Methanol and Dimethyl Ether as Diesel Fuels Substitute in Compression Ignition Engines

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Methanol, when combusted, does not produce smoke, soot or particulates. Parti-culate matter - whether carcinogenic compounds are absorbed onto them, or not - have been identified as a significant health hazard, especially in large cities. Diesel fuel generally produces particles during combustion. This, and the fact that methanol produces very low emissions of NOx because it burns at lower temperatures, makes methanol attractive as a substitute for diesel fuel [123].

Diesel engines are quite different from gasoline engines. Instead of using sparkplugs to ignite the fuel/air mixture in the engine's cylinders, diesel motors rely on the self-ignition properties of the fuel to ignite under specific high-temperature and high-pressure conditions. While a typical gasoline engine has a compression ratio of about 8-9 to 1, a diesel engine has generally a compression ratio in excess of 17 to 1. In the past, such engines were used mainly in heavy-duty vehicles such as buses, tractors, trucks, locomotives and ships. However, in the early 1970s - due to their better fuel economy compared to gasoline engines -diesel engines were increasingly used to power personal automobiles. Today, in western Europe for example, diesel-fueled cars represent about 50% of all cars in operation.

Like gasoline, diesel fuel is composed of many hydrocarbons that have a wide boiling range. The physical and chemical properties of diesel fuel are, however, quite different. Whereas gasoline contains predominantly branched alkanes with three to 10 carbon atoms as well as aromatic compounds to ensure a high octane rating, diesel fuel is mainly composed of straight-chain alkanes with 10 to 20 carbon atoms. A fuel's propensity to self-ignite under high heat and pressure is measured by the "cetane" rating. While diesel fuel has cetane ratings in the range from 40 to 55, methanol's cetane rating is only about 3. With methanol and diesel fuels being practically non-miscible, the possibility of using any blends of methanol and diesel fuel in diesel vehicles must be excluded. In order to overcome the low cetane rating of methanol, diesel motors must be changed and adapted. Additives can be added to increase the cetane rating of methanol to levels close to diesel fuels. These ignition improvers, added in the order of a few percent to methanol, are typically composed of nitrogen-containing compounds such as octyl nitrate and tetrahydrofurfuryl nitrate [123], although many of them are toxic and/or carcinogenic. Non-toxic cetane enhancers based on peroxides and higher alkyl ethers have also been developed. If neat methanol is used, ignition through spark plugs or glow plugs is necessary.

The energy content of methanol on a volume basis is about 2.2 times less than that of diesel fuel [123], which means that the fuel tank must be about twice the size of a conventional diesel fuel tank to provide the same amount of energy. Methanol has a significant vapor pressure compared to diesel fuel. The higher volatility allows heavy-duty engines to start easily in the coldest weathers, thereby avoiding the white smoke which is typical of cold-starts with conventional diesel engines.

As for many other alternative fuels, transit buses have been the main test-field for methanol-powered diesel engines. The Detroit Diesel Corporation (DDC) in particular developed a methanol version of its 6V-92TA diesel engine which in the early 1990s was the lowest emission heavy-duty diesel engine certified by the Environmental Protection Agency (EPA) and the California Air Resource Board (CARB) [123]. Using methanol instead of diesel fuel also allowed a dramatic reduction in particulate as well as NOx emissions. Methanol contains no sulfur; hence, SOx emissions that lead to acid rain are also nearly eliminated. Fleets of buses equipped with methanol-fueled diesel engines were tested in different places around the United States including Los Angeles, Miami and New York [124]. The Metropolitan Transit Authority (MTA) of Los Angeles in particular operated a large fleet of some 330 methanol-powered transit buses for some years in the 1990s (Fig. 11.2). Compared to conventional diesel buses, higher maintenance costs due to some technical problems in the fuel system and engine were experienced using methanol. Similar operating problems were also encountered when these buses were converted to run on ethanol. Most of the difficulties experienced with alcohol use in diesel fuels are believed to be connected with the fuel delivery system and the use of non-compatible materials. Nevertheless, these technical problems are not insurmountable and can be solved with additional studies. Methanol, however, must also compete with other alternative fuels. When considering transit buses, for example, where the bulkiness of fuel tanks is not a major issue, compressed natural gas (CNG) has now become the preferred fuel because of its low emissions, price and widespread availability. Improved diesel engines enabling cleaner operation, particle filters as well as advanced diesel fuels containing less-polluting impurities are also being developed.

Another possibility is to use dimethyl ether, a superior and more calorific fuel than methanol for diesel engines, which can be easily obtained by dehydration of methanol.

Methanol Economy
Figure11.2 Methanol-powered regional transit bus in Denver, Colorado. (Source: Gretz, Warren DOE/NREL.)

Dimethyl ether, DME, the simplest of all ethers, is a colorless, non-toxic, non-corrosive, non-carcinogenic and environmentally friendly chemical that is mainly used today as an aerosol propellant in various spray cans, replacing banned CFC gases. DME has a boiling point of-25 °C, being a gas under ambient conditions. DME however is generally handled as a liquid and stored in pressurized tanks, much like LPG (liquefied petroleum gas), which contains primarily propane and butane, commonly used for cooking and heating purposes. The interest in DME as an alternative transportation fuel lies in its high cetane rating of 5560, compared with 40-55 for conventional diesel fuel and much higher than that of methanol. Therefore, DME can be effectively used in diesel engines, as has been introduced by Haldor Topsoe. Like methanol, DME is clean-burning, produces no soot, black smoke or SO2, and only very low amounts of NOx and other emissions even without exhaust gas after-treatment (Tables 11.2 and 11.3).

