Encyclopedia of Renewable Energy. James G. Speight

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СКАЧАТЬ 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.

      Alcohols – Corrosivity

      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).

      Alloys of tin and lead (Sn+ Pb) shall be avoided as welding material. All bronze parts shall be brass or stainless steel. Steel fuel lines shall be replaced by nylon tube (Nylon 11). Fuel filters used for gasoline are not recommended for the many alcohols. The internal element collapses after the glue that bonds it together is softened by the alcohol. Special filters are necessary. Also, due to the higher flows, filters have to be bigger. The filter body must be made of nylon or Teflon.

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

Alcohols – From Waste

      Alcohols can be made from organic materials by fermentation, and there is the potential for the production of alcohols from organic waste. Historically, the production of methanol, ethanol, and higher molecular alcohols from syngas has been known since the beginning of the 20^th century. There are several processes that can be used to make mixed alcohols from synthesis gas including iso-synthesis, variants of Fischer-Tropsch synthesis, oxo-synthesis involving the hydroformylation of olefins, and homologation of methanol and lower molecular weight alcohols to make higher alcohols. In the context of the Fischer-Tropsch process, depending on the process and its operating conditions, the most abundant products are usually methanol and carbon dioxide, but methanol can be recycled to produce higher molecular weight alcohols.

      With the development of various gas-to-liquid processes, it was recognized that higher alcohols were by-products of these processes when catalysts or conditions were not optimized. Modified Fischer-Tropsch (or methanol synthesis) catalysts can be promoted with alkali metals to shift the products toward higher alcohols. Synthesis of higher molecular weight alcohols is optimal at higher temperatures and lower space velocities compared to methanol synthesis and with a ration of hydrogen/carbon monoxide ratio of approximately 1 rather than 2 or greater.

      In the process, the feedstock enters the process and is converted to synthesis gas with the desired carbon monoxide/ hydrogen ratio, which is then reacted, in the presence of a catalyst, into methanol (CH₃OH), ethanol (CH₃CH₂OH), and higher molecular weight alcohols.

      Thus,

      Stoichiometry suggests that the carbon monoxide/hydrogen ratio is optimum at 2, but the simultaneous presence of water-gas shift leads to an optimum ratio closer to 1.

      As in other synthesis gas conversion processes, the synthesis of higher molecular weight alcohols generates significant heat and an important aspect is choice of the proper reactor to maintain even temperature control which then maintains catalyst activity and selectivity. In fact, the synthesis of higher molecular weight alcohols is carried out in reactors similar to those used in methanol and Fischer-Tropsch synthesis. These include shell and tube reactors with shell-side cooling, trickle-bed, and slurry bed reactors.

      Catalysts for the synthesis of higher molecular weight alcohols generally fall mainly into four groups: (1) modified high pressure methanol synthesis catalysts, such as alkali-doped ZnO/Cr₂O₃, (2) modified low pressure methanol catalysts, such as alkali-doped Cu/ZnO and Cu/ZnO/Al₂O₃, СКАЧАТЬ