Effects of Fuel Aromatic Content on Nonvolatile Particulate Emissions

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Effects of fuel aromatic content on non-volatile particulate emissions of an in-production aircraft gas turbine Benjamin Brem, Lukas Durdina, Frithjof Siegerist, Peter Beyerle, Kevin Bruderer, Theo Rindlisbacher, Sara Rocci Denis, M. Gurhan Andac, Joseph Zelina, Olivier Penanhoat, and Jing Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04167 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 29, 2015

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Environmental Science & Technology

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Effects of fuel aromatic content on non-volatile

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particulate emissions of an in-production aircraft gas

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turbine

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Benjamin T. Brem1,2, Lukas Durdina1,2, Frithjof Siegerist3, Peter Beyerle3, Kevin Bruderer3,

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Theo Rindlisbacher4, Sara Rocci-Denis5 M. Gurhan Andac6, Joseph Zelina6, Olivier Penanhoat7

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and Jing Wang1,2,*

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CH-8600 Dübendorf, Switzerland

Empa Material Science and Technology, Laboratory for Advanced Analytical Technologies,

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ETH Zürich, Institute of Environmental Engineering (IfU), CH-8093 Zürich, Switzerland

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SR Technics, CH-8058 Zürich Airport, Switzerland

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Swiss Federal Office of Civil Aviation, CH-3003 Bern, Switzerland

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General Electric Aviation, D-85748 Garching bei München, Germany

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General Electric Aviation, Evendale, OH 45241, USA

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SNECMA Villaroche center, F-77550, Moissy-Cramayel, France

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*Corresponding author

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Tel. +41 44 633 36 21; Fax: +41 58 765 6963; E-mail: [email protected]


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Abstract

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Aircraft engines emit particulate matter (PM) that affects the air quality in the vicinity of airports

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and contributes to climate change. Non-volatile PM (nvPM) emissions from aircraft turbine

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engines depend on fuel aromatic content, which varies globally by several percent. It is uncertain

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how this variability will affect future nvPM emission regulations and emission inventories. Here

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we present black carbon (BC) mass and nvPM number emission indices (EIs) as a function of

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fuel aromatic content and thrust for an in-production aircraft gas turbine engine. The aromatics

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content was varied from 17.8 % (v/v) in the neat fuel (Jet A-1) to up to 23.6 % (v/v) by injecting

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two aromatic solvents into the engine fuel supply line. Fuel normalized BC mass and nvPM

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number EIs increased by up to 60% with increasing fuel aromatics content and decreasing engine

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thrust. The EIs also increased when fuel naphthalenes were changed from 0.78 % (v/v) to 1.18 %

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(v/v) while keeping the total aromatics constant. The EIs correlated best with fuel hydrogen mass

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content, leading to a simple model that could be used for correcting fuel effects in emission

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inventories and in future aircraft engine nvPM emission standards.

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1. Introduction

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Incomplete combustion produces carbonaceous particulate matter (PM) that is commonly

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referred to as soot or black carbon (BC). These emissions are a nuisance, degrade the

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environment and affect human health and the earth’s climate. Modern civil aviation gas turbine

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engines have combustion efficiencies greater than 98%1 and therefore their global BC mass

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emission is estimated to be minor in comparison to other emission sources2. However, the

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majority of aviation BC mass emissions are estimated to occur during cruising in the upper

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troposphere3 where other direct anthropogenic emissions are absent and their climate impact per

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unit mass is much larger than any other component of aircraft emissions, particularly over polar

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regions4, 5. Cruising BC emissions also provide surface area for chemical reactions and

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microphysical processes that lead to contrail formation and induced cloudiness6.

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The estimates of the aviation BC radiative forcing vary significantly7-9, in particular due to the

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uncertainties in the indirect forcing10-12. In addition to climate impacts, the steady increase in air

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traffic and its related emissions is also a concern for local air quality in the vicinity of airports13-

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17

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gradually received more and more attention regarding their potential health impacts18-20.