Today, DME is produced exclusively by the dehydration of methanol. Being directly derived from methanol, DME can be produced from a large variety of feedstocks: coal, natural gas, biomass, etc. or reductive CO2 recycling (see Chapter 12). A method to synthesize DME directly from syn-gas by combining the methanol synthesis and dehydration steps in a single process has also been developed

[125]. The direct synthesis of DME from CO2 and H2 has also been studied

[126]. The global demand for DME is currently only about 150 000 tons per year, but this could be considerably increased if large quantities of DME were to be needed as fuels.

Table 11.2 Properties of dimethyl ether (DME).

Chemical formula

CH3OCH3

Molecular weight

46.07

Appearance

Colorless gas

Odor

Slightly sweet smell

Chemical composition (%)

Carbon

52

Hydrogen

13

Oxygen

35

Melting point

-138.5 °C

Boiling point

-24.9 °C

Density of liquid at 20 °C

668 kg m-3

Energy content

6880 kcal kg-1

317 kcal mol-1

Cetane number

55-60

Flash point

-41 °C

Autoignition temperature

350 °C

Flammability limits in air

3.4-17%

Table11.3 Comparison of the physical properties of DME and diesel fuel.

DME Diesel fuel

Boiling point (°C)

-24.9

180-360

Vapor pressure at 20 °C (bar)

5.1

-

Liquid density at 20 °C (kg m-3)

668

840-890

Heating value (kcal kg-1)

6880

10150

Cetane number

55-60

40-55

Autoignition temperature (°C)

235

200-300

Flammability limits in air (vol. %)

3.4-17

0.6-6.5

DME vehicles are being developed in many areas of the world. In Europe, according to Volvo, which is running tests on a DME-powered bus and truck, DME is one of the most promising fuels for substituting conventional diesel oil (Fig. 11.3) [127, 128]. It can be used in an ordinary diesel engine equipped with a new fuel injection system producing the same performance, whilst dramatically reducing emissions. Japan, which widely uses imported LPG for domestic applications, has an extensive LPG infrastructure that can be easily adapted to DME. Realizing that future LPG supplies may not meet demand, Japan is studying the DME option not only for the transportation sector but also for electric power generation as well as household and industrial uses [129]. General Electric showed DME to be an excellent fuel for gas turbines, with emissions and performances comparable to those of natural gas [130]. Due to similar combustion characteristics, cooking stoves designed for natural gas can use DME without any modification [131]. Road tests with several DME-powered Japanese trucks and buses led to conclusions comparable to those obtained by Volvo. Isuzu, a well-known diesel engine manufacturer, is taking part in these tests and is confident that diesel oil can be supplanted in the future by improved technologies, including the use of DME. Two major Japanese consortia composed of leading compa-

Japanese Volvo Bus Images
Figure 11.3 DME-fuelled Volvo bus developed in Denmark. (Courtesy: Danish Road Safety and Transport Agency)

nies are currently assessing the economical viability of DME. One consortium, led by the giant NKK Corp, created DME International Corp and DME Development Company to investigate the economics and facilitate the introduction of DME as a fuel with commercial production in the 850000 to 1650 000 tonnes per year range, envisaged to start in late 2006. The other consortium, Japan DME, led by Mitsubishi Gas Chemical, is planning to build a DME plant in Papua New Guinea which could produce up to 3 000 000 tonnes of DME per year. The Japanese government, through substantial financial support from the Ministry of Economy, Trade and Industry (METI) is also committed to the development of mass produced, low-cost DME.

Developing countries in Asia, such as China and India, are also very interested in DME, given their rapidly increasing needs and growing demands for diesel and LPG, as well as their deteriorating air quality. China in particular, because of its enormous reserves of coal, is interested in coal to DME liquefaction technology. In Shandong Province, the construction of a plant to produce 1 million tons of DME per year, and which will also produce 1.5 million tonnes methanol per year, based on coal is under construction. At the Shanghai Jiao Tong University, researchers are developing a transit bus powered by DME, whilst the Chinese Ministry of Science has offered subsidies to produce 30 such buses.

Countries which have low-cost natural gas reserves but are distant from important consuming centers, in the Middle East, Australia, Trinidad and Tobago and others, are also interested in DME as a convenient way to transport energy to markets in highly populated areas. A plant with a production capacity of 800000 tonnes per year of DME for fuel uses is currently under construction in Iran. In the Middle East [3], BP is also seeking for partners for a planned DME plant producing 1.8 million tons per year [132].

Besides DME, dimethyl carbonate (DMC), which has also a high cetane rating, can be blended into diesel fuel at a concentration of up to 10%, thereby reducing the fuel viscosity and improving emissions. In China, plans to produce DMC (mainly from coal or some natural gas resources) on a commercial scale as a diesel additive are under way [132]. Due to a melting point of 3°C, however, neat DMC is not an ideal fuel because of its expected freezing problems at lower operating temperatures. The commercial route to DMC has used the reaction of methanol with phosgene. However, phosgene - being highly toxic - was replaced by the oxidative carbonylation of methanol, developed by EniChem and other companies [133].

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Responses

  • Italo
    Can methanol the used in a diesel engine?
    8 years ago
  • heather
    Which has a higher autoignition temperature diesel or dimethylether?
    8 years ago
  • JOANNA
    What is a chemical formula for dimethyl (diesel)?
    8 years ago
  • irene
    How to make methanol fuel at home?
    3 years ago
  • Amy-leigh
    Is methanol being added to diesel?
    3 months ago
  • Awet
    Why is dimethyl ether added to alcohol fuel with diesel in ci engine?
    2 months ago

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