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Process Monitoring and Control: Ammonia Measurements in Offgases

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Tech ID: 212
Project Overview
In support of the Tank Focus Area, this project is developing and applying tunable-diode-laser (TDL) absorption spectroscopy as a continuous monitor for ammonia in offgases from Savannah River Site (SRS) and Hanford tank wastes. This technology is also applicable to monitoring offgases in thermal treatment processes that are utilized in the MWFA. The robust instrument developed by this project will provide on-line ammonia detection that is (1) sensitive, (2) stable, (3) capable of operation at high temperatures in a radiation environment, and (4) not affected by the presence of high concentrations of water vapor and carbon dioxide in the offgas stream.

Technology Description
The method detects gaseous ammonia using optical absorption by vibrational transitions in molecular overtone and combination modes at wavelengths near 1.55 micrometers. The sensitivity of the near-infrared TDL to ammonia is greatly enhanced by a proprietary method involving high-frequency modulation of the laser beam and phase-sensitive detection. The ultra-high resolution characteristics of the TDL source permit the separation of ammonia absorption features from nearby spectral lines, due to either water vapor or carbon dioxide. This is a very large advantage when compared with lower resolution optical techniques, such as ultraviolet absorption spectroscopy, Fourier transform infrared spectroscopy, or nondispersive infrared analysis. Diode lasers are extremely compact, robust, solid-state devices. Their development by the semiconductor industry allows them to be manufactured by mass production methods, thus greatly reducing the cost of individual diode lasers. Operation in the near-infrared wavelength region permits direct coupling of the laser output into optical fibers, which facilitates transmission of the laser beam and alignment with the offgas stream or extractive sampling volume. This feature also permits the location of laser and electronic modules at a safe distance from toxic, hazardous, or radioactive sources.

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