4d-3: Technologies and approaches for mitigating mercury emissions

With the USEPA issuing a national regulation requiring a minimum of 91% removal of mercury, the need exists for low-cost mercury removal techniques that can be applied to coal-burning power plants. The injection of powdered activated carbon into the ductwork upstream of the particulate control device is the most developed technology for mercury capture. Alternative techniques for mercury capture will also play a role in the near future because of the numerous configurations of air pollution control devices present within the power plants, as well as the many different coals being burned. These methods employ sorbents, catalysts, scrubber liquors, flue gas or coal additives, combustion modification, flue gas cooling, barrier discharges, and ultraviolet radiation for the removal of mercury from flue gas streams. The DOE Mercury Program has been a huge success, spurring continuing development, demonstration, and commercialization of many technologies for the capture of mercury.

An overview of current and alternative technologies for mercury capture from coal-derived flue gas will be provided. In addition, six patent/patent pending methods for mercury as well as carbon dioxide control within coal-derived flue and fuel gases have been recently developed at NETL, and will be discussed. The on-going research needs for mercury control include improved sorbent-flue gas contact, development of poison-resistant sorbents and catalysts, novel sorbent promoters, new scrubber additives for retention of mercury within wet FGD systems, concrete-friendly activated carbons, new continuous measurement methods, byproducts research, development of an ASTM standard lab test for sorbent activity for mercury capture, and exploration of international markets.

Our recently introduced passive air sampler for gaseous mercury (Hg) uses a radial diffusive barrier to control uptake kinetics and sulfur-impregnated activated carbon as a sorbent. An initial outdoor calibration in Toronto revealed highly linear uptake over a one-year period and unprecedented precision. The accuracy of this sampler depends on the extent (i) of the variability of its sampling rate SR (i.e. the volume of air stripped of mercury per unit of time) between different deployment locations and periods, and (ii) to which it will be possible to account for that variability. We addressed this issue two-fold. On the one hand, we measured uptake in passive samplers deployed for up to one year at 22 locations with ongoing active sampling for mercury around the world. The sites in Canada, USA, Australia, China, Germany and Taiwan varied widely in terms of climate (tropical to polar regions) and concentration levels. On the other hand, we quantified in the laboratory how the SR varies with tightly controlled temperature, wind speed and relative humidity. The results in either case confirm that the SR of the sampler varies only to a minor extent and the variability is predictable. When accounting for this variability, the sampler can discriminate even very small concentration differences on the order of 0.2 ng/m3. Successful early applications of the new sampler include (i) the detailed characterization of the spatial and temporal variability of Hg concentration around a major known source, which allows for long term exposure assessment of the local population and for the estimation of a Hg emission rate, (ii) the identification of unknown Hg emission sources within a large urban conglomeration, and (iii) the reliable quantification of the isotopic signature of atmospheric Hg.

Mercury, emitted from coal-fired power plant, is a serious threat to human beings. Mercury from combustion flue gas commonly displays three chemical forms: elemental (Hg0), particulate-bound (Hgp) and oxidized (Hg2+) form. Despite that the oxidized (Hg2+) and particulate-bound (Hgp) mercury can be relatively easily captured by conventional pollution control facilities, it is more challenging to remove Hg0 than other forms of mercury because of its insolubility in water and weak interaction on conventional adsorbent particles.

To suppress the emissions of the mercury in flue gas, especially for the removal of Hg0. Various novel adsorbents based on silver nanoparticles (NPs) functionalized materials, including zeolite derived from fly ash (ZFA), garphene oxide (GO), and mesoporous silica (SBA-15) were prepared by facile chemical methods. And the mercury cold vapour atomic fluorescence spectrophotometry (CVAFS) was applied to explore the mercury adsorption capacity on the adsorbents. The functionalized materials were successfully synthesized and well characterized. Ag NPs were found to be homogeneously deposited on all the substrates. All the adsorbents demonstrate excellent Hg0 adsorption capacity and fast Hg0 adsorption rate at medium and low temperature. Moreover, our adsorbents show excellent cyclic performance. Certain adsorbents could be reused for at least five successive adsorption-desorption cycles without any significant loss in adsorption performance, providing great advantages in practical applications. Our study clearly indicated that our silver NPs functionalized materials could be used as potential adsorbents for mercury emission control in coal-fired power plant.

