Accurate and sensitive metals emissions monitoring with an atmospheric microwave-plasma having a real-time span calibration

摘要

Both high sensitivity and good measurement accuracy are required in a metals continuous emissions monitor in order for it to be acceptable for compliance monitoring. An atmospheric plasma sustained by microwaves with an attached source of a calibrated trace metals aerosol has been shown to be capable of achieving both of these requirements. The microwave plasma is continuous and operates in undiluted stack exhaust for atomic emission spectroscopy of trace metals. The plasma is sustained in a shorted waveguide that is attached to the stack by a short sample line (< 50 cm). It is powered at 1.5 kW, at a frequency of 2.45 GHz. An undiluted stack slipstream is isokinetically drawn into the plasma by a suction pump at a nominal flow of 14l/min. The pneumatic nebulizer attached to the sample line can momentarily, on command, inject a known concentration of metals solution providing a real-time span calibration. The system was tested on the exhaust stack of the rotary kiln incinerator simulator facility at the Environmental Protection Agency (EPA) National Risk Management Laboratory in Research Triangle Park. Three hazardous metals were monitored, lead, chromium, and beryllium. These measurements were referenced to EPA Method-29. A total of 20 spiked stack exhaust tests were carried out. Ten one-hour tests at high concentration (40-60 μg/actual m~3) and ten one and half hour tests at low concentration (10-15 μg/actual m~3). The microwave plasma monitor achieved relative accuracies of approximately 20/100 for lead and beryllium and 40/100 for chromium with a threshold detection capability of < 3 μg/actual m~3 for a time response of ～l min. The relative accuracy deviation from the EPA Method-29 was found to be mostly systematic. This suggests the possibility of using a site-specific calibration to bring the microwave plasma into compliance with EPA's goal of 20/100 relative accuracy to the reference method. Laboratory work is continuing to add mercury, arsenic, and cadmium to the monitored metals. Mercury and arsenic present a particular challenge to achieving high detection sensitivity in undiluted stack exhaust because the plasma is less efficient in exciting Hg and because UV absorption interferes with As detection. This is a problem that is also in common with air ICP and in situ spark plasma methods for continuous emissions monitoring of metals.