ARTICLE pubs.acs.org/est
Comparison of PM Emissions from a Commercial Jet Engine Burning Conventional, Biomass, and Fischer Tropsch Fuels Prem Lobo,* Donald E. Hagen, and Philip D. Whitefield Center of Excellence for Aerospace Particulate Emissions Reduction Research, Missouri University of Science and Technology, Rolla, Missouri 65409, United States
bS Supporting Information ABSTRACT: Rising fuel costs, an increasing desire to enhance security of energy supply, and potential environmental benefits have driven research into alternative renewable fuels for commercial aviation applications. This paper reports the results of the first measurements of particulate matter (PM) emissions from a CFM567B commercial jet engine burning conventional and alternative biomass- and, Fischer Tropsch (F-T)-based fuels. PM emissions reductions are observed with all fuels and blends when compared to the emissions from a reference conventional fuel, Jet A1, and are attributed to fuel properties associated with the fuels and blends studied. Although the alternative fuel candidates studied in this campaign offer the potential for large PM emissions reductions, with the exception of the 50% blend of F-T fuel, they do not meet current standards for aviation fuel and thus cannot be considered as certified replacement fuels. Over the ICAO Landing Takeoff Cycle, which is intended to simulate aircraft engine operations that affect local air quality, the overall PM number-based emissions for the 50% blend of F-T fuel were reduced by 34 ( 7%, and the mass-based emissions were reduced by 39 ( 7%.
’ INTRODUCTION The anticipated growth in commercial air traffic, rising costs of fuel, an increasing desire to enhance the security of energy supply, and potential environmental benefits have recently driven feasibility and viability assessment studies of alternative renewable fuels for commercial aviation applications, with a particular focus on fuels derived from biomass or synthesis from coal and natural gas via the Fischer Tropsch (F-T) process.1,2 Several flight demonstrations of commercial aircrafts burning various blends of conventional jet fuel and either biomass or synthetic F-T fuels have been conducted recently.3 Specifications for aviation turbine fuels are established by American Society for Testing and Materials (ASTM) and United Kingdom Ministry of Defense (MOD). Other specifications for jet fuel exist but these are similar to those of ASTM and MOD. ASTM D16554 includes specifications for Jet A and Jet A-1 fuels used for commercial aviation within the United States. The MOD’s DEF STAN 91-915 outlines the specification for Jet A-1 used in Europe. ASTM recently adopted a new specification D75666 for up to 50:50 blends of synthetic fuel produced from the F-T process and conventional jet fuel. Since 1999, Sasol’s Semi Synthetic Jet Fuel (SSJF), a blend of up to a 50% of synthetic fuel, made from coal by F-T synthesis and conventional jet fuel, has been supplied to the Johannesburg, South Africa Airport.7 Efforts are ongoing to certify Sasol’s Fully Synthetic Jet Fuel (FSJF) for commercial aviation use.8 The United States Department of Defense (DoD) has evaluated the r 2011 American Chemical Society
use of F-T fuels in military gas-turbine and diesel engines as a Battlefield-Use Fuel to reduce reliance on foreign crude oil sources.9 The U.S. Air Force is currently in the process of certifying a 50:50 blend of conventional and synthetic F-T fuel for use in its entire fleet by 2011, and a 50:50 blend of conventional and biofuel by 2013.10 Until recently, almost all of the studies on the performance and emissions characteristics of alternative fuels in gas turbine engines have been limited to military engine applications.11 13 This paper presents the results of a comparison of particulate matter (PM) emissions from a commercial jet engine burning several alternative biomass- (fatty acid methyl ester, FAME) and F-T-based fuels. The Missouri University of Science and Technology (Missouri S&T) along with Aerodyne Research, Inc. (ARI) and Air Force Research Lab (AFRL) at Wright Patterson Air Force Base participated in a field campaign in November 2007 to characterize and compare the PM and gas-phase emissions of a CFM56-7B engine burning several alternative fuels and/or their blends with Jet A1. The measurements were performed at Test Site 3B at the GE Engine Test Facility in Peebles, OH. The results of the physical characterization of PM
Received: June 3, 2011 Accepted: November 1, 2011 Revised: September 29, 2011 Published: November 01, 2011 10744
dx.doi.org/10.1021/es201902e | Environ. Sci. Technol. 2011, 45, 10744–10749
Environmental Science & Technology
ARTICLE
Table 1. Properties of Fuelsa and Fuel Blends Testedb
