Article pubs.acs.org/IECR
Industrial Flare Performance at Low Flow Conditions. 1. Study Overview Vincent M. Torres,†,* Scott Herndon,‡ Zach Kodesh,§ and David T. Allen† †
Center for Energy and Environmental Resources, The University of Texas at Austin, 10100 Burnet Road, Building 133, M.S. R7100, Austin, Texas 78758, United States ‡ Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821-3976, United States § John Zink Company, 11920 East Apache, Tulsa, Oklahoma 74116, United States ABSTRACT: A series of full-scale industrial flare tests were conducted at low flow and low BTU content of flared gases at an industrial test facility. Both a 24” diameter air-assisted flare with a flow capacity of 144,000 lb/h and a 36” steam-assisted flare with a flow capacity of 937,000 lb/h were employed in the testing. Flared gases were mixtures of natural gas, propylene, and nitrogen or natural gas, propane, and nitrogen. Natural gas to propane or propylene ratio was 1:4 by volume for all experiments. Nitrogen was used as a diluent to achieve the desired lower heating values (LHV) for the vent gas. The range of flared gas flow rates was 0.1% to 0.65% of the flare’s design capacity. Flare operation was characterized by measurements of flow rates to the flare, extractive measurements made of the vent gases fed to the flare, extractive measurements made at the end of the flare plume, and remote sensing measurements of the flare plume made by a variety of spectroscopic instruments. Destruction/ removal efficiencies (DRE, fraction of vent gas reacted) of flared species were calculated based on the observed composition of the species in the plume. The tests demonstrated that destruction efficiencies for steam-assisted flares drop dramatically when combustion zone heating values fall below 250 BTU/scf. Air-assisted flares showed a linear drop in DRE as a function of air flow. While the primary focus of the measurements was on DRE, products of incomplete combustion were also measured. Dominant products of incomplete combustion were CO, ethylene, formaldehyde, acetylene, and acetaldehyde. CO represented approximately 24% to 80% (carbon basis) of the total products of incomplete combustion for DRE > 90%. While DREs of 98− 99% were observed in some experiments, many operating conditions produced DREs of substantially less than 99%. Since prescribed methods for estimating emissions would have predicted 98−99% DRE for all the tests, some test conditions resulted in the production of flare emissions multiple times the value that would be calculated using the prescribed estimation methods. In practice, total emissions from flares will depend on both operating conditions and the duration of operation at the various operating conditions.
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INTRODUCTION Industrial emergency flares are safety devices designed to efficiently combust large volumes of gases that are generated during process upsets or during process start-ups or shut downs; these same flares may also be used to combust much smaller flows of gases that occur during routine operations.1 Under high flow conditions, measurements have indicated that flares operate at high combustion efficiencies.2,3 However, a number of field observations have measured flare combustion efficiencies substantially below the target values of 98−99%. 4−7 Generally, these observations of low combustion efficiencies occur at conditions of low flow. There are many parameters that can lead to low combustion efficiency at low flows. One such parameter is the flammability of the vent gas mixture. The conditions that can adversely affect the mixture flammability include too much steam assist or too much inert gas or both. This study focused on the effect of steam assist and low BTU content vent gases on flare efficiency at low flow conditions. Because of the limited data available on combustion efficiencies of full-scale industrial flares operating at low flow, under well controlled conditions, and because of the potential air quality implications of low combustion efficiencies in industrial flaring,8 the Texas Commission on Environmental Quality, © 2012 American Chemical Society
working through the University of Texas, organized field tests to measure flare emissions under controlled flaring conditions. The goal of the tests was to determine the combustion characteristics of full-scale industrial flares under a variety of flare operation conditions and compositions of flared gas (herein referred to as vent gas). The field tests performed for this study were conducted on full-scale industrial flares. Specifically, the flare designs selected were the John Zink Models EEF-QSC-36” Flare Tip with (3) EEP-503 pilots (steam assist) and the LHTS-24/60 with (3) EEP-503 pilots (air assist), with maximum capacities of 937,000 lb/h and 144,000, respectively. The study was conducted at the outdoor flare test facility of the John Zink Company, LLC (Zink), in Tulsa, Oklahoma. Zink is a flare manufacturer whose flare test facility is capable of accommodating a wide range of flare tips, test configurations, and operating conditions. The test program was conducted in September 2010. Special Issue: Industrial Flares Received: Revised: Accepted: Published: 12559