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Current civil aviation BC mass emission inventories are a burgeoning uncertainty because most

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of them have been empirically derived from filter-based smoke number (SN) measurements21, 22

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or combustion models23. SN data have been available through the International Civil Aviation

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Organization (ICAO) engine emission database since 1983, when ICAO began regulating smoke

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emissions of all gas turbine engines. The ICAO SN regulation has conceivably reduced the BC

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mass emissions in the last decades. However, researchers and regulatory agencies need up-to-

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date and accurate aviation emission inventories that include measured BC mass and non-volatile

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PM (nvPM) number data to candidly assess the potential aviation impacts on climate and human

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health. The ICAO SN is expected to be updated by a regulation for BC mass and nvPM number

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emissions for new engines with thrust levels greater than 26.7 kN24. This regulation could

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provide the basis for such new inventories. The SN is considered insensitive to fuel composition

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within the allowed specifications25. Because BC mass and nvPM number measurements are more

. The ultrafine particles (< 100 nm in diameter) emitted by aircraft gas turbines have also

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sensitive than SN measurements, fuel composition effects may need to be considered and

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corrected for.

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Jet fuel has been standardized as Jet A or Jet A-1 (terminology varies), with the main focus on

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operational stability. The Jet A-1 specification26 allows notable compositional variations that

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depend on crude oil feedstock and refining process (e.g. sulfur removal). For example, the Jet A

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fuel used in North America typically contains more total aromatics and naphthalenes than

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European Jet A-127. The link between fuel chemistry and soot emissions has long been the focus

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of gas turbine research, and increased levels of mono-aromatic and naphthalenic hydrocarbons in

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the fuel have been associated with higher soot emissions28-30. Aromatics promote the formation

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of large polycyclic aromatic hydrocarbon (PAH) molecules and the subsequent soot nucleation

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and growth in fuel-rich pockets in the combustion zone31-33. Therefore, engine operating

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conditions, in particular fuel to air equivalence ratio (FAR) and engine combustor temperature

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and pressure, further compound the effect of aromatics on soot emissions34-36. Several studies

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have shown that fuel hydrogen mass content or the fuel hydrogen to carbon ratio anticorrelated

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better with sooting tendencies of fuels than fuel aromatics37, in particular when the naphthalenes

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content was below 5 % (v/v)34, 38. Hydrogen content has strong interdependencies with fuel total

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aromatics and naphthalenes content; on the other hand, it can also account for the chemical

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differences between mono-aromatics and naphthalenes.

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Concerns about energy security and environmental impacts of aviation in recent years have

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resulted in field campaigns that characterized emissions from aircraft engines39-41, helicopter

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engines35,

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blends. An analytical approximation has been developed that relates the BC mass and number

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emissions with fuel total aromatics content and static engine thrust based on some of these

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experimental datasets46. The model developed explained 72% of the variability in number

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emissions and 56% of the variability in BC mass emissions in spite of the many sampling

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systems and technologies that were used to gather the emissions data in the various campaigns. A

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more thorough regression analysis of the various NASA studies data identified the fuel

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naphthalenes content as the determining factor for the magnitude of the nvPM number and BC

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mass emissions47. All these research efforts have covered a wide range of fuel total aromatics

36, 42, 43

and auxiliary power units44,

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burning alternative fuels and alternative fuel

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and hydrogen content with the goal of investigating high blending ratios of potential alternative

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fuels. No study to date has examined in detail the effects of total fuel aromatics content on BC

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mass and nvPM number emissions in the range of actual fuels in use and in the relevant range for

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engine certification (15 % – 23 % (v/v)). Furthermore, no one has systematically tested the

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effects of fuel composition on emissions on an in-production turbofan with a standardized

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sampling methodology as would be used for emissions certification.

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This research addresses these questions through a systematic fuel sensitivity study on emissions

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with a unique aromatic solvent injection system installed in the fuel supply line of an in-

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production turbofan engine. The aims are: (1) to study the effect of fuel total aromatics content

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on the nvPM emissions in an emissions certification-like setting for four levels of total aromatics

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content ranging from 17.8 % to 23.5 % (v/v); (2) to assess the influence of naphthalenes vs.

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mono-aromatics content on the emissions; and (3) to provide a suitable parameterization of the

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measured results that can be used to accurately correct engine nvPM emission data for fuel

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effects. Understanding these aspects is not only necessary for future engine certification, but also

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important for assessing the impacts of the aviation industry on the environment, the climate and

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human health. Potential BC emission reductions could be also estimated, if airlines increasingly

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start blending conventional Jet A-1 with small fractions of alternative fuels.

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2. Experimental

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This experiment was part of the Aircraft Particulate Regulatory Instrumentation Demonstration

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Experiment (A-PRIDE) 7 that was conducted in September and October 2014 in the engine test

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cell of SR Technics at Zürich Airport.

