Final Report: Nanostructured Material Design for Hg, As, and Se CaptureEPA Grant Number: SU833518
Title: Nanostructured Material Design for Hg, As, and Se Capture
Investigators: Wilcox, Jennifer , Casey, Catie , Hudon, Katie , Sasmaz, Erdem , Stewart, Elizabeth
Institution: Worcester Polytechnic Institute , Stanford University
EPA Project Officer: Nolt-Helms, Cynthia
Project Period: July 1, 2007 through June 30, 2008
Project Amount: $10,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2007) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Materials & Chemicals , P3 Awards , Sustainability
The Clean Air Mercury Rule (CAMR) was established by the EPA in 2005 to place a cap on mercury (Hg) emissions from coal-fired power plants in two phases. The first cap to be implemented in 2010 was going to cap emissions at 38 tons/year with the second cap in 2018 at 15 tons/year. Although on February 8th, 2008, the D.C. Circuit vacated EPA’s CAMR, they are reviewing the court’s decisions and evaluating potential impacts. Currently within the U.S., 50 tons Hg/year are emitted, while globally there are 5,000 tons Hg/year. As coal consumption increases worldwide, the need to capture harmful trace elements (TE) of fuel and flue gases to prevent release into the environment becomes increasingly important. Coal, as the most abundant fossil fuel on the planet, at ~ 900 billion metric tons is sufficient to meet current energy demands for nearly 200 years.1 To meet the growing-world energy requirements, driven primarily by developing countries such as China and India, dependency on coal will likely remain prominent likely into the first half of the 21st century. Over 50% of the electricity generated in the U.S. is from coal combustion and currently ~ five hundred full-scale (500 MW) coal-fired power plants with an average age of 35 years exist in the U.S.2
Once released into the atmosphere, Hg emissions accumulate in streams and oceans, where bacteria convert it into an organic form, methylmercury (HgCH3), which can then be ingested by fish. Mercury is known to cause damage to the central nervous system3 and from high exposure can lead to effects such as tremors, mood changes, and slowed sensory and motor nerve function.4 Mercury emissions and its harmful effects have received a great deal of attention, but it is also important to recognize two other similarly-volatile and potentially-harmful species released from combustion sources, i.e., arsenic (As) and selenium (Se). Due to the harmful health effects associated with the compounds of Hg, As, and Se present in the fuel or flue gases of coal-based energy generation, the design of a multipollutant sorbent to capture them becomes essential for not only the environment, but also for human health.
Activated carbon and flue gas desulfurization (FGD) processes are often incorporated to capture TEs (mainly oxidized Hg). Although these processes work fairly well for oxidized Hg capture for combustion applications, they fail for gasification fuel gases since gasification conditions require Hg capture at an elevated temperature, at which point activated carbon-based sorbents to break down. Hence, worldwide there exist no control technologies for Hg, As, and Se species on current coal gasification plants used for energy generation. Also, elemental Hg proceeds through combustion flue gas uncaptured due to its low reactivity. Lastly, TE capture via FGD and activated carbon decrease coal’s sustainability by compromising the recycling of fly ash and FGD waste byproducts. Hence, the primary goal of this investigation is to design a multipollutant sorbent material that will be effective in capturing the TEs, Hg, As, and Se at elevated temperatures of both flue and fuel gases of coal-based energy production. Results of this design project will benefit the planet through the reduction of harmful emissions, and the effective recycling of fly ash and FGD byproducts rather than having to place these waste materials in landfills.
The goal of this research project is to identify potential materials that can be used as multipollutant sorbents using a hierarchy of computational modeling approaches. Palladium (Pd) and gold (Au) alloys were investigated and the results show that the addition of a small amount of Au is able to enhance the sorbent reactivity, although the binding energy was sensitive to the specific local concentration of Au atoms. While this is an interesting finding it would be difficult to optimize the structure for fabrication due to the invariance of the atomic configuration. Further studies showed that using a monolayer of Pd overlayed on a Au substrate enhances binding compared to the PdAu alloys. The use of monolayers not only removes the dependence on the random atomic arrangement, but it may also lead to a higher capacity because of the surface composition uniformity and subsequent increased number of binding sites. Tests were also run for As and Se and although the trend was not as strong, enhanced adsorption was also found for these elements. To further investigate means of improving the sorbent materials, simulations were run on metal dimers-TE complexes using the software package, Gaussian03, an ab initio, or first principles approach. It was determined that iron (Fe) was the best candidate to capture Hg, As and Se, given that nearly all the interactions with Fe were characterized by strong chemisorption with binding energies of over 50 kcal/mol. In addition, at about $5.50/oz Fe is also economically favorable compared to the other sorbent materials considered, for instance the price of Pd is approximately $445/oz. To investigate the adsorption chemistry, the HOMO/LUMO maps and a molecular orbital analysis were carried out for TE species found to be prevalent in coal combustion and gasification environments.