November 20, 2011 February 24, 2012 February 27, 2012 February 27, 2012 dx.doi.org/10.1021/ie202674t | Ind. Eng. Chem. Res. 2012, 51, 12559−12568
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Article
steam injection nozzle for injecting steam inside the body at the center of the tip. This tip is equipped with three natural gas pilots. The total flow rate of natural gas to the three pilots was constant at nominally 10.0 lb/h. The actual gas flow rate was measured continuously during each test series using an orifice primary element with a differential pressure transmitter. Figure 2
Flare operation was characterized by measurements of flow rates to the flare, extractive measurements made of the vent gases fed to the flare, extractive measurements made at the end of the flare plume, and remote sensing measurements of the flare plume made by a variety of spectroscopic instruments. A single blind approach was used to compare the remote sensing technology measurements to the extractive measurements, i.e., the only information provided to the operators of the remote sensing instruments was the flare operating conditions. They did not have access to the extractive plume measurements. This paper provides an overview of the study. The flares used in the study and the extractive and remote sensing measurements are described in detail. A summary of the measurements is provided. Detailed results and the details of the analysis of the extractive sampling are presented in subsequent papers.9,10
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METHODS The methods used in the flare test measurements are described in detail in a Quality Assurance Project Plan (QAPP) prepared for the project. The QAPP was reviewed by the Texas Commission on Environmental Quality (TCEQ) and a technical advisory committee, consisting of industrial personnel with experience in flare operation, environmental regulatory personnel and independent experts. The QAPP and the review documents for the QAPP are available from the TCEQ.11 This section provides a summary of the information described in the QAPP. Flare Configurations. A steam-assisted flare and an airassisted flare were used in the study. The steam flare test equipment consisted of a fuel supply system, fuel metering system, steam supply system, steam metering system, and steamassisted flare. The air flare test system consisted of the same fuel supply system and fuel metering system as the steam flare, along with an air supply and measuring system, and air-assisted flare. The study site was designed so that an unobstructed view of either test flare flue gas was available from all directions within an arc of at least 180° about the centerline of the flare burner. The steam flare operated with a 36” diameter flare tip with tip exit 13 feet above the ground. This flare tip was designed for a maximum flow capacity of 937,000 pounds per hour. The steam-assisted flare tip was a John Zink model EEF-QSC-36”. This tip design has an upper ring for injecting steam around the perimeter of the tip, as shown in Figure 1. It also has a center
Figure 2. Steam-assisted flare in operation, with flare plume extractive sampling device, suspended by a crane, in upper left-hand corner; airassisted flare is located behind the steam-assisted flare.
shows the steam-assisted flare in operation. Operation of the steam flare test equipment consisted of manually adjusting the vent gas control valves until the desired flow rate was achieved. Vent gas flow control was monitored through the use of calibrated orifice plates and transmitters. Mixing of the vent gas components (Tulsa Natural Gas, propylene, nitrogen, and in some cases propane) was accomplished by injecting the components into a mixing manifold. The flow of each component was carefully monitored to achieve the desired lower heating value (LHV) of vent gas for each test. Temperature of the steam was monitored through the use of calibrated thermocouples, and steam pressure was monitored using calibrated pressure transmitters. Steam flow was monitored using an ultrasonic flow meter and was manually controlled by adjusting the steam control valves. A summary of the instrumentation available for the flare system is provided in Table 1. The air-assisted flare operated with a 24” diameter flare tip with tip exit 33 feet above the ground, as shown in Figure 2. This flare tip was designed for a maximum flow capacity of 144,000 pounds per hour. The air-assisted flare tip was a John Zink model LHTS-24/60 with three natural gas pilots. This tip design is segmented into 10 wedge shaped sections, which alternate between air flow and vent gas flow, as shown in Figure 3. Airflow to the air-assisted flare was controlled by adjusting the air blower to operate at a selected point between 10 and 100% of maximum speed. The air flow to the flare, for flow rates up to 20,000 cfm, was calculated based on measurements of air velocities determined using a resistive temperature device. Above 20,000 cfm, a pitot tube and pressure measurement system was used to determine flow rates. Vent gas flow to the flare was measured and controlled using the same system employed by the steam-assisted flare. Table 1 summarizes the measurements that were made for each of the flare tests.
