7.6.4. Current Understanding of Chemical Changes in Turbine and Exhaust Nozzle

Approximately 99.5-99.9% of the molar content of typical commercial engine exhaust consists of N2, O2, CO2, and H2O. The species that compose the remaining 0.1-0.5% exist in trace amounts. This trace exhaust component consists primarily of NOx, CO, unburned HC (including soot), the hydroxy family (HOx, H2Ox), the sulfur oxide family (SOxO), and elemental species such as O. Figure 7-26 provides a general categorization of chemical processes occurring in the turbine and exhaust nozzle. These processes are discussed in greater detail below.

7.6.4.1. Primary Exhaust Constituents (H2O, CO2, N2O2)

Apart from the small effect of reactions involving trace species, changes in major species concentration in the turbine and nozzle flow path are caused by the diluting effect of cooling air. CO2 changes less than a few tenths of a percent as a result of oxidation of CO (increases in H2O from HOx recombination are even smaller). This CO2 fractional increase may grow in the first stages of the high-pressure turbine as more advanced cycles are implemented because associated cycle changes may result in relatively more CO at the entrance to the turbine (Godin et al., 1995, 1997; Leide and Stouffs, 1996). Current small changes and likely future changes in primary exhaust constituents can be predicted with sufficient accuracy (Dryer et al., 1993) for assessment needs, however, and the levels are all relatively easy to derive from measurements.

7.6.4.2. Secondary Combustion Products (NO, NO2, N2O, SO2, CO, stable HC)

Secondary products-such as NO, NO2, and SO2, as well as their oxidative products SO3, HONO, HNO3, and H2SO4-formed via reactions initiated with the reactive radicals OH and O are the principal participants of interest in chemical and microphysical processes occurring soon after emission. Although OH and O are reduced considerably by the engine exit, they continue to play an important role in global atmospheric processes (see Chapters 2 and 3). To understand the processes occurring through the engine, relative and absolute levels of these secondary combustion products, their oxidative products (the acid gases), and the reactive radicals need to be accurately characterized. Emissions indices for NOx, CO, and HC, as measured by ICAO procedures for stages in a standard LTO cycle, are documented (ICAO, 1995b) for most in-use engines as part of the engine certification process; these emissions typically correspond to tens to hundreds of ppmv. SOxO emissions are directly proportional to the level of sulfur in the fuel [a 400 ppmm fuel S level corresponds to an EI(SOxO) of 0.8]. Emissions of metals, whether from impurities in the fuel or engine wear, are much smaller than the emissions discussed here but may be of interest in soot activation and condensation processes (Chen et al., 1998; Twohy et al., 1998). NOx does not change significantly through the turbine and nozzle other than through changes resulting from dilution, although the NO2/NO ratio may shift as a result of increased oxidation. Oxidation of NO and NO2 to HONO and HNO3, respectively, is predicted to be on the order of a few percent or less, occurring largely in the high-pressure turbine (Fahey et al., 1995a; Anderson et al., 1996; Lukachko et al., 1998). Although this change in the NOx level is not significant, changes in HONO and HNO3 represent important changes in trace species of NOy (see below). Ground-based and in-flight measurements indicate that emissions of N2O are also small relative to NOx (Kleffmann et al., 1994; Fahey et al., 1995b). Further validation of NOx chemistry is warranted, but indications are that current models can predict NOx evolution in the turbine and nozzle with sufficient accuracy for assessment needs (Dryer et al., 1993), and measurements of NOx with a few percent accuracy are possible.

For typical civil engines, CO and HC are relatively unchanged through the turbine and exhaust nozzle. However, they can be reduced by up to two orders of magnitude in the turbine and exhaust nozzles (Godin et al., 1995, 1997; Leide and Stouffs, 1996; Lukachko et al., 1998) of advanced cycle military engines, where completion of oxidation in the high-temperature regions of the turbine results in the modest increases of CO2 mentioned above. Measurements of these species CO2 and total hydrocarbons) are routine, and it is possible to measure them to several percentage points accuracy (Katzman and Libby, 1975; Spicer et al., 1992, 1994; Dryer et al., 1993; Howard et al., 1996).

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