Low-Emission, Fuel-Flexible Combustion of Liquid Fuels - Novel Combustion Concepts for Sustainable Energy Development - page 449

For example, rise in air traf c with global business, military, and private demands

has resulted in international recognition of environmental concerns associated with
aviation emissions. In response, the governing organizations are adopting
increasingly stringent regulations to limit emissions from the next generation of
aero-propulsion systems. In particular, nitric oxides (NO X ) emissions remain a
major concern owing to the harmful ozone and smog at ground levels and to the
formation of ozone, acid rain, and contrails in the troposphere (Porter 2007 ). In
addition to reducing emissions, higher fuel ef

ciency will require future aviation
gas turbines with higher pressure ratios. While higher pressure ratio tends to reduce
emissions of soot and greenhouse gas, NO X emissions tend to increase. Since NO X
production is sensitive to reaction-zone temperature, it is necessary to avoid “ hot

spots by burning liquid fuel premixed with air at relatively lean conditions.

In spite of the impressive gains in arenas of ef

”

ciency and emissions, present
combustion systems still rely upon petroleum-derived fuels gasoline, diesel, and jet

fuels. However, existing and future combustion systems for vehicular transportation,
power generation, etc., will increasingly utilize alternative energy sources to achieve
fuel

—

flexibility, while minimizing our dependence on petroleum-based fuels. In par-
ticular, biofuels produced from renewable biomass such as plant or animal matter offer
the possibility of one day becoming carbon neutral, meaning any carbon released
would have been absorbed during the fuel

fl

'

s life cycle. Biofuels also offer other
benefits: allowing energy independence and providing a renewable energy source that
can be around long after the world

s fossil fuel reserves have exhausted.
Biodiesels are among the most widely studied liquid biofuels because of their
similarity to diesel fuels (Graboski andMcCormick 1998 ; Raghavan et al. 2009 ; Song
et al. 2007 ; Bolszo and McDonell 2009a , b , Wang et al. 2011 ; Pan et al. 2009 ; Li et al.
2011 ; Park et al. 2011 ). Biodiesels are produced by transesteri

'

cation of source oils,

such as vegetable oil (VO). The transesteri cation process is necessary to modify the

physical properties of the source oil to produce biodiesel, which can readily replace
diesel fuel in an end-use device. However, transesteri cation requires substantial

energy input and results in glycerol by-product. With increasing biodiesel production,
glycerol can become an unintended waste stream, with its supply far exceeding the
demand in the cosmetics, medical, and chemical industries.
Table 1 lists the key thermo-physical properties No. 2 diesel, biodiesel, straight
VO, and glycerol (Panchasara et al. 2009a , b ). The energy content of biodiesel is
about 12 % less than that of petroleum-based diesel fuel on a mass basis. Biodiesel
is an oxygenated fuel, which contains 10

11 % oxygen by weight. The kinematic
viscosity of biodiesel is about 1.3 times that of diesel. Still, biodiesel properties are
closest to those of the diesel, and thus, biodiesel is the usual replacement for diesel.
In contrast, at room temperature, the kinematic viscosity of VO is over 13 times that
of diesel. VO also has higher auto-ignition temperature and higher speci

-

c heat
capacity compared to biodiesel or diesel. Because of these adverse thermo-physical
properties, VO cannot be used as a diesel substitute in existing liquid fuel com-
bustion systems, and thus, it is processed to generate the biodiesel fuel.
The situation is less favorable when comparing glycerol with diesel. The low
heat value (LHV) of glycerol is only 36 % of that of diesel on mass basis. In