Innovative Approach to Selectively Measure Nitrogen Dioxide from Industrial Processes Over a Wide Linear Dynamic Range

摘要

Heavy pollution immediately and adversely impacts human health, visibility, plants, animals, etc. As the world moves towards accurate measurements of criteria pollutants such as nitrogen dioxide (NO_2), simpler, cost effective and robust approaches are needed. An example of such an approach is to make existing heated-molybdenum chemiluminescence NO_x analyzers selective and specific in the indirect measurement of NO_2 especially emissions from industrial processes. Industrial processes may contribute a significant level of NO_x every day to the global nitrogen oxides burden and hence, there are regulatory frameworks in place to ensure compliance due to their known health effects. To ensure better regulatory compliance, overestimation or underestimation of any the regulated species, such as NO_2 may have legal and health consequences. For the past few decades, most existing NO_x analyzers use a heated molybdenum catalyst to convert NO_2 to nitric oxide (NO). The NO is titrated with excess ozone to produce an equivalent amount of excited state NO_2. The excited NO_2 gives off photons in a process called chemiluminescence and it is detected and quantified. Although the molybdenum converters are robust, they are non-selective and are known to have reduced efficiency under certain process conditions i.e. ammonia interference. Alternatively, photolytic NO2 converters have been demonstrated to be selective for NO_2 conversion to NO and detected using the same chemiluminescence technique. One major drawback to currently known photolytic NO_2 converters is their limited linear dynamic range. This drawback may impact the application of photolytic converters in such analyzers for high source measurements such process stacks and especially to accurately account for NO_2 during plant upset conditions. A novel approach for the conversion of NO_2 to NO using ultraviolet light has been developed and proven through laboratory and field testing. This technology has been specifically designed for application in chemiluminescent NOX analyzers that are typically used for ambient and Continuous Emissions Monitoring applications as a direct replacement for the commonly used heated Molybdenum converters. In this presentation, preliminary field results from a plant site deployment are presented. The results are compared to existing heated molybdenum systems. The photolysis device has been tested as a direct replacement of "heated converters' at reduced pressure (200 mmHg) from 0.03 parts per million (ppm) up to 37.30 ppm at a linear conversion efficiency of 98%. This novel converter only slightly lost conversion efficiency at 37.30 ppm (~94% efficiency) surpassing the performance range of any known photolytic converters at this time. The converter can also be used as a standalone system without any significant decrease in conversion efficiency due to recombination reactions. The advantages, drawbacks, and future applications are discussed.