Afro-Colombians inhabitingPacific rural regions of Colombia havebuilt their subsistence economy onfarming, fishing, and chemical-free artisanal mining. Recently, illegal, small-scale gold mining is increasing in these Pacific rural regions.Many affected rural communities suffer from public health issues and environmental degradationas a result ofirresponsible use ofmercuryamalgamate during mining.Bio-accumulation of mercury potentiallyleads todevastating public health problems onthe nervous, immune, and digestive system for affected local community members. The goal of this project was to developa low-cost portable nanosensor that was durable enough for field use in a variety of applications.To accomplish this, we have developeda laser inscribed grapheneelectrode on a biodegradable polymer for under $1 per electrode. We metalized the graphene electrode with nanometals,such ascopper and platinum andtested for detection of ionic mercury using anodic stripping voltammetry. Theresults show thequantitative limit of detection in lake water was 11.1 ppb, and we are testing other nanometal structures with the aim of measuring Hg2+ as low as 1 ppb (4.5 nM). The response time of the sensor is less than 5 min, and the range extends to 1000 ppb.We are currently comparing the nanoplatinum-graphene sensor to the nanocopper-graphene sensor to create the most effective sensor for thesecommunities.The future work aims to develop methods incorporating metal from waste and to recycle the polymer in an effort to minimize the environmental footprint of the work and extend the sensor to other communities in need of quick detection formercury. These sensors will be used to create mercury heat maps of the local region, providing critical high resolution datasets that will be used to develop public health risk analysis models.

The properties of surface oxygen-containing functional groups (SOFG) on activated carbons was investigated, and their effects on removal of gaseous mercury chloride (HgCl2) was studied. For this purpose, the surface of a coal-based commercial activated carbon BPL was modified by heating in an inert atmosphere and then impregnated with benzoic acid solution. Afterwards, all carbonaceous samples were tested for their HgCl2 adsorption capacities. Nitrogen (N2) adsorption, Fourier Transform Infrared Spectroscopy (FT-IR) and Boehm titration were applied to study the surface characteristics of carbon samples. It was found that after benzoic acid impregnation, the amounts of SOFG improved with the increasing of benzoic acid concentration, especially carboxylic and carbonyl groups. Adsorption experiments showed that higher HgCl2 adsorption capacities were obtained with more carboxyl and carbonyl groups. And the largest capacity obtained in this study was 484.7 lg/g of carbon with carboxyl and carbonyl concentrations of 0.570 mmol/g and 0.706 mmol/g, respectively. This suggests that carboxylic and carbonyl groups play predominant roles in HgCl2 removal. And carboxylic groups are believed to contribute more than that of carbonyl groups in HgCl2 capture process.

