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3 This document describes the results and findings of Airport Cooperative Research Program (ACRP) Project 02-03a, which characterized the emissions of gaseous hazardous air pollutants (HAPs) from idling aircraft engines during three measure- ment campaigns. The document begins with a discussion of the motivation for this project, and a summary of the test plan and approach is presented in Section II. Section III examines the effect of fuel flow on emissions. A method for scaling emission indices to reference values is described in order to enable data comparison of different engine technologies and different volatile organic compounds (VOCs). The relationship between emissions and ambient temperature is discussed in Section IV. Data from this project, as well as from other complemen- tary projects, have been used to develop a model for emission estimates when the engine is operating at near-idle power. The variability observed in the on-wing characterization of multiple engines will be discussed in the context of overall measurement uncertainty. Section V discusses a straight- forward estimation tool that is based on the analysis of mea- sured data trends. The estimation tool has direct applicability for quantifying emission levels in scenario evaluations and airport inventory modeling. The temperature and fuel flow corrections described here have been applied to a series of digital flight data records collected from in-use operations. Additional findings that occurred beyond the initial project statement are also summarized. Section VI includes a discussion of the apparent engine warm-up effect, the near-idle hydrocarbon emission profile, and the effect of fuel properties on emissions. Section VII provides a discussion of the relevance of the project findings to airport practice. Appendix A describes the results of the testing conducted for this project. The test matrix and test procedures developed specifically for the question of addressing near-idle emissions for on-wing engine testing are described in Appendix B. The methods employed for sampling specific VOCs without reactive or absorptive losses in the sampling probe are described in Appendix C. The data collected at the airport fence line (a mix of aircraft emissions and terminal area emissions) and its potential implications are described in Appendix D. The analytical methods used to characterize the engine exhaust and quality assurance procedures are collected in Appendix E. The aircraft exhaust matrix potentially consists of thousands of specific molecules containing carbon, hydrogen, oxygen, nitrogen, and sulfur. Sets and subsets of these emitted com- pounds are denoted by various terms. A variety of terms have been used to describe the species of interest in this docu- ment, including volatile organic compounds, hydrocarbons, un burned hydrocarbons, and partially burned hydrocarbons. Every attempt has been made in this document to keep the language concise and accurate. An important observation drawn from this work and previous measurements is that trends with near-idle engine state and ambient temperature in one volatile organic compound also apply to the compounds in the other classifications. ⢠A hazardous air pollutant (HAP) is defined by the U.S. Environmental Protection Agency (EPA) as a compound that causes or may cause cancer or other serious health effects. EPA lists 187 compounds as HAPs, 15 of which have been identified in aircraft exhaust. ⢠A hydrocarbon (HC) is a chemical compound that contains only carbon and hydrogen. ⢠An unburned hydrocarbon (UHC) refers to the total gas phase organic carbon content in the exhaust as measured by a flame ionization detector. This is the measurement reported by ICAO. ⢠A volatile organic compound (VOC) is defined by EPA as an organic compound that participates in atmospheric photo- chemical reactions. Originally, a VOC was defined based only on volatility, but the current definition makes the term inaccurate for describing the collection of gas phase organic compounds present in aircraft exhaust. Because the term S e c t i o n i Introduction
