Handbook of Cellulosic Ethanol

Fuel Characteristics of Ethanol

As discussed in the earlier section, ethanol was first introduced on a larger scale in the 1970s as a blend in fuel to serve as a replacement for methyl tertiary butyl ether (MTBE). Ethanol has a octane number of 108.6, comparable to 110 of MTBE [7]. Mixing about 10% of ethanol to gasoline can boost the octane number of the fuel [8]. The octane number is a measure of the gasoline quality for prevention of early ignition, which leads to cylinder knocking. Fuels with higher octane numbers are preferred in spark-ignition internal combustion engines (ICEs). An oxygenate fuel such as bioethanol provides a reasonable antiknock value [8]. In addition to a higher octane number, ethanol has broader flammability limits, higher flame speeds and higher heats of vaporization. These properties allow for a higher compres­sion ratio and shorter burn time, which lead to theoretical efficiency advantages over gasoline in an internal combustion engine. A com­parison of important physical properties and fuel characteristics of ethanol, methanol and gasoline is shown in Table 2.1.

Property

Methanol

Ethanol

Gasoline

CH3OH

C2H5OH

C4 - C12

Molecular weight (g/mol)

32

46

~ 114

Specific gravity

0.789 (298 K)

0.788 (298 K)

0.739 (288.5 K)

Vapor density rel. to

1.10

1.59

3.0-4.0

air

Liquid density (g cm-3 at 298K)

0.79

0.79

0.74

Boiling point (K)

338

351

300-518

Melting point (K)

175

129

-

Heat of evaporation (Btu/lb)

472

410

135

Heating value (kBTU gal-1)

Lower

58

74

111

Upper

65

85

122

Table 2.1 Physical properties and some fuel characteristics of ethanol,

methanol, and gasoline [9].

Property

Methanol

Ethanol

Gasoline

CH3OH

C2H5OH

C4 - C12

Tank Design/pressure

15

15

15

(psig)

Viscosity (cp)

0.54

1.20

0.56

Flash Point (K)

284

287

228

Flammability/ Explosion limit (%) Lower (LFL)

6.7

3.3

1.3

(%) Upper (UFL)

36

19

7.6

Auto ignition

733

636

523-733

temperature (K)

Solubility in water

100%

100%

Negligible

Miscible

Miscible

(~ 0.01%)

Azeotrope with water

None

95% Ethanol

Immiscible

Peak flame

2143

2193

2303

temperature (K)

Minimum ignition

0.14

0.23

-

energy in air (mJ)

Research Octane num-

108.7

108.6

91-93

ber (RON)

Handbook of Cellulosic Ethanol

Disadvantages of Ethanol

There are a few disadvantages to ethanol when compared to gaso­line, including: 1. Lower energy density in ethanol is the most signifi­cant disadvantage. A kilogram of ethanol has about 66% …

Handbook of Cellulosic Ethanol

The inevitable decline in petroleum reserves and the rise in demand for oil from rapidly growing economies have caused soaring oil prices, and coupled with climate change concerns have contributed …

Biodiesel Education Grants

Competitive grants are available through the Biodiesel Fuel Education Program (Section 9006) to educate governmental and private enti­ties that operate vehicle fleets, the public, and other interested enti­ties about the …

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