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2.1. Engine Operation

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An in-production, hi-bypass turbofan engine was leased for these measurements. The engine

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used was considered “well run-in” at approximately half of its expected service life before an

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overhaul. Experiments were performed on two consecutive test days in October 2014. On both

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days the fuel total aromatics content was varied with a specifically-built solvent injection system

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connected to the engine fuel line (Fig. 1). The tests started with dry motoring followed by a half 5


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hour warm-up sequence that included minimum idle, 7%, 65% and 85% relative static sea level

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thrust levels of 5 minutes each. After the warm-up, an engine thrust matrix (SI Tab. S1) that

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included the proxy static sea level engine thrust levels of take-off (100%), climb out (85%),

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cruise (65%), approach (30%), taxiing (7%) and minimum idle (3%) running from high to low

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thrusts was performed. Each thrust level included consecutive measurement of the neat fuel (17.8

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% v/v aromatic content), the targeted aromatic content levels and another measurement for the

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neat fuel to account for potential drifts. Between the different aromatic levels at thrusts below

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30%, the thrust was increased to 85% to burn off the previous fuel blend in the fuel line such that

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the test matrix could be executed more rapidly and the nvPM signals could stabilize more

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quickly.

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The engine thrust levels were controlled according to the engine combustor inlet temperature

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(T3, proprietary value) for which the engine manufacturer knows the corresponding thrust levels

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for international standard atmospheric (ISA) conditions (15 °C, 1013.25 hPa). While the actual

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thrust is a function of ambient temperature and pressure, manufacturers commonly control the

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engine by T3 during emissions certification tests and report ISA corrected emissions data. The

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ambient conditions during both test days were fairly stable with temperatures ranging from 15.9

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to 20.1 °C and the pressures from 965 to 967 hPa, respectively.

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2.2. Fuel Parameters and Control of Fuel Aromatics Content

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To increase the fuel total aromatics content, the neat fuel was blended with two petroleum-

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derived aromatic solvents, which encompass the boiling point and molecular weight range of the

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aromatics in Jet A-1. The first aromatic solvent used was the naphthalenes depleted Solvesso

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150ND (ExxonMobil Chemical Inc.). The second one, Solvesso 150 (ExxonMobil Chemical

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Inc.), contained 6 % (v/v) naphthalenes, but otherwise had nearly identical specifications (SI

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Tab. S2). Solvesso 150 was previously used to increase the aromatic content in synthetic Fischer

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Tropsch jet fuel for studying material compatibility and emissions of a helicopter engine43. Two

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diaphragm metering pumps (Sigma type, Prominent Inc.) with maximum feed rates of 400 L/ h

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and 65 L/ h (at the fuel line pressure of 3.2 bars) fed the aromatic solvents into the engine fuel

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supply line from a 1000 L tank (Fig.1). The feed rates of the pumps were chosen so that the

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aromatic solvents could substitute up to 10 % (v/v) of the engine fuel flow at all engine thrusts. 6



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A Coriolis type flow meter (Promass 40E, Endress & Hauser) determined mass feed rates of the

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solvents. The turbine wheel fuel flow gage of the test cell provided the total mass flow of fuel

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and fuel-solvent mixtures to the engine. Fuel samples for offline chemical analysis were drawn at

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a sampling port 7.5 m downstream of the injection point (13.5 m upstream of the engine). For

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each aromatic level a fuel sample was taken when the nvPM levels were stabilized. All fuel

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samples were analyzed for total aromatics content (ASTM D1319), naphthalenes (ASTM

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D1840), and hydrogen mass (ASTM D5291). Moreover, the Jet A-1 fuel, the two solvents, and

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all the fuel blends at the 85 % thrust level were analyzed for the parameters listed in ANNEX 16

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VOL II Appendix 425.

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2.3. Sampling Procedures and Data Analysis

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Engine exhaust was sampled by an Inconel 600 alloy multipoint sampling probe that was

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specifically built for this engine (Fig. 1). This probe covered the engine core exhaust flow with

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12 equal area distributed orifices at a 120 mm horizontal distance from the exit plane. The

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sampling system complied with the Society of Automotive Engineers Aerospace Information

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Report (AIR) 624124. The only difference is in nomenclature: this research project refers to

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nvPM mass as BC mass. The exhaust sampled from the probe was transported through a 5.2 m,

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160 °C heated stainless steel line to the first splitter. The residence time of the sample in this

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undiluted section ranged from 0.6 s at engine idle to less than 0.1 s at 100% engine thrust.

