Two instruments have been designed for airborne remote sensing of landfill methane emissions using the infrared
absorption of reflected sunlight. A gas correlation filter wheel and a length modulated cell have been produced to
discriminate between methane and other interfering species and the performance of the two systems discussed. The two
systems have been interfaced with an Indium Gallium Arsenide (InGaAs) 2D detector array and an Indium Antimonide
(InSb) point detector. The InGaAs array detector response rate was found to be too slow so experiments were done
using the InSb detector. The gas correlation filter wheel has been shown to detect levels of methane equivalent to
200ppmv with a 30m pathlength at the 3.3μm methane band with the InSb point detector. It has been predicted that it
should be possible to detect levels equivalent to 20ppmv over a 30m pathlength at the 1.65μm band with the gas
correlation filter wheel and a fast response InGaAs detector. The length modulated device was found to have far less
sensitivity in comparison to the filter wheel system, but could have enhanced performance with improved design.
The demand for improved radiometric accuracy of the remote sensing instrumentation used for diagnostic applications involving hot gas emission spectroscopy requires regular “in-field” recalibration. The most convenient calibration source for such applications is a large emitting area blackbody capable of operating at temperatures approaching 1000 K which is also compact and portable. NPL in collaboration with the University of Reading have designed and assembled a large emitting area blackbody that meets these requirements. The blackbody design is based on a grooved base that is electrically heated to temperatures up to 1000 K. The base is coated with a high emissivity coating, which does not deteriorate during prolonged heating under atmospheric conditions. This base is enclosed by a specularly reflecting cavity that is water-cooled. Monte Carlo calculations were used to design the shapes of the base and reflective cavity to ensure that despite a cavity depth of 203 mm and a black body aperture diameter of 102 mm, the spectral radiance of the blackbody is known with a 1% uncertainty in the 2.5 μm to 14 μm wavelength range. The presentation will describe the design of the blackbody and the processes used for selecting the black coating of the base and the reflective coating of the specularly reflecting cavity.
Laser-Induced Incandescence studies of graphite aerosols show that great care has to be taken in interpreting Laser-Induced Incandescence (LII) data from gas turbine exhausts. In principle, particle volume fraction, information on particle size, chemical composition, and physical and chemical properties of the exhaust gas is obtainable by LII without the need for a physical probe in the exhaust. However, much about the way in which laser heated particles interact with exhaust gases has yet to be discovered.
As part of the EU funded project AEROJET2, a number of gas turbine engine tests were performed in different facilities around Europe. At Farnborough, UK a Spey engine was used to test a suite of prototype optically based instrumentation designed to measure exhaust gas emissions without using extractive probe systems. In addition to the AEROJET 2 prototype instrumentation, a Bruker Equinox 55 Fourier transform infrared (FTIR) spectrometer was used to obtain infrared spectra of the exhaust plume both in emission and absorption mode.
The Bruker FTIR spectrometer was fitted with a periscope system so that different lines of sight could be monitored in the plume in a vertical plane 25 cm downstream from the nozzle exit and 20 cm upstream of the center line of sight of the AEROJET 2 prototype instrumentation. DERA (now QinetiQ) provided exhaust gas analysis data for different engine running conditions using samples extracted from the plume with an intrusive probe. The probe sampled along a horizontal plane across the centerline of the engine 45 cm downstream of the nozzle exit. The Bruker spectrometer used both InSb (indium antimonide) and MCT (mercury-cadmium-telluride) detectors to maximize the sensitivity across the IR range 600-4000 cm-1.
Typically, CO2 and H2O IR signatures dominate the observed spectra of the plume. However, the engine tests showed that at low power engine conditions spectral features associated with CO around 2147 cm-1 and with hydrocarbons could be observed at around 3000 cm-1. In particular the presence of ethene (C2H2) was detected from observation of its characteristic in and out of plane vibration mode at 949 cm-1. At high engine powers the presence of NO was detected at 1900.3 cm-1. Species concentrations were calculated using a slab model for each line of sight compared against reference spectra. The engine plume was assumed to be symmetric about the centerline. On this basis, data from the extractive sampling gas analysis that had been obtained by traversing the probe across a horizontal plane through the centerline could be compared with non-intrusive measurements made by scanning vertically. Adjustments have been made to account for the 20 cm downstream offset in measurement planes of the probe and the spectrometer behind the nozzle exit.
