Direct measurement of volatile organic compound emissions from

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Direct measurement of volatile organic compound emissions from industrial flares using real-time online techniques: Proton Transfer Reaction Mass Spectrometry and Tunable Infrared Laser Differential Absorption Spectroscopy W. Berk Knighton,*,† Scott C. Herndon,‡ Jon F. Franklin,‡ Ezra C. Wood,‡,∥ Jody Wormhoudt,‡ William Brooks,‡ Edward C. Fortner,‡ and David T. Allen§ †

Montana State University, Bozeman, Montana 59717, United States Aerodyne Research, Inc., Billerica, Masssachusetts 01821, United States § Center for Energy and Environmental Resources, University of Texas, Austin, Texas 78712, United States ‡

S Supporting Information *

ABSTRACT: During the 2010 Comprehensive Flare Study a suite of analytical instrumentation was employed to monitor and quantify in real-time the volatile organic compound (VOC) emissions emanating from an industrial chemical process flare burning either propene/natural gas or propane/natural gas. To our knowledge this represents the first time the VOC composition has been directly measured as a function of flare efficiency on an operational full-scale flare. This compositional information was obtained using a suite of proton-transfer-reaction mass spectrometers (PTR-MS) and quantum cascade laser tunable infrared differential absorption spectrometers (QCL-TILDAS) to measure the unburned fuel and associated combustion byproducts. Methane, ethyne, ethene, and formaldehyde were measured using the QC-TILDAS. Propene, acetaldehyde, methanol, benzene, acrolein, and the sum of the C3H6O isomers were measured with the PTR-MS. A second PTR-MS equipped with a gas chromatograph (GC) was operated in parallel and was used to verify the identity of the neutral components that were responsible for producing the ions monitored with the first PTR-MS. Additional components including 1,3-butadiene and C3H4 (propyne or allene) were identified using the GC/PTR-MS. The propene concentrations derived from the PTR-MS were found to agree with measurements made using a conventional GC with a flame ionization detector (FID). The VOC product (excludes fuel components) speciation profile is more dependent on fuel composition, propene versus propane, than on flare type, airassisted versus steam-assisted, and is essentially constant with respect to combustion efficiency for combustion efficiencies >0.8. Propane flares produce more alkenes with ethene and propene accounting for approximately 80% (per carbon basis) of the VOC combustion product. The propene partial combustion product profile was observed to contain relatively more oxygenated material where formaldehyde and acetaldehyde are major contributors and account for ∼20 - 25% of VOC product carbon. Steam-assisted flares produce less ethyne and benzene than air-assisted flares. This observation is consistent with the understanding that steam assisted flares are more efficient at reducing soot, which is formed via the same reaction mechanisms that form benzene and ethyne.

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INTRODUCTION Industrial gas flares are employed by the petrochemical and chemical process industry for emergency and routine disposal of combustible gases by burning them in a flare.1,2 Current regulations, as stated in the Code of Federal Regulations (CFR) Title 40, Part 60.18,3 require that gas flares operate under smokeless combustion. Smokeless combustion is achieved by the addition of steam or air into the combustion zone of the flare and such flares as referred to as steam-assisted or airassisted flares.1 The destruction and removal efficiency (DRE) of organic gases can be severely affected by operating conditions, particularly for steam-assisted flares. When tabulating volatile organic compound (VOC) emissions for air quality management purposes, it is assumed that flares operating under specified manufacturer conditions achieve DREs of 98% or better.2 The validity of this assumption however, that gas flares operated within the commercial industry using low flow rates routinely achieve DREs of 98% © 2012 American Chemical Society

or better, has been recently called into question. As a result of these concerns, the Texas Commission on Environmental Quality initiated the Comprehensive Flare Study to examine flare emissions using operational full-scale industrial flares. Field tests for the Comprehensive Flare Study were conducted at the John Zink facility in Oklahoma City, OK, in 2010 from September 16th−29th. The primary goal of this study was to evaluate the DRE of the vent gas under different vent gas compositions and steam or air assist conditions. The primary vent gases examined were mixtures of propene or 80% propene 20% Tulsa natural gas (TNG) diluted with N2 to Special Issue: Industrial Flares Received: Revised: Accepted: Published: 12674

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and a series of guy-wires operated by John Zink personnel. The goal was to position the collector down wind of the flare so it was within the center of the plume at a distance far enough removed so all of combustion reactions had ceased. This condition was considered to have been met if the plume temperature was 250 °F (121 °C) or less as measured by a series of three thermocouples mounted at the inlet of the sample probe. The positioning of the probe was facilitated by video camera imagery, which included a visual, an infrared (IR), and two forward looking infrared (FLIR) thermal imaging cameras. The exact location of the collector was monitored and recorded using a global positioning system. An eductor mounted on the end of the sample collector continuously drew approximately 1950 cfm (55,501 L/min) of air through the probe. A flow mixer inserted within the collector was employed to homogenize the constituents within the plume to ensure the sample represented an averaged composition. Sample was extracted via a dilution probe located approximately halfway down the length of the collector. The dilution probe dilutes the sample directly within the tip of the sample probe. Approximately 0.8−1.2 standard liters per minute (slpm) of sample was withdrawn and mixed with an additional 18.7−19.2 slpm of N2 dilution gas prior to its delivery to the analyzers and instruments within the AML. The addition of this extra dilution gas was intended to serve several important functions: (1) To eliminate the condensation of water or any other material within the lines that would have occurred as the sample cooled, (2) To reduce the concentrations of the chemical constituents within the plume to a level where reactions were quenched and the concentrations were within the dynamic range of the trace gas instrumentation used, and (3) To decrease the residence time of the sample within the delivery lines. Flow of the N2 dilution gas was controlled via calibrated mass flow controllers. The actual dilution flow is not critical since all of the concentrations are referenced to the CO combustion tracer (see Herndon et al.4) except during calibration procedures where the calibration gases were delivered to the tip of the dilution probe. The dilution probe used to extract the sample within the collector was mounted via a Swagelok fitting so the dilution probe assembly could be removed between tests. This enabled calibration gases to be introduced directly at the inlet. In this way, the calibrations accounted for any influences that the sample line may have introduced. The sample line between the dilution probe and the AML were constructed from 7.62 m sections of 1.27 cm OD perfluoroalkoxy (PFA) tubing. The sections were connected using Swagelok stainless steel unions. The sample line was wrapped in insulation to shield it from becoming overheated by the flare. The sample line was mated to the main sample line of the AML from which all of the trace gas instrumentation subsampled. The main sample line of the AML was constructed of 1.27 cm OD PFA Teflon tubing. The sample was first pulled through a polytetrafluoroethylene (PTFE) Teflon filter with a nominal pore size of 1−2 μm before it was distributed to the analytical instruments through PFA tubing. All connections to the main sample line were made via stainless steel Swagelok fittings. The flow through the sample line was maintained with two pumps, a Varian triscroll 300 scroll pump and a Vacuubrand MD4 diaphragm pump. Trace Gas Instrumentation. The flare plume constituents were measured using a suite of trace gas instrumentation