fuel tested
fuel identifier
specific
kinematic
EI CO2d
heat of
aromatic
oxygen
gravity @15 °C
viscosityc @ 20 °C (mm2/s)
(g/kg fuel burned)
combustion (kJ/kg)
H/C ratio
content (vol %)
content (wt%)
Jet-A
Jet A
0.803
5.78
3155
43302
1.92
12.3
Jet-A1
Jet A1
0.797
4.27
3155
43300
1.92
18.5
0
20% FAME/80% Jet-A1
20% FAME
0.808
4.74
3045
42000
1.94
14.8
3.4
40% FAME/60% Jet-A1
40% FAME
0.825
5.62
2942
40300
1.94
11.1
6.6
50% F-T/50% Jet-A1
50% F-T
0.776
4.4
3127
43600
2.04
100% F-T
100% F-T
0.755
4.65
3100
44100
2.17
9.25 10:1 could be achieved at the point of sample capture, thereby reducing and/or eliminating the onset of any condensation, agglomeration, and gas-toparticle conversion processes in the sampling system.18 The rake assembly and probes were water cooled to protect them from thermal degradation during testing. The PM samples were conducted to the mobile diagnostic facilities through unheated 19.05 mm o.d. (16 mm i.d.) stainless steel sample lines. The entire sample train from probe tip to diagnostic suite was calibrated for size-dependent line loss as in previous studies.16,19 Sample Analysis. The PM emissions size distributions and total concentrations, associated combustion CO2 concentrations, and atmospheric conditions were measured with state-of-the-art high-frequency, real-time instrumentation. The instrumentation included the Cambustion DMS500, a fast particulate spectrometer to gather real-time PM size distributions, with a fast data acquisition rate yielding size distributions from 5 to 1000 nm at up to a 10 Hz frequency;20 a TSI Condensation Particle Counter (TSI model 3025) to measure total number concentration with a sampling frequency of 1 Hz; a fast-response CO2 detector (Sable Systems model CA-2A with a sampling frequency of 1 Hz) to establish emission factors and quantify sample dilution factors; and a weather station to monitor ambient conditions including
temperature, relative humidity, and pressure (sample frequency 0.2 Hz). Test Matrix. The engine was cycled through a matrix of reproducible engine power settings where for each power setting steady-state emissions and engine data were recorded. The engine power settings selected were as follows: 3%, 7%, 30%, 45%, 65%, 85% and 100% rated thrust. The fuels studied along with relevant physical, thermodynamic, and compositional characteristics are presented in Table 1. Jet A1 has been selected as the reference fuel for this analysis, and thus the PM emissions characteristics of the alternative fuels were compared to those from Jet A1.
’ DATA ANALYSIS Line Loss. Modification of the PM size spectrum due to line loss is an artifact associated with extractive sampling and must be taken into account. In this study a size-dependent line loss function determined from line loss calibrations was applied to the instrument data yielding an estimate of the PM size distributions at the point of entry into the sampling system. The average correction factors applied to the data set for number-based measurements were 42% at 7% power and decreased linearly to 16% at 100% power. The average correction factors applied to mass-based measurements were also found to vary linearly with power yielding an 18% correction at 7% power and a 6% correction at 100% power. The low size cutoff for the PM size distribution data was selected to be 10 nm. Emission Indices. PM number and mass concentrations were converted to number- and mass-based emission indices (EIn and EIm, respectively) to allow quantification of emissions per kilogram of fuel burned.19 The emission indices data are presented in normalized form, achieved by dividing the EIn and EIm values by a fixed normalization factor (same for all fuels), since the emissions data are proprietary. Heats of Combustion Corrections. Each fuel and fuel blend tested has different heats of combustion (Table 1). To accomplish intercomparison between fuels at the same engine power settings, the measured fuel flow rates at each test condition were adjusted to account for these different heats of combustion. The resulting Jet A1 equivalent fuel flow rates assured intercomparisons were made for the same energy per unit time for a given engine power setting. Uncertainty Estimation. Measurement uncertainties in PM parameters are taken to be the repeat measurement standard deviation (1σ) + 5%. These measurement uncertainties are calculated for each parameter for each fuel. A percent difference for PM parameters referenced to Jet A1 was calculated and the 10745
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Figure 1. Normalized EIn size distributions as a function of engine power setting, measured at the engine exit plane for Jet A1 (a) and 100% F-T (b) fuels.