The aim of their work is to develop non-intrusive methods to determine the presence of unburnt hydrocarbons in the emissions from aero-engine combustion systems. In addition to the detection of UHCs for legal certification of aero-engine performance, the consequences of their release into the upper atmosphere has environmental considerations.
The capability of taking non-intrusive species measurements in a jet plume of a modified mid-size low by-pass aero-engine running on a sea level test bed at several thrust levels was demonstrated. Also conventional intrusive measurements were performed with a spatially resolved method using a traversing single-point sampling probe which fulfills ICAO standards. The FTIR spectrometry measurements included both emission and absorption mode with a multi-path reflection compartment as well as the single emission mode. Due to the lack of a common/unique definition for the exhaust plume diameter it was found that the column density was the best measure to compare the different techniques. The FTIR engine measurement results for CO2, CO, and NO have been proven to be in agreement with the intrusive data within plus or minus 30%. Several error sources during the radiometric radiance calibration were identified which lead to uncertainties in the FTIR retrievals, namely (1) incomplete knowledge of the optical surface emissivities, (2) incomplete knowledge and inhomogeneities of the optical surface temperature, and (3) undefined instrumental drifts and non-linearities during the calibration.
A modified mid-size low by-pass aero-engine running on a sea level test bed was used for measurements with non-intrusive demonstrator systems and currently used gas sampling analysis techniques. A novel open-path White mirror system was developed and installed in the test bed to enhance the sensitivity of non-intrusive FTIR spectrometry. A comparison was made of the different measurement techniques at several engine thrust levels i.e. gas concentrations. This included the emission and absorption mode of the FTIR-spectrometers with the multi-path reflection compartment as well as the single emission mode. A new calibration procedure with a hot cell filled with CO (temperatures 300 to 750 K) was developed and used to calibrate the FTIR instruments. Retrieval results from FTIR measurements were obtained by using a rectangular and Gaussian distribution profile of temperature and gas concentrations in the plume. The FTIR measurement results for CO2, CO, and NO have been proven to be in agreement with the intrusive data. The deviations were generally in the order of plus or minus 30%, i.e. comparable to the day-to-day variations of the engine emissions. NO2 could be detected in the absorption mode only.
Fourier Transform Infrared (FTIR) spectroscopy was used to investigate the IR spectral absorption of soot particles from a Palas smoke generator. A TSI Condensation Particle Counter was used to quantify the number of soot particles produced and this was related to the intensity of the IR absorption. The broad band IR absorption increases with soot particle count but quantitative measurements of total soot mass were not obtained because accurate size distributions of the particles were not available. A sample of gas turbine engine exhaust gas was analyzed by Gas Chromatography-Mass Spectroscopy to determine the primary constituent unburnt hydrocarbon (UHC) species. Their relative proportions were measured with a Flame Ionization Detector (FID). These species are predominantly unsaturated C2 to C6 hydrocarbons. The infrared absorption spectrum of the exhaust gas sample was compared with that of combustion products from a laboratory kerosene burner using a multipass White cell. These were also compared with reference spectra and IR spectra of UHCs obtained non-intrusively from gas turbine engine tests. There are IR spectral band shape differences indicating that the relative proportions of the constituent UHCs in gas turbine exhaust are different from those in a kerosene burner plume.
Fourier transform IR (FTIR) spectroscopy for making non- intrusive measurements of gas turbine exhaust gases and laser induced incandescence for measuring soot content are being evaluated in EU Brite EuRam project AEROJET. Soot concentrations in modern aero-engine exhausts are very low with mean particle sizes < 100 nm. The standard extractive filter paper soot measurement gives results expressed in terms of SAE smoke number, typically < 10 SAE for modern engines.
