James G. Speight - Encyclopedia of Renewable Energy

Methanol is produced by a variety of process, the most common are as follows: distillation of wood; distillation of coal; natural gas; and crude oil gas. Ethanol is produced mainly from biomass transformation, or bioconversion. It can also be produced by synthesis from crude oil or mineral coal.

In those countries with large territorial areas, ethanol has been the renewable fuel choice to replace gasoline. The reason is the fact that alcohol is a renewable source of energy. Currently, ethanol is produced from sugar beets and from molasses - a typical yield is 15 to 20 gal of ethanol per ton of sugar cane. Other crops can be used for the production of ethanol. Corn, for example, can yield approximately 75 gal liters of alcohol.

Ethanol (ethyl alcohol, CH 3CH 2OH), also referred to as bioethanol, is a clear, colorless liquid with a characteristic, agreeable odor. Currently, the production of ethanol by fermentation of corn-derived carbohydrates is the main technology used to produce liquid fuels from biomass resources.

Ethanol can be blended with gasoline to create E85, a blend of 85% ethanol and 15% gasoline. Fuel with higher concentrations of ethanol (E95) and pure bioethanol (E100) has been used successfully in Brazil. More widespread practice has been to add up to 20% to gasoline (E20, also called gasohol) to avoid engine changes. E100-fueled and M100-fueled vehicles have difficulty starting in cold weather, but this is not a problem for E85 and M85 vehicles because of the presence of gasoline.

Ethanol has a higher octane number (108), broader flammability limit, higher flame speed, and a higher heat of vaporization than gasoline. These properties allow for a higher compression ratio, shorter burn time, and leaner burn engine, which lead to theoretical efficiency advantages over gasoline in an internal combustion engine. On the other hand, the disadvantages of ethanol include its lower energy density than gasoline, corrosiveness, low flame luminosity, lower vapor pressure, miscibility with water, and toxicity to ecosystems.

The alcohols mix in all proportions with water due to the polar nature of the hydroxyl (OH) group. Low volatility is indicated by high boiling point and high flash point. Alcohols burn with no luminous flame, and methanol produces almost no soot, but the tendency to produce soot increases with molecular weight.

See also: Alcohols – Production, Ethanol, Methanol.

There are some important differences in the combustion characteristics of alcohols and hydrocarbon derivatives. Alcohols have higher flame speeds and extended flammability limits. Also, alcohols produce a great number of product moles per mole of fuel burnt; therefore, higher pressure is achieved.

The alcohols mix in all proportions with water due to the polar nature of OH group. Low volatility is indicated by high boiling point and high flash point. Alcohols burn with no luminous flame and produce almost no soot, especially methanol. The tendency to soot formation increases with molecular weight.

Combustion of alcohol in presence of air can be initiated by an intensive source of localized energy, such as a flame or a spark, and also, the mixture can be ignited by application of energy by means of heat and pressure, such as happens in the compression stroke of a piston engine. The energy of the mixture reaches a level sufficient for ignition to take place after a brief period of delay called ignition delay, or induction time, between the sudden heating of the mixture and the onset of ignition (formation of a flame front which propagates at high speed throughout the whole mixture). The high latent heat of vaporization of alcohols cools the air entering the combustion chamber of the engine, thereby increasing the air density and mass flow. This leads to increased volumetric efficiency and reduced compression temperatures. Together with the low level of combustion temperature, these effects also improve the thermal efficiency by 10%.

Alcohols have higher flame speeds and extended flammability limits than hydrocarbons. Also, alcohols produce a great number of product moles per mole of fuel burnt; therefore, higher pressure is achieved. The higher flame speed, giving earlier energy release in the power stroke, results in a power increase of 11% at normal conditions and up to 20% at the higher levels of a compression ratio (14:1). The power continues to rise steadily as the mixture is enriched to an equivalence ratio of approximately 1 to 4. Because of the low proportion of carbon in alcohols, soot formation does not occur and therefore alcohols burn with low luminosity and therefore low radiation. In conjunction with lower flame temperature, approximately 10% less heat is lost to the engine coolant. The lower flame temperature of alcohols results in much lower NOx (nitrogen oxides) emissions. The wider flammability limits of alcohols permit smooth engine operation even at very lean mixtures. But aldehyde emissions are noticeably higher. For ethanol, emissions are acetaldehydes, and for methanol, emissions are of formaldehydes. Increasing compression ratio from 9 to 14, aldehyde emissions can be reduced by 50%, to a level compared to that for gasoline. An addition of 10% water reduces aldehyde emissions by 40% and NOx by 50%. Addition of 10% water in the alcohol can be tolerated without loss of thermal efficiency.

The oxygen content of alcohols depresses the heating value of the fuel in comparison with hydrocarbon fuels. The heat of combustion per unit volume of alcohol is approximately half that of isooctane. However, the stoichiometric fuel-air mass ratios are such big that the quantity of energy content based on unit mass of stoichiometric mixture becomes comparable with that of hydrocarbon derivatives.

Methanol is not miscible with hydrocarbon derivatives, and separation ensues readily in the presence of small quantities of water, particularly with reduction in temperature. Anhydrous ethanol, on the other hand, is completely miscible in all proportions with gasoline, although separation may be affected by water addition or by cooling. If water is already present, the water tolerance is higher for ethanol than for methanol, and can be improved by the addition of higher alcohols, such as butanol.

The high heat of vaporization and constant boiling point make cold starting difficult with neat alcohols. The problem is not as severe as in case of alcohols blended with gasoline. Ethanol has a constant boiling point of 80°C (176°F). Gasoline which has a high vapor pressure (therefore highly volatile) can be used for cold start.

See also: Alcohols, Butanol, Ethanol, Methanol, Propanol.

Dry methanol is very corrosive to some aluminum alloys, but additional water at 1% almost completely inhibits corrosion. It must be noted that methanol with additional water at more than 2% becomes corrosive again. The same happens with less than 1% water. Nitride and neoprene rubbers, generally satisfactory as elastomers in contact with methanol and polyacetal plastics, are very resistant. Silicon rubber as well as vinyl can be used for gasket material. Ethanol always contains acetic acid and is particularly corrosive to aluminum alloys. Also, certain alloys containing lead are attacked with a general result of the lead being leached out, leaving a porous surface. The same phenomenon exists with alloys of zinc, such as ZAMAC (zinc plus aluminum), and the zinc is leached out as a white zinc oxide, which clogs the small orifices and jets.

Carburetors are normally made of ZAMAC alloy. Experience has shown that if the carburetor is protected with a coat of nickel, the corrosion problem is overcome. The process recommended is electrolysis nickel plating. In this process, the carburetor parts are immersed in a bath of hot nickel, which, due to the low viscosity, covers evenly all the surfaces without clogging the orifices. The floats on the carburetor float-bowl are generally made of porous plastics which are attacked by the ethanol, and the end result is swelling and cracking. It is found that nylon floats arc more durable. A float can be made with thin sheet of brass (0.005 in) or 0.125 mm thickness, molded and welded with pure tin (Sn).