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Approximately 7 standard liters per minute (SLPM) of this flow were then drawn and diluted by

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a factor ranging from 9.1 to 10.3 with synthetic air (purity 5.0) using an eductor dilutor (DI 1000,

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Dekati Inc). The dilution step is critical to minimize coagulation, condensation, and gas-to-

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particle conversion. 25 SLPM of this diluted flow were then transported through a 60 °C, 24.5 m

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temperature controlled line to the PM instrumentation.

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BC mass was determined with a Micro Soot Sensor48 (MSS, Model 483, AVL Inc.) based on the

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photo-acoustic detection principle at a wavelength of 808 nm. This instrument has a limit of

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detection of 1 µg m-3 and was calibrated according to the procedure described in the AIR 6241

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using elemental carbon mass as the reference. To take potential gas phase artifacts on the photo-

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acoustic principle into account, the instrument was zeroed with filtered engine exhaust before

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each measurement cycle. In parallel to the MSS, a particle counter49 (APC, Model 489, AVL 7


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Inc.), a scanning mobility particle sizer (SMPS, Model 3938, TSI Inc.) and a CO2 analyzer

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(Model 410i, Thermo Inc.) measured nvPM number concentrations, particle size distributions,

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and the CO2 concentrations of the diluted line, respectively. The APC instrument has two

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dilution steps with an in-between 350 °C heated oxidation catalyst. After the second dilution, a

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condensation particle counter (CPC) that has a counting efficiency of greater than 50% for

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particle diameters of 10 nm determines the nvPM number concentration. In parallel to the nvPM

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instrumentation on the 25 m, 160 °C heated undiluted line, CO2, oxides of nitrogen (NOx) and

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carbon monoxide (CO) were monitored with a multi gas analyzer (PG-250, Horiba Inc.).

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Hydrocarbons (HC) were also determined on this line with a flame ionization detector (MEXA

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1170 HFID, Horiba Inc.).

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We report emission indices (EIs, quantity of species per mass of fuel burned) and normalized

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emission indices referenced to the neat fuel. The emission index calculation considered CO2, CO,

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and HC species for predicting the fuel burned24. The diluted and undiluted CO2 concentrations

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were used to account for dilution. The change in the fuel hydrogen to carbon ratio when the fuel

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total aromatics content was varied was also taken into account. The formula for the EI

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calculation can be found in the SI. Particle losses in the sampling lines are not considered in the

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results presented. However, estimates of particle loss correction factors in terms of mass and

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number as well as count median diameters and geometric standard deviations of the SMPS

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measured particle size distributions can be found in the SI Tab. S3.

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3. Results and Discussion

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3.1. Fuel Properties

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The solvent injection allowed the changing of the fuel aromatic content with high precision (±

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0.5 % v/v) independent of solvent type at engine thrusts greater than 7 % (Fig. 2a). However, the

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total aromatic content measured downstream of the injection port was on average 17 % lower

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than the targeted total aromatics levels calculated based on fuel and solvent feed rates. At engine

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thrust levels equal and below 7%, the targeted total aromatics contents were not achieved for

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both solvents. 8



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The 17% difference can be explained by the inaccuracies in the fuel flow and solvent flow

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measurements and a possible bias in the determination of the total aromatic content by

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fluorescent indicator adsorption (FIA), which might not be able to resolve all of the aromatic

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species present in the added solvents. Accuracy issues and parallax shifts when interpreting the

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results of the FIA analysis have been reported previously50, 51, nevertheless, the method was used

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in this study for compliance with the engine operating guidelines. At 7 % and minimum idle

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engine thrust, the targeted aromatic concentrations were 23 or 24.5 % (v/v), but the measured

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values lay between 18.5 and 19.5 % (v/v). An explanation for the insufficient aromatic solvent

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injection at these thrust levels could be the calibration of the test cell fuel flow measurement at

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the low fuel flows present at these engine operating conditions. An investigation after the

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experiment indicated that the reported flow rate was up to 8 % lower than actual fuel flow rate at

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engine idle, which resulted in a lower targeted solvent feed rate and subsequently in lower total

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aromatics content in the fuel mixtures than initially planned. Because of this discrepancy, the

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data at engine thrust below 30% were excluded in further analysis. Fig. 2b displays the effect of

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the two different solvents on the hydrogen content measured in the fuel mixtures. The addition of

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Solvesso 150 increased the naphthalenes content from 0.78 % (v/v) (neat fuel) to a maximum of

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1.18 % (v/v) in the mixtures while keeping the total aromatics level approximately constant.

