Coal, an important domestic energy resource, contains trace heavy metals, such as mercury, which can be dispersed through power plant emissions. Sandia researchers are developing a portable, real-time, mercury (Hg) emissions monitor designed to measure the two primary gas-phase, Hg-containing species emitted from coal-fired power plants, mercuric chloride (HgCl2) and elemental mercury (Hg0). This work will enable coal-fired power plant operators and regulators to evaluate and optimize Hg emissions control technologies in their efforts to meet the more stringent requirements for Hg emissions set forth in the EPA's Clean Air Mercury Rule.
Coal typically contains ~100 parts per billion (ppb) Hg. Without appropriate environmental controls on the exhausted flue gas, coal combustion releases this Hg into the atmosphere. Moreover, the effectiveness of these controls depends strongly on the distribution of Hg compounds in the flue gas, which in turn depends on the fuel stock and combustion conditions. The most abundant form of oxidized Hg emitted by most coal-fired boilers, HgCl2, and other forms of oxidized Hg can be efficiently removed by filtration or particle injection. However, Hg0 is much more difficult and costly to collect. Furthermore, HgCl2 and Hg0 undergo different environmental and biological processes upon release into the atmosphere. Accurate measurements of Hg0 and HgCl2 are therefore essential for modeling Hg transport and for evaluating and optimizing the effectiveness of possible Hg control technologies.
With funding from the National Energy Technology Laboratory, CRF researchers Alex Hoops and Tom Reichardt demonstrated photofragment emission (PFE) detection of HgCl2 using a fiber-based, ultraviolet (UV) laser source. In PFE detection of HgCl2, emission is detected from excited Hg0 daughter fragments produced by photodissociation of HgCl2. Hoops and Reichardt previously characterized and quantified the HgCl2 PFE method by evaluating the potential impact of interference gases, determining the dependence of the PFE signal strength on laser irradiance, and examining the effects of collisional quenching by major flue-gas constituents N2, O2, and CO2 (CRF News, Vol. 27, No. 3). These studies were performed with a Nd:YAG-pumped dye laser frequency converted into the UV; although useful for proof-of-concept and optimization experiments, the physical characteristics of this laser preclude its use in real-world field environments. For a field-worthy instrument, the laser source must be compact, rugged, and light-weight, while maintaining high average and peak powers with the requisite spatial beam quality for a stand-off approach. Frequency conversion of pulsed, rare-earth-doped, fiber-based lasers has the potential to meet these requirements for a practical laser source.
Hoops and Reichardt recently used the 213-nm output of a Sandia-built, frequency-quintupled fiber laser (Figs. 1 and 2), developed by Dahv Kliner, Jeffrey Koplow, and Sean Moore, to detect HgCl2 by PFE. The laser system employed a ytterbium-doped fiber amplifier based on the patented “mode-filtering” technique developed by Sandia researchers (CRF News, Vol. 23, No. 2). This technique allows dramatic power scaling of fiber lasers (by more than two orders of magnitude) while maintaining high efficiency and diffraction-limited beam quality (critical for both efficient nonlinear frequency conversion and stand-off detection). The fiber amplifier was seeded by a compact microchip laser operating at a wavelength of 1064 nm and repetition rate of ~6 kHz, and the amplifier output was frequency quintupled to 213 nm using standard nonlinear crystals operating at ambient temperature. A photon-counting approach was incorporated for this low-light measurement, involving the acquisition of less than one photon per laser pulse, which greatly reduced the measurement uncertainty by eliminating electronic noise sources. The high repetition rate of the pulsed fiber amplifier made possible the acquisition of sufficient count levels within measurement times that are reasonable for a real-time device. Hoops and Reichardt measured a quadratic dependence of the HgCl2 PFE on laser irradiance (Fig. 3) in a low-pressure N2 atmosphere, indicating that the photodissociation mechanism involves the absorption of two photons. In addition, they demonstrated the ability to detect 90ppb of HgCl2 using the compact, all-solid-state, fiber laser source in a gas mixture typifying the flue-gas composition, and they extrapolated these results to a detection limit of 0.1ppb for a signal acquisition time of 5 minutes.
These experiments show that the physical and optical characteristics of the frequency-converted fiber amplifier, combined with photon counting, make HgCl2 PFE suitable for a sensitive, real-time, fieldable instrument. In the near future, Hoops and Reichardt will team with Jeff Headrick and Dennis Morrison to develop a portable breadboard instrument for HgCl2 PFE measurements, and initial tests of the instrument will be performed in the CRF’s Multifuel Combustion Laboratory.