Mercury Reaction Chemistry in Combustion Flue Gases From Experiments and Theory

Sunday, November 7, 2010
Hall 1 (Salt Palace Convention Center)
Bihter Padak, Energy Resources Engineering, Stanford University, Stanford, CA

Coal-fired power plants are the greatest anthropogenic source of mercury emissions in the United States. Mercury exists in coal combustion flue gas in a variety of forms depending on the coal type and combustion conditions, i.e., elemental, oxidized, and particulate. 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. Homogeneous oxidation of mercury in the flue gas of coal combustion utility boilers has been studied for many years to understand the speciation of mercury. In spite of a vast amount of experimental studies, supported by modeling efforts, there are still many questions to be answered and the speciation of mercury is not fully understood yet.

During my Master's in the department of Chemical Engineering at Worcester Polytechnic Institute and my PhD in the department of Energy Resources Engineering at Stanford University under the supervision of Prof. Jennifer Wilcox, I have worked on two projects focusing on mercury emissions from coal-fired power plants and have gained experience from both a theoretical and experimental perspective. The first project is investigating gas phase homogeneous mercury oxidation via bromine and chlorine in coal combustion flue gas to understand mercury speciation throughout the utility system. I have designed and built an experimental system consisting of a plug-flow reactor and a burner to generate laminar premixed methane flame to simulate both combustion and the flue gas environment. 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. One needs to be able to make precise mercury measurements to understand its speciation and accurately predict the extents of mercury oxidation. With this goal I have designed and custom-built an electron ionization quadrupole mass spectrometer (EI-QMS) to be able to directly measure mercury species on the order of parts per billion (ppb) in the flue gas. 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 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. With this custom-built instrument, mercury species will be directly measured for the first time for high temperature combustion applications. 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.

Besides the experiments, my second project involves molecular modeling from first principle-based electronic structure calculations. From this study I have published two papers in the journals of Environmental Progress and Carbon in 2006 [1] and 2009 [2], respectively. I have employed ab initio studies to elucidate the binding mechanism of mercury on an activated carbon surface. Activated carbon, when injected into the gas stream of coal-fired boilers, is effective in capturing both elemental and oxidized mercury through adsorption processes. However, the mechanism by which mercury adsorbs on activated carbon is not exactly known and its understanding is crucial to the design and fabrication of effective capture technologies for mercury. The objective of the current study is to apply theoretical-based cluster modeling to examine the possible binding mechanism of mercury on activated carbon. The effects of activated carbon's different surface functional groups and halogens on elemental mercury adsorption have been examined. Also, a thermodynamic approach is followed to examine the binding mechanism of mercury and its oxidized species such as HgCl and HgCl2 on a simulated carbon surface with and without Cl. Energies of different possible surface complexes and possible products are compared and dominant pathways are determined relatively. In addition to studying activated carbon, we can also investigate different materials to find a better sorbent that is more effective and cheaper than activated carbon. Overall, this project will aid in preventing mercury emissions from flue gas.

In addition to the research that I have conducted within graduate school, I also have carried out a summer internship at Niksa Energy Associates LLC (NEA) and finalized NEA's reaction mechanism for the simultaneous oxidation and capture of mercury by chlorine and bromine species. I was able to validate NEA's most recent comprehensive mechanism for Hg/Cl/Br chemistry by interpreting three datasets from full-scale field tests within the measurement uncertainties. In addition to this milestone, I have finalized all values for the rate parameters, identified the dominant reaction pathways, and assessed the relative roles of homogeneous and heterogeneous conversion channels. At the end of three months I drafted a manuscript that was published in the journal, Energy & Fuels in 2010 [3].

Moreover, I have gained experience in teaching throughout my gradute study, which I think forms a strong basis for me to teach in the future. With the knowledge of topics like Combustion, Chemical Kinetics, Quantum Mechanics, Thermodynamics, Spectroscopy, Surface Science, Nanostructure and Characterization, Transport Phenomena, Fluid Mechanics and Fundamentals of Energy Process, I see my background as a good fit for a faculty position in either chemical engineering or another engineering program focused on energy. I am planning to continue doing research on mercury speciation in coal combustion flue gas and also apply my knowledge in combustion to a different application, specifically oxycombustion. Oxycombustion is a new technology developed to reduce carbon dioxide emissions from fossil-based energy resources. In an oxy-fired coal power plant, the coal is burned in an oxygen-rich environment instead of air and the flue gas is recycled back to the boiler, which in the end produces a higly pure carbon dioxide exhaust that can be captured at relatively low-cost and subsequently compressed for transport and sequestration. There is no commercial oxy-combustion plant operating yet because of the high cost of producing oxygen. Currently research is being conducted to develop new techniques for oxy-combustion of coal integrated with the oxygen production process and for testing and scale-up of oxy-combustion systems. As a faculty member I would like to be part of a global research project, which involves both academia and industry to make the oxy-combustion process more efficient.

References [1] Padak, B.; Brunetti. M.; Lewis, A.; Wilcox, J. “Mercury Binding on Activated Carbon” Environmental Progress, 25(4), 319-326, 2006. [2] Padak, B.; Wilcox, J. “Understanding Mercury Binding on Activated Carbon” Carbon, 47(12), 2855-2864, 2009. [3] Niksa, S.; Padak, B.; Krishnakumar, B.; Naik, C. V. “Process Chemistry of Br Addition to Utility Flue Gas for Hg Emissions Control” Energy Fuels, 24(2), 1020–1029, 2010.

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