Figure 1. Steam-assisted flare tip with upper ring steam injectors. 12560
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the flame was partially extinguished (referred to in Table 2 as the snuff point). The procedures used to estimate the ISP and snuff point are described in the study’s final report.16 Each test series consisted of at least three steam to fuel operating ratios, and at least three sets of the steam to fuel operating ratios in each test series were repeated three times. The individual tests are indicated by a number, such as S4.2R3, which represents Steam flare test series number 4 (S4, see Table 2), steam to vent gas flow ratio number 2 (S4.2), and replicate number 3 (S4.2R3). Test series numbers S7 through S14 are identical in principle to S1 through S6, except that vent gas was varied as steam flow was held constant. At least three vent gas flow rates were used for each steam flow. Test series A3-A7 were air-assisted flare tests. As with the steam flare test series, each test series consisted of at least three air to fuel ratios, and at least three sets of conditions in each test series were repeated three times. Extractive Sampling. The flare plume was sampled using the device pictured in Figure 4, consisting of an inlet cone, a sample preparation section, a sample extraction section, and eductor. The inlet face of the cone is 20” diameter tapered to a 12” outlet. The 12” outlet is connected to a 90 degree elbow which in turn is connected to the inlet of the sample preparation section. There are three exposed junction temperature elements located equidistant around the perimeter of the inlet of the 20” diameter cone to measure the temperature of the flue gas as it enters the cone. Attached to the elbow is the sample preparation section, which consists of a 12” diameter Vortab insertion type flow mixer to homogenize the sample. This mixer is 3 feet into the inlet of the 9.5′ long sample preparation section, which consists of a 12” diameter stainless steel straight pipe. The sample preparation section mixes the gases to obtain a uniform composition. At the exit of the sample preparation section is the extractive sampling section. The sampling section is 1.5 feet long and consists of a pitot tube for measurement of the flue gas velocity in the apparatus, an exposed junction temperature element, and two flue gas sampling probes. The flare flue gas sample that was analyzed for emissions composition was obtained from this location. Downstream of the sampling section 7.5 feet is the end of the flare flue gas sampling device where the eductor is attached. The eductor utilizes compressed air to induce a flow through the apparatus. By varying the pressure of the compressed air at the eductor, the flue gas eduction rate is varied. Pipe clamps are used to lift the apparatus with a crane. Rotation of the pipe in the clamps allows orientation of the cone inlet so the inlet plane can be positioned either horizontal to the ground to collect flue gas exiting vertically from the flare, perpendicular to the ground to collect flue gas exiting and traveling horizontally from the flare due to cross winds, or at any angle in-between. During the flare operation, the sampling device cone inlet was positioned outside and downwind of the end of the visible flame, approximately one flame length (see Figure 2). The position of the inlet to the cone was adjusted so that the average temperature, measured by the three inlet temperature thermocouples, was no more than 230 °F. Positioning of the extractive sampler was facilitated through the use of infrared and visible light cameras. Multiple infrared imaging cameras were placed at right angles, for viewing along the axis of the plume and the plume profile. The crane and tethers
Table 1. Summary of Measurements Made of Flare Operating Conditions measurement Feed to Flare flow of propylene to flare flow of natural gas to flare flow of nitrogen to flare flared gas methane, ethane, propylene mixed fuel temperature flare flue gas temperature pilot gas flow
Steam Assist total steam flow to steam assist flare center steam temperature center steam pressure upper ring steam temperature upper ring steam pressure Air Assist air flow
detection limit
precision
orifice plate
na
na
±27 lb/h
orifice plate
na
na
±1 lb/h
orifice plate
na
na
±19 lb/h
gas chromatography
5%
±200 ppmv
thermocouple
±0.01% (100 ppmv) na
±2.7 °F
thermocouple
na
±2.7 °F
integral orifice with differential pressure transmitter
na
±0.18 °F% ±0.18 °F% ±3.0%
ultrasonic flow meter
0.1 ft/sec
±0.2%
±0.2%
thermocouple
na
±0.18 °F%
±2.7 °F
pressure transmitter
na
thermocouple
na
pressure transmitter
na
instrument
thermal mass flow meter Ambient Meteorology wind speed anemometer wind direction anemometer ambient tempersensirion temperature/ ature humidity sensor HT1 relative humidity
±0.18 °F%
accuracy
±0.158 psig
±0.158 psig ±2.7 °F ±0.158 psig
na
±184 lb/h
±1.32% + 459 lb/h
2.2 mph 1° na
0.5 mph 0.5° na
±0.6 mph ±3° ±0.54 °F
1%
1%
±2%
Flare Operating Conditions. Vent gas composition was a mixture of natural gas, propylene, and nitrogen or natural gas, propane, and nitrogen. Natural gas to propane or propylene ratio was 1:4 by volume for all experiments. Nitrogen was used as a diluent to achieve the desired lower heating values (LHV) for the vent gas. This work focused on vent gases with low LHV and low flow rates. The range of vent gas flow rates was 0.1% to 0.65% of the flare’s design capacity. Vent gas flow rates of 0.1% and 0.65% of nominal design capacity were selected as they are in the range of operation for typical flare flow rates (less than 0.5%).12 Table 2 describes the sets of operating conditions employed in the study. The tests are grouped into 21 test series labeled S1−S14 (steam-assisted flare) and A1−A7 (air-assisted flare). Test series S1, S2, A1, and A2 were initial equipment test runs. These test series used pure propylene as vent gas with no nitrogen dilution. The heating value of the vent gas was therefore much higher than the target range of 350−600 BTU/ scf. The remaining test series all used low BTU vent gases and are the focus of analyses presented in this work. For test series S3 through S6, a vent gas flow rate was selected, and steam flows were varied from the incipient smoke point (ISP, an operating condition at which visible smoke just persists beyond the end of the visible flame) to a point where 12561
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Figure 3. Air-assisted flare tip with extractive sampler in background.
