The composition of M100 is 100 percent methanol. The composition of fuel vapors is the same as that of the liquid. M100 contains trace contaminants of water and hydrocarbons (Table 2-4). However, fuel cell vehicles with methanol stream reformers will likely not be able to operate effectively with percent levels of hydrocarbons. Therefore, hydrocarbon free fuel was assumed. Hydrocarbon levels in the ppm range would not affect steam refiners and these levels would have an insignificant impact on fuel-cycle emissions. Measurements of M100 contaminants from vehicle demonstration programs indicate negligible hydrocarbons and typically less than 1000-ppm water. The effect of this level of water on vapor pressure and heating value is negligible.
Table 2-4: Properties of Methanol Fuelsa
aM100 = 100 percent (neat) methanol.
bRVP = Reid vapor pressure.
cHydrocarbon and water level assumed for the purpose of determining fuel-cycle emissions. Lower levels may be required for fuel cell vehicle operation.
2.2.1 Crude Oil
Crude oil contains a mixture of hydrocarbons with a range of compositions. Since crude oil contains light hydrocarbon components it has a higher vapor pressure than diesel. Crude oil is characterized by its API gravity that is inversely proportional to specific gravity. This property determines how “heavy” the oil is and relates to its carbon content and heating value. The properties of typical crude oil are shown in Table 2-1.
2.2.2 Natural Gas
Table 2-3 shows the properties of gaseous fuels. The natural gas compositions are an average of measurements provided by SoCalGas for gas delivered in Southern California. These values resemble closely the weighted average of natural gas composition for ten U.S. cities reported by GRI1 (Liss 1991). While some natural gas supplies can vary significantly in composition, 80 percent (10th through 90th percentiles) of natural gas reported by GRI had a methane content within 88.5 to 96.4 percent. All gas that is currently sold in California is reported to have a relatively
1 Mean composition (vol%) for ten cities in the U.S. was methane: 93.2, ethane: 3.6, propane: 0.8, >C4: 0.5, inerts 2.8.