A review of applications for tunable diode laser spectroscopy (TDLS) instrumentation in process analytics is presented.
We have investigated applications in olefin production suitable for TDLS instrumentation. The possibility to detect
acetylene impurities in different hydrocarbon backgrounds was investigated by TDLS in the 3 micron wavelength region
using novel GaInAsSb/AlGaAsSb DFB lasers. The performance of the TDLS instrument for detection of acetylene
impurities in pure ethylene and in a gas matrix typical of a hydrogenating reactor was investigated more in detail.
Experiments with in-situ measurements of hydrocarbons in an industrial environment using a modified Siemens TDLS
instrument are also discussed.
We will show the results from a Tunable Diode Laser (TDL) spectrometer installation monitoring the O<SUB>2</SUB> concentration and the temperature in an olivine pellet production plant. The spectrometer has been operating continuously for more than two years. In the pelletizing process a reduction of magnetite and sintering takes place at a temperature around 1250 degree(s)C. To achieve a high and predictable quality of the produced pellets the oxygen concentration and the temperature has to be measured in-situ inside the process furnace. A specially designed high temperature sensor was mounted on the furnace wall and an optical fiber was used to carry the probing light from the TDL spectrometer to the measurement point. The TDL spectrometer operates at two absorption lines in the near infrared wavelength region to measure the oxygen concentration and the temperature simultaneously. The temperature is measured using the relative intensity of the two absorption lines and the concentration is calculated from the temperature compensated absorbance. The accuracy of the concentration and temperature measurements at 1 s response time was 0.1 vol.% and 50 degree(s)C, respectively. In order to validate the TDL measurements the pelletizing process furnace temperature was varied between 100 degree(s)C up to 1300 degree(s)C while the oxygen and temperature readings from the TDL spectrometer was recorded. The temperature measurements were also correlated with temperature measurements using thermocouples inside the furnace. The O<SUB>2</SUB> absorption line parameters were determined in a controlled laboratory experiment using a heated measurement path. This work shows that it is possible to build and field a TDL spectrometer to measure O<SUB>2</SUB> and temperature in-situ in a steel making process furnace.
The spectral characteristics of a series of different VCSELs emitting at 850 nm and 762 nm have been examined, to assess the potential of this new kind of semiconductor laser for use in laser spectroscopy and fundamental metrology, particularly for environmental and energy monitoring of combustion processes. Adequate polarization control, which is necessary for laser spectral purity, was provided through manufacture of a dumbbell or 'figure-of-eight' shaped active region of the lasers at 850 nm. Spectral lineshape and linearity of the frequency tuning have been examined with a variety of methods such as optical interferometry and heterodyning. Optical heterodyning was used to measure a minimum linewidth of 90 MHz FWHM for a 762 nm VCSEL during single mode operation. Deviations from linearity of up to some GHz of frequency tuning with current were observed with an original application of a Twyman-Green interferometer, with basic current tuning rates of 58 GHz/mA and 130 GHz/mA for an 850 nm and a 762 nm VCSEL, respectively. The frequency stability for a 762 nm device was measured to be approximately plus or minus 40 MHz. A possible problem for the use of VCSELs in wavelength modulation spectroscopy (WMS) is that due to their extremely high tuning rate with current: ultra-low noise current drivers must be used in order to achieve high sensitivity detection. The WMS absorption spectrum with VCSEL of a number of lines of the oxygen molecule at 760 nm have been registered and studied.
A measurement system optimized for process control in the industrial environment has been developed and successfully commercialized. The system comprises a central unit, which contains all sensitive electronic and electro-optic parts. Fiber optics is used to transport the probing laser light to the measuring points in the process. Extremely rugged sensor heads are used to interface to the harsh industrial environment. Adaptation to the different applications is solely made up by changing the type of sensor head used. Six different process control applications will be presented. Ammonia slip monitoring in the NO(subscript x4/ reduction process in power stations, waste incinerators and heavy-duty diesel engines. Measurement of water vapor and oxygen in municipal waste to energy plants. Monitoring of oxygen and the thermodynamic gas temperature in steel pellets manufacturing. Monitoring HF reduction in a dry scrubber and HF emission from a pot room. Experiences of CO emission peak monitoring to protect electro filter in a chemical waste incinerator. Finally, we will describe measurements of HCI in the raw gas to access the calorific value of waste and to optimize bag-house filter operation.
There is a trend today towards a reduction in target signatures, the signatures becoming increasingly adapted to the background in which the targets operate. In addition, new types of countermeasures are making the task for optical seekers increasingly difficult. One way to increase the capability of detecting low-signature targets in a countermeasure environment is to utilize not only the magnitude of the signature but also its distribution over the spectrum. For collection of information regarding the spectral signatures of targets, countermeasures and backgrounds, a multispectral imaging MWIR sensor has been developed by us. This device utilizes the high frame rate made possible by modern FPA arrays. Such an array has been combined with a rapidly rotating filter wheel, thereby producing images of 128 by 128 pixels in six wavelength bands in the 2 - 5 micrometer region at a frame rate exceeding 30 Hz in each band. The sensor has a field-of-view of 3.7 degrees and a pixel resolution of 0.5 mrad. The sensor has the capability to perform two point correction in real time, thereby compensating for the different dynamic ranges in each spectral band. An extensive measurement program is in progress for gathering data for targets, countermeasures and backgrounds. Selected results from this program are presented.
An industrialized computer-controlled fiber optic laser diode gas analysis system is described. A unique signal processing scheme completely eliminates both non-gas-related transmission variation and long-term drift. Sensitivities better than 1 ppm(DOT)m and 200 ppm(DOT)m is routinely achieved for field-installed systems for ammonia and oxygen, respectively. due to the superior selectivity of laser diodes, interference effects from coexisting gases, such as water vapor in ammonia measurements, are easily avoided.
In this paper a near-JR diode laser system for emission and process control has been designed and tested.
The system utilizes derivative spectroscopy to increase the sensitivity of the measurement due to the
low line strengths of the absorption lines in the 0.7 - 1.7 p.m wavelengths region. A minimum detectable
absorbance of 2 iO-5 was obtained at an integration time of 3 ms. Oxygen was used in the experiments
and the corresponding minimum detectable concentration was 200 ppm. No interference effects was observed
from co-existing emission gases. The system utilizes a unique modulation concept for compensation
of non'gas-related transmission variations in the measurement path. Due to the short response time
the system is very useful for on-line process control.