Table 2. Geometric Mean Diameters for All Alternative Fuels Compared to Those for Jet A1 at Engine Power Settings Corresponding to the LTO Cycle GMDalternative fuel/GMDJet A1 alternative fuel
7%
30%
85%
100%
20% FAME 40% FAME
1.07 0.98
1.12 0.98
1.16 1.02
1.05 0.92
50% F-T
0.93
0.94
1.01
0.93
100% F-T
0.80
0.84
0.95
0.88
fractional uncertainty was estimated by taking the square root of the sum of the squares of the fractional uncertainty in the difference and fractional uncertainty for Jet A1. The fractional uncertainty was multiplied by the percent difference to yield the uncertainty in the percent difference. The resulting uncertainty value was used to ascertain statistical significance of the PM parameters for alternative fuels and blends relative to Jet A1.
’ RESULTS AND DISCUSSION Size Distributions and Emission Indices. The PM emissions intercomparisons for the fuels in this study are based in part on PM emission size distributions data acquired as a function of engine power setting. From these measurements, the geometric mean diameters (GMDs), geometric standard deviations (GSDs), number-based emission indices can be derived and with assumptions on particle sphericity and density (1 g/cm3 was used in this study), the mass-based emission indices can be calculated. For all fuels studied, the PM emission size distribution at all engine power settings was found to be log-normal. For a given fuel, GMD, EIn, and EIm increased linearly with engine power setting from 7% to 100% rated thrust. Figure 1 presents the normalized EIn size distributions as a function of engine power setting, measured at the engine exit plane for Jet A1 and 100% F-T fuels, respectively. The distributions demonstrate a general correlation to both engine power setting and fuel type. These results are consistent with those reported for a T701C military helicopter engine burning a JP-8 and a 100% F-T fuel.13 The geometric mean diameter for the four alternative fuels relative to Jet A1 at engine power settings corresponding to the
LTO cycle are presented in Table 2. The uncertainty in the GMD ratio is 5%, i.e., any difference between the alternative fuels and Jet A1 is statistically significant if it is outside the range 0.95 1.05. The GMDs for the 20% FAME fuel are consistently higher than those for Jet A1. For all other fuels, GMDs were either lower than or equal to those for Jet A1. The most significant differences in GMD are observed for the 50% F-T and 100% F-T fuels. For all fuels studied, EIn and EIm were found to increase with increasing engine power setting, with minimums observed at 7% power and maxima observed at 100% power. Reductions in both EIn and EIm were observed when burning the alternative fuels compared to the baseline Jet A1. The resulting relative changes in EIn and EIm and their uncertainties are summarized in Figure 2a and b. Most of the reductions in number and massbased emission indices were found to be statistically significant (based on the uncertainty estimation) except for the change in EIn for 20% FAME at 7% rated thrust and EIm for 20% FAME at 30% rated thrust. Generally, the measured reductions in PM were largest at idle, and smallest at maximum rated thrust. For low engine power settings, the trend in emissions reduction is 20% FAME < 40% FAME < 50% F-T < 100% F-T, i.e., the emissions reduction is greater as the relative amount of alternative fuel content in the fuel is increased. The overall reduction in PM number and mass reductions over the LTO cycle for the different alternative fuels studied are presented in Table 3. It should be noted that during the Jet A1 measurements the ambient temperature was ∼4 °C and for the other fuels tested it was ∼0 °C. Some of the observed reduction in emissions could be attributed to the change in temperature. A more recent study, AAFEX (Alternative Aviation Fuels EXperiment) specifically examined the impact of temperature on the emissions at the engine exit for a fixed fuel, albeit for a different model of the CFM56 engine and different fuel.21 The AAFEX results suggest that the effects of the 4 °C temperature difference should be small (