control the heat content. A limited number of tests were conducted using 80% propane 20% TNG fuels. Here we describe the analytical methodology and instrumentation employed to identify and quantify the gaseous emissions from the study flares. These include the carbon oxides CO2 and CO, the unburned fuel components (propene and methane), and partially combusted products such as ethyne, ethene, and formaldehyde. To address the challenges of measuring the emissions from operational full-scale flares, while delineating any interfering contributions from these products present within the background ambient environment, a combination of extractive in situ sampling and high time response, 1 Hz, instrumentation was employed. The use of high time response instruments takes advantage of the inherent variability in the sampling of the flare plume, which modulates the component concentrations in time and uses this temporal correlation to quantify the plume constituents relative to a combustion tracer, such as CO. A full description and verification of this methodology is addressed in the companion paper of Herndon et al.4 This manuscript describes how a set of modern online VOC and combustion gas instruments was employed to monitor and quantify in real-time the VOC emissions emanating from an industrial chemical process flare burning either propene/TNG or propane/TNG. To our knowledge this represents the first time the VOC composition has been directly measured in real-time on an operational fullscale flare.

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EXPERIMENTAL DESCRIPTION Test Setup. A full description of the test setup and the flares is provided in Allen et al.5 as well as in the overview companion paper of Torres et al.6 in this special issue. Limited details are provided here. The main elements of the test setup addressed here are the sample collector, the associated sample line, and the gas phase instrumentation housed within the Aerodyne Mobile Laboratory (AML). A photograph of the test setup is shown in Figure 1, which shows the sample collector being held in place during one of the air-assisted flare tests. Visible in the photograph are the sample line and the mobile laboratory. Sample Collector. The flare plume was sampled via a sample collector of similar design to that described by Romano.2 The sample collector was positioned using a crane

Figure 1. Photograph showing the positioning of the sample probe during the testing of the air flare. All of the instrumentation was housed within the Mobile lab. The sample delivery line from the probe to the mobile lab is clearly visible. 12675

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operated continuously at ∼1-Hz. A list of this instrumentation is provided in Table 1 and is described in further detail below.

computed concentration versus the prepared calibration concentration was 0.86 and the reported concentrations have been adjusted accordingly. Formaldehyde was calibrated separately using a permeation device (Kintek 62 ± 8 ng/min at 980 mbar and 298 K). The QCL computed formaldehyde concentration was 6% higher than the concentration calculated from the permeation rate and the molar flow of the N2 dilution gas, but was well within the ±12.9% uncertainty of the permeation device. No adjustments were made to the reported formaldehyde concentrations. Methane and Ethyne. Methane and ethyne were measured using an Aerodyne Quantum Cascade Laser Trace Gas Monitor using a continuous wave room temperature laser system.8 This instrument employs a 210 m path length absorption cell. Methane is measured using the 12CH4 spectral line at 1294 cm−1, but the spectral window also quantified 13CH48. Similarly, ethyne is measured using the absorption feature centered at 1342.35 cm−1. This instrument operates nominally at 1-Hz, where 1400 spectra per second are acquired, averaged and fit to the Voigt profile. The computed concentrations were validated against calibration experiments performed by volumetric dilution a certified gas standard with N2 in the same manner as was described above for CO and ethene. Methane (506 ppm ±2%) and ethyne (10.0 ppm ±2%) were part of a multicomponent standard obtained from Scott Specialty Gases. The slopes of the calibration curves were 0.98 ± 0.008 for methane and 0.97 ± 0.01 for ethyne, and no adjustments were made to the reported concentrations. Propene and Other Selected Volatile Organic Compounds. Propene was measured using Proton transfer reaction mass spectrometry (PTR-MS). PTR-MS is a chemical ionization mass spectrometry technique that utilizes H3O+ as the principal reagent ion.9 H3O+ will react with any molecule having a proton affinity greater than that of water. It is important to note that the primary components of air: N2, O2, Ar, CO2, and the alkanes all have proton affinities less than water and thus do not react with H3O+. Most other organic substances except for acetylene and ethene react with H3O+ via a proton transfer reaction, Reaction 1.

Table 1. Listing of the Compounds Measured and Analytical Method Used during the 2010 Comprehensive Flare Studya compound CO2 CO propene methane total hydrocarbons (HC) ethyne, ethene, formaldehyde methanol, acetaldehyde, benzene butenes, acrolein, propene oxide, propanal, acetone C1−C3 hydrocarbons

measurement technique

instrument

nondispersive IR LiCor 6262 absorbance tunable IR laser Aerodyne QCL Trace Gas differential absorbance Monitor chemical ionization mass Ionicon Analytik PTR-MS spectrometry tunable IR laser Aerodyne QCL Trace Gas differential absorbance Monitor continuous flame California Analytical Model ionization detector 300 HFID Analyzer Speciated Organic Gases tunable IR laser Aerodyne QCL Trace Gas differential absorbance Monitor chemical ionization mass Ionicon Analytik PTR-MS spectrometry gas chromatography/mass spectrometry (GC/MS) gas chromatography with flame ionization detector (GC/FID)

HP 5890 Series II GC Ionicon Analytik PTR-MS Homebuilt interface SRI 8610

a

QCL, quantum cascade laser; PTR-MS, proton transfer reaction mass spectrometer.

