capable of satisfying the oxygen requirements in gasoline and to replace lead (TEL) as an additive. It is made by reacting methanol with isobutene
CH3OHþCH2¼CCH3!CH3OCH2CHCH3
j j
CH3 CH3
(5.5)
The present optimistic predictions of methanol production and demand are shown in Fig. 5.4 where it can be seen that the demand will soon exceed the production. The construction of a full-sized conventional methanol plant (2,000 t/d) takes at least 3 years from start to full time on stream and requires a suitable source of natural gas. Small gas fields which usually accompany crude oil Fig. 5.3 Uses of methanol
5.3 Methanol 75
production are normally vented or flared. The extent of such wasted energy is shown in Fig.5.5, and it is this lost gas which can be readily converted to methanol by the direct partial oxidation process.
The main apparent disadvantage of methanol is its low energy density by volume, 16.6 kJ/L, about half that of gasoline. This does not imply that the fuel tank must be twice as large because if the engine was designed for the fuel, the methanol fuel economy would be close to that of gasoline.
The use of gasohol or methanol (or ethanol) with gasoline (M-85) has become the intermediate stage in the extensive use of alcohol fuels. Because of the possibility of phase separation when water is present in 10% methanol in gasoline, a cosolvent such as tertbutyl alcohol (TBA) must be added.
M-85 does not require a cosolvent, though the emissions are worse than M-100.
Cold starting difficulties with methanol fuel are due to its high heat of vaporization and low volatility. Proper engine design can correct for such deficiencies. Another problem is corrosion due to the acid (formic acid) formed by the oxidation of the methanol, and all fuel and engine components must be carefully selected. The methanol flame is not readily visible, whereas M-85 (15% gasoline) has the advantage of appearing more like a yellow gasoline flame.
Table 5.3 Conversion of methanol into various products (%)
1982 1994
Formaldehyde 31 39
Methylamine 4
Chloromethanes 9
Acetic acid 12 7
Methyl ester 8 6
Solvent 11 7
Fuel-antifreeze 25 2
MTBE 13
Miscellaneous 26
Fig. 5.4 World production of natural gas and the proportion that is wasted by flaring and venting
The aldehyde emission from alcohol fuel must be destroyed by a catalytic after-burner if environ- mental contamination is to be avoided. Methanol has been used in diesel engines, though it has a CN between 0 and 5. However, because it is not readily self-ignited, special engine design must correct for such deficiency. A glow plug or fuel additives can aid self-ignition. Partial conversion of the methanol to dimethyl ether is another approach to the problem which has recently been studied.
A major problem in the use of gasohol is in the materials of construction of the fuel system.
Alcohol tends to dissolve the oxidation products and gums from gasoline, and special precautions must be taken when first switching to gasohol in an old vehicle. This is even more important when pure methanol is used as the fuel. Special gaskets and O-rings are required. However, these present no problem if design and construction are started with such fuel in mind. Besides, being independent of petroleum, alcohol offers other inherent advantages, such as cooler, cleaner combustion; improved power; reduced carcinogens and NOx emission; reduced hydrocarbon emission which can be photochemically activated and smog forming; and smaller and lighter engine designs are possible.
Ethanol and methanol have cetane values from 0 to 5 making them poor compression ignition fuels for a diesel engine. However, alcohol-diesel fuel blends have been used successfully, and emulsifiers have been added to help blend the two components which have limited miscibility.
When methanol is catalytically converted to CO and H2
CH3OH gð Þ !CO gð Þ þ2H2ð Þg DHo¼90:7 kJ=mol (5.6) the resulting synthesis gas has a higher heat of combustion (766 kJ) than the methanol (676 kJ). If the exhaust heat is used to decompose the methanol, then a higher energy fuel is obtained; that is, approximately 20% increase in fuel economy has been proposed. This is valid if only thermal energy is considered. In an internal combustion spark engine, the air and fuel are compressed (which require work), and the combustion then expands the gas, and work is done. Hence, the ratio, R¼moles of gaseous products/moles of gaseous reactant, must be used to normalize differences in fuels.
Assuming N2/O2¼4 and no excess O2:
CH3OHþ1:5O2þ6N2!CO2þ2H2Oþ6N2 (5.7) 8.5 moles!9 moles R1¼1.058
COþ2H2þ1:5O2þ6N2 !CO2þ2H2Oþ6N2 (5.8) 10.5 moles !9 moles R2¼.857, R1/R2¼1.235
Fig. 5.5 World production capacity for methanol and projected world demand
5.3 Methanol 77
The product gases have the same volume in each reaction:
Hence, the additional work of compression (Wc) is essentially due to the PDV orDn RT where Dn ¼2 moles. The remaining question is what is the value of T? We can select 400C as an estimated value of the temperature. This value can be calculated from an adiabatic compression of the gas (mostly air) from T1¼350 K and assuming that CR¼10.
It is worth considering an extension to this argument by considering the increase fuel economy if we remove the nitrogen from the combustion process. Firstly, a higher temperature is obtained, and secondly, less work is done in compressing the gas. If we assume that gasoline is represented by octane, C8H18, then in air, the reaction is
C8H18þ12:5O2þ50N2! 8 CO2þ9H2O þ50N2 (5.9) 63.5 moles !67 moles R3¼1.055
C8H18þ12:5O2! 8 CO2þ9H2O (5.10)
13.5 moles !17 moles R4¼1.259, R4/R3¼1.19
The difference in moles of compression of reaction 5–9 and 5–10Dn is 63.5–13.5 orDn¼50.
Hence,Dn RT¼508.314673¼280 kJ. The heat of combustion of octane is 5,470 kJ/mol.
This extra work of compression is only about 5% of the combustion energy. However, the calculated adiabatic combustion flame temperature for octane in air is about 1,400 K, whereas in oxygen, the flame temperature is over 9,000 K. This represents a substantial increase in energy of combustion due to the removal of nitrogen.
Thus, besides obtaining more heat from the reaction (since the heat capacity of nitrogen absorbs some of the energy to reach the high temperature), the removal of nitrogen can give rise to about 20%
higher fuel economy. This is now being done for stationary furnaces where only the thermal improvement is obtained. It would be interesting to run an automobile on enriched oxygen which can be obtained by using suitable membranes. More will be said about this later.
One aspect which is important to note is that methanol is toxic and its TLV (threshold limit value) is 200 ppm requiring its dispensing in well-ventilated areas.
The ultimately efficient fuel system would be an electric vehicle running on a methanol fuel cell.
Such a system could meet the most stringent environmental emission requirements as well as high energy efficiency. This will be discussed more fully in Chap.9.