A series of quantum chemistry modeling tools were successfully applied to investigate potential multipollutent sorbent materials. This approach represents a shift over conventional chemical engineering methods in which candidate materials are directly fabricated and tested experimentally. Computational techniques to steer subsequent experimental research by investigating and understanding the physical mechanism a priori reduces the amount of expensive experiments (both in terms of cost and time) that are traditionally needed. In addition, the students and researches gain a much more fundamental understanding of the physical and chemical properties that can then be used to make predictions about how to engineer new materials. A clear example of this can been seen in this work, in which simulations of PdAu alloys led to the realization that Pd monolayers would have improved reactivity.
Proposed Phase II Objectives and Strategies:
Based upon the findings of Phase I, a novel PdAu alloy-based sorbent material will be fabricated under the direction of partner, Dr. Hugh Hamilton of Johnson Matthey. This material will be tested in both coal gasification and combustion environments at both bench-and pilot-scales. Testing will take place in the labs of Dr. Wilcox in the Department of Energy Resources Engineering at Stanford University, Dr. Evan Granite of the United States Department of Energy (DOE-NETL), National Energy Technology Laboratory, and Dr. Nick Hutson of the Air Pollution Prevention and Control Division Office of Research & Development at the U. S. Environmental Protection Agency (EPA). At the end of Year 1 and throughout Year 2 of Phase II, the promising materials will be further tested under both gasification and combustion conditions at the Energy and Environmental Research Center in Grand Forks, North Dakota under the direction of Dr. Nick Letz. Additionally, further chemical modeling studies will investigate the use of Fe as a support for nanoparticles, as an alloy, and as a monolayer. In this way a sorbent material will be designed with enhanced selectivity and capacity for TE capture.
From the results and conclusions found through this research, it is suggested that, more in-depth studies be completed for the use of Fe as a multipollutant sorbent. Using Gaussian software limits these calculations to dimers, clusters, and nanoparticles. The characteristics of the Fe sorbent in the bulk should be studied to gain a more accurate prediction of what will happen when the sorbent is used in experimental studies. Using a different software program such as Vienna Ab Initio Simulation Package (VASP) to study the bulk periodic surface chemical properties will be a very good continuation of this work. It would also be beneficial to examine how doping the Fe sorbent with other sorbent materials affects the overall reactivity of the multipollutant sorbent.
The next step in developing this sorbent for the capture of these TE species in coal combustion and gasification processes would be to fabricate the sorbent. Once the sorbent is fabricated, experimental studies will be carried out to verify the predicted trends of Phase I. In addition, during these experiments the affect of potential poisoning species can be tested. For example, sulfur is known to poison Pd and Pt catalysts and chlorine gas has been shown to affect the reactivity of Fe catalysts.5,6 These effects will be taken into account in the modeling investigations carried out in Phase II. With all of this information, TE capture from both flue and fuel gases of coal combustion and gasification, respectively, will be possible.
1MIT Report, “The Future of Coal, Options for a Carbon-Constrained World,” MIT Press (2007).
2US Department of Energy, Energy Information Administration, International Outlook 2006, DOE/EIA0484 (2006).
3S. C. Devito, and W. E. Brooks. "Mercury." Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons, Inc., 2005.
4M. Nowak, and W. Singer. "Mercury Compounds." Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons, Inc, 1995.
5N. S. Nasri, J. M. Jones, V. A. Dupont, A. Williams, A Comparative Study of Sulfur Poisoning and Regeneration of Precious-Metal Catalysts. Energy & Fuels. 12 (1998) 1130-1134.
6Arabczyk, W., Narkiewicz, U., Moszynski, D. “Chlorine as a poison of the fused iron catalyst for ammonia synthesis.” Applied Catalysis A: General. 134. 2. 331-338. 1996.