CO2. A LiCor 6262 nondispersive infrared absorption instrument was used to measure the carbon dioxide concentration. Zero and spans were evaluated by overblowing the inlet with either N2 or span gas. Spans were determined using a gas standard specified to be 1000 ± 20 ppm by the manufacturer (Scott Specialty Gases) and certified to be 994 ppm by an Aerodyne Research Inc. absolute CO2 measurement (accurate to 1%). CO, Ethene, and Formaldehyde. These compounds were measured using two separate Aerodyne Quantum Cascade Laser (QCL) Trace Gas Monitors. Each instrument uses a single multiple reflection gas cell and up to two laser devices. CO and formaldehyde were measured on one instrument, while ethene was measured individually on the second one. These instruments use pulsed quantum cascade lasers and employ Tunable Infrared Laser Differential Absorption Spectroscopy (TILDAS) as the fundamental analytical method for quantifying trace compounds.7 TILDAS is an absolute measurement method where the concentrations of the species of interest are computed by fitting the measured absorption spectrum to the spectral features tabulated in the HITRAN database. The QCL instruments operate nominally at 1-Hz, where 1400 spectra per second are acquired, averaged, and fit to the Voigt profile. These computed concentrations were validated against calibration experiments performed by volumetric dilution of certified gas standards with N2. Flows were set using mass flow controllers and measured directly using a Gilan Gilibrator. Calibration curves were generated by plotting the computed concentrations versus the expected concentrations as determined from the dilution of the standard. The CO standard (303 ppm ±1%) was obtained from Scott Specialty Gases. The ethene standard (10 ppm ±2%) was part of a multicomponent standard from Scott Specialty Gases. The calibration curve for ethene had a slope of 1.01 ± 0.005. For CO, the slope of the

k

H3O+ + R → RH+ + H 2O

(R1)

The proton transfer reaction forms the protonated molecule RH+, which is a stable reaction product in many cases. Quantification of propene is based on the measured ion intensity of the RH+ product ion detected at m/z 43. Quantification of the PTR-MS ion signals is possible directly from first principles, but is most reliably achieved via calibration with certified gas standards. The concentrations deduced for propene, acetaldehyde, benzene, and methanol were evaluated from calibrated response factors. The full details on the calibration procedure for the PTR-MS are provided in the Supporting Information. Minor combustion byproducts for which gas standards were not available were quantified using estimated sensitivity factors. These calibrated response factors are listed in Table 2. The standard equation for quantifying a target compound, designated generically as (R) is shown in eq 1. ⎞ ⎛ 1 ⎞⎛ IRH+ × 106 ⎟⎟ [R] = ⎜ ⎟⎜⎜ ⎝ SR ⎠⎝ IH3O+ + XR IH3O+(H2O) ⎠ 12676

(1)

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test points, the PTR-MS was operated in the multiple ion detection (MID) mode where up to 10 ions were monitored at a dwell time of 50 ms/ion, which kept the response time of the instrument at ∼1 Hz. The intensity of the O2+ impurity ion was not measured in the MID mode. However, during the preliminary flare testing the PTR-MS was operated in full mass spectral mode, which indicated the O2+ level was less than 0.5% of the total H3O+ signal. The O2+ signal was very low because the flare samples were diluted at better than 10:1 with N2. The second PTR-MS was operated under nearly identical conditions except that the drift tube voltage was set to 575 V. The second PTR-MS employed an older drift tube design with a slightly longer drift tube length. The higher voltage produced an equivalent E/N of 128 Td. The PTR-MS technique detects and records the response associated with an ion at a specified mass-to-charge ratio (m/z). The product ions monitored in this study included: m/z 33 (methanol), m/z 41 (propyne or allene, fragment ion from propene), m/z 43 (propene), m/z 45 (acetaldehyde), m/z 55 (1,3-butadiene), m/z 57 (butenes and acrolein), m/z 59 (C3H6O isomers), and m/z 79 (benzene). The compound identities (shown in parentheses above) were established using a second PTR-MS instrument that was deployed with an associated GC. The GC/PTR-MS system was used to determine whether an ion was produced from one or more components. The GC/PTR-MS system was operated in parallel to the primary PTR-MS. Both instruments sampled from the same sample line. The GC/PTR-MS could be operated as either a normal PTR-MS or as the detector for the GC. A previous version of the instrument has been described elsewhere,12 and only a brief description of how this second instrument was operated in the GC-PTR mode is described here. When instructed, a portion of the sample was pulled through a Teflon loop that was immersed in liquid N2, which traps the condensable components in the sample. The trapping time was variable but typically lasted for 2 min. At the conclusion of the trapping cycle, a 6-way valve was used to transfer the contents trapped within the sample loop onto the chromatographic column, a 30 m Restek Rtx-624 capillary column. The column carrier gas was high purity helium with a flow rate of 15 mL/min. Immediately after the 6-way valve was switched, the sample loop was withdrawn from the liquid N2 and was immersed into hot water to desorb the condensable components. A 3-way valve on the PTR-MS was switched so the instrument sampled the outflow from the GC instead of the normal sample line. Inside of the oven, at the end of the column, an additional flow of N2 (∼ 100 mL/min) was added to dilute the helium and meet the flow requirements of the PTR-MS. The GC oven temperature was ramped from 40 °C to 100 at 10 °C/min. A typical GC run lasted about 7 min. Total Hydrocarbons. During the latter portion of the field test, a California Analytical Model 300 Heated Total Hydrocarbon Analyzer was borrowed and integrated onto the Aerodyne Mobile Laboratory sample line. This device uses a continuous flame ionization detector (FID) technique to provide a total gas phase organic carbon measurement. The FID was used to provide a measurement of propane, since propane was not detectable with either the QCL or the PTRMS instrument. This instrument provides a nearly uniform response to all hydrocarbon species, but not to carbons bearing heteroatoms.13,14 This instrument’s calibration was verified using certified propene standards (Airgas 50.06 ppm and 99.99

Table 2. PTR-MS Calibration Factors Used in the Comprehensive Flare Study compound propene methanol acetaldehyde butenes + acrolein C3H6O isomers benzene

standard used

SR (ncps/ ppbv)