4is, however, so prevalent in aircraft emissions literature, we continue to employ it in this document. I.1 Aircraft Engine Emissions at Airports Government agencies and community groups frequently ask airport operators to provide information that enables an assessment of the health impacts of toxic emissions from air- craft and other airport-related sources. Two important catego- ries of pollutants, as classified by the Clean Air Act, are criteria pollutants (comprising particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead) and HAPs (e.g., benzene and formaldehyde). Information on the emission, transformation, and transport of aviation-related HAPs and their health impacts is rudimentary. Without a bet- ter understanding of aviation HAP emissions, airport operators cannot develop accurate emission inventories or adequately respond to the queries from state and local constituencies. The ACRP 02-03 study was undertaken in 2007 to examine and identify gaps in research on airport-related HAP emis- sions and to recommend and prioritize additional research to help understand potential impacts of those emissions. The study findings were published in ACRP Report 7: Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis, which concluded that idling jet engines are a key source of airport-related HAPs at most commercial airports (Wood et al. 2008). Understanding the scale and character of this emission source is a high priority for the airport community. The report recommended a targeted research effort to document the contribution of idling jet engines to HAP emissions. This report describes the resulting research project and the development of a model to estimate emissions from engines operating at near-idle engine states. Testing from earlier measurement campaigns conducted as part of the Aircraft Particle Emissions Experiment (APEX) in 2004 and 2005 (Herndon et al. 2009, Knighton et al. 2007, Timko et al. 2010, Wey et al. 2006, Yelvington et al. 2007) confirmed that emissions of VOCs from modern jet engines, many of which are classified as HAPs, are relatively small at medium and high power settings, but higher at low power settings (engine idle) (Spicer et al. 1994). Notwithstanding the contributions from the APEX measurements, knowledge of VOC emissions from aircraft at idle power settings was still limited. VOC emissions vary as a function of engine state, environmental variables (especially ambient temperature), and engine type. To quantify airport HAP emissions in the context of an airport inventory, data are needed on HAP emission rates as a function of low power settings and ambient conditions. This research project is aimed at arming airport operators with a simple methodology for improving estimates of HAP species in the airport operational context. The test program described here employs on-wing engine testing, pulling air- craft from active commercial service of cooperating airlines. The test matrix used in this project includes the engine state defined in the certification databank (ICAO 2006), but also includes engine states that reflect airport operational conditions developed with advice from airline propulsion engineers. The inclusion of different engines, each with a different maintenance history, implies that there may be variability in the results, particularly when probing an engine state far removed from the optimal combustor design. This project will attempt to âsee throughâ whatever variability is present in the key data trends and develop a simple methodology that can be used to estimate HAP emissions at different ambient temperatures and in-use fuel flow rates at the airport. Although every airport is unique and universal statements are not appropriate, several airport emission inventories report that the greatest source of HAP emissions is idling jet engines (Wood et al. 2008). At power (thrust) settings greater than idle, the combustion efficiency of modern jet engines is high, and emissions of carbon monoxide (CO) and VOCs, which are products of incomplete combustion, are thus small. Many of the VOCs are partially oxidized combustion hydro- carbons and are classified as HAPs (e.g., acetaldehyde). Aircraft engines operating at idle power settings are responsible for the majority of HAP emissions generated during landing/takeoff (LTO) cycles. This point can be illustrated by examining data from the emissions performance databank maintained by the International Civil Aviation Organization (ICAO). In Figure I-1, the emissions data (expressed as unburned hydro- carbon [UHC]) is depicted for an LTO cycle at an airport. The relationship between UHC and the gaseous HAP compounds has been established previously (Herndon et al. 2009) and will be expanded later. In Figure I-1, the emission rate is estimated as a function of time during a hypothetical operation. The estimate depicted by the solid line is based on the tabulated emissions performance data for aircraft equipped with two CFM56-2B24 engines Figure I-1. UHC emissions during a landing/ takeoff cycle.