sensing measurements, are provided in the project final report16 and will be described in separate publications. Destruction Efficiency Calculations. Destruction/Removal Efficiency (DRE) and Combustion Efficiency (CE) are reported for each of the flare tests. DRE is defined by the Texas Commission on Environmental Quality.17 DRE was calculated using eq 1
cables to the ground, manned by operators, allowed the precise positioning of the sampler. Figure 2 shows the sampling system in operation. Chemical Analysis of the Samples Extracted from the Plume. Gas samples extracted from the plume were transferred via sampling lines to two sampling vans. One van collected grab samples and performed a rapid gas chromatography analysis of the flue gas. Target analytes were methane, ethane, and propylene, with a detection limit of 100 ppmv and an accuracy of ±200 ppmv. Typically, two grab samples of plume composition were obtained for each set of test conditions. The first sample was collected at the start of the test when the flare operating conditions were stable, and the next sample was obtained four to five minutes later. The second sampling van contained the suite of measurements shown in Table 3. These measurements were designed to provide rapid time response measurements (1−10 s for most measurements). Measurements were monitored and recorded continuously and allowed for real-time feedback on the positioning of the extractive sampler. Typically, the flare was operated at steady state conditions for 7−10 min at each test condition (e.g., S4.2R3), allowing for the collection of hundreds of data points at each test condition for the continuously monitored parameters. Remote Sensing Measurements. Several remote sensing technologies collected data on the flare plume. These included Infrared Hyper-Spectral Imaging Technology,13 Passive and Active Fourier Transform Infrared (PFTIR, AFTIR) Spectroscopy,7,14 and FLIR GasFindIR Passive Infrared (IR) Cameras (ThermaCam GasFindIR camera).15 These technologies are described in detail in the QAPP.11 Comparisons between the extractive sampling results, reported briefly in this paper and in more detail in Part 2 of this series of papers,10 and the remote
⎛ X plume ⎞ DRE(%) = ⎜1 − ⎟ × 100 X in ⎠ ⎝
(1)
where DRE (%) = destruction and removal efficiency (%), Xplume = mass flow rate of species HC found in the flare plume, and Xin = mass flow rate of species HC entering the flare DRE can be reported separately for each species. For example, in a propylene/methane flame, separate DREs could be calculated for propylene and methane. This work will focus on reporting the DRE of propylene and propane and the calculation of propylene DRE is given as an example, below. The measurements on which propylene DREs are based are (i) the QC-TILDAS extractive measurements for propylene in the plume, (ii) the GC data on the vent gas composition, and (iii) measurements of total carbon, due to the flare (obtained by subtracting background concentrations), in the plume. The details of this method are described in this series of papers9 and in the final report for the flare tests.16 The method is briefly summarized here. Measurements of propylene in the plume, propylene in the flare feed (vent gas), total carbon in the plume due to 12562
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Table 2. Operating Conditions Used in the Flare Testsa steam flare tests steam assist (lbs/h)
vent gas (nominal) test series
flow rate (lb/h)
LHV (BTU/ scf)
center
upper
S1
2342
2149
100% propylene
500
S2
937
2149
100% propylene
500
S3
937
350
500
S4
2342
350
S5
937
600
S6
2342
600
S7
2342 937 2342 937 2342 937 2342 937 2342 937 2342 937 2342 937 2342 937
350
1:4 TNG to propylene ratio diluted to target LHV 1:4 TNG to propylene ratio diluted to target LHV 1:4 TNG to propylene ratio diluted to target LHV 1:4 TNG to propylene ratio diluted to target LHV 1:4 TNG to propylene ratio diluted to target LHV 1:4 TNG to propylene ratio diluted to target LHV 1:4 TNG to propylene ratio diluted to target LHV 1:4 TNG to propylene ratio diluted to target LHV 1:4 TNG to propylene ratio diluted to target LHV 1:4 TNG to propane ratio diluted to target LHV 1:4 TNG to propane ratio diluted to target LHV 1:4 TNG to propane ratio diluted to target LHV air flare test tests
500
ISP to