43 33 45 57

Airgas 50.06 ppm ±2% Apel Riemer 520 ppb ±5% Apel Riemer 490 ppb ±5% none - estimated

9.66 19.6 25.0 25.0

0 1 1 1

m/z 59

Apel Riemer 500 ppb ±5%

37.8

1

m/z 79

Apel Riemer 510 ppb ±5%

20.1

0.1

ion quantified m/z m/z m/z m/z

XR

The term [R] represents the concentration of R in ppbv. SR is the sensitivity factor expressed as normalized counts per second (ncps) per ppbv.10 The term ((IRH+ × 106)/(IH3O+ + XRIH3O+(H2O))) represents the product ion response expressed in ncps, which is the mass spectral intensity of RH+ measured in cps per 1-million reagent ions. This normalization step accounts for any variation in the product ion intensity resulting from changes in the reagent ion intensity. The intensity of H3O+ is too large to measure directly, and its intensity is determined by measurement of the O-18 isotope of this ion detected at m/z 21, which is then multiplied by 500 to correct for the isotopic composition. Measurement of the intensity of H3O+(H2O) is measured directly at m/z 37. Some components react with both H3O+ and H3O+(H2O) while others do not. The XR term is a factor between 0 and 1 that accounts for the reactivity difference between H3O+(H2O) and H3O+ toward R. Additional details on the evaluation of XR are provided in the Supporting Information. Most applications of the PTR-MS are for trace level detection where the substrate concentrations are low, typically much less than 1 ppmv. Under these conditions the reagent ion population (intensity) is not significantly altered by Reaction 1 and can be considered to remain at a constant level. During the Comprehensive Flare Test, however, the propene concentration was often very high (>10 ppmv) and reached levels where the reagent ion intensity was notably depleted, as is depicted in Supporting Information, Figure S3. Under these measurement conditions eq 1 is not valid and a modified formula must be used for accurate quantification. This modified equation is shown in eq 2. ⎛ 1 ⎞⎛ I + × 106 ⎞ ⎟⎟ [R] = ⎜ ⎟⎜⎜ RH ⎝ SR ⎠⎝ ∑ I R iH + ⎠ ⎛ ∑ IR H+ + IH O+ + XR IH O+H O ⎞ i 3 3 2 ⎟⎟ ln⎜⎜ + + X I + I ⎝ ⎠ H3O R H3O H 2O

(2)

The only new term in this equation is the ∑IRiH+ term, which reflects the sum of all product ions. This equation reduces to eq 1 when the sum of the product ion intensity becomes small relative to the total reagent ion intensity. All of the concentrations reported using the PTR-MS in this project were computed using eq 2. The PTR-MS was operated in typical fashion.11 The drift tube pressure was set to 2.05 mbar, and the drift tube temperature was maintained at 40 °C. The drift voltage was 545 V, which corresponds to an electric field to number density (E/ N) ratio of 128 Td (1 Td = 1 × 10−17 V cm2). For the official 12677

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Figure 2. Time series of CO, propene, and acetaldehyde taken on the dilution probe during a steam assist flare test point. Propene and acetaldehyde (CH3CHO) were simultaneously measured using the PTR-MS, while CO was measured with the Aerodyne QCL. Note that the signals are highly correlated demonstrating that components all originated from the flare plume.

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ppm propene ±2%) in neat form and volumetrically diluted with N2. Other Instruments. The flare plume composition of small hydrocarbons C1−C3 was periodically sampled using a SRI8610 gas chromatograph equipped with a flame ionization detector (GC/FID). The GC sampled from a trunk line that pulled ∼1 L per minute from the main sample line. The GC used a heated absorbent trap packed with Carbotrap B to preconcentrate the sample prior to injection onto the GC column. When triggered into the sample mode, a small flow (∼ 40 mL/min) was pulled off the trunk sample line for 30 s. At the conclusion of the trapping period, the adsorbent trap was heated, and a 10-port valve was used to direct the sample onto a 6-ft Porapak-Q 0.125′′ O.D packed column. The column oven was temperature programmed to hold at 30 °C for 1.7 min, ramp at 50 °C per minute to 160 °C and hold at 160 °C for 3.7 min. This configuration allowed for quantification of ethane, propane, propene, and the sum of ethyne + ethene. Methane was observed in the chromatograms, but this compound is not quantitatively retained on the Carbotrap B adsorbent. Ethyne and ethene are not resolved on the Porapak Q column and coelute as a single peak. Sample concentrations were determined as ppmC by integrating the peak areas and converting them to an equivalent concentration using a response factor evaluated for FID using a series of calibrated gas standards (delivered directly or volumetrically diluted in N2). The FID response factor was evaluated by plotting integrated peak versus the delivered hydrocarbon concentration, expressed as ppbC (after correction for dilution and carbon content) and assumed all of hydrocarbons provided a uniform per carbon response. Four gas standards were employed for calibration and to verify retention times. They were Scott Specialty Gases (470 ppm methane, 10 ppm ethene, 10 ppm ethyne ±1%), Airgas (50.06 ppm propene ±2%), Airgas (99.9 ppm propene ±2%), Airgas (100 ppm methane, 100 ppm ethane and 100 ppm propane ±2%).

