(637d) Developing An Instrument for Trace Metal Measurement

Understanding mercury speciation during combustion and how the transformations occur between different forms of mercury is essential to developing an effective control mechanism for removing mercury from flue gas. One needs to be able to make precise mercury measurements to understand the mercury speciation and accurately predict extents of oxidation. An experimental system has been designed and built to simulate coal combustion flue gas and elucidate the homogeneous mercury oxidation during combustion. In this system mercury and chlorine are introduced into a laminar premixed methane-air flame to simulate the flue gas. Cooled flue gas is sampled and sent to the mass spectrometer to analyze the flue gas, with special focus to mercury species. A custom-built electron ionization quadrupole mass spectrometer (EI-QMS) from Extrel will be used in this study. To accurately measure the low concentrations of different mercury species present in coal combustion flue gases, the EI-QMS must be sensitive to concentrations in the ppb (parts per billion) range, which can pose a challenge. To increase the sensitivity, the system has been upgraded to have a supersonic beam and a skimmer is placed after the first orifice. In such a setup, scattering of the molecular beam is avoided and the amount of gas which reaches the ionization region, and subsequently the ion detector, is maximized, thereby improving the sensitivity of the instrument. Also a tuning fork chopper has been implemented in the system along with a lock-in amplifier to enhance the signal-to-noise ratio. In addition, a custom flange has been designed with electrical feedthroughs to be able to heat the orifice in the vacuum chamber to prevent the formation of mercury clusters. After calibration of the mass spectrometer for mercury species, combustion experiments are conducted to speciate mercury in the flue gas environment. A benefit of employing a mass spectrometer is, unlike traditional impinge-based methods, the oxidized forms can be isolated and individually identified because it separates the products based on their mass-to-charge ratio. By directly measuring mercury species accurately, one can determine the actual extent of mercury oxidation in the flue gas, which will aid in developing mercury control technologies.