Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 198

11.3.2 Emissions
The emissions from an airplane are a function of fuel properties, the amount of fuel used, oper-
ating conditions and the combustion efficiency. To evaluate environmental impact, the following
emissions are important for their effects on climate change and public health:

• Carbon dioxide (CO 2 ) is a greenhouse gas that contributes to the warming of the planet. Airlines

account for ∼ 2.5-3% of global CO 2 emissions. Under conditions of complete combustion,
the CO 2 emissions are ∼ 3160 ± 60 g per kilogram of fuel [g/kg] (Lee et al. , 2010). Carbon
monoxide (CO) is also a regulated gaseous emission. Other greenhouse gases are also of
concern with respect to climate change.
• Water vapor (H 2 O) is emitted at a rate of 1230 ± 20 g/kg when fuel is completely burnt (Lee
et al. , 2010). For supersonic aircraft, water vapor is the primary concern for gaseous emissions.
At high altitudes, it can form contrails and promote formation of cirrus clouds. Contrails are
formed when the hot, moist exhaust of the aircraft mix with sufficiently cold ambient air.
Although the detailed mechanisms are not fully established at this time, contrail formation
may have an effect on climate change. Other hydrogen-containing compounds of concern are
hydroxyl radicals (OH) and hydrogen peroxide (H 2 O 2 ). The former is particularly important
in chemical processes that produce sulfuric acid (H 2 SO 4 ), NO x , and ozone (O 3 ).

• Sulfur oxide (SO x ) generation is directly proportional to sulfur content in the fuel. The subscript

x is meant to indicate that this is a class of compounds that contain sulfur and oxygen. Sulfur
dioxide (SO 2 ) is the most abundant sulfur-containing species. It is directly proportional to the
sulfur content in the jet fuel and is mostly produced under the high-temperature conditions of
the combustor (Lee et al. , 2010). These emissions may play a role in the development of acid
rain. The specifications permit an upper limit on sulfur content to 3000 ppm, although jet fuels
are typically in the range of 500-1000 ppm currently (Chevron, 2006). Such compounds may
also play a role through the formation of contrails and affect particulate emissions.

• Nitrous oxide (NO x ) is formed through reaction with atmospheric nitrogen under the high-

temperature conditions in the combustor. Trace quantities of nitrogen bound to the fuel will also
form NO x . At ground level, NO x is of concern because it is toxic and is a precursor to chemical
smog. It is linked to the creation of ozone in the troposphere (ground level to approximately
12,000m). When emitted above the troposphere, NO x is implicated in the destruction of the
stratospheric ozone layer.
• Particulate matter (PM) and unburned hydrocarbons (HCs) are the result of incomplete com-
bustion. Emissions of this pollutant strongly depend on engine design and operating conditions,
but fuel characteristics are also important. At altitude, PMcan act as nucleation sites for contrail
formation. Since the aromatic content in alternative fuels is nearly negligible, the generation of
PM is also significantly reduced in terms of particle size, number density, and total PM mass
(Timko et al. , 2011).

The effects of these pollutants on the atmosphere are compared through their impact on radiative
forcing (RF) of the climate. The mean global surface temperature is directly related to RF. A
positive RF corresponds to a warming effect, which is characteristic of CO 2 and soot emissions.
A negative RF contributes to a cooling effect on the atmosphere, which is typical of sulfate particle
emissions. NO x emissions result in a positive RF due to formation of tropospheric ozone (O 3 ),
while it can induce a negative RF contribution from the destruction of ambient methane (CH 4 ).
This framework provides a way to link the pollutants, so that the net effect on the atmosphere can
be computed as a summation of all of the contributions.
Near ground level, air quality is an issue. At the low power conditions that are typical of
airplane takeoff and landing, fuel is less likely to undergo complete combustion. This leads to
higher emissions of CO, NO x , HC and PM, which may induce smog formation and haze. PMs
are also linked to respiratory distress and increased mutagenic potential (Krahl et al. , 2005) that
could be a factor in the development of lung cancer.