RESULTS The measurements performed during the 2010 Comprehensive Flare Study and their interpretations are discussed in this and several other companion Articles. This manuscript focuses on the analytical methodology and to our knowledge represents the first flare characterization study to report organic gas speciation results from operational full-scale flares. The companion articles address validation of the sampling methodology and carbon balance (Herndon et al.4), and the effect of steam-assist and air-assist conditions on the destruction and removal efficiency of the flares (Torres et al.6). The optical and mass spectrometric techniques employed in this study were chosen because they possess sufficient time response, selectivity, sensitivity, and dynamic range to characterize the flare emissions. In the sections that follow, we will examine the capabilities and uncertainties in the different analytical methods utilized. In particular, considerable attention will be given to the interpretation of PTR-MS ion assignments. Once this is accomplished we will examine how the chemical speciation of the flare emission products is affected by fuel composition and flare type. Representative Measurements. The size and irregular nature of the flare flame make it impractical to consider the capture and sampling of the total flare emissions without altering the conditions of the combustion. As a result, a sample collector is used to capture a portion of flare plume for analysis. Sampling of the plume is challenged by both the irregular nature of the flare flame and the ambient winds, which can shift the position of the plume. Any technique used to characterize the plume therefore needs to have sufficient time response and dynamic range to follow the temporal changes induced by the flare and sampling system. To demonstrate that the instrumentation deployed meets these demands, representative time series measurements for propene, acetaldehyde, and CO are provided in Figure 2. This figure shows the ten-minute test period for a mixture of propene/Tulsa natural gas with the steam-assisted flare (80% propene, 20% Tulsa natural gas, by volume, that was diluted with sufficient nitrogen to reduce the lower heating value to 350 BTU/scf (1.375 × 107 J/m3)). This 12678

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routine, and spectra can be refit. A comparison of the QCL data for the sum of ethene and ethyne was made to that obtained with GC/FID. The sum of ethene and ethyne is examined because these two components are not resolved on the chromatographic column and are detected as a single peak on the GC/FID. To enable this comparison, the ethene and ethyne data were averaged over the 30-s time collection time of the GC/FID. The level of agreement between the two QCLs and the GC/FID is quite good, with the QCL measurements being slightly higher than the GC/FID measurement. A linear least-squares analysis yielded a slope of 1.12 and a correlation coefficient (R2) of 0.94. To our knowledge, the data reported for the five species measured by the TILDAS method are free of any significant spectral interferences. Propane. Propane flares were not part of the initial study and were included as supplementary measurements in the study matrix after the field tests had begun. Since none of the hightime response instrumentation on-board the AML was capable of measuring propane, a method for the determination of propane was developed in the field. To accomplish a determination of propane, a continuous FID instrument was borrowed and integrated onto the AML sample line. The FID technique provides a total hydrocarbon measurement. The propane concentration was extracted from the total hydrocarbon response by subtracting the contributions from the other organic carbon gases that were measured by the AML. There are several caveats to this approach: (1) It assumes that all of the nonpropane gas phase organic carbon is accounted for by the individual species measured by the AML and (2) The FID response to organic species containing heteroatoms is diminished relative to that of compounds containing only carbon and hydrogen.13,14 We have excellent coverage of all of the known decomposition products of propane combustion15 and assume that the sum of the individual of measurements reasonably accounts for all of the nonpropane gas phase carbon. To account for variable response sensitivity of the FID to oxygenated compounds it was assumed that any carbon that was bound to an oxygen did not produce a FID response.14 To validate this approach, several additional experiments were performed. First, we examined how the total speciated sum compared to the FID measurement for several propene flares. A plot of the total speciated sum versus the FID yielded a linear relationship with a least-squares slope of 0.98 and a correlation coefficient (R2) of 0.998. Additionally, a second experiment was conducted in which we used the PTR-MS gas purifier, which contains a heated Platinum catalyst to convert all of the gas phase organic carbon to CO2. A CO2 monitor was used to measure the difference in CO2 concentration ahead of and exiting the catalyst tube. The additional CO2 observed postcatalyst reflects the sum of CO and the total gas phase organic carbon. Similar to the balance achieved by summing the speciated non-CO and CO2 form of carbon described above, the oxidation catalyst data also demonstrates carbon balance. The agreement between the sum of all speciated measurements as ppmC with the oxidation catalyst independently achieved carbon balance to 98%. The oxdiation catalyst results are described more thoroughly elsewhere in this issue.4 Propene, Acetaldehyde, Methanol, Benzene, and other VOCs. Propene and the remaining VOCs were all measured and quantified using the PTR-MS. An underlying assumption with the PTR-MS technique is that a given ion can be used to monitor the concentration of a specified neutral component.

time series data clearly illustrates the turbulent nature of the plume sampling event and the ability of the gas sampling instrumentation to follow the rapid concentration changes. Inspection of the time series measurements shows that the concentration profiles are highly correlated. Each rise and fall in concentration is mirrored by all of the species. Correlation scatter plots of propene and acetaldehyde have been plotted versus CO and provide a more quantitative verification that all of these signals are highly correlated. It is noted that all of the other measured gases, whether they be a fuel component or a combustion byproduct, exhibit similar behavior to that of propene and acetaldehyde, and are highly correlated with CO. The ability of the high time response instruments to capture the inherent variability in the sampling of the flare plume is critical. The modulation of the component concentrations in time can be used to quantify the plume constituents relative to a combustion tracer, such as CO. The slopes of the individual correlation plots can then be used to deduce the carbon fraction for any measured component as shown for propene using eq 3. (see Herndon et al.4) CFpropene = propene(ppmC)/CO

(

)

(propene(ppmC)/CO + CO2 /CO + CO/CO +

∑ other carbon(ppmC)/CO) (3)

The validity of eq 3 depends on accuracy of the individual measurements and on how well the sum of all of the measured forms of carbon (denominator) approximates the true carbon sum. As is described in detail by Herndon et al.4 under most of the conditions quantified in these tests, 97% of the total carbon is accounted for by the fuel components (i.e., propene and methane), CO and CO2 alone. We will now examine the measurements of each of the different chemical species that go into determining the carbon fraction. Quantification and Identification of Flare emission products. Carbon Dioxide. Carbon dioxide was measured using a commercial instrument (Licor 6262) based on nondispersive infrared absorption. These instruments have been used for decades to measure CO2 in combustion research and are not known to have any spectral interferences that would compromise the present measurements. Calibration checks were consistently within 1% of the stated value of certified gas standard. This measurement is considered to be robust. CO, Methane, Ethyne, Ethene, and Formaldehyde. These five components were all measured using the TILDAS technique that employs fitting the measured absorption spectrum using the spectral features from the HITRAN database. The TILDAS method is very effective for the measurement of small molecules that have unique spectral features. Few molecules have totally unambiguous features, and the method requires the inclusion of additional species into the fitting procedure, such as water that has weak overlapping lines and is present in large quantities. At low DREs the large concentration of propene interferes with the determination of the ethene. This required the inclusion of the spectral lines from propene into the fitting procedure for the ethene. A valuable attribute of the TILDAS method is that all of the individual spectra are recorded and saved so any spectral interferences that are discovered at a later date can be addressed by inclusion of the interfering species into the fitting 12679

