Joint Meeting of the Scandinavian-Nordic and French Sections of the Combustion Institute
Copenhagen, 9-10 November 2009
On Developing a Spectroscopic System for Fast Gas
Temperature Measurements in Combustion Environments
V. Evseev1,*, S. Clausen1
1
Technical University of Denmark, Risø National Laboratory for Sustainable Energy,
Roskilde, Denmark
*e-mail of corresponding author: vaev@risoe.dtu.dk
Abstract
Fourier Transform Infra Red (FTIR) spectroscopy techniques are known to provide reliable results for gas
temperature measurements and can be comparatively easily performed on an industrial scale such as a boiler on a
power plant or an exhaust of a ship engine cylinder. However temporal resolution is not high enough to trace fast
temperature variations which are of great importance for complete combustion diagnostics. To eliminate the
above mentioned shortcoming, a new IR spectroscopic-imaging system has been developed at Risø DTU. The
schematic of the system is presented. Results on lab and industrial scale are discussed. System’s flexibility and
changeability which enable applying of a number of other methods for combustion diagnostics are demonstrated.
1 Introduction
Non-intrusive methods for gas temperature measurements are preferable to conventional ones such as based
on sampling or thermocouples which give integrated mean values. Among optical methods laser-based such as
e.g. coherent anti-Stokes Raman spectroscopy (CARS) are known to offer unique features including high
temporal as well as spatial resolution [1] but they involve complicated setup which can hinder applying those
techniques on an industrial scale. A technique called Fourier Transform Infra Red (FTIR) spectroscopy has been
successfully applied for in situ non-contact gas analysis in industrial combustors [2]. But this method being
based on a Michelson interferometer principle necessitates a certain scanning time for an interferogram to be
recorded. This results in disability to resolve fast temperature variations which occur due to turbulent
fluctuations or in an engine exhaust.
A system based on a grating spectrometer and an IR camera has been developed with regard to eliminate the
shortcomings of the above mentioned techniques. Modern grating spectrometers provide sufficient resolving
power. The response time is limited only by the exposure time of the camera which is used to image the
spectrum. The proposed IR spectroscopic-imaging system has been successfully tested on a lab as well as
industrial scale.
2 Schematic of the System
The system consists of a grating spectrometer and an IR camera (see fig.1) connected together optically using
a lens. The lens is focused on the rear focal plane of the spectrometer where the spectrum is also focused by the
spectrometer. The lens acts as an optical adaptor between rear focal plane of the spectrometer and the CCD array
in the camera.
The IR image of the spectrum is a matrix of 512 by 640 elements each of which contains the value
proportional to the energy of incident radiation. The matrix can be represented visually as shown on the lowerleft pane of fig.1. However, a matrix itself is a convenient way of representing the data and those values can be
further processed using various software and procedures. An example of a spectral curve which is a plot of the
values in the matrix along a given horizontal line is shown on the lower-right pane of fig.1.
The radiation from the source under investigation can be carried to the slit of the spectrometer by an optical
fibre. This flexibility makes possible to reach regions inside industrial boilers. Specially designed water-cooled
probes are used in this case to insert and to protect fibre inside a boiler [2].
3 Measurements
The equipment was first tested on a lab scale. In particular, spectral measurements of a flame produced by a
small lab burner with further temperature calculations using spectral emission-absorption method [3] were
performed. The results showed that the developed mathematical model of spectral measurements describes the
measuring process with acceptable accuracy and can be used to develop calibration procedure and for
calculations of spectra from raw measured data.
V. Evseev et al
On Developing a Spectroscopic System for Fast Gas Temperature Measurements...
Figure 1. Schematic of the system
Fast gas temperature measurements have been successfully performed on an industrial boiler of a power
station. The installation and adjustment of the equipment on-site take comparatively short time and can be
completed by one person. A 3-m long water-cooled fibre-optic probe was used to insert and protect optical fibre
inside the boiler. Spectra were recorded at different insertion depths of the probe. The duration of spectra
recording was 10 seconds at each position. The temporal resolution was 1 kHz.
The CO2 emission band at 2350 cm-1 was used for gas temperature calculations from measured spectra [4].
Gas temperature was calculated from Plank radiation law for each wavenumber in the region of the CO2
emission band in which CO2 is a perfect emitter. Standard deviations of results obtained at different
wavenumbers do not exceed 50 degrees and average value is 10 degrees.
The spectral region at 2600cm-1 is the region without any emission or absorption from any gas molecules.
Thus particle temperature was measured using also Planck radiation law for each wavenumber in that region.
4 Summary
The IR camera can be run at frequencies up to 3 kHz. Thus even higher temporal resolution for temperature
measurements can be achieved. Moreover the system has some flexibility which enables, for example,
simultaneous spectral measurements from several lines of sight. It can be done by coupling several optical fibres
onto the slit of the spectrometer. It allows to perform, for example, several different spectral methods for gas
temperature measurements at the same time or to map temperature distribution in a cross-section of a flame
applying some tomography algorithm to spectra taken simultaneously from several positions in that crosssection.
References
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