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Unsaturated hydrocarbons such as mono-aromatics and naphthalenes lose two hydrogen atoms

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per double bond and per ring structure, which results in a significant lower hydrogen mass

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content. The reduction in hydrogen mass due to the added naphthalenes is evident for the

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Solvesso 150 in Fig. 2b. Physical properties of the fuel mixtures varied little with solvent

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injection and solvent type. As the solvent content increased, fuel density increased from 798 kg

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m-3 to 806 kg m-3 and the kinematic viscosity decreased from 3.55 mm2 s-1 to 3.45mm2 s-1,

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consistent with the properties of the neat fuel and solvents (SI Tab. S2).

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3.2 PM Emissions

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The mass and number emission profiles (Fig. 3), are characteristic of this engine type and have

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been reported in previous studies47,

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slightly higher at minimum idle than at 7 % thrust and steadily increase with increasing thrust

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levels above 30 % (Fig. 3a, c). In contrast to the mass, the number EIs show distinctly higher

52-55

. Typically, BC mass EIs from this engine type are

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emissions at idle and peak in both experiments near the 65 % thrust condition (Fig. 3 b, d), likely

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due to particle coagulation at thrust levels greater than 65 % within the engine. The mass and

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number EIs with neat fuel (blue symbols) have nearly identical profiles, indicating stable

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ambient conditions and good reproducibility of the measurements on both test days. An increase

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in both mass and number emissions with increasing fuel total aromatics content is clearly

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distinguishable for both solvents at thrust levels greater than 7 % where the solvent injection

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system worked properly. The increased soot formation with higher total aromatics content is in

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concert with previous studies39,

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aromatic molecules in the fuel act as condensation and addition sites for products of incomplete

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combustion for forming PAHs that subsequently nucleate and carbonize to form soot31, 33. These

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reactions depend on temperature and pressure, local FARs and the residence time under these

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conditions1. In contrast to aromatics, aliphatics have to first undergo fragmentation and

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subsequent aromatic ring formation reactions33. These additional reaction steps are slower

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relative to the aromatics pathway, resulting in less soot formation. In contrast to mono-aromatics,

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naphthalenes should further increase the formation of large PAHs, which subsequently results in

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additional soot inception. However, the difference in the effect of the two solvents on the

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emissions is hard to distinguish in Fig. 3; only slightly higher number EIs are apparent in panel

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(d).

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To investigate the effects of different solvent types and correlation parameters, we looked at the

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relative changes in BC and nvPM number EIs (Fig. 4). The BC mass and nvPM number EIs

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increased by up to a factor of 1.59 and 1.51, respectively, due to the 5.8 % (v/v) increased fuel

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total aromatics content at 30% engine thrust. With increasing engine thrust, the aromatics’

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influence on soot emissions became less pronounced. At 100 % engine thrust, the EIs increased

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by a factor of 1.12 for BC mass and 1.06 for nvPM number. A similar thrust dependence of the

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aromatics’ effect has been observed in measurements of a helicopter gas turbine that was

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operated with different biofuel blends at cruise and idle thrust35. It is surmised that the lower

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combustor temperatures and pressures at 30 % thrust (in comparison to 100 % thrust) result in a

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less efficient combustion of the aromatic species in the rich-burn, quick-quench, lean-burn

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(RQL) combustor design employed in the engine studied. In such a combustor the fuel is initially

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burned and pyrolyzed at a rich FAR with a fast subsequent dilution to overall lean conditions

43, 44

. This dependence has been explained as follows: parent

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where products of incomplete combustion are consumed. This combustor is optimized for low

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NOx, because generally lower flame temperatures can be achieved in comparison to traditional

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designs. Soot emissions are the product of complex reactions and depend on temperature, local

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FAR and residence time in both rich- and lean-burn zones. While the soot oxidation in the lean

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zone is not affected by fuel chemistry, the fuel pyrolyzation and decomposition reactions in the

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rich zone are. The thrust dependence of the aromatics’ effect could be explained by the change in

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local FAR within the rich zone that decreases from rich conditions at 100% thrust to near

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stoichiometric conditions at minimum idle. At 100 % thrust the additional PAH formation due to

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the fuel aromatics as explained previously is miniscule in relation to the formation of fuel

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radicals and products of incomplete combustion that are occurring under such rich FAR

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conditions. At low thrusts, the FAR is near stoichiometric conditions and the fuel aromatics to

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PAH to soot reaction pathways become more relevant for the overall soot emissions. Therefore,

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in addition to the fuel aromatic content, the local FAR in the fuel rich zone is the determining

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variable for the soot emissions of this engine. However, FAR values are proprietary and

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consequently engine thrust was used as a proxy in the regression analysis below.