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For instance, in the present study m/z 43 was used to quantify propene. While there is no doubt that propene will form an ion at m/z 43 in the PTR-MS, what needs to be established is that there are no other components within the flare plume that also will produce this same ion. To verify the ion assignments, a second PTR-MS that had an associated GC was used to examine the purity of the ion signals. Figure 3 shows a set of

grams, regardless of the fuel type or flare condition. While this observation provided strong evidence that there are no spectral interferences, there are other compounds, such as acetic acid, that could be present in the flare matrix, which would not be detectable by chromatography and are known to produce an ion at m/z 43 in the PTR-MS. Acetic acid, as illustrated in Supporting Information, Figure S3 and discussed in the associated text within the Supporting Information, is not thought to compromise the measurement of propene reported here. On the basis of these observations, it is concluded that the m/z 43 signal is a robust indicator of the presence of propene and there are no other compounds in the flare emissions matrix at sufficient levels to interfere with the quantification of propene. Additional confirmation of the accuracy of the propene measurement is provided in Figure 4, which shows a

Figure 3. GC/PTRMS selected ion chromatograms. The first minute of the time series represents the ion intensity monitored in the normal mode which represents the time when the sample was trapped in the cyroloop. At the conclusion of the trapping period the contents of the cryoloop were injected on the column and PTR-MS was switched to monitor the outflow from the GC column. Peak identifications by name are based on retention time matching with known standards. Others are tentatively assigned by molecular weight based on boiling point. See text for additional details.

Figure 4. Comparison of the propene concentrations deduced from the PTR-MS with those obtained with GC/FID. The PTR-MS propene concentrations were averaged over the 30-s time period during which the GC/FID pulled sample through the adsorbent trap.

comparison of the propene concentrations determined with the PTR-MS with those made using a conventional GC/FID. To make this comparison, the PTR-MS measurements were averaged over the 30-s collection time of the GC/FID. The level of agreement between the two instruments is excellent as indicated by the linear regression derived slope, which is 0.98. m/z 41, Propene + C3H4. The majority of the signal intensity of ion signal at this mass originates from the fragmentation of the RH+ ion from propene by loss of H2. The loss of signal from m/z 43 because of this fragmentation does not affect the quantification of propene as this loss of ion intensity is accounted for in the calibration process. Closer inspection of this trace shows presence of a second peak. This peak could be from either propyne or allene as both are known propene pyrolysis products16 and both are expected to elute after propene as they have higher boiling points. The scale on this chromatogram has been enhanced to illustrate the presence of the C3H4. On the basis of the chromatograms, C3H4 represents less than 4% of ion intensity at m/z 41. No attempt was made to quantify the C3H4 concentration. m/z 33, Methanol. The signal at m/z 33 is attributed solely to the presence of methanol. Because the experiments were conducted with N2 dilution, there is no inference from the 17 16 + O O form of O2+ to the ion intensity measured at m/z 33. Only a single peak is observed in the chromatogram. Even

selected ion chromatograms obtained during one of the steam flare tests, S9.4 Run 1,6 burning 80/20 propene/TNG. The identification of the peaks in the chromatograms was restricted to compounds that are known combustion products of the primary fuels, propene, propane, and methane. These assignments were corroborated based on retention time matching with known standards or are tentatively assigned on the basis of boiling point. While the ion chromatograms presented in Figure 3 were obtained on the steam flare using propene/TNG fuel, they are representative of the other fuel and flare conditions. A total of 75 chromatograms were taken, which included tests using propene and propane fuels with steam and air assisted flares. All of the chromatograms show similar features with only the relative intensities of the peaks changing. Thus, the discussion below of the ion chromatograms in Figure 3 is relevant to all of the test conditions studied. m/z 43, Propene. Figure 3 shows that the m/z 43 chromatogram is composed of a single peak, and this peak has the same retention time as that observed with the propene standard. This same result was observed in all of chromato12680

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and acrolein are not significantly affected by this interference for DREs greater than 0.7. m/z 59, C3H6O Isomers. The selected ion chromatogram of m/z 59 appears to have three resolved peaks and have been labeled a, b, c. Three C3H6O isomers, propene oxide, propanal, and acetone have been observed in a propene oxidation study.20 The anticipated elution for these compounds would be propene oxide, propanal and acetone, which would then correspond to a, b, c, respectively. While this identification seems reasonable, it should be considered tentative and the relative distribution of these isomers should not be over interpreted. The chromatogram for m/z 59 shown in Figure 3 is one of the better examples, and most of the GCs appear as a broad unresolved peak. The peak labeled as c (acetone) does seem to predominate in most of chromatograms, which is counter to what has been observed in low temperature propene oxidation studies15,20 that report propene oxide as the predominate form. The low signal intensity and poor quality of the chromatography for this ion make it difficult to draw any conclusions as to how these isomers might be distributed under different fuel or flare conditions. m/z 79, Benzene. The signal at m/z 79 is attributed solely to the presence of benzene. Only a single peak is observed in the chromatogram. The presence of benzene as a flare combustion product is not unexpected. Benzene has been observed in propene pyrolysis experiments.16 Flare Plume Composition versus DRE. While the primary objective of the 2010 Comprehensive Flare Study was evaluating the destruction and removal efficiency of the flaring process, it is also of interest to gain a better understanding of the how the flaring process impacts the combustion byproducts that are produced. In this section we examine the data to see what information can be deduced about the chemical composition of the flare plume with respect to fuel composition, flare type, and DRE. We will focus on CO and the VOC combustion products, as the primary flare components, CO2, propene or propane and methane change in a predictable fashion with DRE as is depicted in Supporting Information, Figure S4. We will start by looking at how the concentration of the combustion byproducts changes as a function of DRE. Concentration will be expressed as a fraction of the total carbon. Figure 5 shows plots of CO, ethene, formaldehyde, and acetaldehyde versus DRE. These plots include all of the test data. The data point marker styles represent the different fuels and flare types, and the marker color reflects the test condition number. Figure 5 contains an abundance of information, and we will start by examining the major features. CO is the most abundant combustion byproduct followed by ethene. The fractional amounts of the combustion byproducts do not continuously increase with decreasing DRE. The production mechanism appears to be similar for all of the components based on the observation that the profiles all exhibit a common shape with respect to DRE. What we mean by this statement is that there is no evidence that combustion byproducts are produced in a sequential fashion. The other components (not shown) exhibit similar profiles. At high DREs their relative fractions are small and increase in a near-linear fashion with decreasing DRE down to DRE ∼ 0.9. Their abundances continue to increase with decreasing DRE up to a DRE of about 0.7−0.6, at which point their carbon fraction reaches a maximum. After reaching a maximum their contribution decreases with decreasing DRE.

