A tunable diode laser stack monitor for sulfuric acid vapor

A field prototype instrument for continuous in situ monitoring of sulfuric acid vapor in industrial smoke stacks was developed. The method of detection is dual wavelength differential absorption in the infrared. Two tunable diode lasers are locked to two specific frequencies that provide analytical and background absorption information. The difference between the analytical and background absorption yields the net absorption due to sulfuric acid. Calibration of the monitor depends upon a precise determination of the differential absorption coefficient for the analytical and background frequencies. The monitoring system was field tested at a 650 MW electrical generation plant burning high sulfur oil of Venezuelan origin. The excess air supplied to the boiler combustion chamber was varied from 0.5 to 1.5 percent. Measured baseline concentration of sulfuric acid ranged from 3 to 15 ppm. As the excess air was stepped up from 0.5 to 1.5 percent, the average measured increase in concentration was 39 ppm.