DOAS for flue gas monitoring—III. In-situ monitoring of sulfur dioxide, nitrogen monoxide and ammonia

Journal article, 1996

A methodology is described for the in-situ detection of NO, NH3 and SO2 in flue gases by DOAS (Differential Optical Absorption Spectroscopy). In order to perform accurate measurements of the concentration it is necessary to compensate for the temperature dependence of the absorption cross-sections as well as for potential deviations from the Beer-Lambert law (nonlinearity effects). From the experimental data in two previous papers, empirical equations were derived for the compensation of the nonlinearity and temperature effects. These were used to compensate obtained concentration values of NO and SO2 retrieved from DOAS spectra that were recorded in a flue gas at 413 K. The measurements of SO2 showed that in a concentration interval of 500–1600 ppm at 413 K, the resulting systematic discrepancies between the DOAS and a conventional reference system decreased from 40 to only 2% when compensating the DOAS data. The maximum random difference was approximately 15%. In the same manner the systematic difference for NO decreased from 23 to 1%, with a maximum random error of 5%, for concentrations between 60 and 160 ppm. The measurements of NH3 demonstrated the versatility of the DOAS technique for time resolved in-situ measurements (<20 sec), and also the feasibility of the technique for measuring several species simultaneously. The measurement methodology developed for NH3 was more complicated than for NO and SO2 and required a larger amount of laboratory calibrations. In the spectral evaluation procedure of NH3 hot bands were utilized for flue gas temperatures above 450 K.