though the methanol signal is quite weak at only a few hundred counts per second, the GC still yields a clearly observable peak. m/z 45, Acetaldehyde. This signal at m/z 45 is attributed to acetaldehyde. CO2 has been reported to provide a weak response in the PTR-MS.17 Because CO2 has a proton affinity less than that of water, the origin of this interference is probably due to the formation of HCO2+ within the expansion region between the drift tube and the entrance lens to the mass spectrometer. The extent to which this interference may occur will vary from instrument to instrument. As gauged by the m/z 45 signal in tests where the flare was burning 100% Tulsa natural gas (a condition where acetaldehyde is not expected to be produced) there is no significant interference from CO2 on the measurement of acetaldehyde. The peak shape for acetaldehyde is not symmetric and exhibits some tailing. This peak shape seems to be more accentuated in the flare samples than in the standards, but (its shape) is a characteristic of analytes that are polar. Closer inspection of the Figure 3 shows that the other components containing oxygen, methanol, acrolein, and C3H6O isomers have similar peak profiles. m/z 55, 1,3-Butadiene + Propene Interference. The selected ion chromatogram of m/z 55 has two peaks. One of these peaks corresponds to 1,3-butadiene. The other peak elutes at the same time as propene. This result indicates this ion is formed from an ion molecule reaction occurring between propene and something else present in the flare matrix. Additionally, we know that the intensity of m/z 55 signal increases in a second order fashion with respect to propene. Anicich et al. report that C3H5+ reacts rapidly with propene to produce C4H7+,18 and this reaction scheme nicely accounts for the behavior described above. On the basis of this observation, we conclude that under high propene concentration conditions that most of the m/z 55 signal is related to propene, and therefore have not attempted to quantify 1,3-butadiene produced during the flaring process. m/z 57, Acrolein + Butenes. The selected ion chromatogram of m/z 57 shows three peaks. The largest peak has been identified as acrolein, C3H4O. A second peak is labeled as C4H8. The specific butene isomer or isomers have not been identified, although propene pyrolysis studies suggest that 1-butene is the major C4H8 species.16 Acrolein and the butenes represent isobars, molecules with the same nominal molecular weight but different elemental compositions, and both are detected at m/z 57.19 It is important to note that the PTR-MS is more sensitive to acrolein than 1-butene by a factor of ∼3:1,19 so the relative amounts of acrolein and butene in the present example are more comparable than that suggested by the ion intensity responses. The relative proportions of the butenes and acrolein change as function of DRE. At high DREs acrolein predominates. Accounting for the difference in detection sensitivity, the contribution of the butenes increases from about 20% at a DRE of 0.98 to about 50% for DREs less than 0.65. Similar to what was observed with m/z 55, a peak is observed at the same elution time as propene, and the intensity of this peak varies with the square of the propene signal (second order). Anicich et al. report that C3H7+ reacts with propene to form C4H9+ with a reaction rate constant that is smaller than for the C3H5+ system.18 Consistent with that result we observe the contribution from this propene interference to the m/z 57 signal is smaller than that observed for m/z 55, which allows us to report data for these compounds. Measurements for butene 12681

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Figure 5. Plots showing the variation in carbon fraction with DRE for a selected set of emission products. Marker styles indicate the different fuel composition and flare types. Marker color delineates the different test conditions. See Supporting Information, Figure S4 for plots of the fuel components, CO2, and the sum of all VOCs.

Figure 6. Plots of VOC combustion product carbon fraction versus DRE for ethene and acetaldehyde for the different fuel mixtures. The results in the left panel are for the steam-assisted flare while those in the right panel are for the air-assisted flare. See text for details. See Supporting Information, Figures S5−S8 for plots of the other combustion products.

While there is considerable variability in the profiles shown in Figure 5, most of this variability appears to be related to differences in the test condition; fuel type, flare type, flare operation. This is demonstrated by the observation that the different test conditions (which contain multiple replicates) all show consistent trends. Such behavior suggests that the compositional differences are real and not measurement artifacts. VOC Emissions Expressed as Product Carbon Fraction. Further interpretation of the results shown in Figure 5 is challenging and generally unproductive. To be quantitative, a better means of examining the data is required. To address this need, we found that more information can be deduced if we examine how the carbon distributes among the VOC emission products. We define a new term, which is referred to as the product carbon fraction (PCF), and compute this term as shown in eq 4 using ethene as an example. PCFethene = CFethene

(

as the smaller markers in Figure 6. Even though this single test condition contributes most of the variability, there is no reason to discount this data as we believe it reflects actual variability and not instrumental variability. The propane data appears to be more tightly constrained, most likely because of fewer test conditions. We expect the propane data would more closely resemble the propene data had the propane flares been studied over the same range of conditions as the propene flares. There is a wealth of information contained in Figure 6 and Supporting Information, Figures S5−S8. One of the goals of this paper is to generate VOC speciation profiles for the different fuel compositions and flare types, steam-assisted and air-assisted. VOC speciation profiles have been generated for two fuels (80/20 mixtures of propene and Tulsa natural gas (TNG) and propane/TNG) with the two different flares (steam-assisted and air-assisted) tested. To accomplish this, it has been assumed the speciation profile is independent of combustion efficiency (DRE > 0.80). This latter assumption while not clearly justified is necessary to tabulate the results. The ethene data for steam-assisted flare in Figure 6 provides compelling evidence that ethene emissions represent between 40 and 50% of VOC combustion product carbon. Similarly with the steam-assisted flare, acetaldehyde appears to contribute between 10 and 18% VOC combustion product carbon. Even though acetaldehyde appears to be enhanced at higher DREs its composition within the flare can be reasonably expressed as the average 12.6% with an uncertainty expressed by the standard deviation 2.4%. The VOC product composition is less uniform with the air-assisted flares and there is certainly more uncertainty for this flare type. The plume PCF of acetaldehyde with air-assisted flares burning propene is reported as 10.0% ± 4.5%. Using the VOC product carbon fraction data for the other components allows us to create VOC composition profiles for the different fuel mixture and flare types. These composition profiles are presented in Table 3 and were generated by averaging the VOC product carbon fraction data for each

