Dimethyl Ether

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Abstract: Dimethyl ether (DME) can be made from a variety of fossil feedstocks including natural gas and coal as well as from renewable feedstocks and waste. When used as a diesel fuel, DME offers PM and NOx emission benefits. Emissions of CO and HC which may increase with DME can be easily controlled by an oxidation catalyst. Energy efficiency of DME is lower than that of diesel but higher than that of methanol/gasoline engines.

Introduction

Dimethyl ether (DME) is the simplest ether, consisting of two methyl groups bonded to a central oxygen atom, as expressed by its chemical formula CH3-O-CH3. DME can be produced from natural gas—providing an alternative way of its utilization, in competition to such technologies as Fischer-Tropsch synthetic fuels—as well as from other carbon-containing feedstocks, including coal and biomass. DME has replaced CFC gases (freons) as an environmentally friendly and safe aerosol propellant, which is one of its major current applications. Potential future uses of DME include an alternative automotive fuel, a substitute for other fuels in power generation and in the household and a source of hydrogen for fuel cells [777]. Worldwide DME production grew from 100,000-150,000 tons per annum in the 1990s [153] to some 200,000 tons in the mid-2000s [1653]. China has developed a large DME production base, which reportedly amounts to almost 9 million tons per annum in installed DME capacity [1655].

With the chemical structure somewhat similar to methanol, DME contains oxygen and no carbon-carbon bonds, thus seriously limiting the possibility of forming carbonaceous particulate emissions during combustion. However, unlike methanol, DME has a high enough cetane number to perform well as a compression-ignition fuel. Also unlike methanol, DME is a gas at ambient temperature and pressure, so it must be stored under pressure as a liquid similar to LPG (liquefied petroleum gas). When used as a diesel fuel, DME provides reduced PM and NOx emissions, but increased CO and HC. Preliminary studies in the 1990s concluded that it should be possible to achieve ULEV emissions using a properly designed DME-based fuel injection system with an HSDI engine and an oxidation catalytic converter in a passenger car [773].

The physical properties of DME (density, viscosity, lubricity, etc.) are so different from the diesel fuel that the entire fuel system must be redesigned [774]. While it seems clear that DME, like perhaps some other alternative fuels, would be able to produce much larger emissions reductions than it is possible with diesel fuel, the emission benefit comes at a price of a specific level of complexity of the fuel storage and injection system, including the need to carry a pressurized fuel tank onboard the vehicle. Furthermore, it is not clear that the apparently inherent emission advantage of DME can offset the fuel’s lack of established supply and fueling infrastructure. From today’s perspective, the DME fuel is more likely to be used in certain niche applications, rather than provide a wide-scale alternative to liquid diesel fuels.

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