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FIGURE 16.40 Calculated ozone production per vehicle mile traveled for various car-fuel combinations. RFG = reformulated gasoline; M85 = 85% methanol, 15% gasoline; E85 = 85% ethanol, 15% gasoline; CNG = compressed natural gas (adapted from Black et al., 1998).

3. Liquefied Petroleum Gas (LPG)

Liquefied petroleum gas is primarily propane but generally also contains significant amounts of olefins, which increase its reactivity substantially (Table 16.14). For example, LPG in the Los Angeles area contains about 2 mol% of alkenes whereas that in Mexico City contains almost 5% (Blake and Rowland, 1995). Hence while it is a significant improvement over gasoline in terms of reactivity (Tables 16.10 and 16.11), the exact amount of improvement is highly dependent on the nature and concentrations of these reactive impurities (Gabele, 1995).

4. Alcohol Fuels and Blends with Gasoline

Methanol and ethanol fuels are used both "neat" and as a blend with gasoline. The terminology "M85" signifies a blend of 85% methanol, 15% gasoline (by volume), and similarly for "E85," used to denote ethanol-gasoline blends. The advantages of using an alcohol-gasoline blend are that the flame is visible (a flame from pure methanol is not), the vapor pressure is higher, which aids in ignition, and there is flexibility in fueling, depending on the availability of gasoline or alcohol fuels.

Methanol is expected to oxidize to formaldehyde, both during combustion and after emission to the atmosphere. As discussed in Chapter 6.H, OH reacts with methanol primarily at the methyl group:

CH3OH + OH ^ CH2OH + H20 (85%), (f7a) -» CH30 + HzO (15%). (17b)

Both CH2OH and CH30 then react with 02 to give HCHO (and HOz). However, as discussed in detail in the National Research Council report (1991) and references cited therein, the results of modeling studies suggest that the amount of HCHO formed by oxidation of methanol in the atmosphere in many locations will not be significant compared to that formed by other processes. (Direct emissions of formaldehyde may be important, however, in some circumstances with low dilution rates such as underground parking garages or tunnels (e.g., see Chang and Rudy, f990b).) Because methanol itself has low reactivity (Tables 16.8 and 16.9), some modest improvements in ozone may result from the use of methanol (e.g., see Dunker, 1990; National Research Council, 1991; Lloyd et al., 1989). It is interesting that the reactivity of the evaporative emissions from M85 is higher than that of the exhaust (Black et al., 1998).

There has been an emphasis on the addition of ethanol to gasoline, due to the availability from grain sources. Ethanol is somewhat more reactive than methanol (Tables 16.8 and 16.9) and forms acetalde-hyde upon oxidation (see Problem 2). Further oxidation of acetaldehyde produces PAN (see Chapters 6 and 7). There is evidence for such an oxidation sequence in Brazil, where extensive use is made of ethanol as a fuel. Thus relatively high concentrations of PAN have been observed in this area compared to those found elsewhere (Tanner et al., 1988; Grosjean et al., 1990). Increases in PAN have also been reported in urban areas such as Albuquerque, New Mexico, using ethanol-gasoline blends for CO reduction (e.g., Gaffney et al., 1997, 1998; Whitten, 1998); concentrations of aldehydes, particularly CH3CHO, were also higher.

Alcohol fuels have also been reported to have higher emission rates of VOC as well as toxics such as HCHO, CH3CHO, f,3-butadiene, and benzene (Gabele, 1995; Black et al., 1998). However, Stump et al. (1996) report decreased emissions of VOC, benzene, and 1,3-butadiene with a gasoline fuel containing 8.8% ethanol but, in agreement with the other studies, increased emissions of HCHO and CH3CHO. Knapp et al. (f 998) tested emissions from 11 vehicles at temperatures from — 20 to +75°F fueled on either gasoline or a blend with 10% ethanol and found the ethanol blend resulted in higher emissions of CH3CHO, in some cases almost by almost an order of magnitude. The changes in the emissions of benzene, HCHO, and 1,3-butadiene were variable with respect to both amount and sign.

An additional problem with alcohol-gasoline blends is the increase in vapor pressure of gasoline in the mixture (e.g., see National Research Council, 1991; Calvert et al., 1993; and Timpe and Wu, 1995). This can contribute to much higher Reid vapor pressures, increasing the relative importance of evaporative emissions.

Table 16.14 shows the VOC composition of the combination of exhaust and evaporative emissions measured on a limited number of vehicles. Similar data have been reported by Gabele (1995). The increased aldehydes associated with the use of alcohol fuels is evident.

Overall, the use of ethanol blends is believed not to be effective in reducing ozone, but may actually increase it (Calvert et al., 1993; Dunker et al., 1996).

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Guide to Alternative Fuels

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