) (CFethene + CFethyne

+ CFformaldehyde +

∑ CFall other VOCs)

(4)

The greatest virtue of using product carbon fraction (PCF) is that it removes the influence of the primary fuel components and major combustion products and allows us to examine how the product VOC composition changes as a function of DRE. This is demonstrated in Figure 6, which shows plots of PCF for ethene and acetaldehyde versus DRE. The data for the steamassisted and air-assisted flares are now shown as separate plots. Representation of the data as PCF, illustrated for ethene and acetaldehyde in Figure 6, appears to reduce much of the variability in the data and allows for a more quantitative interpretation of the VOC product distribution. The other VOCs show similar behavior (Supporting Information, Figures S5−S8). This approach works well for the steam flares, but to a lesser degree for the air flares. There is more variability in the VOC PCF associated with the air-assisted flares. Additionally, one test condition (Air Flare test 6)6 appears to be much different than the others. The data for this test point is shown 12682

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Table 3. Average Flare VOC Composition Expressed as Product Carbon Fraction for the Different Fuels and Flare Types for DREs > 0.80a compound ethyne ethene fomaldehyde methanol propene acetaldehyde butenes + acrolein propene oxide + propanal + acetone benzene a

propene/TNG steam flare (n = 46)

propane/TNG steam flare (n = 10)

propene/TNG air flare (n = 19)

0.148 (0.042) 0.398 (0.028) 0.133 (0.022) 0.0057 (0.0006) N/A 0.126 (0.024) 0.173 (0.019) 0.024 (0.003)

0.029 (0.016) 0.470 (0.010) 0.083 (0.007) 0.0062 (0.0004) 0.344 (0.014) 0.026 (0.007) 0.029 (0.001) 0.009 (0.001)

0.258 0.396 0.098 0.006 N/A 0.100 0.112 0.015

0.021 (0.010)

0.0005 (0.0006)

0.052 (0.037)

propane/TNG air flare (n = 4)

(0.096) (0.048) (0.038) (0.004) (0.045) (0.047) (0.008)

0.064 (0.005) 0.436 (0.014) 0.075 (0.006) 0.0071 (0.0008) 0.356 (0.018) 0.023 (0.003) 0.025 (0.002) 0.009 (0.0005) 0.004 (0.0007)

Compositions are based on averages with standard deviations shown in parentheses.

such relationship presently exists because the plume composition varies (as shown in Figure 5 and Supporting Information, Figure S4) with the specific assist condition used for a given flare. One can, however, use the VOC product composition information in Table 3, along with data plotted for the total VOC carbon fraction in Supporting Information, Figure S4, to estimate the amount of an individual VOC constituent within the flare plume. The plume compositions of CO2 or CO can be directly deduced from Supporting Information, Figures S4 and Figure 5, respectively.

component. The data was restricted to DREs between 0.8 and 1.0. It is recognized that this approach is only approximate; however, it does provide a usable way of describing the VOC composition of the flare emission products. Inspection of the data in Table 3 indicates that VOC product composition is more dependent on fuel composition than the flare type. This is not a surprising result. Propane flares produce more alkenes with ethene and propene accounting for approximately 80% (per carbon basis) of the VOC combustion product. Propene flares produce more oxygenated material where formaldehyde and acetaldehyde are major contributors and account for ∼20−25% of VOC product carbon. Steamassisted flares produce less ethyne and benzene than air-assisted flares. This observation is consistent with the understanding that steam assisted flares are more efficient at reducing soot,1 which is formed via the same reaction mechanisms that lead to the formation of benzene and ethyne.21 Direct comparison of the VOC composition reported in Table 3 with propene or propane combustion studies is beyond the scope of this paper. We do note that all of the VOC products identified here have been reported previously in the oxidation of propene and propane.15,16,20,22−26 Significant amounts of methanol24 and propene oxide20 from propene have been previously reported but were not observed under the flaring conditions studied.

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ASSOCIATED CONTENT

S Supporting Information *

Data and expanded discussion of the following topics: Calibration and quantification of propene by the PTR-MS. Additional plots of the carbon fraction versus DRE for the unburned fuel components, CO2, and total VOCs. Additional plots of combustion product carbon fraction for all species quantified in this study. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

■

Present Address ∥

Department of Public Health, University of Massachusetts, Amherst, MA 01003.

SUMMARY A suite of high time response instrumentation consisting of optical and mass spectroscopic methods was successfully employed to measure the destruction and removal efficiency of propene and propene/TNG mixtures in operational full-scale flares. These instruments provided, for the first time, nearly complete VOC speciation profiles of the emission products formed from flaring propene and propane mixed with natural gas in steam-assisted and air-assisted flares. The product VOC speciation depends primarily on fuel composition and to a lesser extent on flare type. Product VOC speciation is essentially invariant with extent of combustion for DREs > 0.8. For steam flares, the operational conditions such as steam rate and injection position (center or upper) do not appear to significantly influence the VOC combustion product distribution. The operational conditions for the air-assisted flares seemed to exert more influence on the VOC combustion product distribution. While it would be highly desirable to have a simple relationship between species carbon fraction and DRE, no

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the State of Texas through the Air Quality Research Program administered by The University of Texas at Austin by means of a Grant from the Texas Commission on Environmental Quality. We thank Dr. Jim Barufaldi and TRC for assistance with the continuous FID measurements.

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REFERENCES

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