Environmental Engineering Reference
In-Depth Information
94% of transportation energy in 2009, with only 3.4% of transportation energy consumption being
provided by renewables (Davis et al. 2010). This current dependence on petroleum is detrimental
to the U.S. economy and national security, especially as shortages and high oil prices become
increasingly prevalent (EIA 2009b). In 2009, renewable energy represented only 8% of the total
U.S. energy consumption, with biofuels composing 20% of this renewable energy use (EIA 2009b).
Efforts are in place, including the U.S. Renewable Fuel Standard (RFS) 2, which requires 36 bil-
lion gallons of renewable fuels to be used for transportation by 2022. (U.S. EPA 2007a, 2010a) This
will promote the development and incorporation of renewable fuels into the transportation sector
to overcome the current low percentage of consumption, assist with greenhouse gas reduction, and
decrease reliance on imported petroleum. However, biofuel and engine operational challenges must
be understood and overcome to ensure the success of renewable transportation fuels.
Vehicle and engine original equipment manufacturers (OEMs) have developed engines to use
ethanol-gasoline and methyl-ester biodiesel-petroleum diesel blends. These two biofuels have received
significant attention and development in the United States. As of 2010, nearly 8 million E85 (85% vol-
ume ethanal, 15% volume gasoline) ethanol flex-fuel vehicles (FFVs) were estimated to be on the road
in the United States (U.S. DOE 2010b), and automotive manufacturers have committed to making one
half of their vehicles ethanol flex-fuel capable by 2012 (GM 2007). These vehicles are able to utilize gas-
oline-ethanol blends in the range of 0 to 85% ethanol by volume. Additionally, 10% ethanol is approved
for blending with gasoline for use in vehicles with conventional SI gasoline engines, with higher blend
ratios likely being approved soon. Similarly, the use of biodiesel is approved at levels of 5-20% con-
centrations by many of the diesel engine and vehicle OEMs (Cummins 2007). Standards are placed
upon these fuels to ensure they do not significantly deteriorate the IC engine performance or emissions
(ASTM D7467-09a and ASTM D6751-10). However, other fuels, including Fisher-Tropsch (green or
synthetic) diesel (Goodger 1975), dimethyl ether (DME) (Silva-Petrobras 2006), methanol, butanol,
biogas [a mixture of methane (CH
4
) and CO
2
produced via anaerobic digestion of biodegradable matter
(IEA 2005)], and hydrogen (Naber and Siebers 1998) are also notable alternatives for transportation
fuels for use in IC engines (SAE 2007) and other power generation systems including fuel cells.
10.1.2 B
iofuEl
c
haractEriSticS
Combustion fuels are primarily composed of the atomic elements hydrogen, carbon, and oxy-
gen, although trace amounts of other elements, including sulfur and nitrogen, can also be present
(Goodger 1975). These fuels are suitable for SI or CI engines, depending on the fuel character-
istics, with varying requirements for modification of the fuel, engine, control, and aftertreat-
ment systems. SI and CI fuels are subject to detailed specifications (ASTM D4814-10a-SI Fuels,
ASTM D975-10a-CI fuels) to ensure they meet minimum requirements for operation of modern
IC engines. Petroleum hydrocarbon fuels (gasoline and diesel) are a complex mixture of hundreds
of compounds of different molecular structures and atomic weights. The composition is made up
of straight-chained paraffins, cycloparaffins, alkenes, and aromatics. Ranges of atomic weights
result in a nearly continuous distillation curve. Gasoline is composed of C
4
-C
14
molecules with a
50% distillation point of 99°C (Totten et al. 2003). Diesel is composed of higher-molecular-weight
compounds (C
7
-C
24
) with a 50% distillation point of 256°C (Totten et al. 2003). Ratios of the
molecular components affect autoignition (Taylor et al. 2004), combustion, and emissions of soot
(Svensson 2005). Limits are placed on aromatics in fuels because they decrease the hydrogen-
to-carbon ratio and result in high soot emissions (ASTM D975-10a). Regulations as documented
in the standards also require reduced sulfur in the fuel to enable and improve performance of
advanced exhaust aftertreatment systems.
Table 10.2 lists characteristics of petroleum-based fuels and biofuels along with their applica-
tions. Included are the hydrogen- (column A) and oxygen (B)- to carbon ratios, mass density (C),
and specific energy and specific CO
2
emissions. As can be seen in Table 10.2, oxygenated fuels have
a lower energy density. For example, the energy density of gasoline is 44 MJ/kg, whereas that of
