Tunable Diode Laser Absorption Spectroscopy (TDLAS)
Wavelength Multiplexed – Tunable Diode Laser Absorption Spectroscopy (WM-TDLAS) is a well-proven technique which is based on the fact that every molecule has a unique light absorption characteristic or “fingerprint”. TDLAS involves measuring the attenuation due to absorption of a diode laser beam at a very specific wavelength as it passes through a measurement region, such as combustion gases. TDLAS measures the average temperature and/or species concentrations in the gas along the line of sight of the laser beam. At wavelengths slightly different than these absorption lines, there is essentially no absorption.
The laser wavelength is scanned over a small range that encompasses at least one absorption line of the molecule of interest but also includes a region in which there is no absorption. The transmitted light intensities are measured using a photo detector, and the signals are analyzed to obtain the average concentration of the target gas over the beam’s path length.
Since each molecule absorbs light at different wavelengths, a different diode laser is generally required to measure different species. Zolo has developed a proprietary approach that combines several wavelengths into a single laser beam using a sophisticated multiplexer that permits simultaneous measurement of several species with one laser beam (i.e. WM-TDLAS). Other TDLAS technologies require a separate sensor (with only a single laser) to measure each constituent.
TDLAS offers many benefits over conventional sensor such as: fast response time; high temperatures; in-situ; non-evasive; better sampling representation. Many applications have been developed for process, industrial, chemical, refining, gas pipeline and power generation industries to measure critical species in gases. However, Zolo has focused its efforts, in collaboration with experts at Stanford University, on measuring gas concentrations in ultra-harsh combustion environment.
Species concentrations using TDLAS are determined using the absorption equation (Beer-Lambert law). Consequences of Beer’s Law can be subtle, however, the essence of the equation is shown below:
Pabs/Pbaseline= f (So, E”)
The line strength, So, and the lower state energy level, E”, are fundamental physical constants for a particular absorption feature. They describe how strongly the target species absorbs versus wavelength on a particular spectroscopic transition at a particular temperature. They are invariant. There are a number of external resources that provide values for So and E” for the absorption features that the ZoloBOSS system uses to measure species concentrations. The U.S. Government has compiled a spectroscopic database called HITRAN that lists values of So and E” for many transitions of many species. To further improve the accuracy of our measurements and supplement the HITRAN database, Zolo has contracted with Stanford University to independently measure So and E” and validate these constants. Finally, in addition to guidance from HITRAN and measurements by Stanford, Zolo makes independent measurements of So and E” in the calibration facility at Zolo’s factory. Only when measurements from these various sources agree, do we consider the fundamental spectroscopic constants to be known. Once they are known, these constants determine the “fingerprint” of absorption for specific constituents that can then be used to quantify their concentration and, most importantly, the system requires no further calibration.
The laser wavelength is scanned across the targeted absorption feature being careful to include regions of non absorption on both sides of the absorption feature. The regions of non-absorption (baseline) indicate the level of attenuation due to effects other than the target molecule absorption, e.g. dirty windows, ash, soot, etc. This baseline may fluctuate quickly due to effects such as turbulence and variable opacity, but the ratio of the size of the absorption feature to the baseline value does not change, and it is this value that is used to quantify concentrations and temperature. Therefore, the measurement is independent of dust, ash or opacity in the flue gas provided that sufficient light remains to be detected.