Surface Acoustic Wave Mercury Vapor Sensor

This project will design and fabricate a prototype thermal treatment process control instrument for the detection and measurement of gaseous mercury. The proposed sensing element, a chemiresistive film applied to a Surface Acoustic Wave (SAW) resonator, will provide a small, low-power, portable, inexpensive, and accurate means of monitoring mercury vapor over a wide concentration range. The instrument will be designed for operations from 25°C to 500°C for response times of less than 5 seconds, and limits of detection from 200 ppb or less. In the base contract, a prototype SAW sensor will be designed, developed, and fabricated, and then tested in the laboratory under varying conditions. In option 1, the prototype instrument will be fabricated and field tested at a thermal treatment facility chosen as a site representative of DOE needs.

• Optimal film thickness--Determine optimal operating temperature and film thickness from initial parametric testing of multiple prototype sensors.
• Optimal design and fabrication--Sensor Research and Development Corp. (SRD) fabricates "best" sensor prototype for extended laboratory-scale testing. Sensitivities of <1 ppb were recorded, which is well within the limits set up by the EPA for compliance monitoring.
• Field-deployable prototype--SRD completes design and fabrication of field deployable prototype mercury vapor CEM.
• Laboratory testing--SRD completes laboratory acceptance testing of "packaged" sensor. All calibration and temperature compensation issues were resolved.
• Modify gas delivery system--Modify gas delivery system to feed mercuric-chloride-containing simulated flue gas. This is in addition to the preexisting capability to feed elemental mercury vapor.
• Complete lab scale testing--Complete lab-scale evaluations of prototype sensor(s) on simulated flue gases containing mercury compounds. Limits of sensitivity should be established for elemental mercury, as well as mercuric chloride.
• Field test demonstration--Field test mercury continuous emissions monitor (CEM) prototype at the University of North Dakota Energy and Environmental Research Center (EERC) pilot-scale facility on actual flue gas.

After designing and acquiring the gas delivery system for mercuric chloride (HgCl₂ ) last month, the system was set up during the first week of January. After allowing a week's time for the permeation sources to reach equilibrium, tests were begun to assess the sensor's response to HgCl₂ . Two exposure periods were conducted with approximately 50 ppb HgCl₂ in dry nitrogen. These data are an excerpt from a longer test, which measured the sensor's response to HgCl₂ over a wide range of temperatures (25°C to 400°C), and, as in the case of elemental mercury, the response was best at 150°C. The exposure level of 50 ppb is an estimate based on the permeation tube manufacturer's specifications, but further action is in progress to verify the concentration. To facilitate calibration, independent gas concentration analysis will be performed by the Energy and Environmental Research Center (EERC) using an impinger train method similar to EPA Method 101A. In actuality, the newness of the permeation tube and some preliminary measurements with a mass spectrometer indicate that the actual concentration may be significantly lower (10 ppb or less) than the conservative estimate that has been offered. The EERC's results will be presented when available.

In addition to testing the sensor’s response to HgCl₂ , further testing was performed to verify the sensor response to low concentrations of gaseous elemental mercury (Hg 0). These tests were performed to corroborate previous data showing responses to concentrations less than 1 ppb. Tests were run on a different sensor with identical parameters to the first, and the results were similar to those presented before. Once again, the device responded quickly and sensitively to concentrations below 1 ppb. As in the case of HgCl₂ , these concentrations are also being independently verified by the EERC.

The work remaining to be completed under the base contract includes additional interferent testing, as well as more in-depth testing of the sensor's response to HgCl₂ . Additionally, a base phase topical report will be submitted summarizing the work to date.

With about a month remaining in the base contract portion of this project, no problems are foreseen in completing this work in the allotted time and within budget.

Next month's work will involve completing all requirements of the base contract. Interferent testing will be completed, and the characterization of the response to Hg 0 and HgCl₂ will be completed. The base period topical report will be written and submitted to begin the approval process of the base option scheduled to begin March 1. The plans for the following month center mostly on beginning the development of a final prototype instrument to be field tested later in option 1.

PI:  Joshua Caron, Sensor Research and Development Corp., (207) 866-0100

FETC COR: Ron Staubly, (304) 285-4991

November 1998 Report
February 1999 Report
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