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high methane content with typical methane contents above 92 percent, which is within ARB's vehicle fuel specification of 88 percent (vol).

As natural gas demand increases to meet vehicle requirements, California will need to import more natural gas.  This gas will probably be supplied from Canadian and Southwest U.S. sources (Thomason 1993)2.  Canadian gas has higher methane content because hydrocarbons are extracted for LPG use.  An average of 50 percent Canadian gas and 50 percent Southwest gas results in a mixture that is very close to the value in Table 2-3.  Since the exact mix of incremental gas for vehicle fuel is difficult to predict and the composition in Table 2-3 is also representative of U.S. gas as well as possible new gas supplies to California, this composition is used throughout the study.  The compositions in Table 2-3 are also shown as weight percent values so they can be treated consistently with liquid fuel compositions and to allow for calculation of ozone potential on a mass basis.

2.2.3  Landfill Gas and Digester Gas

Landfill gas is produced when organic material decomposes in a landfill.  The organic material converted to methane and CO2 through biological decomposition.  Traces of sulfur containing compounds and chlorinated compounds occur in landfill gas.  Air can also be entrained in landfill gas.  When landfill gas is removed from the center of a landfill the gas has a higher methane and CO2 content.  Gas that is extracted from the periphery of a landfill contains more nitrogen and some oxygen.  Table 2-5 shows the compositions of landfill gas and digester gas which are potential feedstocks for methanol production.  Data from a digester included speciations of hydrocarbons that determined that non-methane hydrocarbons were below 0.1 percent by volume.

Table 2-3:  Composition and Properties of Gaseous Fuels

Property

Digester Gas

Landfill Gas

Carbon (wt %)

44.5

35.9

LHVa (Btu/lb)

7,910

6,350

        (Btu/gal

5,550

45,300

Density (lb/gal)

7.02

7.1

Composition

(vol %)

(wt %)

(vol %)

(wt %)

N2

1.1

1.2

16

16.5

CO2

37.6

62.1

31

50.4

CH4

61.3

36.8

50

29.5

C2H6

0

0

0

0

C3H8

0

0

0

0

C3H6

0

0

0

0

C4H10

0

0

0

0

C5H12

0

0

0

0

O2

0

0

3

3.5

aLHV = Lower heating value.

2 Canadian gas composition (vol%) methane: 96.99, NMHC: 1.46.  Southwestern gas composition (vol%) methane: 91.48, NMHC 6.33).

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