Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
as the fuel/air mass ratio. The dependence of NO x on fuel/air ratio is shown in Figure 7-15. As illustrated, peak NO x formation coincides with peak temperature, which
occurs close to the stoichiometric fuel/air ratio (or equivalence ratio = 1). In current gas turbine engine combustors, there are always some regions of the flame that
burn stoichiometrically, so NO x formation is very strongly linked to combustor inlet temperature.
Table 7-3: Improvements in emissions levels of GE CF6-50E2 engine.
Levels for Levels for
Applicable Original "Low Emissions"
ICAO Production Production
Emission Standard Engine Engine
Smoke (SN) 18.8 6.5 12.5
HC (g kN-1) 19.6 57.8 3.4
CO (g kN-1) 118 97.3 29.8
NOx (g kN-1) 100 58.2 51.6
"Soot" generally refers to particulates in emissions. These particles are composed primarily of carbonaceous material, the sum of graphite carbon and primary organics
resulting from incomplete combustion of carbonaceous material (Novakov, 1982; Chang and Novakov, 1983). "Smoke" refers to combustion emissions particulates that
contribute to a visible plume. The formation of soot and its partial oxidation in gas turbine combustors are very complex processes. Soot is produced mainly in the fuelrich
primary zone of the combustor, then oxidized in the high-temperature regions of the dilution and intermediate zone. A simplified description of the soot production
mechanism is provided in Figure 7-16 (Mullins, 1988).
Estimates of the fleet averaged emission index of soot is 0.04g/kg fuel burned (Döpelheuer, 1997). However, the derivation of this estimate carries considerable
uncertainty and at best is only within a factor of 2. The formation of soot in gas turbine combustors and the precise design features and conditions that can influence
the process continue to be subjects of important ongoing research. Since 1990, there has been a marked improvement in the engine soot and aerosol emissions
database through skilled use of the most modern aerosol measurement techniques (Hagen et al., 1992; Rickey, 1995; Pueschel et al., 1997; Petzold and Schröder,
1998). State-of-the-art instrumentation permits detection of particles as small as 3 nm in diameter (Alofs et al., 1995). These data also show that jet engines emit soot
particles, which have log normal-type size distributions peaking in the 20-30 nm range. Concentrations range between 10 6 and 10 7 particles cm -3 (Hagen et al., 1992;
Whitefield and Hagen, 1995; Schumann et al., 1996; Pueschel et al., 1997; Anderson et al., 1998; Petzold and Schröder, 1998). Number-based emission indices fall
within the range of 10 12 soot particles per kg of fuel burned for current advanced combustors to 10 15 for older type engines (Howard et al., 1996; Whitefield et al.,
1996). A general range for most engines operating in the current commercial fleet is 10 14 -10 15 particles per kg fuel burned (Hagen et al., 1996; Döpelheuer, 1997;
Anderson et al., 1998; Petzold and Schröder, 1998; Petzold et al., 1999).
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