5 operating at 15°C (59°F). The trace denoted by the dashed line is the estimate based on the emissions measurement observed in this work, assuming the entire idle phase is characterized by ground idle with a nominal bleed air demand (rather than the ICAO certification value of 7% thrust). For both cases the area under the particular mode is proportional to the total emissions. This figure demonstrates (for this engine) that the idle phase of airport activity in the standard LTO cycle is responsible for most of the hydrocarbon emissions from aircraft engines. The UHC emission rate for the auxiliary power unit (APU) has been taken from Table 6-1 of Wade (Wade 2002) and assumes a GTCP331-200 APU type. Other APU types that are typically installed on a narrow-body aircraft (Gerstle et al. 1999), such as that presumed by the CFM56-7B24 engine type in the example depicted in Figure I-1, have similar emission levels. The choice of time for APU use during the LTO is arbitrary and is included to provide a sense of how the APU emissions compare with those from the main engines. UHC emissions arising from the idle phase in the example in Figure I-1 account for 94% of total UHC emissions (for the CFM56-7B22). This is fairly typical of the aircraft engines tabulated in the ICAO databank. Figure I-2 depicts a histogram of this calculation (fraction of total UHC emissions from the idle phase during a standard LTO cycle), which shows that the idle phase dominates total UHC emissions for the vast majority of engine types. This project has focused on characterizing selected HAPs, total UHC, and other specific VOCs emitted by aircraft engines during the idle phase. I.2 Anatomy of the High-Bypass-Ratio Turbofan Engine Modern commercial aircraft are powered by engines typically classified as high-bypass-ratio turbofan engines. The combustor uses a turbine-driven fan to draw air into the combustor, and the exhaust outflow goes through the core turbine. A hollow shaft connects the core turbine to the core compressor, and a second shaft passes through the core and is connected to the outer fan. This is the scheme for a two-spool engine. There are variants that use an additional stage of compression and are called three-spool engines. The reason these engines are generally referred to as high-bypass- ratio turbofans is that the majority of the air flow (and hence the thrust) does not go through the combustor region, but rather bypasses the engine core. The generalized schematic depicted in Figure I-3 labels the major pieces of a turbofan engine, where pressure (P) and temperature (T) are noted with a number. For example, in this project, the effect of ambient temperature on the emis- sions of various HAP species would schematically manifest itself as a perturbation to T2, the temperature of the gas coming into the engine. Figure I-2. Histogram of the emissions arising from the idle portion of the ICAO LTO for all engines. Figure I-3. Schematic of turbofan engine. The component pieces (labeled on the lower portion of the figure) of a turbofan engine are depicted with air flow proceeding from left to right. The station numbers T2, P3, T3, and T4 are taken from Aircraft Engines and Gas Turbines (Kerrebrock 1977) and are referred to in subsequent sections of this report.
6I.3 Application of the Current Emission Model to Idle Phase Emissions The Emissions and Dispersion Modeling System (EDMS) was developed by the Federal Aviation Administration (FAA) and the US Air Force to assess air quality impacts of proposed airport development projects (FAA 2006). It has become the preferred computer model for air quality analysis at airports (Anderson et al. 2007). The principal source of emissions data for the aircraft engines is the ICAO emission databank (ICAO 2006). The protocol for emissions performance testing for carbon monoxide, nitrogen oxides (NOx), and unburned hydrocarbons is described in ICAO Annex 16 (ICAO 1993) and SAE Aerospace Recommended Practice (SAE 2006). The ICAO emissions databank tabulates exhaust emissions performance and other engine characteristics at four named conditions that are nominally indicative of operational states at the airport: takeoff, climb-out, approach, and idle. Each of the tabulated emissions values has either been measured at or scaled to the reference pressure (1 atm) and temperature (15°C). To project the certification values to other temperatures and pressures, the EDMS uses the Boeing Fuel Flow Method 2 (BFFM2) (DuBois and Paynter 2006). The premier method for calculating the engine emissions uses established semi- empirical relationships between emissions and combustion pressure and temperature and is known as the P3T3 method. The P3T3 calculations (NEPAIR 2003, Sarli et al. 1975) are based on several parameters that are engine specific and not widely available or published. The BFFM2 approach is sig- nificantly more general and estimates emissions performance using only the engine databank values. The comparisons of NOx emissions predictions based on BFFM2 with those based on the P3T3 method are excellent for a wide variety of engines operating at different ambient temperatures, pressures (altitude), and relative humidity. The comparisons of CO and UHC emissions are reasonable, but there are discrepancies. It is particularly challenging for the BFFM2 to use the logarithmic extrapolation of the emissions data at 7% thrust and 30% thrust to project emissions at the lower thrust values typical of actual operational use (DuBois and Paynter 2006).