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The type of fuel aromatics plays an additional role as shown in Fig. 4 (circles vs crosses in (a)

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and (b)). Substituting mono-aromatics with naphthalenes from 0.78 % (v/v) to up to 1.19 % (v/v)

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at nearly identical fuel total aromatics content results in up to 40 % higher BC mass and up to 30

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% higher nvPM number EIs at the 30 % engine thrust level. The naphthalenes’ effect is

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indistinguishable at 100 % engine thrust, which agrees with the explanations above that fuel

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chemistry plays a less significant role at high thrust levels. However, some of the difference in

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the BC mass EI ratio at 30 % engine thrust could be attributed to the loss in precision of the MSS

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(resolution of 1 µg m-3) at the low BC concentrations (6 to 10 µg m-3 at the instrument) at this

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engine thrust setting. Therefore, further investigation is needed to determine the exact extent of

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the naphthalenes’ effect on BC mass. Detailed tabulated fuel properties and emissions data for

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this study can be found in the SI Tab. S4.

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Using the fuel hydrogen mass content as the correlating variable for the soot EIs (Fig. 4 (c) and

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(d)) reduces the naphthalenes’ effect. In particular the change in nvPM number EIs anticorrelated

290

more favorably at low engine thrust with fuel hydrogen mass content, which confirms the 11


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findings of earlier studies34, 37, 38. However, recent studies46, 47 contradict this finding and identify

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total aromatics content46 or naphthalenes47 as the best correlating variable for soot emissions. It

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is rather difficult to compare our study to these studies because we measured a fairly narrow

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range of aromatic content within Jet A-1 specifications with standardized sampling equipment.

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Former studies have primarily investigated alternative fuel and alternative fuel blends with

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different sampling systems and instruments.

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3.3. Regression Analysis

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A simple model (Eq. 1) was fitted to the experimental data to predict changes in BC mass and

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nvPM number EIs as a function of engine thrust and fuel hydrogen mass content.

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∆‫ = ୶ܫܧ‬൫ߙ଴ + ߙଵ × ‫ܨ‬෠ ൯ × ∆‫( ܪ‬Eq. 1)

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In Eq. 1 ∆EIx corresponds to the percentage change in BC mass or nvPM number EI, α0, α1 are

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fitting parameters, ‫ܨ‬෠ is the percentage of engine thrust and ∆H is the change in hydrogen mass

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content. The model is comparable to the one used by Speth et al.46 that predicts changes in BC

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mass as a function of fuel aromatic content and engine thrust; however, our model uses the

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change in hydrogen mass content as one of the independent variables and can be applied to both

306

BC mass and nvPM number. The method of least squares was applied to determine α0, α1 which

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are provided in Tab. 1.

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The fits were performed for BC mass and nvPM number (indicated as dashed lines in Fig. 4 (c)

309

and (d) and as surface plots in Fig. 5) and for the combined BC mass and nvPM number dataset

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(SI Fig. S1). The model explains the variability in the data, with coefficients of determinations

311

(R2 values) greater than 0.92. It captures the changes in emission indices within ± 5 % for the BC

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mass and nvPM number data sets. An exception is the 30 % thrust point with the lowest

313

hydrogen mass content where the model underpredicts the BC mass emissions by 12 % and the

314

nvPM number emissions by 8%. Combining the BC mass and nvPM number dataset does not

315

improve the fitting further, but could be used as a simplification. We anticipate that this model is

316

valid for engines that use similar technology RQL combustors and burn fuels that are compliant

317

with Jet A-1 specifications. This engine technology is most prevalent in the current fleet. 12



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However, the applicability of the model for the engine thrusts below 30 % and changes in

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hydrogen mass content greater than 0.6 % (m/m) needs further investigation and extrapolations

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should be done carefully.

