TUNABLE DIODE LASER SULFURIC ACID STACK MONITOR

This work was supported by the Environmental Sciences Research Laboratory of the Environmental Protection Agency, Research Triangle Park, NC.Author Institution:Acid rain studies have focused considerable attention on the sources of sulfur oxides. In the combustion of sulfur containing fuels the primary product is sulfur dioxide which can be converted in the atmosphere to sulfuric acid. Direct emission of sulfuric acid may, however, be the major source of local atmospheric concentration of this pollutant. To understand the factors controlling sulfuric acid emissions, an in situ, real time stack monitoring system is needed. We have developed and tested a dual wavelength, tunable diode laser monitor capable of continuously monitoring sulfuric acid vapor concentrations across a stack. Although spectroscopic measurements of species such as $SO_{2}$ and CO are routine, sulfuric acid presents a far more difficult problem. This molecule displays no sharp spectral features in the mid infrared region and is not amenable to detection systems based upon gas filter correlation. In general, infrared methods are limited by interference from the absorption lines of waters, carbon dioxide and other stack gases. Our dual laser monitor selectively avoids interference lines and discriminates against scattering by particulates and dirty optics. Each laser is locked to a sharp absorption line in a low pressure reference gas cell. The locking procedure further provides amplitude modulation of the lasers facilitating phase sensitive detection techniques. Testing of the instrument in the field has proven the concept in the industrial environment. Results relating sulfuric acid vapor generation to combustion air control show that precise, real time measurements are feasible