This work describes the use of a He:Ne laser based test rig, developed at NPL, and a low pressure gas cell containing CO to measure the instrument line shape (ILS) function of a Unicam Research Series FTIR spectrometer, which has been extensively used for the detection and measurement of fugitive gas emissions. During the ILS function measurements minor optical misalignments were introduced into the optical system, and their effect on the measured ILS function observed. The ILS functions obtained using the laser system and low pressure gas cell containing CO were in good agreement, both having a full width at half maximum of 0.3 cm-1. The minor optical misalignments had a significant effect on the ILS function. The initially symmetric function became more asymmetric as the degree of misalignment was increased. Gas concentrations have been retrieved using a symmetric and an asymmetric ILS function, and the differences presented.
Klaus Schaefer, Joerg Heland, Roger Burrows, John Black, Marc Bernard, Gary Bishop, Volker Tank, Erwin Lindermeir, Dave Lister, Robert Falk, Peter Wiesen, Moira Hilton
The environment impact of air traffic and economical aspects require aircraft engines to be developed which have reduced trace gas emissions and, at the same time, increased efficiency. Each new engine must be shown to meet the environmental requirements laid down by regulatory bodies, and exhaust gas measurements must be performed for the certification. The goal of the EC project AEROJET is to demonstrate the equivalence of remote measurement techniques to conventional extractive methods for both gaseous and particulate measurements. The different remote measurement techniques will be compared and calibrated. A demonstrator measurement system for exhaust gases, temperature and particulates including data-analysis software will be regarded as result of this project.
Fourier transform IR (FTIR) spectroscopy has become a powerful analytical tool for the detection and measurement of atmospheric pollutant gases. This work describes the application of concentration analysis techniques to data recorded with a versatile FTIR spectroscopy system, developed at the University of Reading PHysics Department. Spectra were recorded at three separate sites, each possessing a distinct source of atmospheric pollution gases. The two sites monitored in the active mode were a traffic congested town center at rush hour and a dairy farm cow shed. The site monitored passively contained three 5 m high methane burners. The analysis techniques have been designed to provide rapid and accurate analysis of the spectrometer data, without the need for high computing power, thus making analysis possible in the field using a laptop PC. In an attempt to enhance the resolution of the spectral data, and therefore resolve overlapping spectral lines, a super- resolution algorithm has been tested on part of the recorded data. The results of applying the algorithm has been tested on part of the recorded data. The results of applying the algorithm, predominantly an image processing technique, are shown and improvements to the algorithm are discussed. Results from the urban and agricultural sites show that CO, CH4, and NH3 can be measured to a ppm level with a maximum uncertainly of 8 percent.
Techniques for obtaining quantitative values of the temperatures and concentrations of remote hot gaseous effluents from their measured passive emission spectra have been examined in laboratory experiments. The high sensitivity of the spectrometer in the vicinity of the 2397 cm-1 band head region of CO2 has allowed the gas temperature to be calculated from the relative intensity of the observed rotational lines. The spatial distribution of the CO2 in a methane flame has been reconstructed tomographically using a matrix inversion technique. The spectrometer has been calibrated against a black body source at different temperatures and a self absorption correction has been applied to the data avoiding the need to measure the transmission directly. Reconstruction artifacts have been reduced by applying a smoothing routine to the inversion matrix.
Feasibility studies on the use of passive FTIR spectroscopy for the remote detection of atmospheric pollutants have shown that gases may be identified remotely with an optically modified commercial FTIR spectrometer when only a small 7 degree(s)C) temperature difference exists between the gas and a background IR emitter. A correlation technique was used to extract information from noisy (SNR on the order of magnitude 1) emission spectra from which low temperature differential or low concentration gases or mixtures with interferant species were analyzed. Quantitative measurements of gas concentrations were made by first determining the temperature of the remote gas from rotational line intensity distributions. The product of concentration and pathlength was obtained by comparing the remotely detected emission spectra with simulated spectra obtained from computer models at the measured temperature. Laboratory experiments measured the temperature of CO from its emission spectrum to +/- 5% and the concentration also to +/- 5%. Tomographic reconstructions of molecular distributions within a methane flame have recently been obtained.
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