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3.4. Implications

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This research fills an important gap in the understanding of the effects of fuel aromatics on the

323

BC mass and nvPM number emissions of an in-production aircraft gas turbine engine that is

324

representative of the current fleet. The use of a state of the art sampling system allowed us to

325

detect changes in emissions due to changes in fuel aromatics content that are representative for

326

the variability observed within standard jet fuel that is sold commercially27. SN measurements

327

which provide the basis of our current inventories are not sensitive enough to detect such

328

changes in emissions and therefore such inventories can be biased significantly by variations in

329

fuel aromatic content. Therefore, the implications of this research are threefold: (1) The

330

environmental nvPM certification of gas turbine aero engines, which is currently under

331

development, could use the model of this paper to correct for changes in BC mass and nvPM

332

mass emissions induced by the variability in the certification fuel. This standardization might

333

also be necessary to set future nvPM emission regulatory levels. (2) Emission inventories

334

relevant for local air quality and climate could be corrected for variation in fuels. For example,

335

the comparison of two ICAO standard landing and take-off (LTO) cycles with the engine

336

measured in this work, one calculated with 14.3 % (m/m) hydrogen content representative of

337

Zürich and the other one calculated with 13.9 % (m/m) representative of Toronto27, would result

338

in 12 % and 19 % higher LTO BC mass and LTO nvPM number emissions in Toronto compared

339

to Zürich (assuming the model of this study is applicable at 7 % thrust). Assuming that the 65%

340

ground level engine combustor inlet temperature is a valid proxy for the engine combustor

341

condition for cruise56, the BC mass and nvPM number emissions would also be 22% and 15%

342

higher at cruise using the Toronto fuel. Notable reductions in nvPM emissions and potential

343

improvements for local air quality and climate can therefore be made by modestly improving the

344

standard jet fuel refining processes or the oil feedstock used. (3) BC mass and nvPM number

345

emissions from gas turbine engines burning Jet A-1 blended with alternative fuels at low ratios

346

(< 10 % (v/v)) are expected to follow a similar trend with fuel hydrogen mass content as 13


Page 15 of 24


347

observed in this study and should be investigated. To accurately assess these implications,

348

additional research should cover thrust levels lower than 30 %, use fuel innate aromatics, and

349

other engines equipped with further optimized RQL combustor types. In addition, a

350

complementary investigation should be carried out on engines equipped with novel lean burn

351

combustors which might show a different sensitivity to fuel aromatics and hydrogen content than

352

shown here.

353

Supporting Information

354

The following supporting information is available free of charge on the ACS publications

355

website: experimental matrix; fuel and aromatic solvent properties; description of the emission

356

index calculation; estimated particle losses in the sampling system; tabulated measurement data

357

and surface fit of the combined change in mass and number emissions. This information is

358

available free of charge via the Internet at http://pubs.acs.org/.

359

Acknowledgments

360

This research would not have been possible without the support of the Swiss Federal Office of

361

Civil Aviation (FOCA) project “Particulate Matter and Gas Phase Emission Measurement of

362

Aircraft Engine Exhaust”. Support in terms of logistics, analysis, and calibration services and

363

loaner instruments was received from Andrea Fischer at Empa, SR Technics, Transport Canada,

364

the National Research Council of Canada, Intertek AG, Endress and Hauser AG, and TSI Inc. B.

365

Brem further acknowledges the support from the Empa postdoctoral fellowship.

366

References

367 368 369 370 371 372 373 374 375 376 377

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50. Norris, T. A.; Rawdon, M. G., Determination of hydrocarbon types in petroleum liquids by supercritical fluid chromatography with flame ionization detection. Analytical Chemistry 1984, 56, (11), 1767-1769. 51. Bansal, V.; Krishna, G. J.; Singh, A. P.; Gupta, A. K.; Sarpal, A. S., Determination of Hydrocarbons Types and Oxygenates in Motor Gasoline: A Comparative Study by Different Analytical Techniques. Energy & Fuels 2008, 22, (1), 410-415. 52. Herndon, S. C.; Jayne, J. T.; Lobo, P.; Onasch, T. B.; Fleming, G.; Hagen, D. E.; Whitefield, P. D.; Miake-Lye, R. C., Commercial Aircraft Engine Emissions Characterization of in-Use Aircraft at Hartsfield-Jackson Atlanta International Airport. Environmental Science & Technology 2008, 42, (6), 1877-1883. 53. Kinsey, J. S.; Dong, Y. J.; Williams, D. C.; Logan, R., Physical characterization of the fine particle emissions from commercial aircraft engines during the Aircraft Particle Emissions eXperiment (APEX) 1-3. Atmospheric Environment 2010, 44, (17), 2147-2156. 54. Lobo, P.; Durdina, L.; Smallwood, G. J.; Rindlisbacher, T.; Siegerist, F.; Black, E. A.; Yu, Z.; Mensah, A. A.; Hagen, D. E.; Miake-Lye, R. C.; Thomson, K. A.; Brem, B. T.; Corbin, J. C.; Abegglen, M.; Sierau, B.; Whitefield, P. D.; Wang, J., Measurement of Aircraft Engine NonVolatile PM Emissions: Results of the Aviation-Particle Regulatory Instrumentation Demonstration Experiment (A-PRIDE) 4 Campaign. Aerosol Science and Technology 2015, 49, (7), 472-484. 55. Lobo, P.; Hagen, D. E.; Whitefield, P. D.; Alofs, D. J., Physical characterization of aerosol emissions from a commercial gas turbine engine. Journal of Propulsion and Power 2007, 23, (5), 919-929. 56. Döpelheuer, A.; Lecht, M. In Influence of engine performance on emission characteristics, RTO Meeting proceedings, 1999.

