Geosyntec was asked to devise a measurement method that would accurately determine coal-fired power plant flue gas concentrations of sulfur trioxide (SO3) across an array of exhaust duct locations directly downstream of selective catalytic reduction (SCR) catalyst beds. Besides traversing the SO3 concentrations across the entire exhaust area under the catalysts in a high dust environment, continuous measurements of SO3 at low concentrations (single-digit ppmv) were required in order to gauge the effectiveness of a proprietary SO3 mitigation sorbent being injected into the flue gas upstream of the SCR. These measurements also required a high time resolution (on the order of seconds) in an attempt to assess the variability of concentrations at any given sample traverse point.
Geosyntec’s Scope of Services
Geosyntec was already in the process of developing specialized continuous real-time measurement methods for certain flue gas constituents important to electric utilities (such as SO3, ammonia, and various halogens), in addition to the nitrogen oxide, sulfur dioxide, and other combustion byproducts typically present in flue gas exhaust. Therefore, the client requested the field implementation of one of those novel measurement methods, extractive quantum cascade laser (QCL) absorption spectroscopy, in order to meet the aforementioned project objectives. Geosyntec personnel developed the infrared spectroscopic and absorption techniques and conducted the assembly, laboratory testing, and calibrations of the QCL spectrometer for sub-ppmv detection of SO3 prior to field testing. Geosyntec provided the design of field experiments and logistical planning for the project. The field installation and testing of the SCR exhaust duct took place in coordination with conventional chemical impinger testing (controlled condensation sampling, or CCS) conducted by the client.
An array of approximately 36 sampling traverse points were monitored for SO3 concentrations despite the presence of relatively extreme conditions in which to perform multiple point exhaust duct sampling and to conduct analytical measurements (e.g., ambient temperatures exceeding 130°F and little space in which to maneuver). Additionally, statistical calculations were performed to assess exhaust flow variability and possible stratification. The new method achieved favorable detection limits (less than 1 ppmv), and data signal averaging was on the order of 10 seconds per measurement. This enabled real-time monitoring and observation of SO3 changes as sorbent feed rates were varied for optimization.