Development of a Jet-rempi Based Continuous Emissions Monitor for Dioxins

The need for a continuous emissions monitor (CEM) for the determination of dioxins and furans at realistic (parts per trillion or lower) concentrations in real time (minutes) is widely recognized in the waste management community. Such an instrument would enable rapid, and real-time experimentation on incinerators, so as to test and develop control strategies. It would also be extremely useful in laboratory kinetics experiments seeking to understand formation, interconversion, and destruction processes for dioxins and their precursors. Finally, it could be used for continuous compliance monitoring, or rapid, low-cost, facilities certification. The key issues for a dioxin CEM are; overall sensitivity, and selectivity among the many congeners found in real applications, and the ability to detect and distinguish these compounds in the presence of a very complex background. The U.S. EPA Office of Solid Waste (OSW), which regulates hazardous waste treatment processes, has identified continuous dioxin monitoring as a research priority. The OSW recognizes that CEMs offer continuous compliance assurance, compared with infrequent extractive sampling. Their policy provides the economic incentive for waste facilities to use CEMs by eliminating waste feed characterization, compliance testing, and operating parameter monitoring for pollutants when a facility uses CEMs. Although 210 chemically different dioxins can be produced during combustion, it is currently believed that fewer than 20 are toxic enough to warrant monitoring. SRI has initiated a research program to develop a CEM that can be used to study the emission levels of these key dioxins, leading eventually to an improved understanding of the formation of these molecules and to improved means of monitoring and control. A database can be built using emissions from actual waste treatment processes to correlate operating conditions with dioxin formation. Such a database can be used to devise operating strategies to prevent formation of dioxins, and to identify surrogates or indicators that can be monitored more easily than the dioxins themselves, leading to less expensive, more widely implemented, compliance and control strategies. Our approach for the real-time dioxin CEM combines a pulsed gas jet with resonance enhanced multi-photon ionization (REMPI) and time-of-flight mass spectrometry (TOFMS). We estimate that a minimum detectability of approximately 1 ng/m3 can be realized for dichlorinated dioxins using commercial off the shelf technology with minimal development. Using the current one-color REMPI scheme, the sensitivity decreases with increasing chlorination, making detection of the more highly chlorinated dioxins problematic. REMPI will need significant enhancements to reach the sub-ng/m3 detection limits required for a compliance CEM. Potential enhancements include, among others, the use of sample preconcentration, improved pulsed nozzle and ion optics designs, a two-color REMPI scheme, and higher repetition rate laser systems. WM'99 CONFERENCE, FEBRUARY 28 MARCH 4, 1999 Based on previous research with REMPI, and our understanding of dioxin regulatory structure, our primary research thrusts are: (1) determination if tri-, tetra-, and higher chlorinated dioxins will require a 2-color REMPI scheme; and (2) measurement of detection limits for congeners as a function of chlorination, and improvement in the detection limits via system improvements and sampling methods. We are also working closely with other dioxin research groups to ensure that the dioxin mechanistic studies are integrated into our work to determine which congeners should be measured, and at what concentration in order to provide measurements for regulatory compliance and process control.