532 533

18



Page 20 of 24

534 535

Fig. 1 Overview of the experiment: engine fuel supply components and the specifically built

536

aromatic solvent injection system are shown in brown and green, respectively. The sampling

537

system for nvPM and gaseous pollutants in the core flow of the engine exhaust is indicated in

538

orange and red, respectively. A multi-orifice cruciform sampling probe specifically

539

manufactured for this engine was used to collect PM laden exhaust.

19


Page 21 of 24


540 541

Fig. 2 a) Measured fuel total aromatics concentration vs. calculated targeted concentration

542

based on fuel and solvent feed rates. b) Aromatics content vs. hydrogen content; error bars are

543

the stated accuracies of the methods. Blue and red symbols in both panels correspond to

544

Solvesso 150ND (naphthalenes depleted) and Solvesso 150 (6 % (v/v) naphthalenes),

545

respectively.

20


Aromatics Concentration (%v/v) 18 19 20 21 22 23

0.04

0.02

0.00 0

20

40

60

Page 22 of 24

5E+14

a) BC Mass 0.06 Solvesso 150ND

Ei nvPMNumber [kg-1Fuel]

Ei BCMass [g kg-1Fuel]


80

b) nvPM Number Solvesso 150ND 4E+14 3E+14 2E+14 1E+14 0E+00

100

0

20

Relative Thrust [%]

40

60

80

100

80

100

Relative Thrust [%]

Ei nvPMNumber [kg-1Fuel]

Ei BCMass [g kg-1Fuel]

5E+14

c) BC Mass 0.06 Solvesso 150 0.04

0.02

0.00 0

546

20

40

60

80

100

d) nvPM Number Solvesso 150 4E+14 3E+14 2E+14 1E+14 0E+00

0

20

Relative Thrust [%]

40

60

Relative Thrust [%]

547

Fig. 3 BC mass (circles) and nvPM number (crosses) EIs as a function of sea level static

548

engine thrust and fuel total aromatics content (color coding). The top two panels (a) and (b)

549

correspond to the experiment performed with the naphthalenes depleted solvent (Solvesso

550

150ND); the bottom two panels show the experiment performed with Solvesso 150 that

551

contained 6 % (v/v) naphthalenes. Note: the number emissions data presented is a snapshot of

552

the evolving number emissions at approximately 0.3 s after leaving the engine

21


Page 23 of 24


553 554

Fig. 4 Neat fuel normalized EIs for BC mass and nvPM number concentration as a function

555

of fuel total aromatic concentration (a, b), fuel hydrogen mass concentration (c, d) and engine

556

thrust (color coding in all panels). Solvesso 150ND and Solvesso 150 experiments are depicted

557

as circles and crosses, respectively.

558 559

Tab. 1 Model fitting parameters including their standard errors and coefficients of

560

determination. Variable

α0

α1

Adjusted R2

∆EI BCMass

-124.05 ± 5.04

1.02 ± 0.06

0.94

∆EI nvPMNumber

-114.21 ± 3.63

1.06 ± 0.05

0.96

∆EI Combined

-119.31± 3.94

1.03 ± 0.05

0.92

22



Page 24 of 24

561 562

Fig. 5 Surface fitted change in (a) BC mass EIs and (b) nvPM number EIs as a function of

563

sea level static engine thrust and change in fuel hydrogen mass content. Spheres represent the

564

data measured.

565

23


Effects of Fuel Aromatic Content on Nonvolatile Particulate Emissions

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