Mercury pollution from flue gas of coal-fired power plants has attracted worldwide attention in recent years, and elemental mercury (Hg0) is the most challenging form to be removed and often escapes into the atmosphere directly. In this work, a new class of regenerable magnetically responsive nanocomposites is synthesized by a novel, yet simple and robust approach for efficient Hg0 capture. The sorbents (denoted as MagS-Ag) are composed of silica film-coated magnetite Fe3O4, mesoporous silica SBA-15 and supported silver nanoparticles. Each component has a specially designed function. The silica-coated Fe3O4 makes the sorbents easily separated from fly ash for further recovery and regeneration. SBA-15 is a potential scaffold loading highly disperse and small-sized silver nanoparticles due to extremely high specific surface areas, highly uniform mesopores and favorable mass transfer performance. On the basis of well-known amalgamation mechanism, the silver nanoparticles can effectively capture Hg0 at lower temperatures and release it at higher temperatures accordingly. The physical and chemical properties of MagS-Ag sorbents were investigated by various characterization methods. In Hg0 breakthough tests, a complete Hg0 capture by MagS-Ag was achieved at 150 °C, which reflects the typical real flue gas temperature. The sorbents were found to possess a Hg0 capture capacity as high as 5.2 mg·g-1 when the Hg0 breakthrough reached only 1%, extremely higher than present Ag-based sorbents and even comparable with some modified activated carbons. In addition, MagS-Ag were able to achieve a 90.4% Hg0 removal efficiency and 226.1 μg·g-1 capacity when exposed to more stringent flue gas flow for 1 hour with a space velocity of 260,000 h-1. More importantly, the spent sorbents could be effectively regenerated and reused multiple times without any performance degradation. The excellent Hg0 removal capability in combination of facile synthesis, strong tolerance to complicated flue gas, superior thermal stability, and outstanding separation and regeneration performance makes MagS-Ag promising magnetically responsive sorbents for practical application in Hg0 capture from coal-fired flue gas.

Chinese coal-fired plants are now commonly equipped with SCR catalyst for NOx removal. Through injecting NH3, it will catalyze the NO reduce to N2. Many researchers have found the cooperative effect of SCR catalyst on Hg0 oxidation. The SCR catalyst will facilitate the Hg0 oxidized by HCl and O2 during the denitrification process. Some researchers have also established the oxidation mechanisms of Hg0 using experiment and DFT calculation. The mechanisms consistently reflect the fundamental effect of HCl on the Hg0 oxidation. However, before SCR catalyst, NH3 will be injected into the flue gas to reduce NOx. NH3 and NO will play important role in Hg0 process.

NH3 has been certified by experiment as a prohibitive factor on Hg oxidation. Both promotion and prohibition effect of NO on Hg oxidation have been reported in experiment tests. However, the most likely cooperative removal mechanisms have not been fully determined now.

In this paper, quantum chemistry calculation based on density functional theory (DFT) was applied to reveal the influence of NH3, NO on Hg0 oxidization catalyzed by V2O5/TiO2. Firstly, the related V2O5/TiO2 surface model was established. Secondly, the of reduction of NOx and NH3 was calculated. Next, The oxidization of Hg0 under SCR surface was established. Finally, reaction path was established between denitrification and Hg0 oxidization on V2O5/TiO2 catalyst process. Experimental data obtained from previous studies was used to validate some of the simulation results as well.

The emission and migration characteristics of Hg from an ultra low emission (ULE) coal-fired power plant in China was investigated. The flue gas was sampled simultaneously at the inlet and outlet of selective catalytic reduction (SCR) system, low temperature economizer (LTE), electrostatic precipitators (ESP), wet flue gas desulfurization (WFGD), and wet electrostatic precipitators (WESP) by EPA 30B method. The feed coal, lime, limestone slurry, process water, fly ash, bottom ash, gypsum, FGD effluent, and WESP effluent were also sampled. The results showed that Hg concentration in flue gas at the outlet of boilers and stacks was in the range of 4.46-5.17μg/m3 and 0.51-1.22 μg/m3, respectively. The overall gaseous Hg removal efficiencies of existing air pollution control devices (APCDs) ranged from 88.5% to 89.6%. About 41.43%-51.61% of Hg0 was oxidized across SCR systems, and most of Hgp and Hg2+ was captured by ESP and WFGD, respectively. The LTE could significantly affect the distribution and speciation of Hg. The WESP could result in the oxidation of Hg0, which could enhance the removal of Hg. Mass distribution of Hg in the whole system showed that about 70% of Hg present in solid and liquid combustion products and 30% of Hg emitted into atmosphere. The atmospheric emission factor of the power plant before ULE reformation is in the range of 2.18-2.34 g/TJ, which are decreased dramatically to 0.39-0.81 g/TJ after ULE reformation. Thus, the ULE reformation for coal-fired plants is beneficial for the reduction of Hg emission to atmosphere.