found in Good, D.A. and Francisco, J.S., Chem. Rev. 2003, 103, 4999-5023. The article is entitled "Atmospheric Chemistry of Alternative Fuels and Alternative Chlorofluorocarbons." It's a good read.
I'll quote a few bits from that article relevant to DME, merely noting that with respect to
cost, the article was written when the price of oil was around $30/barrel.
3.3. Dimethyl Ether.
Dimethyl ether (DME) is a colorless, nearly odorless, gas at room temperature and atmospheric pressure. DME is quite stable and reacts or decomposes only at extreme conditions. Currently, 150,000 metric tons of DME is produced per year. The DME produced is mostly used as a propellant in aerosol cans. In 1986, a patent described a method for burning a fuel composed of 94-99.9% DME in a diesel engine.21 Early testing of DME as a diesel fuel indicated lowered NOx emissions, essentially zero particulate matter, and lower noise without loss of efficiency. The low emissions of particulate matter are due to the absence of carbon-carbon bonds in the molecular structure. Emissions of hydrocarbons and carbon monoxide are slightly higher for DME-fueled engines unless an oxidation catalyst is used. Emission tests have demonstrated the potential for meeting California’s ULEV standards for medium- and heavy-duty vehicles.23,24 DME has a high cetane number and is thus suited to compression ignition applications. The energy density of DME is higher than that of methanol or ethanol but is significantly lower than that of conventional diesel fuel. The use of dimethyl ether would require 1.8 times the volumetric flow of diesel fuel to obtain the same power output. In addition, the cost of production of DME is currently higher than that of conventional diesel fuel. DME is produced from methanol in dehydration plants. Thus, the cost of DME is roughly 2 times the cost of diesel fuel. Alternatively, DME can be produced directly from synthesis gas generated from various feedstocks ranging from natural gas to coal and biomass.
No mention is made, of course, on the possible use of Olah fuel cells to make methanol, and neither are any economics are included. (The economics of the Olah fuel cell are essentially unknown - and that is a giant caveat.)
As mentioned in the reference, the oxidation of DME in combustion does produce some formaldehyde as well as some methanol and formic acid. The atmospheric degradation pathway of DME, which has a half-life of a few days in air, also includes these common oxidation products.
From a pollution standpoint however, it is clear that DME (via methanol or other means) can be made directly from carbon neutral sources. Then its advantages are incalculable. The Olah fuel cell reduces carbon dioxide to give methanol. The Olah fuel cell is just
one such source, however. Direct hydrogenation of carbon dioxide is also known to work to give either methanol or DME directly. This is in my view, by far, the most useful use to which hydrogen can be put, DME manufacture. (See for instance Bull. Korean. Chem. Soc. 19, 4, 466-71 (1998).)
The processing moreover - unlike the case with ethanol - can be made to have low energy intensity and can be easily modified for continuous flow. This reduces both the pollution profile and the cost (especially on scale up) of manufacture. I note that the bottom of DME cost is certainly not reached since the
application remains small and limited - propellant. That is about to change big time.
With respect to physical properties, DME is superior to natural gas inasmuch as its critical point is above the boiling point of water. Thus it can be shipped and stored as a liquid under pressure and natural gas cannot be without the investment of massive energy for refrigeration.
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