NIR Diode Lasers Measure Coal-Fire Emissions
Brent D. Johnson
Coal-fired power plants have been in operation for nearly a century; nevertheless, some of the processes that occur in their combustion chambers are still not completely understood. In a typical system, gas is siphoned from the furnace into a sampling tube, and measurements are made with nondispersive infrared photometers. But there are a number of problems associated with this technique, not the least of which is a lack of specificity because of the low resolution of optical filters.
Volker Ebert and his colleagues at the Institute of Physical Chemistry at the University of Heidelberg in Germany have developed a method based on tunable diode laser absorption spectroscopy that performs in situ measurement of multiple gas concentrations. The technique could increase the efficiency and length of service of these plants by providing a measurement signal to minimize the deposition of slag on the walls of the combustion chamber.
Two laser beams are collimated and superimposed to form a composite dual-wavelength beam that is sent through the 20-m-diameter combustion vessel of a full-scale 600-MW lignite-fired power plant (see figure). The transmitted laser light is separated into individual wavelengths by a dichroic beamsplitter, and optical narrow-band 10-nm FWHM interference filters are used to reduce thermal background radiation and to prevent crosstalk between the two wavelength channels. Thermoelectrically cooled InGaAs detectors from Hamamatsu convert the 1.56-µm laser radiation, and uncooled silicon photodiodes detect the 813-nm light.
Carbon monoxide is measured at 1.56 µm; however, extracting absolute CO requires the temperature to be known. In many plants, there is no temperature measurement in the combustion vessel because of the cost and long-term drift of the sensors. This is why the team investigated the temperature-dependent line strength ratio of H2O absorption lines at 813 nm, opening up the possibility of correcting the CO signal for small interferences by underlying water absorption lines at 1.56 µm.
The fast wavelength tunability of the laser allows the researchers to separate the severe transmission and emission disturbances in the beam from the weak molecular absorption (typically a few percent). They are particularly interested in focusing on the 1.56-µm band for the detection of CO.
Tuning the laser at a much faster rate than that of the disturbances leads to the assumption that the disturbances are constant during a single scan and separated as a single Ebert said that this is the first time that anyone has been able to perform measurements in the demanding environment within the combustion chamber, but that he and his colleagues encountered several major complications. Most severe was the strongly fluctuating losses in light power caused by dust in combination with strong, background radiation, high temperature and the interfering molecular signals from H2O. Scattering and absorption by dust keeps 99.99 percent of the light that enters the chamber from reaching the detector. With a 15-mW probe beam, only 1.5 mW reaches the detector, and the weakness of the absorption line requires that a variation of only 300 pW in that signal be detected.
Although development and implementation are ongoing, Ebert does not expect that the researchers will come to any definite conclusions regarding reduction of slag or improvement of efficiency for at least several months.
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