Title | How to Measure Flare Vent Gas Composition and Net Heating Value |
Creator | Pearson, D. |
Date | 2015-09-10 |
Spatial Coverage | Salt Lake City, Utah |
Subject | 2015 AFRC Industrial Combustion Symposium |
Description | Paper from the AFRC 2015 conference titled How to Measure Flare Vent Gas Composition and Net Heating Value |
Abstract | Developing regulations and recent consent decrees aim to ensure that assisted flares operate with high combustion efficiency (CE) or destruction efficiency (DE), consequently minimizing emissions of uncombusted materials (particularly volatile organic compounds (VOCs)). Currently, there is no proven technology that can be permanently installed at a plant to continuously and reliably measure flare CE and DE. Due to this limitation, regulations and recent consent decrees rely on the measurement of flare operating parameters such as vent gas composition, vent gas net heating value, vent gas flow rate, and assist media (i.e., steam or air) flow rate. These measurements are used to calculate other flare operating parameters and these calculated parameters are correlated to CE and DE. Several of these parameters have been identified from recent flare tests. The most effective of these parameters are determined "in the combustion zone"-that is, the calculated parameters take into account the addition of steam or air assist. The two parameters that correlate best with CE and DE are:... |
Type | Event |
Format | application/pdf |
Rights | No copyright issues exist |
OCR Text | Show How to Measure Flare Vent Gas Composition and Net Heating Value: A Review of Existing Instrumentation Prepared for the American Flame Research Council 8/25/15 Prepared by Dan Pearson How to Measure Flare Vent Gas Composition and Net Heating Value Page 2 Overview Developing regulations and recent consent decrees aim to ensure that assisted flares operate with high combustion efficiency (CE) or destruction efficiency (DE), consequently minimizing emissions of uncombusted materials (particularly volatile organic compounds (VOCs)). Currently, there is no proven technology that can be permanently installed at a plant to continuously and reliably measure flare CE and DE. Due to this limitation, regulations and recent consent decrees rely on the measurement of flare operating parameters such as vent gas composition, vent gas net heating value, vent gas flow rate, and assist media (i.e., steam or air) flow rate. These measurements are used to calculate other flare operating parameters and these calculated parameters are correlated to CE and DE. Several of these parameters have been identified from recent flare tests. The most effective of these parameters are determined "in the combustion zone" - that is, the calculated parameters take into account the addition of steam or air assist. The two parameters that correlate best with CE and DE are: 1.) Net Heating Value of the Combustion Zone (NHVcz) NHVcz is a calculated term representing the net heating value of all components in the combustion zone. This includes total assist media to the flare tip. The combustion zone is directly above the flare tip and is the point at which all materials mix and combust. The NHVcz is therefore the resultant heat content from the mixture of the vent gas from the flare header and the total assist media. Air inspirated into the combustion zone with steam assist is not included. NHVcz is calculated as follows. π!" β ππ»π!" ππ»π!" = Where: ππ»π!" π!" π!" ππ»π!" = = = = π!" + π!" Equation 1 combustion zone gas net heating value (Btu/scf) volumetric vent gas flow rate (scf/hr) volumetric assist media flow rate (scf/hr) net heating value of vent gas (Btu/scf) ! ππ»π!" = π₯! ∗ ππ»π! Equation 2 !!! Where: ππ»π!" π₯! ππ»π! π = = = = net heating value of vent gas (BTU/scf) concentration of individual component in vent gas (vol fraction) net heating value of pure individual component i (Btu/scf) number of individual pure components in vent gas How to Measure Flare Vent Gas Composition and Net Heating Value Page 3 2.) Lower Flammability Limit of the Combustion Zone (LFLcz) This is the calculated lower flammability limit after assist media has been added. Air inspirated into the combustion zone with steam assist is not included. LFLcz is a calculated parameter and is not the "true" LFL in the combustion zone since gas heating, mixing, and air inspiration are not taken into account. There are many documented instances in flare tests of combustion occurring well below the LFLcz level. There is a direct mathematical relationship between NHVcz and LFLcz - i.e., one may be mathematically transformed into the other when hydrogen is taken into account. LFLcz is calculated as follows. πΏπΉπΏ!" = Where: πΏπΉπΏ!" π!" π!! πΏπΉπΏ!" = = = = = = = = π!" β πΏπΉπΏ!" Equation 3 lower flammability limit of combustion zone gas (volume fraction) volumetric vent gas flow rate (scf/hr) volumetric assist media flow rate (scf/hr) lower flammability limit of vent gas (volume fraction) πΏπΉπΏ!" = Where: πΏπΉπΏ!" x! πΏπΉπΏ! π π!" + π!" 1 ! π₯! ! πΏπΉπΏ ! Equation 4 lower flammability limit of vent gas (volume fraction) concentration of individual component in vent gas (volume fraction) lower flammability limit of individual component i (volume fraction) number of individual pure components in vent gas Once NHVcz or LFLcz is known, CE and DE can be estimated. The calculations show that vent gas volumetric flow rate, assist media volumetric flow rate, and composition are required for these calculations. Another approach is the direct measurement of NHVvg, which can be used to determine NHVcz. This paper evaluates different technologies that can be used to measure vent gas composition or net heating value. How to Measure Flare Vent Gas Composition and Net Heating Value Page 4 Technologies Complex gas mixtures comprise flare vent gas streams. These gas mixtures typically include hydrocarbons, nitrogen, hydrogen, and sulfur containing compounds. As the speed and accuracy of vent gas composition or net heating value measurements improve, the ability of a facility to optimize its flare performance improves as well. Instruments based on the following analytical methods are currently available to characterize vent gas composition or net heating value: 1.) Gas Chromatography 2.) Calorimetery 3.) Mass Spectrometry 4.) Raman and FTIR Spectrometry Gas Chromatograph (GC) At this time, a GC is the instrument most commonly used to measure flare vent gas composition. A GC operates by separating the individual compounds of a complex gas matrix by injection of the sample into a narrow column and subsequently detecting them using a variety of available detectors. The variation in chemical and physical properties of different compounds in a gas mixture cause the individual compounds to elute at different intervals. Several factors can be adjusted to optimize the ability of a GC to separate compounds in a gas mixture. These factors include carrier gas flow rate, the number of columns, column lengths, and temperature. The primary advantages of a GC are that it can measure all vent gas compounds and that many instrument technicians are already familiar with the technology. The primary disadvantage of using a GC is that the analytical cycle is much longer (5-β20 minutes) than any of the other technologies discussed in this report. This long analytical cycle limits the effectiveness of using NHVcz or LFLcz for controlling flare performance. In addition to the long cycle time, the practicality of a GC is also limited by the extensive the calibration requirements necessary to generate quality data. The monthly calibration of a GC requires a wide range of calibration gases. Calibration gases can either be pure component or blends, but all measured components of the vent gas must be contained in the calibration gases. Calorimeter A calorimeter is an instrument that can be used to measure the net heating value of flare vent gas directly. There are different types of calorimeters based on different operating principles for determining net heating value of the sample gas mixture. The two primary types of calorimeters are metered flame calorimeters and residual oxygen calorimeters. How to Measure Flare Vent Gas Composition and Net Heating Value Page 5 The primary advantage of a calorimeter is that it is relatively inexpensive instrument, especially when compared to the other technologies evaluated in this report. Additionally, a calorimeter requires only a single reference gas for calibration. A disadvantage of using a calorimeter for vent gas net heating value measurement is that the vent gas cannot be speciated. The U.S. EPA has allowed the use of a hydrogen adjusted net heating value of 1212 Btu/scf rather than the actual value of 274 Btu/scf when calculating vent gas net heating values from vent gas composition data. Because the vent gas hydrogen concentration is not measured with a calorimeter, flare operators cannot take advantage of this adjustment factor without the use of additional instrumentation. Mass Spectrometer (MS) A mass spectrometer (MS) can be used to measure flare vent gas composition and is currently being used on flare systems at select refineries. This instrument ionizes the sample gas mixture and detects the charged fragments by measuring the mass-β to-βcharge ratio (m/z). The primary advantage of using of using a MS for flare vent gas measurement is the rapid analytical cycle. The short analytical cycle is ideal for controlling flare performance. The primary disadvantages of using a MS is that some instruments have difficulty distinguishing compounds with similar mass-βto-βcharge ratios and sulfur compounds, and some instruments may be limited by the number of compounds that can be measured simultaneously. However, these claims vary between vendors. Additionally, a MS requires the same effort and range of calibration gases as a GC. Raman / FTIR Dual Spectrometer A Raman / FTIR dual spectrometer (Raman/FTIR) uses both Raman and FTIR spectroscopy to measure flare vent gas composition. The Raman Spectrometer uses a laser to excite the molecules of a gas mixture and measures the inelastic scattering of the light to determine composition of the gas mixture. The FTIR spectrometer measures gas composition by collecting high resolution infrared absorbance spectra and comparing them to known references. The primary advantages of using an instrument that combines Raman and FTIR analytical methods are that every vent gas compound can be measured and the fast analytical cycle is ideal for controlling flare performance. A Raman / FTIR uses a Raman spectrometer for measuring homonuclear diatomics (nitrogen, oxygen, hydrogen, etc.) and other hydrocarbons found in percent level concentrations. The FTIR spectrometer can be used to measure compounds found in parts per million level concentrations. An FTIR spectrometer obtains a data point roughly every second while the Raman obtains a data point roughly every minute. How to Measure Flare Vent Gas Composition and Net Heating Value Page 6 In contrast to other analytical methods, a Raman/FTIR requires only two calibration gases (one for each spectrometer) in order to obtain quality data. The Raman/FTIR is the only instrument capable of measuring the full spectrum of components found in flare vent gas with at a speed sufficient to effectively control flare performance. Instrument Summary Table The table below summarizes the key aspects of the four types of instruments discussed above. Instrument Measures Calibration Gases Response Time Gas Chromatograph (GC) All vent gas compounds Either pure component or gas blends; must contain 300-β1,200 seconds all vent gas compounds Calorimeter Vent gas net heating value Single reference gas Mass Spectrometer (MS) Either pure component or gas Hydrocarbon groups, < 1 second per blends; must contain diatomics compound all vent gas compounds Raman / FTIR Dual Spectrometer All vent gas compounds Two calibration gases 3-β30 seconds FTIR : 1 second Raman: 60 seconds Table 1: Instrumentation Summary Summary There is currently no instrument that can be permanently installed at a facility and used to measure flare CE and DE on an ongoing basis. The most effective way to control a flare is to have instrumentation and control logic necessary to calculate either NHVcz or LFLcz. There are several instruments available that are capable of measuring either flare vent gas composition or flare vent gas net heating value. The calibration requirements, components measured, analytical cycle, and cost all vary so the optimum instrument may vary from flare to flare. |
ARK | ark:/87278/s6x96n78 |
Setname | uu_afrc |
ID | 1387830 |
Reference URL | https://collections.lib.utah.edu